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Topic Title: Voltage Optimisers
Topic Summary: Do they effect disconnection times
Created On: 09 August 2017 09:25 PM
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 09 August 2017 09:25 PM
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keithredpath

Posts: 543
Joined: 30 March 2002

If our maximum earth loop tables are based on 230 volts, what happens when you fit a voltage optimiser to the mains supply which stabilises the voltage at 220?

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keithredpath
 09 August 2017 09:45 PM
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geoffsd

Posts: 1601
Joined: 15 June 2010

The maximum Zs now is calculated at 230 x 95%, i.e. 218.5V.
 09 August 2017 10:20 PM
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mapj1

Posts: 9107
Joined: 22 July 2004

And as well as the voltage falling, the equivalent supply impeance will also change. You may imagine it would fall if the transformer were loss -free, as the whole point is that a higher current at a lower voltage is equivalent to a lower current at higher voltage. However, for all but the largerst transformer designs, the transformer internal impedances will likely add more to the Zs than is gained by transforming down the probably already low supply impedance.
An optimiser should not be added without thinking very hard about the effect on pssc and the disconnection times of downstream MCBs and so on.

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regards Mike
 10 August 2017 10:41 AM
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AJJewsbury

Posts: 15632
Joined: 13 August 2003

If our maximum earth loop tables are based on 230 volts, what happens when you fit a voltage optimiser to the mains supply which stabilises the voltage at 220?

A while back we explicitly calculated things based on the transformer's output voltage (Uoc) - which made sense to me as any voltage drop along the supply lines is accounted for by Ze. (If you used just R1+R2 etc and the nominal voltage at the consumer's end, you'd end up underestimating the voltage drop along the supplier's lines during a fault).

If the VO acts during fault conditions like a fixed impedance, I'd say continue to use 230V and account for the VO within your overall Zs values.

Some VOs I think attempt to by-pass themselves when overloaded - so if this happens during a fault too, Zs readings taken during normal conditions might well be erring on the side of safety already.

If however there's any chance that the VO might increase its effective impedance during fault conditions, then all bets are off (start thinking about RCDs)

- Andy.
 10 August 2017 10:53 AM
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broadgage

Posts: 2317
Joined: 07 August 2007

Some VOs do indeed bypass if overloaded, but I believe that this is a thermal trip based on the transformer temperature, so wont react quickly during a short circuit.

Also the ones that I have seen simply reduce the supply voltage by a fixed percentage, they do not regulate in any way.
Therefore if the DNO drop the voltage to about 220 volts as they are entitled to do, then the output could drop to only about 200 volts.
 10 August 2017 11:56 AM
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OMS

Posts: 22312
Joined: 23 March 2004

I've seen them with ferroresonant transformers. I've also seen them with control electronics to provide an anti phase component - so they are accurate to about 1% of set point

I've also seen them with what is effectively an invertor output accurate to about 0.25% of set point

That said, these are generally stable voltage systems rather than voltage optimisers based on basic auto transformers

Voltage stabilisers will usually have a delta primary winding to assist with harmonic problems as well

Regards

OMS

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Let the wind blow you, across a big floor.
 10 August 2017 10:26 PM
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westfield6

Posts: 204
Joined: 12 October 2007

Originally posted by: keithredpath

If our maximum earth loop tables are based on 230 volts, what happens when you fit a voltage optimiser to the mains supply which stabilises the voltage at 220?


If you do that you have been conned. 220 volts is not an optimised voltage in the UK. UK equipment was designed for 240 volts, though these days it seems to be more flexible.
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