Transcript INTRODUCTION Power system harmonic

Chapter 3
Power System
Harmonic Mitigation
INTRODUCTION
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Power system harmonic issues have
existed since the early 1900’s. The earliest
discovered issues were third harmonic
currents produced by saturated iron in
machines and transformers.
Today, there are many harmonic
producing loads, making it increasingly
necessary to address their influence on
power systems.
• Two
concepts must be taken into consideration for
harmonics studies:
1. the nature of non-linear loads.
2. the interaction of harmonic currents and
voltages within the power system.
• Here we will take a study case for corporate center
in Las Vegas that uses over 1000 computers. These
are powered from four 150-kVA delta-wye zig-zag
transformers. SO large amount of third harmonic
current flows on the neutral and phase conductors.
The result of this current is heating of the
conductors as well as the transformer.
•Using blockade device to blockthe flow of the third
harmonic current in the neutral conductor and measure
the energy consumption at the primary of one of the
above transformers over repeated 15 minute intervals,
first with and then without the blockade in the circuit.
Procedure
Figure 1: Single Line of Building Distribution System
•The instrument used is a Yokogawa WT2030, which is
a lab-grade power analyzer connected on the primary
side.
•The Power sight 3000 harmonic analyzer device was
used to measure current and voltage on the secondary.
•A Fluke 87 true RMS digital multimeter was used to
measure neutral to ground voltage at the furthest
outlet from the transformer.
•A Voltech PM100 single phase power analyzer to
measure Blockade device energy consumption.
•Blockade device reduces the Neutral-to-Ground
Voltage 66.30% which is clear by the following table
Energy Measurement Results
•For all these cases, the average energy savings is
2.13%.
•It should be noted that these savings are
dependent on loading and circuit configuration.
Blockade Effect on System
Currents
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The biggest effect will occur on the
secondary of the transformer, in both the
line and the neutral current.
The plots below show the current
waveform and spectrum for the
transformer secondary with and without
the blockade.
The current (especially the third
harmonic) is reduced by 10.2%.
Reducing Harmonics
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Reducing harmonic problems by limiting
the nonlinear load to 30% of the supply
transformer rating.
However, with power factor correction
capacitors installed, resonant conditions
could potentially limit the percentage of
non-linear loads.
The following equation to determine if a
resonant condition is likely to occur at an
undesirable frequency:
If hr is close to a characteristic harmonic, such as
the 5th or 7th, there is a possibility that a harmful
resonant condition could occur.
•Using a Multi-pulse converter -a 12-pulse
configuration - uses twelve diodes instead of six to
eliminate the 5th and 7th harmonics.
•The disadvantages are cost and complex
construction and also affects the overall drive
system efficiency rating because of the voltage
drop associated with the transformer requirement .
Typical Twelve-Pulse AC Drive Components
Pseudo 12-pulse systems: This configuration uses
phase-shifting transformers to cancel 5th and 7th
harmonics at the common bus. For cancellation to occur,
the non-linear loads must be operated simultaneously
and have similar characteristics. One cost-effective
implementation of this concept uses delta-wye isolation
transformers - on a few large VFDs, while using smaller
and less costly line reactors on smaller VFDs.
•Filters can be designed for several nonlinear loads or
for an individual load, as shown
Typical Harmonic Trap Filter Configuration
Solutions to tolerate harmonics
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Larger conductors size are used ,or
taking these problems into
consideration in the initial design
stage.
Larger transformer (derated):In order
to derate a transformer after it is
installed, limits must be placed on
allowed future loading.
K-rated transformer:it transformer
can be used to handle the expected
harmonic currents if the transformer
is being sized before installation.
Conclusions
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Energy savings by using the neutral
current filter.
The neutral current filter eliminates most
of the third harmonic current in the
neutral and phase conductors.
Other solutions for harmonics are effective
only if they are placed very close to the
load.
The neutral current filter does not
dissipate much energy (only about 10% of
that actually saved)