Transcript chapter 7 - Portal UniMAP

EKT 451
CHAPTER 8
Power Quality
Introduction
• Utilities transmit electricity over power lines into
home as an alternating current (AC) wave.
• This is how power travels through wiring and
passes into appliances.
•
•
•
Sometimes, interference in the supply of electricity
affects how the equipment runs.
Many appliances, such as personal computers,
microwave ovens and sophisticated stereo systems,
have sensitive electronics that can be disrupted or
damaged.
Electric systems experience disturbances from natural
or man-made (e.g., lightning, wind, cars hitting power
poles, falling tree branches, ).
Power Interruptions
 Power interruptions are zero-voltage events on one or
more phases and are typically short duration events, the
vast majority of power interruptions are less than 30
seconds.
 Interruptions can be caused by weather, equipment
malfunction, recloser operations, or transmission
outages.
Interruptions - sources
– Electric supply interruptions are almost always
caused by circumstances external to the customer's
facility.
– The leading causes are lightning strikes, damage to
lines usually encountered during inclement weather,
overloaded power lines, and power line contacts such
as birds and trees.
– If working on computer, a momentary interruption can
lose data. Avoid this by using a UPS/battery backup.
Short-term Variations
•
Voltage sags
 Sags are a short-term reduction in voltage, and can cause
interruptions to sensitive equipment.
 Voltage sags typically are non-repetitive, or repeat only a
few times.
 causes of sags are the switching on of large loads, or short
circuits on the power line.
 They are also caused by temporary heavy demand for
electricity that exceeds the utilities capability to meet it.
Voltage Flicker
•
•
•
Voltage flicker is rapidly occurring voltage sags caused by
sudden and large increases in load current.
Voltage flicker is most commonly caused by rapidly varying
loads that require a large amount of reactive power such
as welders.
It can cause visible flicker in lights and cause other
processes to shut down or malfunction.
Transients
•
•
•
A transient is a "momentary change in the voltage or
current over a very short time.
More often, transients are measured in microseconds
rather than milliseconds.
Frequently this transient is called a voltage "Spike."
Transients - sources
•
•
•
Lightning is one of the leading causes of transients on
utility power lines.
The surge currents associated with lightning strikes
interact with the distribution system's impedance creating
voltage transients.
Lightning can also induce voltages on power lines without
even hitting them. The large electric fields generated
during a discharge can couple into the power system,
creating induced transients.
Transients - solutions
•
•
•
•
The solution to transients problems is the installation of
surge protection equipment.
While some utilities install surge suppression equipment
on their distribution lines it is still necessary for the
customer to install equipment in their home or business.
Wiring solutions include using special circuit and
grounding practices.
Properly designed transmission towers and other
distribution system components can minimize lightninggenerated transients.
Harmonics/Voltage Distortion

Harmonic is an integer multiple of the fundamental
frequency (60 Hz).
 So, the 2nd harmonic has a frequency of 120 Hz, the 3rd is
at 180 Hz, the 4th at 240 Hz, and so on.
 Voltage distortion is any deviation from the nominal sine
waveform of the AC line voltage.
 Because of the increased popularity of electronic and other
non-linear loads, these waveforms quite often become
distorted.
Harmonics Distortion
•
Many solid-state motor controls and computer power
supplies can cause the supply voltage to other
equipment to become so distorted that sensitive
electronics will either fail to operate or operate
intermittently.
http://cipco.apogee.net/pq/default.asp
Power Quality Issue
•
•
•
•
Residential area
Heavy Commercial Industry
Light Commercial Industry
Agriculture
Power Quality Issue
• Residential area
 Electronic equipment and motors are two of the most common items
damaged in a home.
 Computers, TVs, and stereos can be damaged by short bursts of high
voltage known as surges.
 Motors like air conditioner compressors and pumps can be damaged by
high voltage surges or long periods of very low voltage.
 Low voltage will cause the motor to slowly burn up.
http://www.sandc.com/products/powerquality.asp
Power Quality Solution
•
Solutions to power quality problems fall into two categories;
•
i) wiring solutions
– use of special circuit
– grounding practices
•
ii) equipment solutions.
– surge suppressors
– power conditioners
– battery backups
– emergency power generators
– voltage regulators