SWITCHGEAR 11KV

Oleh:RAIS BIN MA’AT

Ketua Program Kejuruteraan ElektrikITiDi

DEFINITION

Switchgear is a general term covering switching, interrupting, control, metering,

protective and regulating device with associated interconnections, accessories and supporting structures for use in connection

with the generation, transmission, distribution and conversion of electric

power

FUNCTION• It must safely interrupt the normal working current

as well as the short circuit current• After occurrence of fault the switchgear must

isolate the faulty circuit as quickly as possible i.e keeping the delay to minimum

• It should not operate when an over current flows under healthy conditions

• To facilitate redistribution of loads, inspection and maintenance on the system

TYPES OF CIRCUIT BREAKERInterrupting medium Air, air blast, oil, magnetic blast, gas and vacuum

Service Indoor or outdoor

Operation Gravity opened, gravity closed and horizontal break

Contacts Butt, wedge, laminated flat contact, bayonet,Explosion chamber, deion grid

Action Non-automatic and automatic

Method of control Remote control, manual, pneumatic, electrical, direct control

Tank construction Separate tank for each pole, one tank for all poles

Mounting Panel mounted, rear of panel and remote from panel

SWITCHGEAR CONSTRUCTION

• The control of a power system involves the use of switch gears for making and breaking circuit

• It is of metal clad type where all live parts are completely enclosed in a metal casing filled with insulating medium

• Rigorously interlock to prevent access to live parts

TYPES OF SWITCHGEAR

• Indoor draw-out gear (vertical or horizontal)

• Indoor fixed circuit breaker• Out door draw-out gear

SWITCHGEAR RATINGS

• Rated voltage - kV• Rated current - A• Rated breaking capacities (symmetrical and

asymmetrical) - kA• Rated making capacities - kA• Rated short time current and duration – kA,

second• Rated frequency – 50Hz, 60Hz

RATED VOLTAGE

The rated maximum voltage of a circuit breaker is the highest rms voltage, above

nominal system voltage for which the circuit breaker is designed and is the upper

limit for operation

RATED CURRENT

Is the designated limit of current in rms amperes which it shall be capable of

carrying continuously without exceeding the limit of observable temperature rise (which should not exceed 40 degrees C)

RATED FREQUENCY

50Hz

RATED MAKING CURRENT

The short circuit current where the breaker has the capability to close its contacts

without severe damage

RATED BREAKING CURRENT

• The breaking current of a pole at the instant of contact separation - kA

• Generally expressed in term of MVA and is equal to the product of rated breaking current in kA, rated voltage in kV and a factor which depends upon the number of phases (1 for 1 phase and 3 for 3 phase)

SHORT TIME RATING

Is the current that can be safely applied, with the circuit breaker in its normal

condition, for 3 seconds, if the ratio of symmetrical breaking current to normal current is less than 40, or for 1 second

otherwise

OIL CIRCUIT BREAKER (OCB)

• Oldest type• Oil is used as quenching medium for arc

extinction• Contacts are made to separate in an

insulating oil• Oil is decompose into hydrogen gas by the

arc energy – the gas is used to sweep and compress the arc

ADVANTAGE OF OIL

• The absorption of arc energy in decomposing the oil

• The good cooling properties of the gas formed• Cooling by oil around the gaseous arc path• The ability of cold oil of high electric strength to

flow into the arc space after a current zero• The natural insulation value of the oil, enabling

clearances to be minimised

DISADVANTAGE OF OIL

• It is easily inflammable• It may form an explosive mixture with air• It requires maintenance• Troublesome to handle oil

CLASSIFICATION

• Plain break oil circuit breakers• Self blast or self generated or arc control

circuit breakers• Externally generated pressure oil circuit

breakers or forced blast oil circuit breakers or impulse oil circuit breakers

BREAK TYPE

• Single break• Double break – better choice because spark

is divided into two contacts, last longer

STROKE & WIPE

• Stroke is the distance of the moving contacts at the OCB’s “OFF” position to its “ON” position

• The contact wipe of the OCB is the distance the moving contact travels from the contact touch position to the contact fully “ON”

VACUUM CIRCUIT BREAKER

• Basically it is the same as OCB, the difference is that the contacts are in vacuum bottle instead of oil

• Vacuum has high insulating strength• It is cheap• Need no maintenance

VACUUM CHAMBER/BOTTLE

Made of synthetic material such as urathane foam which is enclosed in an outer glass fibre reinforced plastic tube or of simple glass or porcelain, two contacts, a metal

shield and a metal bellows are sealed inside the chamber. The contacts must be pure and

thoroughly degassed

OPERATING MECHANISM

The lower end is fixed to a spring operated or solenoid operated mechanism, so that the

metallic bellows inside the chamber are moved upward and downward during closing

and opening operations respectively. However there should be sufficient pressure

to allow a reasonable wipe for a good connection between the two contacts

HORIZONTAL MOVEMENT

While the OCB moves vertically, the VCB normally moves horizontally so that the

busbar spouts are at the back of the circuit breaker panel instead of at the top

SF6 GAS CIRCUIT BREAKER

• Instead of vacuum, SF6 (sulphur hexaflouride) gas is used as an insulating medium

• It is an inert gas and non organic• More stable• Slightly higher pressure than atmospheric

pressure (up to 3 bar)

C.B MAINTENANCE

• OCB Maintenance• VCB Maintenance• SF6 C.B Maintenance• Battery Maintenance• Protection circuit testing

OCB MAINTENANCE

• Check contacts for pitting or burn• Pitted copper contact should be smooth by

fine file• Pitted silver contacts should not be filed out• Contacts shall be flushed with oil and wiped

clean using recommended solvent of non-mercurial property

• Burnt silver contacts should be changed

CHECK STROKE

• Checking the stroke is to ensure that the tension rod carries the moving contact the required distance to enable the moving and the fixed contact to make properly and also adequate clearance is maintained between line and isolated section when the OCB is tripped.

• Check tension rod for any sign of cracks or faulting against the guide rail

CHECK WIPE

• Check on contact wipe is to ensure that there is existing sufficient pressure between the moving and the fixed contact when the OCB is at its “ON” position

• Stroke and wipe are always adjusted to the manufacturer’s recommendations

• Tighten bolts and nuts, especially on the tension of the cross-arm

TURBULATOR• Check buffer stacks of the turbulator for sign of

burn and the clearance is maintained for the moving contacts to come through

• Carbon deposits on the stacks should be washed away with oil

• Burned buffer stack should be replaced immediately

• Ensure buffer stacks are rearranged to its original position after cleaning work

BUSHINGS• Inspect internal OCB bushings for cracks• Clean bushing from carbon deposits• Slightly chipped and scratched bushing may be

repaired by good quality varnish as recommended by the manufacturer

• External bushings are to be wiped clean using cotton waste rayon or Tetrachloride

• Bushing terminals should be cleaned and a thin layer of vaseline applied on them

OIL

• New or recycle oil should pass the dielectric strength test of 30kV for 1 minute

• Existing oil in the OCB should pass the dielectric strength test of 14kV

• The oil should be changer to new one after opening under heavy fault condition

• Ensure new and existing oil is not contaminated

MECHANISM

• Check for free movement• Inspect for sign of wear• Tighten bolts and nuts especially on the

moving mechanisms

PHASE BARRIER

• Inspect lining and phase barriers for burns• Change burnt phase barrier• Check that phase barriers are securely held

in place and that they do not hinder any movement of the tension rods or arms when the tank is replaced

OVERSHOOT STOP

• Check overshoot stop and adjust in accordance with the manufacturer’s instruction

OTHERS

• Check protection fuse for continuity and terminal contacts effectiveness

• Clean and lubricated auxiliary switch contacts

• Check termination of pilot cable for tightness and cleanliness

• Replaced faded labels

INSULATION RESISTANCE

• With OCB at “ON” position, insulation resistance of each phase to earth and between phases are to be tested

• With OCB at “OFF” position, the insulation resistance between bushings of the same phases is to be tested

• Test trip the OCB three or four times manually or under remote control to see that the tripping circuit and the OCB are in good working order