do's and don'ts of power factor testingieee and neta standards • neta ats-2017 - section...

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Do's and Don'ts of Power Factor TestingDinesh Chhajer, P.E.Charles Nybeck, PhD

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Agenda• Introduction• Fundamentals and Basics of Operation• Field Challenges & Factors Affecting the Measurement• Power Factor Limitations• Results Analysis• Conclusion

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Introduction• Industrial growth in early 20th

century led to industrial expansion

• Resulted in necessity for methods to determine the condition of HV insulation systems

– Introduction of 2 test methods

• DC insulation resistance test– Used to determine resistance of

insulation system (MΩ)

• AC Power Factor (PF) measurement– Uses leakage current of insulation

system to determine condition of HV insulation system

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Power Factor Theory• Power factor is defined as the ratio of real power

absorbed by a load to the apparent power flowing through the circuit

– 𝑃𝑃𝑃𝑃 = 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝑃𝑃𝑃𝑃𝑃𝑃𝑅𝑅𝑃𝑃𝐴𝐴𝐴𝐴𝐴𝐴𝑅𝑅𝑃𝑃𝑅𝑅𝐴𝐴𝐴𝐴 𝑃𝑃𝑃𝑃𝑃𝑃𝑅𝑅𝑃𝑃

= cos𝜃𝜃

• Commonly used in power systems with the aim of a PF near unity. Opposite for insulation testing, where the ideal situation yields PF of 0

– 𝑃𝑃𝑃𝑃 = 𝐼𝐼𝑅𝑅𝐼𝐼𝑇𝑇

= cos𝜃𝜃

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Power Factor Connection and Modes• To perform PF testing, primary windings

and secondary windings must be shorted independently

– Equal distribution of electric field

• Creating “three-terminal capacitor” used to model transformer insulation system

– CHL, CHG, & CLG

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Power Factor Method Summary (GST-g) (GST-GND) (UST)

• Measure Red • Measure Ground and Red • Measure Ground and Guard Red

Ungrounded Specimen Test Grounded Specimen Test with GuardGrounded Specimen Test

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IEEE and NETA Standards• NETA ATS-2017 - section 7.2.2.B electrical testing

– NETA recommends to perform insulation power-factor or dissipation-factor tests on all windings in accordance with test equipment manufacture’s published data

• IEEE C57.152-2013 is the Guide for Diagnostic Field Testing of Fluid Filled Transformers, Regulators, and Reactors – Section 8

– Provides a diagnostic chart, listing the different tests and diagnostic techniques for each component of a transformer

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Field Challenges and Factors Affecting the Measurements

• Specimen Isolation– Any asset should be isolated from it’s connected network– Parallel capacitances present could mask the actual test results– Insulation systems should not be evaluated in groups or sets– Sectionalize different insulation sections within the asset– Individual measurements performed and analyzed to

truly asses insulation condition

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• Grounding– Ground circuit should be carefully examined before the

start of the test– Single point ground system is recommended to have a

safe and reliable connection – Check for “open ground” circuit test to verify single point

ground system– It is highly recommended to NOT bypass this safety

check before performing the PF measurements.– Asset under test if not truly connected to station ground

can cause floating potential or inaccurate GST (Grounded Specimen Test) mode measurements

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• Lead Connections– Windings should be shorted to create equipotential surface– Shorting done with bare conductor and minimum slack to

avoid stray leakages that could artificially raise the PF values– HV lead should be PD free and checked to avoid any influence

to the recorded measurements– HV lead when connected to bushing terminals should not rest

or touch any of the bushing surface– On Load Tap changer (OLTC) with bypass resistor should be

placed in off-neutral position for testing purposes

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• Guarding– Any section that is not part of intended measuring circuit

should be guarded out through guard leads– Incorrect use of guard lead when compared against the test

mode selection in software could lead to errors– Guard circuit could be used as an advantage when

performing the measurements and reduce interference or unwanted noise signals

– Use of guard circuit when testing four point insulation system such as three winding transformer or three phase motor

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• Test Modes– Three different test modes: UST , GST and GST with

Guard are available for PF measurements– Used for sectionalization of insulation system and

validation of test results– Selection of appropriate test mode for guarding and

isolating external interference and noise– Effective use of red and blue lead to achieve different

configurations– GST test mode could get affected by grounding circuit

(single point grounding system recommended)

Two Winding Transformer

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• Test Voltage– PF results should not be voltage dependent– It is not required to run the test at specimen rated voltage– Test voltage should not exceed the rated winding voltage– Industry practice to perform test at 10 kV test voltage

• Input power supply requirements• High signal to noise ratio

– Trending with historical values and factory/commissioning test results

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• Temperature and Humidity– Temperature and humidity can affect PF measurements– It is recommended Not to perform test under high humidity (exceeds 50%)– Readings can be compensated for temperature but not humidity– Ambient conditions should be recorded– Insulation oil temperature should be recorded from available gauges– It is not recommended to perform PF measurements when the insulation

temperature is at or below 0 ᵒC– Transformer should not be tested immediately after taking offline – The error introduced from temperature compensation would be larger

when specimen is tested farther away (higher or lower) from reference 20 ᵒC temperature

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• Temperature Correction– PF values are temperature dependent and should be corrected

to 20 ᵒC– The correction factor tables are static in nature (based upon

nameplate information). It does not take into account changes taking place in the insulation

– TCF are average values and subjected to introduce error in the corrected values

– It tend to overcompensate for some transformers and under compensate for others

– For those reasons, it is recommended not to use TCF for temperature correction

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• Temperature Correction (ITC)– It is recommended to use Narrow Band Dielectric

Frequency Response (NBDFR) technique for PF correction to 20 ᵒC

– The PF-Frequency test data at a given insulation temperature allows to obtain co-relation between PF and Temperature at nominal frequency using Arrhenius equation

– Method is called as Individual Temperature Correction (ITC) as correction factors obtained are unique to the transformer under test

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• Electrostatic Interference– Transformer under overhead-energized lines can get

influenced by presence of stray capacitances and unwanted stray currents

– HV bushings on a transformer can act as an antenna to attract stray currents into the measurement circuit

– Use 10 kV test voltage to develop high signal to noise ratio– Measure PF at frequencies other than nominal frequency– Use line sync reversal technique with forward and reverse

measurement to cancel interference– Measuring noise signal level before applying the test voltage

and compensating for it during the test

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• Electrostatic Interference– Noise level signal below 15 mA or having a signal to noise

ratio below 1:20 can be effectively neutralized out in the field – Neutralize external interference with effective use of guard

and ground terminals– For UST mode, return lead (R or B) should be farthest away

from overhead energized lines– For GST guard mode, Red or blue lead should be connected

closest to test terminals near the overhead energized lines – When performing C1 measurement (UST mode) on bushings,

the other windings bushings should be shorted and grounded as in UST mode ground act as a guard circuit

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• Bushing C1 and C2 Measurements– Bushings and tap changers contribute to more than

40% of transformer failures– C1 PF test is used to assess the main core insulation

of capacitance graded bushings – C2 capacitance and PF test represents the insulation

between last layer of C1 insulation and the mounting flange

– Bushings should not be tested when present in a wooden shipping crate, lying on wood or in a horizontal position

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• Bushing C1 and C2 Measurements– Recommended to short the bushings to avoid any

cross coupling between the windings before performing the PF test

– For C1 test it is recommended to use 10 kV test voltage. For C2 measurement 500 V or 2000 V is used

– Only the tap cover of bushing under test should be taken off

– The bushings not under test should be grounded to avoid any interference with the measurements

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• Bushing C1 and C2 Measurements– C1 PF values are temperature dependent and should be

corrected to reference 20 ᵒC– It is recommended to use the average of ambient

temperature and insulation temperature of transformer for temperature correction

– NBDFR technique allows most accurate temperature correction of bushing C1 measurements

– C2 insulation although grounded in service is a highly recommended test

– Issues like carbon tracking, increase in moisture content or impurities could be picked up earlier with C2 measurement

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• Surface Contamination– PF measurements can get affected by surface

contamination, humidity and condensation present on the surface of the bushings

– The surface contamination creates a parallel path for the surface loss current to the ground

– Anytime PF value improves by itself or negative PF values, it indicate towards surface loss current

– Repeat the test after cleaning the porcelain surface (top and bottom) with dry lint free cloth using liquid solvent like isopropyl alcohol

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• Surface Contamination– Loss current is resistive in nature and it’s phase angle

relation with respect to reference voltage is close to zero

– The surface loss current could drive the phase angle between voltage and current greater than 90ᵒ causing PF values to turn negative

– Low capacitance circuits would typically have low values of current as current is directly proportional to capacitance

– Specimens with low capacitance values are more susceptible to the surface loss current and can lead to negative PF values

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Averaging Test• Power factor tests the bulk insulation between the

primary, secondary, and ground of a transformer– Includes solid and liquid insulation

• CHL, CHG, & CLG

• Difficulty discriminating between localized problems and widespread general deterioration

– Isolated issues needing immediate attention– General deterioration from typical aging mechanisms

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Borderline Results• A questionable or borderline PF result often leads to the question;

what do I do next?– No specific guidelines are given in standard on what to do when a PF result is

borderline to the acceptance criteria

• A borderline PF measurement results in more question in which other diagnostic tests can be performed to learn more

– Dissolved Gas Analysis (DGA)– Partial Discharge (PD)

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Moisture vs PF Measurement• The presence of moisture in a transformers insulation system can lead

to power loss and can lead to failure

• PF measurements at line frequency have limitations when detecting moisture

– Non-uniform distribution of moisture – Moisture levels in oil are not representative of the moisture content in the paper

insulation

• Dielectric Frequency Response (DFR) measurements can be made to determine moisture content in a HV insulation system

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Narrow Band DFR• Narrow Band Dielectric Frequency

Response (NBDFR)– Provides a more in-depth analysis into

the condition of the insulation system

– Can give more insight into borderline or questionable results

– 505 Hz - 1Hz adds an approximately 3 minutes to a traditional PF measurement

• Allows for earlier action to defects in a HV insulation system

0.1

1

10

1 10 100

%PF

FREQUENCY [HZ]

60 HZ Vs NBDFR

60 Hz PF 60HZ 0.5% Limit NBDFR

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NBDFR Aging

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Results Analysis• IEEE Guideline

IEEE C57.152-2013 “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors”

– The PFs of most older, mineral-oil-filled transformers are also < 0.5% (20 °C), PFs between 0.5% and 1.0% (20 °C) may be acceptable; however, PFs > 1.0% (20 °C) should be investigated.

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Results Analysis• Trending

– PF values from factory 10 kV measurements or commissioning test should be obtained if available

– Only compare the temperature correct PF readings– Readings should be compared overtime with previous historical measurements of

the same asset– It is not recommended to compare one asset PF values with a group of similar

assets– Any PF values on the borderline or outside the IEEE or NETA limits should be

investigated– Seemingly good PF values should be verified with NBDFR results for greater peace

of mind

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Results Analysis• Field Measurements

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Bushing Nameplate PF Measured PFX1 0.21 0.25X2 0.22 0.25X3 0.22 0.29X0 0.22 0.27

Bushing Nameplate PF Measured PFH1 0.23 0.19H2 0.24 0.24H3 0.24 0.23

Case Study NBDFR

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Results Analysis• Correlation

– PF values should be analyzed along with capacitance, watt loss and leakage current– Individual insulation system values should be validated with cross verification tests– Bushings data on the same transformer should be compared with each other– Borderline PF values should be validated with NBDFR measurement– Other tests to consider DGA, PD, DFR etc.– PF test does not directly correlate to moisture and under questionable

measurements full DFR test should be performed– DFR test ran in frequency range 1mHz to 1000 Hz provides moisture content

present in the paper insulation

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Conclusion• PF test can help in identifying insulation health condition• Number of factors can affect the measurement out in the field• Temperature, test voltage, test modes, external interference should be

evaluated before performing the test• Important to follow best practices and avoid common pitfalls• PF test being an average test has limitations and NBDFR test could help

in advanced analysis• Results should be evaluated and analyzed with historical measurements

and IEEE guidelines

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Questions?

[email protected]@megger.com

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do's and don'ts of power factor testingieee and neta standards • neta ats-2017 - section...

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