Various Routine Test of Power Transformer-(Part-4)


(9) Magnetic Balance Test

 Test Purpose:

  • Magnetic balance test of transformer is conducted only on three phase transformers to check the imbalance in the magnetic circuit.

 Test Instrument:

  • Multi meter.
  • Mill Ammeter

 Test Circuit Diagram:

 Untitled

Test Procedure:

  • First keep the tap changer of transformer in normal position.
  • Now disconnect the transformer neutral from ground.
  • Then apply single phase 230V AC supply across one of the HV winding terminals and neutral terminal.
  • Measure the voltage in two other HV terminals in respect of neutral terminal.
  • Repeat the test for each of the three phases.
  • In case of auto transformer, magnetic balance test of transformer should be repeated for IV winding also.
  • There are three limbs side by side in a core of transformer. One phase winding is wound in one limb. The voltage induced in different phases depends upon the respective position of the limb in the core.
  • The voltage induced in different phases of transformer in respect to neutral terminals given in the table below.
  • 415V, Two phase supply is to be applied to any two phases terminals on HV side of Power transformer and voltages in other two phase combination are to be measured with LT open.
  • Sum of the Resultant two values shall be equal to the voltage applied.

 

Applied Voltage (415V) Measured Voltage(V1) Measured Voltage(V2) Result
RY YB BR V=V1+V2
YB RY BR V=V1+V2
BR YB RY V=V1+V2

 

 (10) High Voltage tests on HV & LV Winding:

Test Purpose:

  • To checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

 Test Instrument:

  • High Voltage tester ( 100KV & 3KV)

 Test Circuit Diagram:

UntitledTest Procedure:

  • HV high voltage test: LV winding connected together and earthed. HV winding connected together and given Following HV Supply for 1 minute.
  • LV high Voltage test: HV winding connected together and earthed. LV winding connected together and given Following HV Supply for 1 minute.
  • 433V Winding =3KV High Voltage
  • 11KV Winding =28KV High Voltage
  • 22KV Winding =50KV High Voltage
  • 33KV Winding =70KV High Voltage.

 

(11) Di electrical Test:

Test Purpose:

  • To check the ability of main insulation to earth and between winding
  • To checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

 Test Instruments:

  • 3 Phase Variable Voltage & Frequency Source.
  • Auto Transformer.

 Test Procedure:

  •  The following Dielectric tests are performed in order to meet the transformer insulation strength expectations.
  • Switching impulse test: to confirm the insulation of the transformer terminals and windings to the earthed parts and other windings, and to confirm the insulation strength in the windings and through the windings.
  • Lightning impulse test : to confirm the transformer insulation strength in case of a lightning hitting the connection terminals
  • Separate source AC withstand voltage test: to confirm the insulation strength of the transformer line and neutral connection terminals and the connected windings to the earthed parts and other windings.
  • Induced AC voltage test (short duration ACSD and long duration ACLD ) : to confirm the insulation strength of the transformer connection terminals and the connected windings to the earthed parts and other windings, both between the phases and through the winding.
  • Partial discharge measurement: to confirm the “partial discharge below a determined level” property of the transformer insulation structure under operating conditions.

 Method No 1 (separate source voltage withstand test) Untitled

  • All the terminals of the winding under test should be connected together and the voltage should be applied.
  • The secondary windings of bushing type current transformers should be connected together and earthed. The current should be stable during test and no surges should occur.
  • A single phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the terminals of the winding under test.
  • The test shall be performed on all the windings one by one.
  • The test is successful if no breakdown in the dielectric of the insulation occurs during test.
  • During the Separate source AC withstand voltage test, the frequency of the test voltage should be equal to the transformer’s rated frequency or should be not less than 80% of this frequency. In this way, 60 Hz transformers can also be tested at 50 Hz. The shape of the voltage should be single phase and sinusoidal as far as possible.
  • This test is applied to the star point (neutral point) of uniform insulated windings and gradual (non-uniform) insulation windings. Every point of the winding which test voltage has been applied is accepted to be tested with this voltage.
  • The test voltage is measured with the help of a voltage divider. The test voltage should be read from voltmeter as peak value divided by2. Test period is 1 minute.

 Method No 2 (Induced source voltage withstand test)

Untitled

  • The aim of this test is to check the insulation both between phases and between turns of the windings and also the insulation between the input terminals of the graded insulation windings and earth
  • During test, normally the test voltage is applied to the low voltage winding. Meanwhile HV windings should be keeping open and earthed from a common point.
  • Since the test voltage will be much higher than the transformer’s rated voltage, the test frequency should not be less than twice the rated frequency value, in order to avoid oversaturation of the transformer core.
  • The test shall start with a voltage lower than 1/3 the full test voltage and it shall be quickly increased up to desired value.
  • The test voltage can either be measured on a voltage divider connected to the HV terminal or on a voltage transformer and voltmeter which have been set together with this voltage divider at the LV side. Another method is to measure the test voltage with a peak-value measuring instrument at the measuring-tap end of the capacitor type bushing (if any).
  • Test period which should not be less than 15 seconds.
  • It is calculated according ,Test period=120 seconds x ( Rated frequency / Test frequency )
  • The duration of the test shall be 60 second.
  • The test is accepted to be successful if no surges, voltage collapses or extreme increases in the current have occurred.

 Acceptance Criteria:

  • The test is successful if no break down occurs at full test voltage during test.

Method No 3 Lighting Impulse Test:

  • All the dielectric tests check the insulation level of the Transformer.
  • Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave
  • One impulse of a reduced voltage between 50 to 75% of the full test voltage and subsequent three impulses at full voltage.
  • For a three phase transformer, impulse is carried out on all three phases in succession.
  • The voltage is applied on each of the line terminal in succession, keeping the other terminals earthed.
  • The current and voltage wave shapes are recorded on the oscilloscope and any distortion in the wave shape is the criteria for failure.

Various Routine Test of Power Transformer-(Part-3)


(5) Short Circuit Test

 Test Purpose:

  • The value of the short circuit impedance Z% and the load (copper) losses (I2R) are obtained.
  • This test should be performed before the impulse test-if the later will be performed as a routine test- in order to avoid readings errors

 Test Instrument:

  • Megger or
  • Multi meter.
  • CT ,PT

 Test Procedure:

  • Suitable Low Voltage (3-phase 415V, 50Hz )will be applied to the terminals of one winding (usually the H.V.) with the other winding short circuited with 50 sq. mm. Copper cable. (Usually the L.V.)
  • The applied voltage is adjusted to pass the needed current in the primary/secondary. In order to simulate conditions nearest to full load, it is customary to pass 100%, 50% or at least 25% of full load current.
  • Voltage to be increased gradually till the current in the energized winding reaches the required value (50% to 100% rated current).
  • Measure the 3 Phase line currents at all tap position. If the tap-switch is an Off-Circuit tap-switch, the supply has to be disconnected before changing the tap. A consistent trend in the increase or decrease of current, as the case may be, confirms the healthiness of the transformer.
  • If transformer is equipped with a tap changer, tapping regulations are applied.
  • (1) If tapping range within±5% and rated power less than 2500kAV, load loss guarantee refer to the principal tap only.
  • (2) If tapping range exceeds±5% or rated power above 2500kAV, it shall be stated for which tapping beside the principal tap the load losses will be guaranteed by the manufacturer.
  • Three phase LT supply is applied on HV side of power transformer at normal tap with rated current on HV side and currents measured in all the phases on HV side and phases & neutral on LV side values noted.
  • Readings to be taken as quickly as possible as the windings warm up and the winding resistance increases. Hence, the losses value will increase accordingly.
  • Using appropriate instruments (conventional three watt meter method or digital watt meter with ammeters & voltmeters) measurements of voltage, currents and power can be recorded.

 Untitled

  • Short Circuit Test (Without using CT,PT)
  • To avoid CT’s and PT’s, this method can be used at current levels of 2 to 5 A and measurement of load losses is done at this condition. This measured load loss is then extrapolated to actual load currents to obtain load losses at the operating current.
  • Example: – 11 kV/433 V, 1000 kVA transformer with 5% impedance, the voltage to be applied on H.V. side during load test is estimated below.
  • V. side full load current (I1) = (KVAx1000/1.732xLine Voltage)
  • V. side full load current (I1) =(1000×1000/1.732×11000)=52.5 Amp
  • Line to line voltage to be applied on H.V side for getting 5 A on H.V. side,
  • Line to line voltage to be applied on H.V side Visc= (Line Voltagex1000xZx5/0.866xI1x100)
  • Line to line voltage to be applied on H.V side Visc=(11x1000x5xx/x0.866×52.5×100)=60.5 volts.
  • Since the current drawn on H.V. side is only about 5A in this test, CT’s can be avoided and hence phase angle error is not applicable.

Untitled

  • Short Circuit Test (With using CT,PT)

 UntitledAcceptance Criteria:

  • Measured impedance to be within guaranteed value and nameplate value.
  • Load losses to be within guaranteed values.

 Test can detect:

  • Winding deformation.
  • Deviation in name plate value.

 (6) Open Circuit / No Load Test

 Test Purpose:

  • In this test, the value of No-Load power (Po) & the No-Load current (Io) are measured at rated voltage & frequency.

 Test Instruments:

  • Watt meters.
  • Ammeter , Voltmeter or
  • Power analyses

 Test Procedure:

  • Test is performed at rated frequency.
  • Three phase LT Voltage of 415 V applied on HV side of Power transformer keeping LT open
  • Two voltmeters are connected to the energized winding, one is measuring the voltage mean value and the other is for the Voltage R.M.S value.
  • Voltage applied to winding (usually to H.V. windings).It will be in a range from 90% of winding rated voltage to 110% of the same in steps, each of 5% (i.e. for a 33/11kV transformer, applied voltage values will be 29.7kV, 31.35kV,36.3kV)
  • Readings of watt meters, Voltmeters & Ammeters are recorded to obtain the values of V (r.m.s), Vmean, Po and Io at each voltage step.
  • Test results are considered satisfactory if the readings of the two are equal within 3%. If it’s more than 3%, the validity of the test is subjected to agreement.
  • Measured value of power loss is corrected according to the following formula:
  • Pc=Pm (1+d)
  • D= (Vmean – Vr.m.s) / Vmean
  • Measure the loss in all the three phases with the help of 3 watt meter method. Total no load loss or iron loss of the trf = W1 + W2 +W3

 Test Caution:

  • This test should be performed before the impulse test-if the later will be performed as a routine test- in order to avoid readings errors

 Acceptance Criteria:

  • No Load losses to be within guaranteed values.

 (7) Continuity test:

 Purpose of Test:

  • To know the continuity of windings of the transformer.

 Test Instruments:

  • Megger or
  • Multi meter.

 Test Procedure:

  • Check Continuity of Transformer by using multi meter or by Megger between following Terminals
Transformer P-P P-P P-P Result
HV Side R-Y Y-B B-R Zero Mega ohm or continuity
LV Side r-y y-b b-r Zero Mega ohm or continuity

Test can detect:

  • Open circuit / loose connection of winding

(8) Magnetic Current Test

 Test Purpose:

  • Magnetizing current test of transformer locates the defects in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problem in tap changers.
  • These conditions change the effective reluctance of the magnetic circuit, thus affecting the electric current required to establish flux in the core.

 Test Instrument:

  • Multi meter.
  • Mill Ammeter

 Test Circuit Diagram:

Untitled

  • Three phases LT Voltage of 415 V applied on HV side of Power transformer and currents are to be measured with mill ammeter.
  • The value shall be = (1 to 2 percent of rated full load current of TC / HT KV ) X Voltage Applied

 Test Procedure:

  • First of all keep the tap changer in the lowest position and open all IV & LV terminals.
  • Then apply three phase 415V supply on the line terminals for three phase transformers and single phase 230V supply on single phase transformers.
  • Measure the supply voltage and electric current in each phase.
  • Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position.
  • And repeat the test with keeping the tap at highest position.
  • Generally there are two similar higher readings on two outer limb phases on transformer core and one lower reading on the center limb phase, in case of three phase transformers.
  • An agreement to within 30 % of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory test, there is likelihood of a fault in the winding which needs further analysis.

 Test Caution:

  • This magnetizing current test of transformer is to be carried out before DC resistance measurement.

Various Routine Test of Power Transformer-(Part-2)


(3) Turns Ratio / Voltage Ratio Test:

 Test Purpose:

  • Turns Ratio Test / Voltage Ratio Test are done in Transformer to find out Open Circuited turns, Short Circuited turns in Transformer winding.
  • The voltage ratio is equal to the turn’s ratio in a transformer (V1/V2=N1/N2). Using this principle, the turn’s ratio is measured with the help of a turn’s ratio meter. If it is correct , then the voltage ratio is assumed to be correct
  • This test should be made for any new high-voltage power transformer at the time it is being installed.
  • With use of Turns Ratio meter (TTR), turns Ratio between HV & LV windings at various taps to be measured & recorded.
  • The turn’s ratio is measure of the RMS voltage applied to the primary terminals to the RMS Voltage measured at the secondary terminals.
  • R= Np / Ns
  • Where,
  • R=Voltage ratio
  • Np=Number of turns at primary winding.
  • Ns= Number of turns at secondary Winding.
  • The voltage ratio shall be measured on each tapping in the no-load condition.

 Test Instruments:

  • Turns Ratio meter (TTR) to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
  • Wheatstone Bridge Circuit

 Method No1 Turns Ratio Testing:

 Test Procedure:

  • Transformer Turns Ratio Meter (TTR):
  • Transformer ratio test can be done by Transformer Turns Ratio (TTR) Meter. It has in built power supply, with the voltages commonly used being very low, such as 8, 10 V and 50 Hz.
  • The HV and LV windings of one phase of a transformer (i.e. R-Y & r-n) are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
  • Values are recorded at each tap in case of tapped windings and then compared to calculated ratio at the same tap.
  • The ratio meter gives accuracy of 0.1 per cent over a ratio range up to 1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of the windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistors of the ratio meter.
  • Adjustment of the calibrated variable resistor until zero deflection is obtained on the galvanometer then gives the ratio to unity of the transformer windings from the ratio of the resistors.
  • Bridge Circuit:

Untitled

  • A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %
  • This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable
    transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. The reading on this indicator implies the deviation of measured turn ratio from expected turn ratio in percentage.
  • Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
  • % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected Turns Ration
  • Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current.
  • Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage.
  • But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage.
  • The turn ratio test of transformer also detects high resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

 Test Caution:

  • Disconnect all transformer terminals from line or load.
  • Neutrals directly grounded to the grid can remain connected

 Method No 2 Voltage Ratio Testing:

  •  This test is done to check both the transformer voltage ratio and tap changer.
  • When “Turns Ratio meter” is not available, Voltage Ratio Test is done at various tap position by applying 3 phases LT (415V) supply on HT side of Power transformer. In order to obtain the required accuracy it is usual to use a ratio meter rather than to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
  • At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with precision voltmeter & noted.

 Test Procedure:

  • With 415 V applied on high voltage side, measure the voltage between all phases on the low voltage side for every tap position.
  • First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
  • Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.
  • After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
  • Repeat the same for each of the tap position separately.
  • At other taps values will be as per the percentage raise or lower at the respective tap positions.
  • In case of Delta/Star transformers the ratio measure between RY-rn, YB-yn and BR-bn.
  • Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at normal tap is 33 KV OR 33x√3 = 5.196 ,11 KV / √3 11
  • At higher taps (i-e high voltage steps) less number of turns is in circuit than normal. Hence ratio values increase by a value equal to.5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100
  • Similarly for lower taps than normal the ratio is equal to 5.196 – {5.196 x (no. of steps above normal) x (% rise per each tap)}100

 Test Acceptance Criteria:

  • Range of measured ratio shall be equal to the calculated ratio ±0.5%.
  • Phase displacement is identical to approved arrangement and transformer’s nameplate.
  • The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one half of one percent of the nameplate readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed one half of one percent desired voltage .The ratio test verifies that these conditions are met.
  • The IEC60076-1 standard defines the permissible deviation of the actual to declared ratio
  • Principal tapping for a specified first winding pair: the lesser ±0.5% of the declared voltage ratio
  • or 0.1 times the actual short circuit impedance. Other taps on the first winding pair and other winding pair must be agreed upon, and must be lower than the smaller of the two values stated above.
  • Measurements are typically made by applying a known low voltage across the high voltage winding so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test .For three phase delta/wye or wye/delta transformer, a three phase equivalency test is performed, i.e. the test is performed across corresponding single winding.

 Test can detect:

  • Shorted turns or open circuits in the windings.
  • Incorrect winding connections ,and other internal faults or defects in tap changer

 

(4) Polarity / Vector group Test

 Purpose of Test:

  • The vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

 Test Instruments:

  • Ratio meter.
  • Volt Meter. A Ratio meter may not always be available and this is usually the case on site so that the polarity may be checked by voltmeter.

 Test Circuit Diagram:

 Untitled

 Test Procedure:

  • The primary and secondary windings are connected together at one point.
  • Connect neutral point of star connected winding with earth.
  • Low-voltage three-phase supply (415 V) is then applied to the HV terminals.
  • Voltage measurements are then taken between various pairs of terminals as indicated in the diagram and the readings obtained should be the phasor sum of the separate voltages of each winding under consideration.

 Condition:(HV side R-Y-B-N and LV Side r-y-b-n)

  • R and r should be shorted.
  • Apply 415 Volt to R-Y-B
  • Measure Voltage between Following Phase and Satisfy Following Condition
Vector Group Satisfied Following Condition

Dyn1

Rb=Rn+Bn
Bb=By
Yy<Yb

Dyn11

Ry=Rn+Yn
Yb=Yy
Bb<By

Ynd1

RN=Ry+Yn
By=Yy
Yy<Yb

Ynyn0

Bb=Yy
Bn=Yn
RN=Rn+Nn

Various Routine Test of Power Transformer-(Part-1)


Introduction:

  • There are various Test required on Transformer to conform performance of Transformer.
  • Mainly two types of transformer are done by manufacturer before dispatching the transformer mainly (1) Type test of transformer and (2) Routine test.
  • In addition some other tests are also carried out by the consumer at site before commissioning and also periodically in regular & emergency basis throughout its life.
  • Transformer Testing mainly classified in
  • Transformer Tests done by Manufacturer
  • (A) Routine Tests
  • (B)Type Tests
  • (C) Special Tests
  • Transformer Tests done at Site
  • (D) Pre Commissioning Tests
  • (E) Periodic/Condition Monitoring Tests
  • (F) Emergency Tests

(A) Routine tests:

  • A Routine test of transformer is mainly for confirming operational performance of individual unit in a production lot. Routine tests are carried out on every unit manufactured.
  • All transformers are subjected to the following Routine tests:
  • Insulation resistance Test.
  • Winding resistance Test.
  • Turns Ration / Voltage ratio Test
  • Polarity / Vector group Test.
  • No-load losses and current Test.
  • Short-circuit impedance and load loss Test.
  • Continuity Test
  • Magnetizing Current Test
  • Magnetic Balance Test
  • High Voltage Test.
  • Dielectric tests
  • Separate source AC voltage.
  • Induced overvoltage.
  • Lightning impulse tests.
  • Test on On-load tap changers, where appropriate.

 (B) Type tests

  • Type tests are tests made on a transformer which is representative of other transformers to demonstrate that they comply with specified requirements not covered by routine tests:
  • Temperature rise test (IEC 60076-2).
  • Dielectric type tests (IEC 60076-3).

 (C) Special tests

  • Special tests are tests, other than routine or type tests, agreed between manufacturer and purchaser.
  • Dielectric special tests.
  • Zero-sequence impedance on three-phase transformers.
  • Short-circuit test.
  • Harmonics on the no-load current.
  • Power taken by fan and oil-pump motors.
  • Determination of sound levels.
  • Determination of capacitances between windings and earth, and between windings.
  • Determination of transient voltage transfer between windings.
  • Tests intended to be repeated in the field to confirm no damage during shipment, for example frequency response analysis (FRA).

(D) Pre commissioning Tests

  • The Test performed before commissioning the transformer at site is called pre commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports.
  • All transformers are subjected to the following Pre commissioning tests:
  • IR value of transformer and cables
  • Winding Resistance
  • Transformer Turns Ratio
  • Polarity Test
  • Magnetizing Current
  • Vector Group
  • Magnetic Balance
  • Bushing & Winding Tan Delta (HV )
  • Protective relay testing
  • Transformer oil testing
  • Hipot test

 (A) Routine tests of Transformer

(1) Insulation Resistance Test:

 Test Purpose:

  • Insulation resistance test of transformer is essential to ensure the healthiness of overall insulation of an electrical power transformer.

 Test Instruments:

  • For LT System: Use 500V or 1000V Megger.
  • For MV / HV System: Use 2500V or 5000V Megger.

 Test Procedure:

  • First disconnect all the line and neutral terminals of the transformer.
  • Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance (IR) value in between the LV and HV windings.
  • Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth.
  • Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.
  • NB: It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.
  • Measurements are to be taken as follows:
Type of Transformer Testing-1 Testing-2 Testing-3
Auto Transformer HV-LV to LV HV-IV to E LV to E
Two Winding Transformer HV to LV HV to E LV to E
Three Winding Transformers HV to LV LV to LV HV to E & LV to E
  • Oil temperature should be noted at the time of insulation resistance test of transformer. Since the IR value of transformer insulating oil may vary with temperature.
  • IR values to be recorded at intervals of 15 seconds, 1 minute and 10 minutes.
  • With the duration of application of voltage, IR value increases. The increase in IR is an indication of dryness of insulation.
  • Absorption Coefficient = 1 minute value/ 15 second value.
  • Polarization Index = 10 minutes value / 1 minute value

 Tests can detect:

  • Weakness of Insulation.

 (2) D.C. Resistance or Winding Resistance Test

 Test Purpose:

  • Transformer winding resistance is measured
  • To check any abnormalities like Loose connections, broken strands and High contact resistance in tap changers
  • To Calculation of the I2R losses in transformer.
  • To Calculation of winding temperature at the end of temperature rise test of transformer.

 Test Instrument:

  • The Resistance of HV winding LV winding between their terminals are to be measured with
  • Precision milliohm meter/ micro ohm meter / Transformer Ohmmeter. OR
  • Wheatstone bridge or DC resistance meter.

 Method No: 1 (Kelvin Bridge Method for measurement of winding resistance)

 Untitled

Test Procedure:

  • The main principle of bridge method is based on comparing an unknown resistance with a known resistance.
  • When electric currents flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no electric current will flow through the galvanometer.
  • Very small value of resistance (in milliohms range) can be accurately measured by Kelvin Bridge method whereas for higher value Wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the error is minimized.
  • All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of winding resistance of transformer

 Method No: 2 (current voltage method of measurement of winding resistance) Untitled

Test Procedure:

  • The resistance of each transformer winding is measured using DC current and recorded at a ambient temp.
  • In this test resistance of winding is measurement by applying a small DC voltage to the winding and measuring the current through the same
  • The measured resistance should be corrected to a common temperature such as 75°C or 85°C using the formula: RC=RM x ((CF+CT)/(CF+WT))
  • where
  • RC is the corrected resistance, RM is the measured resistance
  • CF is the correction factor for copper (234.5) or aluminum (225) windings
  • CT is the corrected temperature (75°C or 85°C)
  • WT is the winding temperature (°C) at time of test
  • Before measurement the transformer should be kept in OFF condition at least for 3 to 4 hours so in this time the winding temperature will become equal to its oil temperature.
  • To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings.
  • Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching on and off the current circuit.
  • The readings shall be taken after the electric current and voltage have reached steady state values. In some cases this may take several minutes depending upon the winding impedance.
  • The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its resistance.
  • For Calculating resistance, the corresponding temperature of the winding at the time of measurement must be taken along with resistance value.

 Required Precaution:

  • According to IEC 60076-1, in order to reduce measurement errors due to changes in temperature, some precautions should be taken before the measurement is made.
  • For Delta connected Winding: for delta-connected transformer, the resistance should be measured for each phase (i.e. R-Y , Y-B & B-R) .Delta is composed of parallel combination of the winding under test and the series combination of the remaining winding .It is therefore recommended to make three measurements for each phase to-phase winding in order obtain the most accurate results.
  • For Delta connected windings, such tertiary winding of auto-transformers measurement shall be done between pairs of line terminals and resistance per winding shall be calculated as per the formula: Resistance per Winding = 1.5 X Measured Value
  • For Star connected winding: the neutral brought out, the resistance shall be measured between the line and neutral terminal (i.e. R-N , Y-N,B-N) and average of three sets of reading shall be the tested value. For Star connected auto transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral.
  • For Dry type transformers: the transformer shall be at rest in a constant ambient temperature for at least three hours.
  • For Oil immersed transformers: the transformers should be under oil and without excitation for at least three hours. In case of tapped windings, above readings are recorded at each tap. In addition, it is important to ensure that the average oil temperature (average of the top and bottom oil temperatures) is approximately the same as the winding temperature. Average oil temperature is to be recorded. Measured values are to be corrected to required temperatures.
  • As the measurement current increases, the core will be saturated and inductance will decrease. In this way, the current will reach the saturation value in a shorter time.
  • After the current is applied to the circuit, it should be waited until the current becomes stationary (complete saturation) before taking measurements, otherwise, there will be measurement errors.
  • The values shall be compared with original test an result which varies with the transformer ratings.

 Test Acceptance criteria:

  • DC Resistance Should be<=2% Factory Test.
  • Test Current <10% Rated Current

 Test can detect:

  • Short Turns
  • Loose Connection of bushing
  • Loose Connection or High Contact Resistance on Tap Changer.
  • Broken winding stands

Selection for Street Light Luminar-(PART-4)


(3) Low Pressure Sodium Lamp (LPS):

  • Low Pressure Sodium (LPS) lamp is by far the most efficient light source used in street lighting.
  • LOW Pressure Sodium is not an HID source.
  • IT is a gaseous discharge type lamp, similar in operations to fluorescent lamps.
  • While very efficient (160 lumens/watt), LPS lamps are monochromatic light source.
  • They produce only one light color, a dirty yellow color. That is CRI for LPS is negative.
  • When this type of lamp is first switched on, a small current passes through the gas giving off a faint red discharge.  After several minutes the sodium inside evaporates.
  • This makes colour perception very difficult which means that it is almost solely used for street lighting.
  • Light Color:Bright yellow color light
  • Advantage:
  • The Low Pressure Sodium lamp has the highest lamp efficacy of all sources
  • Disadvantages:
  • Lamps require special ballasts and increase material size as the wattage increases.
  • Large size makes it difficult to obtain good light control in a reasonably sized fixture.
  • For a long time the poor color rendition, when the lamp is on, everything around it looks either orange-yellow, black or shades in between them so LPS lamp made it unpopular for use in other than industrial or security applications.
  • The wattage (energy used) increases as time passes(Age of Lamp increased).
  • Application:Outdoor lighting i.e. street lighting, security lighting, Parking Light

 (4) LED:

  • These are the latest and most energy efficient options for street lighting.
  • Their brightness is much more uniform and can give up to 50% savings over Sodium Vapour lamps.
  • Advantages:
  • Produce less glare and can reduce visual fatigue for drivers and pedestrians.
  • Long and predictable lifetime
  • Reduced maintenance costs
  • Increased road safety
  • Low power consumption
  • Dimming can possible. adjusting to specific light levels
  • Reducing energy consumption and light pollution
  • Flexible, flat and compact lamp design
  • High color rendering (CRI)
  • LED lights are better at focusing light in the downward direction so less light is lost in the air and surrounding environment
  • Disadvantages:
  • Very expensive to buy with longer paybacks.
  • They also LEDs offer the following advantages when used as light sources in street lighting applications.
  • Adequate heat-sinking is required to ensure • long life with high-powered LED.
  • Light Color: LED Produce more natural white / yellow light.
  • Warm up Time: Quick turn on / off .No problem with hot ignition. Turn on / off without time delay

 

Lamp Power (watt) Efficiency (lm/w) Life (Hr) CRI CRI Status
Inductance 100 to 150 100 100000 60 to 70 Good
HPSV 50 to 400 39 to 140 24000 20 to 30 Poor
HPMH 35 to 400 70 to 90 60000 60 to 70 Good
HPMV 50 to 400 35 to 90 100000 40 to 60 O.K
LPSV 18 to 180 100 to 160 200000 Less than 20 Very Poor
Florescent 18 to 57 50 to 80 90000 40 to 90 Good
LED 112 55 500000 20 to 95 Good

 

Advantage & Disadvantage of Luminar

Type of Lamp Advantage Disadvantage
High Pressure Sodium Vapor Lamp (HPSV) Long lamp life,Highestlamp output. High initial cost. Poorcolor rendering, cycles on and off at end of life, not dimmable, cannot use electronic ballast
High Pressure Metal Halide Lamp (HPMH) Moderately long lamp life. High light output.Makes colors look close to natural. High initial cost.
High Pressure Mercury Vapor Lamp (HPMV) Long lamp life, High light output. High initial cost.
Low Pressure Sodium Vapor Lamp (LPSV)   Completely monochromatic,lends no color perception,

shorter life than HPS,

optical control difficult

Florescent Long lamp life, High light output. Low brightness. High initial cost.Frequent switching cuts life,

needs ballast,

Runs poorly in cold temperatures

LED Long life, very efficient, can be dimmable,can offer excellent color quality (w/ less efficiency) Very high initial cost,very sensitive to overheating, requires large heat sinks,

variable color and quality

  Controlling of Street Light Glare /Shielding of Light:

  • As the vertical light angle increases than disability and discomfort glare also increase. To distinguish the glare effects on the driver created by the light source, IES has defined the vertical control of light distribution as follows:
  • The amount of light emitted upward or lower side of laminar and at high or low angle is called shielding of Lights (“Cut off”). It is classified on how much of light is dispersed above the horizontal line of luminaries.
  • The Cutoff means amounts of light above 90 degrees, but it is generally agreed that the light should be no more than the value at 90 degrees, and should be decreasing as the angle increases. In fact, there could be some measurable light emitted at 180 degrees (Zenith
  • There are Four Type of arrangement of Luminaries (1) non cutoff, (2) semi cutoff,(3)cutoff, (4)full cutoff.

 Untitled

 (1) Non-cutoff:

  • Fixture Arrangement:
  • The non-cutoff fixtures usually include the globe-shaped lamps that are mounted on top of lampposts.
  • These lamps distribute their light in all directions.
  • Disadvantages:
  • A major problem is created by the light pollution and glare, as they shoot their light upwards into trees and towards the sky rather than down towards the ground.
  • Non-cutoff fixtures are rarely found on roadways because they tend to blind the driver.

(2) Semi cutoff:

  • Fixture Arrangement:
  • Most of the light can be emitted below 90 degrees but 5% of the light can emitted above 90 degrees of Fixtures and 20 % or less emitted at the 80 degree angle of nadir
  • Advantage:
  • These fixtures do a very good job of spreading the light towards the ground but some up light is possible, though not as serious as non-cutoff fixtures.
  • Semi cutoff fixtures are often mounted on tall poles.
  • This is the most popular street lighting, lighting distribution arrangement. The semi cutoff fixtures usually refer to the cobra heads, but they can also apply to some lamppost-mounted fixtures that do not emit their light upwards.
  • Disadvantages:
  • Little control of light at property line. Potential for increased glare when using high wattage luminaries.
  • Typically directs more light into the sky than cut-off.

(3) Cutoff:

  • Fixture Arrangement:
  • Less than 2.5% of the light can leave the fixture above 90 degrees and 10 % or less emitted at the 80 degree angle of nadir
  • Advantage:
  • This type light gives more light control than semi cutoffs.
  • The cutoff lights have a wider spread of light than full-cut offs, and they generate less glare than semi cutoffs. The cutoff lenses consist of a shallow curved glass (also called a sag lens) that is visible just below the lighting area on the fixture
  • Cutoff fixtures have gained popularity in recent years.
  • Small increase in high angle light allows increased pole spacing.
  • Disadvantages:
  • Allows some up light from luminaries. Small overall impact on sky glow.

(4) Full-cutoff:

  • Fixture Arrangement:
  • These lights do not allow any of the light to escape the fixture above 90 degrees (90 degrees above nadir).
  • Zero light emitted above a horizontal plane drawn through the lowest part of the luminaries, no more than 10% of light emitted at the 80 degree angle above nadir. Also known as “fully shielded.”
  • Advantage:
  • Full-cut offs distribute light in a defined pattern, potentially providing more light on the ground at lower power consumption.
  • Full cutoff luminaries are totally environmentally friendly (causing no light pollution).
  • Limits spill light onto adjacent property, reduces glare.
  • Disadvantages:
  • May reduce pole spacing to maintain uniformity and increase pole and luminaries quantities

Cable Tray Size as per National Electrical Code-2002. Article 392


(I) No of Multi core Cables less than 2000 volts in the Cable Tray

 (A) 4/0 AWG/Kcmill (120 Sq.mm) Cable or Larger Cables:

  • The ladder cable tray: Tray must have an inside available width equal to or greater than the sum of the diameters of the cables, which installed in a single layer.
  • Solid bottom cable tray: the sum of the cable diameters is not to exceed 90% of the available cable tray width.

 (B) Cables Smaller Than 4/0 AWG/Kcmill (120 Sq.mm)

  •  Ladder Type Cable Tray: The total sum of the cross-sectional areas of all the cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as per following Table.
  • Solid Bottom Cable Tray: The allowable cable area is reduced by 22%.
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 7 Sq.inch (4516 Sq.mm)
9 inch (228.6mm) 10 Sq.inch (6451 Sq.mm)
12 inch (304.8mm) 14 Sq.inch (9032 Sq.mm)
18 inch (457.2mm) 21 Sq.inch (13548 Sq.mm)
24 inch (609.6mm) 28Sq.inch (18064 Sq.mm)

(C) 4/0 AWG (120 Sq.mm) or Larger Cables Installed with Cables Smaller than 4/0 AWG (120 Sq.mm)

  • Ladder Type Cable Tray: The ladder cable tray needs to be divided into two zones (a barrier or divider is not required but one can be used if desired) so that the No. 4/0 and larger cables have a dedicated zone, as they are to be placed in a single layer.
  • A direct method to determine the correct cable tray width is to figure the cable tray widths required for each of the cable combinations per steps (2) & (3). Then add the widths in order to select the proper cable tray width.

(D) Multi conductor Control and Signal Cables Only

  • Ladder Type Cable Tray: A ladder cable tray containing only control and/or signal cables may have 50% of its total available cable area filled with cable.
  • Solid Bottom Cable Tray: When using solid bottom cable tray, the allowable cable area is reduced from 50% to 40%.

(II) No of Single conductor Cables < 2000 volts in the Cable Tray (NEC-392.12)

  • All single conductor cables to be installed in the cable tray must be larger than 1/0 AWG (53.5 Sq.mm) and not to be installed with Solid Cable Tray.

 (A) 1000 Kcmill (500 Sq.mm) or Larger Cables

  • The sum of the diameters (Sd) for all single conductor cables to be installed shall not exceed the cable tray width as per following Table.
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(2) 250 Kcmil (120 Sq.mm) to 1000 Kcmil (500 Sq.mm) Cables

  • The total sum of the cross-sectional areas of all the single conductor cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as given in following Table
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(3) 1000 Kcmil (500 Sq.mm) or Larger Cables Installed with Cables Smaller Than 1000 Kcmil (500 Sq.mm)

  • The total sum of the cross-sectional areas of all the single conductor cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as given in following Table
Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm)
6 inch (152.5mm) 6.5Sq.inch (4194 Sq.mm)
9 inch (228.6mm) 9.5 Sq.inch (6129 Sq.mm)
12 inch (304.8mm) 13 Sq.inch (8387 Sq.mm)
18 inch (457.2mm) 19 Sq.inch (12258 Sq.mm)
24 inch (609.6mm) 26Sq.inch (16774 Sq.mm)
30 inch (762mm) 32.5Sq.inch (20968 Sq.mm)
36 inch (914.5mm) 39Sq.inch (25161Sq.mm)

(4) Single Conductor Cables 1/0 (50Sq.mm) to 4/0 (120Sq.mm)

  • These single conductors must be installed in a single layer.
  • Note: It is the opinion of some that this practice may cause problems with
  • To avoid these potential problems due to unbalanced voltages, the individual conductors for this type of cable tray wiring system should be bundled with ties. The bundle should contain all of the three phase conductors with the neutral if used.
Single conductor Size Cable Tray width
152mm 228mm 304mm 457mm 609mm
1/0AWG (50Sq.mm) 10 15 20 31
2/0AWG (70Sq.mm) 9 14 19 29
3/0AWG (9550Sq.mm) 8 13 17 26
4/0AWG (120Sq.mm) 8 12 16
250kcmill (120Sq.mm) 11 18 24
350kcmill (185Sq.mm) 9 14 19
500kcmill (240Sq.mm) 7 11 14
750kcmill (400Sq.mm) 5 8 10
1000kcmill (500Sq.mm) 4 6 8

(III) No of Cables more than 2.1 KV in the Cable Tray

  • The sum of the diameters of all cables rated 2001 volts or over, is not to exceed the cable tray width.

(IV) Barrier Requirements (NEC 392.6)

  • Barrier is used to separate cable systems, when Single Core cables of above and below 600 volts installed in the same cable tray. But when Multi Core type cables over 600 volts are installed in the same cable tray with cables rated 600 volts or less, no barriers are required.
  • The barriers should be made of the same material as the cable tray and height must equal to the depth of the cable tray.

Calculate Size of Conduit


ScreenHunter_01 Jan. 04 20.28

  • Calculate Total Area of Multiple Cables.
  • Calculate Total Area of Conduit for Multiple Cables.
  • Calculate Fill up area of Conduit.
  • Calculate % Fill up area of Conduit.
  • Calculate No of Conduits for Multiple Cables.
  • Calculate Size of Conduits for Multiple Cables.

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