Power Factor Dry Type Transformer Testing

 

12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Maintenance / Testing and Commissioning / Transformers

Performing power factor test on dry-type transformer during commissioning Power factor test The power factor test is a maintenance test used to determine the insulation system dielectric power  loss  by by measuring measuring the the power power angle between an applied AC voltage and the resultant current.

Performing Performing power power factor factor test test on on dry-type dry-type transformer transformer during during commissioning commissioning (photo (photo photo credit: credit: sgb-smit.com) sgb-smit.com)

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Power factor is defined as the ratio of the power dissipated divided by the input volt-ampere multiplied by 100% .

This test may be required required to to be be performed performed during during the the acceptance testing stage to establish a baseline reading for future test comparison.

Figure Figure 11 –– Insulation Insulation System System Equivalent Equivalent Circuit Circuit and and Power Power Factor  Factor Factor  Vector Vector Diagram Diagram

Dielectric Dielectric losses losses in in and and power power factor factor can can be calculated by:

Watts  = = E × IT  × × Cosine Ө   Power factor  =  = Cosine Ө = Watts / (E × IT) Power factor test is performed performed for for detecting insulation deterioration or degradation  usually usually caused caused by moisture , carbonization or other forms of  contaminants of the winding and bushing. Winding distortions results in a change in winding capacitance and short-circuited and partially short-circuited turns result in abnormally high excitation current. Types of transformers that are normally subjected to the power factor test are: https://electrical-engineering-portal.com/dry-type-transformer-power-factor-test

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

1. Two winding transformers 2. Three winding transformer   Auto-transformers rmers 3.  Auto-transfo  Auto-transforme rs 4. Instrument transformers

Other test names names synonymous synonymous with the power factor test are dielectric loss angle, dissipation factor test, tan delta or doble test.

General conditions required for testing transformers are: 1. The unit must be de-energized and isolated from the power system including the neutral connection from ground. 2. Transformer enclosure must be properly grounded  and and applicable applicable to to when when testing spare units. 3.  All terminal of each winding are short-circuit short-circuited ed together including the neutral terminals. This will minimize the effect of winding inductance   during testing. 4. Load tap changer (LTC)  should should be be set of neutral if it has arrester-type elements that are not effectively short circuited in the neutral position.   The power factor factor test test typically typically applies a test test voltage voltage less than the stress working levels of the equipment. Refer to Table 1. Table 1 – Recommended power factor test voltage for dry-type power transformer connected in delta and ungrounded-wye

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Winding

Test Voltage  Voltage  

rating

Line-to-ground, kV

Line-to-line, kV Above 14.4

2 and 10

12 to 14.4

2, 10 and at operating line-to-ground voltage

5.04 to 8.72

2 and 5

2.4 to 4.8

2

Below 2.4

1

Note //

Transformer windings  Transformer windings provided with neutral insulation rating which is less than the line insulation rating should be tested below the neutral insulation rating level. The required tests are noted in Table 2 and its connection is shown in Figure Figure 22 and and Figure Figure 3. 3. The The difference between the high voltage winding tests and the low voltage winding tests  are are the the placement of the test leads and the test voltage levels. Test 3 and test 8 of Table 2 should generate the same value as they both measure the same capacitance capacitance between between the the windings. Table 2 – Power Factor test connection for two winding dry-type transformer 

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Test

Test Energized Ground Grou Gr ound nd G Guar Gu ua ard rd US UST T Measur  Measu sur  r 

number  mode

winding

1

GST

High

Low

 –

 –

CH+CH

2

GST

High

 –

Low

 –

CH

3

UST

High

 –

 –

Low

CHL

4

Calculate Test 1 minus Test 2

CHL

5

GST

Low

High

 –

 –

CL+CH

6

GST

Low

 –

High

 –

CL

7

UST

Low

 –

 –

High

CHL

8

Calculate Test 5 minus Test 6

CHL

HV winding power factor test 1. Isolate transformer  2. Isolate Neutral Connection 3. Install shorting jumpers on H1, H2 and H3 4. Install shorting jumpers on X1, X2, X3 and X0 5.  Apply Specified test voltage at ramp rate in 15 seconds 6. Test at specified voltage for 1 minute 7. Reduce test voltage to zero at ramp rate in 5 seconds

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Figure Figure 22 –– Power Power Factor Factor High High Voltage Voltage Winding Winding Test Test Connection Connection

LVV winding power factor test 1. Isolate transformer  2. Isolate Neutral Connection 3. Install shorting jumpers on H1, H2 and H3 4. Install shorting jumpers on X1, X2, X3 and X0 5. Perform a GST test 6. Perform a Guard test test 7. Perform an UST test 8. Confirm capacitance value from GST test minus Guard test equal UST test

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Figure Figure 33 –– Power Power Factor Factor Low Low Voltage Voltage Winding Winding Test Test Connection Connection

 

Caution //

   Always ground to the previously energized terminal with a grounding stick before making any connection changes to bleed off any electrical charge that may be present. Leave the grounding connected until connection changes is completed and before the start of the next test.

 

Power factor test procedure (Two winding dry-type transformer) 1. Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals. Disconnected cables should have sufficient clearance from the https://electrical-engineering-portal.com/dry-type-transformer-power-factor-test

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

switchgear terminals greater that the phase spacing distance.  

Use nylon rope  to to hold cable away from incoming and outgoing terminals as required.

2. Isolate the neutral bushing connection if  applicable from the transformer grounding bar. 3. Short-circuit all high voltage bushing terminals together. 4. Short-circuit all low voltage bushing terminals and the neutral bushing terminal together. 5. Connect the power factor test set. Refer to Table 2 for the test measuring mode and associated test number. 6.  Apply the specified test voltage levels as noted in Table 2. 7. Record power factor and watts watts loss loss values. values. 8. Repeat step 5 to 7 until all tests are completed   Table 2 -Standard kVA Ratings Ratings for for Dry-Type Dry-Type Transformers

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Single

Three Thre e phase phase Sing Single le phase phase Three Three phase phase

phase 2

6

333

1000

3

9

500

1500

5

15

667

2000

10

30

833

2500

15

45

1000

3000

25

75

1250

3750

37

112

1667

5000

50

150

2500

7500

75

225

3333

open-ended

100

300

5000

—

450

open-ended

167

500

 –

250

750

 –

 

Measuring capacitance and power factor or dissipation factor The condition of the bushings and the overall insulation of power transformers can be investigated by measuring the capacitance and dissipation factor, also known as the tangent delta, or power factor. Aging and decomposition of the insulation, or the ingress of water , increases the losses and thus more energy is turned into heat in the insulation. The level of this this dissipation dissipation is expressed expressed by the the dissipation dissipation factor or power factor.

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

 

Power factor testing In this video you will learn about means to measure the quality of the insulation of power transformers and bushings, such as power factor or dissipation factor  measurement, and measuring capacitance. Possible causes for a reduction in insulation Possible causes for a reduction in insulation quality are also explained.

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

 

TanDelta/Power Factor Testing TanDelta/Power

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12/25/2018

Performing power factor test on dry-type transformer during commissioning | EEP

Substation Reference //  Substation

commissioning commissioning course course – Drytype transformers by Raymond Lee, Technical Trainer 

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (