Study on Performance of Non-Linear Reactive Power Compensation by Using Active Power Filter under Load Conditions

Transcription

1 ELEKTRONIKA IR ELEKTROTECHNIKA, ISSN , VOL. 22, NO. 1, 2016 Stdy on Performance of Non-Linear Reactive Power Compensation by Using Active Power Filter nder Load Conditions Jlian Wosik 1, Artr Kozlowski 1, Marcin Habrych 2, Marian Kals 1, Bogdan Miedzinski 1 1 Institte of Innovative Technologies, EMAG Katowice, Leopolda 31, Katowice, Poland 2 Department of Electrical Engineering, Wrocław University of Technology Wybrzeze Wyspianskiego 27, Wroclaw, Poland marcin.habrych@pwr.ed.pl 1 Abstract The paper analyses and examines the efficiency of non-linear reactive power compensation when se the active power filter controlled by an algorithm based on the theory of CPC. The developed physical model of 3-phase active filter has been presented and reslts of its performance in the system so the linear load and nonlinear RL type are discssed. Particlar attention was pt on its operation in transient conditions. Based on both simlation and investigation reslts appropriate practical conclsions are formlated. Index Terms Active power filter, non-linear reactive power compensation, load conditions. I. INTRODUCTION In modern electric power systems of alternating crrent there is an rgent need to find ways for optimizing working conditions. One of the most important problems is to redce the losses associated with transmission of electric energy. The most of loads in practice are of indctive (RL) type. This implies displacement between voltage and crrent vectors characterized by angle shift. Since, the final consmer is interested in obtaining only the active power (which is the sefl power) ths, nder flow, in feeders, the apparent power vale greater than the reqired the addition power losses are evident. Commonly sed indicator of efficiency of transmission is the power factor vale (cos ) defined nder the known relationship P P cos, S 2 2 P Q where P active power (sefl power), S apparent power, Q reactive power. If therefore, S > P, the energy efficiency is less than maximm de to existence of reactive power component. So striving to improve the efficiency of energy transfer it is reqired to redce or eliminate the participation of reactive power. In electric power systems with sinsoidal waveforms above mentioned problem is solved relatively simply by Manscript received 14 May, 2015; accepted 24 November, (1) application of connected in parallel careflly selected capacitive compensators (bank of capacitors, rotating compensators). Change of parameters of crrent and voltage signals reslts in variation of the capacity vale applied with respective time delay to be introdced by the control system (power factor controller) and time constant of the main circit. Already in 1920, it was fond that the power factor can be less than one for nbalanced circits with sinsoidal crrent and voltage waveforms even if the load is only active [1]. However, it mst be noted that strctre of the compensator controller is relatively easy to achieve when designed for steady state conditions, and becomes more complex for application nder dynamic load changes. Today main problem in electric power systems reslts from the fact that electric energy is extensively consmed by means of electronics converters. These devices draw deformed crrents which can be described by a Forier series i( t) Io 2 Ih sin ht h, (2) h where I o dc component, I h rms vale of h harmonic, h phase shift for h harmonic. In real systems one deals therefore, with stochastic changes of the amplitde of voltages and crrents as well as their freqencies. Therefore, in order to simplify considerations nder designing we assmed following conditions: balanced 3-phase system, constant amplitde and periodicity of waveforms as for qasi steady state. However, in practice crrent waveform can be indicated by high rate of change, p to abot a few ka/ms, as it takes place for example in hoist engines applications powered by thyristor converters. II. THE THEORETICAL BASIS OF REACTIVE POWER COMPENSATION IN SYSTEM WITH DEFORMED VOLTAGE AND CURRENT WAVEFORMS The complexity of the phenomena in circits with nonsinsoidal crrents and voltages did not allow to work ot effective methods of reactive power compensation for more than 90-years despite nmeros attempts. Neither power 19

2 theory of Bdean (1928) nor this of Fryze (1931) gave effective basis for the constrction of compensating devices. Pblication (in 1981 [2]) of theory of instantaneos power (IPT) was a first major achievement in this field. It allowed for determination of the crrents waveforms compensation althogh neither describes nor explains physical phenomena associated with energy exchange in these circits. Another important step forward was the theory of the crrent physical components (CPC) developed and pblished in 1984 [3]. This theory is based on the analysis of crrent and voltage signals in the freqency domain. One phase electric scheme for simplicity of consideration of the CPC theory application is shown in Fig. 1. High harmonics in crrent and voltage are taken into accont with particlar attention to active power flow direction cased by signals deformation. Therefore, phase voltage and crrent as a vector for 3-phase system can be expressed as follows: U R U e R e i I i i R I e R I e i a ha jh 2 1t jh 2 1t b e hb e h, hn hn o U hc a ha jh 2 1t jh 2 1t b e hb e h. hn hn o I hc (3) Next the complex power S h for h order of high harmonic can be calclated Sh Ph jqh, (4) and resltant set N of all harmonics can be divided into two sbsets N A and N B depending on sign of active power of particlar harmonics P h. Ths, if P h 0 (h N A) load is passive, whereas for P h < 0 (h N B), de to nonlinear load the voltage sorce of the system is treated as passive load; i.e. the active power P h is reversely transferred to the sorce. As a reslt the CPC theory introdces the distribtion of the load crrent into 5 components, i.e: i a active crrent related to the transmission of active power to the receiver, i r reactive crrent component de to receiver ssceptance, i h crrent of high harmonics associated with nonlinearity of the receiver (loa, i n nbalanced component, the crrent related to the voltage nbalance sorce, i sc scattering crrent associated with high harmonics in the voltage sorce and presence of load impedance vale z L dependent on freqency. U S i S APF i F Fig. 1. Simplified scheme of circit with active power compensation applied; z L load (receiver) impedance, APF active power filter, US control nit, i L load crrent, i F additional (compensation) crrent, U S, i S network voltage and crrent respectively. Us i i L load Z L Fig. 2. Waveforms of network voltage U s (, network crrent i s = i l (, power factor - cos ( and network crrent high harmonics spectrm ( when spply 3-phase non-linear RL load (2, 10 mh) via 6-plses controlled rectifier at ignition angle = 60 withot active compensation (3 400 V, 50 Hz). Each of these components specified is responsible, in a strctral way, for energy phenomenon occrring in the circit. Both CPC and IPT theory allow for effective reactive power compensation in electric circits with distorted waveforms [4]. It is obvios that the minimization of power losses in the transmission system is related not only with elimination of reactive crrent component i r bt also with elimination of ndesirable other crrents i, i s, i h. Only sch an approach enables efficient minimization of active power losses amont in nbalanced circits and/or in circits with non-sinsoidal crrent and voltage waveforms. 20

3 The essence of the compensation is therefore, based on generation, by the so-called active power filter APF (compensator keying), sch additional crrent component i F that, when injected into the network, allows for existence only active crrent responsible for the transmission of active power from the sorce to the receiver (Fig. 1). and/or resistive-indctive load sch balanced and nbalanced at sinsoidal voltage spply of 3 V 400 V, 50 Hz. The non-linear load modelled sing controlled rectifier. Reslts of simlation stdies confirming the aptness of concept were pblished, inter alia, in [5]. For example in Fig. 2 are shown selected simlation reslts for balanced non-linear RL load controlled by 6-plse thyristor bridge (at ignition angle = 60) withot active compensation whereas, in Fig. 3 with APF applied respectively. One can see the positive impact of the active power filter (APF) manifested in sinsoidal waveshape of crrents drown from the network (Fig. 3(), the total compensation of reactive power (Fig. 3() and fll sppressing of the crrent harmonics (Fig. 3(). Advanced technical measres nowadays, allow for exection of highspeed control systems responsible both for the reliable spectral analysis of the crrent and voltage waveforms, derivation of reference crrent (desire for the network and for generation (in high crrent part of the circit) of sch additional (compensation) crrent component i F to flfil reqirement il if is ia. (5) III. LABORATORY STUDIES OF THE PERFORMANCE AND DYNAMICS OF THE ACTIVE COMPENSATION SYSTEM A. Testing System and Measrement Method Stdies of the efficiency and dynamics of operation of the active power compensator (APF) were performed on physical laboratory model of 10 kva (as illstrated in Fig. 4 and Fig. 5) developed of the compensator controlled by means of an algorithm employed the CPC theory [5] [8]. A block diagram of the developed APF physical model is shown in Fig. 6. In order to determine the APF sitability the laboratory tests were performed in system as in Fig. 7 for a different so linear and non-linear balanced and nbalanced 3-phase loads. Time of signal processing of distorted voltage and crrent waveforms in the control system was valated to be abot 297 µs. Fll reslts of laboratory test nder varios operating conditions can be fond in [9] and [10]. They confirmed the seflness of this APF model both for non-linear reactive power compensation, high harmonics elimination from waveforms of network crrents and spply load balancing. Fig. 3. Waveforms of voltage U s ( and network crrent i s ( as well as power factor vale ( and i s crrent harmonics spectrm ( for non-linear load as in Fig. 2 bt with APF applied. Before constrction of a physical model a mathematical representation of APF has been developed to carry ot appropriate simlation stdies. The compensation effectiveness investigations carried ot for 3-phase resistive Fig. 4. View of the active power filter (APF) developed. 21

4 ELEKTRONIKA IR ELEKTROTECHNIKA, ISSN , VOL. 22, NO. 1, 2016 in a step way in the range of 0o to 50o (by means of the control voltage reglation) and performance of the active compensation system was observed and recorded. B. Investigated Reslts nder Transient States Waveforms of network voltage, network crrent, load crrent and the compensation crrent (APF) were measred and recorded as a response to the change of the α angle of the rectifier. Variation of the control voltage signal with time to change the rectifier angle vale from = 0 to = 50 is shown in Fig. 8.The delay time of this process is arond 4 ms. Fig. 5. Overview of laboratory installation nder testing. =50o U [V] UPI1 is UPU1 15 s Non-linear/ nonsymmetric load il Power sorce 10 5 LF 0 i FAPF Measring system -5 Switching freqency filter PC -10 Control algorithm CF UFd c -15 UFa c 0 IPM ms =0o UPI2 Fig. 8. Voltage waveform of controlling signal of rectifier to change vale of its condction angle from 0o to 50o. Voltage reglator UFd c Fiber optics interface Receiv er Fiber cable Transmit ter Measring system ifref Crrent reglator Gate plse generator ifapf Interface spply Fig. 6. Block diagram of the developed APF physical model. IPMintelligent power modle, CF-DC capacitor, LF-copling reactor, PCmicrocompter, is-spply network crrent, il-load crrent, s-network voltage, ifref-reference crrent of the active filter, ifapf-active filter crrent, UFdc-voltage of the capacity, UFac-AC voltage at otpt of power modle, UPI1, UPI2-crrent measring systems, UPU1-voltage measring system. =variabis Tr is R il P if APF L i Us Fig. 7. Schematic testing system of the effectiveness of the active compensator nder dynamic load, APF active power filter, US control system, R, L different load, P controlled rectifier (α specified angle of delayed condction), is network crrent, if- additional (compensation) crrent, il load crrent. However, the signal processing (in the control systems and in APF) is very fast, bt introdces navoidable time delay vale nder generation of compensation crrent component if with respect to the needs to be related to instantaneos load crrent il vale and its waveform. Therefore, it is of great importance to examine and explain also the inflence of this time delay on efficiency of compensation especially in the case of high dynamics of the il load crrent. Therefore the controlled rectifier was loaded by RL receiver of following parameters: R = 18.2 Ω, L = 14.7 mh. Dring the tests the ignition α angle was changed 22

5 nder rapid change of the condction angle was fond to be satisfactory for application in electric power networks characterized by implemented inertia de to electromagnetic systems applied. The response of the APF for stimlated variation of the non-linear load is within 15 ms and reslts in fast stabilization of instantaneos load crrent vale, load balancing as well as in acceptable sine wave shape of both crrent and voltages in the network. Selected investigated reslts nder rapid change of the condction angle (from 0 to 50 and reverse) of the non-linear load are presented for example in Fig. 9 and Fig. 10 respectively. e) Fig. 9. Variation of voltage and crrent (in network) waveforms (, rectifier crrent (, APF crrent (, and harmonics spectrm of network crrent in transient, e) as a response for rapid change of the angle vale from 0 to 50. IV. CONCLUSIONS The developed active compensation system sing active compensation control algorithm based on CPC theory and employed the active filter (APF) operating at freqency of the 6.4 khz (PWM modlation) was fond to perform properly nder both qasi steady states and rapid variation of the non-liner load parameters. Particlar attention paid on performance in transient showed that after step way changes of the ignition angle vale of the controlled rectifier the compensated vale of the network crrent is qickly stabilized within the time less than one period of basic harmonic (arond 15 ms). For most non-linear loads sch speed and dynamic of performance is satisfactory in practice. In general simlation stdy and laboratory tests confirmed the reliable operation and fll seflness of this type of active compensation devices in electric power networks (of both low and middle voltage) characterized by sfficient inertia de to electromagnetic systems cooperation. Fig. 10. Variation of voltage and crrent in network (, rectifier crrent- (, APF crrent (, in transient as a response for change of the angle vale from 50 to 0. Performance of the active compensation nit in transient REFERENCES [1] W. Y. Lyon, Reactive power and nbalanced circit, Electric World, vol. 75, no. 25, pp , [2] H. Akagi, Y. Kazanawa, A. Nabae, Generalized theory of the instantaneos reactive power in three phase, Circit. Proc. (JIEE- IPEC 1983), Tokyo, 1983, pp [3] L. S. Czarnecki, Consideration on the reactive power in nonsinsoidal sitations, IEEE Trans. Instr. Measr., vol. IM-34, 1985, [Online]. Available: [4] Herbert L. Ginn III, Comparison of applicability of theories to switching compensator, Przegląd Elektrotechniczny, no. 6, pp. 1 10, [Online]. Available: [5] J. Wosik, M. Kals, A. Kozlowski, B. Miedzinski, M. Habrych The efficiency of reactive power compensation of high power nonlinear loads, Elektronika ir Elektrotechnika, vol. 19, no. 7, 2013, pp [Online]. Available: [6] J. Watanabe, E. Akagi, M. Aredes, Instantaneos p-q power theory for compensating nonsinsoidal systems, Przeglad Elektrotechniczny, no. 6, pp , [Online]. Available: [7] S. H. Hosseini, K. Zare, An efficient a-b-c reference frame-based algorithm active power filtering for reactive power compensation nder nbalanced conditions, in Proc. of Int. Conf. on Harmonics and Power Qality, Hong Kong, [Online]. Available: [8] A. Bitolean, M. Popesc, The p-q theory and compensation crrent calclation for shnt active power filters, theoretical aspects and practical implementation, Przeglad Elektrotechniczny, no. 6, pp , [Online]. Available: =7650 [9] J. Wosik, Active power compensation in mining networks, Ph.D. dissertation, Wroclaw University of Technology, [10] J. Wosik, M. Kals, A. Kozlowski, B. Miedzinski, Improvement of the electric energy qality by se of active power filters, in Proc. (ICPEPQ 2013), Bilbao, [Online]. Available: 23