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PPT ON STATCOM for mitigation of voltage dip and voltage flickering problems in Power System.


 PAPER PRESENTATION ON STATCOM for mitigation of voltage dip and voltage flickering problems in Power System. Abstract-- Power quality problem is an occurrence manifested as the non standard voltage, current or frequency deviations that resulted in the failure or mis-operation of the end user equipment. The basic power quality problems in the distribution network are voltage sag (dip), voltage flickering and the service interruptions. In order to mitigate these power quality problems FACTS devices are there to supply/absorb reactive power. In this paper, D-STATCOM is used for voltage dip and voltage flickering mitigation. For this the 66/11 KV distribution system is modeled and simulated using the MATLAB/simulink power system block set. Here D-STATCOM was prepared with six-pulse inverter and the five-level diode clamped inverter. Among these two the five-level diode clamped inverter is the best possible option. Introduction : Power quality in distribution system has been the major issue now-a-days. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of power quality problems like voltage sag, voltage flickering, service interruptions and harmonics. A voltage dip is a short time (10 ms to 1 minute) event during which a reduction in r.m.s voltage magnitude occurs. It is often set only by two parameters, depth/magnitude and duration. The voltage dip magnitude is ranged from 10% to 90% of nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from half a cycle to 1 min. In a three-phase system a voltage dip is by nature a three-phase phenomenon, which affects both the phase-to-ground and phase-to-phase voltages. A voltage dip is caused by a fault in the utility system, a fault within the customer’s facility or a large increase of the load current, like starting a motor or transformer energizing. The high current results in a voltage drop over the network impedance [1]-[2]. Another power quality problem which is discussed in this paper is Voltage flicker, a phenomenon of annoying light intensity fluctuation, caused by large rapid industrial load changes. Erratic variations in reactive power demands lead to fluctuating voltage drops across the impedance of a distribution system which results in voltage flicker. Voltage flicker occurs when large industrial loads, such as Electric arc furnaces, Rolling mills, Large mine hoists, resistance welders, Pumps operate in a weak power distribution system.[3]-[4]. Fig 1. Flickering curve Some of the drawbacks of voltage flicker were explained in [5]. The IEEE Standard 519-1992 [6], which is referenced widely, defines maximum permissible voltage flicker levels with respect to frequency as shown in Fig. 1. The present work is to identify the prominent concerns in this area and hence the measures that can enhance the quality of the power are recommended. Here the mathematical modal of the STATCOM was presented in this work [18].D-STATCOM [7]-[10] is the one of the efficient FACTS controller device for mitigating these power quality problems than the SVC because of the advantages mentioned in [11]. Here the digital simulation is carried out in order to demonstrate the power quality problems i.e. voltage dip, voltage flickering. In this digital simulation we carried out the D-STATCOM modeling with the two different types of Voltage source inverters(VSI).in this simulations , one with six-pulse inverter and another with five-level diode clamped inverter. Multi level inverters are the emerging technology in the voltage source inverters which is having more advantages than six-pulse inverter [12]-[17]. Digital simulations were carried out for both model and results are published in this paper. II. VOLTAGE SOURCE CONVERTERS A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate Power quality problems. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. The converter is normally based on some kind of energy storage, which will supply the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the desired output voltage. The converters employed in the D-STATCOM may be of any number of pulses. Here in this paper we employed the six-pulse inverter and the five -level diode clamped inverter. A. Six-Pulse Inverter The six-pulse voltage source converter is used in both shunt and series compensators. It is basically a converter which converts a DC voltage of a DC capacitor into a three-phase AC voltage through GTO (gate turn-off)/IGBT switches. The magnitude of the AC voltage can be controlled by the firing angle of the switches. A basic three-phase six-pulse converter is shown in Figure. Fig 2: basic six-pulse inverter Six valves, each made up of a GTO/IGBT with a diode connected in anti-parallel, compose this converter. On the DC side, the converter is connected to a voltage source. If only reactive power compensation is required, this source is represented by a small charged capacitor. The converter is connected to the AC grid through a coupling step-up transformer. In a three-phase six-pulse VSC, each GTO is fired and blocked one time per line voltage cycle. In this case, each GTO/IGBT in a single branch conducts during a half-cycle (180 degrees) of the fundamental period. The combined pulses of each leg are 120 degrees phase-shifted. In this way, the converter produces a balanced set of three voltages, as illustrated in Fig 3. These voltages have harmonic components with frequencies of (6k ± 1) f, where f is the fundamental system frequency and k (= 1, 2, 3….). In order to reduce the harmonic contents at the output voltages, the number of pulses can be increased, forming a multi-pulse configuration. Multi-pulse converters are composed by n (n = 2, 4, 8…) six-pulse Fig 3: six-pulse inverter waveforms Bridges connected in parallel on the same DC bus and interconnected in series through transformers on the AC side. The amplitude of these harmonics is inversely related to the pulse number; that is, the amplitude of the kth harmonic of the output voltage wave is proportional to 1/ [Pk±1] and that of the dc supply current to 1/Pk. Where p is the pulse number. Depending on the number of pulses, these transformers and their connections can become very complex. B. Five-Level Diode Clamped Inverter Multilevel inverters include an array of power semiconductors and capacitor voltage sources, the output of which generates voltages with stepped waveforms. The commutation of the switches permits the addition of the capacitor voltages, which reach high voltage at the output, while the power semiconductors must withstand only reduced voltages. The diode-clamped multilevel inverter uses capacitors in series to divide up the dc bus voltage into a set of voltage levels. To produce 'm' levels of the phase voltage, a 'm' level diode-clamped inverter needs 'm-1’ capacitors on dc bus. Fig. 4 shows a five-level diode-clamped converter in which the dc bus consists of four capacitors C1, C2, C3, C4. For dc-bus voltage Vdc, the voltage across each capacitor is Vdc/4, and each device voltage stress will be limited to one capacitor voltage level, Vdc/4, through clamping diodes d1, d2, d3, and d1’, d2’ d3’.out put wave form is shown in the fig Fig 4: five-level diode clamped inverter Switching action is tabulated in Table 1, where switch 'on' is represented by switching state '1' and switch 'off’ is represented by switching state ‘0’. Table 1. Five-level diode-clamped multilevel inverter voltage levels and switching states Output Voltage Switching states Van S1 S2 S3 S4 S1’ S2’ S3’ S4’ Vdc/2 1 1 1 1 0 0 0 0 Vdc/4 0 1 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 -Vdc/4 0 0 0 1 1 1 1 0 -Vdc/2 0 0 0 0 1 1 1 1 Fig 5: output voltage waveform (Van) Advantages and Disadvantages of diode clamped multilevel inverter. a) Advantages: i. When the number of levels is high enough, harmonic content will be low enough to avoid the need for filters. ii. Efficiency is high because all devices are switched at the fundamental frequency. iii. Reactive power flow can be controlled. iv. The control method is simple for a back-to-back inter-tie system. b) Disadvantages: i. Excessive clamping diodes are required when the number of levels is high. ii. It is difficult to do real power flow control for the individual inverter. III. DISTRIBUTION STATIC COMPENSATOR (D-STATCOM) The predecessor of modern solid-state synchronous compensators, the rotating synchronous condenser has been used extensively in the past for reactive shunt compensation both in transmission and distri¬bution systems. Although the rotating condenser exhibits a number of desirable functional characteristics (high capacitive output current at low system voltage levels and an essentially inductive source im¬pedance that cannot cause harmonic resonance with the transmission network), it suffers from a number of operating shortcomings (slow response, potential for rotational instability, low short circuit imped¬ance, and high maintenance) and lacks the application flexibility needed to meet the power control requirements of modern transmis¬sion systems. Fig. 6: Generalized synchronous voltage source C. Structure of STATCOM Basically, STATCOM is comprised of three main parts (as seen from Figure 6, a voltage source inverter (VSI), a step-up coupling transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power transformers can function as coupling reactors. The main purpose of the coupling inductors is to filter out the current harmonic components that are generated mainly by the pulsating output voltage of the power converters. D. Implementation of Synchronous Voltage Source The solid-state synchronous voltage source can be implemented by various switching power converters. However, the switching converter considered here is the voltage-sourced inverter here voltage source inverter may be a six-pulse inverter or the five-level diode clamped inverter as explained in the section II. The reactive power exchange between the inverter and the ac system can be controlled by varying the amplitude of the (three-phase) output voltage produced. That is, if the ampli¬tude of the output voltage is increased above that of the ac system voltage, then the current flows through the reactance from the inverter to the ac system and the inverter generates reactive (capacitive) power for the ac system. If the amplitude of the output voltage is decreased below that of the ac system, then the reactive current flows from the ac system to the inverter and the inverter absorbs reactive (inductive) power. If the output voltage is equal to the ac system voltage, the reactive power exchange is zero. Similarly, the real power exchange between the inverter and the ac system can be controlled by phase-shifting the inverter output voltage with respect to the ac system voltage. That is, the inverter from its dc energy storage supplies real power to the ac system if the inverter output voltage is made to lead the corresponding ac system voltage. By the same token, the inverter absorbs real power from the ac system for dc energy storage, if the inverter output voltage is made to lag the ac system voltage. The mechanism by which the inverter internally generates reactive power can be explained, without considering the detailed operation of the solid-state switch array(s) the inverter is composed of, simply by considering the relationship between the output and input powers of the inverter. The key to this explanation resides in the physical fact that the process of energy transfer through the inverter (consisting of nothing but arrays of solid-state switches) is absolutely direct, and thus it is inherent that the net instantaneous power at the ac output terminals must always be equal to the net instantaneous power at the dc input terminals (neglecting losses). Assume that the inverter is operated to supply only reactive output power. In this case, the real input power provided by the dc source has to be zero. Furthermore, since reactive power at zero frequency by definition is zero, the dc source supplies no input power and there¬fore it clearly play no part in the generation of the reactive output power. In other words, the inverter simply interconnects the three output terminals in such a way that the reactive output currents can flow freely between them. Viewing this from the terminals of the ac system, one could say that the inverter establishes a circulating power exchange among the phases. Although reactive power is internally generated by the action of the solid-state switches, it is still necessary to have a relatively small dc capacitor connected across the input terminals of the inverter. The need for the dc capacitor is primarily required to satisfy the above-stipulated equality of the instantaneous output and input pow¬ers. The output voltage waveform of the inverter is not a perfect sine-wave. E. General Compensation Scheme A shunt-connected solid-state synchronous voltage source, composed of a six-pulse/five level, voltage-sourced inverter and a dc energy storage device, is shown schematically in Figure 7. As explained in the previous section, it can be considered as a perfect sinusoidal synchro¬nous voltage source behind a coupling reactance provided by the leakage inductance of the coupling transformer. If the energy storage is of suitable rating, the STATCOM can exchange both reactive and real power with the ac system. The reactive and real power, generated or absorbed by the STATCOM, can be controlled independently of each other, and any combination of real power generation/absorption With var generation/absorption is possible, as illustrated in Figure 7b. The real power that the STATCOM exchanges at its ac terminals with the ac system must, of course, be supplied to, or absorbed from, its dc terminals by the energy storage device. By contrast, the reactive power exchanged is internally generated by the STATCOM, without the dc energy storage device playing any significant part in it. Fig 7: a) shunt connected synchronous voltage source and b) its possible operating modes for real and reactive power generation When compared to the conventional thyristor-controlled static var compensator, which can negotiate only reactive power exchange with the ac system, the synchronous voltage source clearly has signifi¬cant operating and application advantages. The bi-directional real power exchange capability of the STATCOM; that is, the ability to absorb energy from the ac system and deliver it to the dc energy storage device (large storage capacitor, battery, superconducting magnet) and to reverse this process and deliver power for the ac system from the energy storage device, makes complete, temporary system support possible. Specifically, this capability may be used to improve system efficiency and prevent power outages. Also, in combination with fast reactive power control, dynamic real power exchange provides an extremely effective tool for transient and dynamic stability improve¬ment. F. Reactive Power Compensation Scheme G. If the D-STATCOM is used only for reactive shunt compensation, like a conven¬tional static var compensator, then the dc energy storage device can be replaced by a relatively small dc capacitor, as shown in Figure 8. In this case, the steady-state power exchange between the STATCOM and the ac system can only be reactive, as illustrated in figure given below Fig 8: synchronous voltage source operated as the static condenser. When the STATCOM is used for reactive power generation, the inverter itself can keep the capacitor charged to the required voltage level. This is accomplished by making the output voltages of the inverter lag the system voltages by a small angle. In this way the inverter absorbs a small amount of real power from the ac system to replenish its internal losses and keep the capacitor voltage at the desired level. The same control mechanism can be used to increase or decrease dc capacitor voltage, and thereby the amplitude of the output voltage of the inverter, for the purpose of controlling the var generation or absorption Where ω1 is system frequency. The magnitude of phase voltage at bus 2 (Vi) is directly proportional to the DC voltage across the capacitor Vdc, and therefore can be expressed as (from advanced angles to delayed angles). The active current (id) is small and varies very little with firing angle because it only furnishes the losses in the VSC. FOR THE VOLTAGE DIP: With out D-STATCOM: Here initially the D-STATCOM was not connected to the system and the load of pure inductive of 10MVAR is applied on the system in the time interval of 0.1sec to 0.6 sec. the voltage got dipped from 0.9955p.u to 0.8205p.u. With D-STATCOM: Now if we keep the D-STATCOM in the circuit then the voltage profile at the point of common coupling (PCC) was maintained at 0.9652. Here the excessive reactive power drawn by the load is supplied by the D-STATCOM than by the system. So that instant STATCOM acts like capacitor. Here the control circuit and the six- pulse D-STATCOM was given in the appendix1 FOR THE VOLTAGE FLICKERING: With out D-STATCOM: Here flickering circuit was prepared with the help of R-L load which is periodically operated on the system which will cause the voltage flickering at the bus 1 which is PCC. The magnitude of flickering level with out the D-STATCOM in the circuit is 2.09% which is above the tolerable limits. With D-STATCOM: Here after keeping the D-STATCOM in the circuit the flickering level comes down to 0.68% which is below the thresh hold of objection. Fig13. Subsystem of controller circuit Fig12: D-STATCOM (six-pulse) with controller The 10Mvar D-STATCOM shown in Fig. 11 contains a PWM IGBT inverter, a 5767 μF (approx 5800μf) dc capacitor, and a control system. The PWM Generator with a 3 kHz carrier frequency sends pulses to the IGBT inverter. The D-STATCOM is connected to the11kV Booster bus through an 11/2 kV coupling transformer. The operating principle was explained in the section II. The control subsystem is shown on Fig. 12. The instantaneous current of the D-STATCOM is obtained by abc_to_dq0 transformation. The decoupled d-axis component id and q-axis component iq are regulated by two separate PI regulators. The instantaneous iq reference is obtained from the measurement of reactive current produced by the inductive load. The id current corresponds to the small active power absorbed by the D-STATCOM due to the losses in the transformer and in the inverter. The DC bus voltage is also regulated to compensate for the real power losses. In this direct current control strategy, the reference values ( iqref, idref ), and feedback values (iq , id ) are direct currents signals, therefore the instantaneous current tracing control with no steady-state error can be implemented using PI-control. Fig b: Voltages dip representation because of the sudden switching of inductive loads Fig c: Voltage dip mitigation because othe application of D-STATCOM (six-pulse) Fig d: comparison of with and with out D-STATCOM Fig e: voltage flickering because of the inductive loads likes electric arc furnaces, rolling mills etc… Fig f: voltage flicker mitigation by applying the D-STATCOM (six-pulse) CASE 2: D-STATCOM with three phase five level diode clamped inverter for Voltage Dip & VOLTAGE FLICKERING mitigation. Here for the same system parameters the simulations were performed as follows and the results are shown below as FOR THE VOLTAGE DIP: With out D-STATCOM: Here initially the D-STATCOM was not connected to the system and the load of pure inductive of 10MVAR is applied on the system in the time interval of 0.1sec to 0.6 sec. the voltage got dipped from 0.9955p.u to 0.8205p.u. With D-STATCOM: Now if we keep the D-STATCOM in the circuit then the voltage profile at the point of common coupling (PCC) was maintained at same value of 0.9955 p.u. except some switching transients. Here the excessive reactive power drawn by the load is supplied by the D-STATCOM than by the system. So that instant STATCOM acts like capacitor. Here the control circuit and the six- pulse D-STATCOM was given in the appendix1 FOR THE VOLTAGE FLICKERING: With out D-STATCOM: Here flickering circuit was prepared with the help of R-L load which is periodically operated on the system which will cause the voltage flickering at the bus 1 which is PCC. The magnitude of flickering level with out the D-STATCOM in the circuit is 2.09% which is above the tolerable limits. With D-STATCOM: Here after keeping the D-STATCOM in the circuit the flickering level comes down to 0.29% which is below the thresh hold of objection. The simulation results are shown as follows The simulation results are shown as follows Fig A: Voltage dip because of the sudden application of the inductive loads Fig B: Voltage dip mitigation because of the application of D-STATCOM (five level diode clamped) Fig C: Comparison of voltage dip mitigation with and with out D-STATCOM. Fig D: Reactive power drawn because of flickering loads. Fig E: Voltage Flickering Fig F: Voltage Flicker Mitigation Because Of Five Level Diode Clamped Inverter D-STATCOM Fig G: Comparison of voltage flicker mitigation with and with out D-STATCOM. Performance Comparison between the Six Pulse And the Five Level Diode Clamped Inverter D-STATCOM. TYPE OF INVERTER D-STATCOM STATUS T.H.D. VOLTAGE FLICKRING VOLTAGE DIP Five-level diode clamped inverter Off 12.16% 2.09% 17.50% On 0.29% Approx to zero Six-pulse inverter Off 36.35% 2.09% 17.50% On 0.68% 3.043% CONCLUSIONS Voltage dip and voltage flickering are the two major power quality problems which are frequently seen in the distribution systems. These power quality problems in 66/11KV distribution system are investigated in this paper. The analysis and simulation of a DSTATCOM application for the mitigation of power quality problems are presented and discussed. Here the D-STATCOM was prepared with the 6-pulse inverter and the 5-level diode clamped inverter. The Matlab Power System Block set simulation results shows that the mitigation of the power quality problems (voltage dip and the voltage flickering) done effectively with the five-level diode clamped multilevel inverter D-STATCOM than with the 6-pulse inverter D-STATCOM. REFERENCES : [1] G. Yaleinkaya, M.H.J. Bollen, P.A. Crossley, “Characterization of voltage sags in industrial distribution systems”, IEEE transactions on industry applications, vol.34, no. 4, July/August, pp. 682-688, 1999. [2] Haque, M.H., “Compensation of distribution system voltage sag by DVR and D-STATCOM”, Power Tech Proceedings, 2001 IEEE Porto, vol.1, pp.10-13, Sept. 2001. [3] Peter Ashmole and Paul Amante, “System Flicker Disturbances from Industrial Loads and Their Compensation,” Power Engineering Journal, pp. 213-218, Oct. 1997. [4] W. N. Chang, C. J. Wu, and S. S. Yen, “A Flexible Voltage Flicker Teaching Facility for Electric Power Quality Education,” IEEE Trans.Power Systems, vol. 13, pp. 27-33, Feb. 1998. [5] M. K. Walker, “Electric Utility Flicker Limitations”, IEEE Trans.Industrial Applications, vol. 15, pp. 644-655, Nov. 1979. [6] IEEE Recommended Practices and Requirements for Harmonic Controlin Electrical Power Systems, IEEE Standard 519-1992, 1993, pp. 80-82. [7] Laszlo Gyugyi “ Dynamic compensation of ac transmission lines by solid-state synchronous voltage sources” IEEE transactions on power delivery,vol.9,no.2,pp.904-911,April 1994. [8] Pirre giroux, G.sybille, Hoang le-huy”Modeling and simulation of a D-STATCOM using simulink’s power system blockset” The 27th annual conference of the IEEE industrial electronics society,pp990-994,2001.

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