Institute of Energy Technology Aalborg University Bilag 13 August 2 2006 Comprehensive use of High Voltage AC cables in the Transmission Systems Suggestion for research collaboration and two Industrial PhD studies 1. Introduction The transmission grid has up to present time been laid out as an almost purely overhead line network except for crossing of water, in densely populated areas and where overhead lines would affect national nature interests seriously. The introduction of transmission voltage level XLPE cables and the increasing political and public interest in the environmental impact of overhead lines has resulted in an increasing interest concerning the use of underground cables for the transmission level. As transmission level power cables possess a relatively large shunt capacitance (compared to OHL’s) this sets a demand for steady-state inductive reactive power compensation of the cables capacitive reactive power production. This is today accomplished by means of three-phase shunt reactors directly connected to the line(s). As the cables possesses capacitance and the reactor inductance resonant circuit behaviour becomes an important design feature, especially as the quality factor of the power system is extremely high because of the low overall resistance of the power lines. Switching operations and autoreclosures in the network might easily initiate switching transient voltages and currents leading to non-wanted conditions such as overvoltages. 1 It is seen as an unavoidable development of transmission system layout and mode of operation that cable sections must be included to some degree in future transmission systems and thereby will contribute significantly to the technical performance of the transmission network. Cables for the transmission voltage level possesses significantly different electrical properties than overhead lines with the same capacity, so both steady-state and transient behaviour of such combined transmission systems with cable sections included to a smaller or larger extent, ultimately as a pure cable network, will be different than for todays existing overhead line based transmission systems. This puts up an urgent need for analysis of such combined OHL/Cable transmission systems in order to be able to plan, design and operate such modern power systems, knowing their behaviour in order to assess both costs and reliability of supply. The motivation for the increasing need for insight into the use of AC cables for transmission systems and thereby the motivation for this project can be underlined in the following statements: - Production pattern changes from large conventional power plants to RES (renewable energy sources). - RES is build, where the conditions for the technology are best. For examples are wind farms build, where the wind conditions are the best possible and the environmental impact is small. - Implementation of RES can cause need for reinforcement of the transmission system to the new production units. - There are large public opposition against more overhead lines. - Cables are considered for reinforcement of the transmission system. - The transmission grid is more likely to be used for transit of electric power between countries than originally planned. Todays state of the art for a comprehensive use of cables in the transmission grid is rather limited, especially for the 400 kV voltage level. Examples of existing 400 kV AC cable projects are: Oresund strait 1 Berlin Vienna Vancouver Oslo Oresund strait 2 9 km 2 * 8,2 km 2 * 12 km 2 * 35 km 11,5 km 9 km 1973 1978/79 1979 1983 og 1984 (500 kV) 1981 1985 2 Copenhagen Berlin Madrid Northern Jutland London 33 km 12 km 13,5 km 2 * 14 km 20 km 1997 og 1999 1998 og 2000 2004 2004 2005 Transmission System Operator Energinet.dk are considering an extensive use of 400 kV cables in Denmark to be a possible future development, as can be seen from the table below: 400 kV AC cables Southern ring, 1 system Southern ring, 2. system Northern ring, 1 system Oresund ring, 1 system Total amount Land cable [km] 202 202 60 464 km Submarine cable [km] 39 39 3 28 109 km The amount of possible 400 kV AC cables is very large compared to existing 400 kV cable projects. From an introductory point of view one can foresee many possible problems with the use of cables at transmission level: - Resonance circuits o Impedance of overhead lines are series reactance and some shunt capacitance o Cables has large shunt capacitance and some series reactance o Shunt capacitance in cables are compensated by shunt reactors. Resonance frequency capacitance/compensation is app. 45-47 Hz. o Capacitors and reactors forms various resonance circuits during normal coupling/operation and faulty conditions. - The disconnection problem o At disconnection time of a transmission line, there are stored some energy in shunt capacitors and reactors as well which may be in series connection, as configuration changes during coupling. o After disconnection this energy oscillates in the system until they are damped by losses in the system. o Changes in system configuration due to switching can cause possible non-wanted resonant circuit conditions unless they are damped. 3 o Losses in 400 kV transmission system are low, and the oscillations goes on for a relative long period of time o During the oscillations dangerous over voltages can occur in the system. o Faulted conditions can create unexpected resonant behaviour. - The inrush current problem o When a transmission system is connected to an AC source, the initial current and voltages in the system are zero. o The shunt capacitors are charged through some series reactance and some oscillation phenomena with significant over voltages can occur. o If parts of the iron cores in transformers or reactors are saturated during the inrush, then the transient behaviour becomes very complicated. o Autoreclosures can possibly create unexpected behaviour. The need for new research can be subdivided into two main categories, which are closely interconnected. One package “A” of work containing “close to component” modelling of dynamic behaviour and another package “B” of work containing system studies, i.e. the application of cables to the transmission system. The initial problem of this research project becomes, as a transmission system operator, to be able to perform the transition from today’s OHL based transmission grid to a future transmission grid containing a comprehensive amount of cables without losing the technical confidence of the transmission system in any sense. Is it from a technical point of view possible to include a comprehensive amount of cables in the transmission grid and which countermeasures can be used to control and limit overvoltages to acceptable levels. 2. Description of PhD projects This section describes in more details the ideas and the contents of the research program and the individual PhD projects. This research program is subdivided into packages called A and B. State of the art and literature study for both package A and B will be carried out in a 4 close collaboration between the two candidates in order to create a common foundation for the research program. This includes a close physical location and cooperation between the two candidates in the first period of the research program period. Package A: Modelling of system components The main goal of this PhD project (package A) is to model the components of the transmission system so that a precise simulation model of the transmission system can be used (package B) for planning and design analysis of the future possible integration of cables in the transmission system Recent experience (based on measurements during switching) within Danish transmission system operator Energinet.dk is that such compensated transmission level power lines containing both cables (one or more sections or pure cable lines) and reactors for compensation are capable of generating low-frequent oscillations with a duration of seconds when switched off, see figure 1. Severe switching overvoltages occur as parts of this dynamic oscillatory behaviour. Figure 1: Switching overvoltages of the 400 kV line Trige-Ferslev when switching the line section free of the rest of the grid A Masters Thesis (Kim Søgaard, IET-AAU, 2005) for Energinet.dk has shown that such switching overvoltages are due to resonant conditions of the power lines during switching. The project shows that the 5 dynamic modelling of the components such as reactor, cable and overhead line must be based on an into deep physical modelling of the components in question in order to achieve a sufficiently high degree of precision from the simulations. Especially mutual coupling of the three-phase system seems to play an important role. This precision must be such that simulation results can be verified against transient measurements of switching operations with good results. Areas of work for PhD project in package A - Worldwide state of the art analysis of used design criteria and layout principles of transmission level cable implementations. - An extensive literature study in order to reveal today’s level of knowledge concerning the dynamic behaviour of transmission system components with focus on systems with different degrees of cables included. - An into-deep physically components such as: based dynamic modelling of - Cable section (crossbonding, mutual coupling, screen connection, physical laying in the cable trench, influence of soil...., unsymmetry….) - Compensating device (reactor with teaser, single phase, three phase or FACTS device, unsymmetry, mutual coupling) - Overhead lines (layout, transposition, unsymmetry in both series inductance and shunt capacitance) - Circuit breaker (with focus on the switching of low resonant circuit currents - does the circuit breaker interrupt the current at zero crossing or is the current forced to zero? Circuit breakers with pole spread shall be considered. Measurements plays an important role! - Modelling and verification of non-linear components such as transformers and MOA (metal-oxide-arresters) The modelling shall be closely connected to the actual real-life design of the relevant components and are likely to include elements of electric and magnetic field theory. The models shall be able to reflect correct behaviour for any real-life relevant switching condition. Therefore all possible 6 switching conditions should be laid down as a part of the work (e.g. single or three phase autoreclosure, inrush, switch-off, pole discrepancy....) and thereby shape the basis for the validity of the models. Models should be implementable in commercially available software packages such as PSCAD/EMTDC in order to make the results of the project useful for Energinet.dk - Transient measurements of switching on selected lines with the purpose of a high quality verification of the models described above. The goal is to be able to simulate any relevant transient switching behaviour of the selected lines for verification. - An into-deep physically based study and explanation to the precise origin of the dynamics of the system. It is not sufficient to be able to simulate verified switching conditions. The appearance and characteristics of the transient behaviour must be explained so thorough that this can be communicated to future practical designers of the transmission system. - Countermeasures to lower/avoid non-wanted switching transients should be considered and implemented in the simulation model (e.g. the use of surge arresters, damping countermeasures such as circuit breakers with resistors, switching philosophies (three-phase, single-phase etc...) This project (A) focuses on low-frequent switching behaviour and will not include studies concerning high-frequency behaviour (e.g. lightning transients etc.). Furthermore this project concentrates on the component level and selected, existing OHL/cable transmission lines. The future planning and consequences of an increased degree of cables in the transmission network is not the scope of this project, but is foreseen to be included in package B. Package B: System Analysis The work in package B will be about special design considerations in high voltage transmission systems with a large share of cables. The end goal will be a guideline to special precautions and solutions to avoid resonance problems and dangerous over voltage problems in a transmission network with future increased share of cables. 7 Package B will take a top down approach to the problem. This means that focus in the first hand will be on identifying, understanding and describing physical phenomena that can be expected to cause voltage problems. Preliminary calculations with simple network models shall confirm the expected physical behaviour. The work shall form a basis for specification of the necessary level of detail in models developed in package A. Suggestion of countermeasures and a final system study of a fictitious large expansion of the Danish transmission system with cables are also a part of package B. Areas of work for PhD project in package B - Worldwide state of the art analysis of used design criteria and layout principles of transmission level cable implementations. - An extensive literature study in order to reveal today’s level of knowledge concerning the dynamic behaviour of transmission system components with focus on systems with different degrees of cables included. - Identification and description of resonance phenomena in transmission systems with cables. - Development of future transmission system scenarios with large amount of cables. The scenarios will be developed with the existing Danish transmission system as starting point. - Identifying state-of-the-art analysis methods in high voltage cable projects. - Identifying investigate. the most serious resonance problems to - Suggest various methods to reduce oscillation and over voltage problems. A comprehensive brain storming and valuation of ideas will be carried out. - Investigate the need for new equipment modelling in existing calculation tools and state needed specifications for new models in close cooperation with package A. 8 - List of practical possible advantages and challenges. countermeasures including - Building equipment models in close cooperation with package A. - System studies on the developed transmission system scenarios with use of developed models and assessment of the identified methods to reduce oscillation and over voltage problems. 3. Expected results of the research programme The most original contributions are as follows: - - - - Precise dynamic models of transmission system components validated against real-life dynamic measurements and implemented in commercially available tools as eg. PSCAD. The models should be able to predict the dynamic behaviour of combined OHL/cable transmission systems precisely. (A) An into-deep explanation of the physical origin of the characteristic dynamic behaviour of transmission systems. The explanations should be accessible mostly from a non-simulation approach, i.e. from simple electrophysics.(A) A comprehensive basis for creating a “handbook” for the design and layout of future transmission systems at 400, 150 kV and 132 kV level. (B) Analysis of relevant scenarios with large amount of cables integrated and identification of possible problems and their remedy. Examples are (B): - Harmonic propagation problems Resonance circuit problems The disconnection problem Faulted conditions – autoreclosure 1/3 ph Overvoltage protection design (TOV…?) Inrush current problems Compensation philosophies It is expected that these PhD projects will result in subprojects suited for Masters Thesis works and that the PhD student plays an active role in the supervision of such students. 9 4. Time schedule It is suggested, that two industrial PhD studies is started in connection with package A and B. The industrial PhD initiative is funded by Ministry of Science, Technology and Innovation and the industrial partner in common. Time schedule: Date 1/11 2006 1/2 2007 1/4 2007 31/10 2007 31/10 2008 Milestones A Start Identification of components, which influences resonances Dynamic modelling approach Implementation of models in simulation tools 1/7 2009 Measurements to be used for validation Validation of models and explanations and countermeasures 31/10 2009 End Milestones B Start Identification of most severe problems to investigate Chose of calculation programs Results with simple models and suggestions to more detailed models. Brutto package of possible countermeasures Development and tests of detailed models System studies with detailed models. Resulting netto list of possible countermeasures including advantages and challenges End 10