Is your power transformer installed in area where lightning is frequently occurring? During forensic investigation of your failed transformer, is the damaged region near the top of the winding? If it is true, is there a technical solution to address this observation?

Is lightning impulse a good example of partial discharge. ⤵ What do you think ?

Partial Discharge Measurement and Simulation from CWIEME



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The De-Energized Tap Changer (DETC) of a 3 phase power transformer was set at primary voltage of 67 kV and the OnLoad Tap Changer (OLTC) at secondary winding is set at 13.75 kV as the rated voltage also called the secondary voltage. If you look at transformer nameplate, the neutral voltage is indicated by letter "N"; Calculate the Voltage Ratio at the given DETC & OLTC tap settings. This voltage ratio or turn ratio is your baseline value for a healthy transformer with respect to normal winding turn to turn ratio. If there is no turn to turn short circuit on your primary windings or secondary windings the voltage ratio value or turn ratio value should remained very close to your voltage ratio baseline value. Assuming that the instrument was properly calibrated.

Problem #1, Valid for 3 Φ , Δ - Y only, using voltage measurement to solve for turn ratio

Δ - Y turn ratio / √ 3 = for 3 Φ , Y - Δ turn ratio

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If your primary winding is starting arcing or shorting due to dielectric paper or oil insulation breakdown, you will expect to have a trending of lower turn ratio (a) in your TTR measurement compare to factory test report. It is also indicated by primary voltage reading trending towards lower value. Lower your primary voltage from 67 kV to 49 kV. This will lower your turn ratio from normal value of 8.43 to abnormal value of 6.17 (a = 6.17). When you notice your primary voltage trending down to 49 kV it could be a good indicator for possible primary winding is developing a shorted turn to turn winding condition. Remember the fundamental principle that induced voltage is directly proportional to frequency and number of turns of coil conductors in HV and LV windings. if your winding turn number (N) value is trending downward because of turn to turn shorting, then your voltage measurement will be your confirmation evidence. The fundamental physics principle of induced voltage says if your number of turns of your winding coils is decreasing due to turn to turn fault, then you must expect to see the induced voltage to go down also.

V ∞ f * N [ induced voltage is directly proportional to frequency and number of coil turns ]

Problem #2, Valid for 3 Φ , Δ - Y only

Given three phase turn ratio 8.4396 and Secondary voltage 13.75 kV; Calculate the expected primary voltage

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If your secondary winding is shorted, you will expect to have a higher turn ratio (a) in your TTR measurement compare to factory test report. It is also indicated by lower secondary voltage reading. Use the calculator below to do simulation. What will happen to your secondary voltage if your TTR reading goes up to 11.521?

Problem #3, Valid for 3 Φ , Δ - Y only

Given a three phase turn ratio 8.4396 and Primary voltage 67 kV; Calculate the expected secondary voltage

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Knowing the factory report of voltage ratio (TTR) of the above 3 phase transformer as 8.4396 your baseline for a healthy condition, what do you think happen to the same transformer if the voltage ratio (TTR) from the latest test report is 7.3163 ? Can you estimate the coil location of the Turn to Turn winding short? Can you tell which side of the winding (HV or LV) the Turn to Turn winding short happen? Can you make a generalization about the location of the turn to turn winding short if the TTR test report value is less than the baseline TTR value?

Knowing the factory report of voltage ratio (TTR) of the above 3 phase transformer as 8.4396 your baseline for a healthy condition, what do you think happen to the same transformer if the turn to turn ratio (TTR) from the latest test report is 9.1247? Can you estimate the coil location of the Turn to Turn winding short? Can you tell which side of the winding (HV or LV) the Turn to Turn winding short happen? Can you make a generalization about the location of the turn to turn winding short if the TTR test report value is greater than the baseline TTR value?

Problem #4, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given single phase voltage ratio of 4.8727 and Secondary voltage 13.75 kV; Calculate the expected primary voltage

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Problem #5, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Turn Ratio, a , single phase 5.333 and Primary voltage 69 kV; Calculate the expected secondary voltage

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Assuming from your transformer design review you learned that the primary coil winding turns is 484 for 67 kV tap setting on your DETC and the secondary coil winding turns is 90 to get 12.47 kV and set at Neutral tap setting on your OLTC. Calculate your transformer turn to turn ratio using the given number of turns as your baseline for a healthy condition.

Problem #6, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given primary coils number of turns = 484 and secondary number of turns = 90

Given primary voltage 525 kV and secondary voltage 230 kV ; 3 Φ Y-Y

Calculate the expected Turn to Turn Ratio (TTR).

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Problem #7, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Turn ratio = 5.3778 and Secondary number of turns = 90; Calculate the expected primary number of turns of coil. Now assuming your TTR value a = 5.00, what is the expected number of turns now in primary winding? How many coil turns have been bypass or short circuited from original 484 turns? Is it possible to locate where they are in your primary winding? Possibly with acoustic triangulation methods of diagnostic.

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Problem #8, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Turn Ratio, a , single phase = 5.3778 and Primary number of turns = 484; Calculate the expected secondary number of turns of coil. How can you tell if the turn to turn short circuit happen in primary winding? Answer, usually if the turn ratio value goes down from the baseline value. For example the baseline value of a = 5.3778 then it goes down to 5.00 what will happen when you enter new value by changing the value of 5.3778 to 5.00? The secondary coil number of turns goes up to 96.8 from original coil turn number of 90. At this point, you know immediately the short circuit happens in the primary winding. How? Because you can't add additional turn in your existing secondary transformer coil. It's number will always stay at 90 turns of coil if nothing is shorted on the secondary winding. Now you know the fault happens in primary winding, try changing the number of turns of your primary coil winding until the value of your secondary coil turns is 90. Try 450, you should get 90 on your secondary coil turns. At this time you do the math and you know there were 34 turns in your primary winding that was compromised. It's location could be anywhere in the primary winding side. VERY IMPORTANT: YOU SHOULD BE CAREFUL IF YOUR HAVE DE-ENERGIZED TAP CHANGER (DETC) OR LOAD TAP CHANGER (LTC) because there is a possibility that the tap settings are changed and it will result in different turn ratio. You should have a baseline record of turn ratio at different tap setting.

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Problem #9, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Secondary FLA = 1296.41 and Primary FLA = 241.28;

Given Primary voltage = 525 kV and Secondary = 230 kV ; 3Φ Y - Y

Calculate the expected turn ratio

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Problem #10, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Turn Ratio, a , single phase = 5.3778 and Primary FLA = 241.28; Calculate the expected secondary full load ampere (FLA)

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Problem #11, Valid for 1 Φ only or 3 Φ Y-Y or 3 Φ Δ - Δ

Given Turn Ratio, a , single phase = 5.5334 and Secondary FLA = 1296.41; Calculate the expected primary full load ampere (FLA)

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Problem #12, Valid for 3 Φ only

Given 3 Phase MVA rating = 28 and Secondary voltage = 69 kV; Calculate the expected primary full load ampere (FLA)

3 Phase MVA RATED POWER TRANSFORMER

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Problem #13, Valid for 3 Φ only

Given Secondary FLA = 1296.41 and Secondary voltage = 12.47 kV; Calculate the expected 3 Φ MVA rating of transformer

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Problem #14, Valid for 3 Φ only

Given 3 phase kVA = 1000 and Primary voltage = 480; Calculate the expected primary full load ampere (FLA)

3 Phase KVA RATED POWER TRANSFORMER

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Problem #15, Valid for 3 Φ only

Given Secondary FLA = 2405.70 and Secondary voltage = 240; Calculate the 3 Φ kVA rating of transformer

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Problem #16, Valid for 3 Φ only

Given Primary FLA = 1202.85 and 3 phase kVA = 1000; Calculate the expected primary voltage

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