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Unbalance protection for an 48 MVAR ungrounded wye capacitor. suggestions.

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parramanuel

Electrical
Jan 21, 2012
6
Hello.
I work in an electrical utility as a protection engineer, and we are planning to put in service a 48 MVAR capacitor bank in a 230 kV Bus.



230 kV Capacitor Bank specs:
- 48 MVAR
- Connection: ungrounded wye 

- Each phase of the cap bank have 10 series group each one with 8 capacitor unit in parallel. 

- Each capacitor unit is 200 kVAR ; 13,28kV ; external fuse protected (20T)



 The protection scheme consists of two numerical relays (Main: ABB REF 543 and Backup: ABB DPU2000R). 
Each relay sense the line currents to the cap bank, the residual current and the 230 kV line to ground bus voltages. 


The IEEE standard establish that for an ungrounded wye banks is recommended to use a PT connected to the neutral of the cap bank with an residual overvoltage protection (59N) but we dont have that equipment, i only have the ABB relays to protect the cap bank.



I have read that is also possible to protect the capacitor bank for an unbalance condition using a numerical relay with a neutral overvoltage protection (59N) using the calculated 3U0 from the 3 line to ground voltages measured in the relay, so i can do that with the ABB REF 543 or DPU relay.



In other hand with only 1 unit out of service we have a overvoltage of 1.10 p.u in the remaining units of the group, this is because we only have 8 capacitor units in parallel, so the unbalance protection it has to be adjusted with a sensitive setting.



I really apreciate any comment or suggestion about this topic. 


 
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What type of switch are you using? For 230 kV, most are rated for grounded wye capacitor banks, not ungrounded.
 
The capacitor bank is designed for an ungrounded wye connection, so I suppose that the breaker is capable to work with any trouble, but i'm going to doublecheck that.
I know that the Breaker have a ABB Switchsync that helps to reduce the switching transients.

Regards.
 
Hi parramanuel,

Where I work, a capacitor bank of that size would be arranged as a
double star and fitted with duplicated neutral unbalance protection
(CT installed between the star points).

I don't see how a "3U0" calculated from the line voltages will help, since
the calculated value will be a function of the feeding source rather than
related to the health of the capacitor bank.

If your relays are smart enough, you could perhaps compare the measured
line current to a value calculated from the line voltages. A difference
suggests that the capacitor bank impedance has changed (fault condition).
I mention this approach for academic value more than anything else - I
don't think it will work because the changes in line current won't be
high enough to act upon decisively.

If a can fails short-circuit, are you assured of enough current to blow
the capacitor can fuse?

Do you know the internal make-up of the capacitor cans? Are the internal
elements in series or parallel?

Be sure to set a sensitive and fast earth fault element.

Good luck.

Submonkey
 
parramanuel,
In most places I have used capacitor unbalanced current (ANSI # 60) sensing for ungrounded cap banks and not the #59N voltage method and had used SEL-487V, COOPER iCP440 or
TRENCH-CPR04 relays. It is my preference that #60 is much more sensitive than #59N for ungrounded cap banks. Regarding the breaker you should confirm that (1) during
closing synchronized switching is required or not (2) during opening the bkr should be de-rated or not due to excessive TRVs.
 
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