Unit Substation MV Switchgear with no Main Breaker

[rockman7892]

We are replacing a 2.4kV Switchgear lineup on the secondary of a unit substation with a 5MVA transformer. Primary voltage on transformer is 12.47kV.

The secondary switchgear only requires two feeder breakers for two different feeders. Are there any issues with this switchgear lineup not having a main breaker?

From an NEC standpoint since this is considered a "supervised locations" the NEC does not require a secondary main breaker as long as the primary fuse rating is less than 300% of transformer primary.

From a fault current standpoint this switchgear will have an interrupting rating and short time rating of 25kA for 2-sec. As long as the upstream fuse clears 25kA within 2-sec are there any issues from a fault current withstand standpoint or are there other factors to consider for switchgear withstand without a main breaker.

The only other consideration I can think of is Arc Flash Incident Energy on the entire lineup without a main breaker (no line side & load side calculation) but at 2.4kV the IE may not be tat high. Something that will have to be considered.

 

[RRaghunath]

The transformer itself can withstand a short circuit at its secondary terminals not more than 2s.
Hence, it is to be ensured that the primary fuse would blow in <2s for a fault at secondary terminals/busbar.
This current depends on the transformer %Z and I am sure will be well below 25kA.
What protections do you have for this transformer, is it only primary fuse or is there LV standby earth fault protection (connected to transformer neutral CT)?
Clearing an earth fault at the secondary terminals/busbar needs to be paid attention, as earth faults are not known to yield large currents that can blow primary fuses. This is especially so, if the neutral is resistance earthed.
There is no need for secondary breaker but, the transformer protections need to be wired to trip HV breaker and the two outgoing feeder breakers.
If there is no primary breaker (as it seems from original post), then the primary fuse needs to meet the above said requirements.

Rompicherla Raghunath

 

[unclebob]

The Arc Flash hazard can be easily taken care of by using remote command.

 

[krisys]

If your system is supplying critical (or important) facility, primary fuse protection should be avoided. As you said, 2.4kV is supplying to the large motors. You are exposing the motors to risk of single phasing. (though single phasing protection will be there).

If you are going to replace the 12kV side, better replace it with the breaker. If it is a fuse, then inter-tripping from the down stream, providing the interlock etc. will be a problem. Try to go for the breaker.

 

[rockman7892]

Unit substation will be fed from upstream breaker/relay as well as have a fused switch on the primary of the transformer.

RRaghunath - As you mentioned the transformer will have a "through-fault" withstand so upstream relay and fuse will have to be below let-through damage curve at two seconds as well as be below 2-sec sort time rating of Switchgear bus. Is this correct?

The transformer will be impedance grounded so as mentioned the ground fault sensing on transformer neutral will have to trip upstream breaker. Does it need to trip secondary breakers as well? I would think that neutral CT would be coordinated with zero sequence CT's on feeder breakers and ground fault sensing on neutral CT will only trip upstream breaker? Should ground fault sending on neutral for impedance grounded systems typically trip both primary and secondary main breaker when a secondary main is present?

Without a main would you still suggest having a relay in the main section of switchgear to sense neutral CT as well as phase current to trip upstream breaker? Or would you simply wire neutral CT to upstream relay? I think that having CT's and relay on secondary can lead to faster tripping of main for faults on secondary switchgear? Any other protection elements to consider? Is it worth having bus differential for only two feeder lineup?

Krisys - Why do you say that fuse protection should be avoided for critical facility? Simply because a fuse is more difficult to replace than resetting breaker/relay? As you suggested motors will have single phase protection in motor protection relays.

 

[RRaghunath]

1. The through fault current of transformer is much less when compared to the switchgear rated fault level. Hence, it is important to see that the HV fuses can blow before transformer withstand capability is violated.
2. Secondary side Neutral CT connected earthfault protection will have to isolate the secondary side feeders as well (in addition to primary side breaker). This is to take care in case of uncleared fault in one of the outgoing feeders.
3. Phase O/C protection on secondary of transformer is not a requirement as the primary fuse could do the job. If you could provide, it will be helpful in isolating the transformer even for low current faults and with proper coordination act as a backup for outgoing feeder faults.

Rompicherla Raghunath

 

[krisys]

This is mainly important for LV system. Normally the LV system are not provided with phase failure protection. If the circuit breaker is provided, it ensures the positive opening of all the three poles simultaneously. Thus preventing the possible phase failure.

If the fuse of any one phase ruptures, there will be single phasing and the motors may damage due to single phasing.

Again, now a days the modern motor managers offer comprehensive motor protection package, including the phase failure protection. Hence not to worry.