HV cable insulation resistance test for hybrid vehicle

Figure 1 Toyota Prius, one of the most recognized hybrid cars on the road

Toyota Prius is among the first hybrid cars in mass production. The Toyota Prius is the world's bestselling hybrid car, with cumulative global sales of over 3 million units.  It was designed for fuel efficiency and ultra-low emissions. A hybrid electric vehicle (HEV) is a type of hybrid vehicle and electric vehicle which combines a conventional internal combustion engine (ICE) propulsion system with an electric propulsion system. The presence of the electric power train is intended to achieve either better fuel economy than a conventional vehicle or better performance.

The hybrid control system combines the best operating characteristics of the combustion engine and electric motor depending to the driving condition.  Prius adopts the sophisticated Toyota Hybrid System (THS/THS II*). The system essentially is an energy recovery mechanism which slows down a vehicle or object by converting its kinetic energy to supplement the power of fuel burning. This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted.  This system therefore helps to achieve superior fuel efficiency and reduction of CO₂ emission.

* The THS II developed under Toyota’s “Hybrid Synergy Drive” concept, the refinement of the original Toyota Hybrid System. 

The hybrid system consists of the following main components:

• Gasoline engine – engine runs to drive the wheels during normal driving and at acceleration
• Motor generators (MG) – generate electrical power and recharge HV battery
• Power Split Device – split the torque between the motor generators and engine.
• Inverter system – converts high DC voltage (HV battery) to AC (MG) and vice versa
• HV Batteries – supply electric power to Motor generator during start-off, low speed, acceleration and uphill driving.

The hybrid vehicle operates at high-voltage system up to a few hundred volts. The high voltage system includes the HV battery, inverter assembly and the motor generators. They are connected with the high voltage power cable in orange color. Any leakage in high voltage insulation system may lead to shorter HV battery life. It may be harmful to the human body if accidentally touched. To check the HV cable insulation integrity, the hybrid vehicle service technicians use insulation resistance tester to measure the insulation resistance between the power cable and body ground, and compare the test result with the manufacturer recommended limit. Sometimes, contamination or moisture may lead to low insulation resistance reading. 

Figure 2 Testing the insulation resistance value between the HV power cable and the body ground.  The Keysight U1461A insulation multimeter shows >260 GΩ, indicates the insulation is in good condition.