Faults Of Power System

• 3 major fault occurs usually.• All faults can be solved using

symmetrical component.• We are not going to analyze it

now. Take an overview .

Faults in Power systemFaults Types

◦ Balanced 3Ø faults (Symmetrical). 3Ø direct connect(LLL) 3Ø to ground(LLLG)

◦ Unbalanced 3Ø faults (Asymmetrical). Single line to the ground (SLG) Line to Line (LL) Double line to ground (DLG)

◦ Series Faults. One line open (OLO) Two line open (TLO)

Faults in Power systemFaults Can occur anywhere in

the power system.For the time being consider 33kV

network.Recover faults are more critical.Proper detection of faults helps

to fast recovery.Placing fault indicators over the

network helpful for recover mission.

Fault IndicatorsWhen fault occur, both voltage

and current changes undesirably.By measuring voltage and

current ,can identify faults.Current can measure in various

ways◦By using current shunt◦By using current transformer.◦By measuring magnetic field around

conductor.

Fault IndicatorsCurrent shunt is more suitable for low

voltage applications.We are more prefer for non-contact

method.So current shunt is not suitable.Current transformer is more accurate &

reliable but it suitable up to some extent.It is suitable for bus bars & transformers.Since CT are bulky and main conductor go

through the CT ,its not applicable.

Fault IndicatorsNext method is measure current

by measuring magnetic field.There are several methods to

measure magnetic field.◦Flux gate◦SQUID◦Hall effect◦Magnitoresistance◦Proton precession◦Optical pump

Magnetic Field Sensor

Figure: Magnetic field sensors are divided into two categories based on their field strengths and measurement range: magnetometers measure low fields and gaussmeters measure high fields.

Magnetic Field Sensor

Table: Field Strength Instrument Characteristics

Hall Effect SensorHall sensors uses well known phenomena

called ‘Lorentz force’ ,force acting on moving charge in magnetic field.

To make charge moves ,use constant current.

Hall Effect SensorUnder external magnetic field, charged partials

are moving they tends to concentrate on one side.That creates electrical field perpendicular to

moving direction

Hall Effect SensorHall effect occurs in both

conductors and semiconductors.Compare to conductors,

semiconductors has considerably large VH.

Predominantly use n-type silicon.But there are other materials too.

◦GaAs◦InAs◦InSb

Hall Effect SensorHall voltage is linear function of

◦Applied Current (I)◦Magnetic field (B)◦Separation between sensor points

(d)◦Carrier density (N)

VH =

Hall Effect SensorCompare wit others ,InSb has

large VH.

Hall Effect SensorBut InSb is highly sensitive to

temperature variations.

Hall Effect SensorSince VH is in mV & µV

range ,amplification is required.VH measures between two points.

So it requires differential amplifier.

Hall Effect SensorFinal output is linearly behave

with applied magnetic field.

Hall Effect Sensor

Hall Sensor

Power line

Core

Magnetoresistive Sensors There are two basic types.

◦Giant Magnitoresistance Sensor (GMR)

◦Anisotropic Magnitoresistance sensor (AMR).

Magnitoresistance is the property of a material to change the value of its electrical resistance with an externally applied magnetic field.

Magnetoresistive Sensors Magnetoresistive Sensors available in

market uses MR element in balanced wheatstone bridge configuration.

All four elements are identical but two of them are covered with magnetic shield.

Magnetoresistive Sensors It helps to minimize the effect of

temperature coefficient of resistive elements.

Under external magnetic field only two of them vary accordingly.

It causes the unbalance of bridge.

Unbalanced voltage can be measured using differential amplifier.

Magnetoresistive Sensors

Magnetoresistive Sensors Output voltage is linearly vary

with applied magnetic field.AMR - can identify magnetic pole

typeGMR – equally response to both

poles

Comparison of Magnetic Sensors

• GMR sensors are more suitable for DC current sensing application.

• For AC ,AMR is more prefer than Hall sensor.