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






RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
Amount of MVAR flow as a percentage of the rating would be the same, so your 1369 A looks correct.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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....
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.
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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).
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
All primary equipment was found to be okay when inspected afterwards.
cheers Johnspark
RE: Two LTCs paralleled û one runs to 16L & other 16R - what will happen?
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?