Dietmar Retzmann

Prospects of multilevel VSC technologies for power transmission

Deregulation and privatization are posing new challenges to high voltage transmission and distributions systems. System components are loaded up to their thermal limits, and power trading with fast varying load patterns is leading to an increasing congestion. In addition to this, the dramatic global climate developments call for changes in the way electricity is supplied. Innovative solutions with HVDC (High Voltage Direct Current) and FACTS (Flexible AC Transmission Systems) have the potential to cope with the new challenges. New power electronic technologies with self-commutated converters provide advanced technical features, such as independent control of active and reactive power, the capability to supply weak or passive networks and less space requirements. In many applications, the VSC (Voltage-Sourced Converter) has become a standard for self-commutated converters and will be increasingly more used in transmission and distribution systems in the future. This kind of converter uses power semiconductors with turn-off capability.

State of the Art in Ultrahigh-Voltage Transmission

This paper discusses ultrahigh-voltage (UHV) DC as an efficient solution for bulk power transmission especially of renewable energy. The European policy and legal framework is used to illustrate how this new technology, international electricity market integration, and the distribution of wind and solar natural resources lead to the need for continent-wide internationally coordinated network planning with a view on long-term greenhouse gas reduction. First, the technical and economic aspects of UHV transmission are described, including, e.g., converter configurations and internal and external insulation. Then, UHV transmission is put into the context of other broad current questions of power system development: AC and DC, microgrids, smart grids, power electronics, overhead lines, cables, and gas-insulated lines. The final part discusses the European legal framework for climate protection and for transmission system operator (TSO) cooperation in the new, legally mandated European transmission system operator association ENTSO-E, in particular for joint European grid planning and for R&D. This example shows how UHV technology development can contribute to market integration and economic and environmental goals for continent-wide and internationally coordinated transmission development as one option in international long-term system studies.

SVC PLUS: An MMC STATCOM for network and grid access applications

The use of powerful modular multilevel converters (MMC) for FACTS applications has resulted in the development of new STATCOM products, which have been already applied in a number of projects, due to its superior characteristics and competitive costs. The MMC simplifies equipment design, improves response time and reduces losses. This paper introduces a STATCOM of this type, discussing the characteristics and advantages of this technology for FACTS and grid access applications.

Benefits of FACTS for Power System Enhancement

The performance of power systems decreases with the size, the loading and the complexity of the networks. This is related to problems with load flow, power oscillations and voltage quality. Such problems are even deepened by the changing situations resulting from deregulation of the electrical power markets. The power systems have not been designed for wide-area power trading with daily varying load patterns where power flows do no more follow the initial planning criteria of the existing network configuration. Large blackouts in America and Europe confirmed clearly, that the favorable close electrical coupling might also include risk of uncontrollable cascading effects in large and heavily loaded interconnected systems. FACTS (flexible AC transmission systems), however, provide the necessary features to avoid technical problems in the power systems and they increase the transmission efficiency

Feasibility of HVDC for Very Weak AC Systems with SCR below 1.5

This paper exposes the feasibility to connect high voltage DC transmission (HVDC) systems to very weak AC networks leading to short circuit ratios (SCR) lower than 1.5. The existing literature does not provide solutions for such a low SCR systems and often the HVDC expansions cannot grow enough as expected. The studies developed are for the short-term voltage stability phenomena, where a solution using the PSCAD/EMTDC program for a very weak AC network with SCR=1.0 is presented. The reactive power characteristics are shown since they are important to judge the behavior of solutions for AC/DC interactions

Improvement of power quality with advanced power electronic equipment

The quality of electrical energy supply, especially voltage quality is becoming more and more important in both industrialized and developing countries. An insufficient supply quality can lead to an inadequate quality of products, interruption of important industrial processes and therefore to economic losses. The amount of equipment and industrial processes which are sensitive to insufficient voltage quality, such as harmonics and sags, is increasing. This paper describes the main power quality problems and their solution with IGBT PWM converter-based power conditioning equipment.

Prospects of Voltage-Sourced Converters (VSC) applications in DC transmission systems

Fundamental changes are affecting the power transmission industry. Markets are being deregulated and liberalized, urbanization is continuing around the world accompanied by a constantly growing demand for energy and renewable energy sources are playing an increasingly important role. For these reasons, new and efficient solutions for power transmission are required. This paper will present the prospects and technology issues of the latest developments in Voltage-Sourced Converters (VSC) for advanced power transmission systems. A new technology Modular Multilevel Converters (MMC) will be described and all the benefits of the VSC for transmission systems enhancement will be highlighted.

Stability analysis for large power system interconnections in Europe

A worldwide trend in the development of power systems is to build interconnections with the goal to achieve economical benefits. The interconnections are mostly realized by synchronous links where such solutions are technically feasible and economically justified (AC solution). In cases where the synchronous interconnection is technically not feasible, an HVDC coupling station or HVDC log distance transmission (DC solution) can be used. Finally, the DC solution can also be applied in combination with a synchronous interconnection in order to support the operation of the interconnected systems and thus makes the synchronous AC link more reliable (hybrid solution). The evaluation of the best solution for large system interconnections requires detailed system models for stability analysis. In the paper, examples of very large power system simulations are shown. Simulation results are given for DC and AC interconnections. There are practically no limits for the size of the systems, which can be handled by modern computer simulation tools, e.g. the described advanced simulation program. Finally, the challenges of further system interconnections in Europe are considered on the base of the existing experience in stability analysis.

Smart transmission system by HVDC and FACTS

Nowadays, more than ever before, electric power becomes fundamental to modern societys existence. The power demand for electricity has been growing very fast during the last decades with a high impact on global climate and environmental conditions. The answer is grid access of large amounts of Renewable Energy Sources (RES), e.g. wind and solar technology. This, however, makes the power systems more complex and consequently changes the grid structure: the linear energy chain, consisting of large centralized power plant with excellent control features (“power on demand”), is rapidly becoming a complex power matrix with Dispersed Generation (DG), of which many are installed on medium and even low voltage levels. Such a grid structure must be “Smart”. For this reason the smart transmission system is a big challenge for the necessary system stability and reliability requirements of the Grid Code. Innovative solutions with HVDC (High Voltage Direct Current) and FACTS (Flexible AC Transmission Systems) with LCC (Line-Commutated Converter) and VSC (Voltage-Sourced Converter) technology play an important role in the smart transmission to enable bulk power long-distance transmissions and grid access of RES, to reduce transmission losses and CO2 emissions, to interconnect countries and even continents for an open electricity market and to improve the transmission stability. In this paper, the benefits of the HVDC and FACTS technology for transmission system enhancement are discussed and project applications are presented.

Connecting large offshore wind farms to the transmission network

Wind over the sea is more constant and undisturbed than on land. There are no hills, mountains or valleys in the way to slow wind or redirect wind. This ends in a more constant wind flow and with less direction changes. This improves the energy efficiency of the installed wind turbines. From experiences made in Europe mainly in Germany, the impact of the location of a wind farm can make large differences in annually wind energy generation. Efficiency can differ from 10 – 20 % on land to 70 – 80 % at sea. This high efficiency range is driving the exploration to offshore wind farms, even when the connection to the grid has some very specific requirements: Building offshore and maintaining in an unpleasant ambience with salty air. Especially in Europe where governmental goals are fixed for each member state of the European Union (EU) the development of offshore wind farms is strongly proceeding.