Research On The Carrying Capacity Evaluation Method And Enhancement Strategy Of HVDC Links For Receiving-end Power Grids

In recent years,ultra-high voltage direct current transmis sion technology has been widely adopted in China’s power grid due to its advantages of large transmission capacity and long-distance delivery.The construction and implementation of numerous ultra-high voltage direct current projects have facilitated lar ge-scale cross-regional transmission and effective consumption of power resources,thus efficiently alleviating the power shortage pressure in China’s central and eastern load centers.Consequently,the provincial receiving-end power grids in China’s central and eastern regions present a multi-infeed pattern of UHVDC links.However,the high-density access of UHVDC links makes the operation mode and stability characteristics of the receiving-end power grid increasingly complex.This includes potential risks such as frequency stability,voltage stability,and short-circuit current exceeding standards.In order to meet the development planning needs and ensure the adaptability and access security of the receiving power grid to UHVDC projects,this paper proposes a UHVDC carrying capacity evaluation method for the receiving-end power grids.The resulting evaluation scheme meets the necessa ry requirements for safety and stability,including frequency and voltage constraints,peak regulation capabilities,and possesses the ability to withstand severe contingencies.Furthermore,to address the problem of mismatch between the construction of UHVDC projects and the development cycle of the receiving-end power grid,as well as the issue of short-circuit currents exceeding standards durin g the process of the receiving-end power grid changing from “weak” to “strong” this paper explores coordinated measures to enhance the DC carrying capacity and suppress short-circuit currents in the receiving-end power grid.The proposed measures can prov ide essential theoretical support for ensuring successful DC power absorption and building a robust receiving-end power grid.The main research work of this paper is summarized as follows:(1)Based on the traditional concept of voltage support capability,a dynamic system strength criterion based on voltage sensitivity has b een extended to evaluate the system’s resilience against DC commutation failure faults.This innovative approach takes into account the support effect of all reactive sources.On this foundation,a DC carrying capacity assessment model that considers dynamic voltage support capability for the receivi ng-end power grid is proposed,which determines the maximum number and capacity of DC lines that the receiving-end power grid can accommodate while taking into account multiple anticipated fault safety requirements.To ensure a rapid and effective solution of the model,the original two-stage model is decomposed into a main problem of DC carrying capacity assessment and a safety verification sub-problem considering multiple anticipated severe faults.The generalized Benders decomposition algorithm is employ ed to solve these problems iteratively.The effectiveness of the proposed dynamic voltage support criterion and DC carrying capacity assessment model is verified through simulation analysis of the IEEE RTS-79 system and the Henan power grid planning system.The solution obtained provides the receiving-end power grid with a stronger ability to resist DC fault risks and sufficient peak adjustment margin.(2)A total transfer capability assessment method has been proposed for coordinating sending and receiving-end power grids under frequency stability constraints.The problem is a multi-decision-maker,multi-level optimization model,where the upper-level coordination controller is responsible for solving the main problem with the objective of maximizing the energ y transmitted by the DC line.This includes modeling and coupling relevant constraints for the operational mode of the UHVDC lines,the number of flow adjustments,and correction control strategies.Meanwhile,the sending/receiving-end power grid operators independently solve the sub-problem of optimal flow under frequency stability constraints.This evaluation framework effectively integrates fast/slow active re sources to achieve frequency coordination control.It not only prevents the system’s third line of defense from being triggered after UHVDC bipolar locking failure but also fully utilizes the potential active control measures of the sending/receiving power gr id.Additionally,an objective cascading analysis algorithm based on an accelerated strategy is designed to solve the above model.Compared with traditional objective cascading analysis algorithms,this method has a significantly improved convergence speed.Through simulation analysis of the improved two-zone IEEE RTS-79 system,it is verified that the frequency stability control measures of fast/slow active resource coordination are beneficial for alleviating the primary frequency regulation backup pressur e of the sending/receiving-end power grid.Consequently,providing more reasonable and accurate total transfer capability assessment results.(3)To ensure the safe integration of massive amounts of UHVDC projects,it is essential to strengthen the alternating current grid strength and reactive re source margin of the receiving-end power grid.To address this,a distributed robust planning model that considers coordinated network expansion and phase-shifting transformer configurations has been proposed.Firstly,based on the branch addition method,the impact of high voltage alternating current(HVAC)network expansion and synchronous condenser configuration on the receiving-end power grid’s strength is quantified.Secondly,the main problem of this model has a minimum objective function of line expansion and synchronous condenser configuration cost,coupled with system strength constraints and related investment logic constraints.The dynamic safety verification sub-problem has a minimum objective function of operating costs,taking into account adequate dyna mic reactive power resources under the constraint of UHVDC fault safety requirements to mitigate risks brought by UHVDC bipolar locking faults and enhance the receiving-end power grid’s ability to respond to UHVDC commutation failure faults.To ensure computational efficiency and accuracy,the above model is transformed into a Mixed Integer Second-Order Cone Programming(MISOCP)model and solved using the column-and-constraint generation algorithm.Simulation analysis of the IEEE-39 system and Henan power grid planning system shows that the DC carryin g capacity improvement plan obtained with this method provides a strong receiving-end backbone network and sufficient reactive resource support to safely accommodate massive DC power.This validates the scientific,effectiveness,and practicality of the pr oposed strategy.(4)To address overcurrent issues resulting from the coordination of multiple DC power injections and developments in receiving-end power grids,a short-circuit current constraint model has been proposed that considers superconducting fault current limiters(SFCL)configuration and transmission grid structure optimization.Additionally,a short-circuit current suppression model is proposed that synergistically optimizes SFCL configuration and grid structu re.To ensure efficient modeling solutions,a linearized alternating current flow framework based on the Mc Cormick envelope is adopted.The relaxation error is minimized through iterative structuring until the difference between the linearized model and th e original nonlinear model is within an acceptable range.To effectively solve the above model,the main problem is decomposed into several sub-problems,and a solution strategy based on nested Benders decomposition algorithm is designed.The effectiveness and practicality of the proposed short-circuit current suppression strategy are validated through simulation analysis of the IEEE-39 system and Henan power grid planning system,demonstrating its cost-effectiveness and overcurrent suppression effect.