Component of an HVDC Transmission System

HVDC system has the following main components

1. Converter Station
2. Converter Unit
3. Converter Transformers
4. Filters
   1. AC filter
   2. DC filter
   3. High-frequency filter
5. Reactive Power Source
6. Smoothing Reactor
7. 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. In HVDC transmission a 12-pulse bridge converter is used. 

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

    The AC and DC harmonics are generated in HVDC converters. The AC harmonics are injected into the AC system, and the DC harmonics are injected into DC lines. The harmonics have the following disadvantages.

    1. It causes the interference in telephone lines.
    2. Due to the harmonics, the power losses in machines and capacitors are connected in the system.
    3. The harmonics produced resonance in an AC circuit resulting in over voltages.
    4. Instability of converter controls.

    The harmonics are minimised by using the AC, DC and high-frequency filters. The types of filter are explained below in details.

    • AC Filters – The AC filters are RLC circuit connected between phase and earth. They offered low impedances to the harmonic frequencies. Thus, the AC harmonic currents are passed to earth. Both tuned and damped filters are used. The AC harmonic filter also provided a reactive power required for satisfactory operation of converters.
    • DC Filters  – The DC filter is connected between the pole bus and neutral bus. It diverts the DC harmonics to earth and prevents them from entering DC lines. Such a filter does not require reactive power as DC line does not require DC power.
    • High-Frequency Filters – The HVDC converter may produce electrical noise in the carrier frequency band from 20 kHz to 490 kHz. They also generate radio interference noise in the megahertz range frequencies. High-frequency filters are used to minimise noise and interference with power line carrier communication. Such filters are placed between the converter transformer and the station AC bus.

    • Reactive Power Source

      Reactive power is required for the operations of the converters. The AC harmonic filters provide reactive power partly. The additional supply may also be obtained from shunt capacitors synchronous phase modifiers and static var systems. The choice depends on the speed of control desired.

      Smoothing Reactor

      Smoothing reactor is an oil filled oil cooled reactor having a large inductance. It is connected in series with the converter before the DC filter. It can be located either on the line side or on the neutral side. Smoothing reactors serve the following purposes.

      1. They smooth the ripples in the direct current.
      2. They decrease the harmonic voltage and current in the DC lines.
      3. They limit the fault current in the DC line.
      4. Consequent commutation failures in inverters are prevented by smoothing reactors by reducing the rate of rising of the DC line in the bridge when the direct voltage of another series connected voltage collapses.
      5. Smoothing reactors reduce the steepness of voltage and current surges from the DC line. Thus, the stresses on the converter valves and valve surge diverters are reduced.

      HVDC System Pole

      The HVDC system pole is the part of an HVDC system consisting of all the equipment in the HVDC substation. It also interconnects the transmission lines which during normal operating condition exhibit a common direct polarity with respect to earth. Thus the word pole refers to the path of DC which has the same polarity with respect to earth. The total pole includes substation pole and transmission line pole.