Understanding the Value of Electrical Testing for Power Transformers - PowerPoint PPT Presentation

Understanding the Value of Electrical Testing for Power Transformers Charles Sweetser - OMICRON SEPTEMBER 5 - 7, 2018

Transformers SEPTEMBER 5 - 7, 2018

Diagnostic Testing - OVERALL • DGA • Oil Screen Power Factor / Capacitance • • Exciting Current • Transformer Turns Ratio Leakage Reactance • • DC Winding Resistance • SFRA (Sweep Frequency Response Analysis) DFR (Dielectric Frequency Response) • • Thermal Imaging • Insulation Resistance Partial Discharge • SEPTEMBER 5 - 7, 2018

Transformer Tests Dielectric Thermal Mechanical DGA DGA SFRA Oil Screen Oil Screen Leakage Reactance PF/TD CAP IR PF/TD CAP Exciting Ima DC Winding RES Exciting Ima Turns Ratio Tests DC Winding RES DFR Insulation Resistance SEPTEMBER 5 - 7, 2018

Transformer Test Protocol 9. IR 1. Overall Power Factor and Capacitance 10. DFR 2. Bushings (C1, C2, Hot Collar) 11. SFRA 3. Exciting Current 12. DC Winding Resistance 4. Surge Arresters 5. Insulating Fluids 6. Leakage Reactance 7. Turns Ratio Test 8. Insulation Resistance SEPTEMBER 5 - 7, 2018

Diagnostic Tests 1. Overall Power Factor and Capacitance 2. Bushing Power Factor and Capacitance 3. Exciting Current Test 4. TTR – Transformer Turns Ratio 5. Leakage Reactance (3-Phase Equivalent and Per Phase) 6. DC Winding Resistance SEPTEMBER 5 - 7, 2018

Industry Guides and Standards • IEEE C57.152-2013, "IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors". • ANSI/NETA MTS-2015, "Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems". • IEEE C57.149-2012, "IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers". SEPTEMBER 5 - 7, 2018

Delta-Wye (Dyn1) SEPTEMBER 5 - 7, 2018

Two-Winding Transformer Model • Windings are short-circuited to remove unwanted inductance • CH, CL and CHL insulation systems • CH includes H-C1 • CL includes X-C1 SEPTEMBER 5 - 7, 2018

Overall Power Factor - Test Preparation 1) Ensure that the transformer tank and core are solidly grounded, also connect both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node. 2) Ensure that all bushing surfaces are clean and dry. 3) Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3 and X0. 4) Bond/short the H1, H2, and H3, making sure that they are isolated. We will refer to this point as the “HV” node. 5) Bond/short the X1, X2, X3, and X0 making sure that they are isolated. We will refer to this point as the “LV” node. 6) Document tap-positions, temperatures, humidity, fluid levels, and pressures. SEPTEMBER 5 - 7, 2018

Overall Power Factor - Test Procedure • The test voltages will be limited and should not exceed the line-to- ground rating of the insulation system. • When convenient, Variable Frequency Power Factor Tests will be performed on CH, CL, and CHL insulation components, along with Power Factor Tip-Up measurements. Test Insulation Test Voltage * Test Mode Energize Red LV Lead 1 CH + CHL 10 kV GST HV LV 2a CH 10 kV GST-gA HV LV 2b CH(f) 2 kV (15-400 Hz) GST-gA HV LV 3a CHL 10 kV UST-A HV LV 3b CHL(f) 2 kV (15-400 Hz) UST-A HV LV 4 CL + CLH 7 kV GST LV HV 5a CL 7 kV GST-gA LV HV 5b CL(f) 2 kV (15-400 Hz) GST-gA LV HV 6a CLH 7 kV UST-A LV HV 6b CLH(f) 2 kV (15-400Hz) UST-A LV HV SEPTEMBER 5 - 7, 2018

Overall Power Factor - Expected Results IEEE C57.152 • PF < 0.5% at 20 °C for “new” liquid filled power transformers rated under 230kV • PF < 0.4% at 20 °C for “new” liquid filled power transformers rated over 230kV • PF < 1.0% at 20 °C for “service aged” liquid filled power transformers • PFs between 0.5% and 1.0% at 20 °C warrant additional testing and investigation NETA MTS • PF < 1.0% for liquid filled power transformers • PF < 2.0% for liquid field distribution transformers • PF < 2.0% for dry-type power transformers (CHL insulation) • PF < 5.0% for dry-type distribution transformers (CHL insulation) • PF Tip-Up for dry-type insulation should be < 1.0% Note: Measured values should also be compared to the manufacturer’s published data . SEPTEMBER 5 - 7, 2018

Bushing Power Factor Condenser Bushing with Condensers Bushing with Test Non Condenser Potential Tap Tap Visual Inspection Visual Inspection Visual Inspection C1 Power Factor (60 Hz) C1 Power Factor (60 Hz) Energize Collar Test C1 Capacitance (60 Hz) C1 Capacitance (60 Hz) Infrared Test C2 Power Factor (2.0 kV) C2 Power Factor (0.5 kV) C2 Capacitance (2.0 kV) C2 Capacitance (0.5kV) Advance Power Factor Advance Power Factor Measurements Measurements Power Factor Tip Up Test Power Factor Tip Up Test Infrared Test Infrared Test SEPTEMBER 5 - 7, 2018

Bushing Power Factor – Test Connections C1 C2 Hot Collar SEPTEMBER 5 - 7, 2018

Bushing Power Factor - Expected Results • Bushings shall remain shorted, similar to the overall power factor test. Failure to short the bushing terminals, may result in compromised measurements. • Hot Collar tests are optional; they will not be performed if test taps or potential taps are available. • Test taps and potential taps can be identified, based on the bushing rating, as follows: – Test Taps <= 350 kV BIL – Potential Taps > 350 kV BIL • C2 tests must be performed carefully, ensuring that the “hook” is in the clear, completely. The C1 results should compare well with the nameplate data. C1 Power Factor values should not exceed • 1.5X to 2.0X nameplate data. C1 capacitance should not exceed +/- 5% of nameplate data. • C2 values should compare well with the nameplate or amongst similar bushings. • The hot collar results are analyzed from watts loss. We expect less than 100 mW loss. SEPTEMBER 5 - 7, 2018

Transformer Exciting Current Test Vs 1. Apply Voltage Vs on on primary phase, secondary winding left floating 2. Measure currurent I ex 3. The current required to force ``transformer action´´ (the use of one winding to induce a voltage in the second winding). SEPTEMBER 5 - 7, 2018

Exciting Currents - Analysis Strategy • Confirm Expected Phase Pattern • Confirm Expected LTC Pattern ( For load tap changing transformers ) • Compare to Previous Results SEPTEMBER 5 - 7, 2018

Exciting Current - Analyzing Results Confirming the Expected Phase Pattern: 1. High – Low – High ( HLH ) Pattern  Expected for a 3-legged core type transformer.  Expected for a 5-legged core (or shell) type transformer with a Delta connected secondary winding. 2. Low – High – Low ( LHL ) Pattern  Will be obtained on a 3-legged core type transformer if the traditional test protocals are not followed.  Neutral on high side Wye-configured transformer is inaccessible  Forget to ground 3 rd terminal on a Delta-connected transformer  Expected for a 4-legged core type transformer. 3. All 3 Similar Pattern  Expected for a 5-legged core (or shell) type transformer with a non- delta secondary winding. SEPTEMBER 5 - 7, 2018

Exciting Current Test Results Transformer: Delta – Wye (Dyn1) H2 X2 X0 X1 H1 H3 X3 Test HV Lead LV Lead Ground Float Mode Measure Result 1 H1 H3 H2, X0 X1,X2,X3 UST H1-H3 63.8 mA 2 H2 H1 H3, X0 X1,X2,X3 UST H2-H1 48.6 mA 3 H3 H2 H1, X0 X1,X2,X3 UST H3-H2 64.2 mA SEPTEMBER 5 - 7, 2018

Turn Ratio - Expected Results SEPTEMBER 5 - 7, 2018

Turn Ratio - Expected Results The turn ratio measurement results should be within 0.5% of nameplate markings. SEPTEMBER 5 - 7, 2018

Leakage Reactance SEPTEMBER 5 - 7, 2018

Leakage Reactance • Short circuit LV winding or “winding pairs” • Inject 0.5 - 1.0% of rated current 60 Hz (Line-to-Line) • A variable 280 VAC source is recommended • Measure Series Current and Terminal Voltage RESULT - Z Ω , R Ω , and X Ω • • There are two ways to perform the measurement 1. 3 Phase Equivalent 2. Per Phase SEPTEMBER 5 - 7, 2018

Leakage Reactance - Example Nameplate: 6.85% 69 kV 12.5 MVA Phase V I Z R X L H 1 -H 3 55.22 1.05 51.59 4.38 51.41 136.4 H 2 -H 1 54.68 1.05 51.15 4.37 50.96 135.2 H 3 -H 2 54.46 1.05 50.96 4.46 50.76 134.2 SEPTEMBER 5 - 7, 2018

DC Winding Resistance - Failure Modes A change greater than the criteria mentioned can be indicative of the following: 1. Shorted Circuited Turns 2. Open Turns 3. Defective DETC or LTC (contacts) 4. A Poor Connection Between Terminals Measured SEPTEMBER 5 - 7, 2018

DC Winding Resistance - Case Study SEPTEMBER 5 - 7, 2018