A valve-type lightning arrester is an effective type of lightning arrester for high voltage surges. Here, we shall explore how valve-type lightning arresters work and why they were revolutionary in their time.
What Is a Valve-Type Lightning Arrester?
As stated, a valve-type lightning arrester is a high-voltage surge protection device. It was widely used in power transmission and distribution systems before the development of the gapless ZnO lightning arrester. Like other older and modern types of lightning arresters, its primary job is to safely divert destructive surge energy to the ground.
Unlike earlier spark gap-type arresters, a valve-type arrester is equipped with a series of nonlinear resistor elements, typically made of silicon carbide (SiC). This can be considered an improved version of the spark gap-type lightning arrester. The presence of silicon carbide nonlinear resistors dramatically improves the control over surges.
How Valve-Type Arresters Prevent Arc Sustaining
In both spark gap and valve-type arresters, the air in the spark gaps gets ionized and an arc ignites when the surge voltage exceeds a certain threshold. But in pure spark gap-type arresters, the arc often persists even after the surge is over due to the presence of ionized air in the spark gaps. Because of this ionized air, the normal power-frequency voltage can maintain the arc. Therefore, the spark gaps alone may not extinguish the arc automatically.
This is why valve-type arresters introduce nonlinear resistor blocks (usually silicon carbide, SiC) in series with the spark gaps. When the surge voltage drops below the threshold limit, the nonlinear resistor blocks offer high resistance to the path of current to the ground. Therefore, the arc gets extinguished.
Why Silicon Carbide (SiC) Blocks Need Spark Gaps in Series
Up to this point, we have discussed why nonlinear resistors are required in series with spark gaps in a valve-type lightning arrester. Now we shall discuss why spark gaps are essential in a silicon carbide arrester. That is to say, since silicon carbide blocks are nonlinear resistors, they allow current to flow above the threshold voltage level and block the current below that level. So theoretically, one could make a gapless lightning arrester without using spark gaps.
However, it must be noted that the nonlinearity of silicon carbide is not very sharp. The nonlinearity coefficient of SiC is about 7 to 10. Because of this, even under normal conditions, it allows significant leakage current to flow through it to the earth. But if one or more series gaps are added in series with the SiC blocks, leakage current will not flow.
Basic Construction of a Valve-Type Lightning Arrester
As stated, a valve-type lightning arrester essentially consists of spark gaps and nonlinear resistor elements. Hence, its construction is mainly a combination of spark gaps and nonlinear resistor elements arranged in a specific order to achieve reliable surge protection. The components are:
- Series Spark Gaps
These act as the first line of defense against surges. Construction-wise, there is a set of electrodes arranged with precise air gaps between them. Under normal conditions, the air gaps isolate the nonlinear resistor blocks from the system voltage. During a surge, the high voltage causes ionization of the air, allowing it to appear at the blocks. Due to the influence of high voltage, the SiC blocks become conductive and allow current to flow through. The spark gaps are normally enclosed in a sealed chamber to prevent moisture and ensure reliable breakdown, as moisture reduces the dielectric strength of the gaps and may trigger them even before the surge voltage reaches its threshold. - Nonlinear Resistor Blocks
These are made of silicon carbide (SiC). These resistors exhibit nonlinear resistance, meaning the blocks offer low resistance at high voltages (during a surge) and high resistance at normal voltages (during steady-state). Since each block behaves as an electrical valve, the arrester is called a valve-type lightning arrester. - Grading Ring or Shield
This ensures uniform voltage distribution across the spark gaps and resistors. The grading ring is placed around the arrester housing, particularly in high-voltage units. It primarily prevents the premature breakdown of spark gaps. - Insulating Housing
This isolates the spark gaps and resistor elements from the environment. In addition, it provides mechanical protection and electrical insulation. The housing is normally made of either porcelain or polymer and is sealed against moisture and environmental contaminants. - Terminals
The line terminal, located at the top cover of the arrester, connects to the equipment or power line to be protected. The earth terminal connects to the grounding system, where the surge is discharged. - Grading Resistors
In a valve-type lightning arrester, grading resistors are commonly connected across individual spark gaps in multi-gap arrangements. When several spark gaps are connected in series, the total voltage may not be evenly distributed across each gap due to non-uniform electric field distribution, even when a grading ring surrounds the arrester. The parallel grading resistors help achieve uniform voltage sharing across all spark gaps.
