There are several
techniques available for Flicker Mitigation. However, since flicker is caused
by voltage fluctuations, these methods should be based on reducing such power quality phenomenon. It must be noted that the
effects of voltage fluctuations are dependent on both its amplitude and the
rate of their occurrence. Generally, mitigation measures are focused on
limiting the amplitude of the voltage fluctuations.
Consequently, two
approaches can be implemented for this purpose:
1. Reducing load power
variations, particularly the reactive component
Flicker compensation
devices such as dynamic voltage stabilizers and/or synchronous machines are
installed at the point of common coupling (PCC) to meet this objective.
Synchronous Machines
They
are conventional sources of continuous fundamental reactive power, which can
either be leading or lagging. Also, they can be the source of mechanical energy
when operated as a motor and a compensator.
Synchronous
machines are required to be operated in a closed-loop voltage control system
with fast excitation current control. This is to enable a fast rise time of the
machine’s reactive current.
Dynamic
Voltage Stabilizers
This
is a general term use to describe devices that can control the amount of
reactive power absorbed from or injected into the power system. Subsequently,
the RMS voltage at the PCC can be increased or reduced.
These
flicker-mitigating power quality devices include the following:
- Static Var Compensators (SVC)
- Thyristor-switched Capacitors (TSC)
- Thyristor-controlled Reactors (TSR) w/ Fixed (FC) or Switched Capacitor (TSC)
- STATCOM
- Saturable Reactors
- Dynamic Voltage Regulator (DVR)
Dynamic
voltage stabilizers are a practical solution for the reduction or elimination
of voltage fluctuations. The effectiveness of these power quality devices
depends primarily on their rated power and speed of reaction.
2. Increasing the
short-circuit power level (with respect to the load
power)
Often, this approach can be
applied only at the design stage and are generally expensive. In addition,
these solutions can force the reconsideration of the electrical power system,
basically adding to costs.
Common measures implemented
to increase the short-circuit power are:
- Connecting the load at a higher
nominal voltage level
- Constructing additional line to reinforce existing distribution
line.
- Supplying flicker-producing loads
through dedicated lines.
- Installing series capacitors.
- Separating fluctuating loads from
steady loads (i.e. light or lamps) using separate winding of a 3-winding
transformer.
- Increasing the rated power of the
transformer serving the fluctuating load.
Other Flicker Mitigation
Techniques
In some cases, the strategies
stated below can reduce flicker problems:
a. Use isolating
transformers to separate the supply terminal that feeds the fluctuating loads,
especially from the lighting power supply.
b. Connect the loads to a
phase and feed the lighting systems from the non-disturbed phases. This
solution is applicable for single-phase loads connected at the low voltage or
medium voltage systems.
c. Operate
flicker-producing loads at a time when they cannot disturb people. For example,
operating the load at night since most people are not working at this time.
d. Implement changes in the
operating practice and/or equipment design to minimize voltage fluctuations. An
example is limiting arc furnace transformer taps during the initial meltdown
period, when flicker is more perceptible. This reduces the operating voltage
and decrease PST during these intervals.
e.
Lower the motor starting or inrush current to reduce voltage variations (e.g.
softstart devices).
f.
Supplying the fluctuating loads from a decoupled source of the utility system
(e.g. diesel–electric group).
Illustrative Examples
Adjustable
Speed Drives (ASD)
- Apply soft starter devices.
Welding
Plant
- Connect single-phase welding
machines to different phases from lighting circuits
- Connect single-phase welders to
three-phase system for balanced load distribution between phases.
- Supply the plant from a dedicated
transformer.
Electric
Arc Furnaces (EAF)
- Segregation and preliminary heating
of charge
- Proper functioning of the electrode
control system
- Install series or saturable
reactors
To sum up, it can be said
that voltage fluctuation and flicker mitigation methods are readily available
for implementation. However, the best solution is usually not provided by a
single method. This means that a proper combination of the abovementioned
approaches should be considered.
References:
Hanzelka,
Z. and Bien, A. (2006). Voltage Disturbances - Flicker
Baggini, A. (2008).
Handbook of Power Quality
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