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  Development of dispatching control of the Slovak transmission system
 
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In the beginning of electrification power facilities built up and operated at various locations were nor interconnected. As the time passed by, such small separately operating electrical grids did not meet customers’ requirements on electricity supplies quality and reliability. Therefore small separate grid operators looked for and used all possibilities for interconnection with neighbour areas. This, however, increased requirements on control of the expanding grids and co-operating power plants. In the early stages the grid operation was ensured and controlled by personnel of power plants and switching stations in the scope of agreed rules. After the establishment of the Slovak Power Plants the grids and power plants were interconnected not only by high-voltage lines but also by 110 kV lines. Increased electricity demand surpassed the growth of sources and the situation had to be resolved by administrative measures (consumption diagrams, agreed power capacities), as well as by means of operative control of the transmission system. Therefore in the mid 1946 an emergency service was established within Slovak Power Companies, Bratislava, which was in charge of collection of information about the transmission system operation. The complicated situation in electricity supplies, especially during the winter 1947-1948, resulted in performance of the emergency service on two shifts, and since spring months of 1948 in a continuous three shift duty. In a relatively short time the service was passed to the headquarters of SE, thereby establishing the bases of the dispatching control.
 
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Following a decision about the subsequent reorganisation of the Slovak power industry, in the mid 1948 a decision was adopted about the establishment of the national company Slovak power generation facilities (SEV) in Zilina (in force since January 1, 1949), which were in charge of electricity generation and transmission via 110 kV grids. Activities of the company included the dispatching control on the whole territory of Slovakia. Based on this it was decided on a temporary authorisation to establish and ensure the dispatching control by SE regional headquarters in Zilina as soon as in 1948. This plan was implemented in the first half of October 1948, when a necessary documentation was taken over at the company headquarters in Bratislava, and the three-shift operation of the current Slovak power dispatching (SED) at Zilina was commenced.
 
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After 1949, the SED activities also included the service of protection, and connection and control technology in the extent and force of Slovak Power Generation Facilities (SEV), with the mission of co-ordinating the problem in plants. At this time Slovakia was working in parallel with Moravia (Ostrava region), and it was a part of the Moravian-Slovak system, where the majority was ensured by significantly stronger Moravian region. The power connection between Moravia and Czech was implemented as late as in 1953 by only one 110 kV line, thereby establishing the transmission system of the whole Czechoslovakia. The interconnection of the deficit Moravian-Slovak part of the transmission system with the Czech power generation surplus was useful, however, did not resolve the problem. As soon as in 1950 the Czechoslovak national dispatching was established at the Czechoslovak Power Company, Prague, which worked only as an administrative and statistic office until Bohemia was interconnected with Moravia. After 1953 the national dispatching entered the process as the national control and co-ordination body of the Czechoslovak transmission system, and a comprehensive dispatching three-level control was established – national, regional, and county dispatching centres.
 
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The county companies dealt with control of the high-voltage network and regulation of electricity supplies. Power consumption management, however, was ensured through a step-by-step improving form of daily consumption diagrams, closed monthly. In parallel the improving form of consumption regulation was introduced. The consumption regulation was ensured operatively (and still is) – according to the needs of the transmission system – by the county dispatching centres established at Slovak power companies. Thus the branch company VET dispatching centre was established in 1957, which ensured loading of Vah River hydro-power plants in their hydraulic links.
 
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After the establishment of SEV, SED improved step-by-step and qualified personnel was recruited (there were only two university-level educated staff members – dispatching head and worker in charge of protection at the beginning). In 1954 the operation preparation department was established, which firstly dealt only with daily operation and later also long-term mode preparation of the transmission system operation. There were also departments of operative and control technology and protections established. Important change in SED activities came after the cancellation the Slovak power industry central bodies in 1958, since practically the only common body of the Slovak power industry that remained in the field of the transmission system control was SED, strengthened by electricity sales activities, supplying power plants with fuels; and the activities of control technology division were significantly extended.
 
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In the second half of 50’s and first half of 60’s thermal power plant capacities were loaded as the criterion of the average annual price of electricity generated or average annual specific heat consumption for electricity generation. The criteria were frequently violated due to the balance and subsequent preference or lack of individual fuels. In the second half of 60’s a new criterion for loading the thermal sources was introduced - the method of proportional increase of thermal energy consumption in the fuel per electricity unit generated. SED participated in introducing the method together with power plants as the developers of the consumption characteristics, even in the area of creation and use of variants. The methodology was step-by-step improved (even though it was time to time violated by fuel balance), however, considering no effects of losses in transmission and reactive power transmission. In 80’s the power plant power started to be controlled according to proportional cost increase per generated power unit. In the whole period the participation of SED in the national mode preparation of the Czechoslovak transmission system operation was increasing both in the area of sources as well as the transmission area.
 
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The period from the mid 50’s in the field of mode preparation of operation, in addition to the perfecting the methodology, is characterised with increased use of available computer technology. The use of a unidirectional universal grid model of the Slovak Technical University in Bratislava, and later also the one of SSE Zilina was the first step. After the era of unidirectional grid models there were alternating grids models intensively used till 1965, first at EGU Brno, then at the Czechoslovak national dispatching in Prague, and finally also the SED’s own alternating model. Then there came the period of computer use, first at the national dispatching in Prague, and later at the computer centre in Zilina. Especially the SSE’s computer - since it was at a location immediately accessible for SED, as well as thanks to understanding of SSE, meant very rapid increase of the number of mode status analyses. This brought a considerable improvement in the quality of the transmission system operation preparation, which was more and more complicated. There were about 500 to 600 calculations of the grid operation modes done by the SSE computer, and the results obtained allowed for improved transmission to customers on the 110 kV grid, as well as for ensuring the high quality in the ultra-high grid and minimising transmission losses. in many cases complicated conditions in equipment loading with short-circuit currents were resolved. Construction of a dispatching hall in 1964 was an important acquisition for SED operative control. The new hall included also an extensive network of remote measurement, including telecommunications and their gradual automation. Measurement centre was also commissioned and used till the end of 70’s, when it was replaced by RPP 16 computer launched in 1979.
 
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When commissioning new transmission system equipment, at which top experts of contractors and research institutes participated, extensive tests and measurements were performed that were the basis not only for the transmission system operation, but also for research and construction of devices and equipment of the contractor and designing companies. Direct involvement of SED staff in the test preparation and conduct was a valuable benefit for their professional development and quality improvement of the transmission system control. After the re-establishment of the Slovak power industry bodies - SEP - instrumentation and sales activities were separated from SED. SED continued in improving the quality of the transmission system operation, especially after the 400 kV grid creation that allowed operation of 110 kV nodal systems supplied through the 400/110, 400/220, and 220/110 kV transformation, and connection remained only at the 220 and 400 kV voltage levels. This mode furthermore allowed for simplification of the voltage control in 110 kV lines, as well as reduction of 110 kV line failures and the time required for repairs. The characteristics of the transmission system dispatching control development would not be complete without mentioning modifications at the other dispatching control levels. The growth of the transmission system equipment, as well as organisation relations resulted in the need of taking the control of 110 kV equipment over from SED level to regional dispatching centres that represented continued trend from the mid 70’s. The process is continuous, because a series of generation sources are supply the 110 kV lines, and the mode as well as operative co-operation between SED and regional dispatching must go on, even though at changed level. The co-operation in some cases is forced by the fact that the 110 kV grid supplies own consumption of essential power generation sources.
 
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Regional dispatching centres with major activities in high-voltage transmission were, of course, established in parallel. It should be noted that distribution power companies included regional dispatching centre till 1998 in the following towns:
  • ZSE - Nitra, Trnava, Bratislava, Trencin
  • SSE - Zilina, Banska Bystrica, Liptovsky Mikulas, Rimavska Sobota, Ziar nad Hronom
  • VSE - Spisska Nova Ves, Kosice, Michalovce, Presov,
which close the hierarchy of the three-level dispatching control of the Slovak transmission system.

 
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The entire process of formation and implementation of the transmission system operation control has got recently to a higher level by a rapid increase of qualification of the staff that is involved in the process. The staff has got better conditions provided by the use of a new technology.
 
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On April 1, 1994 The Czech and Slovak Power Dispatching in Prague terminated its activities. The Slovak Power Dispatching in Zilina has taken over the function of the national power dispatching of the Slovak Republic, that is the most important change in dispatching control of the Slovak transmission system