DC Short Circuit Calculations

[sberger3]

Does anyone here know how to perform short circuit calculations for the DC control for a substation? I am very versed on AC calculations using impedance of the trans. etc., but don't know where to start for DC. My thoughts are: since the battery strings for the substation consist of batteries in series, the short circuit would not exceed the capacity of a individual battery, but don't know for sure. Thanks in advance.
Sheldon

 

[roger]

The post above has been moved from another thread.

Roger

 

[zog]

Does anyone here know how to perform short circuit calculations for the DC control for a substation? I am very versed on AC calculations using impedance of the trans. etc., but don't know where to start for DC. My thoughts are: since the battery strings for the substation consist of batteries in series, the short circuit would not exceed the capacity of a individual battery, but don't know for sure. Thanks in advance.
Sheldon

There are some basic rules you can use to estimate battery bank S/C current, 1 of them is 20 times the 3 hour discharge rate. But really it is simply V/Z, but you need to know internal impedence of the cells which requires testing for an accurate value. The 20 times the 3 hour rate rule should suffice, why do you need to know?

 

[broadgage]

You need to measure or otherwise determine the short circuit current that the battery can supply.
This can not be accuratly determined since it varies with the state of charge, ambient temperature, and age of the battery.

Data can sometimes be obtained from the battery manufacturer, or if not available can be determined by measurement.

Fully charge the battery, and disconnect it from everything else.
Accuratly measure the voltage, then connect to the battery a large known DC load, such as a bank of lamps or resistance heaters, and again measure the voltage.
The voltage on load will be lower, and calculation will give the short circuit current.

For example, if with a load of 100 amps, the voltage drops by 5%, then the short circuit current is 2,000 amps.
No great accuracy can be claimed since some of the voltage drop is due to internal resistance, and some due to the running down of the battery.

Large batteries are potentialy dangerous, and proper precautions should be taken when making temporary connections, taking measurements etc.

If the battery is permanently connected to a charger, then the maximum output current of the charger should be added to the battery short circuit current, though the contribution from the charger is normaly trivial.

When selecting fuses or circuit breakers, a very large safety margin should be allowed, since the short circuit curent may increase due to changes in temperature, state of charge, or the subsequent fitting of a different battery.

Here in the UK fuses with a breaking capacity of 40,000 amps at up to 250 volts DC are readily available and should be suitable for almost any battery installation. (although these fuses are not UL listed, I presume that broadly similar products are available, UL listed in the USA)

 

[Besoeker]

Just to add a little to what that other Brit said.....

My experience is that the battery manufacturer should be able to give you a figure for battery internal impedance and cable resistance is usually readily available.

Bussmann make a range of fuses with a high fault interrupting capacity, 500Vdc rated and with UL recognition.

If the batteries are to be used in a substation I imagine they are for switch tripping circuits. If so, you may need monitoring for the status of any protection devices.