by E. Cano, K. Dartawan, R. Austria
One potential impact of interconnecting distributed generation (DG) is the potential sympathetic tripping of overcurrent (OC) protection devices, where a healthy feeder trips unnecessarily for a fault on another feeder. The sympathetic tripping comes from DG with high short-circuit current contribution (typically rotating machines such as Diesel or Gas Turbine units) and can be observed in radial feeders that are fed from a common source.

Also, this issue applies to DG on a lateral-backfeed from the DG to the adjacent lateral circuit.

 

 

 

 

 

 

For example, Figure 1 shows two radial circuits fed from one source. With no DG installed, if a short circuit occurs on Feeder 2, the short circuit will be completely fed from the utility source with no short circuit contribution from Feeder 1. As a result, relay or breaker OC-1 will not respond to the short circuit at feeder 2. The fault is cleared by relay or breaker OC-2 as intended.

In contrast, if a DG is installed on the healthy feeder (Feeder 1), the short circuit contribution from the healthy feeder will not be zero as shown in Figure 2. OC-1 relay operates on the fault current contribution of the interconnected DG. In this case, if the relay at Feeder 1 has faster characteristics than the relay at Feeder 2, as shown in Figure 3, the relay OC-1 may respond to the fault at Feeder 2 un-necessarily and interrupt the loads of the healthy feeder (Feeder 1) before relay OC-2 operated to clear the fault.

To summarize, the sympathetic tripping or bi-directionality issue could potentially occur when all the following conditions are met:

• Two or more radial circuits are fed from a common source
• DG with a relatively high short-circuit contribution is installed in a feeder
• OC relay at the feeder where the DG is installed has faster characteristic than the OC relays on parallel feeders

The following are options to mitigate the sympathetic tripping of OC protection devices due to the interconnection of DG:

1. Re-set the relay settings of OC protection devices of feeders fed from the same source to allow faster response of protection devices at the circuit without DG and slower operation of protection devices at the feeder with DG. However, review coordination of downstream protection devices before implementing.
2. Add a directional element to the upstream protection devices to eliminate tripping on current flow reversal.
3. Install a communication interlock to provide a signal from the faulted feeder that stops the circuit protector on the healthy feeder with DG from responding.

References:

[1] Protection Coordination Planning with Distributed Generation – on-line canmetenergy-canmetenergie.nrcan-rncan.gc.ca/…/2007-149e.pdf

[2] Electric Power Distribution Handbook (Electric Power Engineering Series), Tom Short, 2004, CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431