Embedded Generation
The use of combined heat and power (CHP) plants and renewable energy sources reduces the amount of greenhouse gases released into the atmosphere and helps to alleviate the consequent climate change. The policies of many governments suggest that the proportion of electrical energy produced by these sources will increase dramatically over the next two decades. Unlike traditional generating units, these new types of power plant are usually 'embedded' in the distribution system or 'dispersed' around the network. As a result, conventional design and operating practices are no longer applicable; for example, power protection principles have to be revised and complex economic questions need to be resolved. This book, intended for both students and practising engineers, addresses all the issues pertinent to the implementation of embedded generation. Much of the material was originally developed for the UMIST MSc/CPD course in Electrical Power Engineering so there is a strong tutorial element. However, since this subject is evolving very rapidly, the authors also discuss the technical and commercial consequences of the very high penetration of embedded generation that are to be expected in the years ahead.
Inspec keywords: electric power generation; cogeneration
Other keywords: Electrical Power Engineering; embedded generation; combined heat and power plants
Subjects: Thermal power stations and plants
Modern electrical power systems have been developed, over the last 50 years. Large central generators feed electrical power up through generator transformers to a high voltage interconnected transmission network. The transmission system is used to transport the power, sometimes over considerable distances, which is then extracted from the transmission network and passed down through a series of distribution transformers to final circuits for delivery to the customers. However, recently there has been a considerable revival in interest in connecting generation to the distribution network and this has come to be known as embedded or dispersed generation. The term 'embedded generation' comes from the concept of generation embedded in the distribution network while 'dispersed generation' is used to distinguish it from central generation. The two terms can be considered to be synonymous and interchangeable.
There are many varied types of generating plant connected to electrical distribution networks ranging from well established equipment such as Combined Heat and Power (CHP) units and internal combustion reciprocating engines to more recent types of generation such as wind farms and photovoltaics. In the future some of the many emerging technologies, such as fuel cells, flywheel storage, micro-CHP using small gas turbines or Stirling engines, may become commercially significant. Understanding the interaction of embedded generation with the power system requires an appreciation of the technology of the prime movers, the characteristics of the energy sources and also the conditions under which the embedded generation plant is operated. In deregulated electricity supply systems it is also important to recognise that the owners of embedded generation plant (who will not be the distribution utility in many cases) will respond to pricing and other commercial signals to determine whether to invest in such plant and then how to operate it. This chapter is intended to provide a brief introduction to several of the important embedded generation technologies and to emphasise the multidisciplinary aspects of embedded generation.
This chapter will first reviews the purposes of power flow computations, fault level calculations, stability studies and electromagnetic transient analysis. Then, the principles behind each of these types of studies are explained. Examples of applications to realistic embedded generation installation are presented and the data requirements will be discussed. This chapter should provide the user with the knowledge and understanding that are required to use confidently the software packages designed to perform these system studies.
This chapter shows an embedded generator connected to a radial distribution circuit. The circuit is connected to the transmission system through one or more tap-changing transformers and has loads connected to it at various locations. This arrangement can easily be studied, in as much detail as required (provided data are available), using the computer based methods discussed. However, for many initial, but only approximate, calculations the network may be represented by a single equivalent impedance connected to an infinite busbar. Some sections in the book showed that, in the per unit system, the magnitude of this impedance is simply the reciprocal of the short-circuit level (without the generator connected), although the XIR ratio of the network must also be known. The terms short-circuit level and fault level are synonymous. The simple representation is particularly useful as the data required are usually readily available from the distribution utility and it allows a simple impedance to be added in series to the equivalent circuits used to describe the generators. It is used here to help illustrate various important principles.
The main potential impacts of embedded generation on network power quality are voltage flicker and harmonic voltage distortion. If large numbers of single-phase generators are connected in the future then voltage unbalance may also become significant. The effect of embedded generation may be to improve network power quality by raising the short-circuit level, or cause it to deteriorate by introducing distorted current. The use of embedded generation also increases the amount of relatively large plant on the distribution network and so increases the complexity required in any modelling studies. The conventional techniques for assessing power quality on passive distribution networks are generally adequate for embedded generation, but there is a move to develop international standards and type testing procedures so that embedded generation plant can be assessed for its impact on any distribution network rapidly and with minimum cost. Embedded generation may also suffer from existing poor distribution network power quality, and this aspect needs careful attention, especially for renewable energy schemes in rural areas. It has been found from experience that, at times in rural areas (before the connection of embedded generation), existing distribution network harmonic voltage distortion, voltage flicker and voltage unbalance can all exceed the desired values.
Most protection systems for distribution networks assume power flows from the grid supply point to the downstream low-voltage network. This approach simplifies the problems associated with controlling the voltages at the loads and helps maintain the quality of supply. Protection is normally based on overcurrent relays with settings selected to ensure discrimination between upstream and downstream relays. A fault on a downstream feeder must be cleared by the relay at the source end of the feeder. It must not result in the operation of any of the relays on an upstream feeder unless the downstream relay fails to clear the fault. If this occurs, relays on the immediately adjacent upstream feeders should operate and clear the fault. This will result in a blackout to a part of the network that should not have been affected by the fault.
This chapter addresses the issues relating to the reliability assessment of distribution systems in which local generation is embedded. Superficially it could be expected that conventional approaches used for existing sys tems would suffice. However, this would be an erroneous conclusion for reasons discussed later in this chapter, although the required techniques can be based on developments of existing approaches. To appreciate all the issues involved it is therefore desirable to review existing approaches and their limitations regarding embedded generation before developing approaches and techniques to deal with these systems.
Although a number of technical challenges related to the integration of embedded generation (EG) and power systems have yet to be addressed, this chapter discusses the importance of the integration of EG within a consistent commercial framework. It has been recognised that the present arrangements and mechanisms for pricing of distribution services do not treat EG adequately or systematically. The commercial issues related to charges for losses, connection and use of the distribution and trans mission networks have been the subject of negotiations between local distribution utilities and EG companies, where many different arguments have been used in different contexts to address essentially very similar questions. The technical and network pricing frameworks which are chosen are of considerable consequence to the commercial performance of both network and generation owners and developers. Distribution network operation and planning practices, together with adopted pricing policies, define the level of access available to participants in the electricity market place and therefore make a considerable impact on the amount of generation that can be accommodated. In other words, as adopted technical and commercial arrangements actually dictate the degree of openness and accessibility of distribution networks, it is vitally import ant to establish a coherent and consistent set of rules on both technical and commercial fronts.
Embedded generation offers considerable environmental benefits and so its continued development can be considered to be beneficial for society as a whole. CHP has the obvious advantage of increasing the overall efficiency with which fossil fuels are used, while the use of renewable energy sources is clearly desirable as a way of reducing gaseous emissions from conventional generating plant. It is the policy of most European governments to encourage the development of CHP and the exploitation of the new renewable energy sources. So far these technologies, generally, have been implemented in reasonably large units (>100 kW) and in penetrations that, although starting to become significant, have not yet led to major changes in existing power systems. A number of emerging technologies (e.g. fuel cells, advanced Stirling engines and micro-turbines) have the potential for cost-effective domestic CHP, while it is suggested by some that photovoltaics integrated into the fabric of houses will become economically attractive in the future. If these developments do take place, they would result in extremely large numbers of very small (<5kW) embedded generators connected very widely through the distribution network. Thus, it is realistic to contemplate the probability of a continued increase in the capacity of embedded generation of the types currently in service (e.g. industrial/commercial CHP, wind farms, biomass plants) and the possibility of a significant increase in very small domestic scale units.
