What is the market prospect of charging piles? The most urgent task is to solve the safety problem first! Discussion on the selection of residual current protectors in charging piles!

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What is the market prospect of charging piles? The most urgent task is to solve the safety problem first! Discussion on the selection of residual current protectors in charging piles! [Copy link]

With the explosive growth of the number of new energy vehicles in the past two years, the construction scale of its supporting facilities, charging piles, has also expanded. In the seven years from 2010 to 2017, the number of charging piles in my country has increased from more than 1,000 to 210,000. The growth of the new energy vehicle market is inseparable from the construction of basic charging facilities. How to ensure the safety of electricity use during charging, especially to prevent leakage current from causing harm to life and property, is a problem worthy of attention. Residual Current Operated Protective Devices (RCD) is a leakage protector that is widely used in low-voltage distribution systems to prevent electric shock accidents, leakage damage to electrical equipment and electrical fires. Similarly, in the field of electric vehicle charging, RCD is also widely used as a basic electrical protection device. There are four modes for charging electric vehicles, which are clearly stated in GB/T 18487.1-2015 "Electric Vehicle Conductive Charging System Part 1: General Requirements". Mode 1 uses a charging connection cable to connect the electric vehicle to the AC power grid. The residual current protection mainly relies on the residual current protection device (RCD) in the building distribution box. Since it cannot be guaranteed that all existing building devices are equipped with RCD, this method is very dangerous and has been banned; Mode 2 installs a cable control protection device (IC-CPD) on the charging connection cable. The IC-CPD has a residual current detection protection function inside; Mode 3 uses a dedicated power supply device to directly connect the electric vehicle to the AC power grid, and a control and guidance device is installed on the dedicated power supply device. The dedicated power supply device is the AC charging pile; Mode 4 uses a DC power supply device with a control and guidance function when connecting the electric vehicle to the AC power grid or the DC power grid, that is, the DC charging pile. Here, we mainly discuss the selection of residual current protectors in mode 3 and mode 4 charging piles. It is required in GB/T18487.1-2015 that the residual current protector of AC power supply equipment should adopt type A or type B, which meets the relevant requirements of GB 14084.2-2008, GB 16916.1-2014 and GB 22794-2008. As shown in Figure 1, the schematic diagram of the control and guidance circuit of charging mode 3 is installed with a residual current protector inside the power supply equipment.

Figure 1 Schematic diagram of the control and guidance circuit of charging mode 3

What is a type A or type B residual current protector? my country's residual current protection device (RCD) guiding standard GB/Z 6829-2008 (IEC/TR 60755:2008, MOD) "General Requirements for Residual Current Operated Protectors" divides the product from the aspects of basic structure, residual current type, tripping mode, etc. According to the residual current type, RCD can be divided into AC type, A type, and B type. AC type residual current protector: RCD that ensures tripping for residual sinusoidal AC current that is suddenly applied or slowly rising. A type residual current protector: RCD that includes the characteristics of AC type and ensures tripping for pulsating DC residual current and pulsating DC residual current superimposed with 6mA smooth residual current. B type residual current protector: includes the protection characteristics of A type, in addition, it can also ensure tripping for sinusoidal AC residual current of 1000Hz and below, AC residual current superimposed with smooth DC residual current, pulsating DC residual current superimposed with smooth residual current, pulsating DC residual current generated by two-phase or multi-phase rectifier circuit, and smooth DC residual current.

Figure 2 Internal structure of AC charging pile

At present, due to the high price of type B RCD, most of the AC charging piles in China are equipped with type A residual current protectors. The figure below shows the internal structure of an AC charging pile, using a type A residual current protection device. Can the type A residual current protector meet the leakage protection requirements of the charging pile? Let's analyze the types of residual current that may be generated during the charging process.

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Figure 3 Schematic diagram of the connection between electric vehicle charging facilities, power grid and electric workshop

As shown in Figure 3, during the charging process using an AC charging pile, the AC charging pile and the vehicle coupler are connected to the public power grid. If the insulation in the pile is damaged, power frequency AC leakage current may be generated. In the electric vehicle part, the leakage current that may be generated mainly comes from the leakage of the on-board charger. The general topology of the charger is mainly AC/DC and DC/DC. The following figure shows the main circuit diagram of a common on-board charger.

Figure 4 Schematic diagram of the main circuit of a vehicle charger

The single-phase input AC power of the AC/DC part is first filtered by EMI, and then the 85~265V AC power is rectified into a stable output DC voltage of 400V under the action of the Boost APFC circuit, and a DC input is provided for the subsequent stage. The DC/DC part adopts a phase-shifted full-bridge LLC main circuit to convert the DC voltage of 400V into an acceptable voltage for the battery. When the insulation between the circuit board and the device casing is damaged, a pulsating DC residual current may be generated in the rectifier part, and a DC residual current with a very small ripple factor may be generated in the Boost APFC circuit. Here, Bender's figure is borrowed to explain in detail the generation and harm of DC residual current.

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The single-phase input AC power of the AC/DC part first passes through EMI filtering, and then the 85~265V AC power is rectified into a stable output DC voltage of 400V under the action of the Boost APFC circuit, and a DC input is provided for the subsequent stage. The DC/DC part uses a phase-shifted full-bridge LLC main circuit to convert the DC voltage of 400V into a voltage acceptable to the battery. When the insulation between the circuit board and the device casing is damaged, a pulsating DC residual current may be generated in the rectifier part, and a DC residual current with a very small ripple factor may be generated in the Boost APFC circuit. Here, Bender's figure is borrowed to explain in detail the generation and harm of DC residual current.

Figure 5 Generation of DC leakage in isolated charger

Figure 6 Current waveform of DC fault system of charger

It can be seen that DC leakage may occur in the push-pull full-bridge converter of the DC/DC part. my country's low-voltage power distribution system generally adopts TN power supply. The metal shell of the equipment is connected to the working neutral line. The DC leakage will be fed back to the charging line through the vehicle body and PE line, affecting the current waveform of the entire system. Through the simulation of the equivalent circuit, it is found that the current waveform of the entire system will change, as shown in the figure below. It can be seen that after the DC leakage occurs at the back end, it will also affect the front-stage circuit. The pulsating DC waveform after rectification will be distorted, generating spikes, which will interfere with the back-end circuit step by step, affecting the charging effect and even affecting the battery life. On the other hand, due to the existence of the TN system, this fault will not form a large voltage on the vehicle body, and will cause less harm to the human body. However, if the ground wire of the connection system is missing or the PE wire is disconnected, then this part of the voltage will harm the human body. In fact, there are problems with the connection of the PE wire and ground wire in many places in China, especially in rural areas. The existing type A RCD can only detect pulsating DC leakage without being disturbed by the DC 6mA current, but cannot detect DC leakage and disconnect protection. When the DC leakage is greater than 6mA, the DC residual current will cause the magnetic core to be pre-magnetized, increasing the trip value, causing the type A RCD to fail to operate normally, so it is necessary to use type B RCD for protection! Similarly, inside the DC charging pile, the mains power is converted into high-precision DC power to charge the battery through an off-board charger. The leakage protection of DC charging piles is divided into the AC side and the DC side. In theory, the AC side also needs to add a B-type RCD for protection, and the DC side needs to be equipped with a DC-to-ground insulation monitoring device to detect the insulation of the DC positive and negative poles to the ground. In the foreseeable future, as new energy vehicles enter thousands of households, charging piles will become an indispensable part of people's lives. Therefore, the replacement of residual current protectors in charging piles is very necessary. Only a safe electricity environment can allow everyone to enjoy the convenience brought by new energy vehicles with confidence. Magtron's SoC chip overall solution based on iFluxgate technology has digitally integrated type B leakage protection, and provides a cost-effective type B leakage solution for the technical upgrade of RCCB from traditional AC type/A type to type B, providing better protection for the power safety of charging equipment.