Mikro şebekelerin toprak arızalarına karşı korunması
Microgrid protection against ground faults
Yazar: MUHSİN ALPER MUTLU
Danışman: PROF. DR. ÖMER USTA
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:
İnsan nüfusunun artması, şehirlerin genişlemesi, sanayinin ve teknolojini ilerlemesi ile elektrik enerjisine hiç olmadığı kadar ihtiyaç duyulmaktadır. Ulaşımda kullanılan elektrikli araçlardan, raylı sistemlere, sanayide kullanılan asenkron motorlara; konutlarda kullanılan elektrikli ev aletleri ve aydınlatma sistemlerine, ısıtma ve soğutmada, haberleşme ve finansal aktivitelin hepsinde elektrik enerjisi kullanılmaktadır.  
Günümüzde elektrik enerjisinin büyük bir kısmı termik ve hidro elektrik santrallerinde üretilmekte, gerekli iletim ve dağıtımın yapılması ile son tüketiciye kadar ulaşmaktadır. Elektrik Mühendisleri Odasının yayınlamış olduğu ekim 2017 Türkiye kurulu gücü istatistikleri incelendiğinde hidroelektrik santrallerinin %25, akarsu-hidrolik santrallerin %9, doğalgaz-lng-lpg santrallerinin %33, linyit-taş kömürü-ithal kömür santrallerinin üretimde %23 oranlarında söz sahibi oldukları görülmektedir. Buradan ülke olarak fosil yakıtlara büyük bir bağlılığımız olduğu çıkarımı yapılmaktadır. Ayrıca 31 Mart 2015 tarihinde ülke genelinde yaşanan elektrik kesintisi ile arıza durumunda gerekli önlemlerin alınamadığı ve şebekenin yeterli şekilde korunmadığı görülmüştür. 
Üretiminde fosil yakıtlara olan ihtiyacı azaltmak, arıza durumlarında enerji kesintilerini en aza indirmek ve yeni enerji kaynakları ile şebekeyi tanıştırmak amacıyla dağıtılmış üretim birimlerinin şebekeye entegre edilmesi gerekmektedir. Bu sayede enerji üretimi büyük santraller yerine, küçük güçlerde ve tüketiciye daha yakın noktalarda konumlandırılmış olan mikro şebekeler sayesinde yapılacaktır. Mikro şebekeler ile uzun iletim ve dağıtım hatlarında yaşanan kayıplar azalacak, arızalara daha dayanıklı; işletmesi ve bakımı daha kolay bir şebekeye kavuşulacaktır. 
Bu çalışmada,  her geçen gün sayıları artan mikro şebekelerin, yük akışları, topraklanması, arıza durumları ve korunması konularında çeşitli çalışmalar yapılmıştır. Devlet teşvikleri ile her geçen gün elektrik enerji üretiminde daha fazla pay sahibi olan alçak gerilim rüzgar santralleri, mikro şebekenin kaynağı olarak belirlenmiştir.  Şebeke ile birlikte ve izole çalışma koşulları incelenmiş, topraklama sistemleri tek hat şemaları tanıtılmış ve çeşitli arıza durumları elektrik enerji sistemleri simülasyon programları kullanılarak analiz edilmiştir. Sonuç kısmında alçak gerilim mikro şebekelerin topraklama sistemleri ve dirençleri arasındaki farklılıklar vurgulanmış, kısa devre ve gövde kaçağı gibi arıza durumlarına karşı korunması için çeşitli koruma önerilerinde bulunulmuştur.

Abstract:
Day by day, the world people are living in needs more electrical energy. Consumption of the electrical energy is accelerated by population increase, expanding of the cities and latest developments on industry and technology. Electrical energy is used on transportation by electrical vehicles, in industry by induction motors and in residential buildings by electrical devices and lighting. Moreover, heating and cooling systems, communication and financial activities also use electrical energy.
Today, big portion of the electrical energy is generated by thermal and hydro electric power plants. Transmission  and distribution activities need to be done, in order to satisfy the  necessary energy for the consumers. According to report of the Chamber of the Electrical Engineers, which is published in October 2017, installed power of Turkey consists of 25%, 9%, 33%, 23%, hydro electric power plants, stream power plants, natural gas power plants and coal power plants respectively. It has seen that, energy generation of Turkey is highly depends on fossil fuels. Besides, a recent black out, which was occurred in March 31th of 2015 showed that, necessary precautions are not fulfilled for electrical grid system of Turkey.
Distributed generation needs to be applied on today's traditional grid, in order to decrease the requirement on fossil fuels and integrate the new electrical sources with the electical systems. In addition, interruption time of the electrical energy is going to decrease with the new application. Electrical energy will produced by micro girds instead of big power plants, which are far from the consumers. Not only generation of the energy, but also transmission and distribution losses will decrease with the micro grids. Integration of the micro grids also provide more reliable and well protected electrical network through the country.
Day by day investment on microgrids is increasing. Besides, government supports the investors with the related regulations. Upcoming project  which is called "Areas of Renewable Energy Sources" (ARES) in Turkey is signed by several companies and ARES is going to start in near future. Although, powerful wind turbines are generally integrated on medium voltage levels, low voltage wind turbines are the most common turbine types in terms of consumer demands. A small powered wind turbine can be bought from various sellers in the market and can be easily integrated into houses, schools or rural areas. However, some of the topics should be well studied in order to protect equipment and human life.
In this master thesis, grounding techniques and protection methods of low voltage microgrids are selected as main topics, because of uncertanity between literature and practice. Grounding plays essential roles on equipment life, operation continuity and human life. Inappropriate grounding may cause mechanical stress and insulation problems on electrical equipment during faults. Besides, leakage currents cause electrical shocks on livings. In order to reach correct results, all grounding methods are fully studied. 
Before starting the simulation studies, power and voltage level of all electrical equipment are decided. 20 kw wind turbine is selected source of the low voltage microgrid. Transformer is selected 250 kVA, generator is selected 30 kW and load is selected 25 kW. Since microgrid operates at low voltage, transformer voltage level adjusted at 34.5/0.4 kV. Buses are used for modelling the electrical panels. Cable cross section areas and core numbers are arranged according to nominal currents and grounding schemes. With the help of decided parameters, single line diagram is created on simulation program.
ETAP (Electrical Transient Analyser Program) is selected as simulation program which is widely used by electrical engineers on modelling the power systems. It has several features such as load flows, short circuits analyses, grounding system designs and coordination of protective devices. Lots of calculations easily and quickly can be achieved by ETAP. Besides, all calculations can be reported according to related standards such as ANSI and IEEE. 
Some of the ETAP tools are briefly explained in this thesis. After giving information about the program, load flow studies in grid connected and islanded modes are made. In addition, grounding details of the microgrid is introduced according to practice. Then, several short circuits such as three phase, phase to phase, phase-phase-ground and single phase to ground are made in the system, while changing the grounding system (TN-S, TN-C, TN-C-S, TT and IT) and grounding resistance (directly grounded, 3 ohm grounded). Short circuit results are given in the charts and leakage current paths are introduced with the figures. All results are discuessed at the end of following sections. 
Not only short circuits, but also leakage currents on equipment chassis are investigated for calculating the touch voltages. Grounding system of the microgrid decides the position of the earth conductor. It can be used separately from neutral as PE or combined with neutral as PEN conductors. Loop impedances should be calculated in order to detect the fault currents by the protective devices. 
After both system and equipment grounding simulation results, proper protective devices and their places in the power system are predicted. Nominal and short circuit currents are compared each other and correct values of low voltage circuit breakers and fuses are decided. Simulation results showed that, some of the short circuit currents can not be detected by the protective devices. Besides, some leakage currents cause hazardous touch voltages on electrical equpiment. All cases are examined deeply in their related sections.
To sum up, general concept of microgrid operation, integration and grounding methods are investigated in the literature. Then, simulation program of the project is introduced. Reference system is explained and new low voltage microgrid single line diagram is given. After the decision of the electrical equipment parameters, load flow results are taken both grid connected and islanded modes. Moreover, several short circuit situations are simulated and short circuit currents and voltages are investigated. Proper protective devices are decided and set values are calculated. It has seen that, system grounding types (TN, TT, IT) are not directly effects the short circuit currents. However, system grounding resistance value is essential to detect the short circuit conditions. Besides, Earth electrode and equipment grounding resistance are important to calculate touch voltages. Circuit breakers, fuses and relays should be set according to all short circuit analysis.