5.20.1 Thermal cycling, vibration, shock, and seismic loading
Enclosures shall be designed to withstand all mechanical stresses normally encountered, including thermal cycling, vibration, and shock associated with operation. They shall be designed for seismic loading, if specified.
13 Extracts used from 5.17 of IEC 62271-1 with permission. Copyright © 2007 IEC Geneva, Switzerland. www.iec.ch.
5.20.2 Design pressure
When designing an enclosure, in addition to the maximum filling pressure and maximum design temperature, the following items shall also be considered:
a) The evacuation of the enclosure as part of the gas processing b) The full differential pressure across the enclosure wall or partition
c) In the case of adjacent gas compartments having different operating pressures, the resulting pressure in the event of a leak between the gas compartments (abnormal)
d) The possibility of an internal fault (abnormal)
Components having pressurized enclosures other than metal, such as gas-insulated to atmospheric air bushings, shall conform to the applicable clause of the latest revision of IEEE Std C37.017, IEC 62155 or IEC 61462.
5.20.3 Design temperatures
The maximum design temperature, for purposes of calculating the design pressure of the enclosure, shall be the average temperature of the gas inside the enclosure at rated continuous current, rated maximum ambient temperature, including solar radiation effects at rated gas pressure. This design temperature can be established from existing temperature-rise test data. Solar radiation effect should be evaluated using IEEE Std C37.24.
5.20.4 Calculation methods
Methods for calculating the thickness and construction of the enclosures shall be chosen from established standards for pressurized enclosures of gas-filled, high-voltage switchgear with inert, non-corrosive, low-pressurized gases. CENELEC EN 50052, CENELEC EN 50064, CENELEC EN 50069, or other equivalent national standards may be used. Conformance to local or state codes may also be required and shall be agreed to between the manufacturer and the user. This may include compliance with codes not intended for electrical enclosures such as ANSI/ASME Boiler and Pressure Vessel Code Section VIII, Division 1.2, and ANSI/ASME B31.1.
5.20.5 Pressure coordination
The pressure inside a GIS enclosure may vary from the rated filling pressure level due to different service conditions. Figure 1 shows the various pressure levels and their relationship.
The manufacturer is responsible for choosing the minimum functional pressure for insulation and operation.
The rated filling pressures are related to the alarm pressures and the leakage rate.
Routine test pressure and type test pressure are based on design pressure taking into account material and manufacturing process factors.
Lower pressure Higher pressure
Margin for pressure loss to allow for action
Margin for pressure loss due to gas leakage
Margin for pressure rise due to temperature
Minimum functional pressure
Alarm pressure Rated filling pressure Design pressure of enclosures
Pressure-relief device operating pressure Routine test pressure of enclosures
Type test pressure of enclosures
Burst and rupture pressure of enclosures
Figure 1 —Pressure coordination of enclosures and pressure-relief device
5.20.6 Effects of internal arcing on a GIS enclosure
In order to provide a high degree of protection to personnel, the external consequences of an internal arc shall be limited (by a suitable protective system) to the appearance of a hole or a tear in the enclosure without any fragmentation. The duration of the arc is related to the performance of the protective system determined by the first stage (main protection) and second stage (back-up protection).
Table 5 gives the performance criteria for the duration of the arc according to the performance of the protective system.
Manufacturer and user may define a time during which an arc due to an internal fault up to a given value of short-circuit current will cause no external effects. The definition of this time shall be based on test results or as mutually agreed calculation procedure by manufacturer and user.
The duration of current without burn-through for different values of the short-circuit current may be estimated from a calculation procedure as mutually agreed upon by manufacturer and user.
Table 5 —Enclosure performance criteria for internal arc Rated short-circuit
current
Protection stage Duration of current Performance criteria
< 40 kA rms
5.20.7 Stress under abnormal pressure
Enclosures shall withstand any increase in pressure due to internal arcs which create an abnormal pressure. The abnormal pressure to be withstood is defined as the pressure caused by an internal arc of current magnitude equal to the rated short-circuit current for a minimum duration as specified in Table 5.
Calculation of stress or type tests shall indicate that a rupture of the enclosure will not occur under abnormal pressure. The rupture pressure shall be equal to or greater than 3.5 times the design pressure for cast and cast-welded enclosures and 2.3 times the design pressure for welded enclosures.
5.20.8 Pressure-relief devices
Pressure-relief devices to relieve abnormal pressure shall be set to operate at a pressure not exceeding the routine test pressure. When determining the operating pressure of the pressure-relief device, transient pressure occurring during normal operation of devices (e.g., circuit breaker) shall be considered. Pressure-relief devices shall direct the escaping gases away from the normal path of personnel and shall not exhaust into control cabinets.