High Voltage Insulation Detection Method and Solid State Relay Solution

Significance of high voltage insulation testing

New energy vehicles, charging piles, photovoltaic energy storage, etc. are typical applications of DC high voltage. Under abnormal conditions, such as aging and damaged cables, water ingress in connectors, and structural damage, etc., may lead to reduced insulation and electrified housings. When the insulation between the positive pole and the negative pole of the high-voltage system is reduced, the high-voltage system will form a conductive circuit through the shell and the ground, causing heat accumulation at the contact point, and even causing a fire in severe cases. Therefore, real-time monitoring of the insulation performance of the high-voltage system is of great significance to high-voltage products and personal safety.

What is insulation resistance?

Under certain conditions, the resistance of an insulating material between two conductors. In electric vehicles, good insulation between wiring harnesses has an important impact on vehicle safety. The main index to measure the insulation performance of electric vehicles is the insulation resistance.

Relevant standard requirements for electric vehicles

Chinese standard:

GB/T 18384.1-2015

Electric Vehicle Safety Requirements Part 1: On-board Rechargeable Energy Storage System (REESS)

GB/T 18384.2-2015

Safety requirements for electric vehicles Part 2: Operational safety and failsafe

GB/T 18384.3-2015

Electric Vehicle Safety Requirements Part 3: Personnel Electric Shock Protection

GB/T 18384-2020

Safety requirements for electric vehicles (replaces GB/T 18384.1, GB/T 18384.2, GB/T 18384.3)

QC/T 897-2011

Foreign standards:

UN GTR NO.20 (Global Technical Regulation No. 20)

The human injury caused by electric shock is divided into electric injury and electric shock. Electric injury refers to the direct or indirect injury to the surface of the human body by electric current, in the form of burn (burn) injury, electric branding, skin metallization, etc. Electric shock refers to the injury to the internal organs of the human body (such as the heart, etc.) when the current passes through the human body. It is the most dangerous electric shock injury.

The human body is a "conductor". When it comes into contact with a live conductor, if a current of 40-50mA flows and lasts for 1s, it will cause electric shock damage to the human body. The human body resistance model is complex. When my country formulates the relevant standards and regulations for grounding design, the range of human body resistance is 1000-1500 Ohm. The AC peak value that the human body can withstand does not exceed 42.4V, and the DC voltage does not exceed 60V.

Electric shock is divided into direct electric shock and indirect electric shock. Direct electric shock refers to the electric shock caused by direct contact with the normal live conductor of electrical equipment. The basic insulation design of DC charging points prevents this. Indirect electric shock refers to the electric shock caused by the internal insulation fault of electrical equipment, and the exposed conductive parts such as metal shells that are not charged under normal conditions carry dangerous voltage. The DC charging pile is a Class I device, which can effectively prevent indirect electric contact on the AC side.

How to measure insulation resistance

Including direct method, comparative method, self-discharge method. The direct method is to directly measure the DC voltage U applied across the insulation resistance and the current I flowing through the insulation resistance, and calculate it according to R=U/I. According to the type of measuring instrument, it is divided into ohmmeter, galvanometer and high resistance meter. The comparison method refers to the comparison with the known standard resistance, and the bridge method and the current comparison method are commonly used. The bridge method is a commonly used method in DC charging piles. The self-discharge method is to let the leakage current through the insulation resistance charge the standard capacitor, and measure the charging time and the voltage and charge at both ends of the standard capacitor. The self-discharge method is similar to the signal injection method.

Balanced bridge detection method

As shown in the figure below, where Rp is the positive electrode-to-ground impedance, Rn is the negative electrode-to-ground impedance, R1 and R2 have the same resistance value as a large current-limiting resistor, and R2 and R3 have the same resistance value as a small voltage detection resistor.

When the system is normal, Rp and Rn are infinite, and the detection voltage V1 and V2 are equal. The anode voltage can be calculated by dividing the voltage between R1 and R2, and thus the total bus voltage Vdc_link can be calculated.

When the positive insulation fault occurs, the resistance value of Rp decreases, and Rp and (R1 R2) form a parallel resistance. At this time, the positive voltage divider decreases, that is, V1 is less than V2. According to Kirchhoff’s current law, V1 and V2 can be used at this time. The value of the insulation resistance Rp, the relationship is as follows.

The algorithm is the same when the negative insulation resistance fails.

It can be seen from the above that the balanced bridge method is suitable for the failure of a single pole. When the insulation resistance failure of the positive and negative poles occurs at the same time, there is no way to distinguish the insulation resistance value at this time, and it may occur that the insulation detection cannot be found in time. The phenomenon.

unbalanced bridge detection method

The unbalanced bridge method uses two internal grounding resistors with the same resistance value, and the electronic switches S1 and S2 are opened and closed differently to change the corresponding access resistance during detection, so as to calculate the positive and negative pole-to-ground impedance.

When the switches S1 and S2 are closed at the same time, the bus voltage Vdclink can be calculated as in the balanced bridge method.

When the switch S1 is closed and S2 is open, (R1 R2) is connected in parallel with Rp, and then connected in series with Rn to form a loop, according to Kirchhoff's current law.
When the switch S1 is opened and S2 is closed, (R3 R4) is connected in parallel with Rn, and then forms a series circuit with Rp, according to Kirchhoff's current law.

Therefore, the values ​​of the grounding insulation resistance Rp and Rn can be calculated through the opening and closing sequence of the above three switches. This method requires the measured data to be accurate after the bus voltage is stable. At the same time, the bus voltage will change to the ground when the switch is switched, which requires a certain time interval, so the detection speed is slightly slower. The unbalanced bridge method is commonly used in high-voltage detection. method, here is another insulation detection method.

Detection based on leakage current principle

This detection method shares a voltage sampling point, and the sampling point needs to be set separately for the bus voltage Vdclink, and the existing sampling signal of the system can be used.

Read Vdclink parameters through the system.

Close the switches S1 and S3, and open the switch S2. At this time, Rp is connected in parallel with (R1 R3 R4), and then connected in series with Rn to form a loop, according to Kirchhoff's current law.

Close the switches S2 and S3, and open the switch S1. At this time, RN is connected in parallel with (R2 R3 R4), and then connected in series with RP to form a loop, according to Kirchhoff's current law.

Therefore, the values ​​of grounding insulation resistance Rp and Rn can be calculated by adjusting the opening and closing sequence of the above three switches.

Insulation detection solid state relay SSR

As a semiconductor device, solid state relay SSR has the advantages of small size, no interference from magnetic field, low driving signal, no contact vibration, no mechanical aging, high reliability, etc. It is widely used in the security market, such as passive infrared detection, door lock, alarm Panels, door and window sensors, etc. And smart meter monitoring, including active power, reactive power, task switching, alarm output, execution drive, power consumption limit, etc. It is also suitable for high-voltage insulation detection, sampling and voltage balance as an electronic switch.

Part of the solid state relay product series, the working voltage is 400-800V, the primary side uses an optocoupler drive signal of 2-5mA, and the secondary side uses an anti-series MOSFET. Both AC and DC loads can be used, and the insulation withstand voltage is 3750-5000V to achieve a good one. Secondary test isolation.