voltage drop at secondary terminal of power tranformer but not at the primary

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wilans

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have experience voltage drop significantly (untill activating undervoltage alarm ) at the secondary of power tranformer but not in the primary ( primary remain constant ) during starting an electric motor. transformer rating 20KV/3KV, 10MVA, the motor rating 3KV 600KVA, total load at the transformer is only about30% of the transformer capaity. the mototr is reported in good condition after having tested with PDMA tools. what seems to be the cause???
 
charlie b

charlie b

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Retired Electrical Engineer
This is a long shot, but I'll suggest it anyway. What type of load does the motor serve (i.e., water pump, fan, conveyor, compressor, etc.), and is the motor being started with the load engaged? For example, a centrifugal pump is supposed to be started with its discharge valve shut, for if you start it with that valve open it causes a far worse starting transient.
 
kingpb

kingpb

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SE USA as far as you can go
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The motor has a high starting torque, and low starting power factor whcih requires a lot of system reactance. Impedance of transformer is too high, cable is too small, and not allowing the reactance needed from system.

Of course, that was a total guess.
 
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wilans

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Location
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the voltage dropped to lower than 80% of rated voltage in about 2 seconds even during the motor was uncoupled. any symptoms of transformer failure?
 
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Phil Corso

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Location
Boca Raton, Fl, USA
Wilans...

Was V-Drop experienced phase-to-phase? Phase-to-ground? Single-phase or simultaneously on all 3-phases?

Any corresponding current measurements during "disturbance?"

Was V-drop of 80% measured, or alluded to because of UV relay alarm-setting?

Any observations regarding other apparatuus supplied by same xfmr?

Phil Corso
 
Besoeker

Besoeker

Senior Member
Location
UK
charlie b said:
This is a long shot, but I'll suggest it anyway. What type of load does the motor serve (i.e., water pump, fan, conveyor, compressor, etc.), and is the motor being started with the load engaged? For example, a centrifugal pump is supposed to be started with its discharge valve shut, for if you start it with that valve open it causes a far worse starting transient.
Click to expand...
Longer rather than higher.
 
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wilans

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Location
Asia-Pacific
Hi.. phil,
The measurement was taken on the xfmr secondary BUS not at motor starter. the measurement was phase to phase. the undervoltage relay has been calibrated many times to make sure the setting and function are correct. the transformer resistance test was done and indicating there is an increase in secondary winding resistance ( the design is about 0.0047 Ohm and now is 0.011 ohm ) . impedance test was not done.
 
T

T.M.Haja Sahib

Guest
There may be problem in the transformer taps on the HV side. Change the tapping position suitably and see if the issue resolves.
 
T

T.M.Haja Sahib

Guest
Transformer tap changing should be carried out after de-energising the transformer, if it has no on load tap changer.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Wilans…
T. Sahib makes a good point. If arcing in the primary-ckt is the cause, it could be occurring in the Tap-Changer (usually located in series with the primary winding) then, I suggest a DGA or Dissolved Gas Analysis.
Fortunately, if a TCUL is provided (typical (?) for Xfmrs of the size you cited) the test is rather simple to perform!
Regards, Phil
 
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rcwilson

Senior Member
Location
Redmond, WA
A 600KW motor (about 800HP) will draw 4000 - 8500 kVA during starting, that should not be a large voltage drop on a 10 MVA transformer unless it is already at low voltage prior to motor starting due to low primary voltage or heavy load. Or the high voltage system may not be stiff enough.

Since you say the primary voltage is not changing, indicating the high voltage system may be OK.

What is the transformer impedance? A high impedance (say 10%) will have a higher voltage drop.

Using rule of thumb numbers, the motor starting MVA is about 40-85% of the transformer rating. With a 10% impedance and neglecting phase angles the voltage drop in per unit on the transformer base will be 10% Z x 85% I = 8.5% Voltage drop. If your voltage drop is really 20% look for the reason for the other 11% drop.

What is the voltage prior to starting? It may be 4% low.
What is the high side voltage during starting, maybe it does drop 2%, a value too small to read on many meters.
Check the connections at all points. Check the cable size.
Unless your undervoltage relay is connected to VT's directly at the transformer terminals, it is measuring the voltage drop in the feeder bus , cables and the switchgear and not the voltage at the transformer terminals. Check the setting calculation and the VT ratio. Maybe the 80% trip setting is really only 90% due to using the worn CT ratio. Maybe the setting was calculated phase-phase when the relay is reading phase to ground voltages.

Good Luck!
 
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