Elektrik şebekesi ile paralel çalışan yerel elektrik santrallerinin şebeke koruma sistemlerine etkileri
Effects of local power plants operating in parallel with the electricity grid on grid protection systems
Yazar: İSMAİL SINIRCI
Danışman: PROF. DR. MEHMET ALİ YALÇIN
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Elektrik-Elektronik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Günümüzde enerji ihtiyacının artması ile talebi karşılamak amacıyla dağıtık (embedded) enerji adı ile; GES (Güneş Enerji Santralleri), RES (Rüzgar Enerji Santralleri), HES (Hidroelektrik Enerji Santralleri), KIGS (Kombine Isı ve Güç Santralleri), YH (Yakıt Hücreleri) gibi yerel elektrik santralleri (YES) yaygınlaşmaktadır. Dağıtık (Embedded) santrallerin mevcut şebeke ağıyla entegrasyonu sırasında bir takım istenmeyen durumlar ortaya çıkabilmektedir.
"Şebeke", enerji talep edilen iki nokta arasında elektriğin iletimi için birden fazla yol olduğu anlamına gelir. Besleyici, dağıtım gerilim seviyesine ve iletken boyutuna bağlı olarak 2 MVA'dan 30 MVA'ya kadar güç dağıtabilen küçük bir iletim sistemidir. Elektrik ve Elektronik Mühendisleri Enstitüsü (IEEE), YES'in iletim sistemine yakın yerleştirildiğini ve merkezi generatör setlerinden önemli ölçüde daha küçük (1 kW ila ~10 MW aralığında) olduğunu belirlemiştir.
YES, coğrafi ve maliyet kısıtlamaları, mevcut iletim hatlarının ihtiyacı karşılayamadığı durumlarda trafik sıkışıklığının ve şebeke kayıplarının azaltılması, nüfus artışı ve endüstriyel gelişme nedeniyle iletim hatlarının yeniden yapılandırılması gibi konularda da büyük avantajlara sahiptir. YES ile şebekelerin altyapısı için yapılacak masraflar ertelenebilir ve kaliteli enerji sağlanabilir. YES ile elektrik satmanın yanı sıra şebeke yatırımlarını geciktirme, şebeke kayıplarını azaltma, güç kaynağı güvenilirliğini artırma ve karbon vergisinden kaçınma gibi finansal faydaları da vardır.
Elektriğin sağlanmasını ve tüketicinin ihtiyaç duyduğu elektriğin kalitesini yani güvenilirliğini ve kesintisiz çalışmasını sağlamak önemlidir. Tüketilen elektriğin miktarı ve kalitesi her tüketici için kendi ihtiyaçlarına göre değişmektedir. YES'in dağıtım sistemi üzerindeki olumlu etkisi, sistemde oluşacak kayıplarda azalma, gerilim profilinde düzelme, güç kalitesini iyileştirme, sistemin güvenilirliğini iyileştirme, iletim ve dağıtımı basitleştirmesi ve yeni veya geliştirilmiş iletim ve dağıtım altyapısına yapılan yatırımları geciktirmesidir.
Elektrik tesisatları ve şebekeler istenmeyen bir durumda olabilir ve güç kaynağının devamlılığı için olumsuz sonuçlar doğurabilir. Bu olumsuzluğu ortadan kaldırmak için enerji hatlarının ve bu enerjiyi kullanan cihazların güvenli çalışma koşullarında çalışmasını sağlamak ve bu koşullar altında ortaya çıkan parçayı (arızalı parçayı) şebekeden ayırmak için tasarlanan sistemlere koruma sistemleri denir. Koruma sistemlerinin başlıca işlevlerine seçicilik, hızlı çalışma, güvenilir çalışma, yedek koruma, ekonomik olma, kararlı olma örnek verilebilir. Koruma rölesinin ana görevi, şebekede oluşan arızaları veya acil durumları tespit etmek ve arızalı kısmı sistemin servis verilebilir kısmından izole eden anahtarlama cihazına seçici ve uygun komutlar vermektir.Güç kaynağı sistemlerinde oluşan anormal durumlar yüksek akımların oluşmasına sebep olur. Bu akımlar koruma röleleri ile tespit edilir ve gereken koruma işlevi gerçekleştirilir. Savunma koordinasyonu temel olarak; Güç sistemindeki olası her arıza noktasında, rölelerin hangi sırayla ve hangi gecikmeyle açılacağına karar vermek gerekir.
Dağıtık elektrik üretim tesislerinde koruma sistemleri adalaşma durumunu tespit edebilir olmalıdır. Herhangi bir arıza oluştuğunda ana şebekeden ayrılarak adalaşma durumu meydana gelir. Adalaşma oluştuğunda dağıtık elektrik üretim tesisleri enerji üretmeye devam ettiğinden hatlar da enerjili durumda kalacaktır. Adalaşma durumunun önlenmesi için Vektör Dalgalanma (gerilim değişim oranı tespit) röleleri veya frekans röleleri kullanılabilir. Bu yöntemlerde rölelerin koruma hassasiyeti önem arz etmektedir.
Asenkron motorlar şebeke ile paralel çalışması durumunda asenkron motorlar da herhangi bir kısa devre anında kısa devre akımını etkileyecek bir akım oluşturur.

Abstract:
Electricity is delivered from the generation units, transmission levels and sub-transmissions that make up the transmission system to the distribution system, to the main load station (substation), then to the medium voltage (primary feeder) and low voltage (secondary feeder) levels that make up the distribution system and to the consumer at the lowest voltage level. The term "network" means that there are multiple paths for the transmission of electricity between any two points in the system. Transmission systems are usually networked. Transformers, which convert the energy from high voltage transmission to medium voltage distribution, are the most important equipment of the main load centre. Most distribution systems are both medium voltage (MV) and low voltage (LV) radial.
A feeder is a small transmission system capable of distributing power from 2 MVA to 30 MVA, depending on the distribution voltage level and conductor size. Usually two to twelve branches emerge from the main sub-centre. The feeder, which has a "tree-like" structure, moves from the main loading centre and goes to the consumer in small branches. In the power system, all feeders form a radial system. Some distribution systems do not have a radial structure but have a ring (point secondary) structure. In these systems, the low voltage network is designed as a grid and is fed from several medium voltage networks. The low voltage network is very reliable. In case of failure of the medium voltage network, loads can be easily transferred to another network without interrupting customer services.
There is no generally accepted definition of a local power plant in the literature. Many terms and definitions are used for LPS. For example, the International Council for Large Electricity Systems (CIGRE) defines LPS as all generation facilities with a maximum capacity of 50-100 MW located in distribution systems that were not originally planned as centralised generation units. The Institute of Electrical and Electronics Engineers (IEEE) has determined that HPP are located close to the transmission system and are significantly smaller (in the range of 1 kW to ~10 MW) than centralised generator sets.
The LPS has great advantages in terms of geographical and cost constraints, reduction of traffic congestion and network losses in cases where the existing transmission lines cannot meet the needs, and restructuring of transmission lines due to population growth and industrial development. With the LPS, infrastructure investments of the networks can be postponed and uninterrupted and high quality energy can be provided. In addition to selling electricity, the LPS also has financial benefits such as delaying grid investments, reducing grid losses, increasing power supply reliability and avoiding carbon tax.Distribution systems are not designed to include cash-generating units like transmission systems. For example, adding a RES can change the fault current level. Therefore, local fault-tolerant devices may need to be redesigned. In addition, the resistance (R) of overhead lines commonly used at high voltage is usually smaller than the reactance (X) (X/R>~5). Line resistance has little effect on voltage drop. Line reactance affects voltage drop and line losses. However, in distribution networks, line resistance is usually larger than reactance. Therefore, since line losses are a function of the square of current and resistance (I2R), distribution line resistance significantly affects both line losses and distribution line voltage drop.
It is important to ensure the provision of electricity and the quality of the electricity required by the consumer, i.e. reliability and uninterrupted operation. The quantity and quality of electricity consumed varies according to the needs of each consumer. Today, FACP units are included in the system to meet the electricity needs of distribution systems to a certain extent. In addition to DC units, voltage regulation is performed in the reactive power compensation system with power electronics elements called FACTS elements and STATCOM type SVC active compensators developed in recent years. Therefore, such distribution systems deviate from radiality and the power in the system can also flow towards the source.
The positive impact of LPS on the distribution system is that it reduces system losses, corrects the voltage profile, improves power quality, improves system reliability, simplifies transmission and distribution, and delays investments in new or upgraded transmission and distribution infrastructure. In practice, it is quite difficult to utilise the positive effects of HPP. In order to make the best use of LPS in a distribution system, LPS should be placed in a suitable location with appropriate power, taking into account the characteristics of the distribution system and LPS. If the DC power installed in the power system is not suitable for its location, the advantages of DC may turn into disadvantages such as overvoltage fluctuations or line overload.
Electrical installations and networks may be in an undesirable condition and may have negative consequences for the continuity of the power supply. In order to eliminate this negativity, systems designed to ensure that power lines and devices using this energy operate in safe operating conditions and to separate the part (faulty part) that occurs under these conditions from the network are called protection systems. Examples of elements used in protection systems are breakers, measuring transformers, protection relays. The main functions of protection systems are selectivity, fast operation, reliable operation, backup protection, being economical, being stable. The main functions of protection relays used in protection systems are listed as protection, measurement, control, communication, reporting and alarm.
The main task of the protection relay is to detect abnormal or emergency situations occurring in the power system and to give selective and appropriate commands to the switching device that isolates the faulty part from the serviceable part of the system. Protection relays are connected to power systems on current and voltage transformers. False signals from the transformer may cause false tripping in the protection system. For this reason, current values under normal operating and fault conditions must be outside the saturation zone of current transformers. The selection of current transformers used for measurement and protection purposes is made by considering the following factors. These are insulation level, conversion ratio and secondary load.
Faults in power supply systems cause high currents to flow. These currents are detected by the protection elements and the relevant protection function is performed.Overcurrent protection can be performed by thermomagnetic circuit breaker, moulded case circuit breaker, fuse and overcurrent relay. Overcurrent protection is divided into four according to its operating characteristics. They are instantaneous overcurrent protection, constant-time overcurrent protection, reverse-time overcurrent protection and directional overcurrent protection.
In relays operating in instantaneous maximum current characteristic, when the current amplitude value is equal to or more than the set current value, it operates instantly without time delay. These overcurrent relays used in substations far from the grid have a small amount of current to switch in the circuit, this amount gradually increases as they approach the grid.
The protection function is realised for different current levels with different tripping times. The circuit breaker closest to the fault trips in the shortest time, while the other circuit breakers trip sequentially as they move towards the source of the fault.
As can be seen from the characteristic curve of the reverse-time overcurrent relay, when the current is greater than the set value, the operating time changes in the opposite direction, that is, the operating time shortens as the current increases. Its advantage over constant time overcurrent protection is that it switches on in a shorter time at high currents.
Directional overcurrent relay is used to provide more selective protection in systems where the fault current can be fed from two or more points. For example, in a system consisting of two parallel feeders and a source, if a fault occurs in one of the feeders, both parallel protection elements on the source side will open unless the directional overcurrent relay is used. Thus, while both feeders are de-energised, only the faulty feeder is isolated at opposite ends with the directional overcurrent relay.
Defence coordination is basically; at each possible fault point in the power system, it is necessary to decide in which sequence and with which delay the relays will open. The connection margin, connection speed or connection time interval is defined as the time delay between the primary and backup protections. This time delay is typically used between 0.20 and 0.40 s. To implement selective overcurrent protection in electrical power systems, it is necessary to comply with the conditions for matching the overcurrent protection relay. Otherwise, energy losses will occur in the power system due to unnecessary or incorrect switch-offs.
In the event of a fault in the power supply system; the main protection element closest to the fault must operate, if the main protection cannot perform its functions for any reason, the backup protection must operate, the faulty part must be isolated as quickly and as little as possible and synchronisation current must be provided. Protection relay settings must be calculated and used in such a way that the maximum short-circuit current will not damage electrical equipment. Similarly, they must be sensitive enough to detect the minimum short-circuit current. The protection relays of the equipment they protect under normal operating conditions must allow for dimensions such as current, voltage, etc.
In a radial system, reverse-time overcurrent protection is coordinated in series with each relay, starting from the farthest relay in the system and moving towards the source. The protection equipment is coordinated with each other in terms of current and time. Coordination control between protective devices in an electrical power system must be implemented in a practical and efficient way by analysing the available time characteristics of the protective equipment.Distribution systems utilise multiple generation sources to ensure continuity of power supply. Since the power flow is no longer unidirectional and is also more costly and complex than single-source distribution systems, this needs to be corrected by coordinating protection with the changing system topology.
Protection systems in distributed electricity generation facilities must be able to detect islanding. When any fault occurs, islanding occurs by separating from the main grid. When islanding occurs, the lines will remain energised as distributed electricity generation facilities continue to generate energy. If there is energy in the system, it creates safety risks in line repair. In addition, it causes a decrease in energy quality and damage to the energy source and loads. In order to prevent these situations, the protection system must be activated in a stable and reliable manner and prevent this situation. Vector Fluctuation (voltage change rate detection) relays or frequency relays can be used to prevent islanding. In these methods, the protection sensitivity of the relays is important. If the sensitivity is high, the distributed power plant may be disabled in small frequency or voltage changes. If the sensitivity is low, the relays may not recognise the islanding situation. Adjusting the sensitivity exactly will be healthy in terms of operation.
In the case of asynchronous motors operating in parallel with the mains, asynchronous motors also generate a current that will affect the short circuit current in the event of any short circuit. This current is caused by the rotor moving due to the residual magnetic field effect when there is no voltage in the network. The short-circuit contribution time is usually reset in 0.1 s. According to IEC/EN 60909-0 standard, if there is a motor output of more than 25% of the installed output power, the short circuit contribution of the motors integrated into the system is taken into account.