Voltage Source Converter HVDC Research Articles

Investigating Small Disturbance Stability in Wind Farm Grid-Connected Systems: A Data-Driven Approach

The presence of unknown synchronization characteristics, unclear instability mechanism, and various fault mode evolution laws, lacking corresponding theoretical support and analysis methods and instability criteria, are defined with clear physical concepts. It is still impossible to systematically understand the transient synchronization mechanism of the wind power grid-connected system from the perspective of the whole fault stage. Therefore, this study uniformly reveals its temporary synchronous stability problem and proposes a large/small disturbance adaptive synchronous stability control method, which improves the dynamic characteristics of the wind turbine through the control of the inverter itself to improve the system stability—using different scenarios, such as single doubly-fed wind turbines. The experimental results show that the small disturbance on the AC side significantly impacts the system characteristics, followed by a bit of annoyance on the DC side. The DC side fault will cause a change in system frequency characteristics, especially at the receiving end. However, compared with the Voltage Source Converters-High Voltage Direct Current (VSC-HVDC)system, Modular Multilevel Converters-High Voltage Direct Current (MMC-HVDC) systems operate at a much higher frequency and produce less low-frequency harmonics. This makes them less likely to induce subsynchronous oscillations in the system.

Analysis of Onshore Synthetic Inertia and Primary Control Reserve Contributions of Alternating Current-Side Meshed Offshore Grids with Voltage-Source Converter and Diode Rectifier Unit High-Voltage Direct Current Connections

The increasing use of renewable energy sources in place of conventional generation units is leading to a reduction in onshore inertia and to the development of offshore wind park grids connected by multiple high-voltage direct current (HVDC) connections to the onshore alternating current (AC) grid. For AC-side meshed offshore grids with voltage-source converter (VSC) and diode rectifier unit (DRU) HVDC connections towards onshore grids, this study focuses on the energetic feasibility of synthetic inertia (SI) and primary control reserve (PCR) contributions triggered locally at the onshore converters of both connection types. To this end, the obstacles preventing contributions for VSC HVDC connections and the mechanisms allowing contributions for DRU HVDC connections are identified first. Based on these findings, the article proposes an enhancement of the offshore HVDC converter controls that is continuously active and allows locally triggered onshore contributions at all onshore HVDC converters of both connection types without using communication and requiring only minimal system knowledge. Additional simulations confirm that, although the enhancement is continuously active, the operational performance of the offshore HVDC converter controls for normal offshore grid operation and its robustness against offshore AC-side faults are not affected.

Modular multilevel converter‐based voltage source converter high‐voltage direct current valve operating test circuit

AbstractIt is very important to obtain high reliability in the high‐voltage direct current (HVDC) system operation because of extremely high and huge power capacity. To guarantee the reliability IEC‐62501 standard requires valve test for the voltage source converter (VSC) HVDC system. This paper proposes a test circuit for modular multilevel converter (MMC)‐based HVDC valve. The proposed scheme has good features of adequate test current generation, considerably reduced power supply voltage, simplified test circuit topology, etc., compared to the conventional test circuits. The validity of the proposed scheme is verified through experimental results.

The VSC-HVDC Transmission System Performance Assessment

As the use of renewable resources increases the transfer of power from remote area to consumer is a big challenge. Therefore, Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission is gaining more popularity for integration of renewable energy generators to the consumers using smart grid. Even after many advantages of HVDC transmission system it is not used popularly due to unexpected instability in the system. However recent developments in semiconductor devices and control techniques the HVDC link operation in transmission system becomes easier. In this paper VSC based HVDC system is designed with AC-DC converter model for power flow assessment of both sides of HVDC link. To analyze the proposed model a simulation model of 75km DC line with voltage source converter (VSC) based AC-DC converter model on both side of DC line is designed in a MATLAB environment. HVDC link efficiency is calculated by analyzing power flow through AC lines at both ends of HVDC line.

A Modular DC-DC Converter as a Hybrid Interlink Between Monopolar VSC and Bipolar LCC-Based HVDC Links and Fault Management

High voltage dc (HVdc) transmission has gradually gained its popularity in long distance transmission of power. The possibility of multi-terminal HVdc network has further projected HVDC as a potential mode of power trading across geographical boundaries. The emergence of modular multilevel converter (MMC)-based dc-dc converters has led to the interconnection of HVdc networks across voltage levels, converter technologies and line topologies. This paper puts forward a topological modification in an existing MMC-based dc-dc converter topology, acting as a hybrid interconnect between different converter technologies, for interface of an asymmetric monopolar voltage source converter (VSC) and a bipolar line commutated converter (LCC)-based HVdc links. The proposed operation of the topology is validated experimentally in a down-scaled laboratory prototype. The proposed topology is further investigated for short-circuit fault conditions in any of the poles of HVdc bus and appropriate fault mitigation strategies are adopted for short-cicuit fault in both LCC and VSC bus. The fault mitigation strategies are validated by simulation in Matlab/Simulink platform.

Control strategy used for AC fault ride-through in VSC–HVDC transmission systems

Fault ride-through (FRT) is a very important requirement for power grids based on voltage source converters (VSCs) to improve its operational availability of the AC grid. Grid codes require VSC stations to incorporate fault handling capabilities to prevent disconnection of converter stations from the alternating current (AC) grid for certain fault characteristics. In this paper, an FRT strategy is used in the two converter stations to handle the faults that may occur in the AC part of the grids and ensure their stability. The method improves the performance of the FRT compliance strategy during a voltage drop in the AC side. In addition, a current control that operates simultaneously on positive and negative sequence current is adopted. It also allows the injection of a reactive current to maintain the availability of the high-voltage direct current (HVDC) converter station during a fault. The performance and stability of the suggested FRT control is tested considering the direction of power flow with possible faults in balanced and unbalanced regime. For the validation of the efficiency of the FRT capacity, simulation tests are carried out with MATLAB-Simulink software, and experimental tests with a Smart-grid test bench.

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).

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.

The Impact of Integration of the VSC-HVDC Connected Offshore Wind Farm on Torsional Vibrations of Steam Turbine Generators

For remote offshore wind farms, transmitting power to the main onshore grid via a Voltage Source Converter High Voltage Direct Current (VSC-HVDC) system is the mainstream of power transmission. It is not only cost-effective in long-distance transmission, but also can fully meet the grid side requirements such as black start, voltage support, fault ride through and frequency support. However, it still has some problems, such as the possible impact on the power grid needing to be paid attention to. In this paper, its impact on the torsional responses of turbine generator units neighboring to the onshore side of AC bus is studied by using the DIgSILENT PowerFactory software. It is found that the effects of the Sub-Synchronous Torsional Interaction (SSTI) with onshore controls and the generator de-rating operations can significantly affect the damping ratio of turbine torsional modes, whereas the effects of the machine configurations and the amount of wind farm power integrated can affect the electrical torque disturbance. The most noteworthy is that their effects can be superimposed on each other if these factors act simultaneously, which would lead to increased vibrations and reduce the turbine shaft’ s life. The findings will be helpful for avoiding accidents caused by torsional vibrations when it is going to integrate a VSC-HVDC connected wind farm into a power grid.

Distributed Optimal Power Flow for VSC-MTDC Meshed AC/DC Grids Using ALADIN

The increasing application of voltage source converter (VSC) high voltage direct current (VSC-HVDC) technology in power grids has raised the importance of incorporating DC grids and converters into the existing transmission network. This poses significant challenges in dealing with the resulting optimal power flow (OPF) problem. In this paper, a recently proposed nonconvex distributed optimization algorithm -- Augmented Lagrangian based Alternating Direction Inexact Newton method (ALADIN), is tailored to solve the nonconvex AC/DC OPF problem for emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid systems. The proposed scheme decomposes this AC/DC hybrid OPF problem and handles it in a fully distributed way. Compared to the existing state-of-art Alternating Direction Method of Multipliers(ADMM), which is in general, not applicable for nonconvex problems, ALADIN has a theoretical convergence guarantee. Applying these two approaches to (VSC-MTDC) coupled with an IEEE benchmark AC power system illustrates that the tailored ALADIN outperforms ADMM in convergence speed and numerical robustness.

Large-Signal Stability Analysis of DC Side of VSC-HVDC System Based on Estimation of Domain of Attraction

The existing research on voltage-source converter HVDC (VSC-HVDC) system stability mostly focuses on small-signal stability analysis methods. However, when large-signal disturbances occur, the intrinsic nonlinearity of the VSC-HVDC system becomes inevitable so that the small-signal studies are no longer valid. In this paper, a large-signal stability analysis method of the VSC-HVDC system is proposed based on the estimation of domain of attraction (DOA). This method can not only analyze the asymptotic stability of the operating point, but also answer the question of how large deviations from the operating point can be tolerated by the system. By using Takagi-Sugeno (T-S) fuzzy model and Lyapunov stability theorem, the DOA of the VSC-HVDC System at the operating point is estimated, and the large-signal stability of VSC-HVDC System is analyzed. On this foundation, the influence of system parameters on the large-signal stability is studied. The results can be used to guide the adjustment of system parameters to solve the problem of system instability. The time-domain simulations conducted in Matlab/Simulink are used to test the stability of the system under large-signal disturbance in the range of DOA. The simulation results verify the feasibility and effectiveness of the proposed method.

Practical Fault Current Level Evaluation and Limiting Method of Bipolar HVdc Grid Based on Topology Optimization

This article investigates the relationships between the pole-to-pole fault current and the topology in the bipolar voltage sourced converter (VSC)–high-voltage dc (HVdc) grid. Based on the improved high-frequency equivalent model, it is found that the pole-to-pole fault current value is significantly influenced by the grid topologies, which is different from the situation in symmetrical monopole dc grid. It is also found that only the adjacent converters and the subadjacent converters of the fault lines will impact the fault currents, while the other converters with longer electrical distances basically have negligible contribution to the fault current. According to above mechanism, the simplified index is also proposed in this article to evaluate the fault current level of dc grid efficiently when its topology varies. The proposed index can make evaluations by simple calculations between different topologies and avoid solving complex differential equations one by one when pole-to-pole fault occurs at different lines. Based on the practical evaluation index, the genetic algorithm is used to optimize the dc grid topology to limit the fault current level. Finally, the simulation verification is performed based on the six-terminal bipolar VSC–HVdc grids, and some suggestions on topology design are given.

Decentralized Cooperative Active Power Control for Small-Scale Grids with High Renewable Penetration through VSC-HVDC

Rising renewable penetration has accelerated the volatility and instability of the power grid. A small-scale grid is especially vulnerable. Therefore, flexibility and stability enhancement are required for small-scale grids. The interconnection with the large-scale grid through the voltage sourced converter-high voltage direct current (VSC-HVDC) can be a solution to the aforementioned problems. VSC-HVDC can deliver power bidirectionally and change the direction in a short time. Hereby, the cooperative operation of distributed generations (DG) and a large-scale grid through the VSC-HVDC system is proposed in this paper. The VSC-HVDC will take the role of the main source of the small-scale grid. It determines the grid frequency based on its output power. DGs adjust their output power according to the grid frequency, and then the balance between the demand and the supply is maintained. To verify it, a PSCAD/EMTDC simulation was conducted using actual data from Jeju Island, including transmission lines, loads, and climate. Consequently, by the proposed method, the RE share was improved and the grid was operated stably even though the fault situations.

Current limiting characteristics of a resistance-inductance type superconducting fault current limiter

The superconducting fault current limiter (SFCL) can effectively and quickly limit the fault current, which is appropriate for the voltage source converter high voltage direct current (VSC-HVDC) transmission system to reduce the fault current. A new resistance-inductance type SFCL using the structure of the inductive superconducting coils is presented. The current limiting and inter-turn insulation characteristics of resistance-inductance type SFCL are studied. A simulation model is developed for the comparison of inductive and non-inductive type superconducting coils. The simulation results show that the inductive superconducting coils can significantly reduce the fault current rise rate by more than 42% over the non-inductive superconducting coils. The voltage distribution of the inter-turn channels is calculated by the presented method.Utilizing the voltage distribution, the electric field distribution can be obtained, which is important for the insulation design. Some inductive type pancake superconducting coils were fabricated, and the DC impulse current tests were carried out. The testing results and simulated results have a good agreement, which shows that the inductive type SFCL can effectively limit the fault current.

Performance Enhancement of Doubly Fed Induction Generator–Based Wind Farms With STATCOM in Faulty HVDC Grids

In this study, an investigation of different faults for a wind turbine–based doubly fed induction generator (DFIG) system is studied and the performance using a static compensator (STATCOM) is observed. The DFIG network is connected to a voltage source converter high-voltage dc link with a fault occurring near the wind generator network. The ride through capability of DFIG is promising with STATCOM using the proposed control strategy. The ac and dc voltage and torque oscillations are damped effectively, and improved power flow is observed. The low voltage AC grid fault occurs for an HVDC transmission, and the DFIG performance without and with STATCOM is compared, where the DFIG converter control schemes are developed using the proposed improved field-oriented control (IFOC) method. In this, the reference rotor flux value alters to a new synchronous speed value or a slighter value or a standstill depending on the stator voltage dip due to grid disturbance. This speed variation leads to introducing rotor current at that new rotor slip frequency as there is a change in the rotor speed because of the fault, which further decreases the stator flux dc component. Hence, this dc-offset constituent in the stator flux is alleviated and decays rapidly in scheming the divergence of the speed of the rotor to a new orientation speed with decay in the rotor flux. This operation is done in the inner control scheme of the rotor converter, which is quicker in response to the faults. Apart from this, the stator’s real and reactive power also changes accordingly based on the lookup table mechanism–based closed-loop control action of the pulse generator, and this power change is done in the outer loop. The analysis for DFIG and HVDC operation is verified under different faults without and with STATCOM.

Improved Dynamic Power Flow Model with Frequency Regulation by DFIG Integrated through VSC-HVDC Considering Governor Delay of SG

The doubly-fed induction generators (DFIGs) integrated to the grid through the voltage source converter-high voltage direct current (VSC-HVDC), the cascaded droop control from the system frequency to the DC voltage, then to the active output of the DFIG, was applied to enhance the frequency regulation capability of the power system. The improved dynamic power flow (DPF) model was newly proposed to quantify the frequency response of the coordinated regulation with the inertia of the VSC-HVDC and the DFIGs, and the primary regulation of the synchronous generators (SGs) and the DFIGs. New features of the proposed model include: (i) the SGs’ output in the DPF considering the governor delay, (ii) setting of the virtual inertia of the VSC-HVDC within the DC voltage constraint, and (iii) variable inertia of the DFIGs following changing the kinetic energy of the rotor. The numerical results show the feasibility of the proposed model, and validate the regulation effect and accuracy of the modified inertia of the DFIGs and the maximized virtual inertia of the VSC-HVDC.

Fault protection of multiterminal HVDC networks: Impact of inductance

Multiterminal Voltage Source Converter (VSC) High Voltage Direct Current (HVDC) networks are developing rapidly worldwide, especially in China with urgent needs of long-distance power transmission. Currently, the interrupting capacity of available DC circuit breakers (DCCBs) is a key obstacle to the development of multiterminal VSC HVDC networks. Superconducting fault current limiters (SFCLs) can compensate for the limitation of DCCBs. Conventionally, a resistive SFCL is regarded as essential and sufficient, while an inductive one is less related and optional. However, pure resistance cannot effectively suppress the rising rate of fault current at the very early stage of a fault, resulting in unnecessary blocking of the converter. Adequate inductance is required for such SFCLs to curb rapidly increasing fault currents within the first few milliseconds, which is critical to DCCBs to isolate fault without interrupting other DC transmission lines. In this paper, we compared and analyzed the performances of SFCLs with different resistances and inductances in a four-terminal VSC HVDC network using PSCAD/EMTDC software. The results indicate that even resistance and inductance show different current limiting effects, but they are both necessary to realize effective SFCL function for multiterminal VSC HVDC networks. The outcomes of this paper are of significance in FCL design for multiterminal VSC HVDC networks.

Probabilistic load flow calculation of AC/DC hybrid system based on cumulant method

The operating conditions of the power system have become more complex and changeable. This paper proposes a probabilistic load flow based on the cumulant method (PLF-CM) for the voltage sourced converter high voltage direct current (VSC-HVDC) hybrid system containing photovoltaic grid-connected systems. Firstly, the corresponding control mode is set for the converter, including droop control and master–slave control. The unified iterative method is used to calculate the conventional AC/DC flow. Secondly, on the basis of the probability model of load and photovoltaic output, based on the aforementioned flow results, use correlation coefficient matrix of this paper will change the relevant sample into independent sample, the cumulants of the load and photovoltaic output are obtained; Then, the probability density function (PDF) and cumulative distribution function (CDF) of state variables are obtained by using Gram-Charlie series expansion method. Finally, the mean value and standard deviation of node voltage and line power are calculated on the modified IEEE 34-bus and IEEE 57-bus transmission systems. The algorithm can reflect the inherent uncertainty of new energy sources, and replace the complex convolution operation, greatly improving the calculation speed and the convergence.

Single-End Based Fault Location Method for VSC-HVDC Transmission Systems

A single-end based fault location method is proposed for VSC (Voltage Sourced Converter) HVDC (High Voltage Direct Current) transmission systems. With the proposed method, there is no need for communication system to collect data from different locations and the need for checking data synchronization is avoided. The proposed method is derived from the relationship between the aerial and ground voltage components along the DC transmission system. Bergeron model equations are used for calculating the 1-mode and 0-mode voltages along the transmission system using the measured signals at a single end. The proposed method is generalized for locating both the pole-to-ground and pole-to-pole faults. The profile of the differences between the voltage components is studied with considering the effect of the fault resistance. Artificial neural network (ANN) is trained based on collected samples from different test cases. The proposed single-end based fault location method can successfully locate the fault and determine the fault resistance even with uncertain line parameters. The proposed method is theoretically validated by the help of PSCAD simulation platform. In addition, the effectiveness of the proposed method is verified based on real time measurements obtained from RTDS system.

Modeling and design of an adaptive control for VSC-HVDC system under parameters uncertainties

The growing demand for electricity and the increasing integration of clean energies into the electrical grids requires the multiplication and reinforcement of high-voltage direct current (HVDC) projects throughout the world and demonstrates the interest in this electricity transmission technology. The transmitting system of the voltage source converter-high-voltage direct current (VSC-HVDC) consists primarily of two converter stations that are connected by a dc cable. In this paper, a nonlinear control based on the backstepping approach is proposed to improve the dynamic performance of a VSC-HVDC transmission system, these transport systems are characterized by different complexities such as parametric uncertainties, coupled state variables, neglected dynamics, presents a very interesting research topic. Our contribution through adaptive control based on the backstepping approach allows regulating the direct current (DC) bus voltage and the active and reactive powers of the converter stations. Finally, the validity of the proposed control has been verified under various operating conditions by simulation in the MATLAB/Simulink environment.