Active power filter gradually replaces passive filter in power filtering because of its excellent performance. Because the principle of active filter is to generate a compensation voltage (current) in real time, which is opposite to the harmonic and has the same amplitude, it requires that the compensation voltage (current) generated by the filter must have good tracking and accuracy. PWM (Pulse Width Modulation) control is the most commonly used and effective control method in active power filter control system. When the detection system detects harmonics, it instructs the arithmetic circuit to generate compensation signals according to the detected harmonics, the arithmetic circuit generates PWM signals through the tracking control circuit, and finally the driving circuit generates compensation voltage (current) according to the PWM signals. At present, the main PWM control methods are hysteresis comparison, triangle wave comparison, deadbeat control and one-cycle control. These methods have their own advantages and disadvantages. This paper will discuss several commonly used PWM control methods.
1 hysteresis comparison PWM control mode
Hysteresis comparison control method is to compare the command signal of compensation current (voltage) with the actual current (voltage) compensation signal of inverter, and the difference between them is input into a comparator with hysteresis characteristics, and the output of the comparator controls the switch, so that the output value of inverter can track the reference value of compensation current (voltage) in real time.
The command signal i*c of the compensation current is compared with the actual compensation current signal ic, and the deviation △ic between them is used as the input of the hysteresis comparator. H is used to indicate the ring width of hysteresis comparator. When ∣△ic∣H, the output of hysteresis comparator will flip, and the direction of compensation current ic will change accordingly, so that △ic will decrease, which ensures that the compensation current tracks the change of command current. This control method has simple hardware circuit, belongs to real-time control mode, and has the advantages of fast compensation response, small switching loss, no carrier wave and no harmonic component of specific frequency in inverter output. The disadvantage is that the switching frequency, response speed and current tracking accuracy of the system will be affected by the hysteresis bandwidth. When the bandwidth is fixed, the switching frequency will change with the change of compensation current, which will cause large pulsating current and switching noise.
2 triangle wave comparison PWM control mode
The tracking control method of triangle carrier comparison is the simplest control method.
In this way, the command signal U*c of the compensation voltage is compared with the actual compensation voltage signal Uc, and the deviation △Uc between them is compared with the triangular wave after passing through the amplifier A, and the obtained rectangular pulse is used as the control signal of each switching element of the converter, thus obtaining the required waveform at the output end of the converter. Amplifier A usually adopts proportional amplifier or proportional-integral amplifier. In order to minimize △Uc, a control system with this structure is designed. The biggest advantage of this modulation method is that the switching frequency is fixed, simple and easy, the response speed is fast, and it has good control characteristics for the system with high switching frequency. The disadvantage is that the output waveform contains high-frequency distortion components with the same frequency as the triangular carrier, and the switching loss is large, which is limited in high-power applications. (gentleman's thesis)