MITIGATION OF FLICKER AND VOLTAGE FLUCTUATIONS

Sunday, September 18, 2011

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:


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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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.