Power Switchgear

Topic 4

Power Switchgear

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Switchgears

Outline

• Introduction to power switchgear

• AC power network switching duties (optional)

• Circuit breaker and fuse selection (Sect 7.5 Text)

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Outline


• AC power network switching duties (optional)


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Roles of switchgear

• Load connection and disconnection

• Interrupt fault currents

• Isolate circuits

• The switching device must be able to

– Turn on or interrupt electrical currents

– Switch very low currents (no load currents) and very high currents (fault currents)

– Dissipate energy released at high rate while interrupting high currents

– Withstand transient recovery voltage

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Types of power switching devices

• Circuit breaker (CB)

• Load break switch (LBS)

• Contactor

• Disconnector and isolator

• Earthing switch

• Fuse or fused switch

• Current limiting protector (CLP)

• Static (power electronic) transfer switch (STS)

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Switching devices

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IEC – International Electrotechnical Commission

Switching devices

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Power switching devices

• Mechanical switching devices or fuses in medium voltage (up to 33 kV) and high voltage circuits (66 kV and above)

• Solid-state power electronic switching as a special application (high speed load transfer and unlimited number of switching operations)

• Components of mechanical CB or LBS:

– a set of separable contacts

– a high speed operating mechanism (20 – 100 ms)

– electrical actuator to open (trip) or close the contacts

– arc extinguishing medium and chamber

– electrical insulation to withstand operating voltages

– connecting terminals

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Current interruption

• Arc is drawn when contacts separate

• Arc will extinguishes at current zero-crossing

• Arc will reignite right after the zero-crossing as the voltage builds up

• Arc has to be extinguished to interrupt current

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Current interruption

• Circuit breakers have some means of extinguishing the arc rapidly

– oil (bulk oil, minimum oil volume, small oil volume)

– air

– atmospheric pressure – mainly low voltage CBs

– air blast – high voltage CBs

– SF6 (Sulfur hexafluoride)

– vacuum

– silica (sand) in fuses

– no arc in power electronic devices

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Current interrupting capacity

– CB and LBS • 22 kV circuit breaker

– continuous current rating: 630 A

– fault current interrupting rating: 21 kA

– making current rating: 40 kA peak

– short-time 1 s current rating: 21 kA

• 22 kV load break switch

– continuous current rating: 630 A

– current interrupting rating: 630 A

– making current rating: 40 kA peak

– short-time 1 s current rating: 21 kA

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Low voltage circuit breakers

• Up to 1000 V (3.3 kV)

• Load current rating up to 5 kA

• Fault current interrupting rating up to 100 kA

• Often built-in overcurrent and earth fault protection

• Mostly non-repairable, non-maintainable, replaced after specified number of operations

• Alternative terminology

– Moulded Case CBs (MCCB)

– Miniature CBs (MCB)

– Insulate Case CB (ICCB)

– Air CB (ACB)

– LV Power CB etc.

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Low voltage circuit breakers

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LV CBs – breaking in air

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• Breaking chamber divided by refractory

panels forming arc chutes.

• Arc pushed into chutes by electrodynamic and

thermodynamic forces.

– Insulating plates: arc lengthened within

chutes to achieve maximum length near

current zero-crossing.

– Metal plates: arc divided into short arcs to

maximize voltage drop.

• Arc cools on contact with the refractory

material.

• Cooling and lengthening/dividing increases

arcing voltage.

• Breaking insulation achieved when Varc >

TRV

Blow-out coil

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Main contacts closed –

no current in the coil

Main contacts opened –

current in the coil creates

electrodynamic force

Industrial 400A LV CB

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LV CBs –

I-t curves

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High voltage circuit breakers

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Designed to operate at voltages above 6.6 kV (up to 1,000

kV).

High transient recovery voltages demand fast restoration of

contact insulation. This requires presence of arc interruption

media

• Bulk oil (high oil volume)

• Minimum oil volume (low oil volume, small oil

volume)

• Air blast

• SF6

• Vacuum

Bulk oil circuit breakers

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• Oil vaporised by arc:

80% H2, 20% C2H2

• Hydrogen removes heat,

extinguishes arc

• H2 re-ignition voltage 5-

10 x that of air

• Can be single-tank (MV)

and multi-tank (HV)

• Use oil as main

insulation and as

interrupting medium

• Tank grounded = dead

tank type

Bulk oil CB – simple arc pot

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Arc contained in an

interrupting pot, improved

safety and interruption

capacity

• Contacts closed (a)

• Contacts open (b),

hydrogen bubble

develops, pressure builds

up

• Contacts fully opened

(c), gas escapes blowing

out arc axially

Bulk oil CB – cross baffle chamber

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• Gas pressure blows the arc out radially

Minimum oil CBs

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• Use oil for the

interrupting function only

• Mounted on insulating

supports = live tank type

• Animation

http://www.youtube.com/watch?v=7IynllRcbb4

Oil CBs - disadvantages

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• Inflammable medium – risk of fire

• Risk of explosion in contact with air

• High maintenance (oil testing, refilling)

• Impact on the environment if leaks

• Oil waste

SF6 in a circuit breaker

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• Used from 3 kV up, almost exclusive above 66 kV

• Chemically stable, non-corrosive, non-poisonous,

odourless, colourless at room temperature

• Excellent dielectric and heat transfer properties

• Low dissociation temperature and high dissociative energy

(good for arc quenching)

• Recovers dielectric strength quickly after exposure to arc

• By-products of arced SF6 + moisture = corrosive

electrolytes (compatible materials must be used,

neutralised using lime or sodium carbonate and

bicarbonate)

• Most potent greenhouse agent

• Must be contained and recycled

SF6 circuit breaker

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• Two types of interruption

o puffer: pressure is created by the mechanism

o self-blast (auto-puffer): pressure created by arc

• Two types of constructions

o dead tank

o live tank

Puffer type interrupter

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Self-blast type interrupter

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Arc interruption

http://www.youtube.com/watch?v=z3F-Vpk7eMw&feature=player_detailpage

Live Tank SF6 CB

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Dead tank SF6 CB

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• Type PMG-B, 245 kV, up to 63 kA

Shares of MV CBs by switching medium

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Power vacuum circuit breakers

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• Superior dielectric strength of vacuum

• Super-clean production environment required

• Diffuse-mode arc develops because of molten metals

from cathode spots

• Anode spots appear above 15 kA contributing to arc

• Magnetic field from current is utilised to keep arc in

diffuse state

–transverse field spirals arc on the contact surface

–axial arc spreads arc though the contact surface

• Contact material and construction is critical for the

performance

Arc diffusion by magnetic field

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Vacuum interrupter

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Vacuum interrupter in a circuit breaker

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Vacuum interrupter range (ABB)

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• 3.6 – 36 kV

• 400 – 3150 A

• up to 40 kA

fault current

• up to 30,000

cycles of

switching

operations (a

hundred short

circuit

operations)

Vacuum circuit breakers

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• Compact design

• No impact on the environment

• Very low maintenance, easy to replace

• Lightweight, low energy mechanism

• Inherent apparent insensitivity to high

rate of TRV

• Fast interruption (within half a cycle)

• Limited voltage range (72.5 kV)

• Current chopping can be a problem

Switchgear Summary

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• Oil circuit breakers (bulk and small volume) – old

technology still remaining in service but being replaced

by other arc interrupting media

o reliable, high maintenance, environmental concerns,

extensive civil works required (containment trench)

o bulk oil – largely obsolete, replaced by minimum oil

volume (MOV)

o MOV – low cost if low interrupting capacity required,

voltages up to 145 kV

• SF6 – covers the entire HV range, dominant for voltages

above 66 kV

o low maintenance

o some environmental concerns

o containment and recycling required

Switchgear Summary

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• Vacuum – replacement technology for oil in the MV

range

o reliable, light weight, minimum maintenance but

limited voltage range

• Air blast – older technology, up to the late 1970’s, no

longer manufactured, replaced by SF6

Outline


• AC Power Network Switching Duties (optional)


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Ideal Characteristics

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• From an electrical point of view the ideal characteristics

of an AC circuit breaker are:

• Zero resistance when closed, or passing currents

• Infinite resistance when open

• Smooth transition between these states at a natural

current zero

• Of these points, the last is the most difficult to obtain and

in practice is not fully achieved in any existing switch or

high voltage circuit breaker.

AC Interruption Basics - 1

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• With AC power systems, the arc associated with the

current will obviously pass through zero twice each cycle,

that is every 10 milliseconds for a 50Hz system.

• This creates opportunities for the arc to be extinguished

and the operating principles of power switchgear make

use of techniques to do this.

• The energy dissipated in the switching arc, the contacts

and due to insulation material decomposition is

significant and obviously increases with time if the arc

continues.

• Therefore the arc needs to be extinguished rapidly

otherwise the switching device will be destroyed.


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• With AC, as the instantaneous current decreases towards

the natural zero, the ionization level in the arc also

decreases while the arc resistance increases

• There is a collapse of the arc shortly before the alternating

current reaches its normal zero value at the end of each half

cycle.

• The arc will reignite again when the current flows in the

opposite direction, during the subsequent half cycle,

provided that the conditions across the electrodes are still

suitable for the existence of the arc.


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• The transition time between the two half cycles is greatly

influenced by the medium in which the arc is being

produced and by the characteristics of the external

electrical circuit.

Circuit Breaker Arc Extinguishing Medium

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• Air

• SF6 gas

• Vacuum

• Mineral insulating oil

AC Current Switching – Arc Voltage

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i Arc current – current to be interrupted

es Peak of arc extinguishing voltage

ea Arc voltage during current flow

et Re-ignition voltage

Switching Resistive Current

(unity power factor)

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Transient Recovery Voltage

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System TRV Characteristics

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TRV Capability of CB versus System TRV

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Switching Inductive Current

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Switching Inductive Current

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Breaking Small Inductive Currents

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• Problem is production of high overvoltages if the powerful

arc cooling action of the circuit breaker interrupts, or chops

the current before zero -- referred to as “Current Chopping”

• If the current is chopped at a value ic, then energy stored in L

is exchanged and transferred to stray capacitance C resulting

with high and fast TRV.

• Circuit breaker (or switch) must not chop at too high a

current, the actual value depending on the circuit and its stray

C. The problem is not considered a critical one for HV circuit

breakers above 100kV. It is important for MV applications,

especially those switching MV motors, where the basic

insulation level is relatively low.

Inductive Current Chopping

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Current Chopping

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• Current

chopping can

occur while

disconnecting

unloaded

transformers,

long lightly

loaded lines

and capacitor

banks.

Switching Capacitive Current

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Asymmetry Fault Current – Equivalent Circuit

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v Sinusoidal voltage source = Vpsin(ωt+Φ)

Sw Switch making − an open to closed transition

i Current that flows when switch closes

R, L Circuit Resistance and Inductance

Asymmetrical Current Waveform

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Three-phase Short-circuit Interruption

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Switching Duties Summary

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In practice, each item of switchgear must be capable of a

great variety of switching operations:

• Short-circuit fault interruption. Often the most

demanding duty.

• Short-circuit fault making. Peak current required.

• Load-break duty. Often the least demanding duty but

more frequent.

• Capacitor switching. Gives a high recovery voltage an

can lead to re-striking.

• Line-dropping duty. Very similar to capacitive switching

but can produce higher peak voltages.


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• Inductive switching at low currents. This duty can

produce current chopping leading to very high

overvoltages. It can be found with no-load transformer

switching and the switching of inrush currents, reactor

and motor switching.

• Asynchronous switching of two parts of a system. This

duty is associated with fault currents of modest

magnitude but with high peak voltages.


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• Evolving faults. Where the current to be interrupted is

initially small and increases to a large value during

interruption. This duty has diminished in importance

with the advent of gas and particularly SF6 blast circuit

breakers. It was of especial importance with oil circuit

breakers where the sudden increase of current could lead

to explosion.


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• Parallel switching. Here the switching duty involves two

circuit breakers, one each in a pair of parallel paths. One

path has a higher impedance than the second but is

slower to interrupt. The faster circuit breaker interrupts

leaving all of the current to be taken by the second. This

is a fault of the same kind as the evolving fault but

usually less extreme.

Outline


• AC Power Network Switching Duties (optional)


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Circuit Breaker Selection

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• Modern circuit breaker (CB) standards are based on

symmetrical interrupting current

• It is usually necessary to calculate only symmetrical fault

current at a system location, and then select a CB with a

symmetrical interrupting capability equal to or above the

calculated current

• The maximum symmetrical short-circuit (SSC) current at

the system location in question is calculated from the

prefault voltage and system reactance characteristics

• For X/R (ratio) ≥ 15, the calculated current at the given

operating voltage is used to select CB capacity, otherwise

20% of extra margin is added to CB capacity

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Circuit

Breaker

Ratings

*

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Circuit

Breaker

Ratings

Circuit Breaker Ratings Example

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• At Vmin = 60 kV

(=Vmax/K = 72.5/1.21),

Imax = 1.21(19) = 23 kA

• K = the voltage range factor,

K = 1 for 115 kV or above class CBs

• At the rated

Vmax = 72.5 kV,

the rated short

circuit current

I = 19 kA

Circuit Breaker Selection Example

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Example 7.7

Fuses

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Fuses

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Fuse Time

Response

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Documents

Power Switchgear