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.


  • 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.


  • 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:


  • 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.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

11 Responses to Various Routine Test of Power Transformer-(Part-3)

  1. peter says:


  2. Dear Sir,

    we are installing LV switch board which can be fed by 3Nos of DG set of low voltage with paralleling between three DG set on requirement and infuture momentary paralleling with NEPA grid now the question is no of pole for DG incomer and ACB..can it be three pole or it has to be four pole? can i connect all neutral together? where my PEN has to be earthed?

  3. Vasantha Joshi says:

    Dear Sir the People working on High tension Substations should have first class supervisor License or if he is an B E in Electrical Engineer or Diploma in Electrical engineer need not have first class supervisor license.clarification required urgently

  4. hanumanthappa.A anjinappa says:


    I wanted to know, how to calculate the HT meter cubical constant and energy meter constant….

    Thanks, Hanumanth

  5. Romnick Hilario says:

    Kindly send me a schematic drawing of motor stater connection.this info can gave me a additional knowledge that can help me and my sub-member.regards

  6. Monir Hossain says:

    How can calculated earth wire size for LT Pannel, MDB, DB, SDB and machine earthing? Please send details any link.

  7. mahendra rathod says:

    Good information Dear sir,


  8. lk says:

    dear jignesh, interested in a lucrative middle east job ???

  9. Vishal says:

    Realy I am new learner in electrical field.

    Thank you very much. Your guideline is helpful to me for designing of such type system


  10. Malik Imtiaz says:


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