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Do's and Don'ts of Power Factor Testing Dinesh Chhajer, P.E

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Do's and Don'ts of Power Factor Testing Dinesh Chhajer, P.E Do's and Don'ts of Power Factor Testing Dinesh Chhajer, P.E. Charles Nybeck, PhD tsdos.org Agenda • Introduction • Fundamentals and Basics of Operation • Field Challenges & Factors Affecting the Measurement • Power Factor Limitations • Results Analysis • Conclusion tsdos.org Introduction • Industrial growth in early 20th • DC insulation resistance test century led to industrial – Used to determine resistance of expansion insulation system (MΩ) • AC Power Factor (PF) measurement • Resulted in necessity for methods – Uses leakage current of insulation to determine the condition of HV system to determine condition of HV insulation systems insulation system – Introduction of 2 test methods tsdos.org Agenda • Introduction • Fundamentals and Basics of Operation • Field Challenges & Factors Affecting the Measurement • Power Factor Limitations • Results Analysis • Conclusion tsdos.org 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 – = = cos • 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 – = = cos tsdos.org 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 tsdos.org Power Factor Method Summary (UST) (GST-GND) (GST-g) Ungrounded Specimen Test Grounded Specimen Test Grounded Specimen Test with Guard • Measure Red • Measure Ground and Red • Measure Ground and Guard Red tsdos.org 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 tsdos.org Agenda • Introduction • Fundamentals and Basics of Operation • Field Challenges & Factors Affecting the Measurement • Power Factor Limitations • Results Analysis • Conclusion tsdos.org 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the Measurements • 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 tsdos.org Field Challenges and Factors Affecting the
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