Power-supply System Protection
Power-supply System Protection
a system of measures that prevent and limit the development of breakdowns on power-transmission lines and at power substations. Its objective is to ensure a reliable supply of the proper quality of electric power to consumers. The vast majority of electric power is distributed over a power-supply system for ordinary use. The protection of such systems is of great importance in providing a normal supply of electricity for industry, agriculture, railway transportation, and other consumers, and it is continually being improved. All electrical installations, including independent electrical power sources and their small systems, have some degree of protection.
A power-supply system for general use must be protected from overloading, overvoltages, and short circuits, which are dangerous for the network; from damage to insulating and supporting structures; and from breaks in the wires. Dangerous phenomena also occur as a result of atmospheric disturbances (such as lightning strokes) and changes in the condition of the system itself, such as a breakdown of insulation or the deliberate disconnection of an unloaded transmission line. Faulty insulation may be due to aging of the material or to external causes. The supporting structures (towers, crosspieces, insulator fittings, and so on) break under the action of wind and ice deposits and are subject to corrosion. The wires may be overheated by the current, and they may break as a result of vibration. Breakdowns may be caused by incorrect operation of automatic devices in the system and by errors on the part of the operating personnel. With the extremely large scale of modern electrical systems, which consist of tens of thousands of kilometers of electric transmission lines at various voltages, as well as thousands of electrical substations, it is virtually impossible to avoid dangerous situations. Nevertheless, if a breakdown occurs, the power-supply system protection should minimize its harmful consequences: the damaged element or section of the system must be disconnected as quickly as possible, without affecting adjacent sections, and the consumers must be transferred to a supply from standby sources. However, for economic reasons an uninterrupted supply of electrical power through the automatic connection of standby facilities is not guaranteed to all consumers.
Protection from overloads. Overload protection is provided in systems having voltages up to 1,000 volts (V) by means of fuses or automatic circuit breakers, which disconnect the protected part of the system when the current exceeds a certain value determined by the permissible heating of the wires. Fuses act without any delay, according to the protective characteristics of the fusible link. Automatic circuit breakers have triggers that act either instantaneously or with a delay, depending on the amount by which the line current exceeds the permissible value. In electrical systems having voltages over 1,000 V the transformers are protected against thermal overloading as are certain underground (cable) lines that operate systematically under overload conditions. Aerial lines usually do not require such protection.
Protection from insulation damage. Aerial electrical transmission lines are insulated by the air and by porcelain or glass insulators to which the wires are attached. Underground lines, transformers, and various apparatus are usually insulated with solid or fluid dielectrics that are subject to aging. An insulation breakdown can occur in such devices at the operating voltage; a similar effect may occur in the insulators of aerial lines. The principal method of preventing breakdowns caused by faulty insulation is preventive maintenance—the periodic inspection of the condition of the insulation to detect defects and the timely replacement or repair of the insulating structure. Insulation can be inspected by testing it at a higher voltage or by indirect methods (according to the insulation resistance or the magnitude of the dielectric loss angle, and by measurements of the voltage distribution along a string of insulators and of indications of partial discharges). Insulation defects develop gradually; in many cases they are associated with the penetration of moisture. Preventive maintenance testing reveals insulating elements whose probability of breakdown is higher, which makes possible the avoidance of breakdowns. Preventive insulation maintenance greatly reduces the breakdown rate of electrical installations. In the coastal and plains (desert) regions, as well as near factories, insulators become coated with sea salt, sand, and soot from industrial plants. For such conditions specially designed insulators with a developed outer surface are used, and wet cleaning of the insulators under tension is performed.
Protection from ground fault. Soviet systems for general use with tensions up to 0.38 kV, as well as 110 kV or higher, are operated with a dead grounded neutral wire. Exceptions are made for the permafrost regions, where the installation of grounded gear is difficult. In systems with voltages of 3–35 kV the neutral wire is insulated from the ground or is connected to it through a blowout coil; in this case the system is called an equalized system. A similar practice regarding the conditions of the neutral is followed in other countries. With a ground neutral wire, the connection of even one phase to ground causes a short circuit. Shorting one phase to ground in a system with an insulated neutral wire does not disrupt its operation; therefore, a fast disconnection of the damaged section is not required. However, the voltage on the other two phases with respect to the ground increases under steady-state conditions by a factor of VT, which jeopardizes the insulation and is dangerous for people. Systems having an insulated neutral wire are equipped with devices to signal the presence of ground faults so that the breakdown may be detected and eliminated in a short time (no more than 2 hr). Where required by safety specifications, the damaged section of a system is automatically disconnected. Most ground faults start with a brief breakdown of the insulation caused by an overvoltage and then become arc discharge, which is maintained by the short-circuit current. In very long systems the distributed capacitance of the conductors with respect to the ground is large and the value of the current to the ground when the neutral is insulated reaches tens and hundreds of amperes. For such currents the arc burns for a long time and, as a rule, jumps to an adjacent phase under the influence of the wind and of thermodynamic and electrodynamic effects. A short circuit of one phase to ground becomes a two-phase or a three-phase short circuit that must be rapidly disconnected. The development of breakdowns in systems having a high short-circuit current to ground is prevented by grounding the neutral through a blow-out coil (a Petersen coil). To detect and eliminate damage rapidly is as necessary in a compensated system as in a system with an insulated neutral wire.
Protection from short circuits. The protection of a power-supply system from short circuits is a very important part of the protective measures. Short circuits are the main type of breakdowns in electrical systems both with respect to frequency of occurrence and to the magnitude of harmful consequences.The development of protective measures is proceeding in two directions: the most rapid possible disconnection of the faulty part of the systems and the artificial restriction of the short-circuit current value. A reduction of the time during which short-circuit current acts alleviates the thermal condition of the system elements and facilitates the maintenance of stable parallel station operation. For example, protective relaying, which has a pick-up time of 0.05 sec, is used on 500-kV lines; with a pick-up time of 0.06–0.08 sec for the circuit breaker the total disconnect time is about 0.1 sec. The selectivity of the protection ensures that the largest possible portion of the undamaged system is operational and that the damaged section is disconnected. Among the measures for limiting the magnitude of the short-circuit current are the use of block supply circuits, sectionalization of collecting bars in substations, the series connection of reactors, and an increase in the leakage inductance of transformers. The physical significance of such measures is to increase the inductive reactance of the short circuit, which unavoidably results in difficulty of voltage regulation under normal conditions and an increase in the power losses of the system. In some cases this leads to reduced reliability of the power supply. The artificial limitation of short-circuit current values conflicts with the circuit requirements and the electrical system parameters for optimization of the operating conditions. The conflict can be avoided if the short-circuit current is reduced by means of series-connected limiters whose impedance is negligible under normal conditions but is several times greater under emergency conditions, when most of the phase voltage is across the limiter. The creation of such short-circuit current limiters is possible in principle.
Protection from overvoltage. The protection of power-supply systems from overvoltages includes their protection from atmospheric overvoltages,which occur during a lightning dis-charge into the current-conducting parts of an electrical installation or into the nearby ground, and from internal over-voltages, which are caused by intentional or random changes of system conditions, such as the operation of a circuit breaker or the electrical breakdown of insulation in a section of the system. An overvoltage is a temporary excess of the power of the electromagnetic field on part of the system. The power-supply system is protected by making the insulating structure secure against electrical breakdown by storing or dissipating excess energy. Atmospheric overvoltages are typically of relatively low energy (on the order of millions of joules), of short duration (from fractions to several dozen microseconds), and of high amplitude (millions of volts). Internal overvoltages last for hundredths of a second to several seconds or more. Their amplitude may substantially exceed the operating voltage, and their energy may be as high as tens of millions of joules (in 500-kV electrical installations). The amplitude of internal overvoltages depends on the circuit of the system and the parameters of its elements and of the feeder power stations. In many cases switching operations that change the system parameters can be used for protection for internal overvoltages.
Protection from mechanical damage. Underground transmission lines are protected from the electrochemical corrosion produced by stray currents and, where necessary, from soil corrosion. Earth-moving work near the right-of-way of underground lines is regulated by special rules. The planning of aerial transmission lines and open substations takes into account wind loading and the effect of glaze ice (the icing of wires, with the formation of an ice coating 10–20 mm thick). Heavier icing is also possible with a stronger wind; in such cases the ice on the wires is melted by the electric current. In a light breeze blowing at a constant velocity of 0.5–5.0 m/sec perpendicular to the line, periodic oscillations of the wires may be produced in a vertical plane (so-called wire vibrations). Their frequency varies from 1 to dozens of hertz, and their amplitude does not exceed several centimeters. The vibration is caused by the coincidence of the frequency of the aerodynamic impulses acting on the wire with the natural frequency of the wire’s free oscillations. As a consequence of the vibration, cracks and fractures occur in the wire strands, above all where they emerge from a terminal. Vibration with large amplitude causes failures in the fittings, damage to the insulators and, in some cases, damage to the welded seams of the metal towers. Protection is provided against this kind of vibration by dynamic vibration dampers in the form of cast-iron weights that are attached to the cables at distances of 0.5–2.0 m from the wire terminals and that oppose the vibrations of the wire. With the help of such dampers the vibration amplitude is diminished to a safe value of about 1 mm. At wind velocities of 6 to 20–30m/sec and under icing, wire oscillations are sometimes observed at frequencies between 0.2 and 4.0 Hz of very high amplitude, which may reach several meters (so-called wire dance). As of 1971, no efficient method of protection from wire dance had been developed.
The towers and structures that support the wires are protected from the action of the atmosphere, as well as from the aggressive biosphere (fungi, bacteria, and insects), by impregnation of wooden parts or the application of anticorrosion coatings to metal structures. Special measures are also taken to protect aerial lines from fires along the right-of-way, from falling trees, from snow and rock avalanches, and from springtime ice flows (near rivers). In particular, a protective green belt 20–100 m wide (depending on the value of the operating voltage) is established along the right-of-way of a line.
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V. IU. GESSEN