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MERLIN GERIN TYPE SB6-72 SF6

CIRCUIT BREAKER FAILURES

Presented by

John O Silweya

(BEng, MEIZ, REng)

Senior Maintenance Engineer – Electrical

Copperbelt Energy Corporation Plc - Zambia

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PURPOSE• The aim of this presentation is to share

– our experiences on the violent and unsafe

circuit breaker failures experienced and

– some of the actions we are taking to prevent

these failures and ensure the reliability of these

circuit breakers for continued use on our 66kV

transmission network.

• This presentation also aims to solicit ideas

from others who may have suffered similar

experiences.

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Description of Problem

• 66kV SF6 circuit breakers catastrophic

failure when in the open position

• Two failures of breakers less than 6 years

old at the time of failure

• Evidence that they may have been

conducting even though in the open

position

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CIRCUIT BREAKER DETAILS

The nameplate details for the two breakers are as follows:

Circuit Breaker at Mill Circuit Breaker at Luano

Make Merlin Gerin

Current Rating 800A

Voltage Rating 72000 Volts

Serial Numbers 20000 1855 DIJ 1999 0049

Type SB6 72

Rated Impulse Voltage 325kV

Frequency 50Hz

Rated short circuit breaking current

78kA

Short term current 31.5kA for 3 seconds

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PRINCIPLE OF OPERATION

Parts List Number Description 10 Support Insulator 11 Insulating Connecting Rod 12 Outer Casing Insulator 13 Fixed Contact support 14 Moving contact support 17 Filter 21 Metal envelope (expansion cylinder) 22 Coil 23 insulating spacer (expansion cylinder) 24 Fixed Arcing contacts 25 Moving Arcing contacts 26 Main Fixed contact 27 Moving Main contact 28 Sliding contacts 29 Exhaust Volume

• The opening starts with the opening of main

contacts (26 & 27), followed by opening of

arcing contacts (24 & 25), until it then fully

opens.

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Maintenance History

• SF6 circuit breakers are inspected and

tested annually.

• Primarily insulation resistance and contact

resistance tests.

• Quarterly trip tests.

• Both of the failed breakers had a 100%

compliance to this test regiment

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Maintenance History

of Circuit Breaker at Mill substation

Insulation Resistance - CB Open [GΏ] [ ≥ 1 5 G Ώ ] – t e s t e d @ 5 k V

2004 2005 2006

RΦ YΦ BΦ RΦ YΦ BΦ RΦ YΦ BΦ

U – E 48 45 45 20 19 30 ≥69.5 ≥69.5 ≥69.5

L – E 69 70 70 16 18 15 ≥69.5 ≥69.5 ≥69.5

U - L 70 70 70 23 24 26 ≥69.5 ≥69.5 ≥69.5

Contact Resistance - CB Closed [µΏ] [ ≤ 1 0 0 µ Ώ ]

RΦ YΦ BΦ

2004 25 26 23

2005 19 16 12

2006 92 92 94

Circuit Breaker Timing Tests

Year Opening Time [ms] [(30±3)ms] Closing Time [ms] [(57±3)ms]

RΦ YΦ BΦ RΦ YΦ BΦ

2004 29.6 29.8 29.7 59.8 59.7 59.7

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Maintenance History

of Circuit Breaker at Luano Switching Station

Insulation Resistance - CB Open [GΏ] [≥15 GΏ] – tested @ 5kV

2003 2004 2005 2006 2007

RΦ YΦ BΦ RΦ YΦ BΦ RΦ YΦ BΦ RΦ YΦ BΦ RΦ YΦ BΦ

U – E ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505

L – E ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505

U - L ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505 ≥505

Contact Resistance - CB Closed [µΏ] [≤100µΏ]

RΦ YΦ BΦ

2003 25 30 33

2004 24 35 41

2005 39 42 35.6

2006 40 42 37

2007 39 43 38

Circuit Breaker Timing Tests

Year Opening Time [ms] [(30±3)ms] Closing Time [ms] [(57±3)ms]

RΦ YΦ BΦ RΦ YΦ BΦ

2004 29.6 29.8 29.7 59.8 59.7 59.7

2007 30.5 30.1 29.9 59.9 60.1 60.2

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DESCRIPTION OF FAILURES

1. Nkana line SF6 circuit breaker at Mill

Substation

• The breaker was on a 2.8km 66kV line connecting two substations in a mine area

• On 2nd August 2007, during routine switching, the line was de-energised by opening the SF6 circuit breakers at both ends (bus-bar side on circuit breaker was still live)

• A minute later, the blue phase interrupting column exploded, with porcelain debris being strewn up to 50 - 70 metres away.

• The calculated fault level at the 66kV bus-bar at Mill substation is 19.7kA. Circuit breaker is rated for 31.5kA for 3 seconds.

• Examination of relay traces obtained during routine trip tests two months earlier revealed an anomaly in the blue phase.

• The pictures on the following slides show the waveforms and extent of damage

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Waveform from Relay Tests

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Images of damaged CB at Mill

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• The Luano – Bancroft 2 line is a 37.4km line– was de-energised (bus-bar side on circuit breaker was still live) to

allow the movement of a truck with an abnormally high load.

• As the truck approached the line, it infringed clearances to the“de-energised” blue phase conductor and the conductorsnapped. There were burn - marks on the ground.

• Simultaneously, at Luano substation, the blue phase pole ofthe circuit breaker exploded, resulting into a three phase bus-bar fault, with the debris being strewn over a radius of 100metres.

• The calculated fault level at the point of fault on the Luano –Bancroft No. 2 66kV line is 5kA. Circuit breaker is rated for31.5kA for 3 seconds.

• The pictures show the extent of circuit breaker failure

DESCRIPTION OF FAILURES (cont’d)

2. Bancroft No. 2 line 66kV SF6 circuit breaker at

Luano Switching Station

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Images of Damaged CB at Luano

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DISCUSSION ON POSSIBLE CAUSES OF

FAILURE

• Defects in the interrupting chamber components– Waveform of the Mill line circuit breaker shows the blue phase

was still conducting after it was opened

– The Luano CB may have suffered a defect on account of thehigh number of lightning related trip-outs

• Mechanism failure– Unlikely for the Mill circuit breaker as it was found intact

– Though the bearing housing on the Luano circuit breaker wasfound cracked and the operating linkages were detached, it hasto be established if this was as a result of the explosion or it wasactually the point of failure

• Loss of SF6 gas insulating properties– Can be possible cause, especially in consideration of the high

number of lightning fault clearances of the circuit breaker atLuano (44) and the by-products from the SF6 gasdecomposition

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INTERACTION WITH EQUIPMENT

MANUFACTURER

• The manufacturer, was contacted after the first circuitbreaker failure on 2nd August 2007 to investigate thefailure so that remedial measures could be put in place.

• These efforts did not bear fruit as the manufacturerinstead offered to replace the damaged circuit breakerpole, at CEC’s cost.

• After the 2nd circuit breaker failure of 10th August 2008the manufacturer agreed to send technical experts toinvestigate the causes of the two failures and test theremaining circuit breakers to determine their reliability forcontinued use on the electrical network.

• These tests will be carried out, at CEC’s cost,commencing in mid October 2008.

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PRELIMINARY CONCLUSIONS• The failure experienced on the circuit breaker at Mill

substation is most likely due to a defect in the interruptingchamber, on the insulating separator.

• The failure at Luano Switching Station is thought to beeither due to lack of proper moving contact separation inthe blue phase (either contact jamming or mechanismfailure) or loss of insulating properties of the SF6 gas.These factors, with the fault current caused by thecontact of the abnormal load with the blue phase lineconductor and resulting high fault current, led to thecircuit breaker explosion, originating from the blue phase.

• The fact that the line protection relays operated at Luanolends credence to the thought that the circuit breakerremained in closed position, probably the blue phasecontacts only.

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ACTIONS CURRENTLY BEING UNDERTAKEN

• Procure an SF6 gas analyzer to determine the

condition of SF6 gas in circuit breakers.

• Improve the lightning performance of our

transmission lines

• Downloading trip test traces

• Engage manufacturer in

– investigating the circuit breaker failures to understand

their causes and hence put forth preventive measures

– testing the remaining circuit breakers that are in

service so that their reliability for continued use can

be ascertained.

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