Lightning Protection for a Complex Isolated Renewable and Diesel Power Network Micro Grid

Transcription

1 Lightning Protection for a Complex Isolated Renewable and Diesel Power Network Micro Grid D. Kokkinos 1, G. Ladopoulos 2, N. Kokkinos 1 1: Elemko SA, Tatoiou 90, GR, Metamorphosis, Hellas 2: Fasma Ltd, Trias 18, GR, Athens, Hellas Abstract: In this paper a specific Lightning Protection System (LPS) design for an isolated complex renewable and diesel power network micro grid will be described. This isolated power network was established in 1992 on the Holy Mount Agion Oros for the Simonopetra monastery in northern Hellas. It is composed of a 33kW Hydro generator, a 45kW Solar system, and Diesel generator. contains 936 photovoltaic cells able to deliver 45kW, one small hydro power station generating 33kW and a diesel generator of in order to cover and support the renewable generators. The entire system was active from 1992 and during two years of operation many damages occurred to it due to lightning strokes. In 1994 it was decided to apply lightning protection to it and since then no further damage was reported. Due to the wide space area of the installation a specific external LPS needed to be designed. Additionally due to the sensitive electronics of the control and protection systems of the network, surge protection (Surge Protection Devices - SPDs), equipotential bonding and shielding were also of major importance. The high altitude in addition to the high ground flash density made this system vulnerable to lightning flashes. Ten years now the isolated power network it is protected against lightning. The paper apart from the LPS design, which was used it also aims to evaluate the performance of the LPS that is now in use over this ten years period. Keywords: Lightning protection system, surge protection devices SPDs, Micro-Grid, photovoltaic system, hydro generator, diesel generator 1. Introduction The seven-stored monastery of Simonos Petra or Simonopetra is the most audacious construction, which was build on Athos mountain (see Figure 1), on the southwestern part of the peninsula of Agion Oros in north Hellas and is dedicated to the birth of Christ. The name comes from the founder of the monastery, Hosios Simonas, who lived in Athos in the mid 14th c. The monastery is inhabited by a brotherhood of 60 monks [1]. The Monastery stands at a height of 330 meters on a rocky mountain and the electricity supply comes from a local and isolated from the main grid, network which Figure 1: The Holy Monastery of Simonopetra 2. Site survey Due to the volume and the complexity of the power system it was not possible to have all the sources in a single location. Therefore each individual unit (PV, Hydro, Diesel) was installed in different locations from each other. ICLP

2 To Monastery Td 50 days per Year Control b uilding and main electrical storage/distribution Diesel generator Energy and control cables 1m above ground le el Hydro station 33kW Photovoltaic cells and local distribution panel Water reservoir from various water sources Water storage tank Water pipe 6 Length 1.3km laying on the ground surface Mountain Peak Mountain m Sea level Figure 2 shows a simplified sketch of the entire system. The photovoltaic cells were installed in an open field area on an incline hill. Two 150mm 2 cables deliver the generated energy from the photovoltaic cells to a control building near the monastery. The altitude of the hill where the photovoltaic cells are installed is 650m. At the same altitude of the photovoltaic cells, a water reservoir was developed to collect rainwater, which is flowing through steams down the mountain and also water from local mountain springs. The reservoir is connected with a water tank and depending on the level of the water in the water tank the water is released and driven through a 1.3km water pipe gives the required rotation to the turbine of the hydro station. The altitude difference between water tank and the hydro station is 330m.The generated energy is then delivered to the same control building as the energy from the photovoltaic cells. The hydro and the control building are in a close distance. Finally the diesel generator is installed close to the control building. In the same building are all the required equipment such as regulators, DC/AC converters, batteries and also all the control equipment for the operation of this isolated power system. Figure 3 shows a simplified electrical schematic of the entire system. According to the national standard ELOT 1197 [2] the area has 50 thunderstorm days per year, which is a quite high figure. Figure 2: Schematic configuration of the site standards [3] the LPS should have been designed according to level III. The external LPS design was done according to the Hellenic standard ELOT 1197 [2] equivalent to IEC standard [4]. Reviewing the existing LPS design fulfil the requirements of level III, which should have been initially designed for. Each individual unit was considered separately. Photovoltaic: As shown in figure 4 and 5 the air termination system for the photovoltaic panels is composed of nine, 10m tall masts with an inception rod on the top of each mast. All the masts were interconnected through the earthing system. The earthing system (figure 6) is a combined type B and A according to [2]. All the photovoltaic panels had a local earth and were also bonded to the main earthing system, which has a DC resistance of 1Ohm. The soil where the earthing system was installed has a low resistivity. The total area of the photovoltaic cells is about 416m 2 and the total installations surface is about 2.500m 2. Hydro: The Hydro station was installed indoors and therefore the risk of a direct lightning on the turbine is very low. The size of the building did not cause high risk for a direct lightning stroke either but due to its importance it was decided to install a meshed air terminal with embedded lightning protection rods and two down conductors, which were connecting the air terminal with the earthing system. At each down conductor a type A earthing system was applied. 3. External LPS Protection against direct lightning A risk analysis, prior to the LPS design was not possible to be performed since European and national standards did not exist at that period. Reviewing the installation now and by applying the current national risk analysis Diesel and Control building: Similar to the previous two the external LPS of the building that contains the diesel generator and the control room was designed according to [2] by applying the Faraday cage principle. ICLP

3 36 panels containing 936 P/V cells, 48kW Total area of the photovoltaic cells 416m 2 Total installations surface was about 2.500m 2 Water Reservoir Water Tank Water level probe Water pipe locally earthed through ISG ISG Local electric panel Water pipe 6 & Electric control cable Length 1300m (each) DC Power cables 2 x 150mm 2 Partially underground and partially over ground Length 1800m ISG Control panel of Hydro Hydro station 33kW Hydro station building Diesel generator and control building 40kA 8/20µs Diesel Generator Regulators 400 VDC UPS DC AC Batteries 400 kwh AC DC Main Distribution Panel 230/380 VAC Data Collection & Process Programmable logic control Manual control Indicators of good operation To loads Figure 3: Schematic configuration of the electric layout in the site ICLP

4 4. Internal LPS Protection against overvoltages Total number of masts 10 Group A 36 Photovoltaic panels, 45kW Covering a total area of 2500m 2 Group B 10m 11m Similar to the external LPS each unit was considered individually but some parts of each installation were linked due to the interconnected cabling of the system. In the internal LPS equipotential bonding and surge protection was mainly considered. A general view of the surge protection is shown in figure 3. The maximum discharge current of all the SPDs does not exceed 40kA, 8/20µs. PV cells isolated from lightning protection masts but bonded with the earthing system Local regulators and control switches Perimetrical earthing ring Type Figure 4: Detail of the air terminals that used for the photovoltaic cells protection B Photovoltaic: Data and power surge protection devices (SPDs) were installed at both ends of all the cables, which were connecting each photovoltaic unit and a local regulator situated in a local PV control structure (see figure 7). All the equipment cabinets, cables ducts etc that were installed inside the local PV control structure were bonded with each other and to the earthing system. Link between PV panels and local regulators, set of 36 cables Figure 7: Detail of surge protection between the PV unit and the local regulator Figure 5: Detail showing the actual site, the placement of the air terminations and the earthing system Hydro: All metallic parts including the turbine, the generator, cable ducts etc were bonded directly to the earthing system. SPDs were installed in all the incoming and outgoing cables. The water pipe coming from the water tank was bonded to the earthing system through isolating spark gaps (ISG) to avoid interference with the cathodic protection. Figure 6: Detail showing the placement of the earthing system Figure 8: Detail of surge protection between the Hydro ICLP

5 Diesel and Control building: Inside the control building a horizontal equipotential ring conductor was installed in the perimeter of the building, which was connected to the earthing system. All the metallic equipment such as enclosures, cable screens & ducts etc were bonded on the equipotential horizontal ring conductor. All the control and power cables (incoming and outgoing) were bonded through SPDs to the equipotential ring as well. All the leads connecting the SPDs were kept at the minimum length and their installation position was as close as possible to the under protection equipment. More information about protection against lightning inside structures can be found in [5-7]. Regarding SPD selection and application further details can be found in [8-10]. Metallic enclosures LPZ 1 LPZ 0 LPZ 0 A, B 0 Incoming / outgoing cables Horizontal conductor ring Main LPS earthing system Figure 9: General arrangement of equipotential bondiung for indoor equipment and service entrance lines [5] 5. Discussions & Conclusions The development of a lightning protection system for a complex and isolated power system, which was based on renewable and diesel generators micro grid, was presented. The elaboration of this design solved many problems that were caused due to lightning discharges and improved the reliability of the micro grid system operating conditions. Further development and improvement of the current and future designs is always an issue that should be considered. Prior to the application of the previously described lightning protection system, the micro-grid suffered serious damaged due to lightning discharges. The system is protected over ten years now and no further damage was reported. All the SPDs that were installed were based on varistor technology with discharge current capability varying between 4-40kA. None of them over the ten-year period did ever fail due to an excessive lightning current overstress. provided efficient protection over a long period without having heavy duty SPDs installed. Finally due to increase number of micro grid users [12] the need of a detailed lightning protection is necessary to avoid undesired damage to these installations. Considerations in both technical and financial fields need to be taken when designing the lightning protection system of a micro grid system, risk analysis as suggested by others [13] is a good practice of estimating the need of such an installation. 6. References [1] Hellenic Ministry of Culture [2] Hellenic Standard ELOT 1197, Protection of structures against lightning [3] Hellenic Standard ELOT 1312, Protection of structures against lightning Risk analysis [4] IEC , Protection of structures against lightning, Part 1 General principles [5] IEC , Protection against lightning electromagnetic impulse, Part 1: General Principles [6] IEC , Protection against lightning electromagnetic impulse, Part 2: Shielding of structures, bonding inside structures and earthing [7] IEC , Protection against lightning electromagnetic impulse, Part 3: Requirements of surge protective devices (SPDs) [8] IEC , Surge protective devices connected to lowvoltage power distribution systems, Part 1: Performance requirements and testing methods [9] IEC , Surge protective devices connected to lowvoltage power distribution systems, Part 12: Selection and application principles [10] IEC , Electrical installations of buildings, Part 4-44: Protection for safety Protection against voltage disturbances and electromagnetic disturbances [11] D. Kokkinos, M. Xenakis, S. Vassalos, N. Kokkinos, I. Cotton, Lightning Protection of Air Traffic Control RADAR Systems, 27 th ICLP, Avignon, France, 2004 [12] European sponsored Micro grid project, Project No: NNE [13] A. Kern, F. Krichel, Considerations about the lightning protection system of mains independent renewable energy hybrid-systems practical examples, 26 th ICLP, Cracow, Poland, 2002 This particular case may reinforce conclusions that were derived in other similar cases [11], where an LPS design, which was installed to protect specific equipment on a high altitude in a high ground flash density area, has ICLP