Research On The Optimization Control Of Cascade Power Electronic Transformer And Its Application

With the aggravation of energy crisis and greenhouse effect,the clean energy get lots of attention in electricity production and consumption.The innovation of clean energy utilization technology pushing the new generation grid to develop in a new direction with extensive interconnection,intelligent interaction,flexibility,safety and controllability.The functions and performances of transformer,which act as the joints of power grid,will directly affect the utilization of clean energy.Based on this,the Power Electronic Transformer(PET)which combines power electronic converter technology and high-frequency magnetic technology has been proposed in recent years and received attentions.Compared with traditional power transformers,PET not only own the basic functions of power transmission and voltage transformation,but also own the functions of fault isolation,harmonic isolation,reactive compensation,ac/d c hybrid access,ports expansion and energy management.Therefore,there is a great significance to develop the PET for the new generation grid.And the research of the key technologies for PET is valuable.The key technologies to improve the performance of PET is the main issues in this paper.And the 10kV/0.4kV engineering prototype has been developed.This paper is funded by the National Natural Science Foundation of China “Research on key issues of topology and control of multi-portal power electronic transformer for DC distribution network(51577055)” and the key scientific projects of China Southern Power Grid “Research and application of key technology of intelligent power electronic transformer in distribution network(K-GX2013-031)”.The main work of this paper include five aspects below:(1)The model analysis and voltage balance control method for cascade type power electronic transformer;(2)The unbalanced operation method for the input stage of PET,and its application in multi-port energy router;(3)The dc ripple suppression method with the optimization control at intermediate stage of PET;(4)The quality control method of the output stage of PET;(5)Design and engineering application of 10kV/0.4kV prototype.The research results take full account of the multi-stage series structure in PET,and seek technical perfection and breakthrough.The main innovations of this paper are as follows:(1)A voltage balance control method based on low voltage dc power feed-forward is proposed.The difference of hardware parameters and control accuracy of power electronic transformer will lead to the dc voltage imbalance.The dc voltage imbalance will lead to the power transmission imbalance of intermediate stage,then directly affecting the overall operation efficiency.Based on the structure analysis of classic cascade type power electronic transformer,the circuit model of each part is established.Then,the physical nature of voltage unbalance of cascade power electronic transformer is analyzed.The traditional voltage balance and power balance control strategy is analyzed to clear the complexity.To reduce the complexity of control strategy,a voltage balance control strategy which is based on the low voltage dc bus power feed-forward has been proposed.The proposed control strategy is advantageous to the digital implementation.(2)The operation method of unbalanced condition for PET is proposed,and the method is applied to a modularized energy router.Power transmission block and redundancy operation will lead to an unbalance operation in the input stage,and the operation range will change.The applicability of traditional control strategy for this condition is not clear.In this paper,the the unbalance operation range for input stage of power electronic transformer is analyzed.The control strategy with unit power factor and non-unit power factor operation mode are proposed based on the relationship between voltage and current vectors.Then,the unbalanced operation method of power electronic transformer has been used in an energy router.(3)A strategy for dc ripple voltage suppression in power electronic transformer based on intermediate stage optimization control is presented.Dc ripple voltage will be introduced with the power exchange between DC and AC.The existence of ripple voltage directly restricts the optimization of equipment volume,power density and cost.Based on the analysis of PET topology,the ripple voltage generation mechanism is shown,and the key to suppress the ripple voltage is to adopt the intermediate stage.Then,the average model of the intermediate stage is established,and the dc power and ripple power is decoupled through the linearization process.(4)A harmonic optimization control control strategy which considering the sampling characteristics and capacity limitation of power electronic transformer is proposed.The supply voltage quality of power electronic transformer is closely related to its control strategy.In this paper,the mechanism of voltage quality degradation after the power electronic transformer replaces the traditional transformer access is deeply analyzed.The PCC point voltage quality will degrade when PET is working without harmonic optimization control control.Then,the feasibility of high voltage quality control for power electronic transformer is analyzed,and a direct voltage control strategy is proposed.(5)A 10kV/0.4kV power electronic transformer engineering prototype was developed and its engineering application was realized.The development of engineering prototype is an important way to verify relevant theory,and it is a way to realize the engineering practice.The design method of power electronic transformer is studied.The volume,weight and other factors of power module is considered,and the layout is optimized.The design methods of main controller and man-machine interface of power electronic transformer are studied.The type test items with recognition degree are proposed,and the corresponding type test is completed.The engineering prototype is operated in a 110 kV substation,the detailed operation scheme was formulated.The field operation was smooth and all the indexes met the design requirements.Taking cascade type power electronic transformer as the research object,the input stage,intermediate stage and output stage of PET are studied.The operation stability,cost,power density and voltage quality of PET are improved.The theoretical research methods,research conclusions,engineering prototype design methods and application results can be used as reference for other similar serial multi-stage systems and provide beneficial reference for the development of power electronic transformer.