section7_1-dtt_pt2_iex

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Transformer Excitation

Current Test Procedures

Doble Engineering DiagnosticsToolbox

89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry 2

Abnormal core grounds Winding faults: shorts, open

circuits

LTC problems

Manufacturing defects

The Doble Excitation Current Test has beeneffective in finding:


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1) When an AC Voltage Source, such as a Doble Test Set, is placed on a

transformer, a small current will flow.

2) This small current is the Magnetizing Current: the current required to

magnetize the Transformer core with the Magnetic Fluxmag. ThisMagnetizing Current is the Excitation Current we measure and record.3) This Magnetic Flux will induce a voltage across the secondary

windings: V2

Iexcitation =Imag

V1

mag

+

-V2

+

-


1) When a Load is placed on the secondary windings a current will flow

I2=V2/R2 [ Ohms Law ]

2) The Current I2

will in-turn create an Opposing Magnetic Flux2.

3) The Generator, which regulates voltage at a set level, will provide more

current to maintain the core magnetized equal to the opposing flux

Iexcitation =Imag+I2

I22

V2

+

-

V1

Iexcitation =Imag+I2

+

-

mag+2

R2


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E1

mag+fault

Ifault

X0

X1UST

fault

+

-

Imag+IfHV

LV

H0

H1

1) If a fault develops in the secondary windings, this fault will act as aload across the faulted windings drawing a current Ifault

2) As a result, the Excitation Current will go up due to the opposing flux

created by the fault [fault]

Result: A Fault will cause Excitation Current to Increase


Iexcitation=Imag+Ifault

1) If the secondary winding has a grounded neutral and one of the windingsdevelops a fault to ground, grounded windings will draw a fault current

2) As a result, the Excitation Current will go up due to the opposing flux createdby the fault [fault]

Result: A grounded winding on a transformer with a grounded

neutral will cause the Excitation Current to go up.

H0

H1

fault

X1

Ifault

mag+fault

XoNeutral Ground

Ground Fault

HV

LV

UST

+

-


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When an autotransformer is connected across two taps it acts as aload and the primary current goes up.

Result: When a LTC transitioning device such as a Preventive

Autotransformer is in the bridging position the

excitation current goes up.

Iexcitation=Imag+IAT

H0

H1

AT

X1mag+AT

Xo

HV

LV

UST

+

-

IA


Excitation Current test results present patterns:

Three Phase Transformers: Between phases

(Two Similar Highs and One Low)* Within each Phase

(When Tap Changers are present)**

Single Phase Transformers

Reverse measurement for confirmation

* There are exceptions** Pattern will depend on Tap Changer type and Manufacture


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=

(b) Reluctance Circuit(a) Ferrous Core


For simplicity assume that each section has a reluctance of 1

1

1

1 1

1 1 A B C

In this example we have a three Phase three legged core


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HV

-

P1

P2

A B C

When you energize Phase A the equivalent reluctance circuit becomes:

HV

-

P1

P2

3.75

The same holds true when you energize Phase C


B

For Phase B the equivalent reluctance circuit becomes:

HV

-

P1

P2

2.5

P1

P2


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Benchmark or Base Measurement:

LTC only: all positions NLTC only: all positions LTC and NLTC combination:

all LTC positions with NLTC in

center position; then all positions

in NLTC with LTC in neutral

On a Routine Basis:

NLTC only: as found or positionregularly tapped

LTC only: one full range plus neutraland one position in opposite range

NLTC and LTC combination:NLTC in regularly tapped position or as

found testing the LTC one full range

plus neutral and one position in opposite

range

When Tap Changers are present, test as follows:


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Perform tests at the highest Voltage possible

Test each Phase at the same Voltage

Perform subsequent tests at the same Voltage forcomparison

If a Preventive Autotransformer is included in thetransformer, it might not be possible to excite thatposition of the LTC. In this event, testing might bepossible with the Preventive Autotransformer bypassedor at a lower voltage

If the test set trips, choose a lower Voltage and repeat

all three phases Never allow test voltage to exceed rated L-L for Delta

and L-G for Wye


Be aware of test equipment limitations Have consistent procedures Document special conditions, be specific If the Test set trips, choose a lower Voltage and repeat

all three phases

Example

M4K is rated for 300 mA at 10 kV

10 kV 300 mA

9 kV 333 mA

8 kV 375 mA7 kV 429 mA

6 kV 500 mA

5 kV 600 mA


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=

(b) Reluctance Circuit(a) Ferrous Core

Remember this:

Because test must comply with:

Test at the highest Voltage possible Test each Phase at the same Voltage

Phase demanding the highest current should be tested first



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H1

H2H3

HV

USTI1-2 I1-3

Test Measure

1 H1-H2

2 H2-H3

3 H3-H1

When Low Voltage side is Wye configuration, remember to

ground the Neutral

LV


H1

H0

H2 H3

HV

LV

UST I1-0Test Measure

1 H1-H0

2 H2-H0

3 H3-H0

When Low Voltage side is Wye configuration, remember to

ground the Neutral


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Remember that the Excitation Current test is the only test where

the short circuit jumpers are taken off the bushings

H1

H0

HV

LV

USTI1-0


Check the pattern: two similar high readings and onelow reading is normal (though there are exceptions)

When Tap Changers are present, consider the patternwithin the phase

For three-phase units, compare both high readings withthe following criterion: Readings 50 mA, differences should not exceed 5%

For single phase units both readings should becompared using the same above criterion

Compare Normal test to alternate tests; results shouldbe similar for a winding in good condition


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Compare Normal test to alternate tests; results shouldbe similar for a winding in good condition

Compare readings to previous results. Ensure that thesame voltage was used for both tests for consistentnumerical comparison

If the core is magnetized an irregular pattern (high,medium, low readings) will be present and you will beunable to compare results effectively. A true problemcould be masked, therefore core should be

demagnetized and test should be repeated If unusual results are obtained, consider performing an

alternate test to further investigate

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