Power Systems - short answer questions from AMIE exams (Summer 2019)

 
Give very brief answer the following (2 x 10)

What is the difference between a protective relay and a fuse ?

Fuses are used to protect the electrical system when something goes wrong, and can isolate the faulted parts from the normal parts. Circuit breakers can control and protect the electrical system, but most need a protective relay to determine if a problem exists and tell the circuit breaker to open.

What is meant by dead spot?

A dead spot is a zone that is unprotected. Since adjacent protective zones overlap each other, there are no dead spots in a system due to the overlap of zones. A dead spot may result from an absence of overlaps of adjacent protective zones. It means that if a fault occurs, circuit breakers lying within the zone will not trip. The healthy system may be damaged in this way. This is why adjacent protective zones are overlapped.

Give two methods of protection of transformer.

  • Transformer Overcurrent Protection. Overcurrent protection is commonly used for protection from phase and ground faults. It’s used as primary protection where differential protection is not used. Typically, fuses are used as primary protection for transformers below 10MVA. Above 10MVA overcurrent relays are used as back up along with differential relays as primary protection for transformers.
  • Transformer Differential Protection. Differential Protection provides the best overall protection. However, in case of ungrounded or high impedance grounding, it cannot provide ground fault protection. Differential protection is normally applied to Transformers 10MVA and above.

State the advantages of static relaying.

  • The first and very important advantage is the more reliable and maintenance free operation.
  • Static switches provide much higher speed of operation, which is often desirable in modem machines and processes.
  • Static switching provides comparatively simpler circuit design than relays for complicated processes.
  • These switches can be used in an explosive environment, as they switch ON and OFF without sparking.

State the uses of Ground wires.

Ground wires are extensively used for protection against direct lightning strokes for voltages ranging from 110 kV to 500 kV. They are used on all the important transmission lines and on all sections of transmission lines running through regions which subject to frequent lightning storms.

What are the causes for over voltages?

  • Switching surges
  • Insulation failure
  • Arcing ground
  • Resonance
  • Lightning

What is plug setting and time setting arrangement?

Plug setting
  • Plug setting multiplier is nothing but a ratio between the actual fault current in the relay operating coil to pick up current (the relay current setting). 
  • The plug is a small short link that connects the relay operating coils and resistance. The high number of plug position increases the resistance value, therefore, to operate the relay coils which need a high value of current to energize. At the same time, the lower number of plug positions would be of less resistance, hence to operate relay coil, which needs low current value.
  • The plug position ensures the current setting value of the relay. Plug setting multiplier (PSM) Indicates the severity of the fault. The plus setting multiplier is used only in electromagnetic relays, not in numerical relays.
Time setting
  • The time setting multiplier is nothing but an adjusting or speeds up the tripping mechanism of the relay (it is called as the dial).
  • The dial is nothing but a rotating disc, which rotates when the fault current in the relay coil reaches the pickup current.
  • It means, one end connects with the tripping mechanism and another end connect with the relay operation mechanism.
  • By changing the dial or disc position, we can increase or decrease the tripping time of the relay. 
The total relay operating time= Plug setting Multiplier time (Which is available in the relay) x Time Multiplier Setting

Why distance protection is preferred as primary compared to over current protection for transmission lines?

The distance protection is done by using a distance relay, this relay doesn’t compare the local line current to the current at the far end of the line. The distance relay would compare the local current, with the local voltage in the corresponding phase.

Where Impedance Relay, Reactance Relay and Mho Relays are employed?

  • The Impedance relay is suitable for the phase faults relaying for the lines of moderate lengths 
  • Reactance type relays are employed for the ground faults while 
  • Mho type of relays are best suited for the long transmission lines and particularly where synchronizing power surge may occur.
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