Energy Transition Challenge 2: System Stability

Towards the end of the 19th century, a period in the history of the electrical power systems known as the War of the Currents was drawing to a close. As rivalries tend to do, this dispute involved two groups of people, those that favoured Direct Current (DC) as the future of electric systems and those that thought Alternating Current (AC) was instead the solution.

In the end, also as it often happens in history, the dispute was solved by economics and technology. With the technology available (and recently invented) at the time, the AC systems offered a more cost-effective and technically superior solution to the challenge of building and deploying electrical power systems end-to-end, from generation to consumption.

At the heart of this scheme, is the generation and use of electrical power at a specific frequency. The present-day standards across the world (mostly) being either 50 or 60 Hz.

This frequency not only governs the design of every electrical apparatus connected to it, but also the fundamental theory and models used to analyse and operate the system as a whole. This frequency also dictates the speed at which generators must input energy into the system and the speed of motors that take the energy from it.

Stable vs. Unstable System

At every instant in time, the whole electrical system has to be kept in a fine state of balance between the power being consumed and the power being generated. A system in this condition is known to be in a steady state. In other words, the system is stable.

In this condition, each element comprising this system is operating at the same frequency. In the case of rotating machine generators, this frequency is proportional to their rotating speed.

However, this condition is not maintained at all times. A large electrical system experiences many disturbances. From climatic events to overloads and failures, the system has to adapt to changing conditions.

When the load increases or an electrical fault occurs, the current drawn from the generation sources increases. This places additional mechanical load onto the generators. In other words, this increased demand acts as a breaking force unless something is done about it.

If the increase is gradual and within expectations, the countermeasure to increased load is to increase the generation, either by burning more fuel or adding generators to the system.

When the increase in load is too sudden (i.e. a large fault), generation capacity cannot be increased fast enough to respond to this event. In this case, the mechanical inertia of large rotating generating machines maintains the frequency of the system whilst the fault is cleared.

If these conditions are met when a load increase or fault occur, the system maintains its frequency and therefore it continues to be stable.

However, if the load surpasses the combined generation capacity of the system or the fault is not cleared fast enough, some generators start slowing down and operating at a lower frequency than others.

This places additional electrical and mechanical strains on the system as a whole. All components are no longer operating at the same frequency (or in synchronism) and the system becomes unstable.

In order to prevent damage to components throughout the system, additional measures have to be taken. This typically involves the operation of backup protections or measures to reduce the load. This has the intent to bring the system back into a stable condition.

If the system is not controlled in time, it can lead to a large system black-out. These events tend to have devastating consequences for the community and the economy as a whole.

The "low inertia" of renewables

Now, what does all this have to do with the energy transformation?

Well, as it turns out, some renewable sources of energy such as solar and wind, do not have the same mechanical inertia than the large rotating generating machines.

This means that systems that source their energy from these types of renewables in large proportion, tend to be more vulnerable to instability caused by large faults or sudden increases in load.

Another peculiarity of system with this mix is that whilst load might not suddenly increase, the generating capacity could drop relatively quickly if the solar irradiation reduces quickly. This happens when large clouds pass over the solar panels producing all this power.

What can be done?

All these challenges are well known to system and market operators around the world.

Depending on various factors such as the specific mix of generation sources, the state of the local economy or the particular goals of the political jurisdiction, there are various countermeasures that are being devised and implemented across the globe.

Amongst others, these solutions include:

  • Management of reserve generation capacity

  • Load management

  • Energy storage

  • Economic incentives

This is a good video talking about the effects of large power outages

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