DC Capacitor Bank Protection

[jnemeth21]

I have an application where a bank of capacitors will be used to supply it's stored energy to a DC bus during load transients. The overall system configuration consists of a fuel cell that will be supplying DC voltage to an inverter via a DC bus. The inverter, in turn, will produce 480 VAC, 3-phase utility-grade power to a commercial building in parallel with utility grid power. During very large/sudden load surges (worst case being lost of utility grid power due to an unexpected fault) the fuel cell would have to spool up to the new operating point. The problem is a fuel cell cannot spool-up fast enough to support a minimum DC bus voltage that the inverter requires. Therefore, a capacitor bank would supply the needed transient energy. What is needed at this point in the design is over-current protection for the capacitor bank. I would prefer a CB as opposed to a fuse but I'm open to any suggestions. Here are some system data:

Max. Bus Voltage: 825 Vdc
Min. Bus Voltage: 580 Vdc
Peak Transient Current: 173 A
Max. Transient Time: 20 Sec
Peak Transient Power: 100 kW

 

[DanDel]

Since this is a DC application within a fuel cell design, you may not find any codes to specify your fuse.
I would look at the time-current curves for overcurrent devices in the voltage range you specify. You may have trouble with a fuse if you have a maximum current of 173A and that may last for 20sec. Do you need to limit the 173A to close to 20sec? Is there a lower current limit for a longer time interval, or is this the only limitation?
DC C/Bs with relay protection are available; you may have specify customized protection. If you provide more information, I believe I can help with the protection device specification.

 

[jnemeth21]

The 173 Amps is only peak at the beginning of the 20 second transient. It deceases down from there to zero at the end of the 20 second transient. What additional information would you need to recommend a DC CB? Thank you for your assistance. - John

 

[DanDel]

Overcurrent protection indicates that there is a limit to the current at which your circuit can safely operate during a specified time. Typically, a lower amount of current can exist for a longer time, and a higher amount of current for a shorter time. This describes an inverse time-current curve.
You must determine the initial maximum inrush (or discharge) current within a time frame of 0.004 sec, since a fuse may operate within this window. A C/B may require much longer operation, typically at least 0.05 sec. The discharge transient curve must be established, showing the current decrement from maximum to zero over time. This is would be the maximum operating curve.
A maximum continuous rating of current should be established, based on thermal damage to current-carrying parts.
A maximum fault current should be determined, above which no normal current flow condition would occur, and above which you would want the circuit interrupted with no intentional time delay.
Putting these together on a time-current graph, the protective device should allow current flow in the operating curve, but not allow current flow above the limits set by the continuous and fault ratings.
One other question: Why would you prefer a C/B instead of a fuse for protection?