Short Duration Voltage Variation: Voltage Sag
G4: Parameters
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G2: Area of Vulnerability
G4: Source of voltage sag
G1: Effect of voltage sag
G2: Voltage Sag Indices
G1: Voltage Sag standard
G3: Mitigation technique
voltage magnitude
duration in time or cycle
Starting of induction motor
Short circuit fault in electrical network
Instantaneous 0.5 to 30s
Momentary-30 cycles to 3 secs
Temporary- 3 secs to 1 minute
0.1 to 0.9 pu
Electrical Load changes
Transformer energizing
depth of voltage sag
Loose electrical connection
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DEFINITION
INTENT OF INDICES
Delayed delivery
G3: Equipment Sensitivity to voltage sag
magnitude
device such as under voltage relays, process control, motor drive control etc
sensitive to the min/ max voltage magnitude experience during
sag / swell
other than magnitude and duration
RMS variation performance indices defined are focused on the more common magnitude & duration characteristics
impact are much more difficult to generalize
- phase unbalance during sag event or any transcient oscillation occur during the disturbance
magnitude and duration
important characteristic :-
- duration the rms voltage is below a specified threshold at which the equipment trips.
misoperates or fail when the power supply o/p voltage drop below specified value.
virtually or equip. uses electronic power supplies
TYPE OF INDICES
solution in the manufacturing process itself
Fault on own feeder
Single-event index
Single-site index
System index
Fault on parallel feeder
solution between the process & the public electric grid
solution in the grid
Indicator of the quality and reliability of the services
Method
Compare power quality in different network
Reduced customer satisfaction
Impacted equipment due to Sag
Decrease power delivery
Cost of restarting process
Loss of production
Individual Customer Level
Fault on transmission system
Utility System Level
time between the instant at which the voltage at a particular point on a supply system falls below the voltage sag start threshold
SARFI-x
L to G Fault (less severe but most occurrence)
SARFI-Curve
reduction of the number of fault
L-L-L fault-(most severe but less occurrence)
SARFI-90 - Referring to below than 90% of Vref
SARFI-110 - Referring to above 110% of Vref
the difference between the reference voltage and the residual voltage
Induction motors - Loss of torque
Synchronous motors - Loss of torque, loss of excitation current
Contactors and relays - Nuisance disconnection
Variable speed drives - Loss of control supply to the control supply, loss of torque
IT equipment and control system - Affects CPU & I/O card. Process interruption
High-pressure discharge lamp - reduce voltage <45% during cooling and restart
Reduction of the number of faults
SARFI-CBEMA
SARFI-ITIC
SARFI-SEMI
reduction of the fault clearance
modification of the supply system configuration
voltage stabilizers
improvement in equipment immunity
SEMI F47
Formula:
IEC 61000-2-8 - Environment - Voltage dips and short interruptions on public electric power supply systems with statistical measurement results
ITIC
CBEMA
SEMI F47 & IEC 61000-4-34
DEFINITION
Equipment voltage sag immunity or susceptiblity limit.
Radial System - Voltage Divider Model
replacing overhead lines with cables
the use of insulated conductors on overhead lines
regular tree cutting in the area of the transmission line
determined by:
Total circuit miles of exposure to fault - that can cause voltage magnitude at end-user to drop
fencing against animals
shielding overhead conductors with additional sheath wires
To evaluate the likelihood of sensitive equipment being subjected to voltage lower than its minimum sag ride through capability
increase frequency of overhaul & periodic maintenance cleaning insulators etc
increase insulation level
Failure of voltage regulator
Meshed Systems - Matrix Calculations
(Thevenin’s superposition theorem and node impedance matrix.)
External Factor
Internal Factor
Ohm's law, V= IR, current proportional to voltage, eg: current increase will leads to voltage increase)
P=VI (Current inversely proportional to voltage. eg: current increase, voltage drop)
use of current limiting fuses
Lightning
Crane encroachment
installing generator close to sensitive load
Animal encroachment
increasing the number of substation & busbar
installing current limiting reactors at strategic points of the system
Copper theft
supplying sensitive customer busbar from several substation
Cable fault
Transformer short circuited
energy storage system
UPS
super conduting magnetic energy storage (SMES)
rotating machine flywheel, motor generator system etc
AREA
no energy storing capability
CVT (constant voltage transformer)
Inside area of vulnerability, the voltage will drop below 0.5pu
static fast transfer switching (SFTS)
Outside area of vulnerability, the voltage will not drop below 0.5pu
static generators of the fundamental harmonics current & voltages
acquiring information on system operation
acquiring information equipment sensitivity
determination of the potential effect