24504241 Substaion Earthing

8/3/2019 24504241 Substaion Earthing

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Welcome to the presentation on

SUB STATION EARTHING

ByM.L.SHESHADRI, EE,KPTCL,

A.R.CIRCLE, BANGALORE


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SUB STATION EARTHING AN INTRODUCTIONSUB STATION EARTHING AN INTRODUCTION

HISTORICAL BACKGROUNDHISTORICAL BACKGROUND

Very early use of electricity was with ungrounded system.

The electrical systems were generally localized systems.The first AC Transmission in 1886 paved the way forgrowth of electrical systems.

Fatal accidents were reported when persons came incontact with live parts due to insulation damage.

A need was felt to provide an alternate easy path forfault currents to prevent accidents.


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HISTORICAL BACKGROUND

Earth being a huge mass (mean radius 6371 Km)was thought to be the best sink.

Metal rods were driven into the earth and noncurrent carrying metal parts (enclosures) ofelectrical apparatus were connected to these rodscalled electrodes.

The famous Horse and Dairy accidents. Investigation of these accidents revealed that

potential gradients are the cause of theseaccidents and needs to be addressed to preventelectric shocks.


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SUBSUBSTATION EARTHING AN INTRODUCTIONSTATION EARTHING AN INTRODUCTIONEARTH AS RETURN CONDUCTOR

The electrical behaviour of earth is heterogeneousand it behaves as differently as;

A conductor having resistance. An electrolyte. A dielectric.

The behaviour of a ground electrode buried in soilcan be analyzed by means of the circuit shown

here as soil model.The charging current is negligible in comparison to

the leakage current and the earth can berepresented by a pure resistance for powerfrequency currents.


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SUB STATION EARTHING ANSUB STATION EARTHING ANINTRODUCTIONINTRODUCTIONEARTH AS RETURN CONDUCTOR

The soil resistivity is not affected by a voltagegradient unless the latter exceeds a certain critical

value. If the critical value of gradient (KV/Cm) is

exceeded arcs would develop at the electrodesurface and progress into the earth so as to

increase the effective size of the electrode, untilgradients are reduced to values that the soilmaterial can withstand.

This condition is illustrated by the presence ofgaps in the soil model circuit.


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EARTH AS RETURN CONDUCTOR

The soil resistivity

The concept of Remote Earth.

The earth resistance of an electrode.

The GI electrode as per IS 3043 1966.

The CI electrode as per IS 3043 1987.

The electrode used in KPTCL sub stations.


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THE ELECTRIC SHOCK

Effects of an electric current passing through thevital parts of a human body depend on the

duration, magnitude and frequency of this current.The most dangerous consequence of such an

exposure is a heart condition known as ventricularfibrillation resulting in immediate arrest of bloodcirculation.

Humans are very vulnerable to the effects ofelectric current at power frequency, currents of 0.1Ampere can be lethal.

Still higher currents can be tolerated at high

frequencies.


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THE ELECTRIC SHOCK The most common physiological effects of electric current on thebody, stated in order

of increasing magnitude, are;

1 mA Threshold of perception Just able to detect a slight tinglingsensation in the hands or finger tips.

1 to 6 mA Let Go currents Unpleasant to sustain. Generally do notimpair the ability of the person holding

the energized object to control his

muscles and release it.

9 to 25 mA Painful Difficult or impossible to release energized

objects grasped by the hand. Breathing may

be difficult. The effects disappear when

current is interrupted. Person may respond to

resuscitation. 60 to 100 mA Dangerous Ventricular fibrillation, stoppage of

heart, inhibition of respiration, injury,

burning or death.


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SB = (IB)2 ts

THE ELECTRIC SHOCK

If shock currents can be kept below fibrillating Threshold value by acarefully designed grounding system, Injury or death may beavoided.

The non fibrillating current of magnitude at durations rangingfrom 0.03 to 3 seconds is related to the energy absorbed by thebody is described by the following equation;

Where,

= rms magnitude of the current through the body in Amps.

= duration of current exposure in seconds.

= is the empirical constant related to the electric shock energytolerated by a certain percent of a given population.

SB

ts

IB

IB


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THE ELECTRIC SHOCK

The magnitude and duration of the current conducted through ahuman body at 50 or 60 Hz should be less than the value that cancause ventricular fibrillation of the heart.

Duration formula: The tolerable body current limit that persons can safely withstand

without ventricular fibrillation, with passage of current withmagnitude and duration is determined by the following formula;

For a person weighing 50 Kg the value of SB is 0.0135 as per thefindings ofMr. Dalziel.

Thus, for 50 Kg body weight, K = 0.116Therefore

This results in a value of 116 mA for = 1 second and 367mA for= 0.1 seconds.

IB = K / ts where K = SB

IB = 0.116 / ts

tsts


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THE ELECTRIC SHOCK

Considering the significance of fault duration both in termsof the above equation and implicitly as an accidentexposure factor, high speed clearing of ground faults isadvantageous for two reasons;

The probability of exposure to electric shock isgreatly reduced by fast fault clearing time, in contrastto situations in which fault currents could persist forseveral minutes or possibly hours.

Tests and experience show that the chance of severeinjury or death is greatly reduced if the duration of a

current flow through the body is very brief.

Research provides evidence that a human heart becomesincreasingly susceptible to ventricular fibrillation when thetime of exposure to current is approaching the heart beatperiod, but the danger is much smaller if the time ofexposure to current is in the region of 0.06 to 0.3 seconds.


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THE PURPOSE OF EARTHING

A number of points from Generators to consumers installations are earthed(or Grounded).

In our country we have an effectively grounded system.

The purpose of earthing is:

To provide a low impedance path to facilitate satisfactory operationof protective schemes during fault conditions.

To ensure that living beings in the vicinity of sub stations are notexposed to unsafe potentials under steady state or fault conditions.

To retain system voltages within reasonable limits under faultconditions and ensure that insulation breakdown voltages are notexceeded.

To provide an alternative path for induced current and there byminimize electrical noise in communication circuits.


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THE PURPOSE OF EARTHING

GROUND POTENTIAL RISE (GPR)

During a ground fault the portion of the faultcurrent that is conducted by a sub stationgrounding grid into the earth causes the rise of thegrid potential with respect to remote earth.

The design of earthing system strives to controlthese potential gradients within the sub station forthe safety of personnel as well as to avoid damageto electrical equipment.


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24504241 Substaion Earthing

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