What is Valve Type Arrester?
A valve-type lightning arrester, also known as a gap surge diverter or silicon carbide surge diverter, is a type of lightning arrester that consists of one or multiple gaps connected in series with a current-controlling element. The gaps between the electrodes intercept the flow of current through the arrester, except when the voltage across the gap raises beyond a certain level known as the critical gap flashover.
The critical gap flashover is the point at which the voltage across the gap is high enough to cause an electrical discharge, also known as a spark, to occur. This discharge creates a path for the current to flow through the arrester and away from the protected equipment.
The valve-type arrester uses a series of gaps to provide a high level of protection against lightning strikes and other high-energy transients. The gaps are made with a current controlling element such as silicon carbide. This type of arrester is typically used in high-voltage power systems and is designed to protect equipment from damage caused by lightning strikes, switching transients, and other high-energy events.
Construction of Valve Type Lightning Arrester:
The series spark gaps in the valve type lightning arrester are designed to withstand normal circuit voltage, but when an overvoltage occurs, the gap breaks down and allows the surge current to be redirected to ground through the nonlinear resistor discs. These discs, made of materials such as thyrite or metrosil, have a non-linear resistance that decreases as the current flowing through them increases. This helps to limit the high current flow and protect the circuit from damage.
The spark gaps and nonlinear resistor discs are placed in a tight porcelain container to protect them from atmospheric moisture, condensation, and humidity. The container also helps to ensure reliable protection for the circuit.
The construction of valve type lightning arrester is such that it has a series of spark gaps connected in series with nonlinear resistor discs. The spark gaps are designed to intercept the flow of current through the arrester except when the voltage across the gap raises beyond the critical gap flashover. The nonlinear resistor discs are designed to limit the high current flow and redirect the surge current to ground. Overall, the valve type arrester is an effective device that protects circuits from overvoltage and surge currents caused by lightning strikes.
Working of Valve Type Lightning Arrester:
The working of a valve type lightning arrester can be explained in the following steps:
- At normal system voltage, the valve arrester acts as an insulator and does not allow the breakdown of the air gap assembly.
- When an overvoltage occurs, the series spark gap breaks down, and this causes the surge current to flow through the nonlinear resistors.
- The nonlinear resistors have a low resistance during high surge current, which allows the surge to be directed to the ground instead of the line.
- As the surge current decreases, the nonlinear resistors attain a high resistance, which stops the flow of current.
- The volt-ampere characteristics of a nonlinear resistor are shown in the figure. The static and dynamic characteristics are represented by a dotted line and a dark line respectively.
- A horizontal tangent is drawn to the dynamic characteristic whose intercept gives the residual voltage. The residual voltage is the peak value of the voltage that occurs between the terminals of the surge diverter.
- This voltage is typically between 3 kV and 6 kV, depending on the type of current discharge and rate of change of current.
- These arresters operate very fast and are effective in providing protection for devices like cables and transformers.
Stage of Valve Type Lightning Arrester:
During a lightning strike or other overvoltage event, the surge voltage reaches the transformer and the valve type lightning arrester. Initially, the voltage is below the breakdown value of the series gap in the arrester and it acts as an insulator. However, as the surge voltage increases, it reaches the breakdown value of the series gap and the arrester begins to discharge.
The non-linear resistors, typically made of an inorganic compound like thyrite or metrosil, have a low resistance during high surge current, allowing the surge energy to discharge to ground instead of being transmitted to the terminal equipment. As the voltage decreases, the resistance of the non-linear resistors increases, interrupting the current flow through the arrester.
This process occurs in stages, with the arrester discharging and sealing again as the voltage and current fluctuate. The maximum voltage developed across the arrester terminals and transmitted to the terminal equipment is known as the discharge value of the arrester. This value is typically between 3 kV and 6 kV, depending on the type of current discharge and rate of change of current. Overall, the valve type lightning arrester operates quickly and effectively to provide protection for equipment such as cables and transformers.
Types of Valve Type Lightning Arrester:
Station Type Valve Lightning Arrester:
This type of valve is mainly employed for the protection of the critical power equipment in the circuit of 2.2kV to 400kV and higher. They are designed to handle high energy surges and have a high capacity for energy dissipation. They are typically used to protect power transformers, generators, and other critical equipment in power substations.
Line Type Lightning Arrester:
The line type arresters are used for the protection of substation equipment. They are typically smaller in size and lighter in weight than station type arresters, and are also less expensive. They allow for higher surge voltages across their terminals in comparison to station type arresters and have a lower surge carrying capacity. They are typically used to protect substation equipment such as switchgear, circuit breakers, and other equipment.
Distribution arrester:
These types of arresters are usually mounted on poles and are employed for the protection of generators and motors. They are typically used in distribution systems to protect equipment at the distribution level, such as transformers and distribution lines.
Secondary arrester:
These types of arresters are meant for the protection of low voltage apparatus. They are typically used to protect equipment in secondary systems, such as industrial control panels and other low voltage equipment.
Secondary Arrester:
This type of arrester is designed specifically for the protection of generators and motors. It is designed to protect these types of equipment from high energy surges caused by lightning or other transient events.
Advantages of Valve Type Lightning Arrester:
- Reliability and Effectiveness: Valve type lightning arresters are designed to provide very reliable and effective protection to equipment like cables, transformers and other electrical devices. This is achieved by the use of a series of spark gaps and nonlinear resistor discs, which work together to redirect surge currents to ground, preventing damage to the protected equipment.
- Impulse Ratio: The impulse ratio is the ratio of the breakdown voltage under surge conditions to the breakdown voltage under low-frequency conditions. This ratio is practically unity for valve type lightning arresters, which means that they perform well under both normal and surge voltage conditions.
- Speed of Operation: Valve type lightning arresters have a very fast speed of operation, typically taking less than one second to operate. This is important in protecting equipment from damage during a lightning strike or other high voltage surge event.
- Predetermined Voltage: Valve type lightning arresters are designed to spark over at a predetermined voltage. This allows them to operate only when necessary, reducing the chances of false tripping and reducing maintenance requirements.
- Energy Dissipation: Valve type lightning arresters are designed to dissipate surge energy, which helps to protect equipment from damage. They do this by redirecting the surge current to ground, preventing it from flowing through the protected equipment.
- High Frequency Surge Protection: Valve type lightning arresters can also handle high-frequency surges without any change in their spark-over characteristic. This is important for protection against high-frequency transients that can occur in power systems, such as those caused by switching or other disturbances.
Disadvantages of Valve Type Lightning Arrester:
- One major drawback of valve type lightning arresters is that they may not be able to effectively check the surges of very steep wavefronts from reaching the terminal apparatus. Because valve type arresters rely on a spark gap to conduct the surge current to ground, they may not be able to handle high-energy transients with very fast rise times. This can result in the surge energy reaching the protected equipment and causing damage.
- Another drawback of valve type lightning arresters is that they can be affected by moisture. Moisture can enter the arrester through the porcelain container, and if it does, it can cause the non-linear resistor discs to lose their performance. This can lead to a decrease in the arrester's ability to effectively conduct surge current to ground, and it can also lead to an increase in the arrester's residual voltage. To prevent moisture from entering the arrester, it is important to install it in a location that is protected from the elements, and to ensure that the porcelain container is properly sealed.
Uses of Valve Type Lightning Arrester:
- Valve type lightning arresters are typically used to protect important equipment in power stations that operate at voltages up to 22 kV and higher.
- They are also used in stations that handle voltages up to 66 kV. The main advantage of using a valve type lightning arrester is that it provides very reliable and effective protection to the equipment like cables, transformers and other equipment. The impulse ratio is the ratio of Breakdown voltage under surge condition to the Breakdown voltage under low frequency condition, which is practically unity.
- The speed of operation is also very high, typically taking less than one second. They are designed to spark over at a predetermined voltage, and they can effectively dissipate surge energy and discharge high-frequency surges without any change in spark-over characteristics. However, valve type lightning arresters may have some limitations such as they may fail to check the surges of very steep wavefront from reaching the terminal apparatus, and if moisture enters they may lose their performance.