The Saudi National Grid requires a 17.5kV ring main unit with the following typical features:

· Outdoor enclosure;

· Separate cable withstand voltage test compartment;

· Self-powered protection unit;

· SF6 insulated common box F+F+V (Load Switch + Load Switch + Circuit Breaker) combination;

· Ring main switch unit 630A, outgoing circuit breaker 200A;

· Refer to standard 32-SDMS-11.

The rated voltage of the Saudi National Grid standard ring main unit is 13.8kV, while the equipment rated voltage is 17.5kV. The rated frequency is 60Hz, with a rated short-time withstand current of 21kA for 1 second. The peak withstand current is calculated as 21kA * 2.6 = 54.6kA at 60Hz. It uses SF6 insulation with a common box structure, meets the IP54 protection rating, and is suitable for outdoor installation. The upward hinged cover structure allows for convenient secondary control and protection operations.

Due to the outdoor installation requirement and the need for an instrument room, the switchgear height should not be too large. Domestically, the three-position switch + circuit breaker cabinet is typically arranged in a vertical layout, resulting in a relatively large height. For the Saudi National Grid’s low height product requirement, the three-position switch + vacuum circuit breaker is arranged horizontally, reducing the height. The width requirement is not very strict, and a total width of 1.5 meters is acceptable. The design can refer to the LUCI switchgear dimensions as a basis.

The three-position switch + vacuum circuit breaker arc extinguishing chamber is arranged horizontally as shown in the diagram below.

For the cable withstand voltage test terminal, because the SF6 load switch has a smaller size, but the general gas compartment is larger, the ground contact of the three-position switch is connected to the front bushing of the gas compartment through a copper bar as the cable withstand voltage test terminal. During testing, the load switch is in the "closed" position. The test terminal is connected to high-voltage testing equipment to directly apply high voltage to test the cable's insulation level. After testing, the testing equipment is moved away, and the grounding bus is reconnected to ensure the grounding switch is in the grounded state. The compartment door is closed, and the grounding switch can then be operated to open the circuit, restoring the normal function of the three-position switch.

The copper bus connected to the cable withstand voltage test terminal needs to withstand short-circuit current, ensuring the grounding switch’s closing capability and dynamic thermal stability. Therefore, the copper bar’s cross-section cannot be too small; otherwise, it may be damaged if it cannot withstand 1s of 21kA thermal stability. The cross-sectional area needs to meet the requirements.

For the passive microprocessor protection, such as the WIC1 type protection device, it achieves breaker tripping by a low-energy trip coil. When the external power supply is lost, and the short-circuit current reaches the trip condition, the passive protection outputs an electromagnetic signal to drive the low-energy trip coil, which in turn triggers the circuit breaker to open.

Additionally, the outer casing protection is not implemented through separate ring main units and outdoor boxes. Instead, an integrated design is used, where the external enclosure and ring main unit are one unit. The cable chamber is part of the external enclosure, meeting the sealing and waterproof requirements. The top hinged cover structure allows for easy viewing of the switch status, microprocessor protection device, secondary circuit status, and inspection.

The voltage transformer (VT) integration form is used, where the voltage transformer directly powers the battery. The battery, through the power module, supplies power to the secondary control circuit and the circuit breaker operation circuit, eliminating the need for an external power supply. The secondary control and status circuits communicate via a single line, enabling communication with the external backend management system.