22-05-2014, 11:41 AM
Harmonic Elimination in Single Phase Systems by Means of a Hybrid Series Active Filter
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INTRODUCTION
With the developments of power electronic equipments and nonlinear loads, the power quality has been deteriorating in distribution system. Current harmonics can cause serious harmonic problems in distribution feeders for sensitive consumers. Some technology options have been reported in order to solve power quality issues. Initially, lossless passive filters have been used to mitigate harmonics and compensate reactive power in nonlinear loads. However, passive filters have the demerits of fixed compensation, large size and resonance with the supply system.
Active filers have been explored in shunt and series configurations to compensate different types of nonlinear loads; nevertheless, they have some drawbacks. As a case in point, their rating is sometimes very close to load, and thus it becomes a costly option for power quality improvement. Many researchers have classified different types of nonlinear loads and have suggested various filter options for their compensation. In response to these factors, a series of hybrid filters has been evolved and extensively used in practice as a cost effective solution for the compensation of nonlinear loads. State-of-the-art power electronic technology has enabled engineers to put active filters into practical use. Many shunt active filters consisting of voltage-fed pulse width modulated (PWM) inverters using IGBT or GTO thyristors are operating successfully in all over the world. These filters have provided the required harmonic filtering, reactive power compensation.
POWER QUALITY
The contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds the foundation blocks.
Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations. To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: ‘Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves our natural resources.’ As we are all aware, container crane performance requirements continue to increase at an astounding rate.
Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW – almost double the total average demand three years ago. The rapid increase in power demand levels, an increase in container crane population, SCR converter crane drive retrofits and the large AC and DC drives needed to power and control these cranes will increase awareness of the power quality issue in the very near future.
System Losses
Harmonic currents and low power factor created by nonlinear loads, not only result in possible power factor penalties, but also increase the power losses in the distribution system. These losses are not visible as a separate item on your monthly utility billing, but you pay for them each month. Container cranes are significant contributors to harmonic currents and low power factor. Based on the typical demands of today’s high speed container cranes, correction of power factor alone on a typical state of the art quay crane can result in a reduction of system losses that converts to a 6 to 10% reduction in the monthly utility billing. For most of the larger terminals, this is a significant annual saving in the cost of operation.
Power Service Initial Capital Investments
The power distribution system design and installation for new terminals, as well as modification of systems for terminal capacity upgrades, involves high cost, specialized, high and medium voltage equipment. Transformers, switchgear, feeder cables, cable reel trailing cables, collector bars, etc. must be sized based on the kVA demand. Thus cost of the equipment is directly related to the total kVA demand. As the relationship above indicates, kVA demand is inversely proportional to the overall power factor, i.e. a lower power factor demands higher kVA for the same kW load. Container cranes are one of the most significant users of power in the terminal. Since container cranes with DC, 6 pulse, SCR drives operate at relatively low power factor, the total kVA demand is significantly larger than would be the case if power factor correction
Equipment was supplied on board each crane or at some common bus location in the terminal. In the absence of power quality corrective equipment, transformers are larger, switchgear current ratings must be higher, feeder cable copper sizes are larger, collector system and cable reel cables must be larger, etc. Consequently, the cost of the initial power distribution system equipment for a system which does not address power quality will most likely be higher than the same system which includes power quality equipment.
EQUIPMENT Reliability
Poor power quality can affect machine or equipment reliability and reduce the life of components. Harmonics, voltage transients, and voltage system sags and swells are all power quality problems and are all interdependent. Harmonics affect power factor, voltage transients can induce harmonics, the same phenomena which create harmonic current injection in DC SCR variable speed drives are responsible for poor power factor, and dynamically varying power factor of the same drives can create voltage sags and swells. The effects of harmonic distortion, harmonic currents, and line notch ringing can be mitigated using specially designed filters.
Power System Adequacy
When considering the installation of additional cranes to an existing power distribution system, a power system analysis should be completed to determine the adequacy of the system to support additional crane loads. Power quality corrective actions may be dictated due to inadequacy of existing power distribution systems to which new or relocated cranes are to be connected. In other words, addition of power quality equipment may render a workable scenario on an existing power distribution system, which would otherwise be inadequate to support additional cranes without high risk of problems.
ENVIRONMENT
No issue might be as important as the effect of power quality on our environment. Reduction in system losses and lower demands equate to a reduction in the consumption of our natural nm resources and reduction in power plant emissions. It is our responsibility as occupants of this planet to encourage conservation of our natural resources and support measures which improve our air quality
HARMONICS
The typical definition for a harmonic is “a sinusoidal component of a periodic wave or\ quantity having a frequency that is an integral multiple of the fundamental frequency.”. Some references refer to “clean” or “pure” power as those without any harmonics. But such clean waveforms typically only exist in a laboratory. Harmonics have been around for a long time and will continue to do so. In fact, musicians have been aware of such since the invention of the first string or woodwind instrument. Harmonics (called “overtones” in music) are responsible for what makes a trumpet sound like a trumpet, and a clarinet like a clarinet.
Electrical generators try to produce electric power where the voltage waveform has only one frequency associated with it, the fundamental frequency. In the North America, this frequency is 60 Hz, or cycles per second. In European countries and other parts of the world, this frequency is usually 50 Hz. Aircraft often uses 400 Hz as the fundamental frequency. At 60 Hz, this means that sixty times a second, the voltage waveform increases to a maximum positive value, then decreases to zero, further decreasing to a maximum negative value, and then back to zero. The rate at which these changes occur is the trigometric function called a sine wave, as shown in figure 1. This function occurs in many natural phenomena, such as the speed of a pendulum as it swings back and forth, or the way a string on a violin vibrates when plucked.
Motor Speed Control (Power Control)
Typically when most of us think about controlling the speed of a DC motor we think of varying the voltage to the motor. This is normally done with a variable resistor and provides a limited useful range of operation. The operational range is limited for most applications primarily because torque drops off faster than the voltage drops.
Most DC motors cannot effectively operate with a very low voltage. This method also causes overheating of the coils and eventual failure of the motor if operated too slowly. Of course, DC motors have had speed controllers based on varying voltage for years, but the range of low speed operation had to stay above the failure zone described above.
Additionally, the controlling resistors are large and dissipate a large percentage of energy in the form of heat. With the advent of solid state electronics in the 1950’s and 1960’s and this technology becoming very affordable in the 1970’s & 80’s the use of pulse width modulation (PWM) became much more practical. The basic concept is to keep the voltage at the full value and simply vary the amount of time the voltage is applied to the motor windings. Most PWM circuits use large transistors to simply allow power On & Off, like a very fast switch.
This sends a steady frequency of pulses into the motor windings. When full power is needed one pulse ends just as the next pulse begins, 100% modulation. At lower power settings the pulses are of shorter duration. When the pulse is On as long as it is Off, the motor is operating at 50% modulation. Several advantages of PWM are efficiency, wider operational range and longer lived motors. All of these advantages result from keeping the voltage at full scale resulting in current being limited to a safe limit for the windings.
CONCLUSION
This project presents a fully digitally controlled HSAF for harmonic elimination and reactive power compensation in a single phase system with a control method for series active filter. This method is applicable in both single and three phase systems.
The main advantage of the presented series active filter is that its filter’s power rating is 10% of the load making it a cost-effective solution for high power applications. The performance of the proposed control method is simulated for a HSAF.
The simulation results show the effectiveness of the presented method. Also, to investigate the effectiveness of this method reality, a laboratory prototype 220 V– 2.2 kW HSAF is implemented.