Paper 9 - IJAST Sep 2012

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					                                                                   International Journal of Advances in Science and Technology,
                                                                                                              Vol. 5, No.3, 2012


  Modeling & Simulation of DSTATCOM for
Power Quality Improvement of Diesel Generator
                 K.JYOTHSNA DEVI                                                    S. SRINIVASA RAO
    M-Tech power system control and automation,                           Associate Prof, Advanced Power Systems,
Department Of Electrical And Electronics Engineering,               Department Of Electrical And Electronics Engineering,
P.V.P.Siddhartha Institute Of Technology Kanuru,                    P.V.P.Siddhartha Institute Of Technology Kanuru,
Vijayawada, India.e-mail: kjyothsnadevi88@gmail.com                 Vijayawada, India. e-mail: Ssr.jntuk@gmail.com


Abstract-. This paper presents a novel synchronous reference        converter transformer. The DSTATCOM protects the utility
frame (SRF) controlled DSTATCOM to improve the power                transmission or distribution system from voltage sags and/or
quality in the distribution system. The SRF based                   flicker caused by rapidly varying reactive current demand.
compensation is developed by sensing load currents only,            In utility applications, a DSTATCOM provides leading or
which require for harmonics and reactive power compensation         lagging reactive power to achieve system stability during
due to non-linear loads. The attempt is to come up with a
simple control strategy. Incidentally it is different from
                                                                    transient conditions. The DSTATCOM can also be applied
conventional methods and provides superior performance.             to industrial facilities to compensate for voltage sag and
Nowadays, there are an increasing number of non-linear loads        flicker caused by non-linear dynamic loads, enabling such
which inject harmonics into the system. A three-phase               problem loads to co-exist on the same feeder as more
insulated gate bipolar transistor- (IGBT-) based current            sensitive loads. The DSTATCOM instantaneously
controlled voltage source inverter with a DC bus capacitor          exchanges reactive power with the distribution system
known as a DSTATCOM is used for power factor correction,            without the use of bulky capacitors or reactors. In most
harmonic compensation and for providing required reactive           applications, a DSTATCOM can use its significant short-
power to the load. A model of DSTATCOM connected to a               term transient overload capabilities to reduce the size of the
power distribution system feeding linear and non-linear loads
(diode bridge rectifier with R and R-C) is developed for
                                                                    compensation system needed to handle transient events. The
predicting the behavior of system. Simulation is carried out in     short-term overload capability is up to 325% for periods of
standard MATLAB environment using Simulink and power                1 to 3 seconds, which allows applications such as wind
system block set toolbox. The control of DSTATCOM is made           farms and utility voltage stabilization to optimize the
flexible so that it can eliminate harmonics and provide load        system’s cost and performance. The DSTATCOM controls
balancing by synchronous reference frame controller. The            traditional mechanically switched capacitors to provide
extensive simulation results demonstrate that the DSTATCOM          optimal compensation on a both a transient and steadystate
for the load compensation and an optimal operation of the DG        basis.
set using the synchronous reference frame controller
compensates the harmonics and reactive power.
                                                                              Voltage dips are one of the most occurring power
                                                                    quality problems. Off course, for an industry an outage is
                                                                    worse, than a voltage dip, but voltage dips occur more often
Keywords- D-STATCOM, SRF, Voltage Source Converter                  and cause severe problems and economical losses. Utilities
(VSC).
                                                                    often focus on disturbances from end-user equipment as the
                      I.   INTRODUCTION                             main power quality problems. This is correct for many
                                                                    disturbances, flicker, harmonics, etc., but voltage dips
          When the STATCOM is applied in distribution               mainly have their origin in the higher voltage levels. Faults
system is called DSTACOM (Distribution-STACOM) and                  due to lightning, is one of the most common causes to
its configuration is the same, or with small modifications,         voltage dips on overhead lines. If the economical losses due
oriented to a possible future amplification of its possibilities    to voltage dips are significant, mitigation actions can be
in the distribution network at low and medium voltage,              profitable for the customer and even in some cases for the
implementing the function so that we can describe as flicker        utility. Since there is no standard solution which will work
damping, harmonic filtering and hole and short interruption         for every site, each mitigation action must be carefully
compensation. Distribution STATCOM (DSTATCOM)                       planned and evaluated. There are different ways to mitigate
exhibits high speed control of reactive power to provide            voltage dips, swell and interruptions in transmission and
voltage stabilization, flicker suppression, and other types of      distribution systems. At present, a wide range of very
system control. The DSTATCOM utilizes a design                      flexible controllers, which capitalize on newly available
consisting of a GTO- or IGBT-based voltage sourced                  power electronics components, are emerging for custom
converter connected to the power system via a multi-stage           power applications [3, 4]. Among these, the distribution



    September Issue                                        Page 57 of 132                                     ISSN 2229 5216
                                                                International Journal of Advances in Science and Technology,
                                                                                                           Vol. 5, No.3, 2012

static compensator and the dynamic voltage restorer are          Such configuration allows the device to absorb or generate
most effective devices, both of them based on the VSC            controllable active and reactive power.
principle.                                                           The VSC connected in shunt with the ac system provides
          The increased concern for power quality has            a multifunctional topology which can be used for up to three
resulted in measuring power quality variations, studying the     quite distinct purposes:
characteristics of power disturbances and providing               1. Voltage regulation and compensation of reactive power;
solutions to the power quality problems. In distribution         2. Correction of power factor; and
systems, the power quality problems can reduce the power         3. Elimination of current harmonics.
supplied to the customers from its nominal value. Voltage        Here, such device is employed to provide continuous
sag, harmonic, transient, overvoltage and under voltage are      voltage regulation using an indirectly controlled converter.
major impacts to a distribution system. The utility and the
users are responsible in polluting the supply network due to
operating of large loads. There are many solutions in
mitigating the power quality problems at a distribution
system such as using surge arresters, active power filters,
isolation transformer, uninterruptible power supply and
static VAR compensator. Blazic et. al. proposed a new D-
STATCOM control algorithm which enables separate
control of positive and negative sequence currents and
decoupled control of d- and q-axes current components.                                      Figure. 1 DSTATCOM
Dinavahi et. al use real time digital simulation of power        Figure. 1 the shunt injected current I sh corrects the voltage
electronic system which is a heavily computer intensive          sag by adjusting the voltage drop across the system
operation, and based on VSC D-STATCOM power system.              impedance Z th . The value of I sh can be controlled by
From the studies, it is shown that all these equipments are      adjusting the output voltage of the converter. The shunt
capable in solving power quality problems. The best              injected current I sh can be written as,
equipment to solve this problem at distribution systems at
minimum cost is by using Custom Power family of D-                           I sh = I L – I S = I L – ( V th – V L ) / Z th
STATCOM. Environmental effects also give an impact to                                     I sh /_η = I L /_- θ
the power quality and its reliability. Major concerns on
industrial power quality problems are that they affect the       The complex power injection of the D-STATCOM can be
production, due to sensitive equipment in the industries.        expressed as,
Where there are power quality problems, equipment may                                         S sh = V L I sh *
misoperate or machine may possibly shut down.
Installations by industries such as Adjustable Speed Drive       It may be mentioned that the effectiveness of the
(ASD), switch mode power supplies and high frequency             DSTATCOM in correcting voltage sag depends on the value
switching also affect the power quality (Osborne et al). High    of Zth or fault level of the load bus. When the shunt injected
sensitivity equipment such as high speed motor, super            current Ish is kept in quadrature with VL, the desired
computer, microprocessors and medical instruments may            voltage correction can be achieved without injecting any
also be affected by the power quality problems occurring in      active power into the system. On the other hand, when the
the system                                                       value of Ish is minimized, the same voltage correction can
                                                                 be achieved with minimum apparent power injection into
 II. DISTRIBUTION STATIC COMPENSATOR (D-STATCOM)                 the system.
2.1 Principle of DSTATCOM
                                                                 2.2 Voltage Source Converter (VSC)
A D-STATCOM (Distribution Static Compensator), which
                                                                           A voltage-source converter is a power electronic
is schematically depicted in Fig.1, consists of a two-level
                                                                 device that connected in shunt or parallel to the system. It
Voltage Source Converter (VSC), a dc energy storage
                                                                 can generate a sinusoidal voltage with any required
device, a coupling transformer connected in shunt to the
                                                                 magnitude, frequency and phase angle. The VSC used to
distribution network through a coupling transformer. The
                                                                 either completely replace the voltage or to inject the
VSC converts the dc voltage across the storage device into a
                                                                 ‘missing voltage’. The ‘missing voltage’ is the difference
set of three-phase ac output voltages. These voltages are in
                                                                 between the nominal voltage and the actual. It also converts
phase and coupled with the ac system through the reactance
                                                                 the DC voltage across storage devices into a set of three
of the coupling transformer. Suitable adjustment of the
                                                                 phase AC output voltages [8, 9]. In addition, D-STATCOM
phase and magnitude of the D-STATCOM output voltages
                                                                 is also capable to generate or absorbs reactive power. If the
allows effective control of active and reactive power
                                                                 output voltage of the VSC is greater than AC bus terminal
exchanges between the DSTATCOM and the ac system.
                                                                 voltages, D-STATCOM is said to be in capacitive mode. So,




    September Issue                                     Page 58 of 132                                            ISSN 2229 5216
                                                                     International Journal of Advances in Science and Technology,
                                                                                                                Vol. 5, No.3, 2012

it will compensate the reactive power through AC system
and regulates missing voltages. These voltages are in phase           whereVac= Vtmc-Vmc(n) denotes the error in Vtmc and
and coupled with the AC system through the reactance of               computed value Vtmn from Equation (3) and Kpqand Kiq
coupling transformers. Suitable adjustment of the phase and           are the proportional and integral gains of the second PI
magnitude of the DSTATCOM output voltages allows                      controller.
effectives control of active and reactive power exchanges
between D-STATCOM and AC system. In addition, the
converter is normally based on some kind of energy storage,
which will supply the converter with a DC voltage [10].

2.3 Controller for DSTATCOM
The three-phase reference source currents are computed
using three-phase AC voltages (vta, vtb and vtc) and DC bus           Three-phase quadrature components of the reference supply
voltage (Vdc) of DSTATCOM. These reference supply                     currents (isaqr, isbqr and iscqr) are computed using the output of
currents consist of two components, one in-phase (Ispdr) and          second PI controller (Ispqr) and quadrature unit current
another in quadrature (Ispqr) with the supply voltages. The           vectors (wa, wb and wc ) as
control scheme is represented in Fig. 2. The basic equations
of control algorithm of DSTATCOM are as follows.

2.3.1 Computation of in-phase components of
reference supply current
The instantaneous values of in-phase component of
reference supply currents (Ispdr) is computed using one PI
controller over the average value of DC bus voltage of the
DSTATCOM (vdc) and reference DC voltage (vdcr) as


where Vde(n) = Vdcc-Vdcn) denotes the error in Vdcc and average
value of Vdc Kpd and Kid are proportional and integral
gains of the DC bus voltage PI controller. The output of this
PI controller (Ispdr) is taken as amplitude of in-phase
component of the reference supply currents. Three-phase in-
phase components of the reference supply currents (isadr, isbdr
and iscdr) are computed using the in-phase unit current
vectors (ua, ub and uc) derived from the AC terminal voltages
(vtan, vtbn and vtcn), respectively.


where Vtm is amplitude of the supply voltage and it is
computed as



The instantaneous values of in-phase component of
reference supply currents (isadr, isbdr and iscdr) are computed as
                                                                                       Figure. 2 Control method for DTSATCOM
                                                                      2.3 Computation of total reference supply currents
                                                                      Three-phase instantaneous reference supply currents (isar, isbr
2.3.1 Computation of quadrature components of                         and iscr) are computed by adding in-phase (isadr, isbdr and iscdr)
reference supply current                                              and quadrature components of supply currents (isaqr, isbqr and
The amplitude of quadrature component of reference supply             iscqr) as
currents is computed using a second PI controller over the
amplitude of supply voltage (vtm) and its reference value
(vtmr)




    September Issue                                          Page 59 of 132                                        ISSN 2229 5216
                                                                    International Journal of Advances in Science and Technology,
                                                                                                               Vol. 5, No.3, 2012

A hysteresis pulse width modulated (PWM) current                      three-phase AC source. Three-phase AC loads are
controller is employed over the reference (isar, isbr and iscr)       connected at the load end. DSTATCOM is connected in
and sensed supply currents (isa, isb and isc) to generate gating      shunt and it consists of PWM voltage source inverter circuit
pulses for IGBTs of DSTATCOM.                                         and a DC capacitor connected at its DC bus. An IGBT-
                                                                      based PWM inverter is implemented using Universal bridge
                                                                      block from Power Electronics subset of PSB. Snubber
                                                                      circuits are connected in parallel with each IGBT for
                                                                      protection. Simulation of DSTATCOM system is carried out
                                                                      for linear and non-linear loads. The linear load on the
                                                                      system is modeled using the block three-phase parallel R-L
                                                                      load connected in delta configuration. The non-linear load
                                                                      on the system is modeled using R and R-C circuits
                                                                      connected at output of the diode rectifier. Provision is made
                                                                      to connect loads in parallel so that the effect of sudden load
                                                                      addition and removal is studied. The feeder connected from
                                                                      the three-phase source to load is modelled using appropriate
           Figure 3 Synchronous reference frame controller            values of resistive and inductive components.
         The basic structure of SRF methods consists of
direct (d-q) and inverse (d-q)-1 park transformations, which          3.1 Modeling of Control Circuit
allow the evaluation of a specific harmonic component of                       Figure below shows the control algorithm of
the input signals. The reference frame transformation is              DSTATCOM with two PI controllers. One PI controller
formulated from a three-phase a − b − c stationery system to          regulates the DC link voltage while the second PI controller
the two phase direct axis (d) – quadratic axis (q) rotating           regulates the terminal voltage at PCC. The in-phase
coordinate system. In a-b-c stationary axes are fixed on the          components of DSTATCOM reference currents are
same plane and separated from each other by 120o. These               responsible for power factor correction of load and the
three phase space vectors stationary coordinates are easily           quadrature components of supply reference currents are to
transformed into two axis d-q rotating reference frame. This          regulate the AC system voltage at PCC.
proposed algorithm derivate from a three-phase stationary
coordinate load current iLa, iLb, iLc are convert to id-iq
rotating coordinate current, as follows
     III. MATAB/SIMULINK MODELING OF DSTATCOM
3.1 Modeling of Power Circuit




   Figure. 4 Matlab/Simulink Model of DSTATCOM Power Circuit
                                                                               Figure. 5 Control circuit of DSTATCOM
         Fig. 4 shows the complete MATLAB model of
DSTATCOM along with control circuit. The power circuit                The output of PI controller over the DC bus voltage (Ispdr) is
as well as control system are modelled using Power System             considered as the amplitude of the in-phase component of
Block set and Simulink. The grid source is represented by             supply reference currents and the output of PI controller
                                                                      over AC terminal voltage (Ispqr) is considered as the



    September Issue                                          Page 60 of 132                                     ISSN 2229 5216
                                                                    International Journal of Advances in Science and Technology,
                                                                                                               Vol. 5, No.3, 2012

amplitude of the quadrature component of supply reference
currents. The instantaneous reference currents (isar, isbr and
iscr) are obtained by adding the in-phase supply reference
currents (isadr, isbdr and iscdr) and quadrature supply reference
currents (isaqr, isbqr and iscqr). Once the reference supply
currents are generated, a carrierless hysteresis PWM
controller is employed over the sensed supply currents (isa,
isb and isc) and instantaneous reference currents (isar, isbr and
iscr) to generate gating pulses to the IGBTs of DSTATCOM.
The controller controls the DSTATCOM currents to
maintain supply currents in a band around the desired
reference current values. The hysteresis controller generates
appropriate switching pulses for six IGBTs of the VSI
working as DSTATCOM.
                  IV. SIMULATION RESULTS
         Here Simulation results are presented for four
cases. In case one load is linear RL load, in case two non
linear R load, in case three is R load by SRF controller with
DSTATCOM.

4.1 Case one
           Performance of DSTATCOM connected to a weak
supply system is shown in Fig.6 for power factor correction
and load balancing. This figure shows variation of                            Figure. 6 Simulation results for Linear RL Load
performance variables such as supply voltages (vsa, vsb and
vsc), terminal voltages at PCC (vta, vtb and vtc), supply
currents (isa, isb and isc), load currents (ila, ilb and ilc),
DSTATCOM currents (ica, icb and icc) and DC link voltage
(Vdc) for load changes from 36 kW (three-phase) to two-
phase (24 kW) to single-phase (12 kW) to two-phase (24
kW) to three-phase (36 kW). The response shows that
DSTATCOM balances unbalanced loads either of single-
phase or two-phase type and improves the power factor of
AC source to unity under varying load. Supply currents (isa,
isb and isc), compensator currents (ica, icb and icc) and DC bus
voltage (vdc) settle to steady-state values within a cycle for
any type of change in load.

4.2 Case two
            Balanced three-phase non-linear load is represented
by three-phase uncontrolled diode bridge rectifier with pure
resistive load at its DC bus. Fig. 7 shows the transient
responses of distribution system with DSTATCOM for
supply voltages (vsabc), supply currents (isabc), load currents
(ila, ilb and ilc), DSTATCOM currents (ica, icb and icc) along
with DC link voltage (Vdc) and its reference value (Vdcr) at
rectifier nonlinear load.


                                                                               Figure. 7 Simulation results Non- Linear R Load




    September Issue                                         Page 61 of 132                                           ISSN 2229 5216
                                                                        International Journal of Advances in Science and Technology,
                                                                                                                   Vol. 5, No.3, 2012

At t = 0.16 sec, the DC load resistance is changed from 15                from the supplying network. The simulation results show
to 10 ohm to increase the loading thereby the power                       that the performance of DSTATCOM system has been
absorbed changes from 21 to 30 kW. Consequently, load,                    found to be satisfactory for improving the power quality at
supply and DSTATCOM currents increase to provide                          the consumer premises. DSTATCOM control algorithm is
demanded active and reactive power to the load. The                       flexible and it has been observed to be capable of correcting
increased load on the rectifier reflects in the form of                   power factor to unity, eliminate harmonics in supply
undershoot in DC link voltage. At t = 0.26 sec, the load                  currents and provide load balancing. It is also able to
resistance is changed back to 15ohm and an overshoot is                   regulate voltage at PCC. The control algorithm of
observed now, which settles down within a few cycles due                  DSTATCOM has an inherent property to provide a self-
to action of PI controller. Results show that the supply                  supporting DC bus of DSTATCOM. It has been found that
currents are balanced, sinusoidal and in-phase with the                   the DSTATCOM system reduces THD in the supply
supply voltages.                                                          currents for non-linear loads. Rectifier-based non-linear
                                                                          loads generated harmonics are eliminated by DSTATCOM.
4.3 Case three                                                            When single-phase rectifier loads are connected,
           Balanced three-phase non-linear load is represented            DSTATCOM currents balance these unbalanced load
by three-phase uncontrolled diode bridge rectifier with pure              currents. In this paper both the Adaline and SRF control
resistive load at its DC bus. Fig. 8 shows the load currents,             strategy are used. SRF method is more advantageous and
Compensating currents, supply currents, supply voltages,                  control method is simpler than Adaline technique, where as
DSTATCOM currents (ica, icb and icc) along with DC link                   Adaline method involves the complex fuzzy logic rules.
at rectifier R load .
                                                                                                       V.    REFERENCES

                                                                          [1] A.E. Hammad, Comparing the Voltage source capability of Present and
                                                                          future Var Compensation Techniques in Transmission System, IEEE Trans,
                                                                          on Power Delivery . volume 1. No.1 Jan 1995.

                                                                          [2] G.Yalienkaya, M.H.J Bollen, P.A. Crossley, “Characterization of
                                                                          Voltage Sags in Industrial Distribution System”, IEEE transactions on
                                                                          industry applications, volume 34, No. 4, July/August, PP.682-688, 1999.

                                                                          [3] Haque, M.H., “Compensation Of Distribution Systems Voltage sags by
                                                                          DVR and D-STATCOM”, Power Tech Proceedings, 2001 IEEE Porto,
                                                                          Volume 1, PP.10-13, September 2001.

                                                                          [4] Anaya-Lara O, Acha E., “Modeling and Analysis Of Custom Power
                                                                          Systems by PSCAD/EMTDC”, IEEE Transactions on Power Delivery,
                                                                          Volume 17, Issue: 2002, Pages: 266-272.

                                                                          [5] Bollen, M.H.J.,”Voltage sags in Three Phase Systems”, Power
                                                                          Engineering Review, IEEE, Volume 21, Issue :9, September 2001, PP: 11-
                                                                          15.

                                                                          [6] M.Madrigal, E.Acha., “Modelling OF Custom Power Equipment Using
                                                                          Harmonics Domain Techniques”,IEEE 2000.

                                                                          [7] R.Meinski, R.Pawelek and I.Wasiak, “Shunt Compensation For Power
                                                                          Quality Improvement Using a STATCOM controller Modelling and
                                                                          Simulation”, IEEE Proce, Volume 151, No. 2, March 2004.

                                                                          [8] J.Nastran , R. Cajhen, M. Seliger, and P.Jereb,”Active Power Filters for
                                                                          Nonlinear AC loads, IEEE Trans.on Power Electronics Volume 9, No.1,
                                                                          PP: 92-96, Jan 2004.

       Figure 8. Simulation results   R Load by SRF controller            [9] L.A.Moran, J.W. Dixon , and R.Wallace, A Three Phase Active Power
                                                                          Filter with fixed Switching Frequency For Reactive Power and Current
                        V. CONCLUSION                                     Harmonics Compensation, IEEE Trans. On Industrial Electronics. Volume
                                                                          42, PP:402-8, August 1995.
    DSTATCOM system is an efficient mean for mitigation
of PQ disturbances introduced to the grid by DERs.                        [10] L.T. Moran ,P.D Ziogas, and G.Joos , Analysis and Design Of Three
DSTATCOM compensator is a flexible device which can                       Phase Current source solid State Var Compensator, IEEE Trans, on Indutry
operate in current control mode for compensating voltage                  Applications. Volume 25, No.2, 1989, PP:356-65.
variation, unbalance and reactive power and in voltage
control mode as a voltage stabilizer. The latter feature
enables its application for compensation of dips coming



    September Issue                                              Page 62 of 132                                               ISSN 2229 5216
                                                                   International Journal of Advances in Science and Technology,
                                                                                                              Vol. 5, No.3, 2012

                  AUTHORS PROFILE

          JYOTHSNA DEVI has received her B.Tech (EEE)
          degree from RVR&JC college of engineering, Guntur in
          the year 2009. At present she is pursuing her M.Tech
          degree with the Specialization of power system control
          and automation in PVP Sidhartha Institute of
          Technology, Vijawada. Her area of interest is Power
          Electronics.



            SRINIVASA RAO S has received his B.Tech degree
           in EEE stream from Sir C R Reddy College of
           Engineering in the year 2005 and M.Tech with the
           specialization of APS from J.N.T.University College
           Of Engineering, Kakinada in the year 2009. He is
           having work experience of 6 years. At present he is
           working as Assistant Professor in PVP Siddhartha
           Institute of Technology, Vijayawada




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