Renewable Energy Transmission Research Articles

Sub-synchronous oscillation suppression strategy for virtual synchronous generators based on dual-loop sliding mode control

The virtual synchronous generator (VSG) has synchronous voltage source characteristics that can effectively improve the stability of high-penetration renewable energy power systems. However, the output impedance of VSG using traditional voltage-current inner-loop control has lower damping in the sub-synchronous frequency range. Therefore, when the VSG is connected to a grid with series compensation for long-distance transmission of renewable energy, in the sub-synchronous frequency range, there is a potential interaction between the inductive output impedance of VSG and the capacitive impedance of the line, leading to the occurrence of sub-synchronous resonance (SSR). To address this challenge, the stability of a series-compensated grid-connected system employing voltage-current inner-loop control was analyzed using VSG sequence impedance modeling. A dual-loop sliding mode control strategy (SMC-DL) was proposed to suppress SSR occurrence. This control strategy can significantly enhance the damping of VSG’s output impedance in the sub-synchronous frequency range, and effectively suppress SSR. Compared to traditional control strategies, this control strategy does not require the addition of virtual impedance, avoids voltage offset issues, adapts to grids with different series compensation levels. Finally, the effectiveness of this approach was verified through simulations and experiments.

Protection Scheme Based on Estimated Traveling Wave Arrival Times for Renewable Energy Transmission Line

Protection Scheme Based on Estimated Traveling Wave Arrival Times for Renewable Energy Transmission Line

Large-Signal Stability Analysis Using Takagi-Sugeno Fuzzy Model Theory for Fractional Frequency Transmission System Under Grid Faults

The rapid development of renewable energy requires transmission technology innovation. Fractional frequency transmission system (FFTS) is a promising solution for large-scale renewable energy transmission, and the stability analysis is essential for engineering application. Especially for the grid faults, these large-signal disturbances are inevitable during operation. Thus, the system-level Takagi-Sugeno fuzzy model of FFTS is proposed considering the system parameters, control strategy of modular multilevel matrix converter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M^{3}C$</tex-math></inline-formula> ) and phase locked loop in this study. Subsequently, the Lyapunov function and the large-signal stability domain of attraction (LS-DOA) are calculated expeditiously based on the proposed fuzzy model. Ultimately, the large-signal stability of FFTS is comprehensively analyzed combining with the different-frequency interaction features under grid fault conditions. Compared with the conventional method, the stability strength and affine angle indexes are proposed for quantitative analysis mathematically. The obtained influence law of the system parameters can guide the parameter optimization of FFTS effectively. The effectiveness of proposed methods is verified by the simulation-based study.

Analysis of renewable energy consumption and economy considering the joint optimal allocation of “renewable energy + energy storage + synchronous condenser”

As renewable energy becomes increasingly dominant in the energy mix, the power system is evolving towards high proportions of renewable energy installations and power electronics-based equipment. This transition introduces significant challenges to the grid’s safe and stable operation. On the one hand, renewable energy generation equipment inherently provides weak voltage support, necessitating improvements in the voltage support capacity at renewable energy grid points. This situation leads to frequent curtailments and power limitations. On the other hand, the output of renewable energy is characterized by its volatility and randomness, resulting in substantial power curtailment. The joint intelligent control and optimization technology of “renewable energy + energy storage + synchronous condenser” can effectively enhance the deliverable capacity limits of renewable energy, boost its utilization rates, and meet the demands for renewable energy transmission and consumption. Initially, the paper discusses the mechanism by which distributed synchronous condensers improve the short-circuit ratio based on the MRSCR (Multiple Renewable Energy Station Short-Circuits Ratio) index. Subsequently, with the minimum total cost of system operation as the optimization objective, a time-series production simulation optimization model is established. A corresponding optimization method, considering the joint configuration of “renewable energy + energy storage + synchronous condenser,” is proposed. Finally, the effectiveness of the proposed method is verified through common calculations using BPA, SCCP, and the production simulation model, considering a real-world example involving large-scale renewable and thermal energy transmission through an AC/DC system. The study reveals that the joint intelligent control and optimization technology can enhance both the sending and absorbing capacities of renewable energy while yielding favorable economic benefits.

(Invited) NiO/ β-(Al x Ga1-x )2O3 /Ga2O3 Heterojunction Lateral Rectifiers with Reverse Breakdown Voltage > 7kV

There is significant recent interest in development of Ga2O3 power devices due to their capability for high temperature operation, reduced on-state and switching losses due to lower on-resistance for high voltage devices, and potentially higher frequency switching capability. These are targeted for renewable energy transmission systems, electric vehicle (EV) traction inverter and motor control systems, fast charging stations and more electric aircraft. Since the efficiency of EV powertrain inverters is partially determined by the efficiency of the switching transistors, it is of interest to examine ultra-wide-bandgap semiconductor electronics. The successful development of these power transistors is expected to significantly increase the longevity of a battery charge and the resultant cost of an EV.In this work, NiO/ β-(Al x Ga1-x )2O3 /Ga2O3 heterojunction lateral geometry rectifiers with diameter 50-100 µm exhibited maximum reverse breakdown voltages >7kV, showing the advantage of increasing the bandgap using the β-(Al x Ga1-x )2O3 alloy. This Si-doped alloy layer was grown by Metal Organic Chemical Vapor Deposition with an Al composition of ~20%. On state resistances were in the range 50-2180 Ω.cm2, leading to power figures-of-merit up to 0.72 MW.cm-2. The forward turn-on voltage was in the range 2.3-2.5 V, with maximum on/off ratios >700 when switching from 5V forward to reverse biases up to -100V. Transmission line measurements showed the specific contact resistance was 0.12 Ω.cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based device. Figure 1

Space charge dynamics of epoxy/micro-Al2O3 composites under multi-physical fields

The direct current gas insulated transmission line (DC GIL) technique has emerged as a promising solution to achieve carbon neutrality and enable efficient long-distance transmission of renewable energy. However, the stable and long-term operation of DC GIL insulators poses a significant challenge that requires insulating materials capable of overcoming charge accumulation issues against multi-fields, including electric, temperature, and force. Here, we investigate the space charge dynamics and electrical conductivity of epoxy (EP) resin and EP/micro-Al2O3 composites under various stimuli of electrical, thermal, and mechanical stresses, using two modified pulsed electro-acoustic and electrical conductivity measurement systems with mechanical pressure control. It is found that stronger electric fields and higher temperature conditions have a more significant impact on space charge accumulation, while higher mechanical stress results in more shallow traps in EP composites. Furthermore, the bipolar carrier transport modeling and numerical calculations are performed to rationalize the experimental observations and reveal the mechanistic impacts of multi-physical fields on the space charge behavior of EP composites for DC GIL insulator use.

Large-Scale Renewable Energy Transmission by HVDC: Challenges and Proposals

Renewable energy transmission by high-voltage direct current (HVDC) has attracted increasing attention for the development and utilization of large-scale renewable energy under the Carbon Peak and Carbon Neutrality Strategy in China. High-penetration power electronic systems have gradually formed at the sending end of HVDC transmission. The operation of such systems has undergone profound changes compared with traditional power systems dominated by synchronous generators. New stability issues, such as broadband oscillation and transient over-voltage, have emerged, causing tripping accidents in large-scale renewable energy plants. The analysis methods and design principles of traditional power systems are no longer suitable for HPPESs. In this paper, the mechanisms of broadband oscillation and transient over-voltage are revealed, and analytical methods are proposed for HPPESs, including small-signal impedance analysis and electromagnetic transient simulation. Validation of the theoretical research has been accomplished through its application in several practical projects in north, northwest, and northeast region of China. Finally, suggestions for the construction and operation of the future renewable-energy-dominated power system are put forward.

RPC-based frequency control strategy of LCC-HVDC sending end power system with renewable energy

Long-distance transmission of renewable energy via DC is currently a more economical transmission scheme. As the proportion of renewable energy at the generation end continues to rise, the inertia and damping level of the system is reduced, causing serious frequency-related issues. This paper first analyzes the working principle and control method of the LCC-HVDC system, summarizes the frequency control methods of the sending end power grid and their existing problems, and proposes a frequency control scheme for the LCC-HVDC sending end system based on the concept of rapid power compensation (RPC). Finally, the LCC-HVDC system model integrated with the large-capacity wind farm and thermal power plant is built in MATLAB/Simulink. Under conditions of load power fluctuation and sudden wind speed change, the proposed strategy fully utilizes the spare capacity of the system, effectively improves the frequency indexes of the system, and improves the frequency characteristics of the renewable energy sending end grid.

Capacity configuration of a hydro-wind-solar-storage bundling system with transmission constraints of the receiving-end power grid and its techno-economic evaluation

The hydro-wind-solar-storage bundling system plays a critical role in solving spatial and temporal mismatch problems between renewable energy resources and the electric load in China. An efficient bundling system capacity configuration can improve the consumption level and reduce the renewable energy transmission cost. However, the restriction between economic feasibility and technical transmission constraints of the bundling system is not well understood. This restriction is becoming more and more obvious with the decrease of renewable energy price compensation year by year. This study compares the role of technical and economic indicators of capacity configuration, as well as the constraint relationship between electricity price and transmission constraints. Three screening principles of capacity configuration are proposed to reveal the techno-economic interaction. This paper explores a practical engineering case of Northwest China using a bundling system capacity configuration model. The internal rate of return is assumed as 8% for the bundling system. The results show that reducing the wind curtailment rate can effectively reduce the cost per MWh when the transmission guarantee rate is larger than 90%. Moreover, an integrated feed-in tariff based on market competitiveness is obtained if the wind curtailment rate is controlled by 5%. For example, the integrated feed-in tariff is 463.7 RMB/MWh when the wind curtailment rate is less than 5% and the transmission guarantee rate is larger than 95%. These results verify that the techno-economic interaction of the bundling system offers important theoretical support for selecting techno-economic indicators and capacity configuration.

HVDC Interconnections with SMES Device for PV and Wind Hybrid Power Transmission System

Abstract: The consumption of power shows a steeply increasing behavior in the present scenario which demands the integration of various novel techniques to the existing grid. With the penetration of PV and wind energy units into the grid, the evolution from a generator dominated grid to an inverter dominated grid arises in the near future. The paper discusses the characterization of HVDC convertors with the behavior of SG emulating virtual inertia for frequency stabilization and proper power oscillation damping during transmission of renewable energy based power. The conventional control of HVDC converters is integrated with synchronous power controller with second order characteristics inside power control loop for proper damping essentialities. The functioning of HVDC link is enhanced by means of SMES coil based energy storage operation. The proposed formulation is applied to a four area model in IEEE 14 bus system incorporated with PV, wind and HVDC links. The dynamic behavioral improvements are tested by means of MATLAB simulation.

The propellant role of the mega-grid in regional economic growth: Evidence from China base on the panel smooth transition regression model

One of the secrets for China's economic growth miracle is continuous investment in infrastructure, and the role of power infrastructure is irreplaceable. However, previous studies mostly focused on generation-side indicators rather than grid-side indicators, ignoring the importance of the mega-grid established in China. As a result, this paper is committed to exploring the economic growth pulling effect of power grids with different voltage levels in the nonlinear framework. On the premise of determining the robustness of estimation parameters by instrumental variable method, the panel smooth transition regression (PSTR) model is used to capture panel heterogeneity. To be exact, the regional economic growth driven by extra-high voltage (EHV) and ultra-high voltage (UHV) power grids presents a regime transition accompanied by the growth of regional power generation and electricity consumption per capita. This means that the mega-grid is more conducive to the economic growth of energy bases and load centers. To further build and improve the state-leveled mega-grid into a nationwide basic platform for trans-regional renewable energy transmission and distribution is a key measure to enhance China's sustainable economic growth under the constraints of carbon peak and carbon neutrality targets.

Love thy neighbor (or not): Regionalism and support for the use of offshore wind energy by others

Distributive fairness influences public acceptance of renewable energy technologies. Concerns about distributive fairness contribute to opposition to export of electricity and public preferences for electricity to be consumed near its production. Distributional issues are relevant to offshore wind energy development, where the coastal communities nearest to the developments may see none of the jobs, economic development, or profits associated with development, with undersea cables often delivering the electricity to faraway places. In a survey of coastal residents in the northeast United States, we find differences in support for use of offshore wind energy in their own state. Support for electricity use by other states, however, reveals the effects of subnational regionalism. Residents of Rhode Island prefer the use of electricity produced off their shores by the state of Massachusetts over New York; residents of New Hampshire prefer for Maine to consume energy over Massachusetts. These findings portend conflicts over the transmission of electricity as offshore wind energy development expands in U.S. and has implications for all renewable energy transmission that crosses regional boundaries.

Potential application of Six Sigma method in operation and maintenance management of UHVDC converter station

Abstract The ultra high voltage direct current (UHVDC) technology plays an important role in the long-distance and large-capability transmission of renewable energy. Power grid company has to improve the reliability and efficiency of UHVDC transmission systems to support renewable energy utilization. In this paper, Six Sigma method is considered for improving the operation and maintenance (O&amp;M) performance of UHVDC converter stations. The working tasks for the operation team and the maintenance team are introduced at first, respectively. The limitation of current O&amp;M management is analyzed. A five-stage implementation of Six Sigma method is discussed in detail. The formed negative feedback ensures the sustainability of implementation and realizes closed-loop management. A representative equipment disposal case is used to demonstrate the adaptability and operability of the potential application of Six Sigma method.

Optimal Operation and Control Design of DC Arrester for Renewable Energy Transmission

The trip probability of transmission line under lightning stroke is rapidly increasing under complex weather conditions when large-scale renewable energy is intergrated with modern power grid. Line arrester plays a critical role in reducing lightning damage and economic losses, in which DC reference voltage is a critical parameter to evaluate line arresters. Hence, a set of on-site DC test system of transmission line arresters is developed to realize the DC reference voltage test in this article. The output DC voltage of the test system is continuously adjustable (0–200 kV), while its rated power and the maximum output current are set to 400 W and 2 mA, respectively. Furthermore, the system is powered by a lithium battery instead of a 220/380 V AC energy source. It employs split design and integrating assembly on-site that reduces the total weight by 36.7%, which considerably decreases the demand for the space of test site. Besides, the weights of control cabinet, multiplying cylinder, and energy source are reduced by 60, 54.55, and 33.33%, respectively. Meanwhile, the Bluetooth remote control module is used to effectively ensure the safety of test personnel. This system accomplishes DC reference voltage tests without removing line arresters on site, which can effectively enhance maintenance efficiency and economic benefits.

Climate-friendly and socially inclusive AC-DC renewable energy system with overlay multi-terminal HVDC network (OVANET): Solution with fully distributed optimization and infrastructure combination

A large-scale integration of wind and solar energy relies on a grid that allows for the renewable electric power to securely reach the load centers. While the necessity for the extension of transmission capacity to obtain a climate-friendly and sustainable overall energy system is given by physical evidence, the realization of any extension relies on social acceptance and the support of all involved stakeholders. This is where the overall contribution of this paper comprises three dimensions. Firstly, thanks to the modular multi-level converter (MMC), it has become possible to realize multi-terminal DC overlay networks that lend themselves for connection with AC grids. The full AC-DC integration is at the core of the first dimension and aimed at offering a high flexibility of control when addressing security and efficiency. Secondly, as the AC-DC grid may stretch across multiple countries and therefore also diverse transmission system operator (TSO) control areas, it is important that a desired degree of data privacy and autonomy within those areas can be preserved. This is made possible by the proposed distributed AC-DC power flow optimization based on a modified fully parallel Alternating Direction Method of Multipliers (ADMM). The formulated local physical consensus constraints for boundary variables at the locations of tearing only rely on exchange of information with immediate neighbors. The physical consensus constraints are complemented by the newly introduced operational consensus constraints to prevent conflicting operational control actions. Thirdly, it is shown how social acceptance can be boosted by having the transmission lines share the corridors with other transport infrastructures. Of particular interest in this context are DC cable tunnels along motorways and the joint usage of bridges, leading to a combination of infrastructures. This also readily supports the development of electric vehicle charging stations along motorways. Together, these three dimensions provide the foundations of the proposed OVANET concept of socially inclusive AC-DC grids for renewable energy transmission.

A Topology Analysis-Based MMC-HVDC Grid Transmission Capacity Calculation Method

Modular multilevel converter-based high voltage direct current (MMC-HVDC) has a broad application prospect in renewable energy transmission. With the development of converter capacity, the transmission capacity calculation for MMC-HVDC grids becomes important for power dispatching. The existing method depends on manual work and is suitable for a simple grid. However, as the grid structure and operation modes become more and more complex, it becomes difficult to calculate the transmission capacity of different operation modes for an MMC-HVDC grid. This paper analyzed and simplified affecting factors and basic topologies. On this basis, a topology-based MMC-HVDC grid transmission capacity calculation method is proposed. First, an MMC-HVDC grid is divided into sending end grid, transmission section lines, and receiving end grid. Then the power limits of these three parts are calculated. Finally, the transmission capacity is determined by analyzing the connection mode of these three parts. This method can be adapted to any kind of MMC-HVDC grid and can be easily programmed.

Long-distance renewable hydrogen transmission via cables and pipelines

Intermittency is one of the main obstacles that inhibit the wide adoption of the renewable energy in the power sector. Small-scale fluctuations can be tackled by short-term energy storage system, whereas long-term or seasonal intermittencies rely on large-scale energy management solutions. Besides the supply and demand mismatch in temporal domain, renewable energy sources are usually far away from consumption points. To connect the energy sources to the demand cost-effectively, cable transmission is usually the default option, and considering the long distance, other emerging energy carriers such as hydrogen could be a feasible option. However, there is handful studies on the quantitative evaluation of the long-distance energy transmission cost. This paper investigated the economic feasibility of renewable energy transmission via routes of power cable and gas pipeline. In the direct power transmission case, renewable energy is transmitted via HVDC cable and then converted to hydrogen for convenient storage. The alternative case converts renewable energy into hydrogen at the source and transports the hydrogen in the gas pipeline to consumers. Existing data available from public domain are used for cost estimation. Results show that the improvements of capacity factor and transmission scale are the most cost-effective approach to make the renewable hydrogen economically viable. At 4000 km of transmission distance, renewable hydrogen LCOE of 7 US$/kg and 9 US$/kg are achievable for the corresponding optimum cases, respectively.

Engineering practices for the integration of large-scale renewable energy VSC-HVDC systems

With the continuous development of power electronic devices, intelligent control systems, and other technologies, the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase, while the system losses and costs will gradually decrease. Therefore, it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects. Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects, such as a weak local power grid, lack of support for synchronous power supply, and insufficient accommodation capacity. However, this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems. Based on actual engineering practices that are used worldwide, this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology, while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.

Prospect and key techniques of Global Energy Interconnection Zhangjiakou Innovation Demonstration Zone

In the Background of implementing innovation-driven development strategy and building Global Energy Interconnection, the necessity of building Global Energy Interconnection Zhangjiakou Innovation Demonstration Zone for stimulating economic growth, promoting social development and supporting 2022 Winter Olympics are discussed by analyzing the location advantages of Zhangjiakou and the characteristics of renewable energy in Zhangjiakou. Solutions are put forward in aspects of renewable energy integration in National Wind/Solar/Storage/Transmission Pilot Project, VSC-HVDC transmission of renewable energy and demonstration utilization of renewable energy in the Olympics zone, which could be a reference for problems of large-scale renewable energy in secure integration, outgoing transmission and flexible consumption. And replicable experience for building Global Energy Interconnection will be provided by conducting ±500kV VSC-HVDC Power Grid Demonstration Project, Virtual Synchronous Generator Demonstration Project, Flexible Substation and AC/DC Power Distribution Network Demonstration Project, EV Battery Secondary Utilization Energy Storage Demonstration Project, Smart Grid Demonstration Project of Low Carbon Winter Olympics and other demonstration projects.

Renewable energy transmission by HVDC across the continent: system challenges and opportunities

Rapid development of renewable energy in China is driving a major shift in the characteristics and control requirements of the electricity grid. Since the best renewable energy resources are far away from load centers in the east and southeast, transmission over long distances is required. Over 20 high-voltage DC (HVDC) transmission lines, with a combined capacity exceeding 150 GW, are in operation or are currently under construction. This rapid expansion of new generation and transmission capacities based on power electronics starts to change the characteristics of the grid, especially in areas where they concentrate, creating new stability problems and operational challenges. New system theories and technologies are required to support the development and operation of a future grid that relies more and more on power electronics. This paper highlights the characteristics of power electronics as used in renewable energy generation and HVDC transmission systems, discusses the impacts of these power-electronics-based assets on grid stability and operational requirements, and identifies opportunities for the development of both new system theories and system technologies to support a national energy policy that emphasizes the use of clean energy.