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Voltage Monitoring Relays - what delay?

AJJewsbury

There's been some talk recently of using voltage monitoring relays (e.g. https://uk.rs-online.com/web/p/monitoring-relays/1026131 or https://uk.rs-online.com/web/p/monitoring-relays/0122215), and a suitable contactor, to protect an installation against long duration overvoltages (so not brief surges, more like a single phase installation being fed something approaching 400V due to a broken N in the 3-phase distribution system).

I can see pros and cons to such an approach, but let's go with it for now for the sake of debate...

What I've noticed that all these devices seem to come with a programmable delay (sometimes overridden for large voltage errors, sometimes apparently not) which implies that the installation may have to withstand an overvoltage for some period of time - so the question is how long would we expect things to survive? I can see simple resistive heaters hanging on for a a fair fraction of a minute before overheating becomes catastrophic, filament lamps I suspect won't last anything like as long. What about electronics? or small single phase motors?

I suspect we're treading a fine line here, too short a delay and it'll be tripping out on the slightest glitch (next door's storage heaters switching on, or I suppose these days heat pumps, or a brief fault in some other installation connected to the same distribution system) which wouldn't be ideal even if the installation is only disconnected for a few seconds before being automatically restored. Too long and I presume the risk of damage increases. Where's the happy medium (is there one?)

  - Andy.

Parents

  • The OV/UV thresholds and time delays presented in G98 and G99 would be a good start.

    If the installation has generation (including PV) that can operate in parallel with the grid, then to 'trip' the installation "inside" these limits is likely to contravene ESQCR and any connection agreement.

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  • The OV/UV thresholds and time delays presented in G98 and G99 would be a good start.

    If the installation has generation (including PV) that can operate in parallel with the grid, then to 'trip' the installation "inside" these limits is likely to contravene ESQCR and any connection agreement.

Children
  • in reply to gkenyon

    For those not so familiar. the standards are freely available here G99

    And here G98

    The limits in question for connections at 230V are the same in both standards, but presented slightly differently.




    and

    Personally, I'd like to see a much faster trip time for over-voltages  like 400V appearing where 230 ought to be, as at that point things like the capacitors in most designs of SMPSU will be letting it all hang out within one cycle period of the mains,   but that's my electronics background at work.
    Even at 270V smaller conventional transformers and ballast chokes wound for 230 are likely to be saturating at peak flux, B being proportional to dV/dt and all that, so some very unpleasant currents are likely to be flowing and some very unhappy buzzing noises ensuing - but the damage there involves heating thermal masses of grams to kilos of metal so to survive a few seconds is realistic. Note that for inductors  the most destructive combination is low frequency and high voltage - which does not usually occur together in a conventional rotary generating system.

    Note that surge arrestors typically clamp voltages to well over this sort of level, between 600 and a thousand volts or so but that is much shorter duration, and  the worst has all happened and subsided in a few tens of microseconds, and on that timescale the natural inductance and capacitance of mains wiring tends to limit the energy reaching the most delicate victims (normally the semiconductors, with sub-millisecond fail times for over-voltage).

    Mike

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