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A Giant UPS for the Power Grid

I've never really written much about my job since I've always done pretty mundane stuff. Yes, I always had really cool jobs (well, internships) except the one other full time job that I had - that I'd rather not talk about - but as far as the tech goes nothing was really out of the ordinary. That's changed, however, and the full extent of the coolness of my current job is starting to hit me so hard now I thought I just had to write about it.

Power Grids, Explained.

The world runs on electricity. Everything that you see around you runs on electricity - in some sense having power is almost as important as having drinking water. As it turns out though, if everyone generated their own electrical power according to their own needs, it would be massively inefficient. Power generation works on economies of scale, so it's much cheaper - and more efficient in a thermodynamic sense - to generate a massive amount of power in a specialised location and distribute that power using cabling to all the consumers.

So a nation's electricity infrastructure will consist of a couple of million households or offices or factories and stuff that consume power, a few hundred electricity generation sites, and a tangle of cables that carry power from these power plants to the consumer. This tangle of cables (okay, they're not really a tangle - they're far more organised and heavily engineered) form a nation's power grid.

An ideal power grid is a fully connected graph. That's why it's a grid - there's always a path from every single consumer to every single electricity producer in the system, so that if one power plant fails the consumer is always connected to all the other ones to be able to draw power from them.

However, with so many things connected to the power grid, things start getting complicated.

Let's begin with some high school physics. Joule's Law1 - the first one - says that the heat dissipated by a conductor is directly proportional to the square of the current that passes through it. That means that 1 ampere of current passing through a wire with 1 ohm of resistance will produce 1 joule of heat per second, but 2 amps of current passing through the same conductor will produce 4 joules of heat. A thousand amps of current will... produce enough heat to melt the wire and there will soon be no power grid.

The power grid has to carry tens to hundreds of gigawatts of power.

Notice that I said power, not current. An electrical appliance only cares about how much power it consumes. Power is the product of voltage and current, which means you can carry the same amount of power through a wire by using a low amount of current at an insanely high voltage. High voltages don't melt wires. Also, wires used in power grids have pretty low resistances - usually of the order of 10-10 ohms per meter, but that's still 0.001 Ohms for a 1000 km stretch of wire. If you do the math, a thousand amps of current will produce 10,000 joules of heat every second - or 10 kW of heat. That's a lot of wastage.

So electricity transmission happens at even lower amperages, and insanely high voltages. Common transmission lines operate at anywhere between 345 kilovolts to 750 kilovolts, but there are transmission systems that operate at 1000 kV. A full megavolt. At 1 MV, the current needed to transmit one megawatt of power is just one ampere.

Remember what I said about electrical appliances caring only about the power they consume? I lied... sort of. You can't just connect your television to a one megavolt power line. Potential differences (voltages) have real implications in the physical world - most importantly, if you bring a conductor at a +1 MV potential within a few metres of anything at ground potential (such as yourself), you'll get a brilliant flash of lightning and pretty soon that thing at ground potential will cease to exist. Air is a good insulator, but not enough to prevent an electrical arc between two objects at a million volts of difference.

So just before you deliver power to the consumers, you have to step down the voltage to something a little more sane. In Europe, India and most of the world that sane voltage is 230 volts. At that voltage, the amount of power that a common household consumes does not need a lot of amps to transmit. In the US that voltage is 110 volts, but the USA is a strange country so I will not attempt to explain this.

And here's where things start getting interesting. It turns out - due to something called inductive coupling2 - that you can pretty much convert between low voltage and high current to high voltage and low current, without (theoretically, anyway) any loss in power (i.e., the multiplication product of the voltage and current) by just wrapping the wires around a piece of metal in a specific way. The device used to do this is called a transformer, and transformers are an integral part of every power grid.

There's just one problem. Inductive coupling only works if the voltage in the conductor keeps changing all the time. And this is where alternating current (AC) comes in. With alternating current, the voltage constantly cycles from +230V to -230V, 50 times every second (in the EU, India and other not-strange countries). Okay, it cycles from +325V to -325V because 230V is the RMS voltage, not peak voltage, but that's just splitting hairs.

Remember these numbers. These are national standards. Every wall socket in an European country has to output 230V of AC electricity at a frequency of 50Hz. If it stops doing that, then Bad ThingsTM will happen.

Power Grids, Part 2

In part 1, we learnt that the power grid distributes power from power plants to homes and stuff. We also learnt that the power grid carries power at very high voltages and very low currents, and because it's so efficient to step down voltages at the consumer site using inductive coupling, the whole system uses alternating current.

In part 2, we learn that the power grid has one major drawback - it can only transmit power, not store it. So at any given point in time, all the power plants in the system together produce just as much power as all the consumers together need. No more, no less. All the power that is being produced has to be consumed.

So why can't we just produce more power than is required? Well, here's the deal. Most power generation in power plants is done using Synchronous Alternators. That's a fancy term for generators which generate alternating current at the exact same frequency that they're turning at. This means all the generators in power plants in Europe keep turning at 3000 RPM (50 rotations per second, for AC electricity at 50Hz).

Imagine you are driving your car on a level road, doing 50 KPH. Suddenly, you start going up an incline. Your speed starts dropping, because your wheels now start to turn more slowly. You need to press down harder on the accelerator to make your wheels go faster now, if you want to maintain that 50 KPH speed.

Synchronous Alternators work the same way. The more power you draw from them, the harder it gets to turn them. If whatever is driving that alternator - a diesel engine, a nuclear reactor, a gas turbine or something - does not step up its power output, the alternator will start turning more slowly, and the frequency of the power output will drop. Similarly, if you draw too little power, the driving engine will be turning the alternator with too much power, and it will turn faster than it needs to.

The load on the electrical grid changes every moment, because at any given moment someone is turning something off and someone else is turning something on. Power plants have to adjust their power output every milisecond. Unfortunately, a nuclear reactor, or a steam boiler, or a gas turbine is not like a car engine. You can't just press an accelerator pedal and make it instantly go vroom vroom. Power plants need a lot of time to react to load changes. Tens of seconds to full minutes, sometimes.

Frequency Containment Reserves

This is where the work that I do comes in.

Devices that hook up to the power grid tend to be pretty tolerant about voltage fluctuations, but not about frequency fluctuations. Indeed, some devices (including critical medical devices) rely on the power grid cycling exactly 50 times a second to count time. They measure one second by counting 50 cycles on their input current.

A 100 milihertz deviation of frqeuency is therefore considered a power grid emergency. This means (by EU standards) the power grid frequency can never drop below 49.9Hz and never go above 50.1Hz.

Frequency Containment is the process of controlling the frequency of the power grid. If we see that the frequency is too high, we create load to consume more power from the system and bring the frequency down. If we see that the frequency is too low, we deliver more power into the system (i.e., create a negative load) to bring the frequency of the grid up.

So how does FC become FCR? The R in FCR stands for Reserves. Batteries.

If you bring enough batteries together, you have the capacity to charge and discharge them really fast. Enough Lithium-Ion batteries in one place can be used to draw a massive amount of power from the grid in a very short amount of time. They can also inject a massive amount of power into the grid in just as short a period of time.

And so that's what FCR is. It's a giant uninterruptible power supply for a power grid. It draws or injects power from or into the power grid for the short spans of time that it takes for the power plants to react to changes in power demands and spin their generators faster or slower.

All national power grids have FCR units, typically operated by the power generation companies or the grid operators. In the continental European power grid, however, the FCR services market is open to all players, so there are private players with very innovative control technology competing with established publicly owned infrastructure service providers.

In fact, in the EU, even you as a private person can be an FCR provider. Do you have a giant battery (like a Tesla PowerWall) at home? Install some hardware that reacts to the grid frequency, hook it up to your power line with a grid-tie inverter, and tell your utility company that you'd like to provide local FCR services. TSOs (Transmission Service Operators, a fancy term for power grid operators) theoretically provide daily contracts to individuals who wish to provide FCR services with batteries in their homes. It's not really effective - one PowerWall can't really make much of a dent in the local grid, but in the current EU legal framework it's already possible to do it.

Best of all, as Europe switches to renewable energy, the FCR market is here to stay. Wind turbines and solar panels all generate fluctuating amounts of DC power depending on the sun and the wind, and need grid-tie inverters to convert that into AC power at grid voltages. Electronic inverters have their own set of problems that make maintaining a proper sinusoidal frequency even more difficult under constantly fluctuating loads, so FCR operators will need to provide more and more power balancing capacity.

I hope this was a good explanation of what FCR is. If you have questions or comments, please get in touch! Till next time, tschau!