Presented by,
PREETHA .S
HVDC TECHNOLOGY
o Electric power transmission was originally developed with direct
current.
o However, DC power at low voltage could not be transmitted over
long distances, thus giving rise to high voltage alternating current
(AC) electrical systems.
o With the development of high voltage valves, it was possible to
once again transmit DC power at high voltages and over long
distances, giving rise to HVDC(High Voltage DC) transmission
systems.
o The first commercial HVDC line built in 1954 was a 98 km
submarine cable with ground return between the island of
Gotland and the Swedish mainland.
o Thyristors were applied to d.c. transmission in the late 1960’s and
solid state valves became a reality.
o Today, the highest functional d.c. voltage for d.c. transmission is
+/- 600 kV for the 785 km transmission line of the Itaipu
scheme in Brazil.
o Large AC cable capacitance will limit the maximum possible
underground and underwater cable transmission distance .
o DC allows the use of no DC allows the use of no
environmental impacts.
o In almost all instances DC Transmission has lower losses than AC
Transmission.
o For the DC aerial construction the tower cost and land cost
is lower.
o The D.C. transmission line can have a lower visual profile than an
equivalent A.C. line and so contributes to a lower environmental
impact.
o Some A.C electric power systems are not synchronized to
neighboring networks even though their physical distances
between them is quite small. It is physically impossible to connect
the two together by direct A.C methods in order to exchange
electric power between them.
o If transmission is by submarine or underground cable, the
breakeven distance is much less than overhead transmission. It is
not practical to consider A.C cable systems exceeding 50 km but
D.C cable transmission systems are in service whose length is in
distances of 600 km or greater have been considered feasible.
o HVDC is the unique solution to interconnect asynchronous
systems.
e.g. different grid frequencies.
o HVDC represents the most economical solution for distances
greater than 600 km.
o HVDC is the solution for long submarine transmission.
o A typical HVDC transmission system consists of two converter stations
connected by a transmission path (aerial/underground cable).
o Each converter station acts as either a rectifier (AC‐DC) or
an inverter (DC‐AC)
o The converter uses high speed switching to create a DC voltage from the
AC voltage or an AC voltage from a DC voltage
o Depending upon the design of the HVDC system, filtering of the AC sig
nal may be signal may be required in order to
smooth the resultant waveform
o The integral part of an HVDC power converter is the valve or
valve arm. It may be controllable if constructed from one or more
thyristors in series.
o Electric power flowing between the HVDC valve group and the
a.c. system is ‘three phase’.
o When electric power flows into the d.c. valve group from the a.c.
system then it is considered a rectifier. If power flows from the
d.c. valve group into the a.c. system, it is an inverter. Each valve
consists of many series connected thyristors in thyristor modules.
o No limits in transmitted distance. This is valid for both OH lines
and sea or underground cables.
o Very fast control of power flow, which implies stability
improvements, not only for the HVDC link but also for the
surrounding AC system.
o Direction of power flow can be changed very quickly (bi-
directionality).
o An HVDC link don´t increase the short-circuit power in the
connecting point. This means that it will not be necessary to
change the circuit breakers in the existing network.
o HVDC can carry more power for a given size of conductor.
o The need for ROW (Right Of Way) is much smaller for HVDC
than for HVAC, for the same transmitted power. The
environmental impact is smaller with HVDC.
o VSC technology allows controlling active and reactive power
independently without any needs for extra compensating
equipment.
o VSC technology gives a good opportunity to alternative energy
sources to be economically and technically efficient.
o HVDC transmissions have a high availability and reliability rate,
shown by more than 30 years of operation.
o Converters are expensive.
o Converters require much reactive power.
o Converters generate harmonics, requiring filters.
o Converters have little overload capability.
o Lack of HV dc circuit breakers hampers multiterminal or network
operation.
o For cables crossing bodies of water wider than 32km.
o For interconnecting ac systems having different frequencies or
where asynchronous operation is desired.
o For transmitting large amounts of power over long distances by
overhead lines.
o In congested urban areas or elsewhere where it is difficult to
acquire right of way for overhead lines and where the lengths
involved make ac cables impractical.
o HVDC Light is the newly developed HVDC transmission
technology, which is based on extruded DC cables and voltage
source converters consisting of Insulated Gate Bipolar Transistors
(IGBT’s) with high switching frequency.
o Under more strict environmental and economical constraints due
to the deregulation, the HVDC Light provides the most promising
solution to power transmission and distribution.
o One of the most concerned issues from customers is the
contribution of HVDC Light to short circuit currents.
o Active and reactive power exchange can be controlled flexibly and
independently.
o The power quality and system stability can be improved via
continuously adjustable reactive power support with AC voltage
feedback control.
o Feed AC systems with low short circuit power or even passive
networks with no local power generation.
o SHORT CIRCUIT RATIO
The strength of the a.c. network at the bus of the
HVDC substation can be expressed by the short circuit ratio (SCR),
defined as: ” the relation between the short circuit level in MVA at the
HVDC substation bus at 1.0 per-unit a.c. voltage and the d.c. power in
MW.”
The possible maximum relative short circuit current
Increment (∆Imax) is determined by the short circuit ratio(SCR) as,
ISC :the short-circuit current of the original AC system alone at a 3-ph fault.
ISC_HVDC_L : the short-circuit current of the AC system with converter station
connected and in operation at the same fault.
The maximum possible short circuit increment (∆Imax) due to
HVDC Light is determined by the SCR. It is inversely in
proportional to the SCR and it occurs when the transmission
system is operating at zero active power.
The contribution to the short circuit current is irrelevant to
the fault location if the fault current is evaluated in per unit
with the base value equal to the 3-ph fault current at the
corresponding fault location and without HVDC Light
connected.
HVDC systems remain the best economical and environmentally
friendly option for the above conventional applications. However,
around the world, a quantum leap in efforts to conserve the environment
- are demanding a change in thinking that could make HVDC systems
the preferred alternative to high voltage AC systems in many situations.
The HVDC Light, in contrast to the conventional HVDC which does not
contribute any short circuit current, may contribute some short circuit
current. With HVDC Light, the voltage dip due to distant fault is
possibly reduced and thereby the connected electricity consumers may
suffer less from disturbances.
