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Furnished below is an outline of the theoritical session on tan

By Renee Sullivan,2014-05-29 22:14
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Furnished below is an outline of the theoritical session on tan

    Capacitance and Tan Delta Testing

What is tan delta?

    In an ideal capacitor without any dielectric losses, the insulation current is exactly 90? leading to the applied voltage. For a real insulation with dielectric losses this angle is less than 90?. The angle ;;= 90? - ?;is called loss

    angle. In a simplified diagram of the insulation, Cp represents the loss-free capacitance and Rp the losses. The tan of the loss angle is a measure of the losses in the capacitor and act as a method to evaluate the condition of the capacitor.

     Parallel Circuit

    Training Material for Tan Delta Testing : 06-07/10/2009 Vijayawada SS Page 1 of 11

Phasor Diagram

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Equivalent Series Circuit

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     How losses are generated

    The dipoles in the di-electric align along the Electric Field and changes direction in AC. This movement causes the losses. (For example water molecules).

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Measurement of tan delta

Schering Bridge

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Testing kit block diagram

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Different Modes of Testing

A = Yellow(C1), B = Blue(C2) , Ground Capacitance = C3

    Omicron Kit Eltel Kit

    UST-A Measure C1, Guards C2, C3 1. UST Y

    UST-B Measure C2, Guards C1, C3 2. UST B

    UST-A+B Measure C1+C2, Guards C3 3. UST YB

    GST Measure C1+C2+C3 4. GST YBG

    GSTg-A Measure C2+C3, Guards C1 7. GST BG

    GSTg-B Measure C1+C3, Guards C2 6. GST YG

    GSTg-A+B Measure C3, Guards C1,C2 5. GST G

    Note: For ungrounded objects, use UST-A or UST Y. For simple grounded objects, use GST or GST YBG. In grounded objects, if any other capacitor is connected to HV cable, use GSTg-A+B or GST G with only one cable connected to other end of the capacitor for removing it from the measurement (guarding). Do not connect any LV cable to ground.

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Test on Bushing UST - C1

    Bushings are having a capacitor between the central high voltage connection and tan delta test tap (C1). Since, it is possible to disconnect the test tap from earth, we can measure C1 in UST mode. Connect HV lead to High Voltage connection of the bushing. Connect cable A or Yellow cable to tan delta tap. Make the measurement in UST-A or UST Y mode. In this mode, the effect of High Voltage to Ground capacitance is avoided since it is guarded.

    In a transformer or reactor, the jumpers need not be removed for bushing testing. However, short the bushings of the same winding to avoid effect of winding capacitance(In auto transformer, disconnect neutral, short 400 & 220 KV & N bushings ; short 33KV bushings and earth it while testing 400/220 bushings). Any other winding may be shorted and grounded during the test. While testing one bushing, do not open test tap of other bushings.

Test on Bushing - GSTg - C2

    Bushings are having a capacitor between the tan delta test tap and flange (C2). Since, it is not possible to disconnect the earthing of the flange, only a grounded specimen test can be done for C2. To avoid the effect of bushing high voltage to ground, it is to be guarded. Connect HV cable to tan delta tap and Cable A or Yellow cable to Bushing HV. You can make the measurement in GSTg-A (7. GST BG) or GSTg-A+B (5. GST G). Do not exceed 1 KV during the measurement. Since Cable A (Yellow) or Cable A and B (Yellow and Blue) are guarded, the kit will show C2 capacitance ie., from test tap to ground. Do not connect any LV cable to ground.

Test on Auto transformer with tertiary-UST,GST and GSTg

    In an auto-transformer, there are two windings effectively, ie., HV+IV+N and Tertiary. Before testing the winding, remove all jumpers, disconnect neutral, and short the bushings of the same winding ie., HV+IV+N. Short the 3 bushings of tertiary. Now there are 3 capacitors to be measured :- HV+IV+N to Earth, Tertiary to Earth and HV+IV+N to Tertiary. Out of these, HV+IV+N to Tertiary is to be measured in UST and other two in GSTg mode. During the test, do not open bushing test tap.

    1. HV+IV+N to Tertiary in UST mode

    Connect HV cable to HV+IV+N. Connect LV cable A or Yellow to Tertiary. Test

    in UST-A or UST Y. (By interchanging HV and LV cable, the test values remain

    unchanged).

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    2. HV+IV+N to Earth in GSTg mode

    Connect HV cable to HV+IV+N. Connect LV cable A or Yellow to Tertiary. Test

    in GSTg-A(7. GST BG) or GSTg-A+B (5. GST G). Since HV to Tertiary

    capacitor is guarded in this method, the result will be HV+IV+N to Earth.

    3. Tertiary to Earth in GSTg mode

    Connect HV cable to Tertiary. Connect LV cable A or Yellow to HV+IV+N. Test

    in GSTg-A(7. GST BG) or GSTg-A+B (5. GST G). Since Tertiary to HV

    capacitor is guarded in this method, the result will be Tertiary to Earth.

    Note :

    1. If we avoid guarding in step 2, ie., by doing GST test, the total capacitance

    will be sum of Step 1 and Step 2.

    2. If we avoid guarding in step 3, ie., by doing GST test, the total capacitance

    will be sum of Step 1 and Step 3.

    Test on Reactor - GST

    Since reactor is having only 1 winding, only the winding to earth capacitance can be measured. Isolate neutral and NGR. Remove jumpers on HV side. Short all HV bushings and neutral. Connect HV lead to the shorted winding. Connect LV lead(A or Yellow) to earth. Measure in GST (4. GST YBG). There is no error even if the LV lead is not connected.

Test on CT with test tap - UST

    This test is similar to Bushing C1 test. Connect HV lead to high voltage point of the CT. Connect LV lead to test tap after disconnecting it from earth. Measure in UST.

Test on CT without test tap - GST

    This test is similar to Reactor winding test. Connect HV lead to high voltage point of the CT. Connect LV lead to earth. Measure in GST. There is no error even if the LV lead is not connected. In GST measurements, the jumpers may have to be opened if they are leading to other earthed capacitors.

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Test on CVT - UST and GST

    1. CVTs where Neutral can be isolated UST

    In the case of CVTs where neutral of the EMU transformer can be isolated

    from ground, UST measurements are possible. Isolate HF and Neutral from

    ground. Connect HV lead to high voltage of the CVT and LV lead to HF point.

    Measure in UST mode.

    Individual stack can be measured in UST mode by connecting the test lead to

    the CVT flanges. For the bottom most unit, connect HV lead to the flange and

    LV lead to HF point.

    2. CVTs where Neutral cannot be isolated UST and GST

    In the case of CVTs where neutral of the EMU transformer cannot be isolated

    from ground, UST measurements are not possible for the total capacitor as

    well as the bottom unit. For measuring total capacitance, do not disconnect HF

    or Neutral from ground. Connect HV lead to high voltage point. Connect LV

    lead to ground. Measure in GST mode. It will be required to disconnect the

    jumpers in GST measurements.

    Bottom stack also to be measured in GST mode with HV lead to stack flange

    and LV lead to ground.

    For top and middle stack, measurement can be done in UST mode.

    The total capacitance or bottom stack capacitance test may show negative tan

    delta due to the presence of the EMU transformer. In such cases, the tan delta

    value may be ignored.

Test on Grading capacitor - UST

    This test is similar to Bushing C1 test. Connect HV lead to one end of the grading capacitor. Connect LV lead to middle of the CB. Earth the other grading capacitor. Measure in UST mode.

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