Transcript Slide 1

# Terms and Definitions
# Voltage Sags and
Interruptions
2. Terms and definitions
1
Definitions
• Sag (or dip) is a decrease to a low voltage
(between 0.1 and 0.9 per unit rms) at
power frequency for durations of 0.5 cycle
to 1 minute
• Swell is an increase to a high voltage
(between 1.1 and 1.8 per unit rms) at
power frequency for durations of 0.5 cycle
to 1 minute
2. Terms and definitions
2
Definitions
• Transient refers to an event of duration <
0.5 cycle
• Instantaneous: duration 0.5 cycle to 30
cycles
• Momentary: duration 30 cycles to 3 sec
• Temporary: duration 3 sec to 1 min.
• Sustained or long-duration: duration > 1
min.
2. Terms and definitions
3
Impulsive transient due to lightning
2. Terms and definitions
4
Shunt capacitors may
be installed in
substations or on
poles or in enclosures
2. Terms and definitions
5
Capacitor switching
2. Terms and definitions
6
Long-duration overvoltage caused
by ferroresonance of transformer
2. Terms and definitions
7
What is ferroresonance?
• Resonance of unloaded transformer
(nonlinear magnetizing branch) with a
capacitance
-jXc=1/(jwC)
I
E
-jXc
2. Terms and definitions
VL
8
2.0 Short Duration
• 2.1 Interruption
Short duration sags and swells
2. Terms and definitions
10
Temporary voltage sag caused by induction
motor starting across the line
2. Terms and definitions
11
4.0 Voltage Imbalance
• Voltage Imbalance
Waveform distortion
•
•
•
•
DC offset
Harmonics
Interharmonics
Notching- It is a periodic voltage disturbance
caused by the normal operation of power electronic
devices when current is commuted from one phase
to another.
• Noise- It is defined as unwanted electrical signals
with broadband spectral content lower than 200
kHz superimposed upon the power system voltage
or current in phase conductors, or found on neutral
conductors or signal lines.
2. Terms and definitions
13
Harmonics
2. Terms and definitions
14
Harmonics + interharmonics
2. Terms and definitions
15
Three-phase rectifier driving DC motor
A
B
C
E = k wm
As VB rises above VA, current is
commuted from A to B. While diode B is
turning on, diode A is turning off. A is
shorted to B (for a short time while both
diodes conduct), producing notching.
2. Terms and definitions
16
Waveform distortion due to
notching
2. Terms and definitions
17
Voltage fluctuation
# Voltage magnitude does not normally exceed the
ranges of 0.9 pu to 1.1 pu.
• Series of random or continuous voltage
fluctuations
– Flicker is undesirable results (such as lamps
flickering) from continuous rapid variations in
voltage
– Called voltage flicker (to refer to the voltage
fluctuation)
– Often measured by short term flicker
sensation Pst (IEC standard)
2. Terms and definitions
18
ITI (Information Technology
Industry) curve
• Replaces CBEMA (Computer Business
Equipment Manufacturers Association) curve
# A set of curves representing the withstand
capabilities of computers in terms of the
magnitude and duration of the voltage
disturbance.
• Susceptibility
of
120-volt
computer
equipment to voltage sags and swells
2. Terms and definitions
19
ITI curve
500
400
Voltage in
percent
Vhi t i
Voltage swell intolerable
300
Vlo t i
200
100
Voltage tolerable
Voltage sag intolerable
0
4
1 10
1 10
3
0.01
0.1
1
ti
cy
2. Terms and definitions
10
100
1 10
3
1 10
4
Duration in cycles
20
ITI curve
500
400
Voltage in
percent
Vhi t i
Voltage swell intolerable
300
Vlo t i
200
100
Voltage tolerable
Voltage sag intolerable
0
6
5
4
1 10
1 10
1 10
1 10
3
0.01
0.1
ti
s
2. Terms and definitions
1
10
100
1 10
3
Duration in sec
21
Voltage Sags and
Interruptions
Power Transmission - Distribution
System
4.16 kV, 8.32 kV,
12.47 kV, 13.8 kV,
14.4 kV, 24.9 kV….
46 kV, 69 kV,
115 kV, 138 kV,
161 kV, 230 kV,
….
TRANSMISSION
DISTRIBUTION
Industrial Power System
13.8 kV
Other distribution circuits
115 kV
Sensitive load’s own circuit
Sensitive Load
TRANSMISSION
INDUSTRIAL DISTRIBUTION
Utility Power Distribution System
Utility Distribution System
12.47 kV
Other distribution circuits
115 kV
TRANSMISSION
Customer’s circuit
owned by utility
INDUSTRIAL DISTRIBUTION
with sensitive loads
Industrial Distribution System: Origin of Faults
Causing Misoperation of Production
Equipment
23%
31%
Faults on Own
Distribution Circuit
Faults on Other
Distribution Circuit
46%
Faults on Transmission
System
Example of typical short circuit
Fault at X:
Three-phase short circuit
Single-phase to ground short circuit
Phase to phase short circuit
Double phase to ground short circuit
Three-phase short circuit
Look at the rms value of the ac current
just after the short-circuit occurs
The following phasor diagrams are
qualitative only. Use short-circuit study
for actual values.
Phasor diagram before the fault
Vca
Vcn
Vbc
Ic
Van
Ib
Ia
Vbn
Vab
Normal
Three-phase short circuit
Ic
Ib
Vab
Ia
Three Phase
Short Circuit
Single phase - ground short circuit
Vcn
Vbc
Vca
Van
Vab
Vbn
Ia
Single Phase
to Ground
Short Circuit
Phase to phase short circuit
Vbc
Ib
Vca
Ic
Vab
Phase to Phase
Short Circuit
Phase-phase-ground short circuit
Ib
Vbc
Vca
Ic
Vab
Phase to Phase
to Ground
Short Circuit