While surge arresters installed on a transmission line are termed ‘line surge arresters’ (LSAs), it is not only their location but also their purpose that is decisive in this application. These arresters are commonly used to address lightning or switching related phenomena with the key goal of improving overall reliability of lines. In contrast to the purpose of a station arrester installation that protects local insulation systems such as bushings, cable terminations, machines etc. LSAs act to prevent uncontrolled flashovers of insulators in order to prevent earth faults and short circuits ‘inside’ the power system component, i.e. the transmission line.
This article, based on a paper by Siemens Surge Arrester Division at the 2013 INMR WORLD CONGRESS, explains the benefits offered by LSA installations.
Line arresters improve system reliability by reducing multi-phase and multi-circuit line outage rates and also help control switching overvoltages on EHV lines versus relying on closing resistors or controlled closing schemes. This application of LSAs can be used at all system voltages. There are several different ways to group LSAs, e.g. according to system voltage, problem on the line, type of housing, etc. But the most important is the difference between LSAs without gap (i.e. gapless line arresters) and externally gapped line arresters or EGLAs. Neither type is a recent innovation and both have been well known for years, with EGLAs having a long history of use in Asia. IEC 60099-8 standard (released at the beginning of 2011), was the first global and specific EGLA standard and represented a remarkable step forward for both users and manufacturers of this type of arresters. Both NGLA and EGLA line arrester types have different features that make them more suitable for certain applications.
Line Arrester Application for Lightning
The rated voltage of all line arresters is to be selected so that the lightning and switching surge protective levels are coordinated with (below) the LIWL or SIWL of the line insulation strength respectively. Line arresters can range from IEC Classes 1 to 5, depending on application. In some cases, system studies are needed to determine energy requirements and optimum location on the line. Line arresters are typically either gapless metal-oxide arresters with polymeric housing or of an externally gapped MO arrester design. In the latter case, the arrester body can be in either a porcelain or polymeric housing.
The risk of arrester overloading and subsequent failure due to excessive energy from lightning strokes can be higher for line arrester applications than for arresters applied at substations due to the presence of substation shielding. Since overhead line insulation is generally self-restoring, even overloaded line arresters will facilitate fast reclosing. This can be achieved by isolating the line arrester from the line in the event overloading using a disconnector for gapless line arresters. In the case of externally gapped line arresters, the spark-over voltage of the gap is coordinated with maximum switching overvoltages occurring on the system so that the line can be re-energized without causing a spark-over of the gap.
With self-restoring line insulation, it is not appropriate to use a fixed coordination current to calculate protective margins. Instead, the probability distribution of lightning stroke currents is applied to the line and probability of flashover is calculated. Combined with local ground flash density, this will produce a lightning flashover rate per unit length per year. This flashover rate is the lightning performance metric for the line. For transmission lines, a typical target might be 1 flashover per 100 km per year. For distribution lines, a reasonable target may be somewhat higher. The effect of a flashover depends on protective relaying practices and the possibility of arc quenching by wood. Line performance evaluation can be done accounting for the benefits of line arrester installations.
Lightning energy handling capability can be a major factor when selecting line arresters for certain applications – especially for both improperly installed/placed or even insufficient LSAs along a certain line. The requirements of lightning related energy are typically much more significant for line arresters located on transmission lines than substations due to shielding of substations which prevents direct strokes. Published energy handling capability of arresters is typically based on switching-related tests. However, the lightning impulse discharge capability test will supply the necessary information.
Different Line Arrester Applications
The majority of line arresters presently in use are for improving lightning performance of transmission lines and reducing line-to-ground outage rates. Factors to consider when evaluating lightning performance include soil resistivity, footing resistance, ground flash density, tower dimensions, span length, shield wire locations, insulation levels, etc. Line arresters can be installed on all towers or on only selected towers along a transmission line.
Beside all commonly used ways for improving lightning performance of certain overhead lines (such as extended shielding wires or improved tower footing resistance through additional copper counterpoise or replacing insulators with higher BIL), properly applied line surge arresters can effectively and most economically help reduce line system failure rates.
Insulator Back Flashover Rates
Insulator back flashover rates can be efficiently reduced in case of shielded overhead lines located either in high lightning activity areas or having poor grounding in regard to soil resistivity, footing resistance, tower dimensions, span length, shield wire location, insulator insulation level, etc. Local grounding conditions (soil resistivity and footing design) have a major impact on back flashover performance. These types of outages can be reduced by placing arresters on all phases or only on the phase(s) with lowest coupling factor to the shield wires, is which normally the bottom phase in high footing resistance areas. For applications in high footing resistance areas, it is important to apply the arresters not only on structures in the areas of high footing resistance but also on one or two structures away from the high footing resistance areas. This will prevent flashovers at the low resistance structures caused by the arrester operations at the high footing resistance structures. The higher the footing resistance, the more energy is absorbed by individual line arresters.
Insulator Flashover Rates
Insulator flashovers result from so-called shielding failures mostly observed on unshielded transmission lines and, infrequently, on shielded lines that experience lightning strokes direct to the high voltage conductor. For unshielded transmission or distribution lines, those direct lightning strokes to the phase conductors will be much more frequent than for properly shielded lines, since these lines are simply not protected (shielded) against lightning. In these cases, line arresters can be used to address shielding failure flashovers by applying them on the exposed phases. Line arresters can be installed instead of shield wires on new or unshielded line systems, especially when grounding conditions are bad or on lines with phase conductor arrangements that are triangular where an overhead shield wire is not present. The arresters are used to ‘protect’ the topmost phase from flashover and effectively act similar to a shield wire when the topmost phase intercepts a lightning stroke.
Line arresters can also be installed on all three-phases on one of two three-phase systems in multi-circuit towers to prevent simultaneous double-system faults. This application of line arresters can be effectively used for all system voltages, including EHV.
Underbuilt Distribution Circuit Flashovers
If a distribution line shares a tower or pole with a shielded transmission circuit, the underbuilt distribution conductors are not likely to be struck directly. However, the distribution line is vulnerable to back flashovers, because the coupling between distribution conductors and shield wires is weakest. Insulation strength on the distribution line is also weaker. Once a distribution conductor flashes over, coupling to the transmission conductors will increase and make back flashover less likely on the transmission circuit. The transmission circuit’s lightning performance can improve at the expense of that of the distribution circuit. The situation can be remedied with line arresters on the distribution circuit. Usually arresters are needed at every tower or pole, on at least one phase.
Switching Overvoltage Applications
These events are typically associated with high speed reclosing on EHV transmission lines. Strategically placed surge arresters have been used instead of closing resistors and/or controlled switching schemes to control switching overvoltages along EHV transmission lines. Unlike lightning related applications where arresters can be installed at consecutive structures, arresters to control switching surges are only needed at the end of the line and maybe one or two other points along the line. Arresters at the line ends can be either line arresters installed on the last towers or normal pedestal mounted arresters. For switching overvoltage control, arresters are usually installed on all phases. Line arresters along the line typically require one energy class lower than needed for arresters installed at line ends at the substations. Line arresters for this application are typically used for system voltages of 245 kV and above and are of the gapless type (NGLA), since EGLAs do not handle switching surges. The main advantages with surge arresters for this application are that they will also protect the system against re-strikes of circuit breakers and that they are maintenance free. With increasing use of compact or upgraded line designs, this application is no longer reserved just for EHV levels.
Voltage Upgrades & Other Special Applications
With line arresters installed on every tower it is now possible to design transmission lines with significantly smaller clearances than used traditionally. Line arresters can also be used to upgrade existing system voltages using existing towers and lines, especially for older, not-commonly used system voltages. Today, this becomes more and more important as permission for right-of-way for new lines is under public pressure to be less noticeable. Line arresters can be used either on the top phase(s) to substitute for the shield wire in areas with moderate isokeraunic levels or on all three phases, together with shield wires. In addition, line arresters can be used to protect transmission line structures with insulation levels lower than the majority of the line (e.g. switching structures). Line arresters could also be used on certain open points on the system exposed to voltage surge ‘doubling’. Use of line arresters to intentionally reduce insulation on certain structures results in a possible increase in clearances along the span.
Recommendations for Design
Externally gapped line arresters are recommended for ungrounded systems with low short-circuit currents. For multiple circuits on the same tower, EGLAs have an advantage due to their smaller size and installation across the line insulators. Gapless line arresters (NGLAs) are recommended for all EHV systems and applications with extended substation protection, due to the fact that gaps will cause a steeper rise time of incoming overvoltages. Gapless line arresters are also recommended for all compact line applications since, for compact lines, switching will also be important for lower system voltages.
Selection of Arrester Class
Arrester class is selected depending what outage rate and failure risks are expected for line arresters. For unshielded transmission lines, arresters of IEC classes 2, 3 or 4 are used depending on isokeuronic levels. Typically, line arresters for protection of switching overvoltages use one IEC class lower than for substation arresters due to fact that, for longer lines, gapless line arresters are installed at the midpoint.
Installation of Line Arresters
Line arresters are installed and used under harsher service conditions than other surge arresters. They can be treated rougher during transport out to towers and they are installed with material that typically uses larger dimensions (e.g. bolts) than normally used for other surge arresters. Installed out on towers, line arresters are also more exposed to extreme weather than arresters at substations. Moreover, they are installed in such a way that they may move due to wind and/or line swinging and vibration.
Examples of Line Surge Arresters (LSAs)
Non-Gapped Line Arresters (NGLAs)
Non-gapped line surge arresters offer a high degree of mounting flexibility and operational reliability. Depending on the tower design and the arrangement of insulators and lines, these arresters can either be installed directly on the insulator or on the tower. Thanks to their high energy absorption capability, today’s non-gapped line arresters offer a high level of protection against overvoltages caused by lightning and network-generated switching impulse current overvoltages. A disconnector is installed in series to galvanically isolate the line surge arrester from the line voltage in the unlikely event of a fault or thermal overload. It automatically and immediately disconnects the line surge arrester from the line voltage. This allows the affected overhead line to be re-energized and operated until replacement can be scheduled.
Externally Gapped Line Arresters (EGLAs)
EGLA line surge arresters have an external spark gap placed in series that galvanically isolates the active part of the line surge arrester from the line voltage under normal conditions. In case of lightning, the spark gap is ignited and the dangerous overvoltage is safely discharged through the resulting arc. The active component limits the subsequent current to ensure that the arc is extinguished within the first half-cycle of the operating power-frequency voltage. After this, the line surge arrester immediately returns to standby condition. In this manner, the EGLA line surge arrester prevents all insulator flashovers that would otherwise lead to short interruptions and failures on the power network. EGLAs increase network stability as well as the availability of overhead lines. An additional benefit is that there is no leakage current, because the series gap disconnects the MO blocks, which are the active part, from the system voltage during normal operating conditions. Depending on the topology of an overhead line, (i.e. the arrangement of towers and insulators, attachment options and line voltage) an EGLA can either be attached directly in parallel on the suspension/tension insulators, on the insulator string or on the tower cross-arm. The active component can have either one or two bodies, depending on required system voltage.
The compact design of the EGLA allows installation and lightning protection even on existing towers with very small clearances, as is mostly the case for multi-circuit towers. EGLAs are available to protect overhead lines with system voltages of up to 550 kV.