CHAPTER 1

1.1 Objective of present study:

The present work is an endeavor towards analyzing the different multi-pulse AC

to DC converters in solving the harmonic problem in a three-phase converter system. The

effect of increasing the number of pulses on the performance of AC to DC converters is

analyzed. For performance comparison the major factor considered is the total harmonic

distortion (THD). The effect of load variations on multi-pulse AC to DC converters has

been investigated.

1.2 Organization of thesis:

Chapter 1 deals with the general aspects of power quality and techniques for elimination

of harmonics. It also introduces the objective of the present work.

Chapter 2 deals with literature survey wherein a few topologies used in multi-pulse non–

isolated converters are introduced.

Chapter 3 deals with principle of multi-pulse converters converters.

Chapter 4 presents a brief description of SIMULINK/MATLAB, the software platform

used in this work.

Chapter 5 deals with the modeling of (6, 12, 18, 24, 36 and 48) pulse configurations in

Simulink/Matlab.

Chapter 6 simulation results of proposed converters

Chapter 7 presents the interim conclusion.

1.3 Introduction

About HVDC transmission

The history of electric power transmission reveals that transmission was

originally developed with DC. However, DC power at low voltage could not be

transmitted over long distances, thus it led to the development of alternating current (AC)

electrical systems. Also the availability of transformers and improvement in ac machines

led to the greater usage of ac transmission. The advent of the mercury arc valve for high

power and voltage proved to be a vital breakthrough for High Voltage Direct Current

(HVDC) transmission. These mercury valves were the key elements in the converter

stations and the filtering was done using oil immersed components. The control was

analog and most of the operations were left to the operator. After enough experiments

conducted on mercury valves, the first HVDC line was built in 1954 with a 100 km

submarine cable with ground return between the island of Gotland and the Swedish

mainland. The development of thyristors is another milestone in the development of

HVDC technology. The first solid-state semiconductor valves were commissioned in

1970. The mercury arc valves in the primitive projects were replaced by thyristor valves.

The semiconductor devices like thyristors, IGBTs and GTOs, in conjunction with

microcomputers and digital signal processors have proved to be very effective compared

to older mercury valves. The wider usage of semiconductor technology in present day

HVDC systems has initiated great leaps in the research of power electronics. With

increased demand for high quality power, application of power electronics in the field of

power distribution and transmission systems is attracting wide attention throughout the

world.

Why HVDC?

There are many different reasons as to why HVDC is to chosen instead of ac

transmission. A few of them are listed below. Cost effective HVDC transmission requires

only two conductors compared to the three wire ac transmission system. One-third less

wire is used, thus readily reducing the cost of the conductors. This corresponds to

reduced tower and insulation cost, thereby resulting in cheaper construction. However,

the ac converters stations involve high cost for installation; thus the earlier advantage is

offset by the increase in cost. If the transmission distance is long, a break-even

distance is reached above which total cost of HVDC transmission is less than the ac.

Asynchronous tie HVDC transmission has the ability to connect ac systems of different

frequencies. Thus it can be used for intercontinental asynchronous ties. For example, in

Japan HVDC could be used to connect an ac system operating at 60 Hz with one

operating at 50 Hz. Lower line losses similar to ac transmission, HVDC transmission has

I2R losses too. However, for the same amount of power transfer, DC losses are less due

to the lower resistance of the conductors because of only two-thirds of the conductor

length. The main losses are converter losses that offer better stability and control ensures

low environmental impact and reduces construction time.

1.4 HVDC constraints

Even though HVDC has many advantages, the whole power system cannot be

made DC, because of the fact that generation and distribution of power is ac. So HVDC

technology is restricted to transmission. As no system is perfect, even HVDC

transmission has some disadvantages and drawbacks. A few of them are listed below,

Converter station costs the power electronic converters involve high installation and

maintenance costs. This expenditure offsets the cost savings mentioned as one of the

advantages; for this reason, short overhead HVDC lines are more expensive compared to

ac. Reactive power requirement both the rectifier and inverter in converter stations

consume large amounts of reactive power (VARs). Even though the capacitors used in

the converters supply reactive power to some extent, the rest should be supplied by

additional capacitors or taken from the ac system. Harmonic converters at both ends of an

HVDC system inject a certain amount of harmonics into the ac system.

These harmonics may cause interference to the nearby telecommunication

network and hence need to be filtered. The harmonic frequencies can be suppressed using

capacitors and reactors; however, these increase the cost and complexity of the system,

difficulty in maintenance unlike ac, there are no zero magnitude points in DC

transmission, since the voltage stays constant. The zero crossings help to extinguish the

arc within the breaker when contacts are separated, however in DC transmission; the

voltage stays at a constant level. Faults on the DC line are handled by blocking the

faulted pole and blocking the pole is the same as shutting it off. Thus maintenance of the

lines is difficult and a transmission grid is not practically feasible [1].

1.5 Basic HVDC system configurations

There are many different configurations of HVDC based on the cost and

operational requirements. Five basic configurations are shown in Fig. 1-1. The back-to

back interconnection has two converters on the same site and there is no transmission

line. This type of connection is generally designed for low ratings and is more

economical than the long distance transmission. The converters at both the ends are

identical and can be operated either in rectification or inversion mode based on the

control. The monopolar link has only one conductor and the return path is through the

earth. Generally the use of ground as the return path is restricted to prevent the

underground metallic equipment from being damaged.

Fig. 1.1.1. Back-to-back interconnection

Fig. 1.1.2. Monopolar link

Fig. 1.1.3. Bipolar link

Fig. 1.1.4. Parallel 3-terminal

Fig. 1.1.5. Series connection

Fig. 1.1. Five basic configurations of HVDC transmission

The bipolar link is the most common configuration and has two conductors or

poles. One of the conductors or pole is positive with respect to the other. The current

from the rectifier flows through the positive pole and from the inverter flows through the

negative pole. However, the return path is through the ground and hence the opposite

currents cancel each other and the ground current is practically zero. In the parallel-

connected three-terminal configuration, converters 1 and 2 operate as rectifiers and

converter 3 operates as an inverter. However, by changing the firing angle control and the

polarity of voltage, converters 1 and 2 operate as inverters and 3 as a rectifier. The series

connection, although still unused, is an attractive proposition for small taps because of

comparatively high cost of the full voltage parallel tapping alternative [1].

1.6 Components of HVDC transmission system

1.6.1 The converter station

The converter stations at each end are identical and can be operated either as an

inverter or rectifier based on the control. Hence, each converter is equipped to convert ac

to DC and vice versa. One of the main components of a converter substation is the

thyristor converter is usually housed in a valve hall. As shown in Fig. 1.2, the substation

also essentially consists of converter transformers. These transformers transform the ac

system voltage based on the DC voltage required by the converter. The secondary or DC

side of the converter transformers is connected to the converter bridges. The transformer

is placed outside the thyristor valve hall and the connection has to be made through the

hall wall.

This is accomplished in two ways:

1) With phase isolated bus bars where the bus conductors are housed within insulated bus

ducts with oil or SF6 as the insulating medium,

2) With wall bushings and these require care to avoid external or internal breakdown [1].

Filters are required on both ac and DC sides since the converters generate

harmonics. The filters are tuned based on the converter operation (6 or 12 pulse). DC

reactors are included in each pole of the converter station. These reactors assist the DC

filters in filtering harmonics and mainly smooth the DC side current ensuring continuous

mode of operation. Surge arrestors are provided across each valve in the converter bridge,

across each converter bridge and in the DC and ac switches to protect the equipment from

overvoltages.

Fig. 1.2. HVDC substation configuration

1.6.2 Converter Transformer

The arrangement of the transformer windings depends on the converter

configuration. For example the 12-pulse converter configuration can be obtained with any

of the following transformer arrangements [2]:

Six single-phase, two winding

Three single-phase, three winding

Two three-phase, two winding

Star or delta connections are chosen for different configurations. The entire winding of

the converter transformer is fully insulated, since the potentials across its connections are

determined by the combination of valves conducting at any particular instant. As a result,

the radial leakage fluxes at the end of the windings increase [2]. Because the converter

transformer impedance determines the fault current across each valve, the converter

transformer’s leakage reactance is larger than that of the conventional one. A tap changer

is most critical in HVDC as it reduces the reactive power requirement and the tap-change

range varies from scheme to scheme.

1.6.3 Converter

The role of power electronics in power systems has become highly significant

and had power electronics not been developed, utility applications like HVDC and

flexible ac transmission systems (FACTs) would not be possible at all. The increasing

demand in the quality of power systems necessitates further development of power

electronics, which in turn induces more research in power electronics itself. The

integration of semiconductor devices into the power system has brought improvement in

the system level performance in terms of better voltage control, stability, power quality,

reliability and efficiency.

Converters form the core of the substation and the entire operation depends on the

performance of the converters. Hence, the choice of the semiconductor power device

used in the converter is vital and care should be taken in designing the circuitry. For

HVDC applications, the thyristor has been the choice of device ever since it was invented

in the 1960’s. However, devices like IGBTs and GTOs have been developed and are

being studied for use in HVDC.

Thyristors replaced the mercury arc valves and more predictable performance,

reduced maintenance and no aging were realized. However, it was not available for high

blocking voltages and current ratings required for HVDC applications. The solution was

a series connection of thyristors and this series connection along with the protective and

triggering circuitry is known as a thyristor level. The thyristor level forms the basic

building block of a thyristor valve.

A high voltage thyristor valve is a modular composition of single components in a

series string. The module consists of several components and subsystems such as

Thyristors

Voltage grading and damping circuits

Cooling system

Mechanical and insulating structure

Almost all the HVDC systems to date use line commutated thyristors made from

high purity, mono crystalline silicon. For higher current ratings, the thyristors are

connected in parallel and for higher voltage ratings thyristors are connected in series.

Over the past few decades more sophisticated technologies were developed and the

device ratings were pushed to higher limits. In the last few years silicon carbide has

emerged as a promising material for improved semiconductor devices. The use of Sic is

restricted by the material defects and immature technology; however, in the long term,

thyristors with a blocking voltage of several tens of kilovolts may be feasible. Apart from

voltage and current rating, the control of the thyristor is important. Gate pulse generation

is important for it determines the working of the thyristor and accuracy is a key factor as

it may affect the performance of the whole system.

All thyristors require a snubber circuit connected in parallel to dampen the

voltage overshoot at turn off; this circuit also serves as a means to linearize the voltage

distribution along the series string. Various types of circuits have been suggested in the

past; however, a simple RC connection has evolved as the industry standard.

The major challenge is to find suitable components that support the high voltage

withstand capability of modern thyristors and handle the power losses. A combination of

components would be an immediate solution to this, but this leads to an increase in the

number of components and the thyristor valve would become more susceptible to failure.

So a resistor and a capacitor per thyristor is more safe and efficient. To protect the

thyristors from the high inrush currents when the snubber circuits and external stray

capacitances are discharged at turn on, a nonlinear reactor is connected in series with the

thyristors. The heat losses generated in thyristors, snubber resistors and nonlinear reactors

have a magnitude that requires forced cooling. Deionized water has evolved as the

standard cooling medium because of its superior characteristics. In order to avoid

electrolytic corrosion of metallic parts in the circuit, the cooling circuit is designed such

that the metallic components are made independent of the leakage currents caused by

high voltage stress.

The various components included in a high voltage thyristor valve need to be

mechanically arranged in an insulating structure. In order to avoid damage due to seismic

stresses, suspended design is widely used, especially for high rating HVDC where the

structures are tall. The insulating material used is flame retardant to avoid the risk of fire

due to high voltage across the thyristor valves.

1.6.4 Smoothing Reactors

The main purpose of a smoothing reactor is to reduce the rate of rise of the direct current

following disturbances on either side of the converter [2]. Thus the peak current during

the DC line short circuits and ac commutation failure is limited. The reactor blocks the

non- harmonic frequencies from being transferred between two ac systems and also

reduces the harmonics in the DC line.

1.6.5 AC Filters

Filters are used to control the harmonics in the network. The reactive power consumed by

the converters at both the ends is compensated by the filter banks. For example, in CCC

(capacitor commutated converter) reactive power is compensated by the series capacitors

installed between the converter transformer and the thyristor valves.

1.6.6 DC Filters

The harmonics created by the converter can cause disturbances in

telecommunication systems and specially designed DC filters are used in order to reduce

the disturbances. Generally, filters are not used for submarine or underground cable

transmission but used when HVDC has an overhead line or if it is part of an

interconnecting system. The modern filters are active DC filters and these filters use

power electronics for measuring, inverting and re-injecting the harmonics, thus providing

effective filtering.

1.6.7 Transmission medium

HVDC cables are generally used for submarine transmission and overheads lines

are used for bulk power transmission over the land. The most common types of cables are

solid and the oil-filled ones. The development of new power cable technologies has

accelerated in recent years and the latest HVDC cable available is made of extruded

polyethylene.

1.7 HVDC technology

The fundamental process that occurs in an HVDC is the conversion of electrical

current from ac to DC (rectifier) at the transmitting end and from DC to ac at the

receiving end. There are different ways of achieving conversion with different converter

configurations.

1.8 Selection of Converter Configuration

1.8.1 Introduction

A DC system can be operated with constant voltage or with constant current.

However, it would be a disadvantage to use a constant current system in terms of

additional components required because the supply is taken from a constant alternating

system. The process of instantaneous matching of ac and DC side voltages and currents is

a basic consideration for power conversion. If an impedance free ac network is connected

to a DC network, as the DC voltage is constant, time varying ac voltages will cause

infinite current-level transients. The devices used for switching are capable of matching

the mean values of two voltages and not instantaneous values. Hence, series impedance

should be added to the network so that there would be voltage differences. Now this

impedance can be connected in two different ways and there is again a choice to be made.

As seen in the fig. 1.3, a series impedance Z is connected on the DC side. The reactance

is large enough to make the current coming out of the converter direct current. This DC

current flows as a result of either of the transformer phases that are connected and the

transformer transfers the currents simultaneously to the primary phases of the ac side.

The currents on ac side are directly proportional to the direct currents. This arrangement

is called a current converter.

The proportionality of the fundamental ac side current IN1 to direct current I is given as,

1.732* In1 / I = k (1.1)

The power on DC and ac side is

P= DI= 1.732* Un * In1 *cos (Φ) (1.2)

Where

Un is the rms value of the alternating line-line voltage,

D is the direct voltage,

I is the direct current,

In1 is the fundamental current, Φ is the angle between In1 and Un and cos (Φ) is the

power factor.

From (1.1) and (1.2), it is seen that

D = k * Un *Cos (Φ)

So for a given transformer ratio, the current converter thus has a definite ratio k between

the currents on ac and DC sides and the voltage is dependent on the power factor.

Fig. 1.3. Current converter

Fig. 1.4. Voltage converter

The voltage can still be regulated using the devices and the current ratio remains

unaltered. The converter configuration as shown in fig.1.4, has an impedance Z

connected across each phase of the ac network. These impedances along with the

transformer, control the voltage through the converter and hence the voltage on DC side.

This voltage is transferred across the converter onto the ac side. This type of arrangement

is called a voltage converter.

The fundamental voltage UN1 is proportional to the direct voltage and differs in

phase angle from the network voltage by an angle d [4]. Thus

UN1 /D = k……………………………. (1.3) and

P=DI = (UN * UN1 /X) * sin (d)……….(1.4)

From (1.3) and (1.4) it is seen that

I = k * UN /X *sin (d)………………… (1.5)

X is the reactance on the ac side. For a given transformer ratio the voltage

converter thus has a definite ratio k between the voltages on ac and DC sides and the

current is dependent on the network voltage and the phase angle between the fundamental

components of the voltages on either side of the reactance. The switches can control the

voltages and thus the phase angle and the direct current can be determined. Again there

are three different combinations of voltage and current source converters.

a) Voltage source converters on both ends

b) Voltage source converter on one end and current source on other end

c) Current source converters on both ends.

There are different configurations for the converters used in HVDC and the

conversion process can be done using the following:

Natural Commutated Converters: These are most used in the HVDC systems as of

today. The component that enables this conversion process is the thyristor. The high

voltage for HVDC is realized by connecting the thyristors in series and these form a

thyristor valve. The DC voltage of the bridge is varied by controlling the firing angle of

the thyristor and operated at system frequency (50 Hz or 60 Hz). The control is very rapid

and efficient using natural commutated converters.

Capacitor Commutated Converters: The capacitors are connected in series between

the converter transformer and the thyristor valves. These capacitors prevent the

converters from commutation failure, especially when connected to weak networks. The

more rigid the ac network the less likely will be commutation failures.

Forced Commutated Converters: This type of converters is advantageous in many

ways. For the control of active and reactive power, high power quality etc., the

semiconductor devices used in these converters has the ability to both turn-on and turn-

off. GTO and IGBT are the normally used devices. These types of converters are also

known as voltage source converters (VSC). The operation of the converter is through

PWM and hence changing the PWM pattern can create any amplitude and phase. Since

independent control of both active and reactive power is achieved, VSC is viewed as a

motor or a generator controlling the power transfer in a transmission network.

1.8.2 Converter Operation

The six-pulse converter bridge shown in the fig.1.5 is used as the basic converter

unit of HVDC transmission rectification where electric power flows from the ac side to

the DC side and inversion where the power flow is vice versa. Thyristor valves conduct

current on receiving a gate pulse in the forward biased mode. The thyristor has

unidirectional current conduction control and can be turned off only if the current goes to

zero in the reverse bias. This process is known as line commutation. Inadvertent turn-on

of a thyristor valve may occur once its conducting current falls to zero when it is reverse

biased and the gate pulse is removed. Too rapid an increase in the magnitude of the

forward biased voltage will cause the thyristor to inadvertently turn on and conduct [1].

The design of the thyristor valve and converter bridge must ensure such a condition is

avoided for useful inverter operation.

Fig. 1.5. HVDC operation ([1])

Commutation:

Commutation is the process of transfer of current between any two-converter

valves with both valves carrying current simultaneously during this process [1]. HVDC

converters operate through line or natural commutation process both for rectification and

inversion. The converter operation is defined by the voltage crossings of the ac network

connected at both the ends. The ac network connected should be relatively free of

harmonics. The commutation (transfer of current) takes place when one valve starts

conducting and the current in the other valve begins to fall to zero. The valve starts

conducting only when its forward biased voltage becomes more positive than the forward

bias voltage of the other conducting valve and on receiving a gate pulse. As no system is

ideal, the impedance of the system is not zero and during commutation the current does

not change instantaneously from one valve to another due to the reactance of the system.

The leakage reactance of the transformer windings is also the commutation

reactance as long as the ac filters are located on the primary or ac side of the converter

transformer [1]. The equivalent reactance at the rectifier and inverter is known as the

commutation reactance, Xc. In a practical HVDC transmission system, this commutation

reactance accounts for sub transient reactance of the generator and motors and the

primary, secondary and tertiary leakage reactance of the transformers. The DC reactor

and converter transformer make the DC current smooth and flat. The principle of

operation for both the converters at both the ends is the same; however, the firing angle is

varied for rectification and inversion. If the firing angle is greater than 90 degrees the

converter acts as an inverter and if it is less than 90 degrees it acts a rectifier. Ivr and Ivi

are the non- sinusoidal currents at rectifier and inverter ends respectively and both are

lagging currents.

The higher order harmonics of these currents are filtered and hence the voltages

Ulr and Uli are relatively free from harmonics. Since the thyristors are unidirectional,

power flow reversal is not possible by reversing the direction of current. So, power

reversal is achieved by changing the polarity of the DC voltage.

Converter Bridge Angles: The electrical angles, which describe the converter

bridge operation, are shown in fig. 1.5. Both the converters have these angles, which are

measured in the steady state conditions. These are defined in [1] as:

Delay angle alpha (α): The time expressed in electrical angular measure from the zero

crossing of the idealized sinusoidal commutating voltage to the starting instant of forward

current conduction. This angle is controlled by the gate firing pulse and if less than 90

degrees, the converter bridge is a rectifier and if greater than 90 degrees, it is an inverter.

This angle is often referred to as the firing angle.

Advance angle beta (β): The time expressed in electrical angular measure from the

starting instant of forward current conduction to the next zero crossing of the idealized

sinusoidal commutating voltage. The angle of advance β is related in degrees to the angle

of delay ‘α’ by:

β = 180-α

Overlap angle (µ): The duration of commutation between two converter valves

expressed in electrical angular measure.

Extinction angle gamma (γ): The time expressed in electrical angular measure from

the end of current conduction to the next zero crossing of the idealized sinusoidal

commutating voltage. Gamma (γ) depends on the angle of advance β and the angle of

overlap µ and is determined by the relation

γ=β–µ

1.8.3 Control and Protection

HVDC transmission systems involve (must transport) very large amounts of

electric power and the desired power transfer is achieved by precisely controlled DC

current and voltage across the system. Also in DC transmission the power-flow direction

is determined by the relative voltage magnitudes at the converter terminals which can be

controlled by adopting a firing-angle control scheme. Therefore it is very important and

necessary to continuously and precisely measure system quantities which include at each

converter bridge, the DC current, its DC side voltage the delay angle α and for an

inverter, its extinction angle γ. Each converter station is assumed to be provided with

constant current and constant extinction-angle controls for equidistant firing angle

control. The choice of assigning current control either to the rectifier or to inverter

station, is made considering the investment cost for reactive- power compensation,

minimization of the losses and total running cost. Normally, the line utilization is the best

with minimum reactive-power compensation if the inverter operates on minimum

extinction-angle control while the rectifier operates on constant-current control. The

inverter station maintains a constant extinction angle γ which causes the DC voltage Ud

to droop with increasing DC current Id, as shown in the minimum constant extinction

angle γ characteristic A-B-C-D in fig. 1.5 [1]. If the inverter is operating in a minimum

constant γ or constant Ud characteristic, then the rectifier must control the DC current

Id. This it can do as long as the delay angle α is not at its minimum limit (usually 5

degrees). The steady state constant current characteristic of the rectifier is shown in fig.

1.5 as the vertical section Q-C-H-R. Where the rectifier and inverter characteristic

intersect, either at points C or H, is the operating point of the HVDC system the operating

point is reached by action of the on-line tap changers of the converter transformers. The

inverter must establish the DC voltage Ud, by adjusting its on-line tap changer, to

achieve the desired operating level if it is in constant minimum γ control. If in constant

Ud control, the on-line tap changer must adjust its tap to allow the controlled level of Ud

be achieved with an extinction angle equal to or slightly larger than its minimum setting

of 18 degrees in this case. The on-line tap changers on the converter transformers of the

rectifier are controlled to adjust their tap settings to minimize the reactive-power

consumption subject to a minimum γ limit for maintaining the constant current setting

first (see fig. 1.5).

At the inverter end constant extinction angle minimizes the reactive power and

hence, the tap changer will provide the DC voltage control. During some disturbances,

like ac-system faults, the ac voltage at the rectifier or inverter is depressed and a sag in ac

voltage at either end will result in a lowered DC voltage too. If the disturbance is large,

the converter may not be capable of recovering by itself and it becomes important to

reduce the stress on the converter valves. This is achieved by the controller, which

reduces the maximum current order and is known as a voltage dependent current order

limits (VDCOL). The VDCOL control will keep the DC current Id to the lowered limit

during recovery and only when DC voltage Ud has recovered sufficiently, will the DC

current return to its original 1st order level. There are a number of special purpose

controllers, which can be added to HVDC controls to take advantage of the fast response

of a DC link and help the performance of the ac system. These include ac system

damping controls, ac system frequency control, step change power adjustment, sub

synchronous oscillation damping and ac under voltage compensation.

Power electronic devices are non-linear loads that create harmonic distortion and

can be susceptible to voltage dips if not adequately protected. The most common

economically damaging’ power quality problem encountered involves the use of variable-

speed drives. Variable-speed motor drives or inverters are highly susceptible to voltage

dip disturbances and cause particular problems in industrial processes where loss of

mechanical synchronism is an issue.

THREE-PHASE ac–DC conversion of electric power is widely employed in

adjustable-speeds drive (ASD), uninterruptible power supplies (UPS), HVDC systems

and utility interfaces with non conventional energy sources such as solar photovoltaic

systems (PV), etc., battery energy storage systems (BESS), in process technology such as

electroplating, welding units, etc., battery charging for electric vehicles and power

supplies for telecommunication systems. Traditionally, AC–DC converters, which are

also known as rectifiers are developed using diodes and thyristors to provide controlled

and uncontrolled unidirectional and bidirectional DC power. They have the problems of

poor power quality in terms of injected current harmonics, resultant voltage distortion

and poor power factor at input ac mains and slowly varying rippled DC output at load

end, low efficiency and large size of ac and DC filters.

It is well known that undesirable harmonic line currents may be generated during

a transformer-rectifier combination. The rectification of AC power to DC power itself

may in general produce undesirable current harmonics. These non-linear loads cause

severe current harmonics that may not be tolerated by either a shutdown of the device or

unacceptable powering of the devices.

The great majority of power electronic equipment operates from an ac source but

with an intermediate DC link. Thus a significant opportunity exists to facilitate power

electronics applications by using ac to DC rectifiers that produce low harmonic current in

the ac source. Multi-pulse converters in general and non-isolated multi-pulse converters

in particular can be applied to achieve clean power which is of major interest in higher

power ratings. In general by increasing the number of pulses in multi-pulse converters

THD (total harmonic distortion) and other associated performance parameters can be

enhanced.

1.9 STATE OF THE ART:

A large number of publications have appeared in the field of isolated and non-

isolated multi-pulse converters and multilevel rectifiers many giving new concepts.

In general improvement of power factor and current THD on the AC mains and constant

and regulated DC output voltage on load side can be achieved by

(a) New converter topologies at lower and medium power levels

(i) Multi-pulse connections of converters with or without isolation

(ii) Multi-level connections especially cascaded converters

(b)    With the existing topologies at higher rating by retro-fitting of

(i)  Active filters- shunt or series

(ii)    Passive tuned filters- shunt or series

(iii)  Hybrid filters – a combination of active and passive filters

CHAPTER 2

LITERATURE SURVEY

2.1 General

Large amount of harmonics, poor power factor and high total harmonic

distortion (THD) in the utility interface are common problems when non-linear loads

such as adjustable speed drives, power supplies, induction heating systems, UPS systems

and SMPS are connected to the electric utility. In several cases, the interface to the

electric utility is processed with three-phase uncontrolled diode bridge rectifier. Due to

the nonlinear nature of load, the input line currents have significant harmonics.

Several techniques adopted for elimination of harmonics are conventional

isolated multi-pulse AC-DC converters and non isolated AC-DC converters with phase

shifting transformers.

The increasing use of power electronic based loads (adjustable Speed drives,

Switch mode power supplies, etc) to improve system efficiency and Controllability is

increasing concern for harmonic distortion levels in end use facilities and on overall

power system. The application of passive tuned filters creates new system resonances

which are dependent on specific system conditions. In addition, passive filters often need

to be significantly overrated to account for possible harmonic absorption from power

system. Passive filter ratings must be coordinated with reactive power requirements of

the loads and it is often difficult to design the filter to avoid leading power factor

Operation for some load conditions. Active filters have the advantage of being able to

compensate for harmonics without fundamental frequency reactive power concerns. This

means the rating of active filter will not introduce system resonances that can move a

harmonic problem from one frequency to another.

The active filter concept uses power electronics to produce harmonic current

components that cancel the harmonic current components from the non-linier loads. The

active filter uses power electronic switching to generate harmonic currents that cancel

harmonic currents from a non-linear load. The active filter configuration investigated in

this project is based on the pulse-width modulated (PWM) voltage source inverter that

interfaces to the system interface filter. In this configuration the filter is connected in

parallel or shunt filter for harmonic current cancellation so that the current being supplied

from the source is sinusoidal. Thus the basic principle of shunt active filter is that it

generates a current equal and opposite in Polarity to the harmonic current drawn by the

load and injects it to the point of coupling there by forcing the source current to be pure

sinusoidal.

A number of low-power electronic based appliances such as TV sets, personal

computers and adjustable speed heat pumps generate a large amount of harmonic current

in power systems even though a single low power electronic based appliance, in

which a single-phase diode rectifier with a DC link capacitor is used as utility interface,

produces a negligible amount of harmonic current. Three-phase diode or thyristor

rectifiers and cycloconverters for industry applications. Also generate a large amount of

harmonic current. Voltage distortion or harmonics resulting from current harmonics

produced by power electronic equipment has become a serious problem to be solved in

many countries. Power system harmonics are not a new problem. Due to the widespread

proliferation of nonlinear distorting loads such as power-electronic controlled devices,

the problems caused by harmonics are of increasing importance. Unlike the conventional

load, the power-electronic device controls the flow of power by chopping. Flattening or

shaping the waveforms of the voltage and current. Therefore, harmonics are generated

during the process. These waveform distortions can cause problems for neighboring loads

and they tend to have an overall opposite effect on the quality of electric power. a

concept that can improve the power quality is the active power filter. This type of filters

can meet diverse load conditions. In addition to improve power factor, it also appears to

be an attractive and viable method for reducing voltage and current harmonic distortion

or other power quality problems such as flicker. The active power filter improves the

system power quality by injecting equal-but opposite currents to compensate harmonic

distortion and reactive power. Ideally this active power filter should monitor and

minimize voltage and went distortion of its connected load. In the past some active power

filters were designed based on the conventional IRP theory However, the IRP theory-

based active filter can not compensate the harmonic distortion and does not function

properly. In order to improve the drawbacks of the conventional IRP theory, a new

instantaneous power theory-based algorithm is proposed for the control strategy of the

active filter. Also, for verifying the performance of this algorithm, computer simulations

and experiment are made. From the simulation and experimental test results, it is found

that proposed new instantaneous power theory-based three-phase active power filter is to

be an effective device to reduce harmonic current and to compensate reactive power.

2.2 Power quality problem

The power quality of power supply of an ideal power system means to supply

electric energy with perfect sinusoidal wave form at a constant frequency of a specified

voltage with least amount of disturbances. However the harmonic is one of the major

factor due to which none of condition is fulfilled in practice. The presence of harmonics

disturbs the waveform shape of voltage and current and increases the current level and

changes the power factor of supply and which in turn creates so many problems.

2.3 What are harmonics?

The electricity is produced and distributed in its fundamental form as 50 Hz in

India. A harmonics is defined as the content of signal whose frequency is integral

multiple of the system fundamental frequency. Due to harmonic effect the sinusoidal

wave form is no longer have stand and it become non-sinusoidal or complex wave form.

The complex waveform consists of a fundamental wave of 50 Hz and a number of other

sinusoidal waves whose frequencies are integral multiple of fundamental wave like

2f(100hz), 3f (150 Hz), 4f (200 Hz) etc. Wave having frequency of 2f, 4f, 6f etc are

called the even harmonics and those having frequency of 3f, 5f, 7f etc are called as odd

harmonics. When fundamental frequency is super imposed with high-level harmonics it

results into complex wave and which is non sinusoidal.

2.4 Causes of production of harmonics

There are many cases which are responsible for production of harmonic effect in

power supply system, few of them listed below:

More use of solid-state power converters for industrial drivers.

Use of arc and induction furnaces for steel and non-ferrous plants.

Use of thyristor controlled locomotives.

Use of electronic loads in domestic sectors.

Use of energy conservation devices in both domestic and industrial sectors,

e.g. electronic chokes for florescent light, electronic controllers for motors.

The operation of transformers closure to saturation region for magnetizing curve.

Non-sinusoidal air gap flux in synchronous machines.

Magnetizing current of saturated reactors.

2.5 Effects of Harmonics on Electrical equipments

Few cases in that how electrical equipments and circuits affect due to presence of

harmonics in power supply system.

When complex voltage is applied across circuit containing both inductance and

capacitance, it may happen that circuit resonate at one of the harmonic frequencies of

applied voltage. If it is a series circuit large current will be produced at resonance,

even though the applied voltage due to harmonic may be small. If it is a parallel

circuit then at resonant frequency the resultant current drawn from the supply would

be minimum.

2.5.1 Effect on rotating machines

Pulsating torque may be produces in rotating machines.

Extra audible noise may produce.

The losses in machine increase which result into over heating of motor windings and

reduction in motor’s life.

2.5.2 Effects on power system

Sudden increase in demand reduced capacity of utilization and increased energy

losses.

Increase in neutral current over loading of diesel generator sets, fire hazards due to

burning of over heated cables.

Frequent change due to switchgears and controls.

Amplification of harmonic current in capacitor banks and frequent failure of

Capacitors.

Inaccurate and excess recording by energy meters.

Interface with communication equipments.

2.6 Review of Multi-Pulse Converters

A large number of publications have appeared in the field of multi-pulse

converters, many giving new concepts and verifying their claims by simulations and

experimental work. Paice [1] proposed maximizing the efficiency of a 12 pulse AC-DC

converter based on a hexagonal autotransformer arrangement. Choi [2] in his paper has

presented new autotransformer arrangements with reduced KVA capacities are presented

for harmonic current reduction and to improve AC power quality of high current DC

power supplies. Simulation results are given in the paper. Falcondes and Babri[3] has

proposed a new isolated high power factor 12 KW power supply based on 18-pulse

transformer arrangement .the topology used involves a simple control

strategy .simulations and experimental results are given in paper.

S.Kim Etal [4] has given an analysis and design of a passive and novel

interconnection of a star/delta transformer approach to improve power factor and reduce

harmonics generated by a three phase diode rectifier. Chen Etal [5] has proposed a new

passive 28-step current shaper for three phase rectification .with a phase shifting

transformer on the ac side, per phase input current is shaped into sinusoidal waveform.

Tolbert [6] his work provides the cascade inverter for large automotive drives. Here back

to back diode clamped converter is used, simulation and experimental results are given in

paper. This chapter presented a review of available literature on power quality

improvement pertaining to AC/DC converters. The next chapter presents a detailed study

of multi-pulse converters.

CHAPTER 3

MULTI-PULSE CONVERTERS

As it has been mentioned earlier, there are several techniques primarily adopted

for the mitigation of harmonics in a 3-phase converter and multi-pulse converters fall in

the same category of remedial measures. This technique is discussed, in detail, in the

present chapter.

3.1 Introduction

As it has been emphasized already, AC/DC converters in various drive and other

industrial applications are the root cause for power quality problems. As the research in

high energy physics progresses and as the particle accelerators find many applications in

industrial and medical areas, power supplies with integrated magnetics featuring high

input power quality and better performance are increasing in demand. In non isolated

multi-pulse converters, the windings are interconnected such that the kVA transmitted by

the actual magnetic coupling is only a portion of total kVA. The reduction in kVA rating

of the transformer and a new method to improve the quality of AC input currents by

introducing taps on the interphase reactor has also been proposed in the literature.

3.2 Multi-Pulse methods

The term multi-pulse method is not defined precisely. In principle, it could be

imagined to be simply more than one pulse. However, by proper usage in the power

electronics industry, it has come to mean converters operating in a three phase system

providing more than six pulse of DC per cycle.

Multi-pulse methods involve multiple converters connected so that the harmonics

generated by one converter are cancelled by harmonics produced by other converters. By

this means, certain harmonics related to number of converters are eliminated from the

power source. In multi-pulse converters, it is assumed that the DC link uses a filter such

that any ripple caused by the DC load does not significantly affect the DC current.

Multi-pulse systems result in two major accomplishments namely,

1. Reduction of ac input line current harmonics.

2. Reduction of DC output voltage ripple.

Reduction of ac input line current harmonics is important as regards the impact the

converter has on the power system.

Multi-pulse methods are characterized by the use of multiple converters or multiple

semiconductor devices with a common load.

Phase shifting transformers are an essential ingredient and provide the mechanism for

cancellation of harmonic current pairs, e.g. the 5th and 7th harmonics or the 11th and 13th so

on. Thus for harmonic current reduction the multi-pulse converters are fed from phase

shifting transformers. The phase shift has to be appropriate.

3.3 Zig-Zag Phase shifting transformers

The Zigzag Phase-Shifting Transformer implements a three-phase transformer

with a primary winding connected in a zigzag configuration and a configurable secondary

winding. The model uses three single-phase, three- winding transformers. The primary

winding connects the windings 1 and 2 of the single-phase transformers in a zigzag

configuration. The secondary winding uses the windings 3 of the single phase

transformers and they can be connected in one of the following ways: Y with accessible

neutral Grounded Y Delta (D1), delta lagging Y by 30 degrees Delta (D11), delta leading

Y by 30 degrees. If the secondary winding is connected in Y, the secondary phase

voltages are leading or lagging the primary voltages by the Phi phase angle specified in

the parameters of the block. If the secondary winding is connected in delta (D11), an

additional phase shift of +30 degrees is added to the phase angle. If the secondary

winding is connected in delta (D1), a phase shift of -30 degrees is added to the phase

angle.

3.4 Conclusion:

This chapter presented the intricacies of multi-pulse converters and the

advantages of using phase shifting auto-transformers for providing phase-shifted supplies

to these converters.

CHAPTER 4

SIMULINK/MATLAB AS A TOOL FOR SIMULATION

4.1 Introduction

Simulation is a tool for the understanding of many complex problems. Several

digital simulation packages are commercially available. This chapter presents a

comparison of the salient features of various simulation tools available to model the

electrical drive systems in a digital computer such as PSIM, CASPOC, PSPICE, SABER,

SIMPLORER and SIMULINK/MATLAB.

PSPICE is mainly meant for the simulation of electronic circuits. Modeling of

machines especially with a feedback control loop becomes very difficult in this package.

PSIM and CASPOC take very little time to learn but the micro-modeling of devices is not

possible in this package due to which the accuracy of results is quite limited. SABER and

SIMPLORER are exclusively meant for power electronic and drive system simulations

and they are user-friendly as well. But both these packages are extremely expensive.

SIMULINK/MATLAB is a general-purpose simulation tool with several tool-boxes

embedded in it to enable modeling of complicated control schemes as well. The power

system block set has specifically a large number of components conforming to the needs

of an electrical power engineer.

4.2 MATLAB

The name MATLAB stands for matrix laboratory. Originally it was meant for

providing easy access to the matrix manipulations. Over the years, it has developed into a

tool for high productivity analysis, research and development. MATLAB allows the user

to focus on his technical work and applications rather than on programming details.

MATLAB provides a user-friendly environment to integrate the computation,

visualization and programming. The problems and solutions are expressed in

mathematical notations. MATLAB is an interactive system. The basic data element is an

array, which does not require dimensioning. Thus the technical computing problems, with

matrix and vector formulations, are solved very quickly in MATLAB environment.

MATLAB also provides an extensive library of predefined functions. The advantages of

MATLAB for technical programming are:

Ease of use.

It is supported on many different computer systems. Hence it has platform

independence.

It has an extensive library of predefined functions which make the job easier.

Device independent plotting. MATLAB has many integral plotting and imaging

commands.

4.3 SIMULINK

SIMULINK is a tool-box in MATLAB software that can be used for modeling,

simulating and analyzing dynamical systems. It supports linear and nonlinear systems,

modeled in Continuous time, sampled time or a hybrid of the two. Systems can also

bemultirate, i.e., have different parts that are sampled or updated at differentiates. For

modeling SIMULINK provides a graphical user interface (GUI) for building models as

block diagrams, using click-and-drag mouse operations. With this interface, we can draw

the models just as we would on paper. This is accomplished through the SIMULINK

block library of sinks, sources, linear and nonlinear components and connectors.

MATLAB ODE solver functions implement numerical integration. In this

package, the ode45 solvers used for a non stiff problem and the ode15s solver for a stiff

problem. In a “stiff” problem, solutions can change on a time scale that is very short

compared to the interval of integration.

4.4 Blocks used for simulation

4.4.1 AC Voltage Source

The AC Voltage Source block implements an ideal AC voltage source. Negative

values are allowed for amplitude and phase. A zero frequency specifies a DC voltage

source. Negative frequency is not allowed; otherwise Simulink signals an error and the

block displays a question mark in the block icon.

Parameters

Peak amplitude: The peak amplitude of the generated voltage, in volts (V).

Phase: The phase in degrees (deg).

Frequency: The source frequency in hertz (Hz).

Sample time: The sample period in seconds (s). The default is 0, corresponding to a

continuous source.

Measurements: Select voltage to measure the voltage across the terminals of the AC

voltage source block.

Ground:

The Ground block implements a connection to the ground.

4.4.2 Linear Transformer

The Linear Transformer block model shown consists of three coupled windings wound

on the same core.

Parameters

Nominal power and frequency: The nominal power rating Pn in volt-amperes (VA) and

frequency fn, in hertz (Hz), of the transformer.

Winding 1 parameters: The nominal voltage V, in volts RMS, resistance and leakage

inductance in p.u. The p.u. values are based on the nominal power Pn and on V1.

Winding 2 parameters: The nominal voltage V2 in volts RMS, resistance and leakage

inductance in p.u. The p.u. values are based on the nominal power Pn and on V2.

Three winding transformer: If selected, implements a linear transformer with three

windings; otherwise, it implements a two-winding transformer.

Winding 3 parameters: The Winding 3 parameters parameter is not available if the three

windings transformer parameter is not selected. The nominal voltage in volts RMS

(Vrms), resistance and leakage inductance in p.u. The p.u. values are based on the

nominal power Pn and on V3.

Magnetization resistance and reactance: The resistance and inductance simulating the

core active and reactive losses, both in p.u. The p.u. values are based on the nominal

power Pn and on V1. For example, to specify 0.2% of active and reactive core losses, at

nominal voltage, use Rm = 500 p.u. and Lm = 500 p.u.

Measurements: Select winding voltages to measure the voltage across the winding

terminals of the linear transformer block. Select winding currents to measure the current

flowing through the windings of the linear transformer block. Select magnetization

current to measure the magnetization current of the linear transformer block. Select All

voltages and currents to measure the winding voltages and currents plus the

magnetization current.

4.4.3 Series RLC Branch

The Series RLC branch block implements a single resistor, inductor or capacitor

or a series combination of these. To eliminate the resistance, inductance or capacitance of

the branch, the R, L and C values must be set respectively to zero, zero and infinity (inf).

Only existing elements are displayed in the block icon. Negative values are allowed for

resistance, inductance and capacitance.

Parameters

Resistance: The branch resistance, in ohms (ohms).

Inductance: The branch inductance, in henries (H).

Capacitance: The branch capacitance, in farads (F).

Measurements: Select branch voltage to measure the voltage across the series RLC

branch block terminals. Select branch current to measure the current flowing through the

series RLC branch block. Select branch voltage and current to measure the voltage and

the current of the series RLC branch block.

4.4.4 Synchronized 6-Pulse Generator

The synchronized 6-Pulse generator block can be used to fire the six thyristors of

a six-pulse converter. The output of the block is a vector of six pulses individually

synchronized on the six thyristor voltages. The pulses are generated alpha degrees after

the increasing zero crossings of the thyristor commutation voltages. The synchronized 6-

pulse generator block can be configured to work in double-pulsing mode. In this mode

two pulses are sent to each thyristor: a first pulse when the alpha angle is reached, then a

second pulse 60 degrees later, when the next thyristor is fired.

The pulse ordering at the output of the block corresponds to the natural order of

commutation of a three-phase thyristor bridge. When you connect the synchronized 6-

pulse generator block to the pulses input of the universal bridge block (with the thyristors

as the power electronic device), the pulses are sent to the thyristors.

Parameters

Frequency of synchronization voltages: The frequency, in hertz, of the

synchronization voltages. It usually corresponds to the frequency of the network.

Pulse width: The width of the pulses, in degrees.

Double pulsing: If selected, the generator sends to each thyristor a first pulse when the

alpha angle is reached and then a second pulse 60 degrees later when the next thyristor in

the sequence is fired.

Inputs and Outputs

Alpha deg: Input 1 is the alpha firing signal, in degrees. This input can be connected to a

Constant block or it can be connected to a controller system to control the pulses of the

generator.

AB, BC and CA: Inputs 2, 3 and 4 are the phase-to-phase synchronization voltages Vab,

Vbc and Vca. The synchronization voltages should be in phase with the three phase-

phase voltages at the converter AC terminals. Synchronization voltages are normally

derived at the primary windings of the converter transformer. If the converter is

connected to the delta winding of a Wye/Delta transformer, the synchronization voltages

should be the phase-to-ground voltages of the primary windings.

Frequency: Available only with the discrete version of the synchronized 6-pulse

generator. This input should be connected to a constant block containing the fundamental

frequency, in hertz or to a PLL tracking the frequency of the system.

Block: Input 5 allows you to block the operation of the generator. The pulses are disabled

when the applied signal is greater than zero.

Pulses: The output contains the six pulse signals

4.4.5 Synchronized 12-Pulse Generator

The Synchronized 12-Pulse Generator block generates two vectors of six pulses

synchronized on the twelve thyristor commutation voltages. The first set of pulses,

denoted PY, is sent to the six-pulse bridge connected to the wye secondary winding of the

Y/Y/Delta converter transformer. It is generated alpha degrees after the zero crossing of

the phase-to-phase synchronization voltages. The second set of pulses, denoted PD, is

sent to the six-pulse bridge connected to the delta secondary winding of the converter

transformer. It lags the PY pulses by 30 degrees.

The phase-to-ground A, B and C voltages are provided to the generator and the

two sets of phase-to-phase synchronization voltages required by the two six-pulse bridges

are generated internally.

The ordering of the pulses in the two outputs of the block corresponds to the natural

order of commutation of a three-phase thyristor bridge. When you connect the

synchronized 12-pulse generator block outputs to the pulse inputs of the Universal Bridge

blocks (with the thyristor device), the pulses are sent to the thyristors.

Parameters

Frequency of synchronization voltages: The frequency, in hertz, of the synchronization

voltages. It usually corresponds to the frequency of the network.

Pulse width: The width of the pulses, in degrees.

Double pulsing: If selected, the generator sends to each thyristor a first pulse when the

alpha angle is reached and then a second pulse 60 degrees later when the next thyristor in

the sequence is fired. The double pulsing is applied separately on the two vectors of

pulses.

Inputs and Outputs

alpha_deg: Input 1 is the alpha firing signal, in degrees. This input can be connected to a

Constant block or it can be connected to a controller system to control the pulses of the

generator.

A, B, C: Inputs 2, 3 and 4 are the phase-to-ground synchronization voltages Va, Vb and

Vc. The synchronization voltages should be measured at the primary side of the converter

transformer.

Freq: Available only with the discrete version of the synchronized 6-pulse generator. This

input should be connected to a constant block containing the fundamental frequency, in

hertz or to a PLL tracking the frequency of the system.

Block: Input 5 allows you to block the operation of the generator. The pulses are disabled

when the applied signal is greater than zero.

PY: Output 1 contains the six-pulse signals to be sent to the six-pulse thyristor converter

connected to the Y secondary winding of the converter transformer.

PD: Output 2 contains the six-pulse signals to be sent to the six-pulse thyristor converter

connected to the Delta (D) secondary winding of the converter transformer.

4.4.6 Current Measurement

The current measurement block is used to measure the instantaneous current

flowing in any electrical block or connection line. The Simulink output provides a

Simulink signal that can be used by other Simulink blocks.

Parameters

Output signal: Specifies the format of the output signal when the block is used in a phasor

simulation. The Output signal parameter is disabled when the block is not used in a

phasor simulation. The phasor simulation is activated by a Powergui block placed in the

model.

Set to complex to output the measured current as a complex value. The output is

a complex signal.

Set to real-imag to output the real and imaginary parts of the measured current.

The output is a vector of two elements.

Set to magnitude-angle to output the magnitude and angle of the measured

current. The output is a vector of two elements.

Set to magnitude to output the magnitude of the measured current. The output is

a scalar value.

4.4.7. Voltage Measurement

The Voltage measurement block measures the instantaneous voltage between two

electric nodes. The output provides a Simulink signal that can be used by other Simulink

blocks

Output signal: Specifies the format of the output signal when the block is used in a phasor

simulation. The output signal parameter is disabled when the block is not used in a phasor

simulation. The phasor simulation is activated by a Powergui block placed in the model.

Set to complex to output the measured current as a complex value. The output is a

complex signal.

Set to real-imag to output the real and imaginary parts of the measured current. The

output is a vector of two elements.

Set to magnitude-angle to output the magnitude and angle of the measured current.

The output is a vector of two elements.

Set to Magnitude to output the magnitude of the measured current. The output is a

scalar value.

4.4.8 Universal Bridge

The universal bridge block implements a universal three-phase power converter

that consists of up to six power switches connected in a bridge configuration. The type of

power switch and converter configuration is selectable from the dialog box.

The universal bridge block allows simulation of converters using both naturally

commutated (and line-commutated) power electronic devices (diodes or thyristors) and

forced-commutated devices (GTO, IGBT and MOSFET).

The universal bridge block is the basic block for building two-level voltage-sourced

converters (VSC).

Parameters

Number of bridge arms: Set to 1 or 2 to get a single-phase converter (two or four

switching devices). Set to 3 to get a three-phase converter connected in Graetz bridge

configuration (six switching devices).

Snubber resistance Rs: The snubber resistance, in ohms. Set the Snubber resistance Rs

parameter to inf to eliminate the snubbers from the model.

Snubber capacitance Cs: The snubber capacitance, in farads (F). Set the Snubber

capacitance Cs parameter to 0 to eliminate the snubbers or to inf to get a resistive

snubber. In order to avoid numerical oscillations when your system is discretized, you

need to specify Rs and Cs snubber values for diode and thyristor bridges. For forced-

commutated devices (GTO, IGBT or MOSFET), the bridge operates satisfactorily with

purely resistive snubbers as long as firing pulses are sent to switching devices. If firing

pulses to forced-commutated devices are blocked, only anti-parallel diodes operate and

the bridge operates as a diode rectifier. In this condition appropriate values of Rs and Cs

must also be used. When the system is discretized, use the following formulas to compute

approximate values of Rs and Cs.

These Rs and Cs values are derived from the following two criteria:

The snubber leakage current at fundamental frequency is less than 0.1% of

nominal current when power electronic devices are not conducting. The RC time constant

of snubbers is higher than two times the sample time Ts. These Rs and Cs values that

guarantee numerical stability of the discretized bridge can be different from actual values

used in a physical circuit.

Power electronic device: Select the type of power electronic device to use in the bridge.

Ron: Internal resistance of the selected device, in ohms (ohms).

Lon: Internal inductance, in henries (H), for the diode or the thyristor device. When the

bridge is discretized, the Lon parameter must be set to zero.

Forward voltage Vf: This parameter is available only when the selected power electronic

device is Diodes or Thyristors. Forward voltage, in volts (V), across the device when it is

conducting. Forward voltages [Device Vf, Diode Vfd] this parameter is available when

the selected Power electronic device is GTO/Diodes or IGBT/Diodes. Forward voltages,

in volts (V), of the forced-commutated devices (GTO, MOSFET or IGBT) and of the

antiparallel diodes. [Tf (s) Tt (s)]Fall time Tf and tail time Tt, in seconds (s), for the GTO

or the IGBT devices.

Measurements: Select device voltages to measure the voltages across the six power

electronic device terminals. Select Device currents to measure the currents flowing

through the six power electronic devices. If anti-parallel diodes are used, the measured

current is the total current in the forced-commutated device (GTO, MOSFET or IGBT)

and in the anti-parallel diode. A positive current therefore indicates a current flowing in

the forced-commutated device and a negative current indicates a current flowing in the

diode. If snubber devices are defined, the measured currents are the ones flowing through

the power electronic devices only. Select UAB UBC UCA UDC voltages to measure the

terminal voltages (AC and DC) of the Universal Bridge block. Select All voltages and

currents to measure all voltages and currents defined for the Universal Bridge block.

Assumptions and Limitations

Universal Bridge blocks can be discretized for use in a discrete time step simulation. In

this case, the internal commutation logic of the Universal Bridge takes care of the

commutation between the power switches and the diodes in the converter arms.

Constant

The Constant block generates a real or complex constant value. The block generates

scalar (1x1 2-D array), vector (1-D array) or matrix (2-D array) output, depending on the

dimensionality of the Constant value parameter and the setting of the Interpret vector

parameters as 1-D parameter.

The output of the block has the same dimensions and elements as the Constant

value parameter. If you specify a vector for this parameter and you want the block to

interpret it as a vector (i.e., a 1-D array), select the Interpret vector parameters as 1-D

parameter; otherwise, the block treats the Constant value parameter as a matrix (i.e., a 2-

D array).

Data Type Support

By default, the constant block outputs a signal whose data type and complexity are the

same as that of the block's constant value parameter. However, you can specify the output

to be any supported data type supported by Simulink, including fixed-point data types.

Constant value: Specify the constant value output by the block. We can enter any

MATLAB expression in this field, including the Boolean keywords, true or false, that

evaluates to a matrix value. The constant value parameter is converted from its data type

to the specified output data type offline using round-to-nearest and saturation. Interpret

vector parameters as 1-D.If we select this check box, the Constant block outputs a vector

of length N if the constant value parameter evaluates to an N-element row or column

vector, i.e., a matrix of dimension 1xN or Nx1.

Sample time: Specify the interval between times that the constant block's output can

change during simulation (e.g., as a result of tuning its constant value parameter). The

default sample time is inf, i.e., the block's output can never change. This setting speeds

simulation and generated code by avoiding the need to recompute the block's output.

4.4.9 Scope

The scope block displays its input with respect to simulation time. The scope

block can have multiple axes (one per port); all axes have a common time range with

independent y-axes. The scope allows you to adjust the amount of time and the range of

input values displayed. You can move and resize the scope window and you can modify

the scope's parameter values during the simulation.

When you start a simulation, simulink does not open scope windows, although it

does write data to connected scopes. As a result, if you open a scope after a simulation,

the scope's input signal or signals will be displayed.

If the signal is continuous, the Scope produces a point-to-point plot. If the signal is

discrete, the Scope produces a stair-step plot.

The Scope provides toolbar buttons that enable you to zoom in on displayed data,

display all the data input to the Scope, preserve axis settings from one simulation to the

next, limit data displayed and save data to the workspace. The toolbar buttons are labeled

in this Fig., which shows the Scope window as it appears when you open a Scope block.

This chapter presented a comparison of different software packages available

commercially and brought out the salient features of SIMULINK/MATLAB. Also

various blocks used for simulation and their parameters are described briefly. The next

chapter will present the multi-pulse converters in the SIMULINK environment.

CHAPTER 5

SIMULATION OF CONTROLLED AND UNCONTROLLED MULTI-PULSE AC-

DC CONVERTERS

5.1 General

Normally, multi-pulse converters with isolation use isolating transformers

between the converters and the utility. This is a costly proposition. When isolation

between a utility supply and a rectifier is not required, employing an autotransformer

including a plurality of series and common windings may advantageously reduce the size

and cost of the entire system.

5.2 Use of phase shifting transformers

The auto connected phase shifting transformer discussed earlier in chapter 3 is

ideally suited to provide phase shifted power supplies for converters. For a given phase

shift, the design is simpler and the parts kVA are lower than the equivalent fork

connection.

5.3 Simulation of Uncontrolled Multi-Pulse Converters:

5.3.1 Six-pulse converter (un-controlled)

The six pulse converter bridge shown in Fig. as the basic converter unit of HVDC

transmission is used equally well for rectification where electric power flows from the

a.c. side to the d.c side and inversion where the power flow is from the d.c side to the a.c.

side. Thyristor valves operate as switches which turn on and conduct current when fired

on receiving a gate pulse and are forward biased. A thyristor valve will conduct current in

one direction and once it conducts, will only turn off when it is reverse biased and the

current falls to zero. This process is known as line commutation. An important property

of the thyristor valve is that once it's conducting current falls to zero when it is reverse

biased and the gate pulse is removed, too rapid an increase in the magnitude of the

forward biased voltage will cause the thyristor to inadvertently turn on and conduct. The

design of the thyristor valve and converter bridge must ensure such a condition is avoided

for useful inverter operation.

The characteristic a.c. side current harmonics generated by 6 pulse converters are

6n +/- 1, Characteristic d.c side voltage harmonics generated by a 6 pulse converter are of

the order 6n. a.c. side harmonic filters may be switched with circuit breakers or circuit

switches to accommodate reactive power requirement strategies since these filters

generate reactive power at fundamental frequency. d.c side filters reduce harmonic

current flow on d.c transmission lines to minimize coupling and interference to adjacent

voice frequency communication circuits. Where there is no d.c line such as in the back-

to-back configuration, d.c side filters may not be required. d.c reactors are usually

included in each pole of a converter station. They assist the d.c filters in filtering

harmonic currents and smooth the d.c side current so that a discontinuous current mode is

not reached at low load current operation.

Fig. 5.1. Uncontrolled Six-Pulse Converter

Fig. 5.1.1.Waveforms of input current, output voltage, output current:

Fig. 5.1.2. THD for input current

Fig. 5.1.3.THD for output voltage

5.3.2 Twelve pulse multi-pulse converter (un-controlled)

Twelve pulse converter is a series connection of two fully controlled six pulse

converter bridges and requires two 3 phase systems which are spaced apart from each

other by 30 electrical degrees. The phase difference effected to cancel out the 6-pulse

harmonics on the AC and DC side.

The model for twelve pulse non-isolated converter with is created in SIMULINK

as shown in Fig. The connection diagram fig (5.2) and simulation results are as shown in

fig which show a clear reduction in the harmonic content of the input supply current as

compared to a 6-pulse isolated converter. The purpose of this simulation was to get

familiar with simulation of multi-pulse converters. By making use of delta star

transformer, 30 phase shift is introduced and correspondingly 5th and 7th harmonics are

eliminated.

Continuous

pow ergui

v+-

A

B

C

+

-

Universal Bridge1

A

B

C

+

-

Universal BridgeA

B

C

a2

b2

c2

a3

b3

c3

Series RLC Branch

Scope

i+ -

i+ -

. Fig. 5.2. Uncontrolled twelve pulse converter

Fig. 5.2.1.Output waveform of input current, output voltage, output current:

Fig. 5.2.2THD for input current

Fig. 5.2.3THD for output voltage

5.3.3 Eighteen pulse converter (un-controlled)

In this 18-pulse topology, the magnetic involved is same as that of a 12 pulse

converter. Therefore this topology is comparatively a preferred one. The simulated results

are in close agreement with any result obtained from an 18 pulse converters.

Continuous

pow ergui

A+B+C+A-B-C-

a3

b3

c3

A+B+C+A-B-C-

a3

b3

c3

A+B+C+A-B-C-

a3

b3

c3

v

+

-

A

B

C

+

-

A

B

C

+

-

A

B

C

+

-

Scope

i+ -

i+ -

Fig. 5.3.Uncontrolled eighteen pulse converter

Fig. 5.3.1. Waveforms of input current, output voltage, output current:

Fig. 5.3.2.THD for input current

Fig. 5.3.3.THD for output voltage

5.3.4 Twenty-four pulse converter (un-controlled)

The connection for 24-pulse converter and the corresponding connections are shown

in fig. Two twelve pulse converters phase shifted by 15 degrees from each other, can

provide a twenty four, obviously with much lower harmonics on ac and DC side. Its ac

output voltage would have order harmonics i.e., 23rd, 25th, 47th , 49th harmonics

with magnitudes of 1/23rd , 1/25th , 1/47th ,1/49th ,…respectively, of the phase shift.

One approach is to provide 15 degrees phase shift windings on the two transformers of

one of the two twelve pulse converters. Another approach is to provide phase shift

windings for +7.5 degrees phase shift on the two transformers of one twelve pulse

converter and -7.5 on the two transformers of the other two twelve pulse converters as

shown in the fig. The latter is preferred because it requires transformers of the same

design and leakage inductances. It is also necessary to shift the firing pulses of one

twelve pulse converter by 15 degrees with respect to others.

All four six-pulse converters can be connected on the DC side in parallel, i.e., twelve

phase legs in parallel. Alternatively all four six-pulse converters can be connected in

series for high voltages or two pair of twelve pulse series converters may be connected in

parallel. Each six-pulse converters will have a separate transformer, two with wye-

connected secondaries and the other two with delta-connected secondaries. Primaries of

all four transformers can be connected in series as shown in Fig.. In order to avoid

harmonic circulation current corresponding the twelve pulse order i.e., 11th, 13th, 23rd, 25th

Continuous

pow ergui

A+

B+

C+

A-

B-

C-

a3

b3

c3

A+

B+

C+

A-

B-

C-

a3

b3

c3

A+

B+

C+

A-

B-

C-

a3

b3

c3

A+

B+

C+

A-

B-

C-

a3

b3

c3

v

+

-

Voltage Measurement

A

B

C

+

-

A

B

C

+

-

A

B

C

+

-

A

B

C

+

-

Series RLC Branch

Scope

i

+

-

Current Measurement2

i

+

-

Fig. 5.4.Uncontrolled twenty four pulse converter

Fig. 5.4.1Output waveform of input current, output voltage, output current:

Fig. 5.4.2.THD for input current

Fig. 5.4.3.THD for output voltage

5.3.5 Thirty-Six pulse converter (un-controlled)

Fig. 5.5 Uncontrolled thirty six pulse converter

Fig. 5.5.1Output waveform of input current, output voltage, output current

Fig. 5.5.2.THD for input current

Fig. 5.5.3.THD for output voltage

5.3.6 Forty-Eight pulse converter (un-controlled)

For high power FACTS controllers, from the point of view of the ac systems

even a twenty four pulse converter without ac filters could have voltage harmonics,

which are higher than the acceptable level. In this case a single high pass filter tuned to

the 23rd, 25th harmonics located on the system side of the converter transformer should

be adequate. The alternative of course is go to 48 pulse operation with eight six pulse

groups with one set of transformers of one 24 pulse converters phase shifted converter

from 7.5 degrees or one set shifted by +3.75 and the other by -3.75 degrees logically , all

8 transformer primaries may be connected in series, but because of small phase shift

(7.5) the primaries of the two 24-pulse converters (each with four primaries in series)

may be connected in parallel if the consequent circulating current is acceptable. This

should not be much of a problem because of the higher the order of a harmonic the lower

would be the circulating current. With 48-pulse operation, ac filters should not be

necessary.

Fig. 5.6.Uncontrolled forty eight pulse converter

Fig. 5.6.1.Output waveform of input current, output voltage, output current:

Fig. 5.6.2.THD for input current

Fig. 5.6.3.THD for output voltage

Similarly the simulation results have been obtained for all the above mentioned Un-

Controlled multi-pulse converters with RL load and the comparison of THD values is

made for R and RL loads.

5.4 Comparison of THD for Uncontrolled multi-pulse converters for R and RL loads

NUMBER

OF

PULSES

THD%

R-LOAD

THD%

RL-LOAD

6 30.82

12 15.18

18 1.77

24 1.86

36 0.26

48 0.22

5.5 Simulation of Controlled Multi-Pulse Converters:

For the simulation of controlled multi pulse converters instead of the diode bridge we use the thyristor

bridge and the corresponding pulses are given.

5.5.1 Six-pulse converter (controlled)

Fig. 5.7.Controlled six-pulse Converter

Fig. 5.7.1.Output waveform of input current, output voltage, output current:

Fig. 5.7.2.THD for input current

Fig. 5.7.3.THD for output voltage

5.5.2 Twelve pulse converter (controlled)

Continuous

pow ergui

v+-

v+-

v+-

v+-

g

A

B

C

+

-

Universal Bridge1

g

A

B

C

+

-

Universal BridgeA

B

C

a2

b2

c2

a3

b3

c3

Series RLC Branch

Scope

i+ -

i+ -

30

Constant2

0

Constant1

alpha_deg

A

B

C

Block

PY

PD

Synchronized12-Pulse Generator

Fig. 5.8.Controlled twelve pulse converter

Fig. 5.8.1.Output waveform of input current, output voltage, output current

Fig. 5.8.2.THD for input current

Fig. 5.8.3.THD for output voltage

5.5.3 Eighteen pulse converter (controlled)

Continuous

pow ergui

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer2

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer1

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer

v

+

-

v+-

v+-

v+-

v+-

v+

-

v+-

v+-

v+-

v+-

g

A

B

C

+

-

Universal Bridge2

g

A

B

C

+

-

Universal Bridge1

g

A

B

C

+

-

Universal Bridge

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator2

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator1

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator

Series RLC Branch

Scope

i+ -

Current Measurement1i+ -

Current Measurement

70

0

0

50

0

30

Fig. 5.9.Controlled eighteen pulse converter

Fig. 5.9.1.Output waveform of input current, output voltage, output current:

Fig. 5.9.2.THD for input current

Fig. 5.9.3.THD for output voltage

5.5.4 Twenty four pulse converter (controlled)

Continuous

pow ergui

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer4

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer2

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer1

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer

v

+

-

v+-

v+-

v+-

v+-

v+

-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

g

A

B

C

+

-

Universal Bridge3

g

A

B

C

+

-

Universal Bridge2

g

A

B

C

+

-

Universal Bridge1

g

A

B

C

+

-

Universal Bridge

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator3

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator2

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator1

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator

Series RLC Branch

Scope

i+

-

Current Measurement1i

+-

Current Measurement

0

75

60

0

0

45

0

30

Fig. 5.10.Controlled twenty four pulse converter

Fig. 5.10.1.Output waveform of input current, output voltage, output current:

Fig. 5.10.2.THD for input current

Fig. 5.10.3.THD for output voltage

5.5.5 Thirty six pulse converter (controlled)

Continuous

pow ergui

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer5

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer4

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer3

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer2

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer1

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer

v

+

-

v+-

v+-

v+-

v+-

v+

-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

g

A

B

C

+

-

Universal Bridge5

g

A

B

C

+

-

Universal Bridge4

g

A

B

C

+

-

Universal Bridge3

g

A

B

C

+

-

Universal Bridge2

g

A

B

C

+

-

Universal Bridge1

g

A

B

C

+

-

Universal Bridge

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator5

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator4

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator3

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator2

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator1

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator

Series RLC Branch

Scope

i+ -

Current Measurement1i+ -

Current Measurement

70

0

0

60

50

0

0

40

80

0

0

30

Fig. 5.11.Controlled thirty six pulse converter

Fig. 5.11.1.Output waveform of input current, output voltage, output current

Fig. 5.11.2.THD for input current

Fig. 5.11.3.THD for output voltage

5.5.6 Forty eight pulse converter (controlled)

Continuous

pow ergui

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer7

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer6

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer5

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer4

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer3

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer2

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer1

A+B+C+A-B-C-

a3

b3

c3

ZigzagPhase-Shifting Transformer

v

+

-

v+-

v+-

v+-

v+-

v+

-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

v+-

g

A

B

C

+

-

Universal Bridge7

g

A

B

C

+

-

Universal Bridge6

g

A

B

C

+

-

Universal Bridge5

g

A

B

C

+

-

Universal Bridge4

g

A

B

C

+

-

Universal Bridge3

g

A

B

C

+

-

Universal Bridge2

g

A

B

C

+

-

Universal Bridge1

g

A

B

C

+

-

Universal Bridge

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator7

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator6

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator5

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator4

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator3

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator2

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator1

alpha_deg

AB

BC

CA

Block

pulses

Synchronized6-Pulse Generator

Series RLC Branch

Scope1

i+ -

Current Measurement1i+ -

Current Measurement

60

0

0

52.5

45

0

0

37.5

0

87.5

75

0

67.5

0

0

30

Fig. 5.12.Controlled forty eight pulse converter

Fig. 5.12.1.Output waveform of input current, output voltage, output current

Fig.5.12.2.THD for input current

Fig. 5.12.3.THD for output voltage

5.6 Comparison of controlled multi-pulse converters for R-LOAD AND RL-LOAD

NUMBER

OF PULSES

THD%

R-LOAD

THD%

RL-LOAD

6 35.09

12 15.08

18 13.34

24 7.34

36 4.39

48 3.21

5.7 Simulation of Closed loop six pulse HVDC system

Fig.5.13.Closed six pulse HVDC system

Fig.5.13.1.Output waveforms

5.8 Closed loop twelve pulse converter:

Fig.5.14.Closed loop twelve pulse converter

5.9 Effect of load variation in multi-pulse converters

The effect of different types of loads on R, RL and RC load is analyzed with

reference to THD

5.10 Effect of capacitive load in multi-pulse converters

In general THD current increases for RC load and output ripple voltage decreases.

The same effect is observed on simulated diagrams as shown in the figs. for 6,12,18,24

multi-pulse converters.

5.11 Conclusion

The various multi-pulse configurations, mainly non-isolated were simulated using the

software SIMULIN/MATLAB and the results have been presented in this chapter. The

effect of load variation on different multi-pulse converters reveals that with RL, load

because of inductance there is smoothing effect on current, therefore current THD

decreases; whereas on RC load, the effect of capacitor is to reduce voltage ripple and

gives a smooth DC output. The effect is similar for different multi-pulse converters, i.e. it

increases current discontinuity and hence affecting the harmonic spectrum adversely.

CHAPTER 6

MAIN CONCLUSION AND FUTURE WORK TO BE DONE

The main objective of the present work is to investigate the performance of controlled

and un-controlled multi-pulse converters. These converters are studied in terms of

harmonic spectrum of ac mains current, THD, distortion factor, displacement power

factor and actual power factor in the AC mains. It is concluded therefore that in general

with increase in number of pulses in multi-pulse case the performance parameters of

these converters are remarkably improved.

Future scope:

A back-to-back asynchronous tie comprised of VSC converters employing PWM

may well represent the ultimate HVDC system. Besides controlling the through power

flow, it can supply reactive power and provide independent dynamic voltage control at its

two terminals. The two converters can be paralleled to double the reactive power

capability supplied to one side or the other. The back-to-back converters can be used for

black start or to supply a passive load. Higher voltage designs can be used with

transmission lines or cables to form point-to-point or multi-terminal transmission links.

More sophisticated controls can be used to provide additional network benefits. With the

Eagle Pass project, CSW has realized the system advantages of deploying a VSC based

back-to-back asynchronous Tie with standby dynamic voltage control during network

contingencies. The controlled power transfer capability allows the exchange of power

between the two networks while the voltage control stabilizes the voltage following line

outages especially during peak load periods.

The future scope of work could be the simulation of 18, 24, 36, 48 multi pulse

converter topologies in closed loop.

REFERENCES

[1] D.A.Paice. “Auto connected hexagon transformer for a 12-pulse converter”.

Patent number: 5148357. 1992

[2] Choi dewan, enjeti, pitel “autotransformer configurations to enhance utility power quality

of high power AC/DC rectifier systems.”1996 IEEE

[3] Babri Ivoand Jones, “a new three –phase low THD supply with High –frequency isolation

and 60v/200A regulated DC supply”. 2001. IEEE

[4] S.Kim, Enjeti,”A new approach to improve Power Factor and reduce Harmonics in a

Three-Phase Diode Rectifier Type Utility Interface”IEEE trans.on Industry

appl,Vol.30,No.6,NOV/DEC 1994

[5] Chen and Hong, “A new passive 28-step current shaper for three-phase rectification.”

IEEE transactions on industrial electronics, vol.47, No.6, December 2000.

[6] N.R.Zargari etal, “A multilevel thyristor Rectifier with improved power factor” IEEE

trans.on industry applications, vol.33.No.5, SEPT/OCT. 1997

[7] D.A.Paice, Power Electronic Converter Harmonics- Multipulse Methods for Clean

Power. New York: IEEE Press, 1996.

[8] N.Mohan, TUdeland and W.Robbins, Power Electronics: Converters, Applications and

Design, Second Edition, New York: John Wiley & sons, 1995.