Weak AC System Research Articles

Research on continuous commutation failure suppression strategy for multi-feed DC transmission system considering harmonic characteristic factors

Abstract Regarding the sustained issues of commutation failures in the hybrid double-fed DC system encompassing Line Commutated Converter High Voltage Direct Current (LCC-HVDC) and Modular Multilevel Converter High Voltage Direct Current (MMC-HVDC, from the perspective of harmonics, the harmonics in the voltage waveform of the AC bus on the inverter side are analyzed. Besides, a commutation failure suppression strategy for the hybrid doubly-fed DC transmission system based on the minimum shutdown area is designed, in which the setting value of the shutdown angle of the harmonic is considered in the minimum shutdown area. The reactive power is obtained by attaching the setting value to the reactive power control link of MMC-HVDC, and the simulation analysis in PSCAD verifies that the strategy can make the MMC-HVDC output more reactive power during the fault in the weak AC system. Increasing the shut-off margin of the manifold reduces the potential for continuous commutation failures in LCC-HVDC.

A simplified control parameter optimisation method of the hybrid modular multilevel converter in the medium‐voltage DC distribution network for improved stability under a weak AC system

AbstractTo improve the stability of the hybrid modular multilevel converter (MMC), a simplified dominant mode‐based control parameter optimisation method of the hybrid MMC system is proposed. Firstly, in the medium‐voltage DC distribution network, the small‐signal model of the hybrid MMC is established. Secondly, the influence of a weak AC system on stability is analysed through eigenvalue analysis. Finally, a simplified objective function is designed for eigenvalues of the dominant mode by considering only real parts, and improved small‐signal stability can be achieved by control parameters optimisation. The proposed method optimises all control parameters at the same time, which further reduces the number of algorithm iterations. Simulation results show that by the proposed control parameter optimisation method, the hybrid MMC has better transient performance and reduced disturbance under SCR variation, indicating a significantly improved system stability, and the dynamic response time can be reduced.

Stable operating limits and improvement methods for hydropower and photovoltaic integration through MMC-HVDC systems

This paper addresses the critical need to determine the stable operating limit of modular multilevel converter-based high voltage direct current (MMC-HVDC) systems, particularly concerning the integration of extensive renewable energy sources. To achieve this, the steady-state mathematical model and state-space model of bundled hydropower and photovoltaic integration through MMC-HVDC systems are established. A novel methodology considering steady-state and small-signal stability constraints is proposed to compute the stable operating region of the system. The quantitative assessment reveals that diminishing AC system short-circuit capacities amplify restrictions from small-signal stability constraints, thereby reducing the system's stable operating region. Eigenvalue and participation factor analyses shed light on the pivotal factors affecting small-signal stability in weak AC systems. To expand the system's stable operating region, a supplementary frequency damping control strategy is proposed. The theoretical analysis and calculation results are validated by building a simulation model for the bundled hydropower and photovoltaic integration through MMC-HVDC systems in PSCAD/EMTDC.

Analysis and Dynamic Stabilization of a HighVoltage VSC-MTDC Grid with DC Power Flow Controller Connected to Extremely Weak AC Systems

Analysis and Dynamic Stabilization of a HighVoltage VSC-MTDC Grid with DC Power Flow Controller Connected to Extremely Weak AC Systems

GRID INTEGRATION OF VOLTAGE SOURCE CONVERTER USING POWER SYNCHRONIZATION CONTROL

Renewable Energy Sources (RES) are integrated with electric power systems around the world at an accelerated pace. Due to their intermittent nature, integrating RES into the power grid is a challenging task. Because of their good controllability and superior performance in weak AC grids, the three phase Voltage Source Converters (VSCs) are generally used for this purpose. The grid connected converter has to correctly detect the grid voltage signal during voltage distortion and unbalance condition. In this research work, a control method using power synchronization for grid integration of VSC is developed and simulated. In this control technique, the electricity fed to AC grid from VSC is controlled using the internal synchronization mechanism as like in conventional synchronous machine. Utilizing this type of control, the instability of VSC affected by a standard Phase Locked Loop during weak grid condition can be avoided. Moreover, a VSC terminal could support a weak AC system with strong voltage, just similar to an ordinary synchronous machine organizer. Simulations are done in Matlab/Simulink to validate the system performance of grid attached VSC with the Power Synchronization Control.

Research on Energy Dissipation Technology and AC Overvoltage Suppression of Sending End in UHVDC Transmission

For suppress the amplitude of overvoltage from HVDC sending terminal, an overvoltage suppression scheme based on the principle of controllable energy dissipation was proposed. The overvoltage mechanism of a weak AC system under the fault of the DC system was analyzed, and the method and operating principle of the CSREDD were proposed. Prototype of the develop CSREDD was tested, which proved correctness about the design. The controllable energy dissipation device has been successfully applied to the ±800kV Lu-Gu UHVDC transmission project. The operation results show that the controllable energy dissipation device can effectively suppress the AC overvoltage.

Stability Assessment of a High-Voltage DC Transmission System Using MIMO- and SISO-Based Impedances

Grid-connected inverters used in High-Voltage Direct Current (HVDC) transmission systems, which have become increasingly popular worldwide in recent years, employ a variety of control methods for each frequency band, and resonances can occur between multiple control devices within a device or between multiple devices placed in close proximity. Practical impedance-based data-driven methods, rather than linear approximation models, are becoming widely used for these stability analyses. On the other hand, HVDC transmission systems require multiple domain transformations between synchronous and stationary reference frames. Therefore, in this study, both Multiple-Input Multiple-Output (MIMO) analysis using impedances on the direct-quadrature (dq) synchronous frame and Single-Input Single-Output (SISO) analysis using positive and negative sequence impedances on the stationary reference frame were performed. Consequently, the two unstable frequencies identified in the eigenvalue trajectories for the dq impedance were also identified within a single Nyquist plot for the sequence domain impedance, validating both the unstable frequencies observed in the dq domain. These frequencies suggest that weak AC systems on the generation side of the HVDC transmission system may be causing instability in the DC transmission lines and AC systems on the demand side. Additionally, the results of this study also suggest that the introduction of an advanced inverter control method, called the Virtual Synchronous Generator (VSG) control method, in the grid-connected inverter on the receiving side may mitigate the effects of having weak AC systems on the generation side.

Adaptive integrated control strategy for MMC-MTDC transmission system considering dynamic frequency response and power sharing

Due to the lack of grid inertia caused by the increasing penetration of renewable energy sources, the ability of system to withstand disturbance is reduced. In attempt to address this challenge, an adaptive virtual inertial frequency control (AVIFC) strategy for multi-terminal dc (MMC-MTDC) transmission system considering dynamic frequency response is proposed. Based on the frequency variation characteristics in inertia response and primary response stages, the virtual inertia coefficient can be automatically adjusted. In the inertia response stage, considering the limitation of DC voltage, the virtual inertia coefficient adaptively decreases according to the frequency deviation to reduce the maximum frequency deviation. In the primary response stage, the virtual inertia coefficient decreases to 0 rapidly following the frequency deviation to speed up the frequency recovery. Furthermore, aiming at the problem of coordinated power sharing between converter stations of weak AC systems after disturbance, an adaptive droop control (ADC) strategy based on DC voltage deviation and converter station power margin is studied to realize the optimal distribution of unbalanced power so that DC voltage deviation is reduced, DC voltage regulation time is shortened and the stability of DC voltage is maintained at the same time. Finally, based on PSCAD / EMTDC simulation platform, the control model of a five-terminal MMC-MTDC system is established. Three working conditions, namely the load reduction, the load increment and the converter station out of operation, are selected to verify the effectiveness and feasibility of the proposed adaptive integrated control (AIC) strategy including AVIFC and ADC.

Influence of DC transmission distance on MMC-HVDC stability under weak AC system

The oscillation problem of MMC-HVDC under weak AC system poses a potential threat to the safe and stable operation of power grid, which is related to both of the AC and DC parameters. By comparing the stability of MMC-HVDC under different AC system strength with different DC transmission distance, it is found that when the AC system strength is too weak, there is a risk of instability under the condition of long DC transmission distance, and the correctness of this conclusion is verified by time-domain simulation. Later, the mechanism analysis of this phenomenon is carried out, the result shows that it is related to the interaction of MMC subsystems when the strength of AC system decreases.

Robust Decoupled Outer-Control Design for Single VSC and HVDC Grid With Controller Weak AC System Interconnection

With the increase in number of VSC based HVDC systems, it is inevitable that there would be interconnections with weak ac grids. The paper presents the analytical basis for identifying coupling in the power loops which is dependent on grid inductance and operating point by utilizing the impedance based modeling technique. The formulation is used to define an inverse-based decoupling outer-loop controller for VSC-HVDC system. The concept is extended to HVDC grids and it is shown that the gains obtained using simple inverse may lead to instability. Robust controller tuning with pole-placement and minimization of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_\infty$</tex-math></inline-formula> norm is presented for HVDC grids, which form a restricted feedback system. The results show that the proposed method of decoupling results in effective decoupling of the power loops over a large range of grid inductance values as compared to that with traditional vector control. The system stability is also maintained over a large range of grid inductance values and PLL gains when a sudden variation of parameters is considered. The stability margins can be enhanced by pole-placement. The observations from the small-signal models are confirmed using detailed time-domain simulations. The proposal is expected to be useful in controller design for single VSC as well as HVDC grids, especially, during planing stage.

Comprehensive small-signal stability analysis for master hybrid MMC connected with strong and weak AC systems

The small-signal stability problem of power electronic converters has always been a critical concern for safe operation. This paper conducts a comprehensive analysis of the small-signal stability for the hybrid modular multilevel converter (MMC), which is promising in the application of future over-head-line high-voltage-direct-current transmission. The impacts of the strong and weak AC system connection are comparatively researched through eigenvalue-based analysis. The newly emerging factors that hamper the stability are identified for the weak AC system integration. The work in this paper provides valuable guidance for robust control parameter configuration of the hybrid MMC.

Impact of Strength and Proximity of Receiving AC Systems on Cascaded LCC-MMC Hybrid HVDC System

The cascaded hybrid LCC-MMC inverter is considered as a feasible option to mitigate the commutation failure problem of conventional LCC-HVDC while maintaining bulk power transmission capability. However, the coupling between different types of converters may result in instability when the inverter integrates with weak AC systems. This paper establishes a small-signal model of the cascaded LCC-MMC inverter based hybrid HVDC system and validates the model against PSCAD/EMTDC. Through eigenvalue analysis, the impact of the strength and proximity of receiving AC systems on the small-signal stability is investigated, and the stable zone of the strength of receiving AC systems is identified. Besides, a supplementary coupling mitigation control is proposed to tackle the instability problem under weak AC systems integration. Both analytical results and time-domain simulations demonstrate the validity of the supplementary control.

The harmonic characteristics analysis of offshore wind farms transmitted by the submarine cable based on time domain simulation

The harmonic amplitude may exceed the limit when the large scale of wind farm is connected to the weak AC system through AC submarine cable. The harmonic with high amplitude will endanger the safety of power equipment, and even affect the normal integration of large-scale wind farm. This paper takes an actual offshore wind power connected to the system through AC submarine cable as an example. On the basis of building the model of the system in the PSCAD/EMTDC simulation software. The simulation calculation is systematically carried out. The effect of multiple factors on the harmonic characteristics is analyzed. The preliminary explanation for the harmonic amplitude out of limit is given for the actual large scale of wind farm connected to the weak AC system through AC submarine cable.

Novel grid‐forming control of PMSG‐based wind turbine for integrating weak AC grid without sacrificing maximum power point tracking

Traditional permanent magnet synchronous generator (PMSG)-based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG-based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid-side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC-link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC-link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small-signal analysis of the proposed scheme indicates the DC-link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non-linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid-forming control.

Research on optimizing controller parameters based on transient energy function to improve the stability of weak transmission system

This paper takes the Ximeng-Taizhou UHVDC transmission weak transmission end AC system as the background, and proposes to optimize the low-voltage current limiting control link (VDCOL )to improve the stability of the system. First of all, this paper analyzes the working principle and limitations of traditional VDCOL, and proposes that the short-circuit ratio of the sending end and the voltage of the commutation bus at the receiving end are used as the variables of the VDCOL cut-off voltage to achieve the effect of optimizing VDCOL. Make the cut-off voltage a value that changes with the change of the inverter side commutation bus voltage, which can take into account the recovery speed of the AC and DC systems. The process of constructing the transient energy function considering the dynamic characteristics of the DC system is analyzed in detail, and the real-time digital simulation software RTDS is used to build a single-machine infinite-DC simulation example. Based on time-domain simulation software and transient energy function margin method, the optimization of the low-voltage current-limiting control link in the DC control system is verified to improve the stability of the sending-end AC system.

Control Strategy Optimization of Voltage Source Converter Connected to Various Types of AC Systems

Connecting the voltage source converters (VSCs) to various types of AC systems results in different operation characteristics and core problems associated with traditional control strategies. Therefore, it is necessary to optimize the control strategies of the VSCs according to the types of AC systems. For the VSCs connected to islanded renewable power plants, a voltage/frequency (V/f) droop control strategy is proposed to damp fluctuations of AC voltage and frequency in the island, which is vital for bipolar VSC control. In addition, a multi-branch impedance equivalent method for renewable power plants is proposed, with which large-scale renewable power plants can be modeled accurately in the frequency domain to prevent wide-band oscillation. For the VSCs connected to strong AC systems, smart AC voltage and coordinated frequency transient control strategies are proposed, which can improve AC system transient stability. For the VSCs connected to weak AC systems, the relationship between the system stability and strength is analyzed, and then the control strategy of inner-loop control parameter optimization and outer-loop power limiting (if necessary) is proposed to improve the stability of the allied system. The proposed strategies are verified by both software simulation and field commissioning.

Grid side reactive power support strategy for MMC-HVDC connected to the wind farms based on unloading resistor

In recent years, VSC-HVDC has been widely used in windfarm integration owing to its ability of connecting the weak AC system. And the hybrid modular multilevel converter based HVDC(MMC-HVDC) has attracted more attention because of its DC fault blocking capability. However, most of the researches about MMC-HVDC connected to windfarm are focused on frequency response and wind farm fault ride through, without assisting AC system to maintain the voltage via the converter's reactive power control capacity. Based on this, this paper proposes a mode switch control and reactive power support control scheme for the grid connected inverter connected to windfarms. The switch mode control scheme provides three operation modes for MMC to adapt to different operation situations. The reactive power support coordination control scheme is an advanced control scheme for cooperating converter, unloading resistor and wind farm. With the cooperating control, the PCC voltage sag due to the fault is alleviated while reducing the impact of grid failures on the wind farm. Finally, the presented control strategy is simulated in PSCAD/ EMTDC, which verifies the effectiveness of the scheme proposed.

Coordinated frequency regulation among asynchronous AC grids with an MTDC system

With the increasing penetration level of renewable power, the power systems’ inertias are decreasing. To ensure that the frequency is above the threshold of the under frequency load shedding, one effective way is to share the spinning reserves among asynchronous AC grids through HVDC systems. In this paper, a coordinated frequency regulation scheme that enables spinning reserves sharing is proposed for such hybrid AC/DC grids. This scheme is a communication-free method that consists of the droop control, the inertia emulation control and the frequency safety control. The combined response of the three controls can increase not only the frequency nadir but also the settled frequency of the disturbed grid. In addition, the hysteresis comparators and the power injection limits are adopted in the frequency regulation scheme to reduce the coupling effect among AC grids. Thus, under small and harmless disturbance, the frequency regulation scheme will be inactive; the disturbance in strong AC grids would not affect the weak AC systems. The frequency regulation scheme coordinates well with the automatic generation control of the grids. The feasibility and effectiveness of the proposed frequency regulation scheme are verified through simulation studies.

Comparative Study of Small-Signal Stability Under Weak AC System Integration for Different VSCs

Voltage source converters (VSCs) with self-commutation ability are suitable for interconnecting weak ac systems. This article conducts a comparative study of the small-signal stability characteristics for three typical VSCs, namely, the two-level VSCs (TL-VSCs), the half-bridge modular multilevel converters (HB-MMCs), and the hybrid MMCs (HY-MMCs), under weak ac system integration with special consideration on both inverter and rectifier operation. The frequency responses based on impedance models are compared using the frequency-domain analysis. The oscillation frequencies and mainly participated state variables of the unstable modes are compared using root locus and participation factor analysis in the time domain. Proper parameter retuning approaches for stability enhancement are proposed. The above analysis is adequately validated by electromagnetic simulations.

Analysis and Improvement of the Multiple Controller Interaction in LCC-HVDC for Mitigating Repetitive Commutation Failure

In a line-commutated converter based high-voltage direct-current (LCC-HVDC) system, the inverter control system usually includes constant extinction angle (CEA) controller, constant current (CC) controller, current error controller (CEC), and voltage-dependent current order limiter (VDCOL). However, the interaction influence of multiple controllers may cause repetitive commutation failure (CF). For this purpose, this paper deeply investigates the mechanism and the influencing factors of repetitive CF caused by controller interaction. It is found that the lower limit value of CEA controller can affect the control switching process and thus the probability of repetitive CF. Besides, the minimum gamma measurement (MGM) unit prevents the system from acquiring the real extinction angle in time, which results in the DC current fluctuation during the recovery process. Moreover, the probability of control-induced repetitive CF is much higher when the HVDC inverter is connected to a weak AC system or the AC fault is not serious but not cleared in time. Based on the above analyses, an improved control method is proposed for recovery performance improvement, which is mainly realized by modifying the lower and upper limits of the CEA controller during recovery. Finally, the simulation and experimental results verify the theoretical analyses and the proposed control method.