Abstract: With the rapid development of electric vehicles, the concept of Vehicle-to-Grid (V2G) has been frequently mentioned. Its core idea is to utilize the energy storage of a large number of electric vehicles as a buffer between the power grid and renewable energy sources. When the grid load is too high, the energy stored in electric vehicles feeds power back to the grid; conversely, when the grid load is too low, it stores excess power generated by the grid, preventing waste. In this way, electric vehicle users can buy electricity from the grid when prices are low and sell it back to the grid when prices are high to generate revenue. Furthermore, in the event of emergencies such as war or natural disasters, a large number of electric vehicles can serve as emergency power stations, which is of great significance. Experts have calculated that Beijing's maximum load in August 2016 was 20.77 million kilowatts. If electric vehicles had an output power of 7 kilowatts, 3 million electric vehicles could provide power to the entire city. This article focuses on the safety issues of non-isolated bidirectional chargers in V2G systems.
1. Bidirectional charger topology
One of the key technologies of V2G is the development of bidirectional high-power chargers. For OEMs, on-board chargers require small size, light weight, low cost, and high reliability. Currently, the mainstream charger topology consists of a three-phase uncontrolled rectifier and a high-frequency transformer-isolated DC/DC converter. This type of charger with an isolation transformer is large, has low conversion efficiency, and is expensive. Therefore, using non-isolated chargers is the current mainstream development direction. A new topology for a bidirectional high-power charger is shown in the figure below.
It consists of a front-end three-phase voltage-source PWM rectifier and a rear-end current-reversible chopper circuit. The rear-end current-reversible chopper DC/DC circuit can be understood as a composite circuit composed of a Boost circuit and a Buck circuit. This circuit can not only realize the forward flow of the current, but also realize the reverse flow of the current, thereby realizing the bidirectional flow of energy in the entire charger.
Leakage analysis of 2V2G system
By employing a non-isolated DC/DC topology, the high-frequency transformer is eliminated, improving conversion efficiency and reducing system cost and losses. However, we must consider the issue of leakage current in the entire system. As a complex power electronic device, leakage current is unavoidable in bidirectional high-power chargers. During the design phase, effective control strategies are needed to limit the leakage current to a certain range; otherwise, risks exist for the power grid, the device itself, and even life and property safety. Furthermore, basic protection measures are required to prevent the hazards of leakage current exceeding expectations.
The image above, taken from QC/T895-2011 "Conductive On-board Charger for Electric Vehicles," illustrates a general model of the connection between the power grid and the charger. Power is supplied to the on-board charger via a charging cable. The charger converts the incoming AC power into DC power to charge the battery. When feeding power back to the grid, the battery converts the DC power back into AC power via the on-board charger and feeds it back to the grid through the charging cable. A leakage current protector (RCD) is installed inside the power supply equipment (charging pile) to protect against leakage current during the energy exchange process between the power grid and the electric vehicle. The RCD is the fundamental protection mechanism, making its reliability crucial.
3. Hazards of DC leakage
As we all know, power supply systems include three-phase three-wire and three-phase four-wire systems, and the International Electrotechnical Commission (IEC) specifies them as TT, TN, and IT systems. my country primarily uses the TN system, and this is also the system used for connecting electric vehicles to the power grid. When using this type of bidirectional high-power charger, the limitation of the DC/DC isolation transformer is removed, and the battery gains freedom; it is no longer isolated from the system. Therefore, if an insulation fault occurs on the DC bus during long-term use, leakage current will be fed back to the AC side through the vehicle's grounding PE wire. Taking leakage current at the positive terminal of the battery's DC bus as an example, the leakage current model is shown in the figure below.
As can be seen, leakage current from the positive terminal of the battery's DC bus feeds back to the AC side, forming a loop. This unexpected DC current can affect the entire system. If we simulate the equivalent circuit, we'll find that the entire charging current is distorted, leading to reduced charging efficiency and even shortening battery life. More seriously, if the PE line is broken and the grounding wire is missing, this current could potentially pass through the human body, causing harm. If the DC current enters the power grid, the consequences are even more dire, damaging the entire power distribution network. Therefore, when DC leakage occurs, the circuit must be disconnected and the device inspected. The function of detecting leakage and disconnecting the circuit is naturally performed by the residual current device (RCD).
4. Leakage protection measures
According to GB/ T18487.1-2015 , the residual current device (RCD) in the charging pile should preferably be type B or type A. Type A RCDs ensure tripping for power frequency AC residual current, pulsating DC residual current, and pulsating DC residual current superimposed with a 6mA smoothed DC residual current. Type B RCDs include the characteristics of Type A, and in addition, they can also ensure tripping for sinusoidal AC residual current at 1000Hz and below, AC residual current superimposed with a smoothed DC residual current, pulsating DC residual current superimposed with a smoothed DC residual current, pulsating DC residual current generated by two-phase or multi-phase rectifier circuits, and smoothed DC residual current. It can be seen that only Type B RCDs can provide protection when DC leakage occurs.
However, due to technological and cost constraints, almost all residual current devices (RCDs) in domestic charging piles are currently Type A, which cannot protect against pure DC leakage. In reality, the leakage components in V2G systems are very complex, and both isolated and non-isolated chargers pose a risk of DC leakage. This article focuses on the hazards of DC leakage caused by the battery when using a non-isolated charger solution.
5. Summary
In realizing V2G for electric vehicles, we need to consider how to achieve integration and miniaturization, while also taking into account all components of the entire system. Observing the current situation and future development direction of the electric vehicle field, from the perspective of leakage current protection, we urgently need to upgrade the current Type A residual current circuit breaker (RCCB) to Type B. This is a responsible approach for the entire industry. Magtron's SoC chip solution based on iFluxgate technology provides digital integration for Type B leakage current protection, offering a cost-effective Type B leakage current solution for the technological upgrade of RCCB from traditional Type AC/Type A to Type B, providing reliable protection for the charging and discharging safety of electric vehicles.
References
[1] Hu Long, Luo An, Liu Yuehua, Xie Longyu, Zhuo Yanping, Research on bidirectional high-efficiency high-power-factor electric vehicle charger .
[2] Zhu Yulong, Considerations on the leakage current model of non-isolated on-board charger .
[3] GB/ T18487.1-2015 Electric Vehicle Conductive Charging System - Part 1: General Requirements .
[4] GB22794-2008 Type B residual current operated circuit breakers (Type B RCCB and Type B) for household and similar purposes with and without overcurrent protection
