Dstatcom based voltage regulator for wind turbine driven self excited indu

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
209
DSTATCOM BASED VOLTAGE REGULATOR FOR WIND TURBINE
DRIVEN SELF-EXCITED INDUCTION GENERATOR
Dr. RAJSEH KUMAR AHUJA, PRIYANKA PHAGERIA
YMCA University of Science & Technology, Faridabad
ABSTRACT
Voltage sag is the most important powerquality problems faced by many industries
and utilities.This paper presents the voltage sag mitigation usingDSTATCOM (Distribution
Static Compensator).DSTATCOM is used for reactive power compensation. This paper
presents an analysis of the three-phaseself-excited induction generator (SEIG) with
DSTATCOM as a voltage regulator , which provides fast dynamic response tomaintain
constant voltage at SEIG terminals during severeload perturbations and acts as a source and
sink of reactivepower.Thispaper evaluates vector control algorithm forextracting reference
source currents for voltage regulationat PCC (Point of Common Coupling) for DSTATCOM.
Thevoltage source converter (VSC) is used with DC linkcapacitor as a DSTATCOM.
1. INTRODUCTION
Power Quality problem such as Voltage sag is the most important power quality
problems faced by many industries and utilities. It contributes more than 80% power quality
(PQ) problems that exist in power systems. The majority of power consumption has been
drawn in reactive loads.These excessive reactive power demandincreases feeder losses and
reduces the active power flow capability of distribution system which also affects thevoltage
profile. The reason for this is the majority of loads in the distribution systems are linear
laggingpower factor and nonlinear, balanced and unbalanced loads. Such power quality
problems can be mitigatedby the DSTATCOM (Distribution Static Synchronous
Compensator) at the point of common coupling (PCC). Induction Generators used in stand-
alone as well as grid connected system as recent development has done in pollution free
power generation schemes.When a capacitor bank is connected across the Induction
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 3, May - June (2013), pp. 209-219
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210
Generator then it Known as a Self-Excited Induction generator. It is well known that when
capacitors are connectedacross the stator terminals of an induction machine,driven by an
external prime mover, voltage will beinduced at its terminals . The induced emf andcurrent in
the stator windings will continue to rise untilsteady state is attained, it means reactive power
supplied by capacitor bank is balanced by reactive power absorbed by load. In order for self-
excitationto occur, for a particular capacitance value there is acorresponding minimum speed.
The self-excited induction generator (SEIG) has some advantages over the synchronous
generator, for example:brushless (squirrel-cage rotor), reduced size, rugged and low cost.
However, the induction generator offers poor voltage regulation and its value depends on the
prime mover speed,capacitor bank size and load characteristics. The generated voltage of the
SEIG is mainly depends upon the excitation capacitance values, change in wind velocity and
load conditions. The reactive power requirementby the induction generator can also be
supplied by a group of capacitors. If the capacitance is insufficient,the induction generators
will not build up voltage. The main drawback of the induction generator is need ofreactive to
build up the terminal voltage. In the SEIG, the excitation current is supplied by the capacitors
connected across its terminals. Self-excited induction generators are good candidates for wind
powered electric generation application especially in remote areas, because they do not need
external power supply to produce the magnetic field.
By using the DSTATCOM the voltage across the SEIG remains constant at varying
loads.Thesimulated results demonstrate the use of DSTATCOM as a voltage regulator across
a SEIG network.
2. CONTROL SCHEME
When used in low voltage distribution system, thestatic compensator (STATCOM) is
identified asDistribution STATCOM (DSTATCOM). In generalDSTATCOM is used to
generate or absorb reactivepower.The D-STATCOM is a three-phase and shunt connected
power electronics based device.
When a DSTATCOM is connected across the SEIG then the additional demand
ofreactive power is fulfilled by the DSTATCOM under varyingloads. The DSTATCOM acts
as a source of lagging or leadingcurrents to maintain the terminal voltage constant with
variation in load.The DSTATCOM consists of a three-phaseIGBT (Insulated gate bipolar
transistor) based currentcontrolled voltage source inverter, DC bus capacitor and
ACinductors. The AC output of the inverter is connectedthrough the AC filtering inductor to
the SEIG terminals.The DC bus capacitor is used as an energy storage deviceand provides
self-supporting DC bus.
The control technique to regulate the terminal voltage of theSEIG is based on the
control of source currents (having twocomponents one in-phase and other quadrature). PLL is
used to generate the sin(wt) and cos(wt). The two measurement systems(Vmeas and Imeas)
blocks compute the d-axis and q-axis components of the voltages and currents by executing
an abc-dq transformation in the synchronous reference determined by sin(wt) and cos(wt)
provided by the PLL,this block calculate the mag V and IdIq current.
TheAC terminal voltage (V) is compared with the reference voltage. The voltage error
is processed in the PI controller. The output of the PI controller (lq ref) for AC voltage
control loop decides the amplitude of reactive current required for the system.The DC bus
voltage of the STATCOM is sensed and compared with DC reference voltage. The error
voltage is processed in another PI controller. The output of this PI controller (Id ref) decides

211
the amplitude of active current, which is required by DSTATCOM to maintain constant DC
bus voltage. The quadrature and in-phase components is thesource reference current, which is
compared with the SEIG line current. The error current is processed in PID controller.The
output of this PID controller maintains the primary voltage equal to the reference value. This
VqVd is further used to generate reference voltage for Voltage source converter.
3. MATHEMATICAL MODELLING
Modelling of SEIG
The dynamic model of the three-phase SEIG is developedusing stationary d-q axes
references frame is used because it provides a complete solution for dynamic analysis and
control.
The flux linkages per second taken into account the saturation effect are
Circuit for q axis and q axis of IG
Vqs=rsiqs+ωφds+d/dt(φqs)
Vds=rsids-ωφqs+d/dt(φds)
V’qr = r’ri’qr + (ω - ωr )φ’dr +d/dt(φ’qr)
V’dr= r’ri’dr - (ω - ωr )φ’qr +d/dt(φ’dr)
φqs = Llsiqs + Lm (iqs + i'qr)
φds = Llsids + Lm (ids + i'dr)
φ’qr = L’lriqr + Lm (iqs + i'qr)
φ’dr = L’lri’dr + Lm (ids + i'dr)

212
In the above equations, all the rotor variables are referredto the stator side. The
expression for the electromagnetic Torque of the SEIG is expressed as in per (unit can be
written as:)
Te=(3/2)(p/2)Lm(φdsiqs- φdsids)
The electromagnetic torque balance equation of SEIG isdefined as
Tshaft = TB + J(2/P)p(ωr)
The derivative of rotor speed of the SEIG-
pωr={P/(2J)}(Tshaft-Te)
Modelling of wind turbine
The wind turbine is characterized by no dimensional curves of the power coefficient
Cp as a function of both the tip speed ratio, ë and the blade pitch angle, Â. In order to fully
utilize the available wind energy, the value of ë should be maintained at its optimum value.
Therefore, the power coefficient corresponding to that value will become maximum.
The output power of the wind turbine, can be calculated from the following equation
Pm=.5AρCp(λ,β)V3
wind
where
Pm=Mechanical output power of the turbine (W)
Cp=Performance coefficient of the turbine
Ρ=Air density (kg/m3)
A=Turbine swept area (m2)
Vwind=Wind speed (m/s)
λ =Tip speed ratio of the rotor blade tip speed to wind speed
β=Blade pitch angle (deg)
Pm can be normalized. In the per unit (pu) system we have:-
Pm_pu=.5AρCp_pu(λ,β)V3
wind_pu
generic equation is used to model cp(λ,β). This equation, based on the modelling turbine
characteristics of [1], is:-

213
The non-linear, dimensionless Cpcharacteristic is represented as .:-
Cp(λ,β)=C1(C2/ λi-C3 β-C4)e-(C5/ λi)+
C6λi
With
(1/λi)=(1/( λ+0.08 β))-(0.035/( β3
+1))
where,
C1 = 0.5176, C2 = 116, C3 = 0.4, C4 = 5, C5 = 21, C6 = 0.0068
Modelling of DSTATCOM
Three-phase voltages at the SEIG terminals (va, vb and vc) are considered
Convert this sinusoidal voltage into dq0.
Vd = 2/3*[Va*sin(wt) + Vb*sin(wt-2pi/3) + Vc*sin(wt+2pi/3)]
Vq = 2/3*[Va*cos(wt) + Vb*cos(wt-2pi/3) + Vc*cos(wt+2pi/3)]
V0 = 1/3*[Va + Vb + Vc ]
Amplitude is:-
V=√(Vd2
+Vq2
)
Three phase current at the SEIG terminals(Ia,Ib,Ic) are considered.and convert this current
into Id Iq I0.
Id = 2/3*[Ia*sin(wt) + Ib*sin(wt-2pi/3) + Ic*sin(wt+2pi/3)]
Iq = 2/3*[Ia*cos(wt) + Ib*cos(wt-2pi/3) + Ic*cos(wt+2pi/3)]
I0 = 1/3*[Ia + Ib + Ic ]
The active power component of the source current isId* which is required for the self
supporting DC bus ofDSTATCOM. Moreover, the reactive component isIq* which is
required for maintaining the amplitude of the PCC voltage. The active component of the load
currentId* can be expressed as:-
I* q = kpVdce+ki∫Vdcedt
Vdce=Vdc*-Vdc

214
Vdce error in DC bus voltage.Vdc*and Vdc are the reference voltage and sensed filteredvoltage
of DC bus of DSTATCOM respectively. Kp and Ki are the proportional and integral gains of
the PI controller over the DC bus voltage of DSTATCOM.
I*d=kpVte+ki∫Vtedt
Vte=V*t-Vt
Vte=error in amplitude of the PCCvoltage. Vt* and Vt are reference voltage and amplitude of
PCC voltage respectively. Kpand Ki are proportional and integral gains of the PI controller
over the PCCvoltage.
Now this I*d I*q is compared with Id Iq and the error signal is given to the controller and the
output of controller iusVdVq.
Vd=KpIde+Ki∫Idedt+Kd(d/dt)Ide
Where Ide=Id-I*d
Vq=kpIqe+ki∫Iqedt+kd(d/dt)Iqe
Where Iqe=Iq-I*q
This VqVd is further used to generate reference voltage for Voltage source converter.
4. SIMULATION RESULTS AND DISCUSSION
Simulations have been carried out for a 7.5kW threephase- SEIG with balanced,
unbalanced resistive and reactive at 0.8 PF and nonlinear load. Disconnecting one or two
phases of loads from the SEIG has created unbalanced conditions.
The DC bus voltage of DSTATCOM is selected 700V for the source line voltage of
400 V.
The VSC is connected to the network through the AC inductor of 800mH. For self-
supporting DC bus of DSTATCOM, a capacitor of 6000µF is used. Initially considered linear
reactive load is 3-single phase load.The PCC line voltage is considered 700 V,The simulation
results are taken for the above mentioned load conditions.
A. Performance of DSTATCOM with and Without in Balanced load condition
The SEIG system under balanced load without connecting the FACT device i.e.
DSTATCOM. Here asynchronous machine driven by wind turbine is connected across the
various balanced loads. This asynchronous machine is converted into SEIG by connecting a
capacitor bank.The balanced loads consist of two reactive loads (0.8 lagging) and one
resistive load. The first reactive load is applied at the starting of the simulation, the second
reactive load comes at 2 seconds after starting and the resistive load comes after 2.5 seconds.
Fig. 1 (a), (b) & (c) shows the variations in load voltage, load current and stator
current with respect to variations in applied load and wind speed. It is depicted that at instant
2 sec. as load increases from 7A to 9A the load voltage falls to 290V from 470V. Further at

215
instant 2.5 sec as load increases to 10A the load voltage falls to 300V from 340V. It is also
clear that the voltage and currents are also changing with the variations in wind speeds. The
load voltage drops marginally from 500V to 470V when wind speed changes from 12m/s to
10m/s at instant 1.95 sec. At 2.2 sec. the wind speed increases to 11m/s resulting in increase
in load voltage to 340V from 290V. There are also marginal variations observed in load
current and stator current. This shows that the Voltage across the SEIG not constant, so to
maintain voltage constant across the SEIG DSTATCOM is connected.
Fig. 1 (a) Load Voltage, (b) Load Current, (c) Stator Current and (d) Wind Speed
Now, The DSTATCOM is connecting across the SEIG to improve the Voltage profile.
By comparing the results of with and without DSTATCOM on SEIG system in has
observed that the DSTATCOM maintains the voltage profile of load voltage as constant
irrespective of changes in load and wind speed. The DSTATCOM injects sufficient reactive
power into the system so as to maintain the load voltage constant. It has also observed that
the profile of stator current has also got improved. When the load is increased across the
generator then the reactive power is shared by IG and load so the IG needs some reactive
power to maintain the voltage constant.
This Reactive power demand fulfil by connecting the DSTATCOM across the
generator.

216
Fig. 2 (a) load voltage, (b) load current, (c) stator current and (d) wind speed
B. Performance of DSTATCOM with and without in non-linear load condition
Now connect a non-linear load across the SEIG system without connecting the FACT device
i.e. DSTATCOM
The non-linear load injects the harmonics in system and distorts the currents &
voltages and also de-rates the machine.

217
In Fig. 3 when wind speed is change the voltage across the IG is changes respectively
and when load is increased across the IG the voltage is decreased.to make the voltage profile
constant a FACTS device is connected across the SEIG.A FACTS device DSTATCOM is
connected across the SEIG to improve the voltage profile of the system and also remove the
harmonics in system
By comparing the results of the effect of non-linear load on SEIG system after the
inclusion of DSTATCOM it has observed that it not only fulfils the reactive power demand
of the system but also minimises the harmonics in the system. It is clear from Fig. 4 that the
effects of non-linearity on load voltage and stator current are considerably reduced.
.
In this way DSTATCOM also restores the rating of the machine which has reduced due to the
effects of harmonics.
THD of non-linear load in system without DSTATCOM
THD for Stator voltage = 42.18%

218
THD for stator current = 71.34%
THD of non-linear load in system with DSTATCOM
THD for Stator voltage = 9.39%
THD for stator current = 6.98%

219
5. CONCLUSION
A mathematical model of three-phase self-excited induction generator with
DSTATCOM based voltage regulator under balanced/unbalanced/non-linear loads is studied
in this project. It is concluded from the simulated results that the DSTATCOM acts as an
ideal voltage regulator and load balancing device, which maintains the SEIG voltage constant
and balances the SEIG system at varying balanced, unbalanced and non-linear loads
APPENDIX A
Parameters of machine
Rated power 10 HP(7.5 KW)
Voltage 400V
Frequency 50HZ
RPM 1440
Capacitor Bank 7.5 KVAR
Parameters of wind turbine
Nominal power 7650W
Base power 7650(VA)
Base speed 10
Max power at base wind speed 10m/s
Base rotational speed 1.2 p.u
REFERENCES
[1]. S. S. Murthy, O. P. Malik. and P. Walsh, "Capacitive VAR requirements of self-Excited
induction generators to achieve desired voltage regulation," IEEE Industrial and Commercial
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[2]. M. B. Brermen and A. Abbondati, "Static exciter for induction generator," IEEE Trans, on
Ind. Appl, Vol. 13, No. 5, pp. 422-428, Sep./Oct. 1977.
[3]. Bhim Singh, Sunil kumar, “Modified power balance theory for control of DSTATCOM”
IEEE Transaction 2010.
[4]. K. R. Padiyar, “FACTS Controllers in Power Transmission and Distribution,” New Age
International (P) Limited, Publishers, New Delhi, 2007.
[5]. B.Singh, S.S.Murthy and S.Gupta, “Analysis and design of STATCOM Based Voltage
Regulator for Self-Excited Induction generators,” IEEE Transactions on Energy
Conversion, vol.19, no.4, Dec 2004, pp.783-790.
[6]. B. Singh, P. Jayaprakash, T.R. Somayajulu, D.P. Kothari, A. Chandra, and K. Al-Haddad,
“Integrated three-leg VSC with a zig-zag transformer based three-phase four-wire
DSTATCOM for power quality improvement,” in Proc. of IECON 2008, Nov. 2008, pp. 796
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[7]. Haider M. Husen , Laith O. Maheemed and Prof. D.S. Chavan, “Enhancement of Power
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