High voltage HVDC switches in grid

The Typical Single-Line Diagram of an HVDC Transmission Scheme Using DC Side Switchgear

The typical single-line diagram shown in the figure illustrates an HVDC transmission scheme utilizing DC side switchgear. The following switches can be identified from the diagram:

• NBGS – Neutral Bus Grounding Switch:This switch is typically in the open position. When closed, it firmly connects the converter's neutral line to the station ground pad. If the converter can operate in bipolar mode with balanced current between the poles, resulting in very low direct current to ground, this switch can usually remain closed.
• NBS – Neutral Bus Switch:The NBS is connected in series with the neutral connection of each pole. If a ground fault occurs in one pole, that pole will be blocked, thus protecting the system from the fault.
• GRTS – Ground Return Transfer Switch:The connection between the HVDC conductor and the neutral point includes a high-voltage circuit breaker and the GRTS. The GRTS is used as part of the switching operation to configure the HVDC system for either ground return monopolar or metal return monopolar modes.
• MRTB – Metal Return Transfer Breaker:The MRTB is used in conjunction with the GRTS to transfer the DC load current between the ground return mode (ground loop) and the parallel mode (unused high-voltage conductor).

Explanation

• NBGS: During normal operation, the NBGS is typically kept open to prevent unnecessary ground currents. However, in specific situations, such as when operating in bipolar mode with well-balanced currents between the poles, the NBGS can be closed to provide additional grounding protection.
• NBS: The NBS is used to protect the system from ground faults. When a fault occurs in one pole, the NBS can quickly disconnect the neutral connection of that pole, preventing the fault from spreading.
  GRTS: The GRTS is a critical switching device used to switch between different operating modes of the HVDC system. It works in conjunction with the high-voltage circuit breaker to ensure stability and safety during the switching process.
• MRTB: The MRTB is used to switch the DC load current between ground return mode and metal return mode. This switching operation helps optimize the system's operational efficiency and reliability.
  By coordinating the operation of these switchgear devices, the HVDC system can flexibly switch between different operating modes, ensuring safe, reliable, and efficient system operation.