Unit 1 - PQM
Unit 1 - PQM
Unit 1 - PQM
Q. Why are we concerned more about power quality now a days. (May 16, 5 Marks)
The ultimate reason that we are interested in power quality is economic value. There
are economic impacts on utilities, their customers, and suppliers of load equipment.
The electric utility is concerned about power quality issues as well. Meeting
customer expectations and maintaining customer confidence are strong motivators.
Besides the obvious financial impacts on both utilities and industrial customers, there
are numerous indirect and intangible costs associated with power quality problems.
Poor power quality Increases system losses.
Bad power quality causes serve health hazards.
Poor voltage and high current with harmonics cause heating and high losses.
Poor quality power damages consumer’s equipment’s and affects equipment life.
It is any deviation of the voltage or current waveform from its normal sinusoidal wave
shape.
OR
Standard IEEE1100 defines power quality as “the concept of powering and
grounding sensitive electronic equipment in a manner suitable for the
equipment.”
OR
Any power problem manifested in voltage, current, or frequency deviations that result
in failure or misoperation of customer equipment.
Transients:
Q. Explain the power quality issue like overvoltage, undervoltage, voltage sag and
voltage imbalance.
A variation of the RMS value of the voltage from nominal voltage for a time
greater than 1 min.
Long-duration variations can be either overvoltages or undervoltages.
Overvoltage: It is resulted when there is increase in the rms ac voltage
greater than 100 percent at the power frequency for a duration longer
than 1 min.
Undervoltage: It is resulted when there is decrease in the rms ac
voltage less than 90 percent at the power frequency for a duration
longer than 1 min.
Interruption:
The complete loss of voltage on one or more phase conductors for a time
greater than 1 min.
An interruption is due to equipment malfunctions or loose connections can be
irregular.
Interruptions can be the result of power system faults, equipment failures, and
control malfunctions.
Voltage fluctuation:
Q. Explain the power quality issue like overvoltage, undervoltage, voltage sag and
voltage imbalance.
Flicker:
Variation of input voltage sufficient in duration to allow visual observation of
a change in electric light source intensity.
Quantitatively, flicker may be expressed as the change in voltage over nominal
expressed as a percent.
Purpose of groundings:
The most important reason for grounding is safety. Two important aspects to
grounding requirements with respect to safety and one with respect to power quality
are
1. Personnel safety: Personnel safety is the primary reason that all equipment must
have a safety equipment ground. This is designed to prevent the possibility of high
touch voltages when there is a fault in a piece of equipment. The touch voltage is
the voltage between any two conducting surfaces that can be simultaneously
touched by an individual. The earth may be one of these surfaces.
There should be no “floating” panels or enclosures in the vicinity of electric
circuits. In the event of insulation failure or inadvertent application of moisture,
any electric charge which appears on a panel, enclosure, or raceway must be
drained to “ground” or to an object which is reliably grounded.
2. Grounding to assure protective device operation: A ground fault return path to
the point where the power source neutral conductor is grounded is an essential
safety feature. An insulation failure or other fault that allows a phase wire to make
contact with an enclosure will find a low-impedance path back to the power
source neutral. The resulting overcurrent will cause the circuit breaker or fuse to
disconnect the faulted circuit promptly.
Solution:
NEC Article 250-51 states that an effective grounding path (the path to ground
from circuits, equipment, and conductor enclosures) shall
a. Be permanent and continuous
b. Have the capacity to conduct safely any fault current likely to be imposed on it
c. Have sufficiently low impedance to limit the voltage to ground and to facilitate
the operation of the circuit protective devices in the circuit.
d. Not have the earth as the sole equipment ground conductor
3. Noise control: Noise control includes transients from all sources. This is where
grounding relates to power quality. Grounding for safety reasons defines the
minimum requirements for a grounding system. Anything that is done to the
grounding system to improve the noise performance must be done in addition to
the minimum requirements defined in the NEC and local codes.
The primary objective of grounding for noise control is to create an equipotential
ground system. Potential differences between different ground locations can stress
insulation, create circulating ground currents in low-voltage cables, and interfere
with sensitive equipment that may be grounded in multiple locations. Ground
voltage equalization of voltage differences between parts of an automated data
processing (ADP) grounding system is accomplished in part when the equipment
grounding conductors are connected to the grounding point of a single power
source. However, if the equipment grounding conductors are long, it is difficult to
achieve a constant potential throughout the grounding system, particularly for
high-frequency noise. Supplemental conductors, ground grids, low-inductance
ground plates, etc., may be needed for improving the power quality. These must
be used in addition to the equipment ground conductors, which are required for
safety, and not as a replacement for them.
Good grounding practices:
Figure illustrates the basic elements of a properly grounded electrical system. The
important elements of the electrical system grounding are described further.
The ground rod provides the electrical connection from the power system ground to
earth. The item of primary interest in evaluating the adequacy of the ground rod is the
resistance of this connection. There are three basic components of resistance in a ground
rod:
Electrode resistance. Resistance due to the physical connection of the grounding wire
to the grounding rod.
Rod-earth contact resistance. Resistance due to the interface between the soil and the
rod. This resistance is inversely proportional to the surface area of the grounding rod
(i.e., more area of contact means lower resistance).
Ground resistance. Resistance due to the resistivity of the soil in the vicinity of the
grounding rod. The soil resistivity varies over a wide range, depending on the soil
type and moisture content.
The resistance of the ground-rod connection is important because it influences
transient voltage levels during switching events and lightning transients. High-magnitude
currents during lightning strokes result in a voltage across the resistance, raising the
ground reference for the entire facility. The difference in voltage between the ground
reference and true earth ground will appear at grounded equipment within the facility,
and this can result in dangerous touch potentials.
Problems due to poor grounding:
Some typical power quality problems that are due to inadequacies in the grounding of
electrical systems are:
I. Problems with conductors and connectors;
one of the first things to be done during a site survey is to inspect the service
entrance, main panel, and major subpanels for problems with conductors or
connections. A bad connection (faulty, loose, or resistive) will result in heating,
possible arcing, and burning of insulation. Table summarizes some of the wiring
problems that can be uncovered during a site survey
2) Isolated ground
The noise performance of the supply to sensitive loads can sometimes be improved
by providing an isolated ground to the load. This is done using isolated ground
receptacles, which are orange in color. If an isolated ground receptacle is being used
downline from the panel board, the isolated ground conductor is not connected to the
conduit or enclosure in the panel board, but only to the ground conductor of the
supply feeder (Fig. 10.4). The conduit is the safety ground in this case and is
connected to the enclosure. A separate conductor can also be used for the safety
ground in addition to the conduit. This technique is described in the NEC, Article 274,
Exception 4 on receptacles. It is not described as a grounding technique.
The isolated ground receptacle is orange in color for identification purposes. This
receptacle does not have the ground conductor connected to the receptacle enclosure
or conduit. The isolated ground conductor may pass back through several panel
boards without being connected to local ground until grounded at the service entrance
or other separately derived ground. The use of isolated ground receptacles requires
careful wiring practices to avoid unintentional connections between the isolated
ground and the safety ground. In general, dedicated branch circuits accomplish the
same objective as isolated ground receptacles without the concern for complicated
wiring.
A special case of isolated grounds is used for grounding some hospital equipment.
These procedures are described in the NEC and in the White Book (IEEE Standard
602)