Research On Key Technologies Of LCC-HVDC Control To Enhance The Commutation Failure Suppression Capacity

The line-commutated converter based high-voltage direct-current(LCC-HVDC)has been extensively used in scenarios such as the asynchronous interconnection of cross-regional power grids and the effective utilization of widely distributed renewable energy sources,for the reason that it has the advantages of long-distance bulk power transmission capacity,flexible power flow control ability,relatively low capital cost and also low power loss of the converter stations,etc.With the continuous construction and deployment of LCC-based HVDC and ultra-HVDC(UHVDC)projects,a large-scale hybrid AC/DC power grid has been formed in China.However,considering that the coupling between the AC and DC sides,as well as the sending and receiving ends,has become tighter and much more complicated,if a single fault occurs in the AC power grids or LCC-HVDC converter stations,it might significantly deteriorate and rapidly expand,then causing a series of cascading faults.The commutation failure(CF)is a common malfunction in the LCC-HVDC links.Due to the fact that the thyristors valves without a self-turn-off capability are used in the line-commutated converters,the inverters will be vulnerable to the CFs if fault occurs nearby in the receiving-end AC system.Moreover,on condition that the LCC-HVDC is improperly regulated by the recovery control after onset of the first CF,subsequent or repetitive CFs might happen and would even result in the blocking of the LCC-HVDC link.Given that the power transmission of a heavy-load and crucial HVDC or UHVDC link is entirely interrupted,a wide-range and massive power flow transfer will then take place,and lots of AC transmission lines might be tripped due to the overload protection scheme.Therefore,a fairly simple CF event might be able to initiate a series of severe cascading faults in the large-scale hybrid AC/DC system,and potentially endangering the safety and stability of both sending-and receiving-end power grids.Considering the above-mentioned issues,this dissertation aims at enhancing the CF suppression capacity of the LCC-HVDC link through the key control technologies,and the main contents are as follows:1)The key factors restricting the suppression of first CF at AC fault inception are analyzed,and the optimization and improvement measures for the commutation failure prevention(CFPREV)control are also proposed.Firstly,the mechanism of the first CF is analyzed based on the quasi-steady-state equation,and the characteristics of the DC-side surge current after onset of first CF are also studied by the differential equation-based step response analysis.Secondly,from the perspective of AC fault initiation time,inverter reactive power consumption,and converter plateau effect,the limitation of inverter advancing firing control to suppress the first CF is discussed.Through re-implementation and further analysis of the original CFPREV control scheme,a few measures targeted at optimizing its output characteristics are proposed.Finally,simulation tests are conducted to verify the effectiveness of the proposed improved CFPREV control,as well as the existence of plateau effect in the line-commutated converter.2)The mechanism of subsequent commutation failure(SCF)and the interaction characteristics between the AC and DC sides under the AC fault are analyzed,the SCF onset boundary and its early warning method are also presented.Firstly,on condition of inverter-side symmetrical and asymmetrical AC faults,the SCF mechanism is summarized in terms of the current control ambiguity,DC current oscillation,current error control output fluctuation,phase-locked loop tracking error,etc.Secondly,a switching function method based on the line-to-line voltages is introduced to depict the voltage transfer characteristics of the 12-pulse inverter under unbalanced operating conditions.In addition,the calculation method of the inverter DC-side oscillating current under the asymmetrical AC fault is obtained through the sinusoidal steady-state analysis of DC circuit with due consideration of the interaction with the LCC-HVDC control system.Based on that,the onset boundary and early warning criterion of the SCF are then put forward by taking full account of the regulating risk of control system when using the voltage-time area(VTA)theory and model prediction method.Finally,the validity of the proposed SCF early warning and oscillating current calculation methods are verified in the case studies through simulation.3)The cooperation relationship and regulating characteristics of the CF recovery controllers in the LCC-HVDC link are discussed,and the control strategies,with the ability to perceive the SCF real-time risk,for the LCC-HVDC fault recovery are also provided.Firstly,the cooperation relationship between the inverter constant extinction angle(CEA)control and rectifier constant current(CC)control,which is established via the voltage dependent current order limiter(VDCOL),during the restorations of DC voltage and current after the first CF occurs is briefly discussed.Thereafter,the regulating characteristics of rectifier CC controller is analyzed based on its small-signal transfer function model developed by using the perturbation analysis.Secondly,by improving the CEA control and VDCOL,and taking the real-time SCF risk into account,two fault recovery strategies of the LCC-HVDC link,which are respectively based on the emergency inverter firing adjustment and adaptive DC current order regulation,are thereby presented.Finally,simulation results obtained under various fault conditions indicate that the proposed methods can not only greatly reduce the risk of SCF,but also improve the recovery performance of the LCC-HVDC link.4)The existing defects of extinction angle(EA)measurement during fault transients in the mainstream LCC-HVDC simulation models are revealed,and the EA correction and control methods,which are respectively based on the fictitious end-ofcurrent signals and improved EA pre-filter,are also provided.Firstly,by referring to the overall structure of the actual-measurement-based CEA control,the principles as well as characteristics of those key components in the EA measurement and control system are thoroughly discussed,and the general approach and procedure of the EA measurement in simulations are summarized.Secondly,according to the analysis of EA measurement schemes of the MATLAB/SIMULINK sample model and bespoke CIGRE benchmark model,the defects and corresponding root causes of the invalidation of EA measurement during fault transients are eventually revealed.In the meantime,an EA correction method based on the fictitious end-of-current signals,as well as an EA control method based on the improved EA pre-filter,is proposed.Finally,simulation tests are carried out and the results show that the proposed EA correction method is an effective solution to the EA measurement invalid problem under fault transient conditions,and in contrast to the original CEA control,the presented EA control is also able to avoid the improper regulating behaviors in the LCC-HVDC recovery process.Focusing on the issues of first CF at fault inception and SCF in the following fault-duration period,this dissertation provides related disscussions on the mechanism of first CF and SCF,fault characteristic analyses,improvement of LCC-HVDC control schemes,detailed modeling in simulations,etc.The results and findings also have great theoretical value and substantial technical merits in terms of the enhancement of LCCHVDC immunity against CFs and expansion of security realm in the the hybrid AC/DC power grids.