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Insulation Coordination “the selection of insulation strength”

Insulation Coordination “the selection of insulation strength”. Kiski Valley Substation - 1938. Luxor substation – 1933…………….before lightning arresters. Insulation.

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Insulation Coordination “the selection of insulation strength”

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  1. Insulation Coordination“the selection of insulation strength”

  2. Kiski Valley Substation - 1938

  3. Luxor substation – 1933…………….before lightning arresters

  4. Insulation • Self restoring insulation recovers after a flashover – external such as air switches, insulators, external surfaces of bushings (defined at standard atmospheric conditions) • Non-self restoring insulation does not recover after a flashover – enclosed oil, paper, gas, air systems such as transformers and circuit breakers

  5. Standards http://www.techstreet.com/info/astm. • Voltages: ANSI C84.1- 1995 System Voltages • Voltages: IEEE 1312 – 1987 AC Electrical Systems and Equipment Operating Voltages above 230kv • BILs: ANSI 92 defines BILs for the equipment manufacturers to use • IEEE std 4 – 1995 Standard Techniques for High Voltage Testing • Circuit Breakers: ANSI C-37.06 1997 • Transformers: ANSI C57.12 – 1993 Liquid Immersed Distribution, Power and Regulating Power Transformers • Switches: IEEE C37.34 Standard Test Code for High Voltage Switches • Insulators: ANSI C29.8 & C29.9 Cap and Pin Type & Post Type Insulators • Arresters: IEEE C62.11 Metal Oxide surge arresters for AC Power Circuits • Arresters: IEEE C62.22 Guide for application of Metal Oxide surge arresters for Alternating Current Systems

  6. Impulse Voltages and Equipment Requirements • Over-voltages • Transient Waveshapes • Equipment testing

  7. Over-voltageswhat causes over-voltages on the system? • Lightning • Switching – breakers clearing faults • Increases in power frequency voltage

  8. Transient Waveforms • Lightning - 1.2 x 50 u-second • Front of Wave • Chopped Waves • Switching - 250 x 2500 u-second • 60 hertz waves

  9. Peak or RMS • 60hz voltages are expressed in rmsph-ph • Duty cycle and MCOV are in rms phase-ground • Lightning, chopped wave and switching surges are expressed in peak volts phase to ground: Peak = 2 rmsph-ph 3 Peak = 2 145kv = 118.4kv 3

  10. Lightning Impulses • Very fast rise can cause line insulation to fail • “backflashes” - rise in the 500-2000KV per u-second range and 10-20 u-sec tail (cfo of line - positive polarity waves) • Direct strokes to phase wires (cfo due to negative polarity waves) slightly slower rise and longer tail than backflash • BIL: Basic Impulse Level is the electrical strength of insulation to the crest value of the standard lightning impulse (for dry conditions) • BIL = CFO – 1.28 x of (10% probability of flashover) • Test wave doesn’t reflect actual lightning but its easily reproduced in test lab • Lightning can be positive or negative compared to the earth – most strokes are negative. The insulation withstand can be different for each type

  11. Virtual Zero Virtual front = 1.2 micro-seconds .9 B Virtual tail = 50 micro-seconds .5 .3 A t1 t2 Lightning Impulse 1.2 x 50 u-seconds Tolerances: Front: +/- 30% Tail: +/- 20% BIL = 1.0 v t IEEE std 4-1995 Standard Techniques for High Voltage Testing

  12. + - Impulse Generators: Charging circuit + - Discharging circuit • Impulsegenerators are stacks of capacitors and resistors • Initially the capacitors are connected in parallel and charged up • Then the capacitors are re-connected in series • Then they are discharged to provide the high voltage impulse wave • Resistors are used to shape the front and tail of the wave

  13. Chopped Wave Impulses • Allows a faster front wave to be applied without flashover occurring on the tail of the wave • Chopped wave impulses are standard lightning waves that are shunted or “chopped” to zero on the tail of the wave within 2 or 3 u-seconds • Chopped wave tests are only used in ANSI (not IEC) standards and are meant to model surges created by the flashover of a post insulator or an arrester operation • Stresses transformer turn to turn insulation • Model the need for arresters at the “open breaker”

  14. Chopped Wave Impulse 1.2 x 3 u-seconds 1.1 - 1.29 x BIL BIL = 1.0 v t t IEEE std 4-1995 Standard Techniques for High Voltage Testing

  15. Switching Impulses • BSL: Basic Switching Level is the electrical strength of insulation to the crest value of the standard switching impulse (wet conditions) • BSL = CFO – 1.28 x of (10% probability of flashover) • Switching impulses are longer duration both on the front and the tail (hundreds of u-seconds) • Low line surge impedance (> 138kv overhead lines, also cable circuits) • Re-closing into trapped charge on line

  16. Z0= L/C Switching Surges: Voltage doubles when closing in on an open line = 2 P.U. at open line terminal Assume that High Speed Re-closing traps a negative 1 P.U. charge on the line. Then when the breaker re-closes the maximum voltage at the open end can approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending on damping (R): 3.5 P.U. Trapped charge = -1.0 P.U.

  17. Switching Surges: Insulation strength of air gaps: 1.2x50 lightning 50% Critical Flashover Voltage CFO Positive switching surge Strike Distance At operating voltages above 230kv switching transients become the constraint that drives system design Breakers & switches are only tested for switching surge at 345kv and above. Transformers 115kv & above.

  18. Switching Impulse 250 x 2500 u-seconds Tolerances: Front: +/- 20% Tail: +/- 60% 1.0 v .5 t t1 = 250 u-sec t2 = 2500 u-sec IEEE std 4-1995 Standard Techniques for High Voltage Testing

  19. Power Frequency (60hz) Maximum 60Hz Operating Voltage Nominal + 5% 500kv system is nominal + 10%

  20. System Voltages ANSI C84.1 - 1995

  21. System Voltages ANSI C84.1 - 1995

  22. Power Frequency (60hz) Overvoltages • Rise on unfaulted phases for a line-ground fault (X/R and co-efficient of grounding) • Loss of ground (backfeed) • Ferro-resonance – line or cable capacitance with transformer inductance • Load rejection • Ferranti effect – long unloaded transmission lines • Transformer inrush

  23. Coefficient of Grounding

  24. Potential Rise on the Line Voltage . . Breaker open

  25. Equipment Insulation • Bus Insulators • Air Switches • Circuit Breakers • Transformers • Bushings • Equipment External Clearances

  26. 1.0 BIL/BSL Testing .5 Probability of Flashover .1 CFO KV BIL or BSL = 1.28 of

  27. Equipment BILs (Class one equipment): IEEE 1313.1 1996

  28. Equipment BILs/BSLs for EHV (Class two):

  29. Equipment Insulation60 hz testing: • 60 hz tests – wet tests are done on insulators, air switches and cijrcuit breakers to understand response during rain (IEEE std 4 – 1995) • and dew conditions (ANSI C-29.1 – 1988 section 4.6)

  30. Equipment Insulation • Bus Insulators • Air Switches • Circuit Breakers • Transformers • Bushings

  31. Bus Supports & Air Switches ANSI C37.32 Withstand (10%)

  32. Standard Atmospheric Conditions • Ambient temperature = 20 deg C • Air Pressure = 760mm mercury (1 atmosphere) • Absolute humidity = 11 gms water/Mtr3 air • The test voltages are adjusted as needed when conditions vary from the standard

  33. Equipment Insulation • Bus Insulators • Air Switches • Circuit Breakers • Transformers • Bushings

  34. Bus Supports & Air Switches ANSI C37.32

  35. EHV Switches: Air Switches ANSI C37.32

  36. Air Switch Coordination - Gap flashover - Gap withstand - Insulator flashover - Insulator withstand Make the gap withstand 10% higher than the insulator flashover!

  37. Bus Supports & Air Switches ANSI C37.32

  38. Air Switches Passing the 3+9 test: AP’s switch specification MS 1100-10 requires transmission switches to withstand 15 impulses without flashover for the gap withstand and also flashover for 15 impulses to establish the insulator flashover value.

  39. Circuit Switcher Coordination In the model Mark V circuit switcher the interrupter closes when the switch blade reaches the full open position so the open blade must coordinate with the post insulator. In the series 2000 the interrupter stays Open when the blade opens!

  40. Equipment Insulation • Bus Insulators • Air Switches • Circuit Breakers • Transformers • Bushings

  41. ABB 550 PM

  42. Weakest point is typically the throat shield to ground Circuit Breaker Coordination Breaker closed

  43. ABB 550 PM

  44. ABB 550 PM Failure Doubs DL-55, July 8, 2000 – Tube Flashover

  45. Circuit Breaker Coordination Breaker open Switching surge values are higher for the gap than for phase to ground

  46. Circuit Breaker Design Tests Breaker switching surge withstand is higher across the interrupter than to ground!

  47. Circuit Breaker500kv PM Design Tests • One minute dry withstand 60hz 860kv • Ten second wet withstand 60hz 775kv • Full wave impulse (BIL) 1800kv • Chop wave impulse – 2 u-sec to chop 2320kv • Chop wave impulse – 3 u-sec to chop 2070kv • Switching impulse – terminal to gnd 1175kv (breaker open) • Switching impulse – terminal to gnd 1175kv (breaker closed) • Switching impulse – terminal to terminal 1300kv (breaker open)

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