Download SMJE 2103

SMJE 2103
Overvoltage, Electrical
Insulation and Protection
Devices
Topics Discussion
• Overvoltage
- External overvoltage
- Internal overvoltage
- Temporary overvoltage
• Electrical Insulation
- Conduction and breakdown in dielectric
- Materials
- Damage and failure mechanisms
• Protection Devices
Overvoltage
• Main interruption to the generated power
signal.
• Causes: lightning and switching surges.
• Consequences: damage on equipment and
load.
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 Zo 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.
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:
1. Energization of transmission lines and cable.
2. Reenergization of a line.
3. Load rejection.
4. Switching on and off equipment.
5. Fault initiation and clearing.
Overvoltage
-Some important switching operations which can lead
to switching surges1. Line energization.
2. Reclosing (energization of
a line with trapped
charges).
3. Low voltage side
energization of a line.
4. Energization a line
terminated by an
unloaded transformer.
5. Load rejection at the
receiving end of a line
6. Load rejection at the
receiving end of a line
followed by line dropping
at the sending end
Overvoltage
-Some important switching operations which can lead
to switching surges7. Interrupting lines at noload (line dropping).
8. Switching of transformers
at no-load.
9. Switching reactor loaded
transformers.
10. Switching high voltage
reactors
11. Switching at intermediate
substations.
12. Initiation of a single-phase
to earth fault without a
switching operation.
Overvoltage
-Energization of an Unloaded Transmission Line-
Overvoltage
-Internal• Temporary Overvoltage (sustained 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.
Overvoltage
-Internal• Events leading to the generation of temporary overvoltage;
1. Load rejection
When a transmission line or a large inductive load that is fed from a
power station is suddenly switched off, the generator will speed up
and the bus bar voltage will rise.
2. Ferranti Effect.
When current drawn by the distributed capacitance of the line is
greater than in the load at receiving end with no load. So, voltage drop
keeps on increasing towards the end of the line then, the receiving end
voltage tends to get larger than applied voltage.
3. Ground Fault.
In the case of a line-to-ground fault, systems with neutrals isolated or
grounded through high impedance may develop overvoltage on
healthy phases higher than normal line-to-line voltages
Electrical Insulation failure
• The electrical insulation is stressed by several
factors;
1. Dielectric
2. Thermal
3. Mechanical
4. Chemical
5. Radiations
These factors can produce short and/or long term
degradation
Electrical Insulation
• The electrical ensures current flows only along the
conductors and NOT between individual conductors
or between them to ground.
A week insulation may produce:
• Current leak with local heating up to melting and possibly fire.
• Progressive damage of the leakage path up to a short circuit.
• Unbalance of circulating current (with magnetic field
distortion).
• Autotransformer effect with reduction of magnetic field
• Incorrect functioning of protections
Electrical Insulation failure
• Environmental conditions such as
temperature, humidity, pressure can modify
the dielectric properties of an electrical
system.
• The failure is due to unsufficient properties or
degradation under specific environmental
condition.
Electrical Insulation failure
• In radiation, failure of a dielectric insulation is
due to loss of mechanical properties or the
evolution of gases inside the materials.
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.
An analogy
Material
-Breakdown in GasIn a gas free charge under a sufficiently high force can produce
ionization and avalanche breakdown by hitting other atoms.
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-
Material
-Solid Dielectric-
Failure Mechanisms
• Type of damage and failure mechanisms;
1. Partial discharges.
2. Corona.
3. Electrical treeing
4. Electrical tracking
Failure Mechanisms
-partial discharges• Interfaces air/dielectric such as air bubbles
and de-laminations, represent volumes with;
1. Lower dielectric strength
2. 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 likeelectrodes). We can observe a progressive
evolution of a conducting path.
Failure Mechanisms
-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.
Protection Devices
Four main features of lightning protection;
1) Air terminal
2) Conductors
3) Ground termination
4) Surge protection
Indoor Bonding
Indoor Grounding
Surge Protection for Power
Lightning Protection for
Telecommunication
Tower
Lightning Protection for Roof Mounted
Installations
Lightning Protection for Roof Mounted
Installations