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.