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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 4-1
Part II: Over-voltages and Insulation Coordination in Power Systems
Chapter 4: Traveling waves Chapter 5: Lightning over-voltage protection Chapter 6: Internal over-voltage protection Chapter 7: Insulation coordination in power systems
Instructor: Dr. Jian Li
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
EP
T O
F H
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ON
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CH
ON
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RS
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Lecture 4-2
7.1 Basic principle of insulation coordinationPurposes
Proper electrical clearances and insulation levels are necessary for the design, construction, and operation of electric power systems.
DefinitionsInsulation coordination
The process of bringing the insulation strengths of electrical equipment and buses into the proper relationship with expected overvoltages and with the characteristics of the insulating media and surge protective devices to obtain an acceptable risk of failure.
Withstand voltageThe voltage that an insulation is capable of withstanding without failure or disruptive discharge under specified test conditions.
Basic lightning impulse insulation level (BIL)The electrical strength of insulation expressed in terms of the crest value of a standard lightning impulse under standard atmospheric conditions.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 4-3
7.1 Basic principle of insulation coordinationBasic switching impulse insulation level (BSL)
The electrical strength of insulation expressed in terms of the crest value of a standard switching impulse.
Insulation configurationThe complete geometric configuration of the insulation, including all elements (insulating and conducting) that influence its dielectric behavior. The following insulation configurations are identified:a) Phase-to-ground: An insulation configuration between an energized
part and the neutral or ground.b) Phase-to-phase: An insulation configuration between two phases of
energized conductors or parts.c) Longitudinal: An insulation configuration between energized
conductors or parts belonging to the same phase, which are temporarily separated into two independently energized parts (e.g., open switching device).
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
EP
T O
F H
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GE
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LATI
ON
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G，
CH
ON
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RS
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Lecture 4-4
7.1 Basic principle of insulation coordinationTests to determine insulation levels
Rated power-frequency short duration withstand voltage testPower-frequency long term withstand voltage testSwitching impulse withstand voltage testLightning impulse withstand voltage test
Major overvoltages in power systems with different voltage levels
220 kV and below: lightning overvoltagesEHV systems: switching overvoltagesUHV systems: power frequency temporary overvoltages.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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CH
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RS
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Lecture 4-5
7.1 Basic principle of insulation coordinationInfluences of neutral ground (effective of ineffective) on insulation levels of power systems
Maximum continuous operation voltages (MCOV) as single phase short-circuited
1.1 to 1.15 multiples of phase-to-phase voltages for ineffective neutral ground.1.1 to 1.15 multiples of phase-to-ground voltages for effective neutral ground.
Lightning overvoltagesDecided by maximum operational voltages.
Switching overvoltagesDecided by phase-to-ground voltages for effective neutral ground and by phase-to-phase voltages for ineffective neutral ground.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 4-6
7.2 Approaches to insulation coordinationConventional approaches
Select the surge arrester to be used, considering the MCOV and temporary overvoltages constraints, as well as the type of arrester.Determine the protective level of this surge arrester at a “coordinating current” which is the presumed arrester discharge current for an impulsive (lightning) transient.Determine where the surge arrester is to be located with respect to the protected equipment. Voltage wave behavior can cause voltages to be significantly greater as distance from the arrester is increased.Determine the voltage at the terminals of the protected equipment.Select equipment insulation level.Evaluate voltage protection margins. A minimum 20% margin is desirable.If margins are inadequate, consider alternatives such as different arrester placement, higher insulation level, etc.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 4-7
7.2 Approaches to insulation coordinationStatistical approaches
Based on risk of insulation failure.
Simplified approaches recommended by IEC
s
ws U
UK =
UW : statistical withstand voltageUS : statistical overvoltage
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
EP
T O
F H
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VO
LTA
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LATI
ON
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CH
ON
GQ
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UN
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RS
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Lecture 4-8
7.3 Insulation level selection