Hybrid MMC based multi-terminal DC/DC converter with minimized FBSMs ratio considering DC fault isolation

An isolated high-power multi-terminal DC/DC converter is studied in this paper, based on hybrid MMC configuration consisting of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs). To decrease the investment and power losses, a reduced arm FBSMs ratio (less than 0.5) scheme is adopted. A detailed analysis on the relationship of the DC/DC converter inner AC voltage and the arm FBSMs ratio under reduced DC voltage is presented. Based on this, a control strategy during DC fault is proposed which continues operating the converter connected to the faulty DC side with reactive current absorption. Under the same arm FBSMs ratio, compared to the conventional strategy of blocking the faulty side converter during a DC fault, the proposed unblocking method with reactive current injection can not only achieve greater DC fault current declining rate, but also ensure maximum power transfer between the interconnected healthy DC grids by maintaining a higher inner AC voltage in the DC/DC converter. The two strategies are compared and validated by simulations using PSCAD/EMTDC under different arm FBSMs ratio

Submodule configuration of HVDC-DC auto transformer considering DC fault

This paper studies the submodule configuration of MMC based non-isolated HVDC-DC autotransformer (HVDC-AT) with DC fault blocking capability, including two-terminal and multi-terminal topologies. The operation principle of the HVDC-AT is described. Considering the arm current differences, the total number of required semiconductors for the HVDC-AT is derived and is compared with the MMC based isolated front-to-front (F2F) DC transformer. A full operation process for the multi-terminal HVDC-AT considering DC fault is then presented, including normal operation, fault isolation and continuous operation of healthy converters after fault. The submodule configuration and fault recovery of the multi-terminal HVDC-AT are validated by simulations using PSCAD/EMTDC

Sharing Economy in Local Energy Markets

With an increase in the electrification of end-use sectors, various resources on the demand side provide great flexibility potential for system operation, which also leads to problems such as the strong randomness of power consumption behavior, the low utilization rate of flexible resources, and difficulties in cost recovery. With the core idea of 'access over ownership', the concept of the sharing economy has gained substantial popularity in the local energy market in recent years. Thus, we provide an overview of the potential market design for the sharing economy in local energy markets (LEMs) and conduct a detailed review of research related to local energy sharing, enabling technologies, and potential practices. This paper can provide a useful reference and insights for the activation of demand-side flexibility potential. Hopefully, this paper can also provide novel insights into the development and further integration of the sharing economy in LEMs.</p

An Optimal Integrated Control Scheme for Permanent Magnet Synchronous Generator-Based Wind Turbines under Asymmetrical Grid Fault Conditions

In recent years, the increasing penetration level of wind energy into power systems has brought new issues and challenges. One of the main concerns is the issue of dynamic response capability during outer disturbance conditions, especially the fault-tolerance capability during asymmetrical faults. In order to improve the fault-tolerance and dynamic response capability under asymmetrical grid fault conditions, an optimal integrated control scheme for the grid-side voltage-source converter (VSC) of direct-driven permanent magnet synchronous generator (PMSG)-based wind turbine systems is proposed in this paper. The optimal control strategy includes a main controller and an additional controller. In the main controller, a double-loop controller based on differential flatness-based theory is designed for grid-side VSC. Two parts are involved in the design process of the flatness-based controller: the reference trajectories generation of flatness output and the implementation of the controller. In the additional control aspect, an auxiliary second harmonic compensation control loop based on an improved calculation method for grid-side instantaneous transmission power is designed by the quasi proportional resonant (Quasi-PR) control principle, which is able to simultaneously restrain the second harmonic components in active power and reactive power injected into the grid without the respective calculation for current control references. Moreover, to reduce the DC-link overvoltage during grid faults, the mathematical model of DC-link voltage is analyzed and a feedforward modified control factor is added to the traditional DC voltage control loop in grid-side VSC. The effectiveness of the optimal control scheme is verified in PSCAD/EMTDC simulation software

Operation Modeling of Power Systems Integrated with Large-Scale New Energy Power Sources

In the most current methods of probabilistic power system production simulation, the output characteristics of new energy power generation (NEPG) has not been comprehensively considered. In this paper, the power output characteristics of wind power generation and photovoltaic power generation are firstly analyzed based on statistical methods according to their historical operating data. Then the characteristic indexes and the filtering principle of the NEPG historical output scenarios are introduced with the confidence level, and the calculation model of NEPG’s credible capacity is proposed. Based on this, taking the minimum production costs or the best energy-saving and emission-reduction effect as the optimization objective, the power system operation model with large-scale integration of new energy power generation (NEPG) is established considering the power balance, the electricity balance and the peak balance. Besides, the constraints of the operating characteristics of different power generation types, the maintenance schedule, the load reservation, the emergency reservation, the water abandonment and the transmitting capacity between different areas are also considered. With the proposed power system operation model, the operation simulations are carried out based on the actual Northwest power grid of China, which resolves the new energy power accommodations considering different system operating conditions. The simulation results well verify the validity of the proposed power system operation model in the accommodation analysis for the power system which is penetrated with large scale NEPG

Stability of the Stochastic Model for Power Markets with Interval Parameters

Pertaining to the random nature of demand sides and the range of demand elasticity with suppliers and consumers, a stochastic model for power markets with interval parameters is described to illustrate uncertain external disturbances, which is a generalization of the Alvarado dynamic model, stochastic model, and interval model. The interval stochastic stability criteria of the provided model are investigated by the theory of economics, interval dynamical system, and the theory stability of stochastic differential equations. The conclusions indicate that the demand elasticity stable interval can be calculated and the random excitation intensity does not impact the system stability. Some numerical examples are given to show the applicability and validity of the obtained results from a statistical perspective

Electricity Market Stochastic Dynamic Model and Its Mean Stability Analysis

Based on the deterministic dynamic model of electricity market proposed by Alvarado, a stochastic electricity market model, considering the random nature of demand sides, is presented in this paper on the assumption that generator cost function and consumer utility function are quadratic functions. The stochastic electricity market model is a generalization of the deterministic dynamic model. Using the theory of stochastic differential equations, stochastic process theory, and eigenvalue techniques, the determining conditions of the mean stability for this electricity market model under small Gauss type random excitation are provided and testified theoretically. That is, if the demand elasticity of suppliers is nonnegative and the demand elasticity of consumers is negative, then the stochastic electricity market model is mean stable. It implies that the stability can be judged directly by initial data without any computation. Taking deterministic electricity market data combined with small Gauss type random excitation as numerical samples to interpret random phenomena from a statistical perspective, the results indicate the conclusions above are correct, valid, and practical