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With the rapid development of electric vehicles, the concept of V2G is constantly mentioned. Its core idea is to use the energy storage of a large number of electric vehicles as the buffer of the power grid and renewable energy charging pile. When the power grid load is too high, the electric vehicle is fed to the power grid by the electric vehicle, and when the load is too low, it is used to store the excess power of the power grid. Avoid waste. In this way, the users of electric cars can buy electricity from the power grid when they are low in price, and get profits from selling electricity to the grid when the electricity price is high. At the same time, in the face of emergencies such as war and natural disasters, a large number of electric vehicles can also become emergency power stations, which is of great significance. There are experts accounting that the maximum load of 20 million 770 thousand KW in Beijing in August 2016, if the output power of electric vehicles is 7 kilowatts, 3 million electric vehicles can achieve the whole city to ensure power supply.
One of the key technologies of V2G is the development of bidirectional high-power charger. For the whole vehicle plant, the vehicle charger requires small volume, light weight, low cost and good reliability. At present, the topology of the mainstream charger is composed of three phase uncontrollable rectifier and high frequency transformer isolated DC/DC converter. The charger with isolation transformer is large in volume, low in change efficiency and high in cost, so the non isolated charger is used. It is the main trend of current development. A bidirectional high power charger adopts a new topology, as shown in the following figure.
Fig. 1 a high efficiency high power factor charger topology
The charging pile is composed of a three phase voltage source PWM rectifier and a backward current reversible chopper circuit. The reversible current 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 circuit, but also realize the reverse flow of the current, thus realizing the bidirectional flow of the energy of the whole charger.
Because of the non isolated DC/DC topology, the high frequency transformer is removed, the conversion efficiency is improved, the cost and loss of the system are reduced, but one of the cases we have to consider is the whole system leakage problem. As a complex power electronic device, the problem of leakage is difficult to avoid. It is necessary to limit the leakage size in a certain range by a good control strategy in the design. Otherwise, there are risks to the power grid, the device itself, or the life and property. At the same time, it is also necessary to use a basic protection measure to prevent the leakage of electricity when leakage exceeds expectations.
Figure 2 guidance circuit for vehicle motor input control
The above is the rechargeable pile in the QC/T 895-2011 "electric vehicle conduction vehicle charger", which reflects the general model of the connection between the power grid and the charger, which is powered by the charging cable to the vehicle charger of the electric vehicle. The vehicle charger converts the alternating current into the battery to charge the battery. The DC power is converted to AC through the charger on the vehicle and fed back to the grid end through the charging cable. The internal installation leakage current protector of the power supply equipment (charging pile) has leakage protection on the energy exchange process of the whole power grid and electric vehicle, and the leakage current protector is also known as the residual current protector (RCD). RCD is the basic protection means, so its reliability is very important.
As we all know, the power supply system has three phase three wire system and three phase four wire system. The International Electrotechnical Commission (IEC) provides for TT system, TN system and IT system. The TN system is basically adopted in China, and the connection between electric vehicles and power grids is also adopted. When using this bidirectional high-power charger, the DC/DC isolation transformer is limited, and the battery is first free, and it is no longer isolated from the system. As a result, when the battery is in use for a long time, if there is an insulation fault in the DC bus, the leakage will be fed back to the AC side through the body grounding PE line. Taking the leakage of the positive pole of the DC bus of the battery as an example, the leakage model is shown in the following figure.
Figure 3 electric leakage model of positive pole to ground short circuit in battery
As you can see, the battery DC bus positive pole leakage feedback to the AC side circuit, this unexpected DC will affect the whole system, if we simulate the equivalent circuit, we will find the whole charging current distortion, lead to the reduction of charging efficiency, and even reduce the life of the battery. More serious is that if the PE line is broken and the ground wire is missing, this part of the current may cause harm to the human body through the human body. If the DC current goes into the grid, the consequences will be even more disastrous, which will do harm to the entire distribution network. Therefore, when the DC leakage occurs, the circuit must be disconnected and the device is checked. The function of detecting the leakage and breaking the circuit is naturally done by the residual current protector (RCD).
According to the requirements of GB/T 18487.1-2015, the residual current protector in the charging pile should adopt B or A type. The A type RCD ensures the release of the surplus current, the pulsating DC residual current and the pulsating DC residual current superimposed on the 6mA smooth DC residual current, and the B type RCD contains the characteristics of the A type. In addition, it can also stack the residual current and the residual current of the 1000Hz and the remaining current and the residual current of the AC residual current and the pulsating DC residual current. The flow superposition smoothing residual current, the pulsating DC residual current generated by two phase or multiphase rectifier circuit, smoothing the DC residual current to ensure tripping. It can be found that only B RCD can be protected when DC leakage occurs.
However, due to technology and cost, almost all the current leakage protectors in the country are A type, which can not protect pure DC leakage. In fact, the leakage components in the V2G system are very complex. There is a risk of DC leakage in both isolated and non isolated charger. This paper focuses on the damage of DC leakage caused by the battery when the non isolated charger is used.
In the process of realizing the V2G of electric vehicle, we need to consider how to realize integration and miniaturization, and at the same time, we should take into account the components of the whole system. In view of the current situation and future development direction of electric vehicle, from the view of leakage protection, we need to upgrade the current A type residual current protector to B type, which is a responsible practice for the whole industry.
Magtron based on the overall scheme of SoC chip based on iFluxgate technology, the B type leakage protection is integrated digitally. From the traditional AC type /A type to B type technology, a high performance cost performance B type leakage solution is provided to provide reliable protection for the charge and discharge safety of the electric vehicle.