Over voltage protection in a transmission line is very important as voltage stresses due to over voltages can be very high and they may become dangerous to both the transmission lines as well as the connected equipment and may cause serious damages, unless some protective measures against these over voltages are taken.
Cause of Over voltages in a power system can be generally classified into two main categories as follows:
1.External Over voltages
These over voltages generally originate from atmospheric disturbances which is mainly due to lightning.
The external over voltage may occur due to any of the following causes :
(i) Direct lightning strokes
(ii) Electromagnetically induced over voltages due to lightning discharge taking place near the line (commonly known as ‘side stroke’)
(iii) Voltages induced due to changing atmospheric conditions along the line length
(iv) Electrostatically induced over voltages due to the presence of charge clouds nearby
(v) Electrostatically induced over voltages due to the frictional effects of small particles such as dust or dry snow in the atmosphere or due to change in the altitude of the line
2.Internal Over voltages
Internal over voltage occurs within the power system and can be due to switching surges, fault conditions, or load variations. These over voltages are caused by changes in the operating conditions of the network.
OVERVOLTAGES DUE TO LIGHTNING
How Lightning Strikes Occur
Lightning strikes occur when there is a buildup of electrical charge within a storm cloud. This charge difference can become so great that it overcomes the insulating properties of the air, resulting in a rapid discharge to the ground or another cloud. To protect transmission lines from lightning earth wire or OPGW wire is installed on top of the tower.
Mechanisms of Lightning-Induced Over voltages
Direct Strikes
A direct lightning strike to a transmission line or structure can cause immediate and severe overvoltage, leading to catastrophic damage if not properly managed.
Indirect Strikes
Indirect strikes occur when lightning hits nearby objects, inducing over voltages in adjacent electrical systems through conductive paths or ground currents.
Electromagnetic Induction
Lightning discharges can generate strong electromagnetic fields, inducing over voltages in nearby conductors through electromagnetic induction.
Impact on transmission towers
When a tower of the transmission system is directly struck by lightning, the resistance offered to the lightning current is that of the tower footing and any ground rods or counterpoise wires in parallel. In calculation of overvoltage developed due to lightning in this case, certain assumptions must be made concerning several of the factors such as wave shape and magnitude of lightning current, surge impedance of lightning stroke channel, shape of potential wave at tower top, effect of surges on tower and footing impedance, etc.
All these assumptions can be thrown into the lightning current and tower footing resistance, thus giving V = RI
where, V = voltage across insulation at tower, R = tower footing resistance, and I = lightning current in tower.
For very high towers, such as the ones at river crossings, which are subjected to direct strokes of very high rate of change of current, the tower inductance may be taken into account.
The voltage (v) which may be sufficient to cause insulator flashover is given by:
V=L * di/dt
where, L = tower inductance, and di/dt = rate of change of current due to direct stroke.
In case of indirect strike ,the peak value of the induced voltage surge is given by :
v = E*h
where, E is the mean electric field near the ground (earth) surface under a thundercloud which may vary from 0.5 kV/cm to 2.8 kV/cm, and h is the height of the phase conductor above ground.
Over voltage Protection Against Lightning-Induced Over voltages
Surge Protective Devices (SPDs)
SPDs are crucial for protecting electrical systems from over voltages by diverting excess energy safely to the ground.
Grounding Systems
Effective grounding systems are essential for dissipating the energy of a lightning strike and minimizing the resultant over voltages.
Surge Protective Devices (SPDs)
Function and Importance
SPDs limit the voltage supplied to an electrical device by either blocking or shorting to ground any unwanted voltages above a safe threshold.
Types of SPDs
Voltage-Clamping Devices
These devices, such as varistors, clamp the voltage to a specific level to protect the equipment.
Crowbar Devices
Crowbar devices, like gas discharge tubes, provide a direct path to the ground for excess voltage, effectively “crowbarring” the surge.
Grounding Systems
Role of Grounding in Protection
LM(Lightning mast) towers are also installed in substations to attract lightning towards it and ground them. Height of LM tower is higher than all structures installed in a substation. Grounding provides a low-resistance path for lightning currents to disperse into the earth, reducing the risk of overvoltages.
Other methods of protection
We use surge arrestors to ground excessive voltages in a substation, other than this over/under voltage relays are also installed in a substation to protect equipment from these voltage fluctuations. An over voltage relay is a protective device designed to monitor the voltage level in an electrical system and trip the circuit if the voltage exceeds a predetermined threshold. Its primary purpose is to protect electrical equipment from damage caused by excessive voltage.
How Over-Voltage Relays Detect Abnormal Voltages
Over voltage relays continuously monitor the voltage in the system. When the voltage exceeds the preset threshold, the relay detects this abnormal condition and initiates its operation.
Mechanism of Operation
The relay’s operation involves opening or closing contacts that either trip a circuit breaker or send a signal to other protective devices, depending on the design and application.
Applications of Over-Voltage Relays
Protection of Transformers
Over voltage relays protect transformers from high voltage surges that can cause insulation failure and other damage.
Protection of Generators
Generators are particularly vulnerable to over voltage conditions, and relays help prevent damage to generator windings and other components.
Substation and Transmission Line Protection
In substations and transmission lines, over voltage relays play a crucial role in maintaining the stability and reliability of the power system.
Shielding Techniques
Use of Shield Wires
Shield wires are strung on top of lattice structures in a substation and overhead ground wires/OPGW wires are strung above transmission lines to intercept lightning strikes before they reach the conductors.