This section will explain the influences on scheme design.
There are many interconnected elements in the AUFLS scheme.
Changing just one of them can affect the others and in fact can change how you design the scheme.
These are the elements we are going to look at.
We’re going to start with block size.
Let’s go back to the seesaw and look at what happens to the frequency decline
when you disconnect load from the power system and the AUFLS blocks themselves.
Each time there is a loss in net injection the frequency will fall.
How fast it falls will depend on the magnitude of the disturbance,
what machines are operating on the system on that particular day, and so on.
If we didn’t do anything, the frequency would continue to fall.
It might not be linear, but this illustrates the concept.
Here we have disconnected a small amount of load from the system.
This is like hitting the brakes in your car.
Disconnecting this small load slows down the rate at which the frequency is falling.
If you hit the brakes even harder, by disconnecting a bigger load, then you slow down even more.
Here is a bigger AUFLS block, and you can see it has slowed the fall in frequency even more.
If we shed the exact amount the system needs at the time, the frequency stops falling and levels out.
The generators’ governors pick up and the frequency is returned to 50 Hz.
This is the perfect response.
However, perfect is difficult to achieve,
because the amount of load that is disconnected could be perfect for one situation, but too little or too much for another.
We call the concept we have just explored ‘braking’.
It is a limited analogy, but it helps illustrate that when you disconnect load you are reducing the speed at which the frequency is falling.
When people discuss the ideal AUFLS scheme, they often say how great it would be if we could shed AUFLS blocks in proportion to the event size.
So for a small event, we shed a small block, and for a large event we shed a large block.
As the frequency trigger points of the scheme are set, this raises a good question about the size of your AUFLS blocks and the order you arrange them in.
Let’s explore this concept using some examples.
In this hypothetical example, we know the total amount of load we need to shed to restore balance to the power system.
We divide this into two equal-sized blocks.
Block 1 hits the brakes, slowing the rate of frequency decline.
It’s still falling so the second block trips.
Because exactly the right amount of load was shed, the frequency decline is arrested and the line levels out.
We call this the ‘two equal block’ scheme.
In this second example we start with a smaller AUFLS block.
In other words, we lightly apply the brakes and the frequency continues to fall at a faster rate than in our previous example.
The frequency falls further before the second bigger block triggers,
the frequency decline is arrested, and the line levels out.
We call this design an increasing scheme.
Does it matter that the frequency fell further in the second example?
If we return to our car analogy and imagine that the 47-Hz limit is a brick wall,
we did stop the car but it was a lot closer to the wall than in the equal-block example.
We shed the same amount of load, but were closer to the safe minimum frequency limit.
So the answer is that yes it does matter.
Having distance from the under-frequency limit means we can cover larger events just from how we ‘apply the brakes’.
So in the increasing scheme example, a small block then a big block meant we were very close to the wall.
What about if we flipped the order around and started with a big block, followed by a small block?
In other words, we hit the brakes harder first, then softer.
We call this a ‘decreasing scheme’.
In this example you can see that the first AUFLS block slows the frequency decline more rapidly than the either the equal or increasing-block schemes.
When the second block triggers, the frequency is arrested at a higher level.
The same amount of total load has been shed in all the examples,
but by adjusting the order of the blocks, we can arrest the frequency further away from the limit.
In AUFLS terms, this means we can ensure coverage for larger events, which is a big deal.
The next elements to look at are operating time and range.
People often ask why we don’t simply have as many blocks as possible, so we can match the AUFLS response to any size of event.
Although it sounds great, in practice we have to consider the time it takes for the relays and circuit breakers to operate,
which dictates the range we have to trigger AUFLS.
Let’s look an example where the frequency is falling at a rate of –1 Hz per second.
Our first AUFLS block is set to trigger at 47.9 Hz.
But although it triggers at this point, it takes time to shed.
How much time it takes depends on the configuration each company uses for AUFLS, but the Code requires that it be done within 400 milliseconds.
This means that 400 milliseconds pass from when the frequency crosses the set trigger point.
Here you can see that the frequency continues to decline until the load is actually disconnected.
What this means is that as you move closer to the minimum frequency limit,
there comes a point, at 47.3 Hz, where the last AUFLS block adds little value.
By the time the load is actually disconnected, it’s too late.
This means we have a limited range to trigger AUFLS blocks effectively.
We call this the practical operating range.
The practical operating range for the North Island AUFLS scheme has been identified as between 47.9 and 47.3 Hz.
Let’s talk about what happens when you try to add more blocks in this limited range.
We will go back to our previous example where we compare a decreasing scheme with an increasing scheme.
This time, let’s assume we have three AUFLS blocks at our disposal but we only need to shed two of them to arrest the frequency.
So here is our frequency, falling at a rate of –1 Hz per second.
We start by hitting the brakes hard.
The first block triggers at 47.9 Hz and the load actually disconnects 400 milliseconds later, just before the next trigger point.
The frequency is falling more slowly now but Block 2 still triggers, and 400 milliseconds later you can see the load is disconnected.
The frequency levels out just above the point where Block 3 would be triggered.
Now because we only needed the two blocks, we’ve avoided tripping Block 3
because we were able to arrest the frequency decline before it got to the third AUFLS-block trigger point.
Let’s look at the increasing scheme.
We start by lightly tapping the brakes.
This slows the frequency, but it continues to fall and Block 2 is triggered.
But by the time the load actually disconnects, the frequency is still falling and reaches the point where Block 3 triggers.
This triangle is what we refer to as overlap.
Block 3 is triggered before Block 2 is actually shed.
This is a detailed view of what we could see in the sawtooth analysis, where the frequency shoots up to 52 Hz.
The culprit is usually this overlap, when you are triggering a block before the previous block has operated,
which may result in shedding a block that is not required.
These two green lines compare the effect of overlap for both decreasing and increasing three-block schemes.
This concept in turn affects how many blocks we can effectively fit within our operating range.
Let’s take a quick look at how much overlap exists for two, four, and six-block schemes.
The overlap is highlighted in pink.
The two blocks look good from an overlap perspective, although their large block size means we can still get over-shedding.
With four blocks you can see there’s overlap occurring.
The more blocks you add, the more overlap you need to take into consideration.
For this six-block scheme there is more overlap happening and the potential to shed load without need has increased.
It doesn’t mean that you can’t design around this factor, but you have to be aware of the effect of overlap.
There is a technology that lets you hit the brakes sooner rather than waiting for the frequency to drop below 48 Hz.
If you hit the brakes sooner you get more distance from the wall and can cover larger events.
This technology is called rate of change of frequency – or df/dt for short.
Relays with df/dt capability calculate how quickly the frequency is falling.
The general rule is that the bigger the loss of net injection, the faster the fall.
Let’s look at an example where the frequency is falling fast, at a rate of –1.2 Hz a second.
You can see with a traditional frequency-time based system that Block 4 doesn’t operate quickly enough.
That 400-millisecond operating time is too long and the frequency has fallen below 47 Hz before Block 4 disconnects.
In this example, that would mean ‘lights out’.
Df/dt technology is like a secret weapon that we can use when the frequency is falling at a certain speed or faster.
So let’s see what happens when we add df/dt to the fourth block, in addition to the traditional frequency and time-triggering components.
If the frequency is falling fast enough, the AUFLS block will jump the queue and disconnect the load earlier.
Here you can see the frequency is falling at –1.2 Hz per second.
As it crosses 48.5 Hz, Block 4 disconnects, slowing the frequency decline and giving the other blocks more time to operate.
Df/dt gives us coverage for bigger events and allows us to stop further from the wall.
That’s the point of df/dt and why it’s proposed as a solution for AUFLS going forward.
This shows the factors we’ve just discussed.
They all influence how the AUFLS scheme performs and are interrelated.
With so many factors, you can imagine that there are many ways to design an AUFLS scheme.
Changing how you manage just one of these factors can influence the scheme performance.
So what are the schemes that we actually looked at?
Up the top is our existing scheme, and below are some of the schemes we compared it against.
You’ll notice that they all add up to 32 per cent.
We looked at the total size to see how much load on average is needed by the system while keeping the frequency between 47 and 52 Hz,
and it came in at around 32 per cent.
We started with four blocks of 8 per cent.
Then the increasing scheme and decreasing scheme – each with four blocks.
Next it’s our winner – the large early scheme, consisting of four blocks of 10, 10, 6, and 6.
The last two are our sandwich scheme and finally our six-block decreasing scheme.
Let’s see how two of these, the increasing and decreasing four-block schemes, look on the sawtooth.
Our decreasing scheme will be in green, and our increasing scheme will be in yellow.
We’ll be looking at the two limits: the 47 Hz and 52 Hz.
How do both of these schemes perform from an over-frequency perspective, for a medium-load scenario?
Each of these labelled points in the sawtooth matches where the frequency crosses a trigger point.
So you can see with the decreasing scheme, we never cross the 52-Hz line, which is good.
Now we’ll overlay the increasing scheme, represented by the yellow lines.
This is where you’re hitting the brakes softly at the start.
This sounds great in theory, but you can see how there is greater overlap with bigger events,
and you end up shedding more load than you need with Block 4.
This takes the frequency over 52 Hz.
These two examples show how the same amount of load is shed, but the order makes the difference between staying below 52 Hz or going above.
This analysis shows that for this type of event, the decreasing scheme is performing better than the increasing scheme in terms of over-frequency.
Now let’s compare both the increasing and decreasing schemes from an under-frequency perspective for a low-load scenario like overnight.
Again, we will start with the decreasing scheme, shown here in green.
You can see that the scheme adequately covers events up to a certain size, but at around 800 MW we cross the point of no return, and it’s lights out.
This illustrates the limitation of the decreasing scheme:
it performs very well for any event smaller than 800 MW but not so well for larger events.
Here in yellow we can see the increasing scheme.
It has crossed the point of no return, the 47 Hz line, even earlier.
This shows us that the increasing scheme doesn’t provide coverage for the range of events that the decreasing scheme does.
We can see how ordering the blocks differently makes a big difference.
Not only does the increasing scheme put us at risk from an over-frequency perspective, but it’s even worse from an under-frequency perspective.
This sawtooth analysis allowed us to assess the different scheme designs we discussed earlier.
We could assess each scheme based on the points where it maintains the frequency between 47 and 52 Hz.
Here’s the winning scheme: four blocks of 10, 10, 6, and 6 per cent.
You’ll notice that we have included df/dt on Block 4.
Adding df/dt to the last block of the strongest-performing scheme gives us an additional weapon to deal with events where the frequency is falling fast
(for those large events where it is expected that all four AUFLS blocks may be required).
You will recall that when df/dt triggers, it increases the minimum frequency and gives us more distance from the under-frequency limit.
Although no scheme can give coverage for every event, this winning scheme allows us to stay within 47 to 52 Hz for a wider range of disturbances.
This was the strongest-performing under-frequency scheme.
That wraps up this section, where we’ve looked at the factors influencing scheme design.