transmission- high transient overvoltages

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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,

anita.oommen@eskom.co.za

 Fig. 11: Typical transformer 

 B-H curve