Power Circuit Breakers

Power Circuit Breakers Theory and Operation

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Agenda „

Breaker fundamentals – Types of Breakers – Ratings – Design Tests

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Application & Operations – Application Considerations – Operation Considerations

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1

Introduction „What

does a breaker do? „Circuit interruption „Types of breakers 3

Power Circuit Breakers „

Waits for current zero to extinguish arc

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Typical interrupting times 3-8 cycles

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Attempts to prevent arc re-ignition (The interrupter must build up dielectric faster than the recovery voltage builds up)

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Commutes plasma by removing energy which is in the form of heat 4

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Power Circuit Breakers „

The ability of the breaker to interrupt fault current is determined by two things: – the magnitude of the fault current – the magnitude and rate of rise of the voltage across the contacts after the current goes out (both 60 hertz and transient voltages)

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Interrupters „

Arcing Contacts and medium (oil or gas) flow

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Oil Interrupters:

Recovery voltage

I t Current zeros

Arc goes out

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Types of Breakers: „ Oil „ Air

blast „ Air magnetic „ SF6 gas „ Vacuum „ Others 8

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Circuit Breaker Year of Manufacture 140 120

N um b e r

100 airblast 80

vacuum

60

airmag SF6 OCB

40 20

1999

1994

1989

1984

1979

1974

1969

1964

1959

1954

1949

1923

0

Year 9

Interrupting Mediums „

Oil – Mineral oil is used to extinguish the arc and to insulate the live parts to ground – Generally free breathing – May use a single tank or multiple tanks

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Air Blast – Stored air is used to blow out the arc and cool contacts – Very loud operation 10

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Interrupters „ Oil

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Interrupters „ Oil

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Oil Breaker (Westinghouse 345G)

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Oil Breaker (McGraw Edison CG38)

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Interrupting Mediums „

Air Magnetic – A magnetic field is place perpendicular to the plane of the arc which drives the arc into insulating fins – Sensitive to moisture – Compact

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SF6 (Sulfur Hexa-flouride gas) – Multi-pressure or puffer operation – High speed 15

Interrupters „ Air

Magnetic

Magnetic Field

Insulating Fins

Arcing Contacts

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8

Interrupters „ SF6

Gas

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Puffer Interrupter

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Self-Blast Interrupter

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Gas Breaker (ABB PM)

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10

Gas Breaker (Westinghouse SP)

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Gas Breaker (ABB PM)

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Interrupting Mediums „

Vacuum – Contacts incased in a sealed bottle which is under vacuum – Compact

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Others – – – –

Gas mixtures (N2, SF6 etc.) Water Solid State Super-conductor??? 23

Vacuum

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Vacuum (GE PVDB-1)

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Ratings „Continuous „Interrupting „Close

& latch – momentary „Voltage ratings „Switching scenarios

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Power Circuit Breaker Ratings

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Power Circuit Breaker Ratings „ „

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Continuous (load current)…typically 600,1200, 2000 or 3000 amperes ANSI C37.04 Definition: The established limit of current in rms amps at rated frequency that it shall be required to carry continuously without exceeding temperature limits of its components. Select a breaker with a Continuous Current rating greater than the peak load current.

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Power Circuit Breaker Ratings

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Power Circuit Breaker Ratings „ „

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Interrupting Rating (fault current)…typically 20, 31.5, 40, or 63 kilo-amperes ANSI C37.04 Definition: The highest value of the symmetrical component of the threephase, short-circuit current in rms amps that the circuit breaker shall be required to interrupt at rated maximum voltage and on the standard operating duty. Select a breaker with a fault duty greater than the maximum available fault current. 30

15

Power Circuit Breaker Ratings „ Cannot

forget X/R! „ Standards assume X/R < 17. „ What do you do if X/R > 17? „ Delay opening. „ Increase kA interrupting requirements for the breaker. 31

Power Circuit Breaker Ratings „ „ „

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K Factor: Applies only to OCBs. Defined as the ratio of the rated maximum voltage to the lower limit of the range of operating voltage. The interrupting capabilities are inversely proportional to the operating voltage (i.e. a reduced voltage results in an increase in the current interrupting capability). Reference Engineering Manual Section 2 SI 4.0 – Application Guideline – AC High Voltage Circuit Breakers Based on Short Circuit Capability Criteria. 32

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Power Circuit Breaker Ratings „ Close

and Latch rating is expressed as a multiple of the rated short-circuit current. „ The maximum current for which the breaker will close and latch….stay closed. „ 2.6 x rated short circuit current (Interrupting not Momentary). 33

Power Circuit Breaker Ratings „

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Rated Maximum Voltage is the highest rms phase-to-phase voltage for which the circuit breaker is designed. The upper limit for operation. Typically 15kV, 84kV (25kV, 38kV, 46kV and 69kV), 145kV, 245kV, 362kV and 550kV

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Power Circuit Breaker Ratings „

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Basic Impulse Level (BIL) ,expressed in kV, is the maximum electrical impulse level (lightning strike) that the breaker can be exposed to and not flashover either internally or externally. Actual design BIL does not reflect the maximum impulse that can be generated because the magnitude of the impulse depends on the location of the strike. The designed BIL reflects the insulation coordination practices used in the design of electrical systems (Source: High Voltage Circuit Breakers: Design and Applications) 35

Power Circuit Breaker Ratings „A

circuit breaker is a tie between two networks Load

Source

Load

Source

TRV

36

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TRV

Fault:

z

fault V=0 Transient Recovery

2 per unit

Fault

Voltage

v

i t

37 Breaker opens

Power Circuit Breaker Ratings „

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The ability of the breaker to interrupt fault current is determined by the magnitude of the fault current and also by the rate of rise of the voltage across the breaker contacts as it opens Transient Recovery Voltage (TRV): is the transient voltage that appears across the contacts while interrupting a faulted circuit.

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Transient Recovery Voltage:

TRV

bus

line fault

Initial Rate of rise V

Crest recovery voltage Total TRV = bus - line

TRVbus = 1 - cos t/sqrtLC

TRVline = 1 - cos t/sqrtLC t Time in hundreds of microseconds

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Power Circuit Breaker Ratings „

Standard C37.011 defines the TRV envelope that breakers must meet for rated current (Maximum TRV per standards is 2.4 per unit of rated breaker voltage)

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Power Circuit Breaker Ratings „ Some

breakers were able to clear bus faults with no problem but were unable to interrupt faults a short distance out on the line even though the fault current was less than the bus fault. „ The cause of this becomes clear when the transient recovery voltage is analyzed for both scenarios: 42

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Power Circuit Breaker Ratings „ Short

Line Fault: A fault that occurs a relatively short distance downstream from the circuit breaker on its load side. „ Also known as a kilometric fault since this is generally considered to be the critical distance for maximum severity of the recovery voltage. „ Allegheny specifies full nameplate interrupting for a 90% short line fault 43

Short Line Fault: Vbus

Vline TRV

Vbus

TRV= Vbus - Vline

Vline

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TRV

V Fault i t

Very steep rate of rise for TRV caused by the short length of the transmission line

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V Fault i t

Adding shunt capacitance attenuates the rise 46

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Insert bushing shunt capacitance on line side

Short Line Fault: Vbus

Vline TRV

TRV= Vbus - Vline

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Bushing shunt capacitance

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Power Circuit Breaker Ratings

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Power Circuit Breaker Ratings „

Short Line Fault: – SF6 breakers up to 50 kA don’t require external shunt bushing capacitors. – Most breakers rated at 63 kA require external shunt bushing capacitors to meet Short Line Fault requirements of the standards. – Mainly breakers above 100 kV.

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Power Circuit Breaker Ratings „ 1200pf,

3000pf and 12,000pf are typical values for external bushing capacitors.

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Power Circuit Breaker Ratings „ Operating

Duty Cycle: A series of circuit breaker operations and time delays. „ Defined by ANSI as: O-15 SEC-CO-3 MIN.-CO. „ The first time delay can be reduced to 0.3 seconds for breakers designed for high-speed reclosing. „ 0.3 seconds is the mechanical resetting time. 52

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Power Circuit Breaker Ratings „A

derating factor R is applied to OCBs based on their reclosing duty. „ This does not apply to SF6 breakers.

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Power Circuit Breaker Ratings „ Asymmetrical

Current Rating: Maximum RMS current, at rated frequency, including DC component against which the breaker is required to operate. „ Function of the system X/R „ Standards assume X/R of 17

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Power Circuit Breaker Ratings „ Momentary

Rating:

– Short time current rating – The current, equal to the rated symmetrical short circuit current, that the breaker is required to carry for a specified time. – Time = Allowable Tripping Delay

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Application of Circuit Breakers

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Switching Surges:

Voltage doubles when closing in on an open line = 2 P.U. at open line terminal

Z0= L/C Assume that High Speed Re-closing traps a negative 1 P.U. charge on the line. Then when the breaker re-closes the maximum voltage at the open end can approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending on damping (R): 3.5 P.U.

Trapped charge = -1.0 P.U.

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Breaker Applications „ Closing

resistors are used to reduce the switching surge created at the far end of a transmission line when it is energized – Once breaker is closed the resistor is removed from the circuit. – Not designed to handle continuous load current or fault current. – AP uses closing resistors on EHV breakers – Typical value: 400-450 ohms. 58

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Closing resistors: bypass

Rc Z0= L/C

Rc VS

Z0

V0

V0 = VS

Z0 (Rc + Z0)

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Closing resistors: 2.0 P.U.

Rc Z0

V0

V0B second close V0R first close

Rc > Z0 V0R first close

2.0 P.U.

V0 = VS Z0

V0B second close (Rc + Z0) Rc < Z0

Optimum closing resistor size = surge impedance

2.0 P.U.

V0R first close = V02 second close

Rc = Z0

60

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Breaker Applications „

Grading capacitors. – Applied on circuit breakers with more than one gap per pole – Grading capacitors are applied across the contacts of each interrupter to divide the recovery voltage during interruption – Insures that the first gap to interrupt does not see the whole TRV

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Grading capacitors 500kv PM breaker 62

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