COOLING SYSTEM
The transformer and associated equipment must be put out of service, isolated, earthed and obtain a ‘Permit to Work’ from the Shift Engineer while checking the operation of the fans, pumps, radiators and coolers. The cooling surfaces shall be inspected regularly and cleaned from dust, insects, leaves or other airborne dirt, if required. This is especially important in case of fan cooling. The cleaning is suitably carried out by means of water flushing at high pressure. Precautions should be taken to cover the fan motor so that water may not go inside.
Alternatively, cleaning can be done with cleaning solution and cloth. Normally, no measures are necessary for keeping the internal cooling surfaces clean as long as the oil is in good condition. If, however, sludge formation has set, then the sludge may get deposited on horizontal surfaces in radiators and coolers. In such a case, the radiators and coolers shouldbe flushed internally with clean oil in connection with oil exchange. If the sludge does not loosen, we can firstly flush with petrol etc. and then with oil. This activity is to be carried out in consultation with the manufacturer. The cooler bank should be inspected regularly. All coolers can be cleaned by pulling out the tube packets and thus making them accessible for cleaning.
The fan-motors are provided with permanent – lubricated bearings and double
sealing rings. The motor bearings are axially clamped with spring washers. If the sound
level of the fan increases, re-tighten all mounting supports.
Cooling System-Fans-Controls
i. Fan Controls are designed to operate both manually and automatically. The
automatic function is basically related to load and energisation or both.
ii. Manual Control:
The fan control should be turned “ON” for a brief period to ensure that each stage has sufficient voltage to operate. Fan operation should be observed.
Oil pumps should be checked by observing their flow gauges. In case of any
malfunctions, manufacturers recommendations should be referred to.
iii. Temperature Control:
The temperature bulb should be removed from its pocket/well on the side/top of transformer. The MASTER CONTROL should be set to the automatic position. Using a temperature controlled calibration instrument, the temperature of the bulb should be slowly raised and observed for proper
calibration/operation.
iv. Load Control:
The secondary current of the controlling CT should be checked to ensure that it is operating properly. After shorting out the secondary of CT (if transformer is energized), the secondary lead should be removed from the control circuit, current should then be injected in to the control circuit and the level of this current is varied in order to observe proper operation.
Cooling System-Fans-Visual Inspection:
Visual inspection should be done without taking shut down of transformer to ensure that fans are operating at their design speed, airways are not blocked and guards &blades are not damaged.
Visual check for contamination of motor and fan blades.
Check for build up of moisture in the motor.
Check bearing lubrication.
Check for correct rotation.
Check for unusual noise.
Check for corroding parts.
Cooling Fans- Rotation:
The rotation of the fan blades should be observed to ensure that the
air flow is in the correct direction for the type of device involved. The observations may be
facilitated, if it is performed at a lower than normal speed e.g. during either start up or
immediately after switching off. Corrections to rotation should be made as indicated by
inspection.
Improper Air Flow:
Improper air flow can reduce cooling system efficiency, cause overheating and result in damage to electrical equipment. All fans, which are not running at design speed, should be replaced after stopping the fans. Any obstructions to air flow should be removed and any damaged fan guards or blades should be replaced or repaired.
Check for build-up of Moisture:
Drain holes in the motor are provided at the lowest part of the motor and are fitted with plugs on the totally enclosed motors (wherever applicable).
These plugs should be removed to allow any moisture to drain away and then put back in place.
Check bearing lubrication:
Regular greasing of the bearings should be carried out and, as a general guide, one or two shots from a grease gun should be sufficient at intervals of 1,000 running hours. It will only be necessary to maintain the grease in the bearings.
Note:
An increase in running temperature, after greasing, is to be expected. However, this
will return to normal after a short period. If too much grease is applied and the bearing is
overfilled, the temperature of the bearing will rise considerably due to ‘churning’. This may lead to complete bearing failure.
It is, therefore, recommended that, at least, every two years the bearings and housings be flushed out in white spirit and inspected for wear. Worn parts should be replaced, where necessary , and repacked with grease.
When a motor remains idle for long period, the grease should be inspected regularly.
Checking be done for hardening or separation and, if possible, by rotating the shaft through
90 degree at intervals.
Precautions:
Different grades (or makes) of grease MUST not be mixed. The bearings,
housings and grease pipes must be thoroughly cleaned of the old grease and correctly packed
with new, if a change of grease is required.
Replacement of bearings (to be carried out if unusual noise can be heard):
Cleanliness is of vital importance when handling ball bearings and their lubrication. Special precautions are necessary for the removal of bearings especially when they have been shrunk on to the shaft. Refer manufacturer’s instructions for replacement of bearings.
Visual Inspection:
Carry out a visual inspection on the motor casing and fan guards for early signs of corrosion. Any sign must be treated straight away to prevent further rusting.
Cooling system – Pumps – Visual Checks
The transformer and associated equipment need not be put out of service or isolated while
carrying out visual checks on the pumps.
Obtain a ‘Permit to Work’ from the Shift Engineer.
Obtain keys to the transformer compound and marshalling kiosk.
All results must be recorded in a log for comparison during future tests in service.
Checks to be carried out:
Check for correct rotation.
Check for unusual noises/abnormal vibration- replacement of rotor & bearings.
Check for corroded parts.
Check for electrical problems.
CHECK PROCEDURE
Check for correct rotation:
Provide a supply to the pump and check it for correct rotation.
The pump has a direction and flow indicator which is fitted on the top of the pump.
Check for unusual noises – replacement of rotor bearings:
While the pump is running, check the motor and bearings for unusual noises.
All unusual noises must be investigated further by removing the pump for visual
inspection.
After removing pump from the system, the end play of shaft should be measured. The
manufacturer’s guide should be consulted to determine if excessive bearing wear exists as
indicated by amount of shaft end play observed. Any indicative of wear on the impeller and impeller housing is indicative to excessive thrust bearing wear.
Precautions:
Care must be taken to ensure that damage is not caused either to the windings or the impeller during removal and assembly. Removal of a cooling Pump requires a very precise knowledge of the arrangements of the cooling system. Equipment and cooling pump should be de-energised first. Cooling system surrounding pump should be effectively isolated from the remainder of the equipment’s cooling system. Isolating
valves should be closed and the system drained before the pump is removed. It is
recommended that blanking plates be installed after pump is removed. Pump should
never be run without complete immersion in insulating oil.
Check for corroded parts:
Carry out a visual inspection on the motor casing for any signs of corrosion. This should not occur as the casing is cast iron construction which is painted.
However, any sign of corrosion must be treated urgently to prevent further spread.
Check for Electrical Problems:
The current flowing in each electrical terminal of each pump motor should be accurately measured in running condition of the pump. Any significant imbalance of current between the terminals greater then 15-20% is indicative of the problem with the pump motor. Differences between current ranges for like pumps on the same electrical equipment should be compared. Any significant difference may be indicative or a restriction in the area of the cooling system, where the pump with the higher current drain is located, or a problem with in the pump itself.
Check for Cooling Pump – Oil Flow Gauge:
Oil flow gauge shows only whether there is
oil flowing through the pump or not and it is not indicative of velocity of the oil or condition
of the pump.
After making sure that oil cooling pumps are “ON”, flow gauges should be observed for
indication of flow of oil. If pump is “ON” and no flow is indicated, sending unit may be
defective. If pump is turned “OFF” and flow gauge continues to indicate flow, then gauge is
probably stuck in flow position and sending unit or the entire gauge needs to be replaced. It is normal for gauge to continue to indicate flow for a brief period before indicating off (no
flow), following the turning off of a pump.
This is due to conservation of momentum in
respect of oil.
Precaution:
Lack of flow from oil cooling pumps during operation may be indicative of
imminent failure. Necessary corrective action should be taken immediately.