13-06-2012, 01:52 PM
POWER QUALITY MANAGEMENT
POWER QUALITY MANAGEMENT.doc (Size: 96 KB / Downloads: 37)
ABSTRACT:
The present paper gives a brief introduction to power quality and ways for Energy Conservation. Electric power quality may be defined as a measure of how well electric power service can be utilized by customers. The term Power Quality means di¬fferent things to different people. There is no agreed definition for power quality, it may be defined as the problems manifested in voltage, frequency and the effect of harmonics, poor power factor that results in mis operation/failure of customer equipment. The widespread use of high-power semiconductor switches at the utilization, distribution and transmission levels have made non-sinusoidal load currents more common. Certain types of power quality degradation result in losses and thus losses in transmission and distribution system have come under greater scrutiny in recent years.
This paper outlines the issues relating Reactive Power management to Power Quality and hence Capacitor Demand Meter has been discussed as a Power quality enhancer. The role of capacitor in power Quality Issues proves to be beneficial particularly when energy conservation is outlined.
Introduction:
Electric power quality may be defined as a measure of how well electric power service can be utilized by customers. When wave shapes are irregular, voltage is poorly regulated, harmonics and flicker are present, or there are momentary events that distort the usually sinusoidal wave, and the power utilization is degraded. This is referred as degradation of power quality. The widespread use of high-power semiconductor switches at the utilization, distribution and transmission levels have made non-sinusoidal load currents more common. Certain type of power quality degradation results in losses and thus losses in transmission and distribution system have come under greater scrutiny in recent years. This paper outlines the issues rela¬ting to Power Quality and their impact on Energy Conservation.
Power Quality:
The term Power Quality means di¬fferent things to different people. Power quality is the quality of the electric power supplied to electrical equipment. Poor power quality can result in maloperation of the equipment the electric utility may define power quality as reliability and state that the system is 99.95% reliable.
Ideally the power would be supplied as a sine wave with the amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.
No real life power feed will ever meet this ideal. It can deviate from it in the following ways (among others):
Variations in the peak or rms voltage (both these figures are important to different types of equipment) When the rms voltage exceeds the nominal voltage by a certain margin, a surge is produced. A dip is the opposite situation: the rms voltage is below the nominal voltage by a certain margin.
A sag occurs when the low voltage persists over a longer time period
Variations in the frequency.
Variations in the wave shape - usually described as harmonics
Quick and repetitive variations in the rms voltage. This produces flicker in lighting equipment.
Nonzero low frequency impedance (if the appliance draws more power the voltage drops)
Nonzero high frequency impedance (if the appliance demands a large amount of current or stops demanding it suddenly there will be a dip or spike in the voltage due to the inductances in the power supply line)
rapid spikes and dips and longer term variations in voltage (usually caused by the interaction of other equipment with line impedance).
POWER QUALITY IN POWER DISTRIBUTION SYSTEMS:
Most of the more important international standards define power quality as the physical characteristics of the electrical supply provided under normal operating conditions that do not disrupt or disturb the customer’s processes. Therefore, a power quality problem exists if any voltage, current or frequency deviation results in a failure or in a bad operation of customer’s equipment. However, it is important to notice that the quality of power supply implies basically voltage quality and supply reliability. A voltage quality problem relates to any failure of equipment due to deviations of the line voltage from its nominal characteristics, and the supply reliability is characterized by its adequacy (ability to supply the load), security (ability to withstand sudden disturbances such as system faults) and availability (focusing especially on long interruptions).
Power quality problems are common in most of commercial, industrial and utility networks. Natural phenomena, such as lightning are the most frequent cause of power quality problems. Switching phenomena resulting in oscillatory transients in the electrical supply, for example when capacitors are switched, also contribute substantially to power quality disturbances. Also, the connection of high power non-linear loads contributes to the generation of current and voltage harmonic components. Between the different voltage disturbances that can be produced, the most significant and critical power quality problems are voltage sags due to the high economical losses that can be generated. Short-term voltage drops (sags) can trip electrical drives or more sensitive equipment, leading to costly interruptions of production. For all these reasons, from the consumer point of view, power quality issues will become an increasingly important factor to consider in order to satisfy good productivity. On the other hand, for the electrical supply industry, the quality of power delivered will be one of the distinguishing factors for ensuring customer loyalty in this very competitive and deregulated market. To address the needs of energy consumers trying to improve productivity through the reduction of power quality related process stoppages and energy suppliers trying to maximize operating profits while keeping customers satisfied with supply quality, innovative technology provides the key to cost-effective power quality enhancements solutions. However, with the various power quality solutions available, the obvious question for a consumer or utility facing a particular power quality problem is which equipment provides the better solution.
Power quality varies significantly from one area to another. Some countries have very stable power grids while others are extremely short on capacity.
Power disturbances are caused by the generation, distribution and use of power, and lightning.
A power disturbance can be defined as unwanted excess energy that is presented to the load.
Causes of Power Disturbances:
1. Power disturbance originate both outside and inside customer facilities.
2. Load switching causes surges because of collapsing fields (-e = l * di/dt)
3. Over loaded power distribution systems can cause significant voltage variations between peak and off-peak hours.
4. Significant momentary load changes, such as heavy inrush currents can cause severe voltage variations
5. Black-outs can cause severe voltage surges both on loss and return of power.
6. Circuit-breaker tripping and fuse blowing can cause severe surge voltages
7. Large ups and variable-speed drives can cause various surge voltages inside buildings
Results of Power Disturbances:
1.Sags and under voltages can cause component overheating or destruction
2.Surges and over voltages can cause component overheating, destruction or can trigger other electronic components such as SCR's.
Component overheating reduces the life and deteriorates the real reliability as opposed to the estimated reliability based on steady-state conditions of the product.
False triggering of other components can create nuisance alarm tripping or, worse, can cause overheating or destruction of other electronic components.
The manufacturer of the equipment defines power quality as characteristics of power supply that is required to make his equipment work properly but the customer is the one ultimately affected. While there is no agreed definition for power quality, it may be defined as the problems manifested in voltage, frequency and the effect of harmonics, poor power factor that results in mis operation/failure of customer equipment. According to this approach Power Quality may be co-related with four topics.
Voltage
Frequency
Harmonic distortion
Power Factor
Voltage:
In the context this issue must be viewed from two different directions. The first direction is variation in supply voltage due to the factors arising from transmission and distribution of power. The second direction is variation in voltage within a network due to the characteristics of the loads connected therein. It is well known fact in many other developing Countries that the quality of voltage supplied by the utilities varies widely depending on the type of distribution network and the geographical locations of such networks. The problem of vol¬tage variation in this regard becomes more acute in rural distribution network.
To complicate this problem further the voltage variation is also a fun¬ction of the season of the year, for ex: rural feeders experience the lowest voltages when the drawl of power is the highest, which is invariably in a particular time the year depen¬ding on the agricultural output/crop of that area.
Similarly on particular feeders, which supply highly fluctuating loads of an industrial nature, it is common to find voltage variations beyond permi¬ssible limits. The impact of such voltage varia¬tions is to cause higher energy con-sumption due to a combination of factors.