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 C 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
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-voltages what 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 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 = rmsph-ph 3
Peak = kv = kv

10. Lightning Impulses Very fast rise can cause line insulation to fail
“backflashes” - rise in the KV per u-second range and 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. v t Lightning Impulse 1.2 x 50 u-seconds t1 t2 Tolerances:
Front: +/- 30%
Tail: +/- 20%
Virtual front = 1.2 micro-seconds .9 .3 A B t1
Virtual Zero .5 t2
Virtual tail = 50 micro-seconds
BIL = 1.0 v t
IEEE std Standard Techniques for High Voltage Testing

12. Impulse Generators: Charging circuit Discharging circuit+ -
Charging circuit + -
Discharging circuit
Impulse generators 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. v t Chopped Wave Impulse 1.2 x 3 u-seconds t 1.1 - 1.29 x BIL
IEEE std 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. Switching Surges: Z0= L/C
Voltage doubles when closing in on an open line = 2 P.U. at open line terminal
Z0= L/C
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 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: Strike Distance
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. v t 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 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

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

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

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

29. Equipment BILs (Class one equipment):
IEEE

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

31. Equipment Insulation 60 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)

32. Equipment Insulation Bus Insulators Air Switches Circuit Breakers
Transformers
Bushings

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

34. 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

35. Equipment Insulation Bus Insulators Air Switches Circuit Breakers
Transformers
Bushings

36. Bus Supports &
Air Switches
ANSI C37.32

37. EHV Switches:
Air Switches
ANSI C37.32

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

40. Bus Supports &
Air Switches
ANSI C37.32

41. Air Switches Passing the 3+9 test:
AP’s switch specification MS 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.

42. 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!

43. Equipment Insulation Bus Insulators Air Switches Circuit Breakers
Transformers
Bushings

44. ABB 550 PM

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

46. ABB 550 PM

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

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

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

50. Circuit Breaker 500kv PM Design Tests
One minute dry withstand 60hz 860kv
Ten second wet withstand 60hz 775kv
Full wave impulse (BIL) kv
Chop wave impulse – 2 u-sec to chop 2320kv
Chop wave impulse – 3 u-sec to chop 2070kv
Switching impulse – terminal to gnd kv (breaker open)
Switching impulse – terminal to gnd kv (breaker closed)
Switching impulse – terminal to terminal 1300kv (breaker open)

51. Breaker Applications
Closing resistors are used to reduce the switching surge created at the far end of a transmission line when it is energized
Once breaker is closed the resistor is removed from the circuit.
Not designed to handle continuous load current or fault current.
AP uses closing resistors on EHV breakers
Typical value: ohms.

52. Closing resistors:
bypass Rc
Z0= L/C Rc Z0 VS Z0 V0
V0 = VS
(Rc + Z0)

53. Equipment Insulation Bus Insulators Air Switches Circuit Breakers
Transformers
Bushings

54. Transformer BIL
Levels

55. Transformer Test
Levels

56. Transformers: 138-12kv delta-wye
Gas Space - N2
HV & LV BILs determine
the amount of line end
insulation of
the windings
low
volage
wintding
high
voltage
winding

57. Turn-Turn failure at line end
138kv disk winding – 30/40/50mva

58. Transformer HV BIL determines the distance between
Gas Space - N2
low
volage
wintding
HV BIL determines
the distance between
the high-low windings
and H-L insulation
high
voltage
winding

59. Induced test determines the high-ground insulation
Transformer
Gas Space - N2
Induced test determines
the high-ground
insulation
low
volage
wintding
high
voltage
winding

60. Transformer LV BIL and thru-fault mechanical forces stress low-ground
Gas Space - N2
LV BIL and thru-fault
mechanical forces
stress low-ground
insulation
low
volage
wintding
high
voltage
winding

63. Equipment Insulation Bus Insulators Air Switches Circuit Breakers
Transformers
Bushings

64. Bushings
Condenser type bushings are built to grade the voltage from line to ground evenly.
“creep” and contamination
SF6 weathersheds

65. 500 kV Bushing Type O

66. 500 kV Bushing Type O

67. 500 kV Bushing Type O

68. External Equipment Clearances:
Need to compare external equipment withstands with bus support and air switch clearances
Compare phase-to-ground and phase-to-phase equipment dimensions with the bus design
Equipment manufacturers will set phase-phase and phase-ground dimensions to pass equipment test standards but these are not coordinated with bus dimensions
Typically the clearances between live parts on the equipment are too small! Flashovers will occur at the equipment before the bus.
Need to specify equipment clearances that will coordinate with bus clearances

69. Transformer Clearances:
138-12kv
xfmr
Phase
To phase
To ground
138kv
49 in.
12kv
6.5 in.

70. AP Transformer
Design Clearances:

71. Proper Insulation Coordination
If lightning strikes the phase conductor and it generates enough voltage to flash the insulation I want it flashing over the cheapest, self restoring insulation first and I want it to go to ground

72. Transformer Terminated Lines
Reflected wave
Then close the
xfmr air switch
Initial Switching surge . .
When you close this breaker
a switching transient is created
that doubles at the other end of
the line because the transformer
is open on the low side
Breaker open
Close line
breaker first