WELCOME PRESENTATION ON CB
CIRCUIT BREAKERS IMPORTANCE OF CIRCUIT BREAKER • Circuit breakers are used for automatic isolation of fault in very short duration after receiving the command from relay.If the fault is not cleared in time it may lead to disturbance in the system. • Main Parts Of The Circuit Breaker 1. INTERRUPTING UNIT ,PIR,GRADING CAPACIOR • 2. POLE COLUMN • 3. OPERATING MECHANISM.
MAINTENANCE OF CIRCUITBREAKERS 1. CONDITION BASED – Testing and maintenance will be carried out to know and access the life of equipment. 2. TIME BASED – Inspection of the equipments is carried out in pre determined time like Monthly, Half yearly, Yearly,Two Yearly,Three Yearly.
TESTING OF CIRCUIT BREAKER 1. OPERATING TIMINGS ,ANALYSIS • 2 DEW POINT MEASUREMENT • 3 OPERATIONAL LOCKOUTS • 4 DCR MEASUREMENT • 5 TRAVEL MEASUREMENT • 6 COIL CURRENT MEASUREMENT • 7 PIR TIMTINGS • 8 C&TAN DELTA • 9 STATIC CONTACT RESISTANCE
OPERATING TIMINGS • operating timings are monitored to indicate any problems in operating mechanisms,alignments of main/arcing contacts and also difference between two breaks of the same breaker pole.If these timings are not within order,it may lead to over stressing of one particular interrupting chamber.
PRE INSERTION RESISTANCE Pre insertion resistance timings of CB monitored to indicate any problem in close/open timings of the resistance with respect to the main contacts. PIR time before main contact closing,PIR over lap time with main contact,PIR time before opening of the main contact.
PRE INSERTION RESISTOR • 420 KV circuit breakers in Powergrid’s 400KV transmission lines are presently equipped with 400 ohm insertion resistance • The purpose of the insertion resistors are to reduce transient over voltages during switching operations.
PRE INSERTION RESISTOR • The insertion resistors are required additional maintenance ,and Powergrid found that the insertion resistors are the most vulnerable part of the CB. • Recently number of CBs have failed due to failure of the insertion resistance operation.
PRE INSERTION RESISTOR • Based on above Powergrid wishes evaluated whether the insertion resistor may be eliminated from any of the CB. • A worst model was first created .energizingand single phase reclosing simulations were performed on both single and double circuit lines with line length between 100 and 500km with 390 kv zno arresters.
DEW POINT MEASUREMENT • Dew point measurement of SF6 gas in circuit breakers indicates the change in the value of the dielectric properties of SF6 gas.the dielectric strength of SF6 gas may change due to mixing of moisture and decomposition products of SF6 gas.
EFFECTS OF CONTAMINATION • DECOMPOSITION PRODUCTS ARE CORROSIVE AND MAY AFFECT THE SURFACE INSULATION BY THE FORMATION OF CONDUCTIVE LAYERS ALONG THE INSULATORS. • HUMIDITY/MOISTURE IS VERY DANGEROUS WHEN IT CONDENSES ON THEM IN LIQUID FORM CAUSING REDUCED INSULATION STRENGTH • CUF2 ORIGINATE FROM CONTACT EROSION. • HEALTH RISK DUE TO BYTOXIC PRODUCTS LIKE SO2 ETC.
DEW POINT MEASUREMENT • There are two sources of moisture ingress in SF6 gas after filling in C.B and during o&m: • 1) exudation of moisture contained during manufacturing from insulation materials used in circuit breakers. • 2) Permeation of moisture through sealed sections i.e.gaskets,o rings etc.,
SOURCES OF MOISTURE IN CB • GAS HANDLING – FILLING AND EMPTYING THE CB. AIR MAY BE LEFT DURING EVACUATION WHICH SHALL ADD TO IMPURITIES IN SF6 GAS. EVACUATION TO BE DONE UPTO 1 mbar (CIGRE-WG23) • EXUDATION OF MOISTURE CONTAINED IN ORGANIC INSULATING MATERIALS. • PERMEATION THROUGH SEALED SECTIONS
DEW POINT MEASUREMENT • IF SF6 GAS CONTAINS MOISTURE, IT IS EASILY HYDRATED TO PRODUCE HIGHLY REACTIVE H2SO3 AND HF(HYDROGEN FLUORIDES). • AS THESE CHEMICALS CAUSE DEGRADATION OF INSULATION AND CORROSION IN THE INTERRUPTING CHAMBER, MONITORING OF MOISTURE CONTENT IN SF6 GAS IS VERY IMPORTANT.
DECOMPOSITION OF SF6 GAS ARCS DURING TRIPPING OPERATION LEADS TO A SUBSTANTIAL EROSION OF THE CONTACT AND INSULATION MATERIALS BY THE HOT ARC. THE MAIN CAUSE FOR SF6 GAS DECOMPOSITION IS THE REACTION OF THESE EROSION PRODUCTS WITH FRAGMENTS OF THERMALLY DISSOCIATED SF6 AND OTHER TRACE GASSES SUCH AS OXYGEN AND WATER VAPOUR. (A) Cu + SF6 = CuF2 + SF4 – COPPER POWDER (B) W + 3SF6 = WF6 + 3 SF4 - TUNGSTEN (C) CF2 + SF6 = CF4 + SF4 - ERODED PTFE ( A CF2 POLYMER)
EFFECTS OF CONTAMINATION • DECOMPOSITION PRODUCTS ARE CORROSIVE AND MAY AFFECT THE SURFACE INSULATION BY THE FORMATION OF CONDUCTIVE LAYERS ALONG THE INSULATORS. • HUMIDITY/MOISTURE IS VERY DANGEROUS WHEN IT CONDENSES ON THEM IN LIQUID FORM CAUSING REDUCED INSULATION STRENGTH • CUF2, WO3, WO2F2 ORIGINATE FROM CONTACT EROSION. • HEALTH RISK DUE TO BYTOXIC PRODUCTS LIKE SO2 ETC.
Preventive Maintenance • Checking of oil leaks from Grading Capacitors - M • Dew Point measurement of SF6 gas – 3Y • Checking of oil pressure drop during duty cycle operation check - Y • Checking of auto-starting/stopping of oil pump - Y • CB Operating Timings (Main, PIR, Aux.) - Y
STATIC CONTACT RESISTANCE • Static contact resistance is measured on four wire measurement principle with 100A current .Contact resistance of the main contacts indicates wear out and misalignment of the main contacts .If the contact resistance exceeds the permissible limit this could result in overheating of the contacts.This should be attended by replacing fingers/checking of alignment
DYNAMIC CONTACT RESISTANCE Dynamic contact resistance measurement is the technique for measuring contact resistance during operation (c-o)of circuit breaker.A d.c. Current is injected through the circuit breaker.the current and voltage drop are measured and resistance is calculated. The resistance versus time data provides useful information on the condition of the circuit breaker contacts and is used as a diagnostic tool
CONTACT TRAVEL CONTACT TRAVEL • Transducers and fixtures are attached to the operating rod or interrupting chamber in order to record the contact travel.when CB closes, contact travel is recorded. Contact bounces or any other abnormality is also clearly indicated by the contact travel measurement. • If the contact travel is recorded with DCRM the length of the arcing contact can be monitored
DCRM AND TRAVEL Travel Resistance Current
CIRCUIT BREAKER - 3 YEARLY MAINTENANCE RECORD Dt. of Commissioning.................... MAKE............... RATING.................SL. No. .................Bay Loc. .................. ( i ) Dynamic Contact resistance and contact travel measurement Dynamic contact resistance signature for CO operation for all the breaks of CB to be recorded and compared with the earlier signatures . Minimum CO time should be 300ms.
Case Study • The CB was commissioned in 1993 and was controlling a Transmission Line. • DCRM signatures after closing operation were not stable and wide variations were observed.
Internal Inspection of Suspected CBs • The CB was opened for internal inspection. • Bearing Bush (part no. SG90011024) used for moving main contact to the operating lever had loosened • This caused misalignment and subsequent damage to moving contact Finger Contact Assembly. • Defective Components were replaced
ALARM AND LOCKOUTS • SF6 GAS PRESSURE LOCKOUT • 1.PRESSURE GUAGE CALIBRATION • 2.LOW PRESSURE ALARM • 3. OPERATIONAL LOCKOUT
ALARM AND LOCKOUTS • OPERATIOING SYSTEM • 1. COMPRESSOR START /STOP • 2. CB AUTO RECLOSE LOCK OUT • 3.CB CLOSING LOCK OUT • 4. CB OPERATIONAL LOCKOUT • 5.MECHANICAL CLOSING INTERLOCK • 6.N2 ACCUMULATOR PRESSURE
GRADING CAPACITOR TAN DELTA MEASUREMENT • THE PURPOSE OF DISSIPATION FACTOR MEASUREMENT OF HIGH VOLTAGE INSULATION IS TO DETECT WEAKNESS IN THE INSULATION. • TAN DELTA = Ir /Ic
GRADING CAPACITOR Tan delta indicates the condition of insulation • Chemical deterioration due to time and temperature ,in some cases the deterioration caused by localized over heating. • entry of moisture, carbon deposits,bad oil,dirt. • severe leakage through cracks and over surfaces. • Equipment insulation failure can be anticipated in advance by carrying out tan delta measurement.the value of tan delta vary with temperature, frequency and induction
TAN DELTA • The tan delta value of 60 numbers of grading capacitors in various sub-stations of Powergrid WR found beyond the permissible limits. • Faulty grading capacitors were opened and inspected in the presence of supplier. • It is observed the oil inside the grading capacitor is corbonised and also observed problem in some elements.
DISSIPATION FACTOR Dissipation factor / loss factor (Tan Delta) is defined as the ratio of resistive component (Ir) of current to that of capacitive current (Ic) flowing in an insulating material. DISSIPATION FACTOR Ir Ic Tand = Ir Ic d V
GRADING CAPACITOR • CAPACITANCE MEASUREMENT:- The grading capacitors are having certain capacitance value .This value is need to be measured to ensure the healthiness of grading capacitor for proper voltage gradation.This value may change due to short circuiting of some capacitor stacks, which may lead to the failure of grading capacitor and circuit breaker.
Power factor is the ratio of resistive current to that of total current. POWER FACTOR Ic It Cos q = Ir It q V