LV MV EHV Breaker Maintenance PRESENTED BY PROF

LV, MV & EHV Breaker Maintenance PRESENTED BY PROF. VG PATEL AT THE TATA POWER COMPANY LIMITED, THERMAL POWER STATION, MAHUL, MUMBAI - 400 074 ON 20/12/2018 02 March 2021 VG PATEL 1

BREAKER MAINTENANCE LV, MV & EHV Breaker Maintenance 02 March 2021 VG PATEL 2

BREAKER MAINTENANCE An overview Production units Generator Transformers Overhead lines Underground cables MV/MV distribution substations HV/MV substations Motors 02 March 2021 Industries VG PATEL MV/LV distribution substations 3

BREAKER MAINTENANCE What is a circuit breaker? A circuit breaker is an apparatus in electrical systems that has the capability to, in the shortest possible time, switch from being an ideal conductor to an ideal insulator and vice versa. Furthermore, the circuit breaker should be able to fulfill the following requirements: In the stationary closed position, conduct its rated current without producing • Impermissible heat rise in any of its components. • Able to withstand any type of over voltages within its rating. • The circuit breaker shall, at its rated voltage, be able to make and break any possible current within its rating, without becoming unsuitable for further operation. 02 March 2021 VG PATEL 4

BREAKER MAINTENANCE CIRCUIT BREAKERS It is meant for: Ø Make and break duty Ø To clear the faults automatically within its rupturing / breaking capacity. Ø To be switched on to a fault within its designed parameters. 02 March 2021 VG PATEL 5

BREAKER MAINTENANCE Device to open/close electric circuit Ideal conductor in closed position Ideal insulator in open position 02 March 2021 VG PATEL 6

BREAKER MAINTENANCE Some basic definitions Circuit breaker: Switching and current interrupting device Switchgear: All equipments associated with the fault clearing process Control gear: Used for switching and controlling power consuming devices Fault: A defect in its electrical circuit due to which current is diverted from its intended path 02 March 2021 VG PATEL 7

BREAKER MAINTENANCE What do we mean by fault? Fault is nothing but any kind of abnormality. Whenever current flows in the unwanted device or unwanted place, we call it a Fault. Any undesired thing in the system such as Over voltage or Under voltage condition OR Over / Under frequency condition, we call it a Fault. 02 March 2021 VG PATEL 8

BREAKER MAINTENANCE Electrical system shall tolerate certain degree of faults. Usually faults are caused by breakdown of insulation due to various reasons: system over current (as a result, temperature will increase causing breakdown of insulation), over voltage, lightning, etc. Usually, faults are considered dangerous because of over current that they create. This can damage the apparatus and it endangers the human safety. Consequences of faults: Thermal and electro-dynamic damages to electrical equipments due to heavy fault currents, voltage dip, loss of revenue, loss of production and the worst of all, loss of power system stability. 02 March 2021 VG PATEL 9

BREAKER MAINTENANCE 02 March 2021 VG PATEL 10

BREAKER MAINTENANCE ARC PHENOMENON: ØThe ARC is an ionized gas devoid of a few electrons. Ø Electrons being negatively charged & lighter are attracted towards anode very rapidly compared to slow movement of much heavier positive ions towards cathode. Ø The fast electron movement causes current flow and continuance of the ARC until its extinction. Along with arc energy, CB pole is subjected to huge electromechanical forces. Hence, CB must be designed to contain the arc energy within for safety of operator and surroundings and must be strong enough to withstand electro-mechanical forces. Initiation of an ARC: When breaker contacts begin to separate, an ARC is struck between them due to field and thermal emission of electrons. . 02 March 2021 VG PATEL 11

BREAKER MAINTENANCE I. FIELD EMISSION: The voltage gradient is very high of the order of 107 V/cm at the instant of contact separation when the distance is infinitesimally small, results in electron emission. II. THERMAL EMISSION: The electron emission caused by high temperature generated due to high local current densities of the order of 106 A/cm 2. III. MAINTENANCE OF ARC: The processes of field emission, thermal emission and ionisation maintain the ARC. 02 March 2021 VG PATEL 12

BREAKER MAINTENANCE IV. ARC EXTINCTION: There are two methods of interrupting the ARC current. i. High resistance or rheostatic interruption applicable to L. V. and M. V. CBs. ii. Low resistance interruption or current zero interruption applicable to H. V. A. C. CBs. a. The resistance is increased by: i. 1 Lengthening the ARC. i. 2 Cooling the ARC. i. 3 Splitting the ARC. i. 4 Constraining the ARC. By increasing the resistance, ARC current is reduced to a value where ARC cannot sustain and extinguishes itself. 02 March 2021 VG PATEL 13

BREAKER MAINTENANCE b. Low resistance i. e. current zero interruption. ii. 1 Applicable to H. V. A. C. CBs. ii. 2 Resistance is kept low. ii. 3 At natural current zero, the ARC cannot sustain. ii. 4 Hence, extinguishes itself without being able to re-strike the ARC in spite of high re-striking voltage. 02 March 2021 VG PATEL 14

BREAKER MAINTENANCE n Isolation Basics functions n Control n Protection disconnector contactor circuit breaker 02 March 2021 VG PATEL 15

BREAKER MAINTENANCE Basics functions Isolation Positive break indication Visible break Control Functional Emergency Breaking for maintenance Protection Overload Short circuit Insulation faults 02 March 2021 VG PATEL 16

BREAKER MAINTENANCE Types of Circuit Breakers: Circuit breaker types can be classified according to their characteristics like voltage class, construction type, interrupting type, and structural features. • Low Voltage Circuit Breakers: These breakers are made for direct current (DC) applications and are commonly used in domestic, commercial, and industrial fields. They can be installed in multi-tiers in LV switchboards or switchgear cabinets. Low voltage circuit breakers are usually placed in draw-out enclosures that permit removal and interchange without dismantling the switchgear. Miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) are some common types of low voltage circuit breakers. 02 March 2021 VG PATEL 17

BREAKER MAINTENANCE • Medium Voltage Circuit Breakers: These breakers can be assembled into metal enclosed switchgear line ups for indoor applications, or as individual components for outdoor applications like substations. Medium voltage circuit breakers use discrete current sensors and protection relays, and can be attached into the circuit by bolted connections to bus bars or wires. Vacuum circuit breakers, air circuit breakers and SF 6 circuit breakers are some examples of medium voltage circuit breakers. • High Voltage Circuit Breakers: These breakers help in protecting and controlling electrical power transmission networks. They are solenoid operated and are employed with current sensing protective relays that function through current transformers. 02 March 2021 VG PATEL 18

BREAKER MAINTENANCE LOW VOLTAGE CB: 1. MINIATURE CIRCUIT BREAKER - MCB 2. EARTH LEAKAGE CIRCUIT BREAKER - ELCB 3. MOULDED CASE CIRCUIT BREAKER - MCCB 4. COMBINATION OF MCB & ELCB - ELMCB 5. OFF LOAD CHANGE OVER SWITCH 02 March 2021 VG PATEL 19

BREAKER MAINTENANCE 02 March 2021 VG PATEL 20

BREAKER MAINTENANCE 02 March 2021 VG PATEL 21

BREAKER MAINTENANCE Circuit Breaker Development …the sixties …the seventies & the eighties …the eighties & the nineties Air Blast Oil SF 6 Gas 02 March 2021 VG PATEL 22

BREAKER MAINTENANCE Circuit breaker for 145 k. V 02 March 2021 Circuit breaker for 420 k. V VG PATEL 23

BREAKER MAINTENANCE The Circuit Breaker can be classified on the basis of the arc extinction: • MCB • MCCB • ELCB • FUSES • Isolator • OCB • MOCB • BOCB • ABCB • ACB • SF 6 CB • VCB 02 March 2021 Miniature CB Moulded Case CB Earth Leakage CB (Off Load) Oil CB Minimum Oil CB Bulk Oil CB Air Blast CB Air CB or Air Break CB Sulphur Hexafluoride CB Vacuum CB VG PATEL 24

BREAKER MAINTENANCE CLASSIFICATION OF CIRCUIT BREAKERS a. LV & MV. CBs: I. H. R. C. Fuses (High Rupturing Capacity). II. A. C. B. (Air Circuit Breakers or Air Break CB). III. B. O. C. B. (Bulk Oil Circuit Breaker). IV. M. C. C. B. (Moulded Case Circuit Breaker). b. H. V. CBs: I. O. C. B (Oil Circuit Breaker). I. 1. B. O. C. B. (Bulk Oil Circuit Breaker). I. 2. M. O. C. B. (Minimum Oil Circuit Breaker). II. A. B. C. B. (Air Blast Circuit Breaker) III. V. C. B. (Vacuum Circuit Breaker) IV. SF 6 C. B. (Sulphur Hexafluoride CB) 02 March 2021 VG PATEL 25

BREAKER MAINTENANCE Type Medium Voltage Breaking Capacity ACB Air at atmospheric pressure (430 -600)V, (5 -15)MVA (3. 6 -12)KV, 500 MVA MCB Air at atmospheric pressure (430 -600)V Tank type OCB Dielectric Oil (3. 6 -12)KV MOCB Dielectric Oil (3. 6 -145)KV ABCB Compressed Air (20 245 KV, 35000 MVA upto 1100 -40) bar KV, 50000 MVA SF 6 CB SF 6 Gas 12 KV, 1000 MVA, 36 KV, 2000 MVA, 145 KV, 7500 MVA, 245 KV, 10000 MVA Vacuum CB Vacuum 36 KV, 750 MVA HVDC CB Vacuum, SF 6 Gas 500 KV DC 02 March 2021 VG PATEL 26

BREAKER MAINTENANCE Circuit breaker – Main components 02 March 2021 VG PATEL 27

BREAKER MAINTENANCE Basic Operation of Circuit Breaker Operating Principle: It essentially consists of fixed and moving contacts, called electrodes. When a fault occurs on any part of the system , the trip coils of the circuit breaker get energized through relays and the moving contacts are pulled apart by some mechanism, thus opening the circuit. 02 March 2021 VG PATEL 28

BREAKER MAINTENANCE Basic Operation of Circuit Breaker Contd… When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus enable to continue until the discharge ceases. The production of arc not only delays the current interruption process, but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. The main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. 02 March 2021 VG PATEL 29

BREAKER MAINTENANCE Fuse Vs CB • • • Fuses clear S/C faster Cost is very low Very simple in construction Non reusable type Require switch. Isolator to cut off or switch on supply • Rupturing capacity is lower • Earth fault coordination is difficult 02 March 2021 • • • Takes longer time to clear fault High cost Sophisticated Mechanism Reusable type Require protective relays for fault clearance • Rupturing capacity is higher • Relay coordination is not difficult for all type of fault VG PATEL 30

BREAKER MAINTENANCE Maintenance of Circuit Breakers: Frequency of maintenance depends on the following considerations: Ø Non polluted area Ø Polluted area Ø Highly polluted area Ø Gaseous area Ø Chemical area Ø Acidic area Ø High temp. area Ø Duty cycle Ø Loading pattern Ø Type of Switchgear, etc. 02 March 2021 VG PATEL 31

BREAKER MAINTENANCE MAINT. OF VACUUM CIRCUIT BREAKERS: (Prepare a check list) Quarterly General cleaning, oiling & greasing as per manufacturer’s guidelines. Half yearly Ø All the above Ø Replace worn out parts by original spares. Yearly Ø All the above Ø Take and record I. R. value Ø Measure contact resistance Ø Check for properation 5 yearly Ø Thorough overhauling by complete dismantling & re-assembling. (Caution: vacuum bottle should never be opened) 02 March 2021 VG PATEL 32

BREAKER MAINTENANCE MAINT. OF SF 6 CIRCUIT BREAKERS: (Prepare a check list) Quarterly Ø General cleaning, oiling & greasing as per manufacturer’s guidelines. Ø Check gas pressure, if low, build up pressure. Half yearly Ø All the above Ø Replace worn out parts by original spares. Yearly Ø All the above Ø Take and record I. R. value Ø Measure contact resistance and replace if damaged. Ø Check for properation. Ø Collect gas sample. (IEC 376) & circulate for 10 to 20 hrs. through filters containing activated alumina which absorbs impurities, moisture etc. 5 yearly Ø Thorough overhauling by complete dismantling & re-assembling. 02 March 2021 VG PATEL 33

BREAKER MAINTENANCE MAINT. OF HV AND EHV MOCBs (Prepare a check list) Weekly Ø Check oil level, leakage if any, and take corrective measures. Ø Check Silica gel breather if provided. Monthly Ø Check all the above and heaters. Quarterly Ø General cleaning, oiling & greasing as per manufacturer’s guidelines. Ø Check gas vent & oil dash pot if provided. Ø Check for properation. (Mech. , Elec. , Trips. , Local & Remote, etc. ) Ø Check dielectric strength of oil. Half yearly Ø All the above Ø Replace worn out parts by original spares. 02 March 2021 VG PATEL 34

BREAKER MAINTENANCE MAINT. OF HV AND EHV MOCBs Contd…. Yearly Ø All the above Ø Take and record I. R. value Ø Measure contact resistance and replace if damaged. Ø Check for properation & spring charge motors. 5 yearly Thorough overhauling by complete dismantling & re-assembling by replacing worn out parts by original spares. 02 March 2021 VG PATEL 35

BREAKER MAINTENANCE TIPS 02 March 2021 VG PATEL 36

BREAKER MAINTENANCE CB needs maintenance as it has lot of moving parts and sophisticated mechanism. v. Frequency of maintenance § As per recommendation of manufacturer §Opportunity based maintenance §Condition monitoring based v Maintenance and inspection of CB must be carried out after fault clearance. v Check condition of fixed, moving and arc contacts. v Remove carbon dust accumulated form contacts and arc chute with suitable solvent. 02 March 2021 VG PATEL 37

BREAKER MAINTENANCE v. Do not use sand paper or Amery paper for cleaning and removal of pitting on contacts. v. If required, replace them by new contacts. v. Do not use refurbished contacts. v. Take carbon prints for checking area of contact. v. Lubricate mechanism linkages as per recommendation by manufacturer in manual. v. Use lubricant as per recommendation in manual. v. Carry out maintenance by well trained engineer at manufacturer works or by competent person from manufacturer. v. SF 6 and Vacuum CB bottles are maintenance free. If inspection is felt necessary, call competent engineer from manufacturer. 02 March 2021 VG PATEL 38

BREAKER MAINTENANCE v. Do not over lubricate. v. Measure closing and tripping time after maintenance. v If this timing is higher than recommended value, call qualified and trained service engineer. v Take tripping trial in test position by simulating all trip conditions and by dosing trip contacts of all protective relays. v Measure mili-volt drop across contacts to ensure health of contacts. v For MCCB, ensure that it is suitable for mounting in any direction. v If MCCB used for generator, ensure that it is suitable for reverse current direction. v Inspect contact and measure Mili-volt drop across contacts after clearance of fault before putting it in service. 02 March 2021 VG PATEL 39

BREAKER MAINTENANCE OPEN FORUM 02 March 2021 VG PATEL 40

BREAKER MAINTENANCE THANQ 02 March 2021 VG PATEL 41

BREAKER MAINTENANCE 02 March 2021 Avjo VG PATEL 42