Power Transformers In Parallel

[mbrooke]

What advantage would be reasonable in a scheme using 4 or 5 smaller power transformers operating in parallel vs 2 larger units run normally open as typically seen in the wild?

For example four 12/16/20MVA transformers vs two 40/50/60MVA 115-23kv transformers.

Similarly five 15/20/25MVA vs two 60/80/100MVA 230-34.5kv transformers.

60 and 100MVA loading respectively to allow for failure of one unit during the hottest of days (assume load transfer disabled or not available to other substations).

In running calculations the fault current is about the same for both scenarios.

I know that one large transformer is cheaper in total than many smaller transformers, however, I can not help but notice the capacity of each smaller unit is more utilized per dollar through out the in service life.

This concept tends to apply to generators, underground cables, interconnecting auto transformers, ect but rarely to distribution substation applications. Is there a technical prohibition?

 

[xptpcrewx]

What advantage would be reasonable in a scheme using 4 or 5 smaller power transformers operating in parallel vs 2 larger units run normally open as typically seen in the wild?

For example four 12/16/20MVA transformers vs two 40/50/60MVA 115-23kv transformers.

Similarly five 15/20/25MVA vs two 60/80/100MVA 230-34.5kv transformers.

60 and 100MVA loading respectively to allow for failure of one unit during the hottest of days (assume load transfer disabled or not available to other substations).

In running calculations the fault current is about the same for both scenarios.

I know that one large transformer is cheaper in total than many smaller transformers, however, I can not help but notice the capacity of each smaller unit is more utilized per dollar through out the in service life.

This concept tends to apply to generators, underground cables, interconnecting auto transformers, ect but rarely to distribution substation applications. Is there a technical prohibition?

Reliability and stability are two advantages that come to mind. I would also expect a lower system X/R for the 4-5 smaller units operating in parallel.

 

[mbrooke]

Reliability and stability are two advantages that come to mind. I would also expect a lower system X/R for the 4-5 smaller units operating in parallel.

The lower X/R would in theory increase the AIC of my fused cutouts?

Can you go into detail on stability?

 

[steve66]

If the transformers aren’t identical (same KVA, Z, X, andR), then they won’t share the load equally.

Not usually a problem for 2 transformers if they can be matched.

But with 5 smaller transformers, smaller differences may have a larger effect on load sharing. And if transformer goes bad, you might have to find an exact replacement.

 

[mbrooke]

If the transformers aren’t identical (same KVA, Z, X, andR), then they won’t share the load equally.

Not usually a problem for 2 transformers if they can be matched.

But with 5 smaller transformers, smaller differences may have a larger effect on load sharing. And if transformer goes bad, you might have to find an exact replacement.

Good point, though IMO 20 and 25MVA trafos of 8% Z are relatively commonish? Any idea how much X/R typically varies?


The beauty is if this design were to be used as a design standard by a POCO for every distro substation spares would be plentiful. Or so I would wish.

 

[Hv&Lv]

Good point, though IMO 20 and 25MVA trafos of 8% Z are relatively commonish? Any idea how much X/R typically varies?


The beauty is if this design were to be used as a design standard by a POCO for every distro substation spares would be plentiful. Or so I would wish.

Why?
Why would anyone ever need so many spares?
many substations have transformers that are 30-50 years old. They are changed because loading goes up.

I’m removing two this year for a larger one.
One of them is a 1956 XF that is still reliable. The loads have grown enough to justify changing it to a bigger unit.

How much is (3) 20MVA vs one 60 MVA?

 

[mbrooke]

Why?
Why would anyone ever need so many spares?
many substations have transformers that are 30-50 years old. They are changed because loading goes up.

I’m removing two this year for a larger one.
One of them is a 1956 XF that is still reliable. The loads have grown enough to justify changing it to a bigger unit.

How much is (3) 20MVA vs one 60 MVA?

My point is that if a POCO had 250 substations with 20MVA 115kv-13.8kv transformers vs 60MVA transformers both schemes would have a few spares on hand. Conveyed that a bit poorly.


Typically a larger unit is cheaper per MW, however, a station with two transformers means each doesn't exceed 50% of their FA/FA rating, where as a station with 5 transformers each is loaded at 80% of their FA/FA rating during peak periods.

I know you mentioned a while back that your POCO loads a 2 unit substation to 85%, however, this method involved extensive switching and having to bring in a mobile unit to cover what switching can't.

 

[paulengr]

POCOs use networks and lots of ties. Your scenario already exists,

But the point about X/R and AIC is very important. Using smaller transformers covering smaller areas reduces this. With ties you can easily shift load when one needs to be taken out of service. Industrial examples use main-tie-main and double ended subs usually with open ties. This ultimately ends up in ring bus arrangements geographically distributed. The transmission/distribution lines provide impedance to minimize both AIC concerns and impedance for balancing.

 

[jdsmith]

Historically these 3-5 transformer systems were installed in phases as load grew, or if they were installed all at once it was to limit fault current. Prior to vacuum circuit breakers (70s era) fault current capability cost a lot more than it does today. I don’t have up to the minute pricing, but today’s MV equipment doesn’t carry a huge cost premium as you move from 31 kA to 40 to 50 kA. Last time I checked 63 kA had enough of a premium it was worth avoiding if you could, but many of us use it anyway at 15 kV because we can’t tolerate the weight and space of a higher voltage level.

Part of the consideration is also what type of distribution equipment you want to use at 23 kV. Open air systems are arguably the best if you have space for it - metal clad switchgear and gas insulated switchgear tend to have more problems than an air insulated outdoor station at that voltage. If the client or space considerations force the MV equipment to be indoors a handful of buses with a tie bus and appropriate downstream ties gives you more flexibility to make the inevitable distribution equipment repairs.


Sent from my iPhone using Tapatalk

 

[Terminator5047]

fun fact if you take 6 1phase transformers and wire them in a zig zag pattern you can get 3phase

 

[xptpcrewx]

The lower X/R would in theory increase the AIC of my fused cutouts?

Can you go into detail on stability?

The AIC of a fused cutout is the AIC of the fused cutouts. It does not change based on the system X/R.

Stability from a power generation standpoint because if you lost a small transformer the generator load angle would not shift as much as if you lost a larger one.

 

[xptpcrewx]

fun fact if you take 6 1phase transformers and wire them in a zig zag pattern you can get 3phase

Please explain

 

[mbrooke]

The AIC of a fused cutout is the AIC of the fused cutouts. It does not change based on the system X/R.

Stability from a power generation standpoint because if you lost a small transformer the generator load angle would not shift as much as if you lost a larger one.

I thought AIC was effected X/R, a higher X/R reduced AIC, while a lower X/R increased AIC.


Fortunately S&C has cutouts rated at 16,000 amps for 13.8kv, and 12,000 amps for 23kv. If that is exceeded, companion II backup fuses exist.

 

[mbrooke]

POCOs use networks and lots of ties. Your scenario already exists,

But the point about X/R and AIC is very important. Using smaller transformers covering smaller areas reduces this. With ties you can easily shift load when one needs to be taken out of service. Industrial examples use main-tie-main and double ended subs usually with open ties. This ultimately ends up in ring bus arrangements geographically distributed. The transmission/distribution lines provide impedance to minimize both AIC concerns and impedance for balancing.

Right and right, but not having customers loose service from a failed trafo and not having to open siwtches or reclosers then close in ties has its advantages.

 

[xptpcrewx]

I thought AIC was effected X/R, a higher X/R reduced AIC, while a lower X/R increased AIC.


Fortunately S&C has cutouts rated at 16,000 amps for 13.8kv, and 12,000 amps for 23kv. If that is exceeded, companion II backup fuses exist.

The AIC is the “rating” of fault current an interrupting device can safely interrupt.

A higher system X/R results in a higher magnitude fault duty.

 

[mbrooke]

The AIC is the “rating” of fault current an interrupting device can safely interrupt.

A higher system X/R results in a higher magnitude fault duty.

Right

Any idea why POCOs always use two large transformers in a substation?

 

[xptpcrewx]

Right

Any idea why POCOs always use two large transformers in a substation?

Because it’s good enough is the short answer.

 

[mbrooke]

Because it’s good enough is the short answer.

Can I have the long answer?

 

[xptpcrewx]

Can I have the long answer?

The long answer is along the lines of you are assuming too much by asking why POCO always use two large transformers in a substation...

Not all POCO substations have two transformers so your question presumes something that isn't always the case. Secondly, you haven't specified whether the two transformers were part of the original design, or if a second unit was added due to increased demand. Other factors you haven't explained are: if these units are banked or operated individually, space and weight limitations, cost constraints, availability of parts/equipment, environmental constraints, fault duty, maintenance/repair/ replacement impact on service continuity, reliability/redundancy requirements, bus scheme employed, complexity of protection, and any other system considerations which can have an impact.

What is your technical background if you don't mind me asking?

 

[mbrooke]

I'm very well aware not all POCO subs have two or even three transfomers, ie Con Edison typically has 5-10 transformers in a sub, however the majority of the time POCOs employ a two transformer M-T-M scheme.

The units are to be banked (operated in parallel)