In my first article on WattClarity today, I introduced the analogy of a very long train to help readers visualise some concepts central to operating an interconnected power system. I ended with a description of the Frequency Control ancillary services (FCAS) provided by the engines on the train.
Having all engines in the train controlling the speed has several practical advantages. When load changes occur all of the engines respond together and share the impact. A common issue that occurs on power systems is when a single generator trips due to an internal breakdown. This scenario is easily dealt with when all remaining generators share the load between them; they simply increase their output slightly to compensate.
At the opposite extreme, imagine what would happen if only one generator is able to respond to load changes, all other engines having their throttle locked in a constant position. If the one and only generator which is controlling frequency is tripped, the remaining train cannot respond to the change in load and will slow down until it comes to a halt ( in practice under frequency protection systems would operate and the generators would all trip like falling dominoes, or the system would settle at a lower frequency because the carriages apply less braking force). If it was some other generator then the one and only generator which can respond will have to respond in full, bad luck if it doesn’t have enough capacity. This is known as a lack of spinning reserve.
1) So the question is: – what would cause engines to have their throttles’ locked in a constant position, unable or unwilling to respond to changes in speed?
One obvious case occurs when the engines are running at full power output. At full output they can’t give any more power so cannot respond to reductions in speed, however if the train was going too fast they can reduce their power to reduce the overall speed of the train.
Another case occurs if the control system is simply switched off. There are sometimes sound engineering reasons to do this – e.g. If the control system was mal-operating or under test, but usually switching off control systems is not recommended for very good reasons. Would you drive a car if the brakes and accelerator were disabled?
In the NEM we have effectively created a market for frequency control, like having a market for controlling the speed of our very long train. How does this work?
Some engines in our metaphorical train run flat out and hence cannot increase their power output whereas others running at part load can. Normally all engines running at part load would be able to respond to increase or decreases in speed, and this was the case in the Pre-NEM system.
However, after the NEM came into being, it was decided that frequency control should be dispatched by market mechanisms. In effect generators bid for the right to supply frequency control services and a market clearing house (AEMO) decides to accept or reject the offer based on price.
In theory if the price is too high, than it could be deemed too expensive to accept the offer, and some system frequency control can be compromised. Another way system frequency control can be compromised is for no one to put in a bid, and hence it is entirely possible (in a laissez faire market) for frequency control to be removed entirely. There are safeguards to prevent this extreme situation to be avoided (I hope) – but the fact remains that the frequency control spot market adds a level of uncertainty to the control of an engineering parameter that is unhelpful at best, and is probably outright dangerous.
2) It is important to note that the FCAS market does not control system frequency. Allocating more money to the market does not improve frequency control.
The governor and AGC control systems is the technology that regulates system frequency.
The FCAS market is a spot market operating like the energy market on a 5 minute bidding period. So even if you know which generators are controlling frequency now; that could be change 5 minutes after. Economists will be able to provide what they consider to be good reasons for dispatching FCAS via a spot market, but the author of this article remains unconvinced by the various explanations that have been offered to date.
Instead consider the engineering ramifications. We have a highly complex interconnected system as it is and we effectively decide which machines regulate frequency and which don’t via the random machinations of a spot market.
If the system were a train imagine that sometimes the engine at the back of the train controls the speed, sometimes it’s the middle engine and sometimes it’s at the front of the train.
If the connections between the different carriages of the train were solid and didn’t allow much inter-carriage movement than this might not matter too much, a bit like the difference in handling between a front wheel drive and a rear wheel drive car.
3) This metaphorical train is very long; the connections between carriages are less like solid couplings, more like long and weak springs.
Accordingly changing which engines does the frequency control can have unpredictable results. E.g. there have already been incidents where the system has experienced slow power swing oscillations which appear to be due to control system interactions caused by poor selection of which generators supply frequency control.
Being a long train it is important not just that the speed is controlled, but that the speed is controlled in such a way that oscillations between different parts of the train are kept under control. If the oscillations are too big the weaker connections (springs) between different parts of the train are liable to break – and then we end up with two or three trains instead of just one, and there is no guarantee that each train will control its speed correctly.
At the beginning of this article I mentioned that poor power system understanding has resulted in poor policy decisions and arbitrary ad hoc technical rules. The various recent rulings on system Inertia is a good example of this.
In effect various authorities have decided that it would be a good thing to place lower limits on the amount of inertia for new generation projects. In fact some economics minded but not necessarily engineering minded people even want to set up a spot market for inertia. Without looking too closely at the problem this seems like a good idea. In our train analogy the inertia equates to the mass of the locomotive, a heavier train is harder to accelerate or decelerate and therefore should be easier to keep at constant speed.
4) This simplistic view of a complex power system is misguided.
Consider what would happen to a train connected together by long weak springs which has centres of mass which some rule makers have decided to keep as heavy as possible.
Some event happens which causes a system transient, e.g. a generator trips, a power system fault occurs or someone accidentally pushes the wrong button.
As a result a power system transient occurs which causes a ripple of acceleration/deceleration to propagate the full length of our very long train, like weights bouncing up and down connected together by springs.
What would cause the springs to break? Or to put it another way, would you expect the spring to be more likely to break if the weight of the engines in the train were heavier or lighter?
Those who have been following this discussion up until now will be able to easily see that the springs (which are analogous to transmission lines) are most likely to break when the engines are heavier.
Does this sort of thing happen in real power systems? Yes, and a recent example was the blackout of South Australia. A loss of generation caused by tornados damaging several transmission lines within South Australia resulted in a power swing causing the loss of the weak transmission link to Victoria. If that link had stayed connected there would have been no black out.
What could have prevented that link from breaking?
Well lighter not heavier inertia in South Australia would have helped. The faults on the system caused it to bounce around a lot and a system with lighter inertia would have bounced with higher frequency oscillations, but they would also have calmed down a lot quicker.
The idea is to reduce the stress placed on the weak interconnection to prevent it from tripping (breaking), therefore frequency control in the SA region, or tie line power control would have helped as well, unfortunately both was lacking on the day of the black out.
Various commentators have focused on the rate of change of frequency after the trip of the interconnector and stated (quite correctly and accurately) that the rate of change could be reduced by increased inertia, then maybe a fraction of the system could have been saved by load shedding . In my view for the specific event in question once the interconnector tripped, that was it, the system blackout was unavoidable – but the trip of the interconnector could have been avoided had there been better automated system control before events got to that stage.
5) So where do we go from here?
The power system is a complex beast, taken in its entirety it is the largest and most complex machine on the planet. It is also undergoing a profound transformation in that the traditional methods of generating power using high inertia rotating plant can also be achieved using zero inertia non-rotating plant. This means control of system frequency is more important than ever before, too important to be left to the vagaries of a poorly designed ancillary services market, and too important to be subjected to ad hoc rulings. Accordingly we will be proposing some rule changes to sort this issue out. We believe that a lighter inertia system, far from being the threat to system security that some people currently believe it is could actually make the system more reliable and secure.
The changing power system has obvious ramifications for the need for continuous monitoring and updating of the system – but this should not be necessarily be seen as a threat, rather as an opportunity to make the power systems of the future more reliable and secure than ever before thought possible.
6) The accurate control of system frequency is key to achieving this.
If you are interested in assisting in the drafting of the rule changes to the NEM please get in touch. If you disagree with any of the discussion above, please also get in touch. The aim is to put in place a robust scheme that delivers good system frequency control and improves power system security and reliability.
This article was originally posted in a single piece on LinkedIn here. On WattClarity it has been separated into two for easier readability and as both parts address a different audience, with the first part here.