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
transients.
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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