INSULATION COORDINATION

ContentsIntroductionDefinitionVolt- time CurveBasic Impulse Insulation Level (BIL)Basic Impulse Insulation Level of Substation equipmentSelection of BIL of transformerConclusion

Insulation CoordinationThe selection of suitable values for the insulation levels of the various equipments in any electrical system and its correlation to the insulation of the protective devices used for protection of these equipments against over voltages.

Volt- time Curve

volt time curves of the protecting device and the equipment

Basic Impulse Insulation Level (BIL)Basic Impulse Insulation Level (BIL) are reference levels expressed in impulse crest voltage with a standard wave not longer than 1.2/50 sec wave

Basic Impulse Insulation Level of Substation equipment

BIL values for various system voltages as per Indian Standards

Relative position volt- time curves of substation equipments

Selection of BIL of transformer138 kV transformer with solidly grounded neutralDuring ground fault; the voltage across arrester = 1.05 x 0.74 x 138 = 107.2 kVThe nearest standard rating of LA is 109 kVBreakdown value of the arrester is 400 kVAssume 15% margin and 35 kV insulation between LA & TransformerThe insulation level of the transformer = 400+400 x 0.15+35=495 kV

The characteristics of 109 kV LA

Conclusion The conventional method of insulation coordination requires the evaluation of the highest over voltage and selection of a standard impulse voltage with suitable safety marginBut maintaining a high degree of insulation for infrequent occurrence over voltage is uneconomical It is important to consider the economic viability of the design in terms of equipment cost and service continuityIn modern practice, probabilistic concepts and statistical procedures are used, especially for HV installations.

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