Transcript Transformer Basics

Insulation Coordination
“the selection of insulation
strength”
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Kiski Valley
Substation - 1938
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Luxor substation – 1933…………….before lightning arresters
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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
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Standards
http://www.techstreet.com/info/astm.
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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
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Impulse Voltages and Equipment
Requirements
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Over-voltages
Transient Waveshapes
Equipment testing
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Over-voltages
what causes over-voltages on the system?
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Lightning
Switching – breakers clearing faults
Increases in power frequency voltage
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Transient Waveforms
Lightning - 1.2 x 50 u-second
 Front of Wave
 Chopped Waves
 Switching - 250 x 2500 u-second
 60 hertz waves
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Peak or RMS
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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:
rmsph-ph
Peak
=
2
3
Peak
=
2 145kv = 118.4kv
3
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Lightning Impulses
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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 withstand10
can be different for each type
Lightning Impulse
Tolerances:
Front: +/- 30%
Tail: +/- 20%
1.2 x 50 u-seconds
Virtual Zero
Virtual front = 1.2 micro-seconds
BIL = 1.0
.9
B
Virtual tail = 50 micro-seconds
v
.5
.3
A
t
t1
IEEE std 4-1995 Standard Techniques for High Voltage Testing
t2
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Impulse Generators:
+
-
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
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Chopped Wave Impulses
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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”
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Chopped Wave Impulse
1.2 x 3 u-seconds
1.1 - 1.29 x BIL
BIL = 1.0
v
t
IEEE std 4-1995 Standard Techniques for High Voltage Testing
t
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Switching Impulses
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BSL: Basic Switching Level is the electrical
strength of insulation to the crest value of the
standard switching impulse (wet conditions)
o
– BSL = CFO – 1.28 x f (10% probability of
flashover)
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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
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Switching
Surges:
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 3.5 - 4.0 P.U. for multiple reflections depending on
damping (R):
3.5 P.U.
Trapped charge = -1.0 P.U.
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Switching
Surges:
Insulation strength of
air gaps:
50% Critical
Flashover Voltage
CFO
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.
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Switching Impulse
Tolerances:
Front: +/- 20%
Tail: +/- 60%
250 x 2500 u-seconds
1.0
v
.5
t
t1 = 250 u-sec
t2 = 2500 u-sec
IEEE std 4-1995 Standard Techniques for High Voltage Testing
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Power Frequency (60hz)
Maximum 60Hz Operating
Voltage
Nominal + 5%
500kv system is nominal + 10%
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System Voltages ANSI C84.1 - 1995
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System Voltages ANSI C84.1 - 1995
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Power Frequency (60hz)
Overvoltages
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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
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Coefficient of Grounding
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Potential Rise on the Line
Voltage
..
Breaker open
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Equipment Insulation
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Bus Insulators
Air Switches
Circuit Breakers
Transformers
Bushings
Equipment External Clearances
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Probability of Flashover
1.0
BIL/BSL Testing
.5
.1
CFO
BIL or BSL = 1.28 of
KV
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Equipment BILs (Class one equipment):
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IEEE 1313.1 1996
Equipment BILs/BSLs for EHV (Class two):
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Equipment Insulation
60 hz testing:
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60 hz tests – wet tests are done on insulators, air
switches and cijrcuit breakers to understand
response during rain (IEEE std 4 – 1995)
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and dew conditions (ANSI C-29.1 – 1988 section
4.6)
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Equipment Insulation
Bus Insulators
 Air Switches
 Circuit Breakers
 Transformers
 Bushings
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Bus Supports & Air
Switches ANSI C37.32
Withstand (10%)
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Standard Atmospheric Conditions
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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
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Equipment Insulation
Bus Insulators
 Air Switches
 Circuit Breakers
 Transformers
 Bushings
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Bus Supports &
Air Switches
ANSI C37.32
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EHV Switches:
Air Switches
ANSI C37.32
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Air Switch Coordination
- Gap flashover
- Gap withstand
- Insulator flashover
- Insulator withstand
Make the gap withstand 10%
higher than the insulator flashover!
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Bus Supports &
Air Switches
ANSI C37.32
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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.
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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!
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Equipment Insulation
Bus Insulators
 Air Switches
 Circuit Breakers
 Transformers
 Bushings
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ABB 550 PM
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Weakest point is typically
the throat shield to ground
Circuit Breaker Coordination
Breaker closed
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ABB 550 PM
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ABB 550 PM Failure
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Doubs DL-55, July 8, 2000 – Tube Flashover
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Circuit Breaker Coordination
Breaker open
Switching surge values are higher for the
gap than for phase to ground
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Circuit Breaker Design Tests
Breaker switching surge
withstand is higher across
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the interrupter than to ground!
Circuit Breaker
500kv PM Design Tests
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One minute dry withstand 60hz
Ten second wet withstand 60hz
Full wave impulse (BIL)
Chop wave impulse – 2 u-sec to chop
Chop wave impulse – 3 u-sec to chop
Switching impulse – terminal to gnd
(breaker open)
Switching impulse – terminal to gnd
(breaker closed)
Switching impulse – terminal to terminal
(breaker open)
860kv
775kv
1800kv
2320kv
2070kv
1175kv
1175kv
1300kv
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Breaker Applications
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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: 400-450 ohms.
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Closing resistors:
bypass
Rc
Z0= L/C
Rc
VS
Z0
V0
V0 = VS
Z0
(Rc + Z0)
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Equipment Insulation
Bus Insulators
 Air Switches
 Circuit Breakers
 Transformers
 Bushings
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Transformer BIL
Levels
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Transformer Test
Levels
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Transformers:
138-12kv delta-wye
Gas Space - N2
high
voltage
winding
low
volage
wintding
HV & LV BILs determine
the amount of line end
insulation of
the windings
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Turn-Turn failure at line end
138kv disk winding – 30/40/50mva
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Transformer
Gas Space - N2
high
voltage
winding
low
volage
wintding
HV BIL determines
the distance between
the high-low windings
and H-L insulation
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Transformer
Gas Space - N2
high
voltage
winding
Induced test determines
the high-ground
insulation
low
volage
wintding
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Transformer
Gas Space - N2
high
voltage
winding
LV BIL and thru-fault
mechanical forces
stress low-ground
insulation
low
volage
wintding
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Equipment Insulation
Bus Insulators
 Air Switches
 Circuit Breakers
 Transformers
 Bushings
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Bushings
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Condenser type
bushings are built to
grade the voltage
from line to ground
evenly.
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“creep” and
contamination
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SF6 weathersheds
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500 kV Bushing Type O
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500 kV Bushing Type O
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500 kV Bushing Type O
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External Equipment Clearances:
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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 phaseground 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
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Transformer Clearances:
138-12kv
xfmr
138kv
12kv
Phase
To phase
49 in.
6.5 in.
Phase
To ground
49 in.
6.5 in.
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AP Transformer
Design Clearances:
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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
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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
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