The design of low-voltage (LV) switchgear is a critical aspect of any electrical power distribution system. This article delves into the process of designing a main distribution switchboard, focusing on the specific requirements of a system placed immediately downstream of a 2000kVA MV/LV transformer, supplying three 850A outgoing feeders. We will cover various aspects, including layout considerations, component selection, safety regulations, and relevant documentation, drawing on best practices and referencing available resources like ABB LV switchgear catalogues and Eaton design guides. The emphasis will be on ensuring selectivity and overall system reliability.
1. Project Scope and Requirements:
Our task is to design a main LV distribution switchboard located immediately after a 2000kVA medium-voltage (MV)/LV transformer. This switchboard will serve as the primary distribution point, feeding three 850A outgoing feeders to other downstream distribution switchboards. The specific requirements include:
* Transformer Capacity: 2000kVA MV/LV transformer. This dictates the switchboard's incomer capacity and overall sizing.
* Outgoing Feeders: Three 850A feeders. Each feeder requires dedicated protection and isolation.
* Selectivity: A paramount concern. The design must ensure that faults are isolated to the smallest possible section of the system, minimizing disruption.
* Safety: The design must adhere to all relevant safety standards and regulations (e.g., IEC 60439-1).
* Space Constraints: The physical dimensions of the switchboard must be considered, taking into account available space and accessibility for maintenance.
2. LV Switchgear Layout Design:
The layout of the LV switchgear is crucial for efficient operation and maintenance. A well-planned layout simplifies fault finding, minimizes wiring complexity, and ensures optimal heat dissipation. Several factors influence the layout:
* Incomer: The main incomer from the MV/LV transformer will be the starting point, typically incorporating a main circuit breaker with appropriate fault protection.
* Outgoing Feeders: Three 850A feeders require individual circuit breakers, metering, and potentially other protective devices depending on the load characteristics.
* Busbars: The main busbars must be sized to handle the total current capacity, considering future expansion possibilities. The material and arrangement (e.g., single or double busbar) will depend on the specific requirements and budget.
* Protection Devices: Appropriate overcurrent protection (circuit breakers, fuses) is essential for each feeder. Coordination between these devices is crucial to achieve selectivity. This often involves using different tripping characteristics (e.g., instantaneous, time-delayed) to ensure that only the faulty circuit is isolated. Consideration should be given to incorporating earth fault protection (e.g., residual current devices – RCDs).
* Metering: Energy metering is usually included to monitor energy consumption and facilitate billing.
* Auxiliary Circuits: Space should be allocated for auxiliary circuits, such as control power supplies, lighting, and instrumentation.
* Cable Entry and Termination: Adequate space must be provided for cable entry and termination, adhering to cable management best practices.
3. LV Switchgear Layout Template:
A detailed layout template is essential before physical construction. This template should be a scaled drawing showing the precise location of all components, busbars, wiring, and cable entries. Software tools like AutoCAD or specialized electrical CAD software are commonly used for creating these templates. The template should also include details about:
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