Application of three-phase PWM rectifier in electric vehicle charger

1 Introduction

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The electric vehicle (ev) is driven by the motor [1], while the motor is powered by a rechargeable battery [2], and the electric vehicle has far more requirements for the operating characteristics of the battery than the conventional battery system. With the improvement of battery technology, due to the high voltage and high current in the electric vehicle battery system and the complicated charging algorithm, the charging of the battery becomes more and more complicated [3], which will cause very much damage to the existing power grid. Big interference. Therefore, a charger with high efficiency and low distortion is required [4].

Traditionally, chargers can be divided into two broad categories: linear power supplies and switching power supplies [5] [6] [7]. Linear power supplies have three main advantages: simple design, no electrical noise at the output and low cost. However, the low efficiency of the charging circuit of the linear power supply is a serious disadvantage for the charger. These problems can be solved by using a switching power supply, which is high in efficiency, small in size, and low in cost. Conventional switching power supply chargers use uncontrolled or semi-controlled devices such as thyristors for rectification. Although they can obtain a relatively smooth DC voltage, they also inject a large amount of reactive power and harmonic current into the grid, causing a large impact on the grid. Pollution [8]. With the development of power electronics technology, the three-phase voltage type pwm rectifier (vsr) can solve the power because it has the advantages of controllable power factor, grid-side current approaching sinusoidal, DC-side voltage stability, etc. Low factor, large harmonic current and other issues [9].

However, the switching components of the pwm rectifier consume energy when the voltage and current are not zero [10], and as the switching frequency increases, the loss on the switching device becomes larger and larger [11]. The use of resonant zero-voltage soft switching can solve these problems, and has many advantages: the soft switching of the power switch, the loss during the switching process will be small, which in turn will increase the efficiency of charging and increase the frequency of operation [12] ]. The volume and weight of the charger will also be reduced [13]. Another benefit is that the harmonic content of the voltage and current in the rectifier is reduced after using a resonant [type soft switch [14]. Therefore, when the resonant rectifier and the conventional rectifier operate at the same power level and switching frequency, the EMI problem caused by the resonant rectifier is much smaller [15]. Resonant-type rectification is used to improve the power level, charging efficiency, reliability, and other operating characteristics of the charging [16] machine [17].

Three-phase resonant inverters are widely used in the field of motor speed control [20]. This paper designs three-phase pwm rectifiers based on three-phase inverters. And according to the characteristics of the resonant rectifier, the control method is improved to achieve the lowest distortion (df) and the smallest total harmonic distortion (thd). Applying it to the electric vehicle charger can reduce the pressure of the power factor correction section of the charging station, and because of the soft switching technology, the charging efficiency is not reduced due to the addition of the controllable switching tube, which is the charging machine. Large-scale integration into the grid provides the necessary conditions.

2 Overall topology of the charger

Figure 1 depicts in principle the overall topology of the charger, which includes several main parts:

(1) emi filter: suppressing the influence of high-frequency interference in the AC power grid on the equipment, and shielding the interference caused by the electric vehicle charger to the AC power grid;

(2) Three-phase pwm rectifier: Three-phase pwm rectifier can improve the power factor on the charger, and can reduce the harmonic pollution to the power grid; with the improvement of the power factor, the power factor correction (pfc) pressure of the charging station will be obtained. reduce. Because of its power factor controllable function, it can be applied to the charger or the power factor correction (pfc) of the entire charging station, so it has broad application prospects. This article will mainly design him.

(3) Full-bridge inverter: Invert the DC voltage obtained by rectification into a high-frequency AC square wave, which is used to pass the high-frequency transformer and change the output voltage and current by adjusting the duty ratio;

(4) High-frequency transformer: transmits high-frequency AC power, and can isolate the load from the pre-stage circuit;

(5) Uncontrollable rectifier bridge: AC square wave rectification for high-frequency transformer transmission, used to charge the battery.

The main control in the main circuit is the three main parts of the three-phase pwm rectifier bridge and the full-bridge inverter. However, in terms of improving the power factor and charging efficiency, it is necessary to analyze the operation mechanism of the three-phase pwm rectifier, so The discussion below focuses on how to improve the circuit of the three-phase rectifier and improve the control to meet the requirements.

3 Three-phase pwm rectifier circuit structure and action analysis

Figure 2 shows the circuit structure of a three-phase pwm rectifier with a soft switch. The left half of the circuit diagram is a three-phase pwm rectifier bridge, the right half is a zero voltage switching circuit (zvs), and the buffer is connected in parallel on the switching device. capacitance.

Since the switching frequency of the rectifier is much higher than the grid frequency, the input current and output current of the rectifier can be considered constant during one switching cycle, so that the constant current sources is and il can be used to represent the input current and the output current. Therefore, FIG. 3 can be used as the equivalent circuit of FIG. 2. In FIG. 3, sreg, ds, and cr1 respectively represent the power switch, the freewheeling diode, and the snubber capacitor of the rectifier. Since one of the upper and lower bridge power switching devices of the three-phase rectifier bridge is always turned on, cr1=3cs. The soft switch section includes two switching devices sa1, sa2, two diodes d1, d2, a resonant inductor lr and resonant capacitors cr1, cr2. In the structure of the soft switch, cr1 is the main resonant capacitor, and cr2 is the auxiliary capacitor for inverting the current ilr of the resonant inductor lr. During the period when the main resonant capacitor vcr1 is 0, the power switch of the three-phase bridge operates, and zero voltage operation can be realized, which greatly reduces power consumption.

Through this soft switch structure, the rectifier bridge and the auxiliary switch can be completely placed under the condition of soft switching, and the filter capacitor (electrolytic capacitor) of the DC link can be omitted, the size of the charger can be reduced, and the charger can be extended. The life span plays a great role.

4 achieve unit power factor operation

Using the Kirchhoff voltage law on the AC side of the rectifier, the voltage relationship between the grid voltage, the rectifier bridge voltage drop and the inductor resistance voltage drop can be obtained:

(1) Since the resistance of the distributed resistance r is small, the vector diagram between the voltages can be obtained after ignoring the distributed resistance voltage drop as shown in Fig. 4(a).

Improve the power factor of the system and achieve unity power factor operation. The direction of the voltage and current on the AC side must be consistent (as shown in Figure 4(b)). You can control the magnitude and phase of the voltage drop across the three-phase rectifier bridge. Angle to adjust the direction of the current. Direct current control is used to adjust the voltage drop across the three-phase rectifier bridge. By feeding back the voltage on the DC side of the rectifier and feeding forward the current on the AC side, the magnitude and vector of the regulation can be realized, and finally the direction of the voltage and current on the AC side can be achieved. Consistent and achieve high power factor operation.

5 svpwm application in pwm rectifier

Svpwm is widely used in rectifiers because the maximum output voltage is 15% higher than the spwm modulation method, and the harmonic characteristics are much better than other modulation methods [18], while maintaining the lowest switching frequency [19]. ], but when applying svpwm to a rectifier with a soft switch, the voltage vector sequence during the sampling period needs to be changed.

(2) Among them, the instantaneous space vector is 8 space vectors in the dq coordinate system, as shown in Fig. 5(a), the size is 6, including 6 non-zero vectors v1~v6 and two zero vectors v0, v7 And divide the entire dq plane into six sector areas i~vi.

According to the literature [20], in the three-phase rectifier with soft switch, the best modulation method using svpwm mode is to use the vector action sequence described in Fig. 5(b), and the lowest distortion can be obtained by using this method ( Df) and minimum total harmonic distortion (thd). In the modulation method of Figure 5b, v0, v1, v2 represent a zero vector and two non-zero vectors, respectively. In the same sector, two non-zero vectors alternate as the first action vector in the action time t=2*δθ=2ωts, and the action time of the zero vector is added in the middle of the two non-zero vector action times.

Figure 6 is a control block diagram of a three-phase rectifier, which is divided into three parts: the leftmost one is the soft switching action time and the vector sequence action time control block, which is responsible for generating the resonance control time t1 and the action time t0 of the three voltage synthesis vectors, T1, t2; middle is the generator of soft switch and rectifier igbt gate signal, by receiving the time signal of the controller, generating the igbt gate signal that meets the requirement; the rightmost side is the controlled object three-phase rectifier bridge (VSSR) and soft Switch (zvs) circuit. By controlling the on and off of sa1 and sa2, the switching time of zero voltage is created for the vector action sequence of svpwm. At the same time, according to the improved order of svpwm vector action, the problem of charging efficiency degradation due to the increase of power tube can be greatly reduced.

6 simulation results

For experimental research and analysis, the three-phase pwm rectifier with soft switch was simulated in matlab/simulink. The parameters of the simulation are as follows: the three-phase voltage on the AC side is 380v, the switching frequency is 20khz, and the DC side voltage is set. The value is 450v, the circuit parameters are: cr1=6500μf, cr2=450μf, lr=20mh.

The simulation results are shown in Fig. 7 and Fig. 8: Fig. 7(a) shows the simulation waveform of the DC side voltage. It can be found that the DC side voltage vdc is basically stable at 450v, and the voltage fluctuation range is small, which meets the design requirements. Figure 7(b) shows the relationship between the AC voltage and current on the grid side. After the DC side voltage is stabilized, the voltage and current are always in phase, and the power factor is close to 1. The high power factor of the charger can be realized. The requirement of operation; Figure 8(a) shows the modulation ratio of the voltage. Similarly, his fluctuation range is very small. Figure 8(b) shows the magnitude of active and reactive current, and the reactive current can be seen. Stabilized around 0, the power factor of the rectifier can be close to 1.

7 Conclusion

In this paper, a high-power car charger is designed with switching power supply technology, and the three-phase pwm rectifier is designed in detail. The zero voltage soft switching (zvs) technology and space vector pulse width modulation (svpwm) technology are combined, and the modulation method of svpwm is improved according to the switching conditions of the soft switch, so that the lowest distortion (df) and minimum can be obtained. Total harmonic distortion (thd). Finally, the three-phase pwm rectifier is simulated. The simulation shows that a high power factor can be obtained during the charging process, and the AC side current is close to sinusoidal and the DC side voltage is stable. Since the charger can achieve a high power factor and a low harmonic content, the burden of the power factor correction link of the charging station can be reduced, and the designed three-phase rectifier can be used because of its controllable power factor. As the power factor correction link of the charging station, it provides the necessary conditions for the large-scale use of the charger.

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