INSULATION RESISTANCE (IR) MEASUREMENT Purpose: Insulation resistance (IR) measurement is the simplest and most widely used test to check the soundness of transformer insulation. This test reveals the condition of insulation (i.e. degree of dryness of paper insulation), presence of any foreign contaminants in oil and also any gross defect inside the transformer (like failure to remove the temporary transportation bracket on the live portion of tap-changer part). Test Equipment: Insulation resistance is measured by means of Meggers which are available in 500 V, 1000 V, 2500 V and 5000 V ratings.For transformer windings with voltage rating 430V,500V megger ,430V and above 1000V megger,11 kV and above, 2.5 kV megger shall be used. IR value measurements of EHV transformers shall preferably be done with 5 kV motorized / digital megger. Testing Procedures: IR measurements shall be taken between the windings collectively (i.e. with all the windings being connected together) and the earthed tank (earth) and between each winding and the tank, the rest of the windings being earthed. Before taking measurements the neutral should be disconnected from earth. Following table gives combinations of IR measurements for auto-transformer, three -winding transformer & Shunt Reactor.
For 2 Winding For 3 Winding For Auto- For Shunt Transformer Transformer Transformer Reactor
HV to Earth
HV+IV +LV to Earth
HV+IV to Earth
HV to LV
HV+IV to LV
HV+IV to LV
LV to Earth
HV+LV to IV
LV to Earth
HV to Earth
Where HV- High voltage, IV-Intermediate voltage, LV-Low voltage/Tertiary voltage windings, E- Earth
Minimum Requirement : i) Minimum 200 M ohms, for below 6.6 KV class for 1 min ii) Minimum 400 M ohms between 6.6KV &11KV class for 1 min iii) Minimum 500 M ohms upto 33KV class for 1 min iv) Minimum 600 M ohms upto 132KV class for 1 min v) Minimum 650 M ohms upto 220KV class for 1 min CORE INSULATION CHECK:
(USING 2500V MEGGER) CORE
CORE
TO TO FRAME TANK INSULATION FOR 60 SEC
Connection Diagram:
VALUE
FRAME TO TANK
Measurement table :
Measured between
Temp = ˚C
15 Sec
60 Sec
600 Sec
DAI
PI Value
(Dielectric (Polarization absorption Index) coefficient or 600 sec/60 sec index) 60 sec/15 sec
HV to Earth
HV to LV
LV to Earth PRECAUTIONS: 1. Before testing Temperature must be recorded. 2. There should be no joints in testing cables. 3. Test leads should not touch any live part, 4. Megger body should be earthed (if separate terminal is provided). 5. Surface/terminals should be cleaned 6. IR Measurement should be carried out preferably in dry and sunny weather. 7. Never connect the test set to energized equipment. 8. The ground terminal must be connected first and removed last. 9. After testing with high voltage, test terminals must be grounded before being touch by any personnel. 10. Test leads should be properly screened/shielded.
VOLTAGE RATIO TEST Purpose To determine the turns ratio of transformers Turns Ratio: The Turns Ratio of a transformer is the ratio of the number of turns in a higher voltage winding to that in a lower voltage winding. Test Equipment: Transformer turns ratio (TTR) meter available in convenient portable form. They have a hand crank power supply, with the voltages commonly used being very low, such as 12-120 V and 5060 Hz, so that the test may be performed on a transformer even when the oil is removed. Two windings on one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector, with exciting current also being measured in most cases. Testing Procedures 1. Keep the tap position in the lowest position and IV and LV terminals open. 2. Apply 3 phase 415 V or single phase 230 V supply according to transformer type on HV terminals. 3. Measure the voltages applied on each phase (Phase-Phase) on HV and IV terminals simultaneously. 4. Repeat Steps 2 and 3 for each of the tap position separately.
5. Repeat the steps 1 to 4 after interchanging the voltmeters of HV and IV windings and then average the readings for final calculation of ratio.
HV Side
LV Side
connections
H1
H2
X1
X2
DELTA-DELTA
R-Phase
Y-Phase
R-Phase
Y-Phase
STAR-STAR
R-Phase
N-Neutral
R-Phase
N-Neutral
DELTA - STAR
R-Phase
Y-Phase
R-Phase
N-Neutral
STAR - DELTA
R-Phase
N-Neutral
R-Phase
Y-Phase
Ratio = multiplier x Reading on the ratio dials x (100 + D) 100 % Error = measured value – Actual value Measured value
x 100
Acceptance Critera/ Tolerance on test
: Tolerance permissible is +/-0.5% of Parameter declared ratio on all the taps.
Connection Diagram:
TR-100
H1C H1
X1
H1P X1
Tx X2 X2 H2
H2P
H2C
Measurement table : VOLTAGE RATIO TEST TAP POSITION
RATIO HV/IV
MEASURED RATIO 1R1N-2R1N
1Y1N-2Y1N
1 2 Etc.,
PRECAUTIONS: 1. There should be no joints in testing cables. 2. Test leads should not touch any live part, 3. Surface/terminals should be cleaned 4. Never connect the test set to energized equipment. 5. Test Kit Must be earthed
CALCULATED 1B1N-2B1N
RATIO
VECTOR GROUP AND POLARITY Purpose: To determine the phase relationship and polarity of transformers. Transformer winding ratio, vector group and polarity should be checked before energizing the transformer for the first time on site.Polarity of a transformer is determined by the internal connections and is indicated by the name plate markings. Polarity is of interest if the transformer is to be connected in parallel with one or more other similar transformers. Test Equipment: Voltmeters Testing Procedures: 1. The test shall be conducted with three phase supply (415V, 3 phase supply) on HV Side 2. HV Side R-Phase and LV Side R-Phase will be short. 3. Tthe measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled
Precautions: 1. This testing shall be carried out during Shut down period and all testing shall be done under SFT (Sanction For Test). 2. Ensure the isolation of Transformer for High Voltage &Low voltage side with physical inspection of open condition of the concerned isolators/disconnectors. 3. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing
MAGNETIC BALANCE TEST Purpose This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit Test Equipment: Voltmeters, Clamp meter Testing Procedures 1. Keep the tap in nominal tap position 2. Disconnect transformer neutral from ground 3. Apply single phase 230 V across one phase of Highest Voltage (HV) winding terminal and neutral (call it v1) then measure voltage in other two HV terminals across neutral (call them v2 and v3 respectively). Repeat the test for each of the three phases. 4. Repeat the above test for Intermediate Voltage (IV) winding also HV SIDE
VOLTAGE
MEASURED CURRENT
APPLIED
U-N
V-N
W-N
U-N
Applied
Measured
Measured U
V-N
Measured Applied
Measured V
W-N
Measured Measured
Applied
W
Note : 1. If star means applied Phase to Neutral and Measured Phase to Neutral 2. If Delta means applied Phase to Phase and Measured Phase to Phase 3. From the measurement carried out we ensure that v1 ~= v2 + v3 respect
LV SIDE
VOLTAGE
MEASURED CURRENT
APPLIED
U-V
V-W
W-U
U-V
Applied
Measured
Measured U
V-W
Measured Applied
Measured V
W-U
Measured Measured
Applied
W
Precautions 1. No winding terminal should be grounded; otherwise results would be erratic and confusing. 2. Zero voltage or very negligible voltage induced in other two windings should be investigated.
MAGNETIZING CURRENT TEST Purpose Excitation/ Magnetising current test is performed to locate defect in magnetic core structure, shifting of windings, failures in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core. Exciting/ Magnetising current: is the current required to force a given flux through the core.
Test Equipment: Voltmeters, Clamp meter Testing Procedures The test comprises a simple measurement of single-phase current on one side of the transformer, usually the high-voltage side, with the other side left floating (with the exception of a grounded neutral) APPLIED VOLTAGE IN HV OTHER OPEN
CURRENT MEASURED
1U11V1
U in V mA mA
TAP POSITION
1 2 3 4 Etc.,
1V1W1
1W11U1
in W in mA
Note: 1. Applied HV side 3 phase supply and measured HV side Current 2. Applied LV side 3 phase supply and measured LV side Current 3. If IV is available applied IV side 3 phase supply and measured IV side Current Precautions 1. This test should be done before DC measurements of winding resistance to reduce the effect of residual magnetism. 2. Magnetising current readings may be effected by residual magnetism in the core therefore, transformer under test may be demagnetised before commencement of magnetising current test.
WINDING RESISTANCE MEASUREMENT Purpose Transformer winding resistance are measured in the field in order to check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers as a pre-commissioning checks and compare the measured values with factory test values Test Equipment: Winding Resistance Meter Testing Procedures: 1.
For star connected winding with neutral brought out, the resistance shall be measured between the line and neutral terminal and average of three sets of reading shall be the tested value. If neutral bushing is not available on Star connected windings, take measurement between each phase and ground (if the neutral is grounded), or take readings between pairs of bushings as if it were a Delta connected winding. Repeat in same fashion so that proper comparison can be made.
2. For star connected auto-transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral AT ALL TAPS. The tap changer should be changed from contact to contact so that contact resistance can also be checked. 3. For delta connected windings, such as tertiary winding of auto-transformers, measurement shall be done between pairs of line terminals and resistance per winding shall be calculated as per the following formula: Resistance per winding = 1.5 x Measured value 4. Take the winding temperature reading while doing the resistance measurement. 5. Calculate the resistance at 75°C as per the following formula R75 = Rt (235+75)/(235+t ), Where Rt = Resistance measured at winding temperature t Acceptance Critera/ Tolerance on test
: Tolerance permissible is +/-2% of Parameter declared ratio on all the taps.
Connections: HV Side connections
V+C+
V-C-
DELTA-DELTA
R-Phase
Y-Phase
STAR-STAR
R-Phase
N-Neutral
DELTA - STAR
R-Phase
Y-Phase
STAR - DELTA
R-Phase
N-Neutral
Note: 1. Same connection for LV side also. 2. Always current channel will connected top of bushing terminal. 3. Always Voltage channel will connect below the current channel. WINDING RESISTANCE in m TAP POSITION
1U12U1
1 2 Etc.,
1V12V1
1W12W1
Temp 1U11V1
˚C 1V11W1
1W11U1
1U1-N
1V1-N
1W1-N
Precautions 1. Before testing Temperature must be recorded. 2. As the transformer resistance is Low resistance, the measurement has to be carried out with the help of Kelvin Double Bridge / Transformer ohm meter. Normally winding resistance values 1 ohm or above is measured using Wheatstone Bridge and winding resistance values less than 1 ohm is measured using micro-ohm meter or Kelvin Bridge. 3. To reduce the high inductive effect it is advisable to use a sufficiently high current to saturate the core. This will reduce the time required to get a stabilized reading. 4. It is essential that temperatures of the windings are accurately measured. 5. Care shall be taken that self inductive effects are minimized. Care also must be taken to ensure that direct current circulating in the windings has settled down before the measurement is done. In some cases this may take several minutes depending upon the winding inductance. 6. The winding resistance shall be preferably done when the difference in the top and bottom temperature of the winding (temperature of oil in steady-state condition) is equal to or less than 5˚C. 7. The winding resistance should preferable be carried out last after completion of all other LV tests, as after this test core gets saturated and tests like magnetizing current, magnetic balance etc. carried out after winding test may be affected and indicate a misleading results, if the core is not de-magnetized before carrying out these tests.
CAPACITANCE AND TAN DELTA MEASUREMENT OF BUSHINGS PURPOSE Insulation power factor or dissipation factor (Tan d) and Capacitance measurement of bushing provide an indication of the quality and soundness of the insulation in the bushing. DEFINITIONS / ABBREVIATIONS 1 Bushing An insulating structure, including a through conductor or providing a central passage for such a conductor, with provision for mounting on a barrier, conducting or otherwise , for the purpose of insulating the conductor from the barrier and conducting current from one side of the barrier to the other. 2 Bushing test tap A connection to one of the conducting layers of a capacitance-graded bushing for measurement of partial discharge, power factor, and capacitance values. 3 Capacitance (of bushing) (i) The main capacitance, C1, of a bushing is the capacitance between the high-voltage conductor and the voltage tap or test tap. (ii) The tap capacitance, C2, of a capacitance graded bushing is the capacitance between the voltage tap and mounting flange (gnd.) (iii) The capacitance C, of a bushing without a voltage or test tap is the capacitance between the high- voltage conductor and mounting flange (gnd.) TEST EQUIPMENT For getting accurate results of Tan delta and Capacitance without removing the bushing from the transformer, a suitable test set capable of taking measurement by ungrounded specimen test (UST) method shall be used. It utilises the test tap of the bushing and a Tan delta/Capacitance test set. Both Tan delta and Capacitance can be measured using the same set up.Portable C and Tand Bridge from any reputed manufacturer may be used for field testing.Portable test set include measuring bridge such as Schering bridge or transformer ratio arm bridge, power supply and standard capacitor in one enclosure.
TESTING PROCEDURE · Ensure that test specimen is isolated from other equipments. · Position the test set at least 6 feet (180 cm) away from the test specimen to be tested. · To prevent damage to the test set always set the capacitance multiplier dial to the SHORT position, the capacitance measuring dials to their respective ‘O’ position. · Keep UST-GST switch to UST position. · Keep interference suppressor switches in OFF position. · Connect the ground terminal of the test set to a low impedance earth ground (to Earth mat of the substation). · Connect control unit to the high voltage unit using two 5 feet long shielded cables. Screw the plugs down fully on the receptacles. Connect the low voltage cable with red boot/sheath to the ‘CxL red terminal’ of the test set. Make sure the connector locks to the receptacle. · Connect the external interlock cable to the ‘Interlock terminal of the test set. · Connect the high voltage cable with Black boot/sheath to the high voltage terminal of the high voltage unit. Connect the pig-tail for the outer shield to the black binding post (ground) on the high voltage unit. Screw down the plug shell fully on the receptacle. · With the main breaker switched OFF, plug the input power cord into the test set power receptacle and into a 3 wire grounded power receptacle having the appropriate voltage rating and current capacity. · For 3-Ph auto-transformer, short together all 400kV, 220kV and Neutral (isolated from earth) Bushings. Also short all 33kV Bushings and earth the same. · Measurement of C1 Capacitance and Tand: Connect the crocodile clip of the HV cable to the top terminal of the shorted HV/IV bushings. Unscrew the test tap cover, 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. Connect the LV cable to the test tap (strip/central stud) of the bushing under test to the C & TAN d KIT through a screened cable and earth the flange body. 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
· Measurement of C2 Capacitance and Tand : 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. HV of the bushing is to be connected to the Guard terminal of the test kit. Test to be carried out in GSTg mode at 1.0kV. · For measurement of 33kV Bushing Tan Delta, earth HV/IV Bushings (already shorted). 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. · ALL THE MEASUREMENTS TO BE MADE IN UST MODE ONLY. · 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.
PARAMETER
1U1
1V1
1W1
SL.NO:
SL.NO:
SL.NO:
2 KV
10 KV
2 KV
10 KV
2 KV
10 KV
VOLTAGE(V) CURRENT(mA) CAPACITANCE(pf) DISSIPATION FACTOR POWER (mW) FREQUENCY(HZ) INTERFERENCE (%)
PRECAUTIONS * Test voltage to be applied shall not exceed half of the power frequency test voltage or 10 kV, whichever is lower. It is desirable to have the test set or bridge frequency different but close to operating power frequency; so that stray power frequency currents do not interfere with the operation of the instrument. * Measurements shall be made at similar conditions as that of a previous measurement. The oil-paper insulation combination of bushings exhibit fairly constant tan delta over a
wide range of operating temperature. Hence, effort is to be made for testing at temperature near to previous test and Correction factor need not be applied. * Connection to over head bus at the bushing need be removed, only if the bus line affect the readings appreciably. * Porcelain of the bushings shall be clean and dry before test. Remove any dirt or oil with clean dry cloth. * Test shall not be carried out when there is condensation on the porcelain. Preferably, tests shall not be carried out when the relative humidity is in excess of 75%. * Terminals of the bushings of each winding shall be shorted together using bare braided copper jumper. These jumpers shall not be allowed to sag. Transformer windings not being tested shall be grounded. * Measure and record the ambient temperature and relative humidity for reference. Record OTI and WTI during the measurement. · Safety precautions as recommended by the instrument manufacturer may be followed. The test set is a source of high voltage electrical energy and operator must use all practical safety precautions to prevent contact with energised parts of the test equipment and related circuits. · Do not test a bushing (new or spare) while it is in its wood shipping crate, or while it is lying on wood. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate. Keep the test results as a baseline record to compare with future tests. CAPACITANCE AND TAN d MEASUREMENT OF WINDINGS PURPOSE Dissipation factor/Loss factor (Tan d ) and capacitance measurement of winding is carried out to ascertain the general condition of the ground and inter-winding insulation of transformer and reactors. TEST EQUIPMENT Portable C and Tand Bridge from any reputed manufacturer may be used for field testing.Portable test set includes measuring bridge such as Schering Bridge or transformer ratio Arm Bridge, power supply and standard capacitor in one enclosure.
TESTING PROCEDURE For tan delta capacitance measurement of transformer/reactor winding, the voltage rating of each winding under test must be considered and the test voltage selected accordingly. If neutral bushings are involved, their voltage ratings must also be considered in selecting the test voltage. i) Measurement should be made between each inter winding combination (or set of three phase winding in a 3 phased transformers) with all other windings grounded to tank (UST test mode) or ground the other entire windings guarded (GST L GUARD test mode). ii) For a two winding transformer, measurement should be made between each winding and ground with the remaining windings grounded (GST L GND test mode) iii) For a three winding transformer, measurement should be made between each winding and ground with one remaining winding guarded and the second remaining winding grounded (GST L GUARD test mode) iv) Finally measurement should be made between all windings connected together and the grounded tank. The combination for C & tand measurement of winding is same as that of measurement of IR value. The summery of probable combination is given below:
PARAMETER
HV+IV UST 2 KV
VOLTAGE(V) CURRENT(mA) CAPACITANCE(nf) DISSIPATION FACTOR POWER (mW) FREQUENCY(HZ) INTERFERENCE (%)
/LV HV-IV/LV+G GST 10 KV
2 KV
10 KV
HV-IV/LV GUARD GSTg 2 KV
10 KV
WITH
PARAMETER
LV/HV+IV UST
LV/HV-IV+G GST
LV/HV-IV GUARD GSTg
2 KV
2 KV
2 KV
10 KV
10 KV
WITH
10 KV
VOLTAGE(V) CURRENT(mA) CAPACITANCE(nf) DISSIPATION FACTOR POWER (mW) FREQUENCY(HZ) INTERFERENCE (%)
· Ensure that test specimen is isolated from other equipments. Removal of Jumpers from Bushings is Pre-Requisite for C & Tan _ Measurement of Windings. · Position the test set at least 6 feet (180 cm) away from the test specimen to be tested. · To prevent damage to the test set always set the capacitance multiplier dial to the SHORT position, the capacitance measuring dials to their respective ‘O’ position. · Keep UST-GST switch to UST position. · Keep interference suppressor switches in OFF position. · Connect the ground terminal of the test set to a low impedance earth ground (to Earth mat of the substation). PRECAUTIONS The test set is a source of high voltage electrical energy and operator must use all practical safety precautions to prevent contact with energised parts of the test equipment and related circuits. Following precautions need to be taken: · Never connect the test set to energised equipment. · The ground cable must be connected first and removed last.
· Heart patients should avoid using this H.V. equipment. · The ground terminal of the input supply card (green lead) must be connected to the protective ground (earth) terminal of the line power source. · Keep the high voltage plugs free from moisture, dust during installation and operation. · Adequate clearances (Min 1 foot i.e. 30 cm) are maintained between energised conductor and ground to prevent any arc over. · It should be ensured that test specimen is de-energised and grounded before making any further connection and no person may come in contact with HV output terminal or any materiel energised by the output.
FLOATING NEUTRAL POINT MEASUREMENT 2.0 Purpose This test is conducted to ascertain possibility of short circuit in a winding. The test can also be done during service to ascertain the gradual deterioration or development of fault in winding. Test Equipment Voltmeter,clamp meter Testing Procedures 8.1 Keep the tap in nominal tap position 8.2 Disconnect transformer neutral from ground 8.3 Apply 3 phase 415 Volts in High voltage (HV) winding and measure voltage across each of the IV winding and neutral, then Neutral and ground. 8.4 Apply 3 phase 415 V to the IV winding and measure voltage across each of the tertiary winding terminal and neutral, then neutral and earth. Precautions No winding terminal should be grounded, otherwise results would be erratic and confusing.
TESTS ON BUSHING CURRENT TRANSFORMER INSTALLED IN TURRET OF TRANSFORMER/ REACTOR PURPOSE These tests are carried out to ascertain the healthiness of bushing current transformer at the time of erection and in service. Test Schedule and Frequency: Pre-commissioning tests during initial erection and IR measurements to be done on SOS basis. Isolation required: Ensure the isolation of Transformer from High Voltage & Low voltage side with physical inspection of open condition of the concerned isolators/disconnectors. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing. The secondary circuits of the CTs may be disconnected in the Relay panels in control room. Testing Instruments: Transformer turns ratio meter High resistance measuring Ohm meter with required accessories & Cables 5 kV motorised Meg ohm meter (megger) with associated accessories & cables TESTING PROCEDURE: 1. POLARITY CHECK The lead polarity of any transformer CT/PT) is a designation of the relative instantaneous direction of currents in its leads. Primary and secondary leads are said to have the same polarity when at a given instant the current enters the primary lead in question and leaves the secondary lead in question in the same direction as though two leads formed a continuous circuit. The polarity check can be done by inductive kick with direct current method. In this test, a 6-10 V battery pack is connected momentarily to the primary P1-P2 of bushing CT under test and the momentarily deflection of pointer of voltmeter (Zero centre) or analog multimeter in dc m.Amp selection is connected to secondary is noted. If the connections are made as shown in the diagram then the deflection will be upscale when the battery is connected and downscale when the battery is disconnected, if the polarity is in accordance with the terminal marking. This test is also valid with the battery applied to secondary and the meter connected to primary. Use of mA or mV meter may be made.
Precautions: i. If a bushing CT installed in a power transformer is being tested by connecting the battery to the power transformer terminals, the other windings on the same phase of the power transformer may have to sort0 circuited in order to obtain a stable reading. ii. It is advisable to demagnetise any CT that is tested by impressing DC voltage across a winding. iii. A dangerous voltage may be generated while disconnecting the battery from the transformer winding. Therefore, a knife switch is not used, a hot stick or rubber gloves must be used for connecting and disconnecting the battery. 2. RATIO TEST: A suitable voltage, below saturation, is applied to the secondary (full winding) and the primary voltage is read with a high impedance (20 000 R/V or greater) low range voltmeter as shown in the diagram below:
The turns ratio is approx. equal to the voltage ratio. Saturation level is usually about 1 V per turn in most low- and medium- ratio bushing CTs. At the same time, the overall ratio is being determined, the tap section ratio may be checked with a voltmeter by comparing tap section voltage with the impressed voltage across the full winding. An ammeter is included in the recommended test method as a means of detecting excessive excitation current.
3. EXCITATION TESTS: These tests are carried out during pre-commissioning stage to check if any turns of CT are short-circuited and to establish CT characteristics as well as capability of CT. Excitation tests is made on CTs for comparison with factory test results or previously measured data to determine if deviations are present. To perform the test, an AC test voltage is applied to the secondary winding with the primary open circuited as shown in the diagram:
The test voltage applied to the secondary of the current transformer is varied, and the current drawn by the winding at each selected value of voltage is recorded. Readings near the knee of the excitation curve are especially important in plotting a comparison curve. For current transformers with taps, the secondary tap should be selected to assure that the current transformer can be saturated with the test equipment available. The highest tap which can accommodate that requirement should be used. The selection of instruments is especially important for this test. The ammeter should be an rms instrument. The voltmeter should be an average reading voltmeter. It should be calibrated to give the same numerical indication as an rms voltmeter on sine wave voltage. Any substantial deviation of the excitation curve for the current transformer under test from curves of similar transformers or manufacturer’s data should be investigated. Deviation from expected results may indicate a turn-to-turn short circuit, distortion of test supply voltage waveform, or the presence of a completed conducting path around the current transformer core. INSULATION RESISTANCE TESTS: Insulation resistance between CT secondary and ground is usually checked by the use of conventional test instruments. The neutral ground must be removed and the CT preferably isolated from its burden for this test. Actually, the neutral can be used to test all three phases simultaneously. Meggering core by core (to ground) keeping other cores temporarily grounded. It should be done by 500 V megger for 1 min. The IR value should be compared with those of similar devices or circuits. Readings lower than those known to be good should be carefully investigated. The generally accepted min. IR value is 1 M_. One of the most common reasons for low readings is the presence of moisture. Drying out the equipment and retesting should be considered before it is dismantled.
Precaution: If relays are left connected to the CT during test, the relay manufacturer should be consulted before test values above 500 V are used. Many solid-state relay designs have surge-suppression capacitors connected from input terminals to ground which may be damaged by use of a higher voltage.
