UPS battery isolator shunt trip 1

[raithrovers1]

I was hoping somebody may shed some light on this problem:

We are supplying our customer with an industrial UPS system. The battery (450Vdc) isolator requires an external tripping facility from volt free contacts. The UPS also requires to be able to be black started from the battery supply alone.
We have recommended that the battery isolator is shunt trip. This allows the battery isolator to be closed (after a soft start)and the inverter to be started up when there is no AC power available. The external trip is from a set of volt free contacts. Closed for healthy, open for trip (failsafe). This means that we have had to derive a 24Vdc supply from the AC safe bus output of the UPS using an AC-DC converter to supply the shunt trip. This circuit then uses a relay in series with these contacts to initiate a 24Vdc shunt trip to the battery isolator if the contacts open.
Our customer is stating that a shunt trip battery isolator is not failsafe and therefore can not be used.
If we have to use a U/V coil on the battery isolator then it is extremely difficult to design a system of overriding the U/V coil when we need to close the battery breaker for black start purposes. The only way we can see is to derive the 24Vdc from the 450V battery and have an override switch which applies 24Vdc to the coil of the battery isolator. This would mean having to take extra cables from the battery which would have to be fused as close to the battery terminals as possible for safety purposes. It is made more difficult as the battery isolator is in an Ex enclosure and adding an override switch will be a pain. This solution would be very messy in our opinion.
My main question is if anyone knows of any standards that set down whether using a shunt trip is acceptable? I have designed and worked on many UPS systems where our customer has accepted this shunt trip arrangement before and I know there are numerous systems in the field using shunt trips.
How do I help my customer to understand that using a shunt trip is acceptable if possible?

UPS engineer

http://www.calibrepower.co.uk

 

[rbulsara]

Why fight the customer, when he is paying for it?. I do not think there is any standard. This type of decisions are application specific.

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

Ahh! Here's one standard my old uncle taught me: "Always strive to make the customer happy."

 

[ScottyUK]

You could argue - if you are of a mind to - that the protection systems for virtually all HV plant are energise-to-trip. This is usually because of the large power consumption of the tripping coils and the fact that they are short term rated.

If you go with the customer's plan then Schaefer in Germany make some excellent isolating DC/DC converters which will handle the voltage range you are looking for. If you're UK based then XP Power are the agents.


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

 

[raithrovers1]

The reason we were trying to persuade our customer to use shunt trips was that the system had already started to be built and it was shunt trips that were originally spec'd.
We do try and keep our customers happy (although difficult at times)and we have managed to get a design change and utilise the battery voltage for powering the U/V trip through an override switch.
It would still be interesting to find out more views on shunt trip versus U/V trip and what is actually classed as fail safe.


UPS engineer

http://www.calibrepower.co.uk

 

[ScottyUK]

'Fail safe' is always an interesting subject for debate, especially on emergency installations. What is fail safe for the UPS may cause greater danger by shutting down the loads it feeds. For example, killing power to a DCS in a hydrocarbon facility would probably result in loss of process control and venting of hydrocarbons to flare or atmosphere when the primary protection - normally relief valves - operates. Is that 'fail safe'? Another example would be a firewater pump, which are normally designed to run to destruction rather than protect the pump and watch the facility burn down.

'Fail safe' is often a case of minimising the overall risk rather than protecting individual items as much as possible: some items become sacrificial for the greater good.


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

 

[davidbeach]

Well said Scotty.

 

[burnt2x]

IMO, just ask for a worded logic from the owner on how he expects the controls should do and start from there! If the owner is not well-versed, you could do this by giving them a list of scenarios in bullets, and spaces provided for them to write what they really wanted.
You'll be safer with a document at hand should they opt not to sign-off on the project because they didn't get what they wanted to happen! It cuts argument time. Been there, it worked for me and maybe it will for you.