Synchronizing generators through stepup transformer 8

[Mark Lam]

Hi Guys!
we are doing synchronizing 02 gensets. one is 400v, the other one is 6.3kv genset. we use a step up transformer 400/6.3KV/DY11 to step up the voltage to 6.3kv and doing the synchronizng at 6.3kv side. Attached is single line diagram. The synchronizing controller is easygen 2300. when the controller close the 6.3kv breaker to syn. the 400v breaker at 400v side tripped immediatly.
so, i am thinking about the fault can come from the transformer with the questions as below:
1. The Step up transformer 0.4/6.3kv/DY11 is ok for this application?
2. the phase angle shift between primary winding and secondary winding of the transformer affected to the system when synchronizing?
3. the winding connection of the transformer need to match the winding connection of the gensets (both LV and MV side)?
Can you please give me advices for this case?
Thank you in advanced.

 

[edison123]

Tripped on what? Overcurrent? Differential? Earth fault? Reverse power?

The first rule of synchronization is that the phase sequence of all the machines and the grid, if any, should match at the paralleling point.

I would close the 400 V breaker first and then use the 6.3 KV breaker to synch to the bus. I don't see any CT's and PT's in the SLD.

Muthu

http://www.edison.co.in

 

[Mark Lam]

hi edison,
the lv breaker trip due to overcurrent.
the phase sequence is ok at hv side, we checked already.
the 400v breaker is normally close. then we syn 6.3kv breaker to the bus as you mentioned. the CT's and PT's all are connected from 6.3kv swithgear to the syn controller.
i am just thinking about whether the fault come from the transformer?

 

[edison123]

Don't just go by PT secondaries for phase sequence. I have seen PT secondary phase sequence match but the primaries did not with disastrous consequences. I personally use the same PT with the same phase sequence meter connections by noting the generator phase sequence first, isolating the generator, then back feeding from the bus/grid and see if the sequences match. There are also other methods to check the phase sequences.

If you suspect the transformer, test it first. If trafo is ok, then synch the 400 V generator with the step up trafo first. If the same OC trip happens, then there is an issue with the gen/trafo. If the LV gen synch goes ok, then you can synch the 6.3 KV generator second.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. Mark Lam (Electrical)(OP)14 Jun 22 03:07
"...#0. .we are doing synchronizing 02 gensets. one is 400v, the other one is 6.3kv genset. we use a step up transformer 400/6.3KV/DY11 to step up the voltage to 6.3kv and doing the synchronizng at 6.3kv side.... The synchronizing controller is..... when the controller close the 6.3kv breaker to syn. the 400v breaker at 400v side tripped immediatly.
i am thinking about the fault can come from the transformer with the questions as below:
1. The Step up transformer 0.4/6.3kv/DY11 is ok for this application?
2. the phase angle shift between primary winding and secondary winding of the transformer affected to the system when synchronizing?
3. the winding connection of the transformer need to match the winding connection of the gensets (both LV and MV side)?
#0. The usual notation is with [upper case lettering for HV] and [lower case lettering for LV]. But you stated 0.4/6.3kv/DY11. If 0.4kV is delta and 6.3kV is in star, it should be
Yd 11.
1. Yd 11 is NOT ok. Should be a Yy 0 or a Dd 0, of the same vector group. Attention: the PT should be of correct polarity.
2. Yes
3. For synchronization, observe the following:
a) same vector group,
b) same phase sequence A,B,C ,
c) about the same voltage and with correct polarity at the PT secondaries ,
d) about the same frequency ,
e) two (same phase voltage curves) must be [synchronized].
Che Kuan Yau (Singapore)

 

[Mark Lam]

Hi Che Kuan Yau and Muthu,
Thank you very much both of you for advises and support.
Currently, we are doing the final check for the phase sequence, Step-up transformer and PT as well. after final check we will try to do the synchronizing. If still can not, maybe we need to change the step-up transformer to Yy0 vector group.

 

[edison123]

Mark

I don't see how a transformer's internal connections would affect the paralleling it to the bus. Transformers with many winding configurations are connected to the grid/bus without any issues as long as phase sequences, voltages and frequency match.

Good luck with your commissioning.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. Mark Lam (Electrical)(OP)15 Jun 22 05:09
"...Currently, we are doing the final check for the phase sequence, Step-up transformer and PT as well. after final check we will try to do the synchronizing. If still can not, maybe we need to change the step-up transformer to Yy0 vector group ...".
I would strongly suggest to engage a knowledgeable consultant to look into the problem. It is very very dangerous and damaging to the LV, MV breakers, Generators and the transformer. It is NOT the right place to trail and error , and hoped for the best.
Che Kuan Yau (Singapore)

 

[che12345]

@ Dear Mr. edison123 (Electrical)15 Jun 22 12:50
"...#1. I don't see how a transformer's internal connections would affect the paralleling it to the bus. #2. Transformers with many winding configurations are connected to the grid/bus without any issues as long as phase sequences, voltages and frequency match...".
#1. Consider a Yy 0 transformer. When the HV Aphase is at the peck, the LV Aphase is also at the peck i.e. in synchronism. Now consider a Yy 11 transformer. When the HV Aphase is at the peck, the LV Aphase had passed the peck. But the sym signal is issued when HV and LV Aphase are both at the peck. Actually the LV Aphase had passed the peck. Problem, same Aphase but they are NOT in syn.
#2. For a transformer feeding a load ( Not syn), the vector group is NOT important. The (same phase sequence) is to maintain the [same direction of rotation]; the voltage is to maintain the same current. The transformer does NOT change the (frequency), so the [motor speed] maintained the same.
Che Kuan Yau (Singapore)

 

[RRaghunath]

I still couldn't find where the PTs are located? If they are on either side of 6.3kV breaker, I must say, the trip is due to bad synchronising or due to too liberal synchro settings in the controller.
If one PT is on 400V side of step-up transformer and the other in the 6.3kV switchgear, then we are supposed to use L-L voltage (R-Y) of 400V PT and L-N voltage signal (R-N) of 6.3kV PT.

 

[Mark Lam]

Hi RRaghunath,
Thanks for your advises.
All the PTs are connected on HV 6.3 KV side.
The setting for synchronization on the controller :
- voltage different: 3%
- frequency different: -0. 1 - +0. 15 Hz
- Phase angle: -5 degree to + 5 degree.
We are checking for wiring of PTs, CTs, phase sequence...

 

[RRaghunath]

-0.1Hz to +0.15Hz - the allowable slip between the two machines seems to be 0.25Hz. Is this acceptable? I am used to seeing 0.05Hz. You need to see what is the worst case Synchronising power one machine could be pumping in to the other machine!
It doesn't seem to be the step-up transformer fault as if it were, the 400V breaker would have tripped as the generator terminal voltage is being built-up or you would not have been able to build-up the voltage to rated(you mentioned 400V breaker is normally closed).
Was it the first time the synchronisation is being attempted??

 

[Palletjack]

How are you going to compensate for the 30 degree phase shift in the DY11?.
Where are you taking the sync measurements? Before or after the DY11?.

 

[Mark Lam]

Hi RRaghunath,
The slip frequency is ok for 0.25Hz, sometimes we can set it 0.3Hz. If slip is 0.05Hz, then it is very difficult to synch in.
The 400V breaker tripped due to OC fault after synchronization happened.
Yes, it happened at The First time of commissioning.

 

[Mark Lam]

Hi Palletjack,
Yes, i am thinking about the 30 degree phase shift, we can compensate for 30 phase shift?
We placed the measurement after DY11.

 

[edison123]

che

By your 'theory', the only possible transformer connections for paralleling are y/Y and d/D with the same phase shifts, which is not the case in real life. So, wrong theory.

And thanks for letting me know that the transformer does NOT change the (frequency).

Muthu

http://www.edison.co.in

 

[edison123]

Mark

Have you tried to connect the 400 V gen/trafo to the 6.3 KV bus first without the other 6.3 KV generator?

Muthu

http://www.edison.co.in

 

[beyond86]

You must also check protection setting LV CB.
Do you have protection devices which can make record of currents/voltages? If yes, upload records to this topic. These records will clear picture.

 

[che12345]

@ Dear Mr. edison123 (Electrical)17 Jun 22 03:0
".....By your 'theory', the only possible transformer connections for paralleling are y/Y and d/D with the same phase shifts, which is not the case in real life. So, wrong theory... ".
A1. I mean that yY 0 or dD 0 without any phase shift would be perfect sym at all times.
A2. Two generators with different vector group is going to be problem. If the LV sym signal is taken say Aphase-N1 and the MV signal is taken after the PT from Aphase-N2 with same vector group, there is no problem. Any other ways, with different vector group or taken from different say LV Aphase-N1 but MV is taken from Aphase-Bphase etc is going to be problem. Let us open this issue and listen to others' advice/opinion.

@ Mr. Mark Lam
B. Suggestion:
B1. check that "busbar" and the "incoming" syn voltages are correctly connected on the syn relay and metering (i.e. synscope, DV, Df ),
B2. Always Run and adjust the "incoming" Gen slightly higher speed=frequency and voltage than the "busbar" . The synscope will rotate CCW + direction. The DV and Df should read +,
B3. Syn with the synscope is rotating in the (+ rotation) when it reaches at [ O position].
B4. Both (LV and MV breakers) shall have [reverse-power protection] with [definite time or inverse time delay]. The trip (current) level and time delay is dependent on the type of generator drive.
Che Kuan Yau (Singapore)




















gen signal is connected to the NET, the icming Gen

 

[Palletjack]

Che
1. Consider a Yy 0 transformer. When the HV Aphase is at the peck, the LV Aphase is also at the peck i.e. in synchronism. Now consider a Yy 11 transformer. When the HV Aphase is at the peck, the LV Aphase had passed the peck. But the sym signal is issued when HV and LV Aphase are both at the peck. Actually the LV Aphase had passed the peck. Problem, same Aphase but they are NOT in syn.

Actually, depending on you HV configuration, the LV peak could be before the HV peak.