Fault Current Calculation with CLIP

[x05sween]

When calculating the three phase fault currents in a power network, should a protection device be considered in limiting the current? Let me explain a little more.

At the power station, there are 12 generators that are connected on two separate buses (6 on each) that are inter-tied by a CLIP (current limiting protector). This CLIP works as a fuse and will operate at, lets say, 3kA. This will reduce the fault current at the power station and throughout the network. The clip can also be disconnected in order to operate as two independent buses. There are 2 feeders connected on each side of the bus and do not connect with each other downstream.

So when performing the maximum 3 phase fault calculations, should I be using the fault contribution for only the generators on the same side as the feeder? I am concerned because a busbar downstream is rated at 30kA for 1s and is providing a fault level higher than this when using the 12 generators. I have never previously considered protection devices eg. fuses/CLIP in fault calculations as the calculation is performed first, then the protection devices are chosen to limit this.

Are there any reference to standards that you can think of that might make this clear? My calculations are for Australia, but I have been using IEC 60909 as this is the calculation method in software package PowerFactory.

 

[dpc]

I have worked with a power plant in the US that installed a CLIP device. I don't think you are going to find the answer in any code or standard. I talked quite a bit with the supplier of the product about the issue of downstream breaker ratings. They insist that the device will clear the fault before the first current maximum and will protect the breakers even in terms of the Close and Latch rating. For this facility, this was about the only practical alternative to shutting down the entire facility and spending a large sum of money to replace all of the switchgear. My conclusion was that the CLIP device would protect the breakers IF it operates as promised by the supplier. I suggest you talk with the G&W engineers. My hope is that it never operates and that I never learn if I was right or not.

 

[x05sween]

Thanks dpc. It looks like it comes down to engineering judgement. I have requested some curves of minimum melting and maximum clearing time that should help model it. I hope when they sell their product they are guaranteeing the CLiP will provide stated protection in the occurrence of a fault.

I've found the following in IEC 60909-0 section 2.4 Maximum short-circuit currents;
"When calculating maximum short-circuit currents, it is necessary to introduce the following conditions:
- choose the system configuration and the maximum contribution from power plants and network feeders which leads to the maximum value of short-circuit currents at the short-circuit location, or for accepted sectioning of the network to control the short-circuit current"

Does your interpretation of that include using a CLiP to separate the buses to control the short-circuit current?

 

[dpc]

I think it fits the second part of that sentence.

 

[wroggent]

Isn't this very similar to a series rating?

 

[stevenal]

More like the opposite of a (tested) series rating. Manufacturer claims you can use these devices with downstream breakers on the basis of ratings alone, and absent any series testing.

 

[rcw retired EE]

Slight difference between series rating and using the CLip. In a normal series rating test, the interaction of the arc impedance of the two breakers or breaker and fuse is being tested. The downstream,l ower rated device may add enough arc impedance to reduce the current below the current limiting range of the upstream device resulting in a failure. The upstream device needs the full current to help open the breaker or blow the fuse.

The Clip uses an explosive device triggered by a fast overcurrent or rate of rise sensor. It monitors the current and triggers the explosive charges to blow the bus bar apart when the di/dt and the current magnitude indicate a high level fault is occurring. The Clip is not affected by downstream arcing impedance and can clear before most devices can respond so there is little interaction between it and other components.