Engineer's Notes: insulation coordination
Match stresses to strengths
Insulation coordination means choosing equipment insulation levels and protective devices so that the overvoltages the network can produce never exceed what the insulation can withstand, with a sensible safety margin. Arresters set the stress; the LIWL sets the strength.
Earthing sets the arrester rating
During an earth fault the healthy phases rise towards line voltage on resistance-earthed and unearthed networks. The arrester must ride through that temporary overvoltage for as long as the fault lasts, which is why ratings differ so much between regions.
Distance eats the margin
Surges travel at hundreds of metres per microsecond and reflect at every change of impedance. The voltage at a transformer a few metres from its arrester can be well above the arrester's residual voltage, and the far end of a cable can nearly double it.
Clearances and creepage
Air clearances protect against flashover from impulses; creepage protects against tracking under pollution. Coastal, desert dust and industrial sites need longer creepage, and high altitudes need larger clearances.
Frequently asked questions
Why is a surge arrester needed where an overhead line meets a cable?
The cable has a much lower surge impedance than the line, so a lightning surge reflects at the junction and again at the far end of the cable, where the voltage can almost double. An arrester at the transition clamps the voltage entering the cable and protects the cable and the equipment at its end.
How do I choose the arrester rating Uc?
Uc must be at least the highest continuous phase-to-earth voltage, and the arrester must survive the temporary overvoltage on the healthy phases during an earth fault for as long as the fault lasts. On solidly earthed systems Uc can be close to Um divided by root 3; on unearthed or resonant earthed systems with long faults, Uc should be at least Um.
What is the protective margin?
The ratio of the equipment lightning impulse withstand level (LIWL or BIL) to the arrester residual voltage at nominal discharge current. IEC 60071-2 applies a safety factor of 1.15 for internal insulation, so a margin of at least about 1.2 is usually required, more if the arrester is some distance from the equipment.
Why does the distance between the arrester and the transformer matter?
A surge reflects at the transformer and the voltage there keeps rising until the reflected wave reaches the arrester and comes back. The extra voltage is roughly 2 x steepness x distance / wave speed, so a few metres of separation or long arrester leads can use up the whole margin.
What are the standard insulation levels for 11 kV equipment?
For Um 12 kV, IEC 60071-1 gives a power-frequency withstand of 28 kV and a lightning impulse withstand of 60 kV (list 1) or 75 kV (list 2). Many utilities specify list 2 or higher for equipment connected to overhead lines.
Topics: insulation coordination · surge arrester · Uc · Ur · TOV · residual voltage · protective margin · protective distance · LIWL · BIL · travelling waves · surge impedance · reflection · Bewley lattice · air clearance · creepage · pollution · altitude correction · ground flash density
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