3. Energy balance, imbalance and a definition of grid stability

The previous article described the grid as having no energy storage, and indeed there is no useful storage in the grid itself.  However, there is energy storage in the form of rotating kinetic energy of certain generators, and the quantity of this stored energy is one of the key determinants of stability.  The nature of this store will be expanded upon in the next article about generators.

There are two important performance indicators for a power supply system; frequency and voltage.

Most people would presume that voltage is the key performance indicator of a stable power supply system.  But it isn’t the main one.  Voltage is an important performance indicator and it should of course be kept within acceptable tolerances.  However voltage excursions tend to be reasonably local events.  So while voltage excursions can happen from place to place and they cause damage and disruption, it turns out that voltage alone is not the main ‘system wide’ stability indicator.

The key performance indicator of an acceptably stable power system is its frequency being within a close margin from its target value, typically within 0.5 Hz from either 50 Hz or 60 Hz, and importantly, the rise and fall rate of frequency deviations need to be managed to achieve that narrow window.

So what is special about the rate of change of frequency?  It is that an increasing frequency indicates more power is entering the system than is being taken out.  Likewise, a reducing frequency indicates more is being taken out than is entering.  For a power supply system to be stable it is necessary to control the frequency.  Control systems continuously observe the frequency, and the rate of change of the frequency.  The systems control generator outputs up or down to restore the frequency to the target window.  An also as a backup, other control systems measure the frequency and rate of change of frequency and carry out staged load shedding to help restore the frequency to the target window.

Of course energy imbalances of varying size are occurring all the time.  Every moment of every day the load is continuously changing, generally following a daily load curve.  These changes tend to be gradual and lead to a small rate of change of frequency.  Now and then however faults occur.  Maybe a whole city is disconnected instantly removing say 50 MW from the grid.  Or maybe a generator trips off removing say 100 MW of generation.  A power system has to cope with these changes too.  The rate of change of frequency in these cases is far higher.  These events require a fast response (within a few seconds) if the deviation is to be corrected before system collapse.  If the system can cope with the range of disturbances thrown at it, it is described as ‘stable’.  If it cannot cope with the disturbances it is described as ‘unstable’.

A stable power system is one that continuously responds and compensates for power/ frequency disturbances, and completes the required adjustments within an acceptable timeframe to adequately compensate for the power/frequency disturbances.

In the above discussion I simply stated that changing frequency and power balance are related.  A more detailed explanation of why this relationship occurs will be covered in a later article.  For now though I’ll just state that the rate of change of frequency is directly proportional to the size of the power imbalance and inversely proportional to available ‘rotational inertia’.  Large power imbalances mean a proportionately faster frequency change occurs, and consequently the response has to be bigger and faster, typically within two or three seconds if stability is to be maintained.  If not – in a couple blinks of an eye the power is off – across the whole grid.

The next article will describe some different sorts of generators, and describe why some generators work to provide frequency stability, and some do not.

Index

1. Introduction – Electric Power System Stability
2. Electric current is generated ‘on demand’. There is no stored electric current in the grid
3. Energy balance, imbalance and a definition of grid stability
4. Generator types; synchronous versus asynchronous. What goes on inside the machines?
5. Frequency stability and energy balance. A description of the interaction between frequency and grid energy flow
6. Control of the prime mover, governors, how it is done and why it matters
7. Analysis of an under frequency event