Государственное Образовательное Учреждение
Высшего Профессионального Образования
"Норильский Индустриальный Институт"
РЕФЕРАТ
по английскому языку:
"Анализ проблем оценки качества электроэнергии"
Выполнил:
Прохоров А.Л.
Проверил:
СмирноваА.Т.
Норильск, 2010.
TABLE OF CONTENTS
Introduction
1. Analysis of power quality problems
2. Different kinds of disturbances
2.1 Frequency variations
2.2 Voltage amplitude variations
2.3 Voltage fluctuations
2.4 Flicker
2.5 Voltage dips – short interruptions
2.6 Waveform variation
2.7 Interharmonics
2.8 Unbalance
3. Conclusions
4. My research
ОГЛАВЛЕНИЕ
Введение
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
7. Заключение
8. Список литературы
Introduction
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:
• Frequency;
• Amplitude;
• Waveform;
• 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.
2.2 Voltage amplitude variations
The grid voltage continuously changes because of the commutation of the electrical devices connected to the grid. The voltage variation may be slow or quick depending on whether an overall load progressive change or a step change for a large load is happening. The grid impedance deeply affects the amount of voltage variations as a consequence of load change: the higher the impedance, the larger the variation.
2.3 Voltage fluctuations
A set of quick voltage variations is referred to as voltage fluctuation. The limit between slow and quick variations is not so definite and can range from a few seconds to one minute. Slow variations are assessed through the average value calculated over contiguous intervals of ten minutes.
Rapid variations may be single or repetitive and their amplitude usually does not exceed 6–8% of the nominal voltage. Usually electrical apparatus are able to work even in the presence of this kind of disturbance (in most of the cases corrected by voltage regulation) unless initial voltage is too low. These kinds of variations are caused by variable loads such as welding machines, arc furnaces and mills. Rapid variations of over 10% amplitude irrespective of the duration, are considered voltage dips.
2.4 Flicker
The term flicker is referred to as a systematic or casual variation of the voltage amplitude ranging from 0.9 to 1.1 p.u.. Sometimes the terms flicker and voltage fluctuations are interchangeably used. Notwithstanding, the term flicker is strictly related to the impression of instability of the visual sensation produced by a light whose intensity and spectral distribution are time variant. The amplitude of the voltage variations is usually less than 10% and the behaviour of the electrical apparatus is not affected. Notwithstanding, these small disturbances can result in lightning variations which may affect the human eye. This sensitivity is strictly dependent on the frequency of the phenomena reaching its peak value around 7–10 Hz. In this range, even a 0.3% variation of the rms voltage feeding an incandescent lamp may be perceived.
A perfect flicker compensation is not possible, but an attenuation of this phenomena can be achieved through:
• an increase of the short circuit power;
• a reduction of the reactive power flux;
• a limitation of the motor starting currents.
2.5 Voltage dips – short interruptions
Voltage dips are bi-dimensional electromagnetic distortions which are characterised by the amplitude and duration. Voltage dip means that energy is not properly provided to loads during this event and this could result in different consequences depending on the kind of load. According to International Electrotechnical Commission (IEC) standards, voltage dips are referred to as a sudden reduction of voltage affecting a point of the distribution network below 90% of the reference voltage. This reduction has to be recovered within 60 s. Whenever the voltage falls down to zero the event is classified as a short interruption.
The duration of a voltage dip is the interval between the instant when the voltage falls below the threshold value and the instant when the voltage rises again above the threshold. The depth of a voltage dip is the difference between the reference and the residual voltage.
The starting of large loads and faults on the network are the main causes of voltage dips. Dips caused by starting currents are less deep and longer (up to a few seconds) than the ones caused by faults on the grid (less than one second).
When large loads are switched on, the starting current could be much higher than the steady-state current. Since the feeders and the cable of a distribution system are designed for steady-state operation, the high current value is responsible for a considerable voltage drop.
2.6 Waveform variation
Harmonics If an electric quantity is distorted and periodical it can be split into three terms: the mean value calculated over one period of the considered signal, the fundamental component having the same frequency of the considered signal and the sum of the harmonic components. The amplitude of the harmonics decreases with the frequency. The representation of such amplitudes is referred to as spectrum.
As regards symmetrical waveforms (perfectly matching of the positive and negative half-waves), the even harmonics are nihil. This type of harmonics were common when half-wave rectifiers were used.
Power suppliers provide a 50 Hz sinusoidal voltage, but the current drawn by a load is not always sinusoidal. The current is not sinusoidal anymore when the load impedance varies during one period T(the load voltage/current characteristic is not linear). Such type of loads is referred to as non-linear loads. For example, the magnetising current of a transformer is deformed by a third-order harmonic because of the non-linear magnetisation curve of the machine. Rectifiers (battery chargers, welding machines, etc.), inverters, electronic starters, adjustable speed drives, discharge lamps are other examples of non-linear loads. A distorted current causes distorting voltage drops so that the resulting voltage supplying a circuit will not be sinusoidal anymore. The voltage provided is the transformer voltage minus the voltage drop across the feeder. Thus, the voltage distortion depends on the distance from the transformer and on the line impedance. In short, the voltage distortion affecting the grid at a certain location depends on the value of the short-circuit current of that point. Also, once the grid voltage is distorted a linear load absorbs a distorted current. The presence of such harmonics on the grid is responsible for detrimental effects. Moreover, at higher frequencies, iron losses (hysteresis losses and eddy current losses) as well as the losses in the cables increase. Finally, electronic equipments may experience failures due to the presence of harmonics.
Another aspect which should not be neglected is the resonance issue related to the presence of harmonics in electrical networks. In fact, in this case the amplitude of a specific harmonic may rise up to several times that of normal operation. Consequently this high-value current may seriously damage capacitors and equipments connected to the grid.
In order to prevent this kind of event, the resonance frequency of the grid at a certain point has to be known and, additionally, the insertion of well-fitted anti-resonance coils may be considered to damp the oscillatory phenomena.
2.7 Interharmonics
Interharmonics are particular harmonics whose frequency is not an integer multiple of the fundamental frequency. The analysis of such interharmonics has attracted increasing interest over the last few years since the massive use of power electronic equipments has caused an increment in their amplitude. They can be observed where there is at least a part not pulsating synchronously with the fundamental power system frequency. There are many loads introducing voltage or current interharmonics such as arc furnaces, welding machines and cycloconverters.