Here’s the second article in the series on electric power system stability:
The electric current delivered when you turn on a switch is generated from the instant you operate the switch. There is no store of electric current in the grid. Only certain generators can provide this instant ‘service’.
The three fundamental parts of a power system are:
- its generators, which make the power,
- its loads, which use the power, and
- its grid, which connects them together.
Electricity consumers turn their lights and appliances on and off whenever they want. Factories turn on and off large industrial machinery whenever they want. Distribution companies can even turn whole cities on or off. This level of control at the consumer end and the almost always flawless response of the power system to deliver the required electricity, has led to a mistaken view about how the power system instantly provides this current. People imagine the electric grid must contain a store of electric current, immediately available on the other side the switch.
The mistaken view is that the instantly available on demand electric energy has already been produced and ready and waiting on the other side of their switch. There is voltage on the other side of the switch, but the additional current required for your load is generated only from the instant the switch is closed. There is no store of current in the grid. It’s the generators that instantly provide that current, and only some generators at that.
The current carrying components of an electric grid comprise only three types of things:
- conductors, typically either overhead wires and underground cables,
- transformers and several other sorts and voltage conversion devices, and
- switches, circuit breakers and fuses.
None of these store electric current. They transmit electricity over a distance, convert voltages up and down, and control the flow of current, but they don’t store it.
So if there is no storage in the grid the amount of electric power being put into the grid has to very closely match that taken out. If not, voltage and frequency will move outside of safe margins, and if the imbalance is not corrected very quickly it will lead to voltage and frequency excursions resulting in damage or outages, or both.
The next essay will discuss grid energy balance, imbalance and a definition of grid stability.
Index
- Introduction – Electric Power System Stability
- Electric current is generated ‘on demand’. There is no stored electric current in the grid
- Energy balance, imbalance and a definition of grid stability
- Generator types; synchronous versus asynchronous. What goes on inside the machines?
- Frequency stability and energy balance. A description of the interaction between frequency and grid energy flow
- Control of the prime mover, governors, how it is done and why it matters
- Analysis of an under frequency event
Modern consumers simply take for granted that they throw a switch, and the lights come on. Always.
Looking forward to Lessons #2 >> ∞
You have tried to summarise something which is not as straightforward as it looks. In the very short term, when an additional load is added, there is no increase in input power. An alternating current system just slows down a bit – the frequency drops. All of the previously connected loads reduce power marginally – motors slow down, for example. Governors on turbogenerators sense the speed drop and increase output to restore the frequency, aided by grid control engineer action to increase generation.
Hi Richard, it certainly is tricky to explain, especially in reasonably non technical terms. The article after the next will be talking about energy delivered in transient response from the rotating inertia in a synchronous machine, the delay angle and power curve, and how frequency disturbance relates to system synchronous inertia.
Thank you for your response! I look forward to reading your next installment!
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Black start doesn’t appear to be addressed in these pages; is the topic being developed?
I’m asking because the Victorian government appears blase about its fiasco of 25 Jan 2019 when the lights were only kept on because of the Basslink input without which, since the forced closure of Hazelwood, the system would have gone black.
You are right. I have not covered black-start. However, I would agree with you that it is an important topic. It certainly does not happen by itself. It requires a lot of forward planning and control systems to come together to make it work. The topic is worthy of more work but I am pretty flat out at work at the moment. I will see if I can put together a more detailed answer over the next week or so.
Well thanks but don’t shirk your work duties simply to satisfy the curiosity of some blow in dilettante from the internet such as I am.
All the best.