Research On Converter Optimal Control Strategy For Small Frequency Conversion Hydropower System

Small hydropower is an important water conservancy infrastructure for people's livelihood and clean renewable energy.It plays an important role in safeguarding national economic and social development and improving the quality of life of the people.Vigorously developing small hydropower is in line with China's sustainable development strategy.However,the widely used constant-speed constant-frequency hydropower generation method has large equipment cost,high unit failure rate,and low water energy utilization rate,and it is difficult to meet the requirements of future power grids for efficient,rapid,high-quality,ecological,and low-cost hydropower.The breakthrough in new hydropower technology is particularly eager.This paper innovatively introduces a variable-speed constant-frequency system(referred to as a variable-frequency system)for hydroelectric power generation,which replaces the traditional turbine-speed governor-generator constant-speed constant-frequency unit with a full-power variable-current system based on a permanent-magnet synchronous generator.It can adapt to the time-varying head and perform all-weather power generation,which significantly improves equipment utilization and power generation efficiency.The main research contents are as follows:1)A new hydropower grid-connected converter topology has been introduced to realize variable-speed constant-frequency power generation.The energy conversion system with permanent magnet motor as the core is adopted,and the two-level ACDC converter is adopted to realize the constant-frequency power generation with variable speed,wide voltage and high power range.2)A mathematical model of the variable frequency hydropower system was established.Aiming at the shortcomings of the traditional proportional-integral controllers,such as slow dynamic response,poor anti-disturbance capability and lack of overall optimization ability,the no-beat prediction control strategy of the machineside and grid-side converters is introduced.3)The influence of the internal electromagnetic parameters of the permanent magnet synchronous generator on the control system is analyzed.Aiming at the current control error caused by the motor parameter variation,a new type of deadbeat predictive control based on the disturbance observer is proposed.The observer is constructed using the sliding mode control theory.In order to weaken the influence of buffeting,the quasi-sliding mode method is used on the basis of the approach rate.The observer can simultaneously predict the future value of the stator current and estimate the disturbance caused by the parameter error in real time.The voltage feedforward compensation method is used to eliminate the current static difference,strengthen the operating capacity of the system under variable working conditions,and improve the power generation efficiency.4)The development of the 10 k W frequency conversion hydropower gridconnected system prototype was completed and applied to the actual small hydropower station.The feasibility of the hydroelectric power generation method using the frequency conversion system was confirmed by experiments.At the same time,both simulation and experiment show that the beat-free predictive control based on the disturbance observer still has good dynamic and static performance when the system parameters change.