The DC bus voltage stability control is one key technology to ensure that Active Power Filter (APF) operates stably. The external disturbances such as power grid and load fluctuation and the system parameters changing may affect the stability of APF DC bus voltage and the normal operation of APF. The mathematical model of DC bus voltage is established according to power balance principle and a DC bus voltage piecewise reaching law variable structure control algorithm is proposed to solve the above problem, and the design method is given. The simulation and experiment results proved that the proposed variable structure control algorithm can eliminate the chattering problem existing in traditional variable structure control effectively, is insensitive to system disturbance, and has good robustness and fast dynamic response speed and stable DC bus voltage with small fluctuation. The above advantages ensure the compensation effect of APF.

Recently, power electronic technology has been widely developed and applied. A lot of harmonic and reactive currents inject into the power grid as the power electronic devices and nonlinear loads used in industry. This can cause partial parallel resonance and series resonance in electric power system, power grid, and electrical equipment problems [

The APF power circuit usually consists of voltage source inverter and the drive circuit. It is very important to maintain the stability of DC side capacitor voltage for the whole control system. Low DC side capacitor voltage will reduce the compensation precision of APF. Conversely, high DC side capacitor voltage will make the interference harmonic current of APF increase. Therefore, controlling the DC side voltage of inverter and maintaining it stable have important significance for the APF harmonic current compensation effect [

However, when the device actually operate, the power consumption of switching devices, loading and unloading nonlinear load process, energy fluctuation in the DC side caused by AC side voltage fluctuation, and other factors will make the power grid and DC side capacitor exchange active power. This will lead to bus voltage fluctuation [

Generally, the normal APF DC bus voltage control methods are PI control and fuzzy control. Because of having strong robustness and less depending on the accurate model, the fuzzy control has attracted extensive application [

Variable structure control has the characteristics of low dependence on mathematical model of the controlled object, insensitive to parameter variations and external disturbances [

Figure

Structure diagram of APF converter main circuit.

In the converter, the 1st, 2nd, and 3rd bridge are used to produce compensation current

In [

Assume that the fundamental active current increment of phase A caused by DC bus voltage fluctuation is

Xie et al. [

For the conclusion (1), the on state loss of switching devices can be expressed by

For the conclusion (2), ignore the APF DC bus voltage fluctuation; the switching loss can be expressed as [

An exponential rate reaching law presented in [

A power reaching law presented in [

This is a variable exponential reaching law actually. The switching control part is big at the beginning and will make the system approach sliding mode surface quickly. But the amplitude of switching control is small when it is near the sliding mode surface. Thus the system could converge to sliding mode surface finally. And the system is stable at the origin. The switching band is sector. This reaching law not only could keep the basic requirements of crossing over switching surface step by step at quasi-sliding mode, but also could effectively inhibit or weaken the chattering.

A piecewise reaching law is proposed to design the variable structure controller for DC bus voltage based on the above two kinds of reaching law:

Taking

Select the error of DC bus voltage and its integration as state variables. Let

In order to verify the effectiveness of the proposed piecewise reaching law variable structure controller, we establish the simulation model in MATLAB/SIMULINK. The system parameters are as follows: phase voltage is 220 V (RMS), switching frequency is 10 KHz, DC bus voltage is 700 V, and filter inductor and capacitor are 5 mH and 4700

Figures

Dynamic response curve of DC bus voltage (nonload).

Waves of power grid voltage and current.

It can be seen from Figure

In order to verify the dynamic regulation performance of APF DC bus voltage variable structure controller in the event of external disturbance, the simulation research is carried out on the condition that the power grid voltage and load appear fluctuation.

The DC bus voltage and power grid current response curves under the condition that the power grid voltage dumps 10% and APF operates in stable state with load are given in Figure

Wave of DC bus voltage and power grid current (power grid voltage dumps 10%).

Figures

Dynamic response wave of DC bus voltage (load changes).

Waves of power grid voltage and current (load changes).

It can be seen from Figure

It can be seen from Figure

From the above simulation results and analysis, we can conclude that the dynamic and static performance of the proposed variable structure controller are excellent. The DC bus voltage is stable and thus ensures the current compensation performance of APF. The problem discussed in the third paragraph of Section

In order to verify the effect of the proposed control algorithm, the experiment operates on the existing APF platform. The system uses TMS320F28335 as the control core to execute the control algorithm. The power grid voltage, its frequency, and the filter inductor are 220 V (RMS), 50 Hz, and 5 mH, respectively. As the actual power grid voltage is bigger than 220 V normally, the DC bus voltage is set as 750 V, not as 700 V, in simulation. The load is the same as that of simulation. (Figures

DC bus voltage dynamic response curve.

APF compensation current (nonload).

Waves of DC bus voltage, load current, power grid current, and APF compensation current.

Phase A current spectrum before compensation.

Phase A current spectrum after compensation.

Waves of DC bus voltage and power grid current (load changes).

The DC bus voltage response curve after power on is given in Figure

Figure

The waves of DC side voltage, load current, power grid current, and the output harmonic current of APF under load operation are given in Figure

Figures

In order to verify the anti-interference performance of the improved reaching law variable structure controller at the event of load disturbance, the load is suddenly increased to 2 times of the original. Figure

Figure

Waves of power grid voltage and current after compensation (resistor-inductor load).

The APF DC bus voltage is very important for APF stable and reliable operation. The DC bus voltage mathematical model has been analyzed and established according to power balance. A DC bus voltage piecewise reaching law variable structure controller has been designed aimed at various disturbances. The simulation and experiment results have proved that the proposed control strategy has some good performances, such as maintaining the DC bus voltage stable, no chattering, insensitiveness to external disturbance, quick dynamic response, and small fluctuation. All of these ensured the steady amplitude, sine wave, little distortion, smooth transition process of the power grid current, and the compensation effect of APF.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This research has been supported by National Natural Science Foundation of China (61201410), Natural Science Fund for Colleges and Universities in Jiangsu Province (11KJB470002), and Huaian Science and Technology Plan Project (Social Development, HAS2011049). The authors are grateful to the reviewers for their valuable comments.