Importance of keeping the electricity network frequency stable

Question

The frequency in the electricity network is kept in balance by primary and secondary control among others (see e. g. here). I read that frequencies below 47.5 Hertz are dangerous and can lead to destruction of generators due to resonance oscillations.

So concerning this, I'd like to know:

• If frequencies below 47.5 Hertz can be dangerous for generators, does that mean generators can pass these frequencies (when starting) but they are not a secure operating point?
• What else reasons are there why it is so important to keep the electricity network frequency stable? Is it all about the rotation frequency of electrical drives somehow?

Answer 1

The electricity network has several generators (e.g. power stations, or perhaps a solar panel on your roof)

If all these feed into the same network, they must be in phase or instead of providing power they would be taking it.

Lets freeze the AC used in the network in time and make a small DC model:

Imagine two AA batteries of the same voltage in parallel, + to + and - to -, no problem there.

Now make them 180 degrees out of phase by connecting them + to -, you will have a small explosion.

The +/-0.5 degree limit is how much the network can stand being out of phase due to the resistance and inductance of the connecting cables limiting the power being reflected back by an out of phase node.

Answer 2

Most AC electric motors (induction motors) run at a speed which is a function of the supply frequency and the load on the motor. So as the mains frequency varies, the speed of every induction motor connected to the system varies with it.

At the most basic level, people don't necessarily want their motors to be changing speed all the time, regardless of their load - and lots of load types have a relationship between speed and power which is not linear - fans are a good example of this, where the torque is proportional to the speed, so the power is proportional to the square of the speed. This means that small changes in frequency lead to larger changes in power. It's useful to the network operator that if the frequency drops then the load falls too (falling frequency is a sign that the generators are not keeping up with the load), but you can clearly have too much of a good thing, and it's easy to see how you can end up with everything hunting, as lower frequency caused loads to fall, causing frequency to rise and increasing load, causing frequency to fall, etc., ad infinitum.

Clearly it's easier to set things up to be stable if you only try to do so over a small range of speeds - hence the restrictions.

Besides which, frequency is the one signal of a grid's condition which is immediately available to every generator and every consumer - so consumers can automatically trip-off if the frequency falls too far, in an attempt to help the grid recover from a loss of generation. Generators trip-off if the frequency falls too far, because who wants to be the last person trying to supply a failing power system...

Answer 3

Regards the low frequency running of generators, in my experience there are several modes of resonance that the turbine blades "hit" as they are running up to synchronous speed and the electricity generator companies, when starting up a turbine try and accelerate thru those resonances as quickly as possible hence 47.5Hz is probably a "catch-all" value used to protect all generators.

Many devices use the AC frequency for speed synchronization, timing and clocks. See this from wiki: -

Long-term stability and clock synchronization

Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor. Today network operators regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million. In the synchronous grid of Continental Europe, the deviation between network phase time and UTC (based on International Atomic Time) is calculated at 08:00 each day in a control center in Switzerland. The target frequency is then adjusted by up to ±0.01 Hz (±0.02%) from 50 Hz as needed, to ensure a long-term frequency average of exactly 50 Hz × 60 sec × 60 min × 24 hours = 4,320,000 cycles per day. In North America, whenever the error exceeds 10 seconds for the east, 3 seconds for Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour.

Real-time frequency meters for power generation in the United Kingdom are available online - an official National Grid one, and an unofficial one maintained by Dynamic Demand. Real-time frequency data of the synchronous grid of Continental Europe is available at mains frequency.com. The Frequency Monitoring Network (FNET) at the University of Tennessee measures the frequency of the interconnections within the North American power grid, as well as in several other parts of the world. These measurements are displayed on the FNET website.

Smaller power systems may not maintain frequency with the same degree of accuracy. In 2011, The North American Electric Reliability Corporation (NERC) discussed a proposed experiment that would relax frequency regulation requirements for electrical grids which would reduce the long-term accuracy of clocks and other devices that use the 60 Hz grid frequency as a time base.

Answer 4

The question you are asking is a very important and fundamental one.

Balancing the Real and Reactive power as supplied to a load is very important in electrical power distribution. The electrical supply industry will have forecasted figures for the demand at different times in day. In order to meet these demands without causing fluctuations they may or may not increase the frequencies of the prime movers (such as generators). This is by no way the only means of balancing demand with supply. They will perform power factor correction.

They will use instrument transformers, surge arrestors, shunt reactors, capacitor banks, disconnectors, circuit breakers. But whatever they do they have to ensure that the Rated Frequency (50 or 60 Hz) in the UK and the Nominal voltage is correct at all plants across a network. As the network contains inductive and capacitive sections, changing the overall network frequency is changing the balance of real to reactive power which is changing the power available to the network of users connected to the system.

Changing the frequency means you are also changing your ability to perform corrective actions such as active power injection. I think this is partially why they have all of these active control systems (particularly the instrument transformers) to manage this.

Another good point is that in introductory (per unit) simplifications of three phase signals, we assume that frequency is constant and manage the real/reactive power calculations by considering voltages in and out of phase due to inductive/capacitive motivations.

(CIVIL) [in a capacitive load current leads voltage whereas voltage leads current in an inductive load].

Answer 5

If the normal frequency is 50Hz and the turbine are run at speed corresponding to frequency less than 47.5Hz or more than52.5Hz. The blades of turbine get damage hence a strict limit frequency must be limit