MODs in new substations 2

[Mbrooke]

Are MODs recommended or used in new substations? If so what advantages do they provide?

 

[marks1080]

I've heard good things, but never seen one in person.

 

[MatthewDB]

But now that you mention it I have been thinking about setting an auto open for bay MODs after a line trip.

Is this on ring bus or breaker and a half? Those are the only applications where I see it making sense.

Do you plan on reclosing the breaker after MOD open? Any concern over positive switch opening?

 

[Mbrooke]

Main and transfer. No reclose after the open. Though where would it hold merit in breaker and a half? Positive switching?

 

[marks1080]

I don't think you buy anything by operating the DS after a line automatic.

Some SF6 breakers will want to open their DS's based on SF6 levels... There are some transformer configurations where you may want to open the DS's as part of a protection. Think of a transformer terminated directly on a BUS. If the transformer trips, it will take out the entire bus. An automatic DS operation could potentially speed up restoration of the bus.

Remember that Breakers are 'Protection' devices. DS's are 'Operating' devices. DS's can in no way be relied on to actually open automatically, and any MOD operation in a switchyard really should have someone on the ground to verify correct operation. If you really want to trust in remote DS operation without field personnel confirming the operation you could look at using cameras, but imo that's just adding to much stuff for not much gain. Maybe a good idea in very remote locations.

 

[MatthewDB]

....Though where would it hold merit in breaker and a half? Positive switching?

In both you only get a benefit if the breaker is reclosed after the line switch is open.
The benefit is greater on a ring. With a ring, any other subsequent fault will break the ring.
In a breaker and a half scheme the benefit is the continued access to both busses by the associated line. I can't remember all of the details but one of the West Coast US blackouts impacting LA and Pheonix was from two sequential tree faults on lines blocking access to SVC.

The better fix was to not pair SVC with lines.

 

[Mbrooke]

I don't think you buy anything by operating the DS after a line automatic.

There are SEL relays (and others) that can be configured to open after a breaker has locked out.

Some SF6 breakers will want to open their DS's based on SF6 levels...

I've been thinking about this. My understanding is that once the breaker trips on low low, the breaker will still hold is dielectric properties. Just not be able to interrupt any current.

There are some transformer configurations where you may want to open the DS's as part of a protection. Think of a transformer terminated directly on a BUS. If the transformer trips, it will take out the entire bus. An automatic DS operation could potentially speed up restoration of the bus.

Yup, seen those. Often the transformers are directly attached to the line or at a remote substation without a breaker protecting the transformer. When the transformer faults a transfer trip is initiated. Once the transformer's MOD opens, the line then re-closes. Such philosophy can be taken further into line sensationalizing. However all new substations that I design have the transformer protected with a breaker (or fuse on the smaller units). Thus MODs in this application are rapidly declining for me.

Remember that Breakers are 'Protection' devices. DS's are 'Operating' devices. DS's can in no way be relied on to actually open automatically, and any MOD operation in a switchyard really should have someone on the ground to verify correct operation.

Unfortunately not in my case. MODs are often controlled automatically or via SCADA for a variety of reasons. In ring buss designs once a line locks out its line MOD will open and breakers will then re-close. In split buss stations with 2 lines, one breaker and 2 transformers mods will isolate a faulted line and then re-close the center breaker. In dual feed substations MODs (usually these MODs have SF6 bottles in these cases) are used to open the faulted line and close the normally open line to restore service. All this done automatically. In single breaker double buss applications possessing only a single buss zone a buss fault is dealt with by opening all the MODs on the main bus via SCADA and then closing the mods connected to the auxiliary buss. The breakers are then remotely closed to bring the station back into service before the main buss can be inspected and repaired.


However these concepts are starting to fade in new substations. In my world disconnect switches are starting to become service/isolation switches rather than being part of an integral protection and relaying scheme.

If you really want to trust in remote DS operation without field personnel confirming the operation you could look at using cameras, but imo that's just adding to much stuff for not much gain. Maybe a good idea in very remote locations.

I've though about that. But come serve weather it question visibility.

 

[Mbrooke]

In both you only get a benefit if the breaker is reclosed after the line switch is open.
The benefit is greater on a ring. With a ring, any other subsequent fault will break the ring.
In a breaker and a half scheme the benefit is the continued access to both busses by the associated line. I can't remember all of the details but one of the West Coast US blackouts impacting LA and Pheonix was from two sequential tree faults on lines blocking access to SVC.

The better fix was to not pair SVC with lines.

Thanks, makes sense. Where an SVC is present or critical auto transformer with breaker and a half its best to have its own bay with only 2 breakers.

 

[MatthewDB]

I've heard arguments that the ideal dual voltage transmission substation is a breaker and a half, with two auto-transformers situated between the higher and lower sections. Loss of a bus means loss of 1/2 of the transformer capacity, but no fault can cascade to the loss of both transformers.

Another idea is to put the two transformers on the same bay, but run with MODs on the center breaker normally open. Same limitations on cascading faults, but if a bus is lost, it is possible to return both transformers to service.

 

[davidbeach]

Ok, I'm entirely not following the 1/2 capacity bit. The transformer is either connected on both sides (full capacity) or it isn't (no capacity). How the heck does one come up with half capacity?

 

[Mbrooke]

I think (I think) he is referring to each buss having its own direct transformer connection, and the loss of one buss results in losing one transformer leaving only the other (half the total station capacity) I however do not follow " no fault can cascade to the loss of both transformers."

 

[davidbeach]

A transformer is a position; built the same way as a line position; transformer is connected between two breakers. If one breaker trips for a bus fault the transformer is still connected, no capacity is lost.

 

[Mbrooke]

Correct, but many utilities will attach the transformers directly to the buss. That and its possible if you loose a line on the other side of the center breaker while a bus is out you will also loose a transformer.