Three-dimensional thermal modelling of transformers in transformer room for spatial and temporal failure analysis

Temperature is a key factor for failure analysis of power transformers. Conventionally, transformer failure rate is calculated with hot spot temperature induced from IEEE empirical equations. This article firstly introduces a spatial and temporal related failure model based on three-dimensional thermal simulations of transformer and the related environment. The proposed thermal model is established with computational fluid dynamics for ventilation calculation and heat generation equations for power device simulation. Cooling strategies and mutual heating effect of power equipment are considered for an accurate temperature distribution prediction. By incorporating the three-dimensional thermal model into the service life-dependent and temperature-dependent model, the failure rate of each spatial point in power transformer could be calculated according to Arrhenius theory and Weibull distribution. The simulation results show that the proposed model clearly improves the accuracy of failure analysis and can be used for thermal and ventilation design of transformer room.Published versio

Three-Phase Short-Circuit Current Calculation of Power Systems with High Penetration of VSC-Based Renewable Energy

Short-circuit current level of power grid will be increased with high penetration of VSC-based renewable energy, and a strong coupling between transient fault process and control strategy will change the fault features. The full current expression of VSC-based renewable energy was obtained according to transient characteristics of short-circuit current. Furtherly, by analyzing the closed-loop transfer function model of controller and current source characteristics presented in steady state during a fault, equivalent circuits of VSC-based renewable energy of fault transient state and steady state were proposed, respectively. Then the correctness of the theory was verified by experimental tests. In addition, for power grid with VSC-based renewable energy, superposition theorem was used to calculate AC component and DC component of short-circuit current, respectively, then the peak value of short-circuit current was evaluated effectively. The calculated results could be used for grid planning and design, short-circuit current management as well as adjustment of relay protection. Based on comparing calculation and simulation results of 6-node 500 kV Huainan power grid and 35-node 220 kV Huaisu power grid, the effectiveness of the proposed method was verified

Design of Low-Ripple and Fast-Response DC Filters in DC Distribution Networks

The design and parameter selection of low-ripple and fast-response direct current (DC) filters are discussed in this study with the aim of alleviating the influence of a DC-side low-frequency voltage pulsation on a sensitive load in a DC distribution network. A method for determining the DC filter parameters by using a mofatching most flat response algorithm is presented. The voltage transfer function of the DC-side filter in the DC distribution network is deduced to analyze its voltage transfer characteristics. The resonance peak value of the filter network is an important factor affecting the transfer speed of a filter. A pole-circle-based parameter optimization method is proposed to move the poles of the filter transfer function down and to the left of pole plane for finding the appropriate capacitance, inductance, and damping parameters. This approach effectively restricts the resonance peak value, accelerates the transfer speed, and maintains steady filtering results. Simulation and test results verify that the filter has low resonance value, rapid convergence ability, and an excellent filtering effect

Coordinated allocation of soft open point and converter‐based energy storage systems in PV penetrated active distribution networks

ABSTRACT Soft open point‐based energy storage (SOP‐based ES) can realize the real‐time adjustment of transmission power in space and peak load shaving in time, further promoting the integration of distributed generations (DGs) and decreasing the allocation cost. This article proposed an optimal planning model for coordinated allocation of SOP‐based ES and conventional independent ES systems in PV penetrated Active Distribution Networks (ADN). First, the planning models of SOP‐based ES, battery of ES systems and its lifetime are established. Second, the proposed coordinated allocation model aims to minimize the total planning cost, including power exchange cost, investment cost, operation cost, battery degradation cost and loss cost. In order to solve the uncertainty of PV output and load demand, the stochastic optimization model is established with scenarios reduced by the K‐means clustering technique. Third, the linearization model of the battery degradation cost is derived to transform the optimization model into a mixed integer linear programming (MILP) model. Finally, the presented model is implemented on an IEEE 33‐node distribution system to confirm its effectiveness. The case study results show that the coordinated allocation of SOP and converter‐based ES can effectively reduce the allocation cost of ES and promote the integration of DGs

A Novel Electricity Transaction Mode of Microgrids Based on Blockchain and Continuous Double Auction

The installed capacity of distributed generation (DG) based on renewable energy sources has increased continuously in power systems, and its market-oriented transaction is imperative. However, traditional transaction management based on centralized organizations has many disadvantages, such as high operation cost, low transparency, and potential risk of transaction data modification. Therefore, a decentralized electricity transaction mode for microgrids is proposed in this study based on blockchain and continuous double auction (CDA) mechanism. A buyer and seller initially complete the transaction matching in the CDA market. In view of the frequent price fluctuation in the CDA market, an adaptive aggressiveness strategy is used to adjust the quotation timely according to market changes. DG and consumer exchange digital certificate of power and expenditure on the blockchain system and the interests of consumers are then guaranteed by multi-signature when DG cannot generate power due to failure or other reasons. The digital certification of electricity assets is replaced by the sequence number with specific tags in the transaction script, and the size of digital certification can be adjusted according to transaction energy quantity. Finally, the feasibility of market mechanism through specific microgrid case and settlement process is also provided

Capacity Calculation of Shunt Active Power Filters for Electric Vehicle Charging Stations Based on Harmonic Parameter Estimation and Analytical Modeling

The influence of electric vehicle charging stations on power grid harmonics is becoming increasingly significant as their presence continues to grow. This paper studies the operational principles of the charging current in the continuous and discontinuous modes for a three-phase uncontrolled rectification charger with a passive power factor correction link, which is affected by the charging power. A parameter estimation method is proposed for the equivalent circuit of the charger by using the measured characteristic AC (Alternating Current) voltage and current data combined with the charging circuit constraints in the conduction process, and this method is verified using an experimental platform. The sensitivity of the current harmonics to the changes in the parameters is analyzed. An analytical harmonic model of the charging station is created by separating the chargers into groups by type. Then, the harmonic current amplification caused by the shunt active power filter is researched, and the analytical formula for the overload factor is derived to further correct the capacity of the shunt active power filter. Finally, this method is validated through a field test of a charging station

Load-switching strategy for voltage balancing of bipolar DC distribution networks based on optimal automatic commutation algorithm

The unbalanced DC loads between the positive and negative poles of bipolar DC distribution network leads to the increase of unbalanced current (voltage) and power losses. This affects the efficiency of DC power transmission and also influences the power quality of the DC load. In this article, an unbalanced DC load switching strategy based on automatic commutation switch (ACS) and genetic algorithm (GA) is proposed. The proposed method can automatically adjust the power supply polarity (PSP) of the DC load according to the unbalanced currents in the bipolar network. This article also introduces the configuration and topology of ACS in a bipolar DC distribution network. Furthermore, the ACS and its control diagrams are designed. The state of ACS is represented by the switch state vector, which corresponds to the gene sequence in the GA. Consequently, the calculation algorithm of switch commutation command (SCC) based on the vector gene coding strategy is proposed. In accordance with the optimal SSC, an online unbalanced-load switching strategy is designed. Finally, the simulation is built in MATLAB/Simulink to verify the effectiveness of the proposed method.This work was supported in part by the Natural Science Foundation of China under Grant 51877017 and Grant 52077017

Multi-stage Coordinated Robust Optimization for Soft Open Point Allocation in Active Distribution Networks with PV

To optimize the placement of soft open points (SOPs) in active distribution networks (ADNs), many aspects should be considered, including the adjustment of transmission power, integration of distributed generations (DGs), coordination with conventional control methods, and maintenance of economic costs. To address this multi-objective planning problem, this study proposes a multi-stage coordinated robust optimization model for the SOP allocation in ADNs with photovoltaic (PV). First, two robust technical indices based on a robustness index are proposed to evaluate the operation conditions and robust optimality of the solutions. Second, the proposed coordinated allocation model aims to optimize the total cost, robust voltage offset index, robust utilization index, and voltage collapse proximity index. Third, the optimization methods of the multi-and single-objective models are coordinated to solve the proposed multi-stage problem. Finally, the proposed model is implemented on an IEEE 33-node distribution system to verify its effectiveness. Numerical results show that the proposed index can better reveal voltage offset conditions as well as the SOP utilization, and the proposed model outperforms conventional ones in terms of robustness of placement plans and total cost