Protection of Electricity Distribution Networks (4th Edition)
High quality electrical service is key to power systems around the world, particularly in utilities and industrial facilities. Both voltage and grid frequency must be kept within tight limits to maintain functioning of critical infrastructure. The growing use of renewable power of intermittent character is adding to that challenge. One of the keys to achieving quality service is the protection system, which needs to be reliable, fast and with a good cost/benefit ratio. It consists of various components, including relays, circuit breakers and fuses. An understanding of the different behaviours of these components in the face of malfunctions, and of countermeasures, is needed. This book has established itself as a classic work in its field, allowing the reader to easily follow the ideas explored. It provides an overview of most aspects of electrical protections, with emphasis on distribution systems; but protection of generation and transmission systems are also addressed. For this 4th edition, new topics have been added, such as protection of renewable power plants and transient stability analysis. It offers a thorough revision of the material, particularly the numerical type of relays, protective functions, control, measurement, communications and oscillography features. Most chapters have illustrative examples using MATLAB or PSAT (Power System Analysis Tools). This work remains essential reading for researchers, utility engineers, design, maintenance and consulting engineers as well as for instructors and senior students.
Inspec keywords: distributed power generation; power transmission protection; power engineering computing; power distribution protection; electric fuses; smart power grids; power system control; relay protection
Other keywords: distribution networks; distributed power generation; smart power grids; power system control; relay protection; power transmission protection; power engineering computing; overcurrent protection; power distribution protection; electric fuses
Subjects: Distributed power generation; Power engineering computing; Power system control; Power transmission, distribution and supply; Power system protection; General and management topics; General electrical engineering topics; Distribution networks; Switchgear
- Book DOI: 10.1049/PBPO180E
- Chapter DOI: 10.1049/PBPO180E
- ISBN: 9781839532702
- e-ISBN: 9781839532719
- Page count: 488
- Format: PDF
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Front Matter
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1 Introduction
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With the increasing dependence on electricity supplies, in both developing and developed countries, the need to achieve an acceptable level of reliability, quality and safety at an economic price becomes even more important to customers. A further requirement is the safety of the electricity supply. A priority of any supply system is that it has been well designed and properly maintained in order to limit the number of faults that might occur.
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2 Calculation of short circuit currents
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The current that flows through an element of a power system is a parameter that can be used to detect faults, given the large increase in current flow when a short circuit occurs. For this reason, a review of the concepts and procedures for calculating fault currents will be made in this chapter, together with some calculations illustrating the methods used. Although the use of these short circuit calculations in relation to protection settings will be considered in detail, it is important to bear in mind that these calculations are also required for other applications, e.g. calculating the substation earthing grid, the selection of conductor sizes and for the specifications of equipment such as power circuit breakers.
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3 Classification and function of relays
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A protection relay is a device which senses any change in the signal it receives, usually from a current and/or voltage source. If the magnitude of the incoming signal is outside a preset value, the relay will carry out a specific operation, generally to close or open electrical contacts to initiate some further operation, for example, the tripping of a circuit breaker.
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4 Current and voltage transformers
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Current or voltage instrument transformers are necessary to isolate the protection, control and measurement equipment from the high voltages of a power system, and for supplying the equipment with the appropriate values of current and voltage -generally these are 1 or 5 A for the current coils and 120 V for the voltage coils. The behaviour of current and voltage transformers (VTs) during and after the occurrence of a fault is critical in electrical protection since errors in the signal from a transformer can cause maloperation of the relays. In addition, factors such as the transient period and saturation must be taken into account when selecting the appropriate transformer. When only voltage or current magnitudes are required to operate a relay then the relative direction of the current flow in the transformer windings is not important. However, the polarity must be kept in mind when the relays compare the sum or difference of the currents.
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5 Overcurrent protection
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Very high current levels in electrical power systems are usually caused by faults on the system. These currents can be used to determine the presence of faults and operate protection devices, which can vary in design depending on the complexity and accuracy required. Among the more common types of protection are thermomagnetic switches, moulded-case circuit breakers, fuses and overcurrent relays. The first two types have simple operating arrangements and are principally used in the protection of low-voltage (LV) equipment. Fuses are also often used at LVs, especially for protecting lines and distribution transformers. Overcurrent relays, which form the basis of this chapter, are the most common form of protection used to deal with excessive currents on power systems. They should not be installed purely as a means of protecting systems against overloads - which are associated with the thermal capacity of machines or lines - since overcurrent protection is primarily intended to operate only under fault conditions. However, the relay settings which are selected are often a compromise in order to cope with both overload and overcurrent conditions.
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6 Fuses, reclosers and sectionalisers
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A wide variety of equipment is used to protect distribution networks. The particular type of protection used depends on the system element being protected and the system voltage level, and even though there are no specific standards for the overall protection of distribution networks, some general indication of how these systems work can be made.
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7 Directional overcurrent relays
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Directional overcurrent protection is used when it is necessary to protect the system against fault currents which could circulate in both directions through a system element, and when bidirectional overcurrent protection could produce unnecessary disconnection of circuits. This can happen in ring or mesh-type systems and in systems with a number of infeed points.
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8 Differential protection
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Differential protection functions when the vector difference of two or more similar electrical magnitudes exceeds a predetermined value. Almost any type of relay can function as differential protection - it is not so much the construction of the relay which is important but rather its method of connection in the circuit. The majority of the applications of differential relays are of the current-differential type, but they can also be of the voltage-differential type, operating on the same principle as the current relays; the difference lies in the fact that the operating signal is derived from a voltage across a shunt resistance.
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9 Distance protection
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It is essential that any faults on a power system circuit are cleared quickly, otherwise they could result in the disconnection of customers, loss of stability in the system and damage to equipment. Distance protection meets the requirements of reliability and speed needed to protect these circuits and, for these reasons, is extensively used on power system networks. Distance protection is a non-unit type of protection and has the ability to discriminate between faults occurring in different parts of the system, depending on the impedance measured. Essentially, this involves comparing the fault current, as seen by the relay, against the voltage at the relay location to determine the impedance down the line to the fault. For the system shown in Figure 9.1, a relay located at A uses the line current and the line voltage to evaluate Z = V/I. The value of the impedance Z for a fault at F1 would be ZAF1, and (ZAB + ZBF2) for a fault at F2.The main advantage of using a distance relay is that its zone of protection depends on the impedance of the protected line which is a constant virtually independent of the magnitudes of the voltage and current. Thus, the distance relay has a fixed reach, in contrast to overcurrent units where the reach varies depending on system conditions.
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10 Protection of low-voltage systems
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With the increase in size of industrial plant electrical systems, and the high short-circuit levels encountered on electricity power systems, it is essential that the electrical protection arrangements in any industrial installation are correctly designed and have the appropriate settings applied to ensure the correct functioning of the plant and continuity of supply within the installation. The importance of maintaining continuity of supply to industrial installations cannot be overemphasised, and in this respect, the interconnectors to the public supply system play a vital role. It is crucial that correct coordination is maintained between the protection on the main industrial supply infeeds and the power system supply feeders.
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11 Industrial plant load shedding
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All electrical power systems that contain generation are liable to be subjected to a variety of abnormal operating conditions such as network faults, loss of some or all generation, the tripping of circuits within the system and other disturbances that can result in the reduction in the generation capacity available to the system.
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12 Protection schemes and substation design diagrams
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This chapter considers the combination of relays required to protect various items of power system equipment, plus a brief reference to the diagrams that are part of substation design work. A general knowledge of these diagrams is important in understanding the background to relay applications.
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13 Communication networks for power systems automation
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Protection functionalities have maintained their theoretical foundations because the performance of power systems and their elements is basically the same now as it was when they were first developed. However, the operation of protections has been greatly improved in time response due to the availability of much faster relays based on the numerical technology as well as the wonderful development of communication capabilities that have been attained in recent years. Communications have benefited protection systems remarkably not only by the increase in operation speed but also in the simplification of equipment and hardware. All these improvements have rendered more reliability and better cost/benefit ratio of protection systems. Therefore, reference to modern developments on communications and in particular to the IEC 61850 Standard becomes a must, considering the huge application not only for relay applications but also to all aspects of power system automation handling. With the advent of microprocessor-based multifunction intelligent electronic devices (IEDs), more functionalities into fewer devices were possible, resulting in simpler designs with reduced wiring. In addition, owing to communication capabilities of the IEDs, more information could be accessed remotely; translating into fewer visits to the substation. Microprocessor-based protection solutions have been successful because they offered substantial cost savings while fitting very well into pre-existing frameworks of relay application. A modern IED replaces an entire panel of electromechanical relays with external wiring intact, and internal DC wiring replaced by integrated relay logic.
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14 Installation, testing and maintenance of protection systems
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Although the aim of this book is to provide the basis to guarantee a suitable relay setting procedure in distribution networks, it is felt that some reference should be made to the installation, testing and maintenance of protection systems. No matter how well the relay applications are carried out, a protection scheme is worthless if its performance cannot be guaranteed. It is important to emphasise that a protection scheme covers not only the relays but also the current transformers (CTs) and voltage transformers (VTs) that feed them, and the circuit breakers that isolate the fault when a disturbance occurs. Testing the protection schemes is an essential duty during a project commissioning since most of the failures in service are caused by human factors. This includes wiring and relay programming, which involves protection settings and logic development, and equipment connections, among others. A small percentage of failures during service is due to equipment failures.
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15 Protection of distributed generation systems
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Distributed generation (DG) is located closer to the consumer than big generating plants and does not have any requirements from the transmission networks. In essence, DG is delivering power directly to the connected loads, thus eliminating transmission costs. The arrangement of a distribution system for power delivery has several advantages, including an efficient power dispatch, which is operated by a relatively small staff. Nevertheless, their contribution and effect of system stability during a fault must be studied and considered when the protection schemes are defined. In this chapter, general consideration regarding the protection schemes and suggestion for most typical configurations will be presented.
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16 Impact of stability conditions on protective relaying
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Power system stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact. Power system engineers mainly encountered instability problems related to generator rotor angle stability, with which the whole system cannot keep synchronism after a disturbance. However, with the continuous expansion of modern power systems under more stressed operation conditions, different forms of system instability have emerged. Rotor angle stability refers to the ability of synchronous machines of an inter-connected power system to remain in synchronism after being subjected to a disturbance. The mechanical and electromagnetic torques acting on the rotating masses of each generator balance each other. There is synchronism when the phase angle differences between the internal emf's of the various machines are constant. Following a disturbance, there is an imbalance between the two torques and the rotor speed varies. The rotor angle stability deals with the ability to keep/regain synchronism after being subject to a disturbance.
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References
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Appendix: Solutions to exercises
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Back Matter
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