transmission- high transient overvoltages
TRANSCRIPT
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TRANSMISSION
Costs are reduced due to the absence of
terminal equipment at the tee-off point.
These solutions are commonly used on
distribution networks, but less frequently on the
transmission network. Such a configuration
was ne eded as a temporar y so lu tion
on Eskom’s EHV network due to terminal
equipment rating limits at Alpha substation.
The critical location of Alpha substation
in the secure supply of the Cape corridor
made it necessary to consider non-standard
protection philosophy. The occurrence of a
single phase to ground fault on a transformer’s
HV breaker and the resulting single pole trip
on the EHV line supplying the transformer
resulted in high transient overvoltages in the
network. Changes were subsequently made
to protection operation.
Background
Fig.1 shows the network supplying the Cape
prior to January 2005. The loading on the twoTutuka-Alpha 400 kV lines was such that the
line terminal equipment was being operated
very close to rated limits.
A loss of one of the two Koeberg units or
tripping one of the Tutuka-Alpha lines would
result in loading the remaining line beyond
the continuous current rating of the terminal
equipment. It became necessary to provide
a solution to ensure firm supply to the Cape
under these network conditions. A proposal
was made to provide two new bays at Alpha
substation and turn the Tutuka – Majuba 1 line
into Alpha, thus creating an Alpha - Tutuka 3
line and an Alpha-Majuba line as shown in
Fig. 2.
T h e p ro p o s a l w a s a c c e p t e d f o r
implementation in late 2005. A potential
risk for the operation of the network existed
until this proposal could be implemented.
The scheduled maintenance outages of the
Koeberg units, the first of which was due end
January 2005, as well as the winter loading
would put added strain on this network.
An interim solution needed to be provided inthe configuration of the network in order to
reduce these potential risks. Fig. 3 shows the
interim solution that was proposed.
The Tutuka - Majuba 1 line runs very close to
the 400 kV yard at Alpha. It would therefore
require relatively little effort to tee-off this line
directly onto the 400 kV busbar at Alpha
as shown in Fig. 3. This would provide the
additional network support required at Alpha
under contingency conditions.
Protection considerations
Analys is of the network as it would beconfigured with the proposed interim solution
indicated several risks from a protection
perspective.
The tee-off to Alpha on the Tutuka-Majuba line is
located approximately 3 km from the first tower
at Tutuka substation. The length of the tee-off
to Alpha is approximately 200 m. The tee-off
arrangement introduced potential insecurity of
telecommunication channels and therefore
maximum Zone 1 coverage of the line was
essential in order to provide instantaneous
tripping. The standard Zone 1 setting of 80% at
both Tutuka and Majuba would detect a faulton the Alpha 765 kV busbar. It was therefore
necessary that, for any breaker operation on
the Tutuka - Majuba 1 line, a DTT (direct transfer
trip) signal be issued to Transformer 3 MV
breaker. This would ensure that, for any fault
that the line protection would detect, all three
breakers in the tee arrangement would trip.
The line would ARC (auto-reclose) and restore
supply, provided the fault was not sustained.
However, the transformer breaker would have
to be manually closed after ensuring that it
was safe to do so.
A transformer fault would result in lockout of
the transformer while the line would return to
service. A 765 kV busbar fault would trip both
the line and the transformer. The line would
ARC, but the transformer breaker would be
manually closed.
It was also established that Tutuka would
become unstable after a sustained line
fault of 400 ms duration. Again, due to
the potential insecurity of communication
channels that the tee-off introduced, there
was the possibility of faults at either end of
the line only being cleared in Zone 2 time,conventionally set to 400 ms. Zone 2 times
were therefore reduced to 250 ms at both
the Tutuka and Majuba ends.
High transient overvoltages caused by
single pole line trippingby Anita Oomen, Eskom Transmission
Tapping power transformers off overhead power lines is a cost-effective solution for the supply of growing energy demand.
Fig.1: Network to the Cape, January 2005
Fig. 2: Proposed change to the network
Fig. 3: Interim network configuration
Fig. 4: Damage to blue insulating chamber
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The time frames for the commissioning of this
configuration did not cater for the changes
that were necessary for DTT to be effected in
this set-up. It was therefore decided to use
a non-directional overcurrent element of
the transformer protection relay to provide
tripping of the Transformer 3 400 kV breaker.
It was set with a pick-up ensuring that faults
beyond the Tutuka and Majuba breakers
would not be detected, and a definite time
operation of 200 ms, ensuring co-ordination
for faults on the Alpha 765 kV busbar. Fault
records from the 765 kV network showed a
very low incidence of fault occurrence. It
was therefore decided to accept the risk of
implementing the configuration without DTT.
The line was left on 1 &3 pole Trip and ARC.
Alpha incident of 29 January 2005
At 06h28 on 29 January 2005, the HV breaker
of Transformer 3 experienced a phase to
ground flashover on the blue phase, requiring
its removal from service.
The 765 kV bus-zone protection operated,
tripping Transformer 3 HV breaker (faulted
of up to 400 kV are observed. These
overvoltages disappear as soon as the
breaker poles closes.
In a few cycles of the appearance of the
overvoltage there are distortions in the voltage
and neutral current waveforms typical of the
presence of higher order harmonics. This canbe seen in Fig. 7. The distortions disappear
in about 400 ms. The overvoltage however
continues until the blue phase breaker
poles close on the line, as seen in Fig. 6.
Resonance at a much lower frequency is
also observed in the waveforms.
Analysis of the composition of the voltage
wave fo rm in di ca te s th e pres en ce of
harmonics. There is a significant percentage of
3rd harmonics as well as smaller percentages
of other harmonics as shown in Fig. 8.
Fig. 9 shows the harmonic analysis of the
current waveforms. Again, the wave
comprises the fundamental component
as well as percentages of 2nd and 3rd
harmonics, noticeably in the neutral current
trace.
Overvoltage phenomenon
Considering the nominal ‘pi’ model of a
transmission line, the network can be drawn
as shown in Fig. 10.
Due to normal load flow in the healthy phases
of the line there is unbalanced current flowing
in the earthed neutral of the transformer. A
path exists for these currents to flow in the
open phase of the line due to capacitive
coupling of the line to ground as well as
the magnetic coupling introduced by the
externally connected delta connection of
Transformer 3.
Due to the very high capacitive reactance
of the line, very high voltages develop
during the period of the open phase. This
explains the high overvoltages observed in
the open phase as soon as the line breaker
poles open.
In a few cycles the voltage waveformappears distorted. The flux density inside the
transformer core is given by:
Fig. 5: Fault clearance by protection
Fig. 6: Disturbance record
Fig. 7: Distortions in V and I waveforms
Fig. 8: Harmonic analysis of voltage
Fig. 9: Harmonic analysis of current Fig. 10: Model of network
breaker), Reactor 4, Bus Coupler B and the
Number 1 Busbar 2 Section breakers, effectively
clearing Zone 5 as shown in Fig. 5.
At the same time, protection at both the
Tutuka and Majuba ends of the line detected
the fault in Zone 1 and tripped the blue pole
breaker. After single pole dead time of 1
second the breaker pole was closed returning
the line to service.
Analysis of disturbance records indicates high
transient overvoltages in the open phase
during the single pole ARC dead time. This
is shown in Fig. 6 which shows the currents
and voltages seen on the disturbance record
taken from Tutuka. As soon as the blue phasebreaker opens current disappears in that
phase. At the same time phase voltages
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Where
B m
= maximum flux density
E = applied voltage
K f = form factor of the emf wave
A = cross-sectional area of thecore [mm2]
f = frequency [Hz]
N = number of turns in the winding
The flux density and magnetising force are related by :
When high voltages are applied to a transformer core the flux density
in the core begins to increase. As the core begins to saturate the
transformer is being operated in the non-linear portion C of the B-H
curve as seen in Fig. 11.
The core permeability varies as the transformer is operated in this
portion of the B-H curve. The varying permeability of the core results
in the generation of third harmonics. There is a higher probability of
this phenomenon occurring in single-phase transformers with the
neutral point earthed. The third harmonic currents together with the
fundamental component of current magnetise the core. During this
time the permeability of the core (µr) reduces until it finally becomes
zero, at which point the flux density is at its maximum. At this point,
any variation in the magnetising force will have no effect in changing
the flux density of the core as it has now reached saturation.
Once the core is saturated and µrbecomes zero, the third harmonics
disappear. The appearance and disappearance of these harmonics
is observed both in the neutral current and the phase voltage in the
disturbance record, Fig. 6.
This incident was simulated in Digsilent and Matlab and similarresults were obtained. The high overvoltages thus generated are
detrimental to all equipment insulation, and therefore it is necessary
to prevent their occurrence.
Conclusions
The tee-configuration utilised as a solution to the problem discussed
in this paper serves its purpose as a temporary solution. Protection
performance is however compromised.
Single pole tripping of the line is not advisable for the reasons
discussed in this paper. Protection settings on the line have
subsequently been changed to effect three-pole tripping only.
Direct Transfer Trip of the Transformer MV breaker is essential in order
to isolate the transformer for any line fault, in addition to the Definite
Time Overcurrent tripping already employed.
Acknowledgments
PG Keller for assistance with fault record analysis, and A Perera for
assistance with simulation of the above incident on Digsilent and
Matlab
References
[1] Carolin, T., “Summary of operational philosophy for T-off of Majuba
Tutuka 1 400 kV line onto Alpha 400 kV bus”
[2] Govindasamy, G., ”Alpha Substation, Transformer 3 765 kV Breaker Fail
Incident of 29 Jan 2005”, Investigation Report
[3] Franklin, A. C., Franklin, D. P., ”The J&P Transformer Book”, 11th Edition– 1983
[4] Havunen, I., Korpinen, L., Kähärä, K., Toivonen, E., Hager, T.,
“The Transformer Book”
Contact Anita Oommen, Eskom, Tel (011) 871-3506,
Fig. 11: Typical transformer
B-H curve