The submodule capacitance is a key factor affecting the volume and cost of modular multilevel converter (MMC). The normalized value of the energy stored in an MMC with respect to the rated capacity is taken as the unified index for evaluating the capacitance usage of an MMC. A per-unit model that describes the relationship between energy storage requirement (ESR) value and the fluctuation rate of capacitor voltage is established. Thereby, the evaluation, calculation and analysis of the ESR values of MMCs with different rated parameters can be unified and simplified. Compared with the conventional methods of taking the capacitance value as the design and analysis index, the per-unit model for calculating the ESR value eliminates the influence of various parameters on the calculation results. It is revealed that the ESR value is mainly affected by the base modulation index and the per-unit active and reactive power output. Based on the proposed model, the variation of the ESR value with the modulation index and the power output range is analyzed in detail, which provides a clear and accurate basis for the design and optimization of MMC capacitance usage. The results of digital simulation and experimental results verify the proposed methods.
DC circuit breaker (DCCB) is the key equipment to ensure the safe and stable operation of DC power grid. For the existing DCCB, the cost is too high due to the large number of devices in operation, large current during short-circuit faults, and too much energy absorbed by the arrester. A power electronic multi-port HVDC circuit breaker (MP-DCCB) with current limiting is proposed. The performance of DCCB is studied by improving its topology. The working principle and fault breaking process of the new topology are analyzed theoretically. The relevant parameters and selection basis of the device are given. Finally, the corresponding simulation model is built and verified with PSCAD software. Compared with the conventional DCCB scheme, the fault current is reduced by 48.6%; the absorbed energy, turn-off current and turn-off time of the arrester are reduced by 54.8%, 52.4% and 0.8 ms, respectively. Therefore, the new MP-DCCB has good reliability and economy, and the simulation results verify that the new topology scheme of power electronic multi-port HVDC circuit breaker with current limiting can be applied to the DC power grid.
Aiming at the problem of double-frequency pulsation of DC bus voltage in microgrid under unbalanced AC voltage condition, a control strategy for two-stage AC/DC interface converter with CLLC DC transformer is proposed, which can ensure the three-phase currents balance and suppress the voltage pulsation of DC bus under balanced and unbalanced AC bus voltage. Firstly, the power transmission characteristics of the AC/DC interface converter under unbalanced condition are analyzed, and the control strategy to suppress the negative-sequence current at the AC side is designed. Secondly, the fundamental equivalent model of CLLC DC transformer is established, and its voltage gain and input impedance characteristics are analyzed. On this basis, considering the input voltage pulsation characteristics of CLLC DC transformer under unbalanced conditions, the optimal parameter design of CLLC DC transformer is carried out and a control strategy based on the feedforward of pulsation voltage is proposed to suppress the voltage pulsation of DC bus. Finally, the Matlab/Simulink simulation results show that the proposed control strategy can ensure the three-phase currents balance and suppress the voltage pulsation of the DC bus under both balanced and unbalanced AC bus voltage.
The traditional electricity market transaction is mainly that the electricity generated by power plants dispatched to the customers by dispatch agencies in a centralized manner, according to power supply and demand market and plans. With the rapid development of distributed energy and the need for a large number of power sources being connected to the grid, processing speed of centralized dispatching will become a development bottleneck. The use of distributed energy transactions can effectively solve such problems. Blockchain has the characteristics of decentralization, data openness and transparency. In view of the low reliability of centralized dispatching of distributed energy transactions and the bottleneck of processing speed, combined with the characteristics of blockchain, a distributed energy trading method based on multi-chain cooperative blockchain is proposed firstly. According to the partition of regions, this method solves the problem of slow transaction speed of blockchain system. Then, the distributed energy transaction matrix is proposed by combining the useful workload proof mechanism with the power matching scheduling algorithm, which can maintain the decentralization and solve the problem of computing power waste of workload proof mechanism.
In order to guarantee the information safety and confidential privacy of diverse energy entities among the integrated community energy system (ICES), and satisfy the multiple energy demand for electricity and heat, a decentralized optimal scheduling method is proposed in this paper. Firstly, the mathematical models of electric distribution system, natural gas distribution system and coupling system are established respectively and linearized by certain mathematical method. Secondly, the optimal scheduling model of ICES is developed with the operation cost set as the objective function. By introducing the consensus variable into the scheduling model, the optimal operation can be decoupled on the basis of the decentralized scheduling framework, which is suitable for the multiple entities integration for ICES. Finally, a typical ICES case is utilized to verify the proposed method. It is illustrated that the proposed decentralized scheduling method is able to provide the scheduling scheme same as that by centralized method, which is capable of the satisfaction for operation constraints, the realization of optimal operation and the protection of information safety and confidential privacy.
Zoning planning can make the distribution network planning from large to small, from complex to simple. Independent grid planning can avoid the problem of too much calculation burden in centralized distribution network planning. Therefore, according to the results of grid division, this paper proposes a grid dynamic investment strategy for urban distribution network considering Geographic Information System (GIS). Firstly, this paper establishes a distribution network investment simulation platform based on Matlab-OpenDSS, where Matlab generates distribution network investment strategies, OpenDSS establishes a physical model of the computing grid, and calculates the corresponding investment indicators to correct the feasibility of the investment strategy. Secondly, we clarify the multi-objective and the number of cycles of the investment in the distribution network planning. Then, the NSGA-II algorithm is used to optimize the investment strategy for each cycle, which takes into account the multi-objective requirements of the power network planning scheme, and the investment in the next cycle depends on the optimal investment plan of the previous cycle for measurement, so as to make the dynamic investment more in line with the actual investment and construction process of the distribution network. In the proposed method, a number of planning evaluation indicators are added to the objective function, which can improve the economy and feasibility of the investment strategy; at the same time, dynamic programming is used to divide the total investment cycle into several short-term processes, which is more in line with the needs of investment planning. Finally, a real city distribution network is used as the test system to verify the feasibility of the proposed method.
Under the background of promoting our country’s new round of electric power system reform and building the national unified electricity market, in order to break down inter-provincial barriers and alleviate the power-shortage, abandoned water, wind curtailment and discarded light in some provinces of regional electricity market due to the lack of reserve capacity and the inverse distribution of energy and load, this paper introduces reserve sharing mechanism and coordinated optimization of tie-line, constructs a combined clearing model of energy and reserve for regional electricity spot market, including the security constraints of unit combination and security constraints of economic dispatch. Through combined clearing of energy and reserve for regional electricity spot market considering reserve sharing, the optimal allocation of power resources in a wider range is realized, and the economy and reliability of system operation are improved effectively. Two interconnected IEEE 30-node systems are used to simulate the electricity market transactions between two provinces in the region, and the correctness and effectiveness of the proposed model are verified.
Aiming at the capacity allocation of pumped-storage power stations in the association system of wind power, photovoltaic, thermal power and pumped storage, a bi-level programming of association system model is constructed and a solve loop is proposed. The upper-level model takes the minimum total curtailment of photovoltaic and wind power as the goal to determine the capacity allocation of the pumped-storage power station. The lower-level model aims to maximize the economic and environmental benefits of the association system, while improving the operating conditions of the system and solving the operation scheduling problem of pumped-storage power stations. The targets of the upper and lower-level are solved by the improved grey wolf optimizer (GWO) based on chaos optimization of Tent map. The effectiveness of the model and algorithm is verified by simulation analysis of two typical daily scenes in a certain area in winter and summer. The results show that the constructed bi-level programming model is effective for scientifically determining the capacity of pumped-storage power stations in the association system. And under the condition of satisfying the optimization of operation scheduling, it can improve the operating conditions of the system and realize the expected goal of maximizing economic and environmental benefits.
Large capacity UHVDC technology provides an effective means for long-distance and cross-regional power consumption of the sending-end power grid. However, with the increase of the number and capacity of HVDC, the contradiction of strong DC and weak AC is prominent. The fault of HVDC blocking will lead to an increasing threat to the frequency stability of the sending-end power grid, so the capacity of the HVDC is limited. In order to effectively quantify the extreme feed-out capacity of a single-circuit HVDC for providing references to evaluate the adaptability of the target grid, this paper proposes a method for single-circuit HVDC extreme feed-out capacity evaluation considering the frequency constraints. The proposed method firstly analyzes the frequency stability of the sending-end power grid under the scenario of large capacity HVDC bipolar lockout, and establishes the analytical expression between the transient frequency index and the capacity of HVDC lockout. Then, the second defensive line of power grid emergency control measures, such as the stability-control generator tripping and HVDC emergency active power support, are considered into the evaluation method, which can evaluate the extreme feed-out capacity of single-circuit HVDC that can be supported by the sending-end power grid under frequency stability constraints. Finally, an example simulation analysis is carried out on Sichuan Power Grid in 2030 to verify the effectiveness of the proposed method.
DC distribution network, with its advantage of efficient absorption of direct current energy, will become an important way of connecting the future distributed generators. However, the unbalanced faults of the AC distribution networks will cause the voltage fluctuation of the DC distribution networks, which will harm the normal operation of the DC distribution system. To tackle the problem aforementioned, this paper firstly analyzes the characteristics of active power and DC voltage fluctuations in the case of single-phase-to-ground fault in AC distribution network, especially deduces the expression of instantaneous active power of filter inductance, and points out that the instantaneous active power of filter inductance is the important cause of DC voltage fluctuation. Secondly, an improved phase lock loop (PLL) is proposed to track the phase of positive-sequence voltage. The proposed method only redesigns the compensator of the traditional PLL, which is clear in principle and easy to implement. Finally, a simulation model of typical two-terminal AC-DC distribution system is built in PSCAD/EMTDC to verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.
This paper focuses on load data mining in the field of distribution network planning, on the basis of actual data, the model of response evaluation based on air-conditioning load data mining is established, separating air-conditioning load from power consumption data, which can describe the demand response limits accurately, and the distribution system expansion planning model is also proposed and solved by aiming at minimizing the total cost of the distribution system. The validity of the model is verified by an IEEE 33-node example. The results show that the data mining methodology can be used to evaluate the demand response potential, which enables the distribution network planning scheme to be optimized, the total system cost to be reduced, the investment of distribution system expansion to be delayed, and the overall economic efficiency of distribution system to be improved. The proposed method has important practical significance and function for distribution network planning.
A two-tier model is proposed to control the peak-valley difference of daily load curve of users and to optimize electricity consumption for smart home. This model takes demand-side response as the means and household energy optimization as the strategy to realize the interaction between the power supply end and the power consumption end, and describes the interactive relationship among the price of electricity, incentive mechanism and users’ power consumption behavior. In the environment of time-of-use (TOU) price, the outer model adopts FCM algorithm to analyze the power consumption of users, and designs the power package including incentive subsidy and peak-valley coefficient with the goal of peak-valley load curve clipping. The inner layer model realizes the intelligent management of household appliances applying the power package, simulates the daily load curve of residents before and after the implementation of the power package, adjusts the electricity consumption plan in real time, and enables the daily load curve of users to meet the peak-valley coefficient of the power package. Through simulation, it is verified that the two-tier optimization model proposed in this paper can effectively reduce the peak-valley difference of the daily load curve of users, and the designed power package is practical to a certain extent on the user side, which is beneficial for users to participate more actively in the optimization scheduling of the power grid and meet the requirements of peak load reduction and valley filling of the power grid.
Building an efficient, low-carbon and reliable integrated energy system is an effective means to promote the revolution of energy consumption and the sustainable development of energy. The optimal energy flow of integrated energy system has been widely concerned by academia and industry. In this paper, a new calculation method for optimal energy flow of multi-area electricity-gas integrated energy system is proposed. Firstly, the optimal energy flow model of integrated energy system is established with the objective of total economic optimization of integrated energy system. Secondly, the original model is further transformed into the optimal energy flow convex optimization model of integrated energy system by using second order cone relaxation (SOCR) in view of the non-convex terms of the model introduced by AC power flow constraints and air pressure and flow constraints. Thirdly, considering the inter-regional energy information interaction of multiple integrated energy systems, an adaptive step-size alternating direction multiplier method (ADMM) is proposed to calculate the optimal energy flow of centralized-distributed multi-regional integrated energy system. Finally, a simulation example is given to verify the effectiveness and accuracy of the proposed method.
Load frequency control (LFC) is essential for maintaining the security and stability of the power system concerned. However, the complexity of the differential equation model characterizing the dynamics of an interconnected power system brings a challenge to the design of the load frequency controller. Given this background, a distributed optimal control scheme based on alternating direction method of multiplier (ADMM) is employed to design the load frequency controller for the interconnected power system. The proposed approach can simultaneously optimize the control performance and improve the computation efficiency. Firstly, the mathematical model of LFC is briefed. Then an optimization formulation for designing the distributed optimal control scheme of LFC is presented on the basis of the quadratic performance and matrix sparsification, and solved by using ADMM. Finally, a three-area interconnected power system is employed to demonstrate the feasibility and effectiveness of the proposed LFC approach.
