Electric power industry consulting
Medium voltage 6-35 kV grid usually do not have a solid neutral grounding, and a voltage of 50 Hz may occur on the neutral. Today let’s talk about why it may happen in the cable network even in normal operation, and how this can lead to Temporary Overvoltages (TOV) and damage to equipment. One of
For surge arresters (SA) of 110 kV and above, special grading rings are usually used, which, in normal operation mode, make it possible to equalize the distribution of AC 50 Hz voltage along the SA height. This distribution is important because it guarantees the absence of overheating of the SA elements (varistors), eliminates the risk
The photo shows the grading ring (“screen”) that remained from the damaged 110 kV metal-oxide surge arrester (SA or MOA). I’m not going to discuss now why and where the damaged SA went (flew away or just fell down on some equipment and people’s heads). Now I would like to discuss the SA screen only.
Unfortunately, when checking the resistance of equipment to short-circuit currents, it is necessary to take into account not the time of the main protection (usually less than 0.1 seconds), but the time of backup protection (can reach 0.5-2.0 seconds). Such rules, for example, are applied all over the world when designing cable lines. This means
Currently, there are a lot of switchgears where metal-oxide surge arresters (MOA or SA) are installed nearby other equipment. We can see MOAs very close to the power transformers, measuring transformers, cable terminations. Sometimes MOAs are installed directly on the tank of a power transformer or suspended above it. Unfortunately, operational experience shows that all
Counters of the impulse number were used for the previous generation of MOA – it was gapped valve-type arresters. That generation had a concept of “resource” for them because of a small thermal resource of nonlinear elements connected in series with gap. It was only 20-30 discharges of the gap, after which it was necessary
Under the 50 Hz AC operating voltage there is a AC current inside the MOA. For a serviceable MOA, it is about 0.5 mA (some models can have up to 1 mA). If the current increases, then this is the basis for checking the MOA – this is the reasoning of those who lobby for
The photo shows a monitor (meter) for metal-oxide surge arrester (MOA). The device combines two functions: 1️⃣ To measure the «conduction current» passing through MOA under the influence of the AC operating voltage of the 50 Hz frequency. 2️⃣ To count the «number of impulses» through MOA. Both functions are absolutely useless! I will explain