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# half hourly import

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#### Gilest

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We have a strange problem.
We have a wind turbine and obviously when the wind doesnt blow we have to import electricity.
However intermittently (on a monthly basis) we have import figures that are maybe three times what we would expect based on the maximum load we could generate.
So we have a maximum of 50kw of load and we use this twice a day (dairy farm) and this is made up of various single phase motors, water heaters, cooling compressors.
We wont use all 50kw at the same time but its clustered around milking times and roughly we use it for two hours at a time twice a day (four hours total although water heating happens before milking and cooling continues a little after milking
However half hourly data suggests that sometimes we are importing maybe 150kw (in a half hourly period)
The turbine produces at 690 v and feeds into the grid at 11kv via a transformer. Then the farm is fed through a transformer ( down to 415v from 690 ) i.e three phase although the motors are all single phase.
We cant make head or tail of it as apart from the load not matching the milking times the import is vastly more than we think we could ever use.
Any ideas ?

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#### Lucien Nunes

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Are you saying that for the suspect half-hour periods, the metering is showing energy consumption of 75kWh for that period, as opposed to a peak power demand of 150kW occurring somewhere during that period? If so, and you've only got 50kW of load, and the timing doesn't even correspond to when most of that is operational, then it does seem to be a metering fault. Whose metering is producing these figures?

#### Gilest

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Just looking at import half hourly data for 4th feb this year HH01 says 104KWh and HH02 says 103 KWh.
I assume HH01 is the frst half hour period after midnight.
At midnight the only thing that should be running is a water heater with a 6kw immersion in it.
It is possible that the cooling compressors kicked in briefly and these two are rated at 3.7kw each.
The farm meter which meters all the farm uses whether from the turbine or import has been checked and is ok.
The import /export meter has not been checked and to be honest I am not sure who this meter belongs to. I presume the supplying company (which is Good Energy) but surely if the meter is faulty it should be consistent shouldnt it ?
We wonder if there is any fault condition on the transformer that could cause this intermittently.

#### Shoei

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Is the imported data definitely for a single half-hour period on one day? It isn't (for example) the accumulated consumption in HH01 for a number of days or weeks, as recorded on 4th Feb?
A metering fault is possible as is a transformer fault or massive earth leakage, but with those numbers you'd know it was happening. 104kWh in half an hour implies average consumption of 208kW, if that was being dumped into a fault it would draw attention to itself.
Most likely explanation is misinterpretation of the data, which can be confusing, or a metering problem. Either way a chat to your supplier should sort it out.

#### Gilest

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It’s not an accumulated figure.
I thought a fault of that magnitude would be obvious.
I will check figures and talk to supplier
Thanks

#### marconi

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If I have the right picture in my mind, you have a 3 phase 11kV supply into a transformer (T1) which outputs 690V. The wind turbine is three phase at 690V and is connected to the output of T1.

Also connected to the 690V lines is transformer T2 which outputs 415V 3 phase. The 415V supplies the farm equipment.

I reckon, the farm meter which measures total consumption(C) is connected into the 415V lines.

In order to measure wind turbine generation there is a half-hourly meter either in the lines from the turbine - in which case the meter records generation(G). Or, the half-hourly meter is connected into the 690V from T1, in which case this meter reads import(I). HH01 and HH02 refer to two watthour meters (for the 2 meter method of power consumption in a 3 wire 3 phase supply), and so the total power is HH01 + HH02.

Your settlement(S), ie what you owe the DNO for the electricity they supplied is either S= C-G or S= I.

With this explanation it would not be surprising if you recorded high HH01 and HH02 if wind produced a lot of generation and HH meter was for generation. What is the power of the turbine? I suspect it is the first HH meter configuration you have installed ie: to record generation. Thus S = C-G.

PS: I think these meter only work for one direction of power flow which supports S=C-G

A diagram or pictures would be helpful to clarify the purposes of the meters.

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#### marconi

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You said the meter indicates anomalous import figures. This implies the figures are signed(ie +/-) for import/export. Maybe, the recorded import figures are ignored by Good energy since they are only interested in what the turbine generates.

Electrically, the turbine's alternator when connected to the incoming DNO mains can also motor as well as generate. When it motors it will import energy and run as a big fan. Whether the turbine motors or generates depends on the prevailing wind speed, the voltage difference between the mains and alternator, and fluctuations in wind speed ie: acceleration/deceleration of the rotating blades caused by the wind. The 11kV/690V may be rising in voltage as the network loads reduce or HV transformer taps are changed to such an extent that the alternator voltage is lower than it - thus it motors. This why wind turbines turn when there is no wind. Just some musings.

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#### Gilest

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If I have the right picture in my mind, you have a 3 phase 11kV supply into a transformer (T1) which outputs 690V. The wind turbine is three phase at 690V and is connected to the output of T1.

Also connected to the 690V lines is transformer T2 which outputs 415V 3 phase. The 415V supplies the farm equipment.

I reckon, the farm meter which measures total consumption(C) is connected into the 415V lines.

In order to measure wind turbine generation there is a half-hourly meter either in the lines from the turbine - in which case the meter records generation(G). Or, the half-hourly meter is connected into the 690V from T1, in which case this meter reads import(I). HH01 and HH02 refer to two watthour meters (for the 2 meter method of power consumption in a 3 wire 3 phase supply), and so the total power is HH01 + HH02.

Your settlement(S), ie what you owe the DNO for the electricity they supplied is either S= C-G or S= I.

With this explanation it would not be surprising if you recorded high HH01 and HH02 if wind produced a lot of generation and HH meter was for generation. What is the power of the turbine? I suspect it is the first HH meter configuration you have installed ie: to record generation. Thus S = C-G.

PS: I think these meter only work for one direction of power flow which supports S=C-G

A diagram or pictures would be helpful to clarify the purposes of the meters.
Aha !
I have discovered that the half hourly data I was furnished with as import is in fact export which rather makes sense of the numbers.( a bit of an elementary blunder methinks !)
Nevertheless we do still get intermittent months of high import just not as high as the figures I had suggested.
I have got the right data now so I need to look carefully at it before going any further.
Thanks anyway for your input. I may be back(having double checked I am talking sense )

#### marconi

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Gilchrist: I turned up a few papers on electricity tariffs for dairy farms - there are many others. If your electricity usage is known hour by hour, day by day, you can see if a restricted time tariff might be more economical. Electricity suppliers are using so-called smart meters to encourage folk and businesses to 'even out' the peaks and troughs of demand by variable pricing eg: higher during peak times and much lower during surplus times, and also to reduce their own peak demand to avoid having to invest to in their infrastructure to provide more capacity. There are energy consultants if you don't feel like doing the research yourself but I would presume that like-minded farmers already have a wealth of advice to offer - see farming forum link.

#### Lucien Nunes

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I thought a fault of that magnitude would be obvious.
This is sound logic that many people overlook. You would find it located somewhere near the red truck with flashing blue lights, approximately where the hoses are pointing. 200kW gets very hot very quickly.

Glad you have an explanation for the numbers so far...

#### Gilest

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You said the meter indicates anomalous import figures. This implies the figures are signed(ie +/-) for import/export. Maybe, the recorded import figures are ignored by Good energy since they are only interested in what the turbine generates.

Electrically, the turbine's alternator when connected to the incoming DNO mains can also motor as well as generate. When it motors it will import energy and run as a big fan. Whether the turbine motors or generates depends on the prevailing wind speed, the voltage difference between the mains and alternator, and fluctuations in wind speed ie: acceleration/deceleration of the rotating blades caused by the wind. The 11kV/690V may be rising in voltage as the network loads reduce or HV transformer taps are changed to such an extent that the alternator voltage is lower than it - thus it motors. This why wind turbines turn when there is no wind. Just some musings.

View attachment 50354
This is interesting. I am told categorically that turbines can not create a load. ( motor ?)
How certain are you of this ?
What might the expected consumption be if it can motor ?
I presume it’s related to turbine capacity.
This turbine is rated at 225 kw

#### marconi

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Re #11: I admit I have assumed they were motoring when there was no wind or below the cut-in speed and the rotor was turning. I could not see how they could rotate without them drawing power. I'll do some more research and contact the renewable technology college in my home town to see what I can find out.

#### marconi

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I have asked a university professor at a top university about energy consumption of wind turbines. Meanwhile, I turned this up - I caution it is from a site biased against wind but the remarks made are true enough about how a large wind turbine draws energy from the grid. What matters is the net contribution of energy to the grid by the turbine. You will read that the alternator does sometimes motor and draw power from the grid for a number of reasons including preserving bearings and preventing blade distortion.

It's why I asked were the HH meter was connected - in the lines from the WT or the lines from the secondary of the 11kV/690V transformer.

#### marconi

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If the renewable metering is the same as for my friend's solar PV generation, he has two meters - one for actual total consumption in his household which he pays for at one rate/tariff say 15 p per kWh to his supplier say ScotPower; and another meter for what he feeds into the mains from his solar which he earns money from at another rate/tariff say 11 p per kWh.

In your case you do have a bill from an energy supplier for your actual total consumption on the farm and a payment from Green Energy for what you generate?

#### marconi

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And one last thought for today - there are electrical network stability reasons why the DNO is not that bothered the turbine is connected and motoring, especially if the motoring power is small compared with the generating power. I suspect (can't say without more information) that watthour meters only record for flows in one direction so only generation is recorded not motoring. Whoever said the WT cannot create a load was imprecise; he should have a said it can be a load whose consumption is not recorded by the HH meter.

Anyway something for you to delve into.

#### marconi

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Do you have the Vestas V27 which has a rated output of 225kW and sold to farms in the UK? You could ask them about the energy consumption of this turbine and whether indeed there might be something wrong with yours.

(As I have got interested - and no doubt some others too - would you mind keeping us posted on what you find out please?)

Without boring you from what I have read up it is well understood in the wind turbine industry and users that they do consume energy/power when idle and when turning even if not generating. This is not surprising when one studies the actual technology and engineering rather than the 'for the lay person' idealised description of them which only mentions usually their maximum rated wind generated power.

One of the metrics for renewables is annual energy yield in MWh but I have not bottomed out if this takes into account annual energy consumption.

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#### marconi

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The professor is at Exeter University.

This is the content of my email to him:

Dear Professor Abusara,
Please forgive this 'out of the blue' email but I have a simple question I have not been able to find the answer for myself.
It concerns grid connected wind turbine generators using ac asynchronous alternators, either on or off shore.
I understand how they generate electricity and feed this into the grid when the wind blows. My question is about when there is no or very little wind (below the cut-in speed?) - do the rotors keep turning by being powered by the grid ie: motoring?
Yours sincerely
Dom A-H CEng MIET

#### Lucien Nunes

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Meanwhile, I turned this up - I caution it is from a site biased against wind but the remarks made are true enough about how a large wind turbine draws energy from the grid.
Your caution is well received - I would ignore that piece completely. There are guesses and assumptions, some of which are irrelevant or simply wrong, but all of which purport to demonstrate how wasteful wind turbines are. I do not think the writer has a clear grasp of the technicalities or deliberately confuses them to make his point. I won't bother picking it apart here, nor studying the references although some of those may well be sound.

#### Gilest

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We met these arguments during planning.
It’s amazing what sort of arguments appear !

#### marconi

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What matters is the net contribution over the long term (eg a year) of energy to the grid by the turbine, energy which otherwise would have been generated in full or part by fossil fuels.

I suspect that to some folk wind turbines are rather like pylons - you either see them as objects of beauty with benefits or simply an eyesore on the landscape. Items of beauty to me in both cases.

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#### marconi

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Thinking on the intermittent nature of your charging problem....

The Vesta V27 includes switchable power factor correction under computer control because the generator has two selectable windings depending on the power output..

See para 3.12 and 3.17 of

Some HH meters record kVAh as well as kWh and some energy suppliers charge for reactive or apparent power consumption and not just kWh.

See: https://www.edfenergy.com/sites/default/files/b2b-guide_to_reactive_power.pdf

Thinking more on the intermittent nature of the 'spikes' in half-hourly consumption, it would be very helpful to know:

a. the units and amounts ie: kW? kWh or kVAh?
b. how is the data from the HH meter returned - through the internet automatically?
c. does your electricity bill show a charge for kVAh and/or reactive kVArh. I am making a distinction here between kWh - actual power consumed and kVAh - apparent power which is higher and comprises of actual power plus reactive power which is not a net transfer of energy but rather a tooing and froing between the supply and certain types of load such as motors and generators.

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#### Lucien Nunes

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As an aside that EDF e-guide is pretty rubbish! Its first factual error occurs at the third word and there's a further error before the end of the sentence. It doesn't get much better. Can anybody make sense of this:
Where the first step is 50%, the Local Network Operator does not charge for reactive power for the first 50% of units (kWh). Charges therefore apply when the difference between the total units recorded on the reactive register (kVArh) is less than 50% of the total units consumed (kWh).
I know what it is trying to say, but if I didn't, I'd be more confused than when I started.

#### marconi

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You point out a typo first.

Second you quibble I think about the use of 'difference'. AP = TP + RP when I was at school so RP =AP - TP - a difference. You know why I have written them in bold. AP is apparent power, TP is true power and RP is reactive power.

On the third point you may be tired. An example calculation was provided. But yes, 'more than' instead of 'less than'.

Suggest you contact the author of it.

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#### Lucien Nunes

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Not wishing to sidetrack the thread, my first point was that KVArh is not a unit of power as implied. The second, yes, this is aimed at non-technical people with no knowledge of the underlying maths who are unlikely to interpret correctly the ambiguous 'total power consumed.' Then, apart from the sign issue, the difference between which two variables specifically; the kVArh reading and what?

They were just three of many quibbles picked at random. I could have complained about the meter that consumes 2.5MWh, the number agreement error between 'most electrical equipment' and 'create', the non-sequitur 'reactive power therefore generates...' etc. More importantly, it does not explain reactive power coherently, nor why the charges it attracts are computed in the way they are, nor differentiates between power factor and efficiency (a common source of confusion.) Power factor supposedly 'indicates how effectively electrical power is being used.' Grr.

#### marconi

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Suggest you contact the author of it or re-write it for EDF (for a fee).

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