Short circuit analysis of substations' busbars supplied by double feeder with upgrading of 154 kV transmission network to closed-loop system

Nowadays, power generation, transmission and distribution sectors develop rapidly owing to increasing electric power demand of industrial plants and individual electricity use as a result of technological developments. The number of power plants are increasing to meet the increasing demand of power and new generation power plants and power plants producing power from renewable sources are installed to generate electricity economically. These founded power plants must be far away from the areas where electricity is consumed intensively but quality and stable electric energy has to be reached to consumers. Power transmission system is a main energy connection which provides power exchange between production and consumption centers and balance of supply and demand. Power transmission system has a structure of an interconnected network. Power transmission system grows owing to increasing production and consumption centers which must provide quality, stable and balanced electric energy because production-transmission-distribution chain of power must develop as a balanced manner. New transmission lines and substations are established for providing necessary conditions about power demand and requirements of growing power consumptions areas. The existing transformers' power is rised or new transformer feeders are established in substations where these operations are necessary. In addition, some innovations and necessities are planned and carried out. Quality and reliability of the power demand are increased as a result of these changes in the transmission system. New situations, which will have occured as a result of changes in the transmission systems, should be compared with current situations at power transmission system and some foresights should be represent. In this study, a change carried out in a power transmission network is analyzed. Upgrading of this power transmission network to closed-loop system with establishing a power transmission line between two radial substations supplied from one line feeder is examined. The examination of power system is important in terms of providing operation efficiency and planning about system. Short circuit fault is the most significant fact because short circuit causes power cut, it damages electrical equipment and around the point where it occurs and it affects the consumers are away from short circuit by reducing voltage value in the transmission system. Therefore, short circuit analysis is the most important examination in the power system. Besides, short circuit analysis and calculations are used in some applications about power system. Short circuit calculations are important to determine the value of maximum short circuit current which occurs at the busbar of substation, busbar short circuit power, short circuit strength of electrical equipments, short circuit breaking capacity of circuit breakers and set values of protection devices. These datas are important in terms of efficient operation of transmission system and rapid cleaning of short circuit currents. In this study, short circuit analysis at high voltage and middle voltage bus of substations which must be examined and bus voltage analysis of substations which are sources of the transmission network have been performed with using of the network models designing at MATLAB/Simulink program. Lightning, fail and contamination of insulation, operation fault reasons, mechanical faults are the main factors which cause short circuit. The magnitude of short circuit current depends on the impedance between the source and the point where short circuit occurs, therefore severity of short circuit decreases when short circuit occurs away from the source. For example, substations are powerful sources whose busbars have high power of short circuit and their Thevenin equivalent system impedances are low. Therefore, short circuit currents have great values that occur at the close point of substation. Short circuit currents cause thermal and mechanical damage in power systems. Short circuit must be rapidly and faulted area must be seperate from system not to cut electric power of other consumers for decreasing damage of short circuit currents. The cleaning process of short circuit is fault detecting of relay which gets instantaneous current data of power system and opening of circuit breaker with sending trip command of relay to circuit breaker. Damage of short circuit can minimize or prevent according to this process time. Especially, relays and circuit breakers must clean fault rapidly in high voltage systems. Various types of short circuit faults occur in power system. Short circuit faults are classified as two groups that are symmetrical and unsymmetrical faults. Symmetrical fault is a balanced three phase fault. Frequency of occurrence of balanced three phase fault is low in power systems, but short circuit currents are highest in this fault. Because of this, circuit breaker short circuit breaking capacity is determined according to this fault. Unsymmetrical fault types are single line-to-ground, line-to-line and double line-to-ground fault. The frequency of occurrence of single line-to-ground fault is highest in power systems. In this study, symmetrical components method has been explained and fault types have been described with sequence equivalent circuits of faults. Besides, simulations of single line-to-ground and three phase faults have been carried out in the transmission system that is in this study. Voltage levels of Turkey Transmission System are 400 kV, 154 kV and 66 kV. In Turkey, TEİAŞ is responsible for carrying out operation and development facilities of the transmission network. In this study, 154 kV transmission network has been examined which is in the responsibility area of the 5.Regional Office. There are two 400 kV substations in this transmission network which are source of the network. These substations supply the transmission network with 380/158 kV autotransformers. There are five 154 kV substations in this network. Kaynarca and Karasu Substations are radial because they have one active line feeder. Sakarya and Melen Substations must be fed by one line feeder because of other line feeder of them is connected to radial substation. The transmission network is operated as a open-loop system network. A power transmission line has been built approximately 48 km, but it has not yet commissioned. Power exchange will be carried out between Kaynarca and Karasu Substations with commissioning of this transmission line and the transmission network will upgrade to closed-loop system. Kaynarca and Karasu Substations, Sakarya and Melen Substations, which are fed by single line feeder will be fed by double line feeder with upgrading open-loop system network to closed-loop system network. Firstly, seven substations have been formed as subsystem blocks and they have been connected each other that are the same structure with the real transmission network. Source blocks of Simulink have been added to Adapazarı and Osmanca Substations which are source of the transmission network. Power transformer blocks have been placed to subsystem blocks which represent substations and load blocks, which represent distribution feeders, have been also placed to output of transformers. Furthermore, measurement blocks have been used at specific points to get display of values and oscillographic views. Transmission lines and power transformers datas have obtained and they have been entered blocks. Winding resistance, leakage reactance, magnetization resistance and reactance of transformers have been calculated from short circuit test losses and core losses of the same power rating transformer. Value of resistance and inductance, which are components of impedances from 154 kV busbar Adapazarı and Osmanca Substations seperately back to system , have been written in power source block. The avarage values of power consumption in August 2014 when maximum power demand occured have been written in load blocks. Simulink model of open-loop system network have been formed at first and this model has been run to watch the values of current and voltage from measurement display. Single line-to-ground and three phase faults simulations have been performed at busbars of Kaynarca, Karasu, Sakarya and Melen Substations with using fault block of Simulink. Values of the short circuit currents and oscillographic views have been obtained from results of simulations. Moreover, voltage values of Adapazarı and Osmanca Substations are examined before and during faults. The transmission network model have been upgraded to closed-loop system with adding Kaynarca-Karasu transmission line block. This closed-loop network model have been run without fault and values of voltages and currents have been watched from displays at specific points. Single line-to-ground and three phase faults simulations have been performed at busbars of Kaynarca, Karasu, Sakarya and Melen Substations in this type of network and values of the short circuit currents and oscillographic views have been obtained from results of simulations. Voltage values of Adapazarı and Osmanca Substations are also examined before and during faults. Datas acquired from open-loop and closed-loop network models have been compared. In addition, datas acquired from open-loop and closed-loop network models have been compared. Different operating circumstances have been performed with upgrading network to closed-loop system. Simulink models of Kaynarca, Karasu, Sakarya and Melen Substations have been formed in which these substations are fed by new feeders and the transmission network have been evaluated in this circumstances. Furthermore, model simulations of them have been performed and values of current and voltage have been examined to carry out short circuit analysis. Three phase faults simulations have been performed in Kaynarca, Karasu and Sakarya Substations of which middle voltage busbar are operated by TEİAŞ. Values of fault current and oscillographic views have been obtained from models have been designed in which circumstances of feeding substations by one or double feeder. Maximum fault current simulations have been carried out in open-loop and closed-loop system network with using maximum voltage coefficient cmax according to IEC 60038. The values of Thevenin equivalent impedances have been obtained which are from transformer of substation in which simulations are performed back to network with using maximum fault currents.