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I. Introduction
At present, the energy crisis and environmental pollution are increasing day by day. The development of high-efficiency, energy-saving and zero-emission clean electric vehicles has become an inevitable trend in the development of the domestic and international automobile industry. Compared to conventional fuel vehicles, electric vehicles contain high-voltage components, including motor controllers, power batteries, electric compressors, and DC/DC. These high-voltage components are related to insulation problems, and the working environment of electric vehicles is complicated. Vibration, temperature, humidity, and aging of components can all degrade the insulation performance of the whole vehicle. The positive and negative poles of the power battery form a current loop through the insulation layer and the chassis. When the insulation of the vehicle decreases, the leakage current of the whole vehicle increases. When the leakage current reaches a certain value, the safety of the passenger and the normal operation of the electrical system of the vehicle are endangered. . Therefore, real-time monitoring of the electrical insulation performance of the electric vehicle high-voltage system to the vehicle to ensure that the vehicle is operating in the insulated state is of great significance to ensure the personal safety of the passengers, the normal operation of the electrical equipment and the safe operation of the vehicle.
Second, the relevant standards and regulations
There are many relevant standards and regulations related to electric safety of electric vehicles in China and abroad. The summary is shown in Table 1. These regulations are similar to the safety requirements. The main protection measures are based on high-voltage power battery protection, and relevant protection clauses are derived. There are detailed introductions to the measurement methods of insulation resistance of electric vehicles and related specifications. The standard has a critical index for insulation performance evaluation - insulation strength (Ω/V), and most foreign standards use ≥500Ω/V. ≥100Ω/V. Among them, GB/T 18384.3-2015 electric vehicle safety requirements Part 3: Personnel electric shock protection 6.7 stipulates that at the maximum working voltage, the DC circuit insulation strength ≥100Ω/V; SAE J1766, 4.4.3, is specified under the system nominal voltage , DC circuit insulation strength ≥ 500Ω / V.
Table 1 Standard regulations
  standard name
  Standard
EN
1987-1, 1987-2, 1987-3
FMVSS/CMVSS
No.305
GB
18384.1, 18384.2, 18384.3
ISO
6469-1, 6469-2, 6469-3, 23273-3
SAE
J1766/2344
JIS
D5305-1, D5305-2, D5305-3
Relevant domestic standards for electrical safety of electric vehicle parts include GB/T 18488.1-2015, GB/T 24347-2009 and GB/T 31467.3. Among them, 5.2.7.3 of GB/T 18488.1-2015 specifies that the cold state and thermal insulation resistance of the drive motor controller are not less than 1MΩ (for example, the 540V voltage platform, the corresponding insulation strength is close to 2000Ω/V). 5.6 of GB/T 24347-2009 specifies DC/DC insulation strength ≥500Ω/V. 5.1.5 of GB/T 31467.3 specifies the insulation strength of the power battery pack ≥100Ω/V. From the above national standard analysis, the requirements for insulation strength of each component are quite different, and some even approach the insulation strength requirements of the whole vehicle. In fact, the vehicle high-voltage system contains multiple components, including motors, motor drives, power batteries, PDUs, electric compressors, DC/DC, brakes, and steering controllers. If each component manufacturer stipulates 100Ω/V according to the national standard for insulation strength, the total insulation resistance of each component of the vehicle will be less than 100Ω/V, and the insulation resistance of the vehicle will not meet the national standard insulation requirements.
In combination with the status quo of domestic parts and components, when setting the insulation strength index of the whole vehicle, the insulation strength of the whole vehicle should be higher than the requirements of the national standard, and the insulation strength of each part should be higher than one level of the whole vehicle. In this way, the total insulation strength of the vehicle will not exceed the standard to ensure the safety of the vehicle. This is also why foreign standards set the insulation strength of the vehicle to ≥500Ω/V, which is much higher than the domestic indicator.
Third, the principle of insulation monitoring
The working principle of insulation monitoring mainly includes current sensing method, symmetrical voltage measurement method, bridge resistance method, low frequency signal injection method and the like. Among them, the low frequency signal injection method is the most widely used, and the system topology is shown in Figure 1. A positive and negative symmetrical square wave signal is generated in the interior, and the measurement loop is formed by the insulation resistance RF between the connection terminal of the insulation resistance monitor and the DC high voltage system and the chassis, and the RF is calculated by collecting the voltage divider on the sampling resistor. size.

　　Figure 1 Low frequency signal injection method system topology
When the actual vehicle is in normal operation, the measured insulation resistance includes DC and AC components. The AC component is related to the distributed capacitance and stray inductance of the vehicle system. The equivalent impedance characteristics of the system composed of different components will vary, and will vary with test conditions and test environments. In the electric vehicle system, the electric frequency of the motor is different at different speeds, so the system capacitive reactance and inductive reactance will change with the test speed. The distributed capacitance of the motor winding to the motor housing also changes with the change of the rotational speed and the ambient temperature. Therefore, the impedance characteristics of the system itself change with the operation of the system, and the measured insulation resistance value becomes dynamic.
Fourth, the insulation strength area
Generally, the insulation strength area is divided into five, including a high safety area, a safety area, a general leakage warning area, a serious leakage warning area, and a high-hazard area; the insulation strength monitoring area includes a safety area,
There are three areas of general leakage alarm area and severe leakage alarm area. See Figure 2 for details.

　　Figure 2 Impedance monitoring area definition
The regions are defined as follows:
High-risk areas: Vehicle insulation is seriously damaged and endangers personal safety.
Severe warning zone: red area, there is insulation problem;
General warning area: yellow area, there is slight leakage in this area;
Safety zone: Green zone, there is no insulation problem.
High security area: excellent insulation.
The severe leakage threshold area and the general leakage threshold area are critical areas. In this area, software algorithms can be designed to design alarms according to national standards and vehicle specifications. Different manufacturers in the industry have different settings for the serious leakage threshold and the general leakage threshold in the monitoring area. For example: a well-known domestic electric car
The manufacturer sets the leakage thresholds in the monitoring area to 1MΩ and 5MΩ respectively; there are also component manufacturers whose values are
Set to 200 kΩ and 500 kΩ, respectively.
According to the above mentioned insulation resistance contains DC and AC components, now from these two aspects to 540V
(General voltage range is 420V-630V) The system voltage is taken as an example to evaluate and analyze the 1MΩ and 5MΩ leakage thresholds.
(1) DC component
The corresponding DC leakage currents are approximately 0.5 mA and 0.1 mA, respectively. Its index is much higher than the requirement in IEC/TR2 60497-1 that the human body does not have any sense of threshold DC current of 10 mA.
(2) Communication components
GB/T 18488.1-2015 5.2.8 The maximum voltage of the system is Umax>500V, and the system withstand voltage must meet at least (2Umax+1000)V(AC). The corresponding AC current leakage current is (2*630V+1000V)/1MΩ (, 2*630V+1000V)/5MΩ, which is 2.5mA and 0.5mA. Its index is also higher than the safety indicator of 2mA mA for the human body that does not have any sensation in IEC/TR2 60497-1.
In actual evaluation of the insulation resistance leakage threshold of the whole vehicle, the AC and DC components should be comprehensively considered to ensure the electrical insulation performance of the vehicle and meet the safety requirements of the electric vehicle. If the insulation resistance leakage threshold is set too low, the vehicle safety monitoring and warning margin will be reduced. Therefore, it is more reasonable to deal with the early warning of leakage (increasing the leakage threshold).
Five, insulation monitoring accuracy
The general insulation resistance monitor has a monitoring range of 0-30 MΩ and a wide monitoring range. It is difficult to ensure the accuracy of the full range of the insulation resistance monitor within such a wide monitoring range. The insulation resistance monitor is a monitoring device, not a precise measuring instrument. It only needs to ensure the accuracy of the insulation strength monitoring data of the monitoring area and meet the requirements of the measurement accuracy.
Actually, when designing insulation monitoring products, it is necessary to consider a wide range of ranges and how to be wide.
The monitoring accuracy of the product is guaranteed under the range range condition. Therefore, we need to choose between these two aspects. For the high-hazard and high-safety areas in Figure 2, the monitoring accuracy requirements for these two areas can be lowered. The focus should be on the monitoring accuracy of the area between the severe leakage threshold area and the general leakage threshold area in Figure 2.
Conclusion
Based on the international and domestic related electrical vehicle insulation impedance standards and regulations, this paper discusses and analyzes the insulation resistance monitoring problem of electric vehicles from the perspective of monitoring principle, monitoring area and monitoring accuracy, and draws the following conclusions, which are for OEMs or component manufacturers. Provide reference for insulation monitoring design.
(1) The insulation strength of the whole vehicle should be higher than the national standard of 100Ω/V, and it is recommended that it is not less than 500Ω/V;
(2) In order to ensure the electrical insulation performance of the whole vehicle, it is more reasonable to alarm the leakage in advance;
(3) Focus on ensuring the accuracy of the leakage threshold attachment area monitoring, and other areas can be accurately identified.