Loss Analysis Of The Flexible DC Transmission System

The VSC-HVDC technology is a new direct transmission technology that use self-commutated power electronic devices and PWM technology.It makes up for some defects on the LCC-HVDC,and possess some advantages that the LCC-HVDC does not have.The design of VSC-HVDC system has some advantages of compact structure,modularization,easily movment and expand.It has an important influence on the development of the future power system,especially the renewable energy generation.The loss of VSC-HVDC system plays a vital role in the long-term operating cost and the running performance.Meanwhile,studying the loss of MMC provides a reference value for the design of heat sink and efficiency evaluation of VSC-HVDC system.With the increasing demand for the performance of theVSC-HVDC in engineering practice,the system loss has become an important research direction for scholars at home and abroad.The VSC-HVDC system mainly includes converter valve,converter transformer,reactor,transmission lines.This paper uses a mathematical calculation method to study the loss of VSC-HVDC system,and makes some measures to optimize loss and performance of VSC-HVDC system,as described next.Firstly,this paper calculates the loss of MMC.According to the operating states and working principle of the MMC,the loss of valve is divided into conduction losses,necessary switching losses and additional switching losses.And the specific calculation formulas about the loss are put forward respectively.The loss of the hybrid submodules in the MMC is obtained.And the influence of the capacity,DC voltage and temperature on the loss of valve is analyzed.Secondly,on the basis of MMC loss,two methods to optimize the performance of the VSC-HVDC are proposed.The first way is optimizing the switching control frequency.Based on KVL’s law and KCL’s law,the equivalent circuit of AC harmonic is deduced,and the formula for calculating the harmonic distortion rate of output voltage is derived.Genetic algorithm is used to optimize the loss and output voltage distortion rate of MMC and get the optimal switching frequency.Another way is optimizing the accumulation of the 3rd harmonic.After the modulation voltage is injected into the 3 harmonic,the change of the loss and redundancy of the MMC converter valve is analyzed.The expression of the loss and redundancy of the MMC commutation valve on the injection of the 3 harmonic waves is established.At the same time,genetic algorithm is used to optimize the loss and redundancy,and the optimal value of the 3rd harmonic is obtained.Thirdly,this paper introduces the loss calculation method of converter transformer,reactor and DC line in VSC-HVDC system.The basic composition of the loss of VSC-HVDC system is analyzed,and the loss of each equipment in the system is introduced.The loss of all equipment is added to the loss of the whole system.Finally,based on the optimization method of the MMC,combined with the engineering data,this paper calculates and evaluates the loss of each equipment and the total loss in the VSC-HVDC system.The calculation results show that the loss of the DC line is 2.8% of the rated capacity(8000MW),the loss of the converter station is 1.15% of the rated capacity.It provides some reference value for engineering selection.