POWER QUALITY ENGINEERING QUESTION BANK
UNIT#2-SHORT AND LONG 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.
POWER
QUALITY ENGINEERING QUESTION BANK
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.
Example:
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.
Example:
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. Define nonmaterial inconvenience.
Some inconvenience
cannot be expressed in money. In industrial and commercial environments the non
material inconvenience can also be without contributing to the direct or
indirect costs. A way of quantifying these costs is to look at the amount of
money a customer is willing to pay for not having this interruption.
15. Write the equation for cost per interrupted
KW.
Let ci(d) be the costs of an interruption of duration a for customer i
and Li the load of the customer when there would not
have been an interruption. The costs per interrupted KW defined as
ci(d)
Li
And are expressed in
$/kw. For a group of customers experiencing the same interruption the costs per
interrupted kw defined as the ratio of the total costs of the interruption and
the total load in case there would not have been an interruption.
∑i ci(d)
∑iLi
16. Write the equation for cost per KWh not
delivered.
Cost of an interruption
is proportional to the duration of the interruption. the cost per KWh not
delivered is defined as
ci(d)dLi
The cost per KWh
expressed in $/kwh. For a group of customers the cost per kwh not delivered is
defined as
∑i ci(d)d∑iLi
POWER
QUALITY ENGINEERING QUESTION BANK
17. 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.
18. Write the equation for voltage post fault
period.
Star connected static load,
Three sequence impedances are equal,
YL1=YL2=YL0
Va =0
Delta connected static load,
YL1=YL2
YL0=0
Then Va=-0.5
Motor load,
YL2>YL1
YL0=0
YL2=γYL1
19.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.
20.What are the levels
of weather related outages?
1.Normal weather.
2.Adverse weather.
3.Major storm weather.
21.What are the models
used in reliability.
The overload leads to an outage of the overloaded component,
either immediately or after a certain delay which could depend on the amount of
overload. Second outage can lead to further overloads a cascade effect may
occur.
The overload is assumed to be alleviated through
the shedding of load.
POWER
QUALITY ENGINEERING QUESTION BANK
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PART-B
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1. Explain the origin of short Interruptions?
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(MAY/JUN 09)
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2.Explain the voltage magnitude events due to reclosing and
voltage during interruption.
3.Explain the monitoring of short
Interruptions?
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4. Explain the influence on equipment.
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(NOV/DEC 10)
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5.Explain Single
phase tripping
6.Explain the Prediction of short Interruptions.
7.Explain the Origin of interruptions
8.Explain the Causes of long interruptions
9. Explain the Principles of regulating the voltage and Voltage
regulating devices (NOV/DEC
10)
10.Explain the cost of interruption.
11.Explain Reliability evaluation.
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