Transformer is one of the valuable assets required in the end line of generation, transmission and distribution section of power system. Majorly role by power transformer as a step up or step down voltage to meet the system requirement and current transformer uses in power protection system. [1]
The basic function of transformer is to transform electric power at one voltage to some other voltage either lower or higher level by the concept of electromagnetism. It consist of a core which made-up of thin insulated laminations of electrical steel sheet that carries two windings, which are insulated from each other.[1] A magnetic flux will produce in the core when alternating current (AC) is supply on one of the winding. Then, the varying magnetic flux will be induced into secondary winding. From Faraday's law, the voltage induced will depends on the turn ratio of the transformer. [2]
Figure 1: Basic transformer operation [2]
The formulas above relate to an ideal transformer (zero leakage flux, no winding resistance, infinite permeability and lossless magnetic core) where an assumption of no power losses in the system is made. Practically there will be three main sources of losses, since not all of the magnetic flux produced by the primary winding will link with the secondary winding. There are some points of design consideration to note in order to minimize and further neglect the losses.
Copper losses - winding losses or I2R losses where heat dissipates in the wire winding due to the resistance of the wire. The resistance of the winding must be kept low with suitable cross sectional area and low resistivity so that the copper losses can be minimized.
Hysteresis losses - some energy are consumed when there are physical changes within the core material due to small magnetic domains are reversed when AC reverses on each cycle. To overcome this type of losses, special low reluctance steel is used as the core material.
Eddy current losses - the core is an electrical conductor as well as a magnetic circuit so the current induced in the core will oppose the changes of magnetic field. This eddy current should be as small as possible by dividing the metal core into thin sheets or lamination.
Figure 2: Equivalent circuit of a transformer [3]
R1, R2 - winding losses
L1, L2 - winding magnetisation
Lm1 - core magnetisation
Rc1 - core losses (hysteresis and eddy current losses)
Due to the ways of reducing losses described above, practical transformer can achieved efficiency at about 98% which closely approach the ideal in performance. [2]
Transformer make possible for power generation, transmission and distribution at the most economical level, power utilisation at the most suitable level, provide measurement of high voltage and high current, impedance matching and isolation (insulating one circuit from another or insulating DC circuit from AC circuit). [3]
INSTALLATION SPECIFICATION
There are several important practices should be follow during the installation of for all type of transformer, outdoor, indoor, dry-type and liquid-filled transformer.
Permanent grounding - permanently grounded the transformer with a correct size and proper installed permanent ground once it is placed permanently.
Humidity - do not expose the transformer liquid-filled compartment with excess humidity or rain. If so, dry air should be continuously pumped into the gas space. For the transformers that are shipped with nitrogen in the gas space, at least pump dry air 30 minutes before service personnel can enter the tank. Oxygen concentrations of 19.5% to 23.5% are advised.
Fluid inspection - make sure that the equipment and storage in handling the fluid are clean and dry during the inspection and filtering the liquid prior to refilling the tank. The liquid level should not go below the top of winding when remove them.
Pressure maintenance - sufficient gas pressure must be maintained to allow a positive pressure of 1 psi to 2 psi at all times, even at low temperature cause transformer may be stored outdoor upon delivery.
Inspection and filling - final inspection on the tightness of all electrical connection, bushings, draw lead connection and also electrical tools clearance should be checked, particularly if any work has been done inside the tank.
Loading - after applying full voltage, keep in under observation during the first few hours of operation under load, then check the oxygen content and dielectric strength of the oil after few days and lastly check the temperature and pressure during the first week of operation.
Surge arresters - installed and connected the surge arresters to the transformer terminals with the shortest possible leads. It's a protection from line switching surges and lightning.
Structural consideration - a number of transformers can be mounted on a single pole as long as the total weight within the safe limit and well distributed (balanced).
Mounting - Sub-100kVA single phase distributions transformers are mounted above the secondary mains while the larger kVA on platform or pad mounting.
Protection - lightning arresters and fused cutouts must be installed on the primary side of transformer.
Ground wire - all ground wire should be covered with plastic or wood molding to a point 8 feet above the base of the pole.
Guying of pole - installed properly to avoid damage caused by the strain of the line conductors and pole mounted equipment and severe weather. [4]
The suitable rating for a given maximum apparent power loading of the insulation of MV/LV transformer can be decided by taking account the possibility of improving the power factor of the installation, anticipated extensions to the installation, installation constraints like temperature, and standard transformer ratings
Apparent power kVA
In (A)
237 V
410 V
100
244
141
160
390
225
250
609
352
315
767
444
400
974
563
500
1218
704
630
1535
887
800
1939
1127
1000
2436
1408
1250
3045
1760
1600
3898
2253
2000
4872
2816
2500
6090
3520
3150
7673
4436
Table 1: Standard apparent powers for MV/LV transformers and related nominal output currents [5]
The nominal full load current In on the LV side of a 3 phase transformer is given by
Pa - kVA rating of the transformer
U - Phase to phase voltage at no load in volts (237V or 410V)
For a single phase transformer is given by
V - Voltage between LV terminals at no load
In = kVA x 1.4
The IEC standard for power transformation is IEC 60076
FAILURE MODES
A transformer can fail due to any combination of electrical, mechanical or thermal stresses. Such failures are sometimes catastrophic and almost always include irreversible internal damage.
There are various causes of transformer failures during operation such as electrical disturbance, insulation failure, lightning, inadequate maintenance, design/ manufacturing errors, loose connections, moisture, oil contamination, fire/explosion, line surge and overloading. [6-7]
Insulation failure excludes those failures where there was evidence of a lightning or a line surge. Four factors that are responsible for insulation deterioration are pyrolysis (heat), oxidation, acidity and moisture, but moisture is reported separately. The average age of the transformer that failed due to insulation was 18 years.
Moisture can cause a transformer to fail, that's why it's been highlighted since the installation process. The increase in moisture in power transformers can be caused by floods, leaky pipes, leaking roof and water entering the tanks through leaking bushing or fittings and confirmed presence of moisture in the insulating oil.
Design/manufacturing errors include condition such as loose or unsupported lead, loose blocking, poor brazing, inadequate core insulation, inferior short circuit strength, and foreign objects left in the tank.
Oil contamination is the cause of the failure includes sludging and carbon tracking.
Fire or explosion that happens outside transformer that can be established as the cause of the failure excludes the internal failures.
Line surge includes switching surges, voltage spikes, line faults/flashovers, and other T&D abnormalities. Pay more attention on surge protection or the adequacy of coil clamping and short circuit strength to overcome this type of failure.
Inadequate maintenance and operation includes disconnected or improperly set controls, loss of coolant, accumulation of dirt & oil, and corrosion.
Loose connections happen when workmanship and maintenance in making electrical connection, for instance improper mating of dissimilar metals and improper torqueing of bolted connection.
Overloading due to the transformer experienced a sustained load that exceeded the nameplate capacity.
All these failures may lead to internal fault, short circuit thus overheating of the transformer. This could be dangerous and the safest solution will be replacement of transformer.
The example of the transformer failure statistical analysis presented during conference is the study from [7] shows that the leading cause of transformer failures is insulation failure. The data obtained on 94 cases worldwide and being classified based on the amount of losses that were converted to U.S dollar within 5 year period.
Table 2: Cause of failure analysis [7]
TESTING REQUIREMENT
Testing requirement for fault detection include continuous assessment tests, frequent dissolved gas analysis and oil quality tests, thermography scanning of transformers and electrical connections and on-line monitoring of questionable units. [6]
Routine test of the transformer are dielectric tests (separate-source voltage withstand test), induced voltage test, voltage ratio measurement and check of polarities and connection, no load current and no load loss measurement, winding resistance measurement, short-circuit impedance and load loss measurement and partial discharge measurement.[8]
CONDITION MONITORING PRACTICES
Transformer is the critical asset that should be monitored closely and continuously so that can assess their operating conditions and ensure their maximum uptime. This action is due to major consequence can happen when they fail for instance power outages, personal and environment hazards and also expensive power recovery and replacement.
The condition based monitoring strategy is related to a wide range of on-line condition monitoring applications, which include the detection of partial discharges and insulation degradation, winding deformation diagnosis, monitoring of dissolved gas evolution, classification of hazards and assessing thermal conditions.
There are four main aspects concerning transformer condition monitoring and assessment which are thermal modelling (TM), dissolved gas analysis (DGA), winding frequency response analysis (FRA) and partial discharge analysis (PDA).
Firstly, thermal modelling method by the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE) can be used to predict the zones of hot-spot temperature in a transformer as the sum of the ambient temperature, the mixed top-oil temperature rise above ambient and the hot-spot rise above the mixed top-oil temperature. The two steady-state temperature rises of top-oil and bottom-oil above ambient can be estimated separately. The other improved model rooted on the traditional thermal solutions like a real-time mathematical thermal model which consists of several differential equations and takes into detailed account the influence of weather on thermal behaviours of a transformer. The development method of thermal modelling to get an accurate and meaningful model is highly desired in practice in order to deal with transformer thermal rating. [6]
Secondly, DGA is most widely used as preventive maintenance to monitor on-line transformer operation by technique application on an oil sample. The fluid filled in oil-immersed power transformers serves various purposes like the fluid acts as a dielectric media, an insulator and heat transfer agent. The slow degradation of the mineral oil into certain gases dissolved in the oil in normal operation, however it become more rapidly during electrical fault. Hence, the dissolved gases can be determined quantitatively by DGA by looking at the concentration and the relation of the individual gases to conclude/predict the occurrence of fault and its type. [6]
Thirdly, FRA is a very sensitive technique for detecting winding movement faults caused by loss of clamping pressure or by short circuit forces. Variations in frequency responses may reveal a physical change inside a transformer, for instance winding movement caused by loosened clamping structures and winding deformation due to shorted turns. It is the suitable diagnostic tools to indicate the displacement and deformation of fault. [6]
Lastly, PDA is important to determine the level of insulation degradation by the detection of partial discharge (PD). PD occurs when a local electric field in electrical insulation exceeds a threshold value, resulting in a partial breakdown of the surrounding medium. The detection of PDs can be performed by a variety of techniques, most commonly electrical, acoustical, optical and chemical techniques. There are three types of PD analysis methods, i.e. the time-resolved partial discharge analysis; the intensity spectra based PD analysis and the phase-resolved partial discharge analysis. [6]
