The Hybrid AC-DC Power System Coordinated Control and Operation Technology

A special issue of Electronics (ISSN 2079-9292). This special issue belongs to the section "Industrial Electronics".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 9974

Special Issue Editors

College of Electrical Engineering, Sichuan University, Chengdu 610065, China
Interests: power system stability and control; HVDC technology; DC grids; FACTS
Special Issues, Collections and Topics in MDPI journals
College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China
Interests: HVDC technology; FACTS; renewable power integration
Special Issues, Collections and Topics in MDPI journals
College of Electrical Engineering, Sichuan University, Chengdu 610065, China
Interests: AC–DC hybrid power systems; HVDC; renewable power integration
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

High-voltage direct current (HVDC) technology plays an increasingly important role in modern power systems. With the number of DC converters integrated into traditional power systems, the stability mechanism of AC–DC hybrid systems becomes complex, and their control strategies face coordination problems.

Specifically, the commutation failures of line-commutated converter (LCC)-based HVDC make power interruptions frequent. Additionally, the oscillations risks brought by the voltage-sourced-converter (VSC) deteriorate the impedance characteristic of the nearby grid, and renewable power grid integration leads to power fluctuations. To solve these new problems, some effective coordination controls are needed, and novel operation strategies should also be proposed.

Based on the challenges faced in modern power systems, we encourage contributions addressing hybrid AC–DC power system coordinated control and operation technology in the broadest sense, including but not limited to the stability and control analysis for LCC-HVDC, stability and control analysis for VSC-HVDC, strategies for coordination between DC converters and AC equipment, coordination strategies between FACTS and AC–DC systems, novel control schemes for renewable power, and protection methods for DC grids and hybrid AC–DC systems.

Dr. Baohong Li
Dr. Zheren Zhang
Dr. Qin Jiang
Guest Editors

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Keywords

  • LCC-HVDC
  • VSC-HVDC
  • FACTS
  • power grid stability and control

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Published Papers (14 papers)

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Research

23 pages, 3948 KiB  
Article
A P-Q Coordination Control Strategy of VSC-HVDC and BESS for LVRT Recovery Performance Enhancement
by Zhen Wang, Jialiang Wu, Ruixu Liu and Yu Shan
Electronics 2024, 13(4), 741; https://doi.org/10.3390/electronics13040741 - 12 Feb 2024
Cited by 1 | Viewed by 395
Abstract
Voltage source converter (VSC)-based multi-terminal direct current (MTDC) transmission technology has been a research focus, and the low-voltage ride-through (LVRT) and recovery in receiving-end systems is one of the major problems to consider. A coordinated control strategy for a VSC-MTDC system is proposed [...] Read more.
Voltage source converter (VSC)-based multi-terminal direct current (MTDC) transmission technology has been a research focus, and the low-voltage ride-through (LVRT) and recovery in receiving-end systems is one of the major problems to consider. A coordinated control strategy for a VSC-MTDC system is proposed to improve the frequency and voltage dynamics in the receiving-end system during the LVRT and recovery processes. A battery energy storage system (BESS) plays a significant role in providing frequency and voltage support with its flexible power control capability. During the LVRT process, the BESS can provide reactive current injection and active current absorption to improve system stability in the AC side, and during the recovery process, an adaptive current limitation method is proposed for the BESS converter to dynamically adjust the active and reactive power outputs according to the frequency and voltage deviation severity. Meanwhile, the coordination of the sending-end systems and DC chopper can reduce the power output to avoid DC overvoltage during LVRT, and it can also provide frequency support to the receiving-end system with the DC voltage transmitting frequency information during the recovery process. A simulation was carried out on the MATLAB/Simulink platform, and a three-terminal VSC-MTDC system was used to validate the effectiveness of the proposed strategy. Full article
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16 pages, 372 KiB  
Article
Robust Wind Power Ramp Control Strategy Considering Wind Power Uncertainty
by Bixing Ren, Yongyong Jia, Qiang Li, Dajiang Wang, Weijia Tang and Sen Zhang
Electronics 2024, 13(1), 211; https://doi.org/10.3390/electronics13010211 - 03 Jan 2024
Cited by 1 | Viewed by 658
Abstract
Recent climate change has worsened the risk of extreme weather events, among which extreme offshore wind storms threaten secure operation by inducing offshore wind power ramps. Offshore wind power ramps cause the instantaneous power fluctuation of interconnected onshore grids and may lead to [...] Read more.
Recent climate change has worsened the risk of extreme weather events, among which extreme offshore wind storms threaten secure operation by inducing offshore wind power ramps. Offshore wind power ramps cause the instantaneous power fluctuation of interconnected onshore grids and may lead to unexpected load shedding or generator tripping. In this paper, considering offshore wind power uncertainties, we propose a novel robust coordinated offshore wind power ramp control strategy by dispatching thermal units, energy storage systems, and hydrogen storage systems cooperatively. First, the impact of extreme wind storms on an offshore wind farm output power ramp is analyzed, and the general framework of offshore wind power ramp control is presented based on the two-stage robust optimization considering the dual uncertainties of load demand and wind power. Second, a coordinated wind power ramp control model is established considering the operational characteristics of different ramp control sources such as thermal units, energy storage systems, and offshore wind farms. Third, a robust ramp control strategy is developed using the column-and-constraint generation (CC&G) algorithm. Simulation results show the effectiveness of the proposed ramp control strategy. Full article
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18 pages, 5325 KiB  
Article
Influence Analysis and Control Method of Ultra-Low Frequency Oscillation in a Hydro-Dominant Sending Power System with Wind Power Integration
by Gang Chen, Xueyang Zeng, Huabo Shi, Biao Wang, Gan Li, Qin Jiang, Yongfei Wang and Baohong Li
Electronics 2024, 13(1), 31; https://doi.org/10.3390/electronics13010031 - 20 Dec 2023
Viewed by 505
Abstract
To make clear the influence of renewable energy on ultra-low-frequency oscillation (ULFO) in a hydro-dominant system via an high voltage direct current (HVDC) transmission system, this paper studies the damping characteristics when wind power is integrated into the sending power system. The damping [...] Read more.
To make clear the influence of renewable energy on ultra-low-frequency oscillation (ULFO) in a hydro-dominant system via an high voltage direct current (HVDC) transmission system, this paper studies the damping characteristics when wind power is integrated into the sending power system. The damping torque method is applied for the mechanism study and risk evaluation of ULFO. The study of the inner cause and outer performance proves that ULFO belongs to frequency oscillation rather than conventional power oscillation. According to the theoretical analysis, the suggested control mode is identified to reduce the risk of ULFO. Moreover, a robust controller is designed for wind units based on mixed H2/H robust control theory, and the control method makes use of the fast response ability of the converter without compromising the primary frequency regulation ability. Finally, a simulation model of a hydro-dominant sending power system with wind integration is established in PSCAD Version 4.6 software. The simulations verify the proposed control can suppress ULFO effectively. Full article
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17 pages, 6840 KiB  
Article
A Multi-Terminal Control Method for AC Grids Based on a Hybrid High-Voltage Direct Current with Cascaded MMC Converters
by Lei Liu, Xiaopeng Li, Qin Jiang, Yufei Teng, Mingju Chen, Yongfei Wang, Xueyang Zeng, Yiping Luo and Pengyu Pan
Electronics 2023, 12(23), 4799; https://doi.org/10.3390/electronics12234799 - 27 Nov 2023
Cited by 1 | Viewed by 565
Abstract
The hybrid high-voltage direct current (HVDC) transmission system with cascaded MMC converters has become a promising alternative for possessing the technical merits of both line-commuted converter (LCC) and voltage source converter (VSC), resulting in favorable characteristics and potential control of good prospect. This [...] Read more.
The hybrid high-voltage direct current (HVDC) transmission system with cascaded MMC converters has become a promising alternative for possessing the technical merits of both line-commuted converter (LCC) and voltage source converter (VSC), resulting in favorable characteristics and potential control of good prospect. This paper pays heightened attention to the feasible power and DC voltage control modes of a hybrid HVDC system; characteristics of master–slave control show higher flexibility than the LCC-VSC HVDC system, which demonstrates that the exceptional potential can serve to stability support the AC power grids. To optimize the control effect, besides damping level to attenuate power oscillations, the robustness suitable for various faults is also considered to obtain a multi-objective control problem. A detailed solution is proceeding using the TLS-ESPRIT identification algorithm and H2/H hybrid robust control theory. This motivates multi-terminal controllers in the LCC rectifier and MMC inverters, which immensely improve the stability of both sending and receiving girds at the same time. According to the parameters of the actual hybrid HVDC project, the simulation model is established in PSCAD v4.6.2 software, and proposed control methods have been verified to satisfy damping objectives and perform well in multiple operating scenarios. Full article
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17 pages, 3439 KiB  
Article
Two-Layer Cooperative Optimization of Flexible Interconnected Distribution Networks Considering Electric Vehicle User Satisfaction Degree
by Dai Wan, Wenhui Mo, Junhao Li, Chunzhi Yang, Jinbo Wu, Qianfan Zhou and Yusheng Gong
Electronics 2023, 12(22), 4582; https://doi.org/10.3390/electronics12224582 - 09 Nov 2023
Viewed by 615
Abstract
The scaled access of electric vehicles (EVs) exacerbates load fluctuations in distribution networks, which is not conducive to the stable and economic operation of the distribution networks. At present, user satisfaction degree is generally low. To avoid this problem, this paper proposed a [...] Read more.
The scaled access of electric vehicles (EVs) exacerbates load fluctuations in distribution networks, which is not conducive to the stable and economic operation of the distribution networks. At present, user satisfaction degree is generally low. To avoid this problem, this paper proposed a two-layer cooperative optimization of flexible interconnected distribution networks considering EV user satisfaction degree. First, the EV user satisfaction degree model is established by considering EV users’ charging waiting time, charging power, and other indicators. At the same time, an EV charging mode switching model is constructed based on the number of EVs entering the network and their battery charge state. On this basis, the Monte Carlo algorithm is used to generate the results of the daily distribution of EV loads taking into account the user satisfaction degree, so as to improve the load ratio of the transformer in the distribution network. Further, a two-layer cooperative optimization of flexible interconnected distribution networks considering electric vehicle user satisfaction degree is developed with the daily operating cost of each network as the optimization objective. Finally, a flexible interconnected power distribution network consisting of three power distribution networks is used as an example for validation. The results show that this method is effective in improving EV user satisfaction degree and reducing the peak-to-valley ratio of the system load while taking into account the safe and economic operation of the distribution network, which greatly improves the reliability and economy of the operation of the flexible interconnected power distribution network. Full article
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18 pages, 1697 KiB  
Article
An Integrated Scheme for Forecasting and Controlling Ramps in Offshore Wind Farms Considering Wind Power Uncertainties during Extreme Storms
by Yongyong Jia, Bixing Ren, Qiang Li, Chenggen Wang, Dajiang Wang and Xiaoming Zou
Electronics 2023, 12(21), 4443; https://doi.org/10.3390/electronics12214443 - 29 Oct 2023
Viewed by 559
Abstract
Global warming-induced extreme tropical storms disrupt the operation of offshore wind farms, causing wind power ramp events and threatening the safety of the interconnected onshore grid. In order to attenuate the impact of these ramps, this paper proposes an integrated strategy for forecasting [...] Read more.
Global warming-induced extreme tropical storms disrupt the operation of offshore wind farms, causing wind power ramp events and threatening the safety of the interconnected onshore grid. In order to attenuate the impact of these ramps, this paper proposes an integrated strategy for forecasting and controlling ramps in offshore wind farms. First, the characteristics of wind power ramps during tropical storms are studied, and a general ramp control framework is established. Second, a wind power ramp prediction scheme is designed based on a minimal gated memory network (MGMN). Third, by taking into account the wind power ramp prediction results and wind power uncertainties, a chance-constraint programming-based optimal ramp control scheme is developed to simultaneously maximize wind power absorption and minimize ramp control costs. Finally, we use real-world offshore wind farm data to validate the effectiveness of the proposed strategy. Full article
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16 pages, 1851 KiB  
Article
Research on Reactive Power Coordination Control Strategies of Multi-Infeed Line-Commutated Converter–High-Voltage Direct Current Systems
by Yueyang Yin, Ying Huang, Zheren Zhang, Guoteng Wang, Yijing Zhang and Jianhua Li
Electronics 2023, 12(20), 4262; https://doi.org/10.3390/electronics12204262 - 15 Oct 2023
Viewed by 775
Abstract
For a receiving-end power grid with multi-infeed LCC-HVDC systems, simultaneous commutation failures may seriously threaten the safe and stable operation of the system. To evaluate the impact of commutation failure and improve the voltage stability of the commutation buses in multi-infeed HVDC systems, [...] Read more.
For a receiving-end power grid with multi-infeed LCC-HVDC systems, simultaneous commutation failures may seriously threaten the safe and stable operation of the system. To evaluate the impact of commutation failure and improve the voltage stability of the commutation buses in multi-infeed HVDC systems, this paper proposes a method for evaluating the voltage stability of commutation buses and a reactive power coordination control (RPCC) method for commutation failure of multiple HVDC systems. Firstly, three indicators and the entropy weight method are adopted to comprehensively evaluate the voltage stability of commutation buses. Then, an RPCC method is proposed to resist commutation failure. The proposed RPCC method uses the voltage interaction factor (VIF) to screen out DC systems that are strongly related to dynamic reactive power compensation devices and activates the devices to provide RPCC to the DC systems through an auxiliary controller. Finally, the effectiveness of the proposed method is verified through a practical example of the Jiangsu power grid. Full article
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20 pages, 11052 KiB  
Article
A High-Power Density DC Converter for Medium-Voltage DC Distribution Networks
by Dai Wan, Qianfan Zhou, Xujin Duan, Jiran Zhu, Junhao Li and Hengyi Zhou
Electronics 2023, 12(18), 3975; https://doi.org/10.3390/electronics12183975 - 21 Sep 2023
Cited by 2 | Viewed by 797
Abstract
A DC converter is the core equipment of voltage conversion and power distribution in a DC distribution network. Its operating characteristics have a profound impact on the flexible regulation of distributed resources in an active distribution network. It is challenging for the existing [...] Read more.
A DC converter is the core equipment of voltage conversion and power distribution in a DC distribution network. Its operating characteristics have a profound impact on the flexible regulation of distributed resources in an active distribution network. It is challenging for the existing single-stage conversion topology to meet the requirements of distributed renewable energy connected to a multi-voltage level, medium-voltage grid. It is necessary to study the multistage transform power unit topology further, which can satisfy high reliability, high efficiency, and wide input range. This paper proposes a high-power density DC converter for medium-voltage DC networks with wide voltage levels. It adopts Buck-LLC integrated modular composition. The input ends of the high isolation resonant power unit are connected in series to provide high voltage endurance, and the output ends are connected in parallel to meet the high-power demand and achieve high-power transmission efficiency. The proposed series dual Buck-LLC resonant power unit topology can adjust the duty cycle of series dual buck circuits to meet the needs of different levels of medium-voltage DC power grids. The soft switching problem within the wide input range of all switching tubes is solved by introducing auxiliary inductors, thereby improving energy transmission efficiency. The auxiliary circuit and control parameters are optimized based on the research of each switching tube’s soft switching boundary conditions. Finally, an experimental prototype of a 6.25~7 kW power unit is designed and developed to prove the proposed topology’s feasibility and effectiveness. Great breakthroughs have been made both in theoretical research and engineering prototype development. Full article
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14 pages, 4256 KiB  
Article
Power Equalization Control Strategy for MMCs in Hybrid-Cascaded UHVDC System
by Lei Liu, Yufei Teng, Xiaopeng Li, Yong Tang and Xiaofeng Jiang
Electronics 2023, 12(16), 3532; https://doi.org/10.3390/electronics12163532 - 21 Aug 2023
Viewed by 581
Abstract
Based on the hybrid-cascaded topology of ultra-high-voltage direct current (UHVDC) engineering, this study clarified the mechanism of unbalanced power generation among modular multilevel converters (MMCs) at the inverter side following the fault of the AC system at the rectifying side, and then proposed [...] Read more.
Based on the hybrid-cascaded topology of ultra-high-voltage direct current (UHVDC) engineering, this study clarified the mechanism of unbalanced power generation among modular multilevel converters (MMCs) at the inverter side following the fault of the AC system at the rectifying side, and then proposed the power equalization strategy for MMCs. By performing closed-loop control on the active power deviation between constant-voltage and constant-power MMCs, it was possible to achieve automatic power equalization among MMCs after the occurrence of a fault so as to avoid the detrimental effect of a single MMC’s power fluctuation on the connected AC system. Meanwhile, the control enabling logic was designed to ensure the reliable input and stable exit of the control strategy throughout the disturbance period. Finally, a PSCAD/EMTDC platform was used to simulate various types of faults in the AC system at the rectifier side in order to validate the effectiveness of the proposed power equalization strategy. Full article
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20 pages, 6089 KiB  
Article
The Frequency Control Strategy of a Wind–Storage Combined System Considering Battery SOC
by Xi Wang, Xi Ye, Wei Wei, Yanfeng Wang, Tong Zhu, Chang Liu and Yufan Chen
Electronics 2023, 12(16), 3453; https://doi.org/10.3390/electronics12163453 - 15 Aug 2023
Viewed by 648
Abstract
The wind power capacity has increased a lot recently and the number of close energy storage systems has also rapidly increased. To enhance the frequency stability support ability of such wind–storage combined systems, this paper proposes a virtual synchronous control strategy for a [...] Read more.
The wind power capacity has increased a lot recently and the number of close energy storage systems has also rapidly increased. To enhance the frequency stability support ability of such wind–storage combined systems, this paper proposes a virtual synchronous control strategy for a wind–storage combined system considering the battery state of charge (SOC). The virtual synchronous control is used to make the wind turbine generate more active power when the system is in disturbance. Most importantly, to ensure that the stored energy is used efficiently, the wide-area SOC concept of the energy storage system is considered, which is realized by designing the batteries with additional control that is related to the other batteries’ SOCs. This means that the low-SOC energy storage system’s power shortage can be compensated for by the high-SOC batteries. The above unified control enhances the wind–storage combined system’s frequency supporting ability. According to the simulations, the frequency drop can be suppressed through the help of the virtual synchronous control and the stored energy. Additionally, the instability can also be eliminated when wide-area SOC is considered, such that the proposed efficiency and correctness are finally verified through the simulations. Full article
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18 pages, 3260 KiB  
Article
Coordinated Voltage-Power Control for DC Distribution Networks Based on an Uncertainty and Disturbance Estimator
by Li Lin, Huidan Tan, Xianyu Kong, Yapei Cao, Hao Luo, Yulu Lin and Zhijin Zhang
Electronics 2023, 12(14), 3137; https://doi.org/10.3390/electronics12143137 - 19 Jul 2023
Viewed by 637
Abstract
DC distribution networks are low-inertia systems with a range of uncertainties and disturbances. The traditional droop control widely used in DC distribution networks has a contradiction between the accurate power sharing of power units and voltage deviation due to the presence of line [...] Read more.
DC distribution networks are low-inertia systems with a range of uncertainties and disturbances. The traditional droop control widely used in DC distribution networks has a contradiction between the accurate power sharing of power units and voltage deviation due to the presence of line impedance. To overcome this contradiction and enhance the immunity and tracking performance of voltage-source converter (VSC) current inner loop PI control, this paper proposes a coordinated voltage-power control strategy based on an uncertainty and disturbance estimator (UDE) for DC distribution networks. Firstly, the improved droop control strategy based on the UDE is proposed, which not only avoids the influence of line impedance on the load current sharing, but also achieves voltage stabilization at the set value. Secondly, an improved VSC current inner loop controller based on the UDE is designed to improve the VSC’s tracking performance for the droop output reference value. The UDE control theory is applied to estimate and compensate for the uncertainties and disturbances of the VSC current inner loop control, in order to improve the tracking and immunity of the VSC current inner loop and enhance the DC voltage robustness. Finally, a three-terminal DC distribution network is taken as an example to verify the effectiveness of the proposed strategy. Full article
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23 pages, 2689 KiB  
Article
Frequency Support Control of Multi-Terminal Direct Current System Integrated Offshore Wind Farms Considering Direct Current Side Stability
by Huachun Han, Qun Li and Qiang Li
Electronics 2023, 12(14), 3029; https://doi.org/10.3390/electronics12143029 - 10 Jul 2023
Cited by 1 | Viewed by 700
Abstract
The frequency stability of modern power systems is challenged due to widespread application of large-scale renewable energy systems, of which the offshore wind farm (OWF) is one of the dominant resources. The OWFs are usually integrated into the grid by multi-terminal direct current [...] Read more.
The frequency stability of modern power systems is challenged due to widespread application of large-scale renewable energy systems, of which the offshore wind farm (OWF) is one of the dominant resources. The OWFs are usually integrated into the grid by multi-terminal direct current (MTDC) transmission systems, which makes the energy flow complicated and the frequency control design challenging. A frequency support control method of MTDC system integrated OWFs (referred to as the OWF-MTDC system) is proposed in this paper. First, the wind turbine generation system (WTGS) is controlled to reserve a certain amount of available power according to the real-time wind speed for more comprehensive frequency regulation. Then, the frequency support control of OWFs is designed, and they can release the rotor kinetic energy and reserved power to support the onshore grid frequency. In addition, the virtual inertia control of a modular multi-level converter (MMC) is designed, which can also provide frequency support in an emergency by use of the DC capacitor. To ensure that the frequency control of the OWF-MTDC system does not degrade the stability of the system, a detailed DC impedance model of the MMC-based MTDC systems is developed, considering the constant power control and DC voltage control. Based on the impedance model, the impact of the frequency control coefficients on the DC side stability of the MTDC system is analyzed. Simulation results validate the stability analysis and verify the proposed frequency control method, which can effectively provide frequency support to the onshore power grid. Full article
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13 pages, 4413 KiB  
Article
Adaptive Droop Control of VSC-MTDC System Based on Virtual Inertia
by Congshan Li, Xiaowei Zhang, Ping He, Zikai Zhen and Kefeng Zhao
Electronics 2023, 12(10), 2324; https://doi.org/10.3390/electronics12102324 - 21 May 2023
Cited by 1 | Viewed by 932
Abstract
In order to solve the problem that the voltage source converter based multi-terminal direct current (VSC-MTDC) system cannot provide inertia and participate in frequency modulation after connecting to the AC power grid under the traditional control strategy, an adaptive control strategy based on [...] Read more.
In order to solve the problem that the voltage source converter based multi-terminal direct current (VSC-MTDC) system cannot provide inertia and participate in frequency modulation after connecting to the AC power grid under the traditional control strategy, an adaptive control strategy based on virtual inertia is proposed. First, the relationship between AC frequency and DC voltage was established by a virtual inertia control, allowing the VSC-MTDC system to provide inertia to the AC side. Second, to address the limited inertia coefficient selection due to DC voltage deviation, an adaptive control was adopted. When the DC voltage deviation is small, the inertia coefficient is increased to obtain a better inertial response; on the contrary, the inertia coefficient is reduced to prevent the DC voltage from exceeding the limit. Finally, to solve the problem of insufficient flexibility of the fixed droop coefficient, this paper introduces the power margin of a VSC-station into the droop coefficient to dynamically adjust the distribution ratio of unbalanced power and reduce the DC voltage deviation. The three-terminal VSC-MTDC system was modelled on the PSCAD/EMTDC simulation platform, and the superiority of the control strategy was highlighted in this paper by comparing it with conventional droop control and a fixed virtual inertia coefficient. Full article
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15 pages, 3134 KiB  
Article
The Online Frequency Security Assessment of a Power System Considering the Time-Varying Characteristics of Renewable Energy Inertia
by Zefeng Peng, Yulin Lu, Yingmin Zhang, Wenjun Deng and Qi Zeng
Electronics 2023, 12(10), 2205; https://doi.org/10.3390/electronics12102205 - 12 May 2023
Cited by 4 | Viewed by 744
Abstract
The continuous increase in the penetration rate of renewable energy has led to a decrease in the system’s frequency response capability, which presents great challenges to the safety and stability of the power system. In order to ensure the safe operation of the [...] Read more.
The continuous increase in the penetration rate of renewable energy has led to a decrease in the system’s frequency response capability, which presents great challenges to the safety and stability of the power system. In order to ensure the safe operation of the power system, online frequency safety assessment has become necessary. However, the time-varying characteristics of the virtual inertia HNE of renewable energy stations make it more difficult to accurately predict the lowest point of the system frequency after a disturbance. Based on the general average system frequency (G-ASF) model, this paper proposes a G-ASF-H model that considers the time-varying characteristics of the virtual inertia of renewable energy stations, accurately predicts the lowest frequency point after a system disturbance, and realizes the online frequency safety assessment of the system. Firstly, a unified virtual synchronous generator model is established to identify the virtual inertia time constant of the renewable energy station in real time; then, under the pre-defined frequency safety verification event, the maximum deviation of the system frequency is periodically calculated and judged based on the G model to realize the online frequency safety assessment. The example analysis on the IEEE 10-machine 39-node system shows that the model has a high calculation speed and accuracy under different disturbances or daily load level scenarios and can be used for the online security assessment of new power systems with time-varying virtual inertia characteristics. Full article
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