Power Transformers In Parallel

[xptpcrewx]

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.

Ok well you stated, "why POCOs always use two large transformers..."

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

Also, the number of transformers in a substation is not particularly important or interesting. You are failing to mention if those transformers are banked or not or related by arrangement.

Usually two units in a secondary selective arrangement provides adequate redundancy and capacity at the best cost while keeping complexity to a minimum.

Technical background?

 

[mbrooke]

Ok well you stated, "why POCOs always use two large transformers..."



Also, the number of transformers in a substation is not particularly important or interesting. You are failing to mention if those transformers are banked or not.

Usually two units in a secondary selective arrangement provides adequate redundancy and capacity at the best cost while keeping complexity to a minimum.

Technical background?

Always as in ultra typical. If I had to list every exception my original post would be a lot longer.

I stated that they are going to be operated in parallel.


Is two transformers operated at 50% really cheaper than 5 units operated at 80%? Just seems like the capacity will be more utilized per unit.

 

[xptpcrewx]

Always as in ultra typical. If I had to list every exception my original post would be a lot longer.

I stated that they are going to be operated in parallel.

I started writing my response before you added the part about them being in parallel.

Is two transformers operated at 50% really cheaper than 5 units operated at 80%? Just seems like the capacity will be more utilized per unit.

Who is suggesting 50%? Most utility equipment is operated in an overloaded condition.

 

[mbrooke]

I started writing my response before you added the part about them being in parallel.


Who is suggesting 50%? Most utility equipment is operated in an overloaded condition.

You got a point here

Typically most POCOs over drive one unit for the loss of the other on the count it will not exceed 3 hours and for a reasonable life trade off, wait for a vegabond to come in, do switching into neighboring subs, or a combination of all 3.


Having two units loaded at less than 80% 99.99% of the time is a huge amount of idling asset.

Now, if we go to smaller units in parallel, more asset is utilized without all the extra complexity or outages. Failure of one unit is a 3 cycle blink at most.

 

[paulengr]

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.

You are probably getting into asymmetrical faults which affect AIC as far as X/R.

I was thinking more in terms of the fact that if I have two 3% Z transformers in parallel the load sees 1.5% Z and short circuit current is doubled. An open tie prevents this as does significant impedance between the sources and the tie(s).

I have never found an answer to one question about cutouts…if you over duty a boric acid fuse what does thus really mean? Wouldn’t we just replace the broken fuse clips or at worst the cutout? It’s one thing to overduty a metal clad breaker…it might not be capable of opening or the bus bars can rip apart. But a cutout doesn’t seem like it would be subject to either issue.

 

[mbrooke]

You know more than I do in regards to cutout behavior lol.


The thing is, 4 20MVA trafos have only a bit more fault current than one 60MVA trafo.

 

[xptpcrewx]

You are probably getting into asymmetrical faults which affect AIC as far as X/R.

I was thinking more in terms of the fact that if I have two 3% Z transformers in parallel the load sees 1.5% Z and short circuit current is doubled. An open tie prevents this as does significant impedance between the sources and the tie(s).

I have never found an answer to one question about cutouts…if you over duty a boric acid fuse what does thus really mean? Wouldn’t we just replace the broken fuse clips or at worst the cutout? It’s one thing to overduty a metal clad breaker…it might not be capable of opening or the bus bars can rip apart. But a cutout doesn’t seem like it would be subject to either issue.

Well the cutout would explode and possibly injure anyone on the vicinity. How about partially blowing apart and creatingarcing?

 

[xptpcrewx]

You know more than I do in regards to cutout behavior lol.


The thing is, 4 20MVA trafos have only a bit more fault current than one 60MVA trafo.

I don’t understand how you can make such a generalized statement. Fault currents are very different based on the impedances and X/R ratios involved. To make matters more complex, if there is a tap changer the smaller units could have more impedance per tap change wrt a larger unit.

 

[mbrooke]

I don’t understand how you can make such a generalized statement. Fault currents are very different based on the impedances and X/R ratios involved. To make matters more complex, if there is a tap changer the smaller units could have more impedance per tap change wrt a larger unit.

Assume both units have an 8% Z on base.

More impedance is a good thing, lower fault current. As there are on board tap changers.

 

[xptpcrewx]

Assume both units have an 8% Z on base.

More impedance is a good thing, lower fault current. As there are on board tap changers.

8%Z on base is way different for 20MVA vs 60MVA. Lower fault current does not always produce lower asymmetrical fault current. Again, too many assumptions IMO. What is your experience with short-circuit analysis?

 

[mbrooke]

I don’t understand how you can make such a generalized statement. Fault currents are very different based on the impedances and X/R ratios involved. To make matters more complex, if there is a tap changer the smaller units could have more impedance per tap change wrt a larger unit.

12 MVA base = 301 amps / 0.08 = 3,765 3 phase fault amps x 4 = 15,061

40 MVA base= 1004 amps / 0.08 = 12,551 3 phase fault amps

23kv


12.5 vs 15 is very close- it is possible with some slight tweaks to get the 4 unit option down to 12.5ka for a L-G fault.

 

[xptpcrewx]

12 MVA base = 301 amps / 0.08 = 3,765 3 phase fault amps x 4 = 15,061

40 MVA base= 1004 amps / 0.08 = 12,551 3 phase fault amps

23kv


12.5 vs 15 is very close- it is possible with some slight tweaks to get the 4 unit option down to 12.5ka for a L-G fault.

This isn’t anywhere near how you would calculate the fault duty.

 

[mbrooke]

8%Z on base is way different for 20MVA vs 60MVA. Lower fault current does not always produce lower asymmetrical fault current. Again, too many assumptions IMO. What is your experience with short-circuit analysis?

A lot of this is indeed assumption at this point, the theoretical frame work for a standardization of this scheme is being debated.

 

[mbrooke]

This isn’t anywhere near how you would calculate the fault duty.

It is still a valid general 3 phase fault value. Key word being general.

 

[xptpcrewx]

It is still a valid general 3 phase fault value. Key word being general.

No

 

[mbrooke]

No

Explain.

 

[jdsmith]

The FLA/%Z approximation is probably within 15-20% for small systems where the major part of the source impedance is the transformer we are considering. For larger transformers the impedance of the network upstream of the transformer becomes more important so it is no longer valid to neglect source impedance.

Another concern with this approximation is system X/R ratio and calculation of both symmetrical and asymmetrical fault currents. Transformers 20 MVA and above have a relatively high X/R value, which means the symmetrical and asymmetrical fault currents will differ more than in a system with lower X/R components. This means the dominant factor in selecting circuit breakers or switchgear bus bracing could be the asymmetrical (or momentary) fault current, not the symmetrical fault current.

Also consider that for transformers 10MVA and above it is fairly common to specify the impedance level based on a combination of short circuit, voltage regulation, efficiency, and system stability considerations. Using the C37 default impedance values of 5.75% and 7% or a typical 8% value is missing out on an opportunity to optimize the design. I’ve specified transformers ranging from 1% impedance up to 10% as the most efficient way to meet the design goals on various projects.


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[xptpcrewx]

The FLA/%Z approximation is probably within 15-20% for small systems where the major part of the source impedance is the transformer we are considering. For larger transformers the impedance of the network upstream of the transformer becomes more important so it is no longer valid to neglect source impedance.

Another concern with this approximation is system X/R ratio and calculation of both symmetrical and asymmetrical fault currents. Transformers 20 MVA and above have a relatively high X/R value, which means the symmetrical and asymmetrical fault currents will differ more than in a system with lower X/R components. This means the dominant factor in selecting circuit breakers or switchgear bus bracing could be the asymmetrical (or momentary) fault current, not the symmetrical fault current.

Also consider that for transformers 10MVA and above it is fairly common to specify the impedance level based on a combination of short circuit, voltage regulation, efficiency, and system stability considerations. Using the C37 default impedance values of 5.75% and 7% or a typical 8% value is missing out on an opportunity to optimize the design. I’ve specified transformers ranging from 1% impedance up to 10% as the most efficient way to meet the design goals on various projects.


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What he said.


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[mbrooke]

The FLA/%Z approximation is probably within 15-20% for small systems where the major part of the source impedance is the transformer we are considering. For larger transformers the impedance of the network upstream of the transformer becomes more important so it is no longer valid to neglect source impedance.

Another concern with this approximation is system X/R ratio and calculation of both symmetrical and asymmetrical fault currents. Transformers 20 MVA and above have a relatively high X/R value, which means the symmetrical and asymmetrical fault currents will differ more than in a system with lower X/R components. This means the dominant factor in selecting circuit breakers or switchgear bus bracing could be the asymmetrical (or momentary) fault current, not the symmetrical fault current.

Also consider that for transformers 10MVA and above it is fairly common to specify the impedance level based on a combination of short circuit, voltage regulation, efficiency, and system stability considerations. Using the C37 default impedance values of 5.75% and 7% or a typical 8% value is missing out on an opportunity to optimize the design. I’ve specified transformers ranging from 1% impedance up to 10% as the most efficient way to meet the design goals on various projects.


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You bring up a good point. Is specing a 12/20MVA unit at 8% Z a good idea? C37 defaults would be a better idea?

 

[jdsmith]

You bring up a good point. Is specing a 12/20MVA unit at 8% Z a good idea? C37 defaults would be a better idea?

It depends on a number of things:
- system configuration (transformers in parallel, radial feed, or secondary selective system)
- primary side system stiffness - if we already have fairly low fault current, we probably don’t want to knock it down further with a high impedance transformer
- what kind of load you’re feeding (steady, cyclical, any big motors to start)
- downstream distribution system characteristics - what additional voltage levels are involved, how large is the distribution system, what losses are expected downstream
- power price per kWh to pay for transformer losses
- voltage regulation on the primary
- what voltage regulation is required on the secondary.

Without assessing all of those details, the short answer is 8% sounds high. It might allow you to use 31 kA MV gear, but the voltage regulation and efficiency benefits may justify 6-7% Z even though the switchgear may need to be rated for more fault current.


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