Failure of Power Factor Correction Capacitors 1

[asymptote]

Hi Forum. I have been asked to comment on protection failures following an explosion which destroyed a 6.6kV 400kVAr PFC bank. I can explain the protection issues but would welcome some thoughts on the PFC failure. The site is connected at 33kV via a short 33kV cable from the 132/33kV BSP which has two 90MVA 132/33Yd TX's with YZ earthing TX and liquid earthing resistors. the 33kV fault Z is (100 MVA base) 0.87+j19.42 = 514MVA. On site there are 2 x 33/6.6kV 10MVA Yyo Z 10% with solid earthing operating in parallel. From the 6.6kV BB one OCB feeds via a short cable onto a bus bar cable end box, these bars have two outgoing cct's via OCB's which connect:
1) A 4.4MVA 6.6kV Auto/Star Moving Coil Regulator which is connected directly on to a 6.6/3.8/7.26 Yyo ratio changing TX Z = 1.62% with a 400kVAr Delta PFC solidly connected at the MCR primary. These PFC have now failed three times(over a period of 12 yrs) with suficient force this last time to destroy the building in which they were housed.
2) The second 6.6kV ocb feeds a similar circuit with a 1.9MVA 6600/100/550 volt Yy Moving Coil Regulator and 300kVAr PFC bank.

These MCR's feed the client test bay where a variety of motors can be tested. Due to a lack of understanding the complete system is normally left alive even though no test runs are planned. ie MCR's & PFC alive but unloaded for long periods. Simple plant failure seems to much of a coincident to be realistic. A harmonic survey is in hand, system resonance and ferroresonace need to be considered and any direction in this respect would be welcome.

 

[1x1x1x]

Asymptote,

The parallel resonance frequency of your system can be estimated by the following formula

n = sqrt(Ssc/Qcap)

Based on the given data I calculated the approximately 144 MVA short circuit power @ the 6.6 kV bus
Therefore in your case n = sqrt(144/0.4) =sqrt(360) = 1439 Hz which is very close to 19th harmonics. You also mentioned that normally the capacitors are on the line w/o any test. If the 6.6 kV bus is unloaded and the capacitors are connected to an unloaded (un-damped) bus, the system impedance can be extremely high around the 19th harmonics (1140) Hz, which will cause severe 19th harmonic voltage gain.

 

[reactive]

Sorry to come into the discussion so late.

With one 10 MVA transformer in circuit resonance occurs near to 11th/13th which may be a problem depending on the harmonic sources on the network (you do mention traction nearby) especially as there is often no damping load. However if the transformers are always paralleled, therefore this is unlikely.

Are there current limiting reactors on the banks ? If not switching inrush from one bank to the other could be a problem.

Another potential problem is that the banks are being switched with OCBs which are prone to restrike when switching capacitors (and therefore not suitable for capacitor switching duty). This results in huge high frequency transient overvoltages.

What do you know about the sequence of events leading up to and during the failure (was there any switching, were all transformers in circuit, did it happen at night).

 

[jbartos]

Comment on the previous posting: The medium voltage power factor correction capacitors are usually connected in wye connection rather than delta connection that is used on low voltage levels. Reference: D. Beeman "1955 Industrial Power Systems Handbook"

 

[asymptote]

The conclusion to this event is the PFC expert (Dr. Eng)is of the opinion that the unforunate combination of PFC and light load was a 17th resonate series cct with the PFC's absorbing 76A @ 17th. The watch word seems to be take great care when considering PFC. You can not just "stick it in and hope"
Forum, many thanks to you all for help and interest in this matter. If there are any more devlopments I shall report them to you.