Plugged In: Explaining Electric & Hybrid Vehicle Tech | August 2020
Automotive High-Voltage and Isolation leakage
All technicians engaged in the automotive industry have become accustomed to the standard 12/24V automotive power distribution network. The negative connection of the battery is connected to the vehicle’s chassis and engine block. This method simply uses the body as a current return path to the battery which reduces weight, complexity, and cost, reducing the need for additional ground wiring.
In the case of HEV & BEV high-voltage systems, the chassis of the vehicle cannot be used as the negative connection due to the risk of electric shock. In the event of an unforeseen electrical fault, the system must be able to detect a leakage current and disable the power supply. High-voltage systems (voltages over 60V) have a number of benefits and are a necessary feature of all strong hybrid and EVs due to large power requirements. Since power is a function of voltage x amps, increasing voltage supply provides the ability to create a relatively large amount of power (watts) while keeping the current (amps) relatively low. This has the advantage of reducing wiring / conductor sizes and a reduction in I2R losses (voltage drop) which occur due to wiring/conductor length.
Automotive standard J1766 and US FMVSS 305 standards (electrolyte spillage and electric shock protection) dictate that at least 500Ω/V of isolation resistance must be maintained from high-voltage systems to chassis ground. The HEV/EV control system will be designed to disconnect high-voltage relays and discharge any DC-linked capacitors.
The design performance parameters for the isolation detection circuitry are quite specific and must comply with the following minimum standards:
Voltage measurement accuracy: 0.624%
Isolated leakage current accuracy: 0.621%
ISO_POS accuracy: 0.48%
ISO_NEG accuracy: 0.126%
From the standards / values above, it can be seen that a very small amount of leakage current can trigger a detected leakage event which will result in a disconnection of power supply and render the vehicle unusable.
The Figure 1 circuit diagram shows a typical leakage detection circuit in an inverting op-amp configuration.
Although it is not a concern, nor would it be desirable for the average technician to design such a circuit, it is an advantage to be aware of the function of the circuit and the effect that insulation damage or degradation will have on a vehicle.
Typically, a high-voltage isolation fault code P0AA6 will be set when an isolation break-down is detected. In this situation, the technician must be vigilant when investigating the cause of the fault as the leakage could occur in the vicinity of the battery pack and high voltage may be present despite standard depowering procedures.
Insulation leakages can occur due to a number of factors such as mechanical damage, vibration, excessive temperatures, corrosive vapours, moisture and humidity. Sometimes, a drop in insulation resistance can be sudden, such as when a vehicle is exposed to a flood event. However, it usually drops gradually. A component with poor insulation not only may be dangerous to touch when voltage is applied but may burn out.
Obviously, EV/HEV leakage currents can occur wherever high voltage is present. Since leakage can occur in such things as electric motor windings, invertor/convertor assemblies, high-voltage harnesses, battery packs and management systems, a technician needs the appropriate tooling and equipment to test insulation.
Testing insulation with a standard multimeter is not sufficient since the applied test voltage is in the vicinity of 4 volts. A much higher voltage is required to test insulation performance designed for high-voltage applications.
An insulation tester is usually a hand-held device similar to a multimeter and often includes many multimeter functions such as voltage, resistance etc. The insulation tester will usually allow the user to select several levels of test voltage up to around 1000V or more.
The measured resistance will be a function of the applied voltage and the resultant current. When the voltage is applied there will be three factors influencing the current flow:
1. Capacitance charging current
Current that starts out high and drops as the circuit charges to the applied voltage.
2. Absorption current
Also starts out high and then drops, similar to capacitance charging. However, the effect is due to materials such as insulators and capacitor dielectric material polarising in response to the applied charge.
3. Conduction / Leakage current
The conduction or leakage current is basically the remaining current measured once the capacitance and absorption currents have diminished. For this reason, leakage current tests are performed by leaving the tester connected for a period, usually around one minute. However, always refer to the equipment’s user guide.
Figure 2 demonstrates the characteristics of a leakage test performed over a 10-second period.
It can be seen that the capacitance current reduces quickly whereas absorption current component reduces at a slower rate. Once the capacitance and absorption currents have diminished, the leakage current is the major part of the remaining current which settles at around 10 micro-amps.
As stated earlier, automotive standards specify a minimum insulation resistance of 500Ω/V and with a simple Ohms law calculation the maximum allowable leakage current can be determined. In practice, most systems insulation will exceed the minimum requirement and with a bit of familiarisation using a resistance tester a fault can usually be identified quite easily.
The dangers of high-voltage systems cannot be over-stated as direct contact will result in serious injury. For that reason, strict design standards have been implemented and are adhered to by manufacturers. Similarly, technicians should take care to invest in quality correctly rated test equipment, PPE and training to ensure safe working practices are adhered to.
It is recommended that anyone engaging in HEV/BEV servicing and repair should have accessed appropriately accredited training and be familiar with the various systems encountered.
Until next time, work safe and take care.
12 August 2020