Hysteresis current control three-phase shunt active power filter
1 Introduction With the wide application of power electronic devices, the influence of harmonics and reactive currents generated by various types of non-linear loads on the power grid is increasingly serious. Compared with passive power filters, active power filters can perform dynamic harmonic and reactive power compensation for varying load currents. In various types of active power filters, shunt active power filters can effectively compensate for reactive currents and harmonic currents generated by harmonic current source non-linear loads. It has been extensively studied and has actual products available in parallel. The performance of active power filter depends to a large extent on the speed and accuracy of the acquisition of the current command signal of the active power filter and the control strategy of the inverter output current. So far, it has been applied to parallel active power filter currents. The command signal acquisition method is mainly based on the instantaneous reactive power theory method, the current sample and hold detection method, and the sine wave modulation detection method based on the orthogonality of the current signal spectrum. The first method requires many multipliers and the cost is high. , To achieve a complex circuit; The second method requires high accuracy of the circuit components, and the adjustment is difficult; The third method requires an analog multiplier, and the cost is also high. In addition, the shunt type active power filter can use nonlinearity-based hysteresis. The ring current controller has a simple structure and good robustness but the switching frequency of the common hysteresis current controller depends to a large extent Changes in the output current of the transformer and the voltage on the AC side, the hysteresis width is difficult to set up, and the protection of the inverter is difficult to achieve. Based on the above analysis, this paper proposes an instruction signal acquisition method and the maximum switching frequency using the specified harmonic elimination pulse width modulation technology. Restricted Hysteresis Current Controller Three-Phase Shunt Active Power Filter The following detailed analysis of the working principle and design method of the system 2 The working principle of the system 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 whose output side is connected with a filter inductor to filter the harmonic current caused by the switching action of the inverter power switching element. Three-phase AC supply voltage The three-phase power input current, three-phase active power filter output current, and three-phase load current are as shown.
The three-phase AC power supply voltage is a symmetrical sine wave, and the three-phase non-linear load current is symmetrical. The active filter inverter uses closed-loop output current control. The active filter can be regarded as the current source. The three-phase supply voltage is Us voltage amplitude. The three-phase nonlinear load current can be expressed as visible. The load current can be divided. It is 3 parts.
For the following analysis convenience, when N is 5, the bipolar designated harmonic elimination PWM waveform as shown in a uses the specified harmonic with amplitude 1 to eliminate the PWM signal with a modulation amplitude of /Lp. The waveform of the signal is the same as the shape of the specified harmonic eliminating PWM waveform, except that the signal amplitude becomes /Lp and their spectral distribution must be the same. The first harmonic with non-zero amplitude is at (2N+1) where the load current is assumed. The fundamental frequency of the signal is the same as the frequency of the specified harmonic wave to eliminate the fundamental wave of the PWM wave. The fundamental wave of the specified harmonic wave to eliminate the PWM wave is in phase with the phase a voltage, and the amplitude is 1a. The current iaL wave is shown in b. It can be expressed as (2) Divided into 3 parts. The specified harmonic elimination PWM waveform is iaLM for the load current modulated signal. The waveform shown in c is shown in c. iaLM can be expressed as if the guaranteed load current is not less than 2N times lower harmonics. The lowest frequency of the AC signal is 2 times the fundamental voltage of the power supply voltage, while the DC amount is only 3.2. The detection circuit design passes the above analysis. The specified harmonics are used to eliminate the PWM waveform to modulate and filter the load current of each phase, and the fundamental active current amplitude can be obtained. Value, mining Harmonic elimination PWM specified active current amplitude of the fundamental wave is modulated, filtered, the instantaneous value of each phase can be obtained active load current, so that the signal inverter output current can be obtained.
The required harmonic generation of the specified harmonic elimination PWM waveform circuit as shown in the harmonic generation of the elimination of the PWM waveform generated in the circuit diagram, the detection of the power supply a phase voltage, through the filter circuit to obtain the voltage fundamental component, through the zero comparison The sine wave is converted into a duty cycle of 5 (% square wave signal to do so mainly to eliminate the effect of power supply voltage distortion. After a phase-locked circuit 21 multiplied to obtain a high-frequency pulse signal, the rising edge of the square wave signal and a pulse signal The rising edge coincides with counting the pulse signal to obtain the EPROM M-bit address, and resets the counter according to the a-phase voltage cycle of the power supply. Reading out the table reads the pre-existing EPROM 3-path specified harmonics to eliminate the PWM waveform EPROM bytes as 8 bits, can store 8 different phases of pulse width modulation waveform at the same time.When designing, take the modulation coefficient as 1, the switching angle is 20, according to Table 1 to determine the phase of each pulse, can ensure the specified harmonics to eliminate the PWM waveform in theory The low-order harmonics below 41 times are not conducive to the design of the relevant low-pass filter circuit.The fundamental phase of each PWM signal depends on the phase of the fundamental wave of the power supply a, and the amplitude of the fundamental wave. The amplitude of the current of the supply voltage can be easily changed by changing the EPROM data.The fundamental phase of the PWM signal of each circuit is changed, and the structure and parameters of the circuit filter ArE are the same.The filters AB and C are designed to ensure that the load current before modulation is not more than 37 times. The higher harmonic currents of the 3-way harmonic cancellation PWM signal are modulated by the three-phase current of the load through the analog switch S1S2S, and the sum of the three analog switch outputs is filtered to obtain the peak value of the active current of the fundamental wave of the load.The DC signal is then switched by the analog switch. After S4S modulation, two bipolar designated harmonics are eliminated to generate the PWM signal. After the low-pass filter DE, the active phase current ab of the load is ab-phase compensated and inverted. A specific harmonic elimination PWM technique can be used to broaden the low pass. The amplitude of the passband signals of the filters A~E changes during the processing, and the phase of the fundamental wave of the PWM signal SW1SW2SW3 is balanced by the amplification of the filter. The phase shift of the filter ABC is also considered on the basis of the voltage of the power a phase. This circuit does not have analog multipliers and is low cost.
The above detection circuit can ensure that the load input current is three-phase symmetrical regardless of whether the load is symmetric or not. The detection circuit using the specified harmonic elimination PWM technique is controlled. 3.3 The DC-side voltage control method of the inverter The main circuit of the three-phase parallel active power filter The active power is consumed, resulting in an average value of the voltage on the DC side. The DC voltage is fluctuating. The closed loop of the DC side voltage adjusts the fundamental active current input or output of the shunt active power filter to ensure that the average value of the voltage on the DC side is constant. The stability of the DC voltage is directly related to the change of the fundamental active power and the value of the DC filter capacitor.
A parallel active power filter using a voltage source inverter is used to determine the frequency of the DC side voltage and ensure that the output current of the inverter is fully controllable. Voltage control using traditional PI regulator, as indicated by the dashed line DC voltage given value is 600V3.4 Inverter output current control method Parallel three-phase active power filter command current (including load harmonics reactive current And the active current consumed by the active filter itself is generated using a hysteresis current control method. Hysteresis current control is an effective nonlinear control method for the output current of the voltage source inverter to overcome the disadvantages of the common hysteresis control. A hysteresis current controller with a limited maximum switching frequency is used. One phase of the circuit structure as shown.
Inverter output current shunt active filter control circuit structure diagram The difference between the actual output current of the inverter and the command signal after the hysteresis comparator to form a square wave pulse signal is sampled at a certain frequency through the D flip-flop, then the three-phase PWM signal is obtained. . During the sampling period, the switching state of the inverter remains unchanged, limiting the maximum switching frequency of the inverter. The PWM controller has a simple structure and is insensitive to circuit parameters.
4 The system actually generates a PW/M signal waveform and its step spectrum Y-axis: Harmonic content; X-axis: Harmonic times The specified harmonic cancellation PWM waveform and its frequency spectrum generated by the above method are basically not included as shown. The reason why the harmonics of the 39th or less harmonics are slightly lower harmonics is mainly due to the fact that the frequency of the high-frequency pulse of the synchronization signal generation circuit is not infinite, and there is an error between the switching angle of the PWM waveform actually generated and the theoretical calculation value; in addition, the analog switch may not be Ideal Switch Therefore, the cut-off frequency of the low-pass filter A ~ E in the actual circuit is slightly lower than the theoretical calculation value as the current waveform of the load phase and its frequency spectrum. It is the power supply phase current waveform and frequency spectrum after the active filter is stabilized. After the shunt active filter is working, the harmonic content of the power phase current is greatly reduced, and its waveform is close to sinusoidal. The compensation effect of the shunt active filter is quite obvious: Y-axis: Harmonic content; X-axis: Harmonic times (4丫-axis: 1,15-eighth axis; Axis: /, 5 melons 8 (b) Y-axis: Harmonic content; X-axis: Harmonic times In this paper, the basic working principle of the proposed three-phase shunt active power filter detection circuit and control circuit design are thoroughly analyzed and researched. The specified harmonic cancellation PWM technique is used The detection circuit eliminates the need for analog multipliers 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 non-linear loads
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