SHORT-CIRCUIT ANALYSIS OF UNBALANCED DISTRIBUTION SYSTEMS

Abstract

To improve the reliability and efficiency of the distribution system, an important tool is required, named as Distribution automation (DA). It includes many applications such as distribution network reconfiguration, network optimization, state-estimation, reactive power management, short-circuit analysis etc. In this thesis, one of the application of DA, namely, short-circuit analysis of distribution network, is explored. Short-circuit analysis is an important tool for analyzing the system behavior (system voltage profile and currents) under the short-circuit conditions. Modernize distribution systems have some inherent features, such as radial as well as weakly meshed configurations with several thousands of nodes, untransposed lines, multiphase line sections, unbalanced loads, integrated various types of Distributed Generations (DGs) at any locations etc. Therefore, it becomes necessary to develop the short-circuit analysis algorithm for the distribution network which considers all these special features of the system in the short-circuit study. The information provided by the short circuit studies can be used for real-time applications, such as distribution adaptive relay coordination and settings when feeder reconfiguration is performed automatically and identification of fault locations. The results of short-circuit studies can also be used for the selection of ratings of the protective equipments. It can also be used for the selection of appropriate size of the fault current limiters (required in the network to limit the fault current to a safer value). Nowadays, the distribution systems are changing from one source supplying structure into multi-source supplying structure with participations of distributed generations (DGs). Both conventional and renewable energy resources can energize the DG units. Technologies, based on conventional energy resources, include internal-combustion engines, reciprocating engines, gas turbines, fuel cells, micro-turbines and batteries, while renewable energy technologies included photovoltaic energy conversion system (solar PVs), wind energy conversion systems, small hydro systems, biomass systems, solar-thermal electric systems and geothermal systems. There are so many advantages of the integration of DGs into the distribution network. DG provides an alternative for satisfying the increasing load demand in the network without the need of expansion of distribution system. DG improves the system efficiency by enhancing the system voltage profile and minimizing the number of required voltage regulators and capacitors and reducing feeder i power losses. However, the integration of a large number of DGs into the network introduces so many challenges. One of the problem mentioned in the literature is the violation of original settings of the protective equipments during the short-circuit conditions because of the introduction of additional DG fault current into the network. Generally, the protective devices are designed based on the fault current analysis of the original system without DGs. When DGs are added to the system, they also contribute to the fault current in addition to the grid current. Therefore, the fault current sensed by the protective devices is greater than the original fault current from the grid. It might be possible that the protective devices can get damaged due to this excessive fault current. Even if the increase in fault current does not exceed the rating of installed devices, coordination of the primary and secondary protective devices may be disturbed due to excessive DG fault currents. Therefore, the appropriate short-circuit analysis algorithm is required for the analysis of unbalanced distribution network considering DGs under the fault conditions. In the literature, initially the classical symmetrical component based approach was used for the short-circuit analysis of distribution system. In this approach the phase quantities of the voltage, current and impedances in the distribution system are first converted into their respective positive, negative and zero sequence components and then the short-circuit calculations are performed on these components separately. This approach is advantageous only when all the three sequence components are decoupled from each other. But in case of distribution system, this condition is not true as the mutual impedances between the phases of distribution lines are not equal (since the distribution lines are untransposed). Therefore, the results obtained by this approach are erroneous. To overcome this problem, the phase component based approach was introduced in the literature. In this approach, the short-circuit calculations are directly performed on phase components. Some of the phase component based short-circuit analysis methods are based on the concepts of Thevenin equivalent impedance and bus admittance matrices of the systems, while some are based on [BIBC] (Bus injection to branch current) and [BCBV] (branch current to bus voltage) matrices of the system. In most of these methods, it has been assumed that the load currents are negligible as compared to the fault currents. Therefore, the load currents have been ignored in the calculations of short-circuit currents. Also, the short-circuit analysis methods for the unbalanced distribution system considering ii the effect of DGs are also available in the literature. In these methods, the contribution of DGs into the fault current have been considered during the short-circuit calculations. Generally, the inverter based DG models have been considered in these studies. The appropriate inverter control strategies have been applied to the IBDGs during the faults. Most of these methods are based on dq 􀀀0 sequence component based approach and have carried out only the time domain simulation studies for the analysis of short-circuit faults. However, sequence component based fault analysis methods are not suitable for unbalanced distribution network with single and two phase lines and for distribution lines with unequal mutual impedances. Also, the available short-circuit analysis methods for the unbalanced distribution system with IBDGs have not considered the loads during short-circuit calculations. Hence, the accurate and the efficient short-circuit analysis algorithm is required for the unbalanced distribution system which also includes the effect of loads in the short-circuit calculations. Initially, the short-circuit analysis method, for the unbalanced radial as well weakly meshed distribution system has been developed which considers the effect of loads during short-circuit calculations. The proposed method is based on bus admittance matrix of the system. It is a single iteration method and hence is a less time consuming. This method can also be applicable for the analysis of multiple faults in the distribution system. To demonstrates the accuracy and the effectiveness of the proposed method, it has been tested on modified IEEE 123-bus radial and weakly meshed test distribution system. Subsequently, the proposed method has been extended for the short-circuit analysis of unbalanced distribution system considering IBDGs. Since, with the inclusions of IBDGs in the distribution system, the KCL equations of the network become nonlinear. Hence, to solve these set of non-linear equations, the Newton-Raphson based numerical method has been applied. In this method, initially the current control strategy of the inverters has been applied to the IBDGs and perform the short-circuit calculations to obtain the values of bus voltages, branch currents and inverter currents under the fault conditions. Next, on the basis of obtained values of inverter bus voltages magnitudes, appropriate voltage control strategy has also been applied to the IBDGs and recalculate the voltages and currents under the short-circuit conditions. To validate the proposed method, various short-circuit faults have been simulated on modified IEEE 123-bus test system. Analysis of multiple faults has also been performed on the same test system using the proposed method. iii Further, a novel load flow analysis method for the unbalanced distribution system considering various three-phase transformer models and IBDGs is proposed in this work. The nodal admittance matrix based transformer models (p.u.) have been considered in this approach. This method is based on [BIBC] and [BCBV] matrices of the distribution network. Two modes of operation of IBDGs, namely ”Constant active power mode” and ”Power and voltage control mode”, have been considered in this approach. The proposed method is applicable for the radial as well as weakly meshed distribution systems. The singularity problem for particular types of transformer connections such as, star-grounded/delta (Y g 􀀀 ), star/delta (Y 􀀀 ), delta/star ( 􀀀 Y ), delta/delta ( 􀀀 ) connections etc., has also been addressed in this method. Next, the short-circuit analysis method has been developed for the distribution system considering three-phase transformer models and IBDGs. It is also a Newton-Raphson based approach. The proposed method has been tested on modified IEEE 123-bus test system and the obtained results have been compared with the results obtained by the PSCAD/EMTDC simulink software. A case of multiple faults has also been simulated on the same test system using the proposed method. Furthermore, the method for the load flow analysis of unbalanced three-phase four wire multigrounded radial distribution system has been proposed in this thesis. This method is also based on [BIBC] and [BCBV] matrices of the network. Separate [BIBC] and [BCBV] matrices have been developed for phase, neutral and ground currents and bus voltages. Well established Carson’s formula has been used for the calculation of line impedances of three-phase four wire multigrounded distribution system. A case of isolated neutral has also been simulated using the proposed method. The proposed method has been tested on two different systems, modified three-phase four wire multigrounded IEEE 34-bus and IEEE 123-bus distribution systems. Subsequently, two different short-circuit analysis methods have been proposed for three-phase four wire multigrounded distribution system. One of the proposed method is based on [BIBC] and [BCBV] matrices of the system, while the other one is based on bus admittance matrix [Ybus] of the system. Both of these methods have also considered the effect of loads during the short-circuit calculations. The results obtained by these methods show their accuracy and effectiveness. Finally, the load flow and short-circuit analysis methods have been developed for the threephase four wire multigrounded distribution system considering three-phase transformer models and IBDGs. These methods have been developed separately for two different configurations of iv transformer models, first one is Delta/Star-grounded ( -Yg) and the second one is Star-grounded/Stargrounded (Yg-Yg). First, the load flow analysis method, based on [BIBC] and [BCBV] matrices, has been developed for the two different transformer configurations. Next, two different shortcircuit analysis methods (one is [BIBC] and [BCBV] matrices based, while the other one is bus admittance matrix [Ybus] based method) for both the transformer models have been developed. Again, the current control mode of operation of IBDGs has been considered during the shortcircuit analysis. Both of the proposed short-circuit analysis methods uses the Newton-Raphson based technique. The results obtained by the proposed methods have been compared with the results obtained by PSCAD/EMTDC simulink software which show the accuracy of the proposed methods.