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Knowledge Is Power SM Apparatus Maintenance and Power Management for Energy Delivery

Knowledge Is Power SM Apparatus Maintenance and Power Management for Energy Delivery. Assessing the Magnetic Circuit of a Transformer Jill Duplessis Doble Engineering Company. 2002 Regional Seminar - Denver. Magnetic Circuit of a Transformer. Objectives

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Knowledge Is Power SM Apparatus Maintenance and Power Management for Energy Delivery

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  1. Knowledge IsPowerSMApparatus Maintenance and Power Management for Energy Delivery Assessing the Magnetic Circuit of a Transformer Jill DuplessisDoble Engineering Company 2002 Regional Seminar - Denver

  2. Magnetic Circuit of a Transformer • Objectives • To provide an explanation of what we are learning about a transformer when we perform an exciting current test and a leakage reactance test. • To review which tests Doble recommends that you should be performing & when. • To review how one goes about analyzing results from these tests. • Finally, to reinforce what we learn by reviewing case studies together. 2002 Regional Seminar - Denver

  3. A Transformer Fundamental Principle of Operation Energy Transfer from one electrical circuit to another. Not perfect: • Some energy is lost and dissipated as heat. • Some energy is temporarily stored. 2002 Regional Seminar - Denver

  4. Energy In Energy Out Equivalent Circuit of an Ideal Transformer If the energy transfer process was perfect, we’d be talking about an ideal transformer. Assuming a 1:1 turns ratio, the equivalent circuit of an Ideal Transformer looks like this: Since there are no losses in an ideal transformer, Energy In = Energy Out 2002 Regional Seminar - Denver

  5. R R L R L R DC-1 L- 1 1 L- 2 DC- 2 2 L C UST m R m Equivalent Circuit of a Transformer From an energy transfer point-of-view, the elements in this circuit represent the imperfections in a transformer. Primary winding dc resistance measurement Secondary winding dc resistance measurement Exciting Current and Loss measurement, Zm Leakage Reactance and Loss measurement, ZL Dielectric loss (measured in overall tests is lumped in with Rm) 2002 Regional Seminar - Denver

  6. Losses in a Power Transformer • Practical Transformer vs. Ideal Transformer • Losses occur due to the following imperfections in a transformer: • Windings have resistance DC resistance tests • Real and reactive losses exist in the core Exciting current tests • Physical cores have a finite permeability; exciting current is required to produce magnetic flux Leakage reactance tests • There is magnetic flux leakage • Losses in the dielectric circuit What we measured in the overall tests on a xfmr. 2002 Regional Seminar - Denver

  7. Losses in a Power Transformer Good News Losses in a transformer are specified & controlled. Manufacturer bases price on guaranteed losses. Manufacturer designs adequate cooling for a transformer based on losses. even though losses represent a cost to the user, from a diagnostic perspective, we can use loss info to verify the integrity of the unit. We are looking for evidence of a change in the known losses of the transformer. 2002 Regional Seminar - Denver

  8. Before we get started... A History Lesson in Magnetism: 1820 - Hans Christian Oersted discovered that when an electric current flows through a wire, it causes a compass needle to rotate. i.e. he discovered that an electric current produces a magnetic field. Michael Faraday - his ideas about conservation of energy led him to believe that since an electric current could cause a magnetic field, a magnetic field should be able to produce an electric current. 2002 Regional Seminar - Denver

  9. History of Magnetism Faradaydemonstrated this principle of induction in 1831 with the following experiment: • He moved a coil of wire relative to a magnet & discovered that a voltage was induced in the coil. (but only when relative movement is taking place) Michael Faraday demonstrated the phenomenon of electromagnetism in a series of experiments. • Responsible for the principles by which electric generators and transformers work. 2002 Regional Seminar - Denver

  10. History of Magnetism For example We apply Faraday’s discovery to the arrangement where a magnetic field is associated with the turns of a winding. (This time - we are not moving the winding, it stays stationary. Instead, we vary the magnetic field  same effect.)  Any variation in the strength of the magnetic field will induce a voltage between the terminals of the winding.  No voltage is produced if the magnetic field strength is constant. 2002 Regional Seminar - Denver

  11. History of Magnetism Next  consider a winding through which a current is passed. (Remember that Oersted proved that a current flowing through a winding will produce a magnetic field within the winding & in the space surrounding the winding). So, when the current is varied (as by applying an a.c. source), the strength of the magnetic field produced by the winding will vary. If we place a 2nd winding near this 1st winding, the 2nd winding will enclose some of the magnetic field produced by the 1st. 2002 Regional Seminar - Denver

  12. History of Magnetism  Since the varying magnetic field produced by the 1st winding is “linked” by the 2nd winding, a voltage will be produced between the terminals of the 2nd winding. A Word about Linking: If windings are only in close proximity to each other, linking or coupling between them is not very effective.  a considerable amt. of the magnetic field produced by the 1st wdg. does not link the 2nd wdg. 2002 Regional Seminar - Denver

  13. History of Magnetism How can we improve the linking between windings?  By arranging the windings relative to each other using a structure of magnetic material (the core). The core uses suitable magnetic material (usually silicon-iron) that allows a very high degree of coupling between windings. UNFORTUNATELY... The core material is not perfect. 2002 Regional Seminar - Denver

  14. Electromagnetism Background Recall that one of the properties that differentiates an ideal transformer from an actual transformer sitting in a substation is that ... • Physical cores have a finite permeability  exciting current is required to produce magnetic flux in the core We see this... During an open-circuit measurement in which we observe that a small (usually inductive) current is drawn at the primary terminals even though the secondary terminals are open. 2002 Regional Seminar - Denver

  15. Example of Core Characteristics Permeability (slope) - the ability of a material to conduct flux Illustrates affect of core construction on magnetizing/ hysteresis effects. 2002 Regional Seminar - Denver

  16. 1:1 Iex + E2 + - E1 - Exciting Current Theory When the Secondary Winding Is Open   The current that flows in the primary winding should be sufficient to excite the core. 2002 Regional Seminar - Denver

  17. 1:1 f2 1 Iex ZL f2 + E1 ~ E2 - Exciting Current Theory If Load was Connected to the Secondary + I2 I2 The primary current increases by the value of the secondary current. 2002 Regional Seminar - Denver

  18. 1:1 ff Iex 1 ff HV H1 + If E1 - LV H0 Exciting Current Theory When We Have a Turn-to-Turn Fault on the Secondary During the Exciting Current Test + If The primary current increases by the value of the current through the short-circuited turns. 2002 Regional Seminar - Denver

  19. 1:1 ff Iex f ff HV H1 + If - H0 LV If secondary winding is and one of the windings develops a fault to ground, the primary current will increase by the value of current circulating through the secondary winding and two grounds. Exciting Current Theory Detection of Winding to Ground Fault in the Secondary During Exciting Current Test? + If 2002 Regional Seminar - Denver

  20. 1:1 fa Iex fa HV H1 Ia H0 LV Exciting Current Theory How Do We Detect Fault in the Preventive Autotransformer During Exciting Current Test? + Ia When autotransformer is connected across two taps it acts as a load and the primary current goes up. 2002 Regional Seminar - Denver

  21. Exciting Current Tests Useful in Detecting: • Turn-to-turn winding failure • 1 or more turns completely short-circuited. • 2 or more parallel strands of different turns are short-circuited. • LTC problems • Open circuit, shorted turns or high resistance connections in the LTC P.A., series auto or series transformer • misalignment, mechanical problems, coking and wear of LTC & DETC contacts 2002 Regional Seminar - Denver

  22. Exciting Current Tests Useful in Detecting (cont): • Manufacturing defects. • Abnormal (multiple) core grounds. • Changes in the core characteristics. 2002 Regional Seminar - Denver

  23. H2 L-V Lead L-V Lead H3 H2 Ie(1-2) Ie(1-2) H-V Test Cable Ie(1-3) H3 Ie(1-3) GND Lead I&W Meter GND Lead Guard Point Guard Point I&W Meter Exciting Current Test Procedure - Delta UST Mode Note: For Exciting Current tests performed with the M4000, the charging current (mA) and watts-loss are recorded. 2002 Regional Seminar - Denver

  24. H2 H3 H2 H1 H-V Test Cable H0 Ie(1-0) H1 Ie(1-0) H0 I&W Meter L-V Lead L-V Lead H3 Guard Point Guard Point I&W Meter Exciting Current Test Procedure -Wye UST Mode 2002 Regional Seminar - Denver

  25. Important!!!! Test Measurement Recommendations 2002 Regional Seminar - Denver

  26. Especially Important!!!! Test Measurement Recommendations 2002 Regional Seminar - Denver

  27. Analysis TEST RESULTS ANALYSIS • LTC and phase patterns should be analyzed • It is useful to know whether specimen is capacitive or inductive • Watts loss is always determined by the core • So... • What do we mean by LTC & phase pattern? • What makes a specimen inductive rather than capacitive or vice-versa? 2002 Regional Seminar - Denver

  28. LTC and Phase Pattern • Nomenclature • LTC pattern • The relationship between exciting current (or loss) measurements recorded within a phase as the LTC is moved from one position to another. • 12 LTC patterns • Phase Pattern • The relationship between exciting current (or loss) measurements recorded for all three phases at a single tap position. • 3 Phase patterns 2002 Regional Seminar - Denver

  29. R R L R L R ~ DC-1 L- 1 1 L- 2 DC- 2 2 L C UST m Exciting Current and Loss measurement, Zm R m Capacitive or Inductive Specimen? To understand what makes a specimen capacitive or inductive, we revisit the equivalent circuit of a transformer. I2R loss is much lower than loss in the core we can neglect the energy storage and loss in the leakage channel. Practically all of the magnetic flux is confined to the core.  the impedance encountered by the current is predominantly determined by the reluctance of the core. 2002 Regional Seminar - Denver

  30. I ex I I I V L R C q L V Q I Capacitive or Inductive Specimen? Equivalent Circuit of the Open-Circuit Test reduces to: I C I C R R I ex I L L - Magnetizing Inductance C - Turn-to-turn Capacitance R - Resistance associated with losses in the core & turn-to-turn insulation 2002 Regional Seminar - Denver

  31. Capacitive or Inductive Specimen? • Inductive LTC pattern • Magnetizing current capacitive current in each tap position, so that the resultant measured current is always inductive in nature. • Characteristic of the vast majority of exciting current test results reported for transformers. • Capacitive LTC pattern • Capacitive currentmagnetizing current, at several tap positions. 2002 Regional Seminar - Denver

  32. Analysis for Inductive Specimens For an inductive specimen (majority of xfmrs): • The LTC pattern should be identified by comparing the behavior of the test data with one of the 12 documented LTC patterns. • The LTC pattern should be the same in each of the 3 phases, for both the mA results & the Watts results. • The phase pattern at each tap position should be confirmed. • This pattern should be identical at every tap position, for both mA & Watts measurements. 2002 Regional Seminar - Denver

  33. Analysis for Inductive Specimens • Possible phase patterns: H-L-H Characteristic of: Phase Pattern A • 3-legged core-type transformer • 5-legged core or shell-type transformer that has a delta-connected secondary winding L-H-L Characteristic of: Phase Pattern B • 3-legged core-type transformer that has a wye-connected winding with an inaccessible neutral. • 3-legged core-type transformer with a delta-connected winding if testing two phases of the winding in parallel. 2002 Regional Seminar - Denver

  34. Wye Winding with No H0 Bushing 2002 Regional Seminar - Denver

  35. Wye Winding with No H0 Bushing 2002 Regional Seminar - Denver

  36. Analysis for Inductive Specimens All 3 Readings Similar Phase Pattern C • Characteristic of four and five legged core-type transformers and shell-type transformers with non-delta secondary windings • All 3 Readings Dissimilar (H-M-L) • May be indicative of a magnetized core • May actually be “capacitive” specimen - not inductive after all 2002 Regional Seminar - Denver

  37. Analysis FACTORS OTHER THAN DEFECTS THAT MAY INFLUENCE TEST RESULTS: If capacitance  inductive component, you have a capacitive specimen & analysis changes. • UST capacitance • Test voltage Test results are voltage dependent so data can only be compared if performed at identical voltages. • Residual magnetism • Design and position of LTC • Test Connections 2002 Regional Seminar - Denver

  38. Capacitive LTC Patterns • Experience with Capacitive LTC Patterns • Effects documented as early as 1972 • 1996 - 3rd Component of Exciting Current discussed in detail • Negligible in Low-voltage Transformers • Traditionally, IC Im in High-voltage Transformers • Today, IC may be of same order of magnitude or  Im • Due to Reduced losses & magnetizing power of xfmr cores • Due to high capacitance windings 2002 Regional Seminar - Denver

  39. Capacitive LTC Patterns • Effects of a Strong Capacitive Presence • PHASE PATTERN IS AFFECTED • Depends on Relative Magnitudes of Im and IC in Each Phase • Typically all 3 Phases are Capacitive • CAN RESULT IN ANY PHASE PATTERN • Measured Phase Pattern Accepted as Benchmark • Phase Pattern for Current may Differ from Phase Pattern for Loss 2002 Regional Seminar - Denver

  40. I , I [mA] I L C C I L I < I L C I > I L C 10 5 V [kV] Factors other than defects that can influence test results. Test Voltage 2002 Regional Seminar - Denver

  41. Factors other than defects that can influence test results. RESIDUAL MAGNETISM • Always present, but in most cases has no significant effect on test results. • Majority of problems have a much larger effect (> 50%) on test results than residual magnetism would have • Increases current if specimen is inductive • Increases or decreases current if specimen is capacitive 2002 Regional Seminar - Denver

  42. Example of an LTC Pattern Pattern 1: Test results for all non-bridging positions are equal. Test results for all bridging positions are equal. I 1 N 4R 8R 12R 16R 2002 Regional Seminar - Denver

  43. Pattern 2: Test results for all non-bridging positions are equal. Test results for all bridging positions are equal, except in one or several positions, with all readings in these positions being equal as well. N 4R 8R 12R 16R Example of an LTC Pattern 2002 Regional Seminar - Denver

  44. Exciting Current Testing Case Studies 2002 Regional Seminar - Denver

  45. Exciting Current Case Study 1 Case Number 02-06 Unit Tested: U.S. Transformer, 3Φ two-winding transformer • Δ-Y connected • 20 MVA • 69/12.47 kV • 1978 - vintage • Rewound in 2000 2002 Regional Seminar - Denver

  46. Case Study 1 (# 02-06) • Testing Circumstances: • Tested upon receipt from the manufacturer’s repair facility where it had been completely rewound. • overall insulation tests - acceptable • bushing tests - acceptable • field power factor test on an oil sample from the main tank - acceptable 2002 Regional Seminar - Denver

  47. Case Study 1 - Exciting Current Results 2002 Regional Seminar - Denver

  48. LTC Pattern 4 Pattern 4: Test results represent a series transformer or autotransformer exciting current superimposed on pattern 1. This current changes according to increments in the tap winding. I 1 16R N 4R 8R 12R 2002 Regional Seminar - Denver

  49. LTC Pattern - Phase A 2002 Regional Seminar - Denver

  50. LTC Pattern - Phase B 2002 Regional Seminar - Denver

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