02-07-2012, 11:19 AM
VOLTAGE SAG
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A voltage sag is defined as a decrease in RMS voltage at the power frequency for durations from 0.5 to 30 cycles. Sustained interruptions of longer than 1 min are generally due to permanent faults.
OCCURRENCE
Utility Systems :- Voltage sags that occur at higher voltages will normally spread through a utility system and will be transmitted to lower voltage systems via transformers.
Inside Industrial Plants:- These sags are typically caused by starting large motors or by electrical faults inside the facility.]
PHASE TO PHASE SAGS
Phase-to-Phase sags may be caused by tree branches, adverse weather, animals or vehicle collision with utility poles.
The two-phase voltage sag will typically appear on other feeders from the same substation.
SINGLE PHASE SAGS
The most common voltage sags, over 70%, are single phase events which are typically due to a phase-to-ground fault.
Typical causes are lightning strikes, tree branches, animal contact etc.
It is not uncommon to see singlephase voltage sags to 30% of nominal voltage or even lower in industrial plants.
PHASE SAGS
Symmetrical three-phase sags account for less than 20% of all sag events.
Caused either by switching or tripping of a three-phase circuit breaker, switch or recloser which will create a three-phase voltage sag on other lines fed from the same substation.
Confined to an industrial plant or its immediate neighbours.
CAUSE OF SAG
OPERATION OF RECLOSERS AND CIRCUIT BREAKERS
EQUIPMENT FAILURE
BAD WEATHER
POLLUTION
ANIMAL & BIRDS
VEHICLE PROBLEMS
CONSTRUCTION ACTIVITY
OPERATION OF RECLOSERS AND CIRCUIT BREAKERS
A sub-station circuit breaker or a recloser is tripped, then the line that it is feeding will be temporarily disconnected.
All other feeder lines from the same substation system will see this disconnection event as a voltage sag which will spread to consumers on these other lines .
The depth of the voltage sag at the consumer’s site will vary depending on the supply line voltage and the distance from the fault.
EQUIPMENT FAILURE
If electrical equipment fails due to overloading, cable faults etc., protective equipment will operate at the sub-station and voltage sags will be seen on other feeder lines across the utility system.
BAD WEATHER
Thunderstorms and lightning strikes cause a significant number of voltage sags.
If lightning strikes a power line and continues to ground, this creates a line-to-ground fault. The line to-ground fault in turn creates a voltage sag and this reduced voltage can be seen over a wide area.
High winds can blow tree branches into power lines. As the tree branch strikes the line, a line-to-ground fault occurs which creates a voltage sag.
Broken branches landing on power lines cause phase-to-phase and phase-to-ground faults.
Snow and ice build-up on power line insulators can cause flash-over, either phase-to-ground or phase-to-phase.
POLLUTION
Salt spray build-up on power line insulators over time in
coastal areas, even many miles inland, can cause flashover, especially in stormy weather.
ANIMAL & BIRDS
Animals, particularly squirrels, raccoons and snakes occasionally find there way onto power lines or transformers and can cause a short circuit either phase-to-phase or phase-to ground.
Large birds, geese and swans, fly into power lines and cause similar faults.
VEHICLE PROBLEMS
Utility power lines frequently run alongside public roads.
Vehicles occasionally collide with utility poles causing lines to touch, protective devices trip and voltage sags occur.
CONSTRUCTION ACTIVITY
Even when all power lines are underground, digging foundations for new building construction can result in damage to underground power lines and create voltage sags.
SOLUTION
THE DEVICES SHOULD BE EQUIPPED WITH THE EQUIPMENT WHICH COULD HANDLE THE VOLTAGE SAGS.
SOME OF THE DEVICES WHICH ARE USED FOR THE PURPOSE OF THE PROTECTION OF EQUIPMENTS ARE
FERRORESONANT TRANSFORMER
ONLINE UPS
ACTIVE SERIES COMPENSATORS
MAGNETIC SYNTHESIZERS