What is the principle of high voltage leakage detection in hybrid electric vehicles?

Different from the traditional hybrid vehicle insulation detection model, most hybrid vehicles currently use the low-frequency pulse signal injection method to detect the high-voltage leakage diagnosis technology of the insulation resistance model in real time. The biggest feature of this method is that it is not affected by the DC power supply during measurement, and can be measured when running or stopped. In addition, because the insulation resistance calculation is completely dependent on the common mode potential point sampling value of the only point in the system, the structure of the entire system is relatively simple, the size of the insulation resistance detection hardware is relatively small, and it has good operability in practical applications. Figure 1 shows the schematic diagram of the RC low-frequency pulse signal injection method used by the Toyota hybrid THS-II system to detect the vehicle insulation resistance.

Figure 1 Schematic diagram of THS-II system for detecting insulation resistance

The dotted box in FIG1 is the overall model of the high voltage circuit of the hybrid system. r1 and r3 (22, 24) are the insulation resistances between the positive electrode of the HV battery and the vehicle chassis, r2 and r4 (23, 25) are the insulation resistances between the negative electrode of the HV battery and the vehicle chassis, and the HV battery (12) is the high voltage DC power supply of the hybrid vehicle. The main circuit of the insulation resistance detection system is below the dotted box. The insulation resistance detection system (50) is composed of a detection circuit and a signal processing circuit. The detection circuit includes a coupling capacitor (52) connected in series with the common mode potential reference point (30), a detection resistor (54) and a pulse oscillator generator (56), and the signal processing circuit includes a sampling signal Vx filter (58), an amplifier (60), a peak value detection circuit (62) and a controller (70). The controller (70) has an output in response to the peak value (VX) of the processed superimposed AC frequency signal, and compares it with the voltage of the common mode potential reference point Vx' (30) to calculate the corresponding insulation resistance value. The controller (70) is configured to include an insulation resistance drop detection module (72) for generating a pulse oscillation signal and determining whether the insulation resistance has dropped based on the value detected by the wave peak value detection circuit (62). A common mode voltage change request module (74) is used to balance the common mode potential change of the HVCPU (40) with the insulation resistance so as to accurately detect the insulation resistance value. An insulation resistance detection fault determination module (76) determines whether an insulation resistance detection part or a similar component has a fault by comparing the outputs before and after the common mode potential change.

Here Vp is a 5V square wave signal generated by the pulse oscillator (56), and the signal ground is common to the vehicle chassis. In the figure, /x represents the common mode potential (voltage) of the sampling point. From the knowledge of analog electronic circuits, it can be known that if a pulse signal is injected into the circuit, the DC current source with very small internal resistance in the circuit can be regarded as a short circuit, and the current source with very large internal resistance can be regarded as an open circuit. Generally speaking, the internal resistance of the HV battery is at the mΩ level. When the voltage rises, the internal resistance of the HV battery is still very small. According to this principle, when the pulse oscillator (56) sends a pulse signal, the DC voltage source can be regarded as a short circuit. The simplified circuit is shown in Figure 2.

Figure 2 Simplified circuit

Since the DC voltage source can be regarded as a short circuit, the positive insulation resistance r1, r3 and the negative insulation resistance r2, r4 form a total parallel resistance R, which is equivalent to the insulation resistance. According to the circuit series-parallel equivalent principle, R

During the detection process, the detection resistor Rd and the insulation resistor R form a series circuit. When a pulse voltage is applied to the detection circuit, the sampling voltage Vx is the divided voltage in the circuit. In the formula: Vx—sampling voltage; Vp—pulse oscillation signal voltage; R—insulation resistance value; Rd—detection resistance value; Cc—isolation capacitor value; f-pulse oscillation signal frequency. The relationship between the sampling voltage Vx and the insulation resistance R is shown in Figure 3.

Figure 3 Relationship between sampling voltage Vx and insulation resistance R

It can be seen that the greater the insulation resistance, the greater the sampling voltage Vx, and vice versa, the smaller the insulation resistance, the smaller the sampling voltage Vx. Toyota's hybrid THS-II system converts the Vx value into the HV control ECU data "ShortWaveHighestVal" after processing. This value is between 0 and 5V to indicate the insulation resistance. It can be viewed through the ECU data stream of the intelligent detector. FIG4 shows the characteristic diagram of "ShortWaveHighestVal". When the vehicle is first powered on, "ShortWaveHighestVa" will drop to about 1.5V. The vehicle should be placed in the READY-0N state for a period of time before checking the working condition of the leakage detection circuit. "ShortWaveHighestVal" may drop to about 0V when the drive control system is boosted. It is controlled by the common mode voltage change request module (74), so a fault judgment of reduced insulation resistance should be made when the boost is not performed.

Figure 4. Characteristics of ShortWaveHighestVal5