Application of multiple series inverters in electric vehicles

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Application of multiple series inverters in electric vehicles [Copy link]

Overview

With people's increasing concern for the urban environment, the development of electric vehicles has gained a rare opportunity. In urban transportation, electric buses have become the priority development object due to their large load capacity and high comprehensive benefits. Most electric buses use three-phase AC motors. Due to the high power of the motor, the devices in the three-phase inverter need to withstand high voltage and high current stress. The high dv/dt makes the electromagnetic radiation serious and requires good heat dissipation. The high

-power inverter with multiple series structure reduces the voltage stress borne by a single device and reduces the requirements for the device; reduces the dv/dt value, reduces electromagnetic radiation, and greatly reduces the heating of the device; due to the increase in the type of output level, the control performance is better.

Figure 1 shows a three-phase inverter using multiple series connection. Each unit inverter is a single-phase full-bridge inverter with an independent DC power supply. In electric vehicles, it is provided by an independent battery. Therefore, the danger caused by multiple batteries in series is reduced, and the battery removal is convenient.

2 Multiple series structure

Assume that m unit inverters are connected in series. If the battery voltage on the DC side of each unit inverter is the same, the number of levels that can be combined is m; if the battery voltage of each unit inverter is different, the number of levels that can be combined will increase greatly. Assume that the inverter consists of two unit inverters with battery voltages of Va and Vb (Va>Vb), then the positive levels that can be output are Va+Vb, Va, Va-Vb, Vb. If Va=Vb, the number of positive levels is 2; if Va≠Vb, and Va-Vb≠Vb, the number of positive levels is 4. It can be seen that by properly selecting the voltage ratio, the types of output levels can be increased. The following analysis assumes that the battery voltages are equal.

3 Multiple series inverters

3.1 Complex expression of output vector

Compared with three-phase SPWM technology, the control methods of multiple series inverters include multi-harmonic PWM technology (SHPWM) [1], phase-shift PWM technology (PSPWM) [2], etc. Since electric vehicles have high requirements for the dynamic response of motors, electric vehicles using three-phase asynchronous motors generally use vector control methods and direct torque control methods. In vector control, since multiple series inverters can output multiple PWM levels, the number of switches can be greatly reduced during current tracking control, the harmonics of the output current can be reduced, and the tracking effect can be improved.

When space vector control is used, the inverter output vector can be expressed as follows:

Where: vaN, vbN, vcN are the voltages of the output terminals A, B, C relative to the neutral point N.

3.2 Several states of unit inverter

In the multiple series structure, each unit inverter has three states: forward conduction, reverse conduction and bypass. As shown in Figure 2, when S11 and S14 are turned on, the inverter is in "forward conduction" and outputs a forward voltage; when S12 and S13 are turned on, the inverter is in "reverse conduction" and outputs a reverse voltage; if all the upper bridge arms are turned on or all the lower bridge arms are turned on, the inverter is in the "bypass" state and does not output voltage.
Suppose the state function of the unit inverter is

Taking phase A as an example, assuming that phase A is composed of m unit inverters connected in series and the neutral point voltage is constant, the output voltage of phase A is

Where: Vi is the battery voltage of the inverter unit i.

For a three-phase n-series inverter bridge, the above analysis shows that each single phase can output 2n types of voltages, so the number of space voltage vectors that can be combined in three phases is 8n3. Considering that the output vector must maintain the stability of the neutral point voltage, the number of feasible space vector types in the static coordinate system is 3(2n)(2n-1)+1[3]. For a three-phase double-series inverter bridge, the number of space vectors that can be selected is 37.

3.3 Switch combination selection

The principle block diagram of the switch combination of multiple inverter bridges is shown in Figure 3.

Since multiple inverters have redundant switch states, that is, the same space vector can be realized through multiple switch combinations. This is determined by the characteristics of multiple inverters. Since the switch combination is no longer unique, in order to make the operating frequency of each switch device equal, after selecting the space vector, it is also necessary to perform switch frequency balancing control and select a suitable switch combination.

4 Neutral point offset

For the two-level three-phase inverter shown in Figure 4, taking the voltage of N′ as the reference voltage,
the voltage of the neutral point N is pulsating, and the pulsation amplitude is Ud/6. The waveform is shown in Figure 5 [3]. For multiple inverters, there are multiple output levels. Taking the double inverter as an example, assuming that the DC side voltage of each unit inverter is Ud, the output uUN′, uVN′, uWN′ are step waves, and the levels of the step waves are Ud and 2Ud respectively, as shown in Figure 6. Assume uN=Ud, by

It can be seen that during the operation of the double inverter bridge, the voltage of the neutral point N can be kept constant by properly selecting the output vector.

5 Balanced charging and discharging of batteries

Since the working conditions of electric vehicles change with different driving conditions, the voltage of the motor also fluctuates at any time. For multiple inverters, not all batteries participate in the supply of current. Under low modulation coefficients, only a few batteries contribute current. These batteries discharge faster than other batteries. In

order to balance the discharge of the batteries, some people have proposed to use the method of alternating conduction to balance the discharge of the batteries [4]. When this method is used in the double inverter, the switch waveform distribution is shown in Figure 7.

During battery charging and regenerative braking, the multiple inverter works as a rectifier. When the upper bridge arm or the lower bridge arm of each unit inverter is fully turned on, the battery pack of the inverter is bypassed. Assuming that n inverter bridges are connected in series and i inverters are bypassed, the output voltage is (n-i)Ud. Through the bypass method, the battery pack can be flexibly charged and the torque of regenerative braking can be controlled.

6 Conclusion

Multiple series inverters are suitable for high-power electric vehicle drive systems. The use of multiple series structures can reduce the dangers of multiple batteries in series, reduce the switching stress of devices and reduce electromagnetic radiation. However, the number of batteries required has doubled. The

multiple series structure has greatly increased the types of output voltage vectors, thereby enhancing the flexibility and accuracy of control; at the same time, it reduces the fluctuation of the neutral point voltage of the motor. In order to maintain the balance of power in each group of batteries, it is necessary to ensure that the discharge time of the batteries is consistent during operation. Through the bypass method, the battery pack can be charged flexibly, and the torque of regenerative braking can also be controlled.

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