Download chapter 8 - Power Quality

EKT 451
CHAPTER 8
Power Quality
1
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.
2



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 manmade (e.g., lightning, wind, cars hitting power poles, falling tree
branches, ).
3
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.
4
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.
5
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.
6
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.
7
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."
8
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.
9
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 lightning-generated transients.
10
Harmonics/Voltage Distortion
 because of the increased popularity of electronic and other non



linear loads, these waveforms quite often become distorted.
Voltage distortion is any deviation from the nominal sine waveform
of the AC line voltage.
The distorted waveform repeats itself with some basic frequency.
The sine wave associated with this frequency, which is usually 60
Hz, is called the fundamental.
Harmonic is an integer multiple of the fundamental. So, the 2nd
harmonic has a frequency of 120 Hz, the 3rd is at 180 Hz, the 4th
at 240 Hz, and so on.
11
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.
12
Power Quality Issue




Residential area
Heavy Commercial
Industry
Light Commercial
Industry
Agriculture
13
Power Quality Issue

Residential area
Damaged Equipment
 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.
14
Power Quality Solution


Solutions to power quality problems fall into two
categories;
i) wiring solutions


Wiring solutions include the use of special circuit and
grounding practices
ii) equipment solutions.





surge suppressors
power conditioners
battery backups
emergency power generators
voltage regulators
15