Doğrultucularda ve transformatörlerde harmonikler ve sıradışı harmoniklerin incelenmesi
Examination of harmonics and uncharacteristic harmonics in converters and transformers
- Tez No: 22081
- Danışmanlar: PROF. DR. R. NEJAT TUNÇAY
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 185
Özet
ÖZET Güç sistemlerinde güç elektroniği elemanlarının kullanımının artması har- monik gerilim ve akım seviyelerini arttırmaktadır. 6 ve 12 darben' dönüştürücüler güç sisteminde en yaygın olarak kullanılan güç elektroniği elemanlarıdır. İdeal ça lışma koşullarında bu elemanlar tipik bir harmonik davranışı sergilemektedirler. Ancak bu koşullardan bir veya birkaçının bozulması ile elemanlar sıradışı bir harmo nik davranışı sergilemektedirler. Bu çalışmada alternatif akım sistem dengesizlikleri nedeniyle 6 ve 12 darbeli dönüştürücü devrelerinin sergilediği sıradışı harmonik davranışı incelenmişin Sistem dengesizliği olarak gerilim ve faz dengesizlikleri göz önüne alınmıştır. Sonuçta bu dengesizlikler nedeniyle özellikle 12 darbeli çalışmada önemli büyüklükte 5. ve 7. sıradışı harmonikler elde edilmiştir. Güç sistemlerinde diğer önemli bir harmonik kaynağıda transformatördür. Sistemde çeşitli güç seviyelerinde kullanılan güç ve dağıtım transformatörleri, anma besleme geriliminden daha büyük besleme gerilimlerinde oluşan çekirdek doyması nedeniyle kendi mıknatıslama akımlarında harmoniklere neden olurlar. Harmonik bileşenler içeren mıknatıslama akımı, transformatör kısadevre empedansı nedeniyle transformatör besleme geriliminde harmoniklere neden olur. Mıknatıslama akımı besleme gerilimi ile doğrudan ilgili olduğundan bu kez harmonikli besleme gerilimi mıknatıslama akımına etki ederek mıknatıslama akımındaki harmoniklerin artmasına neden olur. Bu karşılıklı etkileşim sonucu harmonik problemi büyütülmüş olur. Bu çalışmada anma besleme geriliminden daha büyük gerilimlerde transformatör mık natıslama akımı harmoniklerinin nasıl değiştiği incelenmiştir. Sonuçta besleme geri limi anma değerinin üzerinde değerler aldıkça mıknatıslama akımındaki 3. harmoni- ğin önemli ölçüde arttığı görülmüştür. vıı
Özet (Çeviri)
EXAMINATION OF HARMONICS AND UNCHARACTERISTIC HARMONICS IN CONVERTERS AND TRANSFORMERS The levels of harmonic voltage and current on utility systems are increasing. The most important is the proliferation of devices which produce harmonics; solid state power conversion devices are prime examples. These devices find uses at wide ranges of power levels industrially, commercially, and in the home for voltage control, speed control, frequency changing, and power conversion, generally at a lover cost, with incre ased efficiency and reduced maintenance than the devices they replace. The increased usage of shunt capacitors in recent years to improve system operating efficiencies also has a significant influence on harmonic levels. Capacitors don't generate harmonics, but provide network loops for possible local or general resonance conditions. Even though capacitors do not generate harmonics, they can influence the magnitudes of harmonic voltages and currents which occur on the utility system as well as the customer loads. A thorough understanding of harmonics and their effects on capacitors is necessary for the proper application of capacitors in today's power systems. There are many sources of harmonics on an electrical power system. In gen eral, devices with nonlinear operating characteristics produce harmonics, including: - Semiconductor controlled power equipment, such as voltage controllers, rectifiers, inverters, cycloconverters, static var systems. - Transformers - Arc furnaces and arc welding equipment - Rotating machinery VlllUnder balanced system operation, harmonics can be classified as positive, negative, or zero sequence: Positive sequence: 1, 4, 7 Negative sequence: 2, 5, 8 Zero sequence: 3, 6, 9 For unbalanced conditions among the phases, unbalanced system voltage, system impedance, or load, each harmonic can occur in each of the three sequences. These are usually small components of the total harmonic distorsion. Most power systems can absorb large harmonic currents. Most problems occur when power factor capacitors cause the system to be in resonance at a signifi cant harmonic frequency. In radial distribution feeders, the harmonic currents flow from the source of harmonics toward the utility source. Power factor correction capacitors may mo dify that path for one or two of the offending harmonics. The direction of flow is not as easily discerned in transmision networks and can be difficult to determine pre cisely. Industrial power systems are like short distribution feeders with the ma jority of the series impedance coming from transformers. Therefore, the tuning is very sharp. As a result, industrial power systems are more prone to equipment fail ure due to harmonics than are utility systems. Other loads are effective in reducing the magnitudes of harmonic voltages when resonance is present Both motor loads and resistive loads can be effective. Loads have very little effect when the system is not in resonance. A harmonic producing load can affect other loads if significant voltage distorsion is caused. The voltage distorsion is a function of both the system impe dance and the amount of current injected. The mere fact that the load current is dis torted does not always mean there will be adverse effects on other power consumers. If the system impedance is low, the voltage distorsion is usually negligible. IXLimits on the system harmonic voltage distorsion are difficult to translate into limits for individual customers. The system voltage distorsion levels are depen dent on the system impedance characteristics, as well as the harmonic currents in- jacted by individual customers. The fast development of economical solid state power converters has ex panded their applications into process industries. These converters act as highly nonlinear loads in power systems. Naturally the harmonic phenomena are inherent in the use of the converters. If the converter operates under ideally balanced conditions, only the typi cal harmonics of order at the AC and DC side are generated. These harmonics are termed as characteristic or normal harmonics. However system and supply source imbalances are inevitable in any operations. As a result, there are always some un characteristic harmonics generated at both sides of the converter. Usually, imbal ances in the equivalent AC side impedance and in the AC supply voltages only cause asymmetry in the 120 electric spacing of the three phase currents. Each phase current is still symmetrical about the horizontal time axis. Odd harmonic spectra at the AC side of the converter and even harmonic spectra at the DC side are generated due to this imbalance. Usually, uncharacteristic harmonics are of small magnitude unless parallel resonance occurs in the AC system. Although any type of imbalance may occur, zero-sequence component generally do not exist in the AC power system, mainly due to configuration of appa ratus symmetry, and normal operating features. Imbalance can be considered as a negative-sequence component in the AC supply voltage source. In fact, fundamen tal voltage negative-sequence component is, to some extent, inevitable. Typically, a power system is considered balanced if the negative sequence component is less than 5% of the positive-sequence component To specify the source imbalance for the AC voltage, an unbalanced factor which is the ratio of negative sequence component to positive-sequence component can be defined. But, this is not sufficient to describe uncharacteristic harmonic be haviour, because the relative angle between the positive-sequence and the negative sequence components also affects the harmonic generation pattern at both sides of the converter.Some imbalances in the equivalent AC side impedances due to manufac ture tolerance of converter transformer and the AC system equivalent apparent im pedance always exist In practice, these imbalances give rise to uncharacteristic har monic generation. All uncharacteristic harmonics at both sides of the converter in crease with the increase in transformer impedance imbalance. At the DC side, the second order harmonic is dominant But, at the AC side, besides the 3rd order, the 9th and 15th triplen order current harmonics also have noticeable values. These do minant harmonics at both sides usually decrease when the firing delay angle beco mes large. For 12-pulse operation, at the DC side, all even harmonics have notice able values. At the AC side, especially 5th and 7th uncharacteristic harmonics ap pear. The degree of uncharacteristic harmonics caused by AC system and con verter transformer impedance imbalances depends upon design constraints and rea listically achievable tolerance limits. For a particular application, an optimum de sign can be envisaged, where the tolerances are specified to maintain the consequen- tal non-characteristic harmonics to acceptable levels. Triplen harmonics can be minimized by balancing the transformer phase impedances as closely as practicable for each 6-pulse bridge. Unlike other ac/dc system interaction phenomena, the latter problem apears to be reduced with a weak ac system, even if resonant at the third harmonic, but harmonic voltage distorsion may become evident In such a case, the dynamic computation of harmonic interac tion does give more insight into the problem than simple open-loop calculations. In designing the transformers for each 6-pulse bridge of a 12-pulse pair, residual 6-pulse harmonics (5th and 7th) can be rriinimized by giving attention to close matching of the reactances of each transformer. Otherwise the residual har monics could materalize as demanding residual harmonic filters. Valve firing angles are determined by automatic control system in a HVDC system. Random phenomena in the control system causes errors in the valve firing plan. These errors are responsible for uneven firing of the bridge and this in turn causes the direct current of the converter to be distributed between the three phases of the ac system in an uneven manner. XIIndividual valve type of converter control, there is a possibility of abnor mal harmonic generation in the connected ac system. This generation is more likely at heavier converter loadings. The control bus impedance function must be defined and its value calculated as an aid in predetermining the possibility of abnormal har monic generation due to a high-voltage direct current converter installation. If the ac system impedance assumes a large value at some frequency, the converter may excite and sustain a harmonic voltage of that frequency at tha ac bus. The control bus coupling circuit affects the abnormal harmonic generation and consequently it could be used as a tool in correcting some undesirable conditions of abnormal har monic generation. Abnormal harmonic generation can be restricted to a very small value (corresponding to random errors in firing angle control circuiting) if individiu- al valve type of firing angle control is abandoned. The harmonic behaviour of a converting station cannot meaningfully be considered in isolation from the associated ac system. The behaviour can be criti cally dependent upon individual harmonic impedances. Neither the short-circuit ra tio (indicative of the strength of the ac system) nor a source T-representation are suf ficient for harmonic assessment. It cannot be presumed that the operation of a scheme with any existing type of control (even assuming the control to be perfectly accurate) will bi inhe rently immune from harmonic instability. Also, design of filters to optimise the cha racteristic hamonic behaviour may be inadequate or detrimental in restricting non- characteristic harmonics, irrespective of whether imperfections in the firing angles are taken into account or neglected. The nonlinearity of the core saturation distorts the transformer magneti zing current The total harmonic distortion of the magnetizing current depends on the voltage magnitude applied on the transformer. The magnetizing current causes a voltage drop across the primary impe dance and the distribution line series impedance. These impedances consist of the transformer leakage reactance, the primary resistance, and the line wire resistance and series reactance. Since the magnetizing current is distorted, a certain amount of distorsion is added to the voltage waveform. Xll
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