Abstract:With the increasing penetration rate of renewable energy generation, the stable operation of grid-connected converters with grid-following control as the mainstream has been severely challenged. Therefore, the control technologies of grid-forming converters with active support capability for the power grid and strong stability in the weak power grid have received much attention. However, under the conditions of high penetration rate of renewable energy generation, especially in the terminal weak grid, it is difficult for the grid-connected converters to balance stability, economy, and power grid support performance by using a single grid-following or grid-forming mode control. Based on the complementary characteristics of the two modes, scholars at home and abroad have proposed and studied the grid-following/grid-forming hybrid mode control strategy, to make the grid-connected converters operate stably in the weak power grid, maximize the use of renewable energy, and achieve superior power grid support performance. Starting from the basic control structures of grid-following and grid-forming converters, this paper analyzes the complementary characteristics of the control performance of grid-following and grid-forming converters. On this basis, the hybrid mode control schemes of several different technical routes are sorted out, involving a variety of single-machine hybrid control strategies and multi-machine station-level hybrid control strategies. The technical research idea, principle, advantages, and disadvantages of each scheme are elaborated in detail. Finally, the development of grid-following/grid-forming hybrid mode control technologies is prospected.

Abstract:To achieve economy and efficiency in the power collection of the 10 GW-level offshore wind power base from multiple low-frequency AC transmission channels and seamless connection with onshore DC transmission lines, this paper proposes a concept of the large-scale offshore wind power hub and its implementation method. Each offshore low-frequency AC transmission channel is connected to the corresponding interface unit of the power hub after reaching the shore. The core of the interface unit is a simple flexible diode rectifier, which converts the low-frequency AC input of the corresponding channel into DC and integrates it into the DC bus. The flexible diode rectifier interface can provide flexibility and controllability, while maintaining low cost and compactness, allowing each input interface unit to achieve independent black start and power control. The power from all input channels is collected on the DC bus of the hub which can be directly connected to subsequent onshore DC lines for transmission to the load center. This paper proposes and analyzes the topology, basic design principles, and control strategies of the offshore wind power hub. Compared with existing offshore low-frequency AC wind power transmission schemes, the proposed power hub scheme has better economy, reliability, and compactness. The effectiveness of the proposed offshore wind power hub is verified by simulation results.

Abstract:Among numerous offshore wind power transmission schemes, the scheme based on parallel connection of diode rectifier unit (DRU) and modular multilevel converter (MMC) has significant economic benefits, which has gained widespread attention. Compared to flexible DC transmission scheme, the DRU-MMC parallel transmission scheme has smaller volume, lighter weight, and less cost of the offshore converter platform. In this scheme, the DRU transmits most of the power, while the small-capacity MMC provides the power required for the wind turbine start-up, supports voltage and frequency and compensates for harmonic currents introduced by the DRU. An active power distribution strategy and a harmonic compensation strategy are proposed for the DRU-MMC parallel transmission topology, so that the power is reasonably distributed between DRU and MMC. Under the premise of realizing main functions of the MMC, the selection of the minimum capacity of MMC is analyzed. From two aspects of investment and operation, the cost of the offshore converter platform of MMC and DRU-MMC is compared. Finally, simulations are carried out on PSCAD/EMTDC platform to verify the effectiveness of the proposed strategy.

Abstract:Flexible low-frequency transmission has attracted more and more attention due to its superior transmission economy in the transmission scenario of medium- and far-offshore wind power. The existing wind power system with low-frequency transmission adopts constant-voltage and constant-frequency AC current to transmit electric power, which is a simple combination of low-frequency transmission and variable-speed constant-frequency wind power technology. This combination is easy to implement in engineering, but the key factor of the freedom degree of voltage conversion has not been fully considered. In this paper, a variable-voltage operation control method for the wind power system with flexible low-frequency transmission is proposed, which dynamically adjusts the transmission voltage to meet the power generation requirements with different wind speeds. In this operation mode, a matching dynamic minimum DC bus voltage control method for wind turbines is proposed, and a wide-range DC bus voltage frequency support technology is formed for units without DC/DC converters. Furthermore, the influence of variable-voltage operation mode on transient overmodulation problems and power loss is analyzed, and the reason why overmodulation will not occur and the reasons for the decreasing total loss are demonstrated. Finally, the feasibility and the superiority of the proposed control method in terms of system loss, fault transient current and grid frequency support are verified by simulation.

Abstract:The flexible low-frequency transmission system based on modular multilevel matrix converter (M3C) has outstanding advantages in medium- and far-offshore wind power transmission, and asynchronous grids interconnection. However, different from such power electronic converters as modular multilevel converter (MMC), the M3C control strategy may result in significant changes in system fault characteristics. The existing fault equivalence and response characteristic calculation method is not applicable, in which the negative-sequence component of power electronic equipment is equivalent to an open circuit. In this paper, the mapping relationship between the input/output power balance, the bridge-arm power balance (necessary condition for the capacitor voltage balance), and the positive- and negative-sequence voltage/current of M3C after system-side faults, is deduced considering the bridge-arm capacitor voltage balance strategy hierarchically composed of M3C positive-sequence capacitor voltage total average control, negative-sequence current injection control, and circulating current control. Thus, the equivalent models of M3C output voltage/current response characteristics after system-side faults (asymmetrical/symmetrical) are built. On this basis, the accurate calculation method for the voltage/current response characteristics after the system-side faults under the M3C integration is proposed. Finally, based on the PSCAD/EMTDC simulation platform, a simulation model of the offshore wind power flexible low-frequency transmission system is built, and the correctness and accuracy of the proposed fault characteristic analysis method are fully verified through a large number of simulation examples.

Abstract:Offshore wind power to hydrogen (OWPtH) is a vital development trend to improve the economics and the wind power penetration rate of offshore wind farms. Distributed OWPtH is a preferred solution to the hydrogen production in the deep and far sea without expensive high-voltage submarine cables and offshore substations. However, distributed platform hydrogen production capacity differs in different micro-siting schemes due to the wake effect, which further affects the capacity optimal configuration of electrolyzers. Hence, to minimize the levelized cost of hydrogen (LCOH) of distributed OWPtH, a capacity optimal configuration method considering micro-siting is proposed. Firstly, the coupling characteristics between the platforms based on the Larsen wake effect model, and the operation characteristics of wind turbines and electrolyzers in the single platform are modeled and embedded in the proposed capacity optimal configuration model. Secondly, considering the micro-siting of the platforms, the capacity optimal configuration model is decomposed and solved iteratively based on the ideas of decomposition-coordination. Finally, the case studies based on the actual data of deep and far sea in Jiangsu Province of China show that the LCOH of the proposed scheme is significantly reduced compared to the existing scheme. The effectiveness of the proposed OWPtH capacity optimal configuration method is verified.

Abstract:With the proposal of China’s “carbon emission peak and carbon neutrality” strategy, power generation companies not only need to participate in the electricity energy and coal market trading, but also need to participate in the carbon market trading according to the actual carbon emission level. However, due to the characteristics of different trading rules, asynchronous trading timing and multiple trading random elements among electricity-carbon-coal multi-markets, the traditional trading strategies of power generation companies facing a single market are difficult to apply, and a multi-market collaborative trading strategy that takes into account multiple randomness is urgently needed. Considering the uncertainties of multi-market price risks and carbon market verification, the paper proposes a medium- and long-term rolling trading decision model of electricity-carbon-coal for power generation companies. First, the model determines the annual contract electricity volume and coal purchase volume based on pre-forecasted prices for the upcoming year. Then, according to the latest monthly price forecast, the monthly contract electricity volume, monthly coal purchase volume, and carbon quota trading volume in the remaining months of the year are optimized on a rolling basis. Finally, the coal fired power generation companies as the research object for case analysis, are divided into two basic scenarios of carbon buyer and carbon seller for comparative study. The results show that the proposed model can provide strategic guidance for power generation companies to trade in the electricity-carbon-coal multi-market. In the scenario of power generation companies as carbon buyers and carbon sellers, the total profits of enterprises can be increased by about 7.6% and 6.4%, respectively.

Abstract:Current research on 5G base stations as flexibility resources to participate in power grid interaction mostly focuses on the utilization of energy storage resources of 5G base stations, ignoring the regulatory potential of 5G base station communication load. Therefore, a day-ahead interactive operation method for distribution network operator (DNO) and mobile network operator (MNO) based on inter-station migration of communication load is proposed. Firstly, a DNO-MNO dual-layer interactive operation optimization model is constructed. In this model, DNO is an incentive strategy that comprehensively considers the system network loss cost and the incentive cost of the 5G base station, and takes the N-1 safe operation criterion of the system as a constraint and the lowest total system operating cost as an objective. MNO comprehensively considers the power consumption cost of the 5G base station and the grid demand response benefits, and develops the 5G base station power operation plan with the objective of the lowest net operating cost of the 5G base station. Secondly, to solve the problem that the lower-level MNO 5G base station scheduling model has non-convex and nonlinear constraints which are difficult to solve analytically, an algorithm based on feasible domain iteration is proposed to solve the model accurately. Thirdly, the algorithm of DNO-embedded MNO dual-layer interactive model based on the elitist selection genetic algorithm is proposed. Finally, a numerical case is given to verify the effectiveness and practicability of the proposed method.

Abstract:The energy coordination and sharing in multi-microgrid distribution (MMGD) systems are of significant importance for enhancing the economic efficiency of regional grids. Because a single game scheduling strategy is difficult to take into account the interests of various stakeholders, an energy coordinated optimization method based on two-stage game is proposed, which fully considers the realistic scenario of both competition and coordination among stakeholders. In the first stage, a non-cooperative electricity price game model between the distribution networks and surplus microgrids is constructed with the objective of maximizing individual profits, where the system selects different operation modes under various strategy combinations. In the second stage, the MMGD peer-to-peer cooperative game model is established, aiming at minimizing the system transmission loss, applying the Lagrange multiplier method and KKT (Karush-Kuhn-Tucker) condition to deal with the complex constraints, and applying the Shapley value method to ensure the cooperative profit distribution. The simulation results show that the proposed method can balance the individual rationality and collective rationality, and significantly improve the system economy.

Abstract:The integration of a large number of distributed photovoltaic power sources brings great challenges to the safe and economic operation of the distribution system. Aiming at the problem of line losses and power quality in the distribution system, a meshed distribution system architecture based on smart distribution transformer (SDT) is proposed. The coordinated control strategy for the meshed distribution system is constructed by synthesizing the power relationship between source-load and integrated power quality management in the distribution system. Finally, a two-bus system model is established in MATLAB/Simulink for simulation verification. The simulation results show that the proposed meshed distribution system architecture reduces the loss of system power and realizes the function of power quality management compared with other traditional architectures.

Abstract:Power aggregation model remains crucial for the analysis and calculation of new distribution networks. Traditional power sources in the power system are generators with clear mechanisms and similar characteristics, making it easier to establish equivalent aggregation models. As a high proportion of renewable energy sources are connected to the distribution network, their time-varying and non-linear characteristics pose a huge challenge to the equivalent modeling of power sources. First, the model structure of the aggregated generator is constructed, and six parameters of the model are identified based on small disturbances of loads without considering the short-term time-varying characteristic. Then, aiming at the time-varying characteristic of renewable energy sources, assuming that the internal voltage of the traditional generator in the model remains unchanged, the Taylor formula is used to expand the power balance equation and solve the four model parameters related to the time-varying characteristic. Euclidean distance for the above four parameters is constructed and the distance is used to determine whether there is time-varying characteristic inside the power source. If so, the Taylor expansion method will continue to be used for solving. If there is no time-varying characteristic inside the power source and the load meets the small disturbance requirements, six model parameters will be identified again, which can timely correct the error accumulation caused by the time-varying characteristic on the generator side. Subsequent extensions can be continued according to this rule. Finally, the feasibility and effectiveness of the proposed method are verified through numerical cases. By using the above rules, an equivalent power source model can be obtained, which takes into account time-varying characteristic and is suitable for continuous analysis and calculation of high proportion of renewable energy connected to distribution networks.

Abstract:Short-term residential load forecasting can provide real-time and flexible power demand information for virtual power plants, which is helpful for virtual power plants to realize efficient utilization of energy and optimize electricity market transactions. With the increasing prominence of correlation among residential loads, traditional forecasting methods, which primarily rely on time-series forecasting based on individual residential historical load, fail to satisfy the comprehensive demands for load interconnectivity in large-scale virtual power plant. Based on this, a short-term residential load forecasting method based on dynamic association graph attention networks for the virtual power plant is proposed. Firstly, a hybrid correlation analysis method is proposed to describe the linear and nonlinear relationship between residential loads, and a weight pruning threshold mechanism is further proposed to derive the hybrid correlation matrix of residential loads. Secondly, a dynamic association graph structure is constructed based on the hybrid correlation matrix, and a temporal graph attention mechanism is proposed to deeply learn the spatial-temporal association characteristics of residential loads, achieving the objective of short-term residential load forecasting. Finally, the effectiveness of the proposed method is verified by actual residential load data from a specific region.

Abstract:In recent years, extreme ice and snow disasters have led to frequent large-scale power outages in distribution networks, seriously endangering the safety of people’s heating and cooling. At present, the thermal-electric coupling in the integrated energy system is close, and how to improve the resilience of the power system through thermal energy when extreme ice and snow disasters occur is still unsolved. In order to improve the ability of integrated energy system to cope with extreme ice and snow disasters and fully exploit the thermal energy characteristics, firstly, the fault model of distribution network line under extreme ice and snow disasters is proposed. Secondly, combined with the transmission delay characteristic of heat network and the thermal inertia of heat load, a model of integrated energy systems is established. Then, based on the two-stage distributed robust optimization model, a strategy for enhancing the resilience of integrated energy systems is proposed, which uses the thermal inertia of buildings to store thermal energy and allocates thermal storage capacity before disaster as well as closes the interconnection switches and optimizes fault repair after disasters. Finally, a case of thermal-electric coupling is set up for simulation verification. The results show that the resilience enhancement strategy can fully tap the thermal energy potential, balance the economy and robustness, and realize the resilience enhancement of the integrated energy system.

Abstract:The characteristics of negative resistance caused by the phase-locked loop of the grid-following converters are prone to resulting in oscillations in a weak power grid. The improved strategies for the phase-locked loop have been widely studied. In particular, the synchronization control strategy based on the voltage matrix has received much attention due to the lack of phase-locked loops. However, there is no rigorous theoretical proof of its stability. The analysis in this paper indicates that the voltage matrix based control strategy without the phase-locked loop is essentially an open-loop orientation approach based on the arctan calculation of the voltages at the point of common coupling (PCC). This strategy removes the phase-locked loop, but cannot improve stability. Since this strategy does not reduce the negative resistance characteristic brought by voltage perturbation on current control, how to reduce the effect of PCC voltage perturbation on current control is the key to improving the stability of grid-following converter in a weak power grid. Therefore, this paper proposes an adaptive band-pass filter to reduce the bandwidth of the synchronization loop, so as to improve the stability of the grid-following converter in a weak power grid. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed strategy are verified by the impedance model and experimental results.

Abstract:The accurate and effective transient stability assessment for power systems is of great significance for the safe and stable operation of power systems. At present, transient stability assessment methods based on deep learning are faced with problems such as difficulty in time-series feature space representation and serious imbalance of sample categories, which affect the reliability of assessment results. In order to make up for the shortcomings of existing studies, a transient stability assessment method for power systems based on the imbalanced sample enhancement of denoising diffusion probabilistic model (DDPM) is proposed. First, an improved HSV colour model is constructed to process the high-dimensional data in two-dimensional image, so as to visually represent the high-dimensional data and facilitate subsequent training. Then, based on DDPM algorithm, the imbalanced unstable sample space is characterized and learned, and the enhanced samples with the same probability distribution are generated on a large scale to solve the category imbalance problem. Finally, a gradient-weighted class activation mapping convolutional neural network is proposed to construct a transient stability assessment model to improve the reliability and interpretability of the model. The simulation results of IEEE 39-bus system test show that compared with other methods, the proposed model has higher stability discrimination accuracy, and the recognition rate of unstable samples is significantly improved, which verify the effectiveness of the proposed method.

Abstract:The grid-connected converter with photovoltaic and energy storage has the characteristics of low inertia, weak damping, weak voltage regulation ability, and frequent power fluctuations. Large-scale integration of grid-connected converters with photovoltaic and energy storage will affect the safe and stable operation of the power system. For this kind of grid-connected converter with energy storage, this paper proposes a current-based control strategy with embedded synchronous generator support characteristics to enhance the inertial support capability of the system. This strategy simulates the dynamic behavior of synchronous generators and controls the external characteristics of the grid-connected converter output to a controlled current source with synchronous generator control characteristics. It combines the advantages of grid-forming control based on the virtual synchronous generator and grid-following control based on the phase-locked loop, which demonstrates the significant comprehensive advantages in inertia support, fault ride-through, power grid harmonic, and islanding operation. The proposed strategy does not require detection and sampling of the output frequency differentiation of the system, which avoids noise interference caused by frequency differentiation detection. In addition, to address the irreconcilable contradiction between the inertia support capability on the AC side and the voltage stability on the DC side, this paper designs a fuzzy adaptive proportional-integral (PI) control strategy to coordinate the voltage stability on the DC side and the inertia support on the AC side. Finally, the hardware-in-the-loop experimental results verify the feasibility and effectiveness of the proposed strategy.

Abstract:The last section of zero-sequence current protection of AC line adopts 300 A, which has the risk of disordered tripping.Therefore, a new principle of high-resistance grounding distance relay based on zero-sequence reactance line and non-fault phase polarization is proposed. The relay adopts the technical route of phase selection before measurement. The phase selection element combines zero-sequence reactance line and non-fault phase polarization method to form a variety of combined criteria to complete the phase selection. Due to the phase difference between the zero-sequence current at the protection installation site and the zero-sequence current at the fault point, the zero-sequence reactance lines of the single-phase grounding fault phase and the leading phase of the inter-phase grounding fault have aliasing region when the fault point is near the setting point. The large variation of the operation voltage of the non-fault phase is not conducive to distinguishing the two types of faults in the aliasing region, and thus the phase selection element is divided into low-resistance module and high-resistance module. The low-resistance module adopts the zero-sequence reactance line with the downward bias, which is used to identify the near-end and low-resistance short circuit. With the assistance of the low-resistance module, the high-resistance module only needs to deal with the faults near the setting point, which reduces the difficulty in distinguishing the two types of faults. After phase selection, the operation voltage before fault is obtained by non-fault phase polarization method, so as to determine the operation characteristics of the relay. The ability of high-resistance distance relay to withstand the transition resistance is far beyond the requirements of the regulations, which improves the selectivity of grounding backup protection to high-resistance faults.

Abstract:The accurate and reliable detection of high-impedance grounding fault (HIGF) is challenging in the fault handling of distribution networks, and the normal capacitor switching operations can cause interference. Addressing this problem, a disturbance-resistant detection method for HIGFs based on zero-sequence Lissajous curve analysis is proposed in this paper. First, the zero-sequence electrical quantities of HIGFs and capacitor switching disturbances are theoretically derived. There is no regular difference from the perspective of traditional time-frequency domain features between the two, thereby clarifying the cause of the interference. Further, the zero-sequence current and voltage waveforms are reconstructed into zero-sequence Lissajous curves. A quantitative index for the distortion complexity of the Lissajous curve trajectory shape based on the mathematical morphology theory is proposed, and an adaptive starting criterion is designed in combination with the probability distribution law of the zero-sequence Lissajous curve area. A disturbance-resistant detection method for HIGFs in the noise scenario is presented. Finally, the effectiveness and reliability of the proposed method are verified through electromagnetic transient simulation cases and real fault tests in the distribution network.