The efficiency of the active power filters in high power DC drive systems

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1 The efficiency of the active power filters in high power drive systems iedzinski B., Kozlowski A., Wosik J., Kalus. Abstract Reasons for the current and voltage waveform distortion in high power loads are discussed. The ways to reduce the negative impact of such customers on supplying network are presented illustrating them with the results of simulations. Explained the idea of reactive power compensation and limitation of impact of the large power load on supplying network when energize heavy drive systems. Simulation results were verified by tests on physical model and respective conclusions are formulated. Keywords motor, reactive power compensation, high harmonics filter, theory of current physical components (CPC). I. INTRODUCTION In industry the electric drives of a high power are commonly used. In cases where precise regulation of rotational speed is required the motors are often used. They provide good control characteristics and much better energy properties (e.g. smaller active power loss comparing to this in regulatory resistors of ring AC motors). Example of such application are hoisting machinery drives in the mining industry. Supply systems of these engines require voltage source. To the end of the 60s of last century the Leonard drive system was dominant. It was composed of three electric machines: Synchronous motor for driving generator, generator, motor as the key element of the system. sometimes was installed a exciter on a common shaft. Since the late 60s of the last century this system began to modify replacing 2-machines of source ( generator plus AC synchronous motor) by static rectifier. Currently are employed mostly semiconductor, thyristor circuits. However, they reveal negative impact on the supply network resulting in: - increased reactive power consumption drawn by static converters (rectifiers) from supplying network. It is iedzinski Bogdan Institute of Innovative Technologies EAG. Katowice, Poland ( bogdan.miedzinski@emag.pl) Kozlowski Artur Institute of Innovative Technologies EAG. Katowice, Poland ( artur.kozlowski@emag.pl) Wosik Julian Institute of Innovative Technologies EAG. Katowice, Poland ( julian.wosik@emag.pl) Kalus arian Institute of Innovative Technologies EAG. Katowice, Poland ( marian.kalus@emag.pl) particularly onerous at the ignition delay angle of the thyristor up to 90 0 producing so-called reactive power stroke, - power factor deterioration (cos ϕ, tan ϕ) at the connection to the plant what is a commonly used criterion under calculation of the fee charged for electrical energy delivered from the producer, - the occurrence of the so-called commutation kinks in the supply voltage waveform as a result of cyclical arising in time,(depending on number of pulses) two phase short- circuits in rectifier system, - deformation of the sine wave of supply voltage at the connection point to the plant, - generation of electromagnetic disturbances that negatively affect operation of any other devices powered from the same network (control and automation equipment, power protection etc.) II. WAY TO REDUCE THE NEGATIVE IPACT OF CONVERTERS ON POWER SUPPLY Reduction of voltage waveform distortion,from the sinusoidal, and minimizing the content of higher harmonics in current (and voltage) can be obtained by installing passive filters of high current harmonics and by feeding power converters by the phase-shifted voltage values. Depending on the inverter structure in the current drawn from the network occur characteristic harmonics h related to number of pulses: h=nk+/- 1, (1) where: n-number of pulses, k- 0,1,2,3,. For example for 6-pulse rectifier the harmonics are: h= 5,7,11,13,17,19.. whereas, for 12-pulse they are respectively: h=11,13,23,25,35,37.. According to the Fourier series representation of periodic distorted waveform with increasing harmonic number its amplitude decreases.it is worth to know that inaccurate activation of the converter components may cause incidence of other harmonic numbers than resulting from eqn. (1). In practice the number of pulses of the converter is commonly increased through the use of transformers with different vector ISBN:

groups that provide the output voltages shifted in phase. An example of such a supply system is shown in Fig.1.

Application of passive filters of current harmonics results from the fact that non-linear loading (solid-state converters) represents a source of current producing high harmonics that flow into the

(2): f 1 rez = 2π LC (2) Yy0 P2 Dy11 P3 one can limit the amount of current value entering the network. The characteristics of such a filter is presented in Fig.

2 groups that provide the output voltages shifted in phase. An example of such a supply system is shown in Fig.1. Current and voltage waveforms and their harmonics spectra for this case are illustrated in Fig. 2. Application of passive filters of current harmonics results from the fact that non-linear loading (solid-state converters) represents a source of current producing high harmonics that flow into the supply network. So through bypassing of any non-linear load by passive LC filter with resonance frequency specified by eqn.(2): f 1 rez = 2π LC (2) Yy0 P2 Dy11 P3 one can limit the amount of current value entering the network. The characteristics of such a filter is presented in Fig. 3 As one can see for frequency less than resonance value the filter presents capacitive load whereas, for higher-inductive Fig. 1 Simplified scheme of the drive power supply X C X L Z Z = X L X c or Z = X L = ωl R 2 + ( X X ) 2 L c X C 1 = ωc f f r Fig. 3 Passive filter characteristics of the f rez resonant frequency c) d) respectively. It can therefore,compensate inductive current flow for harmonics of number h < h rez.. Its filtration efficiency is dependent on accuracy of tuning. However, there are some factors limiting this efficiency like:- -changes in configuration of the supply network, -change in short-circuit power of the system during the day, -changing of filter parameters with time due to aging (in particular, the capacity change). Current harmonics distributed between the filter and the supply network is illustrated in Fig. 4 e) f) I sh I Lh I Fh Z sh Z Fh Load g) h) Fig. 2 (a-h).current and voltage waveforms and harmonics spectra for drive system as in Fig.1. a,b,c,d at measurement point ; e,f,g,h at measurement point P2 Fig.4 Current harmonics distribution depending on the parameters of the filter and supply network. ISBN:

3 Value of current through the network and filter for given higher harmonics due to non-linear load can be estimated from eqn.(3) I Sh= I Lh zfh z z Sh+ Fh I Fh z Sh = I Lh (3) zsh + zfh In the case of fine tuning of the filter to the resonance frequency occurs: z Fh =0 and I Fh =I Lh Such a situation is, however, onerous in practice since the filter/filters are provided with high harmonics of the network current due to deformed voltage waveform at the power. It may than lead to an overloading of the filter and related thermal destruction of its elements (reactors, capacitors). Therefore, preventive measure used in practice is to tune the filter to the frequency slightly lower than the given harmonic for example, by about 5% thus, to f=0.95frez.. Then, non-zero value of the impedance of the filter ( z Fh 0 ) effectively reduces the value of the current flowing into it from the network.a side effect, however is the deterioration of filtration efficiency and outflow to the network of respective amount of current high harmonics generated by non-linear load(converter). Current and voltage waveforms and their harmonics spectra when apply perfectly tuned filter for required harmonic are presented in Fig. 6 as an example. F11h F13h c) d) Fig.6 Current and voltage waveform in network with non-linear load when apply passive filters (at measurement point ) broad spectrum of harmonics, hence their compensation would require the use of multiple filters or combined filters of a complex structure. In this case for the filter construction consideration the generic algorithm can be employed successfully. In practice however,it is limited to the installation of a few filters of a selected harmonics (usually 2-3) of the highest numbers (amplitude) only to reduce the level of both current and voltage high harmonics. Reactive power compensation by means of sectional static capacity banks or by quasi on-line way using thyristor switches to adjust the capacity do not allow for perfect compensation of the reactive power flows in power system with non-linear loads. Static capacitor banks can be tuned to the needs of the system with a resolution of the power of the smallest degree of compensation step. Speed of this way of compensation is also limited by speed of control unit as well as thyristor switches of particular sections(switching frequency must be limited because of durability requirements). The capacity bank can be overloaded by current harmonics of much higher frequency as well. However, the highest risk of damage occurs during the series resonance. In spite of application of this type of compensation the current waveform in power supply is far from sinusoidal what can be compared from Fig. 7 Fig.5 Compensation system of reactive power and high harmonics of current (11 th and 13 th ) by means of passive LC filters Because, current of the non-linear load is characterized by a ISBN:

P2 The use of very fast thyristor switches for each capacity section or to control inductive current value through the reactance component of the LC system can improve the power balance for the

Hence, this compensation system is also non-linear and represents an additional source of harmonics (current and voltage). For this reason, it is also unfavorable for cooperating another loads.

voltage. This new approach has resulted in the development of a new power theory enabling implementation in practice of so-called active compensation III.

4 P2 The use of very fast thyristor switches for each capacity section or to control inductive current value through the reactance component of the LC system can improve the power balance for the required period of time in a statistical sense. However, the shape of the current flowing through the reactance element is still highly distorted. Hence, this compensation system is also non-linear and represents an additional source of harmonics (current and voltage). For this reason, it is also unfavorable for cooperating another loads. In the recent period there is developed a completely new approach to the problem of reactive power compensation in circuits with non-linear loads resulting in distorted waveforms of both current and voltage. This new approach has resulted in the development of a new power theory enabling implementation in practice of so-called active compensation III. ODERN ETHODS OF REACTIVE POWER COPENSATION Fig.7 Compensation system of reactive power non-linear load by capacitor bank; a- waveform of voltage and current at measurement point phase L1; b- waveform of capacitor bank current at measurement point P2 phase L1 AFF Fig.8 Compensation system of reactive power based on active power filter; scheme, waveform of voltage and current at measurement point The problem of reactive power compensation in circuit with non-linnear loads and therefore, with distorted from sinusoidal waveforms of voltage and current were of interest to scientists and engineers from the end of the nineteenth century, i.e. almost from the beginning of AC application. For many decades, however, failed to create a coherent theoretical basis for such compensation, which would allow for construction of practical compensation systems of high efficiency. In 1971 appeared a concept of the so-called active filters that allow for high harmonics compensation using keying by means of semiconductor devices (SCRs). The problem remained to create a theoretical basis for determining the current reference (desired quantity) for the compensator and the supply network as well. The principal difficult was the incidence of various reasons for current flows( voltage deformation, load nonlinearity) as well as imbalance of the voltage source and asymmetry of the load. In created two independent theories of power in circuits with deformed voltage and current waveforms so-called theory of instantaneous power (IPT) [1], [2], [3] and the theory of the physical component of the current (CPC) [3], [4]. The first one is based on an analysis of current and voltage signals in the time domain and the other on the analysis of the signals in the frequency domain. IPT output theory is based on the transformation of phase voltages and currents in the single phase circuit into the stationary rectangular system p,q in the α and β axes. However,this theory did not allow to compensate the harmonics caused by distorted supply voltage. Further works on its development led to the transformation from p,q system to rectangular coordinate system rotating with angular speed equal to this of phase voltage and current vectors and to its respective adaptation to the various multi-phase circuits without and with the neutral conductor [2]. There is also no physical interpretation of the phenomena of electric energy possible. ISBN:

The CPC theory does not have these disadvantages.

scatter current (new physical quantity). Developed active filter control algorithm (APF) based on the CPC theory has been verified in simulation studies and laboratory tests on physical model.

THE RESULTS OF THE SIULATION OF REACTIVE POWER COPENSATION USING THE ACTIVE POWER FILTER The simulation tests of the effectiveness of compensation in circuit with non-sinusoidal currents and voltages

8 The results of voltage and current waveforms and their harmonics spectra after use of the active power filter are presented in Fig. 8. [5], [6] c) d) AFF e) Fig.

9 odel of system used for the simulation study (control algorithm APF was developed based on the CPC theory [3]) V.

5 The CPC theory does not have these disadvantages.[4] Developed on its basis a control algorithm for active filter allows for compensation: -current higher harmonics, -reactive power of phase shift, -currents due to source imbalance, -so-called scatter current (new physical quantity). Developed active filter control algorithm (APF) based on the CPC theory has been verified in simulation studies and laboratory tests on physical model. U i s i F i L APF u i Fig. 10 odel of system used for the simulation study (control algorithm APF was developed based on the CPC theory [3]) R Load IV. THE RESULTS OF THE SIULATION OF REACTIVE POWER COPENSATION USING THE ACTIVE POWER FILTER The simulation tests of the effectiveness of compensation in circuit with non-sinusoidal currents and voltages and in unbalanced circuits were conducted on a simulation model developed in the software package atlab/simulink. odel adopted for the simulation studies is shown in Fig. 8 The results of voltage and current waveforms and their harmonics spectra after use of the active power filter are presented in Fig. 8. [5], [6] c) d) AFF e) Fig. 11 (a-e) Voltage and current waveforms in system with nonlinear-load and with active power filter, Fig. 9 odel of system used for the simulation study (control algorithm APF was developed based on the CPC theory [3]) V. THE RESULTS OF THE EFFECTIVENESS OF REACTIVE POWER COPENSATION FOR THE PHYSICAL ODEL To conduct laboratory tests a network model composed of power source with non-linear load RL (6th pulse controlled rectifier) has been developed and assembled. Active power filter of 10kVA, operated in open control system, was connected as in Fig.10. The resulting voltage and current waveforms and their harmonics spectra for characteristic point of the system are presented in Fig.11 VI. CONCLUSIONS -Application of passive filters neither allow for effective compensation of high current harmonics nor for compensation of reactive power flows in circuits with non-sinusoidal voltage and current waveforms. -Compensation based on stationary capacity banks employment is not suitable for circuits with non-linear voltage and current waveforms due to high risk of ferro/resonance. -Installing of lag compensators with thyristor switches however, allows you to improve the balance of power and increase power factor value cos ϕ (tan ϕ) but introduces to the system distorted currents.-reactive power compensation by means of active power filter (APF) based on the control algorithm developed on the basis of theory of physical current components allow for successful compensation of reactive power flow of phase shift and in addition to: -elimination of current harmonics in the supply network, -load symmetrization (in case of unbalanced load), ISBN:

6 -elimination of the scatter current occurring for deformed source voltage and when the load impedance is frequency dependent. REFERENCES [1] H. Akagi, Y. Kanazawa, A. Nabae Generalized theory of the instantaneous reactive power in three phase circuits ; Proceedings of the Int. Power Electr. Conf (JIEE IPEC), pp , Tokyo/Japan 1983 [2] A. Ferrero, G. Superti-Furga: A new approach to the definition of power components in three-phase systems under nonsinusoidal conditions. IEEE Trans. Instrum. eas., Vol40, pp , June 1991 [3] L.S. Czarnecki: Dynamic, power quality oriented approach to theory and compensation of asymmetrical system under nonsinusoidal conditions, Europ. Trans. Electr. Power, 5, pp , ETE99 [4] L.S. Czarnecki: Power in Electrical Circuits under nonsinusoidal conditions Wafeform Currents and Voltages, (in polish), Oficyna Wydawnicza Politechniki Warszawskiej, Warszawa 2005 [5] J. Wosik,.Kalus, A.Kozlowski, B.iedzinski,.Habrych: The Efficiency of Reactive Power Compensation of High Power Nonlinear Loads Elektronika Ir Elektrotechnika, vol.19, No7, pp.29-32,2013 [6] J.Wosik,.Kalus, A.Kozlowski, B.iedzinski Improvement of the Electric Energy Quality by use of active Power Filters Proceedings of ICREPQ 13, Bilbao, arch B.iedzinski (Prof.) bogdan.miedzinski@emag.pl Institute of Innovative Technologies EAG, Katowice Poland. Area of his activity are: contact materials, power theory, research of signals by using Rogowsky coil, switches. A.Kozlowski (PhD) artur.kozlowski@emag.pl - Institute of Innovative Technologies EAG, Katowice Poland. Area of his activity are: contact materials, switches, power quality, power electronics, flameproof power supply devices. Head of R&D Department. J.Wosik (PhD) julian.wosik@emag.pl - Institute of Innovative Technologies EAG, Katowice Poland. Area of his activity are: dispatching systems, theory of power, power quality, construction flameproof devices, compensation systems of reactive power and active power filters..kalus (PhD) marian.kalus@emag.pl - Institute of Innovative Technologies EAG, Katowice Poland. Area of his activity are: drive AC and systems and simulation research, converters power. ISBN:

Harmonics Elimination Using Shunt Active Filter

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