doble engineering companyengineering.richmondcc.edu/courses/eus 215/notes/sfra test.pdf · •...

7/7/09

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Diagnostic Testing of

Power Transformer using

Sweep Frequency

Response Analysis

Arturo H. Oropeza

Doble Engineering Company

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

FRA = “Frequency Response Analysis”

FRA is the measurement of a system to a particular input at various frequencies.

• There is no right or wrong answer, just the response

• You put something in and you get something out

Black

Box Input Response

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• FRA represents a systems response

to a sinusoidal input at varying

frequencies

• The output will be the same frequency

but at different phase and magnitude

• FRA has been around for decades in a

variety of industries


Audio

• Speaker performance

• Radio

Telecom

• Communication performance

• Satellite Telemetry

• Cell Phones

• TV

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To distinguish the test methodology or equipment

using a SWEEP approach

Other method is the Impulse technique (IFRA)

Impulse results have poor resolution, lack the range

required for a portable field instrument translating

in poor diagnosis and are usually unrepeatable


For NOW we apply SFRA in

Transformer Condition Assessment (TCA)

Other uses:*

• Wave traps

• Instrument Transformers

• Rotating Machinery

*still studying best application and interpretation for these

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• SFRA is a leading method to analyze the physical winding condition of a transformer

• Will not tell you if the unit is contaminated

• Will not tell you when it will fail

• Provides relative information about winding’s physical condition

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• Dissipation or Power Factor and Capacitance

• Excitation Current

• TTR (Low and High Voltage Doble method)

• Leakage Reactance

• Sweep Frequency Response Analysis (SFRA)

• Dissolved Gas Analysis

These independent diagnostic methods have their place in

ascertaining transformer condition

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– Broad

• Power Factor and Capacitance

• Excitation Current

• TTR (High Voltage and Low Voltage)

• Dissolved Gas Analysis (DGA)

– Narrow

• Sweep Frequency Response Analysis (SFRA)

• Leakage Reactance


• Transformers are subject to immense stresses

during service – impact, nearby or internal faults,

overloads

• These stresses may cause winding deformation,

winding movement or core damage

• Allowing a transformer to enter service without

detecting these types of problem is risking

catastrophic failure of the transformer

• SFRA aids detection and permits diagnosis in a

timely manner.

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Assess Mechanical Condition of

Transformers (Mechanical Distortions)

Detect Core and Winding Movement

Due to:

– Large Electromagnetic Forces From Fault Currents

– Winding shrinkage causing release of clamping pressure

– Transformer Relocation or Shipping


What is the Input?...

• A swept sinusoidal that starts at 20 Hz

and ends at 2 MHz

• Voltage is a constant 10 Volts Peak to

Peak

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What is the Output?...

• A graph of frequency vs. dB

• Logarithmical display: commonly referred

to as a “Bode Plot”

• Linear display


Transformers may be represented by a

network of:

• Resistor

• Inductors

• Capacitors

The combination of these small elements are unique

to a given transformer

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• Impedance of an ideal resistor, capacitor and

inductor Resistance:

Flat response vs. frequency

Inductor:

Increased impedance with

increased frequency; dead

short at low frequency

Capacitor: Reduced impedance with increased frequency; open circuit at low frequency


Inductive roll

off

Capacitive

climb back

Resonance

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• We get a resonance for an inductor-capacitor

(LC) combination

• Changing L or changing C gives a new

resonance

• L and C are dependent on geometry

• Changing R changes the size of the resonance


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• A transformer may be modeled using

lumped parameters

• A simple model is a parallel RLC

circuit

• This has a predictable response as

we explained in previous slides


Low frequency

responses follow

expected form

Three phases of one transformer

High frequency responses

show phase-to-phase

variation which is common

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Low frequency

responses are

different to Wye

windings

Three phases of one transformer

High frequency

responses are similar

phase-to-phase; this is

common


One phase of one transformer

Tested at 9

tap positions Tap winding

responses

show

stepwise

variation

Main

winding

responses

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400kHz - 2MHz:

lead connection

movement

(Windings and/or

LTC/DETC)

2kHz - 20kHz:

winding movement, loose

clamping

20Hz - 2kHz: core

deformation, open

circuits, shorted turns,

residual magnetism

20kHz - 400kHz: Winding

and Tap Changer

deformations

Different frequencies represent different problems


• Interpretation – adequate training

• Rely on experts when needed – there is no substitute for knowledge and experience

• Build a Library of results

• Correlate results from SFRA and other tests

• Subscribe to Users Groups to share experiences (i.e. Doble SFRA User Group)

• Find Resources (i.e. Doble SFRA Resource Center available to Doble users)

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• Poor Grounding

• Absence of Oil

• Magnetized Core

• Comparing similar units


One phase has poor ground

connection during measurement

Three single phase transformers

measured for comparison

Otherwise results were OK; it is important

to make good connections

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With Oil -

in white

Without Oil - in blue

One phase of one transformer

measured with and without oil

Removing oil lowers capacitances and

resonances shift to higher frequencies


Same phase measured

on arrival at site

Results are acceptable - but

need retest after demag for

confirmation

Low frequency

responses show offset

due to core

magnetization

High frequency

responses unchanged

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Two transformers with

successive serial

numbers

High frequency

responses show very

good

correspondence:

indicating very

similar construction

Low frequency

responses show effect of

core magnetization -

variation is identifiable

and acceptable

Results are good


• FRA is a useful and sensitive tool

• SFRA is a reliable and repeatable method of

making FRA measurements

• Can be used routinely or after an incident

• Use as part of factory QA and/or relocation

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Thank you.

Any questions?

Arturo H. Oropeza

Doble Engineering Company

[email protected]

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