The humble arrester is a simple electrical component that serves a critical role: to protect assets against unwanted electrical surges.
Here are 5 important facts about these heroes of electrical power:
1. Arresters are known by a few different terms, such as ‘surge arresters’, ‘lightning arresters’ and ‘surge protection devices’. While these all do the same thing and the terms are used interchangeably, there are some slight differences between certain types. For example, lightning arresters are specifically designed to redirect lightning, however they can also be referred to as surge arresters. What’s more confusing is that surge arresters can be referred to as lightning arresters, even if they’re not designed to redirect lightning!
2. Arresters are categorised by voltage rating. Surge arresters can be rated as low as 30 V, while lightning arresters (and some other types of arresters) can be rated as low as 600 V and as high as 700 kV or more (see Figure 1). This rating is used as the baseline for what is an acceptable voltage for the asset, so if the arrester receives a voltage that exceeds that, it will treat is as a surge.
Figure1: arresters arranged in order of voltage rating
3. Inside modern arresters, there is a metal-oxide varistor (MOV) and this is what reacts to the applied voltage. The body of an arrester is usually made of strong porcelain or similar material, while the ‘fins’ are made of flexible silicone or similar material. These ‘fins’ are known as heat dissipation vents and cool the arrester during operation and keep away rain drops (see Figure 2). Some arresters have a ‘halo’ or other circular structure on top of it that helps capture lightning strikes and prevent them from causing power surges (see Figure 3).
Figure 2: 11 kV arrester without a ‘halo’. You can see the ‘fins’ clearly here.
Figure 3: High voltage 106 kV arrester with a lightning ‘halo’
4. Arresters change their resistance based on the voltage they are subjected to. The graph below shows the typical conductivity characteristics (or V-I curve) of MOV surge arresters at a constant temperature. MOV arresters have nonlinear conduction depending on the applied voltage and temperature they are subjected to.
In the graph below, voltage Uc is the continous operating voltage, Ur is the rated arrester voltage, and Uref is the ‘knee point’ voltage. Beyond the knee point, in zone B, the resistance drops allowing the arrester to conduct higher than normal voltages, preventing damage to the asset.
Upl is the peak permissiable voltage, and it is when the voltage goes beyond this point that damage may occur to the arrester (which could be permanent).
5. If an arrester is damaged due to excessive voltage as described point 4, there are a number of things that can happen as a consequence. Firstly, the arrester itself is at risk of exploding (see Figure 4), secondly, the asset itself could be severely or permanently damaged, and finally, the knock-on effect of this would result in a brown out, or even a complete black-out, for that section of the grid.
It is worth noting that if an arrester is faulty for other reasons, it won’t show any warning signs under normal conditions and puts assets at risk. It’s therefore crucial to test them regularly to ensure they are working correctly.
Figure 4: blown out surge arrester
As you can see from this list, arresters are a simple yet vital component of the power system, ensuring smooth and safe electrical power delivery. As such, it is essential they are maintained and regularly tested to prevent damage to assets and ultimately, loss of power.
All images and graphs used in this post have been taken from the ABB Application Guideline - Overvoltage Protection in Medium Voltage Systems.
Authors: Ahmed El-Rasheed and Léonie Alvey