Short circuit analysis

[aqua1985]

Why are both the maximum and minimum short circuit currents required for relay co-ordination? Isn't the maximum short circuit current sufficient? And while computing minimum short circuit current, do we neglect only the motor contribution or are there other parameters to be ignored?

 

[jghrist]

You need to have proper coordinating time intervals (CTI) at all possible fault currents. Usually, the CTI is smallest at the highest current, but not always. You also want to make sure that the minimum fault at the end of the line will be interrupted. Minimum faults may include some fault resistance. There may be cases where a source (generator or transformer) is out of service that would result in lower currents. Different network configurations (line switching) may result in lower currents.

 

[redfurry]

Maximum faults currents occur for:
- minimum source impedance
- all other infeeds present (motors, generators, etc)

Minimum faults occur for:
- maximum source impedance
- no motors, optional generators, etc.

Note that if you're doing short circuit calculations using the IEC spec there are a bunch of little adjustment factors you use including an assumption of higher line resistances for min fault conditions.

As jghirst mentioned, the simplest reason for considering minimum fault conditions is to ensure your protection can discriminate between minimum fault conditions and maximum load conditions.

Ian Dromey -

http://www.dromeydesign.com

 

[racobb]

In the US and if you are dealing with the REA, they designate the minimum fault as the insertion of 40 ohms into the circuit impedance. I have never subscribed to this approach because in reality you never can know what the fault impedance might be. Accordingly I try to make sure that the curves always have the proper separarion and let it go at that....In other words, I design for the minimum by default as do most people.

As jghrist said, generally you will probably loose separation at the high levels, but that is where you can usually count on the relay operating "dead on" on the curve.


Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[trosepe]

The minimum fault levels are needed to be certain you do not set any overcurrent device higher than the available short circuit current. This is espesially important for arc falsh considerations. If you set an instantanoeus element highr than the circuit can generate during a fault, the I2T energy increases significanly. For Arc Falsh studies, the minimum short circuit values are really more important than the maximum.

 

[pwrtran]

There are tons of reasons you need to take both maximum and minimum fault into account. Some examples:
1) equipment sizing - must capable to withstand or interrupt the maximum fault
2) relay setting - one of all aspects is you need to look into the minimum fault vs the maximum load
3) arc flash - minimum fault results longer clearing time, thus often leads to a higher arc energy

 

[PHovnanian]

Aside from the "minimum fault" level used for arc fault analysis (which probably should be referred to as "maximum power"), a minimum fault level should be considered below which motor starting, transformer and line energization and cold load pickup lie. This will ensure that undesired tripping does not occur due to these events.