BMW 3: Monitoring functions

High-voltage interlock loop

The SME control unit evaluates the signal of the high-voltage interlock loop and checks whether there is an open circuit with this circuit. In the event of an open circuit, the battery management electronics control unit can cause the high-voltage system to perform a quick shutdown.

The principle of the high-voltage interlock loop is familiar from the "Basics of hybrid technology" information bulletin. In the G20 PHEV the high-voltage interlock loop is made up of the high-voltage components pictured below.

The electronics for controlling and generating the test signal for the high-voltage interlock loop is integrated in the battery management electronics (SME). Generating the test signal starts when the high-voltage system is to be started and ends when the high-voltage system has been shut down.

A square-wave AC signal with a frequency of approximately 88 Hz is generated as the test signal by the battery management electronics and fed into the test lead. The test lead has a ring topology (similar to that of the MOST bus). The signal of the test lead is evaluated at 2 points in the ring: in the Electrical Machine Electronics and finally right at the end of the ring, in the battery management electronics. If the signal is outside a fixed, defined range, a disconnection of the circuit or a short circuit to vehicle ground is identified in the test lead and the high-voltage system is switched off immediately.

If the high-voltage interlock loop at the high-voltage safety connector ("Service Disconnect") is disconnected, then the switch contactors are opened directly. In addition, all high-voltage components are switched off.

Isolation monitoring

The isolation monitoring determines whether the isolation resistance between active high-voltage components (e.g. high-voltage cables) and vehicle ground is above or below a required minimum value. If the isolation resistance falls below the minimum value, the danger exists that the vehicle parts will be energized with hazardous voltage. If a second, active high-voltage component is touched, there is danger of an electric shock. There is therefore fully automatic isolation monitoring for high-voltage systems.

The isolation monitoring is located in the safety box. The safety box sends the results to the battery management electronics control unit via local CAN2. The SME control unit evaluates the results of these measurements. The isolation monitoring is carried out by the safety box, while the high-voltage system is active, with a resistance measurement (indirect isolation monitoring). This monitoring is checked at regular intervals of approximately 5 seconds alternately between HV+ and HV- to the reference potential of the body ground. However, it is equally important to identify isolation faults from the high-voltage cables in the vehicle to the body ground. The cable shield is therefore connected to the body ground via the isolation-monitored high-voltage components. This means that isolation faults in the entire high-voltage vehicle electrical system can be identified. The evaluation takes place in the high-voltage battery unit.

The isolation monitoring responds in 2 stages. When the isolation resistance drops below a first threshold value, there is still no direct danger to people. The high-voltage system therefore remains active; no Check Control message is output, but the fault status is naturally stored in the fault memory.

In this way the Service employee is alerted the next time the car is in the workshop and can then check the high-voltage system. When the isolation resistance drops below a second, lower threshold value, this is accompanied not only by a fault memory entry, but also by the appearance of a Check Control message prompting the driver to visit a workshop.

As there is no direct danger for the customer or Service employee by such an isolation fault, the high-voltage system remains active and the customer is able to continue the journey. Nevertheless, the high-voltage system should be checked by BMW Service as soon as possible. In order to identify the component in the high-voltage system which caused the isolation fault the fault must be narrowed down by the Service employee. However, the Service employee does not have to perform a fundamental measurement of the isolation resistance himself - this task is performed by the high- voltage system through the isolation monitoring. When an isolation fault is detected, the Service employee must run through a test plan in the diagnosis system to find the actual location of the isolation fault.

The proper electrical connection of high-voltage components to body ground is an important prerequisite for ensuring that the isolation monitoring functions properly. Accordingly, this electrical connection must be restored carefully if it has been interrupted during repair work.

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