ABSTRACT

In power distribution networks, reactive power is the main cause of increasing

distribution system losses and various power quality problems. Conventionally, Static Var

Compensators (SVCs) have been used in conjunction with passive filters at the distribution level

for reactive power compensation and mitigation of power quality problems. Though SVCs are

very effective system controllers used to provide reactive power compensation at the

transmission level, their limited bandwidth, higher passive element count that increases size and

losses, and slower response make them inapt for the modern day distribution requirement.

Distribution Static Compensator is an important device in correcting power factor,

maintaining constant distribution voltage, and mitigating harmonics in a distribution network.

Prior to the type of control algorithm incorporated, the choice of converter configuration is an

important criterion. The two converter configurations are voltage source converter or current

source converter, in addition to passive storage elements, either a capacitor or an inductor

respectively. Normally, voltage source converters are preferred due to their smaller size, less heat

dissipation and less cost of the capacitor, as compared to an inductor for the same rating.

This paper focuses on the comparative study of the control techniques for voltage source

converter based Distribution Static Compensator, broadly classified as voltage controlled and

current controlled. Under the former, phase shift control is compared with the latter, considering

indirect decoupled current control and regulation of AC bus and DC link voltage with hysteresis

current control. The first two schemes have been successfully implemented for Static

Compensator control at the transmission level for reactive power compensation and voltage

support, and are recently being incorporated to control a Distribution Static Compensator

employed at the distribution end. The operating principles of a DSTATCOM are based on the

exact equivalence of the conventional rotating synchronous compensator. This paper is an

attempt to compare the following schemes of a DSTATCOM for power factor correction and

harmonic mitigation based on: Phase shift control, Indirect decoupled current control, Regulation

of AC bus and DC link voltage.

1

The following indices are considered for comparison–measurement and signal

conditioning requirement, performance with varying linear/nonlinear load, total harmonic

distortion, DC link voltage variation and switching frequency. The paper briefly describes the

salient features of each strategy, with their merits and demerits. A dynamic simulation model of

the DSTATCOM has been developed for various control strategies, in Matlab/SimPower System

environment.

2

1. INTRODUCTION

In power distribution networks, reactive power is the main cause of increasing

distribution system losses and various power quality problems. Conventionally, Static Var

Compensators (SVCs) have been used in conjunction with passive filters at the distribution level

for reactive power compensation and mitigation of power quality problems [1]. Though SVCs

are very effective system controllers used to provide reactive power compensation at the

transmission level, their limited bandwidth, higher passive element count that increases size and

losses, and slower response make them inapt for the modern day distribution requirement.

Another compensating system has been proposed by [2], employing a combination of SVC and

active power filter, which can compensate three phase loads in a minimum of two cycles. Thus, a

controller which continuously monitors the load voltages and currents to determine the right

amount of compensation required by the system and the less response time should e a viable

alternative. Distribution Static Compensator (DSTATCOM) has the capacity to overcome the

above mentioned drawbacks by providing precise control and fast response during transient and

steady state, with reduced foot print and weight [1,3]. A DSTATCOM is basically a converter

based distribution flexible AC transmission controller, sharing many similar concepts with that

of a Static Compensator (STATCOM) used at the transmission level. At the transmission level,

STATCOM handles only fundamental reactive power and provides voltage support, while a

DSTATCOM is employed at the distribution level or at the load end for dynamic compensation.

The latter, DSTATCOM, can be one of the viable alternatives to SVC in a distribution network.

Additionally, a DSTATCOM can also behave as a shunt active filter [4,5], to eliminate

unbalance or distortions in the source current or the supply voltage, as per the IEEE-519 standard

limits. Since a DSTATCOM is such a multifunctional device, the main objective of any control

algorithm should be to make it flexible and easy to implement, in addition to exploiting its multi

functionality to the maximum. Prior to the type of control algorithm incorporated, the choice of

converter configuration is an important criterion. The two converter configurations are voltage

source converter or current source converter, in addition to passive storage elements, either a

capacitor or an inductor respectively. Normally, voltage source converters are preferred due to

their smaller size, less heat dissipation and less cost of the capacitor, as compared to an inductor

for the same rating [6-9]. This paper focuses on the comparative study of the control techniques

3

for voltage source converter based DSTATCOM, broadly classified into voltage control

DSTATCOM and current control DSTATCOM. Under the former, phase shift control [10] is

compared with the latter, considering indirect decoupled current control [11,12] and regulation of

AC bus and DC link voltage with hysteresis current control [13,14]. The first two schemes have

been successfully implemented for STATCOM control at the transmission level, for reactive

power compensation, and voltage support and are recently being incorporated to control a

DSTATCOM employed at the distribution end [15]. The following indices are considered for

comparison – measurement and signal conditioning requirement, performance with varying

linear/nonlinear load, total harmonic distortion (THD), DC link voltage variation and switching

frequency. The paper briefly describes the salient features of each strategy, with their merits and

demerits. The paper also emphasizes the choice of current control technique, as it significantly

affects the performance of a DSTATCOM. A dynamic simulation model of the DSTATCOM has

been developed for various control algorithms in Matlab/SimPower System environment.

2. FACTS DEVICES

4

Flexible AC Transmission Systems, called FACTS, got in the recent years a well known

term for higher controllability in power systems by means of power electronic devices. Several

FACTS-devices have been introduced for various applications worldwide. A number of new

types of devices are in the stage of being introduced in practice.

In most of the applications the controllability is used to avoid cost intensive or landscape

requiring extensions of power systems, for instance like upgrades or additions of substations and

power lines. FACTS-devices provide a better adaptation to varying operational conditions and

improve the usage of existing installations. The basic applications of FACTS-devices are:

• Power flow control,

• Increase of transmission capability,

• Voltage control,

• Reactive power compensation,

• Stability improvement,

• Power quality improvement,

• Power conditioning,

• Flicker mitigation,

• Interconnection of renewable and distributed generation and storages.

Figure 1.1 shows the basic idea of FACTS for transmission systems. The usage of lines

for active power transmission should be ideally up to the thermal limits. Voltage and stability

limits shall be shifted with the means of the several different FACTS devices. It can be seen that

with growing line length, the opportunity for FACTS devices gets more and more important.

The influence of FACTS-devices is achieved through switched or controlled shunt

compensation, series compensation or phase shift control. The devices work electrically as fast

current, voltage or impedance controllers. The power electronic allows very short reaction times

down to far below one second.

5

The development of FACTS-devices has started with the growing capabilities of power

electronic components. Devices for high power levels have been made available in converters for

high and even highest voltage levels. The overall starting points are network elements

influencing the reactive power or the impedance of a part of the power system. Figure 1.2 shows

a number of basic devices separated into the conventional ones and the FACTS-devices.

For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some

explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices

provided by the power electronics. This is one of the main differentiation factors from the

conventional devices. The term 'static' means that the devices have no moving parts like

mechanical switches to perform the dynamic controllability. Therefore most of the FACTS-

devices can equally be static and dynamic.

6

The left column in Figure 1.2 contains the conventional devices build out of fixed or

mechanically switch able components like resistance, inductance or capacitance together with

transformers. The FACTS-devices contain these elements as well but use additional power

electronic valves or converters to switch the elements in smaller steps or with switching patterns

within a cycle of the alternating current. The left column of FACTS-devices uses Thyristor

valves or converters. These valves or converters are well known since several years. They have

low losses because of their low switching frequency of once a cycle in the converters or the

usage of the Thyristors to simply bridge impedances in the valves.

The right column of FACTS-devices contains more advanced technology of voltage

source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated

Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free controllable

voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High

modulation frequencies allow to get low harmonics in the output signal and even to compensate

disturbances coming from the network. The disadvantage is that with an increasing switching

frequency, the losses are increasing as well. Therefore special designs of the converters are

required to compensate this.

7

Configurations of FACTS-Devices:

2.1. Shunt Devices:

The most used FACTS-device is the SVC or the version with Voltage Source Converter

called STATCOM. These shunt devices are operating as reactive power compensators. The main

applications in transmission, distribution and industrial networks are:

• Reduction of unwanted reactive power flows and therefore reduced network losses.

• Keeping of contractual power exchanges with balanced reactive power.

• Compensation of consumers and improvement of power quality especially with huge

demand fluctuations like industrial machines, metal melting plants, railway or

underground train systems.

• Compensation of Thyristor converters e.g. in conventional HVDC lines.

• Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs are used for industrial

applications. Industry as well as commercial and domestic groups of users require power quality.

Flickering lamps are no longer accepted, nor are interruptions of industrial processes due to

insufficient power quality. Railway or underground systems with huge load variations require

SVCs or STATCOMs.

2.1.1. SVC:

Electrical loads both generate and absorb reactive power. Since the transmitted load

varies considerably from one hour to another, the reactive power balance in a grid varies as well.

The result can be unacceptable voltage amplitude variations or even a voltage depression, at the

extreme a voltage collapse.

A rapidly operating Static Var Compensator (SVC) can continuously provide the reactive

power required to control dynamic voltage oscillations under various system conditions and

thereby improve the power system transmission and distribution stability.

Applications of the SVC systems in transmission systems:

a. To increase active power transfer capacity and transient stability margin

b. To damp power oscillations

c. To achieve effective voltage control

In addition, SVCs are also used

8

1. In Transmission Systems

a. To reduce temporary over voltages

b. To damp sub synchronous resonances

c. To damp power oscillations in interconnected power systems

2. In Traction Systems

a. To balance loads

b. To improve power factor

c. To improve voltage regulation

3. In HVDC systems

a. To provide reactive power to ac–dc converters

4. In Arc Furnaces

a. To reduce voltage variations and associated light flicker

Installing an SVC at one or more suitable points in the network can increase transfer

capability and reduce losses while maintaining a smooth voltage profile under different network

conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude

modulation.

SVC installations consist of a number of building blocks. The most important is the

Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide

controllability. Air core reactors and high voltage AC capacitors are the reactive power elements

used together with the Thyristor valves. The step up connection of this equipment to the

transmission voltage is achieved through a power transformer.

9

Fig. 2.1. SVC building blocks and voltage / current characteristic

In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor

Switched or Controlled Reactors (TSR / TCR). The coordinated control of a combination of

these branches varies the reactive power as shown in Figure. The first commercial SVC was

installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in

1979. Since then it is widely used and the most accepted FACTS-device.

2.1.1.1. SVC Using TCR and FC:

In this arrangement, two or more FC (fixed capacitor) banks are connected to a TCR

(thyristor controlled reactor) through a step-down transformer. The rating of the reactor is chosen

larger than the rating of the capacitor by an amount to provide the maximum lagging vars that

have to be absorbed from the system. By changing the firing angle of the thyristor controlling the

reactor from 90° to 180°, the reactive power can be varied over the entire range from maximum

lagging vars to leading vars that can be absorbed from the system by this compensator.

10

Fig. 2.2. SVC Using TCR and FC

2.1.1.2. SVC of the FC/TCR type:

The main disadvantage of this configuration is the significant harmonics that will be

generated because of the partial conduction of the large reactor under normal sinusoidal steady-

state operating condition when the SVC is absorbing zero MVAr. These harmonics are filtered in

the following manner. Triplex harmonics are canceled by arranging the TCR and the secondary

windings of the step-down transformer in delta connection. The capacitor banks with the help of

series reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass

filter. Further losses are high due to the circulating current between the reactor and capacitor

banks.

Fig. 2.3 loss characteristics of TSC–TCR, TCR–FC compensators and synchronous

condenser

11

Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and

synchronous condenser these SVCs do not have a short-time overload capability because the

reactors are usually of the air-core type. In applications requiring overload capability, TCR must

be designed for short-time overloading, or separate thyristor-switched overload reactors must be

employed.

2.1.1.3. SVC Using a TCR and TSC:

This compensator overcomes two major shortcomings of the earlier compensators by

reducing losses under operating conditions and better performance under large system

disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank

will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by the

reactor. In those situations where harmonics have to be reduced further, a small amount of FCs

tuned as filters may be connected in parallel with the TCR.

Fig. .2.4. SVC of combined TSC and TCR type

When large disturbances occur in a power system due to load rejection, there is a

possibility for large voltage transients because of oscillatory interaction between system and the

12

SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC compensator. In the

TSC–TCR scheme, due to the flexibility of rapid switching of capacitor banks without

appreciable disturbance to the power system, oscillations can be avoided, and hence the

transients in the system can also be avoided. The capital cost of this SVC is higher than that of

the earlier one due to the increased number of capacitor switches and increased control

complexity.

2.1.2. STATCOM:

In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic

COMpensator) went into operation. The STATCOM has a characteristic similar to the

synchronous condenser, but as an electronic device it has no inertia and is superior to the

synchronous condenser in several ways, such as better dynamics, a lower investment cost and

lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off

capability like GTO or today IGCT or with more and more IGBTs. The static line between the

current limitations has a certain steepness determining the control characteristic for the voltage.

The advantage of a STATCOM is that the reactive power provision is independent from

the actual voltage on the connection point. This can be seen in the diagram for the maximum

currents being independent of the voltage in comparison to the SVC. This means, that even

during most severe contingencies, the STATCOM keeps its full capability.

In the distributed energy sector the usage of Voltage Source Converters for grid

interconnection is common practice today. The next step in STATCOM development is the

combination with energy storages on the DC-side. The performance for power quality and

balanced network operation can be improved much more with the combination of active and

reactive power.

13

Fig. 2.5. STATCOM structure and voltage / current characteristic

STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either

Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices. The

STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the

STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus

voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es then lagging

or inductive VARS are produced.

2.1.2.1. 6 Pulses STATCOM:

The three phases STATCOM makes use of the fact that on a three phase, fundamental

frequency, steady state basis, and the instantaneous power entering a purely reactive device must

be zero. The reactive power in each phase is supplied by circulating the instantaneous real power

between the phases. This is achieved by firing the GTO/diode switches in a manner that

maintains the phase difference between the ac bus voltage ES and the STATCOM generated

voltage VS. Ideally it is possible to construct a device based on circulating instantaneous power

which has no energy storage device (i.e. no dc capacitor).

14

Fig. 2.6. 6 Pulses STATCOM

A practical STATCOM requires some amount of energy storage to accommodate

harmonic power and ac system unbalances, when the instantaneous real power is non-zero. The

maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of

SVC compensator of comparable rating.

2.1.2.2. STATCOM Equivalent Circuit

Several different control techniques can be used for the firing control of the STATCOM.

Fundamental switching of the GTO/diode once per cycle can be used. This approach will

minimize switching losses, but will generally utilize more complex transformer topologies. As an

alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or IGBT

switch more than once per cycle, can be used. This approach allows for simpler transformer

topologies at the expense of higher switching losses.

Fig. 2.6 equivalent circuit of STATCOM

15

The 6 Pulse STATCOM using fundamental switching will of course produce the 6 N1

harmonics. There are a variety of methods to decrease the harmonics. These methods include the

basic 12 pulse configuration with parallel star / delta transformer connections, a complete

elimination of 5th and 7th harmonic current using series connection of star/star and star/delta

transformers and a quasi 12 pulse method with a single star-star transformer, and two secondary

windings, using control of firing angle to produce a 30phase shift between the two 6 pulse

bridges. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM, thus

eliminating harmonics even further. Another possible approach for harmonic cancellation is a

multi-level configuration which allows for more than one switching element per level and

therefore more than one switching in each bridge arm. The ac voltage derived has a staircase

effect, dependent on the number of levels. This staircase voltage can be controlled to eliminate

harmonics.

2.2. Series Devices:

Series devices have been further developed from fixed or mechanically switched

compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source

Converter based devices.

The main applications are:

• Reduction of series voltage decline in magnitude and angle over a power line,

• Reduction of voltage fluctuations within defined limits during changing power

transmissions,

• Improvement of system damping resp. damping of oscillations,

• Limitation of short circuit currents in networks or substations,

• Avoidance of loop flows resp. power flow adjustments.

2.2.1. TCSC:

Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in

transmission systems. Firstly it increases damping when large electrical systems are

interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), a

phenomenon that involves an interaction between large thermal generating units and series

compensated transmission systems.

16

The TCSC's high speed switching capability provides a mechanism for controlling line

power flow, which permits increased loading of existing transmission lines, and allows for rapid

readjustment of line power flow in response to various contingencies. The TCSC also can

regulate steady-state power flow within its rating limits.

From a principal technology point of view, the TCSC resembles the conventional series

capacitor. All the power equipment is located on an isolated steel platform, including the

Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the

control and protection is located on ground potential together with other auxiliary systems.

Figure shows the principle setup of a TCSC and its operational diagram. The firing angle and the

thermal limits of the Thyristors determine the boundaries of the operational diagram.

Fig. 2.7. TCSC circuit and operation diagram

2.2.2. Advantages

Continuous control of desired compensation level

Direct smooth control of power flow within the network

Improved capacitor bank protection

Local mitigation of sub synchronous resonance (SSR). This permits higher levels of

compensation in networks where interactions with turbine-generator torsional vibrations

or with other control or measuring systems are of concern.

Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between

areas in a large interconnected power network. These oscillations are due to the dynamics

of inter area power transfer and often exhibit poor damping when the aggregate power

transfer over a corridor is high relative to the transmission strength.

2.3. Shunt And Series Devices

17

2.3.1. Dynamic Power Flow Controller(DFC)

A new device in the area of power flow control is the Dynamic Power Flow Controller

(DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched

series compensation.

A functional single line diagram of the Dynamic Flow Controller is shown in Figure 1.19.

The Dynamic Flow Controller consists of the following components:

• A standard phase shifting transformer with tap-changer (PST)

• Series-connected Thyristor Switched Capacitors and Reactors

(TSC / TSR)

• A mechanically switched shunt capacitor (MSC). (This is optional depending on the

system reactive power requirements)

Fig. 2.8. principle configuration of DFC

Based on the system requirements, a DFC might consist of a number of series TSC or

TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of

overload and other conditions. Normally the reactance of reactors and the capacitors are selected

based on a binary basis to result in a desired stepped reactance variation. If a higher power flow

resolution is needed, a reactance equivalent to the half of the smallest one can be added.

The switching of series reactors occurs at zero current to avoid any harmonics. However,

in general, the principle of phase-angle control used in TCSC can be applied for a continuous

control as well.

18

The operation of a DFC is based on the following rules:

• TSC / TSR are switched when a fast response is required.

• The relieve of overload and work in stressed situations is handled by the TSC / TSR.

• The switching of the PST tap-changer should be minimized particularly for the currents

higher than normal loading.

• The total reactive power consumption of the device can be optimized by the operation

of the MSC, tap changer and the switched capacities and reactors.

In order to visualize the steady state operating range of the DFC, we assume an

inductance in parallel representing parallel transmission paths. The overall control objective in

steady state would be to control the distribution of power flow between the branch with the DFC

and the parallel path. This control is accomplished by control of the injected series voltage.

The PST (assuming a quadrature booster) will inject a voltage in quadrature with the

node voltage. The controllable reactance will inject a voltage in quadrature with the throughput

current. Assuming that the power flow has a load factor close to one, the two parts of the series

voltage will be close to collinear. However, in terms of speed of control, influence on reactive

power balance and effectiveness at high/low loading the two parts of the series voltage has quite

different characteristics. The steady state control range for loadings up to rated current is

illustrated in Figure 1.20, where the x-axis corresponds to the throughput current and the y-axis

corresponds to the injected series voltage.

Fig. 2.9. Operational diagram of a DFC

19

Operation in the first and third quadrants corresponds to reduction of power through the

DFC, whereas operation in the second and fourth quadrants corresponds to increasing the power

flow through the DFC. The slope of the line passing through the origin (at which the tap is at

zero and TSC / TSR are bypassed) depends on the short circuit reactance of the PST.

Starting at rated current (2 kA) the short circuit reactance by itself provides an injected

voltage (approximately 20 kV in this case). If more inductance is switched in and/or the tap is

increased, the series voltage increases and the current through the DFC decreases (and the flow

on parallel branches increases). The operating point moves along lines parallel to the arrows in

the figure. The slope of these arrows depends on the size of the parallel reactance. The maximum

series voltage in the first quadrant is obtained when all inductive steps are switched in and the

tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput current decreases (due

e.g. to changing loading of the system) the series voltage will decrease. At zero current, it will

not matter whether the TSC / TSR steps are in or out, they will not contribute to the series

voltage. Consequently, the series voltage at zero current corresponds to rated PST series voltage.

Next, moving into the second quadrant, the operating range will be limited by the line

corresponding to maximum tap and the capacitive step being switched in (and the inductive steps

by-passed). In this case, the capacitive step is approximately as large as the short circuit

reactance of the PST, giving an almost constant maximum voltage in the second quadrant.

2.3.2. Unified Power Flow Controller (UPFC):

The UPFC is a combination of a static compensator and static series compensation. It acts

as a shunt compensating and a phase shifting device simultaneously.

20

Fig. 2.10. Principle configuration of an UPFC

The UPFC consists of a shunt and a series transformer, which are connected via two

voltage source converters with a common DC-capacitor. The DC-circuit allows the active power

exchange between shunt and series transformer to control the phase shift of the series voltage.

This setup, as shown in Figure 1.21, provides the full controllability for voltage and power flow.

The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the

Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits

the practical applications where the voltage and power flow control is required simultaneously.

2.3.2.1. OPERATING PRINCIPLE OF UPFC

The basic components of the UPFC are two voltage source inverters (VSIs) sharing a

common dc storage capacitor, and connected to the power system through coupling transformers.

One VSI is connected to in shunt to the transmission system via a shunt transformer, while the

other one is connected in series through a series transformer.

A basic UPFC functional scheme is shown in fig.2.11.

21

Fig. 2.11. UPFC

The series inverter is controlled to inject a symmetrical three phase voltage system (Vse),

of controllable magnitude and phase angle in series with the line to control active and reactive

power flows on the transmission line. So, this inverter will exchange active and reactive power

with the line. The reactive power is electronically provided by the series inverter, and the active

power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to

demand this dc terminal power (positive or negative) from the line keeping the voltage across the

storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is

equal only to the losses of the inverters and their transformers. The remaining capacity of the

shunt inverter can be used to exchange reactive power with the line so to provide a voltage

regulation at the connection point.

The two VSI’s can work independently of each other by separating the dc side. So in that

case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to

regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as

SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power

low on the transmission line.

The UPFC has many possible operating modes. In particular, the shunt inverter is

operating in such a way to inject a controllable current, Ish into the transmission line. The shunt

inverter can be controlled in two different modes:

VAR Control Mode: The reference input is an inductive or capacitive VAR request. The

shunt inverter control translates the var reference into a corresponding shunt current request and

22

adjusts gating of the inverter to establish the desired current. For this mode of control a feedback

signal representing the dc bus voltage, Vdc, is also required.

Automatic Voltage Control Mode: The shunt inverter reactive current is automatically

regulated to maintain the transmission line voltage at the point of connection to a reference

value. For this mode of control, voltage feedback signals are obtained from the sending end bus

feeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the voltage injected in series with

the line to influence the power flow on the line. The actual value of the injected voltage can be

obtained in several ways.

Direct Voltage Injection Mode: The reference inputs are directly the magnitude and

phase angle of the series voltage.

Phase Angle Shifter Emulation mode: The reference input is phase displacement

between the sending end voltage and the receiving end voltage. Line Impedance Emulation

mode: The reference input is an impedance value to insert in series with the line impedance

Automatic Power Flow Control Mode: The reference inputs are values of P and Q to

maintain on the transmission line despite system changes.

3. D-STATCOM

3.1. INTRODUCTION

In power distribution networks, reactive power is the main cause of increasing

distribution system losses and various power quality problems. Conventionally, Static Var

Compensators (SVCs) have been used in conjunction with passive filters at the distribution level

for reactive power compensation and mitigation of power quality problems. Though SVCs are

very effective system controllers used to provide reactive power compensation at the

transmission level, their limited bandwidth, higher passive element count that increases size and

losses, and slower response make them inapt for the modern day distribution requirement.

Another compensating system has been proposed by , employing a combination of SVC and

23

active power filter, which can compensate three phase loads in a minimum of two cycles. Thus, a

controller which continuously monitors the load voltages and currents to determine the right

amount of compensation required by the system and the less response time should be a viable

alternative. Distribution Static Compensator (DSTATCOM) has the capacity to overcome the

above mentioned drawbacks by providing precise control and fast response during transient and

steady state, with reduced foot print and weight. A DSTATCOM is basically a converter based

distribution flexible AC transmission controller, sharing many similar concepts with that of a

Static Compensator (STATCOM) used at the transmission level. At the transmission level,

STATCOM handles only fundamental reactive power and provides voltage support, while a

DSTATCOM is employed at the distribution level or at the load end for dynamic compensation.

The latter, DSTATCOM, can be one of the viable alternatives to SVC in a distribution network.

Additionally, a DSTATCOM can also behave as a shunt active filter, to eliminate unbalance or

distortions in the source current or the supply voltage, as per the IEEE-519 standard limits. Since

a DSTATCOM is such a multifunctional device, the main objective of any control algorithm

should be to make it flexible and easy to implement, in addition to exploiting its multi

functionality to the maximum.

Prior to the type of control algorithm incorporated, the choice of converter configuration

is an important criterion. The two converter configurations are voltage source converter or

current source converter, in addition to passive storage elements, either a capacitor or an inductor

respectively. Normally, voltage source converters are preferred due to their smaller size, less heat

dissipation and less cost of the capacitor, as compared to an inductor for the same rating . This

paper focuses on the comparative study of the control techniques for voltage source converter

based DSTATCOM, broadly classified into voltage control DSTATCOM and current control

DSTATCOM. Under the former, phase shift control is compared with the latter, considering

indirect decoupled current control and regulation of AC bus and DC link voltage with hysteresis

current control. The first two schemes have been successfully implemented for STATCOM

control at the transmission level, for reactive power compensation, and voltage support and are

recently being incorporated to control a DSTATCOM employed at the distribution end. The

following indices are considered for comparison - measurement and signal conditioning

requirement, performance with varying linear/nonlinear load, total harmonic distortion (THD),

DC link voltage variation and switching frequency. The paper briefly describes the salient

features of each strategy, with their merits and demerits. The paper also emphasizes the choice of

24

current control technique, as it significantly affects the performance of a DSTATCOM. A

dynamic simulation model of the DSTATCOM has been developed for various control

algorithms in Matlab/SimPower System environment.

3.2. Basic Principle of DSTATCOM

A DSTATCOM is a controlled reactive source, which includes a Voltage Source

Converter (VSC) and a DC link capacitor connected in shunt, capable of generating and/or

absorbing reactive power. The operating principles of a DSTATCOM are based on the exact

equivalence of the conventional rotating synchronous compensator. The AC terminals of the

VSC are connected to the Point of Common Coupling (PCC) through an inductance, which could

be a filter inductance or the leakage inductance of the coupling transformer, as shown in Fig. 3.1

Fig.3.1. Block diagram of D-STATCOM

The DC side of the converter is connected to a DC capacitor, which carries the input

ripple current of the converter and is the main reactive energy storage element. This capacitor

could be charged by a battery source, or could be precharged by the converter itself. If the output

voltage of the VSC is equal to the AC terminal voltage, no reactive power is delivered to the

system. If the output voltage is greater than the AC terminal voltage, the DSTATCOM is in the

capacitive mode of operation and vice versa. The quantity of reactive power flow is proportional

to the difference in the two voltages.

25

It is to be noted that voltage regulation at PCC and power factor correction cannot be

achieved simultaneously. For a DSTATCOM used for voltage regulation at the PCC, the

compensation should be such that the supply currents should lead the supply voltages; whereas,

for power factor correction, the supply current should be in phase with the supply voltages. The

control strategies studied in this paper are applied with a view to studying the performance of a

DSTATCOM for power factor correction and harmonic mitigation.

3.3. Control Strategies

Satisfactory performance, fast response, flexible and easy implementation are the main

objectives of any compensation strategy. The control strategies of a DSTATCOM are mainly

implemented in the following steps:

Measurements of system variables and signal conditioning

Extraction of reference compensating signals

Generation of firing angles for switching devices

Fig. 3.2 Schematic diagram of DSTATCOM control

Fig. 3.2 shows the schematic diagram of DSTATCOM control, taking into consideration the

above steps. The generation of proper pulse width modulation (PWM) firing is the most

important part of DSTATCOM control and it has a great impact on its compensation objectives,

26

transient as well as steady state performance. Since a DSTATCOM shares many concepts with

that of a STATCOM at the transmission level, a few control techniques have been directly

implemented to a DSTATCOM, incorporating PWM switching, rather than fundamental

frequency switching (FFS) methods. A PWM based distribution static compensator offers faster

response and capability for harmonic elimination.

This paper is an attempt to compare the following schemes of a DSTATCOM for power

factor correction and harmonic mitigation based on:

1. Phase shift control

2. Indirect decoupled current control

3. Regulation of AC bus and DC link voltage

The performance of DSTATCOM with different control schemes have been studied through

digital simulations for common system parameters, as given in the Appendix.

3.3.1. Phase Shift Control

27

Fig. 3.3 phase shift control

The schematic diagram of phase shift control is shown in Fig. 3.3. In this method, the

compensation is achieved by the measuring of the rms voltage at the load point, whereas no

reactive power measurements are required . Sinusoidal PWM technique is used with constant

switching frequency. The error signal obtained by comparing the measured system rms voltage

and the reference voltage is fed to a proportional integral (PI) controller, which generates the

angle for deciding the necessary phase shift between the output voltage of the VSC and the AC

terminal voltage. This angle is summed with the phase angle of the balanced supply voltages,

assumed to be equally spaced at 120 degrees, to produce the desired synchronizing signal

required to operate the PWM generator. In this scheme, the DC voltage is maintained constant,

using a separate battery source.

28

29

Fig 3.4a and Fig 3.4b show the simulation results obtained using phase shift control for

reactive power compensation and harmonic mitigation for a balanced varying linear load and for

a non linear load respectively. It is observed that the source current and the source voltage are in

phase, correcting the power factor of the system in case of a linearly varying load; whereas,

complete compensation is not achieved in case of nonlinear load (source current THD 24.34%).

The frequency spectrum of the source current for a nonlinear load, before and after

compensation, is shown in Fig 3.5a and Fig 3.5b. Though this strategy is easy to implement, is

robust and can provide partial reactive power compensation without harmonic suppression, it has

the following major disadvantages:

The controller does not use a self supporting DC bus and thus requires a very large DC

source to pre charge the capacitor.

Balanced source supply as rms voltage is assumed and the supply phase angle are

calculated over the fundamental only.

No harmonic suppression and partial compensation is achieved in case of nonlinear loads.

30

3.3.2. Indirect Decoupled Current Control

This scheme is based on the governing equations of advanced static var compensator. It

requires the measurement of instantaneous values of three phase line voltages and current. Fig

3.6 shows the block diagram representation of the control scheme. The control scheme is based

on the transformation of the three phase system to a synchronously rotating frame, using Park's

transformation

31

Fig 3.6. Indirect Decoupled Current Control

Subsequently, when the d axis is made to lie on the space vector of the system voltage, its

quadrature component (v q) becomes zero. The compensation is achieved by the control of id and

iq. This is an indirect current control method, where current error compensation is achieved

indirectly through voltage modulation, in order to incorporate simple open loop sine PWM

modulators, so that fixed switching frequency is achieved. Using the definition of the

instantaneous reactive power theory for a balanced three phase three wire system, the real (p) and

the reactive power (q) injected into the system by the DSTATCOM can be expressed under the

dqO reference frame as:

Since v q = 0, id and iq completely describe the instantaneous value of real and reactive

powers produced by the DSTATCOM, when the system voltage remains constant. The

instantaneous three phase line currents measured are transformed by abc to dqO transformation.

The instantaneous id reference and the instantaneous i  q reference are obtained by the regulation

of the DC voltage and the measured AC terminal voltage measured, using PI controllers. The

inner loop of the DSTATCOM controller consists of a rotating frame current controller, as

shown in Fig 3.6.

32

This controller operates in conjunction with the open loop sine PWM generator. The

three line currents are transformed to dqO reference frame and then compared with the

references obtained from the outer loop. The modulating signals for the rotating frame controller

are given by:

Where K p i, Kn, K P 2, KQ are the gains of the two current PI controllers respectively.

During unity power factor operation for linear nonlinear loads, the quadrature reference current

i q is zero. The modulating signals Ud and Uq are transformed back to the abc frame. The

modulating signal output of each phase is compared with a triangular carrier, which is common

for all the three phases. The switching logic for the phase 4 a' is formulated as:

Where v c t is the instantaneous value of triangular carrier waveform and vc  a is the

transformed modulating signal. S a is the switching logic for the inverter leg corresponding to

phase 'a'. Similarly, the switching logic of the other two phases 'b' and 'c' are formulated as

S b and S c .Thus, instantaneous current tracking control is achieved using four PI regulators. A

Phase Locked Loop (PLL) is used to synchronize the control loop to the AC supply, so as to

operate in the abc_to_dqO reference frame.

Fig 3.7a and Fig 7b show the DSTATCOM response for a linearly varying and nonlinear

load. It is observed from the figure that the transient current reaches a very high value before

reaching steady state. Though complete reactive power compensation and power factor

correction is achieved and the THD in case of nonlinear load is reduced to 13.21%, as shown in

the Fig 3.8a and Fig 8b, the main advantage of this scheme is that it incorporates a self

supporting DC bus and the value of the reference DC link voltage is less, as compared to phase

shift control, due to which the converter switches are less stressed. Also, another advantage of

this scheme is that it operates with fixed switching frequency, which provides a definite

harmonic spectrum, independent of the load.

33

Fig. 3.7

34

Fig. 3.8

35

The disadvantages of this scheme are:

Phase Locked Loop gives erroneous results in case of distorted mains and is applicable

for only three phase systems.

It requires intensive computation, including complex transformations, making the

operation complex.

Harmonic suppression is significantly achieved, but not below the IEEE-519 standards.

Bandwidth is restricted due to the use of sine PWM generator.

During transient condition, the supply current shoots to a very high value

3.3.3.Regulation of AC Bus and DC Link Voltage

Three phase AC supply voltages and DC link voltage are sensed and fed to two PI

controllers, the outputs of which decide the amplitude of reactive and active current to be

generated by the DSTATCOM [22]. Figure 9 shows the block diagram of the implemented

scheme. Multiplication of these amplitudes with the in phase and

Fig. 3.9. Block diagram using regulation of AC/DC link voltage scheme.

Quadrature voltage unit vectors yields the respective component of the reference currents.

When applying the algorithm for power factor correction an harmonic elimination, the

quadrature Component of the reference current is made zero. These reference currents and the

sensed line currents are fed to a hysteresis controller, which is used for tracking control. This

36

hysteresis controller adds a hysteresis band +/−h around the calculated reference current. The

switching is obtained as given below:

If isa>(isa_ref+h), the upper switch of inverter leg corresponding to phase ‘a’ is ON and

the lower switch is OFF.

If isa<isa-ref+h), the upper switch of inverter leg corresponding to phase ‘a’ is OFF and

the lower switch is ON.

The tracking becomes better if the hysteresis band is narrower, but the switching

frequency is increased, which results in increased switching losses. Therefore, the choice of

hysteresis band should be a compromise between tracking error and inverter losses [23]. This

method of tracking current control is simple and robust and it exhibits an automatic current

limiting characteristic. This compensation scheme is multifunctional and can also be effectively

used for load unbalancing and harmonic suppression, in addition to power factor correction and

dynamic voltage regulation. The simulated results of the above control scheme are shown in

Figure 10a and b. The transient period is very short and complete reactive power compensation

and power factor correction is achieved in case of both linear/nonlinear loads. In case of

nonlinear load, the THD of the source current is 2.01%, well below the IEEE-519 standard for

harmonic suppression. The frequency spectrum of the load current, before and after

compensation, is shown in Figure 11 a and b.

The advantages of this scheme are:

• The derivation of switching signals uses a hysteresis controller, which is robust and

simple, with fast dynamic response and automatic current limiting capability.

• The algorithm is flexible and can be easily modified for improved voltage regulation,

harmonic suppression and load balancing.

• The inherent property to provide self supporting dc bus does not require complex

abc_dqO transformations.

• The THD in case of nonlinear loads is well below the IEEE-519 standard limits.

37

4. POWER QUALITY

4.1. INTRODUCTION

The contemporary container crane industry, like many other industry segments, is often

enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and

levels of automation that can be achieved. Although these features and their indirectly related

computer based enhancements are key issues to an efficient terminal operation, we must not

forget the foundation upon which we are building. Power quality is the mortar which bonds the

Foundation blocks. Power quality also affects terminal operating economics, crane

reliability, our environment, and initial investment in power distribution systems to support new

crane installations. To quote the utility company newsletter which accompanied the last monthly

issue of my home utility billing: ‘Using electricity wisely is a good environmental and business

practice which saves you money, reduces emissions from generating plants, and conserves our

natural resources.’ As we are all aware, container crane performance requirements continue to

increase at an astounding rate. Next generation container cranes, already in the bidding process,

will require average power demands of 1500 to 2000 kW – almost double the total average

demand three years ago. The rapid increase in power demand levels, an increase in container

crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to

power and control these cranes will increase awareness of the power quality issue in the very

near future.

4.2POWER QUALITY PROBLEMS

For the purpose of this article, we shall define power quality problems as:

“Any power problem that results in failure or misoperation of customer equipment,

Manifests itself as an economic burden to the user, or produces negative impacts on the

environment.”

When applied to the container crane industry, the power issues which degrade power

quality include:

• Power Factor

• Harmonic Distortion

• Voltage Transients

38

• Voltage Sags or Dips

• Voltage Swells

The AC and DC variable speed drives utilized on board container cranes are significant

contributors to total harmonic current and voltage distortion. Whereas SCR phase control creates

the desirable average power factor, DC SCR drives operate at less than this. In addition, line

notching occurs when SCR’s commutate, creating transient peak recovery voltages that can be 3

to 4 times the nominal line voltage depending upon the system impedance and the size of the

drives. The frequency and severity of these power system disturbances varies with the speed of

the drive. Harmonic current injection by AC and DC drives will be highest when the drives are

operating at slow speeds. Power factor will be lowest when DC drives are operating at slow

speeds or during initial acceleration and deceleration periods, increasing to its maximum value

when the SCR’s are phased on to produce rated or base speed. Above base speed, the power

factor essentially remains constant. Unfortunately, container cranes can spend considerable time

at low speeds as the operator attempts to spot and land containers. Poor power factor places a

greater kVA demand burden on the utility or engine-alternator power source. Low power factor

loads can also affect the voltage stability which can ultimately result in detrimental effects on the

Life of sensitive electronic equipment or even intermittent malfunction. Voltage

transients created by DC drive SCR line notching, AC drive voltage chopping, and high

frequency harmonic voltages and currents are all significant sources of noise and disturbance to

sensitive electronic equipment

It has been our experience that end users often do not associate power quality problems

with

Container cranes, either because they are totally unaware of such issues or there was no

economic Consequence if power quality was not addressed. Before the advent of solid-state

power supplies, Power factor was reasonable, and harmonic current injection was minimal. Not

until the crane Population multiplied, power demands per crane increased, and static power

conversion became the way of life, did power quality issues begin to emerge. Even as harmonic

distortion and power Factor issues surfaced, no one was really prepared. Even today, crane

builders and electrical drive System vendors avoid the issue during competitive bidding for new

cranes. Rather than focus on Awareness and understanding of the potential issues, the power

quality issue is intentionally or unintentionally ignored. Power quality problem solutions are

39

available. Although the solutions are not free, in most cases, they do represent a good return on

investment. However, if power quality is not specified, it most likely will not be delivered.

4.3.Power Quality Improvement:

Power quality can be improved through:

• Power factor correction,

• Harmonic filtering,

• Special line notch filtering,

• Transient voltage surge suppression,

• Proper earthing systems.

In most cases, the person specifying and/or buying a container crane may not be fully

aware of the potential power quality issues. If this article accomplishes nothing else, we would

hope to provide that awareness.

In many cases, those involved with specification and procurement of container cranes

may not be cognizant of such issues, do not pay the utility billings, or consider it someone else’s

concern. As a result, container crane specifications may not include definitive power quality

criteria such as power factor correction and/or harmonic filtering. Also, many of those

specifications which do require power quality equipment do not properly define the criteria.

Early in the process of preparing the crane specification:

• Consult with the utility company to determine regulatory or contract requirements that

must be satisfied, if any.

• Consult with the electrical drive suppliers and determine the power quality profiles that

can be expected based on the drive sizes and technologies proposed for the specific

project.

• Evaluate the economics of power quality correction not only on the present situation,

but consider the impact of future utility deregulation and the future development plans for

the terminal

4.4THE BENEFITS OF POWER QUALITY

Power quality in the container terminal environment impacts the economics of the

terminal operation, affects reliability of the terminal equipment, and affects other consumers

served by the same utility service. Each of these concerns is explored in the following

paragraphs.

40

4.4.1. Economic Impact

The economic impact of power quality is the foremost incentive to container terminal

operators. Economic impact can be significant and manifest itself in several ways:

a. Power Factor Penalties

Many utility companies invoke penalties for low power factor on monthly billings. There

is no industry standard followed by utility companies. Methods of metering and calculating

power factor penalties vary from one utility company to the next. Some utility companies

actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours

consumed. Other utility companies monitor kVAR demands and calculate power factor. If the

power factor falls below a fixed limit value over a demand period, a penalty is billed in the form

of an adjustment to the peak demand charges. A number of utility companies servicing container

terminal equipment do not yet invoke power factor penalties. However, their service contract

with the Port may still require that a minimum power factor over a defined demand period be

met. The utility company may not continuously monitor power factor or kVAR usage and reflect

them in the monthly utility billings; however, they do reserve the right to monitor the Port

service at any time. If

The power factor criteria set forth in the service contract are not met, the user may be

penalized, or required to take corrective actions at the user’s expense. One utility company,

which supplies power service to several east coast container terminals in the USA, does not

reflect power factor penalties in their monthly billings, however, their service contract with the

terminal reads as follows:

‘The average power factor under operating conditions of customer’s load at the point

where service is metered shall be not less than 85%. If below 85%, the customer may be required

to furnish, install and maintain at its expense corrective apparatus which will increase the

Power factor of the entire installation to not less than 85%. The customer shall ensure that

no excessive harmonics or transients are introduced on to the [utility] system. This may require

special power conditioning equipment or filters. The IEEE Std. 519-1992 is used as a guide in

Determining appropriate design requirements.

The Port or terminal operations personnel, who are responsible for maintaining container

cranes, or specifying new container crane equipment, should be aware of these requirements.

41

Utility deregulation will most likely force utilities to enforce requirements such as the example

above.

Terminal operators who do not deal with penalty issues today may be faced with some

rather severe penalties in the future. A sound, future terminal growth plan should include

contingencies for addressing the possible economic impact of utility deregulation.

b. System Losses

Harmonic currents and low power factor created by nonlinear loads, not only result in

possible power factor penalties, but also increase the power losses in the distribution system.

These losses are not visible as a separate item on your monthly utility billing, but you pay for

them each month. Container cranes are significant contributors to harmonic currents and low

power factor. Based on the typical demands of today’s high speed container cranes, correction of

power factor

alone on a typical state of the art quay crane can result in a reduction of system losses that

converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals,

this is a significant annual saving in the cost of operation.

c. Power Service Initial Capital Investments

The power distribution system design and installation for new terminals, as well as

modification of systems for terminal capacity upgrades, involves high cost, specialized, high and

medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables,

collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is

directly related to the total kVA demand. As the relationship above indicates, kVA demand is

inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA

for the same kW load. Container cranes are one of the most significant users of power in the

terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power

factor, the total kVA demand is significantly larger than would be the case if power factor

correction equipment were supplied on board each crane or at some common bus location in the

terminal. In the absence of power quality corrective equipment, transformers are larger,

switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system

and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution

42

system equipment for a system which does not address power quality will most likely be higher

than the same system which includes power quality equipment.

4.4.2. Equipment Reliability

Poor power quality can affect machine or equipment reliability and reduce the life of

components. Harmonics, voltage transients, and voltage system sags and swells are all power

quality problems and are all interdependent. Harmonics affect power factor, voltage transients

can induce harmonics, the same phenomena which create harmonic current injection in DC SCR

Variable speed drives are responsible for poor power factor, and dynamically varying

power factor of the same drives can create voltage sags and swells. The effects of harmonic

distortion, harmonic currents, and line notch ringing can be mitigated using specially designed

filters.

4.4.3. Power System Adequacy

When considering the installation of additional cranes to an existing power distribution

system, a power system analysis should be completed to determine the adequacy of the system to

support additional crane loads. Power quality corrective actions may be dictated due to

inadequacy of existing power distribution systems to which new or relocated cranes are to be

connected. In other words, addition of power quality equipment may render a workable scenario

on an existing power distribution system, which would otherwise be inadequate to support

additional cranes without high risk of problems.

4.4.4. Environment

No issue might be as important as the effect of power quality on our environment.

Reduction in system losses and lower demands equate to a reduction in the consumption of our

natural nm resources and reduction in power plant emissions. It is our responsibility as occupants

of this planet to encourage conservation of our natural resources and support measures which

improve our air quality

43

5. Harmonics

5.1Introduction:

The typical definition for a harmonic is “a sinusoidal component of a periodic wave or\

quantity having a frequency that is an integral multiple of the fundamental frequency.” [1]. Some

references refer to “clean” or “pure” power as those without any harmonics. But such clean

waveforms typically only exist in a laboratory. Harmonics have been around for a long time and

will continue to do so. In fact, musicians have been aware of such since the invention of the first

string or woodwind instrument. Harmonics (called “overtones” in music) are responsible for

what makes a trumpet sound like a trumpet, and a clarinet like a clarinet.

Electrical generators try to produce electric power where the voltage waveform has only

one frequency associated with it, the fundamental frequency. In the North America, this

frequency is 60 Hz, or cycles per second. In European countries and other parts of the world, this

frequency is usually 50 Hz. Aircraft often uses 400 Hz as the fundamental frequency. At 60 Hz,

this means that sixty times a second, the voltage waveform increases to a maximum positive

value, then decreases to zero, further decreasing to a maximum negative value, and then back to

zero. The rate at which these changes occur is the trigometric function called a sine wave, as

shown in figure 1. This function occurs in many natural phenomena, such as the speed of a

pendulum as it swings back and forth, or the way a string on a voilin vibrates when plucked.

Fig 5.1. Sine wave

44

The frequency of the harmonics is different, depending on the fundamental frequency.

For example, the 2nd harmonic on a 60 Hz system is 2*60 or 120 Hz. At 50Hz, the second

harmonic is 2* 50 or 100Hz.

300Hz is the 5th harmonic in a 60 Hz system, or the 6th harmonic in a 50 Hz system.

Figure 2 shows how a signal with two harmonics would appear on an oscilloscope-type

display, which some power quality analyzers provide.

Fig 5.2. Fundamental with two harmonics

In order to be able to analyze complex signals that have many different frequencies

present, a number of mathematical methods were developed. One of the more popular is called

the Fourier Transform. However, duplicating the mathematical steps required in a

microprocessor or computer-based instrument is quite difficult. So more compatible processes,

called the FFT for Fast Fourier transform, or DFT for Discrete Fourier Transform, are used.

These methods only work properly if the signal is composed of only the fundamental and

harmonic frequencies in a certain frequency range (called the Nyquist frequency, which is one-

half of the sampling frequency). The frequency values must not change during the measurement

period. Failure of these rules to be maintained can result in mis-information. For example, if a

voltage waveform is comprised of 60 Hz and 200 Hz signals, the FFT cannot directly see the 200

Hz. It only knows 60, 120, 180, 240,..., which are often called “bins”. The result would be that

the energy of the 200 Hz signal would appear partially in the 180Hz bin, and partially in the 240

Hz bin. An FFT-based processer could show a voltage value of 115V at 60 Hz, 18 V at the 3rd

harmonic, and 12 V at the 4th harmonic, when it really should have been 30 V at 200 Hz.

These in-between frequencies are called “inter harmonics”. There is also a special

category of inter harmonics, which are frequency values less than the fundamental frequency

45

value, called sub-harmonics. For example, the process of melting metal in an electric arc furnace

can result large currents that are comprised of the fundamental , inter harmonic, and sub

harmonic frequencies being drawn from the electric power grid. These levels can be quite high

during the melt-down phase, and usually effect the voltage waveform.

5.2.Effects of harmonics:

The presence of harmonics does not mean that the factory or office cannot run properly.

Like other power quality phenomena, it depends on the “stiffness” of the power distribution

system and the susceptibility of the equipment. As shown below, there are a number of different

types of equipment that can have mis operations or failures due to high harmonic voltage and/or

current levels. In addition, one factory may be the source of high harmonics but able to run

properly. This harmonic pollution is often carried back onto the electric utility distribution

system, and may effect facilities on the same system which are more susceptible.

Some typical types of equipment susceptible to harmonic pollution include: - Excessive

neutral current, resulting in overheated neutrals. The odd triplen harmonics in three phase wye

circuits are actually additive in the neutral. This is because the harmonic number multiplied by

the 120 degree phase shift between phases is a integer multiple of 360 degrees. This puts the

harmonics from each of the three phase legs “in-phase” with each other in the neutral, as shown

in Figure 3.

46

Fig 5.3. Additive Third Harmonics

- Incorrect reading meters, including induction disc W-hr meters and averaging type

current meters.

- Reduced true PF, where PF= Watts/VA.

- Overheated transformers, especially delta windings where triplen harmonics generated

on the load side of a delta-wye transformer will circulate in the primary side. Some type

of losses goes up as the square of harmonic value (such as skin effect and eddy current

losses). This is also true for solenoid coils and lighting ballasts.

- Zero, negative sequence voltages on motors and generators. In a balanced system,

voltage harmonics can either be positive (fundamental, 4th, 7th,...), negative (2nd, 5th,

8th...) or zero (3rd, 6th, 9th,...) sequencing values. This means that the voltage at that

particular frequency tries to rotate the motor forward, backward, or neither (just heats up

the motor), respectively. There is also heating from increased losses as in a transformer.

47

Table 1. Harmonic Sequencing Values in Balanced Systems

- Nuisance operation of protective devices, including false tripping of relays and failure

of a UPS to transfer properly, especially if controls incorporate zero-crossing sensing

circuits.

- Bearing failure from shaft currents through un insulated bearings of electric motors.

- Blown-fuses on PF correction caps, due to high voltage and currents from resonance

with line impedance.

- Mis-operation or failure of electronic equipment

- If there are voltage sub harmonics in the range of 1-30Hz, the effect on lighting is called

flicker. This is especially true at 8.8Hz, where the human eye is most sensitive, and just 0.5%

variation in the voltage is noticeable with some types of lighting. [2]

5.3.Causes

How this electricity is used by the different type of loads can have an effect on “purity”

of the voltage waveform. Some loads cause the voltage and current waveforms to lose this pure

sine wave appearance and become distorted. This distortion may consist of predominately

harmonics, depending on the type of load and system impedances.

Since this article is about harmonics, we will concentrate on those types of sources.

“The main sources of harmonic current are at present the phase angle controlled rectifiers and

inverters.” [3] These are often called static power converters. These devices take AC power and

convert it to another form, sometimes back to AC power at the same or different frequency,

based on the firing scheme. The firing scheme refers to the controlling mechanism that

determines how and when current is conducted. One major variation is the phase angle at which

conduction begins and ends.

48

A typical such converter is the switching-type power supplies found in most personal

computers and peripheral equipment, such as printers. While they offer many benefits in size,

weight and cost, the large increase of this type of equipment over the past fifteen years is largely

responsible for the increased attention to harmonics.

Figure shows below how a switching-type power supply works. The AC voltage is

converted into a DC voltage, which is further converted into other voltages that the equipment

needs to run. The rectifier consists of semi-conductor devices (such as diodes) that only conduct

current in one direction. In order to do so, the voltage on the one end must be greater than the

other end. These devices feed current into a capacitor, where the voltage value on the cap at any

time depends on how much energy is being taken out by the rest of the power supply.

When the input voltage value is higher than voltage on the capacitor, the diode will

conduct current through it. This results in a current waveform as shown in Figure 5, and

harmonic spectrum in Figure 6. Obviously, this is not a pure sinusoidal waveform with only a 60

Hz frequency component.

Fig 5.4. Current Waveform

49

Fig 5.5. Harmonic Spectrum of Current Waveform Shown in Fig 5.4

If the rectifier had only been a half wave rectifier, the waveform would only have every

other current pulse, and the harmonic spectrum would be different, as shown in Fig 5.5.

Fluorescent lights can be the source of harmonics, as the ballasts are non-linear inductors.

The third harmonic is the predominate harmonic in this case. (See Table 3) As previously

mentioned, the third harmonic current from each phase in a four-wire wye or star system will be

additive in the neutral, instead of cancelling out Some of the newer electronic ballasts have very

significant harmonic problems, as they operate somewhat like a switching power supply, but can

result in current harmonic distortion levels over 30%.

50

Table 2. Sample of Harmonic Values for Fluorescent lighting [4]

Low power, AC voltage regulators for light dimmers and small induction motors adjust

the phase angle or point on the wave where conduction occurs. Medium power converters are

used for motor control in manufacturing and railroad applications, and include such equipment as

ASDs (adjustable speed drives) and VFDs (variable frequency drives). Metal reduction

operations, like electric arc furnaces, and high voltage DC transmission employ large power

converters, in the 2-20MVA rating.

This type of 3-phase equipment may also cause other types of power quality problems.

When the semiconductor device is suppose to turn-off, it does not do so abruptly. This happens

under “naturally” commutated conditions, where the voltage that was larger on the anode side

compared to the cathode is now the opposite. This occurs each cycle as the voltage waveform

goes through the sine waveform. It also happens under “forced” commutation conditions, where

the semi-conductor device has a “gate”-type control mechanism built in to it. This commutation

period is a time when two semiconductor devices are both conducting current at the same time,

effectively shorting one phase to the other and resulting in large current transients.

When transformers are first energized, the current drawn is different from the steady state

condition. This is caused by the inrush of the magnetizing current. The harmonics during this

period varies over time. Some harmonics have zero value for part of the time, and then increase

for a while before returning to zero. An unbalanced transformer (where either the output current,

winding impedance or input voltage on each leg are not equal) will cause harmonics, as will

overvoltage saturation of a transformer.

Where to look for them

51

Wherever the aforementioned equipment is used, one can suspect that harmonics are

present. The amount of voltage harmonics will often depend on the amount of harmonic currents

being drawn by the load, and the source impedance, which includes all of the wiring and

transformers back to the source of the electricity. Ohm’s Law says that Voltage equals Current

multiplied by Impedance. This is true for harmonic values as well. If the source harmonic

impedance is very low (often referred to as a “stiff” system) then the harmonic currents will

result in lower harmonic voltages than if the source impedance were high (such as found with

some types of isolation transformers).

Like any power quality investigation, the search can begin at the equipment effected by

the problem or at the point-of-common-coupling (PCC), where the utility service meets the

building distribution system. If only one piece of equipment is effected (or suspected), it is often

easier to start the monitoring process there. If the source is suspected to be from the utility

service side (such is the case when there is a neighboring factory that is known to generate high

harmonics), then monitoring usually begins at the PCC.

The phase voltages and currents, as well as the neutral-to-ground voltage and neutral

current should be monitored, where possible. This will aid in pinpointing problems, or detecting

marginal systems. Monitoring the neutral will often show a high 3rd harmonic value, indicating

the presence of non-linear loads in the facility.

5.4.How do you find harmonics

Hand-held harmonic meters can be useful tools for making spot checks for known

harmonic problems. However, harmonic values will often change during the day, as different

loads are turned on and off within the facility or in other facilities on the same electric utility

distribution system. This requires the use of a harmonic monitor or power quality monitor with

harmonic capabilities (such as shown in Figure 8), which can record the harmonic values over a

period of time.

52

Fig 5.6. Power Quality Monitor with Harmonic Analysis

Typically, monitoring will last for one business cycle. A business cycle is how long it

takes for the normal operation of the plant to repeat itself. For example, if a plant runs three

identical shifts, seven days a week, then a business cycle would be eight hours. More typically, a

business cycle is one week, as different operations take place on a Monday, when the plant

equipment is restarted after being off over the weekend, then on a Wednesday, or a Saturday,

when only a Skelton crew may be working.

Certain types of loads also generate typical harmonic spectrum signatures that can point

the investigator towards the source. This is related to the number of pulses, or paths of

conduction. The general equation is h = ( n * p ) +/- 1, where h is the harmonic number, n is any

integer (1,2,3,..) and p is the number of pulses in the circuit, and the magnitude decreases as the

ration of 1/h (1/3, 1/5, 1/7, 1/9,...). Table 4 shows examples of such.

Table 3. Typical Harmonics Found for Different Converters.

53

5.5. Effects of harmonics

Most electrical loads (except half-wave rectifiers) produce symmetrical current

waveforms, which mean that the positive half of the waveform looks like a mirror image of the

negative half. This results in only odd harmonic values being present. Even harmonics will

disrupt this half-wave symmetry. The presence of these even harmonics should cause the

investigator to suspect there is a half-wave rectifier on the circuit. This also results from a full

wave rectifier when one side of the rectifier has blown or damaged components. Early detection

of this condition in a UPS system can prevent a complete failure when the load is switched onto

back-up power.

To determine what is normal or acceptable levels, a number of standards have been

developed by various organizations. ANSI/IEEE C57.110 Recommended Practice for

Establishing Transformer Compatibility When Supplying No sinusoidal Load Currents is a

useful document for determining how much a transformer should be derated from its nameplate

rating when operating in the presence of harmonics. There are two parameters typically used,

called K-factor and TDF (transformer dereading factor). Some power quality harmonic monitors

will automatically calculate these values.

IEEE 519-1992 Recommended Practices and Requirements for Harmonic Control in

Electrical Power Systems provides guidelines from determining what acceptable limits are. The

harmonic limits for current depend on the ratio of Short Circuit Current (SCC) at PCC (or how

stiff it is) to average Load Current of maximum demand over 1 year, as illustrated in Table 5.

Note how the limit decreases at the higher harmonic values, and increases with larger ratios.

Table 4. Current Harmonic Limits as per IEEE 519-1992

54

For voltage harmonics, the voltage level of the system is used to determine the limits, as

shown in Table 6. At the higher voltages, more customers will be effective, hence, the lower

limits.

Table 5. Voltage Harmonic Limits as per IEEE 519-1992

The European Community has also developed susceptibility and emission limits for\

harmonics. Formerly known as the 555-2 standard for appliances of less than 16 A, a more

encompassing set of standards under IEC 1000-4-7 are now in effect.

5.6. MINIMIZATION OF HARMONICS

Care should be undertaken to make sure that the corrective action taken to minimize the

harmonic problems don’t actually make the system worse. This can be the result of resonance

between harmonic filters, PF correcting capacitors and the system impedance.

Isolating harmonic pollution devices on separate circuits with or without the use of

harmonic filters are typical ways of mitigating the effects of such. Loads can be relocated to try

to balance the system better. Neutral conductors should be properly sized according to the latest

NEC-1996 requirements covering such. Whereas the neutral may have been undersized in the

past, it may now be necessary to run a second neutral wire that is the same size as the phase

conductors. This is particularly important with some modular office partition-type walls, which

can exhibit high impedance values. The operating limits of transformers and motors should be

derated, in accordance with industry standards from IEEE, ANSI and NEMA on such. Use of

higher pulse converters, such as 24-pulse rectifiers, can eliminate lower harmonic values, but at

the expense of creating higher harmonic values.

55

6. ADVANTAGES

• The derivation of switching signals uses a hysteresis controller, which is robust and

simple, with fast dynamic response and automatic current limiting capability.

• The algorithm is flexible and can be easily modified for improved voltage regulation,

harmonic suppression and load balancing.

• The inherent property to provide self supporting dc bus does not require complex

abc_dqO transformations.

• The THD in case of nonlinear loads is well below the IEEE-519 standard limits.

56

7. CONCLUSION

The paper presents the comparative study of three control strategies used for the control

of DSTATCOM, with their relative merits and demerits. The control schemes are described with

the help of simulation results, under linear and nonlinear loads. Simulation results show the

suitability of AC/DC bus voltage regulation for harmonic suppression and reactive power

compensation. A comparison of the three control strategies is shown in Table 1. It can also be

concluded that though conceptually similar to a STATCOM at the transmission level, a

DSTATCOM’s control scheme should be such that in addition to complete reactive power

compensation, power factor correction and voltage regulation of the harmonics are also checked,

in order to achieve improved power quality levels at the distribution end.

57

References:

1. J. Dixon, L. Moran, and J. Rodriguez, Reactive Power Compensation Technologies: State of art Review, Proceedings of the IEEE. Vol. 93.No. 12,2005.

2. J. Dixon, Y. del Valle, M. Orchard, M. Ortizar, L. Moran, and C. Maffrand, A Full Compensating System for General Loads Based on a Combination of Thyristor Binary Compensator, and a PWM-IGBT Active Power Filter, IEEE Trans. On Industrial Electronics. Vol. 18, No. 4, Oct. 2003, pp. 982-9.

3. A. E. Hammad, Comparing the Voltage Source capability of Present and future Var Compensation Techniques in Transmission Systems, IEEE Trans, on Power Deliverv.vol.il. No. 1 Jan 1995.

4. J. Nastran, R. Cajhen, M. Seliger, and P. Jereb, Active Power Filters for nonlinear AC loads, IEEE Trans.on Power Electronics.Vol. 9. No. l, pp. 92-6, Jan. 1994.

5. L. A. Moran, J. W. Dixon, and R. R. Wallace, A three phase Active Power Filter with fixed switching frequency for reactive power and current harmonic compensation, IEEE Trans. On Industrial Electronics. Vol. 42, pp. 402-8, Aug. 1995

6. L. T. Moran, P. D. Ziogas, and G. Joos, Analysis and design of a three phase current source solid state Var Compensator, IEEE Trans, on Industry Applications.Vol. 25. No. 2, 1989, pp. 356-65.

7. D. Shen, and P. W. Lehn, Modeling, analysis and control of a current source inverter based STATCOM. IEEE Trans.on Power Deliverv. Vol.17. No. l, pp. 248-53, 2002.

8. V. Ye, M. Kazerani, and V. Quintana, Current source converter based STATCOM: Modelling and Control, IEEE Trans. on Power Deliverv. Vol. 20. No. 2, pp. 795-800, Apr. 2005.

9. M. Mishra, A. Ghosh, and A. Joshi, Operation of a DSTATCOM in voltage control mode, IEEE Trans, on Power Delivery, vol. 18, No.l, pp. 258-264, Jan. 2003

10. O. A. Lara, and E. Acha, Modelling and Analysis of Custom power Systems by PSCAD/EMTDC, IEEE Trans, on Power Deliverv. Vol. 17. No. l, pp. 266-272, Jan. 2002.

11. C. Schauder, and H. Mchta, Vector Analysis and Control of Advanced Static VAR Compensators.TEE proceedinas-C. Vol. 140, pp. 299-306, July. 1993.

12. J. Sun, D. Czarkowski, and Z. Zabar. Voltage Flicker Mitigation Using PWM-Based Distribution STATCOM, IEEE Conference. 2002 pp. 616- 21.

58

13. B. Singh, V. Mishra, and R. K. P. Bhatt, Performance Analysis of Static Condenser for Article for copyediting Voltage Regulation, Power Factor Correction and Load Balancing, Institution of Engineer’s m. Vol. 84.2003.

14. B. N. Singh, B. Singh, A. Chandra, and K. Al-Haddad, A New Control Strategy of a Three phase Solid State Compensator for Voltage Regulation, Power Factor Correction and Load Balancing, Institution of Engineer’s Vol. 83.2002.

15. V. G. Acha, O. Agelidis, Anaya Lara, and T.J.E. Miller, Power Electronic Control in Electrical Systems. England.Newnes 2002.

16. T. J. E. Miller, Reactive Power Control in Electric Systems, John Wileyand Sons, 1982.

17. N. G. Hingorani, Understanding FACTS IEEE Press, 2002.

18. KK. Sen, STATCOM-Theory. Modeling and Applications, IEEE Conference 1998, pp. 1177-83.

19. P. Giroux, G. Sybille, and H. LeHully, Modeling and Simulation of a Distribution STATCOM using Simulink’s Power System Blockset, IECON’OI. IEEE Industrial Electronics Society, pp. 990-6.

20. J. H. Akagi. Y. Kangawa, and A. Nabae, Instantaneous Reactive Power Compensator Comprising Switching Devices Without Energy Storage Components, IEEE Trans. on Industry Applications. Vol. lA-20. May-June. 1984.

21. F.H. Watanbe, R.Yl. Stephan, and M. Aredes, New Concepts of Instantaneous Active and Reactive Powers in Electrical Systems with Generic Loads, IEEE Trans.on Power Delivery. Vol. S No. 2, pp. 697- 703, April 1993.

22. JB. N. Singh, Bhim Singh, A. Chandra, and Kamal Al Haddad, Digital implementation of an advanced static compensator for voltage profile improvement, power factor correction and balancing of unbalanced reactive loads, Electric Power Systems Research 54(20001101-11).

23. A. Ghosh, and G. Ledwich, Power Quality Enhancement Using Custom Power Devices. Kluwer Academic Publishers, London 2002.

59