MV Switchgear Design: Technical Core Requirements

Medium Voltage Switchgear (MV Switchgear) is the core equipment in MV distribution and transmission systems, undertaking critical functions such as control, protection, metering, and monitoring of electrical circuits. As a critical hub for the grid’s safe operation, its reliability and performance directly determine the power supply quality, operational efficiency, and personnel/equipment safety of the entire power system. Modern switchgear design must strictly address space constraints, environmental impacts, and extreme conditions like short-circuit faults, while ensuring high electrical performance. Therefore, a successful design solution requires a holistic consideration of multiple dimensions, including insulation coordination, thermal stability, dynamic stability, personnel safety protection, and long-term maintainability. Below are the core technical essentials that we must strictly follow and implement during design and manufacturing.

I. MV Switchgear Insulation & Minimum Electrical Clearance

Insulation is the primary factor ensuring the safe operation of medium voltage switchgear. At the start of design, the insulation scheme must be clearly defined, including the choice of insulating medium: air-insulated, SF6 gas-insulated, solid-insulated, or air-composite insulated, etc.

Determining Factors for Insulation Clearance

For air-insulated switchgear, the corresponding air insulation clearance standards must be met. In a slightly non-uniform electric field environment, the Basic Impulse Level (BIL) or Lightning Impulse Withstand Voltage is the decisive indicator for determining the minimum air clearance between live conductors or to the ground.

• Standard Basis: Both IEC and GB/DL standards have clear stipulations for minimum clearance. For example, according to IEC standards, for equipment with a rated voltage level of 17.5kV and a Basic Impulse Level (BIL) of 95kV, if air insulation is used, the minimum clearance must be greater than or equal to 160mm.
• Design Principle: As long as the design clearance is not less than the minimum value required by the standard, the probability of the equipment passing the BIL test is extremely high (over 99%). Conversely, if the design clearance is non-compliant (e.g., domestic GB standards require a minimum clearance of ≥ 125mm for 12kV switchgear, meaning it cannot pass the 95kV BIL test), it cannot be used for 17.5kV systems.
• Other Media: When gas or solid insulation is used, corresponding design standards must also be followed to ensure the insulation distance at critical points exceeds the standard requirement, especially by accurately calculating and controlling the electric field distribution inside and on the surface of the insulators.

II. MV Switchgear Layout and Primary Component Sequence

The layout of the primary system components is key to the long-term performance and maintenance efficiency of the switchgear. The design must strictly follow the current flow in the electrical circuit and the functional sequence of each component. This logical arrangement not only optimizes the internal electric and thermal field distributions, reducing the risk of localized overheating and ensuring insulation performance and thermal stability; a standardized layout also provides clear guidance and standardized procedures for equipment inspection, operation, and maintenance, which is the foundation for ensuring high reliability of the equipment’s operation.

In practical design, the components must be strictly arranged in order according to the current path, forming a clear functional sequence. This sequence starts with the main busbar and the top branch busbar, followed by the connection to the contact box. The core control and protection component, the vacuum circuit breaker, is located in the center of the circuit, completing circuit isolation via the contact box on the other side. Subsequently, the equipment sequentially connects to the current transformer used for monitoring and metering, the grounding switch used for safety operation, and finally connects to the outgoing cable busbar connection and the optional voltage transformer. Strictly adhering to this logical sequence is a prerequisite for ensuring the safe and reliable realization of the MV switchgear’s distribution function.

III. Temperature Rise Control and Heat Dissipation Design Strategy

Temperature rise control is one of the most critical non-transient performance indicators in MV switchgear design. Continuous or excessive localized temperatures not only accelerate the thermal aging process of insulating materials, drastically shortening equipment lifespan, but also lead to increased contact resistance, forming a vicious cycle. Therefore, all designs must strictly adhere to the temperature rise limits for all switchgear components specified by IEC and GB standards, ensuring the equipment can operate stably for a long time at its rated current.

• High Heat Source Identification: High-heat components include the vacuum interrupter chamber of the vacuum circuit breaker and its upper/lower connection terminals, as well as the primary moving and stationary contacts of the draw-out circuit breaker. These areas have higher contact resistance and high current density, making them critical points for temperature rise control.
• Heat Dissipation Channel Design: To prevent heat accumulation and ineffective dissipation, the switchgear design needs to consider air flow. Effective vents or ventilation designs are usually required at the bottom and top of the circuit breaker compartment to form air convection channels that carry heat away.
• Component Location Optimization: When arranging components, the current transformer should be placed slightly further away from the contact box so that the busbar can draw away the high temperature generated at the moving and stationary contact connection, utilizing the busbar as a heat dissipation path.

IV. Verification of Busbar Dynamic and Thermal Stability

The dynamic and thermal stability verification of the busbar is a vital safety performance indicator in MV switchgear design, aiming to assess the structural integrity and electrical reliability of the equipment under short-circuit faults. The design goal is to ensure that the busbar and insulating supports can withstand instantaneous electrodynamic forces (dynamic stability) and rapid thermal stress (thermal stability), preventing permanent deformation or insulation failure, and ensuring long-term operational safety.

• Verification Basis: The design must perform dynamic stability calculations based on the equipment’s rated peak withstand current and thermal stability calculations based on the short-time withstand current.
• Determining Support Spacing: The dynamic stability calculation results are used to determine the minimum spacing between busbar insulating supports. For example, for short-circuit parameters of 31.5kA / 80kA, with a phase spacing of 210mm, the spacing of the insulating supports usually cannot exceed 1m. The design must follow the calculation formula to determine the most conservative design principle.

V. Pressure Release and Protection for Internal Arc Faults

An internal arc fault is the most severe extreme condition in MV switchgear operation, characterized by the instantaneous generation of extremely high thermal energy and pressure shock waves. Therefore, the equipment design must possess the ability to withstand internal arcing to minimize the potential harm to personnel and equipment.

• Pressure Relief Mechanism: The design must consider the internal arc pressure release. The shock wave from the arc explosion must be able to quickly open the pressure relief flap.
• Channel Smoothness: The key is to ensure the smoothness of the pressure relief channel. An optimally designed channel can reduce the time required to open the flap, quickly release the pressure inside the compartment, thereby minimizing the degree of damage to the cabinet structure and ensuring the safety of the operating personnel.

VI. Serviceability and Project Adaptability of MV Switchgear

In addition to electrical performance and safety protection, the switchgear design must also meet high operability and maintainability requirements throughout its life cycle. This requires the design to fully consider the convenience of on-site installation and possess sufficient flexibility to adapt to the customized needs of non-standard projects (special configurations).

• Non-Standard Project Challenges: For non-standard projects requiring multiple windings, multiple epoxy post-type current transformers (CTs), or the mandatory use of window-type CTs to meet PX-class protection requirements, the design must ensure that all operations and maintenance can be performed from the front of the cabinet.
• Earthing Switch Position Optimization: The position of the earthing switch significantly impacts operation.
  • Placing the earthing switch at the very back of the cabinet, while potentially meeting some layout requirements, necessitates lengthening the operating shaft, which significantly increases the operating torque and makes it difficult for the operator to view the earthing switch status from the front of the cabinet.
  • The recommended practice is to maintain the earthing switch’s position as in the original standard switchgear to avoid operational inconvenience caused by a lengthened operating shaft.
• Trolley and CT Placement: To satisfy CT winding requirements, multiple CTs can be placed in the rear of the cabinet. The voltage transformer (PT) trolley, however, should be placed in the front of the cabinet to facilitate inserting the PT trolley after the cable connection is complete and putting it into operation, simplifying the operation procedure.

Summary

The design of Medium Voltage Switchgear is fundamentally a systematic art of balance. The core challenge lies in ensuring the dual reliability of the equipment under rated operation and extreme conditions like short-circuit faults, through precise insulation clearance, optimized heat dissipation control, and strict verification of dynamic and thermal stability, all within a confined space. The ultimate design goal is not only to meet standard electrical performance indicators but also to minimize the risk of faults through comprehensive internal arc pressure relief and human-machine engineering optimization, such as earthing switch operation, thereby comprehensively guaranteeing the efficient operation of the power system and the safety of operating personnel. This requires designers to possess the comprehensive ability to translate theoretical standards into highly reliable and adaptable industrial products.