Electric power industry consulting
Lightning processes are statistical processes. It is hardly possible to study such processes by considering a lightning strike to the line with specific parameters (current, front) or by considering a specific lightning wave coming from the line to the switchgear. To calculate the probability, I normally use ATP/EMTP, which has been changed for the convenience
To protect the transformer from lightning surges, the metal-oxide arrester (MOA) is often placed directly at its bushings, at a distance of no more than 2-3 m. This creates an impression that the transformer will be fine. In fact, this is a false impression, and the effect on the windings depends not only on the
All the experience allows the author to state that the global metal-oxide surge arrester (MOA) industry is in dire need of simple MOA application methods, but, instead it, is developing endless editions of IEC 60099, adding details to an already complex document that contains dozens of MOA characteristics. At the same time, IEC 60099 does not divide
In the energy industry, many processes can not only be calculated on models, but also measured in an operating network. Measurements are important because they allow us to verify the models and, if necessary, adjust them. One of the areas where measurements in the network are practically impossible is impulse processes (lightning and switching). Therefore,
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