Power Quality enhancement of a distribution line with DSTATCOM

Transcription

1 ower Quality enhancement of a distribution line with DSTATCOM Divya arashar 1 Department of Electrical Engineering BSACET Mathura INDIA Aseem Chandel 2 SMIEEE,Deepak arashar 3 Department of Electrical Engineering BSACET Mathura INDIA Abstract- ower quality problem is manifested as a nonlinear voltage, current or frequency that results in a failure or a mis-operation of enduser equipments. In industrial applications, an induction motor may be considered as a linear load in steady state operation. It is now equipped with a diode rectifier and inverter for the purpose of achieving adjustable-speed control, then it is no longer a linear load. Most of the modern equipment behaves as non-linear load injecting a significant amount of harmonic current to the power network. This paper presents a study on the modeling of DSTATCOM used for reactive power compensation on a distribution network. The DSTATCOM is controlled using decoupled theory using the I controller. This model does not represent system harmonics, but the dynamics resulting from control system and power system interactions. The power circuits of DSTATCOM and the distribution network are modeled from the SimowerSystems on a 25 kv network is evaluated and results are discussed for variable load switching. Keywords DSTATCOM, ower Quality, VSC 9 I.INTRODUCTION Electricity suppliers are nowadays concerned about the quality of the power delivered to customers. ower quality is an important issue in the local distribution system with the increased penetration of nonlinear loads due to their harmonic current injection into the grid. With the development of power electronics, several solutions have been proposed to compensate for the fluctuations observed on the distribution networks in order to ensure highest possible power quality for the customers [2]. In early 1980s, Hingorani introduced the term Custom ower Devices for the power quality issues. Mainly Custom ower Devices are classified into three categories by their structures such as Dynamic Voltage Restorer (DVR), Distribution STATCOM (DSTATCOM) and Unified ower Quality Compensator (UQC).. The FACTS devices and Custom ower Devices are introduced to improve the power quality of the current and voltage. The flexible ac transmission technology allows a greater control of power flow. Since these devices provide very fast power swing damping, the power transmission lines can be securely loaded up to their thermal limits. In a similar way power electronic devices can be applied to the power distribution systems to increase the reliability and the quality of power supplied to the customers. The technology of the application of power electronics to power distribution system for the benefit of the customer or group of customers is called Custom ower (C) since through this

2 technology the utilities can supply valueadded power to these specific customers [5]. The custom power devices are basically of two types-network reconfiguring type and compensating type. The compensating devices are used for active filtering; load balancing, power factor correction and voltage regulation. Some of these devices are used as load compensators, i.e., in this mode they correct the unbalance and distortions in the load currents such that compensated load draws a balanced sinusoidal current from the ac system. These ower Quality Devices (Q devices) are power electronic converters connected in parallel or in series with the lines and the operation is controlled by a digital controller [1]-[4].The DSTATCOM is commonly used for voltage sags mitigation and harmonic elimination at the point of connection. The interaction between the Q device and the network is preferably studied by simulation II. DESCRITION OF THE DSTATCOM OERATION DSTATCOM is a shunt device that controls the system voltage by absorbing or generating reactive power. DSTATCOM connected to a typical distribution network represented by an equivalent network Fig.1.The Distribution Static Compensator (DSTATCOM) is a voltage source WM inverter based static compensator (similar in many respects to the DVR) that is used for the correction of bus voltage sags. It is connected (shunt) to the distribution network through a transformer. The dc link voltage is provided by the capacitor C which is charged by the power taken from the network. The control system ensures the regulation of the bus voltage and the dc link voltage. The DSTATCOM is used to regulate the bus voltage on a 25-kV distribution network by absorbing or generating reactive power to the network. This reactive power transfer is done through the leakage reactance of the coupling transformer by using a secondary voltage in phase with the primary voltage (network side).this voltage is provided by a voltage source WM inverter. 25 kv 100 B C 2 MW 0.5 Feed WM Fig.1. DSTATCOM connected to a distribution network The DSTATCOM continuously checks the line waveform with respect to a reference ac signal, and therefore, it provides the correct amount of leading or lagging reactive current compensation to reduce the amount of voltage fluctuations. The major components of a DSTATCOM consists of a dc capacitor, inverter modules, an ac filter, a transformer to match the inverter output to the line voltage, and a WM control strategy. The DSTATCOM operation is illustrated by the phasor diagrams shown in Fig.2. When the secondary voltage (V S ) is lower than the bus voltage (V B ), the DSTATCOM acts like an inductance absorbing reactive power from the bus. When the secondary voltage (V S ) is higher than the bus voltage (V B ), the DSTATCOM acts like a capacitor generating reactive power to the bus. X Q D- STATCOM X Q D- STATCOM BU BU Contr ol Contr ol Fig.2. DSTATCOM as Inductor and Capacitor B 1 10

3 The DSTATCOM has various advantages as compared to conventional Static VAr Compensator (SVC) using thyristors. It is faster and can produce reactive power at low voltage. III. MODELING OF DSTATCOM The DSTATCOM is commonly used for voltage sags mitigation and harmonic elimination at the point of common coupling. The VSC generates a three-phase ac output which is controllable in phase and magnitude. These currents are injected into the ac distribution system in order to maintain the load voltage at the desired voltage reference. We consider here a DSTATCOM connected to a 25-kV distribution line. Fig.3 shows the simulation model of the distribution line with DSTATCOM. The feeding system is represented by a Thevenin equivalent (bus B1) followed by a 25- km feeder which is modeled by a pi-equivalent circuit connected to bus B2. Another feeding system of 5- km is connected between B2 and B3. Both feeders transmit power to loads connected at buses B2 and B3.At bus B2, a 2-MW load is connected, which is a shunt capacitor and helps for power factor correction. The 600 V load connected to bus B3 through a 25 kv/600 V step-down transformer represents a plant absorbing continuously changing currents, similar to an arc furnace, thus producing voltage flicker. The variable inductive and capacitive loads are switched using circuit breakers so that its apparent power varies approximately between 1 MVA and 5 MVA, while keeping a 0.9 lagging power factor and simulation results are evaluated. WM Based Model of VSC DSTATCOM is a voltage source converter connected in shunt with the distribution system by means of a 25 kv/1.25 kv coupling transformer connected to compensate the load current and ensures coupling between the WM inverter and the network. The DSTATCOM output is coupled in parallel with the distribution system through a step-up transformer to maintain isolation between the DSTATCOM circuit and 11 the distribution system. The primary of this transformer is fed by a voltage source WM inverter consisting of a 3 arm, 6 pulse IGBT bridge. The WM inverter is replaced on the AC side with three equivalent voltage sources averaged over one cycle of the switching frequency (1.68 khz)..on the DC side, the inverter is modeled by a current source charging the DC capacitor. The DC current I dc is computed so that the instantaneous power at the AC inputs of the inverter remains equal to the instantaneous power at the DC output. (V a *I a + V b *I b + V c *I c = V dc * I dc ). A µf capacitor is used as dc voltage source for the inverter. LC damped filter is connected at the inverter output to absorb harmonics and resistances connected in series with capacitors provide a quality factor of 40 at 50 Hz. The dc voltage (V dc ) is measured and sent to the controller as well as the three phase terminal voltages (V abc ) and the injected three phase currents (I abc ). V a, V b and V c are voltages at converter output. For the DSTATCOM when the simulation starts, the DC capacitor starts charging. This requires I d component corresponding to the active power absorbed by the capacitor. When the DC voltage reaches its reference value, the I d component drops to a value very close to zero and the I q component stays at the 1 pu reference value. In the case of the DSTATCOM, a constant dc source is provided across the capacitor for charging the capacitor to dc voltage reference value. Fig. 3 Simulink model of DSTATCOM and the distribution network

4 ab d q dro a b dq0 small variations. Another I regulator is responsible for keeping constant the dc voltage through a small active power exchange with the ac network, compensating the active power losses in the transformer and inverter. This I regulator provides the active current reference I d *.The other two I regulators determine voltage reference V d * and V q *, which are sent to the WM signal generator of the converter, after a dq0-to-abc transformation. Finally, V abc * are the three-phase voltages desired at the converter output. Fig. 4 Voltage Controller of DSTATCOM Voltage Controller of DSTATCOM The voltage controller of the DSTATCOM consists of several components: a phase- locked loop (LL), abc_to_dq0 (and dq0_to_abc) transformation and I regulators. The aim of the control scheme is to maintain constant voltage magnitude at the point where sensitive loads are connected, under system disturbances. The voltage controller analyzed in this work is exhibited in Fig.4, which employs the dq0 rotating reference frames. In this figure, V abc represents the three-phase terminal voltages, I abc represents the three-phase currents injected by the devices into the network, V rms is the r.m.s terminal voltage, V dc is the dc voltage measured in the capacitor and the superscripts (*) indicate reference values. LL is used to synchronize the three-phase voltages at the converter output with the zero crossings of the fundamental component of the phase-a terminal voltage. Therefore, the LL provides the angle φ to the abc-to-dq0 (and dq0- to-abc) transformation. There are also four I regulators. The first regulator is responsible for controlling the terminal voltage through the reactive power exchange with the ac network. This I regulator provides the reactive current reference I q *, which is limited between +1 pu capacitive and -1 pu inductive. This regulator has one droop characteristic, usually ± 5%, which allows the terminal voltage to suffer only 12 IV. SIMULATING THE DSTATCOM OERATION Mitigation of Voltage Flicker The voltage of the rogrammable Voltage Source will be kept constant and variable load is enabled so that we can observe how the DSTATCOM will mitigate voltage flicker. The DSTATCOM controller at the Q Regulation mode of operation behaves as floating and performs no voltage correction. Running the simulation and observing on Scope 3 variations of and Q at bus B3( trace 1) in Figure 5. as well as at buses B1 and B3 (trace 2) in Figure 6. Without DSTATCOM, B3 voltage varies between 0.96 pu and 1.04 pu (+/- 4% variation). Now, in the DSTATCOM Controller, change the "Mode of operation" parameter back to "Voltage regulation" and restart simulation. Observe on Scope 3 that voltage fluctuation at bus B3 is now reduced to +/- 0.7 %. The DSTATCOM compensates voltage by injecting a reactive current modulated at 5 Hz (trace 3 of Scope3) and varying between 0.6 pu capacitive when voltage is low and 0.6 pu inductive when voltage is high. DSTATCOM dynamic response The variable load will be kept constant and you will observe the dynamic response of a DSTATCOM to step changes in source voltage. The rogrammable Voltage Source block is used to modulate the internal voltage of the 25 kv equivalent. The voltage is first programmed at

5 1.077 pu in order to keep the DSTATCOM initially floating (B3 voltage=1 pu and reference voltage Vref=1 pu). Three steps are programmed at 0.2 s, 0.3 s, and 0.4 s to successively increase the source voltage by 5%, decrease it by 5% and bring it back to its initial value (1.077 pu). s, the source voltage is decreased by 5% from the value corresponding to Q = 0. The DSTATCOM must generate reactive power to maintain a 1 pu voltage (Q changes from +2.5 MVAr to -2.6 MVAr). Fig. 5 Q Regulation mode of operation Fig. 7 Scope 1 representing DSTATCOM current Fig. 6 Voltage Regulation mode of operation After starting the simulation we will observe on Scope1 the phase A voltage and current waveforms of the DSTATCOM as well as controller signals on Scope2. After a transient lasting approximately 0.09 sec., the steady state is reached. Initially, the source voltage is such that the DSTATCOM is inactive. It does not absorb nor provide reactive power to the network. At, t = 0.2 s, the source voltage is increased by 5%. The DSTATCOM compensates for this voltage increase by absorbing reactive power from the network (Q=+2.5 MVAr on trace 2 of Scope2). At t = Fig. 8 Dynamic performance of DSTATCOM

6 V. CONCLUSION An average model of DSTATCOM has been developed and implemented using of SimowerSystems and presented the case of a ± 2.5 MVAr DSTATCOM connected to a 25 kv distribution network. The obtained simulation results have been demonstrated the validity of the developed model. Average modeling allows a faster simulation which is well suited to the controller tuning purposes. VI. REFERENCES 1) K. K. Sen, STATCOM: Theory, Modeling, Applications, in IEEE ES 1999 Winter Meeting roceedings, pp ) Flexible AC Transmission Systems (FACTS), edited by Y.H. Song and A.I. Johns, The Institution of Electrical Engineers, London, UK, ) K.V. atil, et al., Application of STATCOM for Damping Torsional Oscillations in Series Compensated AC Systems, IEEE Trans. On Energy Conversion, Vol 13, No. 3,Sept 1998,pp ) C.D Schauder, H. Mehta, Vector Analysis Advanced Static VAR Compensators, IEEE roceedings. C, Vol. 140, No. 4, July 1993,pp ) N. G. Hingorani, Introducing Custom ower, IEEE Spectrum, Vol. 32, No. 6, pp , ) T. J. E. Miller Reactive ower Control in Electric Systems, John Wiley, New York,