Studies On Model-Relaxation Based Power System Real-Time Stability Precaution And Control

With the increasing number and types of access devices on the "Source-Network-Load" side,the current power system becomes extremely complex at both physical and mathematical models.For the modern power system,it faces great challenges to construct system-wide models and obtain models' parameters accurately.Therefore,the traditional model-based stability analysis and control methods become extremely unreliable,and are difficult to match the operating state of the real-life power system,directly impacting on the system's safe operation.Considering establishment of the phased measurement unit(PMU)based wide-area measurement system(WAMS),it has become reality to obtain the real-time operation/response information at the normal/disturbance conditions of the actual power system.Fully exploiting the measured information from PMU/WAMS can reduce and even avoid the influence of inaccurate system models and parameters effectively,and then the model-relaxation based,even model-free,stability analysis and control methods under the scheme of real-time analysis,real-time decision-making and real-time matching can be realized.Based on the measured information from PMU/WAMS,this dissertation mainly studies the model-relexation based real-time polymorphic stability precaution and control.It aims to timely predict the system stability status from both fault and operating states,and improve the ability to resist disturbance and maintain polymorphic stability.For the large disturbance fault,the real-time early warning and control of "Load-Source" stability is studied to realize the rapid prediction of status of short-term voltage instability or transient(rotor angle)instability,and improve the active control ability of short-term voltage stability and transient stability.And at the no-fault operating state,the real-time early warning and control of voltage secure operation of the AC/DC power system is studied to realize online monitoring of voltage secure level and improve the coordinated control ability of voltage secure operation,and then make voltage secure operation.Based on these,the main works and innovations of this dissertation are given as follows:(1)For the problems of real-time short-term voltage stability prediction,only relying on online information of power flow and composite load model(CLM),a model-relaxation based prediction method is proposed by combining the mechanism of short-term voltage stability and least square support vector machine(LSSVM)with online learning.The relationship between the short-term voltage stability and stability of the induction motor is discussed,and then the criterion of short-term voltage instability is proposed.A ternary lookup table consisting of contingency,stable equilibrium point and unstable equilibrium point for the induction motor slip is constructed.When suffering an actual fault,the forward Euler method is used to calculate the time series of induction motor slip,and then the LSSVM with online learning is proposed to perform rolling prediction of motor slip.Finally,based on the predicted induction motor slip,the short-term instability status can be predicted early by using the lookup table and the proposed criterion of short-term voltage instability.(2)For the problems of real-time short-term voltage stability control,a model-free adaptive under voltage load shedding(UVLS)control strategy based on fuzzy logic theory is proposed to cope with the problems of short-term voltage instability and fault induced delayed voltage recovery(FIDVR).A multiple-input multiplr-output(MIMO)fuzzy UVLS controller in decentralized scheme is designed.By extracting the key two-dimensional information of the transient voltage trajectory—voltage magnitude deviation and voltage recovery time deviation,as feedback signals of the UVLS controller,it can adaptively determine the three elements of UVLS—the location,the time and the amount of load shedding,and form a closed-loop control strategy.With the proposed UVLS strategy,it can make the post-fault voltage recover above the safety threshold within a specified time at a low cost of load shedding.The proposed UVLS strategy has strong applicability and robustness,and improves the control ability of short-term voltage stability significantly.(3)For the problems of real-time transient stability prediction and control,a model-free method for transient stability prediction and emergency generator-shedding control is proposed.Based on PMU measurements,real-time instability criteria based on the geometrical characteristics of the speed(?)trajectory of the equivalent one machine infinite bus(OMIB)system are derived from extended equal area criterion(EEAC).which can handle the first-and multi-swing stability problems.By using ensemble ELMs with online learning,a new fast trajectory prediction algorithm with an adaptive prediction window is presented,balancing the trajectory prediction accuracy and the ahead of early prediction.By combining the predicted OMIB-? trajectory and the proposed instability criteria,transient stability can be judged accurately,and the instability status can be detected early.For the predicted transient instability,an analytical method of generator shedding is proposed based on the transient energy of the equivalent OMIB system at the critical state to facilitate transient stability recovery with a low cost of generator shedding.The requirements for accurate fitting of the power-angle function relationship of the OMIB system are reduced.The relationship between the wide-area latency and the generator-shedding amount is illustrated,and then a correction method of generator-shedding amount considering a long latency is given.Finally,a closed-loop scheme of transient stability prediction and generator-shedding control is constructed.(4)For the problems of early warning and control of voltage secure operation,a wide-area measurement-based index of voltage secure operation in the AC/DC power system is proposed and a coordinated voltage control(CVC)strategy incorporating the source-grid resources is constructed.First,the equivalent model of converter is build and its equivalent parameters can be calculated by real-time measurements.On this basis,the AC/DC power system is transformed into an equivalent AC power system,and the L-index of voltage secure operation in the traditional AC power system is extended to the the AC/DC power system.Then,an extended L-index of the AC/DC power system is proposed.When the system L-index or voltage magnitude does not meet the safety requirements,a CVC model of AC/DC power systems is established,effectively incorporating DC systems' converters participating in voltage control.The original nonlinear complex problem of voltage control is transformed to a multi-step linear constrained quadratic programming problem,resulting in fast and effective solutions.Finally,three voltage control strategies are designed,including an optimal CVC strategy for L-index correction,an optimal CVC strategy for voltage magnitude correction and an integrated CVC strategy,and a wide-area closed-loop control scheme is formed to ensure voltage secure operation comprehensively.