Published on Feb 12, 2016
The aim of the power system has always been to supply electrical energy to customers. Earlier the consumers of electrical energy were mere acceptors. Interruptions and other voltage disturbances were part of the deal. But today electric power is viewed as a product with certain characteristics which can be measured, predicted, guaranteed, improved etc.
Moreover it has become an integral part of our life. The term 'power quality' emerged as a result of this new emphasis placed on the customer utility relationship.
The fact that power quality has become an issue recently does not mean that it was not important in the past. Utilities all over the world have for decades worked on the improvement of what is now known as power quality. In the recent years, users of electric power have detected an increasing number of drawbacks caused by electric power quality variations. These variations already existed on the electrical system but only recently they are causing serious problems. This is because of the fact that end use equipments have become more sensitive to disturbances that arise both on the supplier as well as the utility side.
End use equipments are more interconnected in networks and industrial processes, that the impact of a problem with any piece of equipment is much more severe. To improve power quality with adequate solutions, it is necessary to know what kinds of disturbances occurred. A power quality monitoring system that is able to automatically detect, characterize and classify disturbances on electrical lines is therefore required.
INCREASED INTEREST IN POWER QUALITY
Power quality is an increasingly important issue for all business. A recent study by IBM showed that power quality problems cost US business more than $15 billion a year. The increased interest in power quality has resulted in significant advances in monitoring equipments that can be used to characterize disturbances and power quality variations. The recent increased interest in power quality can be explained in a number of ways.
¢ Equipments have become more sensitive to voltage disturbances
The electronic and power electronic equipments have especially become much more sensitive to voltage disturbances than their counterparts 10 or 20years ago.
¢ Equipments cause voltage disturbances
Modern electronic and power electronic equipments are not only sensitive to voltage disturbances but also cause disturbances for other customers. E.g. Non-sinusoidal current drawn by rectifiers and inverters.
¢ Technical challenge taken up by utilities
Designing a system with a high reliability of supply at a limited cost is a technical challenge which appealed to many in the power industry and hopefully still does in the future.
¢ Power quality can be measured.
The availability of electronic equipments to measure and show wave forms has certainly contributed to the interest in power quality.
POWER QUALITY- PHENOMENA/DISTURBANCES
Power quality is concerned with the deviation of the voltage from the ideal waveform or the deviation of the current from the ideal waveform. Such a deviation is called a power quality phenomena or disturbances. It is important to first understand the kinds of power quality disturbances that can cause problems with the sensitive loads. Categories of these disturbances must be developed with a consistent set of definitions, so that the measurement equipments can be designed in a consistent manner. Power quality phenomena can be divided into two basis categories.
Steady state variations
A characteristic of voltage or currnt is never exactly equal to its nominal or desired value. The small deviations from the desired value are called voltage or current variations. A property of any variation is that it has value at any moment in time. Monitoring of variations thus has to take place continuously.
Occasionally, the voltage or current deviates significantly from the nominal or ideal wave shape. These sudden deviations are called events. Monitoring of events take place by using a triggering mechanism where recording of voltage or current starts the moment, a threshold is exceeded
Causes of power quality problems
The causes of power quality problems can be many. It is often difficult to point an exact cause for a specific problem. Power quality monitoring equipments comes to aid in such situations. Most of the causes of power quality problem can be divided into two categories
1. Internal causes
Approximately 80% of electrical problems originate within a business facility. Potential culprits may include large equipments start or shut down, improper
wiring and grounding, overloaded circuits or harmonics.
2. External causes
About 20% of power quality problems originate with the utility transmission and distribution system .The most common cause is a lightning strike; other possibilities include equipments failure, vehicle accidents, weather conditions, neighboring business and even normal operation of utility equipments.
STEADY STATE VARIATIONS
This category includes voltage and current variations which are relatively small deviations of voltage and current characteristics around their nominal or ideal values. The two basic characteristics are magnitude and frequency. On average voltage magnitude and voltage frequency are equal to their nominal value but they are never exactly equal. Variations must be measured by sampling the voltage and current over time. Information is best presented as a trend of the quantity over time and then analyzed using statistical methods. An overview of voltage and current variations are given below:
The fast changes or swings in the steady state voltage magnitude are called voltage fluctuation. The change in voltage magnitude can be due to variations of total load of a distribution system, action of transformer tap changers, switching of capacitor banks. If the variations are large enough or in a certain critical frequency range, the performance
of the equipment can be affected
Voltage and current unbalance
Unbalance or 3 phase unbalance is the phenomenon in a 3 phase system in which the RMS values of voltages and phase angles between consecutive phases are not equal. The primary source of voltage unbalance is the unbalanced load. This can be due to an uneven spread of low voltage customers over the three phases but more commonly unbalance is due to a large single phase load.
Harmonic voltage and current distortion
The voltage waveform is never exactly a single frequency sine wave. The phenomenon of having periodic steady state distortions of sine wave due to equipment generating a frequency other than the standard 50 Hz frequency is called harmonic distortion. The non- fundamental components present is called ‘harmonics’. Harmonic distortion of voltage and current result, from the operation of non-linear loads and devices in the power system.
High frequency voltage noise
The non-periodic components in supply voltage can be called ‘noise’. Distinguishing noise from other components is not always simple. An analysis is needed only in case where noise leads to some problem with power system or end user equipments. Electrical noise can be defined as the high frequency interference caused by a number of factors like arc welding or operation of electrical motor.
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