Stability And Power Quality Enhancement Using A Coordinated Capacitive Compensation And Tuned Arm Filter A.M. Sharaf and M. S. El-Moursi Department of ElectricaIKomputer Engineering, University o f N e w Brunswick PO Box 4400-UNB, Fredericton, N.B., Canada, E3B 5A3 Emails :
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[email protected]
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Abstract-This paper presents a nonlinear dynamic error driven error-driven, error-scaled dynamic control scheme for a series scaled error-controller for coordinated static series capacitor capacitor compensator, with (ZnO) lightning I surge arresters compensator SSCC and tuned arm filter TAF, for the enhancement of and an additional switched modulated Tuned Arm filter. The voltage, transient stability, capacity of tie line power transfer and the study system is implemented on digital computer where the power quality. The proposed self adjusting Tri-loop error scaled power system is loaded with both induction motors and controller is based on the load voltage, RMS source current and the dynamic current ripple deviation signals. The proposed study system 6-pulse thyristor converter load and is subjected to two types is a three phase source, a frequency dependent transmission line of disturbances namely, a severe 3-phase short circuit fault model, a nonlinear load comprising an Induction Motor and the and switching of the Induction Motor to study the response of 6 pulse thyrister converter load. The digital simulation is carried out the coordinated SSCC and TAF in reducing system transients, in time domain as well as in frequency domain using the Special voltage fluctuation, total harmonic distortion and enhancing Simulation SoftwarePSCAD/EMTDC. both voltage stability and power quality. Keyword: Voltage stability, capacitor series compensator, modulated power filter, Error driven control.
I. INTRODUCTION Due to environmental and right of way in constructing new transmission lines; need to increase power transfer capability of all existing transmission lines using the concept of flexible ac transmission system (FACTS) received much attention in recent years [ I]-[Z].The electric utility industry and consumers of electrical energy are also facing new challenges for cutting electric energy cost, improving energy-utilization, enhancing electric energy efficiency & demand-side management, improve supply waveforms & power quality, reduce safety hazards to personal and protecting electronic sensitive computer and automatic data processing networks. All nonlinear electric loads fall in either one of two general categories, namely the arc (inrush/saturation) type and converter switching type/ power electronic switching [3]. The growing use of nonlinear electric loads poses a real challenge to power quality issues and harmonic mitigation for electric utilities around the world, especially in the existing era of Unregulated Electricity Market where: competition, supply quality, security and reliability are now key selection issues for economic survival. Network pollution is characterized by the nonlinear electric load-ability to distortimodify and change the voltage and current waveforms due to its inherent nonlinearity. Harmonic and power quality PQ problems [4][9] are usually by product of solid-state converters, industrial rectifiers, switching-mode-power supplies, arc-type load such as ac arc-furnaces. Electric utility supply and power quality problems including harmonics, dynamic and quasi-static waveform distortion are now a top a priority issue for all Equipment Manufactures, Users, Electric Utilities and regulation Agencies. This paper deals with the issues of voltage stability and power quality enhancement using an
0-7803-7906-3/03/$17.0002003 IEEE.
II. DIGITAL SIMULATION MODEL The study power system model comprises the compensation system connected to a large ac power system grid via a long transmission line and equipped with coordinated SSCC bank and modulated power filter TAF. This system is loaded at the receiving end with Induction Motor and the 6 pulse thyristor converter load, as shown in Fig I(a, b). The dynamic series compensator level provided by SSCC and the filtering by the modulated tuned arm filter logically controlled driven controller. The variable structure scheme comprising the SSCC and the TAF are used jointly to improve the Ac system damping, enhance power transfer capacity, reduce harmonic distortion, improve the power quality and ensure adequate voltage stabilization at the load bus. The unified Ac system with the compensation scheme is modeled using the PSCADEMTDC s o h a r e [lo] in order to fully simulate the dynamic response of the unified system under a severe fault at bus 2 and switching motor disturbances. This paper investigates the effectiveness of the Tri-loop controller with the SSCC compensator and modulated filter when the Ac system is subjected to 3 phase short circuit fault for 40 ms. at Bus 2 or switching of Induction Motor, in order to validate the damping effectiveness of the proposed error-driven, error scaled tri-loop dynamic control. scheme and the additional harmonic reduction introduced by the modulatedswitched Tuned Arm Filter for improving voltage stability and power quality, while ensuring the delivery of desired power capacity. The SSCC andthe modulated Tuned Arm Filter are controlled with the same dynamic controller but the sequence of switching is different as the switching order for the IGBT switch of the tuned arm filter TAF is the opposite of that for the SSCC where (S2=S1). The static series capacitor compensation scheme used in this paper is a simple fixed capacitor plus switched capacitor compensation scheme. The system, transformer, transmission lines, load parameters are
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Fig. Ib. Sample study power system functional model using PSCADIEMTDC) software environment.
Fig. la. Single line diagram sample study of the unified power system
given in Table 1. The SSCC switching Logical-Assignment Table is given in Appendix (A). Table 1
The total error signal
e, used lo drive IS the PI controller with
proportional gain (Kp) and intssgral gain (Ki) to determine the control voltage level (Vc). The dynamic capacitor compensation level AC is delermined by the control voltage (Vc) (Refer to Appendix A) and x ,. = (x,,,,,~,XJ.
Fig 2a. Proposed novel error-driven,error -scaled Tri-loop dynamic feed back controller.
A. SSCC switchedmodulated Tuned Arm Filter Controller Figure I shows the proposed modulated Tuned Arm power filter which consists of three capacitors connected in series with inductor and resistor via a 6 pulse diode bridge and the function of this type of modulated power filter is controlled by the electronic switch IGBT. The SSCC and TAF are controlled with same dynamic controller scheme but the sequence 'of the switching is arranged using the pulse width Modulated (PWM) switching strategy. The GTO's switching device used for SSCC switching to change the the dynamic level compensation of SSCC, and the IGBT switch is used for switching or modulating of the TAF. The proposed SSCC&TAF, error-scaled tri loop dynamic controller used the load voltage, current ripples and RMS source current deviation signals. The following equations govem the dynamic controller design as shown in Fig. 2(a ,b).
i' b
b
E.
Cf
It60 )IF
Load voltage Deviation: e
I
=
v n, - y
I
Instantaneous current dev.: e , = i, - i
(1)
RMS sourcecurrentdeviation: e , ( k ) = l k - l i . ,
(2) (3)
Totalerrorsignalatanyinstantk e , = ( e , + e , + e , )
(4)
.
Fig. 2.b. The controlling SSCC compensator scheme using a Fixed capacitor and lwo dynamic capacitor CY stages (Cf, CftCv).
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Figure 2 (a,b) show the SSCC- 3 phase switched controlled series capacitor compensator using the GTO device in each phase to provide the required AC compensation level which is limited to 0.6 p,u, of the tie line equivalent reactance
x,ie,jne . Each phase has only two capacitors one is
fixed (Cf-cI) and the other capacitor (cV=CZ) which is dynamically switched (on or O f f , by the level of the control voltage VC signal derived from the hi-loop dynamic controller to switch the GTO solid state switch. The capacitive compensation level depends on the (load voltage and source current) dynamic excursions. The SSCC compensator has only two dynamic levels to ensure a suitable dynamic compensation at all times. This will also enhance power flow and voltage stability when the ac system is subjected to any large excursion or disturbance such as three phase bolted short circuit fault or switching the Induction Motor. The capacitor hanks are protected by ZnO surge arresters in (phase-to-phase) and (phase-to-ground) configurations to dissipate any excessive dynamic over currents and suppress any transient overvoltage that may occur during the (ordoff) capacitor switching. ZnO surge arresters are installed based on a suitable selection criteria of the rated voltage based on the following calculations of a suitable rated voltage [11]-[12].
Where T O V Temporary overvoltage setting. MCOV maximum continuous operating voltage of Meta oxide lightning arrester LA vs MCOV = Yc 5 1.1(12)
6
Vr2 TO V I 1.2Vc
(13)
In. DIGITAL SIMULATION RESULT To simulate the unified ac radial system, all configuration and parameters of the power system elements as well as load parameters need to be known, The data has been facilitated for this work through the (hvdc research center), Manitoba Hydro h i of~ ~~ l ~ companies, F~~ and ~ ~ ~catalogues illustrates the configuration of the ac study system which is loaded with a large Induction M~~~~ average p,f, 0.8 lag plus a 6-pulse thyristor converter load and is equipped with the SSCC at the receiving end OfTransmission lines near load bus 4. To study the effectiveness of this self-adjusting SSCC capacitor compensator and the modulated Tuned Arm Filter the system is subjected to two types of disturbances, namely, a 3-phase bolted fault with duration 40 ms at bus 2, and switching of the Induction Motor. It is very important to study the effect of these large disturbances on the ac power system without and with the coordinated SSCC and modulated Tuned Arm Filter to assess voltage stability, oscillations, harmonics levels and the power flow transfer. Therefore, this detailed study has been undertaken only for these two specific cases.
The dynamic response of the unified AC system when subjected to 3-phase fault disturbance at bus 2 for 40 ms at time F0.6975 sec. without and with the coordinated (SSCC, controller &TAF) is studied. The unified ac system was simulated using the PSCAD/EMTDC software. This unified ac system is subjected to the three phase short circuit fault at the secondary side of the transformer near to system bus (2) for 40 ms. Figures 3 (a,b,c and d ) show the voltages and currents measurements at the sending end of the feeder and at the load bus in the cases of without and with compensator. Due to this severe disturbance, it is found that excessive oscillations appeared and a transient overvoltage takes place in the system that reached up to 1.956 p,u. , furthermore, using the FFT analysis the waveform of the load voltage, load current is also highly distorted and contain a higher percentage of the 5" and 7'' order harmonics. The analysis of the total harmonic distortion THD and the percentage of the 5" and 7 th order of harmonics are given in Table 2. It is found that the voltage transient stability, power quality and the power transfer levels are severely degraded without the coordinated SSCC scheme and modulated Tuned arm filter. To reduce this transient overvoltages, damp oscillations, enhance transient stability as well as increase the power flow, the new SSCC scheme (with ZnO surge arresters) is installed in the receiving end of the transmission line near the load bus 4. Also for removing any higher order harmonics, the modulated Tuned Arm power Filter is also installed at the load bus which is usually the source of these harmonics. The digital simulation was carried out again under the same short circuit fault condition at bus 2 with the SSCC and the modulated Tuned arm filter. Figures 4 (a, b, c and d) show the voltages and currents measurements at the same measuring points. Figure 5 and 6 shows the power transfer level without and with coordinated compensation. The digital simulation results show that the %Kcdynamic control1er and the Tuned Arm Filter high are effective in *"'ping the system oscillation, enhance the power transfer level , reduce the total hrmonic distonion and improve Power quality. The high is h ~oscillations J ~ are damped f and the ~transient overvoltage ~ reduced from 1.956 p.u to 1.2 p.u which is in permissible limit. Furthermore, the FFT analysis shows that total harmonic distortion (THD) of the load voltages and load currents are greatly reduced and the Percentage of the 5" and 7" order harmonics as shown in 2. Figure shows the comparison between the load load current, THD, Percentage of the 5" and 7" order of harmonic without and with the coordinated sscc and TAF. Table 2
Case (1): Three phase bolted fault at bus 2 for 40 ms. Case (2): Switching of the Induction Motor.
527
~
(a) (b) (C) (4 Fig.3 ( 8 ,b , c and d). The p.u load vollages at bus 4, lerminal voltage at bus 2, the total load current i L and the induction load current i. respectively without (SSCC&TAF).
"I
I
I
(3) (b) (4 (C) Fig.4 (a, b , c and d). The p.u load voltages at bus 4, terminal voltage at bus 2, the total load current iL and the induction load current im respectively with (SSCC&TAF).
indicated the effectiveness of the dynamic controller in damping the system. The ac system was simulated using the PSCADEMTDC software. This unified ac system is subjected to switching of at the inductitm motor at time e0.6975 sec. Figure 8 (a, b, c and d ) show the voltages and currents measurements at the sending #endof the feeder and at the load .bus in the two cases of without and with the coordinated (Compensation, controller & 'Tuned Arm Filter). Due to this severe load disturbance, it is lound that excessive oscillations appeared and a transient overvoltage also takes place in the ac .-*-system that reached up tn 1.61 p.u. The FFT analysis shows that the waveforms of the load voltage, load current is again Fig. 7. Comparison of the load voltage, load current and %THD without and highly distorted and contain high percentage of the 5Ih and 7" with (SSCC&TAF) harmonics. The FFT analysis and the total harmonic distortion as well as percentage of the 5" and 7' orders of harmonics are This case shows the same dynamic response of the unified AC given in Table 3. It is found that the voltage stability, power system when subjected to the disturbance of switching the quality and the power tran:;fer levels are again severely Induction Motor. The dynamic response of the AC system degraded without the SSCC scheme and the modulated Tuned without and with coordinated (SSCC, controller &TAF) arm filter. .,.Dl_^
.*.I/-.T",
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To suppress these overvoltages, damp oscillations, enhance transient stability and increase power flow, The coordinated SSCC scheme (with ZnO surge arresters) with the Tuned Arm Filter is controlled with the same dynamic controller The digital simulation was carried out again under the same motor switching condition at load bus 4 with the coordinated SSCC and the modulated Tuned arm filter. The harmonic analysis has been undertaken using FFT to measure the total harmonic distortion (THD) and the percentage of the of the 5" and 7" harmonics. The distorted wave form as shown in Figure 9. ON LINE FFT
ON LINE FFT
Figures IO (a, b, c and d) show the voltages and currents measurements at the same measuring points. Figure 11 & 12 show the power transfer level without and with coordinated dynamic compensation to show its performance in enhancing the power transfer level. The simulation results show that the self-adjusting SSCC dynamic controller and modulated the Tuned Arm Filter is effective in damping the ac system, enhancing the power transfer level , reducing the total harmonic distortion and improving power quality. The oscillations are also damped and the transient overvoltage is reduced from 1.6 p.u to 1 p.u which is in a permissible limit. Furthermore, the FFT analysis shows that the distorted waveform of the load voltage and load current are some what cleaned and the total harmonic distortion is reduced as well as and also the percentage of the 5" and 7" harmonics as shown in Table 3. Figure 13 shows the comparison between the load voltage, load current, THD,without and with the coordinated SSCC and TAF. Table 3
Fig. 9. FFT blocks analysis far harmonic analysis and THD of the load voltages and load currents wave forms
Fig.%(a ,b ,c and d). The p.u load voltages
at bus 4,
terminal voltage at bus 2, the total load current i i and the induction load current i,reipectively Without tSSCC&TAF).
voltages at bus 4, terminal voltage a1 bus 2, the total load current i L md the induction load current im respectively With (SSCC&TAF).
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0 24
0s
0.72
048
1.2
T i m (sec)
Fig. 12 Power transfer 11:vcIs P&Q with (SSCC&TAF)
Fig. I I . power transfer levels P&Q without (SSCC&TAF)
REFERENCE L.Gpgyi. Solid state control of electric power in AC transmission systems, EECPS, Capri, May 1989. 121 E.Larren, N.Mil1er.S. Nilsson, S.lindgren, Benefits of CTO-based compensation systems for electric utility applications, IEEE Transactions on power delivery, Vo1.7, No.4,Oct.1992, PP.2056-2062. A.M. Sharaf, Pierre KreidYPower Quality Enhancement Using A [3] Unified Switched Capacitor Compensator" CCECE 2003 , Montreal, May 2003 IEEE. R.0.M Powell, '' The design of capacitor components of large high [4] voltage A.C filter networks," IEE conference on High Voltage DC Transmission, Sept 19-23 1966, Manchester, UK No 22. C.D Clark and M.J Jahnson..Brown, "The application of self-tuned [SI harmonic filter 'to HVDC Converters," IEE Conference on High Voltage DC Transmission, Sept 19-23 1966, Manchester. UK N02Z. G.L. Brewer, C.D Clark and AGavrilovic, "Design considerations of 161 AC harmonic filter," IEE conference on High Voltage DC Transmission, Sept 19-23 1966. Manchester, UK, No 22. A.M Sharaf, Caixia Guo and Hong Huang" DistributionRitilizai~" 171 system voltage stabilization ,and power quality enhancement using intelligent sman filter,"UPEC'95, England, UK, 1995. [8] A.M Sharaf, Pierre Kreidi, Dynamic compensation wing switchedtmodulated power fili:ers," proceedings of the 2002 IEEE CCECE, Winnipeg, Manitoba, Canada. [9] A.M. Sharaf, Pielre Kreidi "Power Quality Enhancement and Harmonic Reduction Using Dynamic Power Filters," ELECTRIMACS 2002. Montreal ,Quebec, Canada, August 18-21,2002, [IO] PSCADIEMTDC, Manitoba HVDC Research Center Inc. 244 cree crescent, Winnipeg, Manitoba, Canada R3J 3W1. [I I] Technical Catalogues of General Electric Company (2001). 1121 Mohamed S. E. El-Moursi, F a h i M. H. Youssef and Taher D. Eish, '' Computer Applications About The Switching Overvoltages in Mansaura I I kV Electrical Cmistributian Network" Part II, the 8' lntemational IEEE MIDDLE-EAST Power Systems Conference Mepcon'2001, December 29-31,2001. (I]
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