Throughout the world there is concern over the impact of energy use on the environment (particularly CO2 emissions) and also over the security of fossil fuel supplies. Consequently, governments and energy planners are actively encouraging alternative and cleaner forms of energy production such as renewables (e.g. wind, solar, biomass) and combined heat and power (CHP). The economics and locations of sustainable energy sources have meant that many of these new generators are connected into distribution networks. It is recognized that the information flow and control of distribution networks is inadequate for these future low-carbon electricity supply systems. The future distribution network will change its operation from passive to active, and the distributed generators will be controlled to support the operation of the power system. In many countries this transformation of electricity supply is managed through energy markets and privately owned, regulated transmission and distribution systems. This book discusses the connection of generation to distribution networks and then moves on to consider how sustainable generation can be fully integrated into the operation of the power system. Both technical and economic aspects are addressed. It is written for later-year undergraduate and postgraduate students studying courses on energy. The book has four tutorial chapters (with examples and questions) to provide fundamental material for those without a strong electrical engineering background.
Inspec keywords: power markets; distribution networks; cogeneration; renewable energy sources
Other keywords: distribution networks; information flow; energy production; energy use; fossil fuel supplies; power system operation; low-carbon electricity supply systems; distributed generation; distribution systems; distributed generators; transmission systems; renewable energy; sustainable generation; energy markets; CO2 emissions; combined heat and power; sustainable energy sources
Subjects: Power system management, operation and economics; Energy resources; Thermal power stations and plants; Distribution networks; Distributed power generation
In the early days of electricity supply, each town or city would have its own small generating station supplying local loads. However, modern electrical power systems have been developed, over the past 70 years. Large central generators of ratings up to 1000 MW and voltages of around 25 kV feed electrical power up through generator transformers to a high voltage interconnected transmission network operating at up to 400 kV in most of Europe and 750 kV in North America and China. The transmission system is used to transport the electrical power, sometimes over considerable distances, and it is then extracted and passed down through a series of distribution transformers to final circuits for delivery to the customers. The transmission and distribution circuits are mainly passive with control of the system provided by a limited number of large central generators . From around 1990, there has been a revival of interest in connecting generation to the distribution network and this has come to be known as distributed generation (DG) or the use of distributed energy resources (DER).
With the increasing efforts worldwide to de-carbonise energy supply, a wide variety of generating plant types is being connected to electrical distribution networks. Examples include the well-established technologies of combined heat and power (CHP), wind turbines and photovoltaic systems. In addition there are many newer technologies, such as fuel cells, solar thermal, micro-CHP and the marine renewable technologies as well as flywheel and flow battery storage that are at various stages of demonstrating their commercial viability. In deregulated electricity supply systems the owners of the distributed generation) plant (who will not be the distribution utility in most cases) will respond to pricing and other commercial signals to determine whether to invest in such plant and then how to operate it. As distributed generation displaces large central generating plant, it will increasingly take over the ancillary services (e.g. voltage and frequency control) that are necessary for the operation of the power system and so distributed generation will develop from being a source only of energy to being an integral contributor to the supply of electrical power.
The connection of distributed generators to the network requires understanding the operation and control of different types of generating plant, and often needs studies to evaluate the performance of the power system with the new generation, under both normal and abnormal operating conditions. The types of generators used for distributed generation depend on their application and energy source.
This chapter discusses the fault current and electrical protection scheme in distributed generators. Electrical faults, caused by the breakdown of insulation, are inevitable in any electrical power system. Faults may be caused by mechanical damage to the equipment or created by the degradation of insulation over time. Electrical protection is then used to operate circuit breakers that isolate the faulty equipment rapidly. Distributed generators must be protected against internal electrical faults, with fault current flowing from the network, and conversely the distribution network must be protected against fault current from the distributed generators. Islanded operation of smaller distributed generators is not generally permitted and so this condition is detected by protection systems designed to detect islanding or loss of mains and the generator then disconnected. Finally, the addition of distributed generation to a distribution network may alter the flows of network fault current in subtle ways and so lead to maloperation of conventionally designed distribution network protection systems.
In this context, this chapter describes the concepts and techniques needed for the impacts of distributed generation on the electricity system development to be determined, covering two key areas the: (1) ability of distributed generation to displace the capacity of incumbent conventional generation and contribute to generation capacity adequacy (2) ability of distributed generation to substitute for distribution network capacity and hence contribute to delivery of network security.
For distributed generation to compete successfully with central generation in a competitive environment, network pricing arrangements are critically important. Distributed generation, depending on the technology, operating pattern and exact connection point, may reduce the need for upstream network reinforcements and when located closer to the load, can reduce losses and potentially contribute to increasing local reliability of supply. The vehicle for realising this additional value is efficient network pricing. In general, in a deregulated electrical power system, network prices should send signals to users reflecting the benefits they bring and costs they impose on network operation and/or development. Efficient pricing distinguishes between different locations and between different times of use, thus avoiding cross-subsidies and facilitating a level-playing field between distributed and central generation. For distribution network with distributed generation, it is essential to take account of the contribution that the generation makes to network security, i.e. the ability of distributed generation to substitute for network capacity. Otherwise the value of distributed generation in this regard is not recognised and hence cannot be rewarded.
Distributed generation will continue to increase in importance as many countries move towards de-carbonising their energy systems. The location and size of many renewable energy sources and CHP plants require that they are connected to distribution networks and that effective use is made of existing circuits. This can only be done cost-effectively by integrating the operation of the distributed generation closely into that of the power system and changing the operation of distribution networks from passive to active.
In all public electricity supply systems, the mains voltage alternates at 50 or 60 cycles per second (Hz) and when a load is connected it draws an alternating current. An alternating current or voltage changes its polarity periodically to give a wave like shape on an oscilloscope trace. Sine, square and even saw-tooth waves are used in different electronic circuits, but this chapter considers only the sine wave, which is the shape of the AC mains. A generator produces a three-phase alternating voltage and this voltage is increased (stepped-up) for long distance transmission. The transmission voltages are then stepped-down and the power distributed to loads. Low voltage final distribution circuits typically use four wires (in Europe, three phase wires each at 230 V and a neutral wire that provides the zero volt reference). Single-phase loads (e.g. houses) are connected across two wires (to one phase wire and the neutral wire) and three-phase loads (industry and commercial buildings) are connected to all four wires.
AC electrical machines can act as generators to convert mechanical energy into electrical energy or motors to convert electrical energy into mechanical energy. This chapter discusses synchronous machines, and induction machines. Example problems with solutions as well as actual problems in AC machines were also provided in the end of this chapter.
Power electronic converters are presently used to interface many forms of renewable generation and energy storage systems to distribution networks, while the use of power electronics is likely to increase in the future as this technology is also an important element of SmartGrids and active distribution networks. The development of high power electronic converters benefits from recent rapid advances in power semiconductor switching devices and in the progress being made in the design and control of variable speed drives for large motors.One obvious application of a power electronic converter is to invert the DC generated from some energy sources (e.g. photovoltaics, fuel cells or batteries) to 50/60 Hz AC. Converters may also be used to de-couple a rotating generator and prime mover from the network and so allow it to operate at its most effective speed over a range of input powers. This is one of the arguments put forward in favour of the use of variable speed wind turbines but is also now being proposed for some small hydro generation. Another advantage of variable speed operation is the reduction in mechanical loads possible by making use of the flywheel effect to store energy during transient changes in input or output power. However, large power electronic converters do have a number of disadvantages including significant capital cost and complexity, electrical losses (which may include a considerable element independent of output power) and the possibility of injecting harmonic currents into the network.
The power system converts mechanical energy into electrical energy using generators, then transmits the electricity over long distances and finally distributes it to domestic, industrial and commercial loads. Generation is at a low voltage (400 V to around 25 kV) and then the voltage is stepped up to transmission voltage levels (e.g. 765 kV, 400 kV, 275 kV) and finally stepped down to distribution voltages (e.g. 13.8 kV, 11 kV or 400 V). Each of these conversion stages takes place at a substation with a number of different pieces of equipment to: (a) transform the system voltage (power transformers), (b) break the current during faults (circuit breakers), (c) isolate a section for maintenance (isolators) after breaking the current, (d) protect the circuit against lightning overvoltages (surge arresters) and (e) take voltage and current measurements (voltage transformers VT and current transformers CT). In addition to this primary plant, which carries the main current, secondary electronic equipment is used to monitor and control the power system as well as to detect faults (short-circuits) and control the circuit breakers. Practical AC power systems use three phases that are of the same magnitude and displaced 120° degrees electrical from each other. When the three phases are thus balanced, no current flows in the neutral and so at higher voltages only three phase conductors are used and the neutral wire is omitted. In order to represent the power system in control diagrams and reports, a single line representation is used; the three-phase lines are shown by a single line. Typical single line diagram symbols used for a balanced power system.
