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Chapter: Power Quality : Voltage Sags and Interruptions

Important short questions and answers: Voltage Sags and Interruptions

Power Quality - Voltage Sags and Interruptions - Important short questions and answers: Voltage Sags and Interruptions

Voltage Sags and Interruptions



1.       Define short interruption?

          When the supply is restoring automatically the resulting event is called short interruption.        

2.       Define long interruption?     

When  the  supply  is  restoring  manually  the  resulting  event  is  called  long interruption.         

3.       Define during fault period.   

          The low impedance path between the faulted phase and ground is still present so that the voltage in the faulted phase remains zero or close to zero. It is called the during fault period.         

4.       Define post fault period.       

          The fault has extinguished the short circuit has now an open circuit because the breaker in that phase is still open. It is called the post fault period.   

5.       What are the input data’s for prediction of short interruption?  

          Stochastically predict the number of interruptions experienced by a customer fed from a certain feeder, the following input data is required.


1.     Failure rate per km of feeder, different values might be used for the main and for the lateral conductors


2.     Length of main feeder and of the lateral conductors


3.     Success rate of reclosure if multiple reclosure attempts are used: success rate of first reclosure of the second reclosure.


4.     Position of reclosing breakers and fuses.



6.     What are the assumptions for prediction of short interruption?


1.     The failure rate of main feeder is: 0.1 faults per year per km of feeder.


2.     The failure rate of lateral conductor is: 0.25 faults per year per km of feeder.



3.     The success rate of the first reclosure is 75% thus in 25% of the cases a second trip and reclosure are needed.


4.     The success rate of the second attempt is 10% of the number of faults. Thus for 15% of the faults the second attempt does not clear the fault. Those faults are permanent faults leading to long interruption.


7.     What is the reclosing procedure for prediction of short interruption?


1.     The circuit breaker opens instantaneously on the over current due to fault.


2.     The circuit breaker remains open for a short time 75% of the faults clears in this period.


3.     The circuit breaker closes. If the fault is still present the breaker again opens instantaneously on over current. This is required in 25% of the cases.


4.     The circuit breaker now leaves a longer dead time. Another 10% of the faults clear in this period.


5.     The circuit breaker closes for a second time. If the fault is still present the breaker remains closed until the fuse protecting the lateral conductor has had time to blow.


6.     If the fault is still present after the time needed for the fuse to clear the fault, the breaker opens for a third time and now remains open. Further reclosure has to take place manually and the whole feeder will experience a long interruption.


8.     Define failure.


The term failure is used in the general meaning of the term: a device or system which does not operate as intended.


9.  Define outage.


An outage is the removal of a primary component from the system.


Example: A transformer outage or the outage of a generator station. A failure does not lead to an outage.


10. Define interruption.


The term interruption is the situation in which a customer is no longer supplied with electricity due to one or more outages in the supply. In reliability evaluation the term interruption is used as the consequence of an outage or number of over lapping outages.


11. What are the causes of long interruptions?


1.     Fault occurs in the power system which leads to an intervention by the power system protection. If fault occurs in a part of the system which is not redundant or of which the redundant part is out of operation the intervention by the protection leads to an interruption for a number of customers or pieces of equipment.


2.     A protection relay intervenes incorrectly thus causing a component outage which might again lead to long interruption. If the incorrect tripping occurs in a part of the system without redundancy.



12. Define direct cost


These are the costs which are directly attributable to the interruption.




For domestic customers is the loss of food in refrigerator. For industrial customers is the direct cost consist lost of raw material, lost production, and salary cost during the non productive period. For commercial customers the direct costs are the loss of profit and salary costs during the non productive period.


13. Define Indirect cost


The indirect cost are much harder to evaluate and in many cases not simply to express in amount of money.




A company can lose future orders when an interruption leads to delay in delivering a product. For domestic customer can decide to take an insurance against loss of freezer contents. For commercial customers might install a battery backup. For industrial customers could even decide to an area with less supply interruptions.


14. What is meant by single phase tripping?


Single phase tripping is used in transmission systems to maintain synchronicity between both sides of a line. Single phase tripping is rarely used in distribution or low voltage systems. Not only it requires more expensive equipment, but it will also reduce the chance of a successful reclosure. The fault current continues to flow via the non faulted phases. This reduces the chance that the fault will extinguish and thus increases the number of reclosure attempts and the number of long interruptions.


15.                        What are types of power system reliability?


The reliability of power system protection is split into two aspects,


1.     Dependability


2.     Security


3.     Dependability is the degree of certainty that the protection will operate correctly. The Security is the degree of certainty that the protection will not operate correctly.





16. List out the applications of utility side.


1.     Three independently controlled regulators may be used for better balance between the phase voltages.


2.     The purpose of line drop compensator is to regulate the voltage profile so that it provides the necessary voltage boost at peak load and keep the voltage closer to nominal at lower loads.


17. List out the applications of end user side.


1.     Capacitor application is used for voltage regulation.


2.     The primary motivation is to eliminate utility power factor penalties.


3.     Adding capacitors results in power quality problems.


4.     Harmonic problems.


5.     If power factor correction capacitors are not harmonic sources, they can interact with the system to attenuate the harmonics that are already there.


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