SLOW-FRONT OVER VOLTAGES
Slow-front over voltages has durations of some tens to thousands of microseconds and tail durations of the same order of magnitude, and is oscillatory by nature. They arise generally in the occasions below:
• Line energization/re-energization (circuit breaker closing/opening)
• Fault occurring/fault clearing
• Switching of capacitive/inductive current
• Load rejection
• Distant lightning strike (lightning strike wave front flattened by travelling).
The representative voltage stress is characterized by the representative voltage amplitude and wave shape. Therefore, the representative switching impulse voltage of 250/2500 µs (time to peak 250 µs, and time to half-value on the tail 2.5 ms) has been standardized as the common general concept throughout the world to represent standardized slow-front overvoltages.
During switching of capacitive/inductive current, overvoltages are caused by current chopping, when the current power factor is almost zero. In particular, the following switching operations require special attention:
• Switching of unloaded cables or capacitor banks.
• Inductive current tripping (transformer magnetizing current tripping, for example).
• Arc-furnace load switching.
• Interruption of currents by high-voltage fuses.
The most useful and commonly used method of limiting the slow-front overvoltages is to adopt the resistive tripping/closing breakers. Surge arrester protection against slowfront overvoltages is also significant.
Typically fast-front overvoltages are lightning strikes, although they can originate also from switching operations. Again lightning overvoltages can be classified into direct strikes, back-flashovers and induced lightning strikes. The induced lightning surges occur generally below 400 kV and so are of importance only for lower voltage systems of 100 kV. Back-flashover voltages are less probable on UHV systems of 500 kV or more, due to the high insulation withstanding values. The representative wave shape of fast-front overvoltages is the well-known 1.2/50 µs wave.
Very fast-front overvoltages can originate from switching operations or from faults inside Gas Insulated Sub-Station due to the fast breakdown of the SF6 gas gap and because of nearly undamped surge propagation within GIS, where the average distance between two adjacent transition points is very short. (For example, if 7.5m distance and u=300m/µs is assumed, then the resulting travelling surges repeat 20 times every 1µs, so the natural frequency becomes 20 MHz, and the first-front wavelength (quarter cycle) is 0.0125µs.) However, the amplitudes of the surges would be rapidly dampened and flattened on leaving the GIS, and are relieved to some extent at the external circuit of the GIS bushings.
The overvoltage shape is characterized by a very fast increase of the voltage to nearly its peak value, resulting in a front time below 0.1µs. For switching operations this front is typically followed by an oscillation with frequencies ranging between 1–20 MHz. The duration of very fast-front overvoltages would be less than 2 to 3 ms; however, 20MHz and 3ms means 60,000 times of beating stresses. Furthermore, repetation may occur several times. The magnitudes of overvoltage amplitude depend on the structure of the disconnector and on the adjacent structure of station equipment.
Very fast-front overvoltages are dampened/flattened to some extent, by using the typical countermeasure installation of gap-less arresters, which means insertion of a non-linear high resistance in parallel across the phases and earth. The maximum amplitudes to 2.5 pu can be assumed to be achievable.
Due to faults within GIS, the connected equipment, in particular a transformer is stressed by the overvoltages, which would contain frequencies up to 20 MHz, and the amplitude may exceed the breakdown voltages of the transformer if no effective countermeasure is observed.
For GIS or MTS, disconnector switching is common in substations operation. During the closing and opening of a disconnector, a number of pre-strikes and re-strikes occur due to the low speed of disconnector compared to the circuit breaker. Each strike generates VFFO. Therefore, the switching of the disconnector is the main cause of VFFO. VFFO is characterized by a fast increase of the voltage to its peak value, resulting in a front time below 0.1μs. This front is usually followed by oscillation with frequencies above 1MHz. Dominant frequency components may reach up to 100MHz. VFFO may damage the insulation of GIS, power transformers and voltage transformers. The maximum amplitude is generally less than 2.5 p.u. and may reach up to 2.5 p.u. in special cases. In UHV AC systems, the ratio of the BIL to the system voltage is lower than that in EHV systems. Thus VFFO needs special attention in UHV systems.
The basic criteria for insulation design of overhead transmission lines are given below:
1. Flashovers caused by lightning strikes are considered as fatal phenomena, while damage (damage to conductors, cracks in insulators, etc.) to the transmission line should be avoided.
2. Technically as well as economically balanced insulation distance (clearance) is to be assured in the fundamental design, allowing some extent of failure rate caused by lightning strikes. Also, countermeasures should be adopted as much as possible to reduce the influence and frequency of effects on a substation.
3. Flashover should not be caused by switching surges or by any sustained lower frequency overvoltages.
The above concept is based on the insulation characteristics of atmospheric air and cooling materials of infinite natural circulation type, so that, once broken, insulation would be restored / recovered whenever the surge source disappears (self-restoring insulation characteristics).
The principal countermeasures are:
1. To reduce the probability of lightning strike, and to limit the faulty circuits (in the case of multiple circuit transmission lines) and faulty phases as much as possible. (By adoption of overhead grounding wires and any other effective countermeasure.)
2. To reduce the probability of back-flashover caused by lightning strikes on the overhead shielding wires or on the towers (By surge impedance reduction of towers and overhead grounding wires).
3. Countermeasures to relieve the travelling waves to some extent before reaching the substation terminal point.
4. Insulation-level withstanding against switching surges from the substation.
5. The idea to reduce the probability of simultaneous faults on parallel circuits of the same route.
6. Adoption of auto-reclosing.
• Line energization/re-energization (circuit breaker closing/opening)
• Fault occurring/fault clearing
• Switching of capacitive/inductive current
• Load rejection
• Distant lightning strike (lightning strike wave front flattened by travelling).
The representative voltage stress is characterized by the representative voltage amplitude and wave shape. Therefore, the representative switching impulse voltage of 250/2500 µs (time to peak 250 µs, and time to half-value on the tail 2.5 ms) has been standardized as the common general concept throughout the world to represent standardized slow-front overvoltages.
During switching of capacitive/inductive current, overvoltages are caused by current chopping, when the current power factor is almost zero. In particular, the following switching operations require special attention:
• Switching of unloaded cables or capacitor banks.
• Inductive current tripping (transformer magnetizing current tripping, for example).
• Arc-furnace load switching.
• Interruption of currents by high-voltage fuses.
The most useful and commonly used method of limiting the slow-front overvoltages is to adopt the resistive tripping/closing breakers. Surge arrester protection against slowfront overvoltages is also significant.
FAST-FRONT OVERVOLTAGES
Typically fast-front overvoltages are lightning strikes, although they can originate also from switching operations. Again lightning overvoltages can be classified into direct strikes, back-flashovers and induced lightning strikes. The induced lightning surges occur generally below 400 kV and so are of importance only for lower voltage systems of 100 kV. Back-flashover voltages are less probable on UHV systems of 500 kV or more, due to the high insulation withstanding values. The representative wave shape of fast-front overvoltages is the well-known 1.2/50 µs wave.
VERY FAST-FRONT OVERVOLTAGES
Very fast-front overvoltages can originate from switching operations or from faults inside Gas Insulated Sub-Station due to the fast breakdown of the SF6 gas gap and because of nearly undamped surge propagation within GIS, where the average distance between two adjacent transition points is very short. (For example, if 7.5m distance and u=300m/µs is assumed, then the resulting travelling surges repeat 20 times every 1µs, so the natural frequency becomes 20 MHz, and the first-front wavelength (quarter cycle) is 0.0125µs.) However, the amplitudes of the surges would be rapidly dampened and flattened on leaving the GIS, and are relieved to some extent at the external circuit of the GIS bushings.
The overvoltage shape is characterized by a very fast increase of the voltage to nearly its peak value, resulting in a front time below 0.1µs. For switching operations this front is typically followed by an oscillation with frequencies ranging between 1–20 MHz. The duration of very fast-front overvoltages would be less than 2 to 3 ms; however, 20MHz and 3ms means 60,000 times of beating stresses. Furthermore, repetation may occur several times. The magnitudes of overvoltage amplitude depend on the structure of the disconnector and on the adjacent structure of station equipment.
Very fast-front overvoltages are dampened/flattened to some extent, by using the typical countermeasure installation of gap-less arresters, which means insertion of a non-linear high resistance in parallel across the phases and earth. The maximum amplitudes to 2.5 pu can be assumed to be achievable.
Due to faults within GIS, the connected equipment, in particular a transformer is stressed by the overvoltages, which would contain frequencies up to 20 MHz, and the amplitude may exceed the breakdown voltages of the transformer if no effective countermeasure is observed.
For GIS or MTS, disconnector switching is common in substations operation. During the closing and opening of a disconnector, a number of pre-strikes and re-strikes occur due to the low speed of disconnector compared to the circuit breaker. Each strike generates VFFO. Therefore, the switching of the disconnector is the main cause of VFFO. VFFO is characterized by a fast increase of the voltage to its peak value, resulting in a front time below 0.1μs. This front is usually followed by oscillation with frequencies above 1MHz. Dominant frequency components may reach up to 100MHz. VFFO may damage the insulation of GIS, power transformers and voltage transformers. The maximum amplitude is generally less than 2.5 p.u. and may reach up to 2.5 p.u. in special cases. In UHV AC systems, the ratio of the BIL to the system voltage is lower than that in EHV systems. Thus VFFO needs special attention in UHV systems.
INSULATION DESIGN CRITERIA OF THE OVERHEAD TRANSMISSION LINE
The basic criteria for insulation design of overhead transmission lines are given below:
1. Flashovers caused by lightning strikes are considered as fatal phenomena, while damage (damage to conductors, cracks in insulators, etc.) to the transmission line should be avoided.
2. Technically as well as economically balanced insulation distance (clearance) is to be assured in the fundamental design, allowing some extent of failure rate caused by lightning strikes. Also, countermeasures should be adopted as much as possible to reduce the influence and frequency of effects on a substation.
3. Flashover should not be caused by switching surges or by any sustained lower frequency overvoltages.
The above concept is based on the insulation characteristics of atmospheric air and cooling materials of infinite natural circulation type, so that, once broken, insulation would be restored / recovered whenever the surge source disappears (self-restoring insulation characteristics).
The principal countermeasures are:
1. To reduce the probability of lightning strike, and to limit the faulty circuits (in the case of multiple circuit transmission lines) and faulty phases as much as possible. (By adoption of overhead grounding wires and any other effective countermeasure.)
2. To reduce the probability of back-flashover caused by lightning strikes on the overhead shielding wires or on the towers (By surge impedance reduction of towers and overhead grounding wires).
3. Countermeasures to relieve the travelling waves to some extent before reaching the substation terminal point.
4. Insulation-level withstanding against switching surges from the substation.
5. The idea to reduce the probability of simultaneous faults on parallel circuits of the same route.
6. Adoption of auto-reclosing.
