Catalog

The Residual Current Device (RCD) - How does it work?

I Introduction

As a leakage protector, Residual Current Operated Protective Devices (RCD) are widely used in low-voltage distribution systems to prevent electrical shock accidents, electrical equipment leakage damage and electrical fire. Also in the field of electric vehicle charging field, RCD is widely used as a basic electrical protection device. From 2012 to 2016, global new energy passenger vehicles increased from 116,000 to 751,000, maintaining an average annual growth rate of 75% in the past five years. China's new energy vehicle market has been growing at a high speed since 2014, with a growth rate of over 200% from 2014 to 2015. In 2016, China's new energy vehicle market maintained a growth rate of 63% despite the fraudulent subsidy and policy impact.

The growth of the new energy vehicle market cannot be separated from the construction of basic charging facilities. How to guarantee the electricity safety in the charging process, especially to prevent leakage of current from causing harm to life and property, is a problem that is needed to pay attention to.

II Four charging modes for electric vehicles

There are four charging modes for electric vehicles, as specified in GB/T 18487.1-2015 "electric vehicle conductive charging system part 1: general requirements": 

Mode 1: The residual current protection mainly relies on the residual current protection device (RCD) in the building distribution box. Since it cannot guarantee that all existing building devices are equipped with RCD, this method is very dangerous and has been banned. 

Mode 2: Cable control protection device (ic-cpd) is installed on the charging connection cable. The IC-CPD has the function of detecting residual current. 

Mode 3: Special power supply equipment is used to connect the electric vehicle directly to the AC power grid, and the control guidance device is installed on the special power supply equipment. The special power supply equipment is the AC charging pile.

Mode 4: When the electric vehicle is connected to the AC or DC power grid, the DC power supply equipment with control guidance function is used, that is, the DC charging pile.

Here, we mainly discuss the selection of residual current protector in mode 3 and mode 4 charging piles.

According to the requirements of GB/T 18487.1-2015, the residual current protector of AC power supply equipment should be type A or B, which meets the relevant requirements of GB 14084.2-2008, GB 16916.1-2014 and GB 22794-2008

What is type A or type B residual current protector? The standard GB/Z 6829-2008 (IEC/TR 60755:2008,MOD), the general requirements of the residual current action protector, is divided into the basic structure of the product, the type of residual current and the mode of tripping. 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 with sudden or slowly rising residual sine AC current. Type A residual current protector: RCD that contains AC type characteristics and superimposes 6mA smooth residual current to ensure tripping. B type residual current protector: contains A type of protection features, in addition, also for the remainder of 1000 Hz and the sinusoidal alternating current, AC superposition of residual current smooth DC residual current, pulse DC superposition of residual current smooth residual current, two phase or multiphase rectification circuit of pulsating DC residual current, smooth DC residual current ensure the RCD tripping. At present, due to the high price of type B RCD, most of the AC charging piles in China are installed with type A residual current protector.

The following figure shows the internal structure of the AC charging pile, using the type A residual current protection device. Can the type A residual current protector meet the leakage protection requirements of charging pile? Let's analyze the type of residual current that might be generated during charging.

 Internal Structure of AC Charging Pile

Can the type A residual current protector meet the leakage protection requirements of charging pile? Let's analyze the type of residual current that might be generated during charging. During charging with AC charging piles, AC charging piles and vehicle couplers are connected to the public power grid. If insulation failure occurs in the piles, power frequency AC leakage current may be generated. In the electric vehicle part, the possible leakage current is mainly from the leakage current of the on-board charging machine, and the general topology of the charging machine is mainly AC/DC and DC/DC.

The following figure shows the main circuit diagram of a common on-board charging machine.

Schematic Diagram of Main Circuit of an on-board Charger

The AC/DC single-phase input alternating current is firstly filtered by EMI, and then the AC current of 85~265V is rectified into a stable output DC 400V voltage under the action of Boost APFC circuit, and the DC input is provided for the back stage. The DC/DC section USES the phase-shift full-bridge LLC main circuit to convert the DC voltage of 400V into the acceptable voltage of the battery. When the insulation between the circuit board and the equipment enclosure is damaged, pulsating DC residual current may be generated in the rectified part, and DC residual current with small ripple coefficient may be generated in the Boost APFC circuit.

Here, the figure of Bender is borrowed to illustrate the generation and harm of DC residual current in detail.

Generation of DC Leakage in Isolated Charging Machine

It can be seen that DC leakage may occur in DC/DC partial push-pull full-bridge converter. China's low-voltage distribution system is generally powered by TN. The metal shell of the equipment is connected to the working zero line. Through the simulation of the equivalent circuit, it is found that the current waveform of the whole system will change, as shown in the figure below.

Current Waveform of DC Fault System of Charging Machine

It can be seen that after DC leakage occurs at the back end, the front circuit will also be affected. After the rectification, the ripple DC waveform will be distorted, resulting in sharp spines, which will interfere the back-end circuit step by step, affecting the charging effect and even the battery life. On the other hand, due to the existence of TN system, this fault will not form a large voltage in the car body, which is less harmful to human body. However, if the ground wire of the connection system is missing or PE line is disconnected, this part of voltage will hurt human body. In fact, many parts of China, especially in rural areas, have problems in the connection of PE lines.

Existing type A RCD can only in the detection of pulsating DC leakage from 6 ma DC current interference, and can't detect DCDleakage and disconnect protection, when DC leakage is more than 6 ma, due to the DC residual current will cause core magnetization in advance, make tripping value increases, lead to type A RCD can't normal movement, and therefore must use type B RCD protection!

III Conclusion

Similarly, in the DC charging pile, the municipal power is converted into high-precision direct current through non-on-board charging machine to charge the battery.

DC charging pile leakage protection is divided into AC side and DC side. In theory, type B RCD shall be added to the AC side for protection, and the DC side shall be equipped with DC insulation monitoring device to detect the CD positive and negative insulation detection. In the foreseeable future, with the introduction of new energy vehicles into thousands of households, charging piles will become an indispensable part of people's life. Therefore, it is necessary to upgrade the residual current protection device in charging piles. 

Magtron's overall SoC chip scheme based on iFluxgate technology has achieved digital integration for type B leakage protection, provided A cost-effective B leakage solution for RCCB to upgrade its technology from traditional AC /A type to B type, and provided better protection for electrical safety of charging equipment.

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