Air Blast Circuit Breakers : Hitachi Make (220 kV) | Welcome to Uttar Pradesh Power Transmission Corporation Limited, Government of Uttar Pradesh, India.

Air Blast Circuit Breakers : Hitachi Make (220 kV)

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Sr. No.	Subject
1.	Constructions Feature : Hitachi air blast circuit breaker is a single flow, axial blast type air blast circuit breaker which interrupts fault currents by means of compressed air. Each pole has two horizontal air receivers and three columns of duct-insulators. An interrupter-head consists of two breaks, installed at the top of duct insulator thus one pole unit of breaker has six series breaks, shunt resistors are mounted to equalize the voltage across each break. The isolating switch is placed in between two interrupter columns and operated by the mechanism on centre of the receiver. Valves for operation are mounted on the air receiver and the valve box contains the magnetic valve.- The auxiliary switch and the terminal connectors are placed in the box under the receivers. The control cubicle houses the manual control valve, the pressure gauge etc. All piping and wiring of breaker are gathered into the control cubicle for external connections.
2.	Operation : Opening : When the trip coil of magnetic valve is energized, the compressed air in the air receiver is led into the pilot valve and actuates it, then the air through the pilot valve is led into the main valve. When main valve opens, the air goes up through the duct insulator, flows into the interrupter head and drives the moving contact open. The arc is drawn between the moving and fixed contacts. It is blown into the nozzle and quenched in very short time. On the other hand a portion of air through the main valve is led into the operating cylinder for opening through the exhaust valve. The piston of exhaust valve is pushed to the left and opens the air path to the cylinder. The air drives the isolating switch open The isolating switch is set to open a definite time lag than the opening of the interrupter head. The current through shunt resistor after arc extinction is interrupted by the isolating switch but accompanies no arc as the current is very small. There is interlocking device of the magnetic valve mechanically connected with isolating switch. On completion of opening of the isolating switch the interlocking device resets and locks the magnetic valve for opening allowing the pilot valve, the main valve and the moving contact to reset to the initial position. Then the interrupter heads comes back to the closed position and insulation between the terminals is achieved by the isolating switch. 2-2 Closing: When the closing coil of the magnetic valve is energized, the air is led into the operating cylinder and closes the isolating switch. The magnetic valve is reset on completion of closing by the interlocking devices as in case of opening- The magnetic valve is equipped with an antipumping device to prevent repetition of close-open operation. To absorb shock at the end of motion of isolating switch an oil dashpot is mounted on each pole-unit, For local control of the breaker the manual control valve which is mounted in the control cubicle is pushed by button provided, the air is led to the cylinder in the valve, box and the piston pushes the armature as it is actuated by the coil and breaker operates.

Daily Maintenance :

20 I-01 D	Check air pressure by reading air pressure of common control cubicle. A remarkable decrease in pressure or increase in frequency of compressor running signifies the air leakage in the air system and it requires inspection and remedy.
20 I-02 D	Trip circuit healthy indications should be checked and defects any may be set right immediately.

Yearly Maintenance:

20 I-03 Y	Clean Insulators.
20 I-04 Y	Cleaning of mechanism, application of Hitasol to the contacts of isolating switch.
20 I-05 Y	Drain-out air of air-receivers, clean inside the tank by opening side covers.
20 I-06 Y	Check earthing connection and tighten the same, if required.
20 I-07 Y	Lubricate all bearings, pins, cylinder of all valves after checking.
20 I-C8Y	Tighten all nut-bolts including foundation bolts.
20 I-09 Y	Check oil of dash pot and fill up, if required.
20 I-10 Y	Check all heaters of control cubicle and replace damaged one.
20I-11 Y	Check all control wiring for its tightness.
20 I-12 Y	Check all jumpers and Tighten.
20 I-13 Y	Measure contact resistance and compare with previous values.
20 I-14Y	Check tripping and closing of breaker with local and remote switch both as well as tripping at 50% DC voltage. Replace the coil if required.
20 I-15 Y	Dismantling and inspection of interrupter-head is required, if the breaker has cleared at least one heavy fault current interruption.

Three Yearly Maintenance:

20 I-16 TH	Measure the minimum control current of tripping and closing coil at the rated operating pressure (15 Kg./Cm2.)
20 I-17 TH	Observe the operating pressure of pressure switch in the control cubicle and check by evacuating the pressure of air receiver gradually for checking the low air pressure alarm setting.
20I-18 TH	Check low pressure interlocking and releasing pressure of the low pressure interlocking device on the magnetic valve by raising and lowering the pressure.
20 I-19 TH	Minimum operating pressure to operate Isolating switch may be checked, by lowering the pressure of air receiver and by making low pressure interlocking device inoperative.
20 I-20 TH	Check the pressure drop during one operation, set the pressure of air receiver at rated pressure of 15 Kg/Cm? and close the inlet valve. Observe the pressure drops after a closing or opening operations. When a remarkable difference is observed between the results of the above test and factory tests. Thorough inspection is necessary to find out the points of excessive air leakage and rectify the defect. Tripping Based Maintenance : To be done after every 10 tripping on short circuit current or 2500 tripping on normal current or after every five years. Identify the pole which has cleared the short circuit fault current.
20 I-21 T	Interrupter Head : Dismantle the contact block from the interrupter head. Take out the fixed contact by removing the mounting bolts of the bracket. The moving contact is dismantled by the special tool after removing the mounting knock bolts of the top of the interrupter head. Inspect the surface of bolts, fixed and moving contacts. When the contact is found burnt, file off with smooth cut file. If found loose then replace the contact with new one. Apply hitsol to the sliding surface of the contacts before reassembly. Care should be taken to tighten all bolts equally to avoid partial stresses. Torque wrench should preferably be used.
20 I-22 T	Isolating Switch : The contacts of the isolating switch can be easily inspected without dismantling. When burning is found on contact, grind it smooth with cut file, if burning is excessive, replace the contact. Apply Hitsol to contact. Check the displacement of fixed contacts fingers when the switch is closed. If should be within 0.5 mm.
20 I-23 T	Main Valve and Pilot Valve : Dismantle the cover and clean the valve and valve seat. If required replace valves or valve seat.
20 I-24 T	Magnetic Valve : Dismantle the valve cover and clean the magnetic valve and valve seat. If the valve gets defective, replace with new one.
20 I-25 T	Operation Cylinder : Dismantle the cylinder, clean it and apply Hita-sol before re-assembling.
20 I-26 T	Airing Valve : Check the amount of air flow and keep it on, the measurement can be made guiding the air through the arcing valve into a glass vessel with scale filled with water.
20 I-27 T	Operating Mechanism and Insulator : Lubricate all moving parts of operating mechanism and keep insulators clean.
20 I-28 T	Check control cubicle for all connections and auxiliary contacts. Clean it properly & lubricate moving parts. Clean the auxiliary contacts and replace if found damaged.
20I-29T	General: Repeat all annual checks and three yearly checks.
20 I-30 T	Take advice of firm, if necessary.
	Precautions : Be sure that the inlet valve is closed and the drain valve is open and compressed air is perfectly exhausted from the breaker before inspection. Manual operating handle must not be used on a live breaker. Use it for maintenance only. The first pneumatic operation after reassembling should be done through remote control to avoid injury from spouting of foreign bodies.