Weak AC Grid Conditions Research Articles

A Derivative-Feedback Based Compensation Control to Enhance the Small-Signal Stability for LCC-HVDC System

The small-signal stability of the LCC-HVDC system connected to the weak AC grid is of great concern. This paper derives the SISO model of the LCC inverter station. On this basis, the influence mechanism of AC system strength on system stability is studied via the Bode stability criterion, and the coupling interaction between different control loops is quantitatively evaluated. Results reveal that, as the AC grid becomes weaker, the system stability margin is diminished characterized by the larger phase lag in the phase-frequency response. Also, the interaction between DC voltage controller (DVC) and phase locked loop (PLL) control loops is intensified. Then, a derivative-feedback based compensation (DFC) control is presented to improve the LCC system stability under the weak AC grid condition. Theoretical analysis and simulation results indicate that the presented DFC control can effectively improve the stability and dynamic performances of the LCC system, which allows the system to operate stably even connected to the extremely weak AC grid.

Power coupling analysis and improved decoupling control for the VSC connected to a weak AC grid

Conventional vector control (VC) is widely used in the voltage source converter (VSC) due to its simplicity and power decoupling ability. However, this paper reveals that the VC may result in strong active and reactive power coupling under weak AC grid conditions. The linearized relationship between the power and dq-axis currents is built. On this basis, the power coupling mechanism is revealed, and the power coupling process is presented. Finally, an improved decoupled power control is accordingly proposed based on the feedforward compensation method. The improved decoupling control method can realize more accurate decoupled active and reactive power regulation. Moreover, its design is simple, and thus is easy to be implemented in practice. The power coupling characteristics analysis and the improved power decoupling control are verified through PSCAD/EMTDC simulations and experiments.

Optimization and Configuration of Control Parameters to Enhance Small-signal Stability of Hybrid LCC-MMC HVDC System

This paper investigates the small-signal stability of the hybrid high-voltage direct current (HVDC) transmission system. The system is composed of line commutated converter (LCC) as rectifier and modular multi-level converter (MMC) as inverter under weak AC grid condition. Firstly, the impact of short-circuit ratio (SCR) at inverter side on the system stability is investigated by eigen-analysis, and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis. Then, considering the quadratic index and damping ratio characteristic, an objective function for evaluating the system stability is developed, and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method. The eigenvalue results and the electromagnetic transient (EMT) simulation results show that, with the optimized control parameters, the small-signal stability and the dynamic responses of the hybrid system are greatly improved, and the hybrid system can even operate under weak AC grid condition.

Control and stability analysis of DC microgrid system including wind and solar generation sources and grid‐connected voltage source converter

SummaryThe study system in this paper is a grid‐connected DC microgrid system comprising PV and wind generation sources, resistive and constant power loads (CPLs), and grid‐connected VSC. The VSC keeps the DC link voltage constant, and the wind and PV sources operate in maximum power point tracking (MPPT) mode, and thus, the DC–DC converters related to these sources act such as controlled current sources behaving similar to negative CPLs. The study system suffers from the stability under high CPL penetration and weak AC grid conditions. Hence, this paper investigates and enhances the system stability under both high CPL penetration and weak AC grid conditions. To do this, modal and small‐signal stability analysis of the system is carried out, and then, as the main findings, impacts of the wind and PV sources output powers, CPL power, and AC grid short‐circuit power on the system stability are studied. Next, as the main contribution, VSC control structure is modified by adding stabilizing signals to enhance the system stability under high CPL penetration, reduced output powers of the wind and PV sources, and weak AC grid conditions. At the end, several simulation results verify the theoretical results at different conditions.

Influence of Inverter Controller Parameters on the Small-signal Stability of LCC-HVDC System Based on a Practical Project

Unreasonable control parameters of the Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system may induce small-signal instability. This paper studies the impact of inverter controller parameters on steady response of the LCC-HVDC system under weak AC grid condition. Firstly, according to a practical project, the small-signal model of LCC-HVDC system is established based on the switching function. Then, the eigen-analysis method is adopted to study the impact of the controller parameters on the inverter side on the oscillation mode and damping characteristics of the LCC system, and the correctness of results is verified by PSCAD/EMTDC simulation. The conclusion shows that reasonable controller parameters can improve the stability margin of the system.

Small-Signal Stability Enhancement Approach for VSC-HVDC System Under Weak AC Grid Conditions Based on Single-Input Single-Output Transfer Function Model

The voltage source converter (VSC) system has stability issue under weak AC grid conditions. In this paper, the multi-input multi-output (MIMO) transfer function model of VSC system in frequency-domain is derived. Then, the individual channel analysis and design (ICAD) approach is employed to develop the equivalent single-input single-output (SISO) feedback control model for stability analysis. On that basis, the stability of VSC system and the coupling interaction intensity among the multiple control loops, including the active power control (APC), reactive power control (RPC) and phase-locked-loop (PLL) are investigated. The results show that larger PLL bandwidth increases the coupling degree of active power control (APC) and reactive power control (RPC) closed loops, resulting in the instability of the system under weak AC grid conditions. Finally, a derivative feedback compensation (DFC) approach is proposed to enhance the stability margin for VSC system connected to weak AC grid. The results based on MATLAB-based theoretical analysis and electromagnetic transient (EMT) simulations show that the proposed DFC approach can effectively suppress the instability of VSC system due to the close coupling of APC and RPC closed loops caused by the large PLL bandwidth, thus effectively improve the stability margin of the system.

Comparison study of small-signal stability of MMC-HVDC system in different control modes

The control mode adopted by a modular multilevel converter (MMC) in an HVDC transmission system directly affects both dynamic and steady-state characteristics of the system. This paper, benefiting from a comprehensive dynamic-phasor based state-space model of MMC, presents a comparative analysis on operational stability of the MMC-HVDC station in PV-control mode (i.e., in active-power and AC-voltage-control mode) and PQ-control mode (i.e., in active- and reactive-power-control mode). Firstly, corresponding to the MMC rectifier /inverter in PV- and PQ-control modes, four linearized small-signal models are developed in MATLAB, and validated by electromagnetic transient (EMT) simulations in PSCAD/EMTDC. Then, based on eigen-analysis, the impact of AC-system strength, quantified by the short-circuit-ratio (SCR) and impedance-angle, on the small-signal stability for a MMC rectifier /inverter under PV- and PQ-control are fully compared. Finally, based on the participation factor analysis and parametric sensitivity analysis, this paper digs more deeply into the essential state-variables and critical control-parameters that significantly contribute to the instability of the MMC system under a very weak AC-grid condition. The results show that the stability-margin of the MMC system, either rectifier or inverter, is essentially different when under PV- and PQ-control. This paper provides an effective approach, through adjusting the most sensitive control-parameters, to mitigate the MMC system instability at weak AC-grid conditions, which is a valuable guidance for parametric selection and adjustment in practical engineering.

Comparative Study on Small-Signal Stability of LCC-HVDC System With Different Control Strategies at the Inverter Station

The constant extinction angle control and constant DC voltage control are widely used for the inverter station of Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system in practical engineering. Under different control strategies, the control system parameters would have totally different effects on the stability of the LCC-HVDC system. In this paper, the small-signal models of the LCC-HVDC system with constant extinction angle control and LCC-HVDC system with constant DC voltage control are developed, respectively. Based on the eigenvalue analysis, this paper conducts a comprehensive comparison of the effects of a phase-locked loop (PLL) and LCC controller on the small-signal stability of LCC-HVDC system with different control strategies at the inverter station under weak AC grid condition. Then, the coupling effect between the PLL and LCC controller parameters is investigated when LCC-HVDC system adopts different control strategies. This paper can provide useful guidance for the systematic design and control parameter selection of LCC-HVDC projects.

Small-signal instability and supplementary coordinated damping-control of LCC-HVDC system with STATCOM under weak AC grid conditions

Static Synchronous Compensator (STATCOM) has been utilized to improve the dynamic behaviors of Line-Commutated-Converter based High Voltage Direct Current (LCC-HVDC) system under weak AC grid conditions. However, the close coupling between STATCOM and LCC system could result in system instability especially under weak AC grid conditions, which has not been addressed in the existing literatures. This paper developed a linearized small-signal dynamic model of a LCC system with STATCOM at the inverter busbar, and validated its accuracy by comparing time-domain responses from small-signal model and simulation results from PSCAD/EMTDC. Based on the small-signal model, this paper investigated the impact of STATCOM on the small-signal stability margin of LCC-HVDC by eigen-analysis and participation factor methods. The results show that under weak AC grid conditions, the close interaction between the control system of STATCOM (especially the AC voltage controller) and PLL of LCC system, deteriorates the system stability and may cause oscillation/instability. Then, this paper proposed a novel supplementary coordinated damping-control (SCDC), for LCC-HVDC system with STATCOM under weak AC grid conditions. The results based on the eigen-analysis and electromagnetic transient (EMT) simulations indicate that the proposed SCDC approach can effectively mitigate the oscillations/instability induced by the controller interaction of STATCOM and LCC system, thus effectively improve the stability margin of the overall system.

A novel supplementary frequency-based dual damping control for VSC-HVDC system under weak AC grid

The Voltage-Source-Converter based High Voltage Direct Current (VSC-HVDC) system which is connected to weak AC grid, under the widely used Vector Current Control (VCC), may experience oscillations and even dynamic instability due to the Phase-Locked-Loop (PLL) dynamics. This paper proposes a novel supplementary control, i.e., supplementary frequency-based dual damping control (SFDDC), to enhance the stability margin for VSC-HVDC system under weak AC grid conditions. Based on the tracked frequency from PLL, the SFDDC approach introduces the power-based and AC voltage-based damping components to mitigate the instability induced by the larger PLL gains. A small-signal model of VSC-HVDC system is developed, and validated by the detailed electromagnetic transient (EMT) simulation. Then, the SFDDC dynamics are evaluated and the feasible region of the introduced damping coefficients are calculated. Finally, this paper investigates the maximum available power (MAP) and evaluates the critical short-circuit ratio (CSCR) for VSC-HVDC station, under the proposed control approach. The results show that, by proper selection of the dual damping coefficients within recommended range, the proposed SFDDC approach can effectively mitigate the instability issue induced by PLL, improve the maximum power transfer capability and enable the VSC-HVDC station operate in extremely weak AC grid conditions, even at SCR = 1.0. In addition, without control strategy switching, the over-current can be limited to an acceptable value by the proposed controller during the solid three-phase to ground fault scenario.

Dynamic performance analysis, stability margin improvement and transfer power capability enhancement in DFIG based wind turbines at weak ac grid conditions

Wind farms are usually located in remote areas far away from utility grid and load centers. Hence, long transmission lines, used between wind farms and utility grid, result in a low value for the grid strength at connection point. This paper first studies stability of the grid connected DFIG wind turbine taking the PLL dynamics into account, and then presents an efficient control approach to stabilize the system and to enhance transfer power capability at weak grid conditions. Hence, unified dynamic modeling of the whole DFIG, including dynamics of the PLL, stator flux, rotor current, drive train, dc-link, rotor side converter (RSC) and grid side converter (GSC) current controllers, dc-link voltage and generator speed controllers, and grid is presented. Then, small signal stability of the full system is carried out and impact of the grid short circuit ratio (SCR) on stability of the system is examined. Next, rotor control structure is modified to increase the stability margin and transfer power capability under weak grid conditions. By the modified control structure, the grid SCR is virtually increased to enhance the system performance at weak grid connections. Finally, results of theoretical analyses are verified by time domain simulations conducted in MATLAB-Simulink environment.

A Frequency-Based Synchronization Approach for the VSC-HVDC Station Connected to a Weak AC Grid

This paper proposes a frequency-based synchronization control (FSC) approach for the voltage-sourced converter in rectification and inversion modes of an HVDC station that is connected to a weak ac grid. Similar to the conventional vector current control (VCC) approach, FSC includes a conventional phase-locked loop (PLL), and its outer loop controller generates the current reference for the inner loop. Different from VCC, the inner loop of FSC utilizes a supplementary frequency-based control loop to provide damping for the oscillations due to the PLL and generates the angle/magnitude for the converter ac-side voltage. The inner loop of FSC switches to VCC to limit over-current during fault scenario. This paper develops a linearized dynamic model of VSC-HVDC station, analytically evaluates behavior of the FSC dynamics, and validates the results based on time-domain simulation studies in the PSCAD platform. The eigenanalysis and simulation results show that FSC can effectively mitigate the instability issues of the PLL even under very weak ac grid condition.

Modal analysis of a grid‐connected DFIG‐based WT considering multi‐timescale control interactions

In a grid-connected doubly fed induction generator (DFIG)-based wind turbine system, the multi-timescale control loops of DFIGs work together to keep the output active/reactive power tracking their references. In weak AC grid condition where the grid impedance cannot be neglected, interactions among those control loops are very strong. This study presents a modal analysis of the system containing a DFIG. The DFIG model for analysis considers multi-timescale control and its interactions. The effect of bandwidths of rotor speed control, phase-locked loop, DC-link voltage control and rotor current control on system stability is presented. Interactions among those control loops are explored. The analysis results indicate that the interactions among multi-timescale control are strong and will have an effect on the stability and the dynamics of the DFIG system under weak AC grid condition.

Modeling of Grid-Connected DFIG-Based Wind Turbines for DC-Link Voltage Stability Analysis

The electromagnetic stability issues of the grid-connected doubly fed induction generator (DFIG) system are usually overlooked. This paper presents a reduced order small-signal model that can be used to analyze the stability of DFIGs dc-link voltage control system, especially under weak ac grid conditions. This model neglects DFIG flux and fast current control dynamics. However, the effects of operating points, grid strengths and control loops interactions on system dynamic performance are taken into account. An eigenvalue comparison shows the proposed model holds dominant oscillation mode featured by the detailed model and is suitable for stability analysis of dc-link voltage control system of DFIG. Influence coefficients reflecting control loops interactions are also presented. Application studies of the proposed model show it is suitable for illustrating the effect of grid strength on dynamic performance of the DFIGs dc-link voltage control system. Meanwhile, phase-locked loop (PLL) and rotor-side converter (RSC) active power control (APC)/reactive power controls (RPC) effect on system stability are also explored.