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
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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
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