1 Introduction With the wide application of power electronic devices, the harmonics and reactive current generated by various types of non-linear loads have an increasingly serious impact on the power grid. Compared to passive power filters, active power filters are capable of dynamic harmonic and reactive compensation for varying load currents. Among all kinds of active power filters, the shunt active power filter can effectively compensate the reactive current and harmonic current generated by the harmonic current source type nonlinear load. It has been widely studied and has practical products and parallel type. The performance of the active power filter depends to a large extent on the speed and accuracy of the active power filter current command signal acquisition and the control strategy of the inverter output current. So far, it is applied to the parallel active power filter current. The acquisition method of the command signal is mainly based on the method of instantaneous reactive power theory, the detection method using current sampling and holding and the sine wave modulation detection method based on the orthogonality of the current signal spectrum. The first method requires more multipliers and higher cost. The circuit is complicated; the second method has high precision requirements for circuit components and is difficult to adjust; the third method requires analog multipliers, and the cost is also high. In addition, the parallel active power filter can adopt nonlinear lag. The ring current controller is simple in structure and robust, but the switching frequency of the ordinary hysteresis current controller is largely dependent on The change of the output current and the AC side voltage of the transformer, the hysteresis width setting is difficult, and the protection of the inverter is difficult. Based on the above analysis, this paper proposes a command signal acquisition method and the highest switching frequency using the specified harmonic elimination pulse width modulation technique. Three-phase shunt active power filter with limited hysteresis current controller The following is the working principle and design method of the specific analysis system. 2 System working principle The basic structure of the three-phase shunt active power filter is shown in the figure. The active filter is connected in parallel with the load. The inverter adopts a three-phase bridge voltage source inverter, and the output side is connected with a filter inductor to filter out the harmonic current three-phase AC power supply voltage caused by the switching action of the inverter power switching element. The three-phase power incoming current, the three-phase active power filter output current, and the three-phase load current are as shown.
Let the three-phase AC power supply voltage be a symmetric sine wave, and the three-phase nonlinear load current is symmetrical. The inverter of the active filter adopts the closed loop control of the output current. The active filter can be regarded as the current source. The three-phase power supply voltage is the Us voltage amplitude and the three-phase nonlinear load current can be expressed as visible. The load current can be divided. For 3 parts.
For the following analysis, when the N is 5, the bipolar specified harmonic harmonic elimination PWM waveform is as shown in a, and the specified harmonic is used to cancel the PWM waveform to modulate the DC signal with the amplitude of /Lp. The waveform of the signal is the same as the shape of the specified harmonic cancellation PWM waveform, except that the amplitude of the signal becomes /Lp. Their spectral distribution must be the same. The first harmonic whose amplitude is not zero is located at (2N+1). The frequency of the fundamental signal of the signal is the same as the frequency of the fundamental wave of the specified harmonic elimination PWM waveform. The fundamental wave of the specified harmonic cancellation PWM waveform is in phase with the phase a voltage. The amplitude of the phase 1a is the waveform of the iaL, as shown by b. (2) Divided into 3 parts to specify the harmonic elimination PWM waveform. The signal after modulation of the load current is iaLM. The waveform of the iaLM shown in c can be expressed as if the load current does not contain 2N or less low harmonics. The lowest frequency of the AC signal is 2 times the fundamental frequency of the power supply voltage, and the DC output is only 3.2. The detection circuit design passes the above analysis. The specified harmonic elimination PWM waveform is used to modulate and filter the load current of each phase, and the fundamental active current amplitude can be obtained. Value
The required circuit for specifying the harmonic elimination PWM waveform is as shown in the circuit diagram of the generation of the harmonic elimination PWM waveform. The voltage of the power phase a is detected, and the voltage fundamental component is obtained by the filter circuit. The sine wave is converted into a duty cycle of 5 (% of the square wave signal is mainly to eliminate the influence of the power supply voltage distortion. After the frequency multiplication of the phase-locked circuit 21, the high-frequency pulse signal, the rising edge of the square wave signal and a certain pulse signal are obtained. The rising edge coincides with counting the pulse signal, obtains the M-bit address of the EPROM, and resets the counter according to the voltage phase of the power supply a phase. The byte of the EPROM of the three specified harmonics in the pre-existing EPROM is read by the look-up table. 8 bits, can store 8 pulse width modulation waveforms of different phases at the same time. When designing, take the modulation factor as 1, the number of switching angles is 20, determine the phase of each pulse according to Table 1, can guarantee the specified harmonic elimination PWM waveform in theory It does not contain lower harmonics of 41 times or less, which is beneficial to design related low-pass filter circuits. The fundamental phase of each PWM signal depends on the fundamental phase voltage phase of the power supply a phase, the fundamental amplitude and The amplitude of the current supply voltage can be changed independently by changing the EPROM data. The structure and parameters of the circuit filter ArE are the same. The filters AB and C are used to ensure that the load current before modulation does not exceed 37 times. The high-order harmonic current 3-channel specified harmonic elimination PWM signal is modulated by the analog switch S1S2S for the load three-phase current, and the three analog switch outputs are summed and filtered to obtain the peak value of the load fundamental active current. The DC signal is then passed through the analog switch. After S4S modulation, two bipolar designated harmonic elimination PWM signals are generated. After the low-pass filter DE is passed, the load ab phase fundamental active current ab phase compensation is obtained. The specific harmonic elimination PWM technology can be used to widen the low pass. The amplitude of the passband signal of the filters A~E changes during the processing, and the phase of the fundamental phase of the PWM signal SW1SW2SW3 is balanced by the amplification factor of the filter. The phase shift of the filter ABC is also considered on the basis of the voltage of the power phase a. This circuit has no analog multiplier and is low cost.
The above detection circuit can ensure that regardless of the load symmetry or not, the power input current is three-phase symmetrical. The detection circuit of the specified harmonic elimination PWM technology is used. 3.3 The control method of the DC side voltage of the inverter The main circuit of the three-phase parallel active power filter The active power is consumed, causing the average value of the DC side voltage to change. The DC voltage is fluctuating. The local active loop input or output of the parallel power active filter is locally closed by the DC side voltage to ensure that the average value of the DC side voltage is constant. The stability of the DC voltage is directly related to the fundamental active power variation and the value of the DC filter capacitor.
By using a parallel source active power filter with a voltage source inverter, a fixed-frequency pulse generator with a hysteresis comparator can ensure that the output current of the inverter is completely controllable by properly selecting and controlling the DC-side voltage. The voltage control adopts the traditional PI regulator. For example, the DC voltage given by the dotted line is given as 600V. The control current of the inverter output current is the command current of the parallel three-phase active power filter (including the load harmonic reactive current). And the active current consumed by the active filter itself occurs in hysteresis current control mode. Hysteresis current control is an effective nonlinear control method for the output current of the voltage source inverter. To overcome the shortcomings of ordinary hysteresis control, here Hysteresis current controllers with the highest switching frequency are used. The circuit structure of one of the phases is as shown.
Reverse output current parallel active filter control circuit structure diagram The difference between the actual output current of the inverter and its command signal is formed by the hysteresis comparator to form a square wave pulse signal, which is sampled by a D flip-flop at a certain frequency to obtain a three-phase PWM signal. . During the sampling period, the switching state of the inverter remains unchanged, limiting the maximum switching frequency of the inverter. This PWM controller is simple in structure and insensitive to circuit parameters.
4 The system actually generates a PW/M signal waveform and its step page spectrum Y-axis: harmonic content; X-axis: harmonic times. A specified harmonic elimination PWM waveform and its spectrum generated by the above method are basically not shown. The reason why the harmonics below 39 times have low harmonics is mainly because the high-frequency pulse frequency of the synchronous signal generating circuit is not infinite, and there is an error between the switching angle of the actual generated PWM waveform and the theoretical calculated value; in addition, the analog switch may not be Ideal switch, therefore, the cutoff frequency of the low-pass filter A~E in the actual circuit is slightly lower than the theoretical calculation value to the load phase current waveform and its spectrum, which is the power phase current waveform and its spectrum after the active filter is stable. After the parallel active filter is operated, the harmonic content of the phase current of the power supply is greatly reduced, and its waveform is close to sinusoidal. The compensation effect of the parallel active filter is very obvious Y-axis: harmonic content; X-axis: harmonic order (4 axis: 1,15 octave; axis: /, 5 melon 8 (b) Y-axis: Harmonic content; X-axis: harmonic order This paper deeply analyzes and studies the basic working principle of the proposed three-phase shunt active power filter. The design of the harmonic elimination PWM technology is used. The detection circuit eliminates the analog multiplier and the circuit cost is low. The hysteresis current controller with limited switching frequency is used to control the output current of the inverter. The control circuit has a simple structure and high reliability. The experimental results show that the system designed in this paper can effectively Real-time compensation of harmonics and reactive currents generated by nonlinear loads
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