APPLICATION OF A SHUNT ACTIVE POWER FILTER TO COMPENSATE MULTIPLE NON-LINEAR LOADS

Transcription

1 APPLICATION OF A SHUNT ACTIVE POWER FILTER TO COMPENSATE MULTIPLE NON-LINEAR LOADS Hanny H. Tumbelaka and Chem V. Nayar School of Electrical and Computer Engineering Curtin University of Technology Lawrence J. Borle Electrical and Electronics Engineering University of Western Australia Abstract In this paper, the implementation of a shunt active power filter with a small series reactor for a three-phase system is presented. The system consists of multiple non-linear loads, which are a combination of harmonic current sources and harmonic voltage sources, with significant unbalanced components. The filter consists of a three-phase current-controlled voltage source inverter (CC-VSI) with a filter inductance at the ac output and a dc-bus capacitor. The CC-VSI is operated to directly control the ac grid current to be sinusoidal and in phase with the grid voltage. The switching is controlled using ramptime current control, which is based on the concept of zero average current error. The simulation results indicate that the filter along with the series reactor is able to handle predominantly the harmonic voltage sources, as well as the unbalance, so that the grid currents are sinusoidal, in phase with the grid voltages and symmetrical. 1. INTRODUCTION Non-linear loads, especially power electronic loads, create harmonic currents and voltages in the power systems. For many years, various active power filters (APF) have been developed to suppress the harmonics, as well as compensate for reactive power, so that the utility grid will supply sinusoidal voltage and current with unity power factor [1]-[3]. Conventionally, the shunt type APF acts to eliminate the reactive power and harmonic currents produced by non-linear loads from the grid current by injecting compensating currents intended to result in sinusoidal grid current with unity power factor. This filter has been proven to be effective in compensating harmonic current sources, but it cannot properly compensate for harmonic voltage sources. Many electronic appliances, such as switch mode power supplies and electronic ballasts, are harmonic voltage sources. A voltage sourcing series active power filter is suitable for controlling harmonic voltage sources, but it cannot properly compensate for harmonic current sources [4]. In many cases, non-linear loads consist of combinations of harmonic voltage sources and harmonic current sources, and may contain significant load unbalance (ex. single phase loads on a three phase system). To compensate for these mixed non-linear loads, a combined system of a shunt APF and a series APF can be effective [4]. In this paper, a combination of a grid current forcing shunt APF with a series reactor installed at the Point of Common Coupling (PCC) is investigated to handle the harmonic and unbalance problems from mixed loads (see Figure 1). Figure 1. Active Power Filter configuration

2 2. SHUNT ACTIVE POWER FILTER OPERATION The three-phase shunt active power filter is a three-phase current controlled voltage-source inverter (CC-VSI) with a mid-point earthed, split capacitor in the dc bus and inductors in the ac output (It is essentially three independent single phase inverters with a common dc bus). Conventionally, a shunt APF is controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents are meant to cancel the harmonic and reactive currents drawn by the non-linear loads. However, the reference or desired current to be injected must be determined by extensive calculations with inherent delays, errors and slow transient response. 2.1 Series Inductance A key component of this system is the added series inductance X L (see Figure 2), which is comparable in size to the effective grid impedance, Z S. Without this inductance (or a series active filter), load harmonic voltage sources would produce harmonic currents through the grid impedance, which could not be compensated by a shunt APF. Currents from the APF do not significantly change the harmonic voltage at the loads. Therefore, there are still harmonic voltages across the grid impedance, which continue to produce harmonic currents. The inductance X L takes the place of a series active power filter at significantly less cost, providing the required voltage decoupling between load harmonic voltage sources and the grid. 2.2 Direct Control of the Grid Current In this scheme (see Figure 1), the CC-VSI is operated to directly control the ac grid current rather than it s own current. The grid current is sensed and directly controlled to follow symmetrical sinusoidal reference signals in phase with the grid voltage. Hence, by putting the current sensors on the grid side, the grid current is forced to behave as a sinusoidal current source and the grid appears as a high-impedance circuit for harmonics. By forcing the grid current to be sinusoidal, the APF automatically provides the harmonic, reactive, negative and zero sequence currents for the load, following the basic current summation rule: i grid = i APF + i loads (1) The sinusoidal grid current reference signal is given by: i ref = k v grid-1 (2) where v grid-1 is the fundamental component of the grid voltage, and k is obtained from an outer control loop regulating the CC-VSI dc-bus voltage. This can be accomplished by a simple PI control loop. This is an effective way of determining the required magnitude of active current required, since any mismatch between the required load active current and that being forced by the CC-VSI would result in the necessary corrections to regulate the dc-bus voltage. In the VSI topology used in the APF, the dc-capacitor voltage must be greater than the peak of the ac grid voltage. Figure 2. Circuit equivalent for harmonics 2.3 Ramptime Current Control The performance and the effectiveness of the filter are enhanced by the use of the ramptime current control technique to control the CC-VSI. The principle operation of ramptime current control is based on the concept of zero average current error (ZACE) [7], [8], [9]. In this application, the current error signal is the difference between the actual grid current and the desired/reference grid current waveform. The ramptime control produces switching instants which result in the current error signal crossing zero at intervals of half the desired switching period. Hence the current error signal spends half the time on alternate sides of zero, resulting in an average value of zero, a close following of the reference signal, and a switching period (and

3 hence switching frequency) very close to the desired value. As mentioned, in this application, the CC-VSI is switched so as to regulate the grid current rather than it s own current. Controllability is ensured by the proper relative sizing of the inverter filter inductance L inv and the choice of the dc bus voltage so that the two output pwm states (per phase) will always result a corresponding opposite polarity current error signal slopes. The controllability of the ac grid current is guaranteed since the CC-VSI can be constructed so that its minimum di/dt exceeds the maximum di/dt permitted by the inductance X L. This provides the CC-VSI complete control over the ac grid current. 3. A SHUNT ACTIVE POWER FILTER WITH HARMONIC VOLTAGE SOURCING LOADS 3.1 Compensation for Harmonic Voltage Sources To show a compensation for harmonic voltage sources, a simulation was conducted using circuit constants from the literature [4] based on a threephase ac system with a grid voltage of 400V- 50Hz, a 60kW diode rectifier load with dc filter capacitor, a filter inductance (L inv ) of 0.45mH (5.3%), Z S of 1.8%, and X L of 1.8%, without a high frequency filter. The circuit equivalent from the harmonic point of view is shown in Figure 2. From computer simulation results shown in Figure 3, the three-phase shunt APF successfully forces sinusoidal current from the grid, as shown in Figure 3(a) and 3(b). In doing this, the APF compensates the harmonic voltages because the load harmonic voltage in Figure 3(c) (in spectral performance up to 1kHz) appears across X L in Figure 3(d). These same harmonic voltages appear (in relative proportion to the inductances) in the inverter voltage in Figure 3(e) and across the inverter inductance in Figure 3(f). Thus, the load harmonic voltages do not appear across Z S and load harmonic currents are not created through this grid impedance. Also, assuming the grid voltage harmonics are negligible, the ac grid voltage at the PCC will be sinusoidal. harmonic voltage of CC-VSI yields equal-butopposite voltage on the filter inductance (L inv ) to keep harmonic voltage at the PCC close to zero. This result leads the output harmonic current of the active filter to match the harmonic components of i loads. Figure 4 shows that when X L is reduced to 0.5%, the filter cannot suppress the harmonics properly, so that the grid currents are still distorted and contain significant amount of harmonics. The load harmonic voltage cannot be removed completely by the harmonic voltage on X L, because the inverter cannot produce sufficient harmonic voltage to compensate load harmonic voltage. Then, harmonic voltages still occur across grid impedance. As a result, the inverter loses its controllability; and the compensation by the active filter cannot be accomplished. 3.2 Series Inductance X L There are several ways to determine the size of X L [4], [6]. In [4], it is suggested that the minimum value of X L is 6%. In [6], the X L is used for a different purpose and not related to harmonic voltage type loads. The practical choice of X L is that it should be as small as possible to minimize cost. Furthermore, if the APF can directly force the grid current to be sinusoidal, the voltage at the PCC will have similar characteristics to the grid (except very small fundamental voltage drop and very small phase shift). In order to make the loads operate in the similar operating point to which they were connected directly to the grid, then the size of X L should be chosen close to Z S X S in per-unit value (usually the resistance of the grid impedance is very small compared to its inductance). From the above simulation, it is proven that with the X L = 1.8%, the compensation is successful. The value of X L could be lower than 1.8% provided that minimum di/dt of L inv exceeds the maximum di/dt permitted by the inductance X L. Otherwise, the value of L inv has to be reduced. However, decreasing the Linv will increase the high switching frequency ripple in the ac grid currents. It is apparent that the CC-VSI generates harmonic voltages with the same characteristics as the load harmonic voltages. Moreover, the

4 Figure 3. Simulation results for X L = 1.8%; (a) I grid, (b) I grid spectrum, (c) spectrum of V load harmonics, (d) V on XL, (e) V output CC-VSI, (f) V on filter inductance, (g) V at PCC 4. A THREE-PHASE SHUNT ACTIVE POWER FILTER WITH MULTIPLE NON-LINEAR LOADS By directly controlling the grid current, a threephase shunt APF can be provided for all nonlinear loads at the PCC instead of compensating each load individually. The system is simpler and more efficient because only one current sensor for each phase is located in the grid side. Figure 4. Simulation results for X L = 0.5%; (a) I grid, (b) I grid spectrum, (c) spectrum of V load harmonics, (d) V on XL, (e) V output CC-VSI, (f) V on filter inductance, (g) V at PCC From the preceding explanation, the shunt APF with a series reactor can compensate the harmonic voltage sources in the loads. This filter combination can also succeed for harmonic current sources. In this case, the reactor will function to limit the slope of the falling and rising edges of the load current [6]. For mixed loads, it is practical to provide a series reactor for total loads. The reactor is installed at the PCC and integrated with the APF. The size can be

5 chosen for the possible maximum power of harmonic voltage sources. A three-phase shunt APF has been proven for balanced loads [5]. However, the system may contain significant amounts of load unbalance as in commercial buildings with non-linear singlephase computer type loads. Such loads produce large negative sequence and harmonic currents. Hence, the filter has to inject the inverse of the negative sequence current to balance the unbalanced loads. The shunt APF discussed previously has the ability to balance the asymmetrical current. This is because the CC- VSI is operated to directly control the ac grid current to follow a three-phase balanced sinusoidal reference signal without measuring and determining the negative sequence component. Once the grid currents are able to follow the reference signal, the inverter creates the inverse of the negative sequence currents automatically. At the PCC, all three currents according to (1) are potentially accessible to be directly controlled by the CC-VSI. The system in Figure 1 is tested using computer simulation to verify the concepts discussed in previous sections. The circuit constants are the same as of the above section. The mixed loads are 45kW three-phase diode rectifier with LC filter on dc side and a 15kW single-phase diode rectifier with capacitive filter on the dc side connected phase-to-phase across the ac grid. The three-phase current waveforms of the mixed loads are shown in Figure 5. It shows clearly that the currents are not sinusoidal and are unbalanced. 4.1 Mixed-Type Harmonic Sources And Unbalanced loads three different currents for each phase. For each phase, the current controller is able to force the average current error, which is the difference between the reference signal and the actual current to be zero. Then, the individual phase current can follow its reference signal closely. From Figure 7, it is obvious that phase B of the inverter current is not the same as other two phases, since the single-phase load is connected between phase A and C. Hence, the inverter not only generates harmonics to eliminate the load harmonics but also provide balancing to create the symmetrical grid currents. Figure 5. Three-phase load currents Figure 6. Three-phase grid currents after compensation Figures 6 and 7 show results with several nonlinear loads to demonstrate the validity of the filter. In Figure 6, the shunt active power filter combined with the series reactor is able to successfully compensate the total mixed loads that produce harmonic and unbalanced currents. The grid currents become sinusoidal and in phase with the grid voltage (in this graph, only phase A of the grid voltage is shown). The magnitude is determined by the active power required by the system. Furthermore, the grid currents are symmetrical in magnitude and phase. These currents are balanced because the CC-VSI is able to generate Figure 7. Three-phase output currents of the CC-VSI

6 3.2 DC Bus Figure 8 shows the simulation results of the dynamic condition of the dc-bus voltage. It can be seen that the dc-capacitor voltage is decreased when the load is increased. This is because the active power demanded by the load is higher than that supplied from the grid. The dc-bus has to provide the active power to fulfill the power balance. 4. CONCLUSION This paper proposes the implementation of a three-phase active power filter together with a decoupling reactor in series with the load operated to directly control the ac grid current to be sinusoidal and in phase with the grid voltage. From the simulation results, this system provides unity power factor operation of non-linear loads with harmonic current sources, harmonic voltage sources, reactive, and unbalanced components. 5. REFERENCES Figure 8. Dynamic state of dc-bus when the load is changing; upper graph: load and grid currents - phase A; lower graph: dc-bus voltage Once the transient interval is finished, the dc-bus voltage is recovered and remains at the reference voltage 800V (by using a PI controller), and the magnitude of the grid active currents is fixed at a designated value. At this time, the total active power demanded by the load is supplied from the grid, because the active power filter only supplies the reactive power. This same process will occur when the load is decreased. In this case, the dc-capacitor voltage will increase in a transient state. Hence, the dc bus capacitor must be sized not only to minimize the ripple but also to provide maximum expected power unbalance until the PI loop again achieves steady state. The above result shows that the amplitude of the grid currents is regulated directly by controlling the dc bus voltage, and the calculation process of the grid current amplitude can be eliminated. Figure 8 also shows that the dc-bus contains a ripple voltage at the second harmonic frequency since the system has a single-phase diode rectifier load. [1] M. El-Habrouk, M.K. Darwish and P. Mehta, Active Power Filter: A Review, IEE Proc. Electr.Power.Appl, pp , Sept 2000 [2] B. Singh, K. Al-Haddad and A. Chandra, A Review of Active Filter for Power Quality Improvements, IEEE Trans. on Industrial Electronics, pp , Feb 1999 [3] Fang Zheng Peng, Harmonic Sources and Filtering Approaches, IEEE Industry Applications Magazine, pp , July 2001 [4] Fang Zheng Peng, Application issues of Active Power Filters, IEEE Industry Applications Magazine, pp , Sept 1998 [5] H.L. Jou, Performance Comparison of the Three-phase Active-power-filter Algorithms, IEE Proc. Gener. Trans. Distrib., pp , Nov 1995 [6] Adil M. Al-Zamil and D.A Torrey, A Passive Series, Active Shunt Filter for High Power Applications, IEEE Trans. on Power Electronics, pp , January 2001 [7] L. Borle, Method and control circuit for a switching regulator, U.S. Patent US , granted 1-September-1998 [8] L. Borle, Zero average current error control methods for bidirectional AC- DC converters, PhD thesis, Curtin University of Technology and the Australian Digital Theses Program: [9] L. Borle and C. V. Nayar, Ramptime Current Control, IEEE Applied Power Electronics Conference (APEC 96), March, 1996, pp