Lightning Protection Practice for Large-Extended Photovoltaic Installations

Transcription

1 202 International Conference on ightning Protection (ICP), Vienna, Austria ightning Protection Practice for arge-extended Photovoltaic Installations Dr. Nikolaos Kokkinos Research and Development Department Elemko SA Athens, Greece Dr. Nicholas Christofides Department of Electrical Engineering. Frederick University Nicosia, Cyprus Dr. Charalambos Charalambous Department of Electrical and Computer Engineering. University of Cyprus Nicosia, Cyprus Abstract This paper aims to analyse the lightning protection system (PS) of an isolated large and extended Photovoltaic (PV) installation park. The area where the PV plant operates is characterised by the high ground flash density ( 25 thunderstorm days per year) and the extremely high soil resistivity value (i.e. pure rock with a resistivity of more than 2000 m). The paper includes the PS system design after experimental testing results, which were performed in the laboratory. It also includes solutions to some difficult overcoming problems that were faced during the application of the lightning protection design. The main objective of this on-going work is to address the issues necessary to form a global framework for the lightning protection system (PS) design of isolated large and extended photovoltaic installations - PV parks. In particular, this paper describes the preliminary work on PS system designs with particular emphasis on experimental testing that is performed at EEMKO S H.V laboratory in Greece. This work aims in framing proposals and solutions to overcome challenges and problems that may rise during the installation of lightning protection designs. Keywords-Photovoltaic park, ightning protection system II. SITE SURVEY I. INTRODUCTION In a country like Greece where the sun is shining for most of the year round, the number of photovoltaic (PV) installations has been significantly increasing during the last years. Nowadays, the interest and investment in large scale PV parks in the MWp range is becoming very common. The knowledge however of a proper lightning protection system (PS) design and installation, including surge protection, for such large and extended structure areas (with long cabling loops) is still under research. This is the reason for the development of the new NEEC document; TS : 2009 [] describing application principles of surge protection in PV installations. The investments in such large scale PV parks are considerable and it is merely common sense that investors should choose to adopt a PS for their systems. When compared to the income losses incurred due to a failure or damage resulting from a lightning strike, not to mention the technical and practical difficulties associated with the repairs or component replacements, the cost of a PS system is negligible. It is therefore advisable, not to say self-evident, that a PS is necessary. The particular PV park under study is installed on a mountain peak, flat area, occupying a total surface of around 5,000m 2. In total it contains 80 DC/AC inverters of kw nominal power, operating at 800VDC and 7,300 solar panels of 270Wp nominal power with dimensions of 2m x m each. The PV park is connected to the 2kV medium voltage (M V) distribution system via 8 substations (MV/V). The soil was rocky (>2000Ωm) and the support structure of the PV panels was a combination of concrete reinforced bases embedded in soil and aluminium supports above soil. III. INITIA IGHTNING PROTECTION SYSTEM DESIGN CONSIDERATIONS Due to the high resistivity of the soil, which was not promising for an effective earthing system and in conjunction with the high ground flash density of the area, the design of the lightning and surge protection was considered of high priority. Due to the extensive area coverage of the PV park, the design of the external PS considered both possible cases, the one for an isolated application as well as the one for a non-isolated application design, as per IEC [2].

2 N U C =255Vac Ι n =20κΑ (8/20μs) U p = <.2kV T a = <00ns 40G PE N U C =255Vac Ι n =20κΑ (8/20μs) U p = <.2kV T a = <00ns 40G PE U oc =000Vdc + PE U oc =000Vdc PV U oc =000Vdc PV _ PV Fig. and Fig. 2 illustrate the 2 cases considered when designing the PS. In Fig., for the non-isolated case, the air terminals for every 2 consecutive rows are connected to one to one electrode installed between the 2 rows. In this scenario, it is advised that the distance between the PV frame of each row and the earth electrode does not exceed 3m. Fig. 2, illustrates the isolated case, where the air-terminal of each PV row is connected to a separate electrode directly and not through the PV framework. Each PV frame is also bonded on the same earth electrode but with independent bonding conductors. In addition, the PV park under study was designed utilizing small kw inverters and not central inverters. Therefore, the DC cable loop was small and there was no need to use T+ s on the DC side of the inverter even for the non-isolated PS case. Fig. 4, illustrates the installation of only type s on the DC side resulting from the fact that the lightning loop formation is limited or non-existent Figure 3. Acceptable materials for earthing system depending on the type of foundation used for the PV framework Figure. Design of Type B earth electrode using non-isolated PS. The airterminals of 2 consecutive PV rows are connected to electrode which is installed between them TABE. DETAIED DESCRIPTION OF MATERIAS DEPICTED IN FIG. Type of Foundation of the PV framework Allowed material for earthing system driven into the soil Galvanized steel profile directly buried into the soil Galvanized steel, Stainless steel 2 Steel profile embedded in Copper coated steel, Copper, concrete Stainless steel 3 Reinforced concrete block placed Galvanized steel, Copper coated at ground level not into the soil steel, Copper, Stainless steel 4 Reinforced concrete foundation Copper coated steel, Copper, into the soil Stainless steel Note : Copper conductor may also be tinned Note 2: Aluminum is not allowed to be used into the soil Figure 2. Design of Type B earth electrode using isolated PS. Each isolated air terminal is connected to a separate electrode directly and not through the metal PV framework For the isolated PS it was necessary to provide an earth electrode behind each PV row so as to earth the isolated air terminals directly. For the non-isolated case however, an earth electrode for every 2 consecutive PV rows was necessary, combining the earthing of the air-terminals. For PV system installations in open fields, in order to minimize the cost and at the same time increase the efficiency and life time of the earthing system (by avoiding electrochemical corrosion) it is very important that during the design the type of foundation used for the PV façade framework is taken into consideration. In this particular project, the foundation material was reinforced concrete buried into the soil, therefore copper coated and solid copper earth electrodes were used. Fig. 3 and Table, depict information about the selection criteria of the materials used for the earthing system, depending as mentioned above, on the type of the foundation used for the PV framework or structures. AC electric panel board AC electric panel board >0m m Common earthing system DC Junction Box String String 2 Figure 4. Installation of s for PV application inside a junction box situated at a distance >0m from the inverter at the DC & AC side. Only s are needed on the DC side since there is no lightning loop formation. On the AC side however, T + type s are required due to the cable loop, which may allow the lightning current to flow in a parallel path to earth

3 8899kWhr SURGE PROTECTION kw Meter Inverter AC Input DC & AC Termination Set String DC Input PV Generators String 2 DC Output AC Output Figure 5. DC & AC termination set for optimum protection of the inverter against surges arriving either from the AC or DC cabling For the effective protection of the inverters, a combined protection was developed allowing a combined termination of all DC & AC cables and at the same time providing protection from coupled surges. By using such a combined termination set, the cable lengths are kept to an absolute minimum. However, such a combination should fulfill certain standards [3-7] since coexistence of DC and AC at relative high voltages requires specific isolation distances. Fig. 5, illustrates this arrangement, where termination box is used for both DC and AC cable terminations IV. EXAMINED CASE STUDIES Due to the extensive and long cable loops which are formed by the DC cables, any direct or nearby lightning will cause high induced surges, Fig. 6. Since screening of the DC cables is difficult to provide for in large PV parks, the question that arises is which external PS provides a more suitable and more effective protection against over-voltages induced on the cables. A scaled down experiment was performed in the laboratory in order to obtain measurements and information that would assist in the external PS design of the 2MWp PV park under study. The purpose of the laboratory tests was to evaluate the performance and apply it later on. The scaled down version of the experimental setup in the laboratory was a 2kWp PV system depicted in Fig. 7. The system consists of 9 PV modules connected in series giving a total of 200V output voltage and 0A current. During the impulse experiments the laboratory lights were switched off in order to have zero volts at the DC cable loop, which was approximately 8m long. Figure 6. Effect on the extended wiring of an open field large PV system installation due to a direct or indirect lightning strike Figure 7. PV panels tested in the laboratory and the formation of the DC cable loop (red +ve and black ve) The initial purpose of the test was to investigate which of the following two possible cases would give a lower induced voltage across the DC cable loop of the string. In case A the impulse current was injected directly on the framework of the PV panels through an air termination rod, which was supported on the edge of the PV support framework. In case B the impulse current was injected directly on the on an isolated air termination rod, which was supported on the laboratory floor at a distance of approximately 700mm from the PV support framework. The results are summarized in Fig. 8, Fig. 9 and Fig. 0.

4 25% Case Α Isolated PS at a distance of 0,7m behind the steel frame of the PV structure. The lightning current is driven to earth via a dedicated rod and down conductor. The metallic structure of the PV is only connected to the PS via the earth. T+ s for the DC was not mandatory. Furthermore, due to the fact that the earthing system arrangement was a cost effective solution for the non-isolated case compared with the isolated PS case, the investor decided to use a non-isolated PS for the particular Photovoltaic park. Case Β Non isolated PS, the lightning current is distributed along the metallic frame of the PV structure. The metallic structure is used a natural down conductor. Figure 8. Examining the behavior of isolated and non isolated PS in case of a direct lightning strike and the effect of inducted over-voltages in the DC cabling of a PV installation 25% Case Α The open circuit voltage across the (+) and (-) negative pole of the string was 82V and the injected current was 00kA (0/350μs). Figure 9. Experimental results of scaled down experiment examining the behavior of a non-isolated PS with respect to the induced over-voltages on the DC cabling of a PV string Case Β 00% V. CONCUSIONS The design of a lightning protection system for large scale PV systems may depend on various factors and parameters. It is very important to take into consideration the installation arrangement and design adopted for large scale PV parks as it involves crucial parameters that must be taken into consideration for the effective external and internal PS design. There are numerous large scale PV system designs all of which require a PS that satisfies the unique, in some times, conditions present. The need of an efficient lightning protection system is mandatory for photovoltaic installations by virtue of its preventing nature. Primarily, the need is imperative to prevent any physical damage to structures and life hazards. It is worth noting that the damage of the electrical and/or electronic equipment of a PV installation, due to surges originates from ightning Electromagnetic Impulse ( EMP) as well as from Switching Electromagnetic Impulse (SEMP) [8]. However, literature survey reveals that there is still very little information published regarding the design of lightning and surge protection for large and extended PV installations. In particular, reference [9] comprehensively covers the related scientific background by emphasizing on the aspects of standardisation that should be addressed in the near future. As quoted, the current practice, for protecting PVIs from lightning surges, rests with adopting (partly) protective measures described in standards for conventional low-voltage power distribution systems. 00% The open circuit voltage across the (+) and (-) pole of the string was 390V and the injected lightning current 00kA (0/350μs). It was almost 480% than the non isolated case. This is due to the reason that the current was not distributed but driven 00% through one path creating a high magnetic field. Figure 0. Experimental results of scaled down experiment examining the behavior of an isolated PS with respect to the induced over-voltages on the DC cabling of a PV string It is vital that the PS provide an effective protection against direct or indirect lightning strikes in order avoid the destructive effects of lightning. As previously mentioned, the investment cost is very considerable and thunderstorm can be catastrophic with inestimable financial consequences. The results presented in this paper propose the most cost effective and technically correct solution for the PS design of the large scale photovoltaic system under study. The need however, for deeper and more detailed analysis is required so that amendments are made to the current standard in order to include guidance and regulations for common large scale PV installation practices and examples. The results show that the non-isolated PS will provide lower induced over-voltages on the cabling in case of a direct lightning strike. Additionally, since small inverters were used (therefore no parallel path for the DC to earth) the need of

5 ACKNOWEDGMENT The authors wish to thank the engineering team of BIOSAR SA (GREE) for their contribution to the photovoltaic park project. REFERENS [] TS :2009, Protection of PV installation against overvoltages [2] IEC :200, Protection against lightning Part 3: Physical damage to structures and life hazard [3] EN : ow voltage switchgear and controlgear assemblies Part : Type tested and partially type tested assemblies. [4] EN : ow voltage switchgear and controlgear assemblies Part 3: Particular requirements for low voltage switchgear and controlgear assemblies intended to be installed in places where unskilled persons have access for their use Distribution boards [5] HD : Electrical installations of buildings Part 7-72: Requirements for special installation locations Solar photovoltaic (PV) power supply [6] EN : Insulation co-ordination for equipment within low voltage systems Part : Principles, requirements and tests [7] EN 62446:2009: Grid connected photovoltaic systems Minimum requirements for system documentation, commissioning tests and inspection. [8] IEC Protection against ightning [9] Jesus C. Hernandez, Pedro G. Vidal, Francisco Jurado, ightning and Surge Protection in Photovoltaic Installations, IEEE Transcactions on Power Delivery, Vol. 23, No 4. Oct pp