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A conventional HVDC control and protection system is a key part in guaranteeing the overall performance of the HVDC thyristor-based transmission system. It contributes to safe and steady system operation, provides flexible operating modes and accurate control effects, helps to clear fault and assists in system recovery during system disturbances.
A well-designed control and protection system can maximally exert the advantages of conventional HVDC transmission technology in the following aspects:
Long -distance large-capacity power transmission
Underground or underwater cable transmission
Asynchronous interconnection of an AC system
The PCS-9550 control and protection system is designed based on very powerful and flexible building blocks built with a state-of-art UAPC platform and connected by high performance fiber optic communication links. It brings the biggest convenience to new and retrofit HVDC transmission projects.
The design criterion for the control system is to reach 100% reliability for the transmission system. The NR Electric's HVDC control and protection system is equipped with parallel redundancy at all levels. All system parts that are involved in the power transfer of the HVDC link are made redundant.
The whole HVDC control and protection system is built in hierarchical and distributed structure, which is divided into station level and equipment level. Station level includes SCADA system. Equipment level includes control and protection units, I/O interface and distributed field bus. The station level and equipment level are communicated via SCADA LAN network that is also redundant. While the control and protection units and their I/O interfaces are connected through optical field bus.
A complete PCS-9550 HVDC control and protection system can be divided into the following sub-systems:
Interface with remote control center, e.g. state dispatching center, provincial dispatching center and remote central control.
This sub-system consists of tele-control workstations and Remote Terminal Units (RTUs) for point-to-point communication with remote control centers.
Operator Control System
The operator control system is so called SCADA system. It consists of station LAN, operator workstations (OWS), engineer workstations (EWS), SER terminals, system servers, station synchronization system, etc. The operator control system is used as the man-machine interface during HVDC operation and has the following functions:
Receiving and executing operation orders from operators or dispatch center
Monitoring and processing failure information or abnormal operation status
Event sequence record and event alarm within the whole station
Synchronization and time calibration for all sub-systems within the station
Parameter adjustment for DC control system
History data documentation
Emergent stopping of AC and DC systems
Management of converter station documents and programs
Fundamental training functions
AC/DC Station Control System
AC/DC station control system consists of main computers and distributed I/O. The DC station control is assembled with the pole control system. The main functions of this system are:
Sequence control of the equipment in AC and DC switchyard
Execute trip commands initiated by protections
Open or close filter banks following order from pole control system
Collect and manage the alarm signals of main/auxiliary equipment
DC Control System
The DC control system implements the control of bipolar, single pole and converter unit. It consists of main control computers, distributed field buses and distributed I/O. The redundant system configuration is adopted for each pole to achieve the following functions:
Generate control pulse for converter valves
Maintain the synchronization between control pulse and commutation voltage
Keep interval equal between control pulses
Deblock/block converter valve with smooth and stable startup/shutdown
Regulate triggering angle of converter valves within the designed range to satisfy the ordered amount of power/ current.
Adjust converter transformer tap changers according to the operation status of the AC and DC system
Implement sequence control of HVDC system
DC Protection System
DC protection system ensures the safe operation of the whole HVDC system. It includes pole protection (PPR), DC filter protection (DFP), converter transformer protection (CTP) and AC filter protection (AFP).
The HVDC control and protection system supports connection to electronic measuring devices, which receive digital measurement signals via optical fiber.