Differential relays are not explicitly modeled in EasyPower, but you can use the method described below to include differential functionality for arc flash hazard calculations. The arc flash feature automatically calculates trip times for device functions that are of the type "Differential."
- Examine the physical layout of the substation, the relays and the circuit breakers. Determine which buses are within the differential protection zone. All equipment between the various CTs attached to the differential relay can be considered as being within the differential protection zone. This procedure is only applicable for those devices and equipment that can be protected by the differential relay and the circuit breaker.
- Determine the relay trip time. Typically, differential relays trip very quickly (unless there is an intentional time delay setting). You can look up the response time (or operating time) in the datasheet for the relay. Some common differential relays response curves are also modeled in the EasyPower library. You can plot the TCC to obtain the trip time for the differential relay. Typical response times are in the range of 0.02-0.05s.
- Determine the breaker opening time. The Short Circuit tab of the HV Breaker Data dialog box displays the opening time in cycles for ANSI breakers or seconds for IEC breakers. For ANSI breakers, convert the opening time to seconds.
- Determine if there is an auxiliary relay between the differential relay and the circuit breaker. Note the auxiliary relay operating time.
- The arc duration is the sum of the relay trip time, the breaker opening time, and the auxiliary relay time.
- For those buses that are within the differential protection zone, enter the total arcing duration in the Bus Data dialog box. On the Arc Flash Hazard tab, set the TCC Times for this Bus to User-defined Times. In the Time(s) column, enter the total arc duration in seconds.
If you have modeled the differential function (87) in the Relay Data dialog box, ensure that the Device Function Type is Differential and not Phase OC. If they are accidentally set as Phase OC, then this relay trip time is used for downstream bus faults. This should be avoided.