The switchgear enclosure must be metal (except for ventilation windows and exhaust vents) and must not be made of mesh or similar materials. The metal enclosure must meet one of the protection levels specified in GB 1022.The floor may be part of the enclosure, but if there is a cable trench or cable entry, it must be enclosed and meet one of the protection levels specified in GB 11022. However, the walls of the building cannot be part of the enclosure. Gas-filled compartments must be able to withstand the normal and transient pressures encountered in use. These compartments differ from compressed air containers and similar pressure vessels in terms of the sustained forces they withstand in use. These differences are as follows:
1) Gas-filled compartments enclose the main circuit, preventing contact with live and moving parts. Furthermore, the structure must maintain the rated insulation level at the minimum functional pressure.
2) Gas-filled compartments are typically filled with a dry, stable, inert, non-corrosive gas. To ensure reliable operation of the switchgear, measures are taken to ensure that the gas meeting these conditions has only minimal pressure fluctuations. Since the compartment walls are not subject to corrosion, these factors do not need to be considered when determining the compartment design.
3) The gas pressure during operation is relatively low.
For outdoor high-voltage switchgear, the impact of climatic conditions should also be considered during design.
Gas Compartment Design
- Design Temperature and Design Pressure
The design temperature of a gas compartment is the upper limit of the ambient air temperature plus the temperature rise of the gas flowing through it due to the rated current. Any significant effect of solar radiation should also be taken into consideration. The calculation method for the shell thickness and structure can be selected according to the pressure vessel design specifications. The shell design pressure should be at least the upper limit of the pressure that can occur within the shell at the design temperature. The following issues should also be considered:
⑴ The maximum pressure differential that may occur across the compartment walls or partitions, including the vacuum process that may be used during the inflation process;
⑵ The pressure caused by accidental leakage between adjacent compartments with different operating pressures;
⑶ The possibility of internal failure.To determine the pressure of the shell during type testing and factory testing, the maximum design pressure is expressed as follows: Maximum design pressure (MPa (gauge pressure)) = [rated inflation pressure (gauge pressure) + 0.1] x 1.3 - 0.1.
2. Gas Compartment Sealing
The manufacturer should specify the pressure system used for the gas compartment and the permissible leakage rate for the gas compartment. For gas-filled compartments that require entry into closed or controlled pressure systems, the manufacturer should specify the permissible amount of gas leakage through the partitions. For gas-filled compartments with a minimum operating pressure exceeding 0.1 MPa (gauge pressure), an indication should be provided when the pressure drops below the minimum operating pressure at 20°C. Partitions between gas-filled compartments and liquid-filled compartments (e.g., cable boxes, voltage transformers) should not contain any leakage that could affect the insulation performance of the two media.
3. Pressure Relief for Gas-Filled Compartments
The pressure relief design should minimize the potential danger to the operator from released gases and vapors during normal operation. The pressure relief should ensure that even if an arc ignites at designated points in the enclosure, the resulting burn-through hole will allow the generated pressure to be released.
Covers and Doors
When covers and doors are part of the enclosure, they should be made of metal and, when closed, provide the same degree of protection as the enclosure. Covers and doors should not be made of woven mesh, drawn metal, or similar materials. Covers and doors are classified into two categories based on the different circumstances requiring access to the high-voltage compartment:
- Covers that do not need to be opened during normal operation and maintenance (fixed covers) should not be opened, removed, or moved without the use of tools.
- Covers that need to be opened during normal operation and maintenance (removable covers and doors) should not require the use of tools to open or move them. In armored or compartmentalized high-voltage switchgear, covers and doors may only be opened when the accessible main circuit components within the compartment are de-energized. For box-type switchgear, measures (insertion of safety barriers or other means) should also be taken to prevent operators from contacting live components.
Observation Window
The observation window should meet the protection level specified for the enclosure. The window should be covered with a transparent, flame-retardant material with a mechanical strength similar to that of the enclosure. Sufficient electrical clearance or electrostatic shielding measures should be provided to prevent the formation of dangerous electrostatic charges (e.g., by adding a suitable grounding braid inside the window). The window should be positioned to facilitate observation of the operating equipment within. The insulation of accessible surfaces between live parts of the main circuit and the observation window should be able to withstand the test voltage to ground specified in GB 1022.
Ventilation Windows and Exhaust Vents
Ventilation windows and exhaust vents should be arranged to provide the same degree of protection as the enclosure. Ventilation windows may be made of mesh or similar materials, but they must possess sufficient mechanical strength. The placement of ventilation windows and exhaust vents should also ensure that the oil vapor and steam discharged under pressure do not endanger the operator.
Enclosure Temperature Rise
To protect the operator from burns, the temperature rise of accessible enclosures and covers (including accessible portions of the gas-filled compartment) should be limited to a human-tolerable level. For enclosures or covers that do not require access during normal operation, this limit may be appropriately increased. For inaccessible enclosure areas, the temperature rise should be limited to ensure that the temperature rise of the insulating material within the enclosure does not exceed the permissible value.
