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8th 2013 Asia-Pacific International Conference on Lightning, Jun. 26-28, 2013, Seoul, Korea Introduction of Lightning Protection System Design Concept of the Coal-fired Thermal Power Plants in Korea Yeo, Kyeongkyu#1, Jeong, Gucheol#2 #1,2 Electrical, Instrumentation & Control Engineering Dept., KEPCO E&C 2350 Yonggudaero, Giheung-gu, Yongin-si, Gyeonggi-di, South Korea [email protected] [email protected] Abstract— This paper introduces about the lightning protection system for the coal-fired thermal power plant in Korea. Power generation plant has lots of building or structures for turbine and generator, boiler, coal handling system, FGD(flue gas desulfurization), etc. and also lots of electrical equipment installed outside. Each building should be protected from lightning and the electrical equipment shall not only be protected from the direct stroke of the lightning but also be protected from transferring surge intruded by the lightning stroke away from the equipment at the plant. In this paper the characteristics of lightning protection system for power plant buildings, structures, and electrical equipment will be introduced. And also special check points for lightning protection system of electrical equipment will be introduced. I. INTRODUCTION Considering the importance of the power plant in Korea as industrial energy sources, their operational continuity is very important. The electric power consumption in Korea is at its highest level during summer, and the thunder storms which can give a disturbance to the equipment in the power plant are also most frequent and in high magnitude in a year. Coal-fired thermal power plants are supplying electric power to the base loads with nuclear power plants in Korea. So, the operational security of the coal-fired thermal power plants is very important to the national industry. The appropriate lightning protection system of the plant is critical to ensure the service security of the power plants during thunderstorm weather. In this paper the characteristics of the lightning protection system of the coal-fired thermal power plant in Korea and a specific requirement are introduced. II. CHARACTERISTICS OF THE COAL-FIRED THERMAL POWER PLANT Coal-fired thermal power plant consists of lots of equipment for the electric power generation and several buildings and structures for the installation and the protection of the equipment. Normal buildings for office are also included in the plant area. The major systems for the electric power generation are boiler which generates steam to rotate the turbine, turbine and generator which changes the mechanical power to the electric power, coal handling system which transfers the coal from coal yard and supplies to boiler, FGD which desulfurizes the exhausting gas of boiler and the other system or equipment including electrical equipment. The main electric power system consists of the main generator, step-up transformer, auxiliary transformer and start-up transformer, isolated phase buses, switchgears to supply the electric power to the house loads, high voltage power cables and other low voltage systems. High voltage substation is included to the plant area because the generated electric power shall be transmitted through high voltage transmission lines to the loads which are far from the plant. And the power plant has lots of electronic equipment, too, which is requesting a special consideration for protection from lightning surges. Buildings and structures for each system shall be protected from lightning stoke to protect the building itself and also the equipment installed in the building. The lightning protection system for buildings or structures shall also such designed to include the electrical equipment installed outside, e.g. power transformers, extra-high voltage equipment, etc., to their protection area considering the equipment insulation level. The electrical equipment shall also be protected from lightning surges transferred through conducting paths to ensure the service continuity of electrical equipment in the plant. Lightning protection system for buildings or structures of the coal-fired thermal power plants and the specific design consideration for the lightning protection of the electrical system are introduced in the following sections. - High structure: Stack; - Substation. Lightning protection systems for each categorized building is introduced. A. Normal buildings The design method of the lightning protection system for normal building is same as that for commercial buildings. The lightning protection system consists of air terminals, conductors and down conductors and grounding system. The air terminals are located on the roofs of the buildings or top of each structure to include any facility to the protection zone generated by rolling sphere in accordance with NFPA 780. Each air terminal will be connected each other and connected to the plant grounding system through down conductors. The plant grounding system is designed to meet the requirements of touch voltage and step voltage in accordance with IEEE Std 80-2000, which is quite different from the commercial building. The ground fault current for the design of the grounding III. LIGHTNING PROTECTION SYSTEM FOR BUILDINGS AND STRUCTURES system of the power plant is much higher than that Lightning protection system for buildings at the of the commercial buildings. Some power plant in Korea are installed with coal-fired thermal power plants will be introduced. non-Franklin rod, e.g. DAS(dissipation array The major buildings and structures considered in system) or BDAT(bipolar discharge-dissipation the design of the lightning protection system are as lightning air terminals) as air terminals, even follows: though those air terminals are not mentioned on - Boiler; NFPA. However, the layout of the non-Franklin air - Turbine and generator building; terminals at the power plants is designed with the - Stack; same principle of the Franklin rod in accordance - FGD building and structures; - Structures and buildings for coal handling with NFPA 780. system; B. Stack - Structures and buildings for limestone The stack is the highest structure in the coal-fired handling system; thermal power plant so that it intercepts the - Substation; lightning frequently. According to the layout of the - Other structures and buildings. plant lots of equipment is installed near the stack. The above building and structures can be divided And lots of the failures of the electronic equipment to three categories considering the design installed on and near the stack during the lightning requirements; stroke to the stack have been reported. The special - Normal building: Boiler, Turbine and efforts to prevent such failures are paid during generator building, FGD building and power plant design stage of the lightning protection structures, Structures and buildings for coal system of the stack. handling system, Structures and buildings for The design requirements of air terminals and limestone handling system, Other structures conductors are mentioned on Chapter 6 of NFPA and buildings; 780 and Annex A of IEC 62305. The major difference in the design principle of the lightning protection for stacks or high structures between 780 and IEC is the installation method of the air terminals at the intermediate level of the stack.[1][2] The design method in NFPA 780 is applied for the coal-fired thermal power plant in Korea. The height of the air terminal rod is limited in accordance with NFPA requirements. The air terminals are connected to form a ring connection and connected to the plant grounding system by at least two down conductors. There are such power plants to be protected by Non-Franklin rods, as shown on Fig. 1.[3] Fig. 1 Example of non-Franklin rods installed on the top of the stack The electronic equipment on the stack is protected by SPD. The grounding system near the stack is supplemented with special grounding rods developed to enhance the surge current characteristics of the grounding system. Sometimes supplementary grounding mesh is installed near the stack for such power plant expecting high level of lightning stroke rates. C. Substation Gas-insulated substation(GIS) is normally installed for the power plant in Korea. Because the gas-insulated buses are consisted of enclosures having high conductivity and the ground resistance of the substation is normally very low, no additional lightning protection is necessary except for the equipment regarding take-off tower, which is protected from the direct lightning stroke by grounding wires on the tower top.[4] The generator step-up transformer and the other transformers are installed near turbine building and included to the protection zone of the lightning protection for turbine building. IV. LIGHTNING PROTECTION SYSTEM FOR ELECTRICAL EQUIPMENT The electric system configuration of the power plant shall be thoroughly checked for the appropriate design of the lightning protection. The usual intruding way of the lightning surges affecting the plant equipment is the back flashover of the lightning stroke to the top of the transmission tower. Because the transmission lines are protected from the direct stroke by the overhead wires, the lightning current level stroking the lines directly is usually lower than that of tower top. The magnitude and the waveform of the surge transferred to the electric equipment at the power plant depend on the magnitude and on the location of the stroke and the characteristic impedance of each electric system component. The major electrical equipment of the coal-fired thermal power plant affected from the transferred lightning surges through the transmission lines and the power plant equipment is the main generator, the generator step-up transformer, the auxiliary transformer, and the start-up transformers. Surge arresters are usually connected for the protection of the electrical equipment from lightning surge. The protective level and the continuous operating voltage of the surge arresters regarding the protected equipment and its location are critical to the proper protection of the equipment. The design criteria regarding the selection of the surge arresters for the equipment in the substation are standardized by KEPCO in Korea.[5] However, the design criteria for the protection of the equipment at the power plant are not standardized. The important check points regarding lightning surge protection are determining the location of the surge arresters for the protection of the generator step-up transformer and the characteristics of the surge arresters for the protection of the main generator. The generator step-up transformers are connected to high voltage GIS either by high voltage cables or overhead lines. In case of cable connection, the surge arresters to protect the generator step-up transformer can be installed either at GIS side or at the transformer side according to the lightning surge voltage at the terminal of the transformer. Better protection margin can be expected if the surge arresters are installed at the transformer with additional cost of the supplementary gas insulated buses. If the installation of the surge arresters at the transformer is not allowed due to the space or economic reasons, those shall be installed in GIS side after the careful study regarding the lightning surge at the terminal of the transformer. Fig. 2 shows an example of the main electric power system configuration of typical coal-fired thermal power plant. Even though the lightning surge voltage is reduced to the protective level of the arrester at the GIS, the surge voltage at the transformer can be raised to a dangerous level due to the characteristic impedance difference between GIS and the power cable. [6] Even though the main generator is installed in the turbine building and connected to the transmission line through step-up transformer, the generator is usually protected from the lightning surges by surge arrester due to the transferred surge through the transformer and the importance of the generator in the power plant. The arresters shall be selected with high attention regarding the protective level and the continuous operating voltage of the arresters. The insulation level of the generator is lower than that of the transformer for the same voltage, so that the lower protective level is required for the generator. However, the voltage on the un-faulted phase during ground fault is higher than that expected on the effective grounding system due to the high resistance grounding of the generator. So, the higher continuous operating voltage is required for the arresters to protect the generator than that for the transformer. It is difficult to find an appropriate surge arrester to meet the above requirements simultaneously for some type of generator. 1) One of the surge arresters can be eliminated according to the lightning surge voltage study. 2) Special attention necessary regarding the protective level and the continuous operating voltage of the surge arrester Fig. 2 Example of the electric power system configuration of coal-fired thermal power plant regarding surge arresters V. CONCLUSION The lightning protection system for the coal-fired thermal power plant in Korea is introduced. The specific difference of the protection system compared to normal commercial building is based on the importance of the plant and the high structure, such as stack. And the other specific protection system is regarding the main electric power system. Even though the lightning protection requirements for the substation equipment are standardized by KEPCO in Korea, those for the plant equipment are difficult to be standardized because of the difference of the characteristics of the main equipment of each power plant. Detail lightning surge analysis regarding the generator step-up transformer is required if the arrester cannot be installed at the transformer because of the layout or economic reason. And high attention is requested to determine the protective level and the continuous operating voltage of the arresters for generator. REFERENCES [1] [2] [3] [4] [5] [6] Standard for the Installation of Lightning Protection, NFPA 780, 2011 Edition. Protection against lightning – Part 3: Physical damage to structures and life hazard, IEC 62305-3, Edition 2.0, 2010. M. D. Robinson, “Grounding and lightning protection”, EL- 5036, Power Plant Electrical Reference Series Volume 5, EPRI, 1987 “A Study on the Insulation Coordination of 765kV System”, KEPRI, South Korea, 1995 Selection of Arresters, DS-2531, KEPCO, South Korea, 2008 “Lightning Protection Design Guide-Book for Power Stations and Substations”, CRIEPI , Japan, 1976