Analysis of Harmonic Suppression by Series Active Power Filter

1 series APF system structure and working principle 1.1 series APF system structure The system is mainly composed of harmonic detection circuit, PWM control circuit, drive circuit and main circuit 4 parts, the first 3 parts together to form the APF control circuit. The main circuit adopts three-phase bridge voltage-type PWM inverter composed of IGBT. The inductors L. and Cr are used to filter the switching ripple generated by the PWM inverter. The characteristics of the circuit structure are as follows: the active power filter is used as a voltage source. The current transformer (CT) is connected in series between the power supply and the AC side of a three-phase diode bridge rectifier circuit with a large capacitance, and its output compensation voltage is equivalent to a controlled voltage source for canceling the harmonic voltage generated by the load. APF system structure, as shown.

1.2 Working principle of series APF system One-phase equivalent circuit of harmonic voltage compensation, as shown. Among them, Z is the internal impedance of the power supply; and the fundamental voltage and harmonic voltage of the power supply respectively; ULf and ULh are the fundamental voltage and harmonic voltage of the load respectively; if and ish are the fundamental wave current and harmonic current of the power supply respectively. . When the compensation device is working, the APF is equivalent to a controlled voltage source connected in series with the load and controlled by the harmonic current. It generates a harmonic voltage of K times the harmonic current. Therefore, the APF can wait for harmonics. The effect is a resistor with a value of K. For a fundamental wave, the resistance K is zero and according to the principle of superposition. From Equation (3), it can be known that when K*,ishQ is derived from (1) The above discussion shows that by controlling the APF, the value of K is sufficiently large, and the power supply current is and the power supply voltage do not include harmonic components, thereby achieving a control system for the control method of the star cad6mic 2 series APF system for harmonic compensation. The functional block diagram shows that the control system needs to instantaneously detect the harmonic currents ish in the power supply current and form the compensation voltage command uc so as to drive the six power switching devices in the main circuit, so that the PWM inverter can be generated in real time with ish The proportion of the harmonic voltage uc can only achieve the fundamental impedance of the active filter to zero, exhibit a certain impedance to the harmonics, and then achieve the purpose of real-time tracking and compensation for the changed harmonics. In addition, in order to maintain the PWM inverter, Work, control system should also be able to control Dc side capacitor voltage is kept constant.

2.1 Calculation of Compensating Voltage Instruction According to the working principle of the series APF system, in order to make zero impedance to the fundamental wave and a relatively large impedance to harmonics, the output voltage command of APF should be uC=Kish to three-phase circuit instantaneous Reactive power theory is based on the calculation of ipiq can be drawn three-phase circuit harmonic current ishasbisc is the power supply current instantaneous value, 匕 is the power supply a phase voltage, LPF is a second-order low-pass filter, used to obtain -p- The dc component-p-.PLL is a phase-locked loop circuit that generates standard sine and cosine signals that are synchronized with the supply voltage. That is, according to equations (5) and (6), ip and iq are calculated and filtered by the LPF. The fundamental component of the power supply current, isafisbfisf, is converted by the DC component- and will-and-inverses, ie, the compensation command voltage U* 2.2APF of the series APF control system 1 DC-side voltage control 1 Since the PWM inverter works, The energy loss will cause the DC side capacitor voltage to decrease, in order to ensure that the PWM inverter is positive. +YTO4-05CninicaaemicJourfTleOTonicrubli: Normal operation, Ud must be kept constant For this purpose, the deviation after comparing the reference value of Ud with the feedback value in the feedback loop of 'isb DC voltage is calculated and calculated by PI regulator and harmonic current detection. The subtraction of iP is such that the additional fundamental current components AisafAishf and Airf are added to the calculated harmonic current values ​​isahisbh and ish, so that the voltage command also contains additional fundamental components, and finally the PWM inverter is generated. At the same time as the required harmonic voltage, a certain fundamental wave voltage is generated to interact with the fundamental current of the power supply, and the energy flow in the inverter is controlled to maintain the constant Udc voltage. 2.3 PWM control method for APF system Using the timed instantaneous value comparison voltage tracking PWM control method to obtain the PWM signal, the output voltage of the APF system can track the change of the command voltage signal in real time, as shown.

When the APF system is working, the voltage command signal i and the compensation voltage m generated by the inverter of the series APF system are sent to a comparator for comparison, and the output of the comparator controls the on-off of the main circuit so as to achieve u ( The purpose of real-time tracking of the commanded voltage uC The PWM signal needs at least one clock signal cycle to change once in the timing comparison method.The frequency of the clock signal thus defines the highest frequency of the device operation, thus avoiding the device caused by the high switching frequency. The control method requires only one comparator and flip-flop per phase, and the hardware circuit is simple and feasible.

A harmonic source load was developed to compensate for a phase waveform in the three-phase line current before the series APE system was compensated. It can be seen that the waveform distortion of the three-phase current is serious. After the series APE system is put into operation, the phase The line current waveform is shown in b. It can be seen that the harmonics in the three-phase current waveform are almost completely compensated. That is, the current waveform is almost similar to a sine wave. The total harmonic current content of the power supply before and after the harmonic compensation is measured with the HP35663A dynamic signal analyzer. The THD was 69.97% and 3.

The experimental results show that the series APE system has a good harmonic compensation effect for the three-phase diode bridge rectifier circuit with large capacitance on the DC side. It should be noted that the compensation effect of the series APE system is related to the size of the K value according to (3) In the formula, the greater the K value is, the better the harmonic compensation effect is. The strategy is equivalent to proportional control, but when controlling the second-order system or more, if the K value is too large, the system will oscillate, so the value of K is limited f2). In principle, the harmonics on the power grid cannot be completely eliminated. 5 Conclusion The nature of the harmonic source load in the power grid is not the same. In order to better suppress the effect of the harmonic source of the power grid, different harmonic source loads should be selected. Different types of active power filters. For the harmonics generated by the voltage harmonic source load in the power grid, the three-phase series APE system based on the instantaneous reactive power theory is selected for compensation.

The experimental results of the prototype show that using the series APE system control method and related parameter design methods given in this paper can achieve the purpose of eliminating harmonics and power factor correction, and the parameters and control methods are easier to adjust, thus verifying the series Filtering Principles and Control Schemes of APE System: Correctness and Feasibility (Editor Zhang Qiujuan)

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