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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 September Issue Page 63 of 132 ISSN 2229 5216

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