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COMPUTER MODELLING OF A 245KV 40KA

HYBRID GAS CIRCUIT BREAKER

V. K. Liau, B. Y. Lee, K. D. Song and K. Y. Park

Advanced Power Apparatus Group, P.O. Box 20, Changwon,

641-600, Korea

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Objectives

Modelling of thermal gas-flow simulation in the high current phase of a

245kV class hybrid gas circuit breaker with moving contact has been

performed

PTFE ablation has been taken into account.

Compare the simulation results with the measured experimental results.

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Introduction

In order to reduce the size of the circuit breaker, and to improve the breaking

ability, a hybrid type of GCB has been designed

There is an additional expansion chamber, and the pressure in this expansion

chamber increases rapidly (maximum overpressure of around 1.7MPa) during

the nozzle clogging, and at current zero

The huge pressure results in a strong blowing of the arc and hence helps in the

arc quenching

Computer modelling is used to help in the designing of the geometry of the

circuit breaker.

The volume of the expansion chamber, main and auxiliary PTFE nozzle

geometry are important parameters in designing the circuit breaker.

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Model geometry and assumptions

Assumptions

Arc is axisymmetrical, LTE, turbulent (Prandtl mixing model)

Due to high arcing current and strong radiation, the ablation of

polytetrafluoroethylene (PTFE) nozzle occurs

The mixing of the SF6 gas with the PTFE vapour in the breaker.

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Governing Equations I

*!

x

*x

+*x

x

x

*x

+*x

x

x

*x

Szwzrrvrrrt VV

V 1

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Prandtl mixing length model (turbulent parameter is set to 0.2)

Semi-experimental radiation model is used (40% of the radiation emission from

the arc core was reabsorbed in the re-absorption region )

The filling pressure is 0.6MPa.

Governing Equations II

Current Continuity Equation

Amperes Law

0! NW

zJrBrr

0

1QU !

x

x

ahmQF !E1Rate of ablation of the PTFE nozzle

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Simulation Procedures

Firstly cold flow simulation is performed to obtain the pressure and velocity

distribution before the arcing period.

When the contact starts to separate, the arc is initiated at 2kA by placing a

conducting plasma column with a radius of 3mm between the transparent and

solid contacts.

The whole arcing of the breaker is modeled accordingly to the arcing current

and stroke curves obtained from experimental measurement.

The time-step used in cold flow simulation = 0.2ms, in the arc simulation, the

time-step = 0.02ms, close to the current zero, the time-step is set as 0.01ms.

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Cold Flow Simulation

u i in t an ion C am

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-0.002 0.003 0.008 0.013 0.018 0.023 0.028 0.033 0.038

im ( )

u

i

(

1

a)

im nt Cold a Simulation ult

The simulation and experimental measured pressure rise in the expansion

chamber of the hybrid breaker.

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Initial Stage of Arcing 20 to 23.5ms

The instantaneous current increases from -

kA (at ms) to -

9 (at 1.5ms)

and then reduces to - 6kA (at .5ms).

The temperature within the arc is high at around 19, K during this period.

Temperature at 23.5ms

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Initial Stage of Arcing 20 to 23.5ms

The ablated PTFE vapour from the auxiliary nozzle is very low and mainly

leaks into the flow passage towards the piston chamber.

PTFE concentration at 23.5ms

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Arcing before first current zero 23.5 to 25.8ms

The arcing current continues to reduce from - ! 6kA until current zero (at " 5. # ms)

during this period.The arc temperature and arc diameter reduces.

The arc causes the pressure increase in the expansion chamber and the maximum

pressure rise is $ . % MPa which occurs at the first current zero

Temperature at 25.8ms

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Arcing before first current zero 23.5 to 25.8ms

The ablated PTFE vapour in the auxiliary nozzle flows into the passage towards

piston chamber, where as the ablated PTFE vapour in the main nozzle flows intothe passages towards the expansion chamber.

During this period, the PTFE vapour does not penetrates into the arc

PTFE concentration at 25.8ms

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Arcing until main nozzle being cleared by solid

contact 25.8 to 32ms

The arc current increase from current zero to a maximum of & ' kA at time ( ' ms, and

reduces to ) 7kA at the end of the flat section of the main nozzle.

The arc temperature is high at around 1 0 , ' ' ' K, and the hot gas flows into the expansion

chamber

Temperature at 32ms

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Arcing until main nozzle being cleared by solid

contact 25.8 to 32ms

The PTFE concentration is very high and penetrates into the whole arc.

The PTFE vapour is also responsible for transferring the mass and energy of the hot

gas into the expansion chamberA very high pressure rise in the expansion chamber (around 1. 1 MPa) is achieved.

PTFE concentration at 32ms

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Solid contact in divergent section of main nozzle

32 to 34ms

The arcing current reduces when the solid contact moves into the diverging section of

the main nozzle.

The second current zero is at time 2 3 ms, and he arc temperature is around 1 2 , 4 4 4 Kduring this period.

Temperature at 0.1ms before

current zero

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Solid contact in divergent section of main nozzle

32 to 34ms

Most of the PTFE vapour is blown towards the exhausts and away from the

diverging section of the main nozzle.

PTFE concentration at 0.1ms

before current zero

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Comparison with experimental measurement :

Pressure Rise in Expansion ChamberPressure

0

0.

0.

0.

0.

.

.

.

.

0.02 0.022 0.024 0.026 0.028 0.03 0.032 0.034

time (s)

Pressure(x105MPa)

Experiment Simulation

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Arc Vo g

-1000

-800

-600

-400

-200

0

200400

600

800

0.02 0.022 0.024 0.026 0.028 0.03 0.032 0.034

T ( )

Vo

g

(V)

xp r n S u on

Comparison with experimental measurement :

Arc Voltage

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The simulation of SF6 hybrid circuit breaker has been performed successfully.

The results show good agreement compared with experimental measurements.

The PTFE vapour is mainly responsible for the increase in the pressure in the

expansion chamber. Hence the modelling of the rate of ablation of PTFE nozzle due

to the radiation is very important.

During the second current zero, the high pressure in the expansion chamber

enables strong blowing of the arc.

With the presence of the expansion chamber the hybrid circuit breaker has better

advantage compared with conventional puffer circuit breaker.

Conclusions