Environmental Pollution Caused by Electrical Power Devices

Transcription

1 Environmental Pollution Caused by Electrical Power Devices JOŽE PIHLER Power Engineering Laboratory University of Maribor, Faculty of Electrical Engineering and Computer Sciences Smetanova 17 SLOVENIA Abstract: - This paper deals with environmental pollution caused by the operation of electrical power devices. This pollution can occur either during normal operation or in the cases of failures or mistakes. Such environmental pollution can be reduced and, in some cases, fully prevented by the adequate awareness of individuals, responsible behaviour by the consumers and manufacturers of these devices, and by adequate legislation and standards by governments and professional associations. This paper presents some concrete proposals from the field of medium and high voltage power devices. Key-Words: - electrical power apparatus, devices, insulation media, arc-extinguishing medium, pollution, ecosystems 1 Introduction Electrical power apparatus are basic elements within any electrical power system. They exist in power plants, as 400, 220 or 110 kv transmission networks, connected to 110, 20 or 10 kv distribution networks through substations. The latter are through distribution substations connected to the low voltage electricity networks of consumers. In addition to the abovementioned, there are also many substations with electrical power devices within industrial and other electricity consumers` internal networks. Electrical power devices and apparatus consist of electricity-conducting parts, and insulation-parts, that separate the conducting from the earthed parts. The earthed-parts are those protective elements or housings that prevent people when handling their devices from coming into direct contact with the energised parts of electrical power devices. At the same time their role can also be to prevent failures, such as line to ground faults, throughout the grid or to other devices. The insulation parts can be made of solid, liquid or gaseous dielectric materials. Insulation materials can have an important impact on the functional operation of a device, for example as an arcextinguishing medium. This paper deals with the possibility of replacing electrical power devices` insulation material and thus reducing or preventing environmental pollution. Section 2 describes the impact of electrical power devices` operations on the environment. It presents the insulation, cooling, and arcextinguishing media. Section 3 deals with those methods that could prevent environmental pollution caused due to the operation of electrical power devices. Special emphasis is given to improving environmental impacts during the operation of power devices based on SF 6 gas. Practical possibilities are given especially within the medium of voltage devices and apparatus. Section 4 contains the conclusion. 2 Impacts of electrical power devices` operations on the environment Each electrical power device consists of individual elements, such as switches, fuses, surge arresters, insulation elements (insulators and bushings), switchgear, i.e. the housing where all these elements are mounted, conductors or bus bars connecting conductive parts and insulation medium. Electrical power devices can basically be divided into devices for indoor and outdoor usages. These devices are considerably different with regard to voltage-levels and insulation media. Low voltage devices of up to 1 kv are mainly air-insulated. Medium voltage devices up to 35 kv are air-insulated, as shown in Fig. 1, or are SF 6 insulated. Both low and medium voltage devices for indoor use are placed within areas that are either parts of other buildings or are individual buildings. For outdoor devices single-elements are used that have to comply with the requirements for outdoor usage, especially regarding atmospheric conditions. They ISBN:

2 are placed on various poles or on special constructions. High voltage devices above 110 kv are those devices that are air-insulated as individual elements, and are used outdoor. They can also be SF 6 gas-insulated and used indoors. Switchingoperative devices are needed for performing their principal task interruption of an electrical current the arc-extinguishing medium. The latter can be air, oil, or of various gases. Important elements of electrical power systems are also power transformers and measurement transformers. They also need insulation and cooling media, which can be either air, oil, or various gases. 2.1 Insulation, cooling, and arc-extinguishing media of electrical power devices Electrical devices can have harmful impacts on the environment in those cases of non-operational, normal operation, and malfunctioning. These harmful impacts can be caused by cooling, insulation, and arc-extinguishing media. An example is transformer oil. Conventional transformer oils were inflammable and poisonous. When leaking into the environment, they caused environmental pollution and became a danger to living creatures. Today's mineral synthetic oils, however do have significantly-improved characteristics. Switching devices at all voltage levels interrupt and re-connect electrical circuits. These chambers make sure that the electrical arc is extinguished at a predefined time, which ultimately interrupts an electric current. An essential role is played by the arc-extinguishing medium. There are several methods for extinguishing and various arcextinguishing media. In switch-disconnector and older circuit-breakers this is compressed air. In other switch-disconnectors an important role is played by arc-extinguishing chambers with solid insulation materials. These insulation materials were gasified and ultimately extinguished the electrical arc (Fig. 2). Fig. 2: Switch-disconnector with arc-extinguishing chambers made of solid dielectric materials. Fig. 1: Air-insulated metal-clad switchgear within insulation walls, and a low oil circuit-breaker. SF 6 gas was introduced, as an upgrade of these switch-disconnectors. During the development of circuit-breakers arc-extinguishing chambers firstly phase contained compressed air and transformer oil. In the first phase of development this chamber was a vessel in which the device was submerged. During the development continuation phases, the arcextinguishing chambers were separately, topped-up with oil for each phase. As a result of this development work a low-oil circuit-breaker appeared (apparatus shown in Fig. 1). The essential advantage of such a device is that it is less harmful to the environment due to its significantly-lower quantity of oil. In Europe such devices are still massively used, although they are no longer installed in new buildings. Vacuum circuit-breakers within the middle-voltage area and SF 6 circuitbreakers within medium voltage area followed on low-oil circuit-breakers in Europe. In America, and partly in the UK, these circuit-breakers were the successors of compressed-air circuit-breakers. During the first period of development in Europe low-oil circuit-breakers were deployed at the 110 kv voltage level in the high voltage area. Many of these circuit-breakers are still in operation. SF 6 circuit-breakers became the successors of these devices. Switchgears were, in the early-phases of their usage, almost exclusively air-insulated. In later years, manufacturers started to add various solid dielectric materials composed of duroplast or thermoplast materials as walls for separating ISBN:

3 individual switchgear elements, as shown in Fig. 1. The positive effect was to reduce sizes. On the other hand, the negative effect was ozone production during normal operations, respectively causing burning and fires in the event of failures. SF 6 gas was introduced during the 1990`s as an insulation medium in medium-voltage switchgears, which led to a significant reduction in the devices' sizes. The disadvantage of using SF 6 gas was the pollution of environment, and negative impact on living beings. When speaking about environmental pollution it is necessary to mention the so called electromagnetic pollution. This refers to the impact of electrical and magnetic fields on living beings. There have been many theoretical and practical discussions about this issue. The European Union itself and individual Member States have imposed legal obligations about the maximum permitted values of electrical field strengths and magnetic flux densities in new buildings. This area also includes the operation of electrical power networks, where partial discharges occur in the forms of visible and audible corona. These discharges appear due to the improperly-designed individual elements of devices and conductors. Due to high-frequencies they strongly affect the wellbeing's of nearby residents. Qualities of their lives are greatly reduced. In continuations, a short presentation of today`s most widely-used insulation and arc-extinguishing medium SF 6, is given SF 6 gas as an insulation and arcextinguishing medium Sulphur hexafluoride (SF 6 ) is an almost ideal gas for usage in high voltage switchgear equipment and metal-clad systems. It has excellent electrical (insulation and extinguishing of electric-arcs) and thermal properties. SF 6 gas is produced by a direct chemical reaction between molten sulphur and fluorine, which is produced by the electrolysis of HF acid. The properties of produced gas have to comply with the requirements of IEC [1], which defines the permitted levels of impurities. The annual worldwide production of this gas is between 4,000 and 8,000 tonnes. The main use of this gas is as a filling in switching-devices. The technical advantages of SF 6 insulated switching-devices are generally well-known: insensitivity to environmental impacts (dirt, moisture, access to animals, etc.), safety for people working with them, long-lifespans, very little maintenance needed, small sizes, etc. Pure SF 6 is a non-poisonous and inflammable gas without colour and odour. Its density at 20 C within an atmospheric pressure of MPa amounts to 6,135 kg/m 3. SF 6 has an approximately five-times higher density than air, which means that it tends to accumulate within lower-lying areas, such as cable canals or vessel-bottoms. If larger amounts of this gas accumulate with closed-areas, there is a danger of suffocation. The dielectric strength of SF 6, is under the same conditions, approximately 2.5-times higher than for air, which means that the sizes of the devices can be 2.5-times smaller. At a pressure of 300 kpa, SF 6 gas insulates better than oil. SF 6 gas has certain disadvantages, such as: As insulation medium it is sensitive to appearance of points with high electric field strength, therefore it is very important that surfaces are smooth and high quality processed; As arc-extinguishing medium it forms during the burning phase of electric arc some products that are for long time poisonous for living beings; SF 6 is a greenhouse gas and has the highest greenhouse potential of all greenhouse gases, amounting to 23,900 (which means that 1 kg of SF 6 has the same greenhouse effect as 23.9 tonnes of CO 2 ). In addition to this, SF 6 has a very long atmospheric lifetime, ranging from a few thousands to more than ten thousand years (the longest atmospheric lifetime has CF 4, which stays in the atmosphere for up to 50,000 years). Because of the abovementioned disadvantages and massive use in electrical power devices there have been some initiatives in the recent years to limit the use of SF 6, to prevent its leakage, to capture all SF 6 from decommissioned devices and to replace SF 6 with other gases having lower greenhouse potential. 3 Prevention of environmental pollution caused by electrical power devices Electrical power devices are the most important parts of electrical power systems. They have to operate reliably and continuously. The total prevention of environmental pollution due to their operation is impossible. Nevertheless, it is possible to reduce it and in some cases even to prevent harmful impacts on the environment. The most important thing is to be aware of this fact and to act towards achieving this objective. This is a difficult task in this global world that is dictated to by multinational companies that have only one objective: to maximise their profits. An important ISBN:

4 role in this should be played by governments, professional associations, and nongovernmental organisations (NGOs). A possibility positive action towards the prevention of environmental pollution is discussed below in the case of using harmful SF 6 gas as an insulation and arc-extinguishing medium. The focus of this discussion is on medium-voltage devices. In high-voltage devices for outdoor usage air-insulated devices represent an alternative to SF 6 -insulated ones. This is unfortunately impossible in densely populated areas. High-voltage SF 6 circuit-breakers still have no alternative. 3.1 Mitigation of environmental impacts during the operation of SF 6 -insulated electrical power devices Professional associations, civil, and NGOs in Europe, America and Japan share the same opinion about the harmful impact of SF 6 -gas on the environment. Nevertheless, each sees their own way of solving the problem. Some thoughts and facts are presented in the continuation Governmental level In Europe especially there are increased activities at the governmental level. These activities mostly comprise the adoption of legislation (directives and regulations [2], [3], [4]), the aims of which are to make manufacturers and buyers aware of certain facts regarding SF 6 gas. Some regulations already exist about the requirements for reducing the usage of SF 6 and how to achieve this goal. In some areas the use of SF 6 gas is already prohibited and there are sanctions prescribed for breaching these provisions. The area of usage within electrical power devices is not yet one of them. In the field of standardisation (national, European EN and international IEC) [1],[5],[6],[7] there are already have some valid documents standards that more or less-precisely define the use of SF 6, by giving instructions for the designing of devices using this gas, monitoring of the operations, and for decommissioning after the ends of their lifespans. As is evident from American Standards, at the moment they are in a phase of adopting adequate standards that define this area in detail. Nevertheless, they already have some IEC standards that are more or less recommendations, but give only mild warnings to manufacturers and buyers about the issue of using SF 6 gas [8] Research & Development organisations New devices are being developed and designed especially in the field of medium voltage devices, where gas as an insulation medium is replaced by solid dielectrics and by a vacuum in combination with dry air and with solid insulation materials as an arc-extinguishing medium. In Japan [9] they use a special kind of resin for the fixed and silicone for the moving parts, which has led to a reduction in size for 25 %. Japanese, and some European researchers, [10] studying the possibility of SF 6 gas usage by adding other gases, such as helium, nitrogen, hydrogen, etc., both as arc-extinguishing and insulation mediums. The best results regarding insulation properties are being achieved by helium (Fig. 3). Fig. 3: Dielectric strengths [V/m] of various mixtures of SF 6 and helium in the case of an electrical arc with an arc tube of 2.5 mm Manufacturers of switching-devices and apparatus Manufacturers of switching-devices and apparatus are divided into two groups. In the first group there are manufacturers who still produce mediumvoltage devices with SF 6 as insulation and an arcextinguishing medium, and take the position that there is no need to stop or reduce the usage of this technology. The other group consists of manufacturers who have never used this technology (Northern Europe) or are switching to other technologies, which is clearly the case in Japan and some other parts of the world. An interesting alternative to medium-voltage SF 6 devices is thus being offered by those producers who make dry-air and solid insulation materials as an insulation medium (Fig. 4). Their sizes are even smaller than in the case of SF 6 switchgear blocks Professional associations Professional associations (CAPIEL, EPA ) [11] channel their efforts into making ISBN:

5 manufacturers and buyers aware about the importance of careful and safe-handling regarding SF 6 gas, and the need to reduce unnecessary and adverse emissions of SF 6 gas into the environment. They also place a great emphasis on safely decommissioning these devices after the ends of their lifespans. Fig. 4: RMU switchgear-block with dry-air and solid insulation materials as an insulation medium Environmental associations Environmental Associations claim that the usage of materials such as SF 6 gas should be stopped immediately. 3.2 Mitigation of environmental impacts caused by electrical power devices by other insulation, cooling, and arc-extinguishing media The insulation and arc-extinguishing media of medium voltage circuit-breakers are, in most cases vacuum, as it has replaced the environmentallyproblematic transformer oil. Some manufacturers still manufacture SF 6 circuit-breakers. Even in switch-disconnectors that contained transformer-oil, it has been replaced arc-extinguishing chambers by vacuums. In switchgears, insulation-walls (Fig. 1) made of plastic materials have been replaced by metal-clad cells with metal walls. This is an essential advantage especially in the cases of failures. It is almost impossible for these materials to burn, since any short circuit fault turns into a line-to-ground fault (metal partition wall is earthed), and a protection system within a minimum possible time, switches the device off. Conventional transformer oils in power transformers have been replaced by modern mineral and synthetic oils that are much less harmful to the environment. 4 Conclusions This paper dealt with those harmful impacts to the environment that can be caused by the operation of electrical power devices and apparatus. An important conclusion is that these impacts can be mitigated or even prevented, although these devices keep operating reliably without interruptions. It is very important to take into consideration the research and development achievements of science and those researchers who work for the manufacturers of electrical power equipment. An important role in this can also be played by governments, which have to adopt adequate legislation to incentivise, and in some cases, even force manufacturers and users not to forget the environmental protection aspect of their business decisions. At medium-voltage levels, SF6 gas may be prohibited for use in switchgear. However, there are solutions to the full replacement of SF6 gas as insulation and as an arc-extinguishing medium. In high-voltage switchgear the amount of gas SF6 could be reduce by adding helium and nitrogen. Further improvement could also be achieved by preventing the release of SF6 gas during the operating of circuit-breakers. References: [1] IEC 60376:2005, Specification of technical grade sulphur hexafluoride for use in electrical equipment. [2] REGULATION (EC) No 842/2006 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 17 May 2006, on certain fluorinated greenhouse gases [3] COMMISSION REGULATION (EC) No 1494/2007 of 17 December 2007 establishing, pursuant to Regulation (EC) No 842/2006 of the European Parliament and of the Council, the form of labels and additional labelling requirements as regards products and equipment containing certain fluorinated greenhouse gases. [4] COMMISSION REGULATION (EC) No 1493/2007 of 17 December 2007 establishing, pursuant to Regulation (EC) No 842/2006 of the European Parliament and of the Council, the format for the report to be submitted by ISBN:

6 producers, importers and exporters of certain fluorinated greenhouse gases. [5] IEC/TR ( ) High-voltage switchgear and controlgear - Part 303: Use and handling of sulphur hexafluoride (SF 6 ). [6] IEC 60480: Guidelines for the checking and treatment of sulphur hexafluoride (SF6) taken from electrical equipment and specification for its re-use [7] IEC/TR ( ) Ed. 1.0 Bilingual High-voltage switchgear and controlgear - Part 303: Use and handling of sulphur hexafluoride (SF 6 ). [8] IEEE Std C , IEEE Standard for Gas-Insulated Substations, Sponsor Substations Committee of the IEEE Power Engineering Society Approved December 2, 1993 (Revision of IEEE Std C , includes IEEE Std C37.122a-1991). [9] J. Sato, O Sakaguchi, N. Kubota, New technology for medium voltage solid insulated switchgear, Transmission and Distribution Conference and Exhibition 2002: Asia Pacific. IEEE/PES, Vo. 3 pp , [10] Hiromi Odaka, Masayuki Taki, Hitoshi Mizoguchi, Satoru Yanabu, Discussion about an Application of CF3I gas as a substitute gas for SF 6 gas, ISSN: , vol. 126, no.5, pages: (2006), Japan. [11] CAPIEL - Co-ordinating Committee for the Associations of Manufacturers of Industrial Electrical Switchgear and Controlgear in the European Union. ISBN: