SAFE RUNNING OF POWER CAPACITORS

A - GENERAL
Every capacitor has limits in voltage, current, and power handling capacity; these limits should not be exceeded. For every CELEM capacitor a data sheet is provided indicating the maximum rated voltage rms, the maximum rated current and the maximum kVAr the capacitor can handle. In selecting a capacitor for an application, the highest operating voltage and frequency should be chosen.
For example the CSM 150, 0.33 µF is rated at 700 volts rms, 250 Amperes and 150 kVAr. In examining the graphs it can be seen that the capacitor is ideally suited operating at frequencies between 150 to 250 kHz. This applies to any capacitors whose rated power handling capacity lies in the frequency range as shown on the graph.

If the capacitor is operated at reduced voltage or frequency the kilovars may be computed from the following relationship:

1. For reduced voltage
New kVAr = rated kVAr x (applied volts / rated volts)²
2. For reduced frequency
New kVAr = rated kVAr x (applied Hz / rated Hz)
3. The fundamental formula for kVAr
KVAr = V² x C (µF) x 2 x p x F(kHz) x 10-3


B - POWER CAPACITORS
The evolution of semi-conductors runs parallel with modern capacitors. Both are capable of handling large amount of power in relatively small packages. For correct operation, these components need suitable cooling and be properly terminated. With CELEM water cooled capacitors, the elements inside the capacitors are arranged to be cooled equally and handle equal power.
These capacitors will run trouble free for many years if the required water flow is maintained.
When conduction cooled capacitors are used, 4 items need to be addressed:

1. Cooling
The whole capacitor area should be in contact with the heat sink. The capacitor area should be covered with thermal conductive paste between the capacitor and the heat sink. This is especially required whenever the capacitor is working at its maximum limits. The paste should be applied in the same manner as with power semi-conductors.

2. Conduction losses
Most of the power capacitors are capable of supplying several hundreds of Amperes each. If several capacitors are connected to a common collector, it should be remembered that due to the skin effect, if there is not enough surface area, the portion of the bus bar where the current is collected might get very hot; even though it may be properly cooled.

3. Induction heating of the capacitors
If several conduction-cooled capacitors are assembled between two bus bars, the ones located closest to the output terminals may be induction heated. This practice should be avoided. The correct way is to mount the capacitors on each side of the bus bar or built between the bus bars a low inductance path at the current collector point (see drawing).

4. Stray inductance
Even if the capacitor bank is not connected directly to the work coil, it is a good practice to avoid stray inductance between the capacitor bank and the work coil terminals.
It should be noted that the inductance at a connection is proportional to is length, inversely proportional to its width, and inversely proportional to the distance between the bus bars.


C - CAPACITOR BANK LOSSES
A capacitor bank consists of:
· Capacitors
· Connections between the capacitors and the output terminals.

1. Capacitor losses:
Losses in the capacitors are extremely low, usually = 5 x 10-4 x the reactive power
Therefore, for a 1000 kVAr capacitor bank, the losses are approximately 0.5 kW.
2. Connection losses
In a properly constructed capacitor bank, the losses will be approximately the same as that of the capacitor bank, usually = 5 x 10-4 x the reactive power
Therefore the total losses of the capacitor bank and connections will be 1 x 10-3 of the reactive power


D - WATER FLOW REQUIRED TO COOL A CAPACITOR BANK
Most of the capacitors available to-day employing polypropylene as the dielectric will run safely up to 90° Celsius.
The temperature rise on capacitors built by CELEM is 40° Celsius above the temperature rise of the heat sink and, therefore, the temperature of the cooling water. In the worst-case condition, the temperature of the cooling water is not expected to be above 40° Celsius. Therefore, the maximum temperature rise allowed for the water is 10° Celsius, thus the heat sink temperature is 40 + 10 = 50° Celsius, and the capacitor temperature will be 90° Celsius. This is the maximum allowed temperature. In practice, however, with cold water passing through the cooled heat sink the capacitor temperature will not exceed 40° Celsius.
As previously discussed, in a 1000 kVAr capacitor bank; the total losses are 1 kW. Therefore, for a 10°Celsius rise (18°Fahrenheit) the water flow required is:

in the CGS system LPM (litres per minute) = 14.36 x kW/°C = 1.44 LPM
in the ILS system GPM (gallons per minute) = 6.8 x kW/°F = 0.38 GPM
This of course is the minimum water flow required.

Whenever the capacitor bank is cooled in series with the work coil it must be remembered that the losses of the work coil are at least 10 times that of the capacitor bank.
Therefore, the capacitor bank should be cooled first followed by the work coil.