Earthing of Utility and Industrial Distribution Systems

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Earthing of Utility and Industrial Distribution Systems


Overview:

• Basics of system grounding and protective grounding • Bonding and its role in safety• Ground electrodes, configurations and ground electrode

resistance• Grounding design of electrical substations• Static electricity and grounding needs for safety against

static discharge• Lightning and grounding of lightning protection systems• Surge protection approach• Noise mitigation by proper grounding practices


Objectives of grounding:

• Provides an electrical supply system with a reference to the groundmass (system grounding)

• Protective grounding of electrical equipment enclosures– Makes them safe to persons who may come into contact with them – Enables the flow of fault current in the event of a failure

• Provides a low impedance path for accumulated static charges and surges (lightning protection grounding)

• Helps in mitigating the generation and propagation of noise (grounding of shields and signal reference planes)


Fault in an Ungrounded System:


Effect of neutral (system) grounding:


Ground fault current flow:


Ground electrodes:

• The connection to ground mass is normally achieved by a ground electrode

• Ground electrodes – Use different materials

– Sometimes use buried facilities not specifically meant as electrodes (water piping is an example)

– Adopt different physical configurations

– Follow different designs

– Must conform to local electrical installation standards


A typical ground electrode:

Materials

• Copper

• Copper clad steel

• Galvanized steel

• Copper clad stainless steel


A typical chemical electrode:

• Sometimes called a leach electrode

• Chemical mixtures are added to lower resistance of soil

• Needs regular maintenance


Choice of ground electrode-Factors:

• Need for achieving minimum acceptable earth resistance (1-10 Ohms) appropriate to the installation

• Need to maintain this resistance all round the year in varying climatic conditions

• Type of soil where the grounding is carried out

• Presence of agents that can cause corrosion of elements buried in ground


Grounding for charge/surge control:

• Grounding helps to drain accumulated static charges and makes an equipment safe

• Grounding helps to conduct the energy of a lightning strike to the soil mass

• Energy of over voltage (surge) events is dissipated away from susceptible electrical equipment. Such surges can result from:– Lightning strike on or near electrical conductors– Switching of inductive circuits


Dissipation of Static Charges:

• Figure above shows a charged body A and another body B (isolated from A)

• Figure on top right shows body B earthed and a spark discharge from A to ground and to earthed body B taking place

• Figure on right shows A bonded to earthed body B causing charge to drain away


Lightning Strike:

• Lightning is the result of charge accumulation in cloud systems which causes:– Ionization of air close to the cloud

– Breakdown of the air between clouds or cloud and ground

– Instantaneous dissipation of charge in the form of a lightning strike

– High current flow in the air path

• Lightning strikes to ground may involve: – Tall structures masts or buildings

– Trees or other natural features


Inductors as sources of voltage surge:

Figure on the left shows an inductor fed by DC supply

When the current is interrupted, a voltage spike occurs across the inductor (see below)


Installations with sensitive equipment:

• Grounding of sensitive equipment needs special care because:– They work at very low power and voltage levels – Cannot tolerate even small over voltages or currents– Are prone to interference in the signal lines from adjacent

circuits


AVOID:

• Ground loops

• Voltage spikes/surges from external and internal causes

• Harmonic noise

• Electromagnetic interference


A typical ground loop:


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Earthing of Utility and Industrial Distribution Systems

Engineering

Transcript of Earthing of Utility and Industrial Distribution Systems