Module 2 Flashcards
Micro and Long interruptions
microinterruptions-total interruption of electric supply for a duration of milliseconds to one or two seconds
causes:
-opening and closing of protection devices
-insulation failure, lightning and insulator flashover
consequences:
-Tripping of protection devices
-loss of information and malfunction of data processing equipment
-stoppage of sensitive equipment such as ASD and PCs and PLCs
Long interruptions-total interruption of electric supply for duration greater than 1 to 2 seconds
causes:
-equipment failure in the power system network
-storms and objects(trees,cars),fire,striking lines or poles
-human error,bad coordination or failure of protection devices
consequences:
-stoppage of all equipments
sources of sag and interruption
-faults(short ckt of utility system)
-starting of large induction motor
-sudden increase in load
-line to ground fault untill protective switch operates
-accidents in power lines cause line to ground fault
-transformer energizing
-Faulty equipment: Faulty transformers, breakers, and switches can cause sags and interruptions.
-Human errors: Human errors such as accidents, mistakes in maintenance, or construction work can also cause sags and interruptions.
-Overloading: Overloading the power system beyond its capacity can cause sags and interruptions.
-Cybersecurity threats: Cybersecurity threats such as hacking and malware attacks can also cause power outages.
-Aging infrastructure: Aging power grids and equipment can also cause sags and interruptions.
What is area of vulnerability
it describes all the fault locations that can cause sensitive equiments to malfunction
minimum voltage sag ride through capability-minmum voltage mag an equipment can withstand without misoperation or failure. also called as equipment voltage sag immunity or susceptibilty limit
Equipment sensitivity to sag
Equipment sensitive to only the magnitude of a voltage sag
-voltage relays,process controls,motor drive control,
equipment sensitive to both the magnitude and duration of sag
-electronic power supplies,laptop chargers
equipment sensitive to chara other than magnitude and duration
-some devices affected by the phase unbalance during the sag event ,point in the wave at which the sag is initiated ,transient oscillations
effect of voltage sags
-motor load to start /stop
-digital device reset cause loss of data
-equipment damage or failure
-material spoilage
-lost production due to downtime
-additional costs
-product reworks
-product quality impacts
-cost of investigation into problem
-impacts on customer relations such as late delivery
-loss of sales
Voltage sags due to motor starting
-voltage sag produced by induction motor starting current is one fo the main causes of sensitive equipment dropout
-the use of motor starter reduces the voltage sag depth but increase its duration
-an induction motor draws 6-10 times it’s full load current while starting
this lagging current causes a voltage drop across the impedance of the system
the magnitude of voltage sag depend on
-chara of the induction motor
-strength of the system at the point where motor is connected
Mitigation of voltage sags-different devices
-Dynamic voltage restorer(DVR) or Active series compensator
-distribution static compensator(DSTATCOM)
-solid state transfer switch(SSTS)
-static UPS with energy storage
-backup storage energy supply (BSES)
-ferro resonant transformer
-flywheel and motor generator set
-static var compensator(SVC)
Dynamic voltage restorer(DVR) or Active series compensator
boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition
it injects an equal and opposite voltage of disturbance in series synchronism with the ac main
these are reffered to as active series compensator
available in size ranges from small single phase devices to large devices
Motor starting methods
Autotransformer starters
-have two autotransformer connected in open delta
-Taps provide a motor voltage of 80, 65, or 50 percent of system voltage during start-up.
-Line current and starting torque vary with the square of the voltage applied to the motor
Resistance and reactance starters
-Resistance and reactance starters initially insert an impedance in series with
the motor. After a time delay, this impedance is shorted out. Starting resistors may be shorted out over several steps; starting reactors are shorted out in a single step.
-Line current and starting torque vary directly with the voltage applied to the
motor, so for a given starting voltage, these starters draw more current from
the line than with autotransformer starters, but provide higher starting torque.
part winding starters
Delta-wye starters
Classification of transients over voltage
-impulsive transients
An impulsive transient is a sudden non power frequency change in the
steady state condition of the voltage or current waveforms that is essentially
in one direction either positive or negative with respect to those waveforms.
-oscillatory transients
A sudden, non power frequency change in the steady state condition of voltage or current that is bidirectional in polarity.
An oscillatory transient is a sudden non power frequency change in the steady state condition of the voltage or current
waveforms that is essentially in both directions positive and negative with respect to those waveforms.
