High Voltage Direct Current Link Research Articles

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.

Secondary Frequency Control Considering Optimized Power Support From Virtual Power Plant Containing Aluminum Smelter Loads Through VSC-HVDC Link

The growing number of renewable energy replacing conventional generators results in a loss of the reserve for frequency control in power systems, while many industrial power grids often have excess energy supply due to abundant wind and solar energy resources. This paper proposes a secondary frequency control (SFC) strategy that allows industrial power grids to provide emergency high-voltage direct current (HVDC) power support (EDCPS) for emergency to a system requiring power support through a voltage source converter (VSC) HVDC link. An architecture including multiple model predictive control (MPC) controllers with periodic communication is designed to simultaneously obtain optimized EDCPS capacity and minimize adverse effects on the providing power support (PPS) system. Moreover, a model of a virtual power plant (VPP) containing aluminum smelter loads (ASLs) and a high penetration of wind power is established for the PPS system. The flexibility and controllability of the VPP are improved by the demand response of the ASLs. The uncertainty associated with wind power is considered by chance constraints. The effectiveness of the proposed strategy is verified by simulation results using the data of an actual industrial power grid in Inner Mongolia, China. The DC voltage of the VSCs and the DC in the potlines of the ASLs are also investigated in the simulation.

Study of Discharge Inception and Propagation in Liquid–Solid Insulation System under DC–LI Superimposed Constraints

High-voltage direct current (HVDC) links are starting to become widely implemented thanks to their interesting advantages such as reduced operation losses, the absence of reactive power, which allows energy transport via underground cables over long distances, and improved power control. The latter advantage is very essential for renewable energy resource integration into power grids. However, a thorough understanding of the behavior of insulation systems for HVDC components is critical so as to ensure a more reliable service. Indeed, the existence of the direct current (DC) voltage in HVDC components may induce surface and space charge accumulation that can stress insulation further or even promote discharge inception and propagation. As such, this work focuses on showcasing the effect of surface charge on streamers that develop on the interface of liquid–solid insulation due to the advent of lightning impulse (LI) voltage in the HVDC link. This study was performed using finite-element-based numerical simulations that include a quasi-electrostatic model for surface charge accumulation and an electrohydrodynamic fluid model for streamer initiation and propagation. The geometry used was point–plane configuration where the high voltage is applied to the needle electrode located above the liquid–solid interface. The obtained results suggest that streamer initiation is affected by both the accumulated surface charge density and polarity. For a positive streamer, an accumulation of positive surface charge increases the discharge inception voltage as a result of a weakening in the electric field, while an accumulation of negative surface charge decreases the discharge inception voltage due to an intensification in the electric field. Moreover, streamer travel distance and velocity are also both shown to be affected by surface charge accumulation.

Revealing interactions between HVDC cross-area flows and frequency stability with explainable AI

The transition to renewable energy sources challenges the operation and stability of the electric power system. Wind and solar power generation are volatile and uncertain, and energy sources may be located far away from the centers of the load. High Voltage Direct Current (HVDC) lines enable long-distance power transmission at low losses, both within and between different synchronous power grids. HVDC interconnectors between different synchronous areas can be used to balance volatile generation by leveraging their fast control behavior, but rapid switching may also disturb the power balance. In this article, we analyze the interaction of HVDC interconnector operation and load-frequency control in different European power grids from operational data. We use explainable machine learning to disentangle the various influences affecting the two systems, identify the key influences, and quantify the interrelations in a consistent way. Our results reveal two different types of interaction: Market-based HVDC flows introduce deterministic frequency deviations and thus increase control needs. Control-based HVDC flows mitigate frequency deviations on one side as desired but generally disturb frequency on the other side. The analysis further provides quantitative estimates for the control laws and operation strategies of individual HVDC links, for which there is little public information. Furthermore, we quantify the importance of HVDC links for the frequency dynamics, which is particularly large in the British grid.

Damping Subsynchronous Oscillations Using a High Voltage Direct Current-Based Multi-Modal Damping Controller

Conventional series capacitors in transmission lines are of paramount importance for enhancing power transfer capability. However, the major drawback of series capacitors is the induced subsynchronous resonance phenomenon in power systems. This phenomenon affects nearby turbogenerator shafts and severely limits their reliability. Also, the controls of High Voltage Direct Current links are considered as a potential source of subsynchronous oscillations. This paper investigates the performance of a multimodal damping controller to stabilize unstable torsional modes in combination with a Power System Stabilizer for inertial mode damping. This research uses time-domain simulation-based multimodal damping controller design methods, namely the test signal and phase correction methods. These two methods involve injecting a test signal at a frequency of interest into the rectifier current control loop and measuring its phase difference with the electromagnetic torque, thus providing corresponding compensators to minimize the resulting angle. This work uses Power System Computer-Aided Design for time-domain simulation and Fast Fourier Transforms Analysis to conduct the phase correction method and verify controller performance.

State Estimation Using SCADA and PMU Measurements for Networks Containing Classic HVDC Links

• Hybrid state estimation (HSE) algorithm based on SCADA and PMUs measurements for AC power systems with embedded classic high voltage direct current (HVDC) links. • AC and DC measurements are modeled independently as functions of AC and DC states, respectively. • AC and DC measurements (states) are processed (estimated) simultaneously. • Coupling of AC and DC states is expressed by a set of three nonlinear equality constraints. This paper proposes a hybrid state estimation (HSE) algorithm which employs conventional measurements received from supervisory control and data acquisition (SCADA) system and synchrophasors measured from phasor measurement units (PMUs) for AC power systems with embedded classic high voltage direct current (HVDC) links. DC measurements obtained from the HVDC link are assumed to be sampled at the same rate as AC measurements. Zero injections and AC/DC coupling equations are modeled as equality constraints. By applying the nonlinear weighted least squares (WLS) algorithm with equality constraints, the HSE calculates both AC and DC states simultaneously. The effectiveness of the proposed algorithm is verified by simulations on the IEEE standard 14 and 30 bus test systems. The test results reveal the benefits of the proposed approach in terms of estimation accuracy and convergence speed.

Network Coordination between High-Voltage DC and High-Voltage AC Transmission Systems Using Flexible AC Transmission System Controllers

The strategic intent of the African Union is to develop a “Smart Integrated African Electric Power Super Grid” driven by modern tools and advances in high-voltage direct current (HVDC) engineering and flexible alternating current technology systems (FACTS), which is central in supporting Africa’s sustained economic growth and development. The southern African region, including South Africa, is beset by the critical challenges of perennial load-shedding, which impedes economic growth and aggravates unemployment. This has led to the insecurity of electricity supplies and degraded the quality of life. The parallel operation of high-voltage direct current (HVDC) and flexible AC technology systems (FACTS) controllers is gaining traction as system conditions become more complex, such as weak power networks which requires increased stability requirements, resulting in load-shedding and power outages. These adversely affect business productivity and adversely affect GDP and economic growth. Thus, the application of innovative technologies such as HVDC links can stabilize weak power systems. It is established that HVDC delivery systems reduce losses in long transmission lines transporting bulk power compared with high-voltage alternating current (HVAC) transmission lines for power wheeling. This paper evaluates the parallel operation of the Cahora Bassa 1414 km bipolar HVDC link and a weak parallel 400/330 kV alternating current (AC) link. It demonstrates the use of FACTS controllers to enhance the technical performance of an existing network, such as voltage control, and technical loss reduction. It combines an HVDC line commutated converter (LCC) and HVAC transmission lines, in hybrid notation to increase the voltage stability of the system by controlling the reactive power with a Static Var Compensator (SVC). These modern tools can increase the transmission power controllability and stability of the power network. In this study, HVDC–LCC was used with a setpoint of 1000 MW in conjunction with the 850 MVAr SVC. The results show that the technical losses were reduced by 0.24% from 84.32 MW to 60.32 MW as Apollo 275 kV SVC was utilized for voltage control. The network analysis was performed using DIgSILENT PowerFactory software that is manufactured by DIgSILENT GmbH at Gomaringen, Germany

DE-Algorithm-Optimized Fuzzy-PID Controller for AGC of Integrated Multi Area Power System with HVDC Link

A power system’s nonlinearity and complexity increase from time to time due to increases of power demand. Therefore, properly designed power system controlsare required. Without these, system instability will cause equipment failures, and possibly even cascading events and blackouts. To cope with this, intelligent controllers using soft computing are necessary for real time operation. In this paper, the reheat type three-area thermal power system is considered, and the output scaling factors, gain parameters of fuzzy membership functions, and parameters of fuzzy-proportional integral derivative (FPID) controllers are optimized using a differential evolution (DE) optimization techniqueand integral time multiplied absolute error (ITAE) as objective functions. To improve the limitations of the controller and to enhance stability of the system, high voltage direct current (HVDC) technology is advantageous due to its quickresponse capabilities. In this paper, a HVDC is connected in parallel to the system, revealing that a FPID controller with a HVDC provides better and more accurate resultscompared to a system without a controller. The test results presented in this paper show the proposed controller’s suitability for managing random load changes.

Using Matlab/Simulink Software Package to Investigate Fault Behaviors in HVDC System

Existing studies show that several performance issues will arise in the HVDC link during the three phase-to-ground fault at the side of the inverter and that the DC voltage will oscillate around zero and will not affect the rectifier of the AC system though the inverter of the AC system, and the AC voltages will become zero and the AC currents will show high amplitude as well as minor disturbances. It has also been argued that when the fault is applied on a single-phase to ground fault at the inverter side on the AC side, the voltage will decrease. In this paper, we focus on single line-to-ground fault, double line-to-ground fault, and three phase-to-ground fault at the inverter of the AC system and their behavior on the DC link as well as on the AC system of the rectifier with detailed simulations. A high voltage direct current (HVDC) Monopolar system is modeled using a Matlab/Simulink software package for the research. The results show that during the three phase-to-ground fault at the AC system of the inverter, the DC voltage will increase with a bogus waveform and the currents of the AC system at the rectifier will collapse to zero.At the double phase-to-ground fault level, the DC voltage will experience an increase in waveform while the currents of the AC system of the rectifier will experience different disturbances. At the single phase-to-ground fault level, the DC voltage will remain stable and the rectifier side of the AC system will also experience a stable state for both currents and voltages.

The France–Spain border: A case study advocating for the use of simplified time-domain simulations for steady state calculation

The fast evolution of transmission systems, induced by the massive integration of high voltage direct current links, special protection schemes and renewable energies, has a strong influence on the system’s behaviour and the steady state it reaches after one or several events. The power flow tools that are currently used to calculate steady states after some events are not adapted any more to such changes. A case study advocating for the use of simplified time-domain simulations for steady state calculation instead of fully static tools is presented here. This case study is simulated using DynaFlow, a new simulation tool that does a simplified time-domain simulation whose final state is given as a result. It focuses on the France–Spain border, and it is shown that the dynamic parameters of the special protection schemes have a strong impact on the steady state that cannot be easily captured by classical power flow tools. A sensitivity analysis is also done to illustrate this.

Design of Protection Strategies and Performance Analysis of an HVDC Link During Multiple Disturbances

In High Voltage Direct Current (HVDC) transmission system applications, the control and protection system plays an essential role in the overall performance. This paper aims to give solutions to some performance problems that can occur in the HVDC link. In this paper, ways to mitigate this serious malfunction of the commutation failure process in the operation of HVDC converters are studied and protection functions like the Commutation Failure Prevention (CFPREV) have been used. Furthermore, HVDC transmission systems are vulnerable to DC faults and its protection becomes ever more important with the fast growth in the number of installations. In this context, the DC fault protection function is one of the most important challenges to the development of HVDC transmission systems. The dynamics of the studied power system including the HVDC link are thoroughly investigated in this paper through various simulation scenarios.

Impact of High Voltage Direct Current Link on Transmission Line in Kurdistan Power System

Kurdistan power system is expanded along years ago. The electrical power is transmitted through long transmission lines. The main problem of transmission lines is active and reactive power losses. It is important to solve this issue, unless, the most of electrical energy will lost over transmission system. In this study, High Voltage Direct Current links/bipolar connection were connected in a power system to reduce the power losses. The 132kV, 50 Hz, 36 buses Kurdistan power system is used as a study case. The load flow analysis was implemented by using ETAP.16 program in which Newton-Raphson method for three cases. The results show that the losses are reduced after inserted HVDC links.

Demonstration of Converter Control Interactions in MMC-HVDC Systems

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.

Fault Current and Voltage Estimation Method in Symmetrical Monopolar MMC-based DC Grids

Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current (MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.

Frequency Stability of AC/DC Interconnected Power Systems with Wind Energy Using Arithmetic Optimization Algorithm-Based Fuzzy-PID Controller

This article proposes an intelligent control strategy to enhance the frequency dynamic performance of interconnected multi-source power systems composing of thermal, hydro, and gas power plants and the high penetration level of wind energy. The proposed control strategy is based on a combination of fuzzy logic control with a proportional-integral-derivative (PID) controller to overcome the PID limitations during abnormal conditions. Moreover, a newly adopted optimization technique namely Arithmetic optimization algorithm (AOA) is proposed to fine-tune the proposed fuzzy-PID controller to overcome the disadvantages of conventional and heuristic optimization techniques (i.e., long time in estimating controller parameters-slow convergence curves). Furthermore, the effect of the high voltage direct current link is taken into account in the studied interconnected power system to eliminate the AC transmission disadvantages (i.e., frequent tripping during oscillations in large power systems–high level of fault current). The dynamic performance analysis confirms the superiority of the proposed fuzzy-PID controller based on the AOA compared to the fuzzy-PID controller based on a hybrid local unimodal sampling and teaching learning-based optimization (TLBO) in terms of minimum objective function value and overshoots and undershoots oscillation measurement. Also, the AOA’s proficiency has been verified over several other powerful optimization techniques; differential evolution, TLBO using the PID controller. Moreover, the simulation results ensure the effectiveness and robustness of the proposed fuzzy-PID controller using the AOA in achieving better performance under several contingencies; different load variations, the high penetration level of the wind power, and system uncertainties compared to other literature controllers adjusting by various optimization techniques.

Treating the Impacts of Connecting HVDC Link Converters with AC Power System Using Real-Time Active Power Quality Unit

The High Voltage Direct Current (HVDC) systems have been applied worldwide, due to important roles, technical benefits, and high efficiency. Usually, the HVDC network is a joining link between two separate and different technical properties of HVAC systems, which enhances its use. The joinpoint system that used to connect HVDC and HVAC links is a controlled converter circuit. Despite the importance of use and the significant benefits of the HVDC link, there are negative effects on the power quality of the electrical power of both systems connected on both ends of the HVDC network. The low quality of electric power has been addressed by known methods, whether traditional or modern. But the improvement is usually made with the assumption of load conditions and the need for the system to synthesize it. This research presents an innovative method based on real-time control strategy. This is performed by proposing dSPACE for controlling the Active Power Quality Unit (APQU). The proposed control strategy of the APQU includes a Modified Harmonics Pulse Width Modulation (MHPWM) algorithm in order to mitigate the line current THD and improve the effective power factor of the AC converter sides. The MHPWM is applicable for different nonlinear loads and can be implemented with APQU based on different topologies of H-bridge voltage source inverter. Simulation and practical results have been presented in this paper. The Experimental results are done based on real-time laboratory tests using dSPACE DS1103 board as a controller circuit. The presented results, under different operating loading conditions, show that the APQU provides almost unity power factor and significantly improving THD of the AC supply currents at both sides of the HVDC link controlled converters.

Operation and Challenges of Multi-Infeed LCC–HVDC System: Commutation Failure, AC/DC Power Flow, and Voltage Stability

This paper presents a detailed analysis of commutation failure, AC/DC power flow, and voltage stability of multi-infeed high-voltage direct current (HVDC). The use of HVDC power transmission technology has become common in modern power systems. During the past two decades, HVDC technology has been extensively used for long-distance bulk power transmission to remote areas. Throughout the world, the demand for power has drastically increased in recent years due to industrialization; such situations make HVDC an economic candidate because the distance between power generation plants and load areas is significantly very long. The line-commutated converter (LCC) technology-based HVDC system is well more mature than other available conversion schemes (i.e., voltage source converters), and it is widely used in high-power projects. China had approximately 50 HVDC–LCC links in 2020, and a single LCC-based link with the highest capacity is 12 GW. The installation of several HVDC links in an existing power network has led to a situation where two or more HVDC links terminate in the electric vicinity of each other’s AC network or even in same AC busbar. Such scenarios are termed multi-infeed HVDC system. Multi-infeed HVDC systems bring various challenges related to voltage stability, local and concurrent commutation failure, and AC/DC power flow. Here, the literature available on these phenomena of LCC-based HVDC is discussed for future research. The assumptions and drawbacks of various techniques used for investigating the mentioned phenomena are also highlighted.

Dynamic Modeling of HVDC for Power System Stability Assessment: A Review, Issues, and Recommendations

High-voltage direct current (HVDC) has received considerable attention due to several advantageous features such as minimum transmission losses, enhanced stability, and control operation. An appropriate model of HVDC is necessary to assess the operating conditions as well as to analyze the transient and steady-state stabilities integrated with the AC networks. Nevertheless, the construction of an HVDC model is challenging due to the high computational cost, which needs huge ranges of modeling experience. Therefore, advanced dynamic modeling of HVDC is necessary to improve stability with minimum power loss. This paper presents a comprehensive review of the various dynamic modeling of the HVDC transmission system. In line with this matter, an in-depth investigation of various HVDC mathematical models is carried out including average-value modeling (AVM), voltage source converter (VSC), and line-commutated converter (LCC). Moreover, numerous stability assessment models of HVDC are outlined with regard to stability improvement models, current-source system stability, HVDC link stability, and steady-state rotor angle stability. In addition, the various control schemes of LCC-HVDC systems and modular multilevel converter- multi-terminal direct current (MMC-MTDC) are highlighted. This paper also identifies the key issues, the problems of the existing HVDC models as well as providing some selective suggestions for future improvement. All the highlighted insights in this review will hopefully lead to increased efforts toward the enhancement of the modeling for the HVDC system.

Compensation method for parallel real-time EMT studies✰

The classical solution for computing electromagnetic transients (EMTs) in parallel relies on the propagation delay of transmission lines. The lines are used as decoupling elements to split the network into different tasks. When there is no natural delay or the delay is too short for the selected simulation time-step, other techniques have to be considered. This paper presents one of them. The compensation method is used in this paper for network decoupling for parallelization. A detailed implementation of this method is presented for real-time simulation. Performances are assessed in both offline and real-time environments with distribution and High Voltage Direct Current (HVDC) network test cases. Switching cases are also studied with HVDC power electronics devices. A Hardware-In-The-Loop (HIL) setup for an HVDC link in operation is also considered to validate the proposed method in a real-time environment.