Studies On Security Assessment And Optimal Dispatch Strategy Of The Multi-Infeed LCC-HVDC System

A variety of clean energy resources are distributed across China with huge reserve in scattered locations unevenly.The load center is concentrated along the southeast coast,which is far away from the clean energy resource region.To fulfill the increasing load demand,a large amount of clean energy resources is consumed by load centers through line-commutated converter high voltage direct current(LCC-HVDC)transmission system fed into the receiving end.Consequently,the structure that multiple LCC-HVDC systems fed into the same receiving end has formed,which is called the multi-infeed LCC-HVDC(MIDC)system.The centralized feeding of multiple LCCHVDC systems not only enriches the control means of the system,but also brings new challenges to system operation.This dissertation is focused on the security assessment and optimal dispatch strategy of MIDC system,and the study includes security assessment method,boundary point analysis,day-ahead resilience scheduling and security check.The specific contents are as follows:(1)The existing operating evaluation method conducted by the multi-infeed short circuit ratio(MISCR)indicator may lead to inaccurate operating margin results,since the interactions between active and reactive power of LCC-HVDC systems and AC system on the electrical strength is not considered.Therefore,an operating assessment method based on the equivalent impedance of the LCC-HVDC system is proposed.The equivalent impedance model of the LCC-HVDC system is firstly introduced,and the commutation angle based modeling reflecting the actual status of LCC-HVDC system is realized.The equivalent model of MIDC system under a converter bus is then built.The multi-infeed operating short circuit ratio(MIOSCR)indicator is proposed,and the MISCR is proved to be the special case under the rated status of the MIOSCR.Finally,the multi-infeed operating margin coefficient(MIOMC)indicator is proposed,based on the MIOSCR corresponding to the operating point and boundary point in a specific adjustment direction to evaluate the real-time operating margin of the LCC-HVDC system.The study provides an effective method to evaluate the safety margin for MIDC system.(2)The determination of an operating boundary point for the MIDC system is the foundation of dispatch strategies.The existing maximum available power(MAP)calculation based on DC current model has low efficiency,which may not be suitable to power system dispatch.The equivalent impedance model of the LCC-HVDC system proposed in section 1 reflects the operating status based on the commutation angle.Therefore,the equivalent model is applied into the single-infeed LCC-HVDC(SIDC)system and MIDC system,respectively.For the MAP of the SIDC system and the regional MAP of the MIDC system,the expression corresponding to the critical condition of these two scenarios is proposed.For the MAP of the MIDC system,a calculation model based on the equivalent impedance model is proposed,the number of variables is decreased compared with the traditional one.The calculation efficiency in acquiring the MAP is improved in the study,hence the proposed algorithm can realize quick and accurate calculation of MAP.(3)The output of renewable energy source(RES)is volatile,and faults of AC system and renewable energy may lead to tripping of the RES.Therefore,for the MIDC system with large-scale RES integrated,power shortage may take place,threatening the safe and reliable operation.The adjustment of LCC-HVDC system in balancing the power shortage can reduce the adjustment cost,thus a resilience-considered economic dispatch optimization model of the MIDC system is proposed.Part of the DC power is reserved to meet the contingency and the reservation cost is considered in the day-ahead unit commitment stage,in addition to the start-up cost and power generation cost.An additional cost considering the electrical strength of converter buses is also expressed by the approximate hyperplane of each safety constraint in the commutation angle space.For the MIDC system,a robust day-ahead economic optimization dispatch model considering the above mentioned costs and power shortage led by integration of the RES is established.The voltage support of converter buses and dispatch costs due to power shortage are fully considered in designing plans,and the resilience is improved.The study can provide operators with guidance in acquiring the economic dispatch plan for the MIDC system with large-scale RES integrated.(4)The electrical strength of the converter bus and safety boundary of an HVDC system shall be changed when the key line breaks in the receiving end of the MIDC system,hence the operation plan based on day-ahead dispatch plan conducted by the forecasted data may be unsafe.In order to address the issue,a security check strategy of the MIDC system based on the regulatory domain is proposed.Firstly,within the commutation angle space,the expression of the operating point corresponding to the MAP and converter bus voltage under an adjustment direction is deduced,respectively,and the set of operating points satisfying constraints of HVDC systems in the commutation angle space is defined as a MIDC regulatory domain.A security check optimization model of the MIDC system is proposed,aiming at acquiring the maximum value of the MIOMC corresponding to the converter bus with the smallest MISCR value,and also to improve the operation margin of LCC-HVDC system and resilience of the MIDC system once a contingency takes place.The study can provide operators with guidance in designing the dispatch control strategy under actual status.