VACUUME CIRCUIT BREAKER (VCB)

MAKE	Jyoti Ltd. Vadodara
Type	UY – 30M25
Rated frequency	50Hz
Rated Voltage	36000 Volts(36KV)
Rated Current	1250 Amps
Ms Level Imp	170KV P, PF-75 kv
Rated Breaking current	25 KA
Rated Making Current	63 KA peak
Sr. No.	B-242, Year- 2001
Supply Voltage Closing	110 VDC
Supply Voltage Tripping	110 VDC
Rated short time current	25 KA
Wt of Beaker	375 Kg
Standard	IS:13118/IEC-56
Making Capacity	2.55 x symmetrical ÷ Breaking capacity
Breaking Capacity	1000 x 106/√3 x 33000

ADVANTAGE OF VCB

1.      They are compact, reliable and have longer life

2.      There are no fire hazards

3.      There is no generation of gas during and after operation

4.      They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position

5.      They require little maintenance and are quiet in operation

6.      They can successfully withstand lightening surge.

7.      They have low arc energy

8.      They have low inertia and hence require smaller power for control mechanism.

9.       

DISADVANTAGE OF VCB

1.      Air chopping/current chopping

2.      Vacuum leakage cannot be indentified

3.      Costly

MAKING CAPACITY: Breaker closes on fault condition, contacts does not damage. Current on which a ckt breaker is capable of breaking under specified condition.

BREAKING CAPACITY: Breaking current capacity of circuit-breaker, is the highest value of short circuit current which a circuit breaker is capable of breaking under specified condition.

In the circuit breaker, breaking capacity is less than making capacity: When short circuit occurs, there will be voltage drop. Thus voltage is less, current is also less. So Coil temperature in the circuit breaker = 80⁰.

We are having OCB.VCB with low resistance zero common method. For zero method, contact, material is used.

When breaker is switching on & off, spark takes place & due to hydrogen gas heat produced. To cool this heat and quenching of arc, vacuum is used.

High Vacuum: 10^-5 to 10^-8 Tor

PRINCIPLE OF VACUUM CIRCUIT BREAKER

The production of arc in vacuum circuit breaker and its extinction can be explains as follows:

When the contacts of the Breaker are opened in vacuum (10^-7 – 10^-5 torr) and arc is produced between the contacts by ionization of metal vapor of contacts. However, the arc is quickly extinguished because the metallic vapors, electrons  and ions produced during arc rapidly condenses on the surfaces of the circuit breaker contacts resulting in quick recovery of dielectric strength. The reader May note that the salient feature of vacuum as an arc quenching media. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

MCB ( MINIATURE CIRCUIT BREAKERS)

Ø  “L” – SERIES : GREEN KNOB  MCB – FOR LIGHTING

Ø  “G” – SERIES: BLUE KNOB MCB      - FOR MOTOR

Ø  DC – SERIES  : BLACK KNOB              - FOR DC CIRCUIT

Ø  ISOLATOR     : RED KNOB                   -   

Features:

1.      At the short ckt, it does not allow to flow fault current fully

2.      Contacts get opened within 1 m/sec.

3.      Operating time is less that 5 m/sec.

ELCB

TYPES OF BREAKERS

LOW VOLTAGE:

1.      Miniature circuit breaker

2.      Molded case circuit breaker

3.      Residual case circuit breaker

4.      Power breakers

HIGH VOLTAGE:

1.      Air blast circuit breaker

2.      SF6 – (sulfur)  breaker

3.      Vacuum circuit breaker

4.      Oil circuit breaker

5.      Minimum oil circuit breakers.

NPCC

MAIN INCOMER BREAKER – USED RELAY NPCC – LV

1.      Inverse type Under voltage relay : Type: VAGM – 2 Nos, Range: 44 to 88 Volts

2.      Over Current Relay(Inverse): Type CDG – 2 Nos., Range: 2.5 to 10 Amps

3.      Earth Fault Relay(Inverse): Type CDG – 2 Nos., 0.5 to 2 Amps.

4.      Instantaneous Earth Fault Relay: Type CAG, Range: 0.5 to 2 Amps

5.      Tripping Relay : Type: VAJ, Range 110 to 125 Volts,

6.      Trip Circuit Supervision Relay: Type – VAX MK11

SIEMENS BREAKERS

BASIC PROTECTIVE FUNCTIONS

v  The basic protective functions of the over current release are ensured without additional auxiliary voltage. The required power is supplied by internal transformer of the circuit breaker.

v  To evaluate arc current, the electronic system of the over current release calculates the RMS value.

OVERLOAD PROTECTION

v  The current setting Ir defines the maximum continuous current. The circuit breaker can carry without tripping. The time lag class to determines the maximum duration of an over load without tripping

ETU – 45B – IR (0.4/0.45/0.5/0.55/0.6/0.65/0.7/0.8/1.0) X I_n.

BASIC PROTECTIVE FUNCTIONS OF ACB

v  Over Load Protection – L Tripping

v  Short Time Delay – Short circuit protection – S Tripping

v  Instantaneous short circuit protection – I Tripping

v  Neutral conductor protection – N Tripping

v  Earth fault tripping 1. Alarm setting.

SHORT TIME DELAY:SHORT CIRCUIT TRIPPING – S TRIPPING

v  Over current release ETU 25-B-76B tripping due to the short circuit current Isd can be delayed by the time tsd.

This provides selectivity for the short circuit protection in switch gear with several grading levels.

ETU – 25B, 45B – Isd (1.25/1.5/2/2.5/3/4/6/8/10/12) X I_n.

SETTING FOR tsd

ETU – 45B – tsd=0.02(M)/0.1/0.2/0.3/0.4 Si off

v  The time delay 0.02s is not a grading time. In this protection, the motor protection function is activated.

v  For setting tsd >0.4s the maximum possible setting Isd is reduced automatically according to frame size.

Frame size : 1.5 KA, 20 KA, 30KA. with the setting “off”. For the over current release ETU 45B – 76B is provided to de-activate the short time delay short circuit protection.

If zone selective inter locking ZSI -> is used however, the setting for the time delay tsd is deactivated. If the circuit breaker does not receive any blocking, signal from a down streak circuit breaker, it will trip after 50 ms regardless of the setting for tsd.

MOTOR PROTECTION FUNCTIONS

v  In the circuit breaker position tsd = (M) (0.02), a special protection function for electromotive drive is activated. It prevents the short time delay short circuit tripping. From being activated during the peak inrush current of electric motors. At the same time phase failure protection is activated and time constant for the internally calculated reproductive of the temperature rise and cooling process is switched over from switch gear protection to motor protection.

33KV PANEL RELAY PROTECTION

TYPE OF RELAYS

v  TRIV-VECTOR METER ELT

v  OVER VOLTAGE RELAY TYPE VIC

Definite time – ALStom – 50A (z)

v  INSTENSTENEUS UNDER VOLTAGE RELAY

Setting -> Type VAGM, 44 to 88 Volts

Specified ratings of an AC circuit breaker:

1. Symmetrical Breaker capacity: The breaking capacity in symmetrical condition is known as symmetrical breaking capacity which is generally expressed in terms of MVA and is equal to the product of rated symmetrical breaking current (kA), rated service voltage (kV) and  a factor depending upon the number of phases (1 for single phase and √3 for three phase).

2. Asymmetrical breaking capacity: The breaking capacity in asymmetrical condition is known as asymmetrical breaking capacity which is also expressed in terms of MVA and is equal to the product of rated asymmetrical breaking current (kA), rated service voltage (kV) and a factor depending upon the number of phases(1 for single phase and √3 for three phase).

3. Making capacity: The making capacity is term used in connection with the rating of a switchgear to denote the capability or a switch, circuit breaker or similar apparatus to make a circuit by closing under short circuit condition. Making capacity is approximately equal to 2.55 times of symmetrical breaking capacity.

4. Short time rating: The short time rating of a circuit breaker is that highest permissible value of the current which the fully closed circuit breaker can carry for a specified time of about 1 to 3 seconds without excessive temperature rise and electro-dynamic effect capable of causing injury.

5. Rated normal current: The rated normal current of a circuit breaker is the r.m.s. value of the current which the circuit breaker shall be able to carry at normal frequency and at the rated voltage continuously under specified conditions.

6. Rated service voltage: The rated service voltage of a circuit breaker is the r.m.s. value of the operating voltage at which the circuit breaker shall be able to carry the normal current continuously at normal frequency.

 Rated operating sequence:   O(open)-t-CO(close open)-T-CO    (t=0.3 or 0.5 sec., T=3minutes)

Circuit breaker designed to have a short time rating: In the power system sometimes a very temporary nature of fault persists for a short period of 2 or 3 seconds and after which the fault is automatically cleared. To maintain continuity of supply the breaker should not trip in such condition. Therefore, breaker should be able to carry high current safely for some specified period during running i.e. the circuit breakers should have a short time rating.

Symmetrical fault: In a three phase power system when all the three conductors are brought together simultaneously into a short circuit condition giving rise to equal fault currents with 120⁰ displacement symmetrical currents to flow through the lines then this fault is called as symmetrical fault.

Short circuit kVA: The product of normal system voltage and short circuit current at the point of fault expressed in kVA is called as short circuit kVA.

Unsymmetrical fault In a three phase power system when single line to ground or line to line or double line to ground fault occurs giving rise to unequal fault current with unequal displacement i.e. unsymmetrical currents to flow through the lines then this fault is called as unsymmetrical fault.

Reactor:  A reactor is generally an air cored coil of large number of turns having high value of constant inductive reactance as compared to its ohmic resistance in which magnetic saturation at high current does not reduce the coil reactance.

Making current & Breaking current:

a) Making current of a circuit breaker is the total maximum current peak which occurs during the first cycle immediately after the circuit  is closed on a short circuit.

b) The Breaking current of a circuit breaker is the total maximum current peak that can be safely broken by the circuit breaker at the time of separation of the contacts at its rated voltage.

Is a 50 Hz circuit breaker suitable for 60 Hz supply? Can 12 kV C.B. be used for 6/6 kV application?

a. It is possible only when the breaker is type tested at 60 Hz frequency, otherwise not.

b. A 12 kV circuit breaker can be used for any voltage up to its rated voltage under the condition that the breaking current is within its breaking capacity.

The  difference between low voltage circuit-breaker and high voltage circuit-breaker.

Low voltage circuit breakers are used in indoor control gears for controlling power consuming devices of circuits below rated voltage 1000 volts AC  or 1200 volts DC whereas high voltage circuit breakers are used in both indoor and outdoor control-gears in high voltage supply system. Low voltage circuit breakers operate frequently than high voltage circuit breaker.

Low voltage circuit breakers are more compact than high voltage circuit breakers because of less phase to phase and phase to ground clearances. Testing requirements of high voltage circuit breaker are quite different from those of low voltage circuit breakers.

CLASSIFICATION OF HIGH VOLTAGE CIRCUIT BREAKERS

a) Oil circuit breaker: i) Bulk oil circuit breaker, ii) Minimum oil circuit breaker

b) Oil-less circuit breaker:

i) Air blast circuit breaker,  ii) SF₆ circuit breaker, iii) Vacuum ckt. Breaker, iv) Air break ckt breaker

A) For low & medium voltage installation the following types of circuit breakers are used:

a) Air break with magnetic blow out circuit breaker

b) Bulk oil circuit breaker

B) For high voltage installation the following types of circuit breakers are used:

a) Bulk oil circuit breaker, b) Minimum oil circuit breaker,

c) Air break with magnetic blow out circuit breaker, d) Air blast circuit breaker,

e), Sulphur hexafluoride circuit breaker, f) Vacuum circuit breaker

C) For extra high voltage installation the following types of circuit breakers are used:

66kV: Minimum oil, SF6 and Vacuum circuit breakers

132kV & 220 kV: SF6 and Vacuum circuit breakers.

AIR BREAK CIRCUIT BREAKER: The circuit breakers in which the air at atmospheric pressure is used as an arc quenching medium, is called air break circuit breaker. These are generally indoor type and installed on vertical panels or indoor draw out type switchgears. These are used in DC circuits and AC circuits up to 12 kV. AC air break circuit breakers are widely used in indoor type medium & low voltage switchgears.

BULK OIL CIRCUIT BREAKERS: The bulk oil circuit breakers are large volume oil circuit breakers consisting of contacts and arc control systems arranged inside the earthed metal tank filled with a considerable quantity of insulating oil and operating mechanism mounted outside the tank. These are widely used for the both, indoor & outdoor transformer substation, arc furnace, railway stations, large motors of AC power system.

MINIMUM OIL CIRCUIT BREAKERS: The minimum oil circuit breakers are small volume oil circuit breakers in which the contacts are separated in tube shaped tank of insulating material like porcelain filled with a relatively small quantity of oil and held between the metal end terminal caps for the external circuits. It has having less volume of oil because solid insulation is provided in a minimum oil circuit breaker for insulating the contacts from earth. These are used in high and extra high voltage outdoor substation. Closing and opening operation can be done by manual operation and by remote control operation either by motor drive or pneumatic drive. c

AIR BLAST CIRCUIT BREAKER: The air blast circuit breaker is nothing but a compressed air circuit breaker in which high pressure air is forced on the arc through a nozzle at the instant of contact separation to blow away the ionized medium between the contacts and thereby extinguishing the arc by the blast of compressed air supplied from an air reservoir. Air Blast circuit breaker is operated at the pressure of 20 to 30 kg/cm². These breakers are generally used in outdoor substations of high voltage system specially for railways and inter connected lines where frequent switching operation is the principal requirements.

Merits of these breakers:

i) Air blast circuit breaker operates at high speed maintaining better stability.

ii) Arcing time is very short.

iii) The life of the breaker is increased due to lesser damage to the contact points.

iv) This can be given rapid auto re-closure feature.

v) The possibility of fire hazards is very less.

vi) Due to absence of oil less maintenance is required.

vii) It has very high breaking capacity and service voltage in extra high range can be obtained by connecting more number of units in series.

viii) Frequent operation creates no problems.

Demerits:

i) High capacity compressor plant is required.

ii) The complex design of arc quenching chamber and operating mechanism creates problems of switch over voltages.

iii) Special attention is required for proper operation and maintenance of compressors and its accessories.

iv) Resistance switching is required for reducing switch over voltages and current chopping.

SF₆ CIRCUIT BREAKER:

The circuit breakers in which an inert electronegative gas sulphur hexafluoride(SF₆) is used as dielectric and arc quenching medium, is known as SF₆ circuit breaker. They are similar is design to the air blast circuit breakers with the difference that the blast of air is replaced by that of sulphur hexafluoride gas. These breakers are used mainly in EHV outdoor substation. SF₆ gas used in circuit breaker is transported in liquid form at higher pressure and lower temperature in a sealed cylinder to site. SF₆ gas becomes liquid at lower temperature and higher pressure. At ambient temperature the SF₆ can not be used above 8 kg/cm².

Merits:

i) The dielectric strength is higher than air and oil.

ii) The arc extinction is very rapid.

iii) The contact corrosion is very little due to inertness of SF6 gas. Hence contacts do not suffer oxidation.

iv) The maintenance & replacement of contacts required are minimum.

v) It has ample over load margin.

vi)Since SF6 is non inflammable and chemically stable the decomposition products become non explosive thus eliminating the risk of fire or explosion.

vii) It has noiseless operation.

viii) It has no over voltage problem

ix) The contamination by moisture, dust, sand etc is avoided due to its sealed construction.

x) The requirement of SF6 gas is very small as the same gas is re-circulated in the circuit.

xi) It has ability to interrupt low and high fault current, magnetizing and capacitive current.

xii) It is simple, non toxic, less costly, maintenance free and compact with high interrupting capacity.

Demerits:

i) It has low breaking capacity,

ii) Imperfect joint may cause a leakage at gas.

iii) The presence of moisture in the gas may cause severe failure.

iv) Arced gas is very poisonous to human breathing.

v) It needs special materials for sealing to avoid gas leakage and moisture contamination.

Classification of circuit breaker contact according to their arrangement:

i) Stationary contacts which constitute a permanent electrical connection and do not move in relation to each other during operation.

ii) Switching contacts which make, break or change the connections of circuits carrying an electric current.

iii) Sliding contacts, where one of the contact members slides over the other one without interrupting their electrical contact.

Classification of circuit breaker contact according to their shape of the contact faces:

i) Point contacts in which the current carrying members come into contact only at a single point.

ii) Line contacts in which the current carrying members nominally come into the contact along a line.

iii) Plane contacts in which the current carrying members nominally touch each other over a plane of some configuration.

Reasons of formation of arcs in a circuit breaker.

When the contacts of the circuit breaker are separated during operation high local temperatures are created on the contact surface, which constitute electrons emission from contact surface by thermal emission. The gas or air molecules between contact space become extremely hot and get ionized by thermal ionization and ionization by collision. The normal insulation property of the medium is destroyed and the hot molecules of the medium become conductors of electricity. Thereby due to high speed electron movement arc discharge takes place between the contacts.

Advantages of having oil in the circuit breaker:

i) The transformer oil used in a circuit breaker is a very good insulator and thus reduces the clearance between the live parts and earth.

ii) It absorbs the heat energy of the arc by decomposition of the oil.

iii) The gases so formed by the decomposition of oil(chiefly hydrogen) have good cooling properties.

iv) The oil has a high dielectric strength.

v) The cold oil has the ability to flow into the arc space after the arc current goes to zero.

Disadvantages of having oil in the circuit breaker:

i) Oil is inflammable.

ii) The oil absorbs moisture rapidly. The dielectric strength is reduced by the absorption of moisture and also by carbonization occurred during arcing.

iii) In case of frequent operation of the breaker oil is most unsuitable.

iv) For leakage, losses, replacement and purification of oil it needs regular maintenance.