Types of Surge Arresters
Learn about the most common types of surge arresters used to protect against transient overvoltages and lightning strikes.
Surge arresters introduce shunting resistance to the ground when a surge appears, absorbing energy from the surge without the voltage becoming excessive. They then extinguish the power follow current after dissipating the surge. The most common arrester types in power systems are silicon carbide (SiC) and zinc oxide (ZnO). This article describes these arrester types in more detail.
Characteristics of Different Surge Arrester Types
The first surge arresters provided lightning protection utilizing an air gap connected between the line and the ground. Their main drawback was the requirement of a series linear resistance and a fuse to break the power follow current. Additionally, when the gap sparks over, it creates a fault in the circuit – and an unpleasant outage when cleared by a circuit breaker.
A device able to limit the voltage without producing a power outage is more appealing.
After several generations of surge arresters, the introduction of valve-type silicon-carbide arresters in 1954 was a significant technological advance. The valve element (or valve block) consisted of a non-linear resistor – commonly silicon carbide (SiC) – whose value decreases abruptly as the voltage rises. The name valve block comes from the valving action to the flow of the current.
Silicon carbide arresters allowed for a reduction to the basic lightning impulse insulation level (BIL) of substation equipment, high fault current withstand, and smaller size, with significant economic savings.
Introduced around 1976, modern metal-oxide arresters – typically zinc oxide (ZnO) – do not need gaps and exhibit better handling characteristics for switching surges, reduced current under steady-state conditions, and reduced lead lengths.
Although silicon carbide arresters provided good service for many years, the better performance and improved power system availability make metal-oxide devices a better choice.
There are arresters with different voltage and power levels to best suit the protected equipment’s needs.
Silicon Carbide (SiC) Valve-Type Surge Arresters
SiC valve-type surge arresters employ a non-linear valve element (resistor) made of silicon carbide and inorganic binders. Silicon carbide is a compound of silicon and carbon.
Some arrester applications require that the valve element have a low resistance value during steady-state conditions to deal with particular surge and power system characteristics, creating excessive power losses. Valve-type surge arresters have spark gaps in series with the valve elements to manage this difficulty.
Series spark gaps keep the valve element isolated under steady-state conditions, in order to reduce losses, and they introduce the valve element when a surge emerges from the gap’s sparkover. There is no leakage current flow between the line and earth, allowing the valve design to deal with its voltage-limiting role and energy dissipation capacity only under surge conditions.
The total voltage across the arrester is the gaps’ sparkover level plus the voltage across the valve element. The lower the total voltage, the better the protection level.
SiC arresters also contain current limiting gaps to limit the system follow current. These gaps reduce the energy absorbed during operation, allowing for fewer valve elements, shorter arrester length, and reduced voltage levels. The arrester gaps exhibit drawbacks, like producing transients during the sparkover to engage the valve elements.
Another crucial matter is the arc-quenching ability of the arrester. Arrester design provides creative ways for quenching the arcs created in the gaps, protecting the valve element against the continuous flow of current – the follow current – after the surge is rerouted and steady-state conditions resume.
Figure 1 shows a volt-ampere characteristic for a gapped silicon-carbide arrester.
Figure 1. V-I characteristic of a gapped silicon-carbide surge arrester. Image courtesy of Industrial-electronics.
Figure 2 shows a diagram of a typical 6kV silicon-carbide surge arrester with its components: main gap units, magnetic coil, valve elements, bypass gap, and shunting resistors.
Figure 2. Schematic diagram of a gapped silicon-carbide surge arrester. Image courtesy of General Electric.
The pre-ionizing tips help to initiate the gap’s breakdown when an overvoltage develops. The bypass gap short-circuits the magnetic coil during the surge current transit, placing the surge voltage across the valve element, which presents low resistance at high voltage, and the surge current goes to the ground. The magnetic coil helps to quench the arcs into the main gaps after the surge current passes. The shunting resistors regulate the power frequency voltage across the main gap elements.
Figure 3 shows silicon carbide surge arresters for various voltages.
Figure 3. Silicon-carbide surge arresters. Image courtesy of General Electric.
Metal-Oxide Surge Arresters (MOSA)
A metal-oxide surge arrester contains non-linear metal–oxide resistive disc elements with excellent thermal energy withstand capabilities. Each disc includes powdered zinc oxide material mixed with other metal oxides. This type of surge arrester works like a high-speed electronic switch – opened at steady-state voltages and closed at overvoltages.
Zinc oxide surge arresters are highly non-linear – their non-linear characteristic is much more pronounced than that of silicon carbide – and have low losses under steady-state conditions.
There are three types of metal-oxide arresters:
As with silicon-carbide surge arresters, the first metal-oxide arresters had a gap in series with non-linear resistors. At that time, the resistors’ thermal duty was relatively small and they could not withstand the thermal energy of the leakage current under steady-state conditions, requiring the gap. Gapless arresters appeared around 1980, and their resistors tolerate the constant small leakage current.
Zinc oxide arresters are easy to manufacture, have low cost, and absorb or dissipate large amounts of energy. Nowadays, most arresters employed in new systems or revamps are gapless zinc oxide devices.
Figure 4 shows a gapless zinc oxide surge arrester’s cutaway, containing a single column of ZnO blocks.
Figure 4. Parts of a porcelain-housed gapless zinc-oxide surge arrester. Image courtesy of ABB.
|1||Porcelain insulator||6||Sealing cover|
|2||Venting duct||7||Sealing ring|
|5||Copper sheet||10||Flange cover|
Figure 5 shows a high voltage zinc oxide surge arrester for areas with very high lightning intensity. Note the external grading rings that long arresters regularly require to maintain constant voltage stress along their length.
Figure 5. Zinc-oxide surge arrester. Image courtesy of ABB.
Surge Arrester Classification and Application
Based on voltage rating, protective characteristics, and durability in pressure-relief or fault-withstand characteristics, the classification of surge arresters used in power systems is as follows:
- Station arresters: Provide the best protective levels – lower discharge voltages, higher energy absorption, and more significant pressure relief. Typical applications are large substations and sites with strong surges.
- Intermediate arresters: Have inferior protective characteristics and energy discharge capability. Typical applications are small substations, underground cable protection, and dry-type transformers.
- Distribution arresters: Provide the lowest protective levels and energy discharge capability. They are used in medium voltage networks.
The system and equipment insulation’s voltage withstand ability depends on the surge’s rise time. In this instance, insulation capability is a function of time.
A surge arrester’s protective characteristics are also a function of time; hence, the need for coordinating the insulation and arrester volt-time characteristics to get adequate protection – the insulation coordination procedure.
Insulation coordination compares the system or equipment insulation’s impulse withstand ability with the voltage across the arrester for the selected discharge current, in accordance with the preferred protection level. The choice of insulation levels and coordination practices affects costs considerably. A drop of one level in BIL can reduce major electrical equipment costs by thousands of dollars.
As an example, Figure 6 shows the entire V-I withstand curve for an oil-filled power transformer and the protective characteristics of a surge arrester – front-of-wave sparkover and discharge voltage.
Figure 6. Oil-filled transformer insulation withstand and arrester protective characteristics. Image courtesy of Cooper.
The arrester’s sparkover crest voltage should be below the transformer’s chopped wave withstand. It is safer to compare the arrester’s sparkover with the transformer’s front-of-wave test when the latter is available.
Another comparison is the arrester’s discharge voltage and the 1.2/50 µs impulse sparkover with the transformer’s full-wave test (BIL).
A Review of Surge Arrester Types and Characteristics
The first surge protective devices were the rod gaps. Rod gaps are cheap but have several disadvantages: they may not protect for fast fronts, produce steep surges during sparkover, and generate a fault on every operation – they do not reseal.
Silicon-carbide valve-type surge arresters employ a silicon carbide non-linear valve element and series spark gaps. The spark gaps keep the valve element isolated under steady-state conditions – reducing losses – and activates it when a surge emerges, but they create transients during the sparkover.
Zinc-oxide arresters were introduced around 1976. Zinc oxide is a substitute for silicon carbide. ZnO arresters have a more pronounced non-linear characteristic than SiC and can be used without series gaps due to their small current at nominal voltage. Yet, they are extremely effective at limiting surge voltages.
Most arresters employed today in new systems or revamps are gapless zinc-oxide devices.
There are three classes of power system surge arresters: station-, intermediate-, and distribution-class. Station arresters provide the best protective levels but are more expensive.
Insulation coordination is essential. This coordination compares the system or equipment insulation’s impulse withstand ability with the voltage across the arrester while surge current is being discharged.