Cascaded phase-shifted SPWM converter and its application in active power filter

Since the concept of "flexible AC transmission system" proposed by the American Electric Power Research Institute, the application of power electronics technology in high-power applications has been widely studied in high-power applications such as active power filters, power and switching of power electronic devices. The frequency contradicts each other. Japanese scholar Aldra Nabae et al. proposed a midpoint clamp converter, and developed a diode-clamped multilevel converter. The author proposed a phase-shifted SPWM combined converter. Both technologies have been extensively studied, but there are certain sub-structures of cascading converters that are insufficient for reactive compensation. The N single-phase full-bridge modules are connected in series on the AC side to form a pair of bridge arms. The DC sides are independent of each other. The three bridge arms are connected by a star or a triangle to form a three-phase system. In terms of control, the 3-bed is controlled by a switching frequency. Different modules have different switching angles, which are used to eliminate low-order harmonics, and control DC voltage to adjust the output fundamental voltage. This method has poor dynamic response, inconsistent switching load, and it is difficult to output signals of a wide frequency band, which is not suitable for active filters. For the occasions with high regulation performance requirements, this paper proposes a cascaded phase-shifted SPWM converter, which introduces the advantages of the phase-shifted SPWM combined converter into the cascade-type converter, and does not require transformer cascade. This paper analyzes the modulation method of this converter and constructs a parallel active filter. The simulation proves that the cascaded phase-shifted SPWM converter is an effective means for high-power high-performance power electronic devices, and has broad application prospects. 1 Working principle The topology of the cascaded converter is as shown (constructed in one phase) The phase-shifted SPWM converter is essentially an organic combination of the modulation method of the phase-shifted SPWM technique and the topology of the cascaded converter. The modulation method is as shown.

The single-phase full bridge module j is as shown in (a). The intersection of the triangular wave Tr1(j) whose frequency is /, the period T. and the modulated wave of frequency /m is used as the switching point of the switches SW1 and SW4, and the gate signals of SW1 and SW4 are complementary (eg g1 of (b), G4); triangular wave Tr2(j) and Tr1(j) have the same amplitude and opposite phase, the intersection of Tr2(j) and modulated wave acts as the switching point of switch SW2, SW3, and the gate signal is g2, g3 N of (b) The single-phase full-bridge module uses the same modulated wave, and the phase of the triangular load (N) is sequentially different by T, /(2N). This is called “half-period phase shifting”, taking N=4 as an example, as in (a) Show. The output of each module is as shown in (b). The output of each module is the superposition of the PWM signals generated by the intersection of two triangular waves and the modulated wave, which is a three-logic signal. After superimposing, the output voltage can be seen. The output voltage of the N-module type converter with half-cycle phase shifting method is increased by N times, linearly amplified, and the equivalent switching frequency is increased by 2N times.

Cascaded converters enable high carrier frequency performance at lower device switching frequencies, and require 6N switching transistors for voltage-type phase-shifted SPWM combined converters that do not require a transformer cascaded N-converter unit. Generate N+ 1 level PWM wave; 3N cascading converter consisting of 3N single-phase full-bridge modules requires 12N switching tubes. If the half-cycle phase shift control proposed in this paper is used, 2N+1 level is generated. Output. From the output of the same level, the cascaded converter is the same as the switching tube required for the voltage-type phase-shifted SPWM combined converter. The following analysis of the difference between the maximum switching utilization of the single-phase full-bridge module The maximum voltage Vt and the maximum current It are: lomax is the effective value of the output active current of the AC side of the single-phase full-bridge module, and the insulation of the generator is checked before starting the power. Therefore, the probability of grounding occurring during the startup process is small. Therefore, the 3k protection can be exited before the generator is connected to the grid, and the auxiliary contact of the switch is automatically performed. At this time, the 3k protection block diagram is as shown in (b). 3.4 Other countermeasures and solutions and usage are implemented by software. The protection setting should consider the worst case of the 3rd harmonic under various operating conditions of the unit. The setting is slightly less than the neutral point voltage, so that the amount of action is less than the amount of braking.

After discussion, the 3rd harmonic stator grounding protection of Manwan Power Plant is recommended to use the exit switch auxiliary contact to automatically retract 2 sets of 3k protection, and should have the function of monitoring switch auxiliary contact status. Now 5 units of Manwan Power Plant have been adopted. The exit switch auxiliary contact automatically retracts 2 sets of 3k protection, and has the function of monitoring switch auxiliary contact status. From the operation point of view, the 3k and 3rd machine 3k protection misoperation problems have basically been solved, and we intend to protect the tuning method and Further research on the protection program to find a reliable solution Tang Qingdi, male, the current deputy squad leader of the protection class, engaged in relay protection work.

Tan Jianhua, male, specializing in protection testing, engaged in relay protection work.

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