Temporary Transmission System
Overvoltage
Raj Aggarwal
Introduction
Electrical Transmission systems are designed to withstand overvoltage's
that may may occur for a limited period and limited frequency without
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 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 a
maximum overvoltage 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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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 and
earth, a failure of the insulating strings known as a ‘back flash’.
2
lightning Strikes
•
The various local characteristic impedances will define the amplitude and wave shape
of the travelling wave at the time it stets off. As the wave meets other towers and
particularly line terminations into open isolators and onto cables transformers and busbars there will be instantaneous change in the characteristic impedance.
1. In the case of open isolators or overhead line configurations that increase the
characteristic impedance there is a likelihood of voltage increase.
2. In the case of plant with internal insulating systems much of the voltage wave energy
will be dissipated into this insulation causing permanent damage.
•
In order to get an appreciation of the effects of lightning strikes it is useful to consider
the timescales of events.
•
A lightning surge will travel at nearly the speed of light so its direct effects in terms of
stressing 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 if
multiple flashovers occur due to the travelling wave they appear to occur
simultaneously on the fault recorder records. And in the worst case scenarios multiple
circuit 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 the
line, 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 plant
damage that is irreparable is high and special measure are required.
3
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 equipment
such as transformers, switchgear, cables and bus bars with predominately internal insulation
systems.
•
Often at these points there will be open isolators when the extreme increase in characteristic
impedance will case a flashover
•
Plant components with internal insulating systems will have very high capacitance to earth relative
of overhead lines and a travelling wave incident at these point with steep wave front will disperse
much 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 system
are employed.
•
Typically they will be located very close to transformers or cables connected directly overhead line
circuits.
•
When fitted to HV air insulated (AI) substations these devices typically look like CVT’s but contain
stacks of metal oxide disks designed to conduct at a curtain voltage and absorb the impulse energy.
AC Switchgear performance and transient
overvoltage
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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)
4
Switch gear typical rating (break)
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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 clearance
the 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 dialectic
medium (air or gas) will be dawn in order for the current flow to continue until a current zero
is reached
•
At this point due to intensive cooling of the arc plasma there is an opportunity for the
dielectric media to strengthen, the arc not to establish and the current flow to cease hence
interrupting the circuit.
•
During the arcing stage there is a voltage formed by the arcing across the contacts. Following
a current zero and successful arc extinction this voltage must fall into step with the system
voltage. In order to do this there is a rapid migration of charge. This process causes high
voltage 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 the
point of arc extinction and the system characterises in general, Inductance and capacitance of
the lines and other components that define the natural system frequency response to
impulses.
•
If there was a delayed current zero due to high X/R system impedance such as a generator
and the beaker was not designed for it, the heat due to an extended arcing time may cause
failure, as would a voltage re-strike post arc extinction.
5
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 arc
pre current zero is of secondary importance.
2. The insulation “strength” of the gap post arc extinction must be able to
withstand this attempt to re-strike the arc.
• Design issues
1. 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. If
there 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 breakers
must have sufficient closing force to overcome the magnetic forces
trying to open the contacts
Beaker operation under typical fault conditions
Fault Inception
System Voltage
Collapse of
system voltage
local to fault
Arc extinction
Load Current
Arc Voltage
Fault Current in
phase with arc
voltage
Line
inductance
Re-establishment of
dielectric between
breaker contacts
with system voltage
established across
breaker, the faulted
side of the circuit
being isolated and
effectively earthed
Transient
recovery
voltage across
breaker
contacts
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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 breaker
36 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
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Cables
• Technology
– As with transformers the only practical insulation
system that could be flexible and cope with
complicated shapes was paper impregnated with
oil or some other compound.
– The development of void free polyethylene which
has displaced paper at most commercially
available voltages.
– Moisture ingress into polyethylene caused
significant failure rates at higher voltages
(>300kV). This makes the jointing process very
difficult and has restricted the option of XLPE
cable at super grid voltages.
9
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
10
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.
11
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)
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Transformers
•
Basis of operation is similar to generator but no rotating parts are required
as the magnetic field formed by the primary winding magnetising current
induces a voltage directly into the secondary winding. The flux due to
Current 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 then
magnetising flux (B) required is very low compared to a rotating machine,
so the flux produced by the windings of a transformer must balance or
saturation 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 (dry
transformer). Air provides the cooling in this case.
17
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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