Research On Damping Low Frequency Oscillation Incorperating New Type Of Saturated-core Controllable Reactor

With the rapid expand of ultra-high voltage power grid in China,the regional grids are deeply connected to each other.Hence,the low-frequency oscillation becomes a major threat to the power system stability.Series connected compensation devices play an important role in many aspects including optimizing system-wide power flow,reducing transmission loss,increasing system damping and suppressing the low-frequency oscillation.There are two types of series connected compensation devices:the FACTS device(Flexible AC Transmission System Device)and the controllable reactor.The FACTS device is a mature technology.Due to the limitation of insulation,the FACTS devices are not suitable for ultra-high voltage power grid.On the other hand,the controllable reactor has many advantages based on its structure and features.The controllable reactor has a potential in the power system stability and control which requires more works in the future.In this dissertation,the saturated-core controllable reactor(SCCR)has been studied combining theoretical analysis and simulation.A mathematical model of SCCR is built and the control characteristic of SCCR is investigated.The mechanism of the low-frequency oscillation and the related damping methods are analyzed systematically.The control strategy incorporating SCCR in damping low-frequency oscillation is proposed.The effectiveness of SCCR damping controller is fully researched.The main contents in this dissertation can be concluded as follows:A new type of saturable-core controllable reactor(SCCR)is designed and researched based on the theory of magnetic amplifier.An air gap is inserted in the ferromagnetic core to improve the linearization of the control characteristics and increase the operating range of the SCCR.The magnetization mechanism and the control characteristics are analyzed.The magnetic flux distribution of the reactor is calculated using the three-dimension finite element method(FEM).The flux leakage effect of each part of the reactor core is modeled as flux tubes.On the basis of the above research,the SCCR equivalent model is built incorporating the magnetic flux leakage to calculate the reactance of the SCCR.The influence of the DC bias current on the SCCR reactance with constant AC current and the influence of AC current on the SCCR reactance with changing DC bias current are both carefully investigated.The control characteristics of the SCCR are obtained from the model.The model is validated by comparing the equivalent model simulation results and the prototype test data.A SCCR control strategy in damping low-frequency oscillation is proposed based on the low system damping mechanism.The system state matrix including the SCCR is calculated which is used to design the damping controller.A comparative analysis of the controller's performance is carried out in the single-machine infinite-bus system.The simulation is conducted by Power Systems CAD(PSCAD).The results show than the SCCR reduces the time and amplitude of low-frequency oscillation.A control strategy implementing SCCR to damp the low-frequency oscillation is proposed using wide-area measurement signals.On the basis of small signal analysis,the residues of each system parameter are calculated considering both observability and controllability.A comparative analysis of the controller's performance is carried out incorporated with two input signals including the tie line active power flow and the generator speed.The simulations are conducted by power systems CAD(PSCAD).The results show the effectiveness of controller in damping inter-area oscillations in multiple operating conditions.A SCCR damping controller based on model predictive control method is proposed to damp the low-frequency oscillation.The controller operates on the basis of the system model,incorporates receding optimization and state feedback control method.The trajectory of the control parameter is obtained by adding the SCCR operating range to the algorithm in terms of the system constraint.The optimization in each step is solved by the Hildreth method which avoid the matrix irreversible problem and the algorithm stability problem.An interactive simulation platform consists of PSCAD and Matlab is built to test the multi-machine system with SCCR.By obtaining the predicted trajectory of the control parameter considering the SCCR operating range,the error between the predict value and the actual value is minimized and the negative influence brought by the physical constraints of SCCR is eliminated.The simulation results show the effectiveness of controller designed by model predictive control method in damping low-frequency oscillations in multiple operating conditions.