Üç fazlı, üç iletkenli bir şebekede simetrisiz bir besleme noktasının koşut ve seri simetrileyicilerle simetrilenmesi

Başlık çevirisi mevcut değil.

Tez No: 66628
Yazar: ERGÜN ALKAN
Danışmanlar: PROF. DR. İLHAMİ ÇETİN
Tez Türü: Yüksek Lisans
Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1997
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
Bilim Dalı: Elektrik Mühendisliği Bilim Dalı
Sayfa Sayısı: 51

Özet

ÖZET İncelemede üç fazlı, üç iletkenli bir besleme noktasından beslenen simetrisiz bir yük değişik özadmitanslı, üçgen bağlı bir eşdeğer devre ile, simetrisiz kaynak devresi ise simetrili impedanslı, üç fazlı simetrisiz bir gerilim kaynağından oluşan bir eşdeğer devre ile temsil edilmiştir. Yük eşdeğer devresi simetrili bileşenlerin admitans matrisi ile incelenmiştir. Yükün besleme noktasında oluşturduğu simetrisizliğin yük özadmitanslannın pozitif bileşeninden ileri geldiği ispat edilmiştir. Yük simetrileyicisi yük özadmitanslannın pozitif bileşenini dengeleyerek faz gerilimlerini simetriler. İncelemede etkin güç simetrisizliği, tepkin güç simetrisizliği ve özadmitanslardan birinin farklı olduğu durumda simetrili bileşenlerin admitans matrisi hesaplanmıştır. Kaynak gerilimlerinin simetrisizliğinin besleme noktasında oluşturduğu simetrisizliğin kaynak devresi ile besleme noktası arasına seri bağlanan bir simetrileyici devre ile tamamen giderilebileceği gösterilmiştir. Simetrileyici özimpedanslarm pozitif bileşeninin hat akımlarının pozitif bileşeni ile etkileşimi sonucu besleme gerilimleri simetrilenir. İncelemenin en önemli sonucu indüktanslı veya kapasiteli yükleme durumunda besleme noktasında ölçülen hat akımları ve faz gerilimleriyle yük eşdeğer devresinin nasıl elde edileceğini göstermesidir İncelemede buna ilişkin bir örnek problem de çözülmüştür. vıı

Özet (Çeviri)

SUMMARY In a three-phase, three-wire network the symmetry of the supply voltage in its rated value is the main criterion for the supply and loading quality. Lines, transformers, three-phase induction machines are designed for symmetrical operation. Due to the asymmetry the useful power carried in the lines reduces and the losses in the lines increase. The revolving field which is one of the most important advantages of the three-phase network can not exist if the supply voltage is asymmetrical. Specially in the induction machine, whose negative sequence impedance is very low, a small voltage asymmetry generates a big current asymmetry. Due to the asymmetry overvoltages and undervoltages are formed in the lines. However all the electrical appliances are designed for the rated supply voltage. Several type~öf loads such as computers, lamps of the discharge and fluorescent types, automatic control circuits, televisions are particularly sensitive to undervoltages. Arc furnaces, induction furnaces, steel rolling mills, arc welders, induction welders, large single phase load centers are the largest asymmetrical loads in the network. The symmetry of supply voltage determines directly the efficiency of the energy transmission and the energy conversion. Therefore it is very important both for the supplier and the consumer. The voltage drop on the supply source impedance caused by the negative sequence current components of asymmetrical loads remove the symmetry of the supply voltage. Steinmetz showed that a single-phase resistive load with a conductance of G connected between phases L1-L2 can be transformed into a three-phase symmetrical load by connecting a C element with a susceptance of G/^3 between phases L2-L3 and a L element with a susceptance of -G/V3 between phases L3-L1. The symmetrization method of Steinmetz bases on the fact that the line currents of the assembly formed by L and C elements having zero total susceptance forms a negative sequence current system in the lines if the phase voltages are symmetrical. This method can be easily generalized for three-phase asymmetrical loads. By choosing appropriate L and C elements it is possible to regulate the RMS value and the phase of the negative sequence current system. In this way the asymmetrical load currents can be symmetrized. In the past, as the possibility of controlling the reactive power asymmetrically and fast was very restricted, the applications of the method of symmetrization of Steinmetz were very limited. As it is well known, the possibility of controlling the reactive power in various supply points has great importance in improvement of the supply and loading quality of the network. Static compensators are the modern reactive power generators operating in inductive as well as capacitive zone. With static compensators reactive power can be controlled stepless and very fast in each phase. They can remain connected to the lines even in the case of power failure and they don't have synchronization problem. Nowadays the static compensators are the most suitable implements for applying the symmetrization method of Steinmetz. With them the applications of this method are largely increased. Their structure is simple and reliable. Their losses are low. With these properties they are superior to the synchronous condensers. viiiStatic compensators consist of three main units: 1. A transformer for the connection of the equipment to the high voltage network. 2. Thyristors connected in antiparallel. 3. L, C or L and C elements. The control of the reactive power in inductive range is made by means of a thyristor-controlled reactor or thyristor-switched reactor steps. In thyristor-controlled reactor scheme the control of reactive power is made at the expense of generating harmonics. The control of the reactive power in capacitive zone is made by means of thyristor-switched capacitor steps. If it is desired to control the reactive power in both the inductive and the capacitive ranges a combination of the above mentioned methods may be used. The same result may be obtained by using a thyristor-controlled reactor connected in parallel with a fixed capacitor. In general the asymmetry in a supply point is the resultant of the influences of all the asymmetrical loads in the network. The asymmetry caused by the asymmetrical loads on the load side of the supply point is examined by means of the load equivalent circuit, while the asymmetry caused by the asymmetrical loads on the source side of the supply point is examined by means of the source equivalent circuit.. The source equivalent circuit is represented by its Thevenin equivalent.. The source impedances are considered symmetrical. The linear three-phase load is represented by a delta connected equivalent circuit in which every phase is considered independent from the others. It is easy to calculate the elements of the load equivalent circuit when the phase voltages are symmetrical. However due to the asymmetrical voltage drop on source impedance caused by the negative sequence component of the line currents the phase voltages in a three-phase supply point can not be symmetrical if the source or the load is asymmetrical. Therefore the elements öf the load equivalent circuit should be calculated in the case of asymmetrical phase voltages. However when the phase voltages are asymmetrical there is an infinite number of load equivalent circuits satisfying the circuit conditions in the supply point. Many of these circuits are not equivalent to each other with respect to the asymmetry they contain. To realize a successful symmetrization the load equivalent circuit used in the calculations should contain the same asymmetry as the load. No matter how correct the symmetrization method is, it is not possible to obtain successful results if incorrect equivalent circuits are used. For instance, the symmetrization method of Steinmetz can not be successful if it is applied to incorrect load equivalent circuits. The main objective of this study is to obtain the source and the load equivalent circuits under asymmetrical supply conditions. The load equivalent circuit is examined by means of the admittance matrix of the symmetrical components. This examination gived the following results: 1. The symmetrical components of the line currents are independent from the zero sequence component of phase voltages; 2. The interaction between the zero sequence component of load admittances and the positive sequence component of phase voltages generates a positive sequence current component; IX3. The interaction between the zero sequence component of load admittances and the negative sequence component of phase voltages generates a negative sequence current component; 4. The interaction between the positive sequence component of load admittances and the positive sequence component of phase voltages generates a negative sequence current component; 5. The interaction between the negative sequence component of load admittances and the negative sequence component of phase voltages generates a positive sequence current component. To symmetrize the unbalanced load a delta connected load symmetrizator is supposed to be connected to the supply point. It is showed that: 1. When an asymmetrical load is connected to a symmetrical three-phase supply point, the symmetry of the phase voltages is removed by the positive sequence component of load admittances. The behavior of its zero and the negative sequence components is completely dependent on the positive sequence component. The negative sequence current component relevant to its zero sequence component and the positive sequence component relevant to its negative sequence component are zero if its positive sequence component is zero. For this reason, it is sufficent to balance the positive sequence component of load admittances to symmetrize the supply point; 2. A delta connected load symmetrizator formed by independent L and C elements can completely symmetrize the load. The positive sequence component of the symmetrizator admittances should balance the positive sequence component of load admittances; the other condition which is necessary to determine the admittances of the load symmetrizator is obtained by considering the aimed power factor of the load; 3. The negative sequence component of load admittances does not affect the symmetry of the supply phase voltages. Therefore a three-phase asymmetrical load which does not contain positive sequence component in its admittances is“symmetrical”with respect to the loading quality; 4. If the source supplying the supply point is asymmetrical the negative sequence component of load admittances is independent from its positive sequence component. Therefore both components should be balanced to symmetrize the load. However since a symmetrizator formed by passive elements has only three degrees of freedom it is only possible to symmetrize only three of the active and reactive components of the positive and negative sequence components of load admittances. Therefore when both the source and the load asymmetry are present it is not possible to symmetrize completely the load by controlling the reactive power in the lines. The examination made by means of the admittance matrix of symmetrical components showed that the load asymmetry is determined by the relative magnitude of its admittances' symmetrical components. According to this the admittance matrix of symmetrical components determines the loading quality whatever the supply qualityis. Therefore it represents a fundamental property of the load to which it belongs. Its structure is simple. The admittance matrix of symmetrical components is calculated in the case of active power asymmetry, reactive power asymmetry and when only one of the line-to-line admittances is different. It is showed that in all these three cases the positive and the negative sequence components of load admittances are related to each other and that it is possible to calculate the load admittances if the phase voltages and the line currents are known. The source equivalent circuit is examined by means of the impedance matrix of symmetrical components. To remove the source asymmetry a series compensator is supposed to be connected between the source and the supply point. The examination gived the following results: 1. The asymmetry in the supply point caused by the asymmetry of the source voltages can be removed by using a source symmetrizator formed by passive elements and connected between the source and the supply point; 2. The negative sequence component of source voltages is balanced by the voltage drop due to the interaction between the positive sequence component of symmetrizator impedances and the positive sequence component of the line currents; 3. The compensator does not require active and reactive power. The knowledge of the phase voltages and the line cuirrents in the supply point is not sufficient to calculate the load equivalent circuit if the phase voltages are. asymmetrical. In the case of a general three-phase asymmetrical load the positive and the negative sequence components of load admittances are not related to each other and there is an infinite number of admittance symmetrical component systems satisfying the circuit conditions in the supply point. Each one of these systems corresponds to a particular load equivalent circuit. From these equivalent circuits, those having a different positive sequence component of admittances are not equivalent in the sense of the asymmetry they contain. Therefore a different load symmetrizator corresponds to each one of them. For this reason the admittance matrix of symmetrical component and hence the load asymmetry is undetermined in the case of a general three-phase load. In practice a mixed case of loading is rarely met. The load admittances are in general either inductive or capacitive. Industrial asymmetrical loads are mostly inductive. Therefore it is not required to calculate the admittance matrix of symmetrical components in a general case of loading. It is showed that the active component of the zero sequence component of load admittances has its maximum possible value when all the active power fed to the load is consumed in the lowest line voltage; it has its minimum possible value when all the active power fed to the load is consumed in the highest line voltage. Similarly, in the case of an inductive or capacitive three-phase load, it is showed that the reactive component of the zero sequence component of load admittances has its maximum possible inductive or capacitive value when all the reactive power fed to the load is consumed in the lowest line voltage; it has its minimum possible inductive or xicapacitive value when all the reactive power fed to the load is consumed in the highest line voltage. Hence it is showed that it is possible to determine on the complex plane the area in which the zero sequence component of load admittances is. In the case of an inductive load this area is in the right lower quarter of the complex plane, while in the case of a capacitive load it is in the right upper quarter. The greater the difference in the line voltages is, the larger the area is. Taking in consideration that in general the negative sequence component of the line voltages does not exceed 5% of its positive sequence component, the zero component of load admittances can be calculated with a good approximation. After this the areas in which the positive and the negative sequence components of load admittances are, can be calculated using the circuit conditions in the supply point. Therefore, in the case of inductive or capacitive loading the admittance matrix of the symmetrical components and so the load equivalent circuit can be calculated with a good approximation. The knowledge of the phase voltages and the line currents in the supply point is sufficient for the determination of the admittances of the load symmetrizator. The proposed method of calculation of the load admittances is very useful also for the reactive power compensation as it allows the determination of the reactive component of the zero sequence component of load admittances with a single measurement of the phase voltages and the line currents in the supply point. After the load is symmetrized, the phase voltages and the line currents are measured. In this case the asymmetry of the phase voltages may only be caused by the source asymmetry. The source equivalent circuit can be calculated using the measured valued previous and after the load symmetrization. Then the source asymmetry is removed by the proposed source symmetrizator. A problem is solved as an example to show the method of calculation of the load symmetrizator's admittances and the source symmetrizator's impedances. The solution confirms the success of the proposed methods. xn

Benzer Tezler

Tez No
604461
Alçak gerilim dağıtım sistemlerinde nötr yüklenmesinin incelenmesi
Investigation of neutral loading in low voltage distribution system
VOLKAN AYTEKİN
Yüksek Lisans
Türkçe
2019
Elektrik ve Elektronik Mühendisliği Gebze Teknik Üniversitesi
Elektronik Mühendisliği Ana Bilim Dalı
DR. ÖĞR. ÜYESİ TUBA GÖZEL
Tez No
84984
Lineer olmayan yüklerin bulunduğu alçak gerilim şebekelerinde nötr akımı harmoniklerinin analizi ve simülasyonu
Analysis and simulation of neutral current harmonics in low voltage distribution systems containing nonlinear loads
İHSAN DÖŞEYEN
Doktora
Türkçe
1999
Elektrik ve Elektronik Mühendisliği Yıldız Teknik Üniversitesi
Elektrik Mühendisliği Ana Bilim Dalı
YRD. DOÇ. DR. KENAN UÇKU
Tez No
831844
Comparative study of different MPPT algorithms on 250kW grid-connected PV system
250 kW şebeke bağlantılı sistem üzerinde farklı MPPTalgoritmalarının karşılaştırmalı incelenmesi
ZAINAB LARABI
Yüksek Lisans
İngilizce
2023
Elektrik ve Elektronik Mühendisliği İstanbul Aydın Üniversitesi
Elektrik-Elektronik Mühendisliği Ana Bilim Dalı
DR. ÖĞR. ÜYESİ ABBAS UĞURENVER
Tez No
884680
Solar sulama sistemleri için MPPT tabanlı üç fazlı evirici tasarımı ve uygulaması
Design and application of MPPT based three phase inverter for solar irrigation systems
MUZAFFER ÇAYIR
Yüksek Lisans
Türkçe
2024
Elektrik ve Elektronik Mühendisliği Bandırma Onyedi Eylül Üniversitesi
Elektrik-Elektronik Mühendisliği Ana Bilim Dalı
DOÇ. DR. HARUN ÖZBAY
Tez No
330605
Yenilenebilir enerji kaynakları için tek aşamalı MPPT denetimli çok seviyeli eviricinin gerçekleştirilmesi
Implementation of single stage line interactive multilevel inverter for renewable energy sources with mppt capability
ŞABAN ÖZDEMİR
Doktora
Türkçe
2013
Elektrik ve Elektronik Mühendisliği Gazi Üniversitesi
Elektrik Eğitimi Ana Bilim Dalı
DOÇ. DR. İBRAHİM SEFA

Geri Dön