Switching Surges or Overvoltages in EHV and UHV Substations
Switching Over-Voltages or Surges
Switching overvoltages originate within the substation due to switching, faults, changes in the circuit conditions etc, and these switching surges are significant particularly in the system with voltage ratings above 400kV. Switching surges are of primary importance in designing the substation equipment insulation such as insulation of transformers, circuit breakers, etc. and in designing the insulation co-ordination of the Extra High Voltage (EHV) substations.
While closing the un-loaded transmission lines, over voltages are generated in the circuit breaker. These generated switching overvoltages travel in both the directions. In general, these over voltages are quite common and exist in every circuit breaker operation. However these switching surges will be quite severe during switching of un-loaded transmission lines and during switching of charged transmission lines.
The over-voltages generated by circuit breakers during while closing the transmission lines should be within the specified limits. This feature can be achieved by incorporating pre-insertion resistor, by using point of wave control, by simultaneous closing of lines from both the ends, by using shunt reactors, surge arresters etc. The design or choice of selecting lightning arresters, insulation level fro various substation equipment will be based on switching surges rather than lightning over-voltages in the case of substations with voltage levels above 220kV.
Switching surges are the primary importance while designing the insulation and insulation co-ordination of EHV or UHV substations. Switching surges are caused by the following events:
- Closing of uncharged line
- Closing of charged line
- Auto-reclosure of circuit breakers
- Interruption of short circuit or line faults
- Load shedding at reactive end of the line
The maximum switching over-voltages occur during the closing of the unloaded or charging line. The function of pre-closing resistors is to reduce such over-voltages during enerzisations of the charged/uncharged lines and auto-reclosure operations. The pre-closing resistors remain in circuit for ½ cycle before closing of the main contacts of the circuit breaker. Therefore the main contacts close and shunt the pre-closing resistors.
The value of closing resistors will be equal to surge impedance of the line. However, in practice very resistor value larger than the surge impedance is preferred. The value of pre-closing resistors will be between 400 and 800 ohms per phase, according to surge impedance of the line. The full line current flows through the pre-closing resistors for a short duration. The pre-closing resistors depends on the length of the line, voltage , energy to be dissipated by pre-closing resistors, type of the circuit breakers, permissible over-voltage factor etc,
Below 400kV system voltage, the pre-closing resistors are generally not provided at the peak values are within the limits of the surge arresters used for lightning surges. At 400kV substation voltage levels, pre-closing resistors are employed with every circuit breakers. In circuit breakers for 750kV and above, multi step pre-closing resistors are employed. Pre-closing resistors are fitted with arc extinguish chambers of the circuit breakers.
Methods to Reduce Switching Surges or Switching Over-Voltages:
Switching voltage s are proportional to system voltages and hence they can reach high values in EHV (Extra High Voltage) and UHV (Ultra High Voltage) systems ( voltage levels above 400kV). Some of the switching over-voltages are influenced by the circuit breaker design and arc interruption process. Out of the various causes of switching over-voltages most severe are due to: closing of charging line and operation of auto reclosures
Air blast circuit breakers, minimum oil circuit breakers, SF6 circuit breakers and vacuum circuit breakers behave differently regarding to switching surges. Various methods employed to reduce the over-voltages are listed below
Switching Operation Causing Over-voltages
Mehtods to Reduce Switching Over-voltages
Energizing an uncharged line
High voltage shunt reactors connected to line to reduce power frequency over-voltages
Elimination of trapped charges on the line
Line shunting after opening by means of earth switches
Reduction of current chopping
Resistance switching with circuit breaker
Reducing the switching over-voltages due to closing
SIngle stage pre-closing resistor insertion with circuit breaker. Two stage pre-closing resistors insertion with circuit breaker. Closing resistors in between circuit breaker and shunt reactor
Reducing switching over-voltages by improved switching sequence
Synchronous switching of three poles. Simultaneous operation of the circuit breakers at both the end of the lines
Using Surge Arresters
While Closing of the line and while disconnecting the reactor