As surge protective devices, arresters mitigate the impact of events that might otherwise trigger outages. Monitoring their condition has therefore become part of an industry-wide trend toward greater condition assessment of key network assets. Over most of its service life, an arrester behaves like an insulator, with low leakage current over its insulating surface and extremely low levels through the internal zinc oxide disks. Maintaining such low leakage current is required to ensure the arrester will live up to its normal 20 to 40 year service life. In fact, the only times an arrester does not behave like an insulator is when it is failing or clamping a surge. A number of assessment methods and indicators have traditionally been utilized to reveal signs of deterioration and provide clues to impending arrester failure. These range from fault indicators and disconnectors that indicate complete failure to instruments that can measure small changes in resistive leakage current or power loss in the case of gapless type metal-oxide arresters. The term arrester condition monitoring is generally considered to be on-line and continuous while arrester field testing is off-line and carried out periodically over an arrester’s service life.
Asset management is a growing field in today’s leaner utility environment and one of its most popular methodologies is the use of condition-based maintenance programs. This type of asset management does not rely on a specific maintenance schedule but rather one that is condition-based and more cost-efficient. But for such a maintenance program to work, assets must be constantly monitored. Determining the condition of an arrester asset is a developing field and several methods are available. Still, condition assessment comes with a cost and is therefore most often performed only at critical locations on the network where failure could have most serious ramifications. The goal is to predict imminent failure and have the arrester removed before it fails. Arrester field-testing is another form of asset management and most often carried out to determine if an arrester can be safely re-installed.
Types of Monitors
Such devices count impulses at currents above certain amplitudes or above some combination of amplitude and duration. If the interval between discharges is too short (e.g. less than 50 ms), such counters may not register each current impulse and this is often the case during multi-stroke flash events. Some counters require power follow current, as was generally present through former silicon carbide arresters, and might not count short impulse currents through metal-oxide arresters. Depending on operating principle and sensitivity, a counter will provide some indication of overvoltage events on the system. It can also give information on the number of discharges corresponding to significant energy stresses on the arrester. But electro-mechanical surge counters alone do not offer specific information about the condition of the MOV type arrester other than to register the surges it has encountered above certain amplitude. Still, knowing a surge has taken place on the system can be useful. However since only surges of high magnitude or long duration can degrade an MOV type arrester, number of surges alone does not correlate with arrester relative ‘health’. This type of arrester is designed to withstand thousands of surges as long as these are within its normal operating capability. Repetitive surges alone should not lead to degradation.
For a surge counter to operate properly, the arrester must be isolated from earth by insulators at the base and the counter electrically mounted in series. The surge counter should ideally be located where it can be read from ground level with the arrester in service and installation should be done without significantly lengthening the earth connection or reducing its cross-section. It is important to note that insulators used should be selected so that specified cantilever strength of the arrester is not reduced.
AC Leakage Current Meters
AC leakage meters are generally an accessory for surge counters. When the readout is an analog meter, current being displayed is total current of the arrester. The arrester’s total leakage current is then a combination of capacitive current and resistive current through the disks and over the external housing. If the arrester is equipped with a special ground terminal that isolates these currents, only total internal current is monitored, without interference from external surface leakage current. Another advantage is that no insulators are needed at the bottom if surface leakage current is not of interest. For tall arresters or those located in a seismic region, this could prove a big advantage. Because during steady state metal oxide disks are more like insulators than conductors, they conduct little resistive current but can carry from 2 to 10 ma of capacitive current. However, such a high level of current offers no useful data about actual condition. If an arrester is failing, current shown on the leakage current meter may not even change. Unfortunately, a 5 ma or higher total current (99% of which is capacitive) shrouds the resistive current and eliminates any real detection of a parameter that is a truer indicator of arrester condition. There is therefore no real information provided about condition if using an analog AC meter. If resistive current becomes high enough to affect current reading and become visible on the analog meter, this suggests that the arrester is already in rapid failure mode. It is unlikely to be in this state long enough to be detected by a subsequent inspection.
Measurement of Third Harmonic Current
Newer surge counters that sense third harmonic current offer significantly more information on condition of an arrester than earlier generations designed primarily for silicon carbide arresters. For example, a multi-functional condition assessment tool for arresters not only counts surges down to 10 amps but also time stamps them and holds the data in memory until downloaded. Surge amplitude and time are recorded along with leakage current data. Based on total current, the device calculates the third harmonic of the current – a value that is a very close representation of resistive current. As such, third harmonic current can help accurately assess an arrester’s relative condition. If this current has increased by only a few percent, this is detected and stored in the local database until an operator downloads it using an accompanying hand held device. One such device can be used for many sensors. Recently developed such equipment offers the capability to measure and store data on up to 1000 arresters and is able to provide continuous online surveillance of any unit using modem to PC communication. Another option is the arrester condition monitor (ACM), that offers both local and remote continuous readouts. It devotes its first day to determining all arrester characteristics so that this profile data can be subsequently used to evaluate future condition. This diagnostic tool is already IEC 61850 ready and once fully-integrated station condition monitoring systems become common, it will not require major modification to be included.
Partial Discharge Detection
During the life of a gapless arrester, its internal components will continually be exposed to stresses that can lead to partial discharges. Arresters with some internal air volume (i.e. both porcelain-housed and composite tube designs) typically experience some partial discharge activity during rain, fog and snowy conditions. This is an acceptable condition in most designs. However, these types of arresters should not experience partial discharge under dry conditions. Because internal partial discharge in an arrester is an undesirable condition that can eventually lead to failure of its insulating materials, detection systems have been developed to locate them and give users an early opportunity to rectify the problem. This same equipment is of value for other purposes on the network and can be used for more than just arrester assessment. A wide array of on-line and field oriented PD detection equipment can be applied to arresters. When arresters are manufactured, they must be tested for internal PD. IEC as well as IEEE standards require no more than 10 pico-coulombs be present. Therefore 10 pC should represent a baseline for arrester assessment and any unit exhibiting more than this warrants closer inspection. The real challenge in PD detection is filtering out background noise and there are portable devices able to do this with graphic output able to discriminate between background and real signals.
This form of arrester condition assessment is fast and effective. Within only seconds, an infrared detector can determine if there is a critical arrester condition to deal with at a substation. This is because any arrester in long-term failure mode and nearing its end of life is going to be hot and detectable at a distance of as much as a hundred meters with even simple infrared equipment. While it is typical for arresters to run up to 5°C above ambient, temperature deviations above that should be considered a potential problem. Rarely is an arrester more than 20°C hotter than ambient except in a laboratory. A 10°C difference between two arresters of same design and vintage should be considered a clear indication of maintenance action and the arrester should ideally be removed from the energized circuit. Any arrester between 5° and 10°C from ambient should be monitored and, if on a critical circuit, probably removed from service. An arrester that is 15°C different from other similar units should be de-energized as soon as possible to avoid an outage. If the arrester is porcelain-housed, personnel should not be nearby until de-energization. Unfortunately, although thermal imaging is an effective means of assessing real condition, permanently mounted such devices to measure arrester temperature on a continuous basis are not widely available. One note: there is not necessarily a need for a multi-functional thermal imaging device to detect abnormal temperature of an arrester. Many hand-held devices can measure temperature at any particular spot, even from a distant location.
Off-Line Arrester Field Testing
Off-line field-testing is required if an arrester has been removed or if it is still in service but has been de-energized for some time. Methods to determine if they are worthy of re-installation are typically more demanding than simple on line condition monitoring and therefore arresters should ideally be assessed while in service. The main problem with off-line testing is that to effectively assess an arrester’s condition it must be energized near or above its operating voltage. For MV arresters, this is not difficult, but for units rated 100 kV or higher this is costly. Voltage can be AC or DC but, in either case, if not at or above an arrester’s MCOV, there is only limited data to make an assessment. The optimum off-line field test is to apply an AC voltage to the arrester and measure leakage current. As with on-line monitors, the parameter that matters is resistive leakage current. Total current – predominately capacitive – is not a good indicator of condition and any equipment used must therefore be able to discern total current from resistive current. There does not appear to be stand alone off-line test equipment for arresters above 10 kV. Equipment that measures watts loss of an arrester below its Uc rating will be able to only marginally predict condition. If a large population of arresters is to be assessed, this method might prove effective but is still not optimal. One alternative is to use a standard ‘hi-pot’ tester and this can only be accomplished if the arrester’s Uc rating is below the maximum voltage of the tester. If an AC such tester is used, the most effective means to assess arrester condition is to determine the voltage at which it starts to conduct heavily. This is also referred to as measuring the arrester’s Vref – a term used to quantify the level where an arrester conducts between 1 and 5 ma of resistive current. The methodology is to energize the arrester until it conducts approximately 1mA. If this level is 5-15% above the Uc rating, the arrester is likely sound. Fortunately, if an arrester is off-line at a substation it usually has two ‘partners’ and all should be tested. In such a case, all three units should have the same turn-on point. If not, the unit with a lower value should be removed for more evaluation at a laboratory.
Condition assessment of surge arresters is still a developing area with a number of available options and more still under development. As the smart grid concept evolves, such assessment tools will become mandatory and not only for critical installations. As a result, a new line of devices will likely become available for this task.