Thursday, November 11, 2010

Standard Impulse Insulation Levels for Switchgear Systems (1 kV to 245 kV)

Definitions:

Highest Voltage of a Three-Phase System — The highest rms phase-to-phase voltage which occurs under normal operating conditions at any time and at any point of the system. It excludes voltage transients (such as those due to system switching) and temporary voltage varia­tions due to abnormal system conditions (such as those due to fault conditions or the sudden disconnection of large loads).

Switching Overvoltage — A phase-to-earth or a phase-to-phase over-voltage at a given location on a system due to one specific switching operation, fault, or other cause, the shape of which can be regarded for insulation co-ordination purposes as similar to that of the standard impulse used for switching impulse tests. Such overvoltage are usually highly damped and of short duration.

Lightning Overvoltage — A phase-to-earth or a phase-to-phase overvoltage at a given location on a system, due to one specific lightning discharge or other cause, the shape of which can be regarded, for insula­tion co-ordination purpose, as similar to that of the standard impulse used for lightning impulse tests. Such overvoltages are usuall
y very highly damped and of very short duration.

Conventional Switching (Lightning) Impulse Withstand Voltage — The peak value of a switching (lightning) impulse test voltage at which an insulation shall not show any disruptive discharge when subjected to a specified number of applications of this impulse, under specified conditions. The concept applies particularly to non self-restoring insulation.

Standard 1-minute power frequency withstand voltage
Nominal System Voltage
Rated Maximum System Voltage
Standard 1-minute power frequency withstand voltage
List 1
List 2
List 3
kV rms
kV rms
kV rms
kV rms
kV rms
3.3
3.6
10
-
-
6.6
7.2
20
-
-
11
12
28
-
-
15
17.5
38
-
-
22
24
50
-
-
33
36
70
-
-
44
52
95
-
-
66
72.5
140
-
-
110
123
185
230
-
132
145
230
275
-
154
170
275
325
-
220
245
360
395
460

 Standard Lightning impulse (1.2/50 uS) withstand voltage

Nominal System Voltage
Rated Maximum System Voltage
Standard Lightning impulse (1.2/50 uS) withstand voltage
List 1
List 2
List 3
kV rms
kV rms
kV peak
kV peak
kV peak
3.3
3.6
20
40
-
6.6
7.2
40
60
-
11
12
60
75
95 (IEC only)
15
17.5
75
95
-
22
24
95
125
145 (IEC only)
33
36
145
170
-
44
52
250
-
-
66
72.5
325
-
-
110
123
450
550
-
132
145
550
650
-
154
170
650
750
-
220
245
850
950
1050

List 1

1. When the system is not directly connected to an overhead line, such as when connected through a transformer.
2. When the neutral of the system is grounded solidly or through a small impedance.
3. When the system is provided with a surge protection.

List 2 or List 3

1. When the system is connected to an overhead line.
2. When the neutral of the system is not solidly grounded.
3. When no surge protection is provided.
4. When the installation demands a high degree of security, such as in a generating station.

References:
1. Industrial Power Engineering Applications Handbook by K C Agrawal, Butterworth-Heinemann 2001
2. IEC 60071-1: 2006 - Insulation coordination, Part 1 - Definitions, principles and rules
3. IS 2165-1: 1977 - Insulation coordination, part 1 - Phase to earth insulation coordination, principles and rules

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