Installation, Testing and Troubleshooting of Transformers

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INSTALLATION, TESTING AND TROUBLESHOOTING

OF TRANSFORMERS


Introduction• A number of standards define the functional and testing

requirements for power and distribution transformers.

• Some standards for transformers are:

IEC-60076 Power Transformer

BS 7821-4

BS 7844-2


Kinds of Tests• Routine Tests - Tests to which each individual

transformer is subjected

• Type Tests - Tests made on a transformer (representative of other transformers) to demonstrate these transformers comply with specific requirements not covered by routine tests

• Special Tests - Tests (other than a type test or a routine test), agreed upon by the manufacturer and the purchaser


Routine Tests • Routine tests of a transformer cover the following:

Measurement of winding resistance Measurement of voltage ratio and check of voltage vector

relationship Measurement of impedance voltage/short-circuit impedance and

load loss Measurement of no load loss and current Measurement of insulation resistance Dielectric routine tests Test on on-load tap changer


Special Tests• These tests include:

Dielectric special test Determination of capacitance windings-to-earth, and between windings Determination of transient voltage characteristics Measurement of zero-sequence impedances on three-phase transformers Short-circuit withstand test Determination of sound levels Measurement of harmonics of the no-load current Measurement of the power taken by the fan and oil pump motors Measurement of insulation resistance to earth of the windings, and/or

measurement of dissipation factor (Tan δ) of the insulation system capacitances


General Requirement of Tests• Tests should be made:

– At any ambient air temperature below 50ºC, with cooling water (if required) at any temperature not exceeding 30ºC

– At manufacturer’s works

• All external components and fittings likely to affect performance should be in place

• Tapped windings should be connected on their principal tapping

• The test basis for all characteristics, other than insulation, is the rated condition

• When required, test results should be corrected to a reference temperature of 75ºC


Transformer Tests – Measurement of Winding Resistance

• Measurement of winding resistance is done across the terminals through balanced bridge (Wheatstone Bridge or Kelvin Bridge) configurations

• Sufficient time should be given to ensure resistance reaches a steady state value

• Time taken may be longer if the winding inductance is higher


Measurement of Voltage Ratio

• The voltage ratio should be measured on each tapping

• The polarity of single-phase transformers and vector symbols of three phase transformers should be checked

• Test ensures secondary voltage displacements are as per specifications so the connected protective devices operate correctly


Vector Group Notation

• Internal connection of the HV winding indicated by capital letter D, Y or Z; D for Delta, Y for Star & Z for Zigzag

• Similarly, low voltage winding side indicated by small letter (i.e. d, y, z)

• In the vector group notation, if the first letter is capital, it indicates that higher voltage winding (HV) is placed first

• The clock position indicating the phase angle between the high voltage and the low voltage is placed in the third position.


Vector Group Notation• For example, vector notation Dy11 indicates:

HV (High Voltage) is delta-connected

LV (Low Voltage) is star connected

LV phasor is in 11 o’clock position when HV phasor is in 12 o’clock position, i.e. LV phasor leads the HV phasor by 30º as the direction-of-rotation of vectors is anticlockwise

Thus Dy11 connection is +30º connection

Similarly, Dy1 connection is –30º connection


Vector Presentation of a Dy11 Transformer

• 1U• 11•

2u• • 2v• 2w• 1W 1V


Vector Presentation when 1U & 2U are Shorted

• 1U, 2U• • 2v • • • 2W• • • • 1W 1V


Measurement of Impedance Voltage/Short-Circuit Impedance & Load Loss

• Losses at rated current will be =

2

CurrentTest

CurrentRatedcurrenttestatloss Measured


Measurement of Impedance Voltage/Short-Circuit Impedance & Load Loss (contd…)

• Percentage Impedance will be =

100CurrentTest

CurrentRated

VoltageRated

VoltageTest


Measurement of No Load Loss & Current• Should be measured at rated frequency at a voltage equal to rated voltage if

the test is performed on the principal tapping

• Or should be equal to approximately tapping voltage if the test is performed on another tapping

• If the voltages U’ and U are not the same, the measured value of no-load loss is corrected according to the formula:

where,• P = Corrected Value,

• = Ratio of hysteresis loss to total iron loss,

• = Ratio of eddy current loss to iron loss, and

• K =

2KP

1P

mP

P

2

U'

U


Measurement of Insulation Resistance

• The insulation resistance of each winding, in turn, to all other windings, core and frame or tank connected together, and to earth should be measured and recorded

• The insulation resistance tests are carried out between phases-to-ground, neutral-to-ground, primary-to-secondary with 500 V/ 1000 V/ 2000 V/ 5000 V meggers depending upon the voltage ratings


Dielectric Tests

• Insulation of auxiliary winding

• Separate source voltage withstand test

• Induced over voltage withstand test

• Lightning impulse test


Temperature Rise

• Temperature rise: type test, optional, and carried out on units if client specifies

• Though a type test, the temperature rise within the agreed limits will give a clear condition of the transformer under service conditions

• Basically allows a full current load to be passed through the windings until the thermometer readings reach steady state values


Temperature Rise – Loading Methods

• Loading methods for two-winding transformers: Direct loading method Back-to-back method Short-circuit method

• Loading methods for multi-winding transformers: Short-circuit method


Temperature Rise Limits – For Dry TypeSl No

Part Cooling Method

Insulation Class

Temperature Rise 0C

1 Windings Air, Natural or Forced

A

E

B

F

H

C

50

65

70

90

115

140

2

Cores and other parts

a) Adjacent to windings All

- Same values as for windings

b) Not adjacent to windings All

- Not reach a value that will damage core or adjacent materials.


Temperature Rise Limits – For Oil Immersed Type

Sl No Part

Temperature Rise

External Cooling Medium

Air Water

1 Windings (Temp. Class of Insulation A)

550C, when the oil circulation is natural or forced non-directed.

600C, when the oil circulation is natural or forced non-directed.

600C, when the oil circulation is forced directed.

650C, when the oil circulation is forced directed.

2 Top – Oil

500C, when the transformer is equipped with a conservator or sealed.

550C, when the transformer is equipped with a conservator or sealed.

450C, when the transformer is neither equipped with a conservator nor sealed.

500C, when the transformer is neither equipped with a conservator nor sealed.

3

Cores, Metallic parts and adjacent materials

The temperature should be in no case reach a value that will damage the core itself, other parts or adjacent materials.


Testing on On-Load Tap Changers • Eight complete operating cycles with the transformer not energized, with

100% of rated aux supply voltage

• One complete operating cycle with transformer not energized, with 85% of the rated auxiliary supply voltage

• One complete operating cycle with transformer energized at the rated voltage and frequency at no-load

• Ten tap-change operations with ±2 steps on either side of the principal tapping with as far as possible the rated current of the transformer with one winding short-circuited


Tests on the Transformer• A transformer is supposed to have passed the test if,

No faults result internally during the tests Dielectric tests are successfully repeated No visual defects on windings, supports and tank structure No traces of electric discharges Reactance measurements after the tests are within 2% of the actual

value after each shot


Short - Circuit Test • For a single transformer, number of tests should be three; the

duration of each test = 0.5 second with a tolerance of ±10

• For a three phase transformer, total number of tests should be nine; three tests on each limb; the duration of each test being 0.5 second with a tolerance of ±10%


Magnetic Balance Test

• A delta connected HV Winding of three-phase transformer is given below:

• U V W

•


Characteristics of Insulating OilSl No Characteristic Requirements

1 Appearance The oil should be and transparent and free from suspended matter or sediment.

2 Density at 27˚C, Max 0.89 g/cm3

3 Kinematics Viscosity at 27˚C, Max 27 c-st

4 Interfacial tension at 27˚C, Min 0.04 N/m

5 Pour point Max –9˚C

6 Neutralization Value

a) Total Acidity

b) Inorganic Acidity/ Alkalinity

0.03 mg KOH/g

NIL

7 Flash Point Pensky-Marten (closed), Min 140˚C

8 Corrosive sulphur (in terms of classification of copper strip)

Non-corrosive


Characteristics of Insulating Oil Sl No Characteristic Requirements

9 Electric Strength (break-down voltage), Min.

a) New Insulating Oil

b) After Treatment

30 kV(rms)

if the above value is not attained, the oil should be treated.

50 kV(rms)

10 Dielectric dissipation factor (tan δ) 0.005

11 Specific Resistance (Resistivity)

a) At 90˚C, Min

b) At 27˚C, Min

13×10¹² Ω-cm

500×10¹² Ω-cm

12 Oxidation Stability

a) Neutralization Value, after oxidation, Max

b) Total sludge, after oxidation, Max

0.40 mg KOH/g

0.10 percent by weight

13 Presence of oxidation inhibitor The oil should not contain antioxidant additives

14 Water Content, Max 50 ppm


Testing of Insulating Oil• Dielectric Strength Test

• Total Acidity

• Pour Point Test

• Flash Point

• Viscosity

• Crackle Test

• Resistivity of the Oil

• Interfacial Tension

• SK Value


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Installation, Testing and Troubleshooting of Transformers