Explain the details of HVDC system with functions of different components.
Answers
HVDC Transmission System
Definition: The system which uses the direct current for the transmission of the power such type of system is called HVDC (High Voltage Direct Current) system. The HVDC system is less expensive and has minimum losses. It transmits the power between the unsynchronized AC system.
Component of an HVDC Transmission System
The HVDC system has the following main components.
Converter Station
Converter Unit
Converter Valves
Converter Transformers
Filters
AC filter
DC filter
High-frequency filter
Reactive Power Source
Smoothing Reactor
HVDC System Pole
Converter Station
The terminal substations which convert an AC to DC are called rectifier terminal while the terminal substations which convert DC to AC are called inverter terminal. Every terminal is designed to work in both the rectifier and inverter mode. Therefore, each terminal is called converter terminal, or rectifier terminal. A two-terminal HVDC system has only two terminals and one HVDC line.
Converter Unit
The conversion from AC to DC and vice versa is done in HVDC converter stations by using three-phase bridge converters. This bridge circuit is also called Graetz circuit. In HVDC transmission a 12-pulse bridge converter is used. The converter obtains by connecting two or 6-pulse bridge in series.
Converter Valves
The modern HVDC converters use 12-pulse converter units. The total number of a valve in each unit is 12. The valve is made up of series connected thyristor modules.The number of thyristor valve depends on the required voltage across the valve. The valves are installed in valve halls, and they are cooled by air, oil, water or freon.
Converter Transformer
The converter transformer converts the AC networks to DC networks or vice versa. They have two sets of three phase windings. The AC side winding is connected to the AC bus bar, and the valve side winding is connected to valve bridge.These windings are connected in star for one transformer and delta to another.
The AC side windings of the two, three phase transformer are connected in stars with their neutrals grounded. The valve side transformer winding is designed to withstand alternating voltage stress and direct voltage stress from valve bridge. There are increases in eddy current losses due to the harmonics current. The magnetisation in the core of the converter transformer is because of the following reasons.
The alternating voltage from AC network containing fundamentals and several harmonics.
The direct voltage from valve side terminal also has some harmonics.
Filters