This 3rd edition of High Voltage Engineering Testing describes strategic developments in the field and reflects on how they can best be managed. All the key components of high voltage and distribution systems are covered including electric power networks, UHV and HV. Distribution systems including HVDC and power electronic systems are also considered. In the book, particular consideration is given to recent developments in UHV, AC and DC transmission systems abroad. Recent developments in renewable energy techniques and environmental issues are also discussed and assessed. This new edition gives details of design and testing techniques and considers recent developments in testing and measuring technology and reviews them together with appropriate strategic technological assessments of some applications. The book also looks at UHV, HV and distribution systems both from the point of view of the provider and the user, covering everything from specification and testing to overall system co-ordination. The ongoing dynamic changes which have taken place during the past decade are considered, moving on from the early stages of privatisation and market influences in the UK and abroad, to current strategies aimed at optimising the value of network assets and the effective utilisation of alternative renewable energy sources within network frameworks.
Inspec keywords: power transformers; power transmission; asset management; condition monitoring; testing; switchgear; renewable energy sources; arresters; substations; power cables; insulation co-ordination
Other keywords: network equipment; India; substation condition monitoring strategy; Japan; high-voltage testing; IC technology; power transformers; renewable energy sources; strategic network developments; asset management; high-voltage engineering; insulation co-ordination technology; power cables; ultrahigh-voltage AC transmission substations; switchgear; arresters; China
Subjects: Insulation and insulating coatings; Production facilities and engineering; Energy resources; Conference proceedings; Switchgear; General electrical engineering topics; Protection apparatus; Energy resources and fuels; Power transmission, distribution and supply; Transformers and reactors; Power cables; Substations
Today in all countries in the world that utilise electricity as an efficient source of light and energy, some form of a transmission and distribution system exists. Both systems carry electric current albeit at different voltages and they are connected to each other. They are part of the bulk transport and distribution system essentially delivering electrical energy, converted from primary energy sources, to the end users. The only clear separation between the two systems is based on the perception of their end use and functionality. Transmission systems provide the bulk transport paths for electrical energy from generation centres located close to the primary energy sources to the major load centres within a large geographical area, thus facilitating economic and efficient bulk power transfer. On the other hand distribution systems are concerned with the delivery of electrical energy to individual customers within a smaller geographical area. In this respect, a distribution system may have a number of delivery points to its major load centres, from one or more transmission systems and/or elements of a transmission system. The final structure of the system is dependent upon the magnitude and the pattern of demand within the geographical area. It is also usual for transmission systems to be interconnected to enable shared economic benefits and operational access to generating capacity.
The international standard covering insulation co-ordination is IEC 60071, which provides the definitions, principles and rules for insulation co-ordination for threephase systems having a highest voltage above 1 kV. The collective IEC 60071 document also provides an application guide along with a computational guide to insulation co-ordination and the modelling of electrical networks.
This chapter introduces the subject of gaseous insulation and provides information relating to the application of gaseous insulants to high-voltage systems. It examines atmospheric air and compressed gases and illustrates how, by linking available experimental test data from such sources with a knowledge of the 'effectiveness' of various practical gas-gap clearances, the designer can achieve reliable insulation design. The chapter also briefly discusses the need for extra high-voltage (EHV) and ultra high-voltage (UHV) test areas or laboratories. Evidence is presented of how laboratory studies, on representative insulation systems and electrode arrangements, provide the designer with choices relating to electrical stresses, clearance levels, service performance and testing procedures. Gas-insulated substations (GIS) using sulphur hexafluoride (SF6) gaseous insulation have been used in transmission systems worldwide for more than 45 years. The service reliability of this class of equipment is of paramount importance. In addition, the chapter presents a large amount of experimental breakdown information on SF6 and briefly reviews the application of field computation strategies in support of GIS and other equipment designs. Several of the major factors influencing the insulation design and in-service behaviour and reliability of SF6 gaseous and epoxy resin support insulations, as used in GIS equipment, will also be considered.
Power electronic devices and systems are becoming an increasingly common feature of power systems. Over the years many power electronic products have been developed and can be used to great effect in the alternating current (AC) transmission and distribution system. HVDC is in many respects the ultimate power electronics-based FACTS device which can act within an existing AC network or even replace parts of it. In evaluating the usefulness of HVDC it is important to remember the controllability which it adds to the power system in addition to its bulk power transfer capabilities. HVDC often provides the best of both worlds - support from a neighbouring network without the problems of synchronous interconnection.
Renewable energy will be part of the energy mix for the UK since the government and all political parties have committed themselves to the Kyoto agreement. Renewable energy is from diverse sources and can be captured at a variety of scales - micro to multi-megawatt. Large renewable energy farms could connect to the National Grid; however, most generation schemes will connect at lower voltage levels. The UK best renewable energy resources tend not to be co-located with load, so this will require new distribution and transmission networks. Renewable energy technology has some technical differences to conventional generation, and the volume and cost of connections necessary to achieve the government's targets for renewable generation are driving innovation into electricity networks. A spin-off effect is to evolve the electricity network into a more costeffective, actively managed system with lower capital costs and higher utilisation rates, without compromising safety or reliability.
Almost all power systems contain a mix of overhead line and cable. The balance between the two can vary dramatically. A number of factors govern the choice between line and cable: cost, route availability, environmental aspects and electrical parameters. In terms of initial cost including installation, high-voltage cable systems are almost always more costly than overhead line systems for the same duty. Naturally, therefore, the designer's first choice is overhead line. The cable alternative is selected only when there is an overriding reason to do so.
This chapter first considers the basic principles of current interruption and the influence of the type of fault produced by the connected network. The electrical characteristics of circuit-breakers are described. The formation of an electric arc during the current interruption process is indicated and the significance of the manner in which the arc is controlled and then extinguished is explained. Various means for achieving such control and with different arcing media are considered. Some basic performance limiting factors are discussed. The chapter concludes by discussing some evolving trends in response to various modern requirements such as environmental issues.
This chapter describes the design, development and operation of SF6 switchgear [1-16]. It also describes how the development of generation and transmission has influenced switchgear evolution (see Appendices A and B). Factors that have contributed to the simplicity of design and increased the reliability of SF6 switchgear are addressed and the important features of various manufacturers, designs in first-, second- and third-generation interrupters and improvements in circuit breaker performance are highlighted. It also addresses issues associated with installation and on-site operations and monitoring.
Switchgear is a term used to refer to combinations of switching devices and their interconnection with associated control, measurement and protection equipment. It allows the interconnection of various parts of the electrical network by means of transformers, overhead lines or cables to allow control of the flow of electricity within that network from power station to customer. Switchgear is also designed to be able to safely interrupt any faults that might occur in any part of the network to protect the network itself, associated equipment and operational personnel. It also provides, by means of disconnectors, facilities for isolating sections of the network and, with the provision of earthing switches, to allow the safe application of devices to ensure that the isolated sections of the network are earthed and made safe for maintenance activities or possible fault repair.
This chapter reviews the circuit-breaker designs which are type tested to IEC 62271-200 and IEC 62271-100 for use on distribution voltages up to 52 kV. The design and service experience of different types of commercially available circuit-breakers are considered. The chapter also discusses some special switching duties and focuses on aspects which are necessary for the selection of circuit-breakers for various duties: for example for switching capacitor banks, capacitive and inductive currents, generators, reactors and synchronised switching of transformers with reactors on the secondary side.
The effective management of assets to ensure that the user obtains the optimum life for the plant is becoming more vital as electricity distribution systems are worked harder. Systems and equipment need to be reliable. The life management of plant is concerned with the life of the plant from preconception to final dismantlement (and recycling). In that journey the plant may encounter harsh operating and/or environmental conditions, which may test the limits of the original design. Figure 11.1 shows pictorially the whole operating lifecycle of plant from inception, design, test, commissioning and operating life to refurbishment. During the useful operating life, maintenance testing and monitoring perform an essential element for the user to decide whether refurbishment is an economic option to extend the life of the plant.
A bushing is a device for carrying one or more high-voltage conductors through an earthed barrier such as a wall or a metal tank. It must provide electrical insulation for the rated voltage and for service overvoltages and also serve as mechanical support for the conductor and external connections. The requirements for bushings are specified in IEC 60137: 2008.
Energy, and particularly electrical energy, is a commodity that mankind in general tends to take for granted. We switch on a light or our computer and expect an immediate power flow to energise it. Yet, the steady development and rapid progress that has been made in the transmission and distribution of electrical energy during the past 120 years or more may not have been possible but for the capability of linking generators, transmission lines, the secondary distribution systems operating at a variety of loads, with each at its optimum voltage. This linking of systems at different voltages has relied upon a simple, convenient and reliable device - the power transformer. The unique ability of the transformer to adapt the voltage to the individual requirements of the different parts of the system is derived from the simple fact that it is possible to couple primary and secondary windings of the transformer in such a way that their turns ratio will determine very closely their voltage ratio as well as the inverse of their current ratio, resulting in the output and input volt-amperes and the output and input energies being approximately equal. Coverage in this chapter includes a little of the theory of transformers, the major components involved in their manufacture, and the many different applications of the transformer technology in that are in everyday use.
A transformer is usually considered to be a non-mechanical apparatus for changing the voltage of an alternating current (AC) supply. Transformers are built in a very wide range of sizes from small devices used inside consumer products to very large devices used to connect generators to the national transmission network. This chapter will focus on medium and large transformers. In this context these are considered to be any transformers which are so large or else so complex as not to be manufactured in bulk. This generally includes all transformers rated at more than 2,500 kVA and all transformers which do not include a winding intended for connection to the low-voltage distribution network, but practice varies between countries and between users.
The measurements of voltage and current in high-voltage tests are difficult, because the amplitudes are high and they cannot be measured directly with conventional measuring and recording systems. Furthermore, not only the peak value but also the shape of the signal, particularly at impulse voltage and current, should be measured and evaluated and this requires an adequate recording system.
Tests are generally necessary to demonstrate that the equipment under test fulfils the specified requirements and quality standards. The tests may have different purposes, a type test as check and quality assurance for the design of the equipment and a routine test as check and quality assurance for the manufacturing processes. Further tests could be an acceptance test in the factory or on site to demonstrate the quality of the components under test or to check the integrity of the components after transportation and installation. With the introduction of new technologies or the use of higher voltages so-called prequalification test could be required from the customer in order to check the quality of a complete system under on-site conditions but with higher stresses. Such a prequalification test allows the change of the design and adaption of the system as consequences of the test results, but the test takes time in the range of one year or more. An example is the use of extruded cables up to 250 kV for DC transmission systems. Above all a routine test should show that the equipment is able to withstand the test conditions, which are selected according to the stress during the whole time period of service. That means the test stress should be high enough concerning the sensitivity but low enough to prevent an initiation of undetected defects during the test procedure, which may lead to damage after a certain time of service. Therefore, the test requirements are based on experiences concerning the stress and the behaviour of the tested material during normal operation conditions. Furthermore the following parameters have to be taken into account for the type and/or routine tests: regulations by law, requirements, recommendations, mutual agreements on technical specifications, economy. The following chapters are only related to high-voltage testing requirements and recommendations for type and routine tests, without any consideration of regulations by law, mutual agreement on technical specifications and economic factors.
The partial discharge measurement is a typical non-destructive test and it can be used to judge the insulation performance at the beginning of its service time taking into account the reduction of the performance during the service time by the ageing whereby the ageing depends on numerous parameters like electrical stress, thermal stress and mechanical stress. Depending on the kind of insulating material different limits for the allowed partial discharge value at a given stress are defined in the relevant recommendations. In particular for solid insulation where a complete breakdown damages seriously the test object the partial discharge measurement is a tool for the quality assessment.
The introduction of digital recording system in the high-voltage measuring technique has a great influence on the measuring technique and the evaluation procedures. The formerly used analogue recording instruments were developed for high-voltage measurements and in particular for impulse measurements under very noisy conditions or in other words for high electromagnetic interferences. One of the best measures against the electromagnetic interference was the high signal-to-noise ratio, reached by a high signal voltage level up to 1,500 V for impulse voltage measurements. This high signal level leads also to a high deflection level of about 100 V/cm and requires no amplifier within the analogue oscilloscope.
Professor Norman Allen was the author of the original chapter entitled 'Fundamental Aspects of Air Breakdown' in editions 1 and 2 of High Voltage Engineering and Testing and the accompanying lectures for the IET International Vacation School series. The current author is pleased to acknowledge and incorporate much of his original work including 'Mechanisms of Air Breakdown'. Atmospheric air remains as the main insulant on electricity transmission and distribution systems even though transmission voltages have increased to over 1.2 MV AC and 800 kV DC [þve, -ve]. Such ultra high voltage (UHV) systems may be prone to more lightning strikes and system overvoltages than ever before. Some transmission systems are being developed that will operate at higher altitudes and in higher humidity conditions, under conditions not yet known, or adequately covered by IEC Standards. The electrical stresses that may lead to insulation failure are from two principal sources: lightning surge and switching surge. Direct natural lightning strikes, and also the overvoltages that may be induced from natural lightning, produce the conditions that build up to the breakdown of atmospheric air. Switching operations on high voltage transmission systems can also cause overvoltages (switching surges). These surges can also cause failure of the air insulation.
Transformers and their component parts are critical to the reliable and uninterrupted functioning of all electric power systems. In order to increase availability of critical circuits and to optimise operational management, condition monitoring of power transformers is not only useful, but is fast becoming essential, since utilities are constantly facing the need to reduce costs that are associated with the operation and maintenance of the installed equipment. Condition monitoring equipment that is used to extend the life of a transformer or to prevent catastrophic failure could pay for itself many times over in the lifetime of the apparatus to which it is fitted. Nevertheless, condition monitoring does not come free of charge, and utilities have tight budgets that have to be met. To determine best value for money, life-cycle costing must be applied to each chosen application. The life cost comes from the summation of the installation, the maintenance, the repair and the lifetime operational costs of the equipment. Several options of monitoring equipment covering a wide spectrum of costs are readily available and others are under development. Monitoring equipment that looks at one or two specific parameters is readily available in the marketplace at modest cost. At the other end of the cost spectrum, not only can many parameters be monitored, but an expert system or artificial intelligence system that is capable of generating an estimate of the overall plant condition may be used to interrogate the data and draw conclusions and recommendations. There are several such integrated systems on the market, such as the MS 3000 supplied by Alstom Grid.
This chapter reviews a range of monitoring systems that are currently available and the steps taken to provide the coordinated approach by developing an integrated system to present the 'complete substation picture'.
The present chapter provides examples of some future monitoring technologies, based upon optical fibre sensors and chromatic techniques, which are being researched and evaluated for extending intelligent monitoring capabilities in a traceable manner.
This chapter has discussed the development of smart grids and enhanced transmission and distribution networks of the future, anticipated to be in place within two decades has provided details of additional technical, environmental and economic resource materials. It has also looked carefully into several aspects of renewable energy, wind farms, nuclear energy prospects post the Fukushima nuclear accident in March 2011, some worrying cyber issues, future trends and concerns relating to ever-increasing energy consumer bills, energy subsidies being paid for by UK customers.
