# How do power companies regulate power plant's voltage output?

Whether it be a thermal power station or nuclear power station or a hydroelectric damn its impossible to keep all the dynamos running at a fixed speed.

As such how do power companies regulate the voltage from wildly varying ones to near perfect 110/220 v for household use?

I'm also interested to know how was this done in older times like early 1900s and before.

• All generators in the net are running at the same speed, but it is not a fixed speed. The speed is regulated to hold the frequency very close to the nominal value of 60 or 50 Hz. What you think impossible is done every day round the clock since many decades. – Uwe Nov 8 '16 at 11:45
• @Uwe but how? They have friction brakes or something that continously keep regulating the speed using invasive means? – Allahjane Nov 8 '16 at 22:32
• So many good answers having real hard time choosing one to tick – Allahjane Nov 8 '16 at 22:48

How does some god knows what voltage and frequency generated at a dam ends up as steady 50 hz sine wave household voltage

It's 50Hz when generated - the generators are built with a number of phases and run at a constant speed.

Whether it be a thermal power station or nuclear power station or a hydroelectric damn its impossible to keep all the dynamos running at a fixed speed.

Other way round: before connecting a generator to the grid, it is absolutely essential that it is running at almost the right speed, and in phase with the grid. Once it's locked into the grid, there is a feedback process. Trying to run "ahead" will increase the resistance torque along the shaft, running "behind" will decrease it. This keeps the corresponding turbines running at the right speed.

So when powering up the system, the generator would be spun up with manual control of turbine power levels, then carefully brought into phase, and then a huge switch would be thrown to connect it to the grid.

The 19th-century system for doing this involving lamps is described in https://en.wikipedia.org/wiki/Synchronization_(alternating_current)

(Big exceptions: wind turbines and solar PV panels. But that warrants a whole other question and answer essay. As does "what happens when you don't have the grid to sync to", known as black start.)

Also see

The electricity grid: what keeps small power generators from being 'driven' by large ones?

What happens to excess energy fed into the power grid?

• So you are sort of saying there is nothing between the bulb in my house and the gigantic generator at a nuclear plant except few transformers? As in no voltage regulating resistor network or anything? Mind=blown – Allahjane Nov 8 '16 at 22:42
• Good answer. Janka had a great answer too but had to choose one so chose this one because it is defined in layman terms for beginners like me – Allahjane Nov 8 '16 at 22:50
• Can you imagine the waste of resistive voltage regulation on that scale? No, it's straight through. There are occasional capacitor/inductor elements to maintain phase, and in a few places there are solid state DC-AC converters for international links. – pjc50 Nov 8 '16 at 23:19
• So I get the frequency part. But are generators also designed so that they all output a well defined voltage at the grid frequency? – Allahjane Nov 9 '16 at 1:52

I think it helps you understand when you think of an electrical machine as both a motor and a generator. They are both at the same time, and when they are running on a power grid, they support each other.

Synchronous AC machines running on a grid indeed all run at fixed speed. And they all have a fixed voltage too, which is both set by the grid.

As the machines are mechanically coupled to the turbine, the turbines run at a fixed speed, too. When you put mechanical power on the generator through the turbine, it doesn't run faster but instead the angular displacement between the mechanical rotation angle and the electrical phasor angle increases. Second, the generated voltage increases a little bit over that of the grid offers at the place so the current flow direction is into the grid. That way the generator can transform mechanical power into electrical power and push it into the grid.

As a result, all machines on the grid as a whole are running a little bit faster. The more machines the grid has, the smaller the effect from one single machine. So controlling the power available to the grid can be achieved by regulating the frequency.

A simple dynamo is a synchronous machine, too, but doesn't run on a grid. In that case, the frequency and voltage are variable. It's no-one there who can help out.

In the beginning of AC electrification, grids were weak, only a few machines on them, and frequency and voltage had been fluctuating a lot. But at all times, the frequency and voltage was shared by all machines on the grid. Inherently. That's how synchronous AC machines works.

Before, with DC electrification, there was no frequency for regulation, in was all done by regulating the voltage, which is a bit more tricky, as it heavily depends on the load distribution on the grid.

• I still don't get it. How does some god knows what voltage and frequency generated at a dam ends up as steady 50 hz sine wave household voltage – Allahjane Nov 8 '16 at 3:00
• The frequency is constantly measured. When the frequency decreases because the load in the grid forces all the generators to run a bit slower, some gas turbines on the grid are controlled to burn a bit more fuel, making their generator feed more power into the grid. Then, all the generators speed up again, towards the 50Hz. Voltage measurements on the grid are only used to select which gas turbine power plant is preferred – that one where the voltage is lowest. Voltage can rise or fall in a band of +-10%, while the frequency has a band of only +-0.5% in powerful grids. – Janka Nov 8 '16 at 10:14
• @Allahjane If you already have a strong grid you connect your hydroelectic dam to, there is already a 50 Hz grid which is your "master" and once you connect it will force your generators to 50 Hz speed (and to some extent voltage) and you can feed it with as much power as you want, the grid will still stay 50 Hz. You have voltage regulation at a high level too with tap changers in the transformers but the most busy tap changers are in the end transformers near the customers (10/0.4 kV). – winny Nov 8 '16 at 11:34
• @winny so if one of the plants goes bad and generators go kaput turning at bad frequency then would it be felt by all the other plants? Sort of like a chain reaction de syncing the entire grid and dragging it to a halt – Allahjane Nov 8 '16 at 22:47
• The generators can't get out of sync while connected without self-destruction, but chain failure of regulation is possible when a line drops out : en.m.wikipedia.org/wiki/Northeast_blackout_of_2003 – pjc50 Nov 8 '16 at 23:14

Before you attach the generator on the grid it has to be synchronized by means of regultating the water flow from a dam. Then the generator is switched on the grid. At this time it still doesn't produce any elecricity, becuase it is unloaded and switching the mode of operation from motor to genrator. For example if the water flow is reduced, then it will work as motor and force the flow. If the flow is increased then it turns into a generator, in both situations the speed is equal, only the phase lag is different.
The exact frequency is dictated by the joint of large power plants that are regulating the frequency, the others, smaller power plants just follow.

Without automated phase frequency control and global frequency clocks with an error of < 0.0001 ppm for reference, the grid would slow down in frequency with loading effects on generators and rise in frequency with lower demand. Every generator has some means to control Voltage and Frequency independently with parameters monitored locally but references from the remote interconnect site. Also phase as well, is critical, which is the time integration of frequency error.

The simple way to understand this is the output impedance of the grid is much lower than the demand load impedance such that load changes do not affect the grid voltage too much. This impedance ratio is the same as the regional step load voltage regulation error. Parameters are often measured in "per unit" or p.u. of the nominal values.

Unfortunately in some countries with a loose grid, consumers try to do their own regulation with voltage auto tap changers which then causes more load current to boost voltage and in effect causes a positive feedback loop to the generator making their grid unstable. This can only be improved slowly with bigger transformers and more supply sources to shunt and lower the grid impedance while not exceeding thenratio for short circuit current in any one transformer. The bigger xxx MV transformers must also have a higher Zo of 8 to 10% due to the extreme internal forces limitations, so it is not a simple solution but a steady infrastructure growth to improve power quality.

Thus since the grid is intelligently controlled , every minute an algorithm is computed at an interconnect point and new parameters for each generator are fed back to ensure supply meets demand at the lowest cost with phase/frequency/voltage feedback to each source at connection points.

This in turn affects the power stored on the grid to meet surge demands. It affects RMS voltage, frequency and phase at the end user by many complex interconnected control systems with a regional centralized reference managed by a government regulators group that dictates to all roviders and delivers to all customers, the measureable power standards established in the Industry for each region.

A loose grid in one region might vary 5% in frequency with insufficient redundancy, while a very tight grid might be < 0.0001 PPM, some of the time but vary much more then have a low mean frequency error over a daily cycle. The cost of stability is greater and its tolerance allows more short term cost reduced flexibility but may cost more in maintenance from surges, so changes are made very slowly. Each source may have different rate structures and capacity.