SMJE 1103

Overvoltage, Electrical Insulation and Protective Devices

References & Circuit Simulator

Can be downloaded from;

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1. http://elearning.utm.my/22232/
2. https://people.utm.my/rasli1/

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Circuit simulator: Matlab

Topics Discussion

• Overvoltage
• External overvoltage
• Internal overvoltage
• Temporary overvoltage
• Electrical Insulation
• Conduction and breakdown in dielectric
• Materials
• Damage and failure mechanisms
• Protective Devices

Overvoltage

• Main interruption to the generated power signal.
• Causes: lightning and switching surges.
• Consequences: damage on equipment and load.

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Overvoltage�-External-

• Generated by atmospheric disturbances. Lightning is the most common and the most severe.
• Lightning is produced by nature to maintain a dynamic balance between the positive charged and negative in ionosphere and earth respectively.
• The total potential difference between the two main charge centers may very from 100 to 1000MW.
• The height of the thundercloud dipole above earth may reach 5 km in tropical regions.

Lightning Phenomenon

Facts about Lightning

• A strike can average 100 million volts of electricity.
• Current of up to 100,000 A
• Can generate 54,000 ^oF
• Lightning strikes somewhere of the Earth every second.
• Kills hundreds of people every year.

Lightning Voltage Surges

• The most severe lightning strikes a phase conductor.
• It produces the highest overvoltage for a given stroke current.
• The lightning current magnitudes is rarely less than 10kA.
• For typical overhead line surge impedance Z_o of 300Ω, the lightning surge voltage will have 1500 kV.
• Lightning is always a major source of damage to power system where equipment insulation may break down, under the resulting overvoltage and subsequent high –energy discharge.

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Lightning Voltage Surges

Overvoltage�-Internal-

• Switching overvoltage.
• It become governing factor in the design of insulation for HV system.
• One of the events that would initiate a switching surge in a power network is a switching operations.
• The operations can be classified as follows:
• Energization of transmission lines and cable.
• Reenergization of a line.
• Load rejection.
• Switching on and off equipment.
• Fault initiation and clearing.

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Overvoltage�-Internal-

• Temporary Overvoltage
• This voltage is differ from transient switching overvoltage in that they last for longer duration, typically from a few cycles to a few seconds.
• The classification of temporary overvoltage as distinct from transient switching overvoltage is due mainly to the fact the responses of power network insulation and surge arresters to their wave shapes are different.

Electrical Insulation failure

• The electrical insulation is stressed by several factors;
• Dielectric
• Thermal
• Mechanical
• Chemical
• Radiations

These factors can produce short and/or long term degradation

Conduction in dielectric

• A non-conducting material, in which macroscopic current are mainly due to the displacement current.
• When the electric field is high enough a dielectric may suddenly loose its property of non-conduction, permanently or temporarily, showing an electrical breakdown.

Breakdown in dielectric

• It consist in the abrupt rise of electrical current under the effect of an electric field.
• Its causes depend on the medium, the geometry and the type and amplitude of the electrical field.

Material�-Breakdown in Liquids-

• Insulating liquids in accelerator components are for example in fast magnets pulsers for voltage above 30-50 kV.
• Also in transformer, capacitor, high voltage switches and circuit breakers.

Material�-Breakdown in Vacuum-

• Same phenomena as breakdown in gas with consideration of cleanness of insulating materials.

Material�-Breakdown in Solids-

Failure Mechanisms (Insulation)

• Type of damage and failure mechanisms;
• Partial discharges.
• Corona.
• Electrical treeing
• Electrical tracking

Failure Mechanisms�-partial discharges-

• Interfaces air/dielectric such as air bubbles and de-laminations, represent volumes with;
• Lower dielectric strength
• Concentration of electric field

Failure Mechanisms�-corona-

• In a gas and non uniform electric field, when the breakdown field is exceeded we can have local ionization and discharges.
• The compounds formed during the discharges and the bombardment of ions can degrade nearly insulating materials.

Failure Mechanisms�-electrical treeing-

• In case of diverging electrical field (like in HV cable or geometries with sharp edges, point like-electrodes). We can observe a progressive evolution of a conducting path.

Mechanisms Failure�-electrical tracking-

• Progressive creation of a conductive path along a surface, due to un-sufficient distance between electrodes with respect to material properties of the surface and to environmental condition.

Protective Devices

Four main features of lightning protection;

1. Air terminal
2. Conductors
3. Ground termination
4. Surge protection

Indoor Bonding

Outdoor Grounding

Surge Protection for Power

Lightning Protection for Telecommunication Tower

Lightning Protection for Roof Mounted Installations

Lightning Protection for Roof Mounted Installations

Student’s Activity

• By referring to the presented slides, give your observation in terms of;

i- severe overvoltage

ii- preventive overvoltage

iii- important protective devices

iv- development of insulation