solid dielectric breakdown

Substation Grounding

Proper sub grounding of a substation is important for:

  • It gives a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.
  • It provide safe environment to protect persons in the vicinity of grounded facilities from the danger of electric shocks under fault conditions in substation

Substation Grounding is necessary precaution activity.

Electric Shock – Substation Grounding

When an adequately large current passes through the human body, the heart muscle absorbs high energy causing ventricular fibrillation which can cause death of the person. There are four zone can be identified in below graph

Body current vs Time  Substation Grounding
Body current vs Time

No sensation – Zone 1

Perceptibility that current, but no harmful effect to the human – Zone 2

Muscular contraction and also difficulty in breathing for human – Zone 3

Probability that ventricular fibrillation increases of human – Zone 4

    a: up to 5%

    b: up to 50%

    c: greater than 50%

Safe grounding – Substation Grounding

Requires the interaction of two grounding systems:

  • Intentional ground,

                        consisting grounding system buried at the some depth below the earth’s surface in Substation.

  • Accidental ground,

temporarily established by person exposed to potential gradient in the vicinity of the grounded facility

Basic shock situations – Substation Grounding

Basic shock situations  Substation Grounding
Basic shock situations

There are various types of shock situations which we can identified,

Step voltage – Substation Grounding

The difference in surface potential experienced by a person bridging 1 m with the feet without contacting any other grounded object.

Touch voltage – Substation Grounding

The potential difference between the Ground Potential Resistance (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.

Transferred voltage – Substation

 Special case of touch voltage where the voltage is transferred into or out of the substation and from/to a remote point external to the substation site.

Mesh voltage – Substation Grounding

Maximum touch voltage within the mesh of ground grid.

Metal-to-metal touch voltage – Substation

 The difference of potential between metallic object or structure within the substation sites that can bridged by direct hand to hand or hand to feet contact.

Touch voltage circuit

Touch voltage circuit  Substation Grounding
Touch voltage circuit

VTH = Touch Voltage

Zth = Rf/2 Rf = ρ/4b Empirical Euation

Assume b = 0.08 and Rf = 3ρ then

Zth = Rf/2 = 1.5ρ

Accidental Ground Circuit Step Voltage

Accidental Ground Circuit Step Voltage  Substation Grounding
Accidental Ground Circuit Step Voltage

Estep = IB (RB +ZTH)

Step voltage circuit

Step voltage circuit  Substation Grounding
Step voltage circuit

VTH = Step Voltage

Zth = 2Rf Rf = 3ρ

Zth = 2Rf = 6ρ

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