What are the causes of capacitor bank failure?

[SA07]

Hi
We are a power plant exporting to the grid. We have 3 capacitor bank panels installed on 3 auxiliary transformer loads.

Panel 1 400 Vac 300 kVAR
Panel 2 690 Vac 400 kVAR
Panel 3 690 Vac 300 kVAR

All capacitors in Panel 3 have failed and many fuses have blown. Panel 3 is connected to 4 motors; 2 fans and 2 pumps for a cooling tower.

Panel 1 & 2 are ok.

What can be the cause of all capacitor failure?

All 3 panels are connected to the same common 11 kV busbar through auxiliary transformers.

Can we add any devices to protect the capacitor e.g surge protector?

All 3 panels are installed in a room which is air conditioned. The panels were installed in 2017-2018.

 

[che12345]

Dear Mr SA07
Follow-up No.1
S. "... There are inductor in series with the capacitor. Plz see attached drawing...."

C1. Thank you for attaching the excellently designed circuitry and including detailed data etc.
C2. In sht 1: [Step 1 and 2] are 25kvar each, but Step [3-9] is 50kvar each.
Attn: by default, the regulator is programmed [1:1:1.... ] . Check the programming. In your case is 1:1:2:2:2:2:2:2:2.
C3. In sht 3 : The [sensing voltage] is taken from Y and B line. The secondary is 0-230V polarity shall be y and b to the controller. Check the controller manual for the correct [phase] for the voltage sensing. The voltage phase shall be observed without failed.
Attn: see also C4. below.
C4. In sht 4: The CT is shown on R line without indication of the location/co-ordinate. Maybe at the incoming, which is outside the scope of this board? Check the controller manual for the correct phase and ensure that P1-P2 direction and the S1 and S2 to the correct terminals of the controller.
C5. Check the controller manual the relative position of CT and VT. In most cases CT than PT.
Attn: The relative phase of CT and the phase of the PT shall be observed per the manual.
C6. Check the voltage and the current measurements per my earlier post. Check the running current against the Cap rated current and compare with the fuse current rating.
Che Kuan Yau (Singapore)