Research On Modeling And Control Of Shaft Brushless Doubly-fed Stand-alone Power Generation System

The shaft doubly-fed stand-alone power generation system has become one of the main construction schemes for marine power station due to its good economy performance and low cost which is obtained by sufficiently using the redundant power margin on ships,adopting cheap heavy oil as the fuel for main engines and employing the doubly-fed structure with low power converters.Traditionally,the doubly generator with brush and slip ring is used in the shaft doubly-fed stand-alone power generation system,although its technology has been already very mature,the brush and slip ring are easy to be damaged,the maintenance cost of system is thus increased.To improve the reliability and economy performance of the shaft stand-alone power generation system,the brushless doubly-fed generator is used instead of the brush doubly-fed generator,however,its control system design is rather difficult since the machine’s structure and electromagnetic coupling feature are complex.The current modeling of brushless doubly-fed stand-alone power generation system still follows the method used in the grid-connected application and the influence of load relationship in the shaft stand-alone power generation appliction is not fully considered,the system’s actual control performance thus deviates from the design expectation to a great extent,the workload and difficult of field debugging and tuning are also increased;moreover,the current research mainly focuses on the normal operation conditions,the analysis and controller design under special operation conditions in the ship’s application such as system excitation and load transfer are rarely involved.To overcome these problems,this paper makes further research on modeling and control of the brushless doubly-fed power generation system in the shaft stand-alone power generation application,the following aspects is included:(1)Study on the modeling and control schemes of shaft brushless doubly-fed stand-alone power generation systemTo overcome the problems in the present methods for the design of shaft brushless doubly-fed stand-alone power generation system,the improvement schemes are proposed in this paper.In term of the modeling,the influence of the load and load-side-converter(LSC)is fully considered,so that a mathematical model which can more accurately refect the characteristics of the system can be obtianed;in term of the control,the control system of the shaft brushless doubly-fed stand-alone power generation system is divided into two parts: the linear control part and the variable control reference’s nonlinear control part,which corresponds to the controls under normal operation condition and special operation condition,respectively.Based on this,the idea of design in each control part is proposed: in term of the linear control part’s design,the load information is considered in the controller design,so that the accuracy of the controller design can be improved,the sensitive requirement of too much extra electric quantities for control can be avoided;in term of the nonlinear control part’s design,the control method of changing the control referecnces in some of the linear controllers in the linear control part is proposed,so that the control complexity can be reduced,the switching between different control strategies can be avoided,the system reliability can also be improved.(2)Optimal design of proportinal-integral(PI)controller of shaft brushless doubly-fed stand-alone power generation systemIn term of the linear control part’s design,taking the control winding(CW)current-oriented shaft brushless doubly-fed stand-alone power generation system with a double loop structure as the research object,a new parameter design method for proportinal-integral(PI)controller is proposed in this paper,with which the system’s actual control performance can be more consistent with the design expectation,the workload of field debugging and tuning can be reduced.The control object is built based on the whole model of shaft brushless doubly-fed stand-alone power generation system with load imformation,also combined with the CW current-oriented feature,so that the result of modeling can be simplified and the accuracy of modeling can also be ensured.Based on the system modeling,the inner loop is designed.The influence between controllers in dq channel is fully considered in the design process.In the design process of outer loop,the influence of inner loop controllers is also fully considered,with this design method,the bandwith limitation that the bandwith of outer loop should be far lower than that of inner loop in the traditional double loop design can be eliminated.The controller is designed in the frequency domain,so that the control charactrtistics of the system under different rotor speeds and load conditions can be facilitately analyszed and the stability of the control system under various operation conditions can also be ensured.(3)Design of the forward channel decoupling network(DN)of shaft brushless doubly-fed stand-alone power generation systemTo further improve the dynamic performance of the system,a study on the decoupling control of shaft brushless doubly-fed stand-alone power generation system is researched.Traditional feed-forward decoupling method requires a large number of feed-forward variables,thus the detection cost is high,algorithm is complex and robustness is poor.Based on the aforementioned modeling reaserch,this paper proposes a method by introducing a decoupling network(DN)into the forward channel of the control loop to eliminate the couplings of the system.The original model of shaft brushless doubly-fed stand-alone power generation system is first transformed to a typical Dual-Input-Dual-Output(DIDO)system,so that the DN can be designed by borrowing the classical control theory of forward channel decoupling network method,and then is in series with the original control object to form a new control object,the PI controllers can thus be facilitately designed.The DN method do not require additional sensors,the harware cost of system can thus be reduced,a good adaptability to the rotor speed and load variation can also be obtained.(4)Design of the variable control reference’s nonlinear control part of shaft brushless doubly-fed stand-alone power generation systemTo the special operation conditions such as system excitation and load transfer of the shaft brushless doubly-fed stand-alone power generation system,this paper proposes the design scheme of corresponding nonlinear control part.To overcome the problem that many variables will change at the same time in the process of excitation and makes the excitation control difficult,this paper designs a set of segmented excitation strategy.The excitation process is divided into several stages,only an excitation variable will be changed in each stage,so that the requirement that the system variable and converter power should be regulated according to certain relationship in the process of excitation is satisfied.To the requirement in the shaft power generation application that the load should be uninterruptedly transfered between brushless doubly-fed generator and backup power supply,the transient current impact problem in load transfer is also briefly analyzed in this paper,analysis results show that with the CW current-oriented method,the impact current can be well restrained within safety range during the load transfer process without additional sensors and complicated grid-connected control.The controls of the above two special operation conditions can be easily realized by changing the control reference values of the linear controllers in the linear control part.