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.
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. 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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