power quality improvement by using statcom for three-phase

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Vol.07,Issue.14,

October-2015,

Pages:2874-2881

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Power Quality Improvement by using STATCOM for Three-Phase Load

with Induction Motor Drive KODALI MANOHARA BABU

1, ALLADI ARUN KUMAR

2

1PG Scholar, Dept of EEE, V N R College of Engineering, Ponnur, Guntur (Dt), AP, India.

2Associate Professor, Dept of EEE, V N R College of Engineering, Ponnur, Guntur (Dt), AP, India.

Abstract: Distribution system, as the name suggest, is the

medium through which power is distributed among the end

consumers Distribution systems are comparatively not as

stiff as grid systems, so large starting currents and

objectionable voltage drop during the starting of an

induction motor could be critical for the entire system. Thus

STATCOM is an effective solution for power systems

facing such power quality problems. This report deals with

one of the potential applications of static compensator

(STATCOM) to industrial systems for mitigation of voltage

dip problem. The dip in voltage is generally encountered

during the starting of an induction motor. A dynamic model

of an SEIG-STATCOM system with the ability to

compensate the unbalanced current caused by single-phase

loads that are connected across the two terminals of the

three-phase SEIG under varying loads has been analyzed by

using D-Q frame theory algorithm. This enables us to

predict the behavior of the system under transient

conditions. The simulated results shows that by using a

STATCOM based voltage regulator the SEIG can balance

the current; in addition to that the STATCOM is able to

regulate the terminal voltage of the generator and suppresses

the harmonic currents injected by nonlinear loads. This

paper work aims at developing a STATCOM for induction

machines with reduced voltage dip.

Keywords: Self-Excited Induction Generator (SEIG),

Single Phase Synchronous D-Q Frame Theory, Static

Synchronous Compensator (STATCOM).

I. INTRODUCTION Now a days due to increased power quality problems by

using of switch off/on introduction loads, nonlinear load and

induction motor etc in domestic and industries, power-

quality (PQ) problems, such as harmonics, flicker, and

imbalance have become serious concerns. In addition,

lightning strikes on transmission lines, switching of

capacitor banks, and various network Faults can also cause

PQ problems, such as transients, voltage sag/swell, and

interruption. On the other hand, an increase of sensitive

loads involving digital electronics and complex process

controllers requires a pure sinusoidal supply voltage for

proper load operation. To meet power quality to the

standard limits need some sort of compensation. In few

years back to mitigate the power quality problems in

distribution system by using passive filters like capacitor

banks. Now these research going very fast to mitigate the

power quality problems with help of power conditioning

devices [7]. Power quality and reliability cost the industry

large amounts due to mainly sags and short-term

interruptions. Distorted and unwanted voltage wave forms,

too. And the main concern for the consumers of electricity

was the reliability of supply. Here we define the reliability

as the continuity of supply. The problem of distribution

lines is divided into two major categories. First group is

power quality, second is power reliability. First group

consists of harmonic distortions, impulses and swells.

Second group consists of voltage sags and outages. Voltage

sags is much more serious and can cause a large amount of

damage. If exceeds a few cycle, motors, robots, servo drives

and machine tools cannot maintain control of process.

Transmission lines are exposed to the forces of nature.

Fig.1. Schematic diagram of the SEIG–STATCOM

system feeding single-phase loads.

Furthermore, each transmission line has its load ability

limit that is often determined by either stability constraints

or by thermal limits or by the dielectric limits. Even though

the power quality problem is distribution side problem,

transmission lines are often having an impact on the quality

of the power supplied. It is however to be noted that while

most problems associated with the transmission systems

KODALI MANOHARA BABU, ALLADI ARUN KUMAR

International Journal of Advanced Technology and Innovative Research

Volume.07, IssueNo.14, October-2015, Pages: 2874-2881

arise due to the forces of nature or due to the

interconnection of power systems, individual customers are

responsible for more substantial fraction of the problems of

power distribution systems. Transmission lines are exposed

to the forces of nature. Furthermore, each transmission line

has its load ability limit that is often determined by either

stability constraints or by thermal limits or by the dielectric

limits. Even though the power quality problem is

distribution side problem, transmission lines are often

having an impact on the quality of the power supplied. It is

however to be noted that while most problems associated

with the transmission systems arise due to the forces of

nature or due to the interconnection of power systems,

individual customers are responsible for more substantial

fraction of the problems of power distribution systems.

II. SYSTEM CONFIGURATION AND PRINCIPLE OF

OPERATION

Fig.1 shows the schematic diagram of the STATCOM

compensated three-phase SEIG feeding single-phase loads.

The system consists of an SEIG driven by renewable

energy-based prime mover. The single-phase consumer

loads are connected across ―a‖ and ―c‖ phases of the SEIG.

A two-level, three-leg insulated-gate bipolar transistor

(IGBT)-based VSI with a self sustaining dc-bus capacitor is

used as a STATCOM. The STATCOM is connected at point

of common coupling (PCC) through filter inductors as

shown in Fig.1. The STATCOM regulates the system

voltage by maintaining equilibrium among the reactive

power circulations within the system. Moreover, the

STATCOM suppresses harmonics injected by nonlinear

loads and provides load balancing while feeding single-

phase loads. The unbalanced load currents in a three-phase

system can be divided into two sets of balanced currents

known as positive sequence components and negative

sequence components.

Fig .2 Block diagram of the single-phase synchronous D-

Q theory control algorithm for the STATCOM.

In order to achieve balanced source currents, the source

should be free from the negative sequence components of

load currents. Therefore, when the STATCOM is connected

across PCC, it supplies the negative sequence currents

needed by the unbalanced load or it draws another set of

negative sequence currents which are exactly180◦ out of

phase to those drawn by unbalanced load so as to nullify the

effect of negative sequence currents of unbalanced loads.

III. CONTROL ALGORITHM OF THE STATCOM

Fig.2 shows the block diagram of the proposed single-phase

synchronous D-Q frame theory-based control algorithm for

the three-phase STATCOM. The reference source currents

(i∗sa,i∗sb,i∗sc) for regulating the terminal voltage and current

balancing are computed using a single-phase synchronous

D-Q frame theory applied to the three-phase SEIG system.

A. Single-Phase Synchronous Rotating D-Q Frame

Theory

It is simple to design a controller for a three-phase

system in synchronously rotating D-Q frame because all the

time-varying signals of the system become dc quantities and

time-invariant. In case of a three-phase system, initially, the

three-phase voltages or currents (in abc frame) are

transformed to a stationary frame (α−β) and then to

synchronously rotating D-Q frame. Similarly, to transform

an arbitrary signal ―x(t)‖ of a single-phase system into a

synchronously rotating D-Q frame, initially that variable is

transformed into a stationary α−β frame using the single-

phase p-q theory [28]–[30] and then to a synchronously

rotating D-Q frame. Therefore, to transform a signal into a

stationary α−β frame, at least two phases are needed. Hence,

a pseudo second phase for the arbitrary signal x(t) is created

by giving 90◦ lag to the original signal. The original signal

represents the component of α-axis and90◦ lag signal is the

β-axis component of stationary reference frame.

Fig.3. Stationary α−β frame and synchronously rotating

D-Q frame representation of vector x (t).

Therefore, an arbitrary periodic signal x(t)with a time

period of ―T‖ can be represented in a stationary α−β frame

as

Power Quality Improvement by using STATCOM for Three-Phase Load with Induction Motor Drive



(1)

For a single-phase system, the concept of the stationary

α−β frame and synchronously rotating D-Q frames relative

to an arbitrary periodic signal x(t)is illustrated in Fig.3. The

signal x(t) is represented as vector x, and the vector x can be

decomposed into two components xα and xβ. As the x vector

rotates around the center, its components xα and xβ which

are the projections on the α−β axes vary in time accordingly.

Now, considering that there are synchronously rotating D-Q

coordinates that rotate with the same angular frequency and

direction as x, then the position of x with respect to its

components xD and xQ is same regardless of time.

Therefore, it is clear that the xD and xQ do not vary with

time and only depend on the magnitude of xand its relative

phase with respect to the D-Q rotating frame. The angle θ is

the rotating angle of the D-Q frame and it is defined as

(2)

Where ω is the angular frequency of the arbitrary variable x.

The relationship between stationary and synchronous

rotating frames can be derived from Fig.3. The components

of the arbitrary single-phase variable x(t) in the stationary

reference frame are transformed into the synchronously

rotating D-Q frame using the transformation matrix ―C‖ as

(3)

Where,

(4)

B. Reference Source Currents Estimation Using Single-

Phase Synchronous Rotating D-Q Frame Theory

The main objective of employing a three-phase

STATCOM in a three-phase SEIG-based standalone power

generating system feeding single-phase consumer loads is to

balance the generator currents so that the generator can be

loaded to its full capacity without derating. The control

structure of the STATCOM employs an ac voltage PI

controller to regulate the system voltage and a dc bus

voltage PI controller to maintain the dc bus capacitor

voltage constant and greater than the peak value of the line

voltage of PCC for successful operation of the STATCOM.

The PCC voltages (va, vb, vc), source currents (isa, isb, isc),

load current (il), and dc bus voltage (Vdc) are sensed and

used as feedback signals. Considering PCC voltages as

balanced and sinusoidal, the amplitude of the PCC voltage

(or system voltage) is estimated as

(5)

Consider one of the three phases at a time and then

transform the voltages and currents of that particular phase

into a Stationary α−β frame, then the PCC voltages and load

current in stationary α−β frame are represented as

(6)

(7)

(8)

(9)

(10)

The sinusoidal signal filters based on a second-order

generalized integrator [31] or a sinusoidal signal integrator

(SSI) [32] can be used for creating β-axis signals which are

lagging the original signals. In the present investigation, a

filter based on SSI is used. The SSI filters generate

quadrature signals using system frequency information.

Since the system frequency fluctuates under load

perturbations, a PLL [31] is used to continuously estimate

the system frequency, and the estimated frequency is fed to

SSI filters which makes the proposed control adaptive to

frequency fluctuations, thereby avoids the loss of

synchronization of the STATCOM. Now consider a

synchronously rotating D-Q frame for phase ―a‖ which is

rotating in the same direction as va(t), and the projections of

the load currentil(t)to the D-Q axes give the D and Q

components of the load current. Therefore, the D-axis and

Q-axis components of the load current in phase ―a‖ are

estimated as

(11)

Where cosθa and sinθa are estimated using vaα and vaβ as

follows:

(12)

IlaD represents the active power component of the load

current as the signals belong to the same axis are multiplied

and added to estimate the D-axis component, where as IlaQ

represents the reactive power component of the load current

as the orthogonal signals are multiplied and added to derive

the Q-axis component. Similarly, the D-axis and Q-axis

components of the load current in phase ―c‖ are estimated as

(13)

The negative sign of currents in (11) indicates that the

load current in phase ―c‖ is equal to phase ―a‖ but 180◦ out

of phase. As the single-phase load is connected across the

phases ―a‖ and ―c,‖ D-axis and Q-axis components for

phase ―b‖ are not estimated. The D-axis components of the

load current in phases ―a‖ and ―c‖ are added together to

obtain an equivalent D-axis current component of total load

on the SEIG as

(14)




Similarly, an equivalent Q-axis current component of

total load on the system is estimated as

(15)

The equivalent D-axis and Q-axis current components of

total load are decomposed into two parts namely

fundamental and oscillatory parts as

(16)

(17)

The reason for the existence of the oscillatory part is due

to the nonlinear and single-phase nature of connected loads

in the system. Even if the connected loads are linear in

nature, the D and Q components estimated in (12) and (13)

would still contain oscillatory parts due to the unbalance

caused by single-phase loads. To ensure the power quality,

the reference D-axis and Q-axis components of source

currents must be free from these oscillatory components.

Hence, the signals IlD and IlQ are passed through low-pass

filters (LPFs) to extract the fundamental (or dc) components

as shown in Fig.2. To maintain the dc-bus capacitor voltage

of the STATCOM at a reference value, it is sensed and

compared with the reference value and then the obtained

voltage error is processed through a PI controller. The dc-

bus voltage error of the STATCOM Vdcer at kth sampling

instant is expressed as

(18)

Where Vdcref(k) and Vdc(k) are the reference and sensed dc-

bus voltages of the STATCOM at kth sampling instant,

respectively. In the present investigation, the dc-bus voltage

reference is set to 400 V. The output of the PI controller for

maintaining a constant dc bus voltage of the STATCOM at

kth

sampling instant is expressed as

(19)

Where Iloss is the active power component of the current (or

D-axis current component) that must be supplied to meet the

losses in the STATCOM. Kpd and Kid are the proportional

and integral gain constants of the dc-bus voltage PI

controller, respectively. The source should supply the power

loss component of the current (Iloss) along with the filtered

equivalent D-axis current component of the single-phase

load estimated in (14). In order to ensure balanced and

sinusoidal source currents, the D-axis component of source

currents after compensation must be equal for all the phases

and it should not contain any ripple.

IV. INDUCTION MOTOR

An induction motor (IM) is a type of asynchronous AC

motor where power is supplied to the rotating device by

means of electromagnetic induction. Other commonly used

name is squirrel cage motor due to the fact that the rotor

bars with short circuit rings resemble a squirrel cage

(hamster wheel).An electric motor converts electrical power

to mechanical power in its rotor. There are several ways to

supply power to the rotor. In a DC motor this power is

supplied to the armature directly from a DC source, while in

an induction motor this power is induced in the rotating

device. An induction motor is sometimes called a rotating

transformer because the stator (stationary part) is essentially

the primary side of the transformer and the rotor (rotating

part) is the secondary side. Induction motors are widely

used, especially poly phase induction motors, which are

frequently used in industrial drives. The Induction motor is

a three phase AC motor and is the most widely used

machine. Its characteristic features are-

Simple and rugged construction

Low cost and minimum maintenance

High reliability and sufficiently high efficiency

Needs no extra starting motor and need not be

synchronized

An Induction motor has basically two parts – Stator and

Rotor

The Stator is made up of a number of stampings with

slots to carry three phase windings. It is wound for a definite

number of poles. The windings are geometrically spaced

120 degrees apart. Two types of rotors are used in Induction

motors - Squirrel-cage rotor and Wound rotor.

A. AC Induction Motor

The AC induction motor is a rotating electric machine

designed to operate from a 3-phase source of alternating

voltage. For variable speed drives, the source is normally an

inverter that uses power switches to produce approximately

sinusoidal voltages and currents of controllable magnitude

and frequency. A cross-section of a two-pole induction

motor is shown in Fig.4. Slots in the inner periphery of the

stator accommodate 3-phase winding a,b,c.

Fig .4. 3-Phase AC Induction Motor.

The turns in each winding are distributed so that a current

in a stator winding produces an approximately sinusoidally-

distributed flux density around the periphery of the air gap.

When three currents that are sinusoidally varying in time,




but displaced in phase by 120° from each other, flow

through the three symmetrically-placed windings, a radially-

directed air gap flux density is produced that is also

sinusoidally distributed around the gap and rotates at an

angular velocity equal to the angular frequency, of the stator

currents. The most common type of induction motor has a

squirrel cage rotor in which aluminum conductors or bars

are cast into slots in the outer periphery of the rotor. These

conductors or bars are shorted together at both ends of the

rotor by cast aluminum end rings, which also can be shaped

to act as fans. In larger induction motors, copper or copper-

alloy bars are used to fabricate the rotor cage winding.

As the sinusoidally-distributed flux density wave produced

by the stator magnetizing currents sweeps past the rotor

conductors, it generates a voltage in them. The result is a

sinusoidally-distributed set of currents in the short-circuited

rotor bars. Because of the low resistance of these shorted

bars, only a small relative angular velocity, r, between the

angular velocity, s, of the flux wave and the mechanical

angular velocity of the two-pole rotor is required to produce

the necessary rotor current. The relative angular velocity, r,

is called the slip velocity. The interaction of the

sinusoidally-distributed air gap flux density and induced

rotor currents produces a torque on the rotor. The typical

induction motor speed-torque characteristic is shown in Fig

.5.

Fig.5.AC Induction Motor Speed-Torque Characteristic.

Squirrel-cage AC induction motors are popular for their

simple construction, low cost per horsepower, and low

maintenance (they contain no brushes, as do DC motors).

They are available in a wide range of power ratings. With

field-oriented vector control methods, AC induction motors

can fully replace standard DC motors, even in high-

performance applications.

V. MATLAB/SIMULINK RESULTS

The proposed SEIG–STATCOM system has been

developed and tested simulation at different loads with

induction motor. The simulation results presented in Figs. 6

to 20 demonstrate the performance of the developed system

under steady state as well as dynamic conditions and

induction motor.

Case I: For Linear Load

Fig.6.Simulink circuit for linear load.

Fig.7. Source voltage and current.

Fig.8. Source voltage and load current.

Fig.9.FFT window for source voltage




Fig.10. FFT window for source load current.

Fig.11.Simulation results for active power at source side

and load side.

Case II: For Single Phase Non Linear Load

Fig.12.Simulink circuit for single phase non linear load.

Fig.13.Simulation results for source voltage and source

currents.

Fig.14. Simulation results for source voltage, dc link

current, rms voltage and load current.

Fig.15.Source voltage and compensation currents.

Fig.16.Simulated results for input power factor.

Fig.17.THD analysis for source voltage




Case III: Three Phase Non Linear Load With Induction

Motor

Fig.18.Simulink circuit for three phase non linear load

with induction motor.

Fig.19.Simulation results for line to line voltages.

Fig.20.Simulation results for current, speed and

electromagnetic torque.

VI. CONCLUSION

The proposed method of feeding single-phase loads from a

three-phase induction motor and STATCOM combination

has been tested, and it has been proved that the SEIG is able

to feed single phase loads up to its rated capacity. A single-

phase synchronous D-Q frame theory-based control of a

three-phase STATCOM has been proposed, discussed,

implemented for current balancing of the SEIG system with

an induction motor application to perform motor

characteristics.

VII. REFERENCES

[1] E. D. Bassett and F. M. Potter, ―Capacitive excitation for

induction generators,‖ Trans. Amer. Inst. Elect. Eng., vol.

54, no. 5, pp. 540–545, May 1935.

[2] J. E. Barkle and R. W. Ferguson, ―Induction generator

theory and application,‖ Trans. Amer. Inst. Elect. Eng., vol.

73, no. 1, pp. 12–19, Jan.1954.

[3](2013).[Online].Available: http://www.picohy-dro.org.uk

[4] N. Smith Motors as Generators for Micro-Hydro Power.

London, U.K.: ITDG Publishing, 1994.

[5] S. Khennas and A. Barnett, ―Best practices for

sustainable development of micro hydro power in

developing countries,‖ World Bank, Washington, DC, USA,

ESMAP Tech. Rep. 21640, no. 6, 2000.

[6] H. Rai, A. Tandan, S. Murthy, B. Singh, and B. Singh,

―Voltage regulation of self excited induction generator using

passive elements,‖ inProc. IEEE Int. Conf. Elect. Mach.

Drives, Sep. 1993, pp. 240–245.

[7] L. Shridhar, B. Singh, and C. Jha, ―Transient

performance of the self regulated short shunt self excited

induction generator,‖IEEE Trans. Energy Convers., vol. 10,

no. 2, pp. 261–267, Jun. 1995.

[8] E. Bim, J. Szajner, and Y. Burian, ―Voltage

compensation of an induction generator with long-shunt

connection,‖ IEEE Trans. Energy Convers., vol. 4, no. 3, pp.

526–530, Sep. 1989.

[9] L. Shridhar, B. Singh, C. Jha, B. Singh, and S. Murthy,

―Selection of capacitors for the self regulated short shunt

self excited induction generator,‖ IEEE Trans. Energy

Convers., vol. 10, no. 1, pp. 10–17, Mar. 1995.

[10] L. Wang and C.-H. Lee, ―Long-shunt and short-shunt

connections on dynamic performance of a SEIG feeding an

induction motor load,‖ IEEE Trans. Energy Convers., vol.

15, no. 1, pp. 1–7, Mar. 2000.

[11] M. B. Brennen and A. Abbondanti, ―Static exciters for

induction generators,‖ IEEE Trans. Ind. Appl., vol. IA-13,

no. 5, pp. 422–428, Sep. 1977.

[12] B. Singh and L. Shilpakar, ―Analysis of a novel solid

state voltage regulator for a self-excited induction

generator,‖ IEE Proc.—Generat., Transmits. Distrib. vol.

145, no. 6, pp. 647–655, Nov. 1998.

[13] S.-C. Kuo and L. Wang, ―Analysis of voltage control

for a self-excited induction generator using a current-

controlled voltage source inverter (CC-VSI),‖ IEE Proc.—

Generat. Transmits. Distrib., vol. 148, no. 5, pp. 431–438,

Sep. 2001.

[14] B. Singh, S. Murthy, and S. Gupta, ―STATCOM-based

voltage regulator for self-excited induction generator




feeding nonlinear loads,‖ IEEE Trans. Ind. Electron., vol.

53, no. 5, pp. 1437–1452, Oct.2006.

[15] G. Dastagir and L. A. C. Lopes, ―Voltage and

frequency regulation of a stand-alone self-excited induction

generator,‖ in Proc. IEEE Electr. Power Conf., 2007, pp.

502–506.

power quality improvement by using statcom for three-phase

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