Protection and Relay Co-ordination Studies

Overcurrent phase and earth fault relay coordination is necessary to achieve proper fault identification and fault clearance sequence. These relays must be able to distinguish between the normal operating currents including short time overcurrents that may appear due to certain equipment normal operation (example:  Motor  starting  currents, Transformer  inrush currents) and sustained overcurrent due to fault conditions. During fault  conditions,  these  relays  must  operate  quickly  isolating  the  faulted  section  of  the network only and allowing for continued operation of the healthy circuits. In the event of failure of primary relays meant for isolating the fault within its primary zone of protection, backup relays must operate after providing for sufficient time discrimination for the operation of primary relays.  Hence, the operation of backup relays must be coordinated with those of the operation of the primary relays. The flexible settings of the relays (namely plug or tap setting, the time dial setting and possibly selection of suitable time-current operating characteristics), must be set to achieve the objectives stated in this section.

Once the relays are coordinated, the discrimination in the operation of primary and backup relays and their coordination with the maximum possible load currents will be plotted on the time current characteristics (TCC’s).  The results of the relay coordination will be provided in the form of recommended settings and TCC’s. Relay coordination needs to be evaluated for maximum and minimum fault conditions and for various possible network configurations. Where a network has several levels of primary and backup relay levels, the source end relay operation can become quite delayed due to successive time discrimination at down stream load end coordination levels. In such cases it may be necessary to ensure isolation of fault at the earliest by possibly coordinating the source end relays with much faster dedicated equipment relays in the down stream (example differential protection of transformers).

Relay coordination may need to provide suitable instantaneous settings where it is possible that instantaneous settings can positively discriminate between the faults in the primary and subsequent zones. This is essential to ensure that the instantaneous settings will not act for faults outside its primary protective zone bypassing the needed discrimination between the primary relay to which it is a backup.

The following protection aspects will be covered in the study

• Overcurrent Phase (51, 50) and Earth (51N, 50N) Fault Protection – Relay coordination with High set settings, and definite time releases. Coordination will also consider coordination with maximum load currents, motor starting current and motor starting time; transformer inrush currents, safe stalling time and current of motors, thermal withstand characteristics of the equipments.
• Motor protection
• Transformer protection
• Unit Protection
• Distance Protection

The complete protection system study of the plant will be performed considering the following criterias:

1. Accepted Standard Engineering practices in protection
2. Applicable IEEE standards
3. Information available in the relay application guides, and relay catalogues of the relay manufacturer. This criterion will be generally more used in determining relay settings in comparison to the other methods specified for equipment protections.

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