Research On The Protection Scheme For Transmission Lines Of Current Source Converter

At present,offshore wind power transmission projects usually use Modular Multilevel Converter(MMC)DC transmission to complete the collection of power.However,offshore wind power development is hampered by the MMC offshore converter platform’s huge size and costly cost.Current Source Converter(CSC)has the advantages of high power density and DC fault ride-through without energy storage capacitor at the DC side.In particular,the Fundamental Frequency Modulation-Based CSC(FFM-CSC)reduces the switching frequency of the device,which is especially suitable for offshore wind power transmission.However,the specific fault current mechanism of FFM-CSC has not been clarified,and the protection scheme of transmission line for FFM-CSC is still lacking.The paper conducted research on this aspect:Firstly,the operating principle and control method of FFM-CSC are studied.In this paper,the steady-state mathematical modeling of FFM-CSC is established,and the characteristics of fundamental frequency modulation control method are analyzed.The simulation model of ±500kV/3kA bipolar FFM-CSC DC transmission system is built in PSCAD/EMTDC.The DC line adopts the distributed parameter model to improve the accuracy of the model.The simulation results verifies the correctness of the topology and control method of FFM-CSC.Secondly,the transient modeling of the key equipment of FFM-CSC is carried out.and the mathematical analytic formula of DC fault current is derived.The FFM-CSC converter valve adopts a full-controllable device,the commutation process is extremely short.Moreover,there is a filter capacitor at the outlet of the converter valve,the AC source can be fed into the short-circuit current to the DC side through the parallel capacitor,resulting in a significant difference between FFM-CSC and Line Commutated Converter(LCC)in transient model.Based on the transient model of the system,the mathematical analysis of the initial fault traveling wave is carried out.Then,the time domain expressions of DC fault current is solved with engineering parameters,and compared with the simulation results of PSCAD/EMTDC.Finally,aiming at the DC fault characteristics of FFM-CSC,a DC transmission line protection scheme based on improved recursive wavelet is proposed.The fault characteristics of FFM-CSC are not obvious when the DC fault occurs,and the overcurrent capability of the full-controllable device is poor,so the protection requires both high sensitivity and high rapidity.Improved Recursive Wavelet Transform(IRWT)can fully display the time-frequency domain information of the fault traveling wave.It uses only historical data and does not require the information after the fault time,so it can sacrifice part of sensitivity for higher rapidity.Therefore,it is very suitable for identifying FFM-CSC DC faults.In this paper,the phase angle and amplitude information after improved recursive wavelet transform are used to construct the protection criterion,and the protection algorithm is comprehensively evaluated from three aspects:rapidity,sensitivity and reliability.The simulation results of PSCAD/EMTDC show that the protection can effectively identify the internal and external faults,realize the full line fault detection quickly,and it is robust to high level of interference and high fault resistance.