MV Switchgear Rating 2

[markjp]

Hi, I have a question regarding switchgear hopefully some one can assist please.

We have a project to install a 3-way switch RMU (ring main unit) in an installation with the following power system details:
11kV, 200A maximum load, protection trip setting 200mS, Fault Levels: Ik"=21.7kA 3ph, Ib=13.6kA 3ph, Ip=41.8kA 3ph.

We are looking at the ABB Safelink RMU as a cost effective solution and the specifications given for this unit are:
12kV, 630A Rated Current, Withstand Current 20kArms/3sec, Internal Arc Fault Withstand 20kA, Load Break Switch Short Circuit Making Current 50kA peak.

I am unsure whether the ABB unit meets all the system fault level criteria, in particular the 20kA rating of the ABB unit is less than the 21.7kA system fault level. The I2t of the ABB unit (20kA/3sec) will be greater than the system I2t (21.7kA/0.2s). So does this mean the ABB unit meets the installation power system requirements based on the I2t or other considerations needed.

I use the I2t criteria for cable fault rating, however I'm not sure whether it can be applied to switchgear. I normally specify MV switchgear with kA rating higher than the system fault level, however in this installation the ABB unit is a very cost effective solution for us so I would appreciate any feedback on this issue.

Thanks.

 

[Marmite]

Firstly, are you sure about the fault level calculation, as it is very high for an 11kV network?
If it is correct then your choice of distribution type switchgear will be limited. You can either attenuate the fault level (eg by laying cable)or use primary type switchgear with a higher fault rating.
The Ringmaster range from Schneider has a 21kA rating.
The I2t rating is for through faults. If the switchgear is rated for 20kA then it shouldn't be installed at a location on a system where the fault level is 21.7kA.
Regards
Marmite

 

[ppedUK]

Your fault level of 21.7kA would be quite high if you were on a utility supply at 11kV. And no, this switchgear is NOT suitable for a system fault level of 21.7kA, but I guess you already knew that. You would normally choose equipment with a fault rating at least 10% higher than your system fault level.

Don't confuse Let-Through Energy I2t with the short-time withstand rating of switchgear. You cannot extrapolate the rated withstand current and time for higher fault levels. It is 20KA MAXIMUM for up to 3s. The equipment has to withstand mechanical forces under fault, not just thermal stresses.

Also your protection trip setting of 200mS is not the whole story either when considering I2t, you need to add in relay errors, breaker opening time and arcing time for the upstream breaker and this will give you the overall fault clearance time.

It is also worth pointing out that the internal arc fault withstand time will be a lot less than 3s and you need to take this into account when assessing your upstream breaker fault clearance time if the equipment is in a location close to areas accessible to the public, or located in say a vulnerable area of the plant.

R.M.U.s are rarely rated at anything more than 21kA (most are lower) and even the Schneider 21kA unit Marmite suggested isn't adequate for your application. You need to consider more robust types of switchgear with a suitable fault rating, but it will come at a cost.

 

[ScottyUK]

I've recently been looking at MV switchgear for an installation with unusually high fault levels - as the two other contributors know - and there isn't a huge choice. You are up into circuit breaker territory at that fault level. ABB have the VD4 which is good for 31.5kA and there are a few others. There are plenty of panel builders who can build (e.g.) a VD4 into their own design, so you aren't tied to (e.g.) ABB for the board.



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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

So, for us in the ANSI world, what is an RMU?

 

[ScottyUK]

Hi David,

An RMU is a 'Ring Main Unit' which is a piece of equipment common in British MV distribution systems. They are used in a double-fed ring circuit and normally comprise a pair of switches 'in' and 'out', and a switch-fuse as a feeder in a tee from the ring circuit, all within a common housing. They usually incorporate circuit earthing facilities. Here's a sketch in a Google Books hit - trickier than I thought to find one!

It is typical to operate the 'ring' with an open point which effectively makes it a pair of radial circuits. One advantage is that in the event of a fault on a ring circuit with many RMUs it that it is possible to reconfigure the switches to restore power to all customers fed from the Tee connections by isolating the faulted ring segment.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

So, you have utility owned circuits going through customer owned equipment?

We'll use pad-mount air switches and fuses for sectionalizing of underground primary circuits, but they are either utility owned or behind the meter when customer owned and used on customer circuits. The fused position(s) then generally feed pad-mount transformers in the utility owned case.

 

[markjp]

Hi all , many thanks for your valuable input.

The RMU is to be installed on a remote mine site that has it's own power station, there is no connection to the public network. We are comfortable with the fault levels provided. The power system has been modelled using SKM power tools by a competent consultant. The fault level at the power station has been calculated at Ik"=26.7kA, and the RMU is located about 800m from the power station.

The other RMU we were considering using is the Schneider RM6 which has the following specs: Switch rating 25kA/1s, Tank internal arc withstand 20kA/1s. Not sure if this would be suitable?

What if the incomers of the RMU had fuses, this should reduce the fault level seen on the RMU switch and busbar. Is this an accepatble method for catering for the under rated RMU fault rating?

 

[Marmite]

No. The fault level is the fault level. The straightfoward way to reduce it is to increase the system impedance.
Don't go down the path of trying to fudge things so that your cheap ring main unit will just about do the job, as to do so would be risking catastrophic failure and operator injury/death.
Regards
Marmite

 

[markjp]

Hi Marmite,

Agree with your comments the RMU should be fully rated, I was trying to combine 2 issues into the one post. I was thinking more about other existing downstream RMU's in the network which appear to be under rated. In LV circuits it is common to use fuses which limit the maximum fault current so that lower fault rated circuit breakers can be used downstream. So why can't a similar principal be used for HV equipment.

If we have a feed out of the main fully rated RMU equipped with a fuse then is it okay to have lower rated RMU equipment (totally separate unit located 25m away in this case) downstream provided that the fuse installed is actually able to limit the fault to a level below the RMU rating. Do you think this is feasible?

Regards
markjp

 

[Electic]

There was a product I investigated a few years ago marketed as a current limiting CLIP (some kind of acronym). This might have been very fast acting current limiting fuses in parallel with a current limiting reactor. Such a device could benefit your installation, though I have not had problems specifying switchgear (with PCB's) at higher levels than what you are addressing.

 

[oldfieldguy]

I serviced a few CLIP installations. They're NOT fuses. they're explosive devices that blow when current exceeds a certain value.

One of my clients had a set bypassing a set of reactors, the idea being that with the reactors out of the circuit, voltage drop issues would be minimized, and if there was a fault, a CLIP would open up putting that reactor into the circuit to limit current. Trouble came up when he had a fault and two CLIPs blew on the faulted phase and the third didn't. The resulting plant-wide voltage imbalance was a joy to behold...

Needless to say, he changed his CLIP logic.

Another hazard of the CLIP is that improper maintenance techniques can result in loud and expensive noises as the CLIPs react to test currents.

Still, they ARE an answer to limiting fault current.

Unfortunately it has been some years since I last worked around them and I don't remember who makes them.

old field guy

 

[oldfieldguy]

Oh, never mind... I 'googled' it:

http://www.gwelec.com/product_current/clip.cfm

More than you probably want to know...

old field guy

 

[markjp]

So does this mean fuses can't be used for fault current limiting?

Also can someone please explain what criteria is used for determining if the Internal Arc Withstand rating for switchgear is suitable for an application. From almost all the 12kV equipment I have reviewed the Internal Arc Withstand ratings are lower than the Short Time Withstand ratings. The application we have has 21.7kA fault level, the Schneider RMU we are considering has 25kA fault rating but 20kA internal arc rating, and the ABB unit has 25kA fault rating but only 16kA internal arc rating. How is the internal arc rating assessed? Thanks