REACTOR TESTING

1. CORE-CLAMP-EARTH INSULATION MEASUREMENT:

IR measurement between CL (Core Lamination) to CC (Core Clamp), CL to earth and CC to earth should be checked by 500 V megger and reading should be more than 100 M Ohm for CL-CC & CL-Earth. There is no general insulation requirement on the insulation level for CC-Earth.

The terminal box contains a terminal block with three terminals:

· The terminal marked CL is connected to the core laminations.

· The terminal marked CC is connected to the core clamps.

· The terminal marked G is connected to ground (the tank)

For core-insulation to ground test, remove the cover of the terminal block, Disconnect the   closing link that connects the two terminals CL-G. Apply 1.0 kV direct voltage between CL and CC + G (core grounding strap). The tank shall be grounded during the test.

2. INSULATION RESISTANCE (IR) MEASUREMENT:

Purpose: Insulation resistance (IR) measurement is the simplest and most widely used test to check the soundness of reactor 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 reactor (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 Megger which are available in 500 V, 1000 V, 2500 V and 5000 V ratings.

Precautions:

i)Clean the bushing porcelains by wiping with a piece of dry cloth. 

ii)When using a megger, observe the usual accident preventive rules.

iii)If moisture condensation is suspected on porcelain surface, provide aluminum foil taping below the lower most porcelain shed. (especially for small bushings like Neutral Bushing) Connect the foil to the guard terminal of megger.

iv)Connecting wires from the bushing line lead and tank earth to megger shall be as short as possible without joints and shall not touch tank or each other.

v)Test specimen to be discharged by short-circuiting for a period at least four times as long as test voltage was applied. Before bare hand contact, the absence of voltage shall be confirmed by measurement.

vi)IR measurement should be performed with windings and leads completely immersed in oil. Under no circumstances, tests to be made with the equipment under vacuum.

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 Shunt Reactor. Record date and time of measurement, sl. no., make of megger; oil temperature and IR values at intervals of 15 seconds, 1 minute and 10 minutes. With the duration of application of voltage, IR value increases. The increase in insulation resistance is an indication of dryness of insulation. The ratio of 60 second insulation resistance to 15 second insulation resistance value is called Dielectric absorption coefficient or Index (DAI). For oil filled reactor with class A insulation, in reasonably dried condition the absorption coefficient at 30°C will be more than 1. 3.
Polarization Index Test is ratio meter test, insensitive to temperature variation and may be used to predict insulation system performance even if charging currents (i.e. capacitive, absorption or leakage currents) have not diminished to zero. Since leakage current increases at a faster rate with moisture present than does absorption current, the megaohm readings will not increase with time as fast with insulation in poor condition as with insulation in good condition. This results in a lower polarization index. An advantage of the index ratio is that all of the variables that can affect a single megaohm reading, such as temp and humidity, are essentially the same for both the 1 min and 10 min readings. The polarization index test is performed generally by taking megaohm readings at the following intervals at constant dc voltage: 1 min and then every minute up to 10 min. The PI is the ratio of the 10 min to 1 min megaohm readings. The following are guidelines for evaluating transformer insulation using polarization index values:

Polarization Index Insulation Condition

Less than 1      Dangerous

1.0-1.1             Poor

1.1-1.25           Questionable

1.25-2.0           Fair

2.0 – 4.0          Good

Above 4.0       Excellent

A PI of more than 1.25 and DAI of more than 1.3 are generally considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI less than 1 call for immediate corrective action. For bushings, an IR value of above 10,000 M_ is considered satisfactory.

3.CAPACITANCE AND TAN-d 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.

TEST EQUIPMENT: For getting accurate results of Tan delta and Capacitance without removing the bushing from the reactor, a suitable test set capable of taking measurement by ungrounded specimen test (UST) method shall be used. It utilizes 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 Tan-d Bridge from any reputed manufacturer may be used for field testing. Portable test set include measuring bridge such as Schering Bridge, power supply and standard capacitor in one enclosure. 

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 predation 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. Reactor 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 energized 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.

PROCEDURE: In bushing, conducting layers are arranged within the insulating material for the purpose of uniform distribution of the electric field in the bushing. Bushing test tap is a connection to the last of the conducting layers of a capacitance graded bushing to ground with a disconnector for measurement of partial discharge, power factor, and capacitance values of the bushings. The main capacitance, C1, of a bushing is the capacitance between the high-voltage conductor and the test tap. The tap capacitance, C2, of a capacitance-graded bushing is the capacitance between the test tap and mounting flange (Ground).

The capacitance C of a bushing without a voltage or test tap is the capacitance between the high- voltage conductor and mounting flange (Ground). For measurement of tand and Capacitance of bushings it is strongly advised that shorting of HV terminals of all bushings of the same winding to be made and all other winding bushing terminals be shorted and grounded to avoid influence of capacitance of other windings.

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. The main capacitance (C1) of the bushing i.e., the capacitance between high voltage terminal and test tap is not affected by the surrounding conditions and the accepted deviation from the values measured at factory tests should be less than 10%. The capacitance between bushing test tap and ground is largely influence by the stray capacitances to ground parts in the transformer and hence large deviation in the measured value shall be accepted when compared with the factory test value. Rate of change of tan Delta and capacitance is very important. Rate of rise of tan delta should not be more than 0.001 per year. Capacitance value can be within +10%, -5% in capacitance value.

4.  CAPACITANCE AND TAN-D MEASUREMENT OF WINDING:

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 reactors.

PRECAUTIONS

The test set is a source of high voltage electrical energy and operator must use all practical safety precautions to prevent contact with energized parts of the test equipment and related circuits. Following precautions need to be taken:

· Never connect the test set to energized 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 energized conductor and ground to prevent any arc over.

· It should be ensured that test specimen is de-energized and grounded before making any further connection and no person may come in contact with HV output terminal or any materiel energized by the output.

TESTING PROCEDURE: For tan delta capacitance measurement of 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 with all other windings grounded to tank (UST test mode) or ground the other entire windings guarded (GST L GUARD test mode).

ii) Finally measurement should be made between all windings connected together and the grounded tank.

Maximum values of Dissipation Factor (Tan Delta) of class A insulation e.g. paper insulation, oil impregnated is 0.007. Rate of change of tan Delta and capacitance is very important. The rate of change of tan-d more than 0.001 per year needs further investigation. Capacitance value can be within +10%, -5% in capacitance value. Comparison of test results to those for similar piece of equipment especially those tested under the same conditions shall be done. These tests provide a valuable index of dryness and are helpful in detecting undesirable operating conditions and failure hazards resulting from moisture, carbonization of insulation, defective bushings, contamination of oil by dissolved material or conducting particles improperly grounded or ungrounded core etc.

5.  WINDING RESISTANCE MEASUREMENT:

Purpose: Reactor 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. High resistance measuring ohm meter / bridge or drop of potential method may be used for measuring direct- current resistance.  Current used when making direct- current resistance measurement shall not exceed 15% of the rated current. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.  Measurement shall not be taken untill after steady state values have been reached.  For connection refers manufacturers operating instruction for the testing instrument available at site.  If the drop of potential method is used, following additional precautions should also be taken.  Measuring equipment shall have a very high degree of accuracy.  Greater accuracy may be obtained by the use of potentiometers.   Not less than four pairs of readings for current and voltage shall be taken. The average of the resistance calculated from these measurements shall be considered to be resistance of the winding.

Precautions

1. 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.

2. 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.

3. It is essential that temperatures of the windings are accurately measured.

4. Care shall be taken that self inductive effects are minimized.

5. 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 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.

6.  FREQUENCY RESPONSE ANALYSIS (FRA) MEASUREMENT

Purpose: Frequency Response Analysis (FRA) is made to assess the mechanical integrity of the transformer. Transformers while experiencing severity of short circuit current looses its mechanical property by way of deformation of the winding or core. These changes cannot be detected through conventional condition monitoring techniques such as Dissolved Gas Analysis, Winding Resistance Measurement, Capacitance and Tan delta measurement etc. Sometimes even transportation without proper precaution may cause some internal mechanical damages. FRA measurement, which is signature analysis, provides vital information of the internal condition of the equipment so that early corrective action could be initiated.

Test Principle and Measurement: Short circuit forces can cause winding movement and changes in winding inductance or capacitance in Power Transformers. Recording the frequency response with these changes gives information regarding the internal condition of the equipment. Frequency Response Analysis (FRA) has proved to be an effective tool to detect such changes. Sinusoidal signal output of approximately 2 V r.m.s from the Frequency Response Analyzer is applied and one measuring input (R1) is connected to the end of a winding and the other measuring input (T1) is connected to the other end of the winding. The voltage is applied and measured with respect to the earthed transformer tank. The voltage transfer function T1/R1 is measured for each winding for five standard frequency scans from 5 Hz to 10 MHz and amplitude & phase shift results are recorded. While the low frequency analysis reveals the winding movements, the high frequency analysis reveals the condition of joints. It is ensured that winding which is not under test is terminated in open condition in order to avoid response difference among the three phases. The same procedure is followed on subsequent tests on the same or similar transformer, to ensure that measurements are entirely repeatable. The voltage transfer function T1/R1 is measured for each winding for four standard frequency scans from 5 Hz to 2 MHZ and amplitude & phase shift results are recorded for subsequent analysis

7. TESTING OF BUSHING CT AND NEUTRAL SIDE CT:

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 mAmp 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 demagnetize 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 high impedance (20000 R/V or greater) low range voltmeter as shown in the diagram below:

The turn’s ratio is approx. equal to the voltage ratio. Saturation level is usually about 1 V per turn in most low- and medium- ratio bushing CT’s.

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 TEST (Knee Point Voltage Test):

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 CT’s 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 r.m.s instrument. The voltmeter should be an average reading voltmeter. It should be calibrated to give the same numerical indication as an r.m.s 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.

4. INSULATION RESISTANCE TEST:

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 MΩ. 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.