Elektrik enerji sistemlerinde güç kalitesi

Abstract

Yarıiletken güç elektroniği devrelerinin gelişmesi beraberinde bazı sorunları da getirmiştir. Bu nedenle teknolojinin gelişimine paralel olarak şebekedeki dalga şekli distorsiyonlan da artmıştır. Yarıiletken devreler yapı olarak şebekeden dalgalı akım çektiği için harmoniklere yol açar. Aynı zamanda şebekedeki anahtarlama olayları, kısadevreler, yıldırım düşmesi, kesintiler gibi etkenler de güç kalitesini olumsuz yönde etkiler. Günümüzde güç kalitesi sorunlarının çözümlenmesi hem verimlilik açısından ve hem de ekonomik nedenlerden dolayı bir zorunluluk haline gelmiştir. Bu sorunları çözmek için önce sistemli bir çalışma yapılması gereklidir. Daha sonra ihtiyaca ve elde edilen verilere bağlı olarak değişik yöntemler kullanılarak güç kalitesi problemleri giderilmeye çalışılır.

"Electrical power qualify" is a term which has captured increasing attention in power engineering in recent years. The term broadly refers to maintaining the near sinusoidal waveform of power distribution bus voltages at rated voltage magnitude and/or frequency. Power quality is not a new phenomenon, but what is new is the increasing severity of power quality problems in terms of customer sensitivity. Power quality is a term which has many interpretations, but can be generally be classified as an area that deals with intermittent and steady state voltage and current waveform distrtion from normal operating conditions. Historically, power quality phenomena can be traced to the early parts of this century, as a real world condition that electrical equipment is subjected to in most installations. There are three stages in history of power quaUty which summarize the development and level of interest in the area. The first stage is represented by Steinmetz, who found a solution for third harmonic currents resulting from saturated iron in machines and transformers. In the early 1900's, Steinmetz suggested using delta and undergrounded-wye connections to stop the third harmonics from spreading into the power system. With this problem, harmonics were put aside for several years until the 1930's and 1940's, which represents the second stages in its history. During this period, power and telephone circuits often shared the same paths. Audio interference was (and still is) a common problem because of harmonic currents that coupled inductively into nearby telephone circuits. These harmonics were produced by, among others, arc furnaces, large rectifiers, or power supply rectifiers. Harmonics aroses as an area of study once again. The new problems were lessened by filtering where necessary and by learning to limit the magnetizing current harmonics produced by distribution transformers. vn The third stages started in the late 1950's with the advent of power semiconductor devices. The emerging dependence on power electronic based devices has given rise to a renewed interest in harmonics again, in which turn has broadened to an interest in overall power quality, since power quality includes in addition to harmonics, non-periodic waveform. This renewed interest in power quality is of greater importance today for two reasons: firstly, there are many more harmonic producing technologies today (i.e.: alternate energy converters, static var compensators, motor control devices, flexible AC transmission systems-FACTS, direct energy conversion devices, and so forth), and secondly, there are devices and loads that are sensitive to waveform distortions (i.e.: word-processors, computers, relays, meters and industrial process controllers). A recent study on power system harmonic has identified power quality problems as: i. The failure capacitor banks due to dielectric breakdown or reactive power overload. ii. Interference with ripple control and power line carrier systems, causing misoperation of systems which accomplish remote switching, load control, and metering iii. Excessive losses resulting in heating of induction and synchronous machines. iv. Overvoltages and excessives currents on the system from resonance to harmonic voltages or currents on the network. v. Dielectric breakdown of insulated cables resulting from harmonic voltages. vi. Inductive interference with eommunications systems. vii. Errors in meter reading. viii. Signal interference and relay malfunction, particularly in solid state and microprocessor controlled systems. ix. Interference with large motor controllers and powerplant excitation systems. x. Mechanical oscillations of inductions and synchronous machines. xi. Unstable operations of firing circuits based on zero crossing detecting or latching xii. Excessive heating of transformers due to the frequency dependence core vui xiii. Change in TV picture size and brightness if harmonics affect the peak voltages xiv. Effects on computer and computerized automation production. Electric Power Quality " Harmonics Modelling and Analysis Component models Stochastic Methods Instrument Metering -Measurements Waveform Analysis Voltage Support -Software Solutions -SWCc Passive Filtreler -SVCs Active Filters Fundamental I Concepts I Effects Standards Definitions User Issues Protection Figure 1. Main aspects of electrical power quality IX Figure 1 is a pictorial of the main aspects of electric power quality. There are six areas shown are: Modelling and analysis : Analysis of nonsinusoidal waveforms in power distribution systems may be accomplished by time domain methods; transformed domain methods; simulation of the existing circuit. Time domain metodologies are primarily based on numerical integration techniques. Perhaps the most popular of these techniques is that used in the Electromagnetic Transients Programs (EMTP). This techniques entails a resolution of energy storage elements in power systems (e.g., V s, C's ) to a parallel combination of a fixed resistance and a time-varying current source. The time varying current source is found iteratively and recursively. Under this resolution, the power network may be modeled as a fixed bus conductance matrix, G^, and the injections currents, 1^(0, to the network are calculated at each time step, This metodology is complete and as detailed as the models used to obtain G^t) and I^Ct). Typically data requirements as well as computational requirements are intensive. Effects of power quality problems : The proliferation of non-lineer loads and sources, such as power electronic based equipment, has largely ocurred in the absence of complete standards which limit the harmonic signals that the power system should be able to withstand and that utilities can absorb. This situation is leading to an emerging problem of power quality for both utilities and their customers. The requirements of sensitive loads for "clean power", the characterization of sources of "dirty power", and the establishment of interface guidelines and standards require considerable technical and economic evaluation. Mitigation of power quality problem : Power quality problems span such a wide range of characteristics that it is difficult to summarize and prioritize research areas for this topic. Among these topics are : i. Novel efficient means of voltage regulation- Especially with rapid response time. ii. Optimal passive filter design iii. Practical active filter theory iv. Power electronic solutions to power qualty problems. v. Efficient power conditioning Measurements : Measurements of power quality problems may consist of testing in some or all of the following areas: i. Voltage / current / frequency / N-phase unbalance : Steady state deviations from normal utility voltage, current, frequency and phase operating points can be caused by many factors within the utility grid: customer loadsvariations, improper grounding, tap changing, and insertion of compensation elements. Test standards which may apply are : ANSI C84. 1, ANSI C37. 106, and IEEE 141 ii. Harmonics : No-lineer devices or loads cause voltage and current waveform distortions. These distortions which may be periodic in nature can cause problems. Voltage notching and flicker are also problems that may be caused by by the electronic devices. Test standards which may apply are : IEEE 519-D5, ANSIC57.106, and IEEE 141 XI iii. Energy usage : It is well known that power quality affects the real and the reactive power demans of equipment. This results in high cost of operation, losses, and voltage and current regulation problems. Test standards which may apply are : IEEE 519, IEEE 141, IEEE 389 and IEC 555 iv. Surge tests : Lightning or switching transients can cause damage or misoperation of devices not capable of handling such surges. Test standards which may apply are : ANSI / IEEE C62.4 1, and ANSI / IEEE C37.90. 1 v. Over and under voltage : Sags and swells in voltage over aperiod of several cycles are a phenomenon arising due to phase delays in the power system. Test standards which may apply are : IEEE 141, and ANSI C37