How is the voltage of domestic mains supplies controlled?

[Set Square]

Since I've had rather too many light bulbs 'popping' on turn-on recently, I decided to monitor the mains voltage by leaving my Brennenstuhl volt/amp/watt meter plugged into a socket.

Whereas it sits at just under 240v for most of the time, it has been showing quite a lot of variation - this morning getting up to about 248v for a while.

What are the acceptable limits? Is the voltage controlled locally in each sub-station - or does that just transform it down in a fixed ratio? The thought occurs to me that if power stations generate a constant voltage, the voltage arriving at domestic premises will vary depending on what load is being drawn en-route - and large fluctuations may occur when factories start up and shut down (not that there are any very close to me).

Can anyone throw any light on this subject?

[PhilÃÂÂÂ]

By telling the huge hamster to either slow down or speed up in his wheel

[John Rumm]

Depends on what you mean by acceptable... and to who? ;-)

It ought to be no more than 230 + 10% or less than 230 - 6%, so technically yours is borderline out of spec.

IIUC the transformers have multiple taps that allow a range of voltages to be preselected. The supplier presumably choosing an appropriate tap based on the length and average load of the circuit.

[Mike]

Maximum permissable is 253 volts.

Either there or on the pole top transformer if you have one.

Well it's fixed at installation. But most can be adjusted so that they can ensure the nearest house is under max and the furtherest over min.

< The

Far worse is adverts in Coronation St and times like that. Every kettle in the land suddenly coning on is far worse a load variation than any factory can manage. In fact the contracts with major industrial users of power insist they don't suddenly turn everything on or off in one go.

[Wanderer]

As far as you, the domestic customer is concerned, it happens at primary substations, (33kv/11kv) Current and voltage transformers are connected to relays that detect the load being drawn on the 11kv network and either step or step down the 11kv voltage. The transformers have multiple taps that can be changed on load.

When a line is lightly loaded, the notional supply voltage close to the prmary sub might be about 11,200, to say 10,800 right at the end of the circuit. As the load goes up, the notional voltage increases to allow for voltage drop in the circuit.

11kv to lv transformers normally have five off-load taps, which give a range +5% to - 5% of nominal 11kv. One of the skills in distribution engineering is to work out where along the circuit transformers need to be connected at say 11kv +2 & 1/2%, 11kv + 0% or 11kv -2 & 1/2%.

The theory is that your supply voltage will remain within statutory limits regardless of load on the 11kv system. Where it usually goes wrong is when load has gradually crept up over the years and the circuit hasn't been checked for voltage drop.

Well, you did ask..........

:-))

[John Rumm]

Interesting stuff...

So is this where they would tinker with the voltage if they wanted to shed some load, or would that be done further back in the network?

[kmillar]

Upper limit is 253Volts, although most lightbulbs will burn out quickly at that level.

In the UK the limit is 230 (stated voltage) plus 10%. In my opinion this is an unrealistic level - it gives far too much leeway to the suppliers.

-Km

Set Square wrote:

recently, I

voltage, the

factories start

[John Rumm]

Errm, spotted any inconsistency in those two statements by any chance ;-)

[Wanderer]

Yup, stage 1 and stage 2 load shedding (3% and 6% voltage reduction) is done these days by telecontrol from central control rooms, by making the system transformers lower the tapping. It happens - or used to - much more frequently than you might think, especially during winter months.

Stage 3, rota disconnections - 3 hours off 3 hours on - is also largely done by telecontrol, something we haven't had for some years now.

[Andy Burns]

No, I don't see what you're getting at! 230 + 10% = 253?

[John Rumm]

Yup, brain fart, unable to multiply by 1.1 (somehow came up with 248... think I will go lie down in a quiet room)

[Set Square]

This is really what I was getting at - even if my question wasn't explicit enough. I was interested in knowing whether any automatic *dynamic* adjustments are made - as opposed to leaving transformers on fixed tappings, once installed. It sounds as if dynamic adjustments *are* made.

Could this account for the momentary outages which occur from time to time? Could these be caused by a transformer switching to a different tapping to adjust the voltage?

[Paul King]

Yes. Those dips are called "brown-outs" and are caused when the transformer tappings are changed. Since the switching contacts are "make before break" (otherwise you'd get a very momentary blackout) the transformer experiences a momentary "shorted turns" episode. This causes *HUGE* currents to flow within the (shorted) transformer windings, creating the brown-out.

[Wanderer]

Nope! System transformers are fully capable of changing tap on load with no discernable effect for the customer (other than an increase or decrease in supply voltage). It's a while now since I had anything to do with system transformers, but if memory serves, Ferranti Transformers first came up with a surge divertor mechanism a good few decades ago.

The dips or 'brown outs' are usually caused when automatic switchgear usually operating at 11kv or higher, clears a fault on a circuit that is

*electrically* close to the circuit feeding you, i.e. normally on an adjacent feeder connected to the 11kv busbars at the primary substation.. The fault level or 'available energy' to be dissipated into the fault and the 'closeness' of the electrical interconnection will usually determine how much of a dip you see.

Here's a poser for those interested. Two primary substations, with 11kv inter-connected radial circuits, normally with an open point between them. System transformers can sense load and adjust voltage accordingly. What might happen when the open point is closed, say to carry out maintenance?

[Andrew Gabriel]

Not for at least 50 years (the age of some of my supply distribution books). When mechanical switches were used, a second coil allows for smooth switching with no shorting. SCR's have been used for on load tap switching for perhaps couple of decades now, and their diode properties mean there's no shorting current even without the secondary coil. They do zero voltage switching in any case (not that this mechanism is used to prevent shorting).

[Stefek Zaba]

Yeah, but at least it was an ISO-9001, DoD-STD-2167A *quality-assured* brainfart, right? ;-)

[John Rumm]

Eeeeek, not that please! (20 pages of design document, 150 pages of cross reference later ;-))