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XVth International Symposium on High Voltage Engineering University of Ljubljana, Elektroinštitut Milan Vidmar, Ljubljana, Slovenia, August 27-31, 2007 T2-348.pdf OVERHEAD LINE AND CABLE MAPPING OF THE GREEK ELECTRIC TRANSMISSION SYSTEM, FOR THE STUDY OF TRANSITIONAL OVERVOLTAGES, USING EMTP-ATP N. Lioutas , S. Lazarou* , G Marmidis , E. Pyrgioti , D. Agoris High Voltage Laboratory, University of Patras *Email: [email protected] Abstract: The aim of this paper is an overall description of the Greek Electric Transmission System, which provides all the manufacturing and functional elements that characterize its operation. Overhead line and cable mapping is based on information that is derived from official data and reliable sources. The mapping procedure is presented and at the same time some examples are shown providing some interesting information about the transmission system. Finally, the full map is illustrated and some suggestions are made at the end of the paper for future use. 1 INTRODUCTION The Greek electric transmission system is a part of the interconnected electrical power system, which is operated by the Public Power Corporation. It is mostly composed of overhead transmission lines (there are only a few underground lines) and its total length is estimated to be 11.500 km. More specifically, it is composed of single circuit lines of 66 kV and single and double circuit lines of 150 and 400 kV. 1.1 Manufacturing characteristics Overhead lines are constructed from Aluminium Conductors with Steel Reinforcement (ACSR). In the Greek transmission system 3 types of cables are found depending on their cross sections. ACSR: 336 MCM , 636 MCM and 954 MCM (according to American stylization). The first one is used in the 66kV lines and in the 150kV light lines, the second one in the 150kV heavy lines and the third one in the 400kV lines. Double circuit 400kV transmission lines are the back bone of the transmission system since they transfer electricity from the main generation centre of Greece, which is in northern Greece (70% of the total electricity is produced there) to the south where there is the highest consumption. The underground cables are mostly used to connect the islands to the national grid. 1.2 Functional characteristics Each one of the three conductors in a three phase line is characterized by electric resistance, inductive and capacitance reactance. These parameters are taken from standard tables provided by the manufacturers. For the Greek transmission system all this data as well as the values of reactive and apparent power of the lines, were derived from official catalogues from the Public Power Corporation of Greece and are presented on the following tables. Table 1. Electric characteristics of overhead line and cable of the Greek transmission system [3]. Rated Voltage (kV) (1)Type of line Cross section s of lines (mm2) R (Ω/k m) X (Ω/km) C (nF/km) L/66 1 x 170 0.183 0.400 9.0 150 L/150 1 x 170 0.183 0.446 8.2 150 H/150 1 x 322 0.097 0.422 8.7 150 2H/150 1 x 322 0.097 0.391 9.3 400 E’H’/400 2 x 484 0.031 0.337 10.5 400 2E’/H’/400 2 x 484 0.033 0.318 11.4 (1) 66 Table 2. Rated characteristics transmission system [3] Rated voltage (kV) of the electric (1)Type of line Cross sections of lines (mm2) Rated Apparent Power (MVA) Reactive Power (kVAr/km) 66 L/66 170 12 12 150 L/150 170 54 58 150 H/150 322 57 62 150 2H/150 322 2x62 2x66 400 E’H’/400 2 x 484 500 540 400 2E’H’/400 2 x 484 2x536 2x571 (1) L’: Light type of line, H’: Heavy type of line E’H’: Extra heavy type of line, 2: Double circuit line During the procedure of mapping a very basic tool is going to be used, that is the Electromagnetic Transients program (ATP -EMTP) which is used for electrical simulations and power analysis. The construction of the network is presented in detail 1 2 ATP-EMTP The Alternative Transients Program (ATP) is the most widely used version of the Electromagnetic Transients program ( EMTP ) in the world today --- by far! In no small part, the acceptance of ATP is due to its availability to nearly everyone in the world free of royalty, and its compatibility with the computers of most common interest. EMTP was developed in the public domain at BPA (the Bonneville Power Administration) prior to the commercial initiative in 1984 by DCG (the EMTP Development Coordination Group, with which BPA has had no connection since the expiration of the associated 2.1 Operation The ATP program predicts variables of interest within electric power networks as functions of time, typically initiated by some disturbances. Basically, the trapezoidal rule of integration is used to solve the differential equations of system components in the time domain. Non-zero initial conditions can be determined either automatically by a steady-state phasor solution or they can be entered by the user for simpler components Symmetrical or unsymmetrical disturbances are allowed, such as faults, lightning surges and several kinds of switching operations including commutation of valves. Frequency-domain harmonic anal ysis using harmonic current injection method (HARMONIC FREQUENCY SCAN) and calculation of the frequency response of phasor networks using FREQUENCY SCAN feature is also supported. 2.2 ATP tools ATPdraw: ATPDraw is a graphical, mouse-driven processor that creates *.atp files. The user can construct the digital model of the circuit to be simulated using the mouse and selecting predefined components from an extensive palette, interactively. Once the circuit has been constructed all the necessary input data can be easily inserted in order for the user to proceed to the simulation. ATP Control Center (ATPCC) is an easy to use tool that can manage the different programs of ATP-EMTP such as ATPDraw, PCPlot and every other program that is related to ATP- EMTP runni ng on Windows.PCPlot: Is a program that can design waveforms and graphs. PCPlot receives as input data the ATP_EMTP’s output files *.p14 and draws the equivalent graphs PlotXY: It has the same functionality as PCPlot but it can also support ASCII files. GTPPLOT: The same as the previous two but ,in addition, it can operate in Linux environment as well. Programmer's File Editor (PFE): It is a text editor that is running in windows. With this program the user can control the input and output files of the EMTP. Its evolution has been abandoned as it performs the same actions as notepad. 3 MAPPING PROCEDURE OF THE INTERCONNECTED TRANSMISSION NETWORK OF GREECE During the process of construction of the Greek transmission system it was firstly essential to gather all those elements that characterise its operation. The parameters of the transmission lines are distributed along the length of the circuit and, therefore, i n order to model the lines, models of distributed parameters, composed of ohmic, inductive and capacity reactions, were used. The models required as input the ohmic, inductive and capacitive reactions of positive and zero sequence as well as their lengths. Reactions come from official catalogues from the Public Power Corporation of Greece while their lengths where measured for each line separately with the help of maps from the Ministry of Development of Greece. More than 400 lines were measured. The results are presented in tables depending on the type of each line. Table 3. Total electric characteristics that are used for fault analysis [3] Type of line R+ (Ω/km) X+ (Ω/km) C+ (nF/km) Rο (Ω/km) Xο (Ω/km) Cο (nF/km) L/66 0.183 0.400 9.0 0.395 1.466 5.1 L/150 0.183 0.446 8.2 0.440 1.326 6.3 H/150 0.097 0.422 8.7 0.360 1.309 6.6 2H/150 0.097 0.391 9.3 0.497 2.349 4.1 H’H’ /400 0.031 0.337 10.5 0.295 1.035 7.7 2H’H’ /400 0.033 0.318 11.4 0.485 1.931 5.1 Figure 1. Example of data input in a distributed line model As regards the rest of the network elements, acceptances were made in order for it to be complete. It 2 must not be forgotten that the main aim of this paper is not as much to produce precise simulation results as to display the mapping procedure of the transmission network. So the substations that are connected with the lines are treated as three phase R,L loads. Every high voltage substation has a rated apparent power of 50 MVA at 150kV and every extra high voltage substation 250MVA at 400kV. Both of them have a power factor of 0.9. The substations at their input are star connected. Hence we can easily calculate their resistance and their inductive reactance which are found to be For the 150kV substations: R=405 Ω and ΖL=196,15 Ω For the 400kV substations: R=576 Ω και ΖL=278,97 Ω After all elements have been defined the modeling procedure started. The load values were in Ohms while the values for the lines parameters were in Ω/ m for the capacities.. The main concept of the modeling procedure was to follow the geographical shape of the real transmission system as it is outlined in the official Energy map of the Hellenic Transmission System Operator (HTSO). The modeling was developed gradually separating the system in parts depending on their geographical positions and then connecting the parts together. So for our convenience the system was divided in eight parts. It should also be mentioned that not all the islands are included in the map as there are some special conditions that regulate the energy transmission to them. An example of this separation is shown in the figure. that Peloponnesus has only 150kV single and double circuit overhead transmission lines. This voltage is generated by two thermal power stations (Megalopolis 1 and Megalopolis 2) that are located almost at the centre of Peloponnesus. Power stations are represented by AC sources that give an output of 150 kV at 50 Hz. To the west this part is connected with the island of Zakinthos through an underground cable, to the north with Central Greece and to the north east with Athens. According to the Hellenic Transmission System Operator there is a new 400kV double circuit line that is going to be manufactured which will connect Peloponnesus with the main 400kV transmission system that already exists and connect the northern with the southern Greece. Fig.3: National Information Energy System Peloponnesus (Ministry of Development of Greece) [6] Another geographical part that is worth mentioning is the north-west of Greece. In that territory, as it has already been mentioned, the 70 % of the total electricity production is produced. Due to the large coal reserves that have been found and the morphology of the ground, there are a lot of thermal and hydro power plants. The greater energy demand is in southern Greece and especially in Athens where the heavy industry and the majority of the population is gathered. So there is a great need to transmit the energy to the south. Figure 2. Part of the mapping process (Geographical apartment of Peloponnesus) This part of the grid represents the section shown at the following map of the Ministry of Development of Greece. The transmission lines are shown in green color (150kV). From the observation of the image it is clear Fig.4:Part of the mapping process ( (Geographical apartment of Northwest of Greece) 3 Three 400 kV double circuit lines head to the central and southern Greece and one more is planned to be constructed in the near future, while another two transmit the energy to the north-east of Greece. In addition, two major lines that connect Greece with Albania and FYROM within the scope of the electricity exchange program have their origins in that area. These two characteristic examples represent the steps that were taken during the mapping process in order to complete the whole transmission system. The same procedure was followed for the other parts of the network as well. When all parts were completed they were connected producing in that way the final map that is shown at figure 7. Fig.5:National Information Energy System - Northwest of Greece (Ministry of Development of Greece) [6] Fig.6: Full Greek map [6] At the early stages of the mapping process some problems started to appear. While new lines were being added and the size of the network was increasing, it was not possible for the program to create the *.atp file which is the output of the ATPdraw, meaning that the program was unable to analyze the circuit. That was caused because of the increasing number of the parameters that the program had to calculate. The time step of the simulation time (deltaT) had to be reduced in order to provide more accurate results. So this is a factor that someone should take under consideration while using this program. Another difficulty that has come up, was measuring the length of the lines as it required a relative accuracy and their number was high. Mistakes such as wrong values of the input parameters were inevitable in such a big procedure but they were spotted and corrected. Finally the system was ready as it is presented at the figure below. 4 Fig.7: Full map of the Greek Electric Transmission System 4 CONCLUSIONS After making the previous analysis it is now clear that mapping is a time consuming proce dure that requires the gathering of a big number of parameters as well as plenty of measurements. At the end of this process the readers ha ve at their disposal all the manufacture and functional characteristics that can be met at the Greek transmission system. The greater gain though, is the network itself that has been designed with the help of a powerful Electromagnetic transients program the ATP-EMTP. This network can constitute the basis for a future study for different operating conditions such as transitional overvoltages. It is very important to predict the behaviour of the network under stable and fault conditions so that we can plan measures of action. Some small additions and modifications may be required in order to give precise results. Moreover, it could be very useful in the case of studying the effects that could be caused from the integration to it of a large wind farm or any kind of renewable generation. It is obvious that some changes in this case are essential in order to have reliable results. In conclusion, this design could be used as the bedrock for the study of any electrical additions that can affect the operation of the transmission system and whose results, is essential to be known. 5 REFERENCES Books: [1] ATP – EMTP Rulebook, Canadian/American EMTP User Group [2006] [2] ATP – EMTP Theory Book, BPA, USA. [2006] [3] System Planning Book, PPC, [2006] Web sites: [4] www.desmie.gr: Hellenic Transmission System Operator [5] [6] [2006] www.deh.gr: Public Power Coorporation [2006] www.ypan.gr: Ministry of development [2006] 5