400 kV enerji iletim sisteminde anahtarlama olaylarından kaynaklı geçici aşırı gerilimlerin incelenmesi
Investigation of transient overvoltages in 400 kV transmission system caused by switching actions
Yazar: HABİP ÖNER
Danışman: DR. ÖĞR. ÜYESİ SUAT İLHAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Elektrik İletim Sistemi, ülkemizde 400 kV ve 154 kV gerilim seviyelerinde Türkiye Elektrik İletim Anonim Şirketi (TEİAŞ) tarafından işletilmektedir. Belirtilen bu gerilim seviyeleri, Çok Yüksek Gerilim (ÇYG) ve Yüksek Gerilim (YG) olarak ifade edilmekte olup bu seviyede enerji iletiminin, enerji kaybının optimum sistem koşullarında minimize edilmesi için kullanıldığı birçok veri ve araştırma vasıtasıyla bilinmektedir.
YG iletim sistem işletmesinde sürdürülebilir enerji arz güvenliğinin sağlanması ve enerjinin sürekli ve kaliteli iletilebilmesi için, sistemde kullanılan tüm YG teçhizatın izolasyonunun işletmede olabilecek her türlü sistem koşuluna uygun olarak tasarlanması ve gerek YG iletim hatlarında gerekse Trafo Merkezlerinde(TM) uygun izolasyon koşullarını sağlayacak şekilde tesis edilmesi büyük önem taşımaktadır.
Bu nedenle, YG iletimin gerçekleştirildiği bölgenin yük durumu, iletim hatlarının uzunluğu ve topolojisi, gerilim seviyesi vb. göz önünde bulundurularak sistem analizleri YG teçhizat açısından uygun izolasyon koşullarını belirlemek amacıyla uygun yazılım programları aracılığıyla gerçeklenir.

Elektrik iletim sisteminde gün içerisinde Yük Tevzi Merkezleri tarafından sistem manevraları gereği defalarca tekrar edilen iç anahtarlamalar ise dinamik sistem koşullarının YG techizat üzerinde oluşturduğu etkileti gözlemlemek adına bize önemli veriler sunmaktadır. Şebekede uygulanan bir çok anahtarlama manevrası, gün içerisinde anahtarlamanın yapıldığı anın gerilim seviyesine ve yük durumuna göre farklı geçici durumlara sebebiyet vermektedir.

Özellikle kapasitif devre anahtarlanmasının şebekede karşılığı olan uzun iletim hattı ve kablo anahtarlaması veya küçük endüktif akımların kesilmesi durumuna karşılık gelen reaktör anahtarlaması, yüksek genlikli faz-toprak veya faz-faz arızalarına sebebiyet veren kısa devre olaylarının incelenmesi, YG techizat izolasyonunun her türlü sistem koşulunda optimize etmek ve daha doğru sistem tasarımı yapmak adına oldukça önemlidir.

Enerji sektöründe bir çok alanda olduğu gibi, optimum sistem izolasyon koşullarını sağlamak adına yılların birikimi ile uluslararası elektrik mühendisleri tarafından oluşturulan CIGRE çalışma grupları ve bunların derlenmesi ve yorumlanması ile meydana gelen IEC standartları, elektrik iletim sistemi tasarımında ve sistem işletmesinin dinamik bir şekilde sürekli ve kaliteli sağlanması adına mihenk taşı konumundadırlar.
Bu çalışmada, Türkiye Elektrik İletim Sisteminin bir bölümü olan 28baralı 400 kV iletim sisteminde belirlenmiş, gün içerisinde belirli bir anda elektrik iletim sisteminde oluşan arıza veya anahtarlama koşullarını temsil eden senaryolara bağlı olarak pratikler yazılım ortamında uygulanmış olup elde edilen veriler uluslararası standart ve teknik raporlar referans alınarak yorumlanmıştır.

Abstract:
The increasing need of power indeveloping world is sustained all economically by increasing the transmission voltage levels to higher values as much as we can according to the standards. Thus, huge power plants as well as electrical transmission networks of countries which are bound by neighborhood are interconnected to transmit electrical energy to very long distances effectivelyand to meet power quality level requirements efficiently. 

To meet quality level requirements efficiently, it is highly accepted that having knowledge about electrical system that is operated dynamically is inevitable. Many factors during operation must be considered to not to meet unprecedented events caused by any phenomena. These phenomena may be external or internal effects. While internal affects are considered to be operator-decided system manevours or faults met in the system, external effects generally caused by atmospheric events.

Important phenomena causing overvoltage taking place in power transmission lines are transients due to switching and lightning. As determined by standards, phenomena of lightning overvoltage are significant for lines which their voltage levels are up to 154 kV, whilephenomena of switching overvoltage are significant for UHV lines having 245 kV and higher voltage levels. This kind of overvoltage lead to high stress on insulation of power system equipment that may cause breakdown in the insulators and in the insulation of transformer and generator windings and so resulting in short circuits. Also, different zones of a country might have different withstand characteristics according to their weather conditions or altitudes that would certainly affect the insulation levels as well as attributes of HV equipment while meet by extreme system conditions.Hence, the design phenomena of transmission line and for the selection of insulation withstand levels of transformers and generator windings as well as of circuit breakers, it is significant to work on transient overvoltage before the transmission systems are got in service. 	

In all around the world, getting efficiency in the distribution and transmission energy sector is considered more important by the time passing. New methods have to implemented in order to make electrical transmission system stable as well as efficient that would help to operate system more easily and time-saving. To get this aim, attitude of system before installation ought to be analysed from the point of views of extreme conditions that might be encountered during operation. As known by system operators, it is impossible to make this condition analysis in service, thus software developments have a very significant role in this evaluation and determination process.

Nowadays, qualified electromagnetic transient programs are developed in parallel with the development of computer technology.The transient analysis of transmission systems has been implemented and simulationsof its components with equivalent circuit have been carried out.

In our country, electrical transmission system is operated by Turkish Electricity Transmission Corporation(TEIAS) at the voltage levels of 154 kV and 400 kV. As these voltage levels are High Voltages(HV) and Ultra High Voltages(UHV), it is targeted to minimize the loss of transmitted energy by carrying the energy at these high levels that  has been studied by many articles and papers.

It is extremely important that in order to operate system with sustainable energy supply security as well as with continuous and high-quality conditions, the insulation of HV equipment has to be designed according to different kind of system conditions, also both for HV transmission lines and substations must be installed, with sufficient insulation conditions.

To make the ones stated above, considering the load conditions of transmission zone, topology and length of transmission lines, voltage levels etc., system analysis are made via software to provide suitable and optimum insulation conditions in terms of HV equipment.
The switching actions directed by Load Dispatch Centres during the day to make some manoeuvres to make stable system conditions provide us to observe the effects of dynamic system over HV equipment. Many switching actions made in network causes different transient outputs according to time interval of switching that is significant for voltage levels and load conditions of the electrical transmission system.

Generally, five fundemantal maneuvers take places in Turkish Electrical Transmission System to operate the system in the interval of operation conditions: Energization and de-energization of HV transmission lines, energization and de-energization of undergroung HV cables, energization and de-energization of shunt reactors, energization and de-energization of capacitor banks and energization and de-energization of transformers-auto transformers. In this study, the performance of capacitor banks which doesn't exist in 28 busbar system haven't been evaluated.   According to electrical and magnetic characteristics of equipments to be switched, attributes of sytem simulation results that would effect the  determination of withstand levels changes. This point is very important to have knowledge about equipment as well as electrical system.

Especially, switching the long HV lines and cables which refer to capacitive circuit switching  or breaking the small inductive currents which refer to reactor switching; analysing the short-circuited conditions such as high amplituted phase to ground or phase to phase faults are very significant to optimize the insulation of HV equipments for every kind of system conditions as well as to design better system design.

For the field of power systems, as for other scientific fields, to provide optimum system conditions, organized by intellectual and experienced engineers, CIGRE working groups and IEC standarts which are compilation as well as commentary of technical reports of CIGRE are touchstone for the designing of electrical transmisson system and sustaining less risky, more dynamic and high quality electrical energy.

In developing countries like Turkey, many investments in the field of power transmission sector has been done in recent years. Thus, research and development units have been formed to comprehend and understand the system attitude in any point of view. Especially Turkish Electricity Transmission Corporation (TEIAS) has been taking considerable actions in analysing as well as improving its transmission system.

In this study, as a part of Turkish Electricity Transmission System, some kind of switching scenarios stated above and fault conditions have been applied in 400 kV transmission system with 28 busbars  via PSCAD-EMTDC transient analysis programme and the obtained data have been commented in the light of technical reports of CIGRE and international standarts IEC.