Research On Novel Generalized Active Power Filter

GAPF (generalized active power filter) is a filter with indirect current control strategy. Through outputting positive-sequence fundamental voltages synchronized with the system it can achieve reactive power compensation, harmonic suppression, negative sequence filtering and voltage stabling effectively without the need of load current detection. Although GAPF has many adventages, it also has the following drawbacks and limitations:1. the spectral distribution of the output voltage is not concentrated;2. system side filter reactance is of high power consumption;3. the reactive power compensation precision and phase-locked precision are low;4. does not have active power adjustment ability. This paper makes a study of GAPF and puts forward a series of improving methods for modulation strategy, topology, control method and functionality extension aiming at the drawbacks and limitations of traditional GAPF.Traditional GAPF uses selective harmonic elimination technique (SHEPWM). The principle of this technique is to eliminate selective harmonics according to specific requirements which causes that the residual harmonics are large, scattered, and not easy to filter. In order to improve the spectral distribution of GAPF output voltage and the performance of GAPF, this paper proposes harmonic concentrated optimized PWM technique based on SHEPWM which makes the residual harmonics to concentrate in two adjacent frequencies so they can be filtered by single tuned filter. In order to guarantee the convergence of the iteration, a step-by-step initial value calculation method is proposed. Research results show that harmonic concentrated optimized PWM technique can effectively decrease the capacity and power consumption of system side filter reactance under the same switching frequency or reduce harmonic content of system current.GAPF was proposed using two-level inverter which has high harmonic content in the output voltage and low switching frequency and voltage rating. To suppress the harmonic pollution to system current caused by the output voltage, system side filter reactance needs to be large, leading to high power consumption which is the fatal defect. In order to overcome this defect, a GAPF using new three-level topology is proposed which has better security, realtime performance and higher equivalent switching frequency and can effectively reduce the harmonic content of the output voltage and the capacity and power consumption of system side filter reactance; In order to improve dynamic performance of the device and avoid load voltage fluctuation, a modulation index open-loop control method is proposed, and small signal model and PI parameter setting principles of the controlled object are deduced; To solve the balancing problem of DC side voltages, a quantitative calculation method of small vector duty cycle is proposed which can accurately regulate the voltage of neutral point.Traditional control method of GAPF is modulation index fixed single-loop phase angle control. This method is simple, but the compensation precision is low and over compensation or owe compensation phenomenon can easily appear. To overcome this drawback, this paper proposes two methods which can realize accurate compensation. The quantitative analysis of compensation precision is given; Small signal model and PI parameter setting principles of the controlled object are deduced; the advantages and disadvantages of these two methods are compared. Traditional GAPF is synchronized with system voltages through zero-crossing detection circuit. The dynamic performance of this method is bad and when system voltages are unbalanced or distorted it can not detect the zero-crossing points of the positive-sequence fundamental voltages and the nonlinearity of the system will be enhanced and the stability of the system will be reduced. To overcome these drawbacks, this paper proposes an improved PLL based on dq transformation which can improve dynamic performance and phase-locked precision significantly.Traditional GAPF is composed of DC side capacitor and voltage source inverter. In order to ensure the output voltage amplitude, in steady state it needs to absorb active power from the system to compensate its loss and maintain DC side voltage, so it does not have active power adjustment ability. In order to improve performance and extend functionality, this paper proposes a scheme of combining with battery energy storage system. The topology and operating principle of GAPF/BESS are discussed in detail; the mathematical model of GAPF/BESS in dq coordinates is established; the steady characteristics of GAPF/BESS and GAPF are deduced and compared; the corresponding control methods and modes for charging and discharging are proposed for two different access ways of the battery and the detailed flow chart is given.To verify the efectiveness of GAPF practically, a three-level experimental prototype is built; parameter selection method of passive components is given; methods for narrow pulse elimination and dead time compensation are proposed; according to the theoretical analysis and control methods in this paper, a series of experiments are conducted.