HVDC

HVDC

Energy transport in high voltage:
To power an island, connect an offshore wind farm to the onshore grid, or to interconnect two grids separated by the sea, we can no longer transport the electricity with high voltage underwater lines into Alternating Current beyond 50 km. AC develops, by capacitive effect, a reactive power, which opposes current flow. The HVDC interest (High Voltage Direct Current), which is free of capacitive effect, grows with the transmission distance. In the case of a "far shore" installation, it’s the only solution to transport electrical energy from the wind farm up to the coast.

HVDC Energy Transport

Overhead lines:
The transmission of electrical energy by overhead line HVAC (High Voltage Alternating Current) cannot travel very long distances. Transport in AC develops destabilising reactions by capacitive effect between the line and the ground.
Greater is the length of the line, more important is the phase shift.

 From 1500 km, the phase shift is 90° and synchronisation problems and system stability are encountered.
For these technical reasons, the transport in DC is required.
The use of DC requires significant investment costs (construction of converter stations AC / DC and vice versa). That said, from a distance, an overhead HVDC line transport allows significant gains (e.g., because we need 2 cables in DC, instead 3 in AC). From 600 km, this economy compensates the need having converter stations at both ends to connect with the alternative networks.

The two converter stations

Line losses in AC and DC:

• Capacitance losses (only in AC),  
• Thermal losses, voltage drop,
• Skin effect losses (only in AC).

HVAC Power Line modeled

 
Capacitance and inductance of Power Line:

FACTS (1) (Flexible Alternating Current Transmission System):
It’s a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally a power electronics-based system.

Thermal losses, voltage drop and skin effect:
Physics teaches us that power transmitted over a network is equal to the voltage multiplied by the current intensity.
The voltage drop, in AC lines depends on the circuit impedance (resistance plus reactance).
The thermal losses (Joule effect) due to the current flow in the resistance of the overhead line are proportional to the square of the current.     Pj = R x I² x t
Consequently, to minimise losses, it is necessary to transport electrical energy with the lowest possible intensity thus with higher voltage. This requires transformers that allow switching from one voltage to another easily, but these work only in alternating current (Hence the preponderance of HVAC overhead line systems until now).
Another way to reduce losses is to minimise the line resistance using, for example, bigger conductors. More expensive, it's achievable with underground or undersea cables but inconceivable with overhead lines on account of being overweight.
With alternating current, we are confronted by the Skin effect (2), which further increases the resistance of the conductor.
 The Skin effect is the tendency of the current to flow between the surface and the skin depth (depends on the frequency of the current and electrical and magnetic properties of the conductor). The higher the frequency is, the less the centre of the conductor "serves".
 (This phenomenon is used in the power plant alternators. Since the middle of the conductor is not used, it is hollow, like a pipe, and a coolant is circulated in order to lower the temperature of the alternator and to limit the losses by Joule effect).

All these disadvantages can be considered negligible for conventional length lines. For very long distances the economic weight of these losses becomes very penalising.

The Advantages of HVDC:
Performance is the major advantage of DC power lines. Less energy is lost during transport and there is no need for reactive compensation along the line.
In DC, the cable capacitance appears only during the first powering of the cable or if the voltage level changes; there is no additional current required. For a long AC powered undersea cable, the entire current-carrying ability of the conductor would be needed to offset the reactive current alone.
DC powered cables are only limited by their temperature rise and by Ohm's Law. Although some leakage current flows through the dielectric insulator, this is small compared to the cable's rated current.
Because DC power flows steadily through the wires without changing direction many times each second and through the entire conductor rather than at the surface (skin effect in AC), DC transmission lines lose less power than AC transmission lines.

More power transmitted per conductor with DC line than with AC line, because for a given power rating, the constant voltage in a DC line is lower than the peak voltage in an AC line.

More power transmitted

Smaller Footprint: DC transmission lines require narrower right of way footprints, using less land, than equivalent AC lines.
Fewer conductors than an AC line, as there is no need to support three phases but only one or two.

Smaller Footprint

History:
The first lines of electric power transmission were developed with direct current (at the end of the 19th century). Click to see “War of Currents” between Tesla and Edison.
But, this war was won by AC, which has been the platform for electrical transmission across the world during all the 20th century through use of the transformer that allows increasing and lowering the voltage easily and the development of induction machines (asynchronous motors and synchronous alternators).
But nowadays, due to losses caused by the alternating current transmission on long distance and thanks to advances in power electronics, it has again become interesting to use the DC to transport electrical energy. The HVDC systems use electronic devices like thyristors or IGBTs that were unavailable during the War of Currents era.
 HVDC allows power transmission between unsynchronised AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilise a network against disturbances due to rapid changes in power. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.
Click to see "The war of current"

HVDC scheme types (by AREVA)

AMonopolar and Bipolar HVDC systems (by AREVA):

Standard graphical symbols for valves and bridges

      
Configurations:
The integral part of an HVDC power converter is the valve or valve arm. It may be non-controllable if constructed from one or more power diodes in series or controllable if constructed from one or more thyristors in series. The standard bridge or converter connection is defined as a double-way connection comprising six valves or valve arms. When electric power flows into the DC valve group from the AC system then it is considered a rectifier. If power flows from the DC valve group into the AC system, it is an inverter. Each valve consists of many series connected thyristors in thyristor modules.

(Click to see more configurations)

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