HV Equipment File 4 of 5

POWER CIRCUIT BREAKER (ANSI Std. C37.06)

• A device, that can be able to breaking, making, carrying current both during normal and fault condition • During interruption, fault current flows through circuit breaker contact while it tries to open. This current can not cease suddenly. Therefore high electric field and thermal stress around contact surface occur and cause ionisation of oil or gas nearby and after that arc occurs. The resistance of arc depends on its length, which is important for arc extinguishing method. Gas Flow

Operating Mechanism

Turbulence Electric Arc Gas mixture Radiation

Electric and Magnetic Field

Electrical system

• In most circuit breaker except vacuum type, current interruption needs to increase the length of arc in order to increase the resistance. This arc will extinguish when current passes zero crossing and contact completely open.

Circuit breaker type 1) Vacuum CB – Interrupter is in enclosed chamber with very low air pressure. No ionisation inside, only metal contact evaporation occurs, then it has small contact gap with quick insulation recovery.

Live tank circuit breaker

2) SF6 CB – 3 types – Magnetic Type Interrupter – Puffer Type Interrupter – Self – blast Interrupter

– Magnetic type is used for V < 15kV and low current rating. It uses magnetic force to lengthen ionised gas plasma to mix with normal gas. – Puffer type compresses gas in cylinder and flows through nozzle to increase the arc length and replace ionised gas.

SINGLE PRESSURE PUFFER TYPE INTERRUPTER

1.

Fixed contact

2.

Insulating nozzle

3.

Arcing contact

4.

Compression chamber

5.

Fixed piston

Dead tank circuit breaker

• Nozzle blows gas to arc (20,000K) and dissipate heat by convection and conduction (arc contacts with nozzle wall). • Nozzle before is made of graphite, nowadays by Teflon to reduce/avoid high conductivity metallic contamination of the gas plasma.

Self blast circuit breaker

• Advantages – – – –

Reduction of mechanism requirement. Contact speed depends on required operating time Higher fault current, higher pressure, then easy to interrupt Soft interruption characteristics at low fault current

Circuit Breaker Opening Phase 1. Opening of main contact

Phase 2. Current commutation to arcing contact

i .

Phase 3. Arcing contact opening t

Phase 4. Arcing phenomena

i .

Phase 5. High current phase t

Phase 6. Extinguishing phase

i .

Phase 7. End phase t

Closing operation Arc occurs only at arcing contact.

Interrupting chamber design

CB TECHNICAL DATA - Nominal Voltage - Highest Voltage (1.1 times of Nominal System Voltage) - Rated Continuous Current - Frequency - Duty Cycle

operating cycle of CB is 2 times closing and

opening within 0.3 and 15 s time interval. (CO - 0.3S - CO - 15S - CO)

- Interrupting Current Example CB with 15kVmax and Isc = 37 kA has K Factor = 1.3 means that between voltage 15/1.3=11.5 kV and 15 kV, interrupting rating is directly proportional to operating voltage. Then at 12.5 kV, the rms value of symmetrical interrupting rating = 15/12.5 x 37 = 44.4 kA. This means symmetrical interrupting capacity is higher than S/C rating with ratio of rated max voltage to operating voltage.

- Interrupting Time - Distribution 5 Cycles - Transmission 3 Cycles - EHV 2 Cycles - 1 Minute Power Frequency Test - Impulse Withstand Voltage - Short Circuit Breaking Current - Total Break Time, Total Closing Time - Pre - Closing Resistor in parallel with interrupter to limit over voltage during closing no load line with 6 - 15 ms connecting time. - Voltage Gradient Capacitor for breaker with multi break in parallel with interrupters for equal voltage drop across each interrupters during interruption.

-

3 Pole or 1 Pole 1 Pole mostly for V > 145 kV. Each pole trip independently with one per 1 pole to increase system stability.

Current interruption

• During arcing, thermal race of SF6 with higher heat capacity to absorb thermal energy from arc = mass density x Enthalpy x sound velocity. • Recovery voltage Æ need good dielectric recovery. Dielectric recovery rate > dV/dt of recovery voltage (RRRV).

Transient Recovery Voltage ( TRV )

i(t ) =

α=

V

⎡sin (ωt + θ − ϕ ) − sin (θ − ϕ )e − αt ⎤ ⎢⎣ ⎥⎦ 2 2 2 R +ω L m

ωL R and tan(ϕ ) = R L

1 f = 2π LC

Operating mechanism • • • • •

Manual operation Solenoid operation Motor charge spring Hydraulic Pneumatic

Major failure mode of SF6 single pressure circuit breaker

43 %

Percentage of each failure in HV components

Operating Mechanism

Insulation to ground

45 % Elec. Control & Aux. circuit

6%

Interrupters

HV Components

26 %

25 %

Other causes

4%

51 %

Aux. interrupters, resistors