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THE GARABI 2000 MW INTERCONNECTION
BACK-TO-BACK HVDC TO CONNECT WEAK AC SYSTEMS
John Graham
Birger Jonsson
R.S. Moni
Brazil
Sweden
India
ABB Utilities AB, SE-771 80 Ludvika, Sweden
SUMMARY
The Argentina - Brasil interconnection first
phase of 1000 MW, reported elsewhere, [1] has
recently been increased to 2000 MW total power
delivery. The second phase is very similar to the
first, also employing a back-to-back HVDC
converter at Garabi, in Brasil, close to the border
with Argentina.
A back-to-back HVDC converter can be used
when two asynchronous AC systems need to be
interconnected for bulk power transmission or
for AC system stabilization reasons. In an
HVDC back-to-back station there are no
overhead lines or cables separating the rectifier
and the inverter, hence the DC current can be
kept high and the DC voltage low. The low DC
voltage means that the air clearance requirement
is low, which is in favor of a compact design of
the valve housings. This enabled the modular
back-to-back HVDC concept to be developed.
The back-to-back HVDC converter stations
utilized at Garabi are of the modular type with
Capacitor Commutated Converters (CCC). The
CCC is used to ensure satisfactory performance
at low short circuit levels and to reduce the
impact of reactive power variations.
1 THE GARABI PROJECT
1.1 Introduction
In April 1997, the governments of Argentina and
Brazil signed an agreement to facilitate crossborder energy trading between the two countries.
On May 5, 1998, the Brazilian Ministry of Mines
and Energy, through Eletrobrás, Furnas and
Gerasul, and the Argentinean government,
signed a 20-year contract with CIEN,
“Companhia de Interconexão Energética”, to
import 1000 MW from the wholesale energy
market in Argentina. CIEN is a company of the
ENDESA group.
As the Argentinean system operates at 50 Hz and
Brazil's operates at 60 Hz, the inter-connection is
made through HVDC frequency converters in
back-to-back configuration, located at the Garabi
Station in Brazil, close to the border with
Argentina. Both phases link the 500 kV networks
of Brazil and Argentina through 500 kV
transmission lines, each nearly 500 km in length.
This first phase of the Garabi Project entered
operation in June 2000 and has been reported
elsewhere [1]. In August of this year the second
phase was put into commercial operation, raising
the total transfer capacity to 2000 MW.
The second phase is very similar to the first,
employing the same modular back-to-back
converters,
with
Capacitor
Commutated
Converters (CCC). Both phases link the 500 kV
networks of Brazil and Argentina by 500 kV
transmission lines, nearly 500 km in length,
running from the Rincón de Santa Maria
substation near Yacyretá in Argentina, to the Itá
substation in Brazil. The Argentinean system
operates at 50 Hz and Brazil's operates at 60 Hz,
and the back-to-back frequency converters for
both phases are integrated into the Garabi Station
in Brazil, close to the border with Argentina.
A simple one line diagram for the first phase in
given in fig (1) below:
Figure (1): Garabi HVDC Back-to-back station, Phase 1, Simplified One Line Diagram
It can be seen that the first phase has a
connection with the 230 kV system in southern
Brazil via a transformer station at Santo Ângelo.
This was not foreseen at the time of initiating the
work on the interconnection and was installed by
Eletrosul to reinforce the system in the State of
Rio Grande do Sul.
The phase two interconnection does not connect
at Santo Ângelo, although physically passing
close in order to facilitate a future connection.
Like the Garabi 1 project, the supply of the
second 1100 MW phase also is based on a full
EPC
(Engineering,
Procurement
and
Construction) contract including 500 kV
transmission lines and the substation extensions
in both countries. Engineering included all
studies to satisfy requirements of national
agencies as well as engineering studies to ensure
that the system works satisfactorily under the
wide range of operating conditions. Guarantees
on the electrical performance include availability
greater than 97%. This was possible because
both contracts were based on technical
specifications
with
true
functionality,
concentrating principally on the values of power
to be transmitted and the physical location of the
substations. This meant that each supplier could
bid his most cost effective technical solution as
long as the functional requirements were met and
necessary guarantees were sufficient to fulfill the
clients criteria.
1.2 System aspects
The two electric systems are rather large, yet
they are connected at relatively weak points
within their networks. The Argentinean
integrated system (SADI) has an installed
capacity of about 16000 MW. However, the
Rincón de Santa Maria substation is at the
northeastern extreme, and although adjoining the
Yaciretá generating station, may have a
relatively low short circuit capacity. This can
vary between 9000 and 3000 MVA, depending
on the number of connected generators at
Yaciretá. This has remained virtually unchanged
between the two phases of the interconnection.
The infeed point to Brazilian S/SE
interconnected system was defined at the Itá 500
kV substation for the power purchase. The
integrated Brazilian system has an installed
capacity of 65000 MW, but the Itá substation is
on the southern extension, remote from major
generation, although a power plant of 1450 MW
is coming online with one 290 MW generator
now in operation. At the time set for commercial
operation of the phase one, the short circuit
capacity at Itá was calculated to be about 6000
MVA, dropping to 3500 MVA under
contingency conditions, however now at entry of
phase two the level is 12000 MVA, dropping to
5000 MVA under the worst considered
contingencies.
The location of the converter station, close to the
proposed Garabi Hydroelectric power plant, was
also defined in the power purchase documents
for phase one and so set conditions for the
complete interconnection. This effectively
defined the 50 Hz connection point as Rincón de
Santa Maria, giving a transmission line length of
136 km, duplicated with an identical parallel line
for phase two. However, in the case of the line to
Itá, which has a length of 354 km, the first phase
has a connection at Santo Ângelo, mentioned
above, while the second goes directly, although
the system is designed to permit the connection
to be made. In addition the station at Garabi is
designed to operate with the phase one and phase
two converters either connected in parallel or
operating individually. This gives a set of rather
challenging operating conditions for a converter
station where there is a guaranteed delivery of
two times 1000 MW into a rather weak network.
In order to fulfill all required operating
situations, systems studies were carried out by
ABB together with CEPEL (Centro de Pesquisas
de Energia Electrica) to check compliance with
the requirements of the system operators, both in
Brazil and Argentina [2]. These studies included
load flows over the full operating range at the
given system voltage limits, verification of
reactive interchange at the connection points and
load rejection overvoltages. For the 50 Hz side,
with the relatively short transmission line this
presented little problem. The long 60 Hz line was
a considerable challenge to avoid compensation
at an intermediate point. For the chosen
transmission line parameters, the short circuit
capacity at the Garabi phase two 60 Hz side is
about 2000 MVA, dropping to about 1600 MVA
under contingency conditions. This was met by
using Capacitor Commutated Converters(CCC),
with minimum-sized ConTune harmonic filters.
In this way fixed line reactors are used and the
CCC allows that the converter has characteristics
to absorb or supply reactive power as required by
the system. The converter acts like a static
compensator, giving smooth continuous control
of voltage and power flow. The minimum size of
the ConTune filters help to keep load rejection
overvoltages within limits. However for the
phase two expansion additional switched shunt
reactors are needed on the 60 Hz side to cover all
operating conditions. The resulting system
configuration for the final arrangement with both
phases in parallel is shown in figure 2.
525 kV / 60 Hz
500 kV / 50 Hz
Block 4
Garabí 2
Block 3
Line 2
Tie-breakers
Line 2
Shunt reactor
Tie-breakers
Line 1
Line 1
Garabí 1
Block 2
AC filter
Block 1
Figure (2) Integrated Bus Arrangement
The above diagram appears to be quite different
from the arrangement used for the first phase,
however in fact the two parts are identical with
respect to converter equipment and associated
performance. The apparent difference comes
from the change to a double breaker scheme for
the 500 kV switchyards, compared to the
modified ring bus used in phase one. This gives
greater flexibility when the two phases are
integrated. The second phase maintains the
philosophy of having a filter bank associated
with each converter and using a spare switched
phase to be available for either of the converters.
This allows the use of minimum sized ConTune
filters, which alleviates the effects associated
with line energisation and load rejection when
large shunt capacitance is present. The filter
banks have a total of four branches, two
ConTune for 11th and 13th harmonics, plus two
passive high pass tuned at 24th and 36th. The
inductances of the tunable branches are adjusted
continuously to compensate for temperature and
frequency deviations
The total fundamental frequency rating for the
filter bank is 85 Mvar per 550 MW converter.
The CCC capacitors, as all converter design, are
identical for the two phases, with values of 190
Mvar per 550 MW converter on the 50 Hz side
and 322 Mvar on the 60 Hz side. The larger
value on the 60 Hz side permits supply of
reactive power to the system as required at full
load.
Thus it can be seen that the CCC capacitors not
only fulfill the role of ensuring satisfactory
commutation at low short circuit levels, but also
satisfy steady state requirements of reactive
power. This is illustrated in figures (3) and (4),
and further discussed in item 2 below.
1.2 -
200
CCC
γ´= constant
1.0 -
0.8 -
Conventional
γ = constant
0.6 -
0.4 -
150
Qdc [MVAr]
Ud
With this rather low value the studies also have
to take into account the value of the PLC noise
filters connected on the lines from the station.
These have fundamental ratings of 9 Mvar on the
50 Hz side and 22 Mvar on the 60 Hz.
100
Qmax
Qmin
50
0
-50
0
-
-
0.2 -
0.5
1.0
-100
0
1.5
100
200
Id [pu IdN]
Figure (3) Ud vs Id Characteristics
300 400
Pdc [MW]
500
600
Figure (4) Reactive Power, 60 Hz Side
1.3 Integration
As noted in the earlier paper the Garabi Project employed many new features:
•
•
•
Commutated Converters (CCC) required to permit
operation with low short circuit levels at the inverter
Self tuning AC filters (ConTune)
One redundant AC filter phase
•
•
•
Modular out-door valve enclosures
A new PC based control system, MACH2™
AC breakers with optical CTs (Compact)
These features aided the integration of the two
phases, especially the MACH2 control system
and the 500 kV HPL Compact switching module.
In the first case the powerful computers used for
the control system permit the rearrangement of
the control and protection systems as determined
by the new layout. Further the integrated system
can be tested in the factory to ensure that the
software changes to the existing equipment will
be satisfactory. This test was carried out prior to
shipment of the actual phase two control
equipment. These principles of the MACH2,
namely extremely powerful computers, fully
integrated software for all protection, control and
monitoring functions, plus a comprehensive
factory system test, are not unique to the modular
back-to-back concept, but are utilized in all
HVDC project now supplied by ABB [4].
Secondly the modular switchgear, designed
primarily for ease of installation, can also be
moved rather easily. The HPL Compact
switching module includes all functions
performed by a traditional circuit breaker bay;
HPL circuit breaker, pantograph disconnectors,
earthing switches, digital optical current
transducer, common base frame and pre-wired
control cubicle. The module is mounted on a
single foundation for each phase. This results in
a substantial reduction of required space and
facilitates easy and fast maintenance of the
breaker bay. This has resulted in rather short
outage times being required for the necessary
relocation of the switching modules from the
ring bus in Garabi 1 to the double breaker
scheme used for the integrated station. Figure 5
below shows Garabi 1 on the left with the
original ring bus and Garabi 2 to the right with
the double breaker scheme.
Figure (5): Garabi 500kV yards and ConTune Filter Area
The 500 kV transmission lines and the first
converter block associated with phase two went
into service in April this year (2002). The
complete phase two started commercial
operation in August following extensive system
tests. This is via connection directly to Itá and
the full integration of the two phases has yet to
be completed. However the integration is a
complex process involving both scheduled
outage times and system testing. This process is
on going and programmed for completion only
next year.
give a smooth
voltage steps.
2 MODULAR BACK-TO-BACK
CONCEPTS
2.1 The CCC/ConTune Combination
The series capacitor provides a more
positive resistance characteristic at the
inverter as shown in Figure 3. In a
conventional inverter, an increase in
direct current results in a reduction in
inverter commutating voltage due
largely to a reduction in AC network
voltage. With a series capacitor at the
inverter, increasing current results in an
increasing voltage across the capacitor,
offsetting the drop in the AC network
voltage. It is this factor that permits
stable operation with low short circuit
ratios and better commutation failure
immunity. Another manifestation of this
factor is the increased maximum power
curve [3].
•
The series capacitor’s impedance (dxc)
is typically several times greater than
the transformer’s impedance (dxl),
increasing the total secondary side
impedance and reducing transformer
and valve currents during DC side short
circuits. Even though higher valve
voltage ratings are required, the lower
current stresses allow optimization of
the converter transformer and valve.
•
The series capacitor inserts a leading
phase shift between the transformer
secondary and the valves, compensating
the naturally lagging characteristic of
the converter. The converter’s power
factor, seen from the transformer’s
primary side, can be kept close to unity,
or even become leading, allowing the
converter to generate reactive power as
may be noted in Figure 4. In Figure 4
the Qmin curve indicates reactive power
generation, while the Qmax curve refers
to absorption.
This capacitor commutated converter is coupled
with the ConTune filter in order to give the
maximum benefits from a low shunt connected
capacitance and a controllable reactive power
source in the converter. As described in [3], the
approach offers better electrical performance
than a conventional converter by virtue of the
following factors:
•
Switchable shunt filter banks are no
longer
required
for
reactive
compensation and are replaced by a
series capacitor bank in which the
reactive generation is a function of the
load current flowing through the
capacitor. Small fixed filter banks,
optimized for filter performance, can be
used providing lower load rejection
overvoltages. Figure 7 indicates the
reactive power balance variation for
conventional
and
CCC
based
converters. The CCC based converters
avoiding
•
The converter used in the CCC concept is
characterized by the use of commutation
capacitors inserted in series between the
converter transformers and the converter valves,
see Figure 6. below. It makes it possible to
operate HVDC in very weak networks and
eliminates
the
need
for
synchronous
compensators.
Fig. (6): Single line diagram of a monopolar
station with CCC and ConTune filter.
variation
Fig. (7): Reactive power conditions for a typical conventional converter and for a CCC.
2.2 Modular Construction
The converter valves are in modular housings,
factory assembled and tested, and were shipped
to the site ready for reassembly and operation.
The control equipment and auxiliaries were also
factory assembled and tested to reduce the
installation and commissioning time.
In order to optimize the costs of the thyristor
valves, as well as the DC side equipment, the DC
current is kept as high as the standard thyristor
rating and the valve cooling system can handle
safely. This means that the DC voltage can be
kept fairly low, and is thus chosen to meet the
rated DC power required. The low DC voltage
means that the air clearance requirement is low
Figure(8): Converter Area
The thyristor valves are suspended within the
modular housing and are easily accessible for the
maintenance crew. The housings also contain the
surge arresters connected across the valves. The
valve control units can be seen external to the
valve modules, besides the access door.
Figure 8 also shows the valve cooling modules,
each one close to the converter block. This
houses the water pumps and water treatment
system. The water-to-air closed circuit coolers
are located immediately behind the valve cooling
module.
This modular housing approach is used for all
auxiliary and control equipment. The principle of
factory assembly and testing has been used
extensively to ensure complete and reliable
delivery as well as fast erection times. In
addition a weatherproof location with a
controlled environment is ensured
which favors a compact design of the valve
housings. This has enabled the modular back-toback HVDC concept to be developed.
The thyristor valves are air insulated at
atmospheric pressure and installed in modular
valve housings. In the case of Garabi each valve
module contains two single valves, which means
up to six valve modules per twelve-pulse
converter. The modules are positioned in stacks
of two, giving a quadri-valve type arrangement.
This gives a convenient layout for the use of
single phase, three winding converter
transformer, as can be seen in figures 8 and 9.
Figure (9): Valve Modules
The modular concept has also been used for all
converter bus breakers as described above,
employing compact design, with breaker,
disconnects and optical current transformer
integrated into one unit.
The modular concept used in the station,
extended to converters, auxiliaries and
switchgear has the considerable advantage of
simplifying civil works, the design process being
now restricted essentially to foundations, without
the need for complex interfaces between
buildings and equipment. This results in reduced
costs for civil works and reduced construction
times.
The complete station, with converter capacity of
2200 MW, is constructed on an area of 367m by
670m, giving a very high utilization of land. In
environmentally sensitive areas this is also
considered a great advantage from a permitting
point of view. The ISO 14001 certification states
that the design of the equipment and the
construction at the site are performed in an
environmentally acceptable way.
REFERENCES
1.
J. Graham, G. Biledt, Mauro Baleeiro,
José Ramón Diago, Fco Javier Bugallo;
“Garabi” the Argentina – Brasil 1000
MW Interconnection; VIII Erlac,
Cidade del Este, Paraguay, May 1999.
2.
John Graham, Don Menzies, Geir
Biledt, Antônio Ricardo Carvalho, Wo
Wei Ping, Acacio Wey; Electrical
System
Considerations
for
the
Argentina-Brazil
1000
MW
Interconnection; CIGRE Biennial, Paris,
2000
The use of the CCC/ConTune combination
permits secure operation at low short circuit
levels and improves performance with respect to
reactive power control.
3.
Don Menzies, Hans Eriksson, Fabiano
Uchoas Ribeiro, Commissioning The
Garabi I Back-to-Back Converter
Station, Erlac 2001
The modular design concept simplifies the civil
design and ensures a safe and environmentally
acceptable solution, as well as permitting
reduced delivery times.
4.
Modern Control and Protection System
for HVDC, Torbjörn Karlsson, Mats
Hyttinen, Lars Carlsson and Hans
Björklund
5.
Capacitor Commutated Converters for
HVDC, Tomas Jonsson, Per-Erik
Bjorklund,
IEEE
Power
Tech
Conference, Stockholm, 1995.
3. CONCLUSIONS
The Argentina - Brasil interconnection, having
now entered into commercial operation of the
two 1100 MW phases, demonstrates the
advantages of the modular back-to-back concept.
The HVDC converters at Garabi are notable for
two main characteristics, use of the
CCC/ConTune combination and the construction
using the modular design concept.