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.

 

[waross]

If you are over-correcting the power factor of the motors it may be over-voltage.
In the dim days of antiquity when bulk PF correction was the norm, too much capacity on-line when a plant was idle and the major load was lighting, would lead to over-voltages that would burn out the lamps.
With only four motors, you may be better served by correcting the PF of each motor individually.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[SA07]

The panel is under-designed for the loads. We have installed the capacitor banks to reduce utility bills due to max kVA demand when we import from the utility after a stoppage. Just before synchronizing with the utility we operate only 1 pump. After synchronizing we run the 2nd pump and the 2 fans.

Pump 1 350 kW
Pump 2 350 kW
Fan 1 132 kW
Fan 2 132 kW

In 2020, a pump winding has burnt. Can this cause the capacitors to break down?

 

[cranky108]

Over voltage, age, physical damage, contamatiants, and other. The capacitors should be tested from time to time, for leaks, and change of capacitance.
Also fuses can age.
All electrical equipment requires maintenance, just like the motors need.

 

[EdStainless]

I am with Cranky on this, once one cap starts to leak and run hot the whole set suffers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[thermionic1]

Do you have any power meters on these caps or the MCC that you can provide data from? Can you install a temporary PQ analyzer while the before sync / after sync switching occurs?

 

[esperantes]

What is the age of the installation? Caps last anything between 8-10 years.
Esperantes

 

[SA07]

Installed since 2018. We will install recorder on MCC.

 

[waross]

Are the capacitors online when the motors are not running?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[SA07]

Yes it may happen for a short during. However there is an automatic controller on the panel.

 

[esperantes]

Aging is therefore ruled out. I used to work for a very famous UPS manufacturer. Excessive heat build up is one of the common cause of caps failures. We used a thermal imager to detect deviations in the operating temperature of a capacitor. An increase in the internal temperature is a tell-tale sign the capacitor is starting to fail. Electrolyte leakage also increases temperature and impedance of caps.
Esperantes

 

[cranky108]

Because some oil filled capacitors have internal fuses, a thermal imager is used to detect failed devices. A simple capactance meter can be used to test the can value, what can change as the oil degrades.

Needless to say, be careful with the thermal imager, as capacitors in the sun can give false readings.

 

[EdStainless]

It takes special meters to measure both capacitance and leakage.
Just because caps are new doesn't mean that they match each other.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SA07]

Can we install surge protector or other devices to protect the capacitors in the panel? What type? Can you plz suggest some suppliers?

 

[che12345]

Dear Mr SA07
S1. "... We have 3 capacitor bank panels installed on 3 auxiliary transformer loads.....Panel 3 690 Vac 300 kVAR.... 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...in an air-conditioned room,..."
C1. a) Check all three-phase [voltages] across the capacitor terminals when the system is lightly loaded, or say at night. The measured voltage shall not > the Cap rated voltage,
b) Check all three-phase [current] at the capacitor terminals when the system is under normal operation loading, or say at 9 am and 3pm. The measured current shall not > 1.3 times the Cap rated current, with service voltage is within 1.1 U[sub]N[/sub],
c) If Icap > 1.3 times while voltage is within 1.1 U[sub]N[/sub]; check the system PQ for possible harmonies,
d) If the board is installed with a pf controller, prolong the delay time between switching in from one step to the next step.
S2. "...Can we install surge protector or other devices to protect the capacitors in the panel? What type? Can you suggest some suppliers? ...."
C2. a) Insert inductor in series on each line of the capacitor, if due to excessive harmonics,
b) Use [contactor for capacitor switching] or coil the three capacitor leads 5-6 turns of diameter 200mm = air-core inductor may help if due to switching or back-to-back switching surge.
C3. Refer to IEC 60831-1 or later issue for further details.
Che Kuan Yau (Singapore)

 

[SA07]

Hi Che Kuan Yau

Thanks for advice.

As explained,this panel is always fully loaded. It is under-designed just for 1 motor 350 kW at Plant start up. After synchronizing our alternator with the grid, we do not really need the capacitors.
There are inductor in series with the capacitor. Plz see attached drawing.

 

[SA07]

Drawing

 

[SA07]

Drawing

 

[SA07]

Drawing

 

[SA07]

Drawing