| The power systems have been in continuous development towards more dynamic and complicated due to the electricity demand,the integration of distributed generation resources,entities with dual load-generator behavior,distributed energy storage,new equipment and services,demand-side management and secondary reserve.The operation of power systems is required to be more reliable,efficient and economical.In order to achieve these objectives,it is crucial to monitor the state of the power system accurately in any operating conditions during the process of operation.For that reason,the reliable and robust observation,which specifies the system state,has become more important than before due to new energy market-related requirements.Reliable observation of the system state is essential to guarantee a reliable operation of the power system.To ensure a reliable observation,an important requirement is to obtain sufficient critical measurement information.The critical measurement information mainly depends on the type of measurement,the location of measurement and the distribution of measurement redundancy.These factors are determined by a solution of meter placement.With the remarkable features of Phasor Measurement Unit(PMU),a strategic placement of PMUs,which can determine sufficient number of measurements at the critical locations,can increase the reliability and robustness of observation.This dissertation focuses on improving the power network observability based on the strategic placement of PMUs.The observation of power network is significantly affected by failure events and the change of topology.For enhancing the observability in the condition of failure or/and the change of topology,this study focuses on researching to improve the observation reliability,the robustness of observation,the availability of measurement system,and the effect of measurement redundancy.The main works and results are described as follow:(1)Research on a strategy of PMU placement to obtain the most reliable set of measurements,which can maximize the observation reliability for buses,thereby improving the observation reliability:In the use of PMUs for observing the power network,most buses are made observable through the network connections and the observability of other buses.The reliability of PMUs and connections affect the reliability of measurement system directly.In practice,the reliability value of transmission lines are not the same;the power network is a large-scale system,and then some buses and lines suffer more heavy working conditions,that makes some locations more vulnerable.Moreover,some buses and lines in the power network that have an important impact on the reliability of system because they reflect the character of the power system.The state of the system can be assessed from the information of these elements.That means the characteristics of power network also have certain impacts on the system observation reliability.Therefore,besides the redundant solution,a solution which finds out the important buses and the highly reliable lines to install PMU can enhance the observation reliability for buses,too.A measurement system forms a spanning measurement tree of full rank;a set of measurement involves the nodal voltages and the branch currents.The reliability of the voltage measurements can be considered the same;it depends on the reliability of PMUs and communication links.As for the current measurements,in addition to the reliability of the PMU and communication link,they depend on the reliability of transmission line.In order to find the most reliable set of measurements,we only need to find the set of current measurements having the maximum total reliability.This dissertation focuses on finding out the set of the maximum total reliability current measurements by specifying the most reliable spanning measurement tree.For achieving this target,we proposed a new approach which combines two algorithms:the Prim's algorithm(PA)and the local search algorithm(LSA).The problem of finding the current measurement set having the maximum total reliability is solved in two stages.The first stage will be to identify the most reliable spanning tree.Based on the main tree of important buses,the PA always can find the minimum spanning tree which ensures the number of branches with the highest reliability connected to all buses in the power network.In stage 2,the local search algorithm is applied to optimizes the number and location of PMUs on the spanning tree.This stage eliminates the additional current measurements and also specifies the voltage measurements.At the end,we will obtain a complete set of measurements with the maximum total reliability.The PA fuses with LSA for optimizing both the number of branches of PMU and number of PMUs.It means that the electric power system is observed by a minimum number of PMUs with minimum channels.The effect of this method was demonstrated by simulating on the IEEE 14,24,30 and 57 bus test systems.Through the most reliable measures obtained from the proposed method,the observation reliability value of the buses has been significantly increased,in particular,most buses achieve the highest observation reliability.In addition,the value of the observability security index is also improved.Then the observability of the measurement system has been improved significantly.(2)Research on a strategy of PMU placement to obtain a set of PMUs,which has the lowest total failure rate when deploying on the network.The obtained PMU set can maximize the availability of measurement system;Because one PMU can observe the bus it placed and some adjacent buses.the number of measurements of each PMU is not the same;therefore,the location of PMUs affects the reliability of the measurement system significantly.By selecting the low-unavailability PMU buses,we can improve the observability of the observation system.This study investigated to find a set of PMUs which has the lowest total failure rate when deploying on the network.We developed a method based on the greedy algorithm to achieve this target.The greedy algorithm was improved to enhance the quality of the obtained solution,and to reduce the number of iterations to save the search time.The simulations on IEEE 14-bus,30-bus,57-bus,118-bus test systems,and especially on a large-scale network-the Polish 2383-bus system,were presented for evaluating the feasibility of the proposed approach.The results of simulation showed that the proposed method always finds the PMU set with the lowest probability of failure at an approximation ratio of 1-1/e.Through the obtained PMU set,the observation reliability of the power network has been improved significantly.(3)Research on a strategy of PMU placement for enhancing the effect of measurement redundancy.The obtained PMU measurements can enhance the robustness of observation:The measurement redundancy is considered as a key factor in achieving a successful state estimation implementation.Without redundancy,a state estimator cannot filter out bad measurements and cannot implement when losing measurements.Therefore,a rate of redundancy affects the robustness of state estimation directly.The measurement redundancy implies the measurement reliability;hence,it can be said that in order to obtain a robustness rate of state estimation we need to achieve a minimum value of measurement reliability.We identify the cues that solving the PMU placement problem from the point of view of reliability can enhance the effect of redundancy.Because it can determine the required number of redundancies and their distributions,it will ensure the desired rate of robustness against the failure of elements.Moreover,the redundant measurements are only sufficient for the desired robustness;hence,it not only ensures the robustness but also reduces the installation cost of PMUs.This study proposed a new method based on the observation reliability criteria of the bus to solve the optimal PMU placement problem in the power network.The target of this work is to enhance the effect of measurement redundancies and achieve any desired rates of robustness for state estimation.The dual simplex algorithm is improved and applied to solve the PMU placement problem.The simulations on IEEE RTS 96,14-bus,30-bus,57-bus and 2383-bus test systems were tested for evaluating the effect of the proposed approach.The simulated results showed that the measurement redundancies are distributed actively in low observation reliability locations,which help enhance the ability of observability in vulnerable areas;hence,it increases the effect of redundancy.Moreover,from one value of redundancy ratio that implies a robust rate of state estimation,one desired observation reliability value can be estimated respectively,and then the number and location of PMU can be specified easily.This procedure makes an active solution to achieve the desired robustness for state estimation.Based on the observation reliability of buses we can enhance the effect of redundancy significantly and achieve any desired robust rates of state estimation.(4)Research on a solution of PMU placement,which can ensure the observability with any configurations of topology,thereby maximizing the robustness of observation:In operations,there are usually many optimal topologies that correspond to periods in the time horizon.It is evident that topology switching can have a particular effect on nodal voltages,element currents,and branch flows.Operators can then switch transmission elements to improve voltage profiles or increase capacity transmission.In addition,when a contingency event occurs in the power system,it may cause overloads,cascading failures,voltage violations,or system instability.Transmission switching provides flexibility to the power grid and may be used as a control method for defending against the above problems.For many reasons(outage of lines,maintaining the voltage stability,reducing the loss and cost,or ensuring the security),transmission line switching or bus-bar switching all causes the change of topology of the power network with the larger size and the unpredictable interconnection.With regard to the unpredictable changes in the interconnections of the power system network,optimal location of PMUs has also to be expected in order to maintain the complete network observability.This study investigated a new solution to place PMUs against the unpredictable change of network topology.The proposed solution is to consider the connection change of each bus instead for all topologies of power network.When the placement of PMUs satisfies the observation for all buses with any change of connection,that also means the network is full observable in the condition of the change of system topology.The dual simplex algorithm was improved and applied to solve the PMU placement problem in this study.The proposed approach was tested on the IEEE 14 bus system.Its effect and performance were demonstrated on the simulation results,that is not only much more simple and effective in solving the PMU placement problem in the multi-configuration networks but also ensure the sustainable observation for the contingency cases.Then the observability has been ensured for any conditions. |
PDF Full Text Request