Component of protective system

A protection system consists of many other subsystems which contribute to the detection and removal of faults. As shown in Fig. 1.2, the main subsystems of the protection system are the transducers, relays, circuit breakers, and trip circuit constraining trip coil and battery. The transducers, i.e., the current and voltage transformers (CTs and VTs) are used to reduce currents and voltages to standard lower values and to isolate protective relays from the high voltages of the power system. They constitute a major component of the protection system. Protective relays detect and locate the fault and issue a command to the circuit breaker (CB) to disconnect the faulty element. When a fault occurs in the protected circuit (i.e., the line in this case), the relay connected to the CT and VT actuates and closes its contacts to complete the trip circuit. Current flows from the battery in the trip circuit. As the trip coil of the circuit breaker is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation to disconnect the faulty element. A circuit breaker is a mechanical switching device capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and automatically breaking currents under specified abnormal circuit conditions such as those of short circuits, i.e., faults. It is used to isolate the faulty part of the power system under abnormal conditions. A protective relay detects abnormal conditions and sends a tripping signal to the circuit breaker. A circuit breaker has two contacts—a fixed contact and a moving contact. Under normal conditions, these two contacts remain in closed position. When the circuit breaker is required to isolate the faulty part, the moving contact moves to interrupt the circuit. On the separation of the contacts, the flow of current is interrupted, resulting in the formation of an arc between the contacts. The medium in which circuit interruption is performed is designated by a suitable prefix, such as oil circuit breaker, air-break circuit breaker, air blast circuit breaker, sulfur hexafluoride circuit breaker, or vacuum circuit breaker. Since the primary function of a protection system is to remove a fault, the ability to trip a circuit breaker through a relay must not be compromised during a fault, when the AC voltage available at the substation may not be of sufficient magnitude. In case of a close-in three-phase fault, the AC voltage at the substation can be zero. Therefore the tripping power, as well as the power required by the relays cannot be obtained from the AC system and is usually provided by the station battery. The battery which is permanently connected through a charger to the station AC service floats on the charger during normal steady-state conditions. The charger is of sufficient VA capacity to provide all steady-state loads powered by the battery. The battery should also be rated to maintain adequate DC power for 8-12 hours following a station blackout. For better reliability, EHV stations have duplicate batteries, each Bus bar CT CB Line VT Relay Battery Trip coil Trip circuit Fig. 1.2 Components of a protection system feeding from its charger, and connected to its own complement of relays. Since the severe transients produced by the electromechanical relays on the battery loads during their operation may cause maloperation of other sensitive relays in the substation, or may even damage them, it is common practice, as far as practicable, to separate electromechanical and static equipment by connecting them to different batteries.

 

Need of protective system in power system

 NEED FOR PROTECTIVE SYSTEMS

An electrical power system consists of generators, transformers, transmission and distribution lines, etc. Short circuits and other abnormal conditions often occur on a power system. The heavy current associated with short circuits is likely to cause damage to equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. Short circuits are usually called faults by power engineers. Strictly speaking, the term ‘fault’ simply means a ‘defect’. Some defects, other than short circuits, are also termed faults. For example, the failure of the conducting path due to a break in a conductor is a type of fault. If a fault occurs in an element of a power system, an automatic protective device is needed to isolate the faulty element as quickly as possible to keep the healthy section of the system in regular operation. The fault must be cleared within a fraction of a second. If a short circuit persists in a system for longer, it may cause damage to some important sections of the system. A heavy short-circuit current may cause a fire. It may spread in the system and damage a part of it. The system voltage may reduce to a low level and individual generators in a power station or groups of generators in different power stations may lose synchronism. Thus, an uncleared heavy short circuit may cause the total failure of the system. A protective system includes circuit breakers, transducers (CTs and VTs), and protective relays to isolate the faulty section of the power system from the healthy sections. A circuit breaker can disconnect the faulty element of the system when it is called upon to do so by the protective relay. Transducers (CTs and VTs) are used to reduce currents and voltages to lower values and to isolate protective relays from the high voltages of the power system. The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. It is a device that senses abnormal conditions in a power system by constantly monitoring the electrical quantities of the systems, which differ under normal and abnormal conditions. The basic electrical quantities that are likely to change during abnormal conditions are current, voltage, phase angle (direction), and frequency. Protective relays utilize one or more of these quantities to detect abnormal conditions in a power system. Protection is needed not only against short circuits but also against any other abnormal conditions that may arise in a power system. A few examples of other abnormal conditions are overspeed of generators and motors, overvoltage, under frequency, loss of excitation, overheating of stator and rotor of an alternator, etc. Protective relays are also provided to detect such abnormal conditions and issue alarm signals to alert operators or trip circuit breakers. A protective relay does not anticipate or prevent the occurrence of a fault, instead,  it takes action only after a fault has occurred. However, one exception to this is the Buchholz relay, a gas-actuated relay, which is used for the protection of power transformers. Sometimes, a slow breakdown of insulation due to a minor arc may take place in a transformer, resulting in the generation of heat and decomposition of the transformer’s oil and solid insulation. Such a condition produces a gas that is collected in a gas chamber of the Buchholz relay. When a specified amount of gas is accumulated, the Buchholz relay operates an alarm. This gives an early warning of incipient faults. The transformer is taken out of service for repair before the incipient fault grows into a serious one. Thus, the occurrence of a significant fault is prevented. If the gas evolves rapidly, the Buchholz relay trips the circuit breaker instantly. The cost of the protective equipment generally works out to be about 5% of the total cost of the system.

voltage generation in three phase synchronous generator

 Voltage generation in three-phase synchronous machine generator

The rotor of the alternator is Run at proper speed by its Prime mover. A prime mover is a machine that supplied the mechanical energy input to the alternator. The primers movers used for slow and medium-speed alternators are water wheels or hydraulic turbines. Steam and gas turbines are used as prime movers in the large alternators and run at high speeds. Steam turbine-driven alternators are called Turbo alternators are Turbo generators. As the pole of the Rotor moves under the armature conductors on the stator, the field flux cut the armature conductor. Therefore voltage is generated in this conductor. Voltage is of an alternating nature since poles of alternate polarity successively pass by a given stator conductor. A three-phase alternator has a stator with three sets of windings arranged so that there is a mutual phase displacement of 120 degrees these windings are connected in Star to provide three-phase output.

Excitation system for the three-phase synchronous machine.

Excitation system for the three-phase synchronous machine.

Excitation means the production of flux by passing current in the field winding.Direct current is required to excite the field winding on the rotor of the synchronous machines. for small machines, dc is supplied to the rotor field by a d.c. genrator called exciter.the eciter may be supplied current by a smaller d.c. generator called pilot exciter.the main and th pilot exciters are mounted on the main shaft of the synchronous machine(generator or motor). the main and Pilot exciter are mounted on the main shaft of the synchronous machine (generator or motor). The DC output of the Main exciter is given to the field winding of the synchronous machine through brushes and slip rings. in smaller machines, the plot exciter may be omitted,but this arrangment is not very sensitive are quick acting when change of field current are required by synchronous machine.

For medium size machine AC exciter are used in place of DC exciters.A.C. Exciters are three phase AC generator. The output of an AC exciter is rectified and supplied through bruses and slip rings to the rotor winding of main synchronous machine.

Large synchronous generator with rating of order of few MW requirements become very large. The problem of conveying such amount of power through high speed sliding contacts becomes formidable. At present large synchronous generators and synchronous Motors are using brushless excitation system. Brushless exciter is a a small direct coupled AC generator with its field circuit on the stator and armature circuit on the Rotor. The output of DC exciter generator rectified by solid state rectifiers. The rectifier output is connected directly to the field winding eliminating the use of brushes and slip rings.

A brushless citation system require less maintenance due to absence of brushes and slip rings the power loss is also reduced.

DC required for the field of the exciter itself is sometimes provided by a small pilot exciter. A pilot exciter is a small AC generator with permanent magnet mountain on root shaft  and three phase winding on the stator. Permanent magnet of the pilot exciter produced the field current of the exciter. The exciter supplies field current of the main machine. thy the use of a pilot excited by the excitation of the main generator completely independent of external supplies.