Capacitor bank neutral voltage

[pwrengrds]

This is a ungrounded neutral capacitor bank tied to a 34.5kV main power transformer Y grounded. All the A phase fuses blew, not sure why currently. I'm looking at the event and the neutral voltage before the event was 1.2 kV < -178, likely due to voltage imbalance. After the event, before it tripped the neutral voltage as 2.7kV < -157.

From looking at the event phasors the angle should be be nearer to <180 and the voltage would be about 1/3 phase voltage to ground or 6.5kV. (the angle along the line of IA and IB, the voltage magnitude on a line between Vb and Vc)

The voltage was always balanced (20.2/20.5/20.5 kV), the current before the trip was 129.3/143.8/143.7 amps(14.5 amps lower on A phase which blew fuses). Post fault the current was 0/127.7/128.2 amps

Why is the neutral voltage only 2.7kV?
Neutral voltage is VAZ
The synchro-wave oscillography is attached in adobe.

 

[Kiribanda]

Just wanted to know how your cap bank protection is?
Could you please upload the SLD showing cap bank protection scheme?

 

[pwrengrds]

The fuses that blew were the A phase fuses, which caused a neutral over voltage, sensed by the SEL 487v, causing CS35-1 to open. I added a 3 line to the attachment with notes.

 

[RRaghunath]

If the event had occurred during switching on, it could be wrong fuse sizing that caused fuse to blow on charging inrush.
If it happened when the system was in service, a problem with Surge Arrester in A-phase could have caused the fuse to blow.
Low current in A-phase could be suggesting a problem in some of the elements in the A-phase Capacitor. It could be that there is a developing problem in A-phase capacitor that may be responsible for fuse blowing. The problem could be transient in nature at the moment, which may turn in to a permanent one as the time passes. It may be appropriate to thoroughly test the A-phase capacitor to find more.

R Raghunath

 

[pwrengrds]

The controls had some issues with cycling the cap bank, that likely caused the trip, also could be harmonics on energizing. That's separate issues which will get resolved. In this case it's in relatively steady states for pre and post faults.

My question is much more basic. This isn't critical, I just would like to understand why the neutral went to where it did.

The attached shows the cap running, an event that causing all the A phase fuses to blow on a capacitor bank. Now it's single phasing, just phase B and C. The neutral of the cap bank floats, monitored by Vaz. The source voltage is a grounded Y MPT at 34.5kV. The neutral voltage goes to 2.7kV. Looking at the phasor diagram (provided on attachment) I'm not understanding why it's only 2.7kV. It looks to me that it should be closer to 10kV. Look at the currents and draw a line, and the voltage and draw a line, it crosses at 10kV <180.

 

[7anoter4]

In my opinion, the capacitance of the A branch was a little less than in other phases.
That is the sign of one cell out off work.
However, since the fuse did not blow, then the voltage was still supplied for this cell, too.
A “sick” cell under voltage can short out and blow the fuse.
Ground current in pre-short circuit was 0.1132 A and after short circuit 0.4 A.
Then, if before the short the ground voltage was 1.117kV after it must be 0.4/0.1132*1.117=3.95kV [in my opinion of course]

 

[7anoter4]

Extracting more accurate data from attached diagrams it seems the grounding current before short was 0.1454A and after the short 0.3262 A. Then the after the short VAZ=1.17047*0.3262/0.1454=2.6259 kV

 

[Parchie]

@pwrengrds,
I punched in your capacitor values as well as the series reactor into my spreadsheet and noticed your capacitor voltage rating as under-rated. When designing, the rating of the capacitor should be above the system voltage, considering the voltage fluctuations as well as the peak value of the voltage waveform. The voltage across the capacitor plus the induced voltages from possible harmonics I calculated was 37.7 kV. The fuse rating was correct though (RMS current was 141 A passing through the capacitor with 3rd, 5th, and 7th harmonics present).
I suspect your capacitors failed because harmonics were present in the system and induced more voltage than the capacitors were capable of withstanding. With 3% harmonics on all 3rd, 5th, and 7th harmonics, the additional induced voltage is about 1.8kV per phase. Assuming the MCOVs didn't fail, your reactor-capacitor setup should be evaluated, IMHO.