Reverse Current (67) relay function for 230kV Industrial Substation

[rockman7892]

We are in the process of upgrading existing protection and controls in an industrial customers substation which brings in 230kV transmission voltage and steps it down to 13.8k and 4.16kV through the substation transformers. The existing 230kV SF6 breaker relays had 67 functions for monitoring fault current contribution from the plant to the utility primarily because the plant had local emergency generation at the 4.16kV level that could operate in parallel with the utility.

With the new upgrades the 4.16kv generators will not be able to parallel with the utility. Without this generation is there a need for a 67 function on the 230kV breakers? The plant does have several large synchronous motors (5000 hp) at the 4.16kV level which will contribute fault current to a fault on the 230kV system. Do these synchronous motors justify having a 67 function on the 230kV main breakers for faults on the 230kV system.

Do the 230kV breakers in this substation application typically have 27 and 81 tripping functions?

 

[RRaghunath]

It is standard practice to have detection for utility distress condition and island the plant with its own generation and load. This minimises process upsets or damages to the process.
The criteria for islanding depends on amount of in-house generation vis-a-vis how strong the utility is at the connection point.
Typically, under frequency coupled with df/dt or underfrequency coupled with active power flow towards utility is used to detect utility grid disturbance and island. It is normal to include a small time delay as well to tide over transient conditions.
As you rightly mentioned, pure current based detection is not helpful as reactive current flow needs to be allowed when there is a fault in the utility until the fault is cleared by respective local breaker.

Rompicherla Raghunath

 

[rockman7892]

Thanks RRaghunath

Is is necessary to have a 67 function on the main 230kv breakers when there is no generation source downstream in the plant?

Does fault contribution from synchronous motors warrant a 67 fucnctiin? I know that induction motor fault contribution dissipates within 5 cycles so may not be enough to warrant a 67 function.

 

[davidbeach]

It’s a conversation to have with the serving transmission provider. They’re the only ones that can give you an answer other than “it depends.” Different companies will give different answers.