Design of voltage compensator based on PWM AC-AC conversion

Research shows that more than 90% of disturbances in distribution systems are caused by voltage reduction. Commonly used low-voltage compensation technologies, whether centralized compensation at substations, decentralized compensation for users, or compensation on poles, basically use energy storage devices such as grouped capacitors/inductors, which are relatively expensive.

This article introduces a new type of voltage compensator for important users in distribution systems, that is, a PWM AC-AC converter is installed in the user's autotransformer, and the AC-AC converter is driven by commutation technology. When a disturbance occurs and the voltage is reduced, the device can increase the voltage and keep the load end voltage at the rated value. Energy storage elements such as grouped capacitors/inductors are not used in the design, which has low cost and fast response speed.

1 Design scheme

Figure 1 shows the single-phase structure diagram of this design scheme. The voltage compensation is achieved by superimposing the voltage Vc, which is provided by the PWM AC-AC converter module. When the system is operating normally, the electronic switch of the PWM AC-AC converter acts as a bypass switch to provide a path for the voltage and directly add the voltage Vs to the load. At this time, the voltage Vc is 0. When the power supply voltage Vs is disturbed, the PWM chopper circuit is closed at a high frequency to generate an appropriate voltage Vc to be superimposed on the power supply voltage to maintain the load voltage constant. When the power supply voltage returns to normal, the chopper circuit returns to the bypass mode.

2 Theoretical Analysis

According to Figure 1, the following load voltage expression can be obtained:

Where: Vs is the power supply voltage; Vc is the provided compensation voltage.

Note that under normal working conditions, Uc is equal to 0, so Vload=Vs.

For control purposes, the required load voltage is represented by a constant value Vref. Under normal working conditions, Vs and Vload are both Vref.

When the power supply voltage decreases, Vs changes to the following value:

Where: n is the per-unit value of the voltage amplitude reduction. Figure 2 shows the phasor relationship between the three voltages Vload, Vs, and Vc. At the same time, the voltage Vc is a function of the voltage VL and the load rate of the chopper circuit, that is:

Where: Vs is the value of the power supply voltage converted to the primary side of the transformer; VL = VsN2/N1; D is the load factor of the converter; N2/N1 is the turns ratio of the transformer winding. Then equation (1) can be rewritten as:

From formula (4), when Vload=Vref, the D value can be obtained by the following formula:

Obviously, the maximum value of D is 1. Therefore, the maximum compensation degree that this design can provide is determined by the relative values ​​of the amplitude of the following voltage disturbance:

From formula (6), it can be seen that when the turns ratio N2/N1 is 1, the voltage compensation degree can reach 50%. This method can be adopted in practice because when the turns ratio increases to 2, the compensation degree increases by 16.66%.

3 Circuit Implementation

The voltage compensation control module is shown in Figure 3. According to the characteristics of the system control object, from the perspective of modularization and digitization, the digital control chip TMS320LF2407 is selected to design a PWM implementation method based on DSP.

The topology of the PWM AC-AC converter is shown in FIG4 . The converter output voltage Vc is given by equation (3). The converter shown in FIG4 is composed of four IGBTs (S1a, S1b, S2a, S2b). By operating the on/off mode of switches S1a, S1b, S2a, S2b, the converter output voltage Vc can be in phase with Vs when operating correctly. When the grid voltage is normal, switches S1a and S1b remain closed, and switches S2a and S2b remain open, so the converter output voltage Vc is 0, and the load voltage VL is equal to Vs. This operation mode is called bypass mode. At this time, the power supply is directly transmitted to the user, and the autotransformer is in an open circuit state (i.e., it only absorbs the excitation current). When the grid voltage decreases, the switches S1 (S1a, S1b) and S2 (S2a, S2b) of the converter operate according to the load cycle D shown in equation (5), and the load voltage VL at this time is equal to Vc+Vs.

The IGBT element is driven by a suitable gate signal. This control technology can effectively reduce the loss during switching and eliminate the need for a buffer circuit. The circuit shown in Figure 4 can make the traditional IGBT module widely used in converters. The design shown in Figure 1 (single phase) can be extended to a three-phase system (with or without a neutral point), as shown in Figure 5. Each phase can be controlled independently through each PWM converter module.

4 Conclusion

Under normal working conditions, the PWM converter works in bypass mode, the power of the power supply is directly transmitted to the load, and the autotransformer only absorbs the excitation current. When the voltage drops, the converter superimposes (compensates) the voltage Vc, and at the same time increases a certain output power through the autotransformer. Therefore, the choice of transformer in this design mainly depends on the ability of power change in transient processes. This device can be easily integrated into the distribution transformer that supplies power to important loads. The converter shown in Figure 4 is selected to verify the situation of the three-phase system under various voltage drops. When a single-phase voltage or three-phase voltage of a distribution network power supply drops by 30%, the load voltage can be maintained constant after compensation.