Installing a generator

[Ron Lowe]

But there must ber some kind of import / export metering. That would know.

Hmmm... How does import / export metering work?

I've managed to get my head all confused now. Thanks, guys! Here's my problem...

I have a pair of wires, carrying AC power from a generator to a load. So current flows in one direction, and then the other, alternating 50 times a second. The magnitude of the current is a function of the voltage and load. ( lets just stick with resistive loads for now. )

Now, I stick a black box in the middle.

4 terminals. 2 wires in, 2 wires out.

How can that box determine the direction of energy flow? Clearly, energy is moving from the generator to the load. But electrically, how can I determine the direction of energy ? The box can measure direction of current, but that does not help.

[Ron Lowe]

I guess it must be to do with the phase relationship between the voltage and current.

The generator and grid are in voltage phase ( Remembers EE lecture 20 years ago...the grid,being infinite bus bars, will hold the generator in sync ).

When the grid supplies energy to the consumer, the current in the L will be towards the consumer whilst the V is +ve. ( resistive load ).

When the consumer supplies energy to the grid, the current in the L will be towards the grid when the V is +ve.

Needs further thought for 3ph, but principle probably applies.

[tony sayer]

In article , Roger Mills scribeth thus

I think theres seems to be some confusion here between geniis that provide standby power in case of mains fail, and that requires a changeover isolating switch so that gennie cannot be connected to the mains so as to backfeed it..

The other is where your generation feeds into the mains like say a wind farm..

Begs an interesting question suppose theres a fault on a HV 11 kV local network where one of theses is feeding back, or just the one turbine like that have at the dogs home near Godmanchester how does that -know- when the local system is down when its feeding the supply?....

[Andrew Gabriel]

They are required to detect loss of mains supply and immediately cease generation. I suspect this is done by the part of the circuit that monitors the mains phase and synchronises the inverter output with it.

[Roger Mills]

No, there's no confusion - I know they're not quite the same - which is why I introduced it as "an aside".

The precisely the question I and others have been asking above - but Andrew G seems to have come up with a possible answer to it.

[Dave Liquorice]

And put a decent load on it for an hour or so rather than just start it let it run for 30 seconds then put it away again.

[Andy Wade]

Yes, it's a legal requirement in the ESQCR:

Parallel operation 22. - (1) Without prejudice to regulation 21, no person shall install or operate a source of energy which may be connected in parallel with a distributor's network unless he -

(a) has the necessary and appropriate equipment to prevent danger or interference with that network or with the supply to consumers so far as is reasonably practicable;

(b) has the necessary and appropriate personnel and procedures to prevent danger so far as is reasonably practicable;

(c) where the source of energy is part of a low voltage consumer's installation, complies with British Standard Requirements; and

(d) agrees specific requirements with the distributor who owns or operates the network.

(2) Sub-paragraphs (b) and (d) of paragraph (1) shall not apply to a person who installs or operates a source of energy which may be connected in parallel with a distributor's network provided that sub-paragraphs (a) and (c) of paragraph (1) are complied with; and

(a) the source of energy does not produce an electrical output exceeding 16 amperes per phase at low voltage;

(b) the source of energy is configured to disconnect itself electrically from the parallel connection when the distributor's equipment disconnects the supply of electricity to the person's installation; and

(c) the person installing the source of energy ensures that the distributor is advised of the intention to use the source of energy in parallel with the network before, or at the time of, commissioning the source.

[tony sayer]

All quite "reasonable" and "Practicable"...

Two words which keep me 'learned friends in clover;!..

So how is the actual sensing of the supply failure done?..

Suppose one local 11 kV system fed from one end off the grid..

Other end fed bu local gennie system..

Fault develops in the middle so local gennie tries to supply the load. Load could be very light middle of the night in summer so no storage heating etc. Gennie can cope with that..

So apart from the sudden rise of load how can you determine that the mains network has gone down?...

And I suppose any lines men will obviously see that they earth and short together the affected line before commencing work;!..

[Huge]

Indeed, we've had > 10 cuts today, ranging from 2 or 3 seconds to 90 minutes.

:o)

Half a dozen momentary cuts last weekend.

Longest cut ever - 4 days.

[AJH]

Delta rms V across the meter would reverse if the generator was back feeding?

There is a standard for inverters that can grid intertie for wind turbines and such, something like G80, and these break circuit if the generator becomes islanded.

AJH

[Jim Michaels]

When the installation is "Islanded", it no longer enjoys the frequency stability of the grid and the inverter (or other part of control system) senses this lack of stability and cuts the feed to the mains. This is called "Islanding Protection".

The system would normally continue to supply local loads. Remove SPAMX from email address

[tony sayer]

In article , Jim Michaels scribeth thus

So does the local gennie have an "offset reference" or is it allowed to run at what speed it will?..

So when it looses its "reference signal" in the load its supplying it will drift to where exactly?...

[Dave Liquorice]

Complain or at least report it, there is a marginal fault(*) somewhere that is tripping an auto-recloser. The 2 or 3 second cuts are when it is tripped and automatically reconnects. Get more than X trips in Y seconds and it latches open giving the longer cut until someone comes out and resets it. If it closes without retripping they may or may not decide to investigate what caused it to latch open.

(*) Anything from a leaky insulator to a bit of tree waving in the wind/ weighed down by ice/snow touching the line. The former won't go away and will only get worse. The latter might if it's a small bit of tree as the shorts to earth destroy bits of it.

[Appin]

The message from Huge contains these words:

Granted. And we had seven or eight cuts of several seconds on Saturday night. Leading to the slightly awkward situation of my commercial answering machine and voice mailbox provider reprogramming itself to function entirely in German. Now, I speak German reasonably well, but ..............

It took a bit of effort to find the appropriate menus in German and set the whole machine up all over again. And yes, this time it's on the UPS that serves the telephone exchange, wireless router modem etc.

Hardly enough to demand a generator, though, even at 90 minutes, for my use.

Haven't had a cut that long in the UK. How many have you had of these in the last thirty years? :-)

[Appin]

The message from Jim Michaels contains these words:

Point taken and accepted. And certainly to be present in a gemerating station as it's locked into the grid by the stable frequency of the grid is an experience not to be forgotten (think in terms of a very large bang!). What happens, though, if you've got an extra-stable local source as some people claim the small Honda generators are? (I KNOW the grid is stable, I'm not so convinced about any small generator, no matter how clever they claim to be).

[tony sayer]

On a local comms site five days and for five days the old gennie struggled a bit;!..

Wasn't till we threatened to repair the cables ourselves that EDF became interested. In their finest hour...Not!.

End of October 2002 remember it?....

[Appin]

The message from tony sayer contains these words:

Not particularly :-) So that was once in thirty years?

[tony sayer]

In article , Appin scribeth thus

Well there was quite a wind across East Anglywold that caught the power company right out..

And there was of course the great storm of 87...

So both in the last 30 years;!...

[Appin]

The message from tony sayer contains these words:

Once in 15 years, then? Not convinced that a massive switch-over system with a large generator is justified for me even on a once in 15 years basis, although we haven't had more than two lengthy outages in thirty years and both were due to poor line maintenance more than anything else.

I'm not arguing against sensible provision of backup facilities -- it's more a question of how to go about it in such a way that the provision is commensurate with the real needs rather than trying to maintain life unaltered during a power interruption.

[tony sayer]

In article , Appin scribeth thus

Well this was for a communications and broadcast site so it is justified. There have been a lot of other short duration cuts ..

Thats up to how important the supply is to you.. Fine of you like candlelight but the stuff we maintain isn't steam powered at least not locally;!..