OIL FILLED TRANSFORMER - MAINTENANCE - I
DRYING OUT OF TRANSFORMER
Deterioration of insulation resistance value of transformer is mainly due to ingress of moisture into the windings and insulating materials. In order to improve the insulation resistance at site, following methods are available:
(i) Hot Oil Spraying: In this method about 7% of quantity of oil is heated up to 90-95º C separately and the hot oil is sprayed on to core and windings by means of nozzles in the form of fine spray and simultaneously the transformer is subjected to a high degree of vacuum say less than 5 m bar. The hot oil is collected at the bottom sent through a filter and reheated and sprayed. This process removes moisture from the core and windings. The oil used for spraying should be discarded.
(ii) Flushing Method: In this method, the transformer is put under hot oil circulation up to 60º C. After reaching steady temperature, the entire oil is drained quickly into a separate tank. Immediately after draining the oil, the transformer is subjected to a vacuum as per the guidelines of the manufacturer for 12 hours. During this period of application of vacuum, the transformer oil drained can be filtered to improve the quality. After 12 hours of vacuuming of the transformer, break the vacuum by means of dry nitrogen. Fill the transformer with filtered oil under vacuum. Now the oil in the transformer can be again circulated to raise the temperature of oil to 60º C. Again drain the oil, apply vacuum and repeat the process till you get a good IR value.
Precautions
The diverter switch tank and the main tank should be inter-connected before the above works to equalize the pressure. Otherwise the diverter switch tank may be damaged when the vacuum is applied.
BUCHHOLZ RELAY FUNCTIONAL TEST
Tools and materials required:
Cycle pump or Nitrogen cylinder with 4 kg/cm2 pressure and connecting tubes.
Procedure:
Transformer shall be isolated.
Connect Nitrogen cylinder or cycle pump to the top petcock of Buchholz relay
Open the other petcock
Allow gas to enter the relay
Check and confirm alarm signal is received
Close petcock on gas supply side and release all gases trapped in relay casing
Increase the gas pressure to approx. 2 kg/cm2
Open the test petcock and allow full surge of gas to enter the relay casing
Check and confirm in control room that the Trip signal has been received
Close Buchholz relay petcocks and normalize
BUCHHOLZ GAS ANALYSIS
This is to be done only when the transformer has tripped on account of Buchholz fault or Buchholz alarm has been initiated.
The following procedure should be adopted for testing of gas accumulated in Buchholz relay of power transformers.-
(i) Switch off the transformer when the Buchholz relay alarm rings, indicating the development of an internal fault in the transformer.
(ii) Through the lateral sight hole of the Buchholz relay, the colour and quantity of the gas may be determined.
(iii) Collect a portion of the gas in the test tube and apply a lighted match stick to the test tube to test the combustibility of the gas.
If gas is not combustible, it is mere air.
(iv) Then proceed to carry out the chemical test with a simple gas tester as follows:
The gas tester consists of two glass tubes containing two different silver nitrate solutions which through passage of decomposed gases form two distinguishable precipitates. The tubes must be assembled as indicated in the sketch and tube 1 should be filled with solution prepared by dissolving 5 grams of silver nitrate (Ag NO3) in 100 cc of distilled water.
Tube 2 should be filled with solution prepared by dissolving five grams of silver nitrate (Ag NO3) in 100 cc of watery ammonia solution.
Use of the gas tester is quite simple. Each of the two glass tubes should be filled with corresponding solutions upto the marks. They should be closed by corks fitted with the connecting tubes. Then the gas tester should be screwed on to the test cock of the Buchholz relay. After opening the test cock the collected gas would flow through the solution which would indicate the nature of the fault.
If the gas causes a white precipitate in tube 1 which turns brown under the influence of light, it means the oil has decomposed. Probably a flashover has occurred between bare conductors or between one bare conductor and an earthed part of the transformer.
If the gas causes a dark brown precipitate in the solution in tube 2 it means that solid insulating material like wood, paper, cotton, etc., had decomposed producing carbon monoxide (CO). In this case a leakage in the winding causing an internal short has occurred.
If there is no sedimentation at all the gas is mere air.
MAINTENANCE OF OIL FILLED POWER TRANSFORMERS
GENERAL
As is generally known, a transformer consists essentially of the magnetic core built-up of insulated silicon steel lamination upon which are wound two distinct sets of coils suitably located with respect to each other and termed as primary and secondary windings. Such a combination may be used to step up or step down the voltage. The techniques used in the design and construction of high voltage transformers vary from supplier to supplier. The active parts of a transformer consist of core and windings.
CORE
Core is made from lamination of cold rolled grain oriented silicon steel. The specific loss at operating flux densities in silicon steel is very low.
WINDINGS
Paper insulated copper conductor is used for windings. The conductors are transposed at regular intervals for ensuring equal flux linkage and current distribution.
COOLING
Core and windings are immersed in an oil filled tank. Normally, oil flows through winding and enter cooler or radiator by thermosyphonic effect.
Depending upon the rating, the transformer employs ONAN, ONAF, OFAF and OFWF types of cooling.
ONAN - Oil Natural Air Natural
ONAF - Oil Natural Air Forced
OFAF - Oil Forced Air Forced
OFWF - Oil Forced Water Forced
TANK AND COVER
Steel plates are used for fabricating transformer tanks and covers. They are designed to withstand full vacuum and a positive pressure of 0.3 kg/cm2 above the normal oil head.
CONSERVATOR
Conservator takes care of the expansion and contraction of transformer oil, which takes place due to loading and releasing of load. Modern transformers are provided with separate air shell in the conservator which prevents direct air contact with the transformer oil.
A separate conservator is provided for the on-load top changer diverter switch. Magnetic oil level gauges are provided in the conservator tanks which can give alarm to the operators and isolate the transformer in the event of oil level falling below a preset value.
PRESSURE RELIEF DEVICE
A pressure relief device is provided with an alarm and trip contacts. When excessive pressure is built inside the transformer in the event of severe fault, the pressure relief device releases the excess pressure.
For smaller transformers, an explosion vent is provided with a lighter diaphragm which breaks in the event of increasing internal pressure.
BUCHHOLZ RELAY
This gas and oil actuated relay is provided in the oil pipe which connects the conservator and the main tank. For any internal fault inside the transformer, this relay is actuated. This relay operates on the well-known fact that every type of electric fault in an oil-filled transformer gives rise to gas. This gas is collected in the relay to actuate the alarm and trip contacts.
SILICA GEL BREATHER
Expansion and contraction of oil due to loading causes breathing. External air gets in during the time of contraction. Silica gel absorbs the moisture in the air and prevents moisture entry into the oil.
TEMPERATURE INDICATORS
For continuous measurement of oil and winding temperatures, separate meters are used. These meters have alarm and trip contacts.
BUSHINGS
High voltage connections from the windings pass to the terminal bushings. These bushings are hermitically sealed and filled with oil for EHV transformers. This oil does not communicate with the main transformer oil. A separate oil level gauge is provided for monitoring the oil level in the bushings.
TAP CHANGER
There are two types of tap changers viz., on load and off load. In on load tap changer, tap position changes, when the transformer is energized either through manual mode or auto mode. The OLTC diverter switch has separate oil which needs periodical changing as some amount of arcing takes place during tap changing operations. This has a separate conservator and a Buchholz relay.
PROTECTIONS FOR TRANSFORMER
The following protections are provided normally for a transformer.
(i) Over current protection
(ii) Restricted Earth fault protection
(iii) Over voltage protection alarm
(iv) Over fluxing ( generator transformers )
(v) Surge protection
(vi) Differential protection ( above 5 MVA )
(vii) Oil temperature high protection
(viii) Winding temperature high protection
(ix) Oil level low protection
(x) Buchholz protection
(xi) Pressure relief device
The relays checking and calibration procedures are not covered in this document.
MAINTENANCE
It is essential to carry out regular and careful inspection on the transformer and associated components/equipment and carry out maintenance activities to provide long life to the equipment and achieve trouble-free service.
IN ORDER TO CARRY OUT THE NECESSARY INSPECTION AND MAINTENANCE WORKS, NECESSARY SAFETY PROCEDURES SUCH AS LINE CLEARANCE/EQUIPMENT SHUTDOWN ETC., WILL BE STRICTLY ADHERED TO, WHEREVER NECESSARY.
The frequency of inspection depends on climate, environment, load conditions and also the age of the transformer. The inspection cum maintenance schedule starts with every hour and continues as given below.
HOURLY
The following parameters are to be checked every hour and recorded. If the observed value exceeds the value given by the supplier, immediate remedial action should be taken.
(i) winding temperature
(ii) oil temperature
(iii) load current
(iv) terminal voltage
Normally, maximum allowed winding temperature is 55º C above ambient and oil temperature is 45º C above ambient (actual allowed value may vary from supplier to supplier).
DAILY
(i) Oil level in main conservator
(ii) Oil level in OLTC
(iii) Oil level in bushing
(iv) Leakage of water into cooler (OFWF)
(v) Water temperature (OFWF)
(vi) Water flow (OFWF)
(vii) Colour of silica gel
QUARTERLY CHECKING/ REPLACEMENT
Reconditioning of silica gel breather.
Checking of water cooler functioning
Checking of cooling fans functioning
Gear oil for tap changer mechanism
Checking of cooling pumps and motor functioning
HALF YEARLY
(i) Inspection of all gaskets and joints
ANNUALLY
(i) Protective relays, alarms, meters and circuits to be checked and calibrated
(ii) IR value and Polarisation Index
(iii) Tan delta and capacitance of bushings
(iv) BDV of transformer oil.
(v) Oil resistivity
(vi) Power factor of oil
(vii) Interfacial tension of oil
(viii) Acidity and sludge of oil
(ix) Flash point of oil
(x) Water content of oil
(xi) Dissolved gas analysis
(xii) Replacing of OLTC oil
(xiii) Thermo vision scanning
(xiv) Earthing measurements
(xv) Tan delta and capacitance of winding
ONCE IN FIVE YEARS
(i) Furan analysis (Once in a year after the first 5 years)
(ii) Overhauling of OLTC diverter switch (once in 5 years or after completion of 50,000 operations whichever is earlier)
ONCE IN TEN YEARS
Overhaul, inspection including lifting of core and winding.
ENERGY EFFICIENT MOTOR :-
An "energy efficient" motor produces the same shaft output power (HP), but uses less input power (kW) than a standard-efficiency motor.
Energy efficient motors have the following positive features compared to standard motor:
1) Higher quality low loss laminations for magnetic circuit
2) More & better quality copper in the windings.
3) Better quality insulation
4) Optimised air gap between the rotor and stator.
5) Reduced fan losses.
6) Closer matching tolerances
7) A greater core length
EFFECTS OF HARMONICS ON MOTOR :-
Harmonics increase motor losses, and can adversely affect the operation of sensitive auxiliary equipment. The non-sinusoidal supply results in harmonic currents in the stator which increases the total current drawn. In addition, the rotor resistance (or more precisely, impedance) increases significantly at harmonic frequencies, leading to less efficient operation. Also, stray load losses can increase significantly at harmonic frequencies. Overall motor losses increase by about 20% with a six-step voltage waveform compared to operation with a sinusoidal supply. In some cases the motor may have to be de-rated as a result of the losses. Alternatively, additional circuitry and switching devices can be employed to minimize losses.
Instability can also occur due to the interaction between the motor and the converter. This is especially true of motors of low rating, which have low inertia. Harmonics can also contribute to low power factor.
MOTOR - GOOD MAINTENANCE PRACTICES :-
A checklist of good maintenance practices to help insure proper motor operation would include.
1) Inspecting motors regularly for wear in bearings and housings (to reduce frictional losses) and for dirt/dust in motor ventilating ducts (to ensure proper heat dissipation).
2) Checking load conditions to ensure that the motor is not over or under loaded. A change in motor load from the last test indicates a change in the driven load, the cause of which should be understood.
3) Lubricating appropriately. Manufacturers generally give recommendations for how and when to lubricate their motors. Inadequate lubrication can cause problems, as noted above. Over-lubrication can also create problems, e.g. excess oil or grease from the motor bearings can enter the motor and saturate the motor insulation, causing premature failure or creating a fire risk.
4) Checking periodically for proper alignment of the motor and the driven equipment. Improper alignment can cause shafts and bearings to wear quickly, resulting in damage to both the motor and the driven equipment.
5) Ensuring that supply wiring and terminal box are properly sized and installed. Inspect regularly the connections at the motor and starter to be sure that they are clean and tight.
SELECTION OF MOTOR :-
A. Torque Requirement
The primary consideration defining the motor choice for any particular application is the torque required by the load. The relationship between the maximum torque generated by the motor (break-down torque) and the torque requirements for start-up (locked rotor torque) and during acceleration periods is very important. The thermal loading on the motor is determined by the duty/load cycle. One important consideration with totally enclosed fan cooled (TEFC) motors is that the cooling may be insufficient when the motor is operated at speeds lower than its rated speed.
B. Sizing to Variable Load
Industrial motors frequently operate under varying load conditions due to process requirements. A common practice in cases where such variable loads are found is to select a motor based on the highest anticipated load. In many instances, an alternative approach is typically less costly, more efficient and provides equally satisfactory operation. With this approach, the optimum rating for the motor is selected on the basis of the load duration curve for the particular application. Thus, rather than selecting a motor of high rating that would operate at full capacity for only a short period, a motor would be selected with a rating slightly lower than the peak anticipated load and would operate at overload for a short period of time. Since operating within the thermal capacity of the motor insulation is of greatest concern in a motor operating at higher than its rated load, the motor rating is selected as that which would result in the same temperature rise under continuous full-load operation as the weighted average temperature rise over the actual operating cycle.
Losses in INDUCTION MOTOR :-
Losses are the source of inefficiency in motors, i. e. energy that goes into a motor but does not produce useful work. Losses in induction motors are classified into two types:
1. No-load Losses: These losses are independent of load and incurred even when the motor is idling.
2. Load dependent Losses: Vary as function of motor loading
The losses in a motor are of two types such as fixed i.e. independent of load on the motor and the other variable i.e. dependent on the load.
Fixed losses consist of Iron loss and mechanical loss (friction and windage loss). The iron loss vary with the material and geometry and with input voltage whereas friction and windage losses are caused by friction in the bearings of the motor and aerodynamic losses associated with the ventilation fan and other rotating parts.
Variable losses consist of resistance losses in the stator and in the rotor and other stray losses. Resistance to current flow in the stator and rotor result in heat generation that is proportional to the resistance of the material and square of the current. Stray losses arise from a variety of sources and are difficult to measure directly or to calculate and are generally considered proportional to the square of the rotor current.
THUMB RULE FOR SELECTING CAPACITOR FOR P.F IMPROVEMENT ( To be connected with motor )
The size of capacitor required for a particular motor depends upon the no-load reactive kVA (kVAR) drawn by the motor, which can be determined only from no-load testing of the motor. In general, the capacitor is then selected to not exceed 90 % of the no-load kVAR of the motor. (Higher capacities could result in over-voltages and motor burn-outs). Alternatively, typical power factors of standard motors can provide the basis for conservative estimates of capacitor ratings to use for different size motors |
Utilisation categories for contactors and motor starters
Alternating current:
AC - 1 Non-inductive or slightly inductive
loads, resistance furnaces
AC - 2 Slip-ring motors: starting, switch-off
AC - 3 Squirrel-cage motors: stating, switch-off,
switch-off during running
AC - 4 Squirrel-cage motors: starting, plugging,
reversing, inching
AC - 5A Switching of electric discharge lamp controls
AC - 5B Switching of incandescent lamps
AC - 6A Switching of transformers
AC - 6B Switching of capacitor banks
AC - 7A Slightly inductive loads in household appliances and
similar applications
AC - 7B Motor load for household appliances
AC - 8A Switching of hermetically enclosed refrigerant
compressor motors with manual reset of
overload releases
AC - 8B Switching of hermetically enclosed refrigerant
compressor motors with automatic reset of
overload releases
AC - 53A Switching of squirrel-cage motor with
semi-conductor contactors
Direct current:
DC-1 Non-inductive or slightly inductive loads,
resistance furnaces.
DC-3 Shunt motors: starting, plugging, inching. Dynamic
breaking of d.c. motors.
DC-5 Series motors: starting, plugging, inching. Dynamic
breaking of d.c. motors.
DC-6 Switching of incandescent lamps
Utilization categories for contactor relays
Alternating current:
AC-12 Control of resistive loads and solid state loads
with isolation by opto couplers.
with isolation by opto couplers.
AC-13 Control of solid state loads with transformer
isolation.
AC-14 Control of small electromagnetic loads (≤ 72 VA).
AC-15 Control of electromagnetic loads (> 72 VA).
Direct current:
DC-12 Control of resistive loads and solid state
loads with isolation by opto couplers.
DC-13 Control of electromagnets.
DC-14 Control of electromagnetic loads having economy
resistors in circuit.
Transformers - Definition & Terms
Primary winding: The winding where incoming power supply is connected. Usually this refers to High Voltage side in distribution transformers
Secondary winding: The winding where the principal load is connected. Usually this refers to Low Voltage side in Distribution transformers.
No load loss: The losses taking place in a transformer when only primary winding is energized and all secondary windings are open. They represent constant losses in a transformer.
Dielectric loss: The losses taking place in a stressed dielectric medium (insulation) subjected to stress reversals.
Iron losses: The losses taking place in the magnetic core. There are two types; hysterisis losses and eddy current losses.
Hysteresis losses: This loss depends upon the area of the hysteresis loop, which is depending upon the maximum flux density, the type of material and frequency. It is independent of the waveform
Eddy current losses in core:
This is loss due to circulating currents induced by voltage in the thickness of core laminations. It depends upon thickness of lamination, path resistance which is depended upon the type of material, R.M.S. flux density i.e. waveform and square of frequency
Eddy losses in a conductor:
For a thick conductor, the induced voltage within the conductor cross section due to self linkage and due to current in other conductor varies. The difference in induced voltage in the local path in the thickness of the conductor causes extra eddy current loss. This loss varies with square of current and square of frequency.
Stray losses: All current dependant losses in a winding other than the basic I2R losses. Stray losses include eddy loss in the conductor, eddy losses in structural paths in close
proximity to outgoing conductor and the eddy loss in general in the structural parts. In dry type transformers, the last two mentioned types of stray losses are absent.
Form factor: It is the ratio of the r.m.s. value of a waveform to the average value over one half cycle. For a sine wave the value of form factor is 1.11. For distorted waves with higher peak values, the form factor is higher.
Harmonics: Frequencies other than the main fundamental frequency of current or voltage which are present in a distorted wave as multiples of base fundamental frequency.
Transformer Polarity:
This refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to the way the leads are brought out of the transformer.
Burden: The load on an instrument transformer is referred to as a “burden”.
Short circuit impedance & Impedance voltage:
The impedance voltage of a transformer is the voltage required to circulate rated current through one of the two specified windings; when the other winding is short circuited with the winding connected as for rated operation. The short circuit impedance is the ratio of voltage and current under above conditions. The resistive component of short circuit impedance, gives a parameter for estimating load losses. These losses include eddy current losses in the conductors and structure as a small portion. Their contribution is materially enhanced due to harmonic currents in load. Exact determination by test is difficult and simplified test at low current suffers from the disadvantage of a high multiplying factor; but it is expected to give representative values.
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