Phil Bowley Over Voltages RKA

8/13/2019 Phil Bowley Over Voltages RKA

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Temporary Transmission System

Overvoltage

Raj Aggarwal

Introduction

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

systems.



Insulation Rating

It is useful to consider system insulation under two categories:-

•External insulation, air and solid insulation exposed to the atmosphere

•Internal insulation, typically Oil, or Gas, or Vacuum not exposed to to

atmosphere.

In practice the highest voltages imposed will be as a result of lightning strikes

and switching surges, the first being by far the most common and severe as

all national electricity supply systems will have extensive amounts of

overheard lines, naturally exposed to the atmosphere

The use of an earth wire strung above the main conductors is the most

commonly used method of mitigating the effects of lightning strikes. This

technique is also used over air insulted sub stations if they are in exposed

locations

This effectively creates a ‘Earth Plain’ above the conductors causing any

lightning to strike to earth wire, or the top of a transmission tower, rather than

the conductors

lightning Strikes

A strike on the earth wire will result in a travelling wave along all conduction

paths from the point of strike, which, if at or near a tower will include the tower

itself to its earthed footings as well as in both directions along the earth wire.

The magnitude and character of the wave moving at a little less than C will be

defined by the characteristic impedance of the various conducting paths.

There will be an induced wave in the main conductors running parallel with

the earth wire but at a much reduce magnitude.

If the tower earthing is sound and the strike is not two large and there are no

severe discontinuities in the impedance of the earth wires or conductors the

main external insulating system (the conductor insulating strings) will

withstand the impulse which will dissipate as as it travels

The quality of the tower earthing is of significant importance. If the earthing is

poor the reflected wave will significantly increase the level of the impulse that

sets of down the earth wire, this together with the existing power frequency

voltage at that instant can cause a flashover between the conductor andearth, a failure of the insulating strings known as a ‘back flash’.



lightning Strikes

• 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.

AC Switchgear performance and transient

overvoltage

• Arc quenching and insulation media

• Oil, Air, Vacuum (up to 12kV), SF6

• Design types:

• Metal-clad up to 66kV

• Metal-clad GIS 66kV -750kV

• Open terminal 66KV – 750KV

• Specialised (Generator circuit breakers)



Switch gear typical rating (break)

• 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:

• 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

Load Current

System Voltage

Fault Inception

Fault Current in

phase with arc

voltage

Arc Voltage

Collapse of

system voltage

local to fault

Arc extinction

Transient

recovery

voltage across

breaker

contacts

Re-establishment of

dielectric between

breaker contacts

with system voltage

established across

breaker, the faulted

side of the circuit

being isolated and

effectively earthed

Beaker operation under typical fault conditions

Line

inductance



List of Slide Titles

14 33KV dead tank 33KV SF6 insulated Vacuum breaker

15 132KV dead tank SF6 insulated SF6 breaker

16 500KV GIS Substation

17 500KV AI Substation

24 275KV OHL Suspension tower trident design

25 400KV Double circuit tension tower

26 500KV Single circuit tension tower

27 / 28 / 29 500KV AI substation

30 Voltage surge test on post insulation to failure

31/32 400 KV conventional AI substation

33 High Voltage AI Live tank two break SF6 circuit breaker36 Generator transformer core

37 Generator transformer LV winding

38 Generator transformer HV and Tapping windings

39 Generator transformer following catastrophic failure and fire due to over

heating internal flux shields





Cables

• 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.



Cables (cont’d)

• Advantages/disadvantages over O/H line

– Construction and installation costs of cable at distribution voltages

about 3-6 times equivalent O/H line costs and about 5-10 times in

the case of super grid voltages

– High capacitance of cables requires shunt reactance to be fitted

every 20KM or so to reduce the reactive current required from the

system

– A typical 1000MVA 400kV cable takes 17MVA per KM on open

circuit, so 58KM cable without shunt reactors will run a full load on

open circuit

– A 104MVA 132kV cable only takes 0.5MVA per KM (208KM). So

lower voltage medium runs do not require shunt compensation.

Termination of cable is expensive as cable sealing ends must be

used to transfer insulation system from solid (paper or plastic) to air



Overhead Lines

• The main method of power distribution

– Relatively cheap mainly because air is the insulating media and this has

a very low dielectric loss (unlike cables).

•Problem: – Visual impact.

– Perceived EM radiation causing health problems.



Overhead Lines Cont’d

• Advantages:

– Very long line can be installed up to 150KM without compensation.

– However O/H lines do have inductance so for lines over 150Km series

capacitance may added to enable high power transfers without stability

issues. This can be see by inspection of the basic formula below.

– An alternative and usually the more common approach for line

compensation is the use of the phase shifting transformer which by

means of winding arrangements reduces the natural transmission angle

for a given power transfer and system voltage.

• Power delivery along any circuit is:

– Power(MW)=((V(sending)*V(receiving))/line Impedance)*Sin (Angle

between the voltages)











Transformers

• 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.



Transformers Cont’d

• The impedance of a transformer is formed by the magnetic flux that

does not link the two windings. The leakage flux has a much smaller

effect than the synchronous reactance on a generator. A transformer

can be designed with leakage reactance less than 10% whereas a

typical rotating machine (Synchronous reactance ) is greater than

100%

• The impedance is tuned by using flux shunts with the transformer

casings

• 10%(on rating) reactance means that at full load current 10% of

voltage is dropped across the transformer.





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Phil Bowley Over Voltages RKA