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Chapter: Power Quality : Over Voltages

Important short questions and answers: Over Voltages

Power Quality - Over Voltages - Important short questions and answers: Over Voltages

Over Voltages



1.  Define voltage sag



Sag (dip) a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycles to 1 minute. Voltage sag is defined as a decrease in RMS voltage magnitude lasting from 0.5 to 30 cycles.


2.     Name any four types of sag mitigation devices


1.     Dynamic voltage restorer


2.     Active series compensators


3.     Distribution static compensator


4.     Solid state transfer switches.


3.     What are the causes of over voltages in electric power systems?


1.     Load switching off


2.     Capacitor switching on


3.     System voltage regulation



4.  What is meant by shielding of cables?


Shielding is the use of a conducting and/or ferromagnetic barrier between a potentially disturbing noise source and sensitive circuitry.


Shields are used to protect cables and electronic circuits. It may be in the form of metal barriers, enclosures, wrapping around source circuits and receiving circuits


5.       How power quality problems created by lightning stroke?


The chief power quality problems with lightning stroke currents entering the ground system are


1. They raise the potential of the local ground above other grounds in the vicinity by several kilovolts. Sensitive electronic equipment that is connected between two ground references, such as a computer connected to the telephone system through a modem, can fail when subjected to the lightning surge voltages.


2. They induce high voltages in phase conductors as they pass through cables on the way to a better ground.


6. Define Ferro resonance.


Ferro resonance an irregular, often chaotic type of resonance that involves the nonlinear characteristic of iron-core (ferrous) inductors. It is nearly always undesirable when it occurs in the power delivery system, but it is exploited in technologies such as constant-voltage transformers to improve the power quality.


7.     List the important types of arrestor used in protection of cable.


1.     Under oil arresters


2.     Elbow arresters


3.     Lower discharge arresters


8.     What is shielding?


Shielding is the use of a conducting and/or ferromagnetic barrier between a potentially disturbing noise source and sensitive circuitry. Shields are used to protect cables (data and power) and electronic circuits. They may be in the form of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.


9.  Write the principle of over voltage protection.


The fundamental principles of over voltage protection of load equipment are


1. Limit the voltage across sensitive insulation.


2. Divert the surge current away from the load.


3. Block the surge current from entering the load.


4. Bond grounds together at the equipment.


5. Reduce, or prevent, surge current from flowing between grounds.


6. Create a low-pass filter using limiting and blocking principles.


10.            What are device can be used for over voltage protection



1.     Surge arrester(crowbar & clamping device)


2.     Transient over voltage Surge suppresser


3.     Isolation transformer


4.     Low pass filter


5.     Low impedance power conditioners


6.     Pre-insertion resistors (transmission and distribution)


7.     Pre-insertion inductors (transmission)


8.     Synchronous closing (transmission and distribution)


9.     Fixed inductors (transmission and distribution)




11. What are the various causes of over voltages?


1.     Atmospheric discharges


2.     Switching operations in the public grid and low voltage mains


3.     Electrostatic discharges


4.     Ferro resonance


12. Define transient over voltages?


A transient over voltage can be defined as the response of an electrical network to a sudden change in network conditions either intended or accidental or network simulation.


13. Define impulsive transients?


An impulsive transient is a sudden non power frequency change in the steady state condition of the voltage or current waveforms that is essentially in one direction either positive or negative with respect to those waveforms.


14. Define voltage magnification phenomena?


The highest transient voltages occur at the low voltage capacitor bank when the characteristics frequency of the switching transient is nearly equal to the resonant frequency of the low voltage system and when the switched capacitor is ten or more times the size of the low voltage capacitor.



15. What is the need of surge arrester?


a.     A surge arrester is a protective device for limiting surge voltages on equipment by discharging or by passing surge current


b.     Surge arresters allow only minimal flow of the 50HZ/60HZ power current to ground


16.                        What are the problems associated with Ferro resonance?


1.     Transformer over heating


2.     Audible noise


3.     High over voltages and surge arrester failure


17. How is an over voltage different from swell?


Over voltage: when used to describe a specific type of long duration variation, refers to a voltage having a value of at least 10 percent above the nominal voltage for a period of time greater than 1 minute.


Swell: A temporary increase in the RMS value of the voltage of more than 10 percent of the nominal voltage, at the power frequency, for duration from 0.5 cycles to 1 min.


18.                        What are types of transient over voltages?


Impulsive Oscillatory

19.                        What are the causes of voltage magnification on network?


The voltage magnification will not result in capacitor damage. The problem that usually occurs is the failure or mis operation of sensitive loads in the facility where the low voltage capacitors are installed.


20. Give examples for oscillatory transient over voltages.


Switching operations within the distribution network are a major cause of oscillatory transient over voltages. Such operations include,


1.  Switching of utility capacitor banks



2.     Switching of circuit breakers to clear network faults


3.     Switching of distribution feeders to rearrange the network for maintenance or construction


21.            Mention two important concerns for capacitor bank switching transients.


1. Voltage transients at the capacitor bank substation and neighbouring substations


2.  Power quality impact on sensitive customer loads due to variations in voltage when energizing capacitor banks


22.            Mention the benefits of transmission line surge arresters


1. Reduces the height of transmission lines by eliminating shield wire.


2. Improves outage statistics by eliminating back flash over from the tower ground lead to the phase conductor.


23.            What is the need of low pass filter in transient protection?


1. This LC combination provides a low impedance path to ground for selected resonant frequencies


2. Low pass filter employ principle to achieve better protection even for high frequency




24. Define lighting phenomenon.


Lighting is an electrical discharge in the air between clouds between clouds, between different charge centre within the same cloud, or between cloud and earth.


Even through more discharges occur between or within clouds, there are enough strokes that terminate on the earth to cause problems to power systems and sensitive electronic equipment.


25. List the sources of over voltages.


1. Capacitor switching and lightning all the two main sources of transient over voltages.


2. Switching phenomena on end user facilities also are sources of transient over voltages.


3. Transient over voltages can be generated at higher frequency, medium frequency and also at low frequencies.


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