Transformer Excitation Current Test Procedures Doble Engineering Diagnostics Toolbox
The Doble Excitation Current Test has been effective in finding: • Abnormal core grounds • Winding faults: shorts, open circuits • LTC problems • Manufacturing defects
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1
Iexcitation
= Imag
mag
+
+
V1
V2
-
-
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 Flux mag. This Magnetizing Current is the Excitation Current we measure and record. 3) This Magnetic Flux will induce a voltage across the secondary windings: V2 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
Iexcitation = Imag + I2 + V1 -
1)
+ 2 mag
3
I2
2 + V2 -
R2
When a Load is placed on the secondary windings a current will flow I2=V2/R2 [ Ohm’s Law ]
2.
2)
The Current I2 will in-turn create an Opposing Magnetic Flux
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 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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2
HV UST
E1 + -
Imag+ If
mag+ fault
fault X1
H1
Ifault
LV
X0
H0 1)
If a fault develops in the secondary windings, this fault will act as a load 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 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
Iexcitation=Imag+ Ifault UST HV
H1
mag+ fault
fault
X1
5
Ground Fault
+ -
Ifault LV
H0
Xo
Neutral Ground
1) If the secondary winding has a grounded neutral and one of the windings develops 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 created by the fault [fault]
Result: A grounded winding on a transformer with a grounded neutral will cause the Excitation Current to go up. 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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3
Iexcitation=Imag+ IAT UST HV
mag+ AT
H1
AT
X1
+ -
IA
LV
H0
Xo
When an autotransformer is connected across two taps it acts as a load 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. 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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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
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For simplicity assume that each section has a reluctance of 1 A
1
B
1
1
C
1
1
1
In this example we have a three Phase three legged core 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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When you energize Phase A the equivalent reluctance circuit becomes: P1
P1
HV
HV
3.75
P2
P2
A
B
C
-
The same holds true when you energize Phase C
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For Phase B the equivalent reluctance circuit becomes: P1
P1 HV 2.5
B
P2
P2
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When Tap Changers are present, test as follows: Benchmark or Base Measurement:
On a Routine Basis:
• LTC only: all positions
• NLTC only: as found or position regularly tapped • LTC only: one full range plus neutral and 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
• 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
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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 for comparison • If a Preventive Autotransformer is included in the transformer, it might not be possible to excite that position of the LTC. In this event, testing might be possible with the Preventive Autotransformer bypassed or 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 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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• 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 mA 7 kV 429 mA 6 kV 500 mA 5 kV 600 mA 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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Remember this:
= (a) Ferrous Core
(b) Reluctance Circuit
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 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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HV
H1
UST
Test 1 2 3
I1-3
I1-2
Measure H1-H2 H2-H3 H3-H1
H3
H2 LV
When Low Voltage side is Wye configuration, remember to ground the Neutral 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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HV H1 UST
Test 1 2 3
I1-0 LV
H0
H2
Measure H1-H0 H2-H0 H3-H0
H3
When Low Voltage side is Wye configuration, remember to ground the Neutral 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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HV H1 UST
I1-0 LV
H0
Remember that the Excitation Current test is the only test where the short circuit jumpers are taken off the bushings 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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• Check the pattern: two similar high readings and one low reading is normal (though there are exceptions) • When Tap Changers are present, consider the pattern within the phase • For three-phase units, compare both high readings with the following criterion: – Readings <= 50 mA, difference should not exceed 10% – Readings > 50 mA, differences should not exceed 5%
• For single phase units both readings should be compared using the same above criterion • Compare Normal test to alternate tests; results should be similar for a winding in good condition 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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• Compare Normal test to alternate tests; results should be similar for a winding in good condition • Compare readings to previous results. Ensure that the same voltage was used for both tests for consistent numerical comparison • If the core is magnetized an irregular pattern (high, medium, low readings) will be present and you will be unable to compare results effectively. A true problem could 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 89 Years of Diagnostic Instrumentation and Services for the Electric Power Industry
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