As we work to accelerate the energy transition, we make it important to share internal knowledge within our whole team to facilitate decision-making and to stimulate innovation. During our last knowledge sharing session, we went back to the basics to understand better how power systems work. Are you looking to refresh your knowledge about power systems and to perhaps learn something new about them? Check out these 5 questions and answers!
1. What is a power system?
The illustration below shows a simplified version of a power system, from the power station where electricity is generated to residential and industrial consumers.
To understand what a power system is, let’s start from a common analogy: the blood circulation system in the human body. Larger and smaller arteries carry oxygen-rich blood from the heart to the body, the heartbeat determines the frequency with which the blood is pumped to the rest of the body, and the pressure in the veins could characterize the voltage. In a power system, energy is produced in a power station (the heart) and, through transmission lines (the larger arteries) and distribution lines (the smaller arteries), it gets transported to end-users (residential and industrial customers).
Power systems have significantly evolved over the last century (in the image below you can see how the Swedish grid has evolved, starting from two disconnected power systems to a unified national grid with a complex transmission and distribution system):
But no matter how much power systems may have evolved and growing in complexity over the last century, the basics components of power systems are always these 3:
- Distribution and loads (from simple light bulbs to more complex industrial motors)
Whereas physics books traditionally describe power systems starting from the generator, we prefer to twist the way of seeing things and start from the loads: it’s because of the loads that we have electricity demand, and it’s because of electricity demand that we require a generator. Generators exist to supply loads, not the other way around (going back to our analogy with the human body, you can see it this way: the heart exists because the human body requires oxygen, not the other way around).
2. Why do we use 3-phase AC Systems?
Not differently from any other industry, the electricity industry seeks to maximize output (=transferring as much power as possible) with the lowest costs possible. 3-phase AC systems are one way to achieve this as it allows bulk power transmission over long distances and for distribution.
In a 3-phase system, with three lines you can transfer the same power corresponding to three single-phase systems but without requiring a return line. The main advantages of this are:
- greater power density than a one-phase circuit at the same amperage.
- It is more cost-effective since a three-phase system is balanced; it can be operated without a neutral wire, in cases where a neutral is desired only one wire is sufficient.
3. Is beer head really useless?
The examples of loads we gave in question 1 ranged from simple light bulbs to more complex industrial motors. Let’s take a look at the second one. Industrial motors can have widespread applications, but their basic components are always a stator (which receives power from the grid) and rotor (which rotates to generate the desired outcome, for example, running a turbine). This desired outcome is active power. You can think about it as a beer.
But when you pour a beer you also get the frothy beer head on top of it which, whether you like it or not, it’s not doing any useful work, it just lies there on your beer as a by-product of it. This can be compared to reactive power.
Reactive power is energy being transferred between magnetic and electric fields. Now this might sound a bit abstract so an example is one of the most common loads in our power system: an electric motor (induction type)
the outer part of a motor which receives power is called a stator, the rotating part running the shaft of the motor is called a rotor. The two are not connected mechanically, so they need to be connected in another way: magnetically. So to make industrial motors work they need to be connected in another way: magnetically. Inside a motor, you have a lot of coils (inductors), which generate magnetic fields. These magnetic fields connect stator and rotor, the reactive power a motor draws represents the energy required to magnetize the motor in order to transfer the power (active power) that actually makes the rotor turn.
So beer head turns out to be more useful than you think!
4. What is high voltage?
Power plants are often distant from the loads which they have to supply. These transmission lines have some resistance in them, and we want to minimize these losses to make sure that as much power produced reaches the loads. To reduce this transmission, we must reduce the amount of current in those lines, which is done by increasing the voltage. Once the current reaches the proximity of the loads, resistance becomes less of an issue because it only needs to travel a short distance, so the voltage is lowered.
Once again we can use question 1’s analogy with the blood circulation system in the human body. Thanks to the large arteries, large amounts of oxygenated blood can travel all around the body, also to those organs far from the heart. But these arteries are too large to reach all organs of the body, so they rely on smaller arteries to make sure the oxygenated blood can reach all of the organs.
5. Why can’t we send electricity all around a country?
When moving power through in transmission lines, there are some limitations in stability which become more or less relevant according to the length of transmission lines:
- thermal stability (losses due to resistance in the lines, relevant for lines below 100km)
- voltage stability (the most relevant above 100km)
- angular stability (mostly relevant for very long transmission lines)
Let’s focus on voltage stability, one of the most common limitations on power transmission in Europe (e.g. Sweden). Voltage stability can suffer because the more current passed through a transmission line, the larger the magnetic field coming out of it as a by-product. This magnetic field acts as a resistance to the transmission activities of a power end of the transmission line. The total power transferred depends on the voltage, so if you have a sufficient voltage drop to transfer the same power more current needs to be transmitted and as a result more effects on the voltage. At a certain point, the relationship between the two will not be linear and the voltage drop will be so large that power transfer cannot increase further, your system stability will quickly deteriorate causing widespread and hard to foresee impacts on the rest of the power system. Hence, to avoid this critical operating point standard operating procedure is to set a maximum limit to the amount of power that can be transferred by a certain transmission line.
An interesting consequence of voltage stability limitations is shown in the map (below) of the Swedish transmission lines. Northern Sweden usually has an excess power supply (due to a lot of hydropower plants), whereas Southern Sweden usually has excess demand. While the country would benefit from transferring very large amounts of power from North to South, there is a maximum limit allowed which guarantees sufficient margins for operations. As a consequence, power has to be provided through other paths from more expensive generation/transmission which is reflected in the higher prices (45 EUR) in the southern parts of Sweden and low prices (3 EUR) in the northern parts of Sweden.
You can reach out to me for further questions about power systems at firstname.lastname@example.org!