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In the UK our mains standard is 230 V AC at 50 hertz. So I understand that the generators at the power stations need to spin at exactly 3000 RPM . This is true no matter what the source is (nuclear, coal, gas, hydroelectric etc). Let's use steam as an example here. How do they use the steam to spin the generator at a constant 50 hertz? It doesn't seem plausible to control the speed with the amount of steam passed though to that precise speed. So how do they do it?

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    \$\begingroup\$ The short explanation is that the steam supply is adequate to spin the generator faster than synchronous speed, but the load (the grid) acts as a huge brake holding it back. Apply more steam and it'll just put more power into the grid but remain locked to it (the entire grid will speed up infinitesimally). Increase the load on the grid and it'll slow down, drawing more from each generator. You can see the speed fluctuating in the short term, e.g. at gridwatch.templar.co.uk but the long term average speed remains 50Hz. \$\endgroup\$ – Brian Drummond May 21 '16 at 11:24
  • \$\begingroup\$ It's totally plausible and a reality. The main problem here to address is your learning so I suggest you do some research on the subject. Look up closed-loop control systems and maybe phase-locked-loops. \$\endgroup\$ – Andy aka May 21 '16 at 11:44
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From UK's National Grid:

enter image description here

Figure 1. The graph shows Frequency data to a 15 second resolution over the hour up to 2016-05-21 13:31 BST.

System frequency is a continuously changing variable that is determined and controlled by the second-by-second (real time) balance between system demand and total generation. If demand is greater than generation, the frequency falls while if generation is greater than demand, the frequency rises.

National Grid has a licence obligation to control frequency within the limits specified in the 'Electricity Supply Regulations', i.e. ±1% of nominal system frequency (50.00Hz) save in abnormal or exceptional circumstances. National Grid must therefore ensure that sufficient generation and / or demand is held in automatic readiness to manage all credible circumstances that might result in frequency variations.

There are two types of Frequency Response Dynamic and Non Dynamic Response. Dynamic Frequency Response is a continuously provided service used to manage the normal second by second changes on the system. While Non Dynamic Frequency Response is usually a discrete service triggered at a defined frequency deviation.

To answer your questions:

So I understand that the generators at the power stations need to spin at exactly 3000rpm.

Not constantly. Over a day it averages to exactly 50 Hz. This used to be a requirement to keep all mains powered clocks in synch. I have one on my electrical supply day/night meter.

This is true no matter what source is (nuclear, coal, gas, hydroelectric etc).

Yes, they all run in synch.

Let's use steam as an example here. How do they use the steam to spin the generator at a constant 50 hertz? It doesn't seem plausible to control the speed with the amount to steam passed though to that precise speed. So how do they do it?

It is, in fact, possible, provided there is enough steam being generated to supply the peak load. With a single steam generator supplying an island, for example, the scheduler would plan ahead using weather information, TV schedules (the famous 10 million kettles going on in the big-game half-time), large load user schedules, etc., to have the thermal plant generating enough steam in anticipation. Meanwhile, while demand is low, the plant has to vent the excess steam to atmosphere. Yes, this is wasteful.

In practice, most grids have a mix of base-load thermal plant with other fast response generators such as gas turbine and hydro which can be switched in and out quickly.

The UK also has capability to import power from France and Ireland by underwater DC interconnects. These allow connection of multiple national grids without synchronising problems.

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It is plausible to control the speed just with the amount of steam or water passed through a turbine, due to the immense mass and inertia of all the generating devices spread across the electrical grid.

Although you may think each generator is operating separately to push power into the grid, that is not how it works with multi-phase alternating current. Instead there is a very real comparison between the electric grid, and old-fashioned machines that were tied together in a factory with belts, gears, and line shafts.

Power plant generators are huge and heavy, and the combined mass of their electric rotors and the turbine will together act as a very large flywheel, storing energy in their spinning mass.

When alternating current electric wires are connected between generators, the wiring functions almost exactly as if there were a huge physical shaft, or gear sprockets and a chain, connecting the generators together. So now these spinning generators, which previously were separate, now act as a combined monstrously large flywheel with very high inertia, both spinning together at the same speed.

If one of the generators changes speed, that difference is instantly transferred to the other generators through the electric grid tying them together.

(If you research the history of power transmission, you will discover that before electric power was developed, mechanical force really was transmitted over long distances from multiple power sources, using huge cables suspended on pulleys on poles. So they did actually do it that way at one time, though the distances were short by modern standards.)

Reference:
* On the Development and Transmission of Power: From Central Stations
* Copyright 1894, 312 pages
* Chapter 7, Telodynamic transmission, pages 108-128
* Google books free access: https://books.google.com/books?id=-blKAAAAMAAJ

 

This in fact is how the electric grid recovers from a power failure. Before attempting to restore power to customers, many small power plants will start up and reconnect the grid between each other, combining them together across those long-distance transmission lines, to have a single huge shared spinning mechanical inertia.

When they throw the switch to restore power to a portion of customers, there is an equally huge collection of customer equipment sitting in the "on" position waiting for power, and the power plants need their combined spinning inertia to overcome the load and nearly instantaneously restore power to all those customers simultaneously.

There may be a very brief sag in output as the steam and water to the generators "throttles up" to continue to support the customer load, and then once the correct power plant speed is restored, which is done only through adjusting the amount of pressurized steam or water applied to the turbines, they will activate another collection of customers and repeat the process.

 

Synchronous machines act as a combined motor and generator, with perfect feedback between either role. Although these are sometimes differentiated as being either a generator or a motor, in practice they are both simultaneously.

It acts like a physical gear wheel meshed with other gears, though with even greater torque because every "gear tooth" (pole face) is meshed with a matching rotating magnetic field, acting on all poles simultaneously. This meshing force comes from the energized field windings of the synchronous machine.

If a coal or nuclear power station turns off the supply of steam, or a hydroelectric station turns off the water, a synchronous machine will not slow down as long as the field windings remain energized.

It will immediately begin to act as a motor the moment its rotor starts to lag even slightly behind grid frequency, and it will continue to spin at full speed in synchrony with the grid. It will draw a relatively small amount of power, just to overcome its rotor bearings and air resistance.

 

Large synchronous motors cannot be simply connected to the utility grid while not spinning, because the full output of the grid is reverse-driven into it as a motor and can apply sufficient sudden force to damage the rotor, deform windings, crack concrete foundations, and blow very large fuses.

The effect is very similar to forcing a mechanical manual transmission into gear with the engine at full speed and without using the clutch.

Instead synchronous machines are typically independently brought up to matching speed, and their rotation is then slightly adjusted until their windings exactly align with the grid, and only then are they fully switched into operation.

 

For demonstration purposes, synchrony between the machine's rotor and the grid can be demonstrated with a light bulb connecting one of the machine's phase coils to the grid. The bulb shows the difference in between the grid frequency and the synchronous machine. The bulb glows brightly when the rotor teeth are out of phase and is dark when the teeth are in phase.

The lamp flashes quickly when there is a large speed difference between the rotor and the grid. When the rotor is stopped the lamp flashes at full grid frequency.

The lamp slows the flashing as the rotor accelerates and approaches the correct speed, until the lamp stays continuously dark when the rotor is both at the correct speed and the phase coil is in grid alignment. It is at this point that the power-producing phase coils are fully connected to the utility grid.

Example, from Youtube:

Synchronizing AC generators -- Part 1 (introduction and sync lamps) https://www.youtube.com/watch?v=RGPCIypib5Q

 

Typically very large synchronous machines are wired in reverse from how simple toy generators work, which would have field coils on the outside and stationary, and the phase windings on the inside using slip rings (or a commutator in a DC machine).

For utility-sized synchronous machines, the slip rings would need to be incredibly huge to handle the current produced by the phase windings. So instead, the field coil is on the inside, on the rotating part, and the power producing phase windings on the outside, on the fixed part.

 

To further simplify the design, the inner rotating field coil of a large utility synchronous machine can be indirectly powered by either a rotary transformer, or by a rotary exciter generator occupying part of the rotor.

In either case, no brushes or slip rings are needed at all, and there is no physical connection between the rotor and the surrounding frame, other than at the bearings. This significantly increases reliability and operational life between maintenance periods.

A rotary transformer works like a common electronics transformer, except there are two round loops of wire and two C-shoped pole pieces held side-by-side, with one coil fixed, and the other rotating. At any rotational position, the coils are equally coupled together, and full power can be communicated across the gap at any rotational speed or while stationary. Direct current is typically used for synchronous machine field windings, so alternating current is passed across the rotary transformer and rectified to DC by electronics attached to the spinning rotor shaft.

The other route, the brushless exciter, is wired like a standard AC generator with the field windings on the outside and the phase windings on the inside, again driving a rectifier on the rotor to produce direct current. For this design, the rotor must be spinning for the brushless exciter to produce power to drive the syncronous machine's inner rotating field coil.

 

References:

Rotating Transformer for a Wound Rotor Synchronous Motor by Jiyoung Lee1, Jongmoo Kim, and Byoungchul Woo, Korea Electrotechnology Research Institute, Changwon, Korea, University of Science & Technology http://www.evs28.org/event_file/event_file/1/pfile/EVS28_0540_final.pdf

Rotary Transformer Design for Brushless Electrically Excited Synchronous Machines Masters Thesis by Mattia Tosi, Universit`a of Padua, Department of Industrial Engineering, Master of Science in Electrical Engineering http://tesi.cab.unipd.it/45556/1/Mastersthesis_Tosi_Mattia_1035471.pdf

Synchronous Generators I (presentation slides), EE 340, Spring 2011, Howard R. Hughes College of Engineering, University of Nevada, Las Vegas http://www.egr.unlv.edu/~eebag/Synchronous%20Generator%20I.pdf

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Utility power generators are synchronous machines. When connected to the grid, they must rotate at the rate defined by the grid frequency.

When you increase the amount of steam driving such a generator, the torque goes up and it attempts to drive the entire grid at a faster frequency. However, the actual frequency changes very little if at all, and the major effect is that the generator pushes more current into the grid relative to the other generators.

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    \$\begingroup\$ This is like "they rotate exactly at 3000rpm because they actually rotate at 3000rpm". \$\endgroup\$ – Gregory Kornblum May 21 '16 at 11:35
  • \$\begingroup\$ @GregoryKornblum: Yes. For example, when starting up a generator, it is necessary to spin it up to as close to 3000 RPM as you can get while it is still disconnected from the grid. Monitor the phase alignment between the generator voltage and the grid voltage, and when they're in phase, throw the switch that connects them. From that point on, the rotation of the generator is "locked" to the grid frequency, and the shaft torque determines how much power it adds to the grid. \$\endgroup\$ – Dave Tweed May 21 '16 at 11:56
  • \$\begingroup\$ It's all cool, but i think he asks for the exact feedback mechanism. Say, one generator goes a little faster, where comes the compensating torque from? \$\endgroup\$ – Gregory Kornblum May 21 '16 at 11:59
  • \$\begingroup\$ @GregoryKornblum: That's the point -- one generator cannot "go a little faster". It can attempt to add more power to the grid than it is designed for, but control circuits will detect this excessive current flow and either throttle it back, or in the worst case, disconnect it from the grid. \$\endgroup\$ – Dave Tweed May 21 '16 at 12:02
  • \$\begingroup\$ So it can, at least to the point where error is detectable. And the answer is, the throttle is controlled. And although steam pressure may be controlled relatively slower, there is no actual need to control over each cycle. \$\endgroup\$ – Gregory Kornblum May 21 '16 at 12:05
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There are several very powerfull plants that maintain the frequency by means with radio/atomic clock synchronization, all the others are synced to the mains powerline, they just follow the mains frequency. These powerful plants have the ability to change the frequency if needed (several GVAh per Hz change are needed) enter image description here

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