Effect of a step change in wind output or load on in grid frequency and control mechanisms

Fred,

The frequency is system-wide. That is, all parts of an A/C system will have the same frequency. Any change in frequency is a reflection of a change in the balance between supply and demand. So a drop in frequency is a reflection either of a loss of generation, or an increase in demand.

Actual demand at any time is a aggregation of many millions of demands, with all of us making changes, every time we operate a lift or switch a light or heater, or start or stop some process with motors, etc. So it varies stochastically, from the various changes we all make to demand. These changes can be considered random, but within reasonably defined probability distributions. This makes it feasible to make reasonable predictions of the short term average demand. But there will always be variations from this, and these fluctuations can be significant. Fortunately, we very rarely see synchronised actions that shift the demand by more than about 2% of the total. They do sometimes occur. There is at least a theoretical possibility of many of us all switching kettles on at the same time, and shifting the total demand by 10%. If we chose to set up a synchronising scheme so that millions of us all did something at the same time, the system would fail. But we can be confident that a very rare natural event.

If the imbalance remains, then the frequency will continue to change. So if demand stays more than generation, the frequency will continue to drop indefinitely. But changes in the frequency trigger other changes, both to supply and demand. Some of these changes are inherent, and others actively controlled.

A drop in frequency will generally reduce load. In part this is because motors will slow down, and so will consume less power, but there are other effects, perhaps including a general drop in average voltage, and so reducing resistive load. Nobody really know how much this happens, but there are guesses, and the effect is generally quite small.

A drop in frequency will also reduce generation, as the generators will slow down, and, without changes in torque (ie increased steam or gas pressure), will produce less power. This effect is considerably larger than the effect from load.

These two changes do not balance, so, without correction, the system frequency will continue to decline, and so collapse. The rate of decline is related to the inertia, mostly of generators, but also of load. It is also related to the proportion of generation that is lost, so a system with lots of small generators will decline more slowly than a system with a few large generators. Thus the size of the largest generator capable of instantaneous loss broadly dictates the “spinning reserve” that is needed.

It is this so called “spinning reserve” that has traditionally used governors to make the adjustments in output. When the frequency drops, the reserve generators are required to increase output. They should all do so by the same amount, so they usually share “droop”, commonly set to 4-8%. That is, when the frequency drops by say 1%, the increase in output should be 4-8%.

The governors should all react fast, in the UK within 10 seconds. In Ireland, which is a friskier system, as their generators tend to represent a larger proportion of the total, so it within about 5 seconds. The UCTE system can be slower. They are automatic, so have to respond to different events without knowing any particular cause.

These control actions, as well as covering for imbalance “events” also provide corrections to the stochastic changes in demand. So generators will be adjusting their output as frequency changes, albeit with a deadband, within which they do not react.

The System Operator will usually monitor the output of generators, so will be able to detect how much of the spinning reserve has been “used up”. If it gets a bit short (or long) they will then tend to replenish this by buying or selling in the Balancing Mechanism, so that generators will change their set points, and increase or reduce their output. These orders will not exactly match the changes in demand, so the changes they instruct can also cause a change in frequency, thus adding to the noise in the frequency signal, which is what you see.

So, to come to your question, how much electricity is represented by a given change in frequency, it all depends. Mostly it depends on the overall load on the system at the time. In a heavily loaded system, the loss of a generator will tend to be a smaller proportion of the whole than in a lightly loaded system, so a given frequency drop will correspond to a larger load. Broadly, in the UK, you can reckon that a 0.8Hz drop in frequency (down to 49.2Hz) will correspond to about 1 GW. In a lightly loaded system a 1GW loss might trigger a bigger change in frequency. But much depends on what the frequency response capacity and behaviour is that the time.

In general, spinning reserve is capable of delivering for only a short time: 10 minutes, or 30 minutes, depending. So it needs to be replaced as soon as possible, albeit by the “manual” action of instructing (or purchasing) STOR, at least until slower starting or ramping generators can be fired up.

Fridges are perfect for providing frequency response, as, at least for a while, they can collectively increase or decrease their load. But they do need replenishing, so that they can revert to normal when the STOR or other reserve is in place. This is something that is hard for system operators to get their heads around, as the fridges do this automatically, and not under their instruction!

One final complication about frequency. As I mentioned, averaged over a period of a second or so, the system frequency is a system wide signal. Wherever you are in the system, it will be the same. This does not mean the phase will be the same, so it may be that, when Scotland is exporting electricity to England, it will be ahead of England. Like a spring, the torque applied in Scotland will twist the spring a bit. Conversely, when England is exporting to Scotland, the spring will twist the other way, and England will be ahead of Scotland. When this changes, the spring will wind up or unwind a bit, and the instantaneous frequencies in England and Scotland will be slightly different, for a short while.

This introduces the possibility that generators, responding to the instantaneous frequency, will not all respond at precisely the same time. In the worst case, this can set up an oscillation, with generators in different regions swinging against each other. This makes the control of generators even more exotic and complicated.

Broadly, what this all means is that the control processes set up to handle stochastic variations in demand have the capacity to respond to short term stochastic variations in wind generation. Indeed, the capacity to cope with big frequency variations, which is essential for the safe operation (say) nuclear plant, will be an order of magnitude greater than that needed to cope with wind variation. So short term wind variation adds no new costs.

The bigger difficulty is with slower variations in wind, over hours and days. Broadly, 1 -2 hour forecasts fall with the error range of demand, so it is planning for 8 – 24 – 72 hours ahead that becomes more complicated.

But that is for another post.

David

David Hirst 

!-!?!-Hirst Solutions Limited

Mobile:  +44 7831 405443

 

From: energy-discussion-group@googlegroups.com [mailto:energy-discussion-group@googlegroups.com] On Sent: 20 September 2011 11:28
To: dave Cc: Claverton Grid; Claverton Wind energy group; Claverton AB MAIN GROUP; Kevin
Subject: Re: Effect of a step change in grid frequency

 

Dear Dave and KevinI appeciate your helpful replies which shows that the technical situtation is quite complex.  However, the frequency increases and decreases I am referring to only last a few seconds, so I find it a bit difficult to believe that major loads would be taken in and out of the system to compensate. This is in line with what Ken says, in which it is only after fairly long departures from the grid freqency that the system starts to respond. 

But has anyone got any hard information on how a loss of generating power would affect grid frequency? 

 

Fred

— On Tue, 20/9/11, dave wrote:

From: dave Cc: “Claverton Grid” <gridsupergridetc@claverton-energy.com>, “Claverton Wind energy group” <wind-energy-claverton@googlegroups.com>, “Claverton AB MAIN GROUP” <energy-discussion-group@googlegroups.com>
Date: Tuesday, 20 September, 2011, 9:32

Fred…;.the answer to your question is that the frequency would drop but the drop would be obscured and limited by the automatic under frequency relays that are fitted to all the large disconectable loads…cold stores, steel melting etc as part of Frequency Service (or whatever grid call it these days). 

 

These are all set by grid to a range of frequency, so the further the frequency tries to drop, the more load is switched of automatically.

 

Therefore it is not possible to determine any simple relationship between power loss and frequency drop because it is so heavily damped by the under frequency relays operation.

 

Also of course there are a number of power stations constrained to operated below there maximum output, with a governor controlled by the frequency, so these will also damp out any frequency drop.

 

I expect if you look up Frequency Service on Grids web site it will tell you what the settings of the relays are….something like 49.8 for the first I seem to remembers, then maybe 49.6.etc

 

I expect Alastair will know the actual settings.

 

With a lot of wind, then any drop would be further obscured by the constraining of of a certain proportion to provide the frequency response.

 

kind regards

 

Dave

On 20 September 2011 00:51,

Dear Dave I do not think you have understood the nature of the query. All I am asking is if there is a change in the frequency over a short period, what is the percentage changes in demand or output from the Grid. To give another another example, at about 10.00 pm this evening, the Grid frequency dropped from 50.1 Hz to 49.9 Hz over a six minute period, and then started to climb back.As demand was falling a this time, it presumably means that a generating set was taken off the system. As the load then was around 35 GW, what sort of drop in output did this mean?

 

Perhaps I can turn your point round…..If we lost 1.3 GW by the near instantaneous loss of a large generator in the middle of a summer night when the total demand is 25 GW, what would be the drop in Grid frequency before anyone had the chance to react to it?

 

Fred      

— On Mon, 19/9/11, dave wrote:

, 20:49

Fred… can’t imagine anyone important ie national grid are panicking….the grid is designed to take the instantaneous loss of 1.3 GW being sizewell tripping off. As soon as the frequency starts to drop, up to 2 GW of pre contracted load, on frequency sensitive relays start to cascade of the system until the system is stabilised.

 

Then various STOR participants incluing diesel generators, gas turbines, etc start up all being on line within 20 minutes and the loads re connected.

 

Simples…

 

all described here:

 

http://www.claverton-energy.com/?dl_id=131

 

I cannot imagine wind predictions, deviating from out turn (which is what counts) , at anything like the rate of loss of 1.3 GW instantaneous.

 

If there is excess wind then the grid would simply con strain them off, and use them to  provie spinning reserve.

 

I also sent you a paper some months ago showing how wind turbines can provide a certain of frequency control.

 

I assume generators have some sort of automatic voltage control anyway…you just vary the excitation.

 

Transformer are on auto tap changers all the time to smoothly stabilise voltage lower down the system.

 

All the best

 

Dave

On 19 September 2011 17:53,  wrote:

Dear All There has been a lot of correspondence about the need for standby plant to have sufficient flexibility to cope with the short term vagaries of wind power. In actual fact grid frequency is constantly changing because of changes in demand and what generation equipment is being brought on line.Step changes in frequency will also occur when part of the local distribution system are instantaneously changed from on voltage to another WHY SHOULD THAT BE? POWER TRANSMITTED WONT CHANGE

 

For example at the present time 5.00pm on Monday 25th Sept, the grid frequency changed over a two minute interval by 0.1 hz ( from 49.9 to 50.1Hz)

 

What does this correspond to in terms of power. My guess is that the power output or demand has changed by at least 0.1/50  05 or by 0.2% . However, because a reduction of speed of the generators will reduce the output voltage,I DOUBT IT I ASSUME THEY HAVE AVC ON THE GENERATRORS AS DO DIESEL GENERATORS the change could be as high as160 MW. Has anyone got a better idea than what I am proposing?

 

I am asking this question as there seems to be a certain amount of panic in some quarters CAN YOU SAY WHERE??abou the effect of wind farm fluctuations on the grid…I have seen occasional spikes in frequency up to 0.5 Hz.

 

Fred

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