İletim şebekelerinde geçici hal kararlılık analizi
Transient stability analysis on the transmission system
Yazar: EBRU TAVUKCU
Danışman: PROF. DR. BELGİN TÜRKAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Bu tez çalışmasının temel amacı arıza öncesi kararlı çalışma koşullarına sahip örnek iletim sisteminin dışarıdan etki edilebilecek geçici müdahaleler sonunda (arıza etkisi altında) geçici hal davranışının incelenmesidir. Bu tez çalışmasında MATLAB Matpower (Matlab M-file) ve PowerWorld yazılımı kullanılmıştır. Analiz öncesi her iki programda incelenen IEEE-9 baralı güç sisteminin parametreleri aynı olacak şekilde değiştirilmiştir ve böylece aynı güç sisteminin farklı yazılımlarda kullanılması sağlanmıştır. Statik parametrelerin Matpower ve PowerWorld programında belirlendikten sonra, PowerWorld programında geçici hal kararlılık analizinin incelenmesi için kontrol sistemlerin (senkron generatör, uyarma ve governor) dinamik sistem parametreleri, modelleri ve değişkenleri tanımlanmış ve verilmiştir. İncelenen güç sisteminin potansiyel çalışma koşullarında arıza öncesi yük akışı analizi Matpower programı ile gerçekleştirilmiş ve elde edilen analiz sonuçları aktif ve reaktif güç sistemi kayıpları ile verilmiştir. Dinamik analiz yöntemi ile güç sisteminde müdahale edilmiş çalışma durumlarının elde edilmesi için farklı örnek arızalar oluşturulmuştur. Bunun için öncelikle Matpower ile incelenen potansiyel çalışma durumları (senkron generatörlerin güç sistemine ilettiği toplam mekanik gücün değiştirilmesi) geçici hal kararlılık incelemesi için PowerWorld programında oluşturulmuş ve sonrasında sistemin geçici hal kararlılık analizi sonuçları yük akışı sonuçları ile karşılaştırmalı olarak değerlendirilmiştir. Elde edilen analiz sonuçlarında 75 MW'lık senkron generatörün güç sisteminden devre dışı bırakıldığında 85 MW güçteki senkron generatörün devre dışı bırakılmasından daha fazla iletim kayıpları elde edildiğinin ve arızanın güç sistemine etkisinin - rotor açısı ve hız dalgalanmasının daha fazlı olduğu gözlenmiştir.  Daha sonrasında örnek güç sistemi arızaları ile dinamik geçici hal kararlılık analizi gerçekleştirilerek güç sisteminde elde edilen maksimum arıza yeri ve maksimum arıza durumların güç sistemine etkisi değerlendirilmiştir. Ek analizler ayrıca tez sonunda verilmiştir. Analizi yapılan müdahaleler güç sistemindeki senkron generatörlerde geniş rotor ve açı salınımlarına sebep olduğu için örnek olarak seçilmiş ve incelenmiştir. Bunun için iki temel arıza çeşidi olan senkron generatörlerin ve statik yüklerin açıp/kapatılması ve kısa devre  (3-fazlı dengeli ve faz A-toprak arası tek fazlı) oluşturulması geçici hal kararlılık analizi olarak seçilmiştir. Farklı arıza koşullarında güç sisteminin geçici hal kararlılık incelemesi yapıldıktan sonra elde edilen analizler karşılaştırılmıştır. Analiz sonuçlarında senkron generatörlerin rotor açısının değişimi mekanik ve elektrik yük dengesinin arttıracak arızanın oluşması ile arttığı ve senkron generatörlerin gerilim değişimi de üç-fazlı kısa devrenin oluşması ile arttığı gözlenmiştir. Ayrıca elde edilen analiz sonuçlarında örnek olarak governor kontrol sistemi ile sistem frekansının iyileştirilmesi sağlanmıştır. Yapılan analizlerde güç sistemin farklı çalışma koşullarında analiz edilen süre içerisinde kararlı çalışma koşullarına yaklaştığı gözlenmiştir.

Abstract:
In a power system, the initial conditions of the power system, and the impact of possible disturbances on the power systems determine the stability of the system. The possible disturbances are considered in this thesis as transient disturbances, which occur in a specific period of time such as 0.145 seconds (during 0.30 to 0.445 seconds). The observation of the transient behavior in a power system is carried out by the simulations results of rotor angle, voltage, and frequency fluctuation. 
In another way around, the impact of transient disturbances on a power system is analyzed with the transient stability of the system. The examination of transient stability is realized with the observation of the dynamic behavior of the power system, in which the sample time is selected as 0.0004 seconds in the thesis. Also, the ability of synchronism of a power system under, or after different disturbances is determined by the analysis of the dynamic behavior of power system. 
In this thesis, after the analyzed power system is selected as the modified IEEE 9-bus test system, load flow, and the time domain dynamic simulations are realized. In the selected modified power system, there are three different synchronous generators (the same synchronous generator type but with different parameters) with their different controller systems as governor, and exciter.
The aim of this thesis is to realize different transient sample faults and to analyze the impact of the faults on the power system. The selected fault types are considered as the deactivation of some synchronous generators and creating three-phase balanced short circuit faults in the selected time period. 
Before conducting the transient faults to the test system, the planned future power generation through renewable energy resources in Turkey is given in chapter 1 with introduction considering the objectives and prospects of Ministry of Energy and Natural Resources (ETKB). The given information on the change of energy resources indicates that the desired installed capacity from renewable energy resources is increasing, and expected as 20.000 MW in the end of 2023. While the more renewable energy resources are integrating to the power systems, the more given attention on the potential power system networks failure and manipulation are required to assure the reliability and security of supplying electricity from electrical generation to the end of customers. That's why, the main purpose of the thesis is to analyze the effect of different fault events on the system stability. 
While the load flow analyses are realized by using of MATLAB Matpower (M-file) software, the time-domain dynamic transient analyses are realized by PowerWorld software. After that, MATLAB® is used to adjust the parameters of the governor system to get more stable system frequency results. 
After that, some potential operating conditions of the power system are performed by Matpower m-file software and the results are given with load flow analyses with the active and reactive transmission losses in chapter 5. Then, the same operating conditions are created by PowerWorld software to analyze the power system with dynamic analyses. 
For this purpose, the potential working conditions examined by Matpower (for example the change of total mechanical power of the synchronous generators) are applied to the PowerWorld program as transient steady-state analysis, and the results of the transient stability analyses of the system are then compared with the load flow results. To realize the transient stability analyses, the dynamic parameters, models, and variables of the control systems, which are for synchronous generator, exciter, and governor, are specified as shown in chapter 4. 
But in that case, the different working conditions are considered as a transient fault event, which occurs within 0.145 seconds from 0.300 to 0.445 seconds. Also, the same fault analysis is performed for different cases to see the system's general transient response to the same fault. 
One of the considerable results of the obtained transient stability and load flow analyses is that when the synchronous generator with 75 MW active power is deactivated from the power system, the more transmission losses are obtained rather than deactivating of the synchronous generator with 85 MW active power. That's why, it is summarized in the end of the first simulation part that the more power losses have a large effect on the power system stability by the change and fluctuation of the rotor angle and speed. On the other hand, the exemplary fault cases are selected as switching on/off synchronous generators, and 3-phase balanced short circuit. In the analysis results, it has been observed that the change of rotor angle of the synchronous generators increases with the short circuit faults, which raises the unbalance of produced mechanical and supplied electrical power of the power system.
In the second part of the simulations, by performing transient stability analyses with the exemplary fault cases, the maximum change of rotor angle is specified. After the analyses are performed, the improvements in the system frequency are provided with the governor control system, which is selected as an example in the obtained results. 
After the reheater of the micro-turbine system (which is T5) is arranged according to the limit parameters of the model, the results show that when the mechanical output power of a synchronous generator is changed in the way of increasing, the frequency of the system becomes more stable. With this result, it can be summarized that by arranging output power of a synchronous generator via governor system the desired frequency value is possible to regain after the fault is cleared. 
On the other hand there are many other simulations are performed as shown in the appendix part such as network errors - a single-line short-circuit fault, three-phase balanced short circuit fault (at bus-4), and load fluctuations - switching-off static loads in the power systems, and switching-off static loads and each synchronous generator with new fault cases.  
The obtained results show that when a single-line short-circuit fault occurs in a power system, there is less voltage drop than a balanced three-phase short circuit fault (which is explained as the current during transient fault), and the system reaches its stability in both cases, so the damping torque is enough to return the power system after the faults to equilibrium working conditions. 
When static loads are deactivated from the power system in a defined period (0.30- 0.445 seconds), the swings of synchronous rotor angle are decreased compared with the first analyses (when each synchronous generator is switched-off with a new case study). That is because of the total load demand is decreased and the power balanced of the system is less disturbed than the first analyses. 
With all the analyses, it can be summarized that the load losses are a great issue in a power system to evaluate the transient stability problems, and the understanding of a synchronous generator's behavior during a transient stability helps to control the system with possible faults. After all the analyses power systems security can be improved by integrating these information to Information and Communication Technologies (ICT)