Enhancing the Stability of 10kV switchgear with Degatech’s lightning arresters

Overview of Lightning arresters: Lightning activity can cause line tripping, affecting the stable operation of the power system. In order to reduce the failure rate of distribution equipment under lightning overvoltage, to reduce the trip rate of distribution equipment under lightning strikes, and to improve the stability and reliability of the distribution network, it is necessary to install lightning arresters for distribution in the 10kV power system. However, lightning arresters in 10kV distribution networks can also fail under lightning strikes, and the stability and reliability of the distribution network will be reduced. Therefore, in order to effectively prevent lightning arrester failures caused by overvoltage under lightning strikes and improve the stability of the distribution network, it is necessary to understand the failures of lightning arresters and the measures to solve lightning arrester failures.

1. The working principle of lightning arresters.

The Working Principle of Lightning Arrester The working principle of a lightning arrester, simply put, is that this device is installed on the line or electrical equipment. When the working voltage is normal, the device is insulated from the ground. When lightning invades or the voltage increases, the device changes from insulation to conduction and breakdown discharge with the ground, introducing the lightning current or overvoltage to the ground, limiting the voltage or current, and playing a protective role. After the lightning or overvoltage passes, it quickly returns to the original insulated state from the ground, ready to play the protective role again. Nowadays, zinc oxide ZnO lightning arresters are almost all used. Zinc oxide ZnO lightning arresters are a new type of lightning arrester developed in the 1970s, and they are mainly composed of zinc oxide varistors. Each varistor has a certain switching voltage (called varistor) when it is manufactured. Under normal working voltage (i.e., less than the varistor voltage), the resistance value of the varistor is very large, equivalent to an insulated state. However, under the impact voltage (greater than the varistor voltage), the varistor has a low resistance and is breakdown, equivalent to a short-circuit state. However, the breakdown state of the varistor can be restored; when the voltage higher than the varistor voltage is removed, it returns to a high-resistance state. Therefore, if a zinc oxide lightning arrester is installed on the power line, when lightning strikes, the high voltage of the lightning wave breaks down the varistor, and the lightning current flows to the ground through the varistor, controlling the voltage on the power line within a safe range, thereby protecting the safety of electrical equipment. Degatech electric strictly follows the usage requirements and specifications of lightning arresters in the production process of MV and LV switchgear:

1.1 For 10KV or 35KV switchgear, if there is an overhead line power supply incoming or outgoing line, in order to prevent lightning overvoltage from entering, lightning arresters are installed on the busbar;

1.2 Lightning arresters are installed on the incoming switchgear or sectional cabinet to prevent the incoming line from being an overhead incoming line and introducing lightning overvoltage. Lightning arresters are also installed in the cabinet;

1.3 Vacuum circuit breakers or SF6 circuit breakers with very short opening times are adopted. When opening small currents, operating overvoltage will be generated. Lightning arresters are also installed at the load end of the circuit breaker in the production of the switchgear, and at the same time, overvoltage from the outside is also prevented.

2. Characteristics of Different Types of Lightning Arresters

2.1 Surge arresters utilize the air gap connected between the line and the ground to provide lightning protection. Their main drawback is the need for series linear resistors and fuses to interrupt the power tracking current. Additionally, when the gap sparks, it can cause faults in the circuit, and when the circuit breaker clears, it can cause unpleasant power outages. Devices that can limit voltage without causing power outages are more attractive.

2.2 Valve-type silicon carbide lightning arrester: After the development of several generations of lightning arresters, the valve-type silicon carbide lightning arrester introduced in 1954 was a significant technological advancement. The valve element (or valve block) is composed of a nonlinear resistor (typically silicon carbide (SiC)), and its resistance value suddenly decreases as the voltage rises. The name of the valve block originates from the valve action of current flow. Silicon carbide lightning arresters can reduce the basic lightning impulse insulation level (BIL) of substation equipment, have a high fault current withstand capacity and smaller size, significantly saving costs.

2.3 Modern metal oxide lightning arresters (typically zinc oxide (ZnO): were introduced around 1976, do not require gaps, and exhibit better switching surge handling characteristics, reduced current under steady-state conditions, and shorter lead lengths. Although silicon carbide lightning arresters have provided good service for many years, better performance and improved availability of the power system make metal oxide devices a better choice. There are lightning arresters of different voltages and power levels that can meet the needs of protected equipment.

2.4 Silicon carbide (SiC) valve-type lightning arrester: The SiC valve-type lightning arrester adopts nonlinear valve elements (resistors) made of silicon carbide and inorganic binders. Silicon carbide is a compound of silicon and carbon. Some applications of lightning arresters require the valve elements to have a low resistance value under steady-state conditions to cope with specific surges and power system characteristics, resulting in excessive power loss. To address this issue, spark gaps are connected in series with the valve elements of the valve-type lightning arrester. The series spark gaps keep the valve elements isolated under steady-state conditions to reduce losses, and when a surge occurs during the spark discharge of the gap, they introduce the valve elements. There is no leakage current between the line and the ground, enabling the valve design to exert its voltage-limiting and energy dissipation capabilities only in the case of surges. The total voltage across the lightning arrester is the sparking level of the gap plus the voltage across the valve element. The lower the total voltage, the better the protection level. SiC lightning arresters also contain current-limiting gaps to limit the system follow current. These gaps reduce the energy absorbed during operation, allowing the use of fewer valve elements, shorter lightning arrester lengths, and lower voltage levels. There are some disadvantages of the lightning arrester gap, such as generating transients during spark discharge to engage the valve element.

3. Another key issue is the arc extinguishing capability of the lightning arrester.

The lightning arrester design provides creative methods to extinguish the arcs generated in the gap, protecting the valve elements from the influence of continuous current (follow current) after the surge is rerouted and the steady-state conditions are restored.

Figure 1 shows the volt-ampere characteristics of the gapped silicon carbide lightning arrester. VI characteristics of the gapped silicon carbide lightning arrester.

Figure 1. VI characteristics of the gapped silicon carbide lightning arrester. Image provided by Industrial Electronics.

Figure 2 shows a schematic diagram of a typical 6kV silicon carbide lightning arrester, whose components include: main gap unit, electromagnetic coil, valve element, bypass gap, and shunt resistor. Schematic diagram of the gapped silicon carbide lightning arrester.

Figure 2. Schematic diagram of the gapped silicon carbide lightning arrester. Image provided by General Electric. When overvoltage occurs, pre-ionization helps trigger gap breakdown. The bypass gap shorts the electromagnetic coil during surge current transmission, allowing the surge voltage to pass through the valve element that presents a low resistance at high voltages, and the surge current flows to the ground. The electromagnetic coil helps extinguish the arc entering the main gap after the surge current passes. The shunt resistor regulates the power frequency voltage on the main gap element.

Figure 3 shows silicon carbide lightning arresters suitable for various voltages. Silicon carbide lightning arrester Figure 3. Silicon carbide lightning arrester. Image provided by General Electric.
Metal oxide lightning arrester (MOSA) Metal oxide lightning arresters contain nonlinear metal oxide resistor disc elements with excellent heat energy withstand capacity. Each disc contains powdered zinc oxide material mixed with other metal oxides. The working principle of this type of lightning arrester is similar to a high-speed electronic switch – it opens under steady-state voltage and closes under overvoltage. Zinc oxide lightning arresters are highly nonlinear – their nonlinear characteristics are much more pronounced than those of silicon carbide – and have lower losses under steady-state conditions. Metal oxide lightning arresters are divided into three types: Gapless Series gap Parallel gap Like silicon carbide lightning arresters, each metal oxide lightning arrester also has a gap in series with a nonlinear resistor. At that time, the thermal load of the resistor was relatively small and could not withstand the heat energy of the leakage current under steady-state conditions, requiring a gap. Gapless lightning arresters appeared around 1980, and their resistors can withstand a constant small leakage current. Zinc oxide lightning arresters are easy to manufacture, have low cost, and can absorb or dissipate a large amount of energy. Nowadays, most lightning arresters used in new systems or renovations are gapless zinc oxide devices.

Figure 4 shows a cross-sectional view of a gapless zinc oxide lightning arrester, which contains a single row of ZnO blocks. Parts of a porcelain shell gapless zinc oxide lightning arrester.

Figure 5. Components of a porcelain shell gapless zinc oxide lightning arrester. Image provided by ABB.

1 Porcelain insulator                              6 Sealing cover
2 Ventilation duct                                   7 Sealing ring
3 Spring                                                    8 Indicator plate
4 Desiccant bag                                       9 Zinc oxide block
5 Copper sheet                                        10 Flange cover

Figure 5 shows a high-voltage zinc oxide lightning arrester suitable for areas with extremely high lightning intensity.

Zinc Oxide Arrester
Figure 5. Zinc Oxide Arrester. Image courtesy of ABB.

Arrester Classification and Application
Arrester used in power systems are classified according to voltage level, protection characteristics, and durability of pressure relief or fault tolerance characteristics as follows:
Power station arresters: Provides protection level – lower discharge voltage, higher energy absorption and more significant pressure relief. Typical applications are large substations and locations with strong surges.
Intermediate arresters: Provides poorer protection characteristics and energy relief capabilities. Typical applications are small substations, underground cable protection and dry-type transformers.
Distribution arresters: Provides protection level and energy relief capabilities. They are used in medium voltage networks.
Insulation coordination
The withstand voltage capability of system and equipment insulation depends on the rise time of the surge. In this case, the insulation capability is a function of time.
The protection characteristics of the arrester are also a function of time; therefore, the insulation and arrester voltage-time characteristics need to be coordinated to obtain adequate protection – insulation coordination procedures.
Insulation coordination compares the impulse withstand capability of the system or equipment insulation to the voltage across the arrester at a selected discharge current, depending on the level of protection. The choice of insulation level and coordination practice has a significant impact on cost. A one-level reduction in BIL can reduce major electrical equipment costs by thousands of dollars.

For example, Figure 6 shows the entire VI withstand curve for an oil-filled power transformer and the arrester protection characteristics – wavefront flashover and discharge voltage.
Oil-Immersed Transformer Insulation Withstand Voltage and Arrester Protection Characteristics

Figure 6. Oil-Immersed Transformer Insulation Withstand Voltage and Arrester Protection Characteristics. Image courtesy of Cooper.
The arrester flashover peak voltage should be less than the transformer’s chopping withstand capability. When wavefront testing of the transformer is available, it is safer to compare the arrester flashover test to the transformer’s wavefront test.
Another comparison is the arrester’s discharge voltage and 1.2/50us pulse flashover to the transformer’s full wave test (BIL).

3. Arrester failure mode

3.1 Failure of functional components

The resistor of the arrester will age under long-term operation, and when the leakage current increases or the reference voltage decreases, and when the overvoltage, especially the flashover or breakdown under the lightning overvoltage, occurs, the resistor will fail and cause the arrester to fail. The flashover or breakdown under the lightning overvoltage is the biggest reason for the failure record of the arrester in the distribution network.

3.2 Seal failure

Once there is moisture in the arrester, it is easy to cause leakage tracking, dielectric breakdown and internal short circuit failure. The element that prevents moisture or humid air from entering the arrester is the seal/gasket, but the seal may become fragile over time and cannot play the role of sealing the arrester.

3.3 Increased dirt at the arrester installation

The porcelain sleeve and composite jacket type A arrester is a core-tube structure arrester, which is sensitive to dirt. When the core-tube structure arrester is in a place with a lot of dirt, such as a new factory, road or railway line, the arrester is prone to failure due to dirt.

4. Countermeasures for lightning arrester failure

4.1 Lightning arrester must be reliably grounded

In order to ensure that the lightning arrester can smoothly guide the lightning current into the ground when it is struck by lightning, the lightning arrester must be grounded and reliably grounded. When the lightning arrester is grounded, the connection of each part must be firm and reliable to ensure that the entire system is well grounded. Therefore, not only the grounding bolt of the lightning arrester and the grounding wire of the lightning arrester should be connected directly, but also the connection should be treated by welding and explosion pressure. At the same time, when connecting the bolts, do not use winding, binding and other connection methods. In addition, it is necessary to ensure that the lightning arrester in the distribution system is close to the protected equipment and the grounding end of the lightning arrester and the grounding end of the protected equipment are connected with a very short and straight wire, and then grounded with a grounding rod and a grounding grid. The resistance of the grounding rod and the grounding grid should be small, which can limit the increase of the ground potential of the grounding end and avoid flashover on the high-voltage side of the equipment.

4.2 Improve the withstand voltage of the arrester

When the actual lightning current exceeds the withstand voltage of the arrester, it is easy to cause the arrester to be damaged. Therefore, in order to prevent the arrester from being damaged by the lightning current, in addition to the arrester grounding to smoothly conduct the lightning current, the arrester’s withstand voltage must be increased. Only when the arrester’s withstand voltage is increased can it withstand a larger lightning current without being damaged.

4.3 Strengthen the detection of the arrester

In order to reduce or avoid the occurrence of arrester failures, it is necessary to strengthen the detection and monitoring of the arrester. Not only must the quality of the arrester be ensured during production and installation, but it must also be tested and qualified before use before it can be put into use. During use, the detection of the arrester must also be strengthened to ensure the safety of the arrester. The main diagnostic method for the arrester is infrared diagnosis based on “basic thermal imaging”. Based on the structure and the conduction heat energy path, the thermal field and temperature rise changes of the arrester are analyzed and the relevant measurement results are further referred to to effectively analyze the defects of the arrester and other problems, so as to do a good job in the replacement of the arrester.

4.4 Make good choices of lightning arresters and improve the product quality of lightning arresters

There are many types of lightning arresters, each with its own advantages and disadvantages. The type of lightning arrester should be selected according to the actual situation. Secondly, the quality of the lightning arrester should be ensured before installation. In order to avoid or reduce the failure of the lightning arrester during use, the quality of the lightning arrester needs to be improved. First of all, the quality of the lightning arrester is closely related to the technical and technological level of the manufacturer. To improve the quality of the lightning arrester, it is necessary to improve the manufacturing technology and technological level of the metal oxide lightning arrester to improve the indicators of the key components of the lightning arrester. Not only should the sealing material and performance indicators be improved, but also the sealing structure performance indicators of the product should be improved. At the same time, the moisture resistance of the core component of the lightning arrester, namely the resistor, should also be improved. In addition, the sealing problem of the lightning arrester is also very important. It is necessary to pay attention to and improve the leakage detection level of the lightning arrester seal. Whether it is to improve the production technology or the detection level, it is to ensure the quality of the lightning arrester produced so as to improve the reliability of the lightning arrester during use.

4.5 Install the lightning arrester disconnector

The lightning arrester disconnector is a device that disconnects the lightning arrester from the system and gives a visible mark when the lightning arrester fails. When lightning current passes through a normally operating arrester, the disconnector does not operate. When the arrester fails or is overloaded, the arrester will be permanently short-circuited, causing the system to trip. In order to disconnect the arrester from the ground when the arrester fails or is overloaded, it is necessary to install a disconnector on the ground side of the arrester to ensure that the arrester is disconnected from the ground, thereby avoiding the discharge of the operating voltage at the lower grounding end of the arrester. In addition, the arrester disconnector can also prevent the replaced new fuse from melting again due to the short circuit. It is suitable for installation in places that are difficult to reach or where the overload arrester cannot be replaced quickly.

5. Conclusion

Lightning strikes can cause 10kV distribution network equipment to trip or fail, and the arrester is a device that protects distribution network equipment from tripping or failure due to lightning strikes. However, arresters can also fail and arrester failure is the primary factor in distribution network equipment failure. Therefore, in order to avoid or reduce the failure of lightning arresters, it is necessary not only to strengthen the purchase management of lightning arresters, but also to conduct appearance inspection and temperature measurement of lightning arresters in order to find and replace the problem lightning arresters in time. At the same time, in order to increase the lightning arrester’s ability to discharge lightning current, the lightning arrester should not only use a larger capacity lightning arrester, but also install the lightning arrester on the front end tower of the incoming line and ground it.