(17) LECTURE.4 =TRANSFORMER PROTECTION SECHEMES 2010

TRANSFORMER PROTECTION SECHEMES

INTRODUCTION : NECESSITY OF PROTECTION SCHEMES

Transformers are critical expensive components of the power system. Its failure in service might imply a lengthy and costly outage. Even where spare transformers are available, a lot of time will be spent on transportation, testing and commissioning. A major goal of transformer protection is limiting damage to a faulted transformer.The basic philosophy in protection system design is that any equipment that is threatened with damage by a sustained abnormal condition be automatically isolated.

TRANSFORMER PROTECTION OVERVIEW

The type of protection available for a given transformer, depends on the size, complexity and revenue derived there from. For these reasons, different types of protection schemes are employed for power and distribution transformers.The type of protection used should minimize the time of disconnection for faults within the transformer and to reduce the risk of catastrophic failure.Any extended operation of the transformer under abnormal condition such as faults or overloads compromises the life of the transformer.This implies that the speed of isolation upon the occurrence of a fault is very important.

CAUSES OF TRANSFORMER FAILURE The windings and magnetic core of transformers are subjected to a number of different forces during operation amongst which are : Expansion & Contraction caused by thermal cycling; Vibration caused by flux in the core; Localized heating caused by eddy current in parts of the winding induced by magnetic flux; Impact forces caused by through-fault currents; Thermal heating caused by overloading.

Transformers through-out the power system, experience different levels of through-fault currents in terms of magnitude, duration and frequency.Through-faults in transformers can produce physical forces that causes insulation wear and friction induced displacement in the winding.These effects are cumulative and should be considered over the life of the transformer.

CLASSIFICATION OF FAILURES IN TRANSFORMERSFailures in transformers can be classified into:Winding failures due to short-circuit(turn-to-turn faults; phase-to-phase faults; phase-to-ground & open-circuited winding faults.);Core faults(core insulation failure; shorted laminations; corroded laminations);Terminal failures(open leads; loose connections; short-circuits);Abnormal operating conditions(over-fluxing, over-voltage & over-loading);External faults;On-load tap changer failures(mechanical, electrical & short-circuits).

TRANSFORMER PROTECTION SCHEMESDISTRIBUTION TRANSFORMER PROTECTIONA distribution transformer steps down distribution feeder voltage to the utilization voltage(11kv/415v).For best efficiency, distribution transformers should be operated between 50% and 75% of full load rating.The protection arrangement for distribution transformers is very simple but adequate as long as the devices used are not subjected to any form of abuse.The devices used are:-Fuses Lightning ArrestersTransformer Neutral Earthing.

FUSESFuses are used as primary protection for distribution transformers because of their simplicity & low cost.They are short time devices and can be applied to protect distribution transformers of up to 33kv.Generally, a fuse is designed to snap or rupture when its current rating is exceeded. Fuses ruptures for a variety of reasons. The knowledge of the magnitudes of the transformer primary and secondary current, will help in the correct selection of the primary and secondary fuse ratings. Hence, when properly selected, fuses offer adequate protection for distribution transformers against over load and short circuit currents.

Example:-What are the correct J&P or R.M.U and H.R.C cartridge fuse ratings suitable for a 500KVA 11/0.415kv transformer assuming a 4-way feeder pillar is used with 4 overhead cables.

Solution:Transformer full load primary current = IpIp = KVA = 500 = 26.244 Amps 3 x kv 3 x 11Transformer full load secondary current = Is Is = KVA = 500 = 696 Amps 3 x kv 3 x 0.415

Since a 4-way feeder pillar was used, the secondary current per feeder pillar unit is I4 = 696 Amps=174 Amps 4The nearest value of fuse available is 150 Amps. Therefore 150 Amps can be used per phase in each feeder pillar unit.It is note worthy that continuous overloading is one of the main causes of premature ageing and breakdown of distribution transformer winding insulation.

EFFECTS OF TRANSIENT OVER-VOLTAGESDistribution transformers are subjected to transient over-voltages resulting from the networks to which they are connected.These voltages are either the result of direct or induced lightning strikes on the H.V. or L.V. networks or of surges generated during switching by switchgear operations. These voltages are usually higher than the rated breakdown voltage of the insulation and are usually responsible for the premature ageing of the transformer winding insulation.The cumulative ageing process of the transformer winding insulation eventually leads to its failure in service.

LIGHTNING ARRESTERS AND SPARK GAP PROTECTIONTwo means of over-voltage protection widely used are:Spark Gap Protection &Lightning Arresters.

(1.) Spark Gaps Spark Gap Protection devices are the simplest and least expensive scheme. Voltage limiting across its terminals is achieved by arcing in the air gap.(2.) Lightning Arresters Lightning Arresters provides protection with greater performance but at noticeably higher cost. They are made from voltage dependent materials as soon as the threshold is exceeded, it starts to conduct.

The disadvantages associated with spark gap protection are as follows :The appearance of an earth fault current after spark gap protection operation. This follow-up current whose intensity depends on the networks neutral earthing arrangement, cannot in general extinguish itself spontaneously and requires the intervention of an upstream protection device.Dependence of the level of protection in relation to the steepness of the voltage gradient. This implies that a high over-voltage with very steep gradient does not lead to arcing until a peak value noticeably greater than the protection level is reached.High variations in flash-over level as a function of environmental conditions(humidity, dust, foreign body, etc).

The basic requirements of lightning arresters are:-It should behave as a perfect insulator for the highest system voltage to ground.It should discharge any over-voltage to ground safely.It should restore itself as insulator after discharging the follow voltage.

In order to facilitate this process, the earth resistance must be very low; typically 2 ohms or lower. Transformer 11kv feeder J&P fuse

Location of Lightning Arresters The Lightning Arrester should be located as close as possible to the apparatus or equipment to be protected, particularly if an overhead line terminates in a transformer. During wave propagation phenomena, the transformer represents a point of almost total reflection and the stress that it is subjected to can reach approximately twice the maximum voltage of the incident wave.

TRANSFORMER NEUTRAL GROUNDING

Transformer neutral or star point is usually grounded in order to protect the transformer against the dangerous effects of over-voltages and heavy short circuit current by holding the neutral potential close to ground potential.Neutral Earthing ensures the rapid disconnection of faulty apparatus from the system without undue delay and enables the use of protective relaying(earth fault relays) for fault clearance.The ground resistance must be low(typically 2 ohms or less for distribution substations).In practice, distribution transformer neutral grounding is effected in the feeder pillar by providing a link between the earth bar and the neutral bar. Neutral grounding can be solid, through resistance or reactance or both. The practice in P.H.C.N. is solid grounding.

PROTECTION OF POWER TRANSFORMERS

Power transformers of various sizes and configuration are installed throughout the power system. They are bulk power sources and play an important role in power delivery and the integrity of the power system network as a whole. They operate at full load for best efficiency.Power Transformers are subjected to many external electrical stresses from both upstream & downstream.The consequences of any failure can be very great in terms of operating losses.The transformers must therefore be protected against faults of external origin on one hand and isolated from the network in case of internal faults on the other hand.

POWER TRANSFORMER PROTECTION SCHEMES Protection of Power Transformers does not require the use of Fuses. In its place protective relays and circuit breakers are used because of the magnitude of the voltage and fault current.Power transformer protection schemes can be divided into two categories namely. Non Electrical protectionElectrical protection

NON ELECTRICAL PROTECTION Non Electrical Protection implies that the relay used is not an electrical device but has connections which when actuated can give an alarm and or trip the breaker to isolate the transformer.

THERMAL RELAY PROTECTION In the first category is the transformer oil temperature indicator. This is a simple instrument which indicates the top oil temperature at an instant of time and loading cycle. If the temperature of the top oil is found to be above permissible limit, the transformer is understood to be overloaded.Immediate steps must be taken to ensure that the top oil temperature drops and remains within tolerable limit.The longer a power transformer is allowed to operate above permissible temperature limit, the faster the insulating materials and oil deteriorate and may cause premature failure at an early stage.

The thermal relay with alarm and trip contacts is a very common device for protecting the transformer against high winding temperatures.The thermal relay is immersed in the transformer oil and energized from a current transformer so that it responds to the copper temperature (direct heating effect of load current).The relay can be arranged to close several sets of contacts in succession as the copper temperature climbs with increasing load. The first contact to close can start fans or pumps for auxiliary cooling.The next contact can warn of temperatures approaching the maximum safe limit while the final contacts can trip a circuit breaker to isolate the transformer

BUCHHOLZ PROTECTION

The Buchholz relay is a mechanical safety device sensing the accumulation of gas in large oil-filled transformers and will cause an alarm on slow accumulation of gas or initiate the operation of the transformer breaker to isolate the transformer if the gas is produced rapidly in the transformer oil.

The relay is installed in the pipe between the transformer main tank and the conservator. It responds to internal arcing faults and slow decomposition of insulating materials. It can also detect low oil level in the transformer due to leakage.

It has two elements, a float switch and a combined hinged flap and float switch. Gases generated due to internal failure in the transformer tank cause streams of bubbles which move upwards and towards the conservator tank but is trapped in the buchholz chamber.The trapped gases, displace the oil in the buchholz chamber consequently lowering the upper float.The operation of the switches connected to an external alarm and trip circuit is then initiated for incipient faults and serious faults respectively.Buchholz protection is an important protective scheme for power transformers as it detects fault within the transformer particularly in the incipient stages to avoid major breakdown and sudden failure of power supply.

TYPES OF GASES GENERATED IN TRANSFORMER OILIn oil immersed transformers, different types of gases are generated due to different faults or due to degradation of different materials in the transformer.The major advantage of this gas evolution is that substantial amount of gas is evolved even for very incipient faults or material degradation.Thus, analysis of this gas forms a very important means for monitoring the health of the transformer or for determining the nature of the fault in the event of a tripping due to gas accumulation.The gas can be analyzed on-line(in-situ = that is, as it is being produced inside the transformer oil) in case such systems have been installed on the transformer.

Alternatively, oil samples from the transformer can be taken periodically for analysis in a lab.The periodicity depends on the size and criticality of the transformer.The implication of a few of the gases that may be observed in the oil is as follows:-

Hydrogen : is generated by corona or partial discharge. In conjunction with other gases observed with it, the source of the discharge can be determined.

Ethylene : is associated with the thermal degradation of oil. Trace quantities of Methane and Ethane are generated at 150C. Ethylene is generated in significant quantities at 300C.Carbon Dioxide & Carbon Monoxide : are evolved when cellulose(treated paper) insulation is overheated.Acetylene : is produced in significant quantities by arcing in oil.

ELECTRICAL PROTECTION

Electrical protection is used to protect power transformer from unbalanced short circuit current and over-voltages.When an electrical protection operates, isolation of the power transformer is instantaneous because of the magnitude of current and voltage involved.

DIFFERENTIAL RELAY PROTECTION

The differential protection is an arrangement that covers the transformer and provides the best overall protection for both phase and ground faults. This protection is a current balance scheme. It compares primary and secondary currents of two winding transformers or primary, secondary and tertiary currents of three winding transformers.

The basic requirements that the relay must satisfy are :- The relay must not operate for load or external faults;The relay must operate for severe enough internal faults.

The Differential scheme is a unit protection and the protected zone is exactly determined by the location of the current transformers.

CURRENT TRANSFORMER (C.T.) CONNECTIONSA simple rule of thumb is that the current transformers on any wye(star) winding of a power transformer should be connected delta and the current transformers on any delta winding should be connected wye(star).This is necessary in order to take care of phase shift(usually 30) in the delta windings so that the currents presented to the relay are vectorially the same. For stability in a differential relay scheme, the vectorial difference of the currents must equal to zero so that their effects cancel out.

For this to be achieved, matching current transformers are used in the differential scheme.Any internal fault within the transformer or the difference zone, will bring about a difference between high voltage(H.V) primary currents and low voltage (L.V) secondary currents or between primary, secondary & tertiary currents in case of 3 winding transformers.This imbalance or difference in current will flow in the relay winding to cause operation of the differential relay with the consequent isolation of the transformer.

The relay provides protection against the followings :-Short circuit between phases;Short circuit between windings;Short circuit between phases and earth;Any electrical fault within the protected zone. Trial re-closure is not permitted anytime the differential relay operates. The transformer should be disconnected and properly tested to establish the cause of tripping before restoration in service can be made to prevent subsequent trippings.

RESTRICTED EARTH FAULT (REF)PROTECTION

Restricted earth fault (REF) protection scheme is a current balance scheme involving only the residual currents passing through the phases of one winding and the associated neutral current of the same winding. It is only used to protect a transformer against earth faults close to the neutral of the star winding of a power transformer.

It is based on the principle that if there is an internal fault to earth on any of the windings, the summation of the line currents will no more be the same as the neutral current. Therefore the relay balance will be upset.

Fault detection is confined to the zone between the current transformers and hence the name restricted earth fault.REF protection is fast and can isolate winding faults very quickly, thereby limiting damage and consequent repair cost.To prevent the relay from burning out due to high voltages seen by the relay during a fault, a stabilizing resistor is required. This should be an adjustable resistor which enables it to be used at different earth fault settings.REF relays are high impedance relays and like differential relays, it operates instantaneously and trial re-closure is not allowed until a thorough investigation of the cause of tripping has been carried out.

OVER CURRENT AND EARTH FAULT PROTECTION

Over-current and earth fault relay protection are usually applied on small power transformer since the use of differential relay is not economical.

This protection scheme is used as a back-up for larger power transformers. It does not distinguish between external short circuit, overloads and internal faults.

The scheme comprises of two over current relays on the Red & Blue phases while the earth relay is installed on the yellow phase.The relays are the inverse time type where the time of operation is inversely proportional to the current.These relays also have instantaneous attachments for fast clearing of faults very close to the substation because of the magnitude of fault current involved.

STANDBY EARTH FAULT (SBEF) PROTECTION

The SBEF protection is employed to protect large power transformers against both external and internal faults. It is normally a kind of back-up protection with a relatively long delay.The relay is connected to the neutral current transformer and is meant to coordinate with down stream earth fault relays.Operation of standby earth fault (SBEF) protection relay trips both primary and secondary circuit breakers of the power transformer.

TRIPPING UNIT : D.C. SUPPLY SYSTEMIntroduction : All protection schemes designed to prevent or minimize damage to equipment requires an actuating signal. This signal can be an alternating current(a.c.) or a direct current (d.c.) signal.Alternating current(a.c.) supplies can be used for protection and control systems, but the major problem is that it can disappear when needed most depending on the source of supply. The fact that a.c. cannot be stored makes it less attractive.In practice, a battery bank with a matching charger (rectifier) are the inseparable pair which must be installed for a healthy d.c. system.

The standard arrangement is for the charger to provide d.c. to the standing load and at the same time provide trickle charge or boost charge to the battery bank depending on the discharge condition of the batteries in the bank.The battery bank should come into supply the load during emergency or loss of output from the charger.It is therefore extremely important that the batteries in the bank be fully charged at all times so that in times of prolonged outage to the station or charger, the battery bank can provide the required d.c. signal for protection & control.

Direct Current supply failures have not only led to an outbreak of fire, they have also led to a complete collapse of the power system.In fact it is safer to shut down the station if the battery bank output is not available because without d.c. there is no protection for the bulk of the equipment/apparatus in service in the station.THE CHARGER : The d.c. supply unit is basically a rectifier circuit where an alternating current is converted to direct current. The a.c. is stepped down to the value needed to obtain the required d.c. voltage.

The power rectifier assembly is made up of silicon diodes with the regulator circuits, d.c. output breakers or fuses and the required number of feeder breakers to distribute the d.c. power to the various loads.The arrangement of the charger and the battery banks with the associated d.c. distribution panels is usually the practice in PHCN where the charger is used to continuously charge the storage batteries as well as supply the d.c. loads. The charge rate is usually low in the case of floating(about 0.3 amps or thereabout) and usually high for boosting about 3.5 amps and above depending on the battery capacity.

Supervisory schemes are employed extensively in d.c. supply units to monitor the prevailing condition of the supply unit and to alert in case of unwarranted situations because a highly reliable d.c. system is a requisite for proper operation of a power system.Some supervisory schemes and protection which are in use presently are: A.C. Fail scheme; Charger Failure scheme; Miniature Circuit Breaker(M.C.B) Trip; Battery Earth Fault; Battery Low Voltage scheme; Boast Charge scheme.

While some of the supervisory schemes employ d.c. for its alarm annunciation, it is to be noted that the alarm annunciation for d.c. failure will necessarily be an a.c. scheme(a.c. visual indication and alarm).Generally speaking, if the charger and the d.c. distribution panel are not subjected to any form of mal-operation, they hardly breakdown.However, some form of maintenance is carried out on them, these includes: Periodic check of d.c. voltage output; Panel cleaning and removal of dirt/cobwebs; Check for Charge retention ability; Replace ruptured fuses.

Standard voltage ratings of the d.c. supply unit (Charger) are 30 volts, 50 volts, 110 volts, & 150 volts while the ampere ratings are usually 3 amps, 6 amps, 10 amps, 20 amps, 30 amps, 40 amps, 50 amps.The capacity of the substations, the switching arrangement and the standing load are the factors which decide the size and rating of the supply unit.

STORAGE BATTERIES A battery is an electro-chemical device that is a source of direct current(d.c.) electricity. Some batteries allow recharging while others do not.The primary battery cells are designed for discharge operation and not intended to be recharged. A typical example is the carbon-zinc dry cells used in flashlights and radio sets.The secondary batteries, on the other hand are designed for repeated discharging and recharging or cycling without appreciable decrease in capacity per cycle.The secondary battery can be maintenance free while some others require maintenance.

The condition of a fully charged lead-acid or Nickel-alkaline battery deteriorates if left unattended to, over a period of time, even when left on open circuit.Batteries requires trickle charge to overcome internal losses called self discharge or standing loss which tend to drain the battery, and remain fully charged at all times.The cell voltage is the rated steady open circuit e.m.f of a fully charged cell and is usually about 2.1 volts or 2.2 volts/cell for the lead-acid type while it is 1.2 volts for the alkaline type. The battery bank voltage is normally the sum of the cell voltages in the bank.

The specific gravity of the electrolyte to a large extent determines the status of the cells.The values of the specific gravity when the cell is fully charged is 1.21 0.1 and 1.18 when discharged.The specific gravity is measured with a hydrometer. During charging, the density of the electrolyte increases due to evaporation of water.Evaporation of electrolyte should be made up by adding distilled water occasionally but never acid.

The rate of fall of d.c. voltage depends on the following factors: Current demand of load; Duration of discharge; State of charge of battery at start of discharge; Ageing.

Battery Capacity Ratings : The capacity of the battery is its ability to deliver a given amperage for a given period of time at a giving initial cell temperature while maintaining voltage above a given minimum level.

The Ampere-hour rating is simply the product of the discharge current and a given period of time. It is usually specified at a given definite discharge rate.Thus a battery cell of 250 AH type 10gro E250 at the 10 hour rate of discharge will give 25A output continuously for 10 hours.A given cell usually delivers more total ampere-hours when the discharge rate is decreased.Conversely, the same cell will deliver fewer total ampere-hour when the discharge rate is increased under similar conditions.

Battery Bank Installation : The installation of a battery bank should take cognizance of the followings: Proper battery rack should be constructed. Where steel rack is specified, plastic channels to protect the steel rails against corrosion and to provide insulation should be used. Battery bank should be located as close as possible to the load to avoid excessive voltage drop along the conductor. Appropriate conductor cross sectional area should be used. Battery room to be properly ventilated so as to prevent a build-up of hydrogen gas. Smoking and open flame should always be prohibited in the battery room.

Maintenance of Battery Bank : The most important aspect of battery maintenance is the addition of distilled water to correct electrolyte level. Scheduled checks of electrolytes specific gravity, voltage levels, cell voltage, etc should be noted and recorded in the battery maintenance report sheet. Battery cleaning, greasing of terminals and connections should be done every six months. Battery room should always be kept clean and properly ventilated.

PRECOMMISSIONING PROCEDURES

Transformers are dispatched to various locations either:-In their tanks with sufficient oil to cover the coils, the remaining oil being dispatched separately in sealed steel drums, or alternatively with the full complement of oil in the tanks.In their tanks without oil, all oil being sent separately.

Upon arrival at the site, the transformer is thoroughly examined for possible damage which may have occurred in transit.

Oil leakage from the tanks, cooling fins, bushings etc should be noted and stopped immediately.

Where the transformer was dispatched to the site without oil, as in the case of large power transformers, after installation of the cooling fins, it should be dried out to remove moisture in the transformer tank.When receiving oil which has been dispatched separately, each drum should be inspected and samples tested for the presence of moisture.If necessary, the oil should be dried out before pouring into the transformer tank up to the requisite level as indicated by the oil gauge. The silica gel is then poured into a receptacle (breather) and installed at the oil conservator tank.

TESTS

Various test are carried out on transformers in order to confirm the exact condition of the windings. Even new transformers have been known to fail under test. Perhaps due to storage, haulage & handling conditions.

INSULTION RESISTANCE TEST

This involves the use of a portable 500volts 5,000 volts insulation resistance tester. For a transformer with voltage rating 11/0.415kv, the H.V. winding is tested by applying 5,000 volts at the H.V. terminal with respect to ground. The L.V. side is tested with 1,000 volts.All values obtained are recorded in meg-ohms. Any value below 100 meg-ohms is regarded as bad and is usually seen as a sign of deterioration of the insulation of the windings or ingress of moisture in the oil/ windings.

The results are recorded as follows : HV E = 300 M

LV E = 250 M

HV - LV = 900 M

INSULATION TEST

A 60KV or 80KV D.C insulation tester is used for this purpose. For a transformer with voltage rating 11/0.415kv, 25kv D.C is applied on any of the primary (H.V) winding terminals for 1 minute and 2kv applied on the secondary (L.V) winding terminal for 30 seconds both with respect to ground.

The leakage current is noted if possible. Any considerable drop in the voltage applied isindicative of a fault in the windings of the transformer. The continuity of the windings in the transformer is also checked.

The results are recorded as follows :

T/F windingInitial VoltageTimeContinuityRemarksH.V.25KV1 minuteOkayGoodL.V.2KV30 secondsOkayGood

RATIO TEST

Ratio test is used to check the transformation ratio in the windings of the transformer.

It helps detect any abnormality in the windings.

Transformer ratio test is of 2 types : (i.) Voltage Ratio Test; (ii.) Turns Ratio Test.

VOLTAGE RATIO TEST This is used to check the ratio of voltage transformation in the windings of transformers. This test is usually carried out on Distribution Transformers using a single phase source of a.c. power supply, e.g. Generator. The result of a single phase voltage ratio test of a 500 KVA, 11kv/415v dy11 Transformer is shown below:

ABBCCAabbccaanbncnR215168468.32.65.64.63.61.0Y1132161027.16.80.22.44.62.2B511662172.48.25.81.13.54.6

A three phase voltage ratio test is achieved by using a 3-phase source of a.c. power supply.The difficulty in obtaining a 3-phase source of power supply is usually a set back.The result of a 3-phase voltage ratio test of a 500 KVA, 11kv/415v dy11 transformer is shown below.

ABBCCAabbccaanbncn38838838814.414.414.48.28.28.2

TURNS RATIO TEST : The Ratio Meter MethodThe ratio-meter is used to carry out this test

This is one of the final tests before placing a power transformer in service(especially for new installation).The ratio meter is used to verify that the transformer is connected correctly to give the selected voltages as given on the name plate. With modern electronic ratio-meters, the test is simplified. At switch-on, input transformer vector group at the prompt;Input the test voltage, the tap-position and the secondary voltage. The remainder of the test is automatic with the display indicating ratio, magnetizing current, etc.The ratio test is usually carried out on all tap positions.

The formula used to calculate the nominal ratio is n = X1 Xo x 100 H1 H2

It will be found that the calculated ratio and the ratio reading taken will not match exactly. This slight difference between calculated and actual ratio may be due to the construction of the transformer, the ratio-meter itself and the use of a tap-changer in the windings. The simple ratio of turns or voltage and the related ratio error is not sufficient to detect all possible failures of a transformer winding.

EXCITATION TEST

In this test, a single phase a.c. supply voltage(230 Volts) is applied to the secondary terminals of the transformer with the primary terminals open-circuited.The voltage is applied to all the phases one after the other with the phase voltage at the secondary terminals measured and noted.

Note that the transformer is operated in reverse(i.e. in step-up mode) and a dangerous high voltage will be available at the primary terminals and therefore no measurement can be conducted there.

An excitation test is very powerful and can indicate that a transformer is faulty even when the insulation resistance test, insulation test and ratio test results are okay.

The result of a test on a 500KVA 11kv/415v, dy11 transformer is shown below :

anbncna189136 52b 98196 98c 52136189

TRANSFORMER EARTH RESISTANCE TEST

Transformer or sub-station earth resistance test is usually carried out prior to the commissioning of transformers in service.This is achieved by measuring the resistance to flow of ground current using an earth resistance tester.Typical earth resistance acceptable for distribution substations in 2 ohms. For power transformer substations, this value has to be improved to less than 1 ohm in order to prevent damage to equipment by providing a low impedance path between a fault and the source of ground-fault current. It also help to facilitate the operation of protective devices and minimize the build-up of static charges.

CAPACITY TEST Capacity confirmation test is conducted on a transformer to determine the capacity of a transformer whose rating is not known or whose name-plate is suspicious.At the commencement of this test, a 3-phase voltage ratio test is conducted.The L.V. terminals of the transformer are then shorted. A 3-phase supply is connected to the H.V. terminals and clamp-on ammeters used to determine the values of the circulating currents(H.V. and L.V. sides). It is note worthy that the circulating current in the L.V. windings is not less than 70% of full load current. This is a guide as to the selection of appropriate shorting links.

The results and calculation are as follows for a 500KVA transformer whose impedance is 4% :

1.) 3-Phase Voltage Ratio Test

ABBCCAabbccaanbncn38838838814.414.414.48.28.28.2

SHORT-CIRCUIT TEST RESULT

H.V. Side L.V. Side

R22.0 Amps R610 Amps

Y23.3 Amps Y610 AmpsB23.1 Amps B586 Amps

TRANSFORMER CAPACITY CONFIRMATION FOR PRIMARY SIDE FOR SECONDARY SIDE IN = ISC X VN X Z% IN = 610 X 415 X 4 VT X 100 14.4 X 100 = 703.194 Amps IN = 23.3 x 1100 x 4 Power, P = 3 x 415 x 703.194 388 x 100 = 505.4 KVA = 26.42 Amps > 500 KVA Power, P = 3 x VN x IN = 3 x 1100 x 26.42 = 503.35 KVA > 500KVA

Where : IN = Normal Current ISC = Short Circuit Current VN = Normal Voltage VT = Test Voltage Z = Transformer winding impedance.

MAINTENANCE OF TRANSFORMERS If a transformer is to give long and trouble free service, it should receive a reasonable amount of attention and maintenance.The causes of breakdown of transformers includes :

Faulty design and construction;Incorrect installation;Prolong overloading;Neglect;Wear and Tear and other deterioration;Accidents.

It is standard practice all over the world to draw up maintenance schemes for transformers as they age in service.In recent past, maintenance used to be time based, but the modern trend is condition based in which the performance/condition of a transformers is monitored on a continuous basis.This method is cost effective because limited resources would be utilized for transformers that are in dire need of maintenance.Transformer maintenance consists of regular inspection, testing and reconditioning where necessary.Records should be kept of the transformer performance, giving details of all inspections and tests made and of any unusual occurrences if any.

The reasons for the maintenance of transformers are as follows:-To prolong service life of transformers by preventing premature failures.To minimize power outages and the attendant loss in revenue to suppliers and end-users of electric power.To prevent frequent costly replacement of transformers and the associated long delivery time.To ensure that the useful lives of transformers are fully utilized which translates to good return on investment (R.O.I)

Maintenance can be subdivided into two main parts namely:-

Preventive Maintenance

a. Routine Maintenanceb. Overhaul Maintenance

Breakdown Maintenance

Routine maintenance of transformer include:-Inspecting the transformer for rusts and leakagesRemoving climbers from T/F bodyReplacing moisturized silica gelTesting oil samples obtained from the top and bottom of the transformer tankChecking T/F oil levelTaking T/F load readings and balancing the loadReplacement of vandalized earth conductors.Insulators and bushings cleaned periodically.

Overhaul of Transformers include:-

Complete change of transformer oil and gaskets.Improving on the insulation of the windings.Replacement of damaged bushingsMaintenance of tap changersPurification of transformer oil.

BREAKDOWN MAINTENANCE OF TRANSFORMER

A transformer is confirmed to be defective by a series of tests which include:-

Insulation resistance testInsulation testRatio testExcitation testContinuity test

The repair process of a failed transformer can be time consuming depending on the KVA rating and the availability of materials. It can run into months.

The scope of work involved include:-Un-tanking the transformerDismantling and removing the laminationsRemoval of defective windingsRewinding of the defective windingsTesting to confirm characteristicsRe-assembling of laminationsCouple and oven dryRe-tank dry by vacuuming and or filtrationFinal electrical tests

BREAKDOWN MAINTENANCE OF TRANSFORMER WITH DEFECTIVE TAP CHANGER

Repair is usually by replacement of damaged component parts or a complete replacement of the tap changer with a similar one.The first thing to do is to remove the diverter switch unit (tap changer) with a suitable lifting device.Drain the oil in the compartment and clean the chamber with Electro-clean.Check thoroughly to ascertain the extent of damage. Clean and vanish flashed points on the cylinder housingDry the unit in an oven to improve on the insulation.Re-install and fill with fresh oil.Duration of repair can be up to 2 weeks.

TRANSFORMER DRY-OUT PROCEDURE

It is essential that all transformers be as free as possible from moisture before being placed in service. Due to the hygroscopic nature of the oil and the various insulating materials used, a more or less appreciable amount of moisture will be absorbed by both when they are in contact with the atmosphere.

If samples of oil tested indicate the presence of a small amount of moisture, the oil alone should be dried out, but if a large proportion of moisture is indicated, it would be advisable to dry the transformer itself in addition.

For this the transformer must be un-tanked and allowed to drip-dry before placing it in an oven. Thereafter, the improvement of the winding insulation is monitored by the use of an insulation resistance tester.When standard insulation values are obtained, the transformer is brought out and re-coupled. It is then filled with new oil.This method is only suitable for drying small and medium sized transformers for reason that border on weight and handling. Consequently, vacuuming and hot oil filtration is the most convenient method for drying-out larger transformers.

TRANSFORMER DRY-OUT BY VACUUMIn a vacuum environment, the pressure is lower than atmospheric. Consequently engineering fluids like moisture and volatile liquids evaporate at faster rates and at reduced temperature.The essence of vacuuming a power transformer is to extract as much moisture as possible from the windings.Distribution transformers in the KVA range cannot be vacuumed because the tank cannot withstand vacuum pressures.

When vacuuming a transformer, better results are obtained faster if the hose from the vacuum pump of the machine is connected at or near the top of the transformer as the tank construction will permit.

This dry-out technique is used when the insulation resistance of a transformer tends towards zero meg ohms.The procedure is to drain the entire oil from the transformer into a clean receptacle.

Measure and record the insulation resistance values at regular intervals, say every 4 hours with the vacuum broken. Never measure transformer insulation under vacuum.

Stop vacuuming when a appreciable improvement is noticed. If possible heat the oil in the receptacle and re-introduce into the transformer under vacuum through the bottom valve.

Filter the transformer. Stop when the standard insulation values in excess of 300 meg ohms is achieved with the oil still hot as the heated system cools towards ambient temperature.

The insulation resistance will further improve.

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