Colton Strickland
AGSM 315.503
Due: April 23, 2018
Power Quality Improvement
Abstract
Power quality is a major area of concern for research for any individuals or companies involved in the power sector. It has only been in recent times that technology has been able to keep the power sector virtually free of pollution. Over the past few years, progress has been made toward the work of reduction of harmonic distortion via various applications.
Six-pulse SCR rectifiers are commonly used in the power generation industry, but these systems generate a high amount of Total Harmonic Distortion (THD) in a line’s current.
One solution to this problem would be to use multi-pulse rectifiers—classified as 12, 18, or 24-pulse configurations. Examples of systems where this system could be implemented are high voltage direct current transmission systems, high power battery chargers and load commutated current source inverter powered motor drive.
Two methods exist for the treatment of harmonic issues. These methods are filtering and cancellation. Filtering means that the filters have their own harmonic output to reduce the harmonic disturbance. Power quality has been theorized to be capable of being improved using multi-pulse converters. Increasing the number of pulses reduces the number of harmonic outputs.
Introduction
Power quality is defined as the ability of a system or piece of equipment to function satisfactorily in its electromagnetic environment without intolerable electromagnetic interferences to anything in that environment. Power quality holds a vital role in supplying electricity effectively to consumers in our society. A perfect power supply is one that:
• Is available anytime
• Holds voltage and frequency within proper tolerances
• Has noise free sin wave shape
Electrical power is becoming an increasingly important and essential resource for the entire world, and thus is it is becoming more important to maintain the quality of this energy at each level of its usage in the industry.
Electrical power quality, specifically, is defined by the IEE standards as the “concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment with precise wiring system and other connected equipment”. It is, in practice, the deviation of voltage and currents from the ideal or actual waveforms. Power quality is often decided by the end user receiving said power. In general, if the powered equipment is working satisfactorily with the given supply, then the power is of “good quality”. Poor quality power would be that which fails to power the equipment properly or at all. Some common reasons for poor power quality might be:
• harmonic disturbance
• transient voltages
• voltage fluctuations such as sags or swells
• interruptions—outages or power blinks
Poor power quality can be a serious problem in critical industries. If the power were to go out or provide energy of a quality insufficient to maintain productivity in a bank or inventory control system, the results could be catastrophic for the business involved and any or all their customers.
Concern for Improving Power Quality
In recent years, concern for proper power quality has increased in unison with the growing use of sensitive electronic and computing equipment like personal computers and non-linear loads. A few of the reasons for the growing concern for power quality are:
• Changes in requirements of electric loads and harmonics causing premature equipment failure
• Deregulation of the electricity market
Solving power quality problems requires serious technical knowledge and experience in the electrical field principles. The widespread use of electronic equipment, such as computing equipment, electronic controllers, and energy-efficient lighting have led to drastic changes in the nature of electric loads. It is these specific loads that both cause the major problems of power quality, but are also the pieces of equipment most affected by them.
Solutions to mitigating power quality issues have been found, however. As mentioned earlier, harmonics are a severe issue associated with power quality. These disturbances can cause large power losses in distribution and operation failures for protection devices and industrial processes. These problems often result in high financial losses for various industrial and commercial activities. If harmonics travel through currents to non-linear loads, the common issues are severe overheating and insulation damage that can permanently damage devices and cause loss of power generation and ultimately large blackouts. It is because of these problems that improvement of power quality has been linked to solving the issue of harmonics in line currents. Another approach to improving power quality is the use of pulse width modulation (PWM) that offers some unique advantages:
• Low switching losses
• Good utilization of DC power
• Good linearity in voltage and current control
• Low harmonic content in output voltage and currents
Harmonics
A harmonic is a sin-wave voltage or current with a frequency that is the integer multiple of the frequency at which the supply system is designed to operate. Harmonic distortions are defined by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. A single quantity known as the total harmonic distortion is commonly used to measure the value of a harmonic distortion.
Different methods to eliminate harmonic current exist:
In a system with passive filters, line reactors are often used with capacitors in a rectifier to filter harmonic currents. Connecting capacitors to a line current can, however, magnify harmonic currents to frequencies of harmful levels.
Switching techniques can also be used to eliminate harmonic currents. In high power applications, gate-turn-off types of switches would be used.
The final method of reducing harmonic currents is to use multi-pulse rectifiers. These are commonly based on the designs of phase shifting transformers, which enable certain harmonics to be cancelled from the rectifiers’ line currents. This is a method commonly used in industries where harmonic elimination is required for high power applications.
Multi-Pulse Rectifiers
There are several different types of rectifiers used in the industry. One of the most commonly used is the three-phase 12-pulse rectifier used to convert AC power to DC power. By using a rectifier with a higher number of pulses, the number of input harmonics can be reduced drastically. This would result in a higher power factor and a smoother output.
Rectifiers can be configured in 12, 18, and 24-pulse rectifiers, and are powered by phase shifting transformers with a set number of secondary windings. The main purpose of these multi-pulse rectifiers is to reduce the total line current harmonic distortion. This is achieved through the phase shifting transformers that power them. As the current passes through them, many of the lower-order harmonics that are generated are canceled. In general, the higher the number of pulses in the rectifier, the lower the line current distortion will be. The limit on the number of pulses generally used in multi-pulse rectifiers is 30 due to increased transformer costs and diminishing returns on the improvements to power quality.