Sustained undervolts

We had a power cut the other day, which in itself is not that interesting. However after the initial loss of power we then had a period of about 40 mins of sustained undervolts where we were receiving a stable supply at about 80V. The EDF energy emergency line said it was a fault on an underground cable on the HV network affecting about 650 homes in to local area.

What sort of fault with a underground cable typically results in symptoms like that?

Reply to
John Rumm
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At a guess, one where there is crap interconnection with surrounding network !

You might have been towards the end of a "radial" feed, where the cable fault would normally support the network from the other end, i.e.

sub-------------lots-of-load-----------------you---fault---------sub

...where everything between you can the first sub was basically overloading the network causing volt drop at your location. Without the fault, the backfeed would keep the volts up, and act like a big ring-main :-}

Reply to
Colin Wilson

That did occur as a possibility, but I was surprised how stable the supply was at our end. I would have expected "lots of load" to be quite variable in nature, and hence the supply voltage to fluctuate somewhat.

Reply to
John Rumm

Remember that the 'help' line is staffed by non-technical people, who will come up with all sorts of rubbish on the grounds that the general public won't have much of clue about what's said.

Hmm, faults in underground cables on the hv system tend to be catastrophic in their nature - the faulty circuit is unusable until it's repaired.

Open circuit on an overhead system is a distinct possibility and would lead to three phase transformers single phasing, that could well give rise to the sustained and steady low voltage. A few minutes sketching out a vector diagram will confirm that, but you need to know that distribution transformers are connected Delta/Star hv/lv.

Reply to
The Wanderer

How the hell did I miss that it was a HV fault :-}

Reply to
Colin Wilson

By no means certain anyway. The quality of info from call centre staff is highly suspect.

Reply to
The Wanderer

Loss of one phase on the HV side and never believe what anyone tells you from a call centre;!....

Reply to
tony sayer

Hence the question really, since I was taking what they said with a pinch of salt! ;-)

However I knew it was unlikely to be a LV fault since I can see the sub station (which seems rather too glorified a term for what is just a tranny on a pole) from here and it only feeds the local group of about 8 houses, and judging by the lack of street lights in the distance from some directions it was far more widespread than just our road.

The first call to the call centre said HV fault, but did not specify any more detail (I assumed it was an overhead line at that point), only later did they add the bit about it being an underground cable, which seemed less likely given the typical HV infrastructure in these parts.

Reply to
John Rumm

Ah, yup that does sound more plausible. It would also tie in with the timescale. The "help" centre said that an engineer would be attending as soon as he finished the current job - i.e. there is no one there yet so they were just guessing! About 40 mins later the power went off altogether. Which would tie in with "ok, I am here now, oh look, half the wires are down, we might as well switch off that bit!"

Reply to
John Rumm

Interesting. We also had a power cut (5.5 hours) followed by sustained variable low voltage (192 - 225v) for 6 evenings, varying from 4 - 11 hours. On Sunday it was low from about 1230 pm to 5 am next morning. May not seem a great problem, but the central heating boiler would not operate below 220v (confirmed by Worcester Bosch).

EDF gave various reasons on the reasons for the low voltage (one being an apparent problem with a temp generator). It seems to me that the periods of low voltage corresponded fairly well with maximum load as one would expect. I wondered if the "fault" was perhaps deliberate to avoid straining a supply which had suffered in some way from the previous power cut, with low voltage leading to a lower current throughout the system.

The fault took 6 days to fix.

Reply to
Jeff Layman

How long was your power off, I reckpn It may have been a broken HV jumper, as the power did not trip completley.

Oh the Happy memories of fixing them in the crappy weather.

Regards

Steve

Reply to
Stephen Dawson

That's probably because you were switched to another hv circuit. Generally hv circuits are run as interconnected radial circuits, with numerous isolation points along the circuit. You may be 4-5 km along on your normal feeder, but 7-8km on a temporary hv backfeed, IYSWIM.

Highly unlikely. The hv system gets it's supply through system transformers that have automatic voltage control. As the load increases the system transformers push out more volts, could be upto 11.3 - 11.4kv. System losses and load affect the voltage along the circuit, so at the normal open point the notional voltage may be only 10.8kv. Distribution transformers can be set for a range of input voltages 0, ±2.5%, ±5% of nominal 11kv.

When you're on a long alternative backfeed, the volts start to run out somewhere past the normal open point, so you may be only getting 10.5kv.

Alright it doesn't help you with your c/h boiler, but at least you have some volts whilst the fault is repaired.

Reply to
The Wanderer

About seven hours.

It was wet and windy, but not as bad as it has been in the last couple of weeks.

Still I probably got the better end of the bargain being sat in front of the log fire playing board games by candlelight! ;-)

Reply to
John Rumm

Thanks for the info - explains it all very well.

Main grouch was that we have gas central heating (gas also from EDF), but had to resort to fan heaters when the boiler failed. Cost of latter is 4x former per KWh.

Reply to
Jeff Layman

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