Overvoltage Protection MCQ Quiz - Objective Question with Answer for Overvoltage Protection - Download Free PDF

The Translay scheme is NOT used for lightning protection. The Translay scheme is a differential protection scheme used in feeder protection

Lightning Protection Methods:

Lightning is a high-voltage natural phenomenon that can cause severe damage to power systems. It occurs due to the discharge of electrical energy between clouds or between clouds and the ground. In power systems, lightning can lead to overvoltages, insulation breakdown, equipment damage, and power outages.

Lightning protection schemes are designed to prevent damage to electrical power systems caused by direct or indirect lightning strikes. Some of the lightning protection methods are:

• Overhead ground wires: Used to shield transmission lines from direct lightning strikes.
• Earthing screen: Protects substations by providing a grounded metal mesh to dissipate lightning energy.
• Lightning arrestor: Directs high-voltage lightning surges safely to the ground.

The correct answer is option 4.

Lightning arrester

A Lightning Arrester is a protective device that protects electrical equipment from high voltage surges caused by lightning strikes.

They are usually installed at the top of transmission poles and buildings and provide an easy path for the surge voltage from lightning to flow to the ground.

Types of lightning arrester

Valve-type lightning arrester

A Valve Type Arrester consists of the following main components:

• Series Spark Gaps: These are used to block normal system voltage and operate only when there is an overvoltage.
• Non-linear Resistor Discs: These resistors limit the surge current by reducing their resistance when a high voltage surge appears.

Rod gap lightning arrester

• It is one of the simplest types of Lightning Arresters.
• There is a gap between the end of the two rods. These two rods get connected to the earth and the line directly. The gap gets filled with air.
• When there is a higher voltage on the line, the air ionizes, producing a spark. In this way, the fault current passes to the earth.

Expulsion-type lightning arrester

• The expulsion type arrester is an improvement over the rod gap arrester.
• This type of arrester is also known as a protector tube or expulsion gap and it is commonly limited to systems operating at voltages up to 33kV.
• This arrester consists of a tube made up of very effective fiber, isolating the spark gap and an interrupting spark gap inside the fiber tube.

Multi-gap arrester

• A multigap arrester has multiple air gaps with the help of several cylinders, which are made up of Zinc alloy.
• The top cylinder is connected to the line while the bottom cylinder is connected to the ground with a series resistor.
• In the event of a surge, the air insulation will break down, and the surge will pass through the cylinders and the air gaps to the ground rather than through the shunt resistor.

Lightning surges

• Lightning surges are high-voltage electrical transients caused by lightning discharges that can cause significant damage to electrical and electronic systems.
• These surges are a major concern in power systems, telecommunications, and sensitive electronic equipment due to their intensity and potential for disruption.

Protection Against Lightning Surges:

1. Lightning Arresters: Lightning arresters are installed to divert lightning surges to the ground, preventing them from reaching sensitive equipment. They are placed on power lines, at service entrances, and on telecommunications lines.
2. Surge diverters: Surge diverters, also known as surge arresters or surge protectors, are devices designed to protect electrical systems and equipment from transient overvoltages, such as those caused by lightning strikes or switching surges. Their primary function is to divert or limit the voltage surge to prevent damage to sensitive equipment.
3. Horn gap: A horn gap is a type of electrical protective device used primarily in high-voltage systems to protect electrical equipment from overvoltages, such as those caused by lightning strikes or switching surges.
4. Shielding:​ Shielding sensitive electronic equipment and cables can help reduce the effects of electromagnetic pulses generated by lightning surges.

Horn gap is the most commonly used type of lightening arrester.

Explanation:

Horn Gap Arrester:

• It consists of two horns-shaded pieces of metal separated by a small air gap and connected in a shunt between each conductor and earth.
• The distance between the two electrodes is such that the normal voltage between the line and the earth is insufficient to jump the gap.
• But the abnormal high voltage will break the gap and so find a path to earth.

Additional Information

Expulsion type arrester:

• This type of arrester is also called a protector tube and is commonly used on a system operating at voltages up to 33 kV.
• The figure shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap A A′ in series with a second gap enclosed within the fiber tube.
• The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to the rod gap and the lower electrode to the earth.
• One expulsion arrester is placed under each line conductor. The figure shows the installation of an expulsion arrester on an overhead line.

 

Important Points:

• According to their application, the valve type arresters are classified as (i) station type and (ii) line type.
• The station type arresters are generally used for the protection of important equipment in power stations operating on voltages up to 220 kV or higher.
• The line type arresters are used for stations handling voltages up to 66 kV.

Rod Gap Arrester:

• In such a type of arrester, there is an air gap between the ends of two rods.
• The one end of the arrester is connected to the line and the second end of the rod is connected to the ground.
• The gap setting of the arrester should be such that it should break before the damage.
• When the high voltage occurs on the line, the gap sparks, and the fault current passes to the earth. Hence the equipment is protected from damage.

 

 

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Question:

• Which of the following is NOT an internal cause of overvoltage in power system?

Options:

1. Resonance
2. Lightening
3. Insulation failure
4. Switching surges

Correct Answer:

• The correct answer is option 2.

Solution Statement:

• Lightening is an external cause of overvoltage in power systems. Internal causes include resonance, insulation failure, and switching surges. Therefore, option 2 is the correct answer.

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Expulsion type arrester:

• This type of arrester is also called a protector tube and is commonly used on a system operating at voltages up to 33 kV.
• The figure shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap A A′ in series with a second gap enclosed within the fiber tube.
• The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to the rod gap and the lower electrode to the earth.
• One expulsion arrester is placed under each line conductor. The figure shows the installation of an expulsion arrester on an overhead line.

 

Important Points:

• According to their application, the valve type arresters are classified as (i) station type and (ii) line type.
• The station type arresters are generally used for the protection of important equipment in power stations operating on voltages up to 220 kV or higher.
• The line type arresters are used for stations handling voltages up to 66 kV.

Additional Information 

Horn Gap Arrester:

• It consists of two horns-shaded pieces of metal separated by a small air gap and connected in a shunt between each conductor and earth. 
• The distance between the two electrodes is such that the normal voltage between the line and the earth is insufficient to jump the gap.
• But the abnormal high voltage will break the gap and so find a path to earth.

Rod Gap Arrester:

• In such a type of arrester, there is an air gap between the ends of two rods. 
• The one end of the arrester is connected to the line and the second end of the rod is connected to the ground. 
• The gap setting of the arrester should be such that it should break before the damage.
• When the high voltage occurs on the line, the gap sparks, and the fault current passes to the earth. Hence the equipment is protected from damage.

 

 

Surge diverter:

lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.

Surge absorber:

A surge absorber is a protective device which reduces the steepness of wavefront of a surge by absorbing surge energy.

Switching surges:

The overvoltages produced on the power system due to switching operations are known as switching surges.

• A thyrite lightning arrester has a combination of inverse resistance characteristics and gap
• Thyrite arrester is most common and is mostly used for the protection against high dangerous voltages
• The manufacturing processes have been perfected so that the electrical and mechanical characteristics can be duplicated or varied as desired within practical limits
• It does not follow Ohm's law, for each time the voltage is doubled the current increases 12.6 times
• Thus, it possesses nonlinear characteristics

The most commonly used devices for protection against lightning surges are

Lightning arresters or surge diverters:

• A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground
• It consists of a spark gap in series with a non-linear resistor
• The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap, but a dangerously high voltage will break down the air insulation and form an arc
• The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa
• One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded

Impulse ratio of insulator:

• The impulse ratio of an insulator can be defined as the ratio of impulse peak voltage to the peak voltage.
• It is used to determine the value of flashover or puncture of insulation.
• Generally, the impulse ratio of the insulator is high.

Note:

• Impulse peak voltage is defined on impulse frequency.
• Peak voltage is defined on power frequency.

Impulse Ratio of lighting Arrester:

• The impulse ratio of any lightning arrester is the ratio of the breakdown voltage of the wave of special duration to the breakdown voltage of a 50 Hz wave.
• Generally, the impulse ratio of the lightning assenter is low.
• Lightning arrestors are used to protect against surges and lightning strikes.
• Lightning arresters are composed of materials ZnO or SiC.

Lightning arresters or surge diverters:

• A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground
• It consists of a spark gap in series with a non-linear resistor
• The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap, but a dangerously high voltage will break down the air insulation and form an arc
• The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa
• One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded

Circuit Elements	Symbols
Earthing
Circuit Breaker
Current Transformer
Bus Bar
Potential Transformer
Lightning Arrestor
Fuse
Transformer
Isolator
Tandem Isolator
Auto Transformer

Transient stability: Transient stability is the ability of the power system to maintain synchronism when subjected to a severe transient disturbance.

Transient stability of the system can be improved by

• Increasing the system voltage
• Increase in the X/R ratio
• ​Using high-speed governors on machines
• High-speed circuit breakers help to clear the fault as quickly as possible
• By turbine fast valving
• High-speed excitation
• Use of auto re-closing breakers
• Some of the other ways to improve the transient stability are by employing lightning arresters, high neutral grounding impedance, single pole switching, quick Automatic Voltage Regulators (AVRs).

Overhead ground wires:

• The most effective method of providing protection to transmission lines against direct lightning strokes are using overhead ground wires
• It provides considerable protection against direct lightning strokes on transmission lines
• A grounding wire provides a damping effect on any disturbance travelling along the line as it acts as a short-circuited secondary
• It provides a certain amount of electrostatic shielding against external fields
• Thus, it reduces the voltages induced in the line conductors due to the discharge of a neighbouring cloud

 

Important Point:

Earthing screen:

• The power stations and sub-stations generally have expensive equipment
• These stations can be protected against direct lightning strokes by providing an earthing screen
• It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the substation or power station
• The shield is properly connected to earth on at least two points through a low impedance
• On the occurrence of a direct stroke on the station, the screen provides a low resistance path by which lightning surges are conducted to ground. In this way, station equipment is protected against damage
• The limitation of this method is that it does not provide protection against the travelling waves which may reach the equipment in the station

A varistor is a nonlinear resistor whose resistance decreases as the voltage increases. Therefore, a varistor is a voltage-dependent resistor. It is made of silicon carbide powder and is built in the shape of a disc.

• The zinc oxide-based varistors are primarily used for protecting solid-state power supplies low and medium size voltage in the supply line.
• Silicon carbide varistor provides protection against high-voltage surges caused by lightning and by the discharge of electromagnetic energy stored in the magnetic fields of the large coil.
• The practical voltage range of zinc oxide varistor is as wide as six to several thousand volts.

Silicon-carbide varistors have a high power-handling capability and are used in high-voltage surge (lightning) arresters. These devices tend to draw considerable current in the normal state, and so they are commonly used in series with a gap that provides an open circuit until a surge occurs.