A Novel Automatic Power Factor Regulator

Transcription

1 1 A Novel Automatic Power Factor Regulator Jinn-Chang Wu Abstract A novel automatic power factor regulator (APFR) comprising a conventional APFR and a power converter based protector is proposed in this paper. The APFR can compensate for the reactive power step by step, while the power converter based protector is used to protect the APFR from the harmonic damage. Additionally, a hybrid power switch is used to switch the AC power capacitor to avoid the inrush current of the power capacitor in the process of switching on. Hence, the proposed APFR can compensate the reactive power and avoid the problems of harmonic damage and inrush current of AC capacitor. Computer simulation is made to verify the performance of the proposed novel APFR. The simulation results show that the proposed novel APFR has the expected performance. Index Terms-- power factor, harmonic, power converter I. INTRODUCTION ost loads in the industry power systems are inductive, Mwhich absorb reactive power, and where the phase of its current lags from that of voltage. Hence, the power factor is low, which results in poor power efficiency of the power distribution system; poor voltage regulation in the load side; as well as de-rating of the transmission, substation and distribution facilities. Generally, AC power capacitor is installed to supply a leading reactive power and improve the power factor. Recently, harmonic pollution of the industrial power system has become a serious problem due to the wide use of nonlinear loads [1,2]. The AC power capacitor for power factor correction provides a low impedance path for harmonic current. Hence, AC power capacitors are frequently damaged by harmonics. In addition, this results in the harmonic resonance between the AC power capacitor and the distribution power system, so the AC power capacitor may be damaged due to over-voltage or over-current [3-7]. Furthermore, the over-voltage of AC power capacitor caused by the harmonic resonance may damage the neighboring electric power facilities and even result in public accidents. In order to properly adjust the reactive power provided by the AC power capacitor, the APFR shown in Fig.1 is developed. The APFR consists of several sets of AC power capacitors connected to the utility via switches S 1 through S N. In general, every set of AC power capacitors contain an inductor to suppress the inrush current in the process of switching on. The reactive power supplied by the APFR can be adjusted by switching on/off the set number of AC power capacitors. Although, the APFR can supply an adjustable reactive power to respond to variations of loads, the AC power capacitor is still directly connected to the utility. Therefore, the problem of harmonic damage cannot be solved by the APFR. Figure 2 illustrates a facility based on the power electronic technology to be applied in a distribution power system to compensate for the reactive power [8-10]. This facility is known as the active type reactive power compensator. This active type reactive power compensator uses a power converter via an inductor shunt connected to the utility, and it may provide leading or lagging reactive power. The supplied reactive power can be linearly adjusted to respond to the variation of loads. Hence, the input power factor can be maintained to be close to unity. Meanwhile, the active type reactive power compensator will not result in harmonic resonance, so it can avoid the damage to the harmonic resonance generated by the AC power capacitor. However, the active type reactive power compensator must compensate the full reactive power demanded by the loads, and this requires a large capacity of power converter in the active type reactive power compensator. As a result, it is very expensive and its wide application is limited. Fig. 1 circuit diagram of APFR. J. C. Wu is with the Department of Electrical Engineering, Kun Shan University of Technology, Tainan 710, Taiwan, R.O.C. ( jinnwu@mail.ksut.edu.tw). Fig. 2 circuit diagram of active type reactive power compensator.

2 2 In this paper, a novel APFR is proposed, consisting of a conventional APFR and a power converter based protector shunt connected to the APFR bus, and then connected to the utility via a link inductor. The proposed APFR can supply an adjustable reactive power in response the load variation. The reactive power is primarily compensated by the APFR, and the power converter is used to protect the AC power capacitors of APFR from harmonic damage. To solve the problem of inrush current in the process of switching on the AC power capacitor, a hybrid power switch constructed by an electromagnetic switch shunt connected to a thyristor series connected to a resistor is used to switch the AC power capacitor. The proposed APFR can compensate for the reactive power and avoid the problems of harmonic damage and inrush current of AC capacitors. Hence, its reliability can be improved. To verify the performance of the novel APFR, a computer simulation is made. II. OPERATION PRINCIPLE OF THE PROPOSED APFR Fig. 3 shows the circuit diagram of the proposed APFR, which consists of a conventional APFR and a power converter based protector connected in parallel and then connected to the utility via a link inductor. The proposed APFR can supply an adjustable reactive power to respond to the load variation. The conventional APFR consists of several sets of AC power capacitor, and it compensates the reactive power step by step. Every set of AC power capacitors comprises an inductor and a power capacitor connected in series, and then shunt connects to the APFR bus by a hybrid power switch. The capacity of AC power capacitors switched into the APFR bus depends on the instantaneous reactive power demanded by the loads. The power converter based protector is used to protect the AC power capacitor of the conventional APFR. In contrast, the power converter based protector comprises a power converter serially connected to a filter inductor and an AC power capacitor, and then shunt connects to the APFR bus by a hybrid power switch. A DC power capacitor is placed on the dc side of the power converter to act as an energy buffer. A link inductor is inserted between the utility and APFR bus. This link inductor is used to block the power interference from utility and reduce the power capacity of power converter. Fig. 3 the circuit diagram of the proposed APFR. The APFR supplies the reactive power to respond to load variation by switching on/off a set number of AC power capacitors. The power converter acts as a virtual harmonic resistor to suppress the harmonic effect from nonlinear loads and the distorted utility voltage. From Fig. 3, it shows that an AC power capacitor is connected in series with the power converter in the power converter based protector. The AC capacitor is used to withstand the fundamental voltage of the APFR bus. Since the voltage of the APFR bus is almost the fundamental component in the normal condition, the power rating of the power converter in this power converter based protector can be reduced significantly due to the connected AC power capacitor [11]. Hence, the dc side voltage of the power converter can be kept to below the peak value of APFR bus voltage, and the current rating of the power converter is only the connected AC power capacitor s current. Hence, the power capacity of the power converter is very small as compared with the APFR capacity, and the capacity is dependent on the set number of AC power capacitors and the distortion of load current and the utility voltage. The power converter based protector is expected to suppress the harmonic current injected into the APFR. The conventional control algorithm of the hybrid power filter [12,13] can be used for this purpose. The difference between the proposed power converter based protector and the conventional hybrid power filter is that the current flowing through the link inductor is expected to be sinusoidal in the proposed power converter based protector, whereas the current of the utility side is expected to be sinusoidal in the conventional hybrid power filter. The output voltage v c1 (t) of power converter for harmonic suppression is represented as: vc 1(t) = kiah (t) (1) where i ah (t) is the harmonic component of the APFR current in the link inductor, and k is a constant value. The power converter used in the proposed APFR is used to avoid the power resonance of conventional APFR. If the power converter generates a voltage proportional to the harmonic component of the injecting current shown in (1), it can act as a virtual harmonic resistor k serially connected to the link inductor at the harmonic frequency [12,13]. Hence, the power converter can avoid the harmonic current injecting into the proposed APFR due to the inserted virtual harmonic resistor. Therefore, the power converter can improve the APFR performance and protect the APFR under the conditions of the distorted utility and the neighboring nonlinear load. III. SYSTEM ANALYSIS The harmonic equivalent circuit of the proposed APFR is shown in Fig. 4(a). The APFR impedance can be simplified as Z ah, and the impedance of link inductor and utility impedance are denoted as Z Lih and Z sh. Since the voltage-source power converter generates the voltage shown in (1), it can be regarded as a dependent voltage source, V ch (t). The nonlinear load in the load side is simplified as a current source i Lh (t). The

3 3 Fig. 4 the equivalent circuit of the hybrid compensation system. equivalent circuit of Fig. 4(a) can be simplified to Fig. 4(b) by the current/voltage source transformation, and the following equations can be derived v ( t) = i ( t) Z (2) eqh Lh sh Z eq = Zsh + ZLish (3) From the viewpoint of the harmonic voltage source, the equivalent impedance before applying the power converter ( k=0) can be derived as: Zeqh + Zeqh + Zh = (4) + The harmonic current injecting into the APFR can be derived as: Veqh I Lih = (5) Zh From (4), it can be found that the equivalent impedance Z h is nearly zero at the frequency of power resonance. As seen in (5), the power resonance will result in a very large harmonic current injecting into the APFR, which will damage the APFR. After applying the power converter to the APFR, the independent voltage source, generated the voltage shown as (1), is operated. Then, the equivalent impedance from the viewpoint of the harmonic voltage source can be rewritten as Zeqh + Zeqh + + k Zh = (6) + Comparing (6) with (4), it can be found that the term kz ah is adding in the numerator of (6) after applying the power converter. Hence the equivalent impedance Z h will be amplified under the resonant frequency. This means that the harmonic current caused by the power resonance can be suppressed. The larger k is, and the better suppression effect will be. The frequency response diagram of the injecting current and the power converter current are shown in Fig. 5. The parameters of the proposed APFR system are shown in Tab. 1. In Fig.5, two sets of AC power capacitors and the power converter based protector are applied to the APFR bus. As seen in Fig. 5(a), the injecting harmonic current of APFR is reduced when the gain k is increased. However, Fig. 5(b) shows that the current of the power converter is amplified significantly when the gain k is increased. This is due to that a power resonance occurs in the inner loop Z ah and Z ch of APFR when the power converter is applied. To solve this problem, the tuned frequency of Z ah must be smaller than that of Z ch [14]. Hence, the filter inductor of power converter based protector is reduced to 150uH, and the frequency response diagram of the injecting current and the power converter current are redrawn and shown in Fig. 6. This figure shows that the power converter current is clearly reduced after reducing the filter inductor of power converter based protector, and the injecting harmonic current is not affected. Although the harmonic component of the power converter current is reduced after using a small filter inductor in the power converter based protector, the power resonance still occurs in the inner loop Z ah and Z ch of APFR. To further suppress the power resonance of inner loop Z ah and Z ch, a damping resistor should be inserted into the inner loop Z ah and Z ch. However, the practical resistor cannot be inserted into the inner loop Z ah and Z ch due to the power loss. Hence, the power converter must function not only as a virtual harmonic resistor serially connected to the link inductor but also as a virtual harmonic resistor serially connected to Z ch. To obtain the function of a virtual harmonic resistor serially connected to Z ch, the power converter must generate a voltage proportional to the harmonic current of Z ch branch and it is represented as: vc 2 (t) = k rich (t) (7) where i ch (t) is the harmonic current of the Z ch branch. If the power converter generates a voltage shown as (7), it can act as a virtual harmonic resistor k r serially connected to Z ch at the harmonic frequency. The frequency response diagram of the injecting current and the power converter current after adding the function of virtual harmonic resistor serially connected to Z ch are shown in Fig. 7. As seen in Fig. 7(b), the power converter current can be further reduced when the power converter act as a virtual harmonic resistor serially connected to Z ch at the harmonic frequency. Furthermore, it also can be found that the injecting current is slightly increased when the power converter acts as a virtual harmonic resistor serially connected to Z ch at the harmonic frequency. Hence, a trade-off must be made between the power rating of power converter and the injecting harmonic current of APFR. Tab. I parameters of the proposed APFR Utility 380V, 60Hz Utility inductance 50uH Link inductor 50uH Power capacitor segment L:200uH, C:300uF Utility inductance 50uH DC bus voltage of Power converter 150V Switching frequency of power converter 20kHz

4 4 Fig. 5 the frequency response diagram, (a) injected current, (b) power converter current. The power converter acting as a virtual harmonic resistor will result in real power flowing through the power converter and the variation of dc side voltage in the power converter. To balance the real power flow, the power converter must absorb or regenerate the real power from or into the utility under the fundamental frequency. And then, the dc side voltage of power converter can be maintained at a desired value to operate the power converter normally. Hence, the power converter must generate a voltage proportional to the fundamental component of the Z ch branch current, which can be represented as: vc 3(t) = k f ic1(t) (8) where i c1 (t) is fundamental component of Z ch branch current. The power converter is operated as a virtual fundamental resistor k f when it generates the voltage shown in (8). Considering the function of power resonance suppression and DC bus voltage regulation, the output voltage of the power converter is vc (t) = kiah (t) + k rich (t) + k f ic1(t) (9) IV. HYBRID POWER SWIRCH Conventionally, an electromagnetic switch is used to turn on or off the AC power capacitor to the power feeder at low level voltages such as 220V, 380V or 480V. However, it requires about ten milliseconds or more to actuate the electromagnetic switch, and then a significant transient current may be generated. This is known as the inrush current, which flows through the switch and affects the reliability and life of the switch, as well as shortening the life of the AC power capacitor. In general, the capacity of the electromagnetic switch must be significantly increased to withstand the inrush current of the AC power capacitor at the instant of switching on [15]. However, the electromagnetic switch used in the APFR is switched on/off frequently, and the problem of the Fig. 6 the frequency response diagram, (a) injected current, (b) power converter current. Fig. 7 the frequency response diagram, (a) injected current, (b) power converter current. inrush current is very serious, so the resolution to this problem is very important. A conventional high voltage/current endurable thyristor is also suitable to act as a switch to turn on/off the AC power capacitor from a power feeder [15]. This thyristor can be precisely controlled and connected the AC power capacitor to the power feeder. Hence, it can reduce the inrush current of the AC power capacitor during the process of switching on. However, a significant voltage will drop on the thyristor during the conduction period. This results in a significant power loss on the thyristor switch of the AC power capacitor and the thyristor s temperature increases. In order to avoid the problem of overheating in the thyristor, an additional huge heat-sink and cooling fan are required. Therefore, the overall efficiency of the thyristor used as the AC power capacitor s

5 5 switch is lower than that of a conventional electromagnetic switch. Figure 8 illustrates the circuit configuration of the hybrid power switch. This hybrid power switch includes two switching loops. One is an electromagnetic switch, and the other is a solid-state switch constructed by a thyristor and a resistor connected in series. The resistor in the solid-state switch is very small, and it is used to damping the oscillation caused by the AC power capacitor and the inductor when the set of AC power capacitor is applied to the distribution power system. The solid-state switch of the hybrid switch acts as an auxiliary switch during the transient states of both switching on and switching off processes, and the electromagnetic switch is the main switch in the steady state. In the process of turning on, the solid-state switch is firstly turned on at the zerocrossing point of bus voltage. At the same time, the electromagnetic switch is also actuated, and then it is closed after several milliseconds. The on state dropped voltage of the electromagnetic switch is much smaller than that of the solidstate switch. Hence, the power capacitor current will flow through the electromagnetic switch after the electromagnetic switch is closed. The conduction time of the solid-state switch in the process of switching on is only one to two cycles of the utility. Hence, the heating problem of thyristor and resistor can be neglected. Therefore, the hybrid switch can effectively solve the problem of the inrush current. Since the inrush current of AC power capacitor at the instant of switching on is suppressed by the proposed hybrid switch module, the capacity rating of the electromagnetic switch can be significantly reduced compared with that using a singular electromagnetic switch to turn AC power capacitor on/off. Consequently, this may increase the life and reliability of the electromagnetic switch. V.SIMULATION RESULTS In order to verify the performance of the proposed APFR, a computer simulation is made. The main parameters used in the computer simulation system are also shown in Tab. I. The filter inductor used in power converter based protector is 150uH. Figure 9 shows the current waveforms of the power capacitor at the instant of switching on. In Fig 9(a), the power capacitor is switched on via an electromagnetic switch, and a significant inrush current and current oscillation occurs. Since an inductor is connected in series with the AC power capacitor in every set of AC power capacitor, it can suppress the inrush Fig. 8 the circuit configuration of the hybrid type power switch for APFR. Fig. 9 the current waveforms of the power capacitor at the instant of switching on, (a) by the electromagnetic switch, (b) by the thyristor switch, (c) by hybrid type power switch. current but generates a current oscillation of long duration. The amplitude of inrush current is dependent on the voltage angle that the electromagnetic switch is turned on. Fig. 9(b) shows the power capacitor is switching on via the thyristor switch. It can be found that the inrush current of the AC power capacitor is suppressed by switching at the precise zero crossing point of the APFR bus voltage. However, it still generates the current oscillation. Fig. 9(c) shows the AC power capacitor is switched on via the hybrid power switch, and it shows that almost no transient current appears at the instant of switching on the AC power capacitor. Hence, the hybrid power switch can suppress both the inrush current and the current oscillation, and it is very suitable to be the switch in the APFR application. Figs. 10 and 11 show the simulation results for the proposed APFR before applying the power converter. Three sets of AC power capacitors are switched on, and the load is a phase-controlled current-fed rectifier. As seen in Fig. 10(d), it can be found that the current of APFR contains rich harmonic. Figure 11 shows the spectrum of the utility voltage and APFR current. As seen in Fig. 11(b), the power resonance between the utility impedance and APFR occurs near 420Hz. The power quality is degraded due to the power resonance, and the total harmonic distortion (THD) of utility voltage is 8.6%. Figures 12 and 13 show the simulation results for the proposed APFR after applying the power converter based protector with k=k r =1and two sets of AC power capacitors. As seen in Fig. 12(d), the power converter based protector can effectively suppress the injecting harmonic current of APFR. Figure 13 shows the spectrum of the utility voltage and the APFR current. Figure 13(b) shows that the power resonance at 420Hz is suppressed effectively. Hence the power quality is improved when the power converter based protector is applied, and the THD of the utility voltage is improved to 4.5%. Figure 14 shows the simulation results after applying the power converter based protector with k=1 and k r =0. This indicates that a large harmonic current flows through the power converter because the power resonance occurs in the inner loop Z ah and Z ch of APFR. Comparing Fig. 12(e) with Fig. 14(b), it can be found that the virtual harmonic resistor k r can suppress the power resonance of inner loop Z ah and Z ch of APFR, and it is very consistent with the above analysis. Fig.

6 6 Fig. 14 the simulation result of the proposed APFR under the power converter but k r=0, (a) APFR current, (b) power converter current. Fig. 10 the simulation result before applying the power converter, (a) utility voltage, (b) utility current, (c) load current, (d) APFR current. Fig. 15 the simulation result of the proposed APFR under the load variation, (a) utility voltage, (b) utility current, (c) load current, (d) APFR current, (e) reactive power of utility. Fig. 11 the spectrum, (a) utility voltage, (b) APFR current. Fig. 12 the simulation result of the proposed APFR after applying the power converter, (a) utility voltage, (b) utility current, (c) load current, (d) APFR current, (e) power converter current. VI. CONCLUSION Since nonlinear loads are widely used in modern power systems, the problems of harmonic pollution are very serious. The APFR is widely used to improve the power factor in the distribution power system. However, harmonic amplification may occur due to the power resonance between the power capacitor of APFR and the system impedance under the harmonic-polluted power system. The power resonance will induce a high voltage and high current on the AC power capacitor, and it may damage the AC power capacitor and inductor of the APFR. In this paper, a novel APFR is proposed to supply a reactive power tracing the load variation and avoiding harmonic damage. The proposed APFR consists of a conventional APFR and a power converter based protector. Because the power capacity of the power converter is very small compared with the capacity of APFR, it can be used in practical applications from the viewpoint of cost. The simulation results show that the performance of the proposed APFR is as expected. VII. ACKNOWLEDGEMENT The authors would like to express their acknowledgement to the financial support under NSC E Fig. 13 the spectrum, (a) utility voltage, (b) APFR current. 15 shows the simulation results of the proposed APFR under the load variation. The load is linear and inductive. It indicates that the reactive power supplied by the proposed APFR can effectively trace the load variation. It also shows that the transient performance of the proposed APFR is excellent. VIII. REFERENCES [1] H. L. Jou, Performance Comparison of Three Phase Active Power Filter Algorithms, IEE Proceedings Generation Transmission and Distribution., Vol.142, No.6, 1995, pp [2] W. Mohan, T. M. Vndeland and W. P. Robbins, Power Electronics: Converter Applications and Design, New York: John Wiely and Sons, 1991.

7 7 [3] D. A. Gonzalez and J. C. Mccall, Design of Filters to Reduce Harmonic Distortion in Industrial Power Systems, IEEE Trans. on Industry Applications, 1987, Vol. 23, No. 3, 1987, pp [4] D. F. Miller, Application Guide for Shunt Capacitors on Industrial Distribution Systems at Medium Voltage Levels, IEEE Trans. on Industry Applications, Vol. 12, No. 5, 1976, pp [5] J. R. Harbaugh, J. E. Harder, Important Considerations for Capacitor Applications in the Petroleum and Chemical Process Industries, IEEE Trans. on Industry Applications, Vol. 18, No. 1, 1982, pp [6] A. H. Moore, Application of Power Capacitors to Electrochemical Rectifier Systems, IEEE Trans. on Industry Applications, Vol. 13, No. 5, 1977, pp [7] R. F. Dudley, C. L. Fellers, and J. A. Bonner, Special Design Considerations for Filter Banks in Arc Furnace Installations, IEEE Trans. on Industry Applications, Vol. 33, No. 1, 1997, pp [8] A. Tahri, A. Draou and M. Benghanem, A Fast Current Control Strategy of a PWM Inverter Used for Static VAR Compensation, IEEE IECON, Vol. 1, 1998, pp [9] L. Xu, V. G. Agelidis and E. Acha, Development Considerations of DSP-Controlled PWM VSC-based STATCOM, IEE Proceedings Electric Power Applications, Vol. 148, No. 3, 2001, pp [10] J. E. Hill and W. T. Norris, Exact Analysis of a Multipulse Shunt Converter Compensator or Statcon. I. Performance, IEE Proceedings Generation Transmission and Distribution., Vol.144, No.2, 1997, pp [11] H. L. Jou, J. C. Wu, Y. J. Chang and Y. T. Feng, A Novel Active Power Filter for Harmonic Suppression IEEE Trans. on Power Delivery. (accepted) [12] Fujita, H. and Akagi, H., A Practical Approach to Harmonic Compensation in Power system, Series Connection of Passive and Active Filters, IEEE Trans. on Industrial Applications, 1991, pp [13] F. Z. Peng, H. Akagi and A. Nabae, Compensation Characteristics of the filter system of Shunt Passive and Series Active Power Filter, IEEE Trans. on Industry Applications, Vol. 29, No. 1, 1993, pp [14] H.L.Jou, J. C. Wu and K. D.Wu, Parallel Operation of Passive Power Filter and Hybrid Power Filter for Harmonic Suppression, IEE Proceedings Generation Transmission and Distribution, Vol. 148, No. 1, 2001, pp [15] J. C. Wu, H. L. Jou, K. D. Wu and N. C. Shen, Novel Hybrid Switch to Suppress the Inrush Current of AC power Capacitor, IEEE Trans. on Power Delivery. (accepted) BIOGRAPHIES Jinn-Chang Wu was born in Tainan, Taiwan on He graduated from National Kaohsiung Institute of Technology, Kaohsiung, Taiwan in 1988, and received his M.S. and Ph.D. degree from National Cheng Kung University, Tainan, Taiwan in 1992 and 2000, all in electrical engineering. Since 2002, he has been an associate professor at the Department of Electrical Engineering, Kun Shan University of Technology. His research interests are in power quality and power electronic applications.