Multi-infeed Short Circuit Ratio Research Articles

An energy storage planning method for improving the security of receiving-end system considering the selection of power conversion systems

China's power demand in high load density areas is satisfied by a centralized feed of several large-capacity HVDC links, forming a typical receiving-end system, which is facing problems such as shortage of dynamic reactive power and increased risk of commutation failure. Electrochemical energy storage (ES) has characteristics such as strong dynamic active and reactive power regulation ability, rapid response and flexible control mode, making it a high-quality resource in improving the security of the receiving-end system. Therefore, it is necessary to carry out research on the methods of planning of ES in receiving-end systems. This paper mainly analyzes the role of ES in providing dynamic reactive power support. According to the different control modes of power conversion system (PCS), the ES is divided into grid-following (GFL) and grid-forming (GFM), and multi-infeed short circuit ratio (MISCR) considering both is constructed. The MISCR is taken as an objective function of the ES planning model. At the same time, the method of “accompanying network + model reconstruction” is proposed to realize the linearization calculation of MISCR. In order to compensate for the economic cost of ES investment, its energy storage function should also be used to participate in peak shaving during normal operation. Therefore, an ES planning model considering PCS selection is established, which including ES planning layer, system operation layer and accompanying network layer. Finally, the effectiveness of the proposed planning method is verified by a modified IEEE-39 system.

Optimal transmission switching strategy for multi‐infeed DC receiving‐end power network to prevent over‐limit of short‐circuit current

AbstractA large‐scale AC/DC hybrid power network has recently been constructed in China because of the fast development of high voltage direct current (HVDC) technology. The receiving AC system must have sufficient system strength to guarantee the safe operation of the DC side. The paradox is that raising the AC side system's strength will increase the short‐circuit current (SCC) level, endangering the safety of the system. In order to resolve the contradiction between the system strength of multi‐infeed DC receiving‐end power networks and SCC, this paper proposes an optimal transmission switching strategy. Firstly, the system strength is characterised by the multi‐infeed short‐circuit ratio (MISCR), followed by constructing a comprehensive constraint set that considers the MISCR and SCC simultaneously. Secondly, to optimize SCC with MISCR and N−1 safety constraints, a mixed integer linear programming model is developed. Finally, to enhance the computing performance for large power systems, a new connectivity constraint set and decision space reduction techniques are provided. The proposed method is tested for effectiveness using the IEEE 30‐bus and IEEE 118‐bus systems.

Evaluation of Renewable Energy Hosting Capacity of Grid with Multi-Infeed HVDC

Considering the impact of large-scale renewable energy access to a multi-infeed HVDC receiving-end system on its voltage stability and multi-infeed short circuit ratio, a renewable energy acceptance evaluation method based on an improved holomorphic embedding method is proposed. Firstly, to construct a model containing only an AC system, the DC inverter is simplified into an AC source model; secondly, the sigma index is used to judge the static voltage stability of the system bus and determine the access location of renewable energy, and sensitivity index is used to determine the scheme of replacing conventional units with renewable energy; then, with the voltage crossing the set range as the constraint condition, the analytical expression of each bus voltage obtained by the holomorphic embedding method is used to solve the maximum renewable energy capacity of the system; finally, under the maximum hosting scenario of renewable energy, verify the multi-infeed short circuit ratio at each DC drop point. The effectiveness of the proposed method is verified on an improved IEEE39-bus system.

The Definition of Power Grid Strength and Its Calculation Methods for Power Systems with High Proportion Nonsynchronous-Machine Sources

This paper studies the definition and calculation method of power grid strength in the environment of high-proportion nonsynchronous-machine sources, focusing on the effect of nonsynchronous-machine sources on voltage support strength and frequency support strength. By dividing the nonsynchronous-machine sources into four types, the equivalent circuits of each type under normal state and fault state are derived, respectively. Based on the Thevenin equivalent impedance of the power grid and the equivalent impedance of the connected device, the definition and calculation method of voltage support strength is given, and the new meaning of single-infeed short-circuit ratio and multi-infeed short-circuit ratio in the context of high proportion nonsynchronous-machine sources is presented. Based on the initial frequency change rate and the steady-state frequency deviation of any node in the power grid under the maximum expected active power disturbance, the equivalent inertia lifting factor and steady-state frequency deviation decreasing factor are defined, respectively, to describe the contribution of nonsynchronous-machine sources to the power grid frequency support strength, and the calculation methods of the equivalent inertia lifting factor and the steady-state frequency deviation decreasing factor are given.

Capacity planning of wind generation units in multi-wind-generation DC-connected system based on cooperative game

This paper proposes a cooperative-game-based approach to plan the capacity of wind generation units (WGUs) in the multi-wind-generation DC-connected (MWGDC) system, which considers both the individual economy of each WGU and the system operation security. First, a comprehensive payoff function which considers economic indexes and the multi-infeed short circuit ratio (MSCR) of the HVDC system is designed, with game model established. Then, a cooperation mechanism is introduced among WGUs. Moreover, the comparison among the designed cooperative game, the non-cooperative game and multi-objective optimization is analyzed. In the case study, the results of the test system verify the effectiveness of the proposed capacity planning method.

A Novel Receiving End Grid Planning Method with Mutually Exclusive Constraints in Alternating Current/Direct Current Lines

The large-scale application of high-voltage direct current (HVDC) transmission technology introduces mutually exclusive constraints (MEC) into the power grid planning, which deepens the complexity of power grid planning. The MECs decrease the planning efficiency and effectiveness of the conventional method. This paper proposes a novel hybrid alternating current (AC)/direct current (DC) receiving end grid planning method with MECs in AC/DC lines. The constraint satisfaction problem (CSP) is utilized to model the MECs in candidate lines and then the detailed planning model, in which mutually exclusive candidate lines are described by mutually exclusive variable and constraint sets. Additionally, the proposed planning model takes the hybrid AC/DC power system stability into consideration by introducing the multi-infeed short circuit ratio (MISCR). After establishing the hybrid AC/DC receiving end grid planning model with MECs, the backtracking search algorithm (BSA) is used to solve the optimal planning. The effectiveness of the proposed hybrid AC/DC grid planning method with MECs is verified by case studies.

Analysis of the Operating Margin Evaluation of Multi-Infeed LCC-HVDC Systems Based on the Equivalent Impedance

In China, a structure in which multiple line-commutated converter based high-voltage direct current (LCC-HVDC) systems feed into the same AC system has been formed. For the multiple LCC-HVDC infeed system, the electrical strength and real operating condition evaluation of the receiving AC system are of great significance for system operation. The multi-infeed short circuit ratio (MISCR) is generally utilized to assess the strength based on the rated parameters; hence, it cannot analyze the real operating condition under the non-rated status. In this paper, an LCC-HVDC system is equivalent to a grounded impedance without changing the external characteristics. The multi-infeed operating short circuit ratio (MIOSCR) is then deduced from the equivalent impedance and equivalent nodal analysis. Relationships between the MIOSCR and electrical strength indices of AC and AC/DC systems are analyzed, and the MISCR is deduced as a special case of the MIOSCR under the rated status. The multi-infeed operating margin coefficient (MIOMC) is finally proposed, which can be utilized to evaluate the operating margin of the LCC-HVDC system and provide guidance for designing operating strategies. The proposed MIOSCR and MIOMC overcome the previous shortcomings of inaccuracy about operating status assessment by the MISCR. The results of the case study demonstrate the effectiveness of the MIOSCR and the MIOMC for assessing the operating condition and margin of converter buses.

A Cascaded Converter Interfacing Long-Distance HVdc and Back-to-Back HVdc Systems

This paper proposes a cascaded converter dedicated to a long-distance high-voltage direct current (HVdc) infeed and asynchronous back-to-back interconnection of receiving grids. The cascaded converter consisted of modular multilevel converters (MMCs) in series and parallel connection, meeting the high dc voltage and power demand of the HVdc system. It realizes the hierarchical feeding and asynchronous interconnection of receiving grids, optimizing the multi-infeed short-circuit ratio (MISCR) and improving the flexibility of the receiving grids. The topology and operating characteristics of the cascaded converter are introduced in detail. The MISCR and the maximum power infeed of the cascaded converter-based HVdc systems are analyzed. Various feasible operating modes with online switching strategies of the cascaded converter are studied to improve the operational flexibility of the system. The simulation results verify the effectiveness of the control strategy of the HVdc system embedding the cascaded converter. The dc faults clearing strategy and operating modes switching strategies are also validated.

A Voltage Stability Analysis Method of Multi-infeed HVDC Power Systems

According to the Chinese power network planning, the risk of voltage instability at the receiving-end AC power system will be increased significantly, where there will be more HVDC systems feeding into a same AC grid in the future. At present, the effective voltage stability analysis methods of multi-infeed DC (MIDC) system are few, the most widely used method with most research results is the multi-infeed short-circuit ratio (MISCR) index at home and abroad. The deficiency of MISCR index is pointed out. The relationship between multi-infeed interaction factor (MIIF) and the voltage stability of the receiving-end AC system is discussed. Then a new method joint MIIF and MISCR indexes to evaluate the voltage stability of MIDC system is put forward. Finally, simulation tests and analysis results of a large actual grid prove the feasibility and good performance of the proposed new method.