Анализ проблем оценки качества электроэнергии (150068)Посмотреть архив целиком
Государственное Образовательное Учреждение
Высшего Профессионального Образования
"Норильский Индустриальный Институт"
по английскому языку:
"Анализ проблем оценки качества электроэнергии"
TABLE OF CONTENTS
1. Analysis of power quality problems
2. Different kinds of disturbances
2.1 Frequency variations
2.2 Voltage amplitude variations
2.3 Voltage fluctuations
2.5 Voltage dips – short interruptions
2.6 Waveform variation
4. My research
5. Анализ проблем электроснабжения
6. Различные виды искажений
6.1 Изменение частоты
6.2 Изменение амплитуды
6.3 Флуктуации напряжения
6.4 Доза фликера
6.5 Просадки напряжения
6.6 Изменение формы волны
6.7 Некратные гармоники
6.8 Асимметрия напряжений
8. Список литературы
In recent years power quality issues have captured the attention of many researchers, energy suppliers and customers. The problem of controlling power quality is becoming more relevant because of the widespread use of non-linear and time-varying single-phase or three-phase loads that increasingly affect the operation of distribution networks in residential, commercial and industrial areas. Power quality deterioration is due to transient disturbances (voltage sags, voltage swells, impulses, etc.) and steady-state disturbances (harmonic distortion, unbalance, flicker). The main causes of such disturbances as well as the detrimental effects they have on the electrical systems are described. Among these issues, harmonics are investigated more accurately in this article. Some definitions relevant to non-sinusoidal systems are provided in order to deeply understand the solutions proposed in the remainder. A topic several researchers are interested in is that non-linear loads should not be considered the only cause of the detrimental effects related to power systems. The responsibility for the power quality deterioration should be shared between the supplier and the customer. This kind of information could be provided by indices in a straightforward and simple way. The traditional indices used by the international standards cannot satisfy these specifications. To this purpose, a new index to evaluate the above-mentioned harmonic distortion phenomena in the supply electrical networks is more deeply discussed.
1. Analysis of Power Quality problems
Electricity is an essential aspect of today’s society. That is why such a valuable product rules the majority of the political and strategic choices. Moreover, the industrial and economical development of a country has to be sustained by an adequate amount of electricity produced. To some extent, electricity is the "first" product to be produced. Notwithstanding, over the last few years, the analysis of operation of the electrical systems has proved that the electrical energy should not be considered in terms of "quantity" alone. The "quality" of the supply is a serious issue to be addressed as well (Dugan et al., 1996; Arrillaga et al., 2000). In fact, like other industrial products, electrical energy is required to meet some quality requirements: the reliability of the supply must be known in terms of the continuity of service and the parameters to be kept within some specified standards. On the other hand, unlike other products, electrical energy can undergo a lot of changes starting from where it is produced to the point where it is delivered. In fact, it is generated far from the point of use, is fed to the grid by a variety of generators and arrives to the point of use via several transformers and through many kilometers of overhead and underground cabling. Additionally, networks are managed and maintained by a number of different suppliers. Therefore, it is evident that the quality of the delivered electrical energy is not an easy task to be accomplished. The situation is even more complicated if we consider that there is a lack of exhaustive statistical data on the quality of power the customer can access. In fact, in most of the cases, customers complain about the bad quality of supply causing expensive interruption of the production processes. On the other side, electricity suppliers argue that critical customers have to be directly involved in the hard task of ensuring the quality of power. The supplier should not be expected to deliver high-quality energy to every customer anytime and anywhere on the network. All this would mean huge costs to strengthen the network that only a few customers (in numerical, not consumption, terms) could take advantage of. In fact, there are some aspects that cannot be kept under control by the provider such as weather conditions causing damages and the characteristics of the customer loads which can be responsible for power quality deterioration phenomena. Therefore, the customer should co-operate in guaranteeing a high-quality electrical power flowing through the network.
So, what should we mean by the term "power quality" and what aim should be pursued by all the subjects involved in this matter? An ideal power supply would be the one that is always available, always within voltage and frequency tolerances, and has a pure noise free sinusoidal wave shape. Notwithstanding, electrical grids are normally not capable of providing a supply fulfilling these requirements. In fact, power quality deterioration is usually due to transient disturbances (voltage sags, voltage swells, impulses, etc.) and steady-state disturbances (harmonic distortion, unbalance, and flicker). Each of these power quality problems has a different cause. Some problems are strictly related to the shared nature of the electrical grid. For example, a fault on the network may cause a dip that will affect some customers connected to the grid, and the more severe the fault, the larger the number of users involved. Other problems, such as harmonics, arise from the customer loads and may or may not propagate onto the network and so affect other customers. In most of the cases, industrial customers complain about evident power defects such as interruptions (which range from a few seconds to several hours) and voltage dips or sags where the voltage drops to a lower value for a short duration. In fact, long power interruptions are a problem for all users, but many operations are very sensitive to very short interruptions. Examples of such sensitive operations are the continuous processes, where even short interruptions can lead to the loss of synchronization among different machineries and then stop all production processes.
Although majority of the customers are more sensitive to transient disturbances, since immediate economical losses are involved, there are more underhand problems affecting electrical power such as harmonics and unbalance. In fact, the effects of harmonics on power system apparatus include resonance, reduced operating life of rotating machines, malfunctioning of power system protection devices, errors in power measurements, additional losses, etc. Moreover, unbalance phenomena should also be well monitored, detected and corrected. A machine operating under an unbalanced supply will draw a current with a degree of unbalance. As a result, the three-phase currents might differ considerably and a rise in temperature would take place in the machine. Motors and generators, particularly the large and more expensive ones, may be fitted with protection to detect extreme unbalance and to trip the machine. The behavior of multiphase converters is also affected by an unbalanced supply; this causes an undesirable ripple component on the DC side and non-characteristic harmonics on the AC side. Moreover, the presence of an unbalanced load creates unbalanced current components that cause voltage drops across the source impedance and hence generate harmonic powers flowing backward from the load to the network.
Taking into consideration the above-mentioned aspects, the following section is focused mainly on harmonics whose effects could become more evident in the future.
2 Different kinds of disturbances
It is possible to define some characteristic parameters in order to assess the quality of power delivered from the supply:
• Symmetry of the voltage system.
These characteristics may vary during the normal operation of the electrical system because of load changes, disturbances introduced by user apparatus and fault occurrence. As a consequence, such characteristics may be time variant at each point where energy is delivered and for a given instant they may not be equal at different points in the network. In most of the cases, statistical evaluation of such phenomena may represent an important means to collect information about power quality.
2.1 Frequency variations
Large generators switch-off or important load commutation may lead to transient variations of the frequency, which are quickly compensated through the primary regulation of the generators. Then the power exchanged among interconnected grids is balanced by the generation station, which has to perform the secondary regulation. The primary regulation achieves a null average value for the power exchanges among interconnected grids due to frequency variations. Grid frequency affects the behaviour of motors (speed variations), the performance of some electronic devices where it is used for synchronisation purposes, the losses in magnetic materials and the usefulness of filters to suppress harmonics. Frequency variations are defined in terms of percentage deviation from the nominal frequency.