Interview questions about Power system protection?

Initiative

Power systems whether old or new have some special protection and safety devices to make system free of damages and failure of operation.Many devices are used to achieve this mechanism,few devices in this regard are at hitlist those are as follows,
Fuse
Circuit Breaker
Relays
Residual current devices

Question:Explain the purpose of fuse in a power system?

 Ans:The first and the most common quintessential protecting device is fuse.One should remember that what is fuse, fuse is a simple strip or metal piece that melts upon the passing of heavy current through it.The flow of heavy current through a fuse while melting fuse produce an arc across the gap that is later on extinguished with the help of circuit interruption by any special technique or special method.Fuses can be considered as the weak point of the system that contribute according to their capability in the protecting  power system.

Question:Give two drawbacks of fuses?

Ans:While dealing with fuses in order to protect the power system from damages,two limitations are associated with fuses.Firstly when fault occurs means excess amount of current passes through fuse,it is necessary to replace the fuse.It is obvious that in the moments of fault if fault is at distant place or urgeantly we do not have fuse,this replacement can be a big loss to the power system.. Secondly,fuse alone is an unadequate safety device to protect the system.The reason is that the amount of current that fuse  allows to flow through it is in excess amount so this can be lethal to the animals and human equally.

Question:How two problems of fuse are resolved?

Ans:First problem of replacing the old fuse with new one is sold by using circuit breakers. Typically circuit breaker used for this purpose is miniature circuit breaker. While second problem is resolved by the usage of Residual current devices.

Question:What is a Miniature circuit breaker?

Ans:A miniature circuit collectively combines the tripping mechanism and the mechanism of current flow breakage  in a single unit.In such miniature circuit breakers flow of current is through a solenoid.The magnetic pull of solenoid  helps us in opening beaker contacts on the occasion of excess current flow.

Question:What is Residual current devices?

Ans:A residual current device basically monitors neutral and active lines and performs mechanism of tripping the active line  if it feels a fault or a difference.The requirement of residual device is the availability of a neutral line for each phase and for the proper tripping too.

Question:What is function of a relay?

Ans:The function of relay is the detection of fault and tripping mechanism of breaker contacts.

Question:Give brief introduction of two types of relays?

Ans:An over current relay initiates the tripping process if adequate amount of current exceeds on  any phase.Similarly,a set of differential relay starts tripping phenomenon if collective current between two feels to be leaking to earth.

Types of protection

High-voltage transmission network

Protection on the transmission and distribution serves two functions: Protection of plant and protection of the public (including employees). At a basic level, protection looks to disconnect equipment which experience an overload or a short to earth. Some items in substations such as transformers might require additional protection based on temperature or gas pressure, among others.

Generator sets

In a power plant, the protective relays are intended to prevent damage to alternators or to the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relays have to operate very rarely. If a protective relay fails to detect a fault, the resulting damage to the alternator or to the transformer might require costly equipment repairs or replacement, as well as income loss from the inability to produce and sell energy.

Overload and back-up for distance (overcurrent)

Overload protection requires a current transformer which simply measures the current in a circuit. There are two types of overload protection: instantaneous overcurrent and time overcurrent (TOC). Instantaneous overcurrent requires that the current exceeds a predetermined level for the circuit breaker to operate. TOC protection operates based on a current vs time curve. Based on this curve if the measured current exceeds a given level for the preset amount of time, the circuit breaker or fuse will operate.

Earth fault ("ground fault" in the United States)

Earth fault protection again requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance. If this imbalance exceeds a pre-determined value, a circuit breaker should operate. Restricted earth fault protection is a type of earth fault protection which looks for earth fault between two sets current transformers[4] (hence restricted to that zone).

Distance (impedance relay)

Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If the ratio of voltage to current measured at the relay terminals, which equates to an impedance, lands within a predetermined level the circuit breaker will operate. This is useful for reasonable length lines, lines longer than 10 miles, because its operating characteristics are based on the line characteristics. This means that when a fault appears on the line the impedance setting in the relay is compared to the apparent impedance of the line from the relay terminals to the fault. If the relay setting is determined to be below the apparent impedance it is determined that the fault is within the zone of protection. When the transmission line length is too short, less than 10 miles, distance protection becomes more difficult to coordinate. In these instances the best choice of protection is current differential protection[citation needed.

Back-up

The objective of protection is to remove only the affected portion of plant and nothing else. A circuit breaker or protection relay may fail to operate. In important systems, a failure of primary protection will usually result in the operation of back-up protection. Remote back-up protection will generally remove both the affected and unaffected items of plant to clear the fault. Local back-up protection will remove the affected items of the plant to clear the fault.

Low-voltage networks

The low-voltage network generally relies upon fuses or low-voltage circuit breakers to remove both overload and earth faults.Courtesy of wikipedia...