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KT 80 GB 2000-01 ABB Oy Power transformer
TESTING POWER TRANSFORMERS Test procedures and equipment used for the testing of large power transformers at ABB Oy, Power transformer, Vaasa Works are dealt with in the following sections. The measuring equipment differens from those explained herein. The priciples of routine, type and special tests are however similar and thus this booklet is applicable for testing of distribution transformers too. The electrical characteristics and dielectric strenght of the transformers are checked by means of measurements and tests defined by standards. The tests are carried out in accordance with IEC Standard 76, Power Transformers, unless otherwise specified in the contract documents.
Contents 1. Summary of dielectric tests Pages 4
Routine tests 2. 3. 4. 5. 6. 7. 8. 9. 10. Measurement of voltage ratio and check of connection symbol Measurement of winding resistance Measurement of impedance voltage and load loss Measurement of no-load loss and current Induced AC voltage test (ACSD) Separete-source voltage withstand test Operation tests on on-load tap-changer Lighting impulse test Partial discharge measurement 5...6 7…8 8…11 12…13 14…15 16 17 18...24 25...30
Type tests and special tests 11. 12.. 13. 14. 15. 16. 17. 18. 19. Measurement of zero-sequence impedance Capacitance measurement Insulation resistance measurement Loss factor measurement Measurement of the electric strengh of the insulating oil Temperature-rise test Test with lightning impulse, chopped on the tail Switching impulse test Measurement of acoustic sound level 31 32...33 34 35…36 37 38…41 42…43 44…46 47…48
1. SUMMARY OF DIELECTRIC TESTS According to the Standard IEC 76-3 the dielectric test requirements for a transformer winding depend on the highest voltage for equipment Um applicaple to the winding and on whether the winding insulation is uniform or non-uniform.
Category of winding Uniform insulation Uniform and nonuniform insulation
Highest voltage Um/kV
Um ≤ 72,5
Lightning impulse (LI) (see
clause 13 and 14)
Switching impulse (SI) (see
clause 15)
Long duration AC (ACLD) Not applicable (note 1) Special Routine Routine
Short duration AC (ACSD) Routine
Separate source AC Routine
Type (note 1) Routine Routine Routine
72,5<Um≤170
Not applicable Routine (note 2) Routine
Routine Special (note 2) Special
170<Um<300 Um≥300
NOTE 1 In some countries, for transformers with Um ≤ 72,5 kV, LI tests are required as routine tests, and ACLD tests are required as routine or type tests. NOTE 2 If the ACSD test is specified, the SI test is not required. This should be clearly stated in the enquiry document
2. MEASUREMENT OF VOLTAGE RATIO AND CHECK OF CONNECTION SYMBOL Purpose of the measurement The voltage ratio of the transformer is the ratio of voltages (in three-phase transformers line-to-line voltages) at no-load, e.g., 110000 V/10500 V. The purpose of the measurement is to check that the deviation of the voltage ratio from the specified value does not exceed the limit given in the relevant transformer standard (generally 0,5 %). The connection symbol of the transformer is checked at the same time. Performance and results of the measurement The voltage ratio measurements are carried out by means of a voltage ratio measuring bridge; the error of the bridge is less than ± 0.1 %. The supply voltage is 220 V a.c. The fuctions of the bridge is shown in Fig. 2-1. The voltages of the transformer to be checked are compared to the corresponding voltages of the regulating transformer, which is provided with a decade display unit and located in the gridge casing. When the bridge is balanced, the voltage ratio of the decade transformer is equal to that of the transformer unded test. The result can be seen directly from the numeral display of the bridge. Fig. 2-1. Bridge measurement (of the voltage ratio) T1 transformer to be measured T2 regulating transformer equipped with a decade display, P1 zerosequence voltmeter, U1 supply voltage of the bridge, U2 secondary voltage of the transformer. Since the measuring device is a single-phase bridge, the voltage ratio of a pair of windings mounted on the same leg is measured at a time. It is to be observed that the ratio indicated by the bridge does not alvays correspond to the ratio of the line-to-line voltages. The result depends on the connection symbol of the transformer. For each winding connected to the bridge it is important to observe whether the number of turns relates to the line-to-line or line-to-neutral voltage. For example, 3 the voltage ratio of a 120/21 kV Yd-connecter transformer is 120000: V 21000 = 3.299. The reading obtained from the bridge is to be compared to this value. The connection symbol of the transformer is checked in conjunction with the voltage ratio measurement. When the measuring leads from the transformer
Table 2-2 Determination of the connection symbol Clock hour figure (left). In the report the specified tapping voltage ratios are stated. The voltage ratios are measured for each tapping connection of the transformer. connection symbol (middle) and vector diagram (right).6
are connected to the bridge according to the relevant vector diagram in Table 2-2. the bridge can be balanced only if the transformer connection is correct.
. as well as the measured ratios and their deviations from the specified ratios.
1. U = voltmeter B = DC-supply.
B A Idc A Th V RAB RAC Terminal RBC T
Fig. The measured resistances are needed in connection with the load loss measurement when the load losses are corrected to correspond to the reference temperature.1-1 Circuit for resistance measurement T1 = transformer under test. Measurement of the winding resistances Purpose of the measurement The resistance between all pairs of phase terminals of each transformer winding are measured using direct current. RAC and RBC is measured.
. Th = thermometer. Apparatus and basic measuring circuit The measurement is performed by TETTEX 2285 transformer test system.The resistances are then calculated from Udc and Idc using correction for the error caused by the internal resistance of the voltage measuring equipment The temperature is measured from oil filled thermometer pockets situated in the transformer cover by means of an electronic thermometer connected to the computer. The resistance measurement will also show whether the winding joints are in order and the windings correctly connected.7
3. The principle of the measurement is as follows: The voltage drop Udc caused by the direct current Idc an by the resistance RAB. A = ammeter. Furthermore the corresponding winding temperature is measured.
T4 voltage transformers. Current is generally supplied to the h. T1 step-up transformer.. 4-1 Circuit for the impedance and load-loss measurement.v. value). Apparatus and measuring circuit
Fig.g.. winding and the l.
The supply and measuring facilities are described in a separate measuring apparatus list (Section 20). between which the resistances are measured. the tapping position and the average temperature of the windings during the measurement are stated. Performance of the measurement If the reactive power supplied by the generator G1 is not sufficient when measuring large transformers.1). T3 current transformers. P2 ammeters (r.
G1 supply generator. winding is shortcircuited. The measurements are made separetely for each winding pair (e. P3 voltmeters (r. P1 wattmeters.100 % of the rated current according to the standard 4.The voltage of the supply generator is raised until the current has attained the required value (25.s. and furthermore on the principal and extreme tappings. In the report the terminals.8
Test report The resistance values and the average temperature are calculated.m.
4. T2 transformer to be tested. 1-3 and 2-3 for a threewinding transformer).v. a capasitor bank C1 is used to compensate part of the inductive reactive power taken by the transformer T2.m. MEASUREMENT OF IMPEDANCE VOLTAGE AND LOAD LOSS Purpose of the measurement The measurement is carried out to determine the load-losses of the transformer and the impedanse voltage at rated frequency and rated current. the pairs 1-2. value) C1 capacitor bank. In order to increase the accuracy of readings will be taken at several current values near
.. the connection.s.
1) is shown as a set of curves in Fig.9
the required level.2) (4.2)
IN =( ) Im
IN *Uc Im
. the measurements are carried out on the principal and extreme tappings. It the transformer has more than two windings all winding pairs are measured separately. the power Pkm and the voltage Ukm at rated current are obtained by applying corrections to the values Pc and Uc relating to the measuring current. The corrections are made as follows:
(4. 4-2.3)
(4. The power value correction caused by the phase displacement is calculated as follows: (4. because the windings tend to warm up due to the current and the loss values obtained in the measurement are accondingly too high. The corrections caused by the instrument transformers are made separatley for each phase. Results Corrections caused by the instrument transformers are made to the measured current.1) K δu − δi * tan ϕ ) = Pe * (1 + ) 3440 100 Pc = corrected power Pe = power read from the meters δu = phase displacement of the voltage transformer in minutes δi = phase displacement of the current transformer in minutes ϕ = phase angle between current and voltage in the measurement (ϕ is positive at inductive load) K = correction Pc = P e * (
The correction K obtained from equation (4. because different phases may have different power factors and the phase displacements of the instrument transformers are generally different. voltage and power values. If a winding in the pair to be measured is equipped with an off-circuit or on-load tap-changer. If the measuring current Im deviates from the rated current IN. The readings have to be taken as quickly as possible.
K correction in percent. 4. and the loss values are corrected to the reference temperature 75 °C accordint to the standards as follows. The d.δi phase displacement in minutes. δu .POm
Here Pkm is the measured power.5) Pam = Pkm . The transformer is at ambient temperature when the measurements are carried out. cosδ power factor of the measurement. losses POm at the measuring temperature ϑm are calculated using the resistance values R1m and R2m obtained in the resistance measurement (for windings 1 and 2 between line terminals): (4. 5 *( I1N * R1m + I 2 * R2 m ) 2N
The additional losses Pam at the measuring temperature are (4. and which is corrected to the rated current according to Equation (4. According to the standards the measured value of the losses shall be corrected to a winding temperature of 75 °C (80 °C.δi.
Mean values are calculated of the values corrected to the rated current and the mean values are used in the following. to which the corrections caused by the instrument transformer have been made.2 The correction caused by the phase displacement of istrument transformers.4)
2 POm = 1. The sign of K is the same as that of δu .10
Fig.c. if the oil circulation is forced and directed).2)
c.7) % Rkm = 100 * SN Ukm is the measured short-circuit voltage corrected according to Equation (4. losses POc additional losses Pac load losses Pkc short circuit resistance Rkc short circuit reaactance Xkc short circuit impedance Zkc (PDC) (PA) (PK) (RK) (XK) (ZK)
. The short circuit reactance Xk does not depend on the losses and Xk is the same at the measuring temperature (ϑm) and the reference temperature (75 °C).8)
X km = Z 2 − R2 = X kc km km
When the losses are corrected to 75 °C. d. The losses corrected to 75 °C are obtained as follows: (4.11) Z kc = X 2 + R2 .10) % Rkc = 100 * SN (4.11
The short-circuit impedance Zkm and resistance Rkm at the measureing temperature are U km (4. UN is the rated voltage and SN the rated power. hence (4. 76-1 (1993) Power Transformers. it is assumed that d.c. losses vary directly with resistance and the additional losses inversely with resistance. kc kc Results The report indicates for each winding pair the power SN and the following values corrected to 75 °C and relating to the principal and extreme tappings.9)
Pkc = POm *
ϑ s + 75o C ϑ +ϑ + Pam * s om ϑs + ϑm ϑ s + 75 C
ϑs = 235 °C for Copper ϑs = 225 °C for Aluminium
Now the short circuit resistance Rkc and the short circuit impedance Zkc at the reference temperature can be determined: Pkc (4.6) % Z km = 100 * UN Pkm (4. Literature (4.3).1) IEC Publ.
s. Circuit for the no-load measurement.12
5. 5-1. value).m.11 (reading of a rectifier voltmeter
. The harmonics on the no-load current are also measured on request. T4 voltage transformers. and the results are interpolated to correspond to the voltage values from 90 to 115 % of UN at 5 % intervals. P3 voltmeters (r. The following formula is valid for the iron losses (5. P4 voltmeters (mean value x 1. T2 transformer to be tested. P2 ammeters. The test is usually carried out at several voltages below and above the rated voltage UN. MEASUREMENT OF NO-LOAD LOSS AND CURRENT Purpose of the measurement In the no-load measurement the no-load losses Po and the no-load current Io of the transformer are determined at rated voltage and rated frequency. T3 current transformers. T1 step-up transformer.1)
U' + k2 *U2 P0 = k1 * f * f Po = measured iron losses k1 = coefficient relating to hysteresis losses k2 = coefficient relating to eddy-current losses f = frequency U' = mean value of voltage x 1. P1 wattmeters.11).
The available supply and measuring facilities are described in a separrate measuring instrument list (Section 20). they are generally ignored. The asymmetric voltage at the neutral terminal is also measured in certain cases.
G1 supply generator. Apparatus and measuring circuit
Fig. Performance The following losses occur at no-load iron losses in the transformer core and other constructional parts Dielectric losses in the insulations load losses caused by the no-load current While the two last mentioned losses are small.
100 p1 + k * p2 It is assumed that for oriented sheets p1 = p2 = 50 % Pon = P0 *
The current and power readings of different phases are usually different (the power can even be negative in some phase).m. Then the hysteresis losses correspond to standard conditions. meter will be defferent. the voltage wave shape may somewhat differ from the sinusoidal form. Pon (5. In the test the voltage is adjusted so that the mean value voltmeter indicates the requider voltage value. This is due to the asymmetric construction of the 3-phase transformer. of eddy-current losses to total iron losses The loss value corresponding to standard conditions is obtained from the measured value Po as follows: (5.
. expressed as a percentage. value of a sinusoidal voltage) U = r.3) k= U'
Pon = losses at sinusoidal voltage under standard conditions p1 = ratio. Because the losses are to be determined under standard conditions. it is necessary to apply a wave shape correction whereby the losses are corrected to correspond to test conditions where the supply voltage is sinusoidal. From the noload curve thus obtained no-load losses and no-load apparent power corresponding to voltage values from 90 to 115 % of UN at 5 % intervals are determined and stated.1).2) * ( p1 + k * p2 ) P0 = 100 U 2 (5.s.13
scaled to read the r.m. Results The report shows the corrected readings at each voltage value. Furthermore the no-load current in percentage on the rated current is stated. as well as the mean values of the currents of all three phases.s. A regression analysis is carried out on the corrected readings. but the eddy-current losses must be correcred. The readings of the mean value meter and r. value of the voltage When carrying out the no-load measurement. From (5.m. expressed as a precentage.s. This is caused by the harmonics in the magnetizing current which cause additional voltage drops in the impedances of the supply. of hysteresis losses to total iron losses p2 = ratio. the mutual inductances between different phases are not equal.
The voltage is measured from terminals to earth or between terminals of the low voltage winding using voltage transformers. The duration of the test is
rated frequency *120 s test frequency
The test is successful if no collapse of the test voltage occurs. The test frequency is either 165 Hz or 250 Hz. The other windings are left open-circuited. Alternatively the capasitive taps of the bushings on the high voltage side are used for voltage measurement. tapping leads and terminals. Short duration induced AC withstand test for Uniformly insulated HVwindings The test voltage connection is essentially the same as in service. a. A threephase winding is tested with symmetrical three-phase voltages induced in the phase windings. it is eaerthed during the test. If the winding has a neutral terminal. that the average of the voltage values measured from terminals to earth or between terminals is equal to the required test voltage value. The tapping of the off-circuit or on-load tap-changer is chosen so that in all windings the voltage during the test is as near as possible the rated test voltage. withstand the temporary overvoltages and switching overvoltages to which the transformer may be subjected during its lifetime. The machines and the equipment are described in the test equipment list (Section 20). The voltage is so adjusted. turns. coils. for non-uniformly insulated windings also the insulation between these parts and earth. However.14
. The test voltage is twice the rated voltage. the voltage developed between line terminals of any winding shall not exceed the rated short duration power-frequency withstand voltage. The partial discharges shall be measured if not otherwise agreed. Performance The excitation voltage is applied to the terminals of the low-voltage winding. INDUCED AC VOLTAGE TEST (ACSD) Purpose of the test The object of the test is to secure that the insulation between the phase windings.
frequency. The voltage is measured with a capacitive voltage divider in conjunction with voltmeters responsive to peak and r. The test voltage is adjusted according to this voltmeter. test duration and tapping are stated in the report.m. two sets of tests are required a) b) A phase-to earth test with rated withstand voltages between phase and earth according to standard with partial discharge measurement. P3 voltmeter.
Fig. T1 step-up transformer. P1 ammeter. T2 transformer understest. values. The peak voltmeter indicates the peak value divided by √2.
G1 supply generator. 6-1 is applicable to three-phase transformers if the insultaion level of the neutral terminal is at least one third of the insulation level of the terminals. E voltage divider. L compensating reactor. value) P4 voltmeter (speak value). 6-1. (r. Short duration induced AC withstand test for non-uniformly insulated HVwindings For three-phase transformers. Test circuit for induced overvoltage withstand test on non-uniformly insulated winding of three-phase transformer. During each application the test voltage from terminal to earth is equal to the rated withstand voltage. P2 voltmeter.s. Test report The test voltage. The test voltage is applied to the individual phases in succession. A phase-to-phase test with earthed neutral and with rated withstand voltages between phases according to standard with partial discharge measurement.
The test connection shown in Fig. T4 voltage transformer.m.s.15
b. T3 current transformer.
The peak-voltmeter indicates the peak value divided by 2 . The test voltage is adjusted accordin to this meter. Test report The test voltage.16
7. The test is successful if no collapse of the test voltage occurs.m. The test voltage is applied for 60 seconds between the winding under test and all terminals of the remaining windings. Performanse The test is made with single-phase voltage of rated frequency. P2 voltmeter (r.
G1 supply generator.
. T3 current transformer. P1 ammeter. core and tank of the transormer.b. SEPARATE-SOURCE VOLTAGE WITHSTAND TEST Purpose of the test The object of the test is to secure that the insulation between the windings and the insulation between windings and earthed parts. connected together to earth (Fif. withstand the temporary overvoltages and switching overvoltages which may occur in service. L compensating reaaactor. E voltage divider. Test circuit
The voltage is measured using a capacitive voltage divider in conjunction with voltmeters responsive to r. frequency and test duration are stated in the report. T2 transformer under test. The generators and the equipment are described in the test equipment list (Section 20). T1 test transformer.s value) P3 voltmeter (peak value). and peak values. 7-1 Test circuit for separate-source voltage withstand test. The line terminals of non-uniformly insulated windings are tested by induced test according to Section 6.m. 7-1).
. the following tests are performed at (with exception of b) 100 % of the rated auxiliary supply voltage: a) b) c) d) 8 complete operating cycle with the transformer not energized 1 complete operating cycle with the transformer not energized.17
8. OPERATION TEST ON ON-LOAD TAP-CHANGER After the tap-changer is fully assembled on the transformer. with 85 % of the rated auxiliary supply voltage 1 complete operating cycle with the transformer energized and rated voltage and frequency at no load 10 tap-change operations with ± 2 steps on either side of the principal tapping with as far as possible the rated current of the transformer. with one winding short-circuited.
Fn main spark-gaps. breakdown of the spark-gap F1 is initiated by an external triggering pulse. When F1 breaks down.18
9.. The impulse capacitors Cs (12 capacitors of 750 nF) are charged in parallel through the charging resistors Rc (45 kΩ) (highest permissible charging voltage 200 kV). The basic circuit diagram is shown on Fig. the potential of the following stage (points B and C) rises. 15-1. and since the low-ohmic resistor
. 15-1 Basic circuit diagram of the impulse generator. When the charging voltage has reached the requider value. C1 impulse capacitor Rc charging resistor Rs series resistor Ra low-ohmic discharging resistor for switching impulse. Testing equipment Impulse generator Fig...Fan auxiliary spark-gaps
The impulse generator design is based on the Marx circuit.5 kΩ) and the charging resistor Rc. Fal. LIGHTNING IMPULSE TEST Purpose of the test The purpose of the impulse voltage test is to secure that the transformer insultations withstand the lightning overvoltages which may occur in service. Rb high-ohmic discharging resistor for switching impulse F1. Because the series resistor Rs is of low ohmic value compared with the discharging resistor Rb (4..
Ci input capacitance of transformer. The resistors Ra are connected in parallel with the resistors Rb. In order to obtain the necessary disscharge energy parallel or series-parallall connections of the generator can be used. Concequently the capacitors are discharged in series-connection. F1 spark gaps of impulse generator.
Test circuit Fig.19
Ra is separated from the circuit by the auxiliary spark-gap Fa1.1 MV lightning impulse.
The required impulse shape is obtained by selecting the series and discharge resistors of the generator suitably. LrLp stray inductances. The required voltage is obtained by selecting a suitable number of seriesconnected stages and by adjusting the charging voltage. when the auxiliary spark-gaps break down. The highohmic discharge resistors Rb are dimensioned for switching impulses and the low-ohmic resistors Ra for lightning impulses. Max.
Cr resulting impulse capacitance. 15-2 Equivalent diagram of the impulse test circuit. C1 capacitance of voltage divider. Thus the spark-gaps are caused to break down in sequence. R2 protective resistor.6 MV switching impulse. F2 calibration spheregap.
. with a time dalay of a few hundred nanoseconds. Rsr resulting series resistance. In these cases some of the capacitors are connected in parallel during the discharge. This arrangement is necessary in order to secure the functioning of the generator. 1. test voltage amplitudes: 2. Li transformer inductance. Rar resulting discharge resistance. the potential difference across the spark-gab F2 rises considerably and the breakdown of F2 is is initiated.
The measuring range can be changed by shortcircuiting part of the high voltage capacitors or changing the low voltage capacitor of the divider.20
The front time can be calculated approximately from the equation: (15. C1 high voltage capacitor of voltage divider. 5 * Rsr * (Ci + C1)
and the time to half value from the equation: (15. If necessary the sphere-gap calibration of the measuring circuit can be performed in connection with the testing according to the standard (15. Voltage measuring circuit The impulse shape and the peak value of the impulse voltage are measured by means of an oscilloscope and a peak voltmeter which are connected to the voltage divider (Fig. 15-3).3). E damped capacitive voltage divider.4)
. P2 peak voltmeter.
The measuring circuit is checked in accordance with the standards (15-2) and (15. Rar. P1 oscilloscope.1)
T 1 ≈ 2. R1 damping resistor of voltage divider. Fig. In practice the testing circuit is dimensioned according to experience.2)
T 2 ≈ k * Li * C r
The factor k depends on the quantities Rsr. C2 low voltage capacitor of divider. Li and Cr. Rp terminal resistance of the measuring cable. W measuring cable (=wave impedance = Rp). 15-3 The impulse voltage measuring circuit.
According to the standard IEC 76-3 the resistances of the resistors must be selected so that the voltages at the adjacent terminals do not exceed 75 % of the test voltage and the resistance does not exceed 500 Ω. f neutranl terminal testing. unless the neutral is available) is also tested with an impulse test-sequence applied to the line terminals of the tested winding connected together. the time to half-value can be increased by connecting suitable resistors (Ra in Fig. while the other windings are earthed (three-terminal test. Fig. A delta-connected winding (and star-connected winding.
a and b 1. The other line terminals and the neutral terminal are earthed (singleterminal test. d 2. 15-4a and b).
When testing low voltage windings of high power the time to half-value obtained is often too short (Fig. 15-4b) between the adjacent terminals and earth.
Fig. 15-4 Transormer impulse testing and fault detection connections.21
Transformer testing and fault detection connections The lightning impulse test is normally applied to all windings. e test with transferred voltages. 15-4c).
. The impulse testsequency is applied successively to each of the line terminals of the tested winding. c 3.terminal testing. 15-5). However. Fig.terminal testing.terminal testing.
4d) to the line terminals connected together. 15-5)
.5).22
For delta-connected windings the single and three-terminal testings can be combined by applying the impulse to two line terminals at a time. The two. The front time (T1) and the time to half-value (T2) are defined in accordance with the standard (15. 15.and three-terminal testings are not included in the standard (15. According to IEC 76-3 the line terminals of the low voltage winding are connected to earth through resistances of such value (resistances Ra in Fig.4) (Fig. Certain types of faults give rise to discrepancies in the recorded voltage wave-shapes as well. For fault detection in single-terminal and two-terminal tests the neutral of starconnected windings are earthed via a low-ohmic resistor (Ru). The neutral terminal is normally tested indirectly by connecting a high-ohmic resistor between the neutral and earth (voltage divider Ra. The resistance shall not exceed 5000 Ω. Ru) and by appluying the impulse (Fig. Evidence of insultaion failure arising from the test would be given significant discrepacies between the calibration impulse application and the full voltage applications in recorded current wave-shapes. The fault detection is then based on recording the capacitive current which is transferred to the adjacent winding. 15-4e) that the amplitude of transferred impulse voltage between line terminal and earth or between different line terminals or across a phase winding will be as high as possible but not exceeding the rated impulse withstand voltage. When the low voltage winding cannot in service be subjected to lighting overvoltages from the low voltage system (e. step-up transformers. For fault detection in three-terminal tests and tests on the neutral terminal the adjacent winding is earthed through a low-ohmic resistor. In this case two phases are simultaneously tested in a single-terminal connection and one phase in a test connection corresponding to three-terminal testing.g. 15-4d). tertiary windings) the low voltage winding may (by agreement between customer and manufackturer) be impulse tested simultaneously with the impulse tests on the high voltage winding with surges transferred from the high voltage winding to the low voltage winding (Fig. The impulse test of a neutral terminal is performed only if requested by the customer. test with transferred voltages). while the other line terminals are earthed (two-terminal testing. The current flowing through the detection resistor during the test is rocorded by means of an oscilloscope. Fig. but they can be done if it is so agreed. Performance of the impulse test The test is performerd with standard lightning impulses of negative polarity. 15-4e.
At full test voltage each line terminal is tested with as many impulses as is required by the standard.5 % of the voltage test level and during the 100 % voltage applications. In order to facilitate the detection of possible discrepacies in the oscillographic records. The voltage calibration is performed at 60 % of the voltage test level. is calibrated by means of sphere-gap. The voltage measurement is based on the reading of the peak voltmeter. If required the voltage measuring system. In testing the other terminals the voltage measurement is based on the reading of the calibrated peak voltmeter.
. 15-5 Standard lightning impulse Front time T1 = 1. including the peak voltmeter. Oscillographic records are made of the applied voltage and the voltage across the fault detection resistor Ru during calibration at 62. the oscilloscope attenuation is adjusted such that the curves recorded during the full wave applications can be brought to coincide with those obtained during the calibration.2 µs ± 30 % Time to half-value T2 = 50 µs ± 20 % In practice the impulse shape may deviate from the standard impulse when testing low-voltage windings of high rated power and windings of high input capacitance. in connection with the testing of the first line teminal.23
60-2 (1973): High-voltage test techiques.1) (15.3) by (15. The osccillographic record and measurement records are stored in the archives.24
Unless agreed otherwice different tappings are selected for the impulse tests on the three phases of a three-phase transformer. 76-3 (2000): Power transformers. IEC Publ. Part 3: Measuring devices. Part 2: Test procerudes. where they are available when requider.
Test report The summary of test results is given on a form termed "Report of impulse voltage withstand test on transformer".4) (15. IEC Publ.5) IEC Publ. Part 4: Application guide for measuring devices. 60-3 (1976): High-voltage test techniques. 60-4 (1977): High-voltage test techniques.
. IEC Publ.2) (15. IEC Publ.
Literature (15. 52 (1960): Recommendations for voltage measurement means of sphere gaps. Part 3: Insulation levels and dielectric tests. usually the two extreme tappings and the principal tapping.
Thus discharges take place repeatedly. It is. The ionic discharge following the test voltage. When the field strength reaches its critical value.1 Partial discharges in a gas-filled cavity.
∆Uc voltage strenght across the cavity. which does not bridge the electrodes of the insulation structure. The field strenght of a weak part of the dielectric may exceed the dielectric streght. which causes a breakdown. therefore. Due to the sinusoidal variation of the applied voltage the electrical field strenght increases again after the discharge has been extinguished. 18. and the breakdown remains partial. Ca is the capacitance of the whole insulating gap. and the breakdown is called a partial disharge for the above mentioned reasons. the spark-gap and the capacitrance Cc represent the cavity and the capacitance Cb represents thr dielectric in series with Cc.
The situation is enlightened by the simple anologue circuit of a cavity (Fig.
. (Fig. The remaining whole insulating gap can. a new discharge occurs. to be observed that the weak parts mentioned may form a small portion of the insulation structure only.2). 18-1). 18. withstand voltage stresses corresponding even to the test voltage. Resulting from a partial breakdown the voltage difference across the weak part of the dielectric decreases so much that the discharge current is interrupted. PARTIAL DISCHARGE MEASUREMENT Scope and objeck A partial discharge in an insulating medium is a localized electrical discharge.25
These phenomena result in degradation of the dielectric properties of the insulating medium..2 Analogue circuit of a gas-filled cavity.
The partial discharges do not lead to an immediate breakdown. other effects on the insulating medium: the surface of the dielectric is bombarded by iones. which cause temperature-rise and may result in degrading and chemical changes in the insulating material Chemical changes may give rise to material components.1000 ns.26
When the voltage Uc across Cc has increased enough. which speed up ageing. The discharge magnitude or apparent charge q and the voltage Uc are related by the following equation: (18.. and increase of losses. this change can be measured by means of a capacitive voltage divider and a pulse transformer. the spark-gap ignites. which causes a fast voltage charge at the terminals of the transformer. which may cause destruction of the transformer in service. On the other hand the partial discharges may also be extinguished by the influence of some other degradation products disharges cause high local field strengths near the discharge site. Fig. The capacitance Cc disharges and the voltage difference across the cavity vanishes within 1. however.1)
q = Cb * U c
The discharge gives rise to a current pulse.
. They have. The object of the partial discharges measurement is to reveal the above mentioned weak parts of the dielectric.
18-4 Measurement of partial discharges. G1 feeding generator T1 transformer to be tested T2 pulse transformer T3 step up transformer L1 compensating reactors P1 ammeters Z1 low-pass filters P2 volt-meter (peak value) Z2 terminal resistors of measuring cable P3 oscilloscope W 1 measuring cables P4 volt-meter E capacitive voltage divider Z3 reactance The feeding and measuring instruments used are described on a separate measuring instrument list (Section 20).27
The stability is tested at a voltage equal to half the measurement voltage. Therefore. The reading on the oscilloscope corresponds to the charge qo.and instrument can be connected to the system if necessary. 18-3 Calibration C1 calibration generator.
Fig. The voltage pulse caused by the injected charge is measured by means of an oscillope with the aid of pulse transformers connected to the test to of the bushings. For this reason the stability of the generator voltage control must be tested. The spark. but a narrow. which produces charge pulses of magnitude qo
. The measuring system is basically a wide-band system. The high-voltage side of the stepup transformer is earthed during this measurement. spark-gaps are connected between the high voltage terminals and earth. the voltage on the high voltage side of the transformer under test may rise to an unacceptably high value when connecting the generator to the feeding circuit. Calibration measurement In the calibration measurement (Fig. 18-3) an apparent charge qo is injected between each high voltage terminal and earth.6) IEC 76-3. Stability test Due to internal capacitances.28
Performance of the measurement The measurement is based on observing and evaluating the apparent charge in accordance with the standard (18.gaps are set according to the maximum permissable voltage of the transformers.
If discharges occur.5 Test voltge U1 pre-stress voltage U2 measuring voltage Ui partial discharge inception voltage Ue partial discharge extinction voltage According to the standard (18. the voltage is rapidly reduced to U2 and maintained at this value for the agreed duration of time t mes (Fig. when the occurance of discharges is checked. the results are recorded in order to determine the discharge magnitudes.29
Partial discharge measurement The voltage is increased stepwise.6) IEC 76-3 the test is carried out using the following values of test voltages between line and neutral terminals and test period durations: U1 = Um U2 = either 1. The voltage measurement is carried out at the high voltage side of the transformer to be tested (Fig. The test voltage is increased to the pre-stress voltage level U1 and held there for a duration of 5 seconds.
Fig. If there are discharges at the voltage level U2 the voltage is decreased stepwise after the duration of time Tmes in order to determine the extension voltage. The pre-stress voltage is applied in order to ignite the discharges. During this period the occurence of discharges is being checked at the terminals of the transformer. Thereafter. first up to the measuring voltage U2. 18. 18-5). 18-4).5 Um/ 3 with q < 500 pC t2 = 5 min tmes = 30 min
.3 Um/ 3 with q < 300 pC or 1.
When the test is carried out as a special test. the test procedure can be separately agreed upon.
(18. December. Test report A summary of test results is put down on a form made for this purpose. and is then available when requasted. IEC Publication 270. 1969. Harrold. 76-3 (1980): Power Transformers. Partial discharge measurements. IEC Publ..1) (18. Literature (18.3) (18. November.W.v. The relationship between the picocoulomb and microvolt for corona measurements on h. ELECTRA No. transformers and other apparatus.4) ELECTRA No. IEEE Paper T 72086-2. Brown. Corona measurement on high voltage apparatus using the bushing capacitance tap. and Dakin. Part 3: Insulation levels and dielectric tests.T.D.2) (18. T. 1971. 19. 11. The form is stored in the archives. pp 667-671. IEEE Trans. 1972.6)
. 1968.5) (18.. Power Apparatus and Systems84 (1965). R. R.
. the zero-sequence impedance is about 30. This implies that the measurement is carried out with a test current of 3 x IN. Usually the value corresponding to rated current IN is stated. T2 voltage transformer. 9-1. the zero-sequence impedance is 0. When the transformer has a three-limb core and no deltaconnecter windings. The zero-sequence impedance is measured as function of test current. When the transformer has a delta-connected winding. MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE Purpose of the measurement The zero-cequence impedance is usually measured for all star-connected windings of the transformer. Measuring circuit and performance of measurement
Fig. P2 voltmeter. The zero-sequence impedance per phase is three times the impedance measured in this way. Circuit for zero-sequence impedance measurement
G1 supply generator.60 %. P3 ammeter. T1 transformer to be tested.
. The measurement is carried out by supplying a current of rated frequency between the parallell connected phase terminals and the neutral terminal. T3 current transformer. I test current
The zero-sequence impedance is dependent on the current flowing through the winding.1. Result The zero-sequence impedance is usually given as a percentage of the rated phase impedance.. and when necessary the final result is obtained by extrapolation.8. The zero-sequence is needed for earth-fault protection and earth-fault current calculations. In the test report the zero-sequence impedance values at the principal and extreme tappings are stated. However. this is not always possible in practice since the current must be limited to avoid excessive temperature of metallic constructional parts...31
11.0 times the corresponding short-circuit impedance.
1 is earthed. when winding No. when winding No.10-1 Transformer capacitances. A two-winding transformer is measured as follows: (10. obtained by combining the partial capacitances.1) The capacitance K10 between earth and winding No 1 is measured. a two-winding transformer b three-winding transformer
Because the partial capacitances (C) in Fig. Performance of the measurement All line terminals of each winding are connected together during the measurement. CAPACITANCE MEASUREMENT Purpose of the measurement The purpose of the measurement is to determine the capacitances between the windings and the earthed parts and between the different windings of the transformer. the values of resulting capacitances (K). are measured and the required partial capacitance values are calculated from the measured values. 2 is measured. In addition. 2 earthed.32
12. K10 = C10 + C12 The capacitance K20 between earth and winding No. 10-1 cannot be measured separately.
. Fig. the results are used by the manufacturer for design purposes. The measurement is carried out by means of a capacitance bridge.and three-winding transformers are shown on Fig. 10-1. The capacitance values are needed when planning transformer overvoltage protection and calculating the overvoltages affecting the transformer. The winding capacitances of two.
nk = n * n +1 kpl 2
n = the number of windings Test report The partial capacitances are given per phase. Literature (10.. C12 and C20 are determined by solving the set of equations (10. K12 = C10 + C20
The partial capacitances C10. Sähkö-Electricity in Finland 39 (1966) No..2) (10. p 289. The number nk of partial capacitances (and measurement combinations) is (10.(10..1) Bertula T. For transformers with three or more windings a similar method is used.3)
K20 = C20 + C12 The capacitance K12 from the interconnected windings No..
. 10.3). thus three-phase capacitance values obtained in the measurement are divided by 3. 1 and 2 to earth is mesured.1).293.33
(10. Palva V: Transformer capacitances.
Each winding is measured separately by connecting the voltage between the winding to be tested and earth. while the other windings are earthed. 11-1 Insulation resistance connection
. This meaurement gives information about the condition of the insultation and secures that the leakage current is adequately small.c. The resistance readings R15 and R60 are taken 15 s and 60 s after connecting the voltage. The type of meter used. temperature. the R measuring voltage. as measured at a constant voltage difference across the insulation. R15
Fig. This is a function of the moisture and impurity contents of the insulation and of its temperature such that when these parameters are increased the insultaion resistance.34
13. INSULATION RESISTANCE MEASUREMENT Purpose of the measurement The purpose of the measurement is to determine the leakage current reistance of the insultation. depend on the strenght of the electric field during the measurement and thus on the size and construction of the transformer. Performance of the measurement The insulation resistance is measured by means of an insulation resistance meter at a voltage of 5000 V d. R60 and 60 are stated in the report. R15.
LOSS FACTOR MEASUREMENT Different electrical measurements can be carried out to check the condition of insultaions between transformer windings and between windings and earthed parts.
Fig. 12-1) can be used as the equivalent circuit of the insulation distance 1 to 2 to be measured. The leakage current resistance depends on the measuring voltage.
14. The capacitance Cs of each winding and that of pair-wise connected windings and the loss factor tanδ against earth (connections as in item 10) are generally defined in the measurement. The measuring voltage is usually 5 kV or 10 kV. Parallel-connection or series-connection (Fig. and therefore results obtained for transformers of different sizes cannot directly be compared to each other. The loss factor is primarily a characteristic quantity for the insulation itself. 12-1 Valid for the series-connection: tan δ = ω * C s * Rs
tan δ ω * Cs Obtained for the paralell connection 1 tan δ = ω * C p * Rp 1 Rp = ω * C p * tan δ Rs =
R p = Rs * 1 + tan 2 δ tan2 δ Cp = Cs 1 + tan2 δ
The loss factor tanδ is proportional to the effective resistance Rs of the insulation distance at the AC voltage. The result (leakage current resistance) obtained in the insulation resistance measurement (compare item 11) describes in the first hand the behavior of insulation distances at direct current voltage. The loss factor measurement will be carried out by means of a special measuring bridge and a standard capacitor.
P = U 2 * ω * C p * tan δ
The losses caused by the polarization generated in the insulation of the electrical field are proportional to the square of the voltage. The loss factor tanδ can be used as a standard to be noted that the loss factor is the function of the insulation temperature and humidity content.
. the measuring voltages. The following equation distance to be measured.36
The resistance depends on the dielectric losses of the insulation as well as on the leakage current component caused by the AC voltage. Tanδ corresponding to these losses is independent of the voltage. Results The insulation distances measured. In such cases the tanδ-value does not correctly describe the condition of the insulation in this insulation distance.) or discharges take place. the electrode has an important effect on the size of the displacement angle. the tanδ. If tanδ= increases when the voltage is raised the reason for it may be that the leakage current resistance decreases (humidity. If a rounding electrode made of half-conducting material has been installed on top of the core. capacitance and temperature of the insulation are stated in the report.
1) (13. The electric strength of new treated oil should be at least 60 kV.5 mm apart. The measurement is carried out at 50 Hz. In practice the breakdown voltage is about 70 kV. IEC 296 (1982). measured using an electrode system in accordance with IEC 156 (13. the rate of increase of the voltage being 2 kV/s. MEASUREMENT OF THE ELECTRIC STRENGHT OF THE INSULATING OIL The electric strength of oil is given by the breakdown voltage.
. Oil which does not withstand this voltage may contain air bubbles. dust or moisture.1) IEC 156 (1963) Method for the determination of the elctric strengh of insulating oils.1). Specification for unused mineral insulating oils for transformers and switchgear.37
Literature (13. The electrodes are spherical surfaced with 25 mm radius and are 2. The electric strenght is the average of six breakdown voltage values.
The test is performed as follows: Cold resistance measurement The resistance and the corresponding oil temperature are measured. The supply values and the temperatures of different points are recorded at suitable time intervals. TEMPERATURE-RISE TEST Purpose of the measurement The purpose is to check that the temperature rises of the oil and windings do not exceed the limits agreed on or specified by the standards. Resistances are measured between line terminals e. Apparatus The supply and measuring facilities as well as the measuring circuit are the same as in load loss measurement (Section 4) and in the resistance measurement (Section 3). Determination of the temperature rise of oil The power to be supplied to the transformer is the sum of the no-load losses and load losses on the tapping on which the temperature-rise test is to be performed (generally the maximum loss tapping). In addition thermometers are used for the measurement of the temperature of the oil. The test connection is changed for carrying out the resistance measurement and after the inductive effects have disappeared the resistance-time-curves are measured for a suitable period of time (zero time is the instant of switching off the supply). The resistance is measured between the same line terminals as in the cold resistance measurement. The temperature rise of the windings is determined by the resistance method. The winding temperature is the same as the oil temperature.. When the current has been cut off the hot-resistance measurement is performed. With this power the transformer is warmed up to thermal equilibrium. The supply values and the temperatures are recorded as above. The oil temperature rise above the cooling medium temperature can be calculated from the equilibrium temperatures.g. A-B and 2a-2b. cooling medium and the ambient temperature and further a temperature recorder and Pt-100 resistive sensors are used for the measurement of certain temperatures and for equilibrium control.38
16. Performance of the measurement The test is performed by using the short-circuit method. The resistance of the windings at shut-down are obtained by extrapolating the
. Determination of the temperature rise of winding Without interrupting the supply the current is reduced to rated current for 1 h.
The ambient temperature is measured by means of at least three thermometers.1)
PN θto = Pt
) * (ϑ
− ϑa)
resistance-time -curves to the instant of switching off. For air-cooled transformers with natural air circulation the temperature of the cooling medium is the sama as the ambient temperature. Results The temperature rises are calculated as follows: Oil temperature rise The tempeerature rise of tpo oil Θto is
(14. the top oil temperature is measured from the tube leading to the cooler near the transformer. The temperature rises of the windings above the oil temperature are calculated on the basis of the "hot" and "cold" resistance values and the oil temperature. the water temperature at the intake of the cooler is the reference temperatur. If transformer has separate cooler. Furthermore the temperatures of the oil coming from or going to the transformer are measured and also some other temperatures which may by interesting. The top oil temperature is measured by a thermometer placed in an oil-filled thermometer pocket on the cover or in the tube leading to the coolers. and the power taken by the oil pump and fan motros is measured. For multi-winding transformers the latter part of the temperature rise test is generally carrier out several times in order to determine the individual winding temperature rises at the specified loading combination. If water is used as cooling medium. which are placed at different points around the transformer at a distance defined by the standards approximately half-way up the transformer. The readings of the thermometers mounted on the transformer are checked in connection with the temperature rise test. For forced-air cooled transformers the temperature of the ingoing air is measured. The temperature rises of the windings above the cooling medium temperature are found by adding the temperature rise of oil above the cooling medium temperature to the before mentioned winding temperature rises.
ϑr =
R2 *( ϑ s + ϑ1) − ϑ s R1
ϑs = 235 °C for Copper ϑs = 225 °C for Aluminium R1 = cold resistance R2 = hot resistance ϑ1 = the average temperature of oil during cold resistance measurement
The average temperature rise Θro of the winding above the oil temperature is (14.2)
( P N ) * (ϑ
ϑ 2 − ϑ3 − ϑa) 2
ϑ2 = temperature of oil going into the cooled ϑ3 = temperature of oil coming from the cooler
Temperature rise of windings The average temperature of oil ϑo before the hot-resistance measurement is
ϑ o = ϑ to −
ϑ 2 − ϑ3 2
The average temperature of winding ϑr is (14.40
PN = rated losses Pk 75 °C + Po Pt = power supplied during the test x = exponent according to the standard ϑto= top oil temperature ϑa = cooling medium temperature The average temperature rise Θo of the oil is (14.5)
IN *( ϑ r − ϑ o ) It IN = rated current of the winding It = test current y = exponent according to the standard
θro = (
1 Θro
The winding temperature insicator.6)
Θr = Θo + Θro
The temperature rise Θhs of the hot spot of the winding above the ambient temperature is (14.41
The average temperature rise Θr of the winding above the ambient termperature is (14. if any. E.: Determining the temperature rise in a transformer using the resistance method.1) winding (1974). Sähkö-Electricity in Finland 47 No 1.7)
Θhs = Θto + 1. Kiiskinen. will be adjusted on the basis of the temperature risse Θhs Results The report indicates cold resistance values and the corresponding oil temperature temperatures of oil and cooling medium in thermal equilibrium and the corresponding losses hot resistances at shut-down and the corresponding currents temperature rises calculated from the measuring results
In addition information on the winding combination or combinations involved in the test. the cooling method and the time of delay is given. Literature (14.
. the tapping position.
Performance of the test The test is performed with impulses of negative polarity.. 16-1 Chopped lightning impulse.6 µs
. The duration Tc from the beginning of the impulse to the chopping can vary within the range of 2..2 µs ± 30 % β yc = * 100% < 30% (T2 = 50 µs ± 20 %) α Tc = 2.42
17. Testing equipment For the lightning impulse test the same testing and measuring equipment and the same testing and fault detection connections are used as for the standard lightning impulse test. 161). According to the standard (16. The impulse is chopped by means of a triggered-type chopping gap connected to the terminal to which the impulse is applied.. T1 = 1.. If necessary the overswing amplitude will be limited to the value montioned by means of a damping resistor inserted in the chopping circuit. thus the time Tc from the start of the impulse to the chopping can be ajadjusted (Fig. 16-1).1) the amout of overswing to opposite polarity shall be limited to not more than 30 % of the amplitude of the chopped impulse (Fig. TEST WITH LIGHTNING IMPULSE CHOPPED ON THE TAIL Purpose of the test The purpose of the chopped lightning impulse test is to secure that the transformer insulations withstand the voltage stresses caused by chopped lightning impulse. which may occur in service. 16-1).6 µs (Fig.
Fig. The delay of the choppinggap ignition impulse in relation to the ignition of the impulse generator is adjustable.
76-3 (2000): Power transformers. where they are available when required. In order to make the comparison of fault detection oscillograms obtaine at 100 % voltage are of one same size as calibration oscillograms obtained at 100 % voltage are of one same size as calibration oscillograms obtained at 62. Test report The test voltage values. If necessary the voltage measuring sircuit can be calibrated with the aid of a sphere-gap.1) IEC Publ. Part 3: Insultation levels and dielectric tests.
.5 % voltage. When carrying out the chopped-impulse test. At chopped impulse the fault detection is additionally secured since the test sequence includes the application of two standard impulses after the applicaton of the chopped impulses. tappings and the number of impulses at different voltage levels are stated in the report. The oscillographic records and measurement records are stored in the archives. usually the two extreme tappings and the principal tapping. different tappings are selected for the tests on the three phases of a threephase transformer. impulse shapes. The test with chopped lightning impulse is combined with the test carried out with standard impulse. which causes differences in the fault detection and calibration oscillograms of voltages and winding currents.5 % calibration voltages and 100 % test voltages. The following order of pulse applications is recommended by the standard (16.1) one 62.43
The voltage measurement is based on the peak voltmeter indication. unless otherwise agreed. In this case the fault detection must be based primarily on the recordings obtained at the application of full impulses. At high test voltages (> 750 kV) there is a ssmall delay in the ignitions of the chopping-gap.5 % chopped impulses two 100 % chopped impulses two 100 % full impulses
The fault detection is also for chopped impulses primarily based on the comparison of voltages and winding currents obtained at 62.5 % full impulse one 100 % full impulse one or more 62. Literature (16.
. 17-1. The voltage developed between line terminals during the test is approximately 1. Because the remanent flux can amount to even 70 to 80 % of the saturation flux. The flux density in the magnetic circuit increases considerably during the test. By introducing remanent flux of opposite polarity in relation to the flux caused by the switching impulse. The remanence of opposite polarity is introduced in the core by applying low voltage impulses of opposite polarity to the transformer before each full voltage test impulse. 17-2). A single-phase noload test connection is used in accordance with Fig.5 time the test voltage between line and neutral terminals.44
Fig. which may occud in service.1) the switching impulse test is carried out on each line terminal of a three-phase winding in sequence. between line terminals and earth and between different terminals withstand the switching overvoltages. 17-1 Transformer switching impulse testing and fault detection connections. According to the standard (17. When the core reaches saturation the winding impedance is drastically reduced and a chopping of the applied voltage takes place (Fig. between windings and earth. SWITCHING IMPULSE TEST Purpose of the test The purposet of the switching impulse test is to secure that the insulations between windings. The time to saturation determines the duration of the switching impulse. Performance of the test The same testing and measuring equipment as for the lightning impulse test are used here. the maximum possible switching impulse duration can be increased. the initial remanence of the core has a great influence on the voltage duration.
17. The voltage measuring circuit can be calibrated with the aid of a sphere-gap when required.
Fig.17-2. Thus voltage and current oscillograms obtained at full test voltage and at 62. In order to facilitate the comparison of oscillograms the oscilloscope will be attenuated so that the fault detection oscillograms are of the same size as the calibration oscillograms.45
The test is performed with impulses of negative polarity.
.5 % voltage level will deviate from each other in this respeckt. The requirements on the switching impulse shape given in the standard IEC 76-3 are summarized in Fig. When comparing the fault detection and calibration oscillograms it is to be noticed that the magnetic saturation causes drastical reduction of voltage and increase in winding current and the time to saturation is dependent on the amplitude of the applied voltage.2 Switching impulse Front time T1 > 20 µs Time above 90 % Td > 200 µs Time to the first zero passage Tz > 500 µs Calibration oscillograms of voltages and winding currents are recorded at 62.5 % voltage voltage level for comparison with the fault detection oscillograms recorded at 100 % voltage. At full test voltage each phase will be tested with the number of impulses required by the relevant standard. The voltage measurement is based on the peak voltmeter indication.
and number of impulses at different voltage levels are stated in the report. Part 3: Insulation levels and dielectric tests. Literature (17.
. The test is successful if no sudden collapse of voltage caused by flashover or breakdown is indicated on the voltage oscillograms and no abnormal sound effects are observed. impulse shapes. When the core reaches saturation a slight noise caused by magnetostriction can be heard from the transformer.1) IEC Publ. The fault detection is mainly based on the voltage oscillograms. where they are available when required. Test report The test voltage values. 76-3 (1980): Power transformers.46
In additon disturbances caused by corona discharges in the test circuit may be found on the current oscillograms recorded at test voltage. The oscillographic records are stored in the archives.
i.3).3). (19.2) and (19.5 m. The transformer will be located at the test site so that the free distance from the transformer to reflecting objects is sufficiently large. when the height of the tank is equal to or greater than 2. The measuring equipment is described in a separate list of equipment (Section 20). The sound spectrum analysis of the transformer is carried out by recording the sound band levels automatically as a function of frequency. The measurement is carried out at rated voltage and frequency.1). requirements given in relevant standards.5 m. According to the standards (19.2) and (19.1) or (19.47
19. This is done with the aid of an analyser. A sound spectrum analysis is carried out for the transformer at the customer's request. (19. Before and after the transformer sound level measurement the background noise level is measured. MEASUREMENT OF ACOUSTIC SOUND LEVEL Purpose of the measurment The purpose of the sound level measurement is to check that the sound level of the transformer meets the specification requirements.1).e. a correction for background level will be applied according to standards (19. the measuring plane is located at half the tank height.2).2). or guarantee values given by the transformer manufacturer. If the difference is less than 9 dB(A) but not less than 3 dB(A).1). The measurements are performed using the weightning curve A. (19. Performance of the measurement The measurement is carried out at measuring positions located around the transformer as detailed in the standards (19.
. Preferably the background level should be at least 9 dB(A) below the measured combined sound level. e.3) the microphone position in the vertical direction shall be on horizontal planes at one third and two thirds of one transformer tank height. and when the measurement is carried out in accordance with the standard (19. Mesuring equipment A precision sound level meter complying with standards (19.g.1) and (19.3) is used in the sound level measurements.2) and (19. The sound spectrum indicates the magnitude of sound components (measured at a given band-width) as a function of frequency. The microphone is directed perpendicularly against the surface of the transformer (the principal radiating surface). which is both mechanically and electrically connected to the recorder or with the aid of an octave filter set joined to the sound level meter. When the tank height is less than 2. (19.
regulators and reactors.
Literature (19. TR 1-1980.1) (19.3) NEMA Standards Publication No. Transformers.82. 1976. Teil 1/03. IEC Publication 551.48
Test report The mean value will be calculated from the measurement results.2) (19. Bestimmungen für Transformatoren und Drosselspulen. Measurement of transformer and reactor sound levels. Correctionss for background level and environmental correction are made to the mean value.
. VDE 0532.
2140 . In the no-load voltage or the generators the 5th harmonic is 0.5 MW.49
20. Two identical generators which can be connected in parallel.5 min. squirrel-cage motor
The rotational energy of the unit is 83 MWs. LIST OF EQUIPMENT Rotating machines The most important characteristics of the machines are mentioned.808 .4290 A f = 50 Hz P = 850 kW. Generator: S = 15 MVA U = 10.404 V I = 1240 . The starting time is about 1. The unit can be run continuously only in star-connection (10. Machinery 2. Generators: S = 3 MVA U = 1400 . and a driving motor.2470 . and stopping time (only friction losses) is about 25 min.. and the stopping time (only friction losses) about 60 min.3 min.7 MW.700 .1%. have no effect on the results in transformer tests.9 % and the 7th 1.5/6. however. the generator may be loaded with 135 % current at 100 % voltage (excitation not more than 300 A) and the motor accordingly with 140 % active power. slip-ring motor
The rotational energy of the unit is 18.5 kV). When the outdoor temperature is not higher than 0°C and forced air cooling is used. The starting time is 1.06 kV I = 825/1430 A f = 50 Hz P = 1. Machinery 1. These. slip-ring motor P = 1. In addition there are slot harmonics of the 29th and 31st order..5 MWs.
This voltage is connected to the rotor of the slip-ring motor and the stator then supplies 166 Hz voltage.42 . synchronous motor
The voltage curve of the generator contains the following harmonics: 5th harmonic 1.700 .4 .286 .2 % and the 7th harmonic 1. Generator: S = 1. 10 min.404 V I = 124 . 1 min/460 kW.7 %.808 . Machinery 4. At sinusoidal loading current these values do not increase notably. When it is used as 16 2/3.2.0 %. When the unit is used as a frequency converter the squirrel cage motor acts as a drive-motor and the generator supplies 83 Hz voltage. squirrel-cage motor
The unit is used in two different ways. 60 and 83 Hz
Thermal loading capacity 485 kVA (not for 16 2/3 Hz frequency) Motors: P = 260 kW. Generator: S = 400 kVA U = 1400 . When using the frequency converter these harmonics are 2 %. In the no-load voltage of the generator the 7th harmonic is 1 % and the 19th 0. the slip-ring motor acts as a drive-motor and the generator supplies the voltage. Motor: P = 300 kW. The relation of gearing must be 1:1.700 .8 .50
Machinery 3. 50 or 60 Hz voltage source.429 A f = 50 Hz
Thermal loading capacity 450 kVA.300 -572 A f = 16 2/3. Generator: S = 300 kVA U = 1400 . U = 1.808 .179 A f = 250 Hz
Motor: 870 kW.358 . 10 min. slip-ring motor + gear P = 250 kW.247 .310 . Machinery 5. synchronous motor The unit is mainly used for voltage testing
.214 .2.5 MVS. 50.4.404 V I = 165 .84 kV I = 620 .
Motor: inverter.4 .93 .10000 .119 A/2857 .238 .825 A Zk = 1.200 A/4920 .8660 .35000 .1400 V I = 692 .2 .6.1400 V I = 57. squirrel cage motor.06 .28. Transformer 3.0 . Step-up transformers Tecnical data: Transformer 1.206 .8 kV/3.33.16.4 MWs at a speed corrensponding to 50 Hz.25 .1650 .03 . 10 hrs.72. Transformer 2.7 % Current overload capability 40 %.51
Machinery 6.5 A/1430 .6 .3.346 .833 .6 . The speed of rotation can be regulated by means of an The rotational energy of the unit is 21.5 kV I = 412 .40414 . Generator: S = 10 MVA U = 12 .46 kV I = 481 .10.17320 V/808 . In short-duration single-phase voltage tests the voltage from terminal to earth must not exceed 100 kV.825 A The insulations are dimensioned so that at 83 Hz.1667 A f < 60 Hz P = 1500 kW. S = 15 MVA continuously U = 21 . the highest permissible voltage is 116 kV. S = 2 MVA continuously U = 20207 .5. When the transformer is used in connection with the 250 Hz generator.1429 .
.400 .6.70000 V/808 . S = 6 MVA continuously U = 5000 .2475 A The transformer can be continuously overloaded by 15 % at a 35 °C ambient temperature and by 50 % when using three additional fans.36. 166 Hz or 250 Hz the transformer voltage can exceed the rated value by about 30 % for a short period of time.42 .
50/5 A.200/5 A.1. U = 170/72 . It is also possible to connect the capasitors for seriescompensation. Cl. 72. Instrument transformers Current transformers.5 . S = 5 MVA.5 kV/1.5 2.700 .12370 . 0.404 V I = 722/6190 .52
Transformer 4.8) kV i = 17/40 .56 .1000 .89 A f = 50 and 60 Hz Transformer 6.100 .400 .5 kV Manufacturer: AEG 800 .80/137 .21440 A f = 50 and 60 Hz Capacitor bank The bank comprises 864 units. S = 15 MVA continuously U = 12000/1400 .155 . 0.25/5 A. Star and delta connections of the capacitors are possible.5 .2.50 .48. S = 15 MVA continuosly U = 12/28 . The capacitors are so grouped that the reequired connection is obtained easily.8. 50 Hz.2.36/21 .179 . Cl. Transformer 5.4 (0.8. Cl. 5 pc 4000 . 20 kV Manufacturer: Strömberg
. 15 VA cosϕ = 0.2000 . 30 min.5 .25 . 0.100 .4 µF The rated power of the bank is 216 MVAr. The capacitors can also be used at 60 Hz. 15 VA cosϕ = 0. 15 VA cosϕ = 0.200 .800 .808 .8.10720 .2.275/2060 (3570) A The transformer is mainly used as step-up transformer for partial discharge measurement.97 kV I = 722/309 . 50 Hz. the rated values of which are: Q = 250 kVAr (50 Hz) U = 7300 V C = 15.400 . 20 kV Manufacturer: Strömberg 200 . 50 Hz.10.12.
0. Cl.2. 15 VA cosϕ== 0.5 . 50 Hz Manufacturer: Strömberg 3000 . 50 Hz.25 -12.2.750 .8. 0.2.3 kV 100 V.2.2.5 kV Manufacturer: AEG 35000/100 V. Cl. Cl.8. 15 VA.
. 15 VA cosϕ== 0. 50 Hz Manufacturer: Strömberg
3 pc 3 pc 3 pc 3 pc 3 pc
The errors of the instrument transformers have been measured with burdens corresponding to actual conditions.8. 0.8. 15 VA cosϕ== 0. 50 Hz Manufacturer: Strömberg 10000 .2.5/5 A. Cl.10 .8.5 . insulation level 72.8.20 . 15 VA cosϕ = 0. 50 Hz Manufacturer: Strömberg 20000/100 V. Cl. 0. The corrections for loss measurements are performed using these error curves.380/100 V.1500/100 V.53
50 . Cl. 50 Hz Manufacturer: Strömberg 1500 . 50 Hz.45 kV Manufacturer: Strömberg
Voltage transformers 5 pc 45 . Cl. 0.15 VA cosϕ=0. 15 VA cosϕ== 0. 0.8.2.5.5 -1. cosϕ = 0.2.5000/100 V. 0.
Bernecker + Rainer Norma
0.5 cosϕm = 0. meteers Digital thermometer Pt-100 sensors.11 x mean value) Ammeters Wattmeters Wattmeters D.3 % < ± 0...2 ± 0.m.54
Meters.1 0.2x10-4
ESI TETTEX
Thermocamera equipment AGA Thermovision 782 Scanner AGA Ttermovision 780 Sensitivity district 3 . The following meters are available: Meter Voltmeters (r.1 °C Manufacturer H&B Keithley H&B H&B H&B Siemens Systemteknik
0. value) Voltmeters (1. max.1 % tanδ ± 1.2 0.1 0.s.1 0..1 2.C.65 micron temperature district -20 °C. 5000 V Capacitance meter Lossfactor capacitance bridge Accuracy class 0.2 cosϕm = 1 0. 12 Channels Frequency meter Insulation resistance meter.5.+800 °C (min) Polaroid-camera Video tape recorder Colormonitor
total energy Capacitance per stage Voltage divider. 4231 n = 12 U1 = 200 kV ΣU1 = 2400 kV W k = 15 kJ ΣW = 180 kJ C = 750 nF
. total charging voltage Max energy per stage Max.55
The resistance measuring apparatus Transformer test system Tettex type 2285 Measuring range Resolution Accuracy 1 µΩ … 500 Ω 0. when the duration of the wave front is between 0. Accuracy for chopped-wave measurements: ± 1 % when chopping occurs on the wave tail ..06 % rdg ± 1 µΩ
Impulse testing apparatus Impulse generator.2400 pF The response characteristics and voltage ration of the voltage divider are checked in accordance with the standard IEC 60-3.. 4190 Calibrator. Haefely Number of stages Max.. Haefely Damped capacitive voltage divider Voltage ranges for impulse voltage û = 50 kV. The 1000 mm sphere-gap is provided with a triggering electrode. Calibration sphere gaps Sphere-gaps with sphere diameters of 500 mm and 1000 mm are available..0. 2215 Microphone. Accuracy: ± 1% for full wave measurements. and it can be used as a controlled chopping cap.2400 kV Capacitance C = 600 pF.2.5 µs and time to crest.450 µs and the time to half-value is <3000µs..1 µΩ ± 0..2. 62 The instrument is provided with a digital display unit.5 µs. Peak voltmeter. Brüel & Kjaer. Sound level measuring equipment Precision sound level meter... -3 % when time to chopping is 0. Brüel & Kjaer. Brüel & Kjaer. stage charging voltage Max. Haefely Mod.2 % when the time to chopping is between 0.
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Testing of Power Transformer by Pradeep Singh354 viewsEmbedDownloadRead on Scribd mobile: iPhone, iPad and Android.Copyright: Attribution Non-Commercial (BY-NC)List price: $0.00Download as PDF, TXT or read online from ScribdFlag for inappropriate contentMore informationShow less
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