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D-STATCOM MODELING AND SIMULATIONAPPLYING CONTROL SCHEMES FOR
POWER
QUALITY IMPROVEMENT
Kajal Rathore1*, Dhananjay Kumar1 and Sunil Yadav1
*Corresponding Author: Kajal Rathore,
[email protected]
This paper focuses to design and analyze the Simulink model for
the control of Distributed staticsynchronous compensator
(D-STATCOM). It is important for a distribution system to
havecontrollability, voltage stability and good power transfer
capability. The voltage source converterbased D-STATCOM achieves
voltage regulation in the system by absorbing or supplying
therequired reactive power. The system is modeled using two control
strategies, i.e., direct andindirect control which includes d-q
transform. The comparative analysis of the control strategieshas
been done. The results of simulation are demonstrated and analyzed
using MATLAB.
Keywords: Reactive power compensation, D-STATCOM, dq-model,
Phase shift control, Voltagecontrol, VSC
INTRODUCTIONThe rapidly developing power electronicstechnology
provides an opportunity fordeveloping new power equipment
forimproving the performance of the powersystem. Flexible AC
Transmission Systemtechnology (FACTS) uses the latest
powerelectronic devices and methods to controlelectronically the
high-voltage side of thenetwork. FACTS devices can be used forpower
flow control, voltage regulation, transientstability improvement,
and damping of poweroscillations. FACTS devices can be of shuntor
series or combination of shunt and series
ISSN 2319 – 2518 www.ijeetc.comVol. 4, No. 2, April 2015
© 2015 IJEETC. All Rights Reserved
Int. J. Elec&Electr.Eng&Telecoms. 2015
1 Jawaharlal Institute of Technology, Borawan, Madhya Pradesh
451228, India.
types. The shunt devices can be used forvoltage regulations,
while series devices canbe used for regulation of line impedance
andseries-parallel combination can be used forreal and reactive
power compensation inaddition to regulation of voltage and
regulationof line impedance (Hingorani, 1991).
The family of emerging power electronicsdevices being offered to
achieve these custompower objectives is:
• Distribution Static Compensator (D-STATCOM) that protects the
distributionsystem from the effects of fluctuating
Research Paper
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voltage, voltage sags and swells and non-linear loads,
• DVR that protects a critical load fromdisturbances like sags,
swells, transientsand harmonics originating on theinterconnected
distribution system,
• Unified Power Quality Conditioner (UPQC),a combination of
series and shunt controller,which compensates supply voltage
andload current imperfections in the distributionsystem.
The DSTATCOM is a versatile device forproviding reactive power
compensation in acnetworks. The control of reactive power
isachieved via the regulation of a controlledvoltage source behind
the leakage impedanceof a transformer (Lehn and Iravani, 1998). It
issimilar to a conventional synchronouscompensator, which is
essentially asynchronous generator where the field currentis used
to adjust the regulated voltage. TheDSTATCOM uses Voltage Source
Converter(VSC) to achieve the regulation task. Whenused in low or
medium voltage distributionsystems the STATCOM is normally
identifiedas Distribution STATCOM (D-STATCOM). Itoperates in a
similar manner as the STATCOM(FACTS controller), with the active
power flowcontrolled by the angle between the AC systemand VSC
voltages and the reactive power flowcontrolled by the difference
between themagnitudes of these voltages. As with theSTATCOM, the
capacitor acts as the energystorage device and its size is chosen
basedon power ratings, control and harmonicsconsiderations. The
D-STATCOM controllercontinuously monitors the load voltages
andcurrents and determines the amount of
compensation required by the AC system fora variety of
disturbances.
The D-STATCOM is a shunt device. It shouldtherefore be able to
regulate the voltage of abus to which it is connected. The
operatingprinciple of a D-STATCOM in this mode hasbeen termed as
the D-STATCOM in voltagecontrol mode. This report shows that
eventhough the structure of D-STATCOM used inboth current control
and voltage control modesis the same, its operating principle is
different.In the current control mode it is required tofollow a set
of reference currents while in thevoltage control mode it is
required to follow aset of reference voltages. This paperdiscusses
the reference voltage generationscheme and the control of D-STATCOM
in thevoltage control mode.
BASIC MODEL OF D-STATCOMThe D-STATCOM system comprises of a
VSC,a set of coupling reactors (leakage reactanceof the
transformer) and a controller. The D-STATCOM generates a
controllable acvoltage from the Voltage Source Inverter
(VSI)connected to a dc capacitor (energy storagedevice). The ac
voltage appears behind thetransformer leakage reactance.
The active and reactive power transferbetween the power system
and the D-STATCOM is caused by the voltage differenceacross this
reactance. The D-STATCOM isconnected to the power network at the
Pointof Common Coupling (PCC), where thevoltage-quality problem is
a concern. Allrequired voltages and currents are measuredand are
fed into the controller to be comparedwith the reference. The
controller then performsfeedback control and outputs a set of
switching
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signals to drive the main semiconductorswitches (IGBTs) of the
power converteraccordingly (Introducing Custom Power, 1995).The
basic diagram of the D-STATCOM isillustrated in Figure 1.
WORKING PRINCIPLEOF D-STATCOMD-STATCOM is inserted in
transmission lineto suppress voltage variation due to deviatingload
conditions and control reactive power.Basically it is connected in
shunt totransmission line with Point of CommonCoupling (PCC) in
phase with system voltage.It can compensate for inductive and
capacitivecurrents linearly and continuously. Figure 2shows the
vector diagram at the fundamentalfrequency for capacitive and
inductive modesand for the transition states from capacitive
toinductive and vice versa. The terminal voltage(Vbus) is equal to
the sum of the inverter voltage(Vvsc) and the voltage across the
couplingtransformer reactive VL in both capacitive andinductive
modes. I mean that if output voltage ofDSTATCOM (Vvsc) is in phase
with bus terminalvoltage (Vbus) and (Vvsc) is greater than Vbus
D-STATCOM provides reactive power to system.
And if Vvsc is smaller than, D-STATCOMabsorbs reactive power
from power systemVbusand Vvschave the same phase, but actuallythey
have a little phase difference to componentthe loss of transformer
winding and inverterswitching, so absorbs some real power
fromsystem.
Figure 2 is DSTATCOM vector diagrams,which show inverter output
voltageVI, systemvoltage VT, reactive voltage VL and line currentI
in correlation with magnitude and phase .Figures 2a and 2b explains
how VI and VTproduces capacitive or inductive power bycontrolling
the magnitude for inverter outputvoltage VI in phase with each
other.
MATHAMATICAL MODELINGOF D-STATCOMModeling Equations are given
as:
scsaa
faf VVdtdiLIR ...(1)
Figure 1: Basic Block Diagramof D-STATCOM
Figure 2: Vector Diagram of D-STATCOM(a) Capacitive Mode (b)
Inductive Mode
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Kajal Rathore et
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cbsbb
fbf VVdtdiLIR ...(2)
ccscc
fcf VVdtdiLIR ...(3)
Here,
Vsa, Vsb, Vsc : Voltage at the PCC
Vsa, Vcb, Vcc : Inverter output voltage
Lf : Filter inductance
Rf : Equivalent filter resistance
C : DC Link Capacitor
Designing Equations
a) CSTAT VIQ 3
b) MaVVDC 3/22
c)pps
DCAC Iaf
MaVL
123
d) Capacitor designing
2 12/3 DCDCLSDC VVTIVC Here,
IC = D-STATCOM Line Current
V = Line Voltage
Ma = Modulation Index
VDC = DC Link Voltage
QSTAT= Power Rating
LAC = AC Inductor
a = Over Current Factor (1.2)
Park’s Transformation: This block performsthe abc to dq
transformation on a set of three-phase signals. It computes the
direct axis Vd,quadrature axis Vq, and zero sequence V0quantities
in a two axis rotating reference frameaccording to the following
transformation:
3/2sin3/2sinsin*3/2 tVtVtVV cbad...(4)
3/2cos3/2coscos*3/2 tVtVtVV cbaq...(5)
cba VVVV *3/10 ...(6)
where = rotation speed (radian/second) ofthe rotating frame.
This transforms threequantities (direct axis, quadrature axis
andzero-sequence components) expressed in atwo axis reference frame
back to phasequantities. The following transformation isused:
0cossin VtVtVV qda ...(7)
03/2cos3/2sin VtVtVV qda ...(8)
03/2cos3/2sin VtVtVV qdc ...(9)
where = rotation speed (radian/second) ofthe rotating reference
frame.
VARIOUS CONTROL SCHEMESSatisfactory performance, fast
response,flexible and easy implementation are the mainobjectives of
any compensation scheme. Thecontrol strategy of D-STATCOM is
mainlyimplemented in the following steps (Masandet al., 2006; and
Singh, 2006):
Figure 3: Simplified Single Line Diagramof D-STATCOM
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• Measurements of system variables andsignal conditioning
• Extraction of reference compensatingsignals
• Generation of firing angles for switchingdevices
Basically all the above steps will be appliedin every control
scheme but methods ofmeasurement of signal variables, extraction
ofreference, and generation of firing pulses willbe different for
different control schemes. Twocontrol schemes are implemented
andcompared here are Phase Shift Control andDecoupled Current
Control.
Phase Shift ControlIn this control algorithm the voltage
regulationis achieved in a D-STATCOM by themeasurement of the RMS
voltage at the loadpoint and no reactive power measurementsare
required. Figure 4 shows the blockdiagram of the implemented scheme
(Masandet al., xxxx).
necessary phase shift between the outputvoltage of the VSC and
the AC terminalvoltage. This angle is summed with the phaseangle of
the balanced supply voltages,assumed to be equally spaced at 120
degrees,to produce the desired synchronizing signalrequired to
operate the PWM generator. In thisalgorithm the DC voltage is
maintainedconstant using a separate dc source.
Decoupled Current Control p-qTheoryThis algorithm requires the
measurement ofinstantaneous values of three phase voltageand
current. Figure 5 shows the block diagramrepresentation of the
control scheme. Thecompensation is achieved by the control ofdirect
axis and quadrature axis currents and .Using the definition of the
instantaneousreactive power theory for a balanced threephase three
wire system, the quadraturecomponent of the voltage is always zero,
thereal (P) and the reactive power (Q) injectedinto the system by
the D-STATCOM can beexpressed under the dq reference frame as:
qqdd IVIVP ...(10)
qddq IVIVQ ...(11)
Since Vq = 0, id and iq completely describesthe instantaneous
value of real and reactivepowers produced by the D-STATCOM whenthe
system voltage remains constant.Therefore the instantaneous three
phasecurrent measured is transformed by abc to dq0transformation
block. The decoupled d-axiscomponent id and q-axis component iq
areregulated by two separate PI regulators. Theinstantaneous id
reference and theinstantaneous iq reference are obtained by
thecontrol of the dc voltage and the terminal
Figure 4: Block Diagram of Phase ShiftControl
Sinusoidal PWM technique is used whichis simple and gives a good
response. Theerror signal obtained by comparing themeasured system
RMS voltage and thereference voltage, is fed to a PI controller
whichgenerates the angle which decides the
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voltage measured. Thus, instantaneouscurrent tracking control is
achieved using fourPI regulators. A Phase Locked Loop (PLL) isused
to synchronize the control loop to the acsupply so as to operate in
the abc to dq0reference frame.
MODELLING ANDSIMULATION RESULTSIn this work, the performance of
VSC basedpower devices acting as a voltage controller
is investigated. The D-STATCOM model canbe seen in Figure 6
which comprised ofdistribution bus, a VSC and the controller.
Thecontroller scheme is applied to generate theswitching pulse for
VSC.
Phase Shift ControlIt does not have a self supporting dc bus
andrequires a separate dc source to pre-chargethe dc side capacitor
and maintain its voltageduring the operation of D-STATCOM.
Itassumes that the supply side voltage isbalanced and without
harmonics, sincefundamental wave form is used to obtain thephase
angles of the supply wave form. Thereis no provision for harmonic
suppression in
Figure 5: Block Diagram of DecoupledTheory Based Control of
D-STATCOM
Figure 6: Simulink Model for Phase ShiftControl
Figure 7: Simulink Model of Controllerfor Phase Shift
Controller
Figure 8: 3-Phase Source Voltage at PCC
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Figure 9: Voltage of SeparatePre-Charged Capacitor
Figure 10: Inverter Output Voltage
Figure 11: Compensated Output Voltageand Current
Figure 12: Total Harmonic Distortion(Linear Load) (a) Before
Compensation
(b) After Compensation
Figure 13: Total Harmonic Distortion(Nonlinear Load) (a)
Before
Compensation (b) After Compensation
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case the load connected to PCC is nonlinear.This method results
in generation of activepower by the VSC along with the var.
Decoupled Current ControlThe advantageous features of this
scheme are:
It incorporates a self supporting dc bus. Theactive and reactive
power control achievedthrough and control are decoupled from
eachother. The dc bus control or regulation aredecoupled from the
ac bus control like voltageregulation or power factor correction
and loadbalancing.
Switching of devices of VSC is done at fixedfrequency. Thus
switching losses can be
limited within the rating of the devices. Thistype of control is
inherently linear and robustand uses PI or PID controls.
The results can be seen for variousparameters like voltage,
current before andafter applying the controller for
compensation.
Figure 14: Simulink Model for DecoupledCurrent Controller
Figure 15: Source Current Waveform
Figure 16: Before Compensation (a) LoadVoltage and Load Current
(Linear)(b) Load Voltage and Load Current
(Nonlinear)
Figure 17: After Compensation (a) LoadVoltage, Load Current
(Linear), (b) Load
Voltage, Load Current (Nonlinear)
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Kajal Rathore et
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These Power devices provide solutions topower quality at the
medium and low voltagedistribution network level. This
projectpresents the detailed modeling of one of thecustom power
products, D-STATCOM ispresented using instantaneous P-Q theory
andphase shift theory used for the control of D-STATCOM are
discussed. These controlalgorithms are described with the help
ofsimulation results under linear loads andnonlinear loads. It was
observed thatundersized capacitor degrades all threeaspects. On the
other hand, an oversizedcapacitor may also lead to a PWM control
witha sluggish response but it will reduceD-STATCOM harmonic
generation and
Figure 18: Total Harmonic Distortion(Linear Load) (a) Before
Compensation
(b) After Compensation
Figure 19: Total Harmonic Distortion(Nonlinear Load (a) Before
Compensation
(b) After Compensation
CONCLUSIONDetailed modeling is presented and resultsare
discussed with different control studies.
1. Reactive power Average Partialcompensation
2. Performance Contains Not Satisfactoryunder nonlinear
undesiredloads harmonics
3. Applicable for Yes Nosingle phasesystems
4. Harmonic Less Averagecompensation
5. PWM switching Fixed Fixedfrequency
6. Self supporting No YesDC Bus
7. Generation of Sine PWM Sine PWMfiring pulses
8. Power factor 0.8 to 0.94 0.8 to 0.94Linear Load (linear)
(linear)
9. Power factor 0.8 to 0.92 0.8 to 0.95Non Linear Load
(Nonlinear) (Nonlinear)
10. Response time Less More
11. Control scheme Easy Complex
Table 1
S.No Parameters
PhaseShift
Control
DecoupledCurrentControl
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transient overshooting. It is concluded that aD-STATCOM though
is conceptually similar toa STATCOM at the transmission level;
itscontrol scheme should be such that in additionto complete
reactive power compensation,power factor correction and voltage
regulationthe harmonics are also checked, and forachieving improved
power quality levels at thedistribution end.
FUTURE WORK• This work presents a detailed modeling and
analysis of one of the custom power deviceD-STATCOM.
Instantaneous DecoupledCurrent Control or instantaneous p-q
theoryand phase shift control is discussed andverified through
detailed simulations bydeveloping the models in MATLAB
Simulinkusing the Simpower system control toolboxes. Now we are
posing a challenge tocomplete if the remaining control
strategieswhich include the synchronous frame theory,regulation of
Bus and DC link voltage, andANN based Adaline theory. These
controlstrategies are implemented and studied indetail through
various simulations then itwould be of immense help for the real
timeimplementation of the D-STATCOM acrossall over the globe.
• If thrown light on other custom powerdevices like the Dynamic
VoltageRegulator (DVR), and Unified Power QualityConditioner
(UPQC), applying differentstrategies then we can bring a revolution
inthe control of power in the distributionsystems.
REFERENCES1. Hingorani N (1991), “FACTS—Flexible ac
Transmission Systems”, in Proc. IEE 5th
Int. Conf. AC DC Transmission, Vol. 345,pp. 1-7, Conf. Pub.,
London, UK.
2. “Introducing Custom Power”, IEEESpectrum, Vol. 32, June, pp.
41-48.
3. Lehn P and Iravani M (1998),“Experimental Evaluation of
STATCOMClosed Loop Dynamics”, IEEE Trans.Power Delivery , Vol. 13,
October,pp. 1378-1384.
4. Masand D, Jain S and Agnihotri G (2008),“Distribution Static
CompensatorPerformance Under Linear and NonlinearCurrent Regulation
Methods”, J. ElectricalSystem, Vols. 4-1, pp. 91-105.
5. Masand D, Jain S and Agnihotri G (2006),“Control Algorithms
for Distribution StaticCompensator”, Dept. of
ElectricalEngineering, Maulana Azaad NationalInstitute of
Technology, Bhopal, MP, India.
6. Pierre Giroux, Gilbert Sybille and HoangLe-Huy (2001),
“Modeling and Simulationof a Distribution STATCOM UsingSimulink’s
Power System Block Set”,IECON’01, The 27th Annual Conferenceof the
IEEE Industrial Electronics Society.
7. Singh B (2006), “A Comparative Study ofControl Algorithms for
D-STATCOM forLoad Compensation”, Department ofElectrical
Engineering, Indian Institute ofTechnology, Hauz Khas, New
Delhi.