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Protection Philosophy Now a days protection system, which evolved from a vertically integrated utility structure, was designed to protect equipment, not the power system. The protection philosophy is to protect equipment because any disturbance would be brief and service would be quickly restored. Protection system design is a much an art as it is a science. There are IEEE standards for suggested best design practices for all normal protection functions. For protection systems we have relays for each protection funtion. A relay has two failure modes. It can trip when it should not trip (a false trip) or it can fail to trip when it should trip. The two types of reliability have been designated as “security” and “dependability” by protection engineers. Dependability is defined as the measure of the certainty that the relays will operate correctly for all faults for which they are designed to operate. Security is the measure of the certainty that the relays will not operate incorrectly. Relays are used for protection. A zone of protection refers to a region in a power system with a defined set of equipment. A protection system consisting of one or several relays is made responsible for all faults occurring within the zone of protection. Protection of transmission lines: Transmission lines can be protected on overcurrent. The activation value of an overcurrent relay is set between the maximum load current and the minimum fault current expected on the line. When a distance calculation is involved, impedance is calculated and used as the triggering or “pickup” quantity. Pilot relaying is used to protect a line when high speed protection of an entire line is desirable. Tie lines are usually protected by impedance relays but may be protected on overfrequency as well. Because the transmission system is a three-phase system (whereas most homes use only one-phase), relays must deal with a number of possible faults. In a three-phase transmission system, there are three conductors (one for each phase) and a ground wire on the towers (in addition to the actual ground below). A short circuit between any two of these conductors caused by lightning, a tree, a failed insulator, etc. is a fault. Most faults are between a single conductor and ground (that is, a phase to ground fault). Even so, a protection system must be able to contend with all possible faults combinations. More than one three-phase line can be supported by one set of towers. In most situations the relays must decide whether a fault is on the line the relay is protecting or on an adjacent line. In the former case the circuit breaker should open. In the latter case, the relay should not send a signal to the breaker. Signals from the remote end of the line can make this determination easier. Transformer protection: Small transformers are usually protected by fuses or overcurrent relays. Large transformers (2.5 MVA or greater) are protected by circuit breakers operated by percentage current differential relays, overcurrent relays, temperature and pressure sensing equipment. Electrical side of generator protection Generally, protection of generators is not a system protection function but rather a plant protection function. Generators are protected against rotor heating caused by unbalanced stator currents and against loss of field current. There may be protection against reverse power, inadvertent energization, and out-of-step-protection as well. Frequency protection is backup protection for mechanical overspeed or underspeed conditions since speed is proportional to frequency. Bus protection: Bus protection is designed to protect substations against fault conditions. Differential relaying is the primary tool.
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