Capacitor Bank Hazards

[dlmartin]

I am involved in a project involving the addition of capacitor banks (fixed & switched) to 14.4k and 7.2k distribution lines. We have done the research to properly size and place the capacitor banks. I do not have any experience with power capacitors and am very concerned with dangers associated with them. Can anyone point out the most common dangers with capacitor banks? Thanks.

 

[Skogsgurra]

There are several risks.

1 Voltage after switch-off. There shall be resistors or reaktors/coils(often primary of VT:s) that discharge the capacitors within a a minute or two. There shall also be warnings telling the operators not to touch the capacitors or the bus-bars before the time has elapsed and a grounding/shorting device has been attached.

2 Resonance risk. If you have a large proportion of controlled rectifiers or VFDs, there is a very real risk that you get a resonance between transformer reactance and capacitor at one of the harmonic frequencies that exist in the plant. If you know that there are variable drives or voltage controlled heating or electrostatic precipitators or anything that produces harmonics, please find a knowledgeble consultatnt to check the resonance risk. You may be without capacitors very soon if you haven't checked this.

3 Switching a capacitor is like connecting to a short circuit for a few hundred microseconds. The breakers are likely to weld together if they are not designed for this duty. You can also add current-limiting reactors to avoid this problem. The worst situation is when half of the capacitor bank has been swithed in and you switch the other half. There is not much to limit the switched current in such an instant. The first thing you see is probably the direct acting overcurrent (the >>I) tripping. That is a warning that you shall take seriously.

 

[RalphChristie]

Read through this for more information:

http://www.pppl.gov/eshis/ESHD_MANUAL/safety/es6.0.pdf

http://www.stephstuff.com/esafe/StoredEnergy.htm

 

[040358]

What protections you have applied for capacitor banks?
Generally following are used for capacitor bank protections.
1. Bank overcurrent protection
a) Using bank fuses or
b) Using O/C relays
2. Over-voltage protection
3. Unbalance protection

Capacitor bank protection equipment must guard against seven basic conditions:
1. O/C due to capacitor bank bus faults
2. System surge voltages
3. O/C due to individual capacitor failures
4. Continuous capacito unit over-voltages
5. Discharge current from parallel capacitor units
6. Inrush current due to switching
7. Arc-over within capacitor rack

 

[040358]

What protections you have applied for capacitor banks?
Generally following are used for capacitor bank protections.
1. Bank overcurrent protection
a) Using bank fuses or
b) Using O/C relays
2. Over-voltage protection
3. Unbalance protection

Capacitor bank protection equipment must guard against seven basic conditions:
1. O/C due to capacitor bank bus faults
2. System surge voltages
3. O/C due to individual capacitor failures
4. Continuous capacito unit over-voltages
5. Discharge current from parallel capacitor units
6. Inrush current due to switching
7. Arc-over within capacitor rack

 

[busbar]

 
You may want to obtain IEEE Std 1036-1992 …Application of Shunt Power Capacitors

“Guidelines for the application, protection, and ratings of equipment for the safe and reliable utilization of shunt power capacitors are provided. This guide applies to the use of 50 and 60 Hz shunt power capacitors rated 2400 Vac and above, and assemblies of capacitors. Applications that range from simple unit utilization to complex bank situations are covered.”

Listed at shop.ieee.org/ieeestore/Product.aspx?product_no=SS15644