Example: injection exclusion with multiple interconnected incoming feeders

The PLC can have the following characteristics:

• Number of digital inputs: is the number of circuit breakers for incoming feeder and for coupling. These inputs can be self-powered by the PLC or powered by an external power supply.

• Number of digital outputs: is the number of devices minus 1. These digital outputs can be electromechanical or solid-state outputs.

• Processing cycle is equal to 0.1 s or less.

Using a basic PLC to manage device exclusion allows to:

• Monitor continuously each part of ungrounded power system.

• Have a short response time to detect insulation faults.

• Be compatible with complex power systems with high number of incoming feeders and couplings.

There are two methods to determine the logic driving the injection inhibition input of each device:

Method 1: Give a priority to each device using indexes of incoming feeders. In this example:

• Priority of device no.1 is 1 (highest priority)

  This device is always injecting, its injection inhibition input is left unwired.

• Priority of device no.2 is 2

  This device is always injects into the system except when there is a closed path between itself and one device with a higher priority, that is, device no.1 in this case. The closed path occurs when D2, C1 and D1 are closed.

  Therefore, Injection inhibition of device no.2 = D2 AND C1 AND D1

  The representation in Ladder PLC programming language is as follows:

  

• Priority of device no.3 is 3

  This device is always injects into the system except when there is a closed path between itself and one device with a higher priority, that is, device no.2 and device no.1 in this case. The closed path occurs when:

  • (D3, C2, and D2) are closed, or

  • (D3, C2, C1, and D1) are closed.

  Therefore,

  • Injection inhibition of device no.3 = (D3 AND C2 AND D2) OR (D3 AND C2 AND C1 AND D1)

  • Injection inhibition of device no.3 = (D3 AND C2) AND (D2 OR (C1 AND D1))

  The representation in Ladder PLC programming language is as follows:

  

• Priority of device no.4 is 4(lowest priority)

  This device is always injects into the system except when there is a closed path between itself and one device with a higher priority, that is, device no.3, device no.2, and device no.1 in this case. The closed path occurs when:

  • (D4, C3, and D3) are closed, or

  • (D4, C3, C2, and D2) are closed, or

  • (D4, C3, C2, C1, and D1) are closed.

  Therefore,

  • Injection inhibition of device no.4 = (D4 AND C3 AND D3) OR (D4 AND C3 AND C2 AND D2) OR (D4 AND C3 AND C2 AND C1 AND D1)

  • Injection inhibition of device no.3 = (D4 AND C3) AND (D3 OR (C2 AND D2) OR (C2 AND C1 AND D1))

  The representation in Ladder PLC programming language is as follows:

  

Method 2: Use a truth table.