Two MV feeders for double ended MV switchgear

[m sleem]

When we have more than two feeders coming from the utility (may be two different substations) & feeding double ended MV switchgear at two sections, the two sections feeding an interconnected loop (no of RMU+XFMR), my question regarding the requirements for this loop when both sections are energized & we've two different circuits from different sources, is it mandatory to keep an opened breaker in this loop?
If no, do we need a synchronization panel to manage the parallel operation?

 

[templdl]

When we have more than two feeders coming from the utility (may be two different substations) & feeding double ended MV switchgear at two sections, the two sections feeding an interconnected loop (no of RMU+XFMR), my question regarding the requirements for this loop when both sections are energized & we've two different circuits from different sources, is it mandatory to keep an opened breaker in this loop?
If no, do we need a synchronization panel to manage the parallel operation?

Why would you want to do so or is this a 'what if' question?
You also have to consider back feeding either on of the utility supplies should both utility breakers be closed and the tie breaker closed all at the same time.

 

[petersonra]

isn't the whole purpose of having two feeds so you are less likely to lose power?

if the breakers are not all closed simultaneously, you have potential to lose power to part of your plant until the tie breaker closes.

I am not real familiar with this kind of setup so I just don't know.

 

[GoldDigger]

isn't the whole purpose of having two feeds so you are less likely to lose power?

if the breakers are not all closed simultaneously, you have potential to lose power to part of your plant until the tie breaker closes.

I am not real familiar with this kind of setup so I just don't know.

If you have a setup with both sources active and paralleled (instead of feeding different groups of circuits), you would at a minimum need to have a network protection (reverse power detection) relay on each service to prevent backfeeding and messing up the POCO side if one source fails upstream.
Another way to get similar results, but still suffering a momentary interruption, would be to use a ATS-like arrangement to switch from one source to the alternate.
(The lights in the stadium would go out, of course, and take a while to restart....)

 

[m sleem]

This is my basic question, where we've two circuits from two different sources, how about the phase shift angle and Vrms ? do we need a synchronization panel at that time?

 

[GoldDigger]

This is my basic question, where we've two circuits from two different sources, how about the phase shift angle and Vrms ? do we need a synchronization panel at that time?

A synchronization panel typically describes equipment that has active control of one or more of the parallel power sources to force synchronization, not just detect it and switch when synchronized.

If both sources are from the same utility and you have an assurance that POCO will not change the phase relationship between the two as part of any equipment changes, then you could (with POCO consent and coordination) parallel them, with the use of protective relays as I mentioned before. Otherwise your parallel link could compromise the stability and segmentation (fault isolation) of the POCO network.

 

[m sleem]

A synchronization panel typically describes equipment that has active control of one or more of the parallel power sources to force synchronization, not just detect it and switch when synchronized.

If both sources are from the same utility and you have an assurance that POCO will not change the phase relationship between the two as part of any equipment changes, then you could (with POCO consent and coordination) parallel them, with the use of protective relays as I mentioned before. Otherwise your parallel link could compromise the stability and segmentation (fault isolation) of the POCO network.

For the end user, what is the effect? especially when we have very critical equipments.

 

[templdl]

For the end user, what is the effect? especially when we have very critical equipments.

Was this stated in the OP? If it was the responses may have been different as if GoldDigger should have know this all along.

 

[kingpb]

Since you have no way to control the voltage angle of either sources, synchronization is not possible, however utilizing a sync check relay at the point of interconnection would be in order.

Paralleling the sources is not really an issue provided the proper equipment is in place. As stated earlier you need to be able to disconnect from one source so you don't feed a fault on the system from the opposite source, but this is easily handled with relays.

If you feed from one source and leave the other one open, there is a fast transfer capability that Basler makes that works very well, that even allows motors, in most cases, to stay engaged. Otherwise you would need to do a break before make.

 

[m sleem]

..... but still suffering a momentary interruption, would be to use a ATS-like arrangement to switch from one source to the alternate.......

This is good idea actually, i've seen the asco ATSs alternate the source without any interruption by using internal synchronization unit. But the challenge is, do we have the same for MV?

 

[__dan]

When we have more than two feeders coming from the utility (may be two different substations) & feeding double ended MV switchgear at two sections, the two sections feeding an interconnected loop (no of RMU+XFMR), my question regarding the requirements for this loop when both sections are energized & we've two different circuits from different sources, is it mandatory to keep an opened breaker in this loop?
If no, do we need a synchronization panel to manage the parallel operation?

If you have a dual primary feed, any closing of one feed to the other would most likely be done by the utility with utility equipment, if that fit the utility plan. On the customer side, most likely would be an open transition switch with keys and mechanical interlocks to prevent closing both mains and the tie at the same time.

For the critical uninterruptible loads, one of the options would be full online motor generators, rotary UPS, with the MG outputs paralleled. Depending on the size, you may be able to get the MG's with primary voltage motors.

Interruptible loads would be through an open transition switch

 

[kingpb]

If you have a dual primary feed, any closing of one feed to the other would most likely be done by the utility with utility equipment, if that fit the utility plan. On the customer side, most likely would be an open transition switch with keys and mechanical interlocks to prevent closing both mains and the tie at the same time.

Why? MV equipment, i.e. vacuum circuit breakers on the customer side are quite suited to this with electrically operated breakers. We do fast transfer all the time.

Also, I don't think the utility is going to do this for you. They will give you two feeders, its going to be the customers responsibility to develop the design to their satisfaction. A sync check relay would keep them from closing out of phase. Again, quite common.

 

[John120/240]

When we have more than two feeders coming from the utility (may be two different substations) & feeding double ended MV switchgear at two sections, the two sections feeding an interconnected loop (no of RMU+XFMR), my question regarding the requirements for this loop when both sections are energized & we've two different circuits from different sources, is it mandatory to keep an opened breaker in this loop?
If no, do we need a synchronization panel to manage the parallel operation?

What type of facility would require feeds from two different sub stations ? Hospitals, correctional facilities, all rely on back up generators as far as I know. We have a FAA Air Route Traffic Control Center that utilize generators.

 

[GoldDigger]

What type of facility would require feeds from two different sub stations ? Hospitals, correctional facilities, all rely on back up generators as far as I know. We have a FAA Air Route Traffic Control Center that utilize generators.

Backup generators would generally require start up time, possibly load shedding, and a host of economic considerations that would make an alternate feed preferable.
Include large stadiums, etc. in your list. Just having half of your power is safer than having only backup and emergency lighting. However in most cases that would be a split feed to split loads with a normally open crossover bus rather than a constant paralleling.

 

[SG-1]

As stated before by others, this is very common in medium voltage. The only thing I would like to add is that many utilities do not like their lines being paralleled for any length of time. This could cause large amounts of current to flow across your bus between the two lines. This could cause your protection to open a Main Breaker.

What is your normal mode of operation ?
M1 closed & M2 closed with the TIE open ?
M1 closed & M2 open with the TIE closed ?
M1 open & M2 closed with the TIE closed ?

You have two options for transferring load from one Main Breaker to the other:

1. Open transition transfer ( Incoming lines are never in parallel. Load loss can occur, because both sources are open at the same time for a few cycles. In other words, the in-service Main is opened, then alternate Main is closed.

2. Closed transition transfer ( Both Mains are closed together, then one of them is opened. All this can take place in just a few cycles. )

 

[m sleem]

As stated before by others, this is very common in medium voltage. The only thing I would like to add is that many utilities do not like their lines being paralleled for any length of time. This could cause large amounts of current to flow across your bus between the two lines. This could cause your protection to open a Main Breaker.

What is your normal mode of operation ?
M1 closed & M2 closed with the TIE open ?
M1 closed & M2 open with the TIE closed ?
M1 open & M2 closed with the TIE closed ?

You have two options for transferring load from one Main Breaker to the other:

1. Open transition transfer ( Incoming lines are never in parallel. Load loss can occur, because both sources are open at the same time for a few cycles. In other words, the in-service Main is opened, then alternate Main is closed.

2. Closed transition transfer ( Both Mains are closed together, then one of them is opened. All this can take place in just a few cycles. )

Sometimes we have two feeders are energized & working in parallel, i've seen medical cities supplied by two MV feeders from one utility, the MV switchgear feeding 8 loops where each loop receiving power from each section of the switchgear. By this connection, do we need to disconnect one breaker in between this loop either at the first or medium or at the end to avoid the short circuit?

As long as the service conductors supplied by two different transformers, we've two different circuits with different parameters.

 

[__dan]

Sometimes we have two feeders are energized & working in parallel, i've seen medical cities supplied by two MV feeders from one utility, the MV switchgear feeding 8 loops where each loop receiving power from each section of the switchgear. By this connection, do we need to disconnect one breaker in between this loop either at the first or medium or at the end to avoid the short circuit?

As long as the service conductors supplied by two different transformers, we've two different circuits with different parameters.

If you have a loop circuit primary, that's a single primary circuit that can be fed from either end. The expectation would be that all the taps of the loop are synched to each other. You may parallel as part of an engineered solution, if the utility allows. Taps A and B will always match each other.

If you have a true dual primary feed, A and B are redundant and would be a tie of two different systems. The utility would be able to show statistical outage studies that show primary A and B are isolated from coincident outages. A is up when B is down and the reverse. Because A and B have such different origin, A and B mostly may not match. It is for the utility to decide if the switching transition is open or closed. I would expect to see open transition switching. If it's a large load on the UPS, then the MG set rotary UPS with the outputs paralleled are an option. The diesels tie into the MG sets as part of the MG package.

 

[SG-1]

Sometimes we have two feeders are energized & working in parallel, i've seen medical cities supplied by two MV feeders from one utility, the MV switchgear feeding 8 loops where each loop receiving power from each section of the switchgear. By this connection, do we need to disconnect one breaker in between this loop either at the first or medium or at the end to avoid the short circuit?

As long as the service conductors supplied by two different transformers, we've two different circuits with different parameters.

If I understand correctly your MV switchgear has two incoming lines ( feeders from utility ) Main A & Main B.
The switchgear has a Bus A & Bus B.
The TIE is open.
Then a feeder from Bus A is supplying energy to the same loads as a feeder from Bus B. Both of these feeders normal operation is with their breaker closed forming a loop & a connection to the other Bus.

 

[mbrooke]

The requirements would depend on the utility along with what your willing to spend on protective coordination. A double feeder scheme can be either have an open or closed tie breaker as well as make before break or always break before make transition. Operating in a closed loop is always far more complicated then an open loop.

In an open loop with break before make things are easy and the sources do not have to have an identical phase angle, frequency voltage ect. A loss of one supply circuit will cause your normally closed source sectionalizing breaker to open on a loss of voltage response while the tie point breaker then closes after the source breaker opens restoring the buss. The tie breaker is usually coordinated to trip first should a buss fault occur or the buss its closing into is faulted. Interlocks are put in place to prevent the tie breaker from closing into a live buss as well as while the source breaker is still closed. Once the source supply feeder gets restored the tie quickly opens and the source sectionalizing breaker closes. There is a brief power outage during this transition. This type of scheme is the simplest and perhaps the most reliable plus its definitely something pocos prefer for customer owned feeders.

The other is a normally closed scheme and it can get complicated. Both feeders need to be in near perfect synchronization, no large amount of current can flow between them when the tie breaker is closed. These schemes are the easiest to coordinate generally when both feeders originate from the same poco substation buss; ie, in essence the feeder cables are running paralleled. Fault current is higher both in the feeder cables and all branches originating from the customer owner gear, this is especially true if both feeders are fed from separate poco transformers. Relaying is done thorough differential comparison logic. Cts measure and compare directional flows of current and initiate a trip command to the two breakers between the fault when the abnormality takes place. Getting this logic to work right takes good analysis. Often the poco needs to know about this type of set up as well since when they are coordinating their breakers they need to be aware of how to take that closed scheme into account. Usually their engineers will work in conjunction with you so nothing gets missed. If other loads are attached to the feeders before they reach the customer the poco may discourage the idea since it can increase the fault current on their own equipment. POCO may also discourage it if they have to add anti reverse flow logic to their reclosers or breakers, since normally pocos operate MV in radial not anticipating reversed flows. (the exception is large city networks but back flow is prevented via a network protector on the LV side of the step down transformer)

However, when done right the only interruption experienced is the voltage sag from a cable fault or protective devices in the substation clearing from faulted equipment. Once the feeder is restored there is no transfer interruption, which is beneficial to data centers, hospitals and the like. Closed transition set ups are usually done on large facilities and its not uncommon for the poco to own the primary switch gear either though I have seen it go both ways.

 

[Julius Right]

In all power stations I saw each low voltage MCB [and MCC also] is double ended supplied from different source. The policy is the possibility to supply both sections from a single source but usually each section is separate connected with its source. The bus coupler breaker is open but a [very rapid] ATS ?break-before-make can catch all the equipment still functioning.