With the explosive growth in the number of new energy vehicles in the past two years, the scale of construction for their supporting facilities, namely charging piles, has also expanded. The growth of the new energy vehicle market is inseparable from the construction of basic charging infrastructure. Ensuring electrical safety during charging, especially preventing leakage currents from endangering lives and property, is a matter of concern.

Residual Current Operated Protective Devices, commonly known as RCDs, are widely used in low voltage distribution systems to prevent electric shock incidents, electrical equipment leakage damage, and electrical fires. Similarly, in the field of electric vehicle charging, RCDs are widely used as basic electrical protection devices.

There are four modes of electric vehicle charging, as clearly defined in GB/T 18487.1-2015 “Electric Vehicle Conductive Charging System Part 1: General Requirements.” Mode 1 involves using a charging connection cable to connect the electric vehicle to the AC grid, where residual current protection mainly relies on the residual current protective device (RCD) in the building distribution box. Since not all existing buildings are equipped with RCDs, this mode is very dangerous and has been prohibited. Mode 2 involves installing an In-Cable Control and Protection Device (IC-CPD) on the charging connection cable, which has an internal residual current detection protection function. Mode 3 uses dedicated power supply equipment to directly connect the electric vehicle to the AC grid, with a control guiding device installed on the dedicated power supply equipment, i.e., the AC charging pile. Mode 4 involves connecting the electric vehicle to the AC or DC grid using DC power supply equipment with control guiding functions, i.e., the DC charging pile. Here, we mainly discuss the selection of residual current protective devices in charging piles for Modes 3 and 4.

GB/T 18487.1-2015 requires that the residual current protective devices for AC power supply equipment should use Type A or Type B, in compliance with GB 14084.2-2008, GB 16916.1-2014, and GB 22794-2008. Figure 1 shows the control guiding circuit diagram for charging mode 3, with a residual current protective device installed inside the power supply equipment.

General Schematic For Level 3 Charging

Figure 1: Control guiding circuit diagram for charging mode 3.

What are Type A or Type B residual current protective devices? China’s guiding standard for Residual Current Protective Devices (RCD), GB/Z 6829-2008 (IEC/TR 60755:2008, MOD) “General requirements for residual current operated protective devices,” classifies RCDs based on basic structure, type of residual current, and tripping mechanism. RCDs can be divided into Type AC, A, and B. Type AC RCDs ensure tripping for suddenly applied or slowly rising sinusoidal AC residual currents. Type A RCDs include the characteristics of Type AC and ensure tripping for pulsating DC residual currents and pulsating DC residual currents superimposed with a 6mA smooth residual current. Type B RCDs include Type A protection features and additionally ensure tripping for sinusoidal AC residual currents up to 1000Hz, AC residual currents superimposed with smooth DC residual current, pulsating DC residual currents superimposed with smooth residual current, and pulsating DC residual currents generated by two-phase or multi-phase rectifier circuits, along with smooth DC residual current.

Currently, due to the high cost of Type B RCDs, most AC charging piles in China are installed with Type A residual current protective devices. The following figure shows the internal structure of an AC charging pile using a Type A residual current protective device.

Internal structure of an AC charging pile.

Figure 2: Internal structure of an AC charging pile.

Can Type A residual current protective devices meet the leakage protection requirements of charging piles? Let’s analyze the types of residual currents that may be generated during the charging process.

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Figure 3 shows the connection diagram between electric vehicle charging facilities, the public grid, and the electric vehicle during the use of AC charging piles. If there is insulation damage inside the pile, it may produce a sinusoidal AC leakage current. In the electric vehicle part, the main source of leakage current is from the onboard charger, which typically consists of AC/DC and DC/DC sections. The following figure shows the main circuit diagram of a common onboard charger.

 

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Figure 4: Main circuit diagram of an onboard charger.

In the AC/DC section, single-phase AC input is first passed through EMI filtering, then rectified into a stable DC output of 400V under the action of a Boost-type APFC circuit, providing DC input for the subsequent stage. The DC/DC section uses a phase-shifted full-bridge LLC main circuit to convert the 400V DC voltage into a voltage acceptable for the battery. If there is insulation damage between the circuit board and the equipment casing, pulsating DC residual currents may be generated in the rectification section, and small ripple DC residual currents may be generated in the Boost-type APFC circuit. The following Bender diagram illustrates the generation and hazards of DC residual currents.

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Figure 5: Generation of DC leakage in isolated chargers.

It can be seen that DC leakage may occur in the push-pull full-bridge converter of the DC/DC section. China’s low-voltage distribution system generally uses a TN supply system, where the equipment metal casing is connected to the neutral working line. DC leakage will feedback to the charging circuit through the vehicle body and PE line, affecting the entire system’s current waveform. Simulation of the equivalent circuit shows that the current waveform of the entire system will change, as shown in the following figure.

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Figure 6: DC fault system current waveform of the charger.

After the occurrence of DC leakage at the back end, it also affects the front-end circuit, distorting the pulsating DC waveform after rectification, generating spikes, and causing interference to the downstream circuit, affecting the charging effect and even the battery life. On the other hand, due to the presence of the TN system, this fault will not create a high voltage on the vehicle body, posing little danger to humans. However, if the connection to the system ground is missing or the PE line is disconnected, this voltage can harm humans. In fact, many areas in China, especially in rural areas, have problems with PE line connections. Existing Type A RCDs can only detect pulsating DC leakage without being affected by a direct current of 6mA, but cannot detect and disconnect protection for DC leakage. When DC leakage exceeds 6mA, the DC residual current causes pre-magnetization of the core, increasing the tripping value and preventing the Type A RCD from operating normally. Therefore, Type B RCDs must be used for protection!

Similarly, inside a DC charging pile, a non-vehicle charger converts mains electricity into high-precision DC for battery charging. DC charging pile leakage protection is divided into AC and DC sides. Theoretically, the AC side also needs to add Type B RCDs for protection, and the DC side needs to install a DC ground insulation monitoring device to detect the insulation status 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, it is essential to upgrade the residual current protective devices in charging piles. Only a safe electricity environment can let everyone enjoy the convenience brought by new energy vehicles with peace of mind.