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Insulation Coordination

These electrical characteristics are used to determine protective margins for insulation levels in use.

These two figures illustrate a 34.5 kV effectively grounded system, 150 kV BIL, protected with 22 kV MCOV Type PDV arresters.

Figure 1 shows equipment protected with a PDV100-Optima arrester Catalog Number 213722 and Figure 2 shows the same equipment protected with a PDV65-Optima arrester Catalog Number 213272.

The protective margins are calculated using the following formula:

The insulation levels are obtained from the manufacturer of the equipment being protected. The arrester discharge voltages are obtained from the table on page 8.

For example, the protective margin for the BIL is determined by first finding the arrester discharge voltage at the impulse current level selected. In this example, the 10 kA discharge current is used as representative of typical lightning stroke currents discharged through the arrester. The 10 kA 8/20 discharge voltage of catalog number 213722 is 85.5 kV. The percent protective margin using the formula is:

The protective margins for other impulse currents are found in a similar fashion. In the case of the PDV65-Optima arrester, the 10 kA 8/20 discharge voltage of the arrester is 97.2 kV. This results in a protective margin of 54 percent.

These examples include many simplifying assumptions. Not included are the effects of faster rates of current rise on the discharge voltage, reduced insulation levels due to various factors, and line and ground leads.

For example, if the effect of lead length was included in the calculations and 1.6 kV per foot of lead length added to the discharge voltage of the arrester. With two feet of line lead and two feet of ground lead, 6.4 kV is added to the published discharge voltage of the arrester. Therefore, the discharge voltage of the PDV100-Optima is effectively 91.9 kV, resulting in a reduction of protective margin to 63 percent at 10 kA. In the case of the PDV65- Optima arrester, the 97.2 kV discharge voltage of the arrester is added to the 6.4 kV from the leads resulting in equipment seeing 103.6 kV reducing the protective margin to 44.7 percent.

The chopped wave strength of the insulation being protected is typically coordinated with the .5 μsec discharge voltage of the surge arrester. The heavy duty PDV100-Optima uses a 10 kA current while the PDV65-Optima catalog value is for a 5 kA peak current.

The switching surge insulation level of the equipment is coordinated with the 500 ampere switching discharge voltage of the surge arrester.

The effects of faster rates of rise and of reduced insulation strength due to aging effects would result in further reduced protective margins.

Industry standards recommend minimum margins of 20 percent for the chopped wave and BIL levels and 15 percent for switching surge protection.

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