Selection of residual current device in EV charging pile / module

June 17 , 2020

With the explosive growth of the number of new energy vehicles in the past two years, the construction scale of its supporting facilities and charging piles has also expanded.

The growth of the new energy vehicle market is inseparable from the construction of basic charging facilities. How to ensure the safety of electricity used in the charging process, especially to prevent the leakage current from harming life and property, is a problem worthy of attention.

As a leakage protector, the residual current protector is widely used in low-voltage power distribution systems to prevent electric shock accidents, electrical equipment leakage damage and electrical fires. Also in the field of electric vehicle charging, RCD is also widely used as a basic electrical protection device.

Our zero phase current transformers Type A / Type B can be used for EV charing module.

There are four modes of electric vehicle charging, which are clearly stated in GB/T 18487.1-2015 "Electric Vehicle Conductive Charging System Part 1: General Requirements".

Mode one, the electric vehicle is connected to the AC grid using a charging connection cable. The residual current protection mainly depends on the residual current protection device (RCD) in the building distribution box. Since it is not guaranteed that all existing building devices are equipped with RCD, this kind of The method is very dangerous and has been banned

Mode two, the on-cable control and protection device (IC-CPD) is installed on the charging connection cable, and the IC-CPD has a residual current detection and protection function

Mode three,use special power supply equipment to directly connect the electric vehicle to the AC power grid, and install a control guide device on the special power supply equipment. The special power supply equipment is the AC charging pile

Mode four, when the electric vehicle is connected to the AC grid or the DC grid, a DC power supply device with a control guidance function, that is, a DC charging pile is used.

Here, we mainly discuss the selection of the residual current protector in the charging piles of mode three and mode four.

According to GB/T 18487.1-2015, 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, it is the schematic diagram of the charging mode 3 control guide circuit, and the residual current protector is installed inside the power supply equipment.

What is type A or type B residual current protector? China's Residual Current Protection Device (RCD) Guiding Standard GB/Z 6829-2008 (IEC/TR 60755:2008, MOD) "General Requirements for Residual Current Action Protector" from the basic structure of the product, residual current type, trip Methods and other aspects have been divided.

According to the type of residual current, RCD can be divided into AC type, A type, and B type. AC type residual current protector: RCD that ensures tripping of residual sinusoidal AC current that is suddenly applied or slowly rising. Type A residual current protector: It includes AC type characteristics and superimposes 6mA of pulsating DC residual current and pulsating DC residual current to smooth the residual current to ensure tripping RCD.

Type B residual current protector: includes type A protection features, in addition, it can superimpose the sine AC residual current of 1000 Hz and below, the AC residual current superimposed smooth DC residual current, the pulsating DC residual current superimposed smooth residual current, two-phase or more The pulsating DC residual current and smooth DC residual current generated by the phase rectifier circuit ensure the tripped RCD.

At present, because the price of type B RCD is too expensive, most domestic AC charging piles are installed with type A residual current protectors. So can the type A residual current protector meet the leakage protection requirements of the charging pile? Let's analyze the type of residual current that may occur during charging.

During the charging process using the AC charging pile, the AC charging pile and the vehicle coupler are connected to the public power grid. If the insulation is damaged in the pile, power frequency AC leakage current may be generated.

In the electric vehicle part, the possible leakage current mainly comes from the leakage of the on-board charger. The general topology of the charger is mainly AC/DC and DC/DC.

The AC/DC part of the single-phase input AC power is first filtered by EMI, and then under the action of the Boost APFC circuit, the AC power of 85~265V is rectified into a stable output DC 400V voltage, and 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 an acceptable voltage for the battery. When the insulation between the circuit board and the device casing is damaged, pulsating DC residual current may be generated in the rectifying part, and DC residual current with a small ripple factor may be generated in the Boost APFC circuit.

The following picture illustrates the generation and harm of DC residual current in detail (for reference)

It can be seen that DC leakage may occur in the DC/DC part of the push-pull full-bridge converter. China's low-voltage distribution system generally uses TN power supply, the metal shell of the equipment is connected to the working neutral line, and the DC leakage will pass through the body and the PE line Feedback to the charging line will affect 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 following figure.

On the other hand, due to the existence of the TN system, this kind of fault will not form a large voltage on the body, which is less harmful 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 in the connection of PE lines and ground lines in many parts of the country, especially in rural areas. Existing A-type RCD can only be detected by pulsating DC leakage without interference from DC 6mA current, and cannot detect DC leakage and disconnect the protection.When DC leakage is greater than 6mA, the core will be pre-magnetized due to DC residual current , Making the trip value increase, resulting in A type RCD can not operate normally, so you must use B type RCD for protection!

Similarly, in the DC charging pile, the off-board charger converts the commercial power into high-precision DC power to charge the battery. The DC charging pile leakage protection is divided into AC side and DC side. Theoretically, the AC side also needs to add B-type RCD for protection. The DC side needs to be installed with a DC ground insulation monitoring device to detect the DC positive and negative pole ground insulation detection.

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 the residual current protector in the charging pile is very necessary, and only safe use Only the electric environment can let everyone enjoy the convenience brought by new energy vehicles.

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

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