Download 11 – Power Conditioners and UPS 1

POWER CONDITIONERS AND UNINTERRUPTABLE POWER SUPPLIES (UPS) Part 1
POWER CONDITIONERS:
Power conditioners provide an effective way of suppressing some or all of the electrical
disturbances other than power outages and frequency deviations from 60 Hz. Some of these
power conditioners are listed below:
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METAL-OXIDE VARISTORS – provide protection against voltage spikes
ELECTROMAGNETIC INTERFERENCE FILTERS – help to prevent the effect of chopped
waveform on the equipment as well as to prevent the equipment from conducting
high-frequency noise into the utility grid
ISOLATION TRANSFORMERS WITH ELECTROSTATIC SHIELDS - provide galvanic
isolation and filter voltage spikes
FERRORESONANT TRANSFORMERS – provide voltage regulation as well as filtering
of voltage spikes; also partially effective in filtering noise
LINEAR CONDITIONERS – used in many sensitive applications to supply clean power
For voltage regulation, an electronic tap changing scheme using triacs as shown below
can be used. In this case triacs are used to replace mechanical contacts and they also allow
current flow in both directions.
UNINTERRUPTABLE POWER SUPPLIES (UPS):
For supplying very critical loads such as computers controlling important processes,
some medical equipment and the like, it may be necessary to use UPS. UPS provide protection
against power disturbances as well as voltage regulation during power line voltage sags and
swells. They are also excellent in terms of suppressing incoming line transient and harmonic
disturbances.
Generally, an ideal UPS should be able to simultaneously deliver uninterrupted power
and provide the necessary power conditioning for the particular power application. Therefore,
an Ideal UPS should have the following features:
1. Regulated sinusoidal output voltage with low total harmonic distortion (THD) and
independent from the changes in the input voltage or load
2. Zero transition time from normal to back-up mode and vice versa
3. Low THD sinusoidal input current and unity power factor
4. High reliability
5. High efficiency
6. Low EMI and acoustic noise
7. Electric isolation
8. Low maintenance
9. Low cost, weight, and size
Obviously, there is not a single configuration that can provide all of these features. Different
configurations of UPS systems emphasize on some features mentioned above.
BASIC BLOCK DIAGRAM OF UPS:
RECTIFIER – is used for converting single-phase or three-phase ac input into dc, which supplies
power to the inverter as well as to the battery to keep it charged; in the normal mode of
operation, the power to the inverter is provided by the rectifier.
Rectifier arrangements:
a. In a conventional arrangement shown below, a phase-controlled rectifier is used
b. It is also possible to use a diode rectifier bridge in cascade with a step-down dc-dc
converter as shown below
c. When an electrical isolation from the mains is required, it is possible to use a dc-dc
converter with a high frequency isolation transformer as shown below
d. Another rectifier arrangement is shown below, where the bulk of the power supplied to
the inverter flows through the diode bridge and only the power required for charging of
the battery bank flows through a single-phase phase controlled thyristor rectifier. The
voltage Vcharge can be controlled in magnitude and polarity for proper charging of the
battery bank. Thyristor T1 normally remains off; it is turned on in the event of a power
outage.
(As a classroom activity next meeting, you will be divided into 5 groups.
1. Each group is assigned one power conditioner topic to research on.
2. Each group should submit a brief written report to the instructor about the
topic assigned, and at the same time, this written report will serve as handouts
to your classmates during the time your group will deliver your report to the
rest of the class.
3. Your group’s written report will serve as your assignment #1 for midterm and
will be graded according to the amount of information you have researched
and according to how well you have organized the information about your
topic.
4. Your group’s oral report will serve as your quiz #1 for midterm and will be
graded according to your visual aides, mastery, and delivery.)