Outlines
Evolution of HVDC
Components of HVDC
Comparison of HVDC and HVAC
Types of HVDC
Advantage and Disadvantage of HVDC
 The first commercial application of HVDC was started in the 1950s with 20-MW Gotland
submarine-land project, based on mercury arc valves designed and built in Sweden.
 Then, in the 1960s came a major evolution through General Electric's introduction of silicon-
controlled rectifiers (now known as thyristors).
 Since then, there has been a steady increase in the application of HVDC transmission.
 The first HVDC system using thyristor valves was the Eel River scheme commissioned in
1972, forming a 320 MW back-to-back converter dc interconnection between Canada and
Quebec.
 The remarkable developments of thyristors have resulted in a more widespread use of HVDC.
HVDC Transmission System and Its Components
 We know that AC power is generated in the generating station. This should first be converted
into DC. The conversion is done with the help of rectifier.
 The DC power will flow through the overhead lines. At the user end, this DC has to be
converted into AC.
 For that purpose, an inverter is placed at the receiving end.
HVDC Transmission Components
 Major components are: Transformers, Converters, Harmonic filters, Smoothing reactors,
Electrodes, Dc Lines, Ac Circuit Breakers and Reactive power sources.
HVDC Transmission Components
 Converters:
They are main components of HVDC transmission
Each HVDC line has at least two convertors
Sending end convertor working as rectifier and receiving end working as invertor
Several thyristors are connected in series and/or parallel to form a valve to achieve
higher voltage/current ratings
 Depending on the firing angle, each converter can operate as either a rectification mode or an
inverter mode.
 The valve bridges consist of high-voltage valves connected in a 6-pulse or 12-pulse
arrangement as per the requirement.
 Various requirements are of major concern while designing and selecting the rating.
HVDC Transmission Components
 Converter Transformers: Especial made transformers are used to capable of the transient
effect and harmonics due to the switching of thyristor valves.
 Since fault current due to fault across the valve is predominantly controlled by a transformer
impedance, its leakage impedance is higher than the conventional transformer.
 On-line tap-changing is used to control the voltage and reactive power demand
Smoothing Reactors:
 As its name, these reactors are used for smoothing the Dc current output.
 It also limits the rate of rise of the fault current the case of Dc line short circuit.
HVDC Transmission Components
 Harmonic Filters: Converters generate harmonic voltages and currents on both Ac and Dc
sides. These harmonics may cause overheating of capacitors and nearby generators, and
interference with telecommunication systems. Therefore filters are used on both sides.
 Dc Lines: They may be overhead lines or cables. Except for the number of conductors and
spacing required, Dc lines are very similar to Ac lines.
 Reactive Power Sources: Dc converters inherently absorb, 50% of active power transferred,
reactive power. Reactive Power Sources are therefore provided near to the converters.
 Earth Electrodes: The earth resistivity at upper layer is higher and electrodes cannot be kept
directly on the earth surface.
Classification of HVDC Links
 HVDC links may be broadly classified into the following categories:
 Monopolar links
 Bipolar links
 Homopolar links
 In the monopolar links, Single conductor is required and water or ground act as the return
path. If the earth resistivity is high, metallic return is used.
 In the bipolar links, Double converters of same voltage rating, one positive and the other
negative, are used in each terminal.
 If one pole is isolated due to a fault in its conductor, the other pole can operate with ground.
 In the homopolar links, It consists of more than two conductors which is having equal
polarity generally negative.
 If one pole is isolated due to a fault in its conductor, the other pole can operate with ground.
Advantages of HVDC transmission
 No reactive power loss
 No stability problem
 No charging current
 No skin and Ferranti effect
 Power control is easier
50Hz
 Less corona loss and radio interference
 Ground can be used as return
 Requires less space compared to AC
 Asynchronous operation is possible
 Cheaper for long distance transmission
 No switching transient
 No compensation problem
 Fast fault clearing time
60Hz
Converter
Disadvantages of HVDC transmission
 Cost of terminal equipment is high
 Introduction of harmonics
 Blocking of reactive power
 Point-to-point transmission
 Limited overload capacity
 Huge reactive power requirements at the converter terminals (50-60% of the Active power)
The reactive power demand can be supplied from:
 AC filters
 Shunt capacitors
 Static compensators like SVC
 Synchronous condensers