MadSci Network: Engineering

Re: What are the rough metrics of a power plant generator (see comments)

Date: Fri Oct 16 17:02:18 2009
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Engineering
ID: 1255658897.Eg

Some of these questions are quite difficult to answer. Though I am an electrical engineer and know a bit about generators, I have never actually worked in the field of high power electrical generation.

However I have found some answers for you.

First, some basics -- AC power generation is most often by an AC synchronous generator. The generator can be turned by steam turbines, gas turbines, windmills, reciprocating engines etc.

Synchronous engines are so named because their rotating speed and the frequency of the electrical current are matched. This is an inherent characteristic of the machine. It is to do with the way the stator locks onto a rotating magnetic field. I could go into how synchronous generators work, but that is really another topic. For now just accept that synchronous generators/motors speed is matched to the frequency of the elctrical circuit.

Also understand that there is no fundamental difference between a motor and a generator. The operation is the same. One however will consume electrical power will the other will produce it. Basically it is just a matter of current flow through the stator windings.

For a single generator the speed it spins at determines the frequency it will output, if it is acting as a motor, it spins at a speed determined by the frequency of the electrical circuit it is connected to.

So if you attach an unpowered (so no driving turbine connected to the generator) synchronous AC engine to a 50Hz power grid, it will spin (as a motor) at a speed determined by the frequency of the circuit. It will however be drawing power from the grid.

The relationship is N = (120 * F)/P, where N is rotor speed in rpm, F is frequency in Hz and P is the number of poles.

So for a two pole generator driving a 50Hz grid the speed of the generator would be 3000 rpm. In very large generators the generator is usually directly driven from the turbine. In smaller generators there is often speed reduction gearing.

Okay, we will deal with speed regulation question first.

In a large power grid the generators are all connected in parallel. Because of this, their speed is dictated by the frequency of the grid. Even if a generator had zero torque being applied to it from it's driving turbine it would still spin at the synchronous speed. However, the generator would be consuming power (it would be an synchronous motor). It would be like any other load on the grid.

So all the generators connected will be "locked" together at a set frequency -- as long as the power supplied to the generators (from their turbines) matches the power consumed on the grid.

I suppose you need to look at how that frequency first got established. When the very first generator started, or when another is brought online, it must first be spun up to the speed that matches the grid frequency. This is done by controlling the speed of the turbine (or other driving device). When there are lots of turbines online a single turbine should not change the frequency of the entire grid. Once the turbines are all connected together they lock in synchronisation.

Now it is possible to lower that frequency by putting large loads onto the grid. If the load starts exceeding the grid capacity frequency will change (it will lower). However whilst the generating capacity of the grid matches the grid load, frequency will be stable. As the load on the grid increases the mechanical power into the turbines, that drive the synchronous generators, needs to increase. This extra power into the generators is seen as an increase in current out from the generator with speed locked at the synchronous speed.

There will still be changes in frequency, especially with large load changes on the grid. However individual loads on the grid are normally much smaller than total grid capacity. Small changes in frequency are controlled by either using load control devices on the generators, or by speed control systems on the turbines, or both.

For a good but fairly technical discussion of frequency control see, and see also

For single generators driving an electrical load not connected to other generators, or the grid, the frequency will be controlled by controlling the speed of the synchronous generator. So say on an aeroplane, the electrical generators are connected by mechanical gearing to the jet engines. A hydro mechanical unit, often called a constant speed drive, will keep the generator running at the same constant speed, even though the speed of the driving jet engines changes.

Voltage regulation is done in two methods. At the generator there will be an AVR. Automatic voltage regulator. The generator voltage output, at a fixed RPM, is set by the rotor (field) voltage. The AVR is an electronic device that detects changes in output voltage and changes the field voltage accordingly. The grid will also have multitap transformers that will change the voltage in the circuit as loads change.

The discussion I linked to earlier also discusses AVR and the relationship between excitation (the field) and the output voltage.

Now to the actual metrics you wanted. This is a bit harder. I have had no direct contact with large power systems and the internet is a bit short on specific information. The best I can do is tell you when I have seen large power station generators the entire machine was many metres across. I have been told there are many many kilometers of wiring inside. The rotor would be approximately a metre or two across.

Generally they do not use permanent magnets. The armature (the field winding in a synchronous machine) is wound with coils of wire and a DC voltage is applied. When it spins this sets up the rotating magnetic field in the stator windings that causes the AC output. The field RPM sets the frequency of the stator out, and field RPM and voltage determines the stator voltage (but remember RPM is fixed, so stator voltage becomes controllable by field voltage).

Generator voltages are usually in the 11kV to 22Kv range.

Power output depends on the machine. Obviously you can buy small portable machines with only a few Watts of power. Large turbine driven generators can be up to 600MW.

I have also found a book on large generator maintenance if you want to read up on that.

Geoff Klempner, Isidor Kerszenbaum
Handbook of Large Turbo Generator Operation and Maintenance, second edition
2008, John Wiley

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