NEW TECHNOLOGIES IN HVDC CONVERTER DESIGN
Alf Persson
Lennart Carlsson
Mikael Åberg
ABB Power Systems, Sweden
ABB Power Systems, Sweden
ABB Power Systems, Sweden
1. SUMMARY
HVDC technology took a big step forward around 20 years
ago when thyristor valves succeeded the mercury arc valves
previously used. The converter station concept introduced
at that time, however, has remained practically unchanged
since then.
The time has now come for a further major advance in
technology. The introduction of new concepts will change
whole approach to building an HVDC station. Even though
this innovation may not be quite as significant as when
thyristor valves were introduced, the new features will
greatly improve the operating characteristics of HVDC
transmissions and reduce the size and complexity of
converter stations.
The new generation of converter stations is now likely to
include some of the following features:
- a new type of converter circuit, the capacitor commutated
converter (CCC)
- actively tuned AC filters
- air insulated outdoor thyristor valves
- active DC filters.
Figure 1: Single line diagram of a monopolar station with
CCC and ConTune® AC filter.
Consequently, many other main circuit components, in
addition to the filter components, such as switching
equipment, CTs, etc, must be installed. The filters will
therefore take up considerable space in a converter station.
2.2 De-coupling of filtering and reactive power
supply
2. CAPACITOR COMMUTATED CONVERTERS
The development of new, effective AC filters, described
in a separate section of this paper, makes it possible to
perform the filtering function through a single filter bank
with small Mvar rating. The new AC filter thus allows decoupling of the functions of the filtering and the reactive
power generation to a large extent. In this situation, the
traditional way of generating the required reactive power
would be to install a number of shunt capacitor banks.
However, during the last few years, another concept, the
capacitor commutated converter, abbreviated CCC, which
provides a much more interesting solution, has been studied
and developed.
2.1 General
2.3 The CCC concept
In a conventional HVDC converter the consumption of
reactive power is typically around 0.5 p.u. of the active
power. This reactive power requirement is in most cases
fully compensated for locally by installation of shunt AC
filters in the converter station. Requirements for permitted
reactive power unbalance, or AC voltage changes upon
filter switching, in many cases result in splitting of the
installed reactive power into several filter/shunt banks.
The electrical diagram of a CCC is shown in Fig. 2.
Keywords: converter circuit, capacitor commutated
converter, actively tuned AC filter, outdoor thyristor valve,
active DC filter.
This converter is characterized by the use of commutation
capacitors inserted in series between the converter transformers and the valve bridge. This circuit has been
proposed in several previous papers, see, for example, Refs.
1 and 2, as a method of obtaining a self-commutated
converter.
Figure 2: Capacitor Commutated Converter.
However, the ABB approach does not aim at achieving a
self-commutated converter: instead, it provides reactive
power compensation proportional to the load of the
converter. The need of switchable shunt capacitor banks
for reactive power compensation is thereby eliminated.
Since the AC filters are necessary only from the point of
view of filtering harmonics, the shunt-connected reactive
power generation can be minimized. In the ABB solution
the size of the commutation capacitor is chosen so that the
full load reactive power consumption of the converter is
compensated by the reactive generation of the small high
performance AC filter. Fig. 3 compares the reactive power
conditions.
Conventional converter
Figure 4: Remote single phase to ground fault in the inverter AC network.
2.5 Improved dynamic stability
The contribution to the commutation voltage from the
commutation capacitors results in positive inverter
impedance characteristics for an inverter operating at minimum commutation margin control. An increase in direct
current therefore results in a DC voltage increase rather
than the opposite, which is the case for conventional
inverters with commutation margin control. The dynamic
stability of an inverter will thus be dramatically improved
with a CCC.
CCC
Figure 5: Ud/Id characteristics.
Figure 3: Reactive power conditions for a typical conventional converter and for a CCC.
2.4 Sturdily constructed and resistant to disturbances
The commutation capacitors improve the commutation
failure performance of the converter. The capacitors
introduce a source of commutation voltage in addition to
the AC bus voltage which, if proper control functions are
included, can be used to minimize the risk of commutation
failures. Typically, a CCC can tolerate a sudden 15-20%
voltage drop without developing a commutation failure.
The improved inverter performance as described above
results in more economical solutions, particularly for
HVDC schemes feeding weak systems and for HVDC schemes using very long DC cables.
Fig. 6 shows the MAP (Maximum Available Power) curves
for a conventional converter and a CCC for SCR = 2. As
can be seen, the CCC is in a very stable situation while the
conventional converter is close to the stability limit.
The diagrams also show that the load rejection overvoltage
which occurs upon pole tripping or commutation failures
is reduced from 1.5 to 1.2 p.u. as a result of the small size
of the shunt-connected filters for the CCC. The small shunt
filters will also reduce the risk of low order harmonic
resonances on the AC side.
2.7 Effects on other equipment
Introduction of commutation capacitors results in different stresses on the other equipment compared to a
conventional HVDC converter. The main influence from
the capacitors is a considerable reduction of valve shortcircuit currents. This is due to the voltage drop across the
commutation capacitor varistors. On the other hand, a
somewhat higher peak voltage across the valve, as well as
higher extinction voltage steps, will be obtained compared
to conventional HVDC.
Figure 6: Maximum power curve for conventional and
Capacitor Commutated Converters, SCR = 2, g=17º.
2.6 The capacitor in the CCC concept
In principle, it would be possible to locate the capacitors
on the AC side of the converter transformers, as proposed
in Refs. 3 and 4.
The voltage contribution from the commutation capacitors
will support the commutation of the direct current from
one valve to another; i.e., the overlap angle will be reduced
compared to a conventional HVDC converter. The reduced
overlap angle will result in somewhat higher AC harmonic
currents and the reduced overlap will, in combination with
the higher extinction voltage step, give somewhat increased
generation of harmonics on the DC side compared to
conventional HVDC. The increased harmonic production
of a CCC is of the order of 20 % and can be coped with by
using high performance filters on both the AC and DC
sides.
However, it was deemed that it would not be possible to
completely avoid ferro-resonance problems and certain
other drawbacks using this concept. The location of the
capacitors between the converter transformers and the
valve bridge results in full control of the capacitor currents
and complete elimination of the risk of ferro-resonance.
Figure 7:
Commutation
Capacitor.
A key component in a CCC is the
commutation capacitor. The steady
state operating voltage of the
commutation capacitor is defined by
the direct current. The capacitors must
be protected against overvoltages by
parallel ZnO varistors. The voltage
stresses on the capacitors, as well as
the energy requirements made of the
parallel varistors, are relatively low
compared to the installed capacity,
and consequently the commutation
capacitors can be of compact design.
Figs. 7 and 8 show a typical layout
for a commutation capacitor with its
varistors, and the voltage of the
commutation capacitor in normal operation.
Figure 8: Commutation capacitor voltage.
Figure 9: Valve short circuit current.
With the location of the commutation capacitors on the
valve side of the converter transformer, the rating of the
converter transformer can be reduced by reducing the nominal phase-phase voltage on the valve side; i.e., the
reactive power flow through the transformer is minimized.
2.8 Impact on station design
The elimination of switched reactive power compensation
equipment will simplify the AC switchyard and minimize
the number of circuit-breakers needed, which will reduce
the area required for an HVDC station built with CCC.
2.9 CCC - a fully developed concept
The CCC concept has been thoroughly studied in both digital simulation programs and in the HVDC simulator over
the last few years. Design rules for the CCC have been
developed and verification of the CCC concept in a high
power test circuit will be finalized at the beginning of 1996.
3. CONTINUOUSLY TUNED AC FILTERS
3.1 General
HVDC converters produce current harmonics on the AC
side and voltage harmonics on the DC side. For a 12-pulse
converter, AC-side harmonics of the order 12n±1 are
created. A typical filter set-up consists of 11/13 and HP24
filters.
To obtain good performance, low impedance tuned filters
often need to be provided for the lowest characteristic
harmonics; i.e., the 11th and 13th.
Filters have two important characteristics: impedance and
bandwidth. Low impedance is required to ensure that
harmonic voltages have a low magnitude. A certain
bandwidth is needed to limit the consequences of
filterdetuning.
The conventional filter reactor design has been modified
by inserting a core and a control winding. A DC current in
the control winding affects the permeability of the core
and thus changes the inductance of the reactor. No
mechanically moving parts are needed. Fig. 10 shows the
basic design of the reactor.
3.2. Control of tuned AC filters
A simplified diagram of the filter control is shown in Fig.
11. The phase angle between the voltage and current of
the harmonic is used as an input signal to control tuning.
The regulator is a PI-regulator and a small standard 6pulse controlled rectifier is used as amplifier to feed the
control winding of the reactor. The power needed to feed
the control winding is around 1 kW per phase.
Detuning of conventional filters is caused by network
frequency excursions and component variations, e.g.
capacitance changes due to temperature differences.
A filter in which tuning can be adjusted to follow frequency
variations and component variations offers several advantages:
- the filter can be designed with a high Q-factor to provide a low impedance for the harmonics
- automatic tuning will ensure that all risks of resonances
and current amplification phenomena are eliminated,
implying that the ratings of the AC filter components
can be reduced.
ABB has developed and field-tested a new method to
achieve continuous automatic tuning of an AC filter. The
concept is based on orthogonal magnetizing of an iron core
in the filter reactor. The reactor inductance is controlled
by a direct current creating a field perpendicular to the
main axis of the reactor.
Figure 10:
Variable reactor.
The permeability of magnetic
materials can be changed by
applying a transverse DC magnetic field. This permeability
controlling field has to be oriented
perpendicular to the main flux
direction and has the effect of
lowering the permeability by
”destroying” favourably oriented
magnetic domains. A transverse
DC field is able to reduce the
permeability by several orders of
magnitude without affecting the
linearity of the magnetizing process. Because of the linearity no
additional harmonics are produced.
Figure 11: AC filter tuning control.
3.3 Operational experience
A test installation of an 11th harmonic ConTune® filter was
made in the Lindome station of the 300 MW Konti-Skan 2
HVDC transmission in 1993. The filter has the same
generated reactive power, 11.6 MVAr at 132 kV, as the
original filter. Fig. 12 shows the test installation.
Figure 12: Test installation of a ConTune® filter in
Lindome.
The filter was designed to accommodate frequency
variations and component variations that represent the
detuning (+2, -3 Hz).
A comparison of the performance of the passive and active
tuned filters shows that the 11th harmonic distortion was
reduced from around 0.026% with the passive filter to around 0.010% with the ConTune® filter with its Q factor of
around 200. The converter was in both cases operating
under the same conditions.
It should be noted that the original AC filters in Lindome
have a high quality factor for the 11th and 13th filter, Q=65,
while a typical value is 30-40. Hence, the distortion with
the passive filter was already very low.
The filter performance measured at the test installation
shows that the ConTune® concept is an appropriate solution. The test installation has been in operation now for
more than two years and operating experience has been
good. Commercial installation of a ConTune® filter is
already in progress at the Celilo terminal of the Pacific
Intertie.
4. OUTDOOR HVDC VALVE DESIGN
* Platform with support insulators.
The platform for a single valve housing or a number of
valves is of the same design as used for series capacitor
banks.
* Communication channel.
An important new element needed for the outdoor valve
design is a communication channel. It consists of a
composite insulator for DC application which is used for
fibre optics, cooling water and ventilation air between the
valve housing and earth.
* Valve base electronics.
The valve base electronics can be located very close to a
single valve or be common to a number of valves. The
valve control and opto interface are included in the valve
base electronics.
* Valve cooling including air-cooled liquid coolers and
cooling control.
The most suitable solution as seen today for the valve
cooling is to have a cooling system serving one pole, i.e.,
12 valves. In most cases the cooling system will be a closed
single-circuit system with a coolant consisting of a mixture
of water and glycol for anti-freeze purposes.
4.1 General
4.3 Operational experience
The outdoor air-insulated thyristor valve is a new
component, made possible by the development of high
power thyristors. It gives increased flexibility in the station layout; eliminates the need of a valve hall, including
its subsystems; reduces the equipment size; and makes it
easier to upgrade existing stations. Future relocation of an
HVDC station will also be simpler when outdoor HVDC
valves are used.
A test valve in the Konti-Skan 1 HVDC link has given
operational experience of a valve designed for 275 kV DC
voltage since June 1992. The operation of the test installation has been very successful, and has provided a basis for
further development. The ongoing development is aiming
at an outdoor valve design for 500 kV DC voltage.
5. ACTIVE DC FILTERS
The outdoor valve unit is built as a single valve function;
consequently, 12 units are needed for a 12-pulse convertor.
Inside the outdoor valve unit, the electrical configuration
is of traditional design with air-insulated thyristor modules and reactor modules, and the ambient conditions for
these components being the same as for a valve hall solution.
4.2 Elements of the outdoor valve
The basic elements of the outdoor valve are:
* Valve housing.
The encapsulation of the valve is made of steel or aluminium. The insulation medium inside the housing is air at
atmospheric pressure. The size of the valve housing has
been chosen to make transportation of a complete and
assembled valve possible on roads and railways. The length
of the valve housing is a function of the DC voltage for the
valve.
* Active part with thyristor and reactor modules.
The modules are of water-cooled design, similar to the
modules used for an indoor installation.
5.1 General
Demands regarding permitted interference levels from DC
lines have become increasingly stringent in recent years.
To fulfill these requirements using passive filters a number
of large parallel branches are necessary.
A more attractive solution is therefore to use an active DC
filter in combination with a small passive DC filter branch.
5.2 Operating principles
The principle of the active filter is to inject a current via
the passive DC filter into the DC circuit as shown in Fig.
13.
Figure 13: Circuit diagram of
active DC filter.
The current to be injected is formed from the measured
harmonics on the DC line. A control system calculates the
amplitude and phase angle of a signal that is injected via a
power amplifier into the DC circuit to eliminate the
harmonics on the DC line.
As can be seen from the layout, four phases of the
ConTune® 11th, 13th and passive high pass filters are
included. The fourth phase is added for redundancy reasons
in case of a filter outage and can be connected to each of
the three phases .
In Fig. 14 the harmonic content on the DC line is shown
for a typical installation, both with and without the active
filter in operation. As can be seen from the figure, the active
filter reduces the harmonic content considerably.
7. CONCLUSIONS
Several new concepts which will result in a new generation of HVDC converter stations have been developed over
the past few years. Capacitor commutated converters,
actively tuned AC filters and outdoor thyristor valves are
three of the most important new features. Active DC filters, optical current transducers, fully computer-based
converter controls and deep hole electrodes are other
important elements. These technological advances will
result in improved operating characteristics, reduced
complexity and smaller area requirements for future HVDC
converter stations.
8. REFERENCES
Figure 14: Harmonic current content on a typical DC line.
5.3 Operational experience
The first prototype was commissioned in December 1991.
Commercial installations have been in operation since
autumn 1993 and autumn 1994 for the Skagerrak and Baltic
Cable HVDC schemes, respectively. Today active DC filters is a standard solution for HVDC transmissions with
stringent DC filtering requirements.
1. Reeve J, Baron JA and Hanley GA, Oct 1968, “A
Technical Assessment of Artificial Commutation of HVDC
Converters with Series Capacitors”, IEEE Trans. on PAS;
Vol. PAS-87, No. 10, pp. 1830-1840.
2. Gole AM and Menzies RW, “Analysis of Certain Aspects
of Forced Commutated HVDC Inverters.”
3. Nyati S, Atmuri SR, Gordon D, Koschnik V and Matur
RM, April 1988, “Comparison of Voltage Control Devices
at HVDC Converter Stations.” IEEE Trans. on Power
Delivery; Vol. 3, No. 2,
6. NEW CONVERTER STATION DESIGN
Through utilizing the features described in this paper a
major impact on the design of converter stations is foreseen. An example of a HVDC converter station for a monopolar scheme incorporating the features described in this
paper is shown in Fig. 15.
Figure 15: Possible layout of
converter station
with new features included.
4. Woodford DA, Zheng F, May 1995, “Series
Compensation of DC Links.”, CIGRE Symposium, Power
Electronics in Electric Power Systems, Tokyo.