Compact SF6 insulated ring main units are widely used in urban and rural power grids, wind power stations, medium-voltage switching stations, factory power distribution, and commercial buildings worldwide. Therefore, they are also power distribution products managed by power companies in various countries, and different regions have different requirements. This article analyzes these application requirements.
After electricity from urban and rural power grids is transformed to 24/12kV via high-voltage primary substations, numerous regional secondary substations are needed to distribute the power to user terminals. SF6 fully insulated ring main units, as core products for secondary power distribution, have a wide range of applications and are used in large quantities. The safety and reliability of the ring main units directly affect the stability of the power distribution network. Although typical SF6 fully insulated ring main units can meet application requirements, some countries and regions have formulated special requirements based on safety and application considerations.
The SF6 ring main unit is a completely sealed system; all its live parts and switches are enclosed within a stainless steel housing. The entire switching device is unaffected by external environmental conditions, thus ensuring operational reliability and personal safety, and achieving maintenance-free operation. By selecting expandable busbars, any combination can be achieved, realizing full modularity. The extended busbar is fully insulated and shielded, ensuring high reliability and safety. It complies with standards such as IEC62271-1, EC62271-100, IEC62271-200, IEC60265, and IEC60480.
Environmental Requirements
1. High Humidity Areas
In high humidity areas, condensation frequently occurs. While the primary circuit, sealed within the gas chamber, remains unaffected, the operating mechanism and secondary circuits require protection. Special attention must be paid to condensation within the fuse compartment. For example, an industrial user in Australia had a ring main unit installed in an outdoor enclosure. It was powered on in the morning and shut off at night when there was no load. One day, when replacing a fuse, severe corrosion was found on the fuse holder. Because the fuse compartment cover and body were completely sealed, meeting IP67 requirements, and the silicone rubber compression ensured the high-voltage fuse could withstand the power frequency withstand voltage between itself and the cabinet, it was impossible for moisture to enter the fuse compartment, leaving the user puzzled.
Assuming the temperature is 20 degrees Celsius, relative humidity is 80%, and dew point is 16.4 degrees Celsius during fuse installation, and the installation process is lengthy, with the fuse compartment sealed after installation, ideally completely isolated from the outside environment, the conditions for condensation inside the enclosure are as follows: Assuming the ambient temperature is 25 degrees Celsius and relative humidity is 60% during fuse installation, and the initial air temperature and humidity inside the enclosure are the same as the ambient temperature and humidity during fuse installation, as shown in the table below, the condensation temperature is 16.7 degrees Celsius. Since the equipment is shut down at night, the ambient temperature is only 5-10 degrees Celsius, and the dew point temperature for condensation is always lower than the ambient temperature. The inside of the fuse compartment must be in the area with the lowest air temperature. The area with the lowest temperature inside the compartment is the fuse cover. Therefore, the fuse catch on the fuse cover reaches the dew point temperature, causing condensation. This cycle repeats, resulting in silver plating oxidation and severe corrosion of the fuse holder. Therefore, ring main units must consider this application scenario, keeping the high-voltage fuse compartment dry. Humidity conditions must be met when replacing fuses to minimize exposure time. If necessary, circuit breaker cabinets should replace combined electrical control cabinets.
2. High-Altitude Areas
For gas-insulated switchgear, since the main energized circuits are all housed in sealed gas-filled boxes, and external connections use solid insulation, they are unaffected by atmospheric pressure on the external insulation. For gas-insulated switchgear, the strength of the gas box is the primary consideration. In South American countries like Chile, the altitude is generally around 3500 meters. For SF6 compact ring main units, the impact of altitude is mainly reflected in changes in atmospheric pressure. At an altitude of 3500 meters, the atmospheric pressure is 0.065 MPa. Assuming the filling pressure is 0.13 MPa absolute pressure, at an altitude of 1000 meters, the pressure difference inside and outside the gas box is 0.04 MPa.
However, at an altitude of 3000 meters, the pressure difference reaches 0.065 MPa. Under these conditions, the gas box will expand, potentially leading to rupture and leaks. The general practice is to use a reinforced air box, strengthen the pressure relief valve, sealing rings, and other structures, and appropriately reduce the inflation pressure while ensuring insulation. It's necessary to consider not only actual operating conditions but also whether the transportation will pass through high-altitude areas, using low-pressure or no-pressure transportation to ensure the product's airtightness and prevent damage to the air box's strength.
Safety Requirements
1. Internal Arc Fault Rating and Pressure Relief Methods
For overseas customers, resistance to internal arc faults in switchgear is mandatory, as human safety is paramount. Ring main units (RMS) must pass internal arc fault tests, including the cable compartment and gas box, which must pass an AFL 20kA 1s test. Generally, only the front and sides of the AFL are required to meet the standard, considering wall-mounted installation; rear protection is usually not required. Many RMS units are installed in separate transformer substations or outdoor enclosures; therefore, the pressure relief methods mainly include the following:
Cable Trench Pressure Relief: The internal arc pressure in the RMS gas box and cable compartment is directly released into the cable trench through a pressure relief channel at the rear of the cable compartment. Some designs seal the rear of the cable trench with a dedicated pressure relief system, but this reduces the size of the cable trench, making installation difficult.
Top-Rear Pressure Relief: Pressure is released through the upper part of the rear channel. After exiting, the airflow travels along the top of the cabinet, having already traveled a long distance. This significantly reduces the impact of the combustion flame and minimizes damage to equipment and personnel. It avoids releasing pressure into the cable trench, which could damage cables, or releasing the arc pressure directly from the top of the gas chamber to the top of the switch room, potentially injuring personnel in front of the cabinet or causing further damage to other equipment.
Bottom-Level Buffer Pressure Relief for Ring Main Units: Some European countries, such as Belgium, require ring main units to be pressure-relieved in this way. As shown in the diagram, the switchgear has a common base. The base space acts as a buffer against the arc, rapidly reducing pressure and energy before releasing it through a 200x200mm opening at the rear, minimizing damage to people and equipment.
2. Cable Withstand Voltage Test
According to IEC 62271-200, switchgear and controlgear can be designed to allow testing while cables are connected to them. This can be performed using dedicated test connections or cable terminations. In this case, the switchgear and controlgear should be able to withstand the rated cable test voltage specified in the standard applied to the parts still connected to the cable, while the rated voltage is applied to those sections of the cable. The main circuit is designed to remain energized during cable testing.
