20-09-2012, 01:18 PM
HVDC Light, a tool for electric power transmission to distant loads
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SUMMARY
HVDC Light is a newly developed technology for
electric power transmission by HVDC based on
Voltage Source Converters. This has many
interesting characteristics that make it a very
promising tool for transmission of electric power to
distant loads, where no other transmission is possible
or economic. The technology is presented here and
its application to a pilot transmission, which is now
operating in a commercial network since March
1997. Special emphasis is given to the possibility to
serve the loads in a connected AC network without
own generation.
New DC power cables based on a modified triple
extrusion technology and a specially designed DC
material have been developed. DC power cables with
ratings 2 x 25 MW at 100 kV can be accomplished
weighing only 1 kg/m per cable. Such cables can be
installed at low cost by e.g. ploughing technique and
aerial cabling. Larger cables can transmit much more
power.
INTRODUCTION
The HVDC technology has been successful to connect
AC networks that for technical or economical reasons
cannot be connected by AC transmission.
The present technology uses circuits with PCC (Phase
Commutated Converters) and is based on thyristor
valves with semiconductor devices that can be turned
on by a positive gate pulse when the main voltage is
positive. To turn off the thyristors need a negative
voltage across the main terminals. This is normally
achieved by commutating the current to the valve in
the next phase.
Thereby the present technology has inherent
weaknesses, which to some extent limit the use of
HVDC as the means to overcome these weaknesses
are relatively expensive. These are the need for
rotating machines in the receiving network and the
risk of commutation failure, which means that for
some cycles there is no transmission of power.
These weaknesses can be overcome by using Voltage
Source Converters (VSC) which have now been developed
for high voltage application. The Hellsjön
Project is the world’s first VSC HVDC transmission.
It is rated 3 MW and ±10 kV DC. The link is in
operation in a commercial network since the
beginning of March 1997 between Hellsjön and
Grängesberg in central Sweden on a 10 km long decommissioned
AC line. The operation experience has
been entirely positive. The transmission performs as
predicted, both during steady-state and transient
conditions.
VSC TECHNOLOGY AND PULSE WIDTH
MODULATION (PWM)
In industrial drives the PCC (Phase Commutated
Converter) technology which is used in HVDC is now
almost totally replaced by VSC (Voltage Source
Converter) technology. The fundamental difference
between these two technologies is that VSC need
components that can switch off the current and not
only switch it on as is the case in PCC.
As in a VSC the current can be switched off, there is
no need for a network to commutate against. In
HVDC-applications it could then be of interest to use
VSC technology in order to supply passive networks,
that is areas which lack rotating machines or
networks that does not have enough power in the
rotating machines (too low short circuit power).
By use of higher switching frequency components it is
possible to use Pulse Width Modulation (PWM)
technology. Then only one converter is needed and
the AC voltage is created by switching very fast
between two fixed voltages. After low pass filtering
the desired fundamental frequency voltage is created.
In this case it is not necessary to have a transformer
for the functioning of the converter. See figure 1.
IGBT
From the above it appears advantageous to shift from
present Phase Commutated Converter Technology for
HVDC to VSC and PWM. Why has this not
happened a long time ago?
The correct answer is that there have not been
semiconductor components available that have been
good enough for the task.
In this respect the IGBT is a very interesting
component, as it is a MOS-device and the power need
for the control of the component is very low and can
be fed from the snubber circuits. This makes series
connection possible with good voltage distribution
even at switching frequencies in the kHz range.
There is a fast development of the IGBT and
components for the voltage of 2.5 kV has recently
become available in the market and soon higher
voltages are expected. The market for IGBT also
increases very fast which add to the knowledge base
of the technology itself and makes it an interesting
component for small scale HVDC applications.
CABLES
The new HVDC Light cables have insulation of
extruded polymer. Until now, the cables used for
HVDC transmission and distribution, have been
paper insulated cables, low pressure oil filled cables
(LPOF) or mass impregnated non draining cables
(MIND). There are several drawbacks with these
designs. The LPOF cable needs auxiliary equipment
to maintain the oil pressure and can not be easily
installed. The MIND cable has limitations in the
operating conductor temperature. There are of course
also environmental oil spill concerns that are
associated with the LPOF cable. Paper insulated
cables are not feasible for aerial cables because of
sensitivity to repeated bending. HVDC Light cables
are laid in pairs with antiparallel currents and thus
eliminating magnetic fields.
DISTANT LOAD APPLICATIONS
Electrical systems are mostly built as meshed
networks with multiple interconnections between
various loads and generation stations. In such a
network the power can be exchanged via different
routes and the cost of power can be considered
common to the all loads in the network. There are,
however many places, small cities, villages, mines
etc., that are located far from any network. Such a
place we call a distant load. The supply of power to a
distant load can be made by a radial transmission
from a meshed network or by local generation.
For small loads below 150 MW, local generation has
been necessary, for distances beyond what has been
possible to reach with an AC transmission.
Traditional HVDC has not been cost effective in this
power range, because it did not have the technical
possibilities to feed power into an isolated load
without synchronous machines. HVDC Light will
now provide an excellent alternative for power
transmission to small distant loads (see Figure 6).
CONCLUSIONS
The development of power semiconductors,
specifically IGBT and extruded DC cables led to
that small scale HVDC in combination with cables
can offer a number of new applications to serve the
needs of utilities.