T emporary Transmission System Overvoltage Raj Aggarwal Introduction Electrical T ransmission systems are designe d to withstand overvo ltage's that may may occur for a limited period and limited frequency witho ut sustaining damage to equipment Over voltages typically occur due to the following reasons:- •Naturally occurring lightning strikes (in presentation) •Switchgear operation under particular circumstance (in presentation) •Operational erro rs and control equipment faults (discussion only) •Poor or faulty Earthing arrangements (discussion only) •Resonance (discussion only) All transmission equipment will have a no rmal opera tional v oltage and a maximum overvo ltage rating which will be defined by the Basic Impulse Level (BIL) of the equipment. This is well below the voltage typically caused by lightning strike so mitigating measures must be taken to limit the impact of lightning It is impractical to design insulating systems to withstand lightning voltage impulse levels of typically 6MV . The BIL is typically 1MV for 400KV systems.
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Electrical Transmission systems are designed to withstand overvoltage'sthat may may occur for a limited period and limited frequency withoutsustaining damage to equipment
Over voltages typically occur due to the following reasons:-
•Naturally occurring lightning strikes (in presentation)
•Switchgear operation under particular circumstance (in presentation)•Operational errors and control equipment faults (discussion only)
•Poor or faulty Earthing arrangements (discussion only)
•Resonance (discussion only)
All transmission equipment will have a normal operational voltage and amaximum overvoltage rating which will be defined by the Basic ImpulseLevel (BIL) of the equipment. This is well below the voltage typicallycaused by lightning strike so mitigating measures must be taken to limit theimpact of lightning
It is impractical to design insulating systems to withstand lightning voltageimpulse levels of typically 6MV. The BIL is typically 1MV for 400KV
• The various local characteristic impedances will define the amplitude and wave shapeof the travelling wave at the time it stets off. As the wave meets other towers andparticularly line terminations into open isolators and onto cables transformers and bus-bars there will be instantaneous change in the characteristic impedance.
1. In the case of open isolators or overhead line configurations that increase thecharacteristic impedance there is a likelihood of voltage increase.
2. In the case of plant with internal insulating systems much of the voltage wave energywill be dissipated into this insulation causing permanent damage.
• In order to get an appreciation of the effects of lightning strikes it is useful to considerthe timescales of events.
• A lightning surge will travel at nearly the speed of light so its direct effects in terms ofstressing the insulation around all the parts of the system connected to the point of
strike can be considered instantaneous from a power frequency point of view.
• As the wave hits various discontinuities and the associated insulation is stressed then ifmultiple flashovers occur due to the travelling wave they appear to occursimultaneously on the fault recorder records. And in the worst case scenarios multiplecircuit tripping can take place with a resulting disruption to the system
Effects of lightning Strikes on Power Networks
• Hopefully a lightning strike will not result in the failure of the external insulation
however a poorly earthed tower or ‘super’ strike as they are sometimes called can
cause such a failure and the transmission line will be tripped out of service
• If the failure is such that no damage has occurred to the insulation then as soon a the
line is de-energies the dielectric strength will return and it would be safe to return theline, all effects of the lightning having long gone. This is normally done automatically via
an ‘auto reclose’ system’
• Auto reclose systems are usually categorised as “high Speed’ or ‘Low Speed’ and within
these categories either ‘single phase’ or ‘three phase tripping’. The number of reclose
attempts will normally be limed to two following a fault and 3 Phase reclose not
attempted at all at a Generator Substation due to risk if out of synch closure.
• Most external insulation systems have ‘arcing horns’ fitted with the aim of diverting the
power arc fro the surface on the insulator and avoiding damage to the porcelain
•Where an external insulating system interfaces with an internal one, the risk of plantdamage that is irreparable is high and special measure are required.
Protection of electrical equipment against voltage
surges• As mentioned in the previous slide there is a very high likelihood of voltage surge increases at the
connection points between overhead line system with external insulation and other equipmentsuch as transformers, switchgear, cables and bus bars with predominately internal insulationsystems.
• Often at these points there will be open isolators when the extreme increase in characteristicimpedance will case a flashover
• Plant components with internal insulating systems will have very high capacitance to earth relativeof overhead lines and a travelling wave incident at these point with steep wave front will dispersemuch of its its energy into this insulating system with potentially damaging effects.
• In order to mitigate these effects the use of voltage surge arrestors at strategic points on the systemare employed.
• Typically they will be located very close to transformers or cables connected directly overhead linecircuits.
• When fitted to HV air insulated (AI) substations these devices typically look like CVT ’s but containstacks of metal oxide disks designed to conduct at a curtain voltage and absorb the impulse energy.
• 11kV:- up to 50KA (952MVA). Typical for industry 13KA (250MVA).
• Make rating is 2.55 times break.
• 33kV:- up to 31.5KA (1800MVA)
• 66kV:-up to 31.5KA (3600MVA)
• 132kV:-up to 40KA (9145MVA)
• 275kV:- up to 50KA (23816MVA)
• 400kV:- up to 63KA (43648MVA)
• All types are subject to the same basic principles of fault current
interruption
• Fundamentally alternating current interruption in an inductive circuit will
draw an arc until the current falls to zero and for a successful clearancethe resulting voltage rise across the gap must not break down the
establishing dielectric strength
• The only way to interrupt a DC arc is to force a current zero by
developing a sufficiently high arc voltage
Switching Voltage Transients
• When a breaker opens whilst carrying alternating fault current an arc in the primary dialecticmedium (air or gas) will be dawn in order for the current flow to continue until a current zerois reached
• At this point due to intensive cooling of the arc plasma there is an opportunity for thedielectric media to strengthen, the arc not to establish and the current flow to cease henceinterrupting the circuit.
• During the arcing stage there is a voltage formed by the arcing across the contacts. Followinga current zero and successful arc extinction this voltage must fall into step with the systemvoltage. In order to do this there is a rapid migration of charge. This process causes highvoltage transients across the contacts known as the “Transient Recovery Voltage” (TRV)
• The shape and characteristic of this TRV dependent on the phase of the system voltage at thepoint of arc extinction and the system characterises in general, Inductance and capacitance ofthe lines and other components that define the natural system frequency response toimpulses.
• If there was a delayed current zero due to high X/R system impedance such as a generatorand the beaker was not designed for it, the heat due to an extended arcing time may causefailure, as would a voltage re-strike post arc extinction.
• Fundamental Requirements:1. The “quenching” media must be able to remove the energy during
the TRV. This is the critical function and the cooling of the power arcpre current zero is of secondary importance.
2. The insulation “strength” of the gap post arc extinction must be able towithstand this attempt to re-strike the arc.
• Design issues1. Air Blast and most SF6 breakers have a period of time during its
opening phase when the effect of arc extinction is at a maximum. Ifthere is no current zeros occurring within this time the breaker will fail
2. If high current starts to flow on the breakers closing then the breakersmust have sufficient closing force to overcome the magnetic forcestrying to open the contacts
• Technology – As with transformers the only practical insulation
system that could be flexible and cope withcomplicated shapes was paper impregnated withoil or some other compound.
– The development of void free polyethylene whichhas displaced paper at most commerciallyavailable voltages.
– Moisture ingress into polyethylene causedsignificant failure rates at higher voltages(>300kV). This makes the jointing process verydifficult and has restricted the option of XLPEcable at super grid voltages.
• Basis of operation is similar to generator but no rotating parts are requiredas the magnetic field formed by the primary winding magnetising currentinduces a voltage directly into the secondary winding. The flux due toCurrent flowing in the secondary winding opposes the primary flux
resulting in power flow through the transformer
• No air gap means that the magnetising force (H) to produce thenmagnetising flux (B) required is very low compared to a rotating machine,so the flux produced by the windings of a transformer must balance orsaturation will occur
• Insulation of most transformers above 10MVA is by paper immersed in oil.Oil also provides the cooling with air or watre proving the oil cooling
• For smaller transformers epoxy resin is often used to provide insulation (drytransformer). Air provides the cooling in this case.