Proving a Transformer 3

[CuriousElectron]

Hi All,
I've got a situation where I need to check that an existing dry-type 100kVA transformer which was collecting dust in a storage is fit for duty.
What kind of tests should I run apart from performing the visual inspection?

Thanks,
EE

 

[waross]

Megger test followed by a high-pot at twice rated voltage plus 1000 Volts.
There are newer tests but this was a standard for many generations.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thermionic1]

The generally accepted minimum tests should be Megger (DC Insulation Resistance) and TTR (Ratio Test). I'm assuming this is a dry type and if so, check that the core is grounded in a single place and that the single ground hasn't been mistaken for a mounting screw. The Megger should be Pri-Gnd, Pri-Sec & Sec-Gnd. Some will blow the dust off with dry N2, but exercise care as to not damage the insulation. Some will gently vacuum the dust. Pay attention to the condition of the taps on the no load taps for proper torque.

 

[prc]

After cleaning you may conduct Capacitance and tan delta test also.

 

[che12345]

CuriousElectron
Q. "... I need to check that an existing dry-type 100kVA transformer .... is fit for duty. What kind of tests should I run apart from performing the visual inspection?"
A. some basic procedures:
a) [vacuum] instead of [blowing off] the dust as it will be disperse into the space and deposit on other equipment. Dry cleaning on windings and insulators if any,
b) visual inspection for any cracks, corrosions and torque all connections,
c) insulation resistance with 500Vdc: pri-frame, sec-frame and pri-sec . For voltage < 1kV , the insulation resistance shall > 1M Ohm. If lower, heat up with heater lamps or by short-circuit circuit current with low voltage,
d) high-voltage test with AC voltage at [2 x rated voltage + 1000V] for one minute between pri-frame, sec-frame and pri-sec

 

[che12345]

Dear Mr. CuriousElectron
Q. "... I need to check that an existing dry-type 100kVA transformer .... is fit for duty. What kind of tests should I run apart from performing the visual inspection?"
A. some basic procedures:
a) [vacuum] instead of [blowing off] the dust as it will be disperse into the space and deposit on other equipments. Dry cleaning the windings and insulators if any,
b) visual inspection for any cracks, damaged insulation, corrosions and torque all connections,
c) insulation resistance test with 500Vdc: pri-frame, sec-frame and pri-sec . For rated voltage < 1kV , the insulation resistance shall > 1M Ohm. If lower, heat up with heater lamps or by short-circuit circuiting current with low voltage,
d) high-voltage test with AC voltage at [2 x rated voltage + 1000V] between pri-frame, sec-frame and pri-sec. There shall be [no break-down] for a duration of [one minute]. Ignore the value of leakage currents. There is some defects if the leakage current > the max setting of the test set!,
e) temporally connect a single-phase/three-phase (low voltage) supply to the [high-voltage] terminals. Measure the voltage at the [low-voltage] terminals. Check that the [voltage ratio] is as per the name-plate rating,
f) for a 100kVA transformer, it "may?" (not) required to conduct the [Capacitance and tan delta test], as the test set cost and the lobour charges would not justify it.
Che Kuan Yau (Singapore)

 

[prc]

If health of the unit is in doubt, most relevant test is single phase, low voltage excitation current measurement. This will ensure no inter turn shorts or core Damage.
In case of high humidity blow with dry air just before charging.