Metal oxide surge arresters have high reliability and will function as intended almost indefinitely, assuming that they have been well designed, properly manufactured and operated within their specified range of applied voltage, temporary overvoltage, surge magnitude and surge energy. Nonetheless, arresters do occasionally fail in service. This implies that one or more of these variables has been violated or that the arrester may have been damaged during handling or installation. Failure in such cases usually means that the arrester has suffered an internal short circuit, placing a fault, e.g. line-to-ground fault, on the system. Almost invariably, this will result in operation of a circuit-interrupting device such as a breaker, recloser, fuse, etc., thereby causing a disruption in service. If the arrester is equipped with a disconnector, the disconnector should operate when fault current flows following arrester short circuit. This will isolate the arrester either from the ground or from the line, depending on where the disconnector is installed. Then, upon re-energization, the failed arrester is no longer in the circuit electrically and service can be continued albeit with a reduced level of surge protection.
Station arrester failure will typically cause a system lockout until either that portion of the substation can be by-passed or until the arrester is physically removed from service. While it is desirable to avoid arrester failure at all times, the consequences of station arrester failure are typically far more severe than is the case for distribution arresters. Typically, there will be considerably greater disruption in service and the cost of replacing a station arrester is orders of magnitude higher. Steve Brewer of Hubbell Power Systems in the United States outlines the methods to monitor condition of arresters installed in a power system. He also presents the basis behind each as well as comparative advantages and drawbacks.
Goals of Field Testing
A program of field testing arresters is undertaken by users with the goals of determining if an arrester is nearing end of life, predicting when end of life will occur and evaluating if the arrester is still providing protection to the insulation. The first two of these objectives can be reasonably achieved but the third is not yet practical in a field environment. Fortunately, with today’s MOV arresters and all the design tests that must be passed, there is virtually no duty that can impact an arrester’s ability to protect.
To better understand end of life testing, it is necessary to know how a surge arrester behaves under power frequency conditions – not only when the arrester has aged but also when it is tested in the factory prior to being shipped. Under normal line-to-ground power frequency voltages, surge arrester current is nearly all capacitive. Fig. 1 shows the relationship between voltage and current on a new surge arrester
As voltage on the arrester increases, current waveform becomes much more resistive in nature. This characteristic is also true if voltage remains constant but internal temperature of the arrester increases for some reason. Given this, monitoring the resistive component of current over time is key in efforts to evaluate the internal state of an arrester and to predict its ultimate demise. MOV blocks exhibit a negative temperature coefficient and, as such, the resistive component of current increases as block temperature rises. Figs. 2 and 3 demonstrate these characteristics.
Almost without exception, arrester failure results in a complete short-circuiting of the unit inside its external housing. While an arrester could flashover externally without its external air gap being compromised (e.g. by contact with wildlife or vegetation), the MOV blocks inside typically prevent terminal-to-terminal voltage reaching a point where external flashover will occur. In all scenarios, failure is ultimately the result of dielectric breakdown whereby the internal structure of the arrester has deteriorated to the point where it is unable to withstand the voltage applied to its terminal – whether this be normal system voltage, temporary power frequency overvoltage, e.g. following external line faults or switching, or lightning or switching surge overvoltages. There are a variety of ways an arrester could reach such a state, although their rate of occurrence is rare.
Probably the most common root cause of arrester failure is moisture ingress to the interior. This suggests that it was not well designed or not properly manufactured or that it may have been physically damaged by external force that compromised its sealing system.
Temporary Overvoltage (TOV)
Under normal operating conditions where an arrester is energized at its maximum continuous operating voltage (MCOV), temperature of the MOV blocks rises to just slightly above ambient. A point is soon reached where the heat being generated by the blocks is perfectly matched by the heat the arrester dissipates into surrounding air. If the power frequency voltage across the arrester increases, e.g. as a result of a system disturbance, fault or switching operation, the MOV blocks conduct more current and begin to rise in temperature. If the overvoltage is of sufficient magnitude, the heat generated by the MOV blocks will always be greater than what can be dissipated and there is potential thermal runaway.