The overvoltage or transients or surge voltages are the voltages whose values are much more than the normal voltages. These are of short duration which can cause malfunction or failure of equipment in the system depending upon the severity of the voltage level.
The overvoltage with a level above twice the standard peak voltage of the system will cause degradation of electronic components and insulation in the equipment of the system. The main causes of overvoltages in power systems are classified as,
- Internal causes, and
- External causes.
Overvoltage due to Internal Causes :
These causes are due to some abnormal conditions generated in the circuit itself. These abnormal conditions not only change the system's parameters but also damage the circuit under severe conditions. Internal causes of overvoltages are mainly due to,
- Switching surges,
- Insulation failure,
- Arcing ground, and
- Resonance.
Switching Surges :
Switching surges are the overvoltages produced on a power system due to switching operations. The study of switching surges is important for designing the insulation of winding and for insulation coordination of EHV lines. Some of the causes for the production of switching surges are listed below,
- Switching of an Unloaded Line or Open Line - Switching surges due to switching of an unloaded line arise due to the traveling wave phenomenon being originated and due to the sudden application of a voltage in the line as soon as the circuit breaker at the sending end is closed. Successive reflection of the voltage wave causes overvoltages in the line.
- Switching of a Loaded Line - Surges are also produced during the switching operation of a loaded line. Let us consider a loaded line that is suddenly interrupted which results in a voltage setup of 2Zn i across the switch. Where i is the initial value of current at the time of opening of the line and Zn is the natural impedance of the line. If Vp is the phase voltage and Vs is the voltage across the switch, then the maximum value of voltage to which a line may be subjected is,Vm = Vp + Vs kV
- Reactor Switching - Reactor switching may cause current chopping. Current chopping is the phenomenon of current interruption before the natural current zero is reached. Current chopping results in the production of high voltages due to the transfer of energy in inductance (1/2 LI2) to capacitance (1/2 CV2) across the contacts of the circuit breaker. Overvoltages due to current chopping can be reduced by resistance switching.
- Short-circuits and Line-faults Interruption - In a circuit breaker, after clearing faults a transient restriking voltage appears which may cause overvoltages.
- Auto-reclosing of Circuit-breaker in Long EHV Lines - High-speed three-phase auto-reclosure generates surges in long lines due to closure on the trapped change of the line. The surge voltages are influenced by the non-simultaneous reclosure of all the three-phase poles.
- Opening of Only One Phase due to Occurrence of Fault Cable Charging Circuit Switching OFF - In a 3-phase system, when a fault occurs on any one phase, then the phase voltage of the remaining two phases will increase beyond its normal value until the fault is cleared. This rise in voltage across the healthy phases appears as a surge.
Insulation Failure :
The overvoltages are also caused due to the insulation failure between line and ground or the grounding of the conductor in a power system.
Arcing Ground :
Arcing ground is commonly caused in long transmission line operations on high voltages due to oscillation. The high voltage surges produced due to the arcing ground can damage the power system apparatus severely. The voltage produced due to arcing grounds is nearly 3 to 4 times the power frequency voltage. Arcing grounds can be avoided by earthing the neutral.
Resonance :
In an electrical system when inductive reactance of the circuit becomes equal to capacitive reactance, resonance takes place. High voltage surges are produced in the power system due to resonance in transmission lines. Resonance rarely occurs as the capacitance is very small. In the case of cables, resonance may occur due to the 5th harmonic component.
Overvoltage due to External Causes :
External causes of overvoltages in a power system are mainly due to lightning. The other factors which cause overvoltages are as follows,- Indirect lightning strokes such as lightning discharge taking place near the line result in electromagnetically induced overvoltages.
- Different atmospheric conditions along the line length will cause overvoltages.
- Overvoltages are produced due to friction between the atmospheric particles such as dust, dry snow, etc.
- Overvoltages are produced due to lightning between the two nearby clouds.
In general, the majority of faults in the power system are transient in nature and caused due to switching operations or lightning surges. However, the surges cause due to lighting strokes have a very high voltage of million volts, which can cause the insulation breakdown and further damage the components of the power system.
The high voltages produced can also cause disturbances in neighborhood transmission lines. Hence, it is necessary to protect all the lines and equipment connected to the network in power systems.
The underground cables are not affected by lightning surges and can be protected against lightning surges. The protection of overhead lines from lightning surges is done by running one or more wires above the overhead lines, which is at ground potential. These wires are also called ground wires.
Switching surges do not affect systems where operating voltages are below 230kV. However, switching surges cause damage to power systems where the operating voltage is above 230kV and it is worse for operating voltages above 700kV. Hence protection system should be employed.