D. POVH*, D. RETZMANN, E. TELTSCH U. KERIN, R. MIHALIC
Siemens AG University of Ljubljana
Germany Slovenia
SUMMARY
The driving force for the development of power systems is the on-going increase of electrical power
demand. Therefore, power systems developed from the regional to national systems. To achieve
technical and economical advantages, they extended further to large continental systems by applying
interconnections to the neighboring systems. The liberalization in power industry additionally supports
system interconnections to enable the exchange of power among the regions or countries and to
transport cheaper and more ecologically suitable energy over long distances to the load centers.
However, with an increasing size and complexity of the interconnected systems, congestion and
transmission bottlenecks are coming up, and this reduces the system reliability. Power systems have
not been designed for wide-area power trading with daily varying load patterns where power flows do
not follow the initial planning criteria of the existing network configuration. Large blackouts in
America and Europe confirmed clearly, that the grids are already close to their limits. If transmission
of large power blocks through the interconnected systems is needed, system enhancement will be
essential.
Problems can be solved by use of HVDC (High Voltage Direct Current) transmission for system
interconnection or by using FACTS (Flexible AC Transmission), which can both effectively control
power flow through the interconnection independent on system conditions and it can additionally
enhance the system stability (e.g. damping of power oscillations). A significant advantage of the
HVDC interconnection is that power can be transmitted directly between two locations without
overloading the existing system. Therefore, HVDC can essentially improve the reliability of complex
interconnected systems. Furthermore, HVDC is a firewall against cascading disturbances and in this
way, it prevents blackouts. For these reasons, in some parts of the world, HVDC or hybrid
interconnections, consisting of AC and DC interconnections, became already the preferred solution.
In the paper, a benchmark model of an interconnected system has been developed, similar to existing
and future systems. With this benchmark model, the application of HVDC, integrated into the AC
system, has been studied. It is shown that the HVDC transmission alternative offers important cost
advantages compared to the needed additional AC lines to transmit power through the system. In
addition, the advantage of FACTS to improve the stability of the system after faults and to control
power flow is shown. Some realized and future HVDC transmissions, integrated into the AC system,
are presented. The advantages of the solutions are discussed.
KEYWORDS
System Interconnection, Complexity, Reliability, Improvement by HVDC and FACTS, Study
Examples, Realized and Planned Projects
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1. INTRODUCTION
The development of power systems follows the requirements to transmit power from generation to the
consumers. With an increased demand for energy and the construction of new generation plants, built
first close to and then at remote locations from the load centers, the complexity of power systems has
grown.
Power systems have developed first to isolated small regional grids, later to the national and finally to
large, internationally interconnected networks using high voltage levels. The goal of power industry
has been to establish arrangements for power exchange with neighboring partners. This is the main
stimulation towards the extension of interconnected systems to gain the well known advantages such
as sharing generation reserves, using more economic energy resources, and to increase the reliability
of the system. These trends have been further intensified through the liberalization of the energy
markets. The system operators should ensure transmission of power between any of the power
producers and the load consumers according to the delivery contracts, without diminishing the
reliability of the system.
High complexity of such large interconnected systems has been reached in Europe (UCTE, NORDEL
and IPS/UPS systems), and the grids of India and China as well as in South America will also reach a
similar high density and complexity soon [1, 5]. However, with the increasing complexity of power
systems, the reliability of power supply has been diminished as already shown by a number of large
blackouts in different parts of the world. Studies have shown that the probability for large blackouts is
much higher than theoretically expected [2].
Reason is, that fault sequences leading to a blackout do not only result from statistical failures. Human
errors, insufficient investments, lack in maintenance and systematic errors in planning and operation
play an essential role, leading to cascading effects after system faults or overloads. Large
interconnected power systems with relatively weak interconnections incline stability problems. These
problems can not be completely avoided because of too high complexity of the systems. However, the
implementation of the HVDC and FACTS technology into the large interconnected system can enable
a better power flow control and improve operational conditions, thus reducing the probability for large
outages.
2. ENHANCEMENT OF INTERCONNECTED POWER SYSTEMS
The possibilities for enhancement of complex interconnected power systems to decrease the
probability of large blackouts and to enable an increasing power exchange among the different
systems inside the large interconnected network are:
• The simplest way is to build new additional AC lines between some of the subsystems to
strengthen the interconnection. However, this method would be only a provisional solution as
congestion and bottlenecks can occur after local outages or due to changing requirements for
power transmission routes to other locations. An example for such problems is depicted in [3]
with the analysis of the existing UCTE system.
• Building a new, superposed higher AC voltage level as “backbone”, which enable an essential
increase of power flows among the subsystems. This solution is, however, not possible in high
density populated areas due to right-of-way limitations and environmental restrictions. In
some developing countries where the networks are still isolated or underdeveloped this is,
however, the preferable solution.
• The use of HVDC back-to-back schemes instead of, or in addition to a weak AC
interconnection between the subsystems. Advantage of this solution compared to the
additional AC lines is that no additional technical problems can be expected as the HVDC
doesn’t depend on the technical parameters of the subsystems. Fast control of the HVDC
further enables control of load flow and, if needed, active damping of power oscillations. The