Research on a Practical Control Strategy of Three-Phase Active Filter

Abstract: A practical control strategy for a three-phase active power filter (APF) is introduced. The main circuit topology of the APF adopts a three-phase four-wire voltage-type converter. Its control circuit is divided into two parts: the generation of phase current reference and hysteresis control pulse modulation. The phase current reference is in phase with the phase voltage, and the amplitude is determined according to the energy balance between the power supply, the active filter and the load. The hysteresis control pulse modulation makes the phase current follow the reference within a differential band. When the switching ripple is filtered out, the three-phase current is a symmetrical current with high power factor, low distortion. The experimental results show that the active filter based on this control strategy can not only simultaneously complete the three functions of reactive power compensation, harmonic suppression and three-phase current balance, but also has simple control, high reliability and good compensation effect. These advantages make it have broad application prospects.

Keywords: Three-phase active filter, hysteresis control, pulse modulation, reactive power compensation, harmonic suppression, three-phase current balance

An Improved Control Method for Three－ phase Active Power Filter

Abstract:In this paper a three－ phase active power filter with an improved control method is described in detail. Its topology of main circuit adopts the three－ phase four－ wire voltage－ source converter with split－ capacitors. The control circuit generates current references for mains currents to follow with dynamic hysteresis－ band current controller. The proposed active power filter may provide the following multifunction at the same time:reacive power compensation,harmonic suppression and cureent balance in three－ phase power system. Experimental results prove that the control method has the following features:simple control circuit, high system reliability and good ability of reactive power compensation, harmonic suppression and current balance.

Keywords:Three－ phase active power filter Reactive power compensation Harmonic suppression Current balance Current reference Dynamic hystersis－ band current control

1 Introduction

With the continuous development of power electronics technology, more and more power electronic devices are widely used in various fields. However, the inherent nonlinearity of power electronic devices makes its impact on the mains, such as harmonic pollution and input power factor problems, increasingly prominent. In the past, we used passive filtering networks to solve harmonic problems. However, the filtering characteristics of passive filtering networks are non-selective, and they also have some disadvantages such as large size, easy to cause resonance, and poor compensation characteristics. In recent years, many static VAR compensator topologies have been proposed to solve the power factor correction problem. However, some static VAR compensators themselves will generate low-order harmonics. Moreover, their dynamic response cannot meet the requirements of some special loads.

To this end, a series of active power filter (APF) schemes have been proposed to solve harmonic and reactive power compensation and improve power factor. For example, Akagi [16] proposed an active filter using a multi-voltage source converter and a delayed PWM scheme. In his subsequent paper [17], he described in detail the control circuit based on the instantaneous reactive power theory and discussed its transient response characteristics. Enslin [19] determined the compensation current by detecting and calculating the reactive power of the load and injected it into the power grid. Enslin and Hayafune [20] used a microprocessor to calculate the compensation current and generate a switching signal. Jou H.L. [24] determined the compensation current by calculating the fundamental active part of the load current and injected it into the power grid. All of the above active filter control methods require detecting the waveform of the load current and then performing corresponding processing to generate a reference for the compensation current. Wu J.C. and Jou H.L. [25] proposed a simplified single-phase active filter control method in 1996. This control method detects the waveform of the incoming power current and only generates a power current reference, not a compensation current reference. Therefore, the incoming power current is the directly controlled quantity.

The practical three-phase active filter control strategy described in this paper is an expansion and improvement of the "simplified control method" and is applied to the three-phase active filter. It is not only convenient and reliable to implement, but also the three-phase active filter after the dynamic current bandwidth control can not only realize the compensation of reactive power and the suppression of harmonic current, but also balance the three-phase current on the transmission line. Therefore, it can improve the power quality and the transmission efficiency of the transmission line.

2 Main circuit structure and working principle of three-phase active filter

Figure 1 Main circuit of three-phase parallel active filter

The main circuit of the active filter adopts a three-phase four-wire voltage-type converter, as shown in Figure 1. The DC capacitors C1 and C2 on the DC bus of the voltage-type converter are used as energy storage elements, and the capacitance of C1 and C2 is equal. Switches S1 and S4 form a half-bridge converter of phase A. When the bridge arm is turned on and off, the voltage on the inductor L1 causes ifa to rise or fall linearly, thereby controlling the size, phase and waveform of ifa, and thus the input current ia of phase A can be controlled. This converter can work in the rectification state or in the inversion state. Similarly, the B-phase half-bridge converter composed of switches S3 and S6 and the C-phase half-bridge converter composed of switches S5 and S2 can control the input current of phase B and phase C respectively, and they can also work in the rectification and inversion states. Inductor L0 and capacitor C0 form a low-pass filter, which is used to eliminate the switching ripple generated by the converter.

The basic idea of ​​the control strategy of the practical three-phase active filter discussed in this article is: set three phase current references for the three-phase input phase current that are in phase with the phase voltage, contain only the fundamental wave, and have equal amplitudes to each other, and control the converters of each phase to make the phase currents ia, ib, ic follow the references within a difference band. Because the reference is a sine wave with equal amplitude and in phase with the phase voltage, when the switching frequency band is filtered out by an additional passive filter, the phase currents iA, iB, iC are symmetrical currents with high power factor, low distortion, and high power factor.

If the loss of the active filter is ignored, the converter neither absorbs nor releases energy. In steady state, the energy consumed by the load is equal to the energy provided by the power supply, that is, the input and output power remain balanced. The capacitor on the DC bus is an energy storage element, so the converter can exchange energy with the power supply and the load. The reactive power required by the load is obtained by exchanging with the active filter. Reactive power is only exchanged between the active filter and the load. The asymmetric power required by the load is provided to the load by the power supply after the energy exchange between the phases of the converter.

If the input and output power are out of balance, the unbalanced energy will be supplied or absorbed by the DC capacitor, causing a change in the capacitor voltage. If the active power supplied by the power supply is less than the load requirement, the DC capacitor voltage drops. After the control circuit detects the capacitor voltage drop, it will simultaneously increase the amplitude of the current reference of each phase to increase the active power supplied by the power supply. On the contrary, when the DC capacitor voltage rises, the amplitude of the current reference of each phase will be reduced at the same time, and finally the supply and demand of energy will be balanced. The average value of the capacitor voltage on the DC bus can provide information on whether the power is balanced. We use feedback control to determine the amplitude of the reference current to achieve a balance between input and output power.

3 Control block diagram Figure 2 shows the control block diagram of the active filter. In the control of the active filter, the quantities to be detected are: phase current ik, phase voltage uk, and voltages Uc1 and Uc2 on the upper and lower DC capacitors. The three-phase incoming line voltage is detected to generate a unit sine wave Sk in phase with the phase voltage. Uref is the expected value of the set capacitor voltage. The level R obtained by proportional integration of the error e between it and (Uc2 + Uc1) is used to adjust the amplitude of the reference to balance the energy between the power supply and the load. Feedback to the current balance setting circuit generates a fine-tuning level Tk to fine-tune the reference amplitude so that the three-phase current reference amplitudes are completely equal. This makes the three-phase power on the incoming line symmetrical when the three-phase voltage is symmetrical. The difference between Uc2 and Uc1 is proportionally integrated to obtain an error ε, which is added to the current reference so that the phase current has an adjustable DC component to ensure that the average values ​​of the voltages on C1 and C2 are equal during operation. (This will be discussed in detail later). Therefore, the current reference is determined by four quantities: Sk, R, Tk and ε: (1)

The reference current floats up and down by δ to form a bandwidth of 2δ. We use the hysteresis control pulse modulation method to control the opening and closing of the upper and lower arms of the half-bridge converter, so that the phase current ik rises and falls within the difference band, thereby realizing the three major functions of the active filter.

Figure 2 Control block diagram

Assuming that the system voltage is constant and the load is constant, the load current i1k (k=a, b, c) is stable. i1k can be an asymmetric, nonlinear current, containing reactive power and harmonics. The phase current ik is the sum of the load current i1k and the compensation current ifk. Therefore, if the compensation current ifk can be controlled, the phase current ik can be controlled. The typical working mode of the hysteresis control pulse modulator is shown in Figure 3.

Switching devices S1 and S4 form a half-bridge converter for phase A. By adjusting the reference amplitude of the phase current, the average values ​​of Uc1 and Uc2 are kept constant. In order for the active filter to work properly, Uc1 and Uc2 must be greater than the peak value of the input voltage.

Open S4 and close S1. If ifa>0, ifa flows through S4 to discharge C2, Uc2 decreases, ifa increases, and ia increases; if ifa<0, ifa flows through the lower bridge arm reverse diode to forward charge C2, Uc2 increases, ifa increases, and ia increases. Therefore, ifa and ia increase when the lower bridge arm is turned on (dia/dt>0). When ia increases to slightly greater than +δ, turn off S4 and open S1. If ifa>0, ifa flows through the upper bridge arm reverse diode to forward charge C1, Uc1 increases, ifa decreases, and ia decreases; if ifa<0, ifa flows through S1 to discharge C1, UC1 decreases, ifa decreases, and ia decreases. Therefore, when the upper bridge arm is turned on, ifa and ia decrease (dia/dt<0). When ia drops to slightly less than -δ, S4 is turned on again and S1 is turned off, and so on. By turning on and off the upper and lower bridge arms of this half-bridge converter, the phase current ia can follow the phase current reference change within a difference band 2δ, as shown in Figure 3.

The working principle of the hysteresis control pulse modulator is explained above using phase A as an example. The main circuit of the three-phase active filter is a three-phase half-bridge type. The hysteresis control pulse modulation of each phase is relatively independent. Because the reference current (k=a, b, c) is a three-phase symmetrical sinusoidal fundamental wave, and the main current ik (k=a, b, c) tracks the reference current under dynamic current bandwidth control, after passive filtering, the switching frequency is easily filtered out, leaving only the required three-phase symmetrical fundamental wave, so the current on the transmission line at the input end is three-phase balanced, there is no zero-sequence current flowing on the neutral line of the power supply, and the zero-sequence current required by the nonlinear load is all provided by the active filter.

In the process of the upper and lower bridge arms of the converter being turned on in sequence, whether ifk flows into or out of the converter, it will cause fluctuations in UC1 and UC2. Table 1 summarizes the voltage changes on capacitors C1 and C2 caused by various conditions of current ifk.

Table 1 Capacitor voltage UC1 and UC2 changes

Figure 3 Typical operating mode of hysteresis control pulse modulator

When ifk>0, UC1 increases and UC2 decreases, but their change values ​​are not equal. This is because the value of difk/dt is affected by the instantaneous value of the AC phase voltage. The same is true when ifk<0.

When ifk contains a DC component, even if it is very small, the charge will accumulate on the DC capacitor, causing UC1 and UC2 to lose symmetry. If a controllable small DC component ε is added to the reference current, the difference between the average values ​​of the upper and lower capacitor voltages can be controlled and limited to an acceptable range. The DC component ε is determined by the proportional integration of the difference ΔU (ΔU=UC2-UC1) between the upper and lower capacitor voltages.

It can be seen that after adding the DC component ε (-δ<ε <δ), the values ​​of the upper and lower bands of the reference current are changed, but the entire bandwidth value (2δ) and the switching frequency determined by the bandwidth are not changed. The influence of the DC component ε on the capacitor voltage is as follows: The fluctuation of the DC capacitor voltage will also change with the change of the amplitude and bandwidth of the reference current. In addition, (UC1+UC2) and (UC2-UC1) will not only have the ripple of the switching frequency, but also the ripple applied to the neutral current i0.

4 Selection of inductor L1 and operating frequency

When the upper bridge arm is turned on, ifa decreases; when the lower bridge arm is turned on, ifa increases:

Given uA, determine the average value of UC1 and UC2, the maximum and minimum operating frequency of the half-bridge converter depends on the value of inductance L1 and difference band 2δ. L1 and difference band 2δ are both inversely proportional to the frequency.

After comprehensively considering the design of the passive filter network and the frequency characteristics of the device, we can make the half-bridge converter work within a certain frequency band by selecting the values ​​of L1 and difference band 2δ.

5. Circuit Simulation

Use PSPICE software to simulate the above three-phase active filter.

(1) Phase voltage effective value: 110VAC

(2) Average voltage of UC1 and UC2: 180VDC

(3) Inductor L1: 3mH

The load current i1a and the A-phase current ia are obtained as shown in FIG4 .

As can be seen from Figure 4, except for the switching ripple, the A-phase current ia is a sine wave, which is in phase with the phase voltage. The high-frequency switching ripple can be easily filtered out by an external passive network.

6 Experimental results

Three-phase input voltage: 110VAC

DC bus voltage: 370VDC

Figure 4 Simulated A-phase load current and main circuit current

Inductor L1: 2.5mH

Compensation power: 2.5kVA

Working frequency band: 3k to 7k

LC passive filter network: L0=500μH, C0=30μF

The nonlinear load circuit is shown in FIG5 , its nonlinear current is shown in FIG6 , and the main circuit current after compensation is shown in FIG7 .

The THD of the A, B, and C phase currents measured by the HP35663A dynamic signal analyzer are 3.9%, 3.8%, and 3.9% respectively. The power factors of the A, B, and C phases measured by the power factor meter are 0.999, 0.997, and 0.998 respectively. The asymmetry of the three-phase current is 2.1%.

Figure 5 Nonlinear load used in the experiment

Figure 6 Non-linear load current

Figure 7 Main circuit current after compensation

7 Conclusion

In summary, the practical three-phase active filter has the functions of reactive power compensation, harmonic suppression and balanced incoming line current for the mains. Experiments have proved that its compensation effect is good, and it can automatically compensate according to the demand of the mains for reactive power and harmonics, and the compensated reactive power is not limited by the system voltage. It is simple to control and easy to implement, and has broad application prospects.