In accordance with the advantage of conventional hysteretic current control method, this paper advances a novel control method for active power filter (APF). In conventional hysteretic current control, the hysteretic band (HB) is fixed and actual compensating current is limited in a fixed hysteretic band. When the voltage source inverter (VSI) works in high-frequency state, as the switching frequency changes the problems such as increasing switching losses and audible noise will appear. In response to this problem, the constant frequency hysteretic current control method based on variable hysteretic band is advanced in this paper. Firstly, the connection between hysteretic band and switching frequency must be found correctly. Then, the variable hysteretic band current controller is designed according to the connection. Finally, the Matlab simulation results show that the switching frequency of VSI is held nearly constant and the proposed controller can track reference current well. The problems of increasing switching losses and audible noise which happened in high-frequency state can be resolved in conventional hysteretic current control.
Active power filter (APF) is a very useful tool for eliminating harmonic pollution in power systems. As compared with the conventional passive filters, APF has many significant advantages such as good controllability, fast response and high control accuracy etc. APF can also compensate non-characteristic harmonics, which makes it extremely attractive in certain circumstances. With the development of power electronic technology, APF finds its wide use in the modern industry.
Fig.1 is a simple system with a VSI-based APF. The three phase load current vector Li with its harmonic current to be compensated is measured and its harmonic component is denoted as reference current vector * c i .APF output current vector c i should be controlled to track the reference current vector. If the error * c c Δ= − ii i is larger than a certain tolerance, the current control of the APF will be activated. The current controller will determine the VSI switching operation to yield proper terminal voltage for reducing the errorΔi . The APF drive circuit will then realize the decision. It is clear that the current control block in Fig. 1 is significant to APF performance. It should respond quickly and determine optimal voltage space vector correctly in order to reduce the current error efficiently and in the meantime it is better to keep constant VSI switching frequency for safe operation.
There are various current control methods proposed for active power filter configurations, but in terms of quick current controllability and easy implementation the hysteretic band current control method has the highest rate among other current control methods . And the switching frequency varies constantly within a power frequency cycle. In principle the increasing inverter operation frequency helps to get a better compensating waveform. However, there are device limitations and increasing the switching frequency causes increasing switching losses, audible noise and other related problems. In this paper, the control of switching frequency is realized by introducing a constant frequency hysteretic current control algorithm. The main aim of this study is to investigate the effects of hysteretic bandwidth to the switching frequency of APF. The hysteretic band current controller changes the hysteretic bandwidth as a function of reference compensator current variation to optimize switching frequency. In this paper, the p q i i − theory for detecting current is first briefly reviewed. Secondly, the constant frequency hysteretic current controller is described. Thirdly, simulation results are presented followed by the conclusion.
Ⅲ. CONSTANT FREQUENCY HYSTERETIC CURRENT CONTROLLER
The hysteretic band current control method is popularly used because of its simplicity of implementation, among the various PWM techniques , . Besides fast-response current loop and inherent-peak current limiting capability, the technique does not need any information about system parameters. However, the current control with a fixed hysteretic band has the disadvantage that the switching frequency varies within a band because peak-to-peak current ripple is required to be controlled at all points of the fundamental frequency wave , .
Therefore, the constant frequency hysteretic current control method based on variable HB is advanced in this paper. The principle of this method is shown in Fig.3 and the control technology of HB is added to conventional hysteretic current controller.
Firstly, the switching state of VSI in hysteretic current control is analyzed. Conveniently, take the case of phase A. Fig.4 is the schematic diagram of the switching state of phase a controlled by hysteretic controller. In Fig.4, ca i is the feedback current and it is the actual compensating current and the namely output current of VSI.