Electric power transmission was originally developed with direct
current. The availability of transformers and the development and
improvement of induction motors at the beginning of the 20th Century, led to
greater appeal and use of ac transmission.
Power transmission using HVDC lines is more cost effective than with
AC lines, but requires more complicated and expensive interconnections
(converters) at each end of the line. The application of HVDC power
transmission is therefore limited to transmission of large energies over long
distances.
Moreover, HVDC is the only practical option when transporting
electric energy through cables over long distances. Another special
application of HVDC is as a link between AC networks of different frequencies,
or between AC networks that become unstable when tied togethter directly.
The HVDC link is here short and the term "back-to-back" is used to characterize
such a configuration.
Dc transmission became practical when long distances were to be
covered or where cables were required. Originally, mercury arc valves were
used in the converters. Thyristors were applied in the late 1960’s and solid state
valves became a reality.
The fundamental process that occurs in an HVDC system is the
conversion of electrical current from AC to DC (rectifier) at the transmitting end,
and from DC to AC (inverter) at the receiving end. There are three
ways of achieving conversion:
•Natural Commutated Converters. Natural commutated converters are most used in
the HVDC systems as of today. The component that enables this conversion process is the
thyristor, which is a controllable semiconductor that can carry very high currents (4000 A)
and is able to block very high voltages (up to 10 kV).
•Capacitor Commutated Converters (CCC). An improvement in the thyristor-based
commutation, the CCC concept is characterised by the use of commutation capacitors
inserted in series between the converter transformers and the thyristor valves. The
commutation capacitors improve the commutation failure performance of the converters
when connected to
weak networks.
•Forced Commutated Converters. This type of converters introduces a spectrum of
advantages,
e.g. feed of passive networks (without generation), independent control of active and
reactive
power, power quality. The valves of these converters are built up with semiconductors with
the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source
Converters).
The components of an HVDC transmission system
To assist the designers of transmission systems, the components that comprise the
HVDC system, and the options available in these components, are presented and
discussed. The three main elements of an HVDC system are: the converter station at the
transmission and receiving ends, the transmission medium, and the
electrodes.
The converter station: The converter stations at each end are replica’s
of each other and therefore consists of all the needed equipment for going from
AC to DC or vice versa. The main component of a converter station are:
Thyristor valves
VSC valves
Transformers
AC Filters and
Capacitor Banks
DC filters
Transmission medium
For bulk power transmission over land, the most frequent transmission medium
used is the overhead line.This overhead line is normally bipolar, i.e. two conductors with
different polarity. HVDC cables are normally used for submarine transmission. The most
common types of cables are the solid and the oil-filled
ones. The solid type is in many cases the most economic one. Its insulation consists of paper
tapes impregnated with a high viscosity oil. No length limitation exists for this type and
designs are today available for depths of about 1000 m. The self –contained oil-filled cable
is completely filled with a low viscosity oil and always works under pressure. The maximum
length for this cable type seems to be around 60 km.
The development of new power cable technologies has accelerated in recent
years and today a new HVDC cable is available for HVDC underground or submarine
power transmissions. This new HVDC cable is made of extruded polyethylene, and is used in
VSC based HVDC systems.
Load voltage regulated by thyristor phase control.
Red trace: load voltage
Blue trace: trigger signal.
Thyristors can be used as the control elements for phase angle triggered
controllers, also known as phase fired controllers.
Phase control (PFC), also called phase cutting, is a method of pulse width
modulation (PWM) for power limiting, applied to AC voltages. It works by modulating a
thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of
conduction at a predetermined phase of the applied waveform.
Despite alternating-current being the dominant mode for electric power
transmission, in a number of applications HVDC is often the preferred option.
•Undersea cables (Ex: 250 km Baltic Cable between Germany and Sweden)
•Increasing the capacity of an existing power-grid in situation where additional
wire are difficult or expensive to instal
•Allowing power transmission between unsynchronised AC distribution systems.
•Reducing the profile of wiring and pylons for a given power transmission
capacity
•Stabilising a predominantly AC power-grid
Long undersea cables have a high capacitance.This causes AC power
to be lost extremely quickly in reactive and dielectric losses, even on cables of a
modest length.
HVDC can carry more power per conductor,because for a given power
rating the constant voltage in a DC line is lover than the peak voltage in an AC
line.This voltage determines the insulation thickness and conductor spacing.This
allows existing transmission line corridors to be ued to carry more power into an
are of high power consumption,which can lower the costs!
The disadvanteges of HVDC are in conversion,switching ,control
,availability of link capacity and maintenance.
HVDC is less reliable and has lower availability than AC systems, mainly due
to the extra conversion equipment. Single pole systems have availability of about
98.5%, with about a third of the downtime unscheduled due to faults. Fault
redundant bipole systems provide high availability for 50% of the link capacity, but
availability of the full capacity is about 97% to 98%
The required static inverters are expensive and have limited overload
capacity.At smaller transmission distances the losses in static inverters may be bigger
than in an AC transmission line!
In contrast to AC systems ,realizing multiterminal systems is complex,as is
expanding existinf schemes to multiterminal systems.
High voltage DC circuit breakers are difficult to build because some
mechanism must be included in the circuit breaker to force to zero!
Operating a HVDC scheme requires many spare parts to be kept,often
exclusively for one system as HVDC systems are less standardized than AC systems
and technology chandes faster!
Costs vary widely depending on the specifics of the project
such as power rating, circuit length, overhead vs. underwater route,
land costs, and AC network improvements required at either terminal. A
detailed evaluation of DC vs. AC cost may be required where there is
no clear technical advantage to DC alone and only economics drives
the selection.
For an 8 GW 40 km link laid under sea, the following are approximate
primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC
link (exclude way-leaving, on-shore reinforcement works, consenting,
engineering, insurance, etc.)
•Converter stations = 110 milion of euro
•Subsea cable+installation= 1 milion / km
So for an 8 GW capacity ,40 km lenght the cost will be of
almost 750 milion of euro! But there are requiered another
300 milion for the other works!
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Iceland-UK(1100MW)
Uk-Ireland
Norway-UK(1200MW)
GermanyNorway(600MW)
Norway-Denmark
Finland to Sweden
Vyborg
Estonia to Finland
Norway to Netherlands
Denmark to Norway
Denmark to
Sweden(600MW)
Sweden to Sweden
Germany-Germany
Denmark to Denmark
None
Sweden to Poland
Germany to Sweden
Denmark to Germany
Ireland to Uk
UK to
Netherlands(1000MW)
France to Uk
Italy to France
Italy to Italy
Spain to Spain
Till 28 is EuroMed