M MTEKP PROPOW WER TRANSF R FORME ER TES STING MT TEKPRO Technolo ogies Pvt t. Ltd, Ne ew Delhi -110048 8 Phone:- +91 P 1 11 4617 3333, Fax:-+91 11 41825662 Email:- info@m mtekpro.com, shonal@ @mtekpro.c com, Web:-m mtekpro.co om m[T Type text] [Type tex xt] [Typ pe text] Power Transformer Testing 2 Testing of Power Transformers 1. Verification of voltage Ratio and vector Group or phase displacement and polarity. MTEKPRO Technologies Pvt. Ltd, New Delhi. (Measured turn ratio) . . which means the turn ratio between the winding couples which have the parallel vectors in the vector diagram can be measured. Ltd. The aim of measurement is.(Designed turn ratio) % Deviation = (Designed turn ratio) * 100 The % deviation of the turn ratios should be ≤ 0. determining the conditions of both the windings and the connections and examining the problems (if any). During measurement. MTEKPRO Technologies Pvt. New Delhi. The value read on this error indicator shows the deviation of the transformer from real turn ratio as %. only turn ratio between the winding couples which have the same magnetic flux can be measured. confirming the no-load voltage ratio given in the customer order Specifications. 3 1. Turn Ratio Measurement 1. it is changed until a balance occurs on the % error indicator.Power Transformer Testing 1. applying a phase voltage to one of the windings using a bridge (equipment) and measuring the ratio of the induced voltage at the bridge.2 Turn Ratio Measurement Bridge method Measurement of turn ratio is based on. The measurements are repeated in all phases and at all tap positions. sequentially. Theoretical turn ratio = HV winding voltage / LV winding voltage The theoretical no-load turn ratio of the transformer is adjusted on the equipment by an adjustable transformer. The measurements are made at all tap positions and all phases.1 Purpose of measurement The no-load voltage ratio between two windings of a transformer is called turn ratio.5 %. g.b’ correlation has to realized. MTEKPRO Technologies Pvt. Fig: . .Connection group representation and measuring The order of the measurements: 1) .voltage between B and B’ is measured 5) .voltage between phases (e.A short circuit is made between C and n 4) . Taking the other phases as reference for starting. Representing as vectors. AC) is measured 3) . in order to be Dyn 11 group. Also. For example. A. the bridge can be balanced. the LV end shows 11 o’clock (after 330⁰). same principles can be used and also for determining the other connection groups. Determining the connection group is valid only in three phase transformers. If the vector directions of the connection are correct. same principles will be helpful. the input and output windings of a multi-phase transformer are connected either as star ( Y ) or delta( D ) or zigzag( Z ).1.3 phase voltage is applied to ABC phases 2) . in Dyn 11 connection group the HV winding is delta and the LV winding is star and there is a phase difference of 330⁰ (11x30⁰) between two windings.c’ > AB > B. The high voltage winding is shown first (as reference) and the other windings follow it. Ltd. checking the connection group or polarity is possible by using a voltmeter. Direct current or alternating current can be used for this check.voltage between A and c’ is measured As seen from the vector diagram. The connections about the alternating current method are detailed in standards. While the HV end shows 12 (0). An example of this method is shown on a vector diagram below. New Delhi.2 Determining the Vector Group Power Transformer Testing 4 Depending on the type of the transformer. The phase angle between the high voltage and the low voltage windings varies between 0⁰ and 360⁰. the HV winding is represented as 12 (0) hour and the other windings of the connection group are represented by other numbers of the clock in reference to the real or virtual point. Power Transformer Testing 5 MTEKPRO Technologies Pvt. New Delhi. . Ltd. potential differences appear between the open terminals and are used to determine the vector group.3: Polarity test and connection test on three-phase transformer Using one voltmeter MTEKPRO Technologies Pvt.1.3.3 Polarity test 1.1 Polarity test using voltmeters Single-phase transformers Power Transformer Testing 6 For single-phase transformers the polarity can be either additive or subtractive. Ltd.3. Figure 1.3. if the phase displacement is correct. New Delhi. The low voltage winding is connected in series with the high voltage winding.2 Figure 1. This is done by connecting a terminal from the low voltage side to a terminal on the high voltage side.2: Connection for subtractive polarity test.3.3.1.3. see figure 1. Polyphase Transformers.3. . For additive determination of polarity. either in phase or in opposite phase. see figure 1. see figure 1. The vector group must be checked for three-phase transformers.1: Connection for additive polarity test And for subtractive determination of polarity.3. Figure 1. When a three-phase supply is connected to the high voltage winding. The polarity is shown on a polarized voltmeter connected to the low voltage side.3. Ltd. New Delhi.4: Basic analog ratio bridge circuit MTEKPRO Technologies Pvt.4. .2 Polarity check using DC current.3. see figure 1. Figure 1.1. Power Transformer Testing 7 This method establishes the polarity of single and three-phase transformers by briefly switching on a DC current source at the high voltage winding.3. New Delhi. Ltd.Power Transformer Testing 8 Testing of Power Transformers 2. Winding Resistance Measurement. . MTEKPRO Technologies Pvt. Measurement with voltmeter and ammeter The measuring circuit is shown in figure 2.1 Purpose of the test Winding resistance serves a number of important functions like: Power Transformer Testing 9 • Providing a base value to establish load loss. New Delhi. The resistance is calculated from the readings in accordance with Ohm’s Law.2.1. however. For temperatures within the normal operating range of a transformer the following relationship between resistance and temperature is sufficiently accurate: C+Ø2 R 2 = R1 C+Ø1 Where: R1 = resistance at temperature Ø2 R2 = resistance at temperature Ø1 Ø = temperature in °C C = constant which is a function of material type IEC [1] specifies: C = 235 for copper = 225 for aluminum Principle and methods for resistance measurement:There are basically two different methods for resistance measurement: namely. 2. the so-called “voltmeterammeter method” and the bridge method. 2. • Providing a basis for an indirect method to establish winding temperature and Temperature rise within a winding. Resistance RX is calculated according to Ohm’s Law: RX=U/I MTEKPRO Technologies Pvt. Temperature dependence It should be noted that the resistivity of the conductor material in a winding – copper or aluminum – is strongly dependent on temperature. • Inclusion as part of an in-house quality assurance program. today digital devices such as Data Acquisition Systems (DAS) with direct resistance display are being used more and more. Winding resistance is always defined as the DC-resistance (active or actual resistance) of a winding in Ohms [Ω]. .2 General. Winding Resistance Measurement. Ltd. like verifying electric continuity within a winding. “Voltmeter-ammeter Method” The measurement is carried out using DC current. This measurement may be performed using conventional analog (rarely used nowadays) or digital meters. Simultaneous readings of current and voltage are taken. DC-current is made to flow through RX and RN and the corresponding voltage drops are measured and compared. Ltd.1: Voltmeter .ammeter method measuring circuit Figure 2. figure 2.Power Transformer Testing The advantage of this method is the simplicity of the test-circuit. On the other hand. the voltage drop across the unknown winding resistance RX. which have relatively high resistance values.2. New Delhi. . The bridge is balanced by varying the two resistors Rdec and RV. A balanced condition is indicated when the galvanometer deflection is zero. this method is rather 10 inaccurate and requires simultaneous reading of the two instruments. Figure 2. compared to a voltage drop across a known resistance RN (standard resistor). at which time the following relationship holds: Rdec RX = RN Rv The influence of contact resistances and the connection cable resistances (even of the connection between RX and RN) can be neglected.2: Kelvin (Thomson) Bridge method MTEKPRO Technologies Pvt. “Resistance measurement using a Kelvin (Thomson) Bridge” This measurement is based on the comparison of two voltage drops: namely. depending on the measuring current value (magnetizing current). .2 times higher than the crest value of the magnetizing current to be sure to saturate the iron core 11 Figure 3. Winding inductance is strongly dependent on current and displays the following characteristic for transformers. As the measuring circuit time-constant is given by the relation L/R. Therefore.3: Equivalent circuit of a Winding Figure 3. New Delhi. the current-time characteristic differs quite significantly when switching on the DC-source.4: Inductance of transformer winding as a function of the current. Ltd. Minimum value: The lower limit of the DC-current is given by the following considerations: The measuring circuit for all resistance measuring methods consists of a DC-source and a transformer winding fixed around an iron core as represented by the following equivalent circuit. the DC measuring current should be at least 1. MTEKPRO Technologies Pvt. see figure 3. the DC-current should be limited to a maximum 10% of the rated current of the corresponding winding.4.Power Transformer Testing Value of the DC-current of measurement Maximum value: To avoid an inadmissible winding temperature rise during the measurement. Power Transformer Testing 12 Testing of Power Transformers 3. Ltd. MTEKPRO Technologies Pvt. . New Delhi. Magnetic Balance and Magnetising Current Measurement. no winding terminal should be grounded. . Ltd. when the center phase is excited then the voltage induced in the outer phases is generally 30 to 70% of the applied voltage. Left side phase AN Voltage applied at left side phase Voltage applied at central phase Voltage applied at right side phase 230 V 115 V 50 V Central phase BN 180 V 230 V 180 V Right side phase CN 50 V 115 V 230 V MTEKPRO Technologies Pvt.Power Transformer Testing 13 Magnetic Balance Test on 3-phase Transformers This test is conducted only in three-phase transformers to check the imbalance in the magnetic circuit. The test shall be performed before winding resistance measurement. Evaluation Criteria The voltage induced in the center phase is generally 50% to 90% of the applied voltage on the outer phases. Zero voltage or very negligible voltage with higher excitation current induced in the other two windings should be investigated. New Delhi. otherwise results would be erratic and confusing. However. The test voltage shall be limited to maximum power supply voltage available at site. The voltage induced in different phases of transformer in respect to neutral terminals given in the table below. In this test. then the winding needs further analysis. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase. Results between similar single-phase units should not vary more than 10 % . The set of reading for current measurement in each of the tap position should be equal. the test shall be conducted with 230 V. MTEKPRO Technologies Pvt. Measure phase to phase voltage between the IV terminals and current on each of the IV terminals. minimum and max. Three-phase transformers are tested by applying Single-phase 10 kV voltage to one phase (HV terminals) and keeping other winding open circuited and measuring the current at normal. If the measured exciting current value is 50% higher than the value measured during precommissioning checks. New Delhi. then there is likelihood of a fault in the winding which needs further analysis. Measurement of Magnetization Current at Low Voltage For 3-phase transformers. Therefore. shifting of windings. failures in turn insulation or problem in tap changers. Apply 1phase 10kV supply on IV terminals. Ltd. Results compared to previous tests made under the same conditions should not vary more than 25%. the test shall be conducted either with 415 V. tap positions. Unequal currents shall indicate possible short circuits in winding. 3-phase (neutral grounded) or 230 V. Keep the tap position in normal position and keep HV and LV terminals open. . For single phase transformers.Power Transformer Testing 14 EXCITING/ MAGNETISING CURRENT MEASUREMENT This test should be done before DC measurements of winding resistance to reduce the effect of residual magnetism. 1 phase (preferred). transformer under test may be demagnetised before commencement of magnetizing current test. The availability of test data of normal condition and faulty condition results help us to analyze the problem in future. This test is performed to locate defect in magnetic core structure. in case of three phase transformers. Magnetising current readings may be effected by residual magnetism in the core. If the measured exciting current value is 50 times higher than the value measured during precommissioning checks. The identical results confirm no damage due to transportation.The test values on the outside legs should be within 15 % of each other. and values for the centre leg should not be more than either outside for a three-phase transformers. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. MEASUREMENT OF SHORT CIRCUIT IMPEDANCE MTEKPRO Technologies Pvt. Ltd. . New Delhi.Power Transformer Testing 15 Testing of Power Transformers 4. ensure the isolation of the same prior to commencement of testing The measurement is performed in single phase mode. The test shall be conducted with variac of 0-280 V. The acceptable criteria should be the measured impedance voltage having agreement to within 3 percent of impedance specified in rating and diagram nameplate of the transformer. Ltd. The conductors used for short-circuiting one of the transformer windings should have low impedance (less than 1m-ohm) and short length. 10 A. . Variation in impedance voltage of more than 3% should be considered significant and further investigated. New Delhi.Power Transformer Testing MEASUREMENT OF SHORT CIRCUIT IMPEDANCE This test is used to detect winding movement that usually occurs due to heavy fault current or mechanical damage during transportation or installation since dispatch fro the factory. The contacts should be clean and tight. Ensure the isolation of Transformer from High Voltage & Low voltage side with physical inspection of open condition of the concerned isolators/disconnectors. This test is performed for the combination of two winding. precision RMS voltmeter and ammeter. The one of the winding is short circuited and voltage is applied to other winding. The formula for calculating the percentage impedance with current and frequency correction is: 16 Where: Vtest = Test voltage Vrated =Rated voltage Itest = Test current Irated = Rated current ft = Test frequency fr = Rated frequency MTEKPRO Technologies Pvt. In case tertiary is also connected. The voltage and current reading are noted. Measurement of dissipation factor (tanδ) of the insulation system capacitances. MTEKPRO Technologies Pvt.Power Transformer Testing 17 Testing of Power Transformers 5. Ltd. . New Delhi. Ltd. • The standard capacitor CN. on the other hand defines the insulation power-factor as the ratio of the power dissipated in the insulation in watts. that the power-factor is helpful in assessing the probable condition of the insulation when good judgment is used [51]. Insulation power-factor is usually expressed in percent [51]. IEEE [51]. The significance of the power factor figure is still a matter of opinion. however. In order to reduce the influence of external disturbances. New Delhi. Experience has shown. coaxial cables must be used for the connection between CX (the transformer under test) to the bridge and also between standard capacitance CN and the bridge. • The various liquids and insulation materials used in transformers result in Large variations in insulation power factors [51]. Measurement of power-factor values in the factory is useful for comparison with field power-factor measurements and assessing the probable condition of the insulation. similar to the insulation resistance test. which represents the transformer under test whose power-factor (or tanδ) and capacitance are to be measured. to the product of the effective voltage and current in volt-amperes (corresponding to the apparent power) when tested using a sinusoidal voltage. which must be a HV capacitor with very low dielectric losses. Conventional Schering-Bridge Figure shows the measuring circuit for the insulation power-factor measurement of a two-winding transformer using a conventional Schering-bridge. • The Schering-Bridge casing contains resistors R3. Normally its capacitance is between 100 pF and 10 nF. It has not been feasible to establish standard power-factor values for the following reasons: • There is little or no relationship between power-factor and the ability of the Transformer to withstand the prescribed dielectric tests. adjustable capacitor C4 and galvanometer G. R4 and r. The measuring circuit / the measuring procedure [51] Measurement using a bridge The method is based on comparing the capacitance CX (transformer under test) with a well-known capacitance CN (standard capacitor). 18 General IEC defines the power factor as the ratio between the absorbed active power to the absolute value of the reactive power. allows certain conclusions to be drawn concerning the condition of the transformer insulation. • The variation of power-factor with temperature is substantial and erratic. Instrumentation The Schering-Bridge test circuit consists of three main parts: • The unknown capacitance CX. . This corresponds to tanδ.Power Transformer Testing Measurement of dissipation factor (tanδ) of the insulation system capacitances The purpose of the measurement The insulation power-factor test. MTEKPRO Technologies Pvt. Power Transformer Testing 19 Figure: Measuring circuit for the measurement of power factor and winding Capacitances. the insulation power factor tanδ will be: MTEKPRO Technologies Pvt. When the bridge is balanced. For a 50 Hz measurement. Ltd. 1000/π or 10000/π etc. New Delhi. . with R4 = 1000/π and C4 in nF. to simplify the calculation: 100/π. the unknown capacitance CX and tanδ can be calculated using the following equations: In most bridges the following resistance values are used for R4. in ohms. Power Transformer Testing A modern tanδ bridge with current comparator and microprocessor This bridge uses basically the same measuring principle as described above. Connect the LV cable to the test tap (strip/central stud) of the bushing under test to the C & TAN δ KIT through a screened cable and earth the flange body. • For measurement of 33kV Bushing Tan Delta. MTEKPRO Technologies Pvt. New Delhi. Figure 18. or while it is lying on wood. Hence. The oil-paper insulation combination of bushings exhibit fairly constant tan delta over a wide range of operating temperature. The currents are balanced in a comparator (more-winding differential transformer) and quadrature current is injected to balance the losses. short together all 400kV. • For 3-Ph auto-transformer. 20 MEASURING METHODS: CAPACITANCE AND TAN δ MEASUREMENT OF BUSHINGS C & Tan δ measurement of bushings shall be done at 10kV with fully automatic test kit so as to have reliable test result.0kV. For the unknown capacitance Cx. the standard capacitor CN and the connections between transformer and bridge are the same as mentioned above for the conventional Schering Bridge. . Ltd. Apply HV lead of the Test kit to shorted 33kV Bushings and connect LV lead of the test kit to Test tap of the Bushing under test. Keep the test results as a baseline record to compare with future tests. Also short all 33kV Bushings and earth the same. Repeat the test for all Bushings by changing only LV lead connection of the kit to test tap of the Bushing which is to be tested. HV of the bushing is to be connected to the Guard terminal of the test kit. Measurement of C1 Capacitance and Tanδ: Connect the crocodile clip of the HV cable to the top terminal of the shorted HV/IV bushings. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate.1 b shows the measuring circuit for dissipation factor and capacitance measurement with a modern tan _ measuring bridge with incorporated microprocessor. Insert a pin in the hole of the central test tap stud by pressing the surrounding contact plug in case of 245 kV OIP Bushing and remove the earthing strip from the flange by unscrewing the screw (holding earth strip to the flange body) in case of 420 kV OIP Bushing. • Do not test a bushing (new or spare) while it is in its wood shipping crate. Measurement of C2 Capacitance and Tanδ : HV lead to be connected to the test tap of the bushing under test (if required additional crocodile type clip may be used) and LV of the kit to be connected to the ground. 220kV and Neutral (isolated from earth) Bushings. earth HV/IV Bushings (already shorted). Test to be carried out in GSTg mode at 1. Unscrew the test tap cover. • Measurements shall be made at similar conditions as that of a previous measurement. effort is to be made for testing at temperature near to previous test and Correction factor need not be applied. • It is to be ensured that Test Tap points are earthed immediately after carrying out the measurements for that particular Bushing and earthing of test tap to be ensured by carrying out continuity test. Ltd. • For ICTs (Auto-Transformers): Shorting of all three phase Bushings (400kV&220kV) and neutral to be done.Power Transformer Testing • It is to be ensured that C& Tan δ measurement of bushings and testing of turrets carried out before 21 installation. 400kV. • Ensure that test specimen is isolated from other equipments. Removal of Jumpers from Bushings is PreRequisite for C & Tan δ Measurement of Windings. This will prevent installation of bushings having C& Tan δ values beyond permissible limits. mode Cap Symbol Test lead Connection Remarks MTEKPRO Technologies Pvt. CAPACITANCE AND TAN δ MEASUREMENT OF WINDINGS The combination for C & tanδ measurement of winding is same as that of measurement of IR value. New Delhi. 220kV and neutral Bushings to be shorted Capacitance and Tan δ measurement of windings should be done in following combinations: Test Test Winding Combination No. In case of single phase. . The summery of probable combination is given below: Auto-Transformer (Two Test Mode winding) HV + IV to LV HV + IV to E LV to E UST V GSTg GSTg Shunt Reactor HV to E Test Mode GST 3 winding Test Transformer HV to LV1 HV to LV2 LV1 to LV2 UST UST UST Mode HV to Ground GSTg LV1 to Ground GSTg LV2 to Ground GSTg Table: Combination for C & tanδ measurement of winding for various Transformers/ Shunt Reactor. 3. • For Reactors: All 400kV and neutral Bushings to be shorted. HV-IV/LV UST CHL HV lead of test kit to HV/IV bushings of transformer LV lead of test kit to LV bushing of transformer 22 2. Ltd.Winding combination for C & tan δ measurement for auto transformer. Measure the Capacitance and tan Delta in GST mode. • Measurement inter-check can be done by calculating C1= C2-C3 & C4= C5. of test in above table.doLV to be Guarded 4 HV-IV/LV UST CHL LV lead of test kit to HV/IV bushings of transformer HV lead of test kit to LV bushing of transformer 5 LV/ HV-IV +G GST CHL +CLG LG -doHV to be Guarded 6. New Delhi. Neutral connection with earth/ NGR to be isolated before the test. HV-IV/ LV+G HV-IV / LV with Guard GST GSTg CHL +CHG C HG -do. no.C6 & DF1=C2DF2-C3DF3 / C2-C3= C4DF4-C5DF5 / C4-C5 Where C stands for capacitance and DF for dissipation factor or tan δ and attached suffix (1…6) denotes the sr. .Power Transformer Testing 1. LV/ HV-IV with Guard GSTg C LG -do- Table :. HV of the test kit to be connected to shorted Bushings and LV of the test kit to be connected to Earth connection. MTEKPRO Technologies Pvt. Ltd. New Delhi. MTEKPRO Technologies Pvt.Power Transformer Testing 23 Testing of Power Transformers 6. . Measurement Insulation Resistance. IR is normally measured at 5 kV DC or lower test voltage. dryness. 24 Figure: Principal measuring circuit for the insulation resistance measurement IEEE Std. therefore. the dielectric absorption is the ratio of IR values measured after 60 sec and 15 sec. present trend is to monitor oil characteristics MTEKPRO Technologies Pvt. Whereas. New Delhi. transformer oil.25 are considered satisfactory for a transformer when the results of other low voltage tests are found in order. Insulation resistance is commonly measured in megohms. When insulation resistance falls below specified value. 60 sec and 600 sec. A PI of more than 1.Megger tests .00 [50] also specifies the insulation resistance measurement between core and earth. The test is conducted with the help of mega-ohmmeter. The IR value of transformer is dependent on various factors such as configuration of winding insulation structures.are performed to determine the insulation resistance from individual windings to earth or between individual windings. it can often be brought back to the required value by cleaning and drying. Ltd. that variations in insulation resistance can be caused by numerous factors including: design. and cleanliness of parts.Power Transformer Testing Measurement of insulation resistance Insulation resistance tests .5 Kv DC for a duration of 1 minute. PI of less than 1 calls for immediate corrective action.3 and dielectric absorption factor of more than 1. Insulation resistance shall be measured after the intervals of 15 sec. C57. IR is proportional to the leakage current through/over the insulation after capacitive charging and absorption currents become negligible on application of DC voltage. Insulation resistance varies with the applied voltage. Any measurement comparisons should always be carried out at the same voltage. The polarization index (PI) is defined as the ratio of IR values measured at the intervals of 600 and 60 seconds respectively. This ratio is independent of temperature and gives more reliable data for large power transformers. Polarization index (PI) is useful parameters for logistic interpretation of IR test results. but the test voltage should not exceed half the rated power-frequency test voltage of transformer windings. . temperature. It should be stated.12. an IR value of above 10000 M-ohms is considered satisfactory. especially of bushings. For bushings. (MΩ). It shall be measured after complete assembly of the transformer at a level of at least 0. Knowledge of the insulation resistance is of value when evaluating the condition of the transformer insulation. atmosphere condition etc. New Delhi. E. three -winding transformer & Shunt Reactor. . MTEKPRO Technologies Pvt. Following table gives combinations of IR measurements for auto-transformer. Before taking measurements the neutral should be disconnected from earth. It may be note that no national/international standards specify minimum insulation resistance values of 25 transformers. During IR measurement. we must ensure following conditions: • Transformer is disconnected from other associated equipment • Bushings are cleaned and free of moisture • Transformer tank and core are properly grounded • Both ends of winding under test are short-circuited. LV-Low voltage/Tertiary voltage windings. the rest of the windings being earthed.Earth Unless otherwise recommended by the manufacturer the following IR values as a thumb rule may be considered as the minimum satisfactory values at 30°C (one minute measurements) at the time of commissioning. Insulation resistance varies inversely with temperature and is generally corrected to a standard temperature (usually 20 °C) using table (Source: BHEL instruction Manual) as given below. IV-Intermediate voltage. with all the Windings being connected together) and the earthed tank (earth) and between each winding and the tank. The value of IR may be very low under heavy fog or humid conditions. Ltd. Power Transformer Testing Measuring Methods:IR measurements shall be taken between the windings collectively (i.e. For Auto-transformer For 3 winding transformer HV + IV to LV HV + LV to IV HV + IV +LV to E For Shunt Reactor HV to E HV + IV to LV HV + IV to E LV to E Where HV-High voltage.for judging the condition of dryness of the transformer and not to rely solely on absolute values of IR. an IR value of above 10. MTEKPRO Technologies Pvt.Power Transformer Testing 26 PI= R10 / R1 (dimensionless). New Delhi.25 and DAI of more than 1. For bushings. PI less than 1 calls for immediate corrective action.000 MΩ is considered satisfactory. Ltd.3 are generally considered satisfactory for a transformer when the results of other low voltage tests are found in order. Where PI is Polarisation Index and R is resistance The following are guidelines for evaluating transformer insulation using polarization index values: A PI of more than 1. . Power Transformer Testing 27 Testing of Power Transformers 7. . Ltd. New Delhi. MTEKPRO Technologies Pvt. CHECK LIST FOR ENERGISATION OF TRANSFORMER. if provided • Conformity of connection to the lines (no tensile stress on the terminal heads) • Bushing CT secondary terminals must be shorted and earthed. 4. BDV. Check the bushings: • Oil level (bushings fitted with sight-glasses) • Adjustment of spark-gaps /arcing horn –gaps. if any • Signaling of positions 10. Check the whole assembly for tightness and rectify where necessary. flanges. Check that the silica gel in the breather is blue and that there is oil in the breather cup (oil seal) 6. Check that CC-CL-G are shorted 7.local and remote electrical operation as well as manual Operation and parallel operation. Moisture content. • On-load tap changer: valves open 5. radiators. tank cover and the cooling devices including headers. 2. resistivity & tan δ at 90°C and IFT ) MTEKPRO Technologies Pvt. Ltd. Level should correspond to 35º C mark on oil level gauges for BHEL transformers 8. bushing caps. Release air at the high points. Check the quality of the oil: • Draw off a sample from the bottom of the tank • Carry out DGA and oil parameters test (i. like oil communicating bushings. Check that the valves are in the correct position: • Tank: valves closed and blanked • Cooling circuit: valves open • Conservator connection: valves open • By-pass: valves open or closed as the case may be. of the transformer. • Neutral bushing effectively earthed 9. Check the oil level in the main conservator and the conservator of on-load tap changer. 3. if not in use. Check the general appearance and retouch the paint work if need be. pumps. New Delhi. expansion bellows as per manufacturer’s recommendation. turrets.Power Transformer Testing CHECK LIST FOR ENERGISATION OF TRANSFORMER/ REACTOR PRELIMINARY CHECKS 1. expansion joints etc. buchholz petcock. 28 . Check the on-load tap changer: • Conformity of the positions between the tap changer control cubicle and the tap Changer head • Adjustment of the tap-changer control cubicle coupling • Electric and mechanical limit switches and protective relays • Step by step operation.e. Air release should be resorted from low points to high points. Check the insulation of the auxiliary circuits in relation to the ground by 2 kV megger for 1 min. waste etc. over-current protection. pieces of clothes. Check the direction of installation of buchholtz relay. The transformer should always be soaked for few hours under constant care i. New Delhi. depending upon the particular moment in the cycle. Check the operation of the buchholz relay and the surge protective relay of the tap-changer : • Alarm and tripping • Protections and signals interlocked with these relays 5. Measure the supply voltages of the auxiliary circuits 8. CHECKING OF AUXILIARY AND PROTECTIVE CIRCUITS 1. Check temperature indicator readings and their calibrations 2. . 12 Check that equalising link between OLTC tank and Main tank is removed 13 Extraneous materials like tools. The initial magnetising current at the time of switching will be very high. the relay time delays 9. Check that there is efficient protection on the electric circuit supplying the accessories and tightness of all electrical connections 10. over-fluxing protection etc. if necessary. Ltd. keep it energised for twelve hours. After the inspection / tests are completed. should be located and corrected. Check the earthing of the tank and auxiliaries like cooler banks at two places. The transformer hum should be observed for any abnormality. Check the differential protection. the transformer may be energised from the incoming side on NO LOAD. Check the setting and working of the mercury switches of winding and oil temperature indicators and presence of oil in the thermometer pockets • Follow the same procedure for the thermal replicas 3. 6. restricted earth fault protection. control and adjust. drain and fill with filtered oil upto desired level. 7. Check the oil of OLTC chamber.e. Check the heating and lighting in the cubicles 11. Should the gas prove to be inflammable. are in service and settings are as per CC/Engg recommendations. After that it may be checked for gas collection.before energisation. earthing rods. should be removed before energisation. try to detect the cause which may probably be an internal fault? If the breaker trips on MTEKPRO Technologies Pvt. Check the cooling system : • Check the direction of installation of oil pumps • Check the direction of rotation of the pumps and fans • Check the working of the oil flow indicators • Check the setting of the thermal overload relays • Go through the starting up sequences. if not good. Power Transformer Testing 29 11. 4. Excessive vibrations of radiator parts etc.