High Voltage Direct Current Link Research Articles

MW-Scale High-Voltage Direct-Current Power Conversion for Large-Spacecraft Electric Propulsion

This paper proposes a megawatt (MW)-scale high-voltage (HV) electrical power-conversion element for large-spacecraft electric propulsion (EP) systems. The proposed scheme is intended for long-term and crewed missions, and it is driven by a nuclear electric propulsion (NEP) that acts as a heat source. The scheme includes (i) A two-rotor generator (TRG), (ii) A rectification stage, and (iii) An isolated dual output DC-DC (iDC2) converter. The TRG is a high-reliability electric machine with two rotors, a permanent magnet rotor (PMR), and a wound field rotor (WFR). The PMR has a fixed flux and hence back-EMF, while the back-EMF due to the WFR is controlled by injecting a direct current (DC) into the WFR winding. The total TRG output voltage, which is the sum of voltages due to the PMR and WFR, is controlled over a prescribed region of spacecraft operation. The output of the TRG is rectified and connected to the input of the iDC2 converter. The iDC2 converter uses a three-winding transformer, where the primary winding is fed from the rectified output of TRG, the secondary winding processes the propulsion power to an electric thruster via a high-voltage DC (HVDC) link and a tertiary winding that is connected to the spacecraft’s low-voltage DC (LVDC) power system. Three controllers are proposed for the system: an HVDC voltage controller, an HVDC current controller that controls the voltage and current processed to the thruster, and an LVDC controller that adjusts the current to the LVDC system. Detailed analytical models for the TRG, iDC2 converter, and controllers are developed and verified via simulations under different conditions. The analytical studies are further validated via results from a laboratory prototype.

Adaptive inter-area power oscillation damping from offshore wind farm and MMC-HVDC using deep reinforcement learning

The coordination of the offshore wind farm (OWF) and high-voltage direct current (HVDC) links to provide ancillary power oscillation damping (POD) services may become a mandatory requirement from the transmission system operators in the near future. However, the performances of the POD controllers (PODCs) are vulnerable to the power system uncertainties and random communication delays of the wide-area signals. To address these issues, this paper proposes the design of the coordinated PODCs by using the deep reinforcement learning (DRL) method. The DRL-based PODCs employ one of the state-of-the-art DRL algorithms, proximal policy optimization, which can learn to adapt to the system uncertainties and time-varying communication delays by interacting with the power system continuously. A detailed simulation model of an OWF connected to the IEEE-39 bus AC grid via the modular multilevel converter based HVDC link has been built as a simulation platform, and the efficacy of the proposed DRL-based PODCs has been validated across a broad spectrum of operating conditions, disturbances, and communication latency.

Optimal Reactive Power Flow of AC-DC Power System with Shunt Capacitors Using Backtracking Search Algorithm

In this paper, it is proposed that a two-terminal high voltage direct current (HVDC) be integrated into the power system. Line-commutated converter (LCC)-HVDC is used because of its ability to reduce line losses, which improves overall system efficiency. Shunt capacitors also aid in voltage maintenance by compensating for the reactive power demand. In essence, limiting voltage drops in electrical networks promotes a more efficient power transmission and distribution by lowering resistive losses. In power system investigations, it was discovered that the HVDC link and SCB exist separately. So, for the first time, the backtracking search algorithm (BSA) is used to solve the optimal reactive power flow (ORPF) of a power system with a HVDC link and shunt capacitor banks (SCB). Although BSA simulations on a modified IEEE 30 bus yielded successful results, ABC was also utilized for comparing the outcomes of different methods. Overall, three separate cases of the modified IEEE 30 bus system were examined. When the acquired results are compared to other methods, the suggested algorithm is found to be better at concerning effectiveness as well as performance.

Optimal Estimation of Under-Frequency Load Shedding Scheme Parameters by Considering Virtual Inertia Injection

Under-frequency load shedding (UFLS) schemes are the latest safety measures applied for safeguarding the integrity of the grid against abrupt frequency imbalances. The overall inertia of electrical power systems is expected to decrease with an increased penetration of renewable energy as well as elements connected through power electronic interfaces. However, voltage source converter-based high voltage direct current (VSC-HVDC) links can provide virtual inertia through a control loop that allows for a reaction to occur at certain frequency fluctuations. This paper evaluates a UFLS scheme that considers the injection of virtual inertia through a VSC-HVDC link. A genetic algorithm (GA) is used to determine the location of the UFLS relays, the activation threshold of each stage, the delay time and the percentage of load shedding at each stage. It was found that the virtual inertia causes the nadir to delay and sometimes reach a greater depth. Furthermore, the implemented GA approximates the frequency response to the limits set with the constraints, reducing the load shedding but achieving a steeper nadir and a lower steady-state frequency level than traditional UFLS. The simulations were performed using the IEEE 39-bus test system.

Application of an optimal tilt controller in a partial loading schedule of multi-area power system considering HVDC link and virtual inertia

Maintaining the reliability of the modern power system is becoming increasingly challenging due to the continuous growth in its size and complexity. One of the most significant challenges PS faces is frequency regulation, which is essential for maintaining a stable and reliable operation of PS. This article investigates the automatic generation control (AGC) of a multi-area PS considering partial loading scheduled in a hydro and thermal generating unit. A bird swarm algorithm (BSA) based two-degree of freedom tilt integral derivative (2DOF-TID) controller has been suggested. The superiority of the optimal 2DOF-TID controller has been selected by comparing the system dynamics with other conventional controllers. Also, the ascendancy of suggested BSA has been shown over the grasshopper algorithm and particle swarm optimization techniques. The investigation on partial loading schedules for the hydro-thermal system showed that the system's dynamic response deteriorates when loading on hydro units exceeds their nominal loading. Also, system dynamics are enhanced when thermal units are loaded beyond their capacity. The investigations are also conducted to analyse the effect of high voltage direct current (HVDC) link and virtual inertia emulation with an energy storage system. The studies with alternating current (AC)/HVDC and virtual inertia revealed that dynamic responses are better with the latter when compared with AC and HVDC links alone.

Interconnecting industrial multi-microgrids using bidirectional hybrid energy links

Sharing and exchange energy among nearby industrial microgrids are crucial, especially with high energy requirements for their production targets and costly energy storage systems that may be oversized for their operations. Facilitating energy exchange can provide an economic advantage for industrial production by utilizing cheaper energy sources and reducing production costs. This manuscript presents an efficient approach for transferring large energy packets with minimal energy losses using high-voltage direct current (HVDC) energy transmission. The manuscript methodology focuses on implementing an industrial multi-microgrid using a modular multilevel converter. This converter utilizes two power link channels: a three-phase AC and an HVDC link, creating a hybrid energy transmission between microgrids. When a substantial amount of energy to transfer, the HVDC method enhances overall efficiency by reducing copper losses and mitigating issues associated with the AC link, such as harmonics and skin effects. The modular multilevel converter topology offers high flexibility and the use of fewer converters. Additionally, the HVDC link eliminates distance restrictions for energy transfer between industrial microgrids. A case study illustrates the functionality of this topology, demonstrating optimized power transfer and decreased energy losses. This methodology allows industrial microgrids to enhance energy efficiency and productivity while minimizing operational costs.

Adjustments for an HVDC link fault detector: practical considerations

In this work, practical considerations regarding the use of a fault diagnosis scheme based on a distributed adaptive estimator for the high-voltage direct current (HVDC) transmission line within a simplified model of the DC link are presented. Due to transients and steady-state ripple caused by unmodeled nonlinearities in the HVDC system's conversion part, parameter adjustments of the distributed estimator, including gains and thresholds, are performed to successfully determine the fault magnitude and position within the line through simulation studies. To validate the proposed scheme, numerical simulations of the diagnostic scheme are compared with two HVDC links: a simplified one and the CIGRE (International Council on Large Electric Systems).

Determination of modular multilevel converters admittances and their impacts on HVDC power system stability

This paper deals with the determination of the complex admittances and trans-admittances of power converters in general and in particular of two modular multilevel converters (mmcs) connected by a high-voltage direct current (hvdc) link to two distinct ac systems. With the term trans-admittances we mean the cross admittance “seen” between the points of common coupling of the two mmcs in the two ac grids. These quantities show how the hvdc link couples the two seemingly separated ac systems. The accurate computation of admittances and trans-admittances requires careful consideration of several frequency harmonics and frequency up-conversion/down-conversion phenomena through the hvdc link. To determine them, we exploit the periodic small-signal (pac) analysis that we briefly introduce from a theoretical perspective. Results show how ac and dc instabilities can be induced by mmcs and how they propagate through the hvdc link.

A new cyber-security approach for load frequency control of hybrid interconnected renewable power systems

Countries worldwide are embracing the trend toward smart grids, depending on advanced technology and communication networks. However, the smart grid is more vulnerable to cyberattacks than its traditional counterpart, especially the false data injection attack (FDIA), which aims to add false data into the transmitted signals to cause undesirable behavior. This article proposes a new scheme to detect and estimate the FDIA on load frequency control (LFC) of modern interconnected power systems considering renewable energy sources (RESs) and high voltage direct current (HVDC) link. The proposed scheme is based on the Kalman filter (KF) and artificial neural networks (ANNs). The KF is used to detect the FDIA in the transmitted system signals and indicates the possibility of an attack in each transmitted signal, where the role of ANNs is to estimate the transmitted signal before attacking. Hence, based on KF and ANNs, the proposed scheme can ensure the detection of false data and remove the attack simultaneously. The proposed cyber-security scheme is considered a thorough solution that can effectively reduce the impact of the FDIA on any of the four transmitted signals within the two-area interconnected power system, all while simulating the virtual inertia control strategy. The simulation results of the studied low-inertia interconnected power system are carried out using MATLAB/Simulink® software. The simulation results successfully demonstrate the proposed scheme's efficacy, accuracy, and acceptability to detect and eliminate the FDIA on the LFC of low-inertia interconnected power grids.

Frequency Control of a Realistic Dish Stirling Solar Thermal System and Accurate HVDC Models Using a Cascaded FOPI-IDDN-Based Crow Search Algorithm

Unbalancing the real power in power system leads to fluctuation in system frequency which can cause the several negative effects on the performance and reliability of the interconnected power system. Therefore, to deal with this, the load frequency control (LFC) of a three-area asymmetric thermal power system integrated with a solar thermal power plant (STPP), a realistic dish-stirling solar thermal system (DSTS), and an accurate high voltage direct current (HVDC) link are presented in this work. For the suggested system, a novel cascade controller called fractional-order proportional-integral and integral-double-derivative with filter (FOPI-IDDN) is designed. By minimising a newly proposed performance index called the HPA-ISE and adjusting the controller and other system model parameters using a metaheuristic method called the crow search algorithm (CS). When comparing the system dynamics, it was found that the suggested FOPI-IDDN controller outperformed the FOPI, PIDN, and FOPIDN controllers. The findings of this study show that HPA-ISE shows approximately 30% and 60% improvements in settling time (ST) and peak overshoots (POS) for frequency response, and 32% and 18% improvements for the tie power responses in terms of ST and POS over ISE criteria. Also, studies on different area capacity ratios have shown that a system connected to a greater capacity ratio operates better. The realistic DSTS system with fixed and recurring insolation in area 1 and area 2 outperforms the others, according to experiments using different DSTS insolation. Also, it is discovered that the parallel AC-AHVDC link study is superior to the AC and HVDC connection research. Moreover, it seems from the sensitivity study that the CS-optimized FOPI-IDDN controller improvements obtained under normal settings are consistent across a wide range of changes.

Evaluation of inertia resource for securing nadir frequency in HVDC interconnected system with high penetration of RES

For power systems to manage a variety of renewable energy sources while securing system stability and reliability, more resources are required. However, due to the interaction of the frequency response resources, determining the appropriate resources for frequency stability is challenging. In the Jeju system, where two HVDCs control the frequency primarily, this paper suggests a closed-form frequency response prediction model that represents the response characteristics of a high-voltage direct current link (HVDC). The Jeju system has reached its limit as a result of the increased penetration of renewable energy, which has led to increasingly frequent renewable energy curtailments. To ensure the frequency stability of the Jeju power system, it is thought to be necessary to construct a flywheel-connected synchronous condenser. The impact of the inertia constant and capacity of a flywheel-connected synchronous condenser is examined using the proposed closed-form frequency response prediction model. Case studies are presented to demonstrate the feasibility of the proposed method. PSSE simulations are performed with the HVDC user-defined model which manufactured by General Electric (GE) and the Jeju Island power system in Korea. The suggested approach can be successfully applied to choose appropriate resources and plan resources for frequency stability.

Hybrid bipolar VSC-DRU HVdc connection for offshore wind farms

This paper presents a hybrid bipolar high voltage direct current (HVdc) link for offshore wind farms (OWFs) connection. Voltage source converter (VSC) technology is used in one of the offshore poles, using a diode rectifier unit (DRU) in the other one. Therefore, economic and power-loss advantages of the DRU over the VSC are leveraged by changing one of the HVdc poles conventional technology. In the proposed control strategy, VSC reactive power is used to control OWF ac grid frequency, allowing the DRU commutation and active power transfer. VSC active power is used for OWF ac grid start-up and hybrid bipolar HVdc link power flow control. OWF ac grid voltage magnitude is controlled while the DRU pole is blocked, and it is automatically accommodated to the required value when the DRU is conducting. Moreover, VSC is also used as an active filter for the main DRU current harmonic components, thus removing the huge passive ac filters from the offshore platform. Poles balanced and unbalanced power flow control strategies are proposed for the hybrid bipolar HVdc, considering both bipolar with neutral return and rigid bipolar configurations. Simulation results validate the proposal which aims reducing cost and size of the OWF HVdc connection.

Analysis of VSC-HVDC support capability for power grids with large-scale renewable energy and multi-infeed HVDC links

Henan Power Grid is located in the north of Central China Power Grid. During its energy transition, thermal power units will be decommissioned on a large scale. Therefore, the scale of the new energy grid connection and regional external power will be further greatly increased to match the load growth demand. Under this background, Henan Power Grid plans to introduce the third HVDC (High Voltage Direct Current). Based on the actual data and the current operation status of Henan Power Grid, this paper proposes an analysis strategy about VSC-HVDC (Voltage Source Converter HVDC) to improve the support capability of the receiving power grid with multi-infeed HVDC. In recent years, VSC-HVDC technology has emerged with the strengths of flexible power regulation and strong voltage support. This paper firstly analyzes the VSC-HVDC power transmission characteristics and clarifies the mechanism of the VSC-HVDC power regulation. Then, based on the actual planning of Henan power grid, the influence of two feed-in schemes (LCC-HVDC (Line Commutated Converter HVDC) and VSC-HVDC) on the voltage stability of power grid and the number of subsequent commutation failures of other conventional DC are compared and analyzed under the three-phase short-circuit fault of AC lines near HVDC terminal location by using PSD-BPA simulation software. The results show that the VSC-HVDC scheme can effectively alleviate the voltage fluctuation (duration within 1 s and fluctuation number within 2 times) after the fault and restrain the number of subsequent commutation failures of conventional DC(with 2 times and even none).

Application of artificial hummingbird algorithm in a renewable energy source integrated multi-area power system considering Fuzzy based tilt integral derivative controller

This paper demonstrates the application of a novel artificial hummingbirds’ algorithm (AHA) and fuzzy (F) based tilt-integral-derivative (F-TID) controller in a renewable energy source integrated multi-area power system for the first time. Also, the chest time constant of steam turbine are calculated based sliding pressure mode operation of the superheater and it has been found that the chest time constant of steam turbine depends on plant loading. The system dynamics with the proposed controller are compared to those of existing controllers such as proportional-integral-derivative (PID) and TID, and it is revealed that the system with proposed F-TID controller has better system dynamics. The comparison of system dynamic with F-TID considering grasshopper algorithm (GHA), bird swarm algorithm (BSA) and AHA yield that the system dynamic with AHA outperformed. Further, a Friedman test has been also performed to validate the proposed algorithm. The study on various loading of thermal unit considering sliding pressure mode of operation revealed that, when loading increases toward or above its rated value the system dynamic improves. The effects of dish-stirling-thermal-system (DSTS) and wind-turbine-system (WTS) integration on system performance are also analyzed, and it has been found that the integration of renewable energy sources supports in a generation and it shows approximately 6% improvement in frequency profile along with 8% improvement in tie-power responses. The comparative analysis of system dynamics with AC tie-line, conventional high voltage direct current (HVDC), and accurate model of HVDC link reveals the better dynamics performance of the latter. Furthermore, the robustness of proposed F-TID controller is evaluated by sensitivity analysis and it is inferred that the proposed controller provides stables responses under varied loading condition.

Resilient virtual synchronous generator approach using DC-link capacitor energy for frequency support of interconnected renewable power systems

Modern interconnected power systems with many renewable energy sources (RESs) are more susceptible to disturbances than conventional power systems because they lack the inertia constant. Therefore, this study presents an advanced frequency control strategy for interconnected power systems considering high RESs penetration relying on the virtual inertia control (VIC) strategy-based high voltage direct current (HVDC) link, thus improving the dynamic frequency performance of future power systems. The HVDC link has been used as a DC capacitor which stores the electrostatic energy. This stored energy can supply the earliest active power support in automatic generation control system operation. Moreover, this paper develops the conventional VIC strategy based on the HVDC link by considering the dynamic behavior of a virtual rotor that mimics the inertia and damping properties of actual synchronous machines, thus stabilizing future renewable power systems. Also, this paper proposes a novel frequency control scheme based on the inertia emulation-based HVDC link and a virtual synchronous generator, which mimics the virtual rotor and virtual frequency control loops, to improve further the frequency regulation of low-inertia modern interconnected power systems. The parameters of the virtual synchronous generator scheme are optimally adjusted utilizing particle swarm optimization. The utility and superiority of the proposed VIC strategies based on the HVDC link are confirmed by contrasting them with the conventional VIC strategy relying on the HVDC link in the studied interconnected power system against various load/renewables interruptions, uncertainties, and physical restrictions. MATLAB/Simulink® software was used to simulate the results of the investigated system.

Optimal coordination of spinning and standing reserve with energy payback of thermostatically controlled loads

Thermostatic loads are expected to support the transition to low-carbon systems by providing an array of ancillary services. The delivery of energy-intensive services requires the thermostatic appliances to payback the deployed thermal energy via extra power consumption at a later time. This is made available by conventional generators providing contingency reserve. Previous works acknowledged the interplay between secondary response and contingency reserve. However, the structure of the payback period was fixed and did not fully consider the capabilities of generators providing contingency reserve, e.g., modelling only spinning generators while neglecting standing units. This paper introduces a novel methodology that allows to optimally coordinate the energy recovery of thermostatic loads and the allocation of contingency reserve among spinning and standing units. To do so, the commitment times and structures of secondary response and contingency reserve can be optimally chosen depending on system needs and current availabilities at each time step. The model is applied to the Great Britain network, which is connected to the power system in Continental Europe via a high-voltage direct-current link. Results illustrate the techno-economic advantages stemming from the proposed methodology.

Artificial Intelligence-based Control Techniques for HVDC Systems

The electrical energy industry depends, among other things, on the ability of networks to deal with uncertainties from several directions. Smart-grid systems in high-voltage direct current (HVDC) networks, being an application of artificial intelligence (AI), are a reliable way to achieve this goal as they solve complex problems in power system engineering using AI algorithms. Due to their distinctive characteristics, they are usually effective approaches for optimization problems. They have been successfully applied to HVDC systems. This paper presents a number of issues in HVDC transmission systems. It reviews AI applications such as HVDC transmission system controllers and power flow control within DC grids in multi-terminal HVDC systems. Advancements in HVDC systems enable better performance under varying conditions to obtain the optimal dynamic response in practical settings. However, they also pose difficulties in mathematical modeling as they are non-linear and complex. ANN-based controllers have replaced traditional PI controllers in the rectifier of the HVDC link. Moreover, the combination of ANN and fuzzy logic has proven to be a powerful strategy for controlling excessively non-linear loads. Future research can focus on developing AI algorithms for an advanced control scheme for UPFC devices. Also, there is a need for a comprehensive analysis of power fluctuations or steady-state errors that can be eliminated by the quick response of this control scheme. This survey was informed by the need to develop adaptive AI controllers to enhance the performance of HVDC systems based on their promising results in the control of power systems. Doi: 10.28991/ESJ-2023-07-02-024 Full Text: PDF

Reliability Assessment of Interconnected Power Systems with HVDC Links Considering Frequency Regulation Process

With various components and complex topologies, the applications of high-voltage direct current (HVDC) links bring new challenges to the interconnected power systems in the aspect of frequency security, which further influence their reliability performances. Consequently, this paper presents an approach to evaluate the impacts of the HVDC link outage on the reliability of interconnected power system considering the frequency regulation process during system contingencies. Firstly, a multi-state model of an HVDC link with different available loading rates (ALRs) is established based on its reliability network. Then, dynamic frequency response models of the inter- connected power system are presented and integrated with a novel frequency regulation scheme enabled by the HVDC link. The proposed scheme exploits the temporary overload capability of normal converters to compensate for the imbalanced power during system contingencies. Moreover, it offers frequency support that enables the frequency regulation reserves of the sending-end and receiving-end power systems to be mutually available. Several indices are established to measure the system reliability based on the given models in terms of abnormal frequency duration, frequency deviation, and energy losses of the frequency regulation process during system contingencies. Finally, a modified two-area reliability test system (RTS) with an HVDC link is adopted to verify the proposed approach.

Fault Ride Through and Intermittency Improvement of Renewable Energy Integrated MMC-HVDC System Employing Flywheel Energy Storage

Modular multilevel converter (MMC)-based high voltage direct current (HVDC) transmission networks integrate remotely located distributed renewable energy resources (RER). The intermittent nature of RER and symmetrical and asymmetrical AC-side low-voltage faults produce operational difficulties. It reduces the MMC’s ability to transfer the rated power, which raises the voltage of the HVDC link. Therefore, the MMC-HVDC network’s fault ride-through (FRT) capabilities and the power fluctuation brought on by RER intermittency are potential challenges to the efficient integration of renewable energy resources. In response, this article suggests an energy control scheme for the flywheel energy storage of the PV-wind-MMC-HVDC system in order to regulate the HVDC-link voltage during low voltage faults at the point of common coupling (PCC) of the AC grids and to address the problem of power fluctuation caused by intermittent RER and sudden load changes. The suggested method eliminates the dynamic braking resistor (DBR) from the HVDC link and seamlessly integrates the RER without actively reducing renewable energy power during low voltage faults. To test the effectiveness of the proposed control method for the flywheel energy storage in reducing excess energy in the HVDC link, symmetrical and asymmetrical low voltage faults have been conducted. In addition, changes in wind speed, solar radiation, and temperature have been made to validate the flywheel energy management system’s performance. A real-time digital simulator (RTDS) and dSPACE-based controller hardware in loop (CHIL) configuration with a complete system have been used to simulate and evaluate the entire system.

Energy-balancing dual-port grid-forming control for VSC-HVDC systems

This work examines energy-balancing dual port grid-forming (GFM) control for high-voltage direct current (HVDC) transmission. In contrast to the state-of-the-art, HVDC converters controlled in this way do not require assigning GFM and grid-following roles to different converters. Moreover, this control enables primary frequency control and inertia support through HVDC links. A detailed stability and steady-state analysis results in conditions on the control gains such that i) the overall hybrid dc/ac system is stable, ii) asynchronous ac areas are quasi-synchronous, and iii) circulating power in cyclic topologies is avoided. Finally, a high-fidelity case study is used to illustrate and verify the analytical results.