01. TRANSFORMER GENARAL DATA
This format contains the general technical data of the transformer.Data shall be filled in the format shall be filled as per the details from the nameplate, valve schedule plate, factory acceptance test report, approved drawings etc.
02. FINAL CHECKS
This document contains the items to be checked and reconfirmed prior to energisation of the transformer. These checks are conducted after completion of all tests as a reconfirmation. All items specified in this format are verified and comments if any are reported.
03. INTIAL CHECKS
These are the checks that to be verified prior to start of any site acceptance tests. These are for ensuring the fitness of the transformer to be tested at site. The checks include ensuring that the transformer was received safely at site on the basis of the safe receipt report and shock recorder data. With the available details, verification is to be done on perfect completion of site processing (errection,asembling,oil processing and etc).Also the availability of records including specifications, approved drawings their catalogues, site note file etc.at site office to be checked.
04. SITE PROCESING DETAILS
These are checks to ensure whether the oil processing at site was completed either as per the procedure furnished by the manufacturer or based on any relevant documents/standards. The safe and healthy condition of oil purifier, vacuum pump, pressure gauges etc. used during oil processing are to be ensured. The record of the insulation resistance values taken at regular intervals during oil filtration site has to be verified. Also the degree of vacuum maintained during processing and the condition of the hot oil circulation are verified. The ppm, BDV and tan delta capacitance values of the oil as per test report are verified.
05. VISUAL INSPECTION
These checks include physical verification of the components and accessories fixed on the
transformer, marshalling kiosk etc.This is general check for compliance of the different parts of the transformer with approval list and specifications. All the times listed in the parts list and given in general outline arrangement drawings are verified.
06. INSULATION RESISTANCE TEST OF WINDING, CORE&CLAMP
Standards: IEC60076-2000(Part1), IEEE62
General Insulation resistance measurements are performed to verify that the state of dryness of the insulation of the various winding and the core are of acceptable values. Insulation resistance testing may also reveal important information about concealed damage of bushings.
Insulation resistance is the volume and surface resistivity of the insulation involved. Insulation resistance of transformers and reactors are measured using insulation tester and the value expressed in mega ohms. The output voltage from the insulation tester is DC.The value of insulation resistance (IR value) depends on the design (Voltage class, type etc.).Temperature of oil, dryness of internal parts, cleanness of parts (especially bushing porcelains, terminals), and condition of oil atmosphere conditions (humidity) etc.IR value varies with voltage applied for measurement and hence compression may always be done with measurement carried out at same voltages.IR value is inversely proportional to the temperature.
Before starting of the measurement, the temperature of the oil must be noted; which shall be almost the same as the reference ambient temperature. Measurements are made at 15th, 60th and 600th seconds. Ensure that the measured value at 60th seconds is not less than 2000meg.ohm.Polarization index (PI) value, which is the ratio of 600th to 60th seconds IR value is determined. PI value gives a rough guide to the condition of the insulation properties; especially the dryness of the windings and internal parts of the transformer. PI value ≤1.0 indicates bad/unsatisfactory, ≥1.3 satisfactory, ≥1.5 good and, ≥2.0 very good condition of the dryness/insulation properties inside the transformer.
The new generation insulation testers have inbuilt provisions for discharging the voltages through the testing equipment itself, which takes place within a few seconds after switching off. However, as the voltage generated for measurement is off the tune of 500V-5000V, earthing may be done
externally before removing the connection as a satisfactory precaution.
7.1 Insulation resistance of winding
Aim
a) To determine the insulation resistance of winding to earth and between windings.
b) To evaluate the condition of the transformer insulation.
Test preparation
Insulation tester (Megger) must be off 5KV range having valid calibration. The earthing of the main body/tank, core/clamp etc. cleanness and dryness of the bushing porcelain/terminals and tightness of the connections are checked. The temperature of the oil inside the transformer (OTI reading) is noted. Check the leveling of the megger; if required and access of the supply unit.
Care may be taken to see that test leads do not touch among themselves and with the
transformer body.
Test procedure
Terminals of all HV bushings and HVN bushings are shorted together as shown in the figure.
Similarly all terminals of LV bushings are shorted together. The test voltages is selected as 5000V in the megger.IR values of individual winding to earth and between windings are measured at 15th, 60th, and 600th seconds. Compare the measured IR values with the factory results. Also evaluate PI values.
Connection for two winding transformer is as shown in the figure.
1. HV-LV+EARTH
1. LV-HV
Precautions/Safety
a) Transformer and bushings under test shall be thoroughly cleaned with dry cloth.
b) During the measurement the test area shall be demarked with warning tape.
c) Megger shall be placed at leveled platform.
d) Sagging of the connections and leads shall be avoided.
e) Infinity and zero of the megger shall be checked prior to test.
f) Sufficient time shall be given for discharge of megger before taking the next measurement.
The test circuit must be discharged by short circuiting for a period which shall be at least
four times the voltage application time; if the connection is made by bare hands. However
in practice at least a discharge time equal to the voltage application time shall be given.
g) Measurement at higher voltage shall not be done while the transformer is under vacuum.
Interpretation
a) A high value of polarization index indicates the insulation is very good. PI value <1
indicates immediate corrective action required.
b) There are no specific absolute values of acceptable insulation resistance; however
reference should be made to previous test history to establish a trend.
7.2 Insulation Resistance of Core and Clamp
Aim
To determined the insulation resistance of core, core clamp and between core& clamp.
Test preparation
Identify the core and core clamp bushings, insulation tester (Megger) shall be of 500V range having valid calibration. Ensure proper earthing of the transformer main body. Disconnect the earthing from core and core clamp bushings. Check the cleanness & dryness of the core and core clamp bushing porcelains/terminals and tightness of the connections. Note the temperature of the
oil inside the transformer (OTI reading).Check that the megger is placed on a leveled surface and access of the supply input. Care has to be taken to see that test leads do not touch among themselves and with the transformer body.
Test procedure
The test is conducted at 500V.IR values are measured at 60th second for core to earth, clamp to earth and core to clamp. The measurements are repeated after the application of 2KV.
Interpretation
a) Single point earthing of core is ensured by good IR value between core system and earth.
b) Typical IR value for a new transformer shall be >1000MΩ.
c) For a transformer in service.IR value core to earth >100MΩ indicates normal conditions of insulation.IR value is between 10&100MΩ indicates core deterioration.IR value <10MΩ
indicates generation of inadvertent destructive circulating currents which shall be investigated.
08.2KV TEST ON CORE AND CLAMPING STRUCTURE
Standards: IEC60076-2000, Clause-II
Aim
To ensure that the insulation is capable to withstand the specified test voltage.
Test preparation
The test setup consist of a source transformer of sufficient capacity in order to test voltage of 2KV AC, single phase slide arc, voltage transformer,multimeters etc.Ensure the input supply,connecting leads, tightness of the connection etc before application of test voltage. Test durationis 60 seconds.
Test procedure
The voltage is varied from minimum to the specified test voltage of 2KV rms and maintained for 60 seconds. During this period, there should not be any collapse in voltage or sudden increase in leakage current. The leakage current is also noted with the clamp meter.
2KV test is conducted between core & core clamp together to tank (earth) and between core and core clamp for one minute each.
Test condition
Core to tank (Clamp connected to Tank earth)
Clamp to tank (Core connected to Tank earth)
09. RATIO AND VECTOR GROUP CHECK
Standards: IEC60076-2000, Part1
9.1 Voltage Ratio Measurement
General
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. The voltage ratio of a transformer is the ratio of the rms terminal voltage of a higher voltage winding to the rms voltage of a lower voltage winding under specified conditions of load. When the transformer is open circuit, for all practical purpose its voltage and turns ratio may be considered equal.
Ratio error is the ratio of the difference between ratios of the measured value and the actual (design/declared) value to the actual ratio. The maximum allowable ratio error for principle tap is ±0.5% or ± 1/10 of the actual percentage impedance; whichever is less. For the other taps the desired maximum ratio error is ±0.5%; even though it is not purely binding.
Aim
a) To ensure the voltage ratio of the transformer are designed/manufactured as per the specifications/requirements.
b) To ensure the voltage ratio errors within acceptable limit.
Test preparation.
Ensure proper earthing of main tank, core and clamp, availability of stable variable 3 Phase or single phase input supply, voltmeters and/or Ratio Bridge meter etc. Also ensure that the neutral is not connected to earth.
Test Procedure for voltage ratio measurement.The voltage ratio shall be determined by two methods.
a) Direct measurement of the voltages using voltmeter
b) Measurement using Ratio Bridge
a) Direct measurement of the voltages using voltmeter
The 3 phases or single phase voltage is applied to the high voltage side and induced voltage across the low voltage side is measured. The result is determined from the two measured voltmeter readings. In a star windings transformer the applied and measured voltage shall be checked between line and neutral. Measurements are made for all phases at all taps positions.
Percentage ratio error is determine as [(measured ratio-declared ratio)/Declared ratio] x100 Arrangement for the measurement of ratio error of a two winding Transformer with HV star and LV delta as shown.
b) Ratio measurement using Ratio Bridge.
The applied and measured voltages are compared in a bridge by the null indication method. This is a direct measurement of the ratio error and hence do not require any calculation. This can be done either with 3phase or single phase bridge. Measurements are made for all phase at all taps positions.
9.2 Polarity /Vector group check
General
The individual winding of poly phase transformer can be connected in star, delta or Zig Zag as per design requirements. The phase displacement between windings may be 0°-360° according to the connection methods.
The vector diagram of the high voltage winding is placed on a clock face so that the tip of the vector 1U is at 12 O’ clock and the vector diagram of the low voltage winding is placed on top with the same phase orientation; then the direction of vector 2U identifies the clock number of the vector group.
For a three phase transformer, the phase angle of the intermediate and low voltage winding is referred to the high voltage winding for the vector group. For zig zag connections the winding half closet to the terminals determines the terminal markings.
Aim
To ensure the phase displacement or vector group of the transformer is
designed/manufactured as per the specification.
Test Procedure for polarity/Vector group
1) Single phase Transformer
For a single phase transformer, the polarity shall be either additive or subtractive. Polarity shall be checked either by
a) Voltmeter method or
b) DC kick method
(a) Volt meter method
I. Subtractive polarity
Terminals 1.1(HV) and 2.1(LV) are shorted. The applied voltage across HV (V1), the induced voltage on LV in the opposite direction (V3) and the voltage between the terminals 1.2(HV) and 2.2(LV) i.e., V3 are measured. If V2 is less than V1, the polarity is subtractive.
II. Additive polarity
The low voltage winding is connected in series with the high voltage winding in series. Terminals 1.1(HV) and 2.2(LV) are shorted. Also terminals 1.2(HV) and 2.1(LV) are shorted.
The applied voltage across HV (V1), the induced voltage on LV in the opposite direction (V3) and the voltage between terminals 1.2(HV) and 2.1(LV) ie, V3 are measured. If V2 is greater than V1 the polarity is additive.
(b) DC Kick method The positive of the DC source is connected the HV terminal 1.1 through a switch K and negative is connected to HV terminal 1.2.The LV terminals 2.1 AND 2.2 are connected to positive and negative of a center of zero galvanometer. While closing the switch K if the galvanometer gives a clock wise (positive) kick and a clock wise (negative) kick while opening the switch, the polarity is subtractive. For additive polarity, the direction of kick will be reverse to that of subtractive under the same conditions.
(2) Three Phase Transformer
For the phase three transformer, the polarity/phase displacement shall be verified by checking vector group. The vector diagram for a three phase, two winding transformer with HV star connection (YN) and LV delta 1 O” clock direction (dl) is shown below.
Terminals 1U (HV) and 2U (LV) are shorted. Thus HV U phase and LV U phase are brought
together in phase. A balanced three phase 400V shall be applied across the HV winding and induced voltages across the terminals for the following combinations are measured.
a) 1U –1V
b) 1V-1W
c) 1U-1W
d) 1W-2u
e) 1W-2v
f) 1U-N
g) 2u-2v
h) 2v-N
The vector is confirmed as Y nd1, if the measured voltages as per the above combinations satisfy the following.
i. a=b=c
ii. dNe
iii. f=g+h
OLTC Continuity Check
General
On load tap changer is a device for changing the tapping connections of a winding, suitable for operation while the transformer is energized or on load condition. OLTC is normally connected at the neutral end of the winding. For large capacity Power Transformers three separate OLTCs are provided one for each phase: according to the design.
Aim
To ensure that the Transformer windings are not open circuited during OLTC operation.
Test preparation Ensure availability of 3 phase or single phase variable supply, analog type voltmeter etc. connect the voltmeters tightly across the line terminals of HV.
Test procedure
The OLTC continuity check shall be done using either 3 phase of single phase voltage supply.
However the 3 phase supply is preferred as the test can be done simultaneously on all the three phases. Voltage is slowly raised across the low voltage winding of the transformer in order to get a readable value in the voltmeters connected across the line and neutral of the HV winding. The OLTC is operated for one complete cycle (minimum tap-maximum-minimum tap). If there is any winding break/discontinuity through the OLTC, the respective voltmeter will indicate zero reading.
during the OLTC operation at that particular tap. For ensuring OLTC continuity, there should not be any zero indication in the voltmeters during the OLTC operation.
10. MEASUREMENT OF MAGNETISING CURRENT
Standards: IEC60076-2000
General
The exciting current test is very useful in locating problems like the magnetic core structure,shifting of windings, failure in the turn-to-turn insulation or problems in the tap changer. These conditions result in a change in the effective reluctance of the magnetic circuit, which affects the current required to force a flux through the core.
Low voltage exciting current is measured by this test. The pattern and value of the magnetizing current gives an idea of the magnetic circuit of the built-up core and winding interconnection.
These values are considered as reference for all future measurements and Said to be the base of transformer diagnostic tool. The measured value at site shall be compared
with the previous test results at factory. As the value varies accordingly to the applied voltage,
comparisons may be done with the same applied voltage. Usually 400V, 3phase balanced supply is applied on the HV terminals and the corresponding magnetizing currents are noted. For LV, the voltage is suitably reduced in order to limit the induced voltage on the HV terminals. The repeatability of magnetizing current readings is an indication of the healthy magnetic circuit in the transformer.
The magnetic currents will be in the tune of milli ampears.For a star connected winding the
magnetizing currents in the middle phase (V Phase) will be less than the other two phases and the magnetizing currents on U&W phase will be almost equal and >V phase. For delta connected transformers the magnetizing current in W phase will be < U&V phases and magnetizing currents of V phase will be <U phase, but slightly >W phase. For D11 delta connected transformers the magnetizing currents in the U phase will be <V&W phases and magnetizing currents of V phase
<W phase, but slightly >U phase.
The test results may be none confirming as a result of residual magnetisms in the core due to DC applied. Hence this test has to be carried out before applying any DC on the transformer.ie, before the winding resistance measurement. The core will be demagnetized once the high voltage AC is fed to the transformer.
Aim
a) To ensure that there is no shorting in the core material.
b) To ensure no inter turn shorting of winding.
c) To ensure no loose contact or loose connection in the winding or core.
Test preparation
Stabilized 3 phase variable input supply, voltmeters and ammeters with valid calibration must be available. The neutral of the transformer must be open. For comparison of test results, ensure that the test voltage at factory is same as that the site. Confirm the tightness of all connections made.
Test procedure
The 3 phase supply is fed to the transformer with milli ammeters in series to each pahse.With LV winding open circuited. Voltage on HV is varied from minimum to the applied voltage of 400V and magnetizing currents are measured at all taps. While Applying on LV, care has to be taken to restrict the voltage in safe limits; if required as the HV/LV ratio is high and induced voltage on HV is more. Also the phase sequence of the applied voltage shall be maintained.
A test circuit of two winding transformer (Star/Delta) for measurement of magnetizing current is shown in below.
Precaution/Safety
a) The test shall be performed at highest possible test voltage without exceeding the voltage rating of excited winding.
b) The instrumentation shall exclude to the possible extent exclude from the measurement of the capacitive currents between the excited winding and other windings, the core or the
tank.
c) For the purpose of comparison, the subsequent test shall be performed at the same test voltage and similar test connections.
Interpretation
a) The residual magnetism results in the measurement of higher than the normal exciting current.
b) There is no widely accepted field method for distinguishing between the effect of residual
magnetism and effect of a problem present in the transformer.
c) The measured values are compared with previous test results and if the values fall within
+150%, it is usually considered as normal.
11. WINDING RESISTANCE MEASUREMENT
Standards: IEC60076-2000, Part1, Clause-10.2
General
Winding resistance is defined as the DC resistance of the winding and its value is applied voltage divided by delivered current expressed in ohms. Measurement is made by applying a DC current to the specified winding. In order to compensate the resistance of the lead wires a four wire system is established as two wires each for potential and current connections.
Winding resistance provides the base value to establish the load loss. It forms an indirect base to establish the winding temperature and winding temperature rise from the resistance
measurement made during heat run test on transformer.
For preventing core saturation and raise of temperature in the winding during the measurement, the test current should be limited to a maximum of 10% of the rated current of the specified winding. Also this current should be at least 1.2 times of the crest value of the magnetization current.
Winding resistance is proportional to the temperature and reference is75ºC.Resistance measured at a temperature t1ºC (R1) may be converted to R@75ºC.Resistance at 75ºC as
R@75º= [(K+75 ºC)/ (K + t1ºC)] X R1
Where K = Temperature co efficient of winding material
(Copper = 235 & Aluminum = 225)
R1 = Measured Resistance & t1 Ambient temperature
Aim
a) To determine the winding resistance of HV winding for all phases at all taps and that of LV winding for all phases.
b) To confirm the FAT report values are matching with the site tested values within the tolerance limits.
c) For verifying the electrical continuity in a winding and to ensure absence of loose contact of leads connection at OLTC, bushing terminals.etc.
Test Preparation
A Constant current DC source, switch and leads with sufficient cross sectional area.DC ammeter, voltmeter or measuring bridge with suitable current rating and leads with sufficient cross sectional area.
Test procedure
Winding resistance can be measured either as phase to phase or phase to neutral. The winding for which the resistance is to be measured must be connected in the circuit and other phases and windings shall be
kept open circuited. After switching on the DC voltage source, reading shall be taken only after the current reaches a steady state condition as otherwise the switching may cause induced voltage which in turn
influences resistance vaue.Measurement of winding resistance for HV winding at all phases and all tap positions and that of LV windings for all phases are to be carried out. Also the oil temperature shall be noted.
Two methods basically used to measure the winding resistances.
a) Voltmeter –Ammeter method
This method is also known as VI method and the measured resistance is determined by ohm’s law as the voltage drop in the winding (Voltmeter reading) divided by the DC current flowing through the winding (Ammeter reading). Connections shall be given as shown in the figure.
The voltmeter leads must be independent from the current leads and shall be connected as close as possible to the winding terminal in order to prevent possible voltage drop. After the applied DC current reached its steady state, the voltmeter and ammeter readings are simultaneously noted.
The voltmeter has to be disconnected from the circuit before switching on and switching off the DC supply has the sudden off scale damping may damage it.
b) Bridge or micro-ohm meter.
2. LV WINDING RESISTANCE
Voltage drop across the unknown resistance Rx compared to the voltage drop across a known resistance RN.
DC current is injected through the Rx and RN.Corresponding voltage drops are measured
and compared.Varieing the value of resistances Rdec and Rv the bridge shall be balanced showing null deflection in the galvanometer will be RN X Rdec.
Rv
Where
Rx = Winding Under test.
RN = Standard resistor
Rdec = Decade resistor
Rv = Variable Resistor
G = DC source
A Wheatstone bridge is preferred for resistance values ≥1Ω and a Kelvin bridge or a micro
ohmmeter for resistance values ≤1Ω
While using a micro ohmmeter only a range selection is to be made and the measured
reading is displayed directly on the measurement.
The average value of the winding resistance of three phases at each tap is corrected to
75ºC
Precaution/Safety
The voltage and current leads shall be independent and shall be connected as closely as possible to the winding terminals in order to avoid lead resistance.
Readings shall be taken only after the current and voltage values become stable.
If the current is suddenly switched off, a high voltage is generated in the winding. The
current should be switched off by a suitable isolated switch before any personnel
contact the circuit.
Interpretations
i. A tolerance of ±3% is generally acceptable for the measured values at site over the factory
test report values.
ii. Similarly the measured values between phases of transformers of identical design also may vary with in ±3%..
1. Since the overall measurement accuracy is around 1%, changes of ±2% of the short circuit
impedance with the factory measured value is usually considered significant.
2. Changes of more than ±3% of the short circuit should be considered as a significant and hence not acceptable.
13. MEASUREMENT OF TAN DELTA AND CAPACITANCE OF WINDING.
Standards: IEC60076-2000, Part1, Clause-10.1.3, IEEE62
General
Tan delta
Dielectric loss is the power dissipated by the insulation when subjected to an alternating voltage,Tan delta or dissipation factor is the ratio of absorbed active power to the absolute value of the of the reactive power expressed in percentage. It is ratio of resistive current to capacitive current flowing through the insulation.
Low value of dielectric loss indicates good condition of insulation, aging of dielectric media,Deteriorations/contaminations of insulation, physical damage due to electrical stress/out side forces etc, results in the increase of dielectric loss and thereby increase dissipation factor.
Dissipation factor varies with environmental conditions like temperature, relative humidity and precipitation etc at the time of testing. It is also dependent on dryness of the transformer
insulation (Oil and insulation materials), external, and cleanness of the porcelain, applied voltage and influence of stray parameters during measurement. Tan Delta is reported at a reference temperature of 20ºC.
Capacitance Capacitance is defined as charge per unit current, i.e. C=Q/V.It is usually expressed in Pico farad.
It is dependent on the characteristics of dielectric material; conductor configuration etc.It does not vary considerably with the voltage and temperature.
Tan delta and Capacitance shall be measured simultaneously using tan delta and capacitance measuring bridge or Volt-ampere watt method bride.
Aim
1. To ensure the condition of transformer insulation.
2. To check any physical damage /shifting of
the transformer windings/internal parts during
transportation.
Test preparation
All windings shall be shorted separately with proper shorting leads and tightness checked. Proper earthing of core/clamp and main tank are checked. Common earthing for the testing equipment and transformer is ensured. Arrangements for the oil and ambient temperatures and humidity are
made.
Test Procedure
Measurement may be done in the following modes in Megger DELTA, Dobble equipments.
Ground mode (GND/GST)
Measures the insulation between the HV probe and Ground (earth) as well as between
HV probe and LV leads.
Guard mode (GRD)
Measure the insulation between the HV and ground (earth); leakage through the
insulation between HV probe and LV leads by passes the meter.
Un Grounded specimen(UST)
Measure the insulation between the HV probe and LV leads; leakage through the
insulation between HV probe and ground (earth) lead by pass the meter.
If n is the number of independent windings the number of possible measurement in GST
(Grounded specimen) mode is given by
(n+1)! / 2! *(n-1)!
For n>1, Hence for a two winding transformer, there shall be 3 measurements possible.
Before the test, bushing terminals of the same winding shall be shorted together and connection given accordingly as per the circuit shown in below. Tan delta and capacitance between the following combinations shall be measured. Test voltage at site is limited to 10KV for HV and 5KV for LV windings.
HV- EARTH
LV-EARTH
HV+LV-EARTH
voltage is applied and tan delta and capacitance are measured. This is to reduce the time delay for stabilizing the final test results and to reduce the extent of damage in case of a fault. Then the actual test voltage is applied and measured repeated for the different combinations. Ambient temperature, oil temperature and humidity are noted. The conversion factor 20ºC is applied for
tan delta value and compared with factory test results.
Between each pair of winding and each winding to earth constitute discrete capacitances. Atypical transformer having two independent winding has capacitances. Atypical transformer having two independent winding has capacitances as shown below
Measurement by shunting CL (Shorting LV and E) gives (CH+ CL).Shorting of HV to E gives (CHL+CL).By shorting HV to LV gives (CH+ CL).Solving three simultaneous equation CH, CL,CHL.Corresponding tan delta in combination is also obtained.
Precautions/Safety
1. All bushings terminals and the bushings shall be thoroughly cleaned with dry cloth to avoid surface leakage current.
2. During measurement the test area shall be demarked with warning tape.
3. Sagging of the connections shall be avoided.
4. Measurement at higher voltage shall not be done while the transformer is under vacuum.
5. Electrostatic interference; especially due to nearby EHV lines may lead to unreliable results.
Interference suppressor circuits along with shielded cable may be used to avoid this.
6. Ensure the tightness of the test tap in HV winding.
Interpretation
1. Dielectric factor for oil filled transformer in good condition should not exceed 0.5% at 20
ºC.
2. Periodic tests done in service will indicate whether the aging of insulation is normal or
rapid.
3. Diagnostics tests on suspect or failed equipment may be helpful in locating fault or reason for failure.
4. Test results on new equipment provide a benchmark for failure comparison.
14. MEASUREMENT OF TAN DELTA AND CAPACITANCE BUSHING.
Standards: IEC60076-2000, Part1, Clause-10.1.3, IEEE62
Aim
To ensure the condition of bushing
insulation.
General
Bushing test tap is used to for this
measurement and hence measurement
is possible only in condenser type
bushing having test tap provision.
Test preparation
Same setup for measurement of tan
delta and capacitance measurement of winding shall be used for this measurement with the measurement lead in test tap of the respective bushing. Test voltage is 10KV.
Test procedure
The voltage shall be given parallel to all bushing in HV and LV at a time or separately in HV and LV or one by none. Test voltage shall be applied at the bushing line terminal and measurement lead from test tap of bushing. Each of the condenser bushing tan delta and capacitance C can be measured in UST mode.
After verifying the connection and set up is switched on.Initally around 20% of the test voltage is applied and tan delta and capacitance are measured. This is to reduce the time delay for stabilizing the final test results and to reduce the extent of damage in case fault.
Then the actual test voltage is applied and measurement repeated for all condenser bushings.
Ambient temperature, oil temperature and humidity are noted. The conversion factor for 20°C is applied for tan delta value and compared with factory tested values. The site test results must be comparable with the factory test values.
Precautions.
The bushing terminals and test tap under test shall be thoroughly cleaned with dry cloth to
avoid surface leakage currents.
If the bushings are shorted together, the test tap of the bushings that are not under
measurement must be tightly closed.
During the measurement the test area shall bed demarked with the warning tap.
Measurement at higher voltage shall not be done while the transformer is under vacuum.
Electrostatic interference; especially due to near b y EHV lines may lead to unreliable
results. Interference suppressor circuits along with shielded cable may be used to avoid
this.
After testing/measurement ensure the test tap tightness.
Interpretation
a) Dielectric factor for condenser bushings in good condition should not exceed 0.7% at 20°C.
b) Test results on the new bushings provide a bench mark for future comparison.
15. CALIBRATION OF TEMPERATURE INDICATORS
Standards: IEC60076-1, 2000
Aim
To ensure that the proper working of temperature indicators according to the actual sensing of temperature of the transformer oil and winding.
General
The temperature indicators are used to measure the transformer oil and winding temperatures accurately. The indicator used for measuring oil is called oil temperature indicator (OTI) and for winding is called winding temperature indictor (WIT).Separate contacts are provided for alarm, trip and cooler start/off functions. These indicators contribute as protective devices for transformer giving timely warnings and action against possible faults arising due to rise of temperature of oil/winding.
Following formula are generally adopted for alarm and trip contact setting, but some
manufacturers are using thereon formulas
OTI
Alarm: Maximum top oil temperature rise during factory test +ambient average +5ºC.
TRIP: Alarm+10ºC.
WTI
Alarm: Maximum top oil temperature rise during factory test + (Winding gradient*1.3)
+ambient average +5ºC.
TRIP: Alarm+10ºC.
Test preparation
A standard oil bath with valid calibration status and sufficient dimensions for fully immersing the sensing bulb of OTI/WTI is selected for the test. Transformer oil at ambient temperature is filled in the bath up to the required level. This test is started with minimum temperature onwards and reading taken at multiples of 20ºC up to 120 for OTI and 120ºC for WTI.Calibration of indicators shall be done simultaneously. Do not disturb any factory settings already made in the meters by
the manufacture at factory.
Temperature readings between standard bath and indicators shall be ±2ºC at all calibration steps.
Temperature reading between indicators is given a maximum allowance of 2ºC.
Test procedure
The oil bath is set at the required temperature. Minimum 45minutes shall be given for stabilization of each set temperature and temperature reading of the indicators and standard bath are noted down. Close monitoring is required during the test to check whether any indicator needles goes
abnormal fast/slow or is getting struck. Compare the measured and standard temperature reading and verify that it is acceptable or not as per the tolerance limits. Factory set alarm and trip contacts shall be checked with the setup. The bath temperature is brought to the required level and contact operations are checked with the help of multi meter.
Precautions/Safety
1. Ensure the capillary tube of indicators is not damaged while making preparation for test.
2. Ensure the sensing bulb of indicators does not have any contact with the walls of the bath
and is free in oil.
3. Care shall be taken against possible fire while the bath temperature rises up to 120 ºC
4. The bath shall be kept at the same height at which the indicators are located to avoid
effects due to head variations.
Interpretation
1. If the difference in the temperature readings between standard bath and the indicator is
>2 ºC at any of calibration steps, that indicator is rejected.
2. If temperature reading between indicators is more than 2ºC, the indicators are rejected.
16. HOT SPOT GRADIENT CHECK OF WINDING TEMPERATURE INDICATORS
Aim
To ensure the set gradient in WTIs either as per the factory temperature rise test results or as per the values given in the factory acceptance test report.
General
Winding gradient is the difference between winding temperature and average oil temperature.
Hot spot winding gradient is the hot spot developed in the winding and is 1.3 times the winding.
Winding temperature is depending upon the load current. Hence that secondary current of WTI BCT is taken in to account for gradient checking. While passing the BCT secondary current through the heater coil of WTI, heat and gas will be produced in the capillary tube of the
indicators. This will expand further due to heat conveyed and push the needle to indicate winding temperature at the particular load current.
Test preparation
The same set up used for calibration of indicators shall be used for this test. Set the hot spot gradient shall be checked at 80ºC of oil temperature. Hot spot gradient shall be checked first at ONAF rating, since this is full load continuous rating and there upon extended for other rating like ONAN, and 1.2 times of ONAF (LTEC).
Calculate the BCT current required for each rating. Put the sensing bulb of local and remote WTI in the oil bath. Digital multi meter with valid calibration status is used to measure the milli ampere output of transducer for RWTI.
Test procedure
Switch on the bath and set the reading at 80ºC and is kept for 45minutes for stabilization of the temperature. Check and note down the readings of oil bath, indicators and milli ampere output of the remote WTI.The current corresponding to ONAF rating is injected to the heater coil of the indicator and kept for minimum 45 minutes. The difference of WTI reading after injecting BCT current and bath temperature gives the hot spot gradient in the local WTI.
Take the indicator reading and milli ampere output of the remote WTI.The difference of measured milli ampere output of the transducer before and after injecting BCT current and multiplied with ratio of full scale reading of WTI to maximum output of transducer gives the hot spot gradient.
Compare the measured hot spot gradient with factory test results and set values in the indicators.
The factory set values and measured values must be more or less the same difference should be less then 2ºC in any case for acceptance of test.
17. RATIO CHECK OF CURRENT TRANSFORMER FOR WTI
Aim
To ensure that the ratios of BCTs for WTI are correct and manufactured as per the specifications.
General
This test is done to verify the primary to secondary current ratio of WTI BCTs without connecting a burden and hence this is only a rough ratio check. The principle is that primary ampere turns and secondary ampere turns in the BCT are same. A separate winding called the test winding provided in addition to the secondary winding, is used for the measurement and easy checking of ratio. If test windings are not available, the test can be done during short circuit impedance
measurement.
Test preparation
The test can be done either during short circuit impedance measurement or with the test winding and the test setup can be decided accordingly. For test with test winding, single phase dimmer start and it accessories are required. When measured through test winding.HV and LV are kept open.
Test Procedure
BCT secondary is connected to ammeter. The BCTs secondary current and primary applied current shall be measured as shown below. Tap positions shall be noted during the measurement.
Data and calculation for checking the BCT ratio as an example is given below.
Design Ratio Voltage Vector Accuracy Class
HV---- 400/5 112.2Kv at tap 19 YN 3.0
LV-----3500/5 12Kv d1 3.0
From the above data, HV/LV ratio is = 9.35A
Applied HV current = 2.80A
Corresponding current in LV =2.8*HV/LV Ratio = 26.18A
HV BCT
If measured HV BCT secondary current when HV current is 2.8A is 35 milli amperes (0.035A),
then the BCT secondary current, when HV is 400A = (400/2.8)*0.035 = 4.99A
Hence the percentage error for HV BCT = (4.999-5.0)/5*100 = -0.02% ok.
Thus the measured ratio is 400/4.999 against design ratio of 400/5A
HV BCT
If measured LV BCT secondary current when LV current is at 26.18A is 37 milli amperes
(0.037A), then the BCT secondary current,
when LV is 3500A= (3500/26.18)*0.037 = 4.946A
Hence the percentage error for HV BCT = (4.946-5.0)/5*100 = 1.08% ok.
Thus the measured ratio is 3500/4.946 against design ratio of 3500/5A
Measurement with Test Winding
Since the transformer lead conductor serves as the primary of BCT, the primary turn of the BCT is normally designed with a single turn.
Data and calculations to check the BCT ratio as an example is given below.
Let HV BCT design ratio is 400/5 and test winding current is 10A.
Assume HV turns =1, BCT secondary turns =80 and test winding turns =40
Primary ampere turns = Secondary ampere turns
Primary ampere turns = 400 *1=400
Secondary ampere turns=5*80=400
Test winding ampere turns =10*40=400
This means Primary ampere turns=Secondary ampere turns=Test winding ampere turns
If 10A is applied to test winding, the secondary winding will read 5A.Keeping the HV and LV
winding open applied 10A to the test winding and measure the secondary current. If the
measured secondary current is 4.96A, then the BCT measured ratio can be taken as 400/4.96
19. OPERATION CHECK OF ON LOAD TAP CHANGER
Aim
To ensure the proper operation of /performance of On Load Tap Changer and it’s compliance with the requirements.
General
On Load Tap Changer is device, used for enabling the transformer to serve a constant voltage in the load dispatch side (Grid) as per the input supply or demand variation by changing the tappositions on the transformer on loaded condition. Performance of operation of OLTC can primarily confirmed by the operation from MDU.
Test Preparation
Earthing of transformer and MDU body is ensured. The supply input and phase sequence to the MDU is checked.Multimeter and 500V megger is also required.
Test procedure
Insulation resistance of motor and wiring to MDU is checked. Under voltage and over voltage setting are done the voltage monitoring relay and its performance checked. The handle operation of OLTC, electrical & hand operation limits at extreme tap, step by step operation, limits of over, travel, starting & running current of motors, tap position indications in MDU/AVR,rotations of handle per step of operation, time for tap changing at normal tap setting of MPR&Voltage monitoring relay etc has to be checked.
20. TAP POSITION INDICATION CHECK
Aim
To check the tap position indications on OLTC, MDU, AVR, DCS and SCDA are same at every tap.
Test preparation
Ensure the availability of MDU input supply and phase sequence.
Test procedure
The OLTC tap shall be operated in raise or lower mode. The tap position indications are
simultaneously checked at OLTC, MDU, AVR, DCS and SCADA to confirm that all tap position
indications are same either in raise or lower mode.
22. FAN MOTOR CHECKS
Aim
To ensure the performance of the fan motor.
Test preparation
Ensure all wiring is completed at marshalling kiosk. Check the supply input of fan motor and
phase sequence. Availability of digital multimeter, 500V megger, stop watch etc are to be required.
Test procedure
Switch on individual fans one by one and ensure correct direction of rotation, i.e. it should be in air intake directions to the radiators. Check the starting and running current of individual motor.
The time taken to trip the over load setting shall be noted after blocking the rotor of each fan.
The over load setting shall be done as per the manufacturer instructions.
23. OPERATION CHECK OF COOLING EQUIPEMENT
Aim
To ensure the performance of cooling equipments.
Test preparation
Ensure all the wiring is completed at marshalling and supply is put and phase sequence of fan motors. Also check the availability of digital millimeters.
Procedure
Energize the marshalling kiosk. Check the manual and auto control of fans from HV and LV WTI by selecting respective switches and selection modes. Check the tripping of MCB and fan control switchs.Check the cooling fan interlock with fire deluge operation. Terminal block numbers ferrules, grouping of fan etc.shall are checked with respective drawings.
23. b Settings
Aim
To ensure operation of cooling fans are according to the start /stop temperature setting at WTI.
General
Start /Stop set values of cooling fans are given by the manufacturer according the factory
temperature raise test results and these values are normally set in each of the indicators. Fans
may be group in to two, say group 1 and group 2.
Test preparation
Ensure availability of standard oil bath with accessories, single phase supply and digital oil bath.
Test procedure
HV and LV WTI sensing bulbs are inserted in the oil bath carefully. Transformer oil shall be filled in the oil bath kit up to the required level. Switch on the oil bath and set required temperature in the oil bath for start of group 1 fans. Check the start operation of the group 1 fans when the set value of the temperature is attained in the oil bath. Then set the stop value at oil bath and check the stopping of cooling fans when the set temperature is attained in the oil bath.
The procedure is repeated to check the operation of the cooling fans in group 2.
24. PROTECTIVE DEVICE WIRING AND OPERATION CHECKS
Aim
1. To ensure correct operation of protective devices at DCS/SCADA before energisation of the substation.
2. To check the wiring and terminals identification at MK/LCC/RCP/RP/etc are as per the approved drawings
Test preparation
Check the availability of approved wiring drawing with terminal details and digital multi
meter.
Test procedure
Alarm and trip contacts of each protective device shall be checked at
MK/LCC/RCP/RP/DCS/SCS/SCADA etc by operating the respective devices.Buchhloz relay
shall be checked by applying nitrogen (N2) with pressure.
26.0 CHECK ON AUTOMATIC VOLTAGE REGULATOR (AVR)
Aim
To ensure the operation and performance of AVR as per the specifications and
documents/requirements.
General
Automatic voltage control shall be initiated by a voltage-regulating relay. The relay shall operate
from the nominal reference voltage stated in requirements, derived from a circuit mounted LV voltage transformer. The relay shall be insensitive to frequency variation from 47 to 51 Hz. The relay shall also incorporate an under voltage blocking facility, if the reference voltage falls below 80% of the nominal value and with automatic restoration of control when the reference voltage raises to 85% of the nominal value. The relay band width shall preferably be adjustable to any value between1.5 and 2.5 times the transformer tap step%; the nominal setting being twice the transformer tap step%.
Test Preparation
Ensure the single-phase supply input and earthing for the AVR panel. Check the readiness of the MDU and DCS &SCADA connections. Availability of digital multi meter technical specifications,drawings, documents etc. are also ensured.
Operation
a) Put AVR in auto mode and MDU in remote and check the following
1. No operation possible from MDU, AVR DCS and SCADA.
2. Auto operation takes place with variation in VT secondary voltage. When tap raise
command is given, VT secondary voltage reduces and vice versa.
b) Put AVR in manual mode and MDU in remote and check the following.
1. Auto operation not possible
2. Manual operation possible from AVCR, DCS, SCADA.
c) Put MDU in local position and check the following
1. Operation possible for MDU
2. Operation not possible from AVR, DCS and SCADA.
d) Put MDU in remote position and check the following
1. Operation not possible from MDU
e) Put MDU and AVR in remote and check the following
1. Operation possible from AVR
2. Operation not possible from DCS and SCADA.
f) Put AVR in supervisory and MDU in remote and check the following.
1. Operation not possible from MDU and AVR
2. Operation possible from DCS and SCADA.
g) Under and over voltage blocking of MDU
1. No operation possible from any point.
AVR Display
Ensure that AVR display has parameters like Voltage, Current, Frequency, Tap no.,
Master/Follower/independent mode, paragramer functions (Breaker status).
Detailed test procedures of AVR test with following formats are given below
Formats no: CDT/PT/ST/21 : Check on AVR
CDT/PT/ST/22 : Tap changer and cooler alarm
CDT/PT/ST/23 : AVR parallel operation
CDT/PT/ST/24 : AVR parallel operation
TESTING PROCEDURES OF REMOTE TAP CHANGER CUBICLE (RTCC) CONSISTING OF
AUTOMATIC VOLTAGE REGULATOR RELAY (AVR)
Following are the components of RTCC site testing:-
Installation and Drawing checks.
Operation, Indication and Alarm Checks on each AVR relay.
Parallel Operation, Indication and Alarm checks
Communication with DCS/SCC
AVR-Modes of operation
R/L Control (Hand) & AVRs in Independent mode.
R/L Control (Auto) & AVRs in Independent mode.
R/L Control (Hand) & AVRs in Parallel Manual mode.
R/L Control (Auto) & AVRs in Parallel Manual mode.
R/L Control (Hand) & AVRs in Parallel Auto mode.
R/L Control (Auto) & AVRs in Parallel Auto mode.
1. Installation and Drawing checks.
1. a Drawings and document verification
The following are checked and verified.
Approved drawing and letter of approval
Engineering comments and their compliance
Statement/reply of the contractor against the comments.
Period of warranty.
O & M manual.
Inspection report at manufacturer’s site.
Marked up drawing.
Suggested modification on the drawing to rectify the problems, if any
1. b Installation inspection
The following checks are carried out on the AVR.
AVR identification including the serial no, model no. etc. corresponding to all the
transformers. i.e. IDT1, IDT2, IDT3.
Front Panel arrangement, module disposition of devices and MCBs used, terminal block
marking and arrangement, ferruling, neatness and proper dressing of wiring,
functioning of heater and its controllers, lamp testing, panel earthing etc. are checked
with approved drawing for conformity.
Also ensure that sources of AC & DC power supply are as per drawing.
2. Operations, Indication and Alarm Checks each AVR Relay.
Following procedures are to be carried out in each AVR. Also take Safety precautions before
starting the test.
2. a R/L Control (Hand) & AVRs in Independent mode.
Switch on AVR & MDU control supply, DC supply and MDU motor supply, Identify and
verify the Terminals where CB status input are connected.
All the CB status inputs are temporarily short circuited.
Select paragrammer screen of all AVRs and ensure that all CB status are closed in all
AVRs.
Switch off control supply of AVR1 and observe whether there is any status disturbance in the paragrammer of other AVRs.
Keep MDU in Local and ensure that the tap positions are same in MDU, OLTC, and AVR
Display and Bus Bar diagram.
Keep AVR in “Hand” (manual) and Independent mode and ensure the following
Indications/operations.
o Check that Tap Changer in Local indication lamp glows.
o REMOTE/SUPRY try color LED off and MDU local LED glows.
o LED in the Hand push button glows and Raise/Lower operation not possible. Also
Independent LED glows.
o AVR VT supply absent message in the status line.
o Keeping MDU in Remote ensure that tap
operation is not possible from MDU
o Keeping AVR in “REMOTE” ensure that “REMOTE / SUPRY” try color LED glows in
green and MDU local LED off. Also LED in the “HAND” push button glows.
o Raise/Lower operation possible from AVR and Independent LED glows.
o AVR VT supply absent message in the status line.
o Give Raiser/Lower commands from AVR and ensure simultaneous tap raising/lowering of the tap positions at MDU, OLTC, AVR display & Paragramer.
During Raise/Lower operation ensure following indications and status:
o Check that no display of alarm in the status line and alarm LED indication is received
during these operations.
o “Tap change in progress” message will be displayed in the status line as long as the
operation is in progress.
o Independent display in status line and corresponding green LED in the Raise/Lower
push button glows.
During tap operation ensure that “Tap 0” display is not coming at any point.
Bring the OLTC in extreme tap positions and observe the message Tap changer in extreme
position in the status line is displayed and tap operations are blocked at extreme positions.
Check “T.C. Incomplete” alarm in the status line when tap operation is stopped midway.
This can be simulated by switching off the MDU control supply during tap operation in
progress. Reset TC Incomplete alarm by doing further tap operations after switching on the
MDU control supply.
Ensure “TC motor supply faulty” alarm in the status line by switching off the MCB of TC
motor supply at LVAC panel. Ensure “TC control supply faulty” alarm in the status line by
switching off the MCB of TC control supply at AVR panel
Put the AVR in “SUPRY” and ensure the following:
o Put the AVR in “SUPRY” and ensure that Red color LED of REMOTE/SUPRY push
button glows.
o Raise/lower operation not possible from AVR panel.
o Tap Changer in “Local” indication lamp off.
o MDU local LED off
o LED in the “Hand” push button glows
o Independent LED glows.
o AVR VT supply also absent message in the status line.
2.b R/L Control (Auto) & AVRs in Independent mode:-
Identify the Terminals where VT secondary connections are to be given.
Disconnect the wires from the terminal after noting the TB. And ferrule Nos.
Connect the output wires of a variac of suitable rating at these terminals.
Ensure that TB links are properly inserted.
Set the output voltage of the variac at 110 V
Note down the present tap position
Put the AVR in AUTO mode.
Ensure that Raise/Lower operation from the AVR panel not possible manually.
Keep AVR in REMOTE position.
Also ensure following Indications and display.
o LED in the Hand push button off.
o LED in the “AUTO” push button glows.
o Measuring value display“11KV” in the home screen.
o Voltage deviation display approximately zero.
o Deviation pointer in balanced position.
o Time bar remains unfilled.
o AVAR VT supply absent alarm message vanished
o No alarm display and alarm indication.
o REMOTE/SUPRY try color LED glows in green.
Gradually raise the voltage output of the variac and observe the voltage deviation in the
home screen display. Also, observe the deviation pointer moves upwards.
Increase the voltage by 4% of the (band width) the reference level (110 V) and ensure the following:
o Time bar in the home screen display starts filling and the end of filling, LED in the
“Lower” Command push button starts glowing.
o Tap change in progress message in the status line as long as the operation is in
progress.
o After Green LED of lower command goes off, tap position is seen reduced by one
tap.
o Tap position in MDU, OLTC, AVR display and programmer are same.
o As soon as the lower tap operation starts, bring the variac out put back to 110V.
o Repeat the above automatic tap lowering for two or three tap operations.
Decrease the voltage by 4% of the (band width) the reference level (110 V) and ensure
the following:
o Time bar in the home screen display starts filling and at the end of filling. LED in the
“Raise” Command push button starts glowing.
o Tap change in progress message in the status line as long as the operation is in
progress.
o After Green LED of Raise command goes off, tap position is seen increased by one
tap.
o Tap position in MDU, OLTC, AVR display and paragramar are same.
As soon as the Raise tap operation starts, bring the variac out put back to 110 V.
Repeat the above automatic tap raising for two or three tap operations.
Keep the variac out put at 110 V and ensure no alarms.
Reduce the voltage by 20% of the reference value and ensure the following:
o Red LED glows against “<U”.
o Under voltage message in the status line.
o Automatic tap raising operation is seen blocked from AVAR.
o Ensure that in manual mode both raise and lower operations are blocked when AVR
is in under voltage conditions.
o Put AVR in AUTO mode.
Bring the variac back to 110 V and increase the output by 20% of the reference value and
observe following:
o Red LED glows against “>U”.
o Over voltage message in the status line.
o High speed automatic taps lowering command issued by the relay.
o Bring the variac out put less than 30% of the reference value and observe that all
alarm indication vanished except “AVAR VT supply absent” message in status line.
o Put the AVR in SUPRY mode and in AUTO.
o Ensure that automatic tap operation and signaling take place in SUPRY mode also as
described above.
3. Parallel Operation, Indication and Alarm checks.
3. a Parallel Manual R/L control in Manual:-
Identify and verify the Terminals where CB status inputs are connected.
All the inputs are temporarily short circuited.
Select all AVRs in Hand mode.
Keep all IDTs in the same tap positions.
Ensure that tap position at MDU, OLTC, AVR display and Programmer is same.
Select parallel control of all AVRs in manual.
Select AVRs in REMOTE.
Select AVR1 in Master and others in Followers. (Ie. M F F).
Ensure the following:
o MASTER LED of AVR1 glows.
o FOLLOWER LED of AVR 2 & 3 glows.
o LED against parallel symbol glows in all AVRs.
o Master message in the status line display of AVR1.
o Follower message in the status line of AVR 2 & 3.
o No alarm display and alarm indications in any AVRs.
o All circuit breakers are in closed status in programmers of all AVRs.
o Programmer shows the correct indication of the parallel selection.
o Raise/Lower the tap from the master AVR and ensure proper parallel tap operation
in all IDTs. Check taps position at MDU, OLTC, AVR display and Programmer of all
IDTs.
o Ensure that tap operations are not possible from the AVRs selected as Followers.
o Select AVR2 as Master and Others as Followers and do the tap operations as done in
AVR1 and ensure satisfactory operations and indications. ( F M F)
Then select AVR3 as Master and others as Followers and repeat the procedure as
mentioned above. ( F F M)
Do tap operations and indication checks on all possible combinations as suggested in the
test format for OLTC Operation Checks-Parallel Manual mode.
Ensure that operation shall not be possible from followers AVRs.
Parallel operation shall not be possible from any of the AVRs if more than one master are
selected.
Parallel operation shall not be possible from any of the AVRs if Followers are selected
without any master.
Switch off AVRs one by one and observe “Failure Par Control” message in the status line of other AVRs.
Operate any of the AVRs in Independent mode and keep one-step tap difference with other AVRs.
Put this AVR again in parallel mode and then observe that Raise/ Lower tap operation takes
place in this AVR to bring it to the same tap position as that of others.
Again operate the same AVAR in Independent mode and create one step tap difference
and Switch off the MDU control supply.
Then put this AVR again in parallel mode and then observe that Raise/lower command is
initiated from the AVR but tap operation is not taking place at MDU and OLTC.
Mean time ensures an “Out of Step” alarm message in the status line of each AVR after a
time delay.
Repeat this check in other AVRs also.
Create two-step tap difference and put the AVR in parallel mode and observe an “out of
step” alarm message immediately.
Select AVRs in SUPRY mode and ensure that operations are not possible from AVR panel.
3. b Parallel Manual – R/L control in Auto:
Select all AVRs in REMOTE.
Then connect variac out put to the VT input of all AVRs as done earlier.
Keep the variac out put at 110V.
Select all AVRs in AUTO mode.
Select Parallel control of all AVRs in Manual mode.
Select M F F mode
By varying variac out put ensure tap operation as done earlier.
Ensure proper parallel operation. I.e. Follower AVRs follow the Master.
Ensure indications and tap position at various locations.
Ensure no alarm display and alarm messages at this condition.
Create various alarm conditions as described earlier.
Ensure Parallel operation blocking on alarm conditions.
Ensure that VT wiring is done correctly such that a cable coming from IDT1 VT is
connected AVR1, IDT2 VT to AVR2 and IDT3 VT to AVR3 respectively.
3. c Parallel AUTO-R/L control in Manual:
Disconnect all shorting wires from the TB of CB status.
Identify the CB open/close status condition input terminals in LCC and 11 KV panel.
Insert all LV CB in test position and close all the CBs including HV CBs.
Observe the programmers of all AVRs for correct bus bar status.
Select AVTRs in REMOTE and in HAND.
Select all AVRs in Parallel AUTO mode.
Observe AVR1 automatically selected as Master and others as Followers (MFF).
Mean time checks all indications and alarms for proper display
Keep the Master IDT out of parallel control by opening respective CBs and then observe
that AVR of another IDT, who’s CAN BUS ADDRESS, is low, is selected automatically as
Master. Men time the AVR of the IDT which is out of parallel control becomes independent.
Repeat the check as per the test format for Parallel Auto Mode and observe that correct
selection and indications take place automatically.
Raise/Lower operations at each combination shall be done in HAND mode and tap
positions, indications and display shall be checked for its correctness as described earlier.
Create various alarm conditions as described earlier.
Ensure Parallel operation blocking on alarm conditions.
3. d Parallel Auto –R/L control in Auto:
Keep AVRs in Parallel Auto and Tap control in AUTO mode.
Keep AVR1 in Master and other in Followers.
Check Raise/Lower operation by varying the variac output by +/- 4% of the reference
voltage.
Ensure that Parallel control is satisfactory in this mode also.
Ensure tap positions, indications and display for its correctness as described earlier.
Ensure that in this mode Raise/Lower operation is not possible by HAND.
Create alarm conditions in AVRs and ensure blocking of parallel operation.
28.0 OLTC PARALLEL OPERATION
Parallel Manual mode and Parallel Auto mode.
Aim
To ensure the performance of OLTC parallel operation through AVR at parallel manual mode with master, follower, and independent selection independently, parallel auto mode with master, follower, and independent selection automatically with Breaker status and same operation with auto according to the reference voltage of VT secondary voltage.
General
Tap position of the transformer shall be the same. MDU must be always in remote position. AVR is put in manual and remote mode. Breaker operation shall also be done form remote. The first Transformer is the master second will be follower and third will be independent at normal operating condition.
Test Preparation
Ensure designation of AVR1 for IDT1, AVR2 for IDT2 and AVR3 for IDT3.
Ensure all MDU are in remote position.
Ensure all OLTC are in same tap position.
Ensure the readiness of all MDU for operation.
Ensure the supply input for all AVR.
Ensure the readiness of DCS and SCADA
Ensure the readiness of 132kV & 11 kV GIS.
Ensure the clearance from the concerned departments for the operation of GIS(Breaker
closing and opening)
Procedure
Parallel-Manual Mode
Switch on all AVR supply.
Select all required parameters in all AVR.
Put all AVR in hand /manual and remote mode.
Manually select the first AVR in master and follower on second and third.
With the above condition the operation of OLTC shall be checked from AVR, DCS and
SCADA and operation will be given below.
Similarly the balance operation mentioned in the documents shall be checked by manually
selecting the required mode in each of the AVR at AVR, DCS and SCADA.
MFF: MFF means AVR1 is the master of IDT1, AVR2 and AVR3 are the followers of IDT2,
IDT3 for the master.
MFI: MFI means AVR1 is the master of IDT1, AVR2 is follower of IDT2 for the master and
AVR3 is the independent for IDT3
Master: (M) Master means the OLTC operation is possible from the master only.
Follower: (F) Follower means the OLTC operation not possible form the follower but
operation followed by the master.
Independent: (I) Independent means the operation of OLTC can be done individually.
Parallel Auto Mode:
Select required parameters in all AVR
Put all AVR in hand (manual) and remote mode.
Check the breaker status.
If the entire breaker are closed as shown in figure, the scheme will be configured in such
way that the IDT1 as master and IDT2 & IDT3 are followers.
As the above condition, open Bus Section one (BS1) breaker
IDT1 will be separated from the circuit and it became independent
IDT2 became master
IDT3 became follower
All operation with the above condition shall be checked at AVR, DCS and SCADA.
Similarly all the operation mentioned in document shall be checked.
Auto Operation check with VT secondary voltage
Put AVR in auto instead of had (manual)
Give the VT secondary voltage with slide are to the terminals where the VT secondary
connections are to the AVRT after disconnecting the wires actually coming from the VT secondary side.
Set the voltage 110V
Varying the voltage from 110 V to below and above in line with approved set values for
lower and raise operation of the OLTC.
With the above raise/lower conditions, the operation as stated in parallel auto mode shall
be repeated and ensure the performance.
29. OIL LEAK TEST
Aim
To ensure that the transformer is fully leak proof.
General
Oil leak test is done to ensure any oil leakage through welds, gasket joints, devices and any point of the transformer. During the continuous full load operation, the temperature of oil goes up. So the oil viscosity will be reduced and oil level increased in the conservator and hence there are more chances for oil leak. This condition is simulated by applying additional pressure over the normal oil head for a particular period constantly. During this keeping time if there is no oil leakage, it is presumed that oil leakage will not occur during continuous normal loading. Dry nitrogen (N2) is normally used for applying pressure.
At test pressure of normal pressure plus 34N/m² shall be applied at top of oil in the conservator or 69N/m² at bottom of the tank whichever is greater, constantly for 48Hrs.During this period no oil leakage or ingress in to normal oil free spaces shall occur. It is suggested to apply the pressure at maximum ambient temperature so that we can reduce the applied pressure during a whole
keeping period to avoid creating excess pressure which may cause the PRD operation.
Test preparation
Ensure the rubber bag and rapture relay are normalized (If provided)
Ensure the main tank and OLTC (if applicable) tanks are equalized.
All the bolts and nuts are fully tightened and torque checked.
Ensure new gaskets are used during erection.
Ensure no oil trace at any where the transformer.
Ensure proper cleaning of the transformer and its surroundings.
Ensure the availability of nitrogen cylinder with regulator and pressure gauge.
Ensure all gauges have proper valid calibration status.
Ensure the availability of thermometer for ambient temperature.
Ensure valve between main buchholz relay and conservator and between HV cable (if
applicable) box/boxes and conservator are open.
Ensure all radiator valves are open.
Ensure the valve between OLTC and conservator is open (If OLTC is provided)
Ensure the PRD operating pressure.
Test procedure
Remove OLTC and Main tank breather; connect the output of the nitrogen cylinder parallel to the OLTC main tank breather line so as to apply the pressure both tank at time.
Pressure gauges shall be connected at bottom (P1), at top of main tank (P2) and one (P3) for the applied pressure measurement.
Note down the pressure due to normal head at bottom and top of the tank.
Note down the ambient and oil temperature.
Slightly open the N2 cylinder valve and regulated N2 shall be allow passing and pressured the normal oil haed.When the pressure reaches at as stated above by simultaneously monitoring the pressure gauges P1, P2 and P3 and maintaining continuously for 48hrs.
During the pressurizing, ensure the pressure at tank top (P2) is less than the operating
pressure of PRD.
During the keeping time, every one hour, the ambient temperature, top, bottom and
applied pressure shall be note down.
During the keeping time there should not be any oil leakage.
Test observation shall be reported and recorded.
30. OIL BDV TEST
IEC60076-1, 2000
Aim
To ensure the electrical strength of the insulating oil and BDV of the oil each stage meets the requirements/Specifications.
General
This is one of the points to ascertain of oil. Sphere gap method with a gap of 2.5mm in oil is normally used. The BDV of oil at above test condition shall be greater than 70KV.
Following precautions shall be taken during oil sampling.
It is suggested to take the oil sample directly to the BDV kit set. Special tools, gloves etc shall be used.Dry, clean and good atmospheric condition is better for taking sample. Avoid talking during sampling. Do not air bubbles in the sampling oil. Care shall be taken to avoid entry of moisture, dust metal parts etc to the sampling oil. Required quantity of oil shall be taken in the kit at a time for test. After sampling, immediately cover it properly, to avoid entry of air moisture through the
sampling portion of the transformer as well as the sampling oil.
Test preparation
The BDV kit with valid calibration status placed in a clean and closed room. Ensure the availability of supply unit.
Test procedure
Precautions shall be taken during sampling as stated above. The cleaned BDV kit vessel shall be washed two to three times with the oil to be tested. Check any air bubbles in the sampling oil.
The oil sample shall be kept carefully inside the kit and covered. Minimum 15 minutes shall be
given as a settling time. Six set reading shall be taken at an appropriate intervals and reported
average. The average value will be greater than 70Kv.Ensure that none of the six readings is less than 60Kv.If the test values are not satisfactory, decision shall be taken for taking sample again for a retest or further oil processing is required to improve the values. After test, the kit shall be switched off and clean the oil vessel.
Following oil sample shall be tested.
Processed oil before filling in to the transformer.
After hot oil circulation of the oil-----Main tank top and bottom.
HV Cable box
31. TEST ON TRANSFORMER OIL (OIL CHARCTRESTICS)
IEC600296 - 2003
Aim
To ensure the properties and characteristics of transformer oil
General
This is purely an oil manufacturer’s test report which contains data to meet our
requirements/Specifications. If oil manufacturers test report is incomplete, test shall be done at independent laboratory before conducting any oil processing. Following are the properties and characteristics of oil.
Kinetic viscosity
Flash point
Pour point
Dielectric Dissipation factor
Appearance
Density
Interfacial tension
Neutralization value
Corrosive sulphur
Total sulphur content
Water content
Total acidity
Sludge
Dissipation factor @90ºC
Induction period
Total sulphur content
DGA
Break down Voltage
36. SWEEP FREQUENCY RESPONSE ANALYSIS
IEC60076-1, 2000
Aim
To ensure any possible damages (Physical shifting/deformation etc) of the transformer winding,core or internal parts have occurred during transportation or after any failure.
General
Frequency response analysis FRA is a valuable tool to verify the geometric and mechanical
integrity of electrical apparatus, especially transformers, providing accurate and repeatable measurements. This is a proven field diagnostic technique capable of detecting damage arising due to dislocation/movement of core and winding during transportation or after occurrence any field.
FRA is based on the principle of measurement of steady electrical response against a steady
sinusoidal input given to the test object.
The basis of the FRA technique is that the impedance of the transformer is a function of the resistance, inductance, capacitance, and applied signal response. The resistance is related to the physical construction of the winding .Capacitance and inductances are related to the construction and geometry of the windings. Deformation and movements have an effect on both inductance and capacitance that may be reflected in the resulting frequency response.
There are two basic methods to conducting FRA test; low voltage impulse method and
sweep frequency response analysis SFRA
Low voltage impulse method relies on indirect measurement of the frequency response. An
impulse or a set of impulse with assumed frequencies of interest is applied to the winding that is under test. These frequencies are disentangled using signal analysis techniques (FFT) to produce a frequency response plot by converting time domain to frequency domain. This technique is dependent on good impulse signals, fast signal processing and analysis. However these produce limited or poor response at lower frequencies, up to few hundred Hz, where much information
about internal construction of the transformer in terms of inductance is gained.
SFRA measurement is done with the frequencies being swept in a wide range; usually from 20Hz to 2MHz.This is the simple direct measurement in frequency dimain.Also a direct relationship exists between geometric configurations and RLC network constituted by winding and core assembly identified by its frequency dependent transfer function. Measuring the frequency may
be performed by applying a simple varying frequency signal to the transformer that covers
frequency range of interest. Sweep frequency method has far higher repeatability and consistency over impulse methods which are prone to variation due to lead arrangement and impulse shape.
Normally a factory FRA measurement is taken as reference (Signature) and later on site
measurement (after errection at site) is compared with the reference measurement. In practice measurement after erection at site is taken as reference for the future measurements done at site during the operational period of transformer.
Possible damages in the winding are indicated in relevant ranges of frequencies by shifting of
resonance points or by the introduction of new resonance, not being present in the reference
measurement.
The comparison of the traces belonging to different phases or different transformer of same design (Sister Unit) may be useful, but with laminations.
A shifting of the resonant point (If any) of the open circuited admittance is noted. The difference (ie, the shifts of resonant points or the introduction of new resonance not present in the reference measurement) is evaluated as an indication of normal variation or faults in the winding
The plots of admittance (Reverse of impedance) verses frequency indicate a series of resonance with the small value and anti resonance with high value. The resonance with low admittance represents a parallel resonance circuit. The resonance with high admittance represents a series resonant circuit.
Test preparation
Measuring the frequency response of a winding with in a transformer, or of a transformer transfer functions from winding to winding, gives a finger point for that winding or transformer. The process requires measurement of both input and output signals, which are then radioed to give
the response. This reduces the effect of the measurement of three lead systems involving input measured input and measured response.
Signals are applied and measured with respect to ground. The amplitude and phases of the two signals (S Measurement and R Measurement), are measured to determine the relative amplitude and phase shift changes between them. The basic measurement is of the attenuation and phase shift of signal after having passed through the winding from the input to the output terminals.
The test can also include voltage transfers between windings i.e. applying a signal to one winding of a transformer and measuring the response at another winding to determine the amplitude change and phase shift of the signal having been transferred along a winding, or form one winding to other Transformer under test should be completely reenergized and isolated from the power system.
This shall be solidly earthed which is common to FRA test equipement.FRA is performed as
response to electrical measurement at the terminals connected to the starting and end of any winding. The test leads must be first connected to the FRA instrument and then to the transformer bushings.
Connect the color code cable to the instrument.
1. Yellow : Excitation source cable
2. Block : Measurement cable
3. Red:Refernce cables
The main cable clamps red and the block are connected to bushings terminals accordingly. Also connect the ground clips for each cable, red and block to the corresponding bushing flanges. Test set up and actual test shall be done as per the manufacturer’s manual.
Test procedure
Transformer tap number, earthing details, ambient temperature etc.are noted. Frequency range is selected as 20Hz to 2Mz.The test connections are verified and measurements done for the following combinations. The plot names are assigned wit reference to terminal markings as given below.
1U1N-LVoc, 1VN-LVoc, 1W1N-LVoc
1U1N-LVsc, 1V1N-LVsc1W1N-LVsc,
2U2V-HVoc, 2V2W-HVoc, 2U2W-HVoc,
The measured magnitude and phase plots are compared with corresponding reference plots
(Signature).If any abnormality/deviations in the measured plot is noticed, tightness and
correctness of connections may be confirmed before finalizing the test results.
Precautions and safety.
1) FRA measurement is sensitive to DC magnetization of the core and hence it has carried before application of any DC voltage. If any DC measurement is carried out before FRA
measurement, it should be properly demagnetized.
2) The resultant plot may distort from the actual one in case of loose connection.
Interpretation
1) The exact comparison of the FRA measurement plots at site with the factory measurement can be made only if the measuring equipements, measuring connection, setup, measuring leads etc are the same. However idetintical test equipments with the same connection and setup may give almost the same results.
2) Frequency response plot provides a finger print for a transformer. Fingerprints from similar transformer have common features.
3) The core condition, in principle, cannot be checked by FRA method. However
magnetization of core affects low frequency results.
4) A Clear shifting of first resonance usually below 1000Hz indicates inter turn connection
failure.
5) Clear shifts of resonance or new resonance of the lower harmonics in the range 1000Hz to
10 kHz indicate the axial movement of bulk winding or radial hoop bucking.
6) Shifts of resonance or new resonance in the range of 10 kHz to 100 kHz indicate
movements of small winding parts e.g. tap winding.
