Two LTCs paralleled û one runs to 16L & other 16R - what will happen?

[Manindemand]

An incorrect switching order was issued by dispatch that would have left two 50 MVA LTC Transformers tied together through the 13.8 kv bus tie switch. Fortunately the field techs caught the error before carrying out the order.

They were speculating about the damage that this could cause if the LTCs began to buck and boost and went to 16 lower and 16 raise.

The XFMRs are 115000Delta/13800Wye and high sides are connected to the same high line. Low sides go through 2000 amp Main VCBs then to distribution buses with five 1200 amp Feeder VCBs on each bus

The load is low this time of year so I don’t think overloading one LTC is an immediate problem. And I know that opening the bus tie switch would cause severe arcing.

I know there would be a lot of circulating current between the LTCs. What problems can this circulating current cause during low load season?

What problems could you have during peak load season? Worst case scenarios please.

Thanks in advance.

 

[jbartos]

Suggestion: It depends on the LTC protection scheme that might include the LTC circulating currents. What if there is an LTC malfunction?

 

[hsj]

If the transformer and LTC are identical then the circulating current would be momentary. After paralleling there should be no circulating current.

The circulating current during low voltage period will be lower than high load period. Because during low load period the bus voltages on either side of tie breaker would be close to each other.

There would be no harm in closing tie breaker. Breakers are designed for 40% out of phase switching.

 

[Manindemand]

These are two identical LTCs that normally run independently and do not have circulating current protection.

After maintenance the switching order would have left them tied together (in auto mode) through a bus tie air break switch (not breaker).

Even though they were on 16L and 16R the buses kv would still be 8kv (per phase) but the one on 16R would hog the load. This is why you could overload one LTC in peak load season.

I assumed there would be a lot of circulating VARs.

 

[busbar]

With both transformers set on the same hi-side tap, there would be a 20% voltage mismatch between the secondaries: +10% and –10%. That may result in fairly large VAR demand on the 115kV bus [with no secondary load.] It would be an interesting experiment.

 

[stevenal]

Worst case, if nothing is overloaded, would be poor efficiency.

 

[dpc]

I ran a quick model in EasyPower - with one transformer at 10% buck (12.42 kV) and the other at 10% boost (15.18 kV), the transformer in boost is consuming 60.5 MVAR and pushing

 

[dpc]

Sorry about the first post - hit the send button by mistake. As I was trying to say - I modeled this situation in EasyPower, assuming ANSI standard transformer impedance of 9%.

The boosting transformer is consuming 60.5 MVAR and sending 54 MVAR to the bucking transformer. The bucking transformer is sending 49 MVAR back into the system. This corresponds to about 2300 A of circulating current at 13.8 kV.

So both transformers would be overloaded by the circulating current alone, neglecting any load current.

 

[jbartos]

Suggestion to the previous post. LTCs may be overloaded too besides the transformers.

 

[Manindemand]

dpc - looks like you used 50 MVA as base MVA. They are 30/40/50 MVA @ 65 deg C. Shouldn't you use the 30 MVA for this calculation?

 

[Manindemand]

I forgot to mention - I came up with j1369 Amps of circulating current (reactive) with no load at all. This was using 30 MVA base in the calculation

 

[dpc]

Your first post said only 50 MVA.

Amount of MVAR flow as a percentage of the rating would be the same, so your 1369 A looks correct.

 

[Manindemand]

I'm sorry, I forgot to give all the data in my first post.

Beckwith gives an example to calculate the loop impedance of two 12/16/20 MVA transformers with no load. They use Base 12 MVA in their example.

http://www.beckwithelectric.com/infoctr/appnotes/App11.pdf

I calculated a hypothetical situation where both the 30/40/50 MVA transformers had 30 MVA of load. They are 9.17%Z by the way.

If one was on 16R and the other on 16L:

Transformer #1 would have 53.16 MVA
Transformer #2 would have 33.40 MVA

#1 Main VCB would probably trip due to load. Then #2 Main would trip for the same reason. It would try to carry 60 MVA and then trip.

So a major metropolitan station would be off line - hospitals, malls, traffic signals etc...

Someone please double check my calculations. Thanks.

 

[Manindemand]

I forgot to note - I used a 90 % PF to calculate original 30 MVA load on each LTC before they ran away to 16R and 16L.

 

[NormGA]

I have seen exactly this situation caused when an auxiliary switch contact in one of the LTC's kept it from running, while the control on the other unit continued to see low voltage. Each LTC had its own control and there was no paralleling protection. The transformers ran this way for over an hour with no harm done. What happened as a result was educational to me: The real load, watts that is, divided evenly between the two transformers. This taught me that when tying circuits in the field you can not shift watts by changing the voltage at either end. There was an awful lot of circulating current which caused the reactive on one unit to be lagging, while leading on the other. The magnitude of this circulating var load was identical on the two transformers. The net reactive load was the reactive load that existed on the banks before the problem began. In other words, the net external effect was not noticeable, but the transformers saw an awful lot of internal load and were starting to heat up. This taught me that when tying circuits in the field you can shift vars by changing the voltage, but the best you can do is minimize var flow. It also gave me the idea that if you are commissioning parallel transformers you can generate your own current for verifying some of your relay inputs if you can not get real load.

 

[Manindemand]

The data below was recorded by our SCADA host system in 5 min increments. It occurred while we were setting up the parrelling controls for two new 30/40/50 MVA LTC XFMRs (at another station). During check out they would not both regulate on the same step (or even close). We found we had to roll polarity on one of the Aux CTs to get them to regulate together. At one time they were about 10 steps apart but I don't know if SCADA recorded data at that exact time.

The combined MVA when near the same step is 23 and goes to 29 when apart but the combined MW doesn't change.


LT1 KV LT1 MVA LT1 MW LT1 MVar LT2 KV LT2 MVA LT2 MW LT2 MVar

8.03 -13.56 -10.94 -8.03 8.11 -10.82 -10.82 -0.07
8.01 -13.56 -10.94 -8.03 8.09 -10.82 -10.82 -0.07
8.04 -14.27 -10.94 -9.17 8.11 -10.87 -10.82 1.04
8.02 -11.99 -10.94 4.90 8.10 -16.96 -10.82 -13.07
8.00 -11.49 -10.77 3.95 8.08 -16.15 -10.98 -11.84
8.05 -11.31 -10.77 -3.43 8.13 -11.87 -10.98 -4.50
8.02 -11.31 -10.77 -3.43 8.10 -11.87 -10.98 -4.50

We don't get SCADA Amp readings from the Main VCBs. How much real current would the overcurrent relays in the Main VCBs see? Is there really a risk of them tripping?

 

[Manindemand]

NormGA - the aux switch you mentioned sounds like the 43R sw in a Waukesha LTC. We had a similar problem.

 

[NormGA]

I am assuming that the low side bus was carrying load while the readings above were made, then: It appears to me that the polarity of your Watts reading on LT1 is reversed. If Watts is reversed, what about vars? Note that by assuming the negative MW readings are really positive, the real power is shared equally by the two transformers (close enough anyway), which matches my observation in my previous post. By assuming LT1 Watts and vars to be positive, and LT2 Watts and vars as shown, my slide rule says the total bus load is 21.76 MW, -8.17 mvars, with 4.9 mvar circulating for the worst case. I hope someone will correct me if I am wrong, or made a wrong assumption.

You mentioned tying the transformers with a switch. I hope this a rated load break switch. If not you will have problems unless the bus loads just happen to be identical.

I learned years ago that it is almost impossible to get even two identical LTC's to operate correctly in parallel. There are too many variables. We use station service transformers for potential and the ratios are never the same. Load on the station service bus adds another variable, and you can go on and on. A couple of steps is about as close as you're gonna get. For the problem I cited above, I converted the two LTC's to a master/slave scheme. Problem solved (but added potential for others).

 

[johnspark]

Manindemand, on our system, an event occurred where one transformer was on tap 5, and the other was on tap 17. Something was wrong with the (new) On Load Tap Changer relay circuitry to allow this to occur. That's another story... What happened was that one of the transformers tripped out on overload. Load was successfully serviced during this time, and bus volts on the low side had been okay, in other words circulating current was VAR current.

All primary equipment was found to be okay when inspected afterwards.

cheers Johnspark

 

[Manindemand]

NormGA,
Yes, the low side buses were carrying load. That station feeds underground network only. It has two 13.8 buses (1A & 2A) and each UG circuit has two feeder breakers. Each LTC has two main breakers. All four main breakers were closed, which is how the LTCs were paralleled.

Both #1 LTC watts and #2 LTC watts are negative, which indicates watts flowing out of the transmission grid. This would be normal for distribution transformers since customer load is constant. Notice the polarity only reverses on the VAR flow.

Yes, the bus tie switch (mentioned in my original post) is a load break. We still have a few stations without load break bus tie switches though.

johnspark,
Thank you for sharing that event. That's a good example that confirms excessive circulating current will trip a main breaker. I'm assuming it was the main breaker that tripped. Is this correct?