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Fault Model Of Inverter Based Distributed Generation And Distribution System Protection

Posted on:2017-03-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y SiFull Text:PDF
GTID:1222330485482339Subject:Power system and its automation
Abstract/Summary:PDF Full Text Request
With the development of renewable energy sources and the need for energy conservation, a large number of distributed generations (DGs) are connected to the distribution system, which makes the tranditional radial network with single-terminal source change to active distribution network with multi-terminal structure. With the integration of various types of DGs, the fault characteristic of the active distribution network is quite different from the traditional distribution system. The existing protection and control schemes of tranditional distribution system face new challenges.Connecting to the grid with inverter is an important form of renewable energy. In order to study the fault characteristics of inverter based distributed generation (IBDG), it is needed to study the related issues such as structure and control mode. The IBDG is mainly composed of power source section and the inverter section, which should be both taken into account. The fault characteristic of IBDG is mainly influenced by its control mode, which is quite different from the traditional power source. In addition, as there exist much works for symmetrical fault analysis of IBDG, the asymmetrical fault needs to be addressed.The protection schemes for active distribution network should consider the impact of DGs. There are many types of DGs which have very different fault characteristics. With the integration of a large number of DGs, the impact on the distribution network fault characteristics is also increasing, the traditional protection schemes based on current values are difficult to set. The studies of protection schemes applicable to active distribution networks are needed. Moreover, the range of the distribution network is quite wide, it is also needed to consider the economic issues of the proposed schemes. Therefore, proposing a protection scheme which can both accurately identify faults and reduce the investment at the same time, is significant for active distribution network.This paper studies modeling of the IBDG with both power source and inverter, a piecewise linearization based equivalent model for photovoltaic (PV) array and a fault model for IBDG are deduced. A pilot line protection theory for active distribution network which only uses current information and transmits logical judgement results is proposed. Then a breaker-reducing protection scheme for active distribution network, on the premise that the fault can be identified accurately, is proposed. The main research contents and contributions are shown as follows:(1) To study PV distributed generation, a piecewise linearization based equivalent model of PV array is proposed. According to the requirements of applying PV array in system analysis and simulation experiments, to perform simplified equivalent of PV array’s non-linear I-U characteristics, a piecewise linearization based equivalent method is put forward. The proposed method describes the I-U characteristics of PV array by relatively simple linear models and corresponding equivalent circuits. By piecewise linearization, the non-linear relationship of PV array is represented by multi-sectional linear equations. Each linear equation describing corresponding section of the characteristic curve is represented by an equivalent circuit. Finally, an equivalent circuit representing the I-U characteristics of PV array, which contains multi-parallel branches, is proposed. Using PSCAD software, a simulation model containing piecewisely linearized equivalent circuits is constructed, simulation results are compared with the results from mathematical analytical calculation. The results show that the multi-sectional piecewisely linearized model can represent the non-linear I-U characteristics of PV array pretty well. (2) The fault characteristics of IBDG and fault calculation of distribution system with IBDG are studied. The fault characteristics of IBDG are mainly determined by control strategies. To achieve good control performance under asymmetrical fault, dual-variable decoupling control is adopted. The control strategies under asymmetrical fault are studied. Considering the control objective of suppressing the IBDG negative sequence current during asymmetrical fault, the fault characteristics of IBDG under asymmetrical fault and low voltage ride through are studied. Then PSCAD simulation software is applied to build a simulation model of IBDG, to validate its fault characteristics. On this basis, the fault analysis method of distribution system with IBDG is studied, and validated by simulation studies. (3) A line pilot protection theory for active distribution network based on directions of current phase angle variation is proposed. Considering the changes of power flow directions and fault locations, this paper analyses the variation characteristics of fault current phase angles. It is deduced that when an internal fault occurs, the current phase variation directions of the both ends are different; when an external fault occurs, the current phase variation directions of the both ends are the same. A line protection scheme employing positive sequence currents based on this theory is proposed. This protection scheme can identify the internal fault by only use the directions of current phase angle variation, which is with low requirements for the communication channel, and does not require current synchronous sampling. As voltage values are not required to identify the fault location, the investements of voltage transformers are saved compared with the tranditional directional protection. Several simulation models are built by PSCAD simulation software to verify this line protection throry. The line protection theory can accurately identify the faults of different locations, different fault resistances and different fault types.(4) This paper proposes a breaker-reducing protection scheme for the whole active distribution network, and a determination method for tripping breakers. The protection scheme adopts smart terminal unit (STU) to realize main protection functions of line protection, node protection and load protection, as well as backup protection function for breaker failure. A large number of STUs are installed to fulfill accurately fault identification; by reducing the amount of breakers to meet the actual demand of distribution system and reduce the investments. The STUs determine the tripping breakers by the associated matrices for protection which are obtained at the normal operation status. This paper details the formation of the matrices for protection. The associated matrix for main protection is obtained by topology searching, and the associated matrix for backup protection can be obtained by computing the associated matrices for main protection. They are used for the main protection functions and the backup protection of breaker failure. When the topology of the distribution network changes, the processing methods for different situations of STU are discussed. Case studies of a complex wiring distribution system explain the action procedure of the protection and breakers. It shows that when a fault occurs, this protection scheme can determine the tripping breakers accurately to cut the fault section. When a breaker refuses to act, this scheme can also act accurately.This paper studies the fault model of IBDG, then a line protection theory based on directions of current phase angle variation is proposed, also a protection scheme for the whole active distribution network is present. The works done in this paper enrich the researches of active distribution network protection, and have important supports for building reliable protection scheme for active distribution network.
Keywords/Search Tags:Distributed generation, inverter based source, fault analysis, active distribution network, relay protection
PDF Full Text Request
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