3 6 field experience in sfra measurement on generator transformers

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SFRA generators and transformers

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ng Field Experience in SFRA iMeasurement er ne on Generator iTransformers ng E Conference 2009 le EuroDoble ob D A. Saravanakumar M. Boltze S. MarkalousDoble LDIC Germany Doble LDIC Germany Doble LDIC Germany

Outline Introduction ng ri Principle of SFRA ee Interpretation Strategy gin En Sample results ble Do Conclusion

2

Diagnostic Testing Method Monitoring g Sensor development Data acquisition, noise suppression, filtering Diagnosis Interpretation of monitored data Benefits f Reduced risk of failure, better asset management Better maintenance strategy

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DTCM MethodsInsulation System Partial discharge Dissolved gas analysis Top oil temperature Winding/core arrangement

Degree of polymerisation Furan analysis

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En Transfer function method le

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Leakage reactance

Low voltage impulse test

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Sweep Frequency Response Analysis (SFRA)

Recovery voltage Insulation resistance tan() or loss factor

4

LVI and TF method1 Exc citation - v(t) 1.2s / 50s LV Impulse (V)

Tested winding Non-Tested winding (shorted)

0.8 0.6 0.4 0.2 0 0 5

v(t)

Magnitude

0.2 0.1 0

Phase (deg)

in(t)

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En leRespons - in(t) se 0 -5 -10 00.30.4Magnitude

ne gi100 200 100 200

ng ri e300 400 300100 0 -100 -200

500

600

700

800

900

Neutral Current (mA)

400 500 time (s)

600

700

800

900

Time domain data deconvoulted in frequency domain (TF method) 200

Direct frequency domain measurement (SFRA) 200Phase (deg)

0.3

100

In( f ) H( f ) = V( f )

0.2

0

0.1

-100

0 0

500

1000 Frequency (kHz)

1500

2000

-200 0

500

1500 1000 Frequency (kHz)

2000

5

Principle of SFRA Apply spectrally pure sinusoidal signal Measure excitation and response Compute and store Gain/phase Repeat measurement at next discrete frequency di t f Direct frequency domain measurement Better immunity to noise and interference Sensitive in detecting winding/core movement An Draft on IEEE Guide for performing SFRA exists

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Practical difficulties Ensure spectral purity of source p p y Source harmonics exist & introduce problems Excitation level low; SNR problem for on-site Influence of cable length, bushing capacitance Unshielded connecting leads cause problems

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Typical Response of windingMagnitude Admittance Y(f)

Phase

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+ 90

- 90

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SFRA and Natural frequencies Several factors affect natural frequency Winding type, position, arrangement, clearance and terminal connection t i l ti Terminal connection is the only factor that can be externally manipulated Some natural frequencies are non-excitable under certain terminal connections Knowledge of all natural frequencies indicates highest sensitivityEquivalent circuit representation of a two winding single phase transformer

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Influence of Terminal Conditions1 0.81

(a) - Excitation with respect to ground HVOC/Y(f)

1 0.8

2

(b) - Excitation with respect to ground HVSC/Y(f)

Mag (p.u)

3

Mag (p.u)

0.6

0.6

1

0.4

4

7

0.4

0.20 10.8

2

5

8

6

(c) - Excitation with respect to neutral HVOC/Y(f)

Mag (p.u)

0. 06

Mag (p.u)

2

0.4

ob D3

e l

ng E

0.2

ne i0 1

ng ri e3451

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(d) - Excitation with respect to neutral HVSC/(Yf)

0.8 06 0.

0.4

0.20 0

1

0.25001500 1000 Frequency (kHz)20000 0

500

1500 1000 Frequency (kHz)

2000

Fig. 1 Measured amplitude frequency response pertaining to terminal connection where non tested windings are (a), (c) open circuited and floating (b), (d) shorted and floating10

Pole-Zero50

-5

-0.1

-0.09

-0.08

5

(Mrad/s)

D -0.09

le ob-0.07

-0.06

En

-0.05 -0.04 (Mneper/s)

ne gi

ng ri e-0.02 -0.01 0 0.01

(Mrad d/s)

-0.03

0-5-0.1-0.08-0.07

-0.06

-0.05 -0.04 (Mneper/s)

-0.03

-0.02

-0.01

0

0.01

Fig. 2 Pole-Zero representation corresponding to Fig. 1(a) and (d)11

All System Function Terminal Condition1

Mag M

10.50 1

2

3

5 4

6

7 8 9 10

1

2

3

4

5

Mag

6

0.5 0

7

0 1

12

Mag

0.50 1

3

4

5

ob D6 7

En le88 9

9

10

ne gi1110

ng ri e11

12

Ig( )/Ip( )

Vp( )/Ip( )

Ip( )/Vp( )

12 3

Vsn( )/Vp( )4 5

Mag

0.50 0

100

200

300

400 500 600 Frequency (kHz)

700

800

900

1000

12

Cable Terminations1 0.8Mag (p.u) )

1

(a)

0.6 0.4 0.20 1 0.8 2 3 4 5

6

8

1

Mag (p.u u)

0.6 06 0.4 0.2 0 0 100

2

D 3 200 4

le ob5 6 300

ng E77 400

ne i

ng ri e

Vout(f)/Vin(f)

9

10

(b) Vout(f)/Vin(f)

500 600 Frequency(kHz)

700

800

900

1000

Measured amplitude frequency response pertaining to terminal connection where non g p ) ( ) tested windings are open circuited (a) 50 (b) 1 M termination13

Dynamic Range1.5 3 (a)

Mag (p.u)

1

1 4 2 5 6

0.5

0 1.5 Mag (p.u) 1 0.5 0 0 1 2

4 100

3

ob D5 200 6

En le7 8 7 400 8

ne gi9 (b)

ng ri e

Vout(f)/Vin(f)

Vout(f)/Vin(f)

9 700 800 900 1000

300

500 600 Frequency(kHz)

Measured amplitude frequency response from a single phase two winding transformer (a) with dynamic range (b) without dynamic range14

DOBLE SFRA EQUIPMENT Voltage 10 V p-p (at 50 impedance) Frequency range 10 Hz to 10 MHz Accuracy 1 dB to -70 dB 70 Input/Output impedance 50 Number of sample 1250 (logarthmically placed) Three lead measuring system

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Current practice (Doble)

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- Exciting Phase / Line end - Response measured - Neutral (1 phase) - Phase (3 phase/ ) - Neutral (3 phase/ Y) Non N tested winding d i di - Open circuited - Short circuited Non tested terminals are floating16

Test and Analysis At factory Before and after any destructive testing (like short circuit strength assesment test) Routine test ( ase e data) out e (Baseline At Field Routine test (after installing) Before and after relocation After any major event Analysis With baseline data Phase to phase Sister unit measurements

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General GuidelineScreen captured from DOBLE SFRA ANALYZER (M5200) SC response

Magnitude

Phase

D

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OC response

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ng ri eCore Influence Bulk movement (Core/winding) Winding geometry Bushing, lead movement and so on

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Characterstics of windingsOpen circuit response measured from 60 MVA, 128.36 / 20 kV transformer

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H2 / Middle phase H3 / Outer phase

Comparison of outer phases (H1 and H3)

Comparison of middle phase with outer phase (H2 and H3)

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Measurable ParametersOpen circuit responseExciting current

DC winding resistance Leakage reactance

ob DInductance

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Short i it Sh t circuit response

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Certain type of faults With and without oil spectrum remains same with increase in natural frequencies Bad ground shows notable differences at high frequencies (f>1 MHz)

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Core Magnetization

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Interpretation strategy (phase - phase)Open circuit f

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