AC-DC Grid Research Articles

Optimization Techniques for Hybrid AC-DC Grid: A Review

Optimization Techniques for Hybrid AC-DC Grid: A Review

Experimental evaluation of virtual synchronous generators with high regulation performance to enhance AC–DC grid interconnections

In the recent years there has been a significant increase in the interest surrounding the interconnection of AC and DC networks due to its potential to improve the power systems reliability. Despite the potential advantages, however, the AC and DC grid coexistence can pose challenges if not appropriately managed. In this scenario, this paper presents an enhanced control scheme enabling the use of Virtual Synchronous Generators (VSGs) as elements of interconnection between AC and DC networks. The proposed solution enables VSG operation with both fast dynamics for DC voltage regulation and slow dynamics for AC inertia emulation. This approach accommodates the contrasting requirements arising from the distinct mechanisms of DC voltage regulation and AC frequency regulation. Simulation and experimental results obtained with a 8 kVA three-phase converter prototype are provided to validate the effectiveness of the proposed technique and prove its better performances with respect to the solutions currently presented in literature.

Data-Driven Emergency Frequency Control for Multi-Infeed Hybrid AC-DC System

With the continuous development of large-scale complex hybrid AC-DC grids, the fast adjustability of HVDC systems is required by the grid to provide frequency regulation services. This paper develops a fully data-driven linear quadratic regulator (LQR) for the HVDC to provide temporal frequency support. The main technical challenge is the complexity and the nonlinearity of multi-infeed hybrid AC-DC (MIDC) systems dynamics that make the LQR intractable. Based on Koopman operator (KO) theory, a Koopman eigenpairs construction method is developed to fit a global linear dynamic model of MIDC systems. Once globally linear representation of uncontrolled system dynamics is obtained offline, the control term is constituted by the gradient of the identified eigenfunctions and the control matrix <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{B}$</tex-math></inline-formula> . In case that <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{B}$</tex-math></inline-formula> is unknown, we propose a method to identify it based on the verified Koopman eigenfunctions. The active power reference is optimized online for LCC-HVDC in a moving horizon fashion to provide frequency support, with only locally measurable frequency and transmission power. The robustness of the proposed control method against approximation errors of the linear representation in eigenfunction coordinates is analyzed. Simulation results show the effectiveness, robustness and adaptability of the proposed emergency control strategy.

The Cost and Benefit of Enhancing Cybersecurity for Hybrid AC/DC Grids

As critical interfaces of AC grids and DC grids inside a hybrid AC/DC grid, the voltage-sourced-converter (VSC) has been demonstrated to be vulnerable to false data injection (FDI) cyber-attacks. As a result, the cyber-attack-induced AC grid frequency deviations and DC grid voltage deviations threaten the secure operation. To enhance cybersecurity in a not only feasible but also cost-effective manner, this paper proposes a cost-benefit-based cyber-defense strategy for a hybrid AC/DC grid. First, this paper establishes a spatial-temporal dual cyber-attack evaluation model, in which the cyber-attack-induced frequency and voltage deviations are modelled in both a spatially and temporally dual manner. Then, the proposed cost-benefit-based cyber-defense strategy is modelled as a VSC commitment problem to achieve the trade-off between maximizing the cyber-defense benefits and minimizing the cyber-defense costs. The VSC commitment problem is then mathematically convexified into a mixed-integer second-order cone programming (MISOCP) problem, which could be efficiently solved in an event-triggered manner against unfolding cyber-attack events. Simulation results on a test hybrid AC/DC grid verified the feasibility and the cost-effectiveness of the proposed cyber-defense strategy.

Control Scheme for AC-DC Grid Connected Hybrid Energy System for Power Quality Improvement

Abstract: Renewable electricity assets inclusive of air, solar, water, biomass, and many others. Are crucial for sustainable development and social development, in addition to assembly power wishes through the usage of renewable electricity resources. Complete renewable electricity such as wind strength is needed in the energy device to lessen environmental influences. The project proposes the STATCOM manage plan for grid-connected wind electricity structures to increase strength excellent. The bang-bang controller changed into advanced for STATCOM primarily based at the hysteresis cutting-edge manipulate scheme. STATCOM has been merged with common coupling to reduce electricity excellent problems. The STATCOM manage scheme implemented for the grid related wind strength era machine used to enhance the power quality is simulated the usage of the MATLAB / Simulink inside the electricity device block set. This proposed scheme ensures an increase in energy great in production output.

Influencing factors and propagation mechanisms of sub-synchronous oscillations in complex AC–DC grid

The modern power system exhibits typical “double-high” characteristics as a result of the extensive use of new energy sources like wind and photovoltaics that are connected to the electrical grid, and as a result of this development, sub-synchronous oscillations are becoming an increasingly significant issue. There is an urgent need to research the main influencing factors and propagation mechanism of SSO in complex AC–DC grid since they have a wide range of propagation properties. In this article, the propagation mechanism based on the sub-synchronous power propagation divider coefficient is proposed, and a quantitative analysis approach based on the maximal information coefficient (MIC) of the influencing factors of sub-synchronous oscillation propagation is clarified. The results demonstrate that the power flow on AC transmission is the key factor affecting the propagation of the SSO power’s AC component, the AC transmission resistance characteristics at the frequency of the sub-synchronous oscillation is the main factor affecting the propagation of SSO power’s DC component, and the sub-synchronous power propagates in the complex system according to the diversion coefficient of circuit branch. The efficiency of the method and mechanism proposed in this paper is confirmed by the simulation results.

Two-stage robust optimal operation of AC/DC distribution networks with power electronic transformers

Power electronic transformers (PET) are a new type of power electronic equipment with a multi-port flexible dispatch function, which can play the role of a power hub in a system composed of multiple AC-DC hybrid distribution grids for interactive sharing of power in multiple regions. In this study, a two-stage robust optimization operation model of a hybrid AC-DC distribution network with PET is proposed based on PET power transmission and transformation characteristics. The stochastic uncertainty of the distributed renewable energy output in the AC-DC grid is handled by a two-stage robust optimization method to determine the minimum total system operation cost under the worst case of distributed renewable energy output. Finally, a constrained column generation algorithm is used to solve the two-stage robust optimization model in the min-max-min form and verifies the validity of the model in this study.

Service Restoration of AC–DC Distribution Grids Based on Multiple-Criteria Decision Analysis

Modern distribution grids are transforming due to the increasing amount of load and generation based on DC technologies, whose interconnection is facilitated by the upcoming AC-DC distribution networks. Their fault management is challenged by new issues related to the power transferred among AC and DC sub-networks. In light of this, the present paper proposes a novel Service Restoration (SR) algorithm, specifically tailored for AC-DC distribution grids, that optimizes the re-energization by establishing priorities of disconnected bus groups and computing, as candidate solutions, hierarchical combinations of normally open and normally closed switches. To overcome the limitations of existing approaches, a Multiple Criteria Decision Analysis (MCDA) is proposed to combine and objectively prioritize different SR goals (i.e., the use of telecontrolled switches, the minimization of power losses, and the applicability of the proposed solutions in a defined time horizon), and ultimately make the grid operator benefit from the possibility to flexibly tune different operational objectives. The proposed algorithm allows to effectively discriminating competing solutions, i.e. whence and how to re-energize disconnected buses, and complies with the time requirements for field implementation. The results prove the crucial role of DC sub-networks, associated to the control of power injection from AC-DC converters, in enhancing the SR by improving its targets and increasing the number of re-energized loads. Ultimately, the effect of MCDA comparison parameters on the SR outcomes is quantitatively investigated via global sensitivity analysis, whose adoption is recommended for supporting the grid operator in the algorithm implementation.

Coordinated control strategy of multi-type power coupling system participating in primary frequency regulation of AC-DC hybrid power grid

Wind power, photovoltaic (PV) power, thermal power and hydropower are connected to AC-DC power grid through the same power coupling point, which is common in North China. Such physical properties make it possible for multiple types of power sources to form a coupling system and behave as a large integrated power supply. How the coupling system participate in the primary frequency regulation (PFR) is a problem worthy of investigation. However, there is a lack of research in this field by now. In this paper, a coordinated control strategy of coupling system participating in PFR is proposed, that is, each power source separately measures the frequency and carries out PFR; the central controller of the coupling system is responsible for receiving related information of each power source, calculating and issuing the key parameters for each power source, such as the frequency control gain. A test system is established based on the actual regional power grid in Liaoning province, China. The simulation results under typical operating conditions verify the effectiveness of the proposed control strategy, which can realize the coordination of multiple power sources on the basis of ensuring the overall frequency regulation rapidity of the coupling system.

Simulation framework for automatic load frequency control studies of VSC-based AC/DC power grids

This paper introduces an efficient simulation framework for automatic load frequency control (ALFC) studies of VSC-based AC/DC power grids. It is aimed at slow transients caused by large megawatt imbalances occurring either in the AC systems or DC grid. These studies involve the coordinated action of the primary and secondary control loops of power plants, ones requiring long simulation times ranging from various tens of seconds to minutes. To address these overly challenging AC/DC power system simulations, the developed modelling framework resorts to suitable linear models, i.e., VSC dynamic controls are devised to conform to the slow dynamics featuring the ALFC. The most fundamental dynamics of AC systems, DC grid and VSC stations are compiled with reasonable precision. This is demonstrated using a three-terminal, multilevel VSC-based HVDC system implemented in the PSCAD simulation package for comparative purposes; both solutions are shown to concur very well because root mean squared errors inferior to 3% are obtained. The practicality of this effective simulation framework is also demonstrated by studying a five-terminal VSC-based transmission grid facing large power imbalances imposed by load disconnections and VSC outages.

Machine Learning Based Transient Stability Emulation and Dynamic System Equivalencing of Large-Scale AC-DC Grids for Faster-Than-Real-Time Digital Twin

Modern power systems have been expanding significantly including the integration of high voltage direct current (HVDC) systems, bringing a tremendous computational challenge to transient stability simulation for dynamic security assessment (DSA). In this work, a practical method for energy control center with the machine learning (ML) based synchronous generator model (SGM) and dynamic equivalent model (DEM) is proposed to reduce the computational burden of the traditional transient stability (TS) simulation. The proposed ML-based models are deployed on the field programmable gate arrays (FPGAs) for faster-than-real-time (FTRT) digital twin hardware emulation of the real power system. The Gated Recurrent Unit (GRU) algorithm is adopted to train the SGM and DEM, where the training and testing datasets are obtained from the off-line simulation tool DSAToolsTM/TSAT®. A test system containing 15 ACTIVSg 500-bus systems interconnected by a 15-terminal DC grid is established for validating the accuracy of the proposed FTRT digital twin emulation platform. Due to the complexity of emulating large-scale AC-DC grid, multiple FPGA boards are applied, and a proper interface strategy is also proposed for data synchronization. As a result, the efficacy of the hardware emulation is demonstrated by two case studies, where an FTRT ratio of more than 684 is achieved by applying the GRU-SGM, while it reaches over 208 times for hybrid computational-ML based digital twin of AC-DC grid.

Comparative Assessment of Multi-Port MMCs for High-Power Applications

Multi-port converters can interconnect different dc and ac systems while reducing semiconductor requirements and losses by eliminating redundant power conversion stages. Modular multilevel converter (MMC)-based multiport systems are well suited for application in mixed ac-dc grids containing high-voltage dc (HVDC) and medium-voltage dc (MVDC) systems. This paper conducts a detailed comparative assessment of multi-port dc-dc-ac MMCs for high power applications. Four representative topologies are chosen for study due to their contrasting internal power processing characteristics. Three different network scenarios are investigated that include HVDC and MVDC applications, covering several different power flow cases. The multi-port MMCs are compared in terms of losses, semiconductor effort, internal energy storage and magnetics requirements. The results are extensively discussed and general conclusions are summarized.

Adaptive control strategy of solid state transformer with fast dynamic response and enhanced balance performance

Solid state transformer (SST) is designed as a power transmission device between AC grids and DC grids. However, the power fluctuation causes unbalanced power sharing among dual active bridge (DAB) converters and large voltage fluctuations on dc buses. Here, an adaptive virtual impedance control (AVIC) strategy is proposed to improve the balance performance of SST. Adaptive virtual impedance controller gets the same dynamic response for DAB converters, which matches dynamic impedance of DAB converters. Thereby the power balance can be achieved even the load changes. Meanwhile, an adaptive feedforward control scheme is proposed to reduce the influence of load fluctuation on DC bus voltage. The adaptive feedforward scheme also considers the parameter mismatch, thereby the balance of SST will not be destroyed. Moreover, the balanced power sharing achieved by adaptive controller eliminates the difference between actual transmission power and compensation power in each rectifier, which further suppresses the intermediate capacitor voltage fluctuations. Finally, simulation and StarSim experimental results verified the correctness and effectiveness of the proposed control strategy.

Distributed optimal voltage control strategy for AC grid with DC connection and offshore wind farms based on ADMM

A distributed optimal voltage control (DOVC) strategy based on the alternating direction method of multipliers (ADMM) is proposed for the AC grid with voltage source converter (VSC) DC connection and offshore wind farms (OWFs). The proposed DOVC strategy achieves global optimal power distribution inside the AC grid with DC connection to minimize the power losses while minimizing the bus voltage deviation from the rated value as well as power losses inside several OWFs. The optimal power flow (OPF) of the AC grid with VSC DC connection combined with model predictive control (MPC) is relaxed by using the semidefinite programming (SDP) relaxation technique. The MPC-based voltage optimization problem for the OWF is formulated by considering the dynamics of WTs. The ADMM is used to decompose the large-scale strongly coupled optimization problem, which is calculated in AC-DC grid and OWF controllers in parallel while guaranteeing global optimality. Compared to conventional control methods, the control performance of the system is enhanced. A modified IEEE-30 bus system with DC connection incorporating three OWFs consisting of 288 wind turbines (WTs) is used to evaluate the proposed DOVC strategy.

The use of analytical converter loss formula to eliminate DC slack/droop bus iteration in sequential AC-DC power flow algorithm

In conventional sequential AC-DC power flow algorithm, DC slack bus iteration (SBI) or droop bus iteration (DBI) is used to update converter loss of the DC slack or droop buses in each AC-DC power flow iteration. The use of SBI or DBI causes additional computational burden on top of the iterative solution of AC-DC power flow, reducing the computational efficiency of the power flow. This letter proposes a novel method to eliminate the SBI or DBI while preserving the accuracy of power flow algorithm. Moreover, a generalized converter loss formula for modular multilevel converters with different submodule types, i.e., half-bridge, full-bridge or mixed half- and full-bridge is derived to eliminate the SBI or DBI. The proposed algorithm is validated using two interconnected AC-DC grid consisting of the IEEE 39-bus AC grid and a 6-terminal DC grid and the IEEE 300-bus AC grid integrated with a 10-terminal DC grid, respectively.

A new design of control & power management strategies of hybrid ac-dc microgrids toward high power quality

The micro grid idea provides for the lack of several reversing switches to unitary AC-DC grid that enables connection and charges (loads) to the electrical systems with changeable regenerative AC and CC sources. Safe operation and gadget safety involve digital integration with utilities/grid through power converters. Enhanced client reliability, decreased input losses, local voltages are supported, and waste heat efficiency increased, voltage drop or interruptible supply of electricity can be customized to satisfy their unique customer demands. Work at present Analyses the performance in grid tie mode of hybrid AC/DC systems. Here are PV systems, PV systems, For the construction of microgrids wind turbine generators and batteries are employed. Convert procedures for the correct coordination of AC sub-grids to DC subs-grids have also been established for converters. MATLAB/SIMULINK environment results are generated.

Enabling hybrid energy storage systems in VSC‐based MTDC grids for decentralized fast frequency response control in low‐inertia AC/DC systems

This paper studies the hybrid energy storage system to provide frequency support for the interconnected AC grid through MTDC systems interfacing renewable resources. A hybrid energy storage structure was created using a supercapacitor (SC) and battery storage systems (BESs). To coordinate the sharing of power between SC and BESS, an improved droop controller based on virtual inductance, capacitance, and resistance gain (VICRC) is suggested. Meanwhile, the droop controller's steady-state deviation of the DC link voltage is immediately suppressed. The SC will be in charge of compensating the high-frequency demand, whereas the BESS will be in charge of compensating the low-frequency power demand, thanks to a decentralized energy management process. In addition, the suggested method aids in the rehabilitation of the SC's SOC. Finally, the simulation results with detailed models of the wind farms and AC–DC grid converters are performed on IEEE tests systems in Simulink/SPS and the results extensively discussed to evaluate the proposed structure.

Modeling and simulation of PSS performance in the AC-DC grid for small disturbance stability

Modeling and simulation of PSS performance in the AC-DC grid for small disturbance stability

Congestion-aware robust security constrained unit commitment model for AC-DC grids

Due to the increasing penetration of wind energy, their natural volatility may cause power flow congestions by blocking some key transmission interfaces. To further reduce energy violations induced by uncertainty issues and ensure the system reliability, the controllability of High Voltage Direct Current (HVDC) system should be exploited in AC-DC grids. By leveraging the congestion management capabilities of HVDC, we propose a congestion-aware robust security constrained unit commitment (SCUC) model, to preemptively consider the plausible congestion circumstances in a more adaptive way in the day-ahead (DA) market. In our model, the conventional uncertainty budget sets are modified by the proposed congestion-aware factors, where the typical operational characteristics of traditional corrective actions and HVDC regulations in real-time (RT) market are formulated adaptively. Moreover, the feasibility region of proposed problem is further pruned by the optimality cut planes where more pertinent real-time information are included, thus the local optimal solution is eliminated and the superior economic benefits are obtained in our model. To solve this mix-integer linear problem (MILP), the model is divided into three parts including the DA decision model, the traditional corrective dispatch model, and the HVDC regulation dispatch model. Correspondingly, an improved three-level Benders Decomposition algorithm is developed to solve the model. The enhanced performance of the proposed model was validated in a modified IEEE 118-bus test case. The results show that, in our model, the flexible regulation of HVDC system optimally allocate the operational burdens between DA and RT markets, thus the congestion-aware robust SCUC model further captures the economic benefits when ensuring the model’s robustness.

Power Decoupling Control of MMC and Small-Signal Stability Analysis of AC/DC Distribution Network With Renewable Energy

AC/DC hybrid distribution network can realize the high penetration utilization of renewable energy and modular multilevel converter (MMC) is the key equipment to connect AC grid and DC grid. In this paper, the direct modulation based on the virtual resistor and the reference value of dc-bus voltage is adopted, and instantaneous-value model of three-phase half-bridge MMC is derived by introducing differential-mode and common-mode component representation. Then, the decoupling control of AC active power and reactive power is designed based on the linear active disturbance rejection control theory (LADRC). The total disturbances including the coupling term and capacitor voltage fluctuation can be estimated by extended state observer (ESO), and then cancelled exactly. Based on the simplified average-value model of MMC, the small signal stability analysis of three-terminal AC/DC distribution network with 26-level MMCs corresponding to the Tangjiawan-Jishan1-Jishan2 demonstration project is carried out by root locus and bode diagram method, which gives the guidance for the choice of the main circuit component and controller parameters, and shows that the virtual resistor greatly reduces the resonant peak of AC/DC distribution network. Simulation and experiment results verify the effectiveness of the power decoupling control strategy of MMC and the AC/DC distribution network can realize complex multi-directional power flow.