There is a wide variety of insulating material options and environments in which insulator sets are used. So insulator set designs can vary considerably and may need to be individually tailored for specific applications. Even the smallest differences in design can affect the power arc behaviour of a string. Hence, physical testing is the most reliable way to assess a string’s power arc behaviour.
Over the last five years, KEMA Laboratories, Prague has carried out over 160 such power arc tests on insulator sets of various types. These tests were performed according to the IEC 61467:2008 standard. This standard applies to insulator strings or sets comprised of ceramic, glass or composite materials, and that will be mounted on metallic poles or towers and used in AC overhead lines with a nominal voltage above 1000 V.
The standard defines methods, parameters, circuits etc. for power arc tests on both insulator sets and short strings. Different test arrangements are allowed within the standard, and the choice of test arrangement depends entirely on the final application of the insulator sets according to customer requirements. As such, the test circuit and series should be chosen based on factors such as the geometry and type of insulator, its position in the line and the type of tower it is to be used with. Figure 1 shows a typical test arrangement for a V-string set with composite insulator units. This test arrangement comprises balanced supply and return circuits.
The main verification test performed is the mechanical failing load test (MFLT). This is performed on the insulator units to ensure they can withstand mechanical forces after a power arc is applied. Insulator sets may also be required to undergo a dry power frequency flashover (DPFF) test to ensure the insulator does not suffer punctures at voltages below the flashover voltage. Additional electrical tests may be performed on the fittings and conductors within the insulator set to verify their withstand capabilities.
66% of components tested successfully completed a full test sequence. Just one set suffered a separation failure during the arc test. DPFF tests were not performed on an additional 35 sets and no MFLT was performed on an additional 10 sets. These 45 sets are not included among those sets that successfully completed testing even if no failure occurred during those tests carried out.
The configuration of the insulator, tower and current return paths are subject to an agreement between the utility and the manufacturer. The results from the test are valid for that configuration only. For other applications and configurations the test needs to be repeated.
Long-rod insulator sets
For long-rod sets, three particular potential issues were identified. The metallic parts of the insulator unit can be prone to melt if not well designed. Furthermore, poor design can lead to the sheds near these metallic parts breaking. Finally, the arc can cause metal to evaporate from the set’s protective fitting, causing puddling.
Composite insulator sets
The silicone rubber used in composite insulator exhibits a very high resistance to power arc testing. However, for these sets, the critical point is the where the fibre-glass core connects with the metal end fittings.
Cap-and-pin insulator sets (glass or porcelain)
Cap-and-pin insulators exhibit very high mechanical resistance after power arc tests. The main areas of concern for these sets are the possibility of sheds breaking and, for glass sets, the cooling of the cap-and-pin units.
Protective fittings: load-bearing fitting protection and arc direction fittings
Power arc tests put much greater mechanical and thermal strain on these fittings than short-circuit tests, and this must be taken into account when designing the protective fittings of insulator sets of all kinds. There is also a danger of material from these fittings melting onto the insulator units. Care should also be taken to ensure that protective fittings do not move during arcing, which can lead to arc root sitting on the load-bearing fittings.
Protective fittings: corona rings, grading rings, etc.
These fittings are not primarily designated for arc currents, but they influence the position of the arc root and how it moves. As with other protective fittings, risk factors here include material melting onto the insulator units and movement of the fittings causing the arc root to move away from its intended position. In addition, changes to the contours of these fittings can lead to excessive corona discharge and radio noise.