SB6 Circuit-Breaker Design

Technical information 3d edition - 19 jan 00

TC/INT- Battaglia Terme Unit + PM

1

1.1 Description of the circuit-breaker

The SB6 circuit-breaker has three independent poles with an integrated operating mechanism which use the SF6 as a driving fluid. Each of these poles stands on a structure and is electrically connected to the control cabinet. The cabinet is placed next to the poles (see fig.1) Each pole is divided into 2 compartments which are filled with SF6.

1. The HP (High Pressure) compartment has a interrupting chamber and a supporting column. This

compartment is inflated with SF6 at the rated pressure. 2. The LP (Low Pressure) compartment integrates the pole operating mechanism. It is inflated with SF6

at low pressure.

Normally for each pole, a density switch controls the density of the SF6 gas of the HP compartment. The HP and LP compartments are tightened by double gaskets on each SF6-air sealing point.

fig. 1 SB6 lay out

Main data

2

Rated voltage kV 123 145 170 245 BIL kVp 550 650 750 1050 (1200*) Power frequency w/stand kVrms

230 275 325 460 (530*)

Rated frequency Hz

50-60

Rated current A up to 3150 (and 50C ambient) Rated breaking capacity kArms

31,5 40

Operation Single or three phase Duty cycle O 0,3s CO 1 or 3 min CO

3

Total break time ms

< = 50ms

International Standards

IEC

(*)solutions availables for extended insulation withstand requirements

HP (Rated Pressure) COMPARTMENT

This HP compartment has: a well-proven interrupting chamber which applies the principle of self-blowing puffer and it

includes: a stationary contact a mobile contact

main and arcing functions are separated a pressure limiting device ( as an option ) a drying device (molecular sieve)

a supporting column which includes:

an insulated rod which connects the mobile contact with the piston of the operating mechanism.

a filter to prevent decomposition products dust generated by the switching in the SF6 penetrating the operating mechanism.

a HP/LP tightness system

Note: the porcelain elements for both the chamber and the supporting column have been standard designed with creepage distances for different pollution levels according to IEC815. INTERRUPTION PRINCIPLE Fig. A During each tripping operation , the self blast interrupting unit produces a blow of gas between the arcing contacts by a piston integrated into the moving contact. The gas contained inside the puffer cylinder is compressed by the relative movement of cylinder itself in respect of the fixed piston , and forcibly expelled between the contacts. This is known as the self blast effect. The arc core reduces the flow of gas causing an increase in puffing pressure proportionate to the current to be interrupted until the arc is extinguished. SB6 family basic chamber is mechanically thermally and dielectrically designed and type tested for short circuit currents up to 40kA in full compliance with IEC International Standards conditions. Fig. B shows the main components of the breaking parts of an SB6 interrupting unit :

4

1. fixed arcing contact 2. moving arcing contact 3. fixed main current carrying contact 4. moving main current carrying contact 5. puffer piston 6. current transfer contact 7. PTFE nozzle Fig. B

7

5

3

4

2

1

6

LP COMPARTMENT

This compartment is the driving part of the pole. It includes the following components:

a double effect piston which acts as main actuator two groups of valves: one for the opening and one for the closing. Inside each group, a servo-valve drives the main valve, which constitutes the power stage. The opening device is supplied as standard with one tripping coil. A second opening coil is also

available . As a standard, closing and opening coils are rated at 110-125 Vdc; other voltages are

possibles. A bistable (toggle) mechanical safety device (spring lock) firmly keeps the position on opened

or closed. After each operation, a compressor will take care to restore the rated pressure in the

interrupting chamber (HP compartment). This compressor is powered by a single-phase ,230V 50Hz or 115V 60Hz ,supply

In case of DC supply, a static converter is inserted between the power source and the compressors.

An auxiliary contact set which is mechanically connected to the mobile contact. Low voltage plug-in connectors which provide feed-through for signals and power for the

operating mechanism.

5

A mechanical position indicator to show the equipment position (open or closed) directly

connected to the main kinematic chain.

6

Fig. 2 pole section

Closing coil

Tripping Coil(s)

Auxiliary Contacts & mechanical indicator

Compressor

Molecular sieves

Upper terminal

Fixed contact assembly

Upper porcelaininsulator

Moving contact assembly

Rated pressure volume

Lower terminalplate

Insulating rod Filter

Lower Porcelain insulator

Bistable (toggle) device

Closing Valve group

Opening Valve group

Double effect jack

Low pressure volume

View port of mechanical gas filling valve

Pole simplified scheme Rated pressure SF6 Low pressure SF6

Operating cylinder

Mechanical positionindicator

Directly drivenaux.switches

Closing valves group

Tripping valves group

Driving rod

1.2 Operation principle of the circuit-breaker

Each circuit-breaker pole has an integrated operating mechanism driven by SF6 fluid. The high pressure SF6 of the interrupting chamber is used for insulating the live parts, for extinguishing the arc and for operating the circuit-breaker. The driving energy, which is needed to move the contacts, is produced by the pressure difference between the HP and the LP compartments. The HP SF6 is used to move a piston which is mechanically and axially connected to the circuit-breaker mobile contact. Two groups of valves are designed to convey the pressurised SF6 onto one of the piston faces in order to open or close the breaker. The pressure drop which appears in the interrupting chamber after each operation is compensated by a fridge-type sealed compressor, which collects the SF6 in the LP compartment and introduces it into the HP compartment in order to restore the rated pressure. Normally the volumes and pressures are designed to allow, even without the compressor contribution, a stored operation cycle O CO CO.

7

The density switch lodged inside the LP compartment is connected to the HP one and has the task of controlling the SF6 pressure in the interrupting chamber and of driving the compressor. It is equipped with 3 contacts: 1. Compressor driving contact 2. LP closure inhibition contact

3.LP opening inhibition contact A 4th contact can be provided optionally for second trip circuit opening inhibition or reclosure inhibition . A single temperature - compensated pressure switch is sufficient to control both poles compartments (HP & LP) since it has to survey a fixed quantity of gas introduced in the pole . Fig. 3 operation principle of SB6 , gas control and recovery

A

1 1

B 1

18 8

1910

97

2022 11

M

T/P=0

7

8

9

1.3 Procedures for opening and closing

A double effect jack, controlled by two similar groups of valves (solenoid valve + servo-valve + main valve) for switching-on and -off, moves the mobile contact. This jack is driven by HP SF6 which, after the operation, expanded into the LP compartment.

The control valves comprise one set for the closing commands and one set for the opening one. Each set of valves includes a one way solenoid operated valve (or electrovalve) (fig 4 ref A - G) and two pneumatically operated series connected valves ,one called servovalve (B B1) and the other one called main valve (C E). Comparing the two sets of valves , while the electrovalves and servovalves are equal and interchangeables , the main ones are different having ports of different size . In fact during on opening operation the energy is higher then during closing. The servovalve acts as a seal in device which once correctly started by a command from the electrovalve (excitation lasting for several ms) assure the right pneumatic feeding and operation timing independently from electric command duration.

As an option on additional set of opening valves is provided in case an undevoltage opening device is required (see fig.7 ref D1- D2)

The position is kept by means of the bistable (19): at the end of the stroke, the jack is depressurised and the mobile contact is kept in position by the effort produced by the leaf springs of the bistable system.

All the HP SF6, which is required by the driving procedure, passes through the filter (6) which retains the dusty decomposition products that are generated by the electrical arc in the chamber which eventually fall nearby.

1.3.1. Opening sequence [ reference: fig. 4 and 5]

The opening command voltage energizes the solenoid valve (A), the moving armature is picked up against the load of a spring , causing the HP to push back the servo-valve (B) which depressurises the back side of the main valve piston (C).

The main valve (C) commutates and sends HP SF6 , coming from the filter ( 6 ) by conduit ( H ), into the chamber of cylinder and on the upper side of jack piston(D) which is forced downwards causing main contacts to open.

The lower chamber of the jack is kept in communication with the low pressure through the LP gate of the main closing valve (E).

When the voltage to the solenoid valve drops or when near the end of the operation , a contact of the auxiliary switch opens and de-energizes it , its core is released and the moving armature is brought again to its rest positon

At the end of the jack stroke, the solenoid valve is depressurised by a tube which communicates with the low pressure causing the servo-valve to recover its rest position and allowing HP pressure to built up again on the back side of main valve piston .The main valve (C) commutates again and closes the HP gate towards the jack, opens its LP gate and the upper part of the jack is depressurised.

When the jack is depressurised, the bistable (F) which has passed its mid (dead) point, generates a force which firmly keeps the pole in a stable opened position.

10

G

C

F

Opening

valves groupH

E

D

B

A closing valves

group HP compartmentLP compartmentB1

Fig. 4 SB6 main kinematic scheme and principle operating mechanism functions identification Fig. 5 opening operation (partial sequence)

11During opening :op.coil

energized, op. servo-

valve and main opening

Closed steady

state position ;

ready to open

End of opening :op.coil

de-energized, op.

servo-valve and main

12

1.3.3 Closing sequence [ reference: fig. 6 ]

The closing command voltage enegizes the solenoid valve (G) ), the moving armature is

picked up against the load of a spring , causing the HP to push back the servo-valve (B1) which depressurises the back side of the main valve piston (E).

The main valve (E) commutates and sends HP SF6 , coming from the filter ( 6 ) by a conduit ( H ),into the lower chamber of the jack piston (D) which is forced upwards causing main contacts to close.

The upper chamber of the jack is kept in communication with the low pressure through the LP gate of the main closing valve (C).

When the voltage to the solenoid valve drops or when near the end of the operation , a contact of the auxiliary switch opens and de-energizes it , its core is released and the moving armature is brought again to its rest positon

At the end of the jack stroke, the solenoid valve is depressurised by a tube which communicates with the low pressure causing the servo-valve to recover its rest position and allowing HP pressure to built up again on the back side of main valve piston .The main valve (E) commutates again and closes the HP gate towards the jack, opens its LP gate and the lower part of the jack is depressurised.

When the jack is depressurised, the bistable (F) which has passed its mid (dead) point, generates a force firmly keeps the pole in a stable closed position.

1.3.4 Opening sequence via undervoltage opening device (optional)(reference : fig. 7)

Conventional solenoid operated control electrovalves , such as those described before , operates only against energization lasting max hundredths of a second required to initiate a closing or a opening operation , whereupon they remain de-energized for long period of time. When sometimes a breaker control scheme requires the automatically opening on a low or zero auxiliary feeding voltage , an undervoltage tripping control device have to be provided. Of course , such electrovalve is purposely designed to remain energized indefinitely without overheating. An operating mechanism that meets the above requirements has all the control valves described before plus an undervoltage electrovalve (D1) and a monostable valve (D2) connected so as to by pass the conventional opening electrovalve. The monostable valve is a normally closed spring biased , pilot operated valve that opens as soon as its pilot cylinder is set under HP pressure but, even if control pressure would remains applied , recloses automatically after a time metered by a timing orifice. This valve has the task of transforming into a transient the pressure step caused by the de energization of the undervoltage electrovalve and that , would not be removed up to its re-energization. De energizing the coil of D1 causes its armature to shift to closed position under the load of its return spring . This allows the HP gas to pass into the body of D2 moving the plunger against its return spring . The opening of D2 allows the HP gas to enter the opening servovalve and push back its pilot cylinder and the opening sequence to proceed as in the case of previously described conventional opening. At the end of the delay time metered by orifice in D2 bottom plunger , the reclosure of the monostable valve cuts the HP gas connection to the pilot cylinder of the opening servovalve which can in its turn to reclose. Obviously , at the end , of the opening , D2 remains under pressure until after D1 is energized again or its plunger is kept closed by a manual lock ; this last locking feature is an additional option.

Yet this does not prevent the circuit breaker from being operated by conventional electrovalves if their control voltage has not discontinued.

FigOpened steady

state position ;

ready to close . 6 closing operation (partial sDuring closing :closing coil

energized, clos. servo-

valve and main closing

valve commutated 13

equence) End of closing :cl.coil

de-energized, cl. servo-

valve and main closing

valve returned to initial

position ;jack

depressurized.

D1 first stage undervoltage electrovalve D2 2nd stage undervoltage

seal-in valve

Start of opening

sequence : voltage

dropping on uv. coil

Normal condition (underv.coil excited)

Fig. 7 opening operation via undervoltage opening device (partial sequence)

Commutation

of D2

Commutation of

opening

servovalve

Reclosing

of D2

14

End of opening

sequence and ev

re-energizing of uv.co

entual

il

1.4 Keeping the SF6 pressure level in the HP compartment

The manufacturing tolerances are such that the possible bleeding of SF6 from the HP compartment to the LP compartment is kept below an acceptable and controlled level. This seldom causes the SF6 density switch to start the compressor in order to restore the rated pressure in the HP compartment. The SF6 is delivered into the HP compartment through an oil separator and a reverse-locking valve. Since there are not dynamic SF6 - air seals but only double static ones , the reliability of gas tightness towards the environment is greatly improved.

1.5 Emergency operations

In case of failure of the auxiliary control voltage, the operation of the breaker can be electrically initiated

thanks to the stored energy of a capacitor-rectifier combination that can be included in the control

cabinet. This enables the three-phase operation (tripping or optionally closing) without auxiliary voltage.

This device can be fed either by AC or DC source ;it permits by means of interlocking , to operate the

circuit breaker with quenching and driving gas pressure into normal range.

Optionally the restoring of the operating energy of the mechanism is also possible, in case of

emergency, by the use of a portable inverter to be operated with a car battery (12 24V).

In case of failure of the auxiliary voltage, the voltage taken by the car battery is converted, by this

portable converter, into ac which can be used, through a suitable connection in the control cabinet, to

operate one compressor at a time until the rated pressure, and eventually to recharge the capacitor.

The operation of the breaker is then possible either by auxiliary DC supply or by stored energy.

15