If you have been involved in trying to establish a new wind farm or solar farm on the Australian Grid you have probably heard about 'system strength', 'synchronous condensers', 'control instability' and “reactive power”.
These are technical terms which should be well defined – unfortunately they are vague and subject to misinterpretation and often misused to game market outcomes. Hopefully this article will shed a little bit of light and avoid the heat that often characterizes discussions surrounding these topics.
All the concepts listed in the first sentence revolve around so called 'system strength' – but what is this exactly? In the power system context, it is an electrical engineering concept, but it is probably best understood by analogy with civil engineering – in particular, structures like bridges.
As a convenient abstraction, bridges have two types of strength – horizontal and vertical. The horizontal span of a bridge will resist stretching or buckling as load is placed on it.
For example, the carriage way of the Sydney Harbour bridge being made of substantial beams of steel is considered 'strong' – whereas a thin rope strung across a gulley is considered 'weak'. One can carry several lanes of traffic and trains whereas the other may struggle to safely support a single person.
This is the first type of strength – the ability of the bridge to safely carry load across an unsupported span without bending or buckling. Translating this to electrical engineering it is the ability of a transmission line to carry power from one point to another.
A strong transmission line is one that operates at very high voltage (say 500 kV) and therefore can carry 1,000's of MW long distances without experiencing excessive voltage drop. On the other hand, a 'weak' transmission line may operate at a relatively low voltage (say 220 V) and can only carry 10's of kW a dozen house blocks.
Does connecting generation synchronous or asynchronous, traditional or renewable affect this 'horizontal' system strength? Not in the slightest way. The only way this type of system strength can be improved is by improving the transmission system, by increasing the conductor size, or more effectively by changing the transmission voltage to a higher value.
However, there is another form of strength on both bridges and power transmission networks. In the case of bridges, it is the pylons or supports, in the case of power transmission networks it is given the confusing term for non-experts of 'reactive power'.
Adding more supports to a bridge increases its 'vertical strength' whilst doing nothing for its 'horizontal strength'. However, depending on where you add the additional supports the average length of the span will be decreased. This will lead to a stronger structure which is now able to carry heavier load simply because the maximum span is now shorter.
How does this analogy carry over to electrical engineering?
In power engineering the transmission system is supported by generators and other sources of reactive power such as capacitor banks, SVCs or Statcoms. Connecting new additional sources of generation to an existing transmission system does not make it weaker – just the reverse – it makes it stronger.
This is the case regardless of whether the generation is synchronous or asynchronous, traditional or renewable. It helps support the power system wherever it is connected – and if the generation is not there the system will be weaker than if the generation is there.
To be sure synchronous machines can generally contribute higher levels of reactive power (i.e. provide more support) than can asynchronous power electronics which are typically used by wind farms and solar farms, however this misses an important point.
We do not need synchronous machines to be connected to a part of the grid just because you want to connect an asynchronous machine where nothing existed beforehand.
Adding a generator – any type of generator, is already making the grid stronger than it was before.
If you have been following this discussion so far, and you are aware that many new renewable projects are installing synchronous condensers you might be wondering why is it that people are spending possibly $20 million dollars to install a piece of equipment to increase system strength.
The reasons given by various industry players justifying the installation of synchronous condensers at the connection point of the new wind or solar farm do not appear to this author to be convincing.
Reactive power is needed to support power system voltage in the same way that the pylons of a bridge support its spans. However, adding generation is like adding pylons – you don't make a bridge weaker by adding pylons that then requires you to add more pylons in the same locale.
And yet this seems to be what many projects are effectively doing by adding generation and then adding synchronous condensers to the same connection point.
On the other hand – if you take generation off the system (for example when Northern Power station and Hazelwood were decommissioned) – you will
make the system weaker and you may need something to support the voltage which was previously achieved using the large traditional synchronous generator power station.
But you need it where
you took the power station away from, not where you have added the new renewable plant.
It is like if you remove a pylon from a bridge, you need to replace it with an equivalent located nearby, you will weaken the overall structure if you replace it with pylons at the other end of the bridge.
Ironically, the market as it is currently operating seems to be weakening the system by putting reactive support in the wrong locations and providing no incentive or rationale for installing reactive support where it will become needed when large fossil fuelled power stations are decommissioned.
The next major blackout event could be caused by a voltage collapse on the system, exacerbated by having the reactive support located at new renewable projects but not where it is needed which is generally near where the load centres are.
I have drawn a line under the discussion above – because the article will now look at another aspect of the system strength issue – control system instability.
Control system instability can arise in a weak power system in the same way that it can arise in any feedback control system.
If your feedback control system has an unfortunate set of time delays and gains it may oscillate uncontrollably from one state to another – we have all had the experience of a high pitched shrill from a public PA system, or more contemporaneously, the confusing echo caused by a Skype call to a remote conference room with loud audio.
When this happens with generation systems it is referred to as control system instability (or sometimes oscillatory instability).
Its relationship with weak power systems is easy to explain – a weak system's voltage responds to changes in reactive power more than a strong system's voltage does. This is like turning up the gain on your feedback control system. To fix it you need to either:
Make the system stronger – for example by installing a synchronous condenser
Or you adjust the gain down internal to the control system or redesign its internal feedback control structures.
One might cost the project about $ 20 million – the other is an adjustment to the control system software which is typically much more cost effective.
Synchronous condensers can provide more fault current to the system than inverter connected equipment can, and it may turn out that they are desirable for this reason. However this author has yet to come across this as a justification for such a development.
Bruce Miller is an Electrical Engineer and senior advisor at Advisian. This article was first published on his LinkedIn
page. Reproduced with permission of the author.