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.

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  • 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.

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  • 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:

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  • 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.
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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:

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  • 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
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