T. Davies

3 – Current and voltage transformers for protection

Publisher Summary The current transformer is regarded as a device that reproduces a primary current at a reduced level. A current transformer designed for measuring purposes operates over a range of current up to a specific rated value, which usually corresponds to the circuit normal rating, and has specified errors at that value. On the other hand, a protection current transformer is required to operate over a range of current which is many times the circuit rating and is frequently subjected to conditions that are greatly exceeding those which it would be subjected to as a measuring current transformer. Under such conditions, the flux density corresponds to advanced saturation. Voltage transformers are generally protected by HRC fuses on the primary side and fuses or a miniature circuit-breaker on the secondary side. As they are designed to operate at a low flux density, their impedance is low and, therefore, a secondary side short-circuit produces a fault current of several times rated current. It is important that a voltage of the correct magnitude and phase angle is presented to directional earth-fault relays and the earth-fault elements of impedance relays. As an earth-fault can be any one of the three phases, it is not possible to derive a voltage in the conventional manner. The solution is to use the residual or broken delta connection.

9 – Motor protection

Publisher Summary Motor protection is essential to safeguard motors and their cables from damage caused by overheating. Most motor protection relays detect the conditions of overloading, stalling, and single-phasing that cause overheating. There has been a complete change-over from thermal to electronic and microprocessor-based relays for motor protection, ranging from simple overload relays to comprehensive relays, which not only detect all fault and abnormal conditions but can also report, to a remote location, the state of the motor while running. This chapter describes a large number of thermal motor protection relays that are also in use. Before discussing the protection, the chapter provides a short description of the operation of the squirrel-cage induction motor. One of the most important relays for the detection of abnormal conditions is the overload relay, which is applied to the protection of motors. Compared to the overcurrent relay, which under fault conditions is required to detect a current that is many times the normal current, the overload relay must be capable of accurately measuring the current that is only slightly greater than the nominal full-load current. The function of the overload relay is to prevent the overheating of equipment. The operating time of a typical motor overload relay is of the order of two minutes at twice full-load current.

5 – Time-graded overcurrent protection

Publisher Summary The principal electromechanical relay used for time-graded overcurrent protection is the inverse-time relay, which is an induction relay in which torque is proportional to I2. This relay has a range of current settings, usually 50 to 200%, of nominal current in 25% steps. The setting is generally selected by the position of a plug in a plugbridge that determines the number of active turns on the operating coil and therefore the current setting. The relay operating time can also be varied. At the maximum time setting, the disc has to travel through 180° before contact is made. By moving the disc reset position closer to the contact-making position, the operating time can be reduced. There is an adjuster, known as the time multiplier, with a calibrated scale of 0.1 to 1.0, which is used to set the disc reset position. In some cases, a 0.05 position is marked. This setting should never be used because of the small contact gap.

11 – Control circuits

Publisher Summary The DC circuits that are associated with protection are, in the main, tripping circuits, which are an essential link between the protection and the circuit-breaker. In an automatic control circuit, equipment is in continuous operation and any fault is discovered quickly. On the other hand, a tripping circuit may operate only once, and at a time where conditions are abnormal, and hence, there is no risk of failure. Therefore, reliability is the vital requirement, which means that the performance of the circuit must be completely predictable. Simplicity in design allows this objective to be achieved. Alternatively, the positive side of each trip circuit can be fused on the basis that any wiring fault only renders one trip circuit inoperative. This approach is particularly sound if used in conjunction with trip circuit supervision relays. Many protection relays have auxiliary contactors that are used either to increase the number of relay contacts or to reinforce the protection contact. Auxiliary contactors are usually attracted-armature relays and because of their snap action, the contact force produced is considerable.

10 – Generator protection

Publisher Summary The AC generator needs protection against a number of conditions, some of which require immediate disconnection and some may be allowed to continue for some time. The conditions that require immediate disconnection are connected with insulation failure, whereas the conditions that may be allowed to continue for some time are generally associated with unsatisfactory operating conditions. Of all the items of equipment which make up a power system, the generator is unique in that it is usually installed in an attended station and is therefore subject to more or less constant observation. Some of the unsatisfactory operating conditions could be dealt with by an operator, whereas if the generator were not attended, tripping would be the only course of action. To reduce the possibility of damage, earth-fault current is usually limited by earthing the generator neutral point via a resistor, reactor, or transformer.

6 – Unit protection

Publisher Summary Protection schemes that operate on the principle of discrimination by comparison are known as unit schemes as they protect only the unit with which they are associated and do not provide the backup protection that all discrimination by time schemes provide. Most unit schemes are based on the Merz-Price principle, according to which, if the current flowing into the protected unit is the same as the current leaving, then the fault is not in the protected unit and the protection should not trip. If there is a difference in either phase or magnitude between input and output, then the fault is in the unit and the protection should trip. The scheme depends on the relay being connected to the centre point of a balanced system. When the current in the two current transformers is not the same, the difference between the CT secondary current is passed through the relay. There will always be differences, however slight, in the magnetizing characteristic, which leads to instability in the scheme during fault conditions.

1 – Simple protection devices

Publisher Summary Protection devices are built with the aim of ensuring the maximum continuity of supply, which is achieved by determining the location of a fault and disconnecting the minimum amount of equipment necessary to clear it. When a fault occurs, a number of relays will detect it but only the relays directly associated with the faulty equipment are required to operate, which is achieved by discrimination. There are three methods of discrimination: time, comparison, and magnitude. Relays that discriminate by time, such as Inverse Definite Minimum Time Relays (IDMT) or impedance relays, protect the equipment with which they are associated and also act as a backup protection for other relays. The major disadvantage is that there is a delay in the removal of the fault which increases damage to the faulty equipment and increases the possibility of damage to healthy equipment carrying the fault current. Relays that discriminate by comparison, such as Differential Feeder Protection or Earth Fault Relays, protect only the equipment with which they are associated and are therefore known as unit protection. The basic principle of unit protection is that if the current entering and leaving the unit is the same then it is healthy, if there is a difference then the unit is faulty. Relays that discriminate by magnitude, such as High Set Overcorrect Protection, can only be used where there is a large change in fault current as is the case between the primary and secondary windings of a transformer. Discrimination by time is the basis for many simple protection devices, the time delay being in general inversely proportional to current level.

7 – Transformer protection

Publisher Summary Possibly the most important item of equipment in an industrial power system is the transformer. It ranges in size from large incoming units that deliver power at the distribution voltage to those for low voltage utilization and for lighting systems. As in all protection schemes, the cost of the transformer has to be related to the value of the equipment which it is protecting. However, while specifying a scheme, the economic effect of the loss of the unit and the cost to repair a major breakdown should also be taken into account. Because of its static nature, the power transformer can be regarded as a reliable unit. Nevertheless, there is a possibility of failure because of internal faults and because of subjection to stresses from external sources, which could cause the internal fault condition. Most power transformers are provided with tappings so that the overall transformation ratio may be varied to suit the voltage requirements of the system.

4 – Fault calculations

Publisher Summary It is necessary to know the fault conditions for predicting the performance of a protection scheme. This chapter focuses on the process of determining the fault level during a short-circuit, which requires the knowledge of impedance of various components of the power system and the ability to calculate current in every part of the system. For fault calculations, it is usually sufficient to take into consideration only reactance rather than resistance. Typical impedance values can be attributed to all components of a power system in the absence of definite information. Transformer impedances are usually easy to determine as the value is marked on the rating plate. The impedance of generators is usually of secondary importance as most distribution systems have a much higher in-feed and fault contribution from the public electricity-supply system.

8 – Feeder protection

Publisher Summary Feeder protection should maintain optimum speed of operation under fault conditions. In many circumstances, the requirement is met by the use of a unit scheme of differential protection for a particular section of feeder. Differential protection is principally of the longitudinal form in which measuring equipment situated at each end of a feeder section is interconnected by means of pilot wires to compare the magnitude and phase angle of the current entering and leaving the feeder. Transverse differential schemes, in which currents in two or more parallel feeders are compared at one line end only, are suitable only where certain restrictive system conditions are met and are therefore rarely used. Many schemes of differential protection for feeders are based on the Merz-Price system. In practice, pilot connections in the role of CT leads would present an excessive burden and so a further stage of current transformation is introduced; it is also desirable for the purpose of tripping to have a relay at each line end. Most of the protective systems that are in use at present are derived from either the circulating current scheme or the balanced voltage scheme. The circulating current scheme has a drawback in that the current circulates even under load conditions. As this current can cause interference on telecommunication circuits, the balanced voltage scheme is preferred. The pilot wires are cross-connected in the balanced voltage scheme. It means that as the voltages at the two ends are the same, no current will circulate under balanced conditions.