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EnergySpectrum - Issue 1, Vol. 5, 2010 www. energyspectrum.net Specialised Transformer Use for Active Power Flow Control in the Electric Power System Michal Kolcun1), Daniel Hlubeň2), Alexander Mészáros 3), Ľubomír Beňa 4), Martin Duch 5) Technical University of Kosice, Faculty of Electrical Engineering and Informatics, Department of Electric Power Engineering, Mäsiarska 74, 041 20 Kosice, Slovak republic, http://web.tuke.sk/fei-kee/kee-s.html 1) tel: +421 55 602 3550, email: [email protected] 2) tel: +421 55 602 3559, email: [email protected] 3) tel: +421 55 602 3562, email: [email protected] 4) tel: +421 55 602 3561, email: [email protected] 5) Slovenská elektrizačná prenosová sústava, Obchodná 2, Žilina, Slovenská Republika, www.sepsas.sk, email: [email protected] ABSTRACT The article deals with possibilities of the specialized transformer (PST – phase shifting transformer) to power flows control in interconnected electrical power system. The main goal is to find out an influence of installation of PST on international lines and losses of the transmission system (TS) of the Slovak Republic. Keywords: Power flow, HVDC, PST, FACTS, transmission system 1 INTRODUCTION Transmission system of Slovak republic is a part of Union for the Co-ordination of Transmission of Electricity (UCTE). The operation and dispatch control of electric power systems requires resources for power flow control in networks but periodic labours by the absence of these facilities. In electrical power system of west Europe and USA development of the systems requires using active elements for the power load flow controlling in network before now. 2 TRENDS IN THE ELECTRIC POWER SYSTEMS COOPERATION With reference to interconnected transmission networks operation, the actual tendencies of the electric power systems cooperation have the following character: increasing electric energy transits on big distances in case of the participation of more electric power systems in transit, increasing capacities of the electric energy international exchanges in term of exportation from the sources, as well as in term of powers transit, increasing operating exploitation of the transmission elements, mainly international lines of interconnected electric power systems. This advanced form of the using of elements causes less reserves in case of the line surcharge. In background, the international lines were used mainly to increase the operation electric power ISSN: 1214-7044 system dependability in given area. Nowadays is overrides big business using, at what networks and their interconnections were not conceptually constructed, increasing differences between physical and business electric energy flows with the negative consequence to loss. These differences are still more expanding in last years, it is exists relatively big unstableness and time changes of the transmission size. These processes are not possible to well predict, the networks operation is often adapted (by nonstandard solutions too) to the business events, on these conditions, in any cases during the operation are beginning to detect networks bottlenecks, which can to be limiting factor for the desired business changes. Sequentially, these situations can to cause the risk of the fail and breaking of the electric energy supply in areas. The classic solution of the networks development (networks bottlenecks elimination), relative with the reinforcing and building of the new lines, is no wear as sufficient and quickly in continuity with the problems to obtain of the new corridors and in connection with environmental problems. Therefore, there are often sought the ways, which would to enable at least regionally to affect the negative functioning of the therein before presented processes for the transmission networks operation [2]. 3 THE TOOLS FOR INFLUENCING OF POWER FLOW IN THE SLOVAK TRANSMISSION SYSTEM The tools for the power flow control in the Slovak transmission system are the following: changing of the sources operation, controlling of the consumption, electrical network reconfiguration, earmarking of the supply area, earmarking of the source operation. 7 EnergySpectrum - Issue 1, Vol. 5, 2010 However, these possibilities are insufficient from the point of view of increasing demands. For the reasons mentioned in chapter 1, in last date it is increasing the need for the technical application facilities, which can effectively affect largeness and direction of the transmitted flows in networks. In west-European systems and USA are already in the operation some facilities, which can by its using greatly to serve for the dispatching and business needs of the networks operators [1], [2]. Faster development of The market with FACTS and HVDC equipment for load flow control is expected in the future, as a result of the deregulation and liberalization in the power industry. The market in the HVDC field is further progressing fast. A large number of high power long distance transmission schemes using either overhead lines or submarine cables, as well as back-to-back projects have been put into operation or they are in the stage of installation [5], [6]. PST 4 SPECIALIZED TOOLS FOR POWER FLOW CONTROL IN ELECTRIC POWER SYSTEM HVDC and FACTS HVDC (High Voltage Direct Current) system was commercially first used 50 years ago. Since then a growing number of transmission systems have been constructed around the world. HVDC system has a number of properties that makes it different from AC transmission. The most important are: the two stations that are not synchronized or does not even have the same frequency, can be connected to together via HVDC system power can be transmitted over very long distances without compensation of the reactive power, only two conductors are needed (or even one conductor if the ground or the sea is used as return) for HVDC compared to three conductors of alternating current transmission system [3]. Converter stations HVDC are very good controllable and able to change quickly amount and direction of transmitted power flow with influence on AC network. FACTS (Flexible AC Transmission Systems), based on power electronics have been developed to improve the performance of long distance AC transmission. Later, the technology has been extended to the devices which can also control power flow. world-wide are available Excellent operating experiences. FACTS technology became mature and reliable. Theoretically, the PST can be considered a sinusoidal AC voltage source with controllable amplitude and phase angle. Function of PST can be described through the current distribution over parallel lines, see Fig. . The „natural” current distribution depends on the impedance of the lines. This distribution may be rather inefficient, if Zline1 and Zline2 are extremely different. the introduction of an additional voltage source generates a circulating current , which equalizes the currents. Fig. 2 Current distribution over the parallel lines without and with PST Because the main part of the line impedance (on high voltage levels) is inductive reactance, inserting a voltage in phase with or opposite to the line voltage (changing the magnitude of the voltage) will have an impact mainly on the reactive part of currents (reactive power flows). The boost voltage with a phase angle perpendicular to the line voltage (creating a phase shift) influences mainly the real part of currents (real power flows) [4]. VFT Fig. 1 Simplified model of the line between two nodes of electrical network P12 = U1 ⋅ U 2 ⋅ sin (ϑ1 − ϑ2 ) (W;V,V, Ω) X 12 (1) The main idea of FACTS can be explained by the basic equation for transmission (1). Power transmitted between two nodes in the systems (Fig. 1) depends on voltages at both ends of the interconnection, the impedance of the line and the angle difference between both systems. Different FACTS devices can actively influence one or more of these parameters and control the power flow through the interconnection. ISSN: 1214-7044 One of the possible technical elements, which might be the cheaper equivalent of HVDC in the future, is Variable Frequency Transformer. This device, when compared with HVDC back-to-back provides unparalleled flexibility for utilities and transmission system developers to create viable economic business models to meet the everchanging energy markets. The VFT provides a simple and controlled path between electrical grids, while retaining many of the inherent virtues of an AC interconnection. This permits power exchanges that could not previously be accomplished, due to technical constraints such as asynchronous boundaries or congested systems. The low grid interaction of the VFT, in terms of harmonics, control interactions, and impact on nearby generators, allows the installation and operation to be decoupled from other grid issues. 8 EnergySpectrum - Issue 1, Vol. 5, 2010 The VFT system, based on a combination of hydrogenerator and transformer technologies, consists of a rotary transformer, for continuously controllable phase shift at any angle. This with a drive system and control that adjusts the angle and speed of the rotary transformer to regulate the power flow through the VFT. Smooth power control comes from regulating the torque through the drive system. Rotational speed is dictated by the difference in grid frequencies and generally will be below 3 rpm. Power transfer through the rotary transformer is a function of the torque applied to the rotor. If torque is applied in one direction, then power flows from the stator winding to the rotor winding. If torque is applied in the opposite direction, then power flows from the rotor winding to the stator winding. If no torque is applied, then no power flows through the rotary transformer. Regardless of power flow, the rotor inherently orients itself to follow the phase angle difference imposed by the two asynchronous systems, and will rotate continuously if the grids are at different frequencies. However, if the power grid on one side experiences a disturbance that causes a frequency excursion, the VFT will rotate at a speed proportional to the difference in frequency between the two power grids. VFT is designed to continuously regulate power flow with drifting frequencies on both grids. Reactive power flow through the VFT follows conventional ac-circuit rules. It is determined by the series impedance of the rotary transformer and the difference in magnitude of voltages on the two sides. Unlike power-electronic alternatives, the VFT produces no harmonics and cannot cause undesirable interactions with neighbouring generators or other equipment on the grid. [11]. The power losses before PST consideration in Slovak transmission system are following: - power losses on lines 220 kV: 2,731 MW - power losses on lines 400 kV: 19,770 MW - power losses on transformers: 0,724 MW Total power losses: 23,225 MW Fig. 4 The regulation effect of PST added into line 404 5 APPLICATION OF PST ON SLOVAK TRANSMISSION SYSTEM In order to simulate power flows in the Transmission system of the Slovak Republic, it is necessary to have a model of the whole transmission system of UCTE. In this simulation the model of UCTE (29th November 2007 at 12.30 o’clock) was used to simulate power flows on boundary lines. This model contains 5784 nodes (from this 982 generators, 3262 loads), 7798 lines, 1102 transformers [7]. Transformer PST was added separately into each of international 400 kV lines of Slovak transmission system (Fig. 3). After individual simulation, the regulation effect and losses was calculated, see Fig. 4 - Fig. 17. Fig. 5 Power losses in Slovak transmission system (PST in line 404) Fig. 6 The regulation effect of PST added into line 424 Fig. 3 International400 kV lines of Slovak TS ISSN: 1214-7044 9 EnergySpectrum - Issue 1, Vol. 5, 2010 Fig. 7 Power losses in Slovak transmission system (PST in line 424) Fig. 8 The regulation effect of PST added into line 440 Fig. 9 Power losses in Slovak transmission system (PST in line 440) ISSN: 1214-7044 Fig. 10 The regulation effect of PST added into line 448 Fig. 11 Power losses in Slovak transmission system (PST in line 448) Fig. 12 The regulation effect of PST added into line 449 10 EnergySpectrum - Issue 1, Vol. 5, 2010 Fig. 13 Power losses in Slovak transmission system (PST in line 449) Fig. 14 The regulation effect of PST added into line 477, line 478 switched off Fig. 15 Power losses in Slovak transmission system (PST in line 477, line 478 switched off) Fig. 16 The regulation effect of PST added into line 497 Fig. 17 Power losses in Slovak transmission system (PST in line 497) As we can see: PST added into 404 affects mostly the power transmissions in profile SK-PL, PST added into 424 affects mostly the power transmissions in profile SK-HU, PST added into 440 affects mostly the power transmissions in profile SK-HU, PST added into 448 affects mostly the power transmissions in profile SK-CZ, PST added into 449 affects mostly the power transmissions in profile SK-UA, PST added into 477 (or 478) affects mostly the power transmissions in profile SK-CZ, PST added into 497 affects mostly the power transmissions in profile SK-HU. The putting of PST into transmission line and the angle changing between primary and secondary voltage causes the increasing of the power losses in whole interconnected electric power system [7]. 6 CONCLUSION In case that power should be transmitted through a meshed system, undesired load flow occur which loads other parts of the system. This can lead to bottlenecks in the system. In such cases specialised devices (FACTS, HVDC, PST, VFT) could help to improve the situation. The article analyses influence of PST installation on international lines ISSN: 1214-7044 11 EnergySpectrum - Issue 1, Vol. 5, 2010 and losses of the transmission system of the Slovak Republic. ACKNOWLEDGEMENTS This work was supported by Scientific Grant Agency of the Ministry of Education of Slovak Republic and the Slovak Academy of Sciences under the contract No. 1/4072/07 and by Slovak Research and Development Agency under the contract No. APVV-0385-07 and No. SKBG-0010-08. 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