Arresters are essentially devices to divert lightning and switching surges and can sometimes become overloaded to the point of failure. Whenever this occurs, it’s extremely useful to properly determine the reason why the arrester (or companion arresters) became inoperative. This will allow utility maintenance engineers to determine whether the end-of-life event was simply the arrester performing its expected protective function or if there are other issues that need to be resolved and that might be system wide.
Here is where forensics can play a valuable role, just as in television dramas where specialist police investigators try to identify the real culprit behind a crime. This article, prepared by INMR Columnist Jon Woodworth, offers guidance on how best to search for and establish the root cause of an arrester’s overload.
Terminal overload is basically any event whereby an arrester is stressed beyond its design capability. The arrester may or may not even be in one piece at that point and electrically represents an open or short circuit.
While such a situation could be regarded as failure to perform its function, this is not necessarily the case. For example, should a long-serving arrester that protects a high value asset be terminally overloaded during an overvoltage event, this can hardly be termed failure. In fact, this scenario should really be called a success.
However, if an arrester experiences a terminal overload within days of energization and with no related surges, then it’s more likely an indication of some form of failure. Forensic analysis is then the only real means to properly determine which of these scenarios really applies.
In most cases, the underlying reason for conducting this type of analysis is that several arresters of similar design experience terminal overloads and the affected power utility needs to find out if the problem is system wide. If system wide and therefore a failure scenario, a different mitigation strategy would likely be needed compared to a situation where it was only an isolated event. Indeed, one of the main purposes of the analysis is to determine if there are separate power system issues to resolve that are not directly related to arresters.
A companion arrester is one of similar vintage and style that could be located on the same phase close to the overloaded arrester or nearby on a separate phase. The importance of studying these during a forensic analysis is high and, in this regard, it is preferable that the companion arrester not have been similarly overloaded or blown. This is because any arrester that has experienced terminal overload has much of the forensic evidence obscured by the damage from the power frequency fault current. This fault current typically flows off the system to earth along the arrester and raises the temperature of the various components past their melting points. Obtaining a suitable companion arrester therefore becomes a key ingredient in identifying the root cause of the problem.
Relevant System Data
Collecting system data relating to the overload event is beneficial to any analysis but is often the most difficult part of the forensic examination. Such data should include:
1. System voltage;
2. Neutral configuration of the source transformer (i.e. grounded, floating, impedance grounded);
3. Magnitude of available fault current;
4. Location of the arrester;
5. Other equipment on the same phase, at the same substation or on the same line (i.e. capacitors, switches, breakers, transformers, inductors) and its status during and after the overload;
6. History at that location (e.g. other overloads during past years);
7. Switching or lightning activity at that time or during prior weeks;
8. Performance history of that arrester vintage and design applied on the system;
9. Existence of any other forensic analysis data that might offer clues to the root cause.
In assembling the above, it’s useful to avoid irrelevant or inaccurate information and rely as much as possible on those directly involved in the response to the overload. The persons picking up the pieces usually have the most information such as the arrester’s exact condition following the event. It is also valuable to know if the overloaded arrester was removed only after a severe rainstorm.
Routines to observe so as to make each analysis as effective as possible include the following:
• Gather as much system data and from as many sources as possible. Use groups of people who can offer different input on the situation.
• Inspect the arresters in question and record as much data as possible from tags, shipping documents and nameplates.
• Collect original catalogues and literature on the arrester.
• Assemble a comprehensive set of photographs of the received parts.
• Assuming a complete arrester unit (overloaded or companion) is available, perform full-scale electrical tests such as Vref, watts loss, PD and leakage.
• If sealed with an internal air volume, take a sample and conduct a gas analysis before the arrester is disassembled.
• Disassemble the arrester carefully, labeling all parts and with a camera constantly in play that’s capable of close ups with high-resolution images. Involve others in this process to ensure nothing is overlooked.
• If the parts are not too damaged, run more electrical tests and go through a checklist of clues
• Once testing and physical examination is complete, let the parts sit in place at least until all the photos have been reviewed – ideally on a big screen and with as high a magnification as possible.
• Write the forensic report, listing potential root causes. Eliminate any causes deemed unlikely based on the photos and tests.