Voltage Sag

Voltage Sag

 What is Voltage Voltage Sag ? 

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Sag ( dip ) is the reduction in rms voltage or current at the power frequency with a magnitude between 0.1 to 0.9 pu within the time duration of 0.5 seconds  1 minute. Sources ! "ault "ault ( short circuit system) Starting large loads ( electric motors # arc furnaces # etc )

voltage sag waveform

 Voltage sag Sources Short $ircuit on lines %ower transmission or distribution systems  Starting of large motor  Starting of large loads such as arc furnace 

Short circuit sources  &ightning stro'e  ver voltage (lightning surge  switching surge) will caused electrical strees on isolator that e*ceed of brea'down voltage strength and lead to isolator brea'down   +egradation of insulation (present of pollutants materials# corrosion# moisture # aging factors# etc.) resulting in decreased brea'down voltage strength of insulator  ,echanical factor (short circuit between two conductors through contacts a foreign ob-ect such as an animal  bird or equipment )

 Short circuit sources

Lightning strike

Lightning Strike Direct lightning strikes on transmission line will cause the voltage wave form of the voltage transient impulses with steep front time and will propagate towards the substation. /nsulator through which these waves will e*perience a high electric field stress

he present of pollutant in line transmission insulator

ne of the main problems that have been found is the effect produced by pollution in the insulators of electric lines. his pollution is one of the main causes of flashover in the insulators and finally will cause short circuit

/nsulator washing

The eect of short circuit 1.

2

4 6

t the point of fault  +estructive arcing insulators and conductors  "ire /n a circuit faulted  3lectrodynamic force can result in deformation of the busbar and cable disconnection igh temperatures can cause damage to the insulator /n another circuit close with a circuit of interrupted  Sag voltage  malfunction control and monitoring circuit  System instability or loss of synchroni7ation on synchronous machine

Starting of large motor

Starting of large motor

Starting large motor  Starting of induction motors can draw a current of 8 s  d 10 times the full load current # it is because at the time of starting of induction motors require a certain amount of energy to produce a magnetic field in overcoming the moment of inertia

Starting of induction motor (for example)  4 %hase 50 %  680   80 induction motor

 Arc Furnace

 Arc Furnace

Arc furnace is operated in a way to connect an electric short circuit at a stack of metal resulting in arcing to the temperature exceeded 10 thousand der. used to melt the metal pile

Fault on power transmission & distribution sstems :0; faulted on overhead lines  Stri'e lightning  "lashover (insulation failure)  $ontact with the birds

Short circuit

Short circuit

Short circuit cause current flow in line became very high because only limited by impedance line only.

For example ! voltage sag calculaton

oltage Sag deeper when short circuit fault location point is getting closer to the point load

The tpe of short circuit 

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Symmetric short circuit !  ( 4 phase shor circuitt)