Regional Technical Seminar SHORT CIRCUIT FORCES
Short Circuit Forces Wallace Exum Electrical Design Engineer wallace.exum@spx.com
Agenda 1. What is Short Circuit 2. Types of Faults 3. How to Calculate Short Circuit Current 4. Relationship between Current, Magnetic Field and Forces 5. Short Circuit Failure Modes 6. Non-standard Requirements 7. Types of Transformer Applications with Special Fault Duty SPX Transformer Solutions, Inc. August 31, 2018 3
What is Short Circuit SPX Transformer Solutions, Inc.
Normal Transformer Operation Normal Circuit An AC source supplies power to a given load (i.e. a city). A complete circuit has a source, with power entering a load and returning to the source. Amount of current that flows is directly related to the load on the transformer. 1000 amps 1000 amps SPX Transformer Solutions, Inc. June 20 th, 2018 5
12500 amps What is a Fault? System Fault An un-intended electrical connection made between two energized components having different voltage potentials. Results in some (or all) of the current bypassing the intended load. Currents are typically very high due to low fault impedance SPX Transformer Solutions, Inc. June 20 th, 2018 6
Types of Faults SPX Transformer Solutions, Inc.
Types of Faults Basic Types of Faults in Power Systems Line-to-Ground (Most Common) One or more conductors make electrical contact with ground Example: Wildlife or Lightning. A lightning strike causes a flashover. The stroke between the line and ground causes ionization of the air (a low impedance path to ground). SPX Transformer Solutions, Inc. August 31, 2018 8
Types of Faults Basic Types of Faults in Power Systems Line-to-Line Two different phases come into direct or indirect contact with each other Example: A bird with a large wingspan touches two conductors simultaneously SPX Transformer Solutions, Inc. August 31, 2018 9
Types of Faults Basic Types of Faults in Power Systems Double Line-to-ground Three Phase (least common) Similar to Line-to-Line but when all three phases make contact with each other Example: A falling tree on a transmission line SPX Transformer Solutions, Inc. August 31, 2018 10
Types of Faults IEEE Standards for Transformers and Short Circuit: C57.12.00-2015 Section 7 Liquid-immersed transformers shall be designed and constructed to withstand the mechanical and thermal stresses produced by external short circuits under the conditions specified in 7.1.3, 7.1.4, and 7.1.5. The external short circuits shall include three-phase, single line-toground, double line-to-ground, and line-to-line faults on any one set of terminals at a time. Duration of faults shall be limited to 2 seconds unless otherwise specified by the user Transformer components such as leads, bushings, LTCs, de-energized tap changers, and current transformers that carry current continuously shall comply with all the requirements of 7.1.3 and 7.1.4. However, when not explicitly specified, LTCs are not required to change taps successfully under short circuit conditions. The temperature of the winding conductor (copper) during short circuit shall not exceed 250C C57.12.90-2015 Section 12 Impedance variation shall not exceed 2% for transformers <= 30MVA (ONAN) Impedance variation shall not exceed 1% for transformers > 30MVA (ONAN) Transformers shall be designed to withstand all types of faults SPX Transformer Solutions, Inc. August 31, 2018 11
Types of Faults IEEE Standards for Transformers and Short Circuit: IEEE Standard C57.109 Sample I 2 * t curve for a Category IV transformer. Category IV transformer > 10,000 KVA single-phase > 30,000 KVA three-phase SPX Transformer Solutions, Inc. August 31, 2018 12
How to Calculate Short Circuit Current SPX Transformer Solutions, Inc.
How to Calculate Short Circuit Current C57.12.00 Section 7 defines both symmetrical and asymmetrical current Symmetrical Current Asymmetrical Current Isc symmetrical SC Current (A, rms) Ir rated current (A, rms) Zt transformer impedance for same voltage tap and MVA as rated current (Ir) Zs system impedance in per unit on the same MVA base for rated current (Ir) Make sure Ir and Zt are on the same MVA base SPX Transformer Solutions, Inc. August 31, 2018 14
How to Calculate Short Circuit Current EXAMPLE: Assume we have a 30MVA transformer with a 69kV primary and a rated secondary current of 1000 amps. The tested load losses are 72 kw, and the tested impedance is 8.0%. Find Isc and Isc(pk,asym). Symmetrical Current with Zs Symmetrical Current without Zs 1000 0.08 + Z s Z s = MVA T MVA S = 30 9800 = 0.31% 1000 0.08 + 0.0031 = 12,034 A 1000 0.08 + 0 = 12,500A Difference is almost 500A or 4% Note: Zs is derived from C57.12.00-2015 Table 14 if not specified from customer. SPX Transformer Solutions, Inc. August 31, 2018 10
How to Calculate Short Circuit Current EXAMPLE: Assume we have a 30MVA transformer with a 69kV primary and a rated secondary current of 1000 amps. The tested load losses are 72 kw, and the tested impedance is 8.0%. Find Isc and Isc(pk,asym). Asymmetrical Current without Zs I SC pk asym = 2.702 12,500 I SC pk asym = 33,775 A Note: Forces are proportional to current-squared. Therefore the forces from this example will be (33,775 amps / 1000 amps) 2 = (33.775) 2 = 1140 x normal forces 1. Find %R Load Loss (kw) %R = 100x KVA T 2. Find X/R X R = Z T %R = 8% 0.24% = 33.33 = 100x72 30,000 = 0.24% K = 1 + e tan 1 33.33 + π 2 1 33.33 sin(tan 1 (33.33)) 2 K = 2.702 SC Current is much greater than normal load current SPX Transformer Solutions, Inc. August 31, 2018 11
How to Calculate Short Circuit Current EXAMPLE: Assume we have a 30MVA transformer with a 69kV primary and a rated secondary current of 1000 amps. The tested load losses are 72 kw, and the tested impedance is 8.0%. Find Isc and Isc(pk,asym). C57.12.00-2015 Table 13 K = 1 + e tan 1 33.33 + π 2 1 33.33 sin(tan 1 (33.33)) 2 K = 2.702 Peak factor can be calculated or more easily looked up in table SPX Transformer Solutions, Inc. August 31, 2018 12
How to Calculate Short Circuit Current COMPARISON EXAMPLE: Assume we have a 30MVA transformer with a 69kV primary and a rated secondary current of 1000 amps. What is the symmetrical current considering only the transformer impedance of 8%. What is it at 6% Symmetrical Current, 8% Zt Symmetrical Current, 6% Zt 1000 0.08 + 0 = 12,500A 1000 0.06 + 0 = 16,667A SPX Transformer Solutions, Inc. August 31, 2018 18
How to Calculate Short Circuit Current Impedance Consequences Standard Impedances are tabulated in C57.12.10-2010 Section 4.6.1 Standard Impedances are classified by HV BIL and whether or not the unit has an LTC Standard Impedances are tabulated for the self-cooled (ONAN) rating SPX Transformer Solutions, Inc. August 31, 2018 19
Relationship Between Current, Magnetic Field and Force SPX Transformer Solutions, Inc.
Relationship Between Current, Magnetic Field and Force Resulting Magnetic Field Direction (CW) Current Flow Derived from Maxwell s MMF Law SPX Transformer Solutions, Inc. August 31, 2018 21
Relationship Between Current, Magnetic Field and Force Effect of Many Turns Fields at inner/outer edges add together. One uniform magnetic path results Magnetic field (B) intensifies with # turns (N) or the current (I). B NI F (NI) 2 Forces are proportional to the amount of current, squared SPX Transformer Solutions, Inc. August 31, 2018 22
Relationship Between Current, Magnetic Field and Force Left hand rule Resulting Magnetic Field Direction (CW) F Resulting Force Direction F - Force B Flux I - Current Current Flow (I) Leakage Magnetic Field (B) Remember TIM and the FBI SPX Transformer Solutions, Inc. August 31, 2018 23
Relationship Between Current, Magnetic Field and Force Weaker Magnetic Field Weaker Magnetic Field Arrows represent FORCE and its relative magnitude & direction Max LV COIL Strongest Magnetic Field HV COIL SPX Transformer Solutions, Inc. Min Magnetic Field Radial Distance August 31, 2018 24
Relationship Between Current, Magnetic Field and Force A net magnetic force also results between two coils (i.e. HV to LV), because the two coils are essentially two huge electro-magnets that repel each other. Repulsive Force Directions Between Magnets or Coils Summative force between these coils could be millions of pounds The inner coil experiences net inward radial crushing compressive forces The outer coil experiences net outward radial expanding type forces SPX Transformer Solutions, Inc. August 31, 2018 25
Relationship Between Current, Magnetic Field and Force INDICATES FORCE DIRECTION Mag field leaks out radially whenever there is an axial spreading out of turns in a coil. The larger the axial spread of turns, the more radial the field becomes Axial Location of DETC Taps Finite Element Analysis of Leakage Field Between Coils SPX Transformer Solutions, Inc. August 31, 2018 26
Short Circuit Failure Modes Photo Courtesy of A-Line E.D.S SPX Transformer Solutions, Inc.
Short Circuit Failure Modes Axial Radial Combined Beam Bending Beam Bending Spiraling Keyspacer Compression Buckling Telescoping Conductor Tilting Hoop Facture of Axial Support SPX Transformer Solutions, Inc. August 31, 2018 28
Short Circuit Failure Modes Ribs Cooling duct Radial spacers Paper insulated conductor SPX Transformer Solutions, Inc. August 31, 2018 29
Short Circuit Failure Modes Due to Axial Forces Axial Beam Bending & Keyspacer Compression Photo Courtesy of A-Line E.D.S SPX Transformer Solutions, Inc. August 31, 2018 30
Short Circuit Failure Modes Due to Axial Forces Conductor Tilting SPX Transformer Solutions, Inc. August 31, 2018 31
Short Circuit Failure Modes Due to Axial Forces Fracture of Axial Support Photo Courtesy of A-Line E.D.S SPX Transformer Solutions, Inc. August 31, 2018 32
Short Circuit Failure Modes Axial Forces - Design Criteria Fracture of Axial Support Coil Supports Limited on deflection and flexural strength Verticals/ Flitch-plates are designed to some % of yield strength Active-Part is pre-stressed (clamped) Keyspacer Compression Stress must be no more than 11,600 PSI (80 mpa) for Paper Covered Conductor (Much less than facture limit) Stress must be no more than 17,400 PSI (120 mpa) for Paperless Conductor (such as netting CTC) Axial Beam Bending Stress must be no more than some % of yield strength Conductor Tilting Stress must be no more than some % critical tilting limit SPX Transformer Solutions, Inc. August 31, 2018 33
Short Circuit Failure Modes Due to Radial Forces Beam Bending SPX Transformer Solutions, Inc. August 31, 2018 34
Short Circuit Failure Modes Due to Radial Forces Buckling Photo Courtesy of A-Line E.D.S SPX Transformer Solutions, Inc. August 31, 2018 35
Short Circuit Failure Modes Due to Radial Forces Hoop SPX Transformer Solutions, Inc. August 31, 2018 36
Short Circuit Failure Modes Radial Forces - Design Criteria Radial Beam Bending and Hoop Stress must be no more than some % of yield strength Buckling Designed for Free and Forced Buckling Stress must be no more than some % of critical buckling limit Critical buckling limit depends on the slenderness ratio of the winding and the yield strength of the conductor SPX Transformer Solutions, Inc. August 31, 2018 37
Short Circuit Failure Modes Due to Combined Forces Spiraling Telescoping Photo Courtesy of A-Line E.D.S SPX Transformer Solutions, Inc. August 31, 2018 38
Winding Temperature During a Short Circuit Calculated on basis that all heat is stored (heats up too quickly to radiate heat to equilibrium) Insulation Temperature not to exceed 250 C for copper 200 C for EC grade aluminum Copper or Aluminum Method defined on IEEE C57.12.00-2000 section 7.4. Insulated Conductor SPX Transformer Solutions, Inc. June 20 th, 2018 39
Winding Temperature During a Short Circuit Approximate method: Tf = (SΔk)2 t Km + TOR + Ta Tf = final winding temperature at end of a short circuit ( C) TOR = maximum top liquid temperature rise over ambient temperature ( C) Ta = ambient temperature ( C) SΔk = winding current density at symmetrical short circuit current (W/dm 2 ) t = short circuit duration (s). Km = 156 for copper / 73 for EC grade aluminum SPX Transformer Solutions, Inc. June 20 th, 2018 40
Non-Standard Requirements SPX Transformer Solutions, Inc.
Non-Standard Requirements Short Circuit Current shall only be limited by the transformer impedance. Windings shall be designed for infinite bus condition (no system impedance). Any pre-fault operating voltage greater than 1.0 per unit rated voltage Any customer specified asymmetrical peak factor (Ex: 2.762) Any customer specified minimum offset of the winding s electrical centers Any customer specified buckling limits Example: Stress must be less than 35% of yield strength for Magwire Customer specifies CTC for all windings subjected to compressive forces References to IEC 60076-5 Annex A Fault duration longer than 2 seconds Special fault duty (C57.12.00-2015 Section 7.1.5.5) Specific lead support requirements (max span, material, density, etc.) SPX Transformer Solutions, Inc. August 31, 2018 42
Non-Standard Requirements - EXAMPLE EXAMPLE IS A TWO WINDING NON-LTC TRANSFORMER (DYN1). 12/16/20MVA 69KV 26.4KV 8.0% IZ FOUR CASES 0.25% OF WINDING OFFSET, 0% OVERVOLTAGE 0.50% OF WINDING OFFSET, 0% OVERVOLTAGE 0.50% OF WINDING OFFSET, 5% OVERVOLTAGE 0.50% OF WINDING OFFSET, 10% OVERVOLTAGE SPX Transformer Solutions, Inc. August 31, 2018 43
Non-Standard Requirements - EXAMPLE STRESS (PSI) FOR THE FOUR DIFFERENT CASES Low Voltage (Inner Winding) Radial Axial Beam Beam Keyspacer Hoop Buckling Bending Bending Compression Conductor Tilting 0.25% Wdg Offset, 0% Overvoltage 5437 5437 12311 12886 6935 1555 0.50% Wdg Offset, 0% Overvoltage 5435 5435 12307 12962 7439 1667 0.50% Wdg Offset, 5% Overvoltage 5993 5993 13569 14291 8201 1838 0.50% Wdg Offset, 10% Overvoltage 6577 6577 14892 15684 9000.6 2018 SPX Transformer Solutions, Inc. August 31, 2018 44
Non-Standard Requirements - EXAMPLE STRESS (PSI) FOR THE FOUR DIFFERENT CASES High Voltage (Outer Winding) Radial Axial Beam Beam Keyspacer Conductor Hoop Buckling Bending Bending Compression Tilting 0.25% Wdg Offset, 0% Overvoltage 5073 0 0 6348 2906 631 0.50% Wdg Offset, 0% Overvoltage 5071 0 0 6428 3098 673 0.50% Wdg Offset, 5% Overvoltage 5591 0 0 7087 3416 742 0.50% Wdg Offset, 10% Overvoltage 6136 0 0 7778 3749 814 SPX Transformer Solutions, Inc. August 31, 2018 45
Non-Standard Requirements - EXAMPLE PERCENT INCREASE FOR THE FOUR DIFFERENT CASES Low Voltage (Inner Winding) Radial Axial Beam Beam Keyspacer Hoop Buckling Bending Bending Compression Conductor Tilting 0.25% Wdg Offset, 0% Overvoltage - - - - - - 0.50% Wdg Offset, 0% Overvoltage -0.04% -0.04% -0.03% 0.59% 7.27% 7.20% 0.50% Wdg Offset, 5% Overvoltage 10.23% 10.23% 10.22% 10.90% 18.26% 18.20% 0.50% Wdg Offset, 10% Overvoltage 20.97% 20.97% 20.96% 21.71% 29.79% 29.77% SPX Transformer Solutions, Inc. August 31, 2018 46
Non-Standard Requirements - EXAMPLE PERCENT INCREASE FOR THE FOUR DIFFERENT CASES High Voltage (Outer Winding) Radial Axial Beam Beam Keyspacer Conductor Hoop Buckling Bending Bending Compression Tilting 0.25% Wdg Offset, 0% Overvoltage - - - - - - 0.50% Wdg Offset, 0% Overvoltage -0.04% - - 1.26% 6.61% 6.66% 0.50% Wdg Offset, 5% Overvoltage 10.21% - - 11.64% 17.55% 17.59% 0.50% Wdg Offset, 10% Overvoltage 20.95% - - 22.53% 29.01% 29.00% SPX Transformer Solutions, Inc. August 31, 2018 47
Non-Standard Requirements - EXAMPLE Changing the offset of electrical centers of the windings primarily affects the axial forces Changing the pre-fault overvoltage condition increases the Peak Factor (K) by that same percentage thereby increasing the peak current. Both axial and radial forces are affected. 5% Overvoltage will increase the short circuit forces by about 10% 10% Overvoltage will increase the short circuit forces by about 21% SPX Transformer Solutions, Inc. August 31, 2018 48
Questions SPX Transformer Solutions, Inc.