MitigationofVoltageDipandVoltageFlickeringby MultilevelD...

Hindawi Publishing CorporationAdvances in Power ElectronicsVolume 2012, Article ID 871652, 11 pagesdoi:10.1155/2012/871652

Research Article

Mitigation of Voltage Dip and Voltage Flickering byMultilevel D-STATCOM

M. S. Ballal, H. M. Suryawanshi, and T. Venkateswara Reddy

Visvesvaraya National Institute of Technology, Nagpur, India

Correspondence should be addressed to H. M. Suryawanshi, hms [email protected]

Received 3 May 2012; Accepted 19 July 2012

Academic Editor: Neville Watson

Copyright © 2012 M. S. Ballal et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The basic power quality problems in the distribution network are voltage sag (dip), voltage flickering, and the service interruptions.STATCOM is a Flexible AC Transmission Systems (FACTS) technology device which can independently control the flow ofreactive power. This paper presents the simulation and analysis of a STATCOM for voltage dip and voltage flickering mitigation.Simulations are carried out in MATLAB/Simulink to validate the performance of the STATCOM. A comparison between the six-pulse inverter and the five-level diode-clamped inverter is carried out for the performance of 66/11 KV distribution system.

1. Introduction

STATCOM has got a much more widely application dueto the advent of the concepts of the smart grid and themicrogrid and the rapid development of new energy and thedistributed generation. The Distributed Static Compensator(D-STATCOM) becomes the tendency of the reactive powercompensation and the power quality control in distributingnetworks at the present time. It is of great significance toenhance the power quality and keep the relay protectiondevices working normally as it can make a comprehen-sive compensation to voltage fluctuation, voltage flicker,and three-phase unbalance. The output harmonic in D-STATCOM comes to our attention, as it is a member of thepower electronic devices. The relationship between powerquality and distribution system has been a subject of interestfor several years.

The application of a high-voltage multilevel inverter ina 13.8 kV distribution system Static Synchronous (SSC) isexamined in [1]. The capability of the multi-level inverterto limit device voltage stress makes it suitable for high-voltage power conversion. A capacitor voltage balancingcontroller maintains operation of the multi-level inverterSSC under conditions of phase voltage imbalance shown inEMTP simulations. The concept of power quality describesthe quality of the supplier voltage in relation to the transient

breaks, falling voltage, harmonics, and voltage flicker. Utilitydistribution networks, sensitive industrial loads, and criticalcommercial operations suffer from various types of powerquality problems like voltage sag, voltage flickering, serviceinterruptions, and harmonics. The various characteristicsof voltage sags experienced by customers within industrialdistribution systems are described in [2]. The influence of theinduction motor load on the characterization of voltage sagsis discussed. It came to know that during a fault, an inductionmotor operates as a generator for a short period of time andcauses an increase in sag magnitude. A microcomputer-basedvoltage flicker teaching facility consists of a single- and three-phase voltage flicker generator, measurement devices, and alamp test system is described in [3]. Information obtainedby the facility includes understanding the causes and theinfluences of voltage flicker, voltage flicker measurement, andthe realistic illuminating flicker phenomena.

Topologies like diode-clamped inverter (neutral-pointclamped), capacitor clamped (flying capacitor), and cas-caded multicell with separate dc sources are presented in[4]. The relevant control and modulation methods arediscussed with multilevel sinusoidal pulse width modulation,multilevel selective harmonic elimination, and space-vectormodulation. However, only the fundamental principle ofdifferent multilevel inverters has been introduced systemat-ically. A voltage dip is a short-time (10 ms to 1 minute) event

2 Advances in Power Electronics

during which a reduction in r.m.s voltage magnitude occurs.It is often set only by two parameters, depth/magnitude, andduration. The voltage dip magnitude is ranged from 10% to90% of nominal voltage (which corresponds to 90% to 10%remaining voltage) and with a duration from half a cycleto 1 min. In a three-phase system a voltage dip is by naturea three-phase phenomenon, which affects both the line-to-ground and line-to-line voltages. A voltage dip is caused bya fault in the utility system, a fault within the customer’sfacility or a large increase of the load current, like startinga motor or transformer energizing. The high current resultsin a voltage drop over the network impedance.

A STATCOM configuration is described in [5] thatcombines the advantages of the diode clamped, d.c. voltagereinjection VSC, and soft switching concepts. This schemeuses asymmetrical switches instead of diodes in a com-mon clamping switching circuit for the three phases ofthe converter bridges. The theoretical analysis, backed byEMTDC simulation, is carried out. A method of modellingand control strategies for fast load voltage regulation usingSTATCOMs is described in [6]. Another power qualityproblem is voltage flicker, a phenomenon of annoying lightintensity fluctuation, caused by large rapid industrial loadchanges. Erratic variations in reactive power demands leadto fluctuating voltage drops across the impedance of adistribution system which results in voltage flicker. Voltageflicker occurs when large industrial loads, such as electric arcfurnaces, rolling mills, large mine hoists, resistance welders,and pumps operating in a weak power distribution system.The modelling strategy, similar to that used for field-orientedcontrol of ac machines, defines the bus voltage magnitudeand reactive current input from the STATCOM on an instan-taneous basis. The particular coordinate transform used alsofacilitates extraction of liberalized system dynamics withthe help of circuit simulators having analogue behaviouralmodelling capabilities. In order to accurately determine thefrequency response of a multilevel converter, it is necessaryto consider the dc voltage polluted with ac ripple, for theduration of conduction periods. Analytical formulae for asingle cell and for a generic cascade multilevel converterwhere the number of cells is a parameter are presented in [7].The testing is performed in digital simulation in PSCAD. Ananalytical framework for minimizing losses and harmonicsis presented for the cascaded-type-based multilevel staticsynchronous compensator (STATCOM) with square-wavecontrol being discussed in [8]. The focus lays on low losses,low-voltage total harmonic distortion (THD), and low dcvoltage ripple. Digital simulations are used in PSCAD.

Variation on the typical MPC control scheme for a H-StatCom which provides excellent current tracking performancewhile simultaneously trading off the voltage balancingcharacteristics with the switching losses is presented in[9]. The scheme consists of a dead-beat current controllerthat has been integrated with heuristic models of thevoltage balancing and switching loss characteristics. Systemconfiguration and a control method for a multivoltagecascade converter in order to reduce power loss and volumeof a 6.6-kV transformer less D-STATCOM are proposed in[10]. Downscaled STATCOM model verification tests are

executed restricted to rates at 220 V and 10 kVA. Some of thedrawbacks of voltage flicker were explained in the literature[4–8] and the IEEE Standard 519–1992, which is referredwidely, defines maximum permissible voltage flicker levelswith respect to frequency as shown in Figure 1.

In this paper, two D-STATCOM controllers based onsix-pulse inverter and five-level diode-clamped inverter areproposed. Both strategies are simulated using MATLAB/Simulink models. Simulation results confirming the effec-tiveness of the control schemes to impose a linear STATCOMdynamics are presented.

2. Dynamic Model of D-STATCOM

D-STATCOM is one of the most recent FACTS devicesfor power transmission shunt compensation. It is shunt-connected device which was developed as a static VARcompensator where a Voltage Source Converter (VSC) isused instead of controllable reactors. The STATCOM canbe seen as a current source since it is connected in shuntwith the distribution system and the load. By controllingthe magnitude and the phase angle of the output voltage ofthe VSC, both active and reactive power can be exchangedbetween the distribution system and the STATCOM. Being ashunt-connected device, the STATCOM mainly injects reac-tive power to the system [3]. Radial system with STACOMis shown in Figure 2. Rd is included to represent small lossesin the switching devices of VSC. The space vector diagramfor voltages and currents is shown in Figure 3, where αβ axesrepresent stationary reference frame and dq axes representsynchronous rotating reference frame. The equivalent circuitof the tie transformer between bus voltage Vt and Vi isrepresented by R and L. The circuit equation in respect ofstationary reference frame can be written as below:

Ldi

dt+ Ri = Vt −Vi. (1)

The stationary reference frame in terms of synchronouslyrotating reference frame is illustrated by the following equa-tion:

Vte− jλ = vtd + jvtq. (2)

This can be elaborated with angle δ in respect of Figure 3and thus it is written as

Vie− jλ = vid + jviq = Vie

− jδ = vi cos δ − jvi sin δ. (3)

Corresponding current equation is given as

ie− jλ = id + jiq. (4)

The relationship between current and voltage equation isshown by the following equation:

Le− jλ di

dt+ Re− jλi = Vte

− jλ −Vie− jλ. (5)

Advances in Power Electronics 3

Vol

tage

flu

ctu

atio

n (

%)

0

1

2

3

4

5

Border lineof visibility

Border lines ofirritation

House pumps

Sump pumps

A/C equipment

Theatrical lighting

Domestic refrigerators

Oil burners

Single elevator

Hoists

Cranes

Y-delta changes on

elevator motor generator sets

X-ray equipment

Arc furnacesFlashing signsArc welders

Manual spot weldersDrop hammers

SawsGroup elevators

Reciprocating pumpsCompressors

AutomaticSpot welders

Solid lines composite curves of voltage flicker studiesby General Electric Company, General Electric reviewAugust 1925; Kansas City Power and Light Company,Electrical World, May 19, 1934; T and D Committee, EEI,October 24, 1934, Chicago; Detroit Edison Company;West Pennsylvania Power Company; Public Service Companyof northern Illinois.

Dotted lines voltage flicker allowed by two utilities,references Electrical World November 3, 1958 andJune 26, 1961.

1 2 3 6 10 20 30 1 2 4 6 10 20 30 60 2 3 4 6 10 15

Fluctuations per hour Fluctuations per minute Fluctuations per second

Figure 1: Flickering curve.

Vt

R L

Vi

Bus 1 Bus 1

VSIidc

Rd

ic

iRd

Vdc

Figure 2: D-STATCOM configuration on the radial system.

β

λ

δ

q

d

V

α

i

ϕ

ω1

Figure 3: State vector diagram.

Substituting (2), (3), and (4) into (1) and rearranging, volt-age equations for real part in d-axis and for imaginary partin q-axis are

Ldiddt

+ Rid = utd −mVdc cos δ + Lω1iq,

Ldiqdt

+ Riq = utq −mVdc sin δ − Lω1id,

(6)

where ω1 is system frequency. The magnitude of phase volt-age at bus 2 (Vi) is directly proportional to the DC voltageacross the capacitor Vdc and therefore can be expressed as

Vi = mVdc. (7)

The value of m is proportional depending on the type ofVSC. If the dc current (idc) is defined as the sum of capacitorcurrent (ic) and resistor current (iRd), the power flows intoVSC and is described as

p = Vdcidc = 32

(vdid + vqiq

). (8)

From (3), (7), and (8), the dc current is given as

idc = 32m(id cos δ − iq sin δ

)= C

dVdc

dt+Vdc

Rd. (9)


Equations (6) and (9) form a state equation for STATCOM:

d

dt

⎡⎢⎣idiqVdc

⎤⎥⎦

=

⎡⎢⎢⎢⎢⎢⎢⎣

− 1T1

ω1 −m

Lcos δ

−ω1 − 1T1

m

Lsin δ

32m

ccos δ −3

2m

csin δ − 1

T1

⎤⎥⎥⎥⎥⎥⎥⎦

⎡⎢⎣idiqVdc

⎤⎥⎦

+

⎡⎢⎢⎢⎢⎢⎣

1L

0

01L

0 0

⎤⎥⎥⎥⎥⎥⎦

[vtdvtq

],

(10)

where T1 = L/R, T2 = RdC.Linearization of (10) around the operating firing angle,

δ0, gives a set of linear equations as shown in:

d

dt

⎡⎢⎣idiqVdc

⎤⎥⎦

=

⎡⎢⎢⎢⎢⎢⎢⎣

− 1T1

ω1 −m

Lcos δ0

−ω1 − 1T1

m

Lsin δ0

32m

ccos δ0 −3

2m

csin δ0 − 1

T1

⎤⎥⎥⎥⎥⎥⎥⎦

×⎡⎢⎣idiqVdc

⎤⎥⎦

+

⎡⎢⎢⎢⎢⎢⎣

1L

0m

LVdc0 sin δ0

01L

m

LVdc0 cos δ0

0 0 −32m

c

(id0 sin δ0 + iq0 cos δ0

)

⎤⎥⎥⎥⎥⎥⎦

×⎡⎢⎣vtdvtdδ

⎤⎥⎦.

(11)

The characteristic equation of the system described by (11)is

S3 +{

2T1

+1T2

}S2 +

{2

T1T2+

1T2

1+ K + ω2

1

}S

+

{1

T21T2

+K

T1+ω2

1

T2

}= 0,

(12)

where K = (3/2)(m2/LC).The characteristic equation is not a function of firing

angle. Hence, firing angle does not affect the position ofcharacteristic roots on the complex plane.

2

2

1.5

1.5

1

1

0.5

0.5

0

0

−0.5

−0.5

−1

−1

−1.5

−1.5−2−2

Firing angle (deg)

(pu

)

idiqVdc

Figure 4: Steady-state response.

2.1. Stability Test of STATCOM Model. The stability of STAT-COM can be tested with Routh-Hurwitz criterion. By assign-ing p, q, and r to represent the coefficients of s2, s1, and s0,respectively, (12) becomes s3 +ps2 +qs+r = 0 and the Routh’sArray equation can be written as

s3 1 qs2 p r

s1 q − r

ps0 r

. (13)

Substitute p, q, and r to determine the element in the s1 row:

q − r

p= 4

T1+

2T2

+2T2

T21

+ KT2 + KT1 + 2ω21T2 ≥ 0.

(14)

Examination of all elements in the first column of Routh’sarray reveals that all elements are positive, and the STAT-COM is a stable system. Therefore, the values of resistors,inductors, and capacitors in the STATCOM equivalent circuithave no effect on stability.

2.2. Steady-State Analysis. Equation for the steady state oper-ation of STATCOM can be obtained from the dynamic modelby setting all derivative terms to zero. After transformationinto d-q reference frame, voltages and current become DCquantities, that is, Vtd = Vt, vtq = 0, id = Id, iq = Iq.Rearranging and assigning X for ω1L, steady state equationbecomes

⎡⎢⎢⎣

−R X −m cos δ−X −R m sin δ

3m cos δ −3m sin δ − 2Rd

⎤⎥⎥⎦

⎡⎢⎣IdIqVdc

⎤⎥⎦ = −

⎡⎢⎣Vt

00

⎤⎥⎦. (15)


source Step-downtransformer

PCC

Group of sensitive andinductive loads

Heavy industrial loads likearc furnaces, rolling mills

and mine hoists

Cdc

Six-pulse

inverter

Couplingtransformer

Three-phase ac

(a)

ABCABCABC

ABC

ABC

ABC

ABC

ABC

abc

abc

A

B

C

a

b

c

abcMag

Phase Scope 3

Three-phase source 1

abc

sin cos 0

Freq

wt

Sin Cos

A

B

C

a

b

c

A B C

a b c

D-STATCOM

Flickering circuit

Discrete,

Discrete3-phase PLL2

Powergui

A

B

CN

YY

Ts =5e−005 s

Vabc (pu)

dq0 (Iq ref )

Iq ref

(Iabc b1)

(b)

Figure 5: (a) Circuit implementation of the D-STATCOM with the six-pulse inverter. (b) MATLAB Simulation model for D-STATCOMwith 6-pulse converter for mitigation of voltage dip and voltage flickering.


0 0.2 0.4 0.6 0.8 1 1.20123456789

10

Time (s)

Rea

ctiv

e po

wer

(V

ar)

Reactive power drawn by the inductive load×106

Figure 6: Reactive power drawn at the time of applying the induc-tive load.

Solving for Id, Iq, and Vdc, the solutions are

Id = R + (3/2)m2Rdsin2δ

R2 + x2 + (3/2)m2 ∗ R∗ RdVt , (16)

Iq = −x + (3/2)m2Rd sin δ cos δR2 + x2 + (3/2)m2 ∗ R∗ Rd

Vt, (17)

Vdc = (3/2)mRd(R cos δ + x sin δ)R2 + x2 + (3/2)m2 ∗ R∗ Rd

Vt. (18)

The equations of direct current, quadrature current,and capacitor voltage do not contain capacitor. Hence, thesize of dc capacitor does not affect STATCOM steady stateperformance. Especially, the quadrature current, which isreactive current (iq), does not depend on the size of dccapacitor. The steady state performances were calculated withthe following parameters: R = 0.01 pu, X = 0.15 pu, Rd =128 pu, C = 0.013 pu, and m = 4/π. The plot of activecurrent id, reactive current Iq, and capacitor voltage Vdc asa function of firing angle is shown in Figure 4.

At steady state, the reactive current (iq) is a linear func-tion of firing angle within this operating range. With ad-vanced firing angle (δ is negative), the reactive current flowsinto the STATCOM and vice versa. The capacitor voltage(Vdc) increases linearly with firing angles (from advancedangles to delayed angles). The active current (id) is small andvaries very little with firing angle because it only furnishesthe losses in the VSC.

3. Simulation Studies

Verification of the proposed control strategies was accomp-lished through simulation studies using a detailed model of athree-phase STATCOM implemented in MATLAB/Simulink.The test data taken for simulation is given in the Appendix.For easy comparison of the performance of a STATCOMunder the proposed control schemes, results concerning thesix-pulse inverter are presented first.

3.1. Simulation Results for STATCOM with Six-Pulse Inverter.In this case, the D-STATCOM is prepared with six-pulse

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Time (s)

Voltage dip because of inductive loads

Line

-to-

line

volta

ge (

pu)

Figure 7: Voltage dip representation because of the sudden switch-ing of inductive loads.

Time (s)

Lin

e-to

-lin

e vo

ltag

e (p

u)

0

0.2

0.4

0.6

0.8

1

1.2

1.4Mitigation of the voltage dip because of 6-pulse inverter

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Figure 8: Voltage dip mitigation because of the application of six-pulse inverter.

inverter. The complete 66/11 KV utility distribution system isshown in Figure 5(a). The circuit consists of a utility systemand a step down transformer to step-down the voltage levelto distribution side. In this system, all kinds of loads likeindustrial, commercial, domestic, and even sensitive loadscan be connected to the point of common coupling (PCC). Inthis paper, heavy inductive load is connected to the system forthe simulation purpose. The reactive current (Iq) mentionedin (17) is injected at PCC for the mitigation of voltagedip and voltage flickering. The MATLAB Simulation modelfor D-STATCOM with 6-pulse converter for mitigation ofvoltage dip and voltage flickering is shown in Figure 5(b).The ratings of the system are specified in the above section. Inorder to analyze the system for the power quality problems,simulation was done as described in following cases.

Case 1 (voltage dip). Initially, the D-STATCOM is not con-nected to the system and the load of pure inductive of10 MVAR is applied on the system in the time interval of0.1 sec to 0.6 sec as shown in Figures 6 and 7. The voltagegot dipped from 0.9955 p.u to 0.8205 p.u. Now, the D-STATCOM is connected in the circuit; the voltage profileat PCC is maintained at 0.9652. Here, the excessive reactive


Time (s)

Lin

e-to

-lin

e vo

ltag

e (p

u)

Voltage dip because of the inductive loads

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Without D-STATCOMWith D-STATCOM

Dip due to suddenincrease of inductive

load

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Figure 9: Comparison of with and without D-STATCOM with six-pulse inverter.

power drawn by the load is supplied by the D-STATCOMthan by the system. So at that instant STATCOM acts likecapacitor. The response of control circuit with the six-pulseD-STATCOM is shown in Figures 8 and 9.

Case 2 (voltage flickering). The flickering circuit preparedwith the help of R-L load which is periodically operated onthe system causes the voltage flickering at the PCC (bus1)as shown in Figure 10. The magnitude of flickering levelwithout the D-STATCOM in the circuit is 2.09% which isabove the tolerable limits. After keeping the D-STATCOM inthe circuit, the flickering level comes down to 0.68% whichis below the threshold of objection shown in Figure 11. The10 MVAR D-STATCOM contains a PWM IGBT inverter,5800 μF dc capacitor, and a control system. The PWMgenerator with a 3 kHz carrier frequency generates pulsesfor the IGBT inverter. The D-STATCOM is connected to the11 kV bus through an 11/2 kV coupling transformer.

The instantaneous current of the D-STATCOM is obtain-ed by abc to dq0 transformation. The decoupled d-axiscomponent id and q-axis component iq are regulated bytwo separate PI regulators. The instantaneous iq reference isobtained from the measurement of reactive current producedby the inductive load. The id current corresponds to thesmall active power absorbed by the D-STATCOM due tothe losses in the transformer and in the inverter. This isreferring to (15)–(17). The DC bus voltage is also regulatedto compensate for the real power losses. In this direct currentcontrol strategy, the reference values (iqref, idref) and feedbackvalues (iq, id) are the dc signals; therefore the instantaneouscurrent tracing control with no steady-state error can beimplemented using PI control. FFT transformation of six-pulse inverter is depicted in Figure 12.

3.2. Simulation Results for STATCOM with Five-Level Inverter.In this case, the D-STATCOM is prepared with five-level

Lin

e-to

-lin

e vo

ltag

e (p

u)

Voltage flickring due to large industrial loads1

0.99

0.98

0.97

0.96

0.950.94

0.93

0.92

0.910.9

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (s)

Figure 10: Voltage flickering because of the inductive loads likeselectric arc furnaces and rolling mills.

Time (s)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2

Line

-to-

line

volta

ge (

pu)

Voltage flicker mitigation using the six-pulse D-STATCOM

Figure 11: Voltage flicker mitigation by applying the D-STATCOMsix-pulse inverter.

3

012

−3−2−1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Time (s)

0 5 10 15 20 25 30

Harmonic order

0

5

10

15

20

Selected signal: 9.835 cycles

Fundamental (50 Hz) = 0.2177, THD = 36.35%

Mag

of

fun

dam

enta

l (%

)

Figure 12: FFT transformation of six-pulse inverter.


Three-phase acsource

Step-downtransformer PCC

Group of sensitive andinductive loads

Heavy industrial loads likearc furnaces, rolling millsand mine hoists

Leakage reactance

Five-level diode-clampedinverter

(a)

3-phase source

Transformer

Bus

CB

ScopeScope

Scope

Scope

Control

Control

Control

Control

Load

Load

Display

Display

Display

Display

Display

Display

Bus

Powergui

Inverter

(b)

Figure 13: (a) Voltage dip because of the sudden application of the inductive loads. (b) MATLAB Simulation modal for D-STATCOM withfive-level inverter for mitigation of voltage dip and voltage flickering.


Voltage dip because of inductive loads

Lin

e-to

-lin

e vo

ltag

e (p

u)

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (s)

Figure 14: Voltage dip because of the sudden application of theinductive loads.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Voltage dip mitigation D-STATCOM

Time (s)

Lin

e-to

-lin

e vo

ltag

e (p

u)

Figure 15: Voltage dip mitigation by D-STATCOM five-level diodeclamped.

diode-clamped multilevel inverter. The complete 66/11 KVutility distribution system is shown in Figure 13(a). In thissystem, also all kind of loads like industrial, commercial,domestic, and even sensitive loads can be connected tothe point of common coupling (PCC). Here again, heavyinductive load is connected to the system for the simulationpurpose. The MATLAB Simulation modal for D-STATCOMwith five-level inverter for mitigation of voltage dip and volt-age flickering is shown in Figure 13(b). In order to analyzethe system for the power quality problems, simulation wasdone as described in the previous section with similar cases.

Case 1 (voltage dip). Initially, the D-STATCOM is not con-nected to the system and the load of pure inductive of 10MVAR is applied on the system in the time interval of 0.1 secto 0.6 sec as shown in Figure 14. The voltage got dipped from0.9955 p.u to 0.8205 p.u. Now, the D-STATCOM is connectedin the circuit; the voltage profile at the point of commoncoupling (PCC) is maintained at same value of 0.9955 p.u.This is except some switching transients. Here, the excessivereactive power drawn by the load is supplied by the D-STATCOM than by the system. Therefore, at this instantSTATCOM acts like capacitor.

Case 2 (voltage flickering). The flickering circuit is preparedwith the help of R-L load which is periodically operated on

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Comparison of with and without D-STATCOM

Time (s)

Lin

e-to

-lin

e vo

ltag

e in

P.U

.

Figure 16: Comparisons of voltage dip mitigation with and withoutD-STATCOM having five-level inverter.

0

0.5

1

1.5

2.5

2

Rea

ctiv

e po

wer

req

uir

emen

t (p

u)

−0.5

×10−3

0 0.2 0.4 0.6 0.8 1 1.2

Time (s)

Figure 17: Reactive power drawn because of flickering loads.

the system cause the voltage flickering at the bus1 (PCC).The magnitude of flickering level without the D-STATCOMin the circuit is 2.09% which is above the tolerable limits.The voltages dip mitigation is shown in Figure 15 and thecomparison is shown in Figure 16 for five-level inverter.The reactive power and voltage flickering are shown in Fig-ures 17 and 18, respectively. After keeping the D-STATCOMin the circuit the flickering level comes down to 0.29% whichis below the threshold of objection as shown in Figure 19 andthe comparison is depicted in Figure 20. The output voltageand FFT of five-level diode-clamped multilevel inverter areshown in Figure 21. The complete STATCOM control systemscheme is implemented and the performance is checkedfor both types of inverters. Output voltage is mitigated byboth the STATCOM circuitries and its results are depictedin relevant figures. In these cases, the voltage flicker isalso mitigated and improved voltage waveform is obtained.Furthermore, it is clearly observed from Table 1 that themitigation of the power quality problems (voltage dip andthe voltage flickering), done effectively with the five-level


Table 1: Performance comparison between the six-pulse and the five-level diode-clamped inverters.

Type of inverter D-STATCOM status T.H.D. of Vab Voltage flickering Voltage dip

Five-level diode-clamped inverterOff

12.16%2.09% 17.50%

On 0.29% 0.00005%

Six-pulse inverterOff

36.35%2.09% 17.50%

On 0.68% 3.043%

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (s)

1

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91

0.9

Lin

e-to

-lin

e vo

ltag

e (p

u)

Voltage flickring due to large industrial loads

Figure 18: Voltage flickering.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (s)

1

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91

0.9

Lin

e-to

-lin

e vo

ltag

e (p

u)

Mitigation of voltage flicker

Figure 19: Voltage flicker mitigation because of five-level diode-clamped inverter D-STATCOM.

diode-clamped multilevel inverter D-STATCOM than withthe six-pulse inverter D-STATCOM.

4. Conclusion

Voltage dip and voltage flickering are the two major powerquality problems which are frequently seen in the distri-bution systems. These power quality problems in 66/11 KVdistribution system are investigated in this paper. Theanalysis and simulation of a D-STATCOM application forthe mitigation of power quality problems are presented anddiscussed. Here, the D-STATCOM was prepared with the six-pulse inverter and the five-level diode-clamped multilevel

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Time (s)

1

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91

0.9Li

ne-

to-l

ine

volt

age

(pu

)


Voltage flicker mitigation comparison

Figure 20: Comparison of voltage flicker mitigation with and with-out D-STATCOM.

FFT window: 10 of 60 cycles of selected signal3210−1−2−3

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Time (s)

Fundamental (50 Hz) = 1.702, THD = 12.16%

0 5 10 15 20 25 30 35 40 45 50

0.090.080.070.060.050.040.030.020.01

0

Harmonic order

Mag

Figure 21: FFT transformation of five-level diode-clamped inverter.

inverter. A systematic approach for designing a nonlinearinternal controller for the converter has been developed. Forthe STATCOM with five-level inverter, the mitigation of thepower quality problems is effective. The strategy has beenvalidated with extensive MATLAB Simulation results.


Appendix

The following test data is considered for MATLAB Simula-tion.

Three-phase source (utility):

100 MVA, 66 KV, Rs = 0.8029Ω, Ls = 16.85 MH.

Transformer rating:

100 MVA, 66/11 kV, Rl = 0.087Ω,

Xl = 0.3267Ω, both Rl, Xl referred to the primary.

Inverter rating:

+/− 10 MVAR, 11 kV on secondary side of trans-former.

Vdc = 3400 V is the rated dc bus voltage for six-pulseinverter.

Vdc = 4500 V is the each DC source in five levelinverter without transformer.

Ron = 0.01 ohm.

Xls = 0.1936Ω referred to 11 kv side.

References

[1] C. Hochgraf and R. H. Lasseter, “A transformer-less static syn-chronous compensator employing a multi-level inverter,” IEEETransactions on Power Delivery, vol. 12, no. 2, pp. 881–887,1997.

[2] G. Yalcinkaya, “Characterization of voltage sags in industrialdistribution systems,” IEEE Transactions on Industry Applica-tions, vol. 34, no. 4, pp. 682–688, 1998.

[3] W. N. Chang, “A flexible voltage flicker teaching facility forelectric power quality education,” IEEE Transactions on PowerSystems, vol. 13, no. 1, pp. 27–33, 1998.

[4] J. Rodrıguez, J. S. Lai, and F. Z. Peng, “Multilevel inverters: asurvey of topologies, controls, and applications,” IEEE Trans-actions on Industrial Electronics, vol. 49, no. 4, pp. 724–738,2002.

[5] Y. H. Liu, J. Arrillaga, and N. R. Watson, “A new STATCOMconfiguration using multi-level DC voltage reinjection forhigh power application,” IEEE Transactions on Power Delivery,vol. 19, no. 4, pp. 1828–1834, 2004.

[6] A. K. Jain, K. Joshi, A. Behal, and N. Mohan, “Voltage regu-lation with STATCOMs: modeling, control and results,” IEEETransactions on Power Delivery, vol. 21, no. 2, pp. 726–735,2006.

[7] D. Jovcic and R. Sternberger, “Frequency-domain analyticalmodel for a cascaded multilevel STATCOM,” IEEE Transac-tions on Power Delivery, vol. 23, no. 4, pp. 2139–2147, 2008.

[8] R. Sternberger and D. Jovcic, “Theoretical framework forminimizing converter losses and harmonics in a multilevelSTATCOM,” IEEE Transactions on Power Delivery, vol. 23, no.4, pp. 2376–2384, 2008.

[9] C. D. Townsend, T. J. Summers, and R. E. Betz, “Multigoalheuristic model predictive control technique applied to acascaded H-bridge statcom,” IEEE Transactions on Power Elec-tronics, vol. 27, no. 3, pp. 1191–1200, 2012.

[10] K. Sano and M. Takasaki, “A transformer less D-STATCOMbased on a multi voltage cascade converter requiring no DCsources,” IEEE Transactions on Power Electronics, vol. 27, no. 6,pp. 2783–2795, 2012.

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