Two LV bus bars with Transformer Different Phasing / Phase Rotation
Two LV bus bars with Transformer Different Phasing / Phase Rotation
(OP)
Hi,
We have come across an issue where we need to connect two 132/11kV power transformer LV (11kV) bus bars in parallel through an interconnector. The vector group of both is YN0yn0 however the HV bushing of one of the transformers is connected as Red phase to C Bushing, Yellow Phase to B Bushing and Blue Phase to A bushing of the transformer. Red, Yellow and Blue are network or system phases whereas A, B and C are transformer HV bushings. LV Connections are a, b, c following the HV A, B, C which means on LV side c will be red, b will be yellow and a will be blue. Now the other transformer has different configuration i.e on HV side Red to A, Yellow to B and Blue to C which means on LV side a is Red, b is Yellow and c is Blue. When the transformers are run independently this is fine however if one of the transformers is out of service and the LV bus bars are interconnected, the phasing will be different. We can't change the HV phasing due to site constraints. We can change the LV cable connections so the question is will this solve the problem? I have attached a diagram as well. Your help is much appreciated.
We have come across an issue where we need to connect two 132/11kV power transformer LV (11kV) bus bars in parallel through an interconnector. The vector group of both is YN0yn0 however the HV bushing of one of the transformers is connected as Red phase to C Bushing, Yellow Phase to B Bushing and Blue Phase to A bushing of the transformer. Red, Yellow and Blue are network or system phases whereas A, B and C are transformer HV bushings. LV Connections are a, b, c following the HV A, B, C which means on LV side c will be red, b will be yellow and a will be blue. Now the other transformer has different configuration i.e on HV side Red to A, Yellow to B and Blue to C which means on LV side a is Red, b is Yellow and c is Blue. When the transformers are run independently this is fine however if one of the transformers is out of service and the LV bus bars are interconnected, the phasing will be different. We can't change the HV phasing due to site constraints. We can change the LV cable connections so the question is will this solve the problem? I have attached a diagram as well. Your help is much appreciated.






RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Thanks for your response. During the normal operation when the bus bars are running separately with the interconnector breaker open, its OK, however when one of the transformer is out of service, and we have to close the interconnector breaker, the previosuly Red phase on, say, GT5 will now be Blue phase from GT3, in my opinion reversing the previous phasing and the direction of motors. If we swap LV cables on GT5 to make a - blue and c - red to match GT3, we will still have the same problem when we connect the two LV Bus Bars.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
If you have accessible 11kV connections you can make sure the two sides are in phase using phasing sticks, or potentially by comparing the two VT outputs if you have 3 phase 11kV VT's on the switchgear, or even auxilary transformer secondary outputs if fitted. The first option is the one to go for if you can as it gives a definitive answer.
Regards
Marmite
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Thanks for your suggestions.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Regards
Marmite
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
I am also thinking on the same lines as the results suggest it. My biggest worry is if we need to alter the HV side of GT5 which would be a disaster as it is all built and bus bar connected in a tight old yard. I am not very good with vector diagrams and phase rotation. Is there a way to make sure, on paper, by a vector diagram that after this LV cross on c-b terminals for GT5, it will work in correct phase sequence. I am trying to get a porper phase sequence meter but have to report back in black and white with a vector diagram as well.
Appreciate if someone could help.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Regards
Marmite
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Phase rotation is determined by the system, and cannot be altered with transformer connections. Diagram indicates it is RYB, but even if incorrect, all that matters is that motors turn in the right direction.
With wye/wye transformers you can make any corrections needed on the low side, leaving the high side alone.
Of course if you swap phases at one of the transformers, the loads on that bus will also need to be swapped. (assuming motor rotation is correct now, and only a single swap was done)
Whether or not differential relay CT secondaries need to be swapped depends on CT location. If using bushing CTs and you have stability now, no changes need to be made.
I'm concerned with swapping VT secondary connections around to check for phasing. This depends entirely on how things are wired, and we already know that at least one drawing is wrong. I would have some qualified people use a set of phasing sticks across the open switch prior to closing it.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
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If we learn from our mistakes I'm getting a great education!
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Isolate both GT3 and GT5 from 132kV grid.
Force closure of "interconnector" and GT3 and GT5 breaker.
Find a smart way to supply 480V AC to the primary side (132kV) of GT3 or GT5. You should get 40 V AC on the 11 kV Side (assuming 30kW iron losses per transformer you need something like 140Amps at 480 Volts to energize both transformers).
With your multimeter check each phase of GT3 vs to GT5 at 132 kV Side (480Volts) . When you have a phase match you should read zero in you voltmeter. When phase doesn't match you should read 480V.
In this way you check (with one test) both primary and secondary crossings if any.
PS : By the way, it seems from your drawing that GT3 and GT5 has opposite phase rotation. GT3 has a-b-c rotation and thus can be matched only with one of the combinationa a-b-c or b-c-a or c-a-b. But GT5 has a-c-b rotation which is opposite to GT3.
(opposite rotations : c-b-a / b-a-c / a-c-b)
Recersing the LV side of the transformer i believe should do the job. Reversing high side would make the installation perfect. I would go for matching primaries and avoid tricks to secondaries.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
GT3 132
A-Blue
B-Yellow
C-Red
GT3 11kV
a-Yellow
b-Blue-earthed
c-Red
GT5 132
A-Red
B-Yellow
C-Blue
GT5 11kV
a-Red
b-Yellow
c-Blue
What we think we need to do (and thank God it doesn't include any 132kV modifictions)
GT5 11kV (Cable Swap)
a-Red
b-Blue
c-Yellow
Secondary Side
VT
a-Red
b-Blue-earthed
c-Yellow
We will swap the GT5 Differential CT connections Yellow-Blue (SIEMENS Duobias Relay) as well as LV Directional Overcurrent ( MiCom P142 Relay). Also a setting change for GT5 AVC (Microtapp Relay). Not sure about the LV power transducer (Alstom iSTAT500) but I presume if we are using L1, L2 and L3 in the same sequence it doesn't check the phase rotation etc. Will give Alstom a call.
Lesson learnt; confirm the phasing and rotation in early stage, never rely on visual connections to assume phasing (as there was no drawing or info available).
Could you please send me links to literature about phase rotation and how to draw it?
Many thanks for your responses. It really help to confirm our understanding.
RE: Two LV bus bars with Transformer Different Phasing / Phase Rotation
Also looks like may end up with a system that does not phase across the switch. This will force you to drop load before making the transfer. If correctly phased, you can instead do a make before break transfer in order to remove a transformer from service while keeping the load on.
I think you may be off track with your suggested differential relay reconnections. See my post above.