High Voltage Engineering power point pre
- 2. UNIT I OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
- 3. Causes of over voltages and its effects on power system • The main causes of overvoltage in power systems are – Internal causes, and – External causes.
- 4. Internal causes • These causes are due to some abnormal conditions generated in the circuit itself. • These abnormal conditions change the system's parameters • Under severe conditions they also damage the circuit
- 5. Internal causes • Switching surges, • Insulation failure, • Arcing ground, and • Resonance.
- 6. Switching Surges • overvoltage produced on a power system due to switching operations • Study helps designing the insulation of winding and insulation coordination of EHV lines
- 7. Switching Surges • Switching of an Unloaded Line or Open Line – Switching surges are due to traveling wave phenomena. Successive reflection of the voltage wave causes overvoltage. • Switching of a Loaded Line – line that is suddenly interrupted results in a voltage setup of Vs = 2Zn i across the switch. then the maximum value of voltage to which a line may be subjected is, Vm = Vp + Vs kV
- 8. Switching Surges • 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. Overvoltage due to current chopping can be reduced by resistance switching. • Short-circuits and Line-faults Interruption – In circuit breakers after clearing faults a transient restriking voltage appears which may cause overvoltage.
- 9. Switching Surges • Auto-reclosing of Circuit-breaker in Long EHV Lines – auto-reclosure generates surges in long lines due to closure on the trapped charge of the line. – 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 – phase voltage of the remaining two phases will increase beyond its normal value until the fault is cleared.
- 10. Insulation Failure : • overvoltages are also caused due to the insulation failure between • line and ground • grounding of the conductor in a power system.
- 11. Arcing Ground • caused in long transmission line operations on high voltages due to oscillation • high voltage surges produced due to the arcing ground can damage the power system apparatus severely • The voltage produced is nearly 3 to 4 times the power frequency voltage • can be avoided by earthing the neutral
- 12. Resonance • 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 the case of cables, resonance may occur due to the 5th harmonic component