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HIGH VOLTAGE
Insulation Coordination
Assistant Professor Suna BOLAT KRÖGER
Eastern Mediterranean University
Department of Electric & Electronic Engineering
Insulation coordination
The term Insulation Co-ordination was originally introduced to
arrange the insulation levels of the several components in the
transmission system in such a manner that an insulation failure,
would be confined to the place on the system where it would
• result in the least damage,
• be the least expensive to repair,
• and cause the least disturbance to the continuity of the
supply.
The present usage of the term
• Insulation co-ordination now comprises the selection of the electric
strength of equipment in relation to the voltages which can appear on the
system for which the equipment is intended.
• The overall aim is to reduce to an economically and operationally
acceptable level the cost and disturbance caused by insulation failure and
resulting system outages.
• To keep interruptions to a minimum, the insulation of the various parts of
the system must be so graded that flashovers only occur at intended
points.
• With increasing system voltage, the need to reduce the amount of
insulation in the system, by proper co-ordination of the insulating levels
become more critical.
Terminology
•
Nominal System Voltage: It is the r.m.s. phase-to-phase voltage by which a
system is designated
•
Maximum System Voltage: It is the maximum rise of the r.m.s. phase-to-phase
system voltage
•
Insulation Level: For equipment rated at less than 300 kV, it is a statement of
the Lightning impulse withstand voltage and the short duration power
frequency withstand voltage.
For equipment rated at greater than 300 kV, it is a statement of the Switching
impulse withstand voltage and the power frequency withstand voltage.
•
Protective Level of Protective Device: These are the highest peak voltage
value which should not be exceeded at the terminals of a protective device
when switching impulses and lightning impulses of standard shape and rate
values are applied under specific conditions.
Method of insulation coordination
• In order to avoid insulation failure, the insulation level of
different types of equipment connected to the system has to
be higher than the magnitude of transient overvoltages that
appear on the system.
• The magnitude of transient over-voltages are usually limited
to a protective level by protective devices.
• Thus the insulation level has to be above the protective level
by a safe margin.
• Normally the impulse insulation level is established at a value
15-20% above the protective level.
Insulation level
Protection level
• Correlation of the insulation strength of electrical
equipment with the characteristics of protective
devices such that the insulation is protected from
excessive overvoltages
Overvoltage protection
• Rod and horn gaps: Flash over with overvoltage Fault current flows - Outage- Poor protection for short ,
high impulses
Overvoltage protection
• Silicon carbide gapped lightning arresters:
Nonlinear resistor in series with gap - gap flashes over current and voltage limited by resistor - power arc goes
out at current zero - magnetic blow out coils.
Conclusion
• Insulation Coordination encompasses all aspects of the power
system and attempts to ensure uninterrupted supply of power
under the worst overvoltage and environmental conditions.
• Surge arresters play an important role in reaching this goal.
Overvoltages
• Switching overvoltages
U [kV]
t [s]
Overvoltages
• Lightning overvoltages
U [kV]
t [s]
Lightning
• An electrical discharge occurring in a big electrode seperation
with high current, high voltage
Thunder clouds
cumulonimbus
Diameter: about 10 km
Height: about 14-15 km
Average electric field on earth
• Good weather:
100 V/m = 0,1 kV/m = 0,001 kV/cm
• Thunderstorm:
15-20 kV/m
• Field under the thunder cloud:
1000 -10000 V/m = 1 kV/m -10 kV/m
Charge formation
Lightning types
•
•
•
•
Cloud to ground
Cloud to cloud
Inside cloud
Cloud to sky
Cloud to ground lightnings
•
•
•
•
Positive downward
Negative downward
Positive upward
Negative upward
Properties of lightning
•
•
•
•
•
•
•
Voltage: 1 MV – 1000 MV
Current: 3 kA – 400 kA (in practice ave: 18 kA)
Power: P = U * I : MW (BIG!)
Energy: W = P * t : J or Ws (SMALL)
Temperature: 25000 – 30000 ° K (hotter than sun!)
Thickness: 1 – 10 cm
Charge: 0.1 – 10 C
Effects of lightning
•
•
•
•
•
•
•
Thermal effect (Joule los, heat, fire, melting)
Thermodynamic effect, electrodynamic effect (deformation)
Electromagnetic effect (induction)
Acoustic effect (thunder)
Visual effect (flash)
Electrochemical effect (Ozone, NO)
Effects on living (dangerous burns, muscular contraction,
fibrilation, step voltage)
Impulse voltage
Power = U x I
= 100 kV x 20 kA
= 2000 MW
(MegaWatt)
Energy = P x t
=2000 MW x 10 µs
=200 Ws = 200 J
Impulse voltage
Impulse current
Lightning rod
(French: Paratonneire, Old synonym: Siperisaika)
It is a set-up for protection against effects of lightning.
Research done by Buffon, Romas and Franklin is resulted in the invention
of "Franklin rod" still used today.
The system consists of a metal rod, a wire connecting it to the ground
conductor and the ground conductor that distributes the current.
Louis Mertens developed a method in 19th century, called a Faraday
cage. The building to be protected is surrounded by conductors.
Later, using radioactive elements at the sharp edges, modern protection
devices are developed. Today, radioactive lightning rods are abandoned
and replaced with non-radioactive rods in order to avoid a possible
radioactivity.
Protection wire,
Protection line,
protective earth conductor
protection angle
protection zone
Michael Faraday (1791-1867) Faraday cage