The surge diverter is a protective device that protects the system by limiting transient overvoltages through the diversion of surge energy into the earth. Under normal operating conditions, the surge diverter is not electrically involved and has extremely high impedance. However, when a transient overvoltage occurs on the system, the device responds by changing the electrical properties, thereby reducing the impedance to a very low level.

In medium and high voltage levels, surge diverters are referred to as surge arresters, but the term ‘diverter’ is still commonly applied within the industrial field and in the mining sector. In all instances, however, the functionality or purpose remains the same, i.e., for the purpose of protecting the overall insulation systems against voltage levels considered to be beyond the norm.

Contrary to the case where surge protection devices are usually installed, this device, being a medium- and high-voltage surge diverter, is usually installed in appropriate points in the network, unlike the case where surge protection devices, which are usually used for low-voltage surge, are installed in the switchboard and consumers.

Why transient overvoltages occur in MV and HV networks

Overvoltages in medium and high voltage systems are produced in various ways. Lightning is often considered one of the most obvious causes, especially in overhead distribution and transmission systems. A direct lightning strike or even a strike very near the system may generate extremely high voltage impulses.

Another significant cause of transient overvoltages is switching operations. In power systems, circuit breakers, isolators, and contactors are often subjected to switching surges owing to their switching actions, particularly when they disconnect inductive loads like transformers, machines, and reactors. With the increasing use of switching devices owing to modernization and automation, switching surges are becoming extremely significant for new power systems.

Earth faults, ferroresonance, capacitor bank switching, and network reconfiguration may also cause transient voltage conditions. Although these voltage events may not reach the highest values that occur during lightning strikes, repeated voltage stress from these causes can accumulate to stress the insulation.

The operating principle of surge diverters

The surge diverter works on the principle of nonlinear resistance. During normal system voltage, the diverter exhibits very high resistance and attracts negligible current. For transient overvoltages, resistance falls rapidly and allows surge current through the diverter to the earthing system.

The modern surge diverter relies mainly upon the technology of metal-oxide varistor. In manufacture, such varistors are prepared from zinc oxide blocks that possess highly nonlinear voltage-current characteristics. The response time is incredibly fast-a diverter can act within microseconds after a surge event.

Once the transient has passed, the system voltage returns to normal levels, and the diverter goes back into its high-impedance state. This type of self-restoring behaviour means that surge diverters can operate repeatedly without the need for replacement after each event, provided their thermal and energy limits are not exceeded.

Construction and materials used in MV and HV surge diverters

Medium- and high-voltage surge diverters are engineered for harsh electrical, mechanical, and environmental stresses.

Internally, surge diverters comprise multiple blocks of metal oxide varistors arranged in a stack and mounted in an insulating enclosure. The enclosure material can vary from porcelain to a polymeric casing depending on service conditions and functional use.

Porcelain housed surge dividers have long been in use in high voltage substations and outdoor applications. They provide good mechanical strength and stability but may be heavier and more prone to mechanical damage.

The polymeric housing design in surge diverters has become a trend, especially in the distribution network and in industrial applications. This is attributed to the overall lightness, superior performance in a polluted or coastal setting, and minimization of explosive failure through safer venting.

Additionally, there is housing for the pressure relief systems, which provide for the safe release of interior pressure in the event of a catastrophic failure, causing the diverter to fail in a controlled fashion rather than rupturing violently.

Medium-voltage surge diverters and their applications

Typically, medium-voltage surge diverters are used for systems rated between 1 kV and 33 kV. This range includes a vast majority of the electrical distribution system, industrial plants, mining plants, and commercial facilities within Australia.

One major application is in the protection of transformers. Medium-voltage transformers are often exposed to transient overvoltages owing to their insulation system and long cable or line distribution system. The surge protectors, which are normally installed on their terminal ends, assist in controlling the surge and hence protecting the insulation system of the transformers.

Switchgear protection is another major application. In medium voltage switchboards, there are sensitive parts such as circuit breakers, current transformers, and voltage transformers. Transient overvoltages can create insulation tracking, partial discharges, and degrade insulation over time.

Medium voltage motors and generators may also be protected against surges, especially for installations that involve frequent switching or have long-feeder-cable configurations. Severe voltage wavefronts occur for these installations, and this can cause insulation failure.

High-voltage surge diverters in transmission and substation systems

In the high voltage range above 33 kV, high voltage surge diverters are utilized by systems such as the subtransmission or transmission grid systems. They require immense levels of energy handling and are exposed to extreme voltage strain.

In substations, surge dividers are connected to line entrances, transformer bushings, bus bars, and other key locations where surge dividers are necessary. Surge dividers are designed to coordinate with the level of insulation of the equipment, thus limiting the voltage surge to an acceptable level by the insulation system.

Sometimes, surge diverters can be used on towers or poles to protect against outages caused by lightning strikes. By charging a line before a lightning strike occurs, there can be less flashover between the line and the insulators, thus reducing line trips.

In Australia’s extensive electrical transmission network, where often sparsely populated terrain is crossed by electrical cables, high-voltage surge diverters are an essential element in raising network reliability and decreasing maintenance costs due to the effects of lightning strikes.

Insulation coordination and the role of surge diverters

Insulation coordination is a method of selection of the level of insulation and other protective devices to ensure that electrical equipment can withstand overvoltages without failure. Surge diverters are an important part of the process of insulation coordination for medium and higher voltage equipment.

In effect, surge diverters limit the voltage that equipment is exposed to during a transient, making it possible to optimize the level of insulation without over-specifying the equipment, which is expensive in the long run.

Australian practice for insulation coordination follows international standards for IEC principles and takes into account lightning impulse levels, switching impulse levels, and temporary overvoltage levels to select different characteristics of surge diverters like residual voltage, energy levels, and maximum voltage ratings for operation.

Earthing considerations for effective surge diversion

The effectiveness of a surge diverter is measured by the earthing system to which it is connected. Under a surge condition, large currents have to be dissipated into the earth without creating threatening touch or step voltages.

In medium voltage switchgear and installations, as well as high voltage systems, the design of the earthing systems aims to provide low-impedance paths to the surge currents, which are achieved using earth grids.

If the earthing process is not suitable, higher levels of ground potential can occur, which can undermine the function of the diverter and increase safety concerns. Australian conditions require consideration of soil resistivity, moisture, and seasonal factors for effective earthing systems for surge protection.

Environmental and operational factors affecting surge diverter performance

Surge diverters installed outdoors are likely to experience various environmental stresses. Pollution and/or salt, exposure to UV, and temperature extremes are environmental stresses.

In coastal and industrial environments, contamination may cause surface leakage currents and degradation of insulating materials. However, polymeric casing materials are normally preferred for use in such environments owing to their hydrophobic nature and ability to withstand pollution build-up.

Factors like frequent changes in switching, high levels of faults, and raised levels in the system may also cause further stress to the surge diverters.

Electrical devices must be selected with the correct thermal and energy margins to achieve the required life.

Installation practices for MV and HV surge diverters

Accurate installation is one of the essential factors in the correct functioning of surge diverters. The location of the surge diverter in relation to the equipment that requires protection also influences its performance. Surge diverters should be as close as possible to equipment terminals for optimal results.

The connection conductors are to be short, straight, and secure. There can be a reduced capability of the diverter to clamp fast-rising surges if there are sharp bends or excessive conductor lengths.

Typically, surge diverters in medium- and high-voltage switchgear assemblies are mounted in specific brackets or cable compartments. With respect to outdoor applications, clearances and support are of great importance, considering wind, vibration, and, if applicable, seismic conditions.

Maintenance and condition monitoring

Modern surge diverters are maintenance-free, and that does not mean they should be forgotten about altogether. They should still be inspected visually to look for signs of damage, contamination, or mechanical stress.

Some high voltage surge diverters have condition monitoring attributes such as leakage current detection or a surge counter. Some of these condition monitoring attributes can prove invaluable to the asset manager in terms of getting a clear view of the operating history of the high voltage surge dividers.

In Australia, for instance, surge diverters may be replaced as part of asset renewal programs rather than at fixed time periods. This reflects an understanding that service conditions, and not just age, influence performance and reliability.