Newton-Raphson yöntemi ile rüzgar santrali için eniyilenmiş şebeke entegrasyonu
Optimum grid integration of wind power plants by using Newton-Raphson method
Yazar: EMİN ÖZKÖSE
Danışman: PROF. DR. AHMET CANSIZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Dağıtım şebekeleri = Distribution networks

Özet:
Güç sistem analizinin en önemli konularından olan yük akışı analizi enerji sisteminin sağlıklı çalışması için yapılacak olan ilk analizlerdendir. Bu analiz sayesinde enerji sistemindeki gerilim, güç, enerji kaybı gibi temel bilinmeyenler hesaplanabilir.
Elektrik şebeke sistemindeki kayıplar gün geçtikçe daha önemli hale gelmektedir. Enerji kayıplarının en aza indirilmesi için tüm sisteme en kısa yoldan enerjinin aktarılması gereklidir. Bunun yanında yenilebilir enerjiyi şebekeye entegre etmek kadar, doğru noktadan da bağlantının yapılması önemlidir. İyi bir analiz yapılmadan bağlantısı yapılan herhangi bir üretim düzeneği enerji sisteminde kararlılık sorunlarına neden olabileceği gibi, gereksiz yere hatlarda enerji kaybına da neden olabilecektir. Hat kayıplarının azaltılması yönünde yapılacak olan çalışmalar enerji iletim maliyetlerinin düşürülmesi, karbon salınımının azaltılması, hatların daha verimli kullanılması gibi sonuçları doğuracaktır.
Bu tezde elektrik şebekesine yeni entegre edilen rüzgar santrallerinin, en düşük enerji kaybını gerçekleştirecek şekilde bağlantılarının nasıl yapılabileceği incelenmiştir. Bunun için öncelikle Newton- Raphson yük akışı yöntemi incelenmiş, bu yöntem temel alınarak bir yazılım geliştirilmiştir. Geliştirilen program özellikle yenilenebilir enerjinin şebekeye entegrasyonunun en iyilenmiş (optimum) şartlarda sağlanabilmesi için tasarlanmıştır.   Piyasada DigSilent, Etap gibi ticari yazılımlar olduğu gibi çeşitli ücretsiz yazılımlar da mevcuttur. Geliştirilen yazılım ile rüzgar türbinleri tiplerine göre ayrılabilmiş, rüzgar türbini her bir baraya teker teker bağlanıp rüzgar türbinin uç gerilimleri değiştirilerek sistem davranışı analiz edilebilmiştir.
Örnekleme amacıyla ele alınan çeşitli rüzgar santrali tipleri için rüzgar santralinin bağlantısının en uygun hangi baradan hangi şartlarda gerçekleştirilebileceği incelenmiş ve sonuçlar tartışılmıştır.

Abstract:
Electrical losses of the grids are getting more important every day. To achieve minimum losses at the grid, it is expected that the electrical energy should be provided to the grid via the shortest path. This work discusses the best way to achieve minimum line losses in case of a new wind turbine connection to grid.

Load flow analysis is used to determine unknown state variables of the electrical system. These unknown variables are voltage amplitudes and voltage angles. Other parameters of the system can also be calculated by finding unknowns. Line losses, active and reactive forces in buses, reactive powers produced by generators are the values that can be calculated from these voltage values. The analysis is performed in steady state. This analysis is necessary for the future expansion of the system as well as the control of the existing system. When the load analysis is performed, the buses may have voltages over the desired limit. The voltages below the limit cause flickers in the lighting, the engine speed decreases, and the pumps do not produce the desired pressure. An increase in voltage may cause the devices to burn. In addition, high voltage may result in overheating of the lines and operation near the stability limit.

In order to find unknown variables in the electrical network, known ones are needed. Active and reactive consumptions of loads are taken as known values. On the production side, the active power of the source is considered to be known. Load flow analysis provides voltage values of buses from known values

Power flow analysis that is required for a well-working power system is one of the major topics of power system analysis and design. Thanks to this method, the unknowns of a power system such as voltage, power or loss of energy can be calculated.

Generators, loads and transmission lines are used and analyzed for power system load flow analysis. 

Generators usually have active power output (P) and voltage amplitude | V | externally controlled. The most important part of the generator excitation system can be considered as an automatic voltage regulator (AVR). The voltage regulator measures the voltage at the generator output and takes corrective measures.
Voltage regulators are designed to take action even at the slightest voltage change. Therefore, time delay control is not used.
Each active power supply generator does not have to control (stabilize) the busbar voltage. For example, a wind farm with an induction generator with squirrel cage does not have a voltage stabilizing feature. Although the DVR (Dynamic Voltage Regulator) can be used as the voltage stabilizer in the squirrel cage induction generator, the result should be investigated whether the generator connected to the bus is capable of voltage stabilization.

Loads are active and reactive power elements. Therefore, the active and reactive consumption of the loads are known constants. The voltage at the terminals varies according to the current state of the network. For this reason, they are the values to be calculated.

Transmission lines are generally power consuming elements such as loads. The transmission lines can be modeled in various ways (nominal-pi, nominal-T) according to their length. It also shows a capacitive effect (Ferranti effect) if the line is not used. The lines connect the visible forces in the buses via their admittance and shunt admittance.

In wind turbines, induction generators are widely used. In an induction machine operating as a generator, the power source is the mechanical power received from the rotor. The stator of the induction machine consists of three-phase windings and is spread around the stator. Due to the current flow on these windings, a rotating magnetic field is formed around the rotor.
The interaction between the rotor current and the stator flux causes the rotor to rotate. However, this rotation is normally smaller than the synchronous rotation speed. If the rotor is connected to a wind turbine and rotated faster than the synchronous speed, the direction of the induced current and torque in the rotor is in the opposite direction to the motor operation. In this case the machine works as a generator. The mechanical power of the turbine is converted to electrical power and the load connected to the stator begins to feed. Since the machine is connected to the network, it will supply power to the network.

If the stator is designed with a 3-phase excitation current system, the machine connected to the wind turbine and the gearbox will initially start as a motor and increase its speed in the direction of capturing the synchronous speed. When the wind speed acts on the generator shaft at a level exceeding the synchronous speed, the induction machine will automatically switch to the generator operation and the obtained electrical power will be transferred to the grid via stator windings. Since the machine operates in parallel with the mains, excitation current is supplied from the mains. However, by connecting an external capacitor to the machine, the excitation current required by the machine can be provided without the need for a network.

Energy losses of the electrical grid are getting more important within the time. To minimize energy losses, it is certain that one must feed the loads via the shortest path available. As much as integrating renewable energy to the grid, it is also important to connect these renewables to the grid from the right point. Without making a good analysis, any energy generation unit may cause stability problems and also unnecessary wasting of the useful energy. Preventing the waste of energy has positive effects like decrease in cost of electricity transportation, decrease in carbon emissions, and effective use of electrical lines. 

This thesis examines the methods for making the connection of a new wind turbine to the grid at the bus that causes minimum energy loss. To achieve this, first Newton-Raphson load flow method is analyzed, then a new software developed based on this method. The developed software is designed to integrate the wind turbines to the grid at the optimum circumstances. There are commercial products such as DigSilent, Etap and free products such as Power System Toolbox.  By developing a new software, one can select the wind turbine type, integrate wind turbine to the buses and change terminal voltages automatically and analyze the system behavior instantly. Then, connection possibilities to the grid of wind turbines are examined. 

This thesis discusses the optimum connection conditions and optimum connection bus place for different wind turbine types.