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52 E L EC TR I C AL CONNEC T I ON
AU T UMN 20 1 6
Although details of the compensation
will vary between commercially available
equipment, in each case the fundamental
current is extracted by some means – fast
Fourier transform (FFT), notch filter, etc.
Pulse width modulation, adjustable
per phase, basically allows adjustment
of power factor per phase and also the
balancing of phases.
For many installations, phase balancing
is a very important feature requiring
a dynamic response, in that phase
loading fluctuates in many if not most
installations.
There is little or no detail in suppliers’
technical literature on how phase
balancing is achieved. One method is
possible by way of control protocols
converting unbalanced currents into
symmetrical components. See Figure 2
for a brief description of symmetrical
components.
The negative sequence controller injects
capacitive or inductive negative sequence
currents of opposite phase to that of the
load negative sequence currents.
As a result, the network sees
symmetrical load current and phase
voltages without exchanging power with
the network. Note: the zero component
flows only in a four-wire distribution
system or otherwise in delta windings of
transformers.
It is advisable to examine phase
balancing features closely and seek
practical verification if that feature is
crucial for an installation.
OTHER COMPENSATION
The use of thyristors and GTOs as
switching elements for capacitors – and
sometimes parallel connected inductors
– is well established in MV and HV
transmission.
The STATCOM had its start in MV
distribution, and more recently it has
come into use in LV networks. The term
static var compensator is generally
reserved for equipment switching
passive components.
CAPACITOR-BASEDCORRECTION
Metallised polypropylene (MPP)
capacitors have greatly improved
capacitor bank reliability compared
with earlier types using polychlorinated
biphenyl (PCB) dielectric material.
MPP capacitors are durable and
generally offer a self-healing feature.
When they short out, the conductor area
surrounding the shorted area vaporises,
thus removing the shorted circuit. The
capacitor continues operating, with
slightly lower capacitance.
The self-healing feature is convenient,
but if capacitors continue operating
beyond their design constraints they
start to have multiple shorts and can lose
capacitance more rapidly.
Capacitor banks are generally
connected in delta circuits, using power
factor as measured on one phase as the
basis for power factor correction.
Specialised contactors are sometimes
employed with early-make contacts in
series and a current-limiting resistor to
help ‘form’ the capacitor, followed by the
later main contact closure.
Some contactors employ the use of
small, air-core inductors to limit inrush
current. However, this technology
increases the response time of power
factor correction equipment. Zero
voltage cross-over thyristor (triac)
control diminishes inrush current and
improves response time.
PREVENTIVEMAINTENANCE
Temperature degradation, in terms of
component value and reduced lifetime, is
a serious problem.
It should be dealt with by regular
preventive maintenance testing, including
capacitance measurement.
Twice yearly check-ups, including
verification of capacitance values, are
recommended. But this is rarely done
in practice, even though it is desirable
for continued effective operation of the
equipment.
Problems occur with frequently
exercised capacitor banks, particularly
in regard to inrush current. This can be
several orders of magnitude larger than
under steady-state conditions, thereby
causing contacts to weld.
SELECTINGCORRECTIONEQUIPMENT
Significant harmonics, load unbalance
and power factor variations must be
taken into account.
It is a very good idea to carry out a
thorough power quality survey before
specifying power factor correction
equipment.
Specifications for new installations
may describe in detail the equipment to
be quoted on, but they lack information
on power quality at the site. This adds to
potential future problems such as voltage
regulation, resonance and imbalance.
For existing installations a power
quality survey should be conducted over
a reasonably long period. This will allow
as many possible variations in electrical
parameters as possible to be captured and
contrasted against incoming power line
conditions. Problems in power quality at
that stage cannot easily be compensated.
Although not a direct power factor
issue, phase balancing is very important.
Many types of load – in particular
induction motors – respond badly to
phase imbalance.
Figure 3: Capacitor lifetime versus
temperature.
Temperature in centigrade
250%
200%
150%
100%
50%
0%
0
600
700
800
900
Percent of rated life
200%
100%
50%
25%