Talking about the Leakage Problem in Electric Vehicle V2G System

GW MB10F 0.8A 1000V rectifier bridge

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

Photocoupler

With the rapid development of electric vehicles, the concept of V2G is constantly mentioned. The core idea is to use the energy storage of a large number of electric vehicles as a buffer for power grids and renewable energy. When the grid load is too high, the electric vehicles store energy to the grid. Feeding, and when the grid load is too low, it is used to store excess power generation in the grid to avoid waste. In this way, electric vehicle users can buy electricity from the grid when the electricity price is low, and sell electricity to the grid when the electricity price is high. At the same time, in the event of unexpected events such as wars and natural disasters, a large number of electric vehicles can also become emergency power stations, which is of great significance. Some experts have calculated that Beijing's maximum load in 2016 is 20.77 million kilowatts. If the output of electric vehicles is 7 kilowatts, 3 million electric vehicles can achieve power supply throughout the city.

One of the key technologies of V2G is the development of two-way high-power chargers. For the OEM, the on-board charger requires small size, light weight, low cost and good reliability. At present, the topology of the mainstream charger consists of a three-phase uncontrollable rectifier and a high-frequency transformer isolated DC/DC converter. A charger with an isolation transformer is bulky, has low conversion efficiency, and has high cost. Therefore, the use of a non-isolated charger is currently the mainstream development direction. A two-way high-power charger uses a new topology as shown below.

Figure 1 shows an efficient high power factor charger topology

It consists of a three-phase voltage-type PWM rectifier in the front stage and a current reversible chopper circuit in the latter stage. The current reversible chopper DC/DC circuit of the latter stage can be understood as a composite circuit composed of a Boost circuit and a Buck circuit, which not only realizes the forward flow of the circuit, but also realizes the reverse flow of the current, thereby realizing the whole Two-way flow of charger energy.

Due to the non-isolated DC/DC topology, the high-frequency transformer is removed, the conversion efficiency is improved, and the system cost and loss are reduced. However, one situation that we have to consider is the leakage problem of the entire system. As a complicated power electronic device, the two-way high-power charger is difficult to avoid. It is necessary to limit the leakage current to a certain range through a good control strategy at design time, otherwise it is for the power grid or the device itself or life and property. Safety is risky. At the same time, it is also necessary to use a basic protection method to prevent the leakage of electricity when the leakage exceeds expectations.

Figure 2 vehicle motor input control leading circuit

The above picture is taken in QC/T 895-2011 "Conductive Car Charger for Electric Vehicles", which reflects the general model of the connection between the power grid and the charger, and supplies power to the on-board charger of the electric vehicle through the charging cable. The connected alternating current is converted into direct current to charge the battery. When feeding the grid, the battery converts the direct current into alternating current through the onboard charger to feed back to the grid through the charging cable. The power supply equipment (charging pile) is internally installed with a leakage current protector to protect the energy exchange process of the entire power grid and the electric vehicle. The leakage current protector is also called a residual current protector (RCD). RCD is the basic protection, so its reliability is crucial.

We all know that the power supply system has three-phase three-wire system and three-phase four-wire system. The International Electrotechnical Commission (IEC) stipulates TT system, TN system and IT system. China basically adopts the TN system, and the connection between the electric vehicle and the power grid is also adopted. When using this two-way high-power charger, the limitation of the DC/DC isolation transformer is lost, and the battery is first freed, and it is no longer isolated from the system. Therefore, if the insulation of the DC bus occurs during long-term use of the battery, the leakage will be fed back to the AC side through the ground PE line. Taking the battery DC bus positive leakage as an example, the leakage model is shown in the figure below.

Figure 3: Battery short circuit leakage to ground model

It can be seen that the battery DC bus positive leakage feedback to the AC side constitutes a loop. This unexpected DC current will affect the whole system. If we simulate the equivalent circuit, we will find that the entire charging current is distorted, resulting in charging efficiency. Reduce or even reduce battery life. More seriously, if the PE wire is disconnected and the ground wire is missing, this part of the current may pass through the human body and cause harm to the human body. If DC current enters the grid, the consequences are even more unimaginable, which will cause harm to the entire distribution network. Therefore, when DC leakage occurs, the circuit must be disconnected, the device is inspected, and the function of detecting leakage and disconnecting the circuit is naturally performed by a residual current protector (RCD).

According to the requirements of GB/T 18487.1-2015, the residual current protector in the charging pile should be of type B or type A. The A-type RCD ensures the tripping of the residual current of the power frequency AC, the residual current of the pulsating DC, and the residual current of the pulsating DC residual current of 6 mA. The B-type RCD includes the characteristics of the A-type, in addition, the sinusoidal AC residual of 1000 Hz and below. Current, AC residual current superimposed smooth DC residual current, pulsating DC residual current superimposed smooth residual current, pulsating DC residual current generated by two-phase or multi-phase rectifier circuit, smooth DC residual current to ensure tripping. It can be found that only B-type RCD can be protected when DC leakage occurs.

However, due to the technology and cost, at present, almost all the leakage protectors in the piles in China are type A, which cannot protect pure DC leakage. In fact, the leakage component in the V2G system is very complicated. There is a risk of DC leakage in both the isolated and non-isolated chargers. This paper focuses on the use of the non-isolated charger solution. DC leakage hazard.

In the process of implementing electric vehicle V2G, we need to consider how to achieve integration and miniaturization, and also take into account the various components of the entire system. Observing the status quo and future development of the electric vehicle field, from the point of view of leakage protection, we urgently need to upgrade the current type A residual current protector to type B, which is responsible for the entire industry.

Magtron's overall SoC chip solution based on iFluxgate technology digitally integrates B-type leakage protection, providing a cost-effective B-type leakage solution for RCCB's upgrade from traditional AC/A to B-type technology. It provides reliable guarantee for the safety of electric vehicle charging and discharging.

references

[1] Hu Long, Luo An, Liu Yuehua, Xie Longyu, Zhuo Yanping, research on two-way high-efficiency high-power factor electric vehicle charger.

[2] Zhu Yulong, consideration of leakage model of non-isolated car charger.

[3] GB/T 18487.1-2015 "Electrical Vehicle Conductive Charging System Part 1: General Requirements".

[4] GB 22794-2008 Type B residual current operated circuit breakers (type B RCCB and type B RCBO) without and with overcurrent protection for household and similar purposes.