Subsynchronous Control Interaction Research Articles

Characteristic analysis and mitigation strategy for SSCI in series-compensated DFIG-based wind farm controlled by a virtual synchronous generator

The proliferation of virtual synchronous generator (VSG) technology within series-compensated double-fed induction generator (DFIG)-based wind farms is substantially hampered by the attendant risk of subsynchronous control interaction (SSCI), resulting in a significant research deficiency on systematic control interaction analysis and the development of mitigation strategies. The paper proposes an advanced active disturbance rejection control (ADRC) framework, incorporating real-time compensation mechanisms to mitigate the inadequate suppression efficacy attributable to the VSG's diminished output impedance. Initially, the mathematical expression for the VSG output impedance is rigorously deduced, and the positive damping attributes of the VSG in relation to SSCI are elucidated from the perspective of underlying mechanistic principles. Subsequently, the suppressive mechanism of SSCI by the ADRC is revealed in the context of VSG involvement, and the consequent augmentation of SSCI attributed to PI control is systematically derived. In immediate succession, the quanta of oscillation and inductive cross-coupling are encapsulated as the system's aggregate disturbance, thereby streamlining the ADRC to its primary order configuration, permitting the utilization of an extended state observer (ESO) for the dynamic estimation of said disturbance. Furthermore, a fractional-order filter function is instituted to engineer an augmented ESO, which refines the output voltage of the grid-side converter. Concurrently, a meticulous discourse on the rectification strategy for the proposed ESO parameters and its stability ensues. Ultimately, the efficacy of the mechanism analysis, alongside the robustness of the proffered control strategy for SSCI mitigation under diverse perturbation conditions, is corroborated via impedance evaluation and time-domain simulation.

Robust H-infinity and μ-synthesis controllers to mitigate sub-synchronous control interaction in DFIG wind farms considering time delay

Robust H-infinity and μ-synthesis controllers to mitigate sub-synchronous control interaction in DFIG wind farms considering time delay

Analysis and mitigation of SSCI when integrating wind power to series compensated lines

As more power electronics are introduced into the power system, its stability is impacted, e.g., through undesired interactions. One such interaction is called sub-synchronous control interaction (SSCI), an example being an interaction between a DFIG wind farm and a series compensated line. In this paper, two methods are used to assess the risk of SSCI: the reactance crossover method, and the Nyquist criterion. The analysis is performed on three case studies: one system based on the IEEE First Benchmark System, and the other system is modelled as a typical Swedish transmission system with two different degrees of series compensation. Both methods predict SSCI in all three case studies, with the Nyquist criterion being able to predict the oscillation frequency more accurately. To mitigate the sub-synchronous oscillations, a PV farm is implemented and placed in parallel to the wind farm. The performed simulations show that it is able to damp the oscillations successfully in all case studies.

A Robust UDE-Based 2DOF Controller for Mitigating Sub-Synchronous Control Interaction in Wind Power Plants

Sustaining large amount of power flow in the wind power plant, series capacitors are incorporated with the transmission line system which can cause subsynchronous control interaction (SSCI) in the doubly-fed induction generator (DFIG). This SSCI effect can lead to system instability. To damp SSCI in DFIG, several traditional controllers are involved but it leads to poor damping response. To solve this issues, a new controller based on the uncertainty and disturbance estimator and two degree of freedom (UDE-2DOF) controller is proposed. The proposed controller gives the controlled output based on the input control signal like reference and actual value of the d-q axis rotor current. With this evaluation, the proposed controller damp out the SSCI in a series-compensated DFIG. Then, the effectiveness of the proposed control scheme is verified in the MATLAB platform. Both eigenvalue analysis and time-domain simulations are carried out for analyzing the system stability and SSCI damping. The results indicate that the proposed method can mitigate the SSCI within 2.5 sec. Furthermore, the variation in wind speed and the level of series compensation plays an important role on the overall system’s stability especially at low wind speed (7 m/sec) and high compensation level under 45%.

A data-driven method for SSCI identification using synchrophasor measurements

In recent years, subsynchronous control interaction (SSCI) frequently occurred due to the rapid development of inverter based resources. Online monitoring and identifying SSCI are thus of great importance for the safe operation of power systems. This paper proposes a data-driven method for SSCI identification using synchrophasors generated by phasor measurement units (PMUs). The challenges of this work mainly result from two factors: 1) SSCI may simultaneously involve a sub-synchronous oscillation (Sub-SO) mode and a super-synchronous oscillation (Sup-SO) mode. The spectra of these two modes would be aliased due to the limited reporting rate of PMUs. 2) The reporting filter in PMUs significantly weakens the oscillation signal as the frequency of SSCI is generally far away from the nominal frequency. To tackle these challenges, the influence of Sub-SO and Sup-SO on spectra of synchrophasors is analyzed first. It is revealed that the positive and negative spectra are linearly correlated with Sub-SO and Sup-SO modes. Several features are then extracted via the analysis of the spectra which can reflect the SSCI features and a data-driven identification method is further proposed through a fast reduced kernel extreme learning machine (RKELM) model. Compared with the traditional classification techniques, the RKELM algorithm based on kernel function has stronger generalization performance and higher accuracy while ensuring shorter training time. This significantly improves the accuracy and efficiency of SSCI identification, especially under noise conditions. Both simulations and field tests demonstrate the effectiveness and usefulness of the proposed method.

Improved methodology for damping sub‐synchronous oscillation in a series‐compensated DFIG‐based wind farm

AbstractThe interaction between the wind farm and the series‐compensated transmission line may lead to the sub‐synchronous oscillation (SSO) phenomenon which affects the system stability and equipment safety. Among the presented methods for damping the SSO phenomenon, the supplementary damping controller (SDC) is the most prevalent method because of its low cost, effectiveness, and simplicity. In all previous studies, one input control signal is fed to the SDC for mitigating the SSO. Here, two input control signals (active power and reactive power deviation) are applied to the SDC for enhancing the system stability and damping the SSO quickly. The ability of the proposed method for damping the SSO has not been investigated before. The proposed SDC is embedded into the q‐axis of the rotor‐side converter (RSC) inner current loop. The modified IEEE first benchmark model (FBM) is used to analyze the performance of the proposed method under different compensation levels, variable wind speeds, and sub‐synchronous control interaction (SSCI). Small‐signal stability using the eigenvalue approach is carried out, where the impact of the proposed method on sub‐synchronous mode, and super‐synchronous mode is investigated. The results successfully prove the superiority of the proposed method for damping the SSO under various operating conditions compared to some previous methods, where the least overshoot and faster convergence have been achieved by the proposed method in all studied cases.

Gate‐controlled series capacitor: A new methodology for mitigating sub‐synchronous resonance in a series‐compensated DFIG‐based wind farm

AbstractSeries‐compensated transmission lines can lead to a sub‐synchronous resonance (SSR) phenomenon at high compensation levels, where the system lacks stability. Gate‐controlled series capacitor (GCSC) is widely used to damp the SSR. Here, an effective and new methodology for damping the SSR is presented. The proposed method aims to alleviate the SSR phenomenon based on estimating both the frequency and the maximum value of the voltage magnitude signal rather than using the Proportional Integral controller. The appropriate turn‐off angle corresponding to the maximum value of the voltage magnitude signal is applied to the GCSC on the condition that the estimated frequency of the voltage magnitude signal is within the range of the SSR frequency. If the estimated frequency of the voltage magnitude signal is out of the range of the SSR frequency, the steady‐state turn‐off angle (1130) is applied to the GCSC. The capability of the proposed method for damping the SSR under various compensation levels, various wind speeds, and sub‐synchronous control interaction (SSCI) is validated by time‐domain simulation and eigenvalue analysis. Compared to the presented latest method for SSR damping using GCSC, all the obtained results demonstrate that the oscillations converge faster when the proposed method is activated.

Analysis and Damping of Sub-Synchronous Oscillations in a 250 MW DFIM Hydro Unit Connected to Series Compensated 765 kV Transmission Lines

Increasing energy consumption has required a greater need for connections between the grid and nonconventional energy sources. Recently, doubly fed induction machine (DFIM) fed hydro power units are increasingly connected to the power system due to the requirement of large energy storage option and cost-effective converter solution in the variable speed units. On the other hand, stability related issues are raised in the power system due to the sub synchronous control interaction (SSCI) that occurs between radially connected DFIM, and series compensated transmission line. This article focuses on analysis of resonance stability in a doubly fed induction machine (DFIM) based hydropower system. The controller of the rotor side converter is tuned using a dampness optimization approach. Additionally, optimized SSCI damping controller in the grid side converter is designed to attenuate the sub-synchronous oscillation (SSO) of the DFIM. Time domain simulation analysis is conducted for this research to validate the SSCI mitigation in the DFIM-based hydro units. In MATLAB/Simulink environment, the efficacy of the proposed system with 250 MW DFIM and a series compensated 765 kV extra high voltage transmission line is tested. To scale down the model and check SSO in the 2.2 kW DFIM system, a practical demonstration is carried out in the Lab.

HPF-LADRC for DFIG-based wind farm to mitigate subsynchronous control interaction

• The HPF-LADRC is introduced to replace the traditional PI controller on the wind turbine's rotor side converter. Firstly, the estimate and feedback of the system's subsynchronous oscillations are implemented using linear extended state observer (LESO) in the LADRC. • An HPF is added to the linear state error feedback (LSEF) in the LADRC to mitigate low-frequency oscillation. • Time-domain simulations and impedance model-based analysis are performed to validate the proposed suppression technique for SSCI under different wind speeds, compensated levels, and the number of in-service DFIGs. Doubly-fed induction generator (DFIG)-based wind farms interact with series capacitors in transmission lines, leading to sub-synchronous control interaction (SSCI). The SSCI's oscillation frequency is affected by the system's operating conditions. Therefore, this paper proposes the linear active disturbance rejection control (LADRC) based on the high-pass filter (HPF) to mitigate SSCI under various conditions. The HPF-LADRC is introduced to replace the traditional PI controller on the wind turbine's rotor side converter. Firstly, the estimate and feedback of the system's subsynchronous oscillations are implemented using linear extended state observer (LESO) in the LADRC. Then, an HPF is added to the linear state error feedback (LSEF) in the LADRC to mitigate low-frequency oscillation. Finally, time-domain simulations and impedance model-based analysis are performed to validate the proposed suppression technique for SSCI under different wind speeds, compensated levels, and the number of in-service DFIGs. The results demonstrate that the proposed HPF-LADRC increases the system's low-frequency band dampness and enhances its disturbance resistance.

Real-time Subsynchronous Control Interaction Monitoring Using Improved Intrinsic Time-scale Decomposition

In recent years, subsynchronous control interaction (SSCI) has frequently taken place in renewable-connected power systems. To counter this issue, utilities have been seeking tools for fast and accurate identification of SSCI events. The main challenges of SSCI monitoring are the time-varying nature and uncertain modes of SSCI events. Accordingly, this paper presents a simple but effective method that takes advantage of intrinsic time-scale decomposition (ITD). The main purpose is to improve the accuracy and robustness of ITD by incorporating the least-squares method. Results show that the proposed method strikes a good balance between dynamic performance and estimation accuracy. More importantly, the method does not require any prior information, and its performance is therefore not affected by the frequency constitution of the SSCI. Comprehensive comparative studies are conducted to demonstrate the usefulness of the method through synthetic signals, electromagnetic temporary program (EMTP) simulations, and field-recorded SSCI data. Finally, real-time simulation tests are conducted to show the feasibility of the method for real-time monitoring.

Mitigation of Subsynchronous Control Interaction in DFIG-Based Wind Farm Using Repetitive-PI

Mitigation of Subsynchronous Control Interaction in DFIG-Based Wind Farm Using Repetitive-PI

Performance evaluation of SSCI damping controller based on the elastic energy equivalent system

This paper proposes an Elastic Energy Equivalent System (EEES) for evaluating the performance of sub-synchronous control interaction (SSCI) damping controller. The proposed EEES can evaluate the dynamic energy exchange and dissipation of the SSCI damping controllers under various operating conditions, and pinpoint the sensitive control parameters at different oscillation frequencies in advance. First, inspired by the vibration of spring-damper system, we establish the EEES to imitate the SSCI, so as to explore the elastic energy composition of the power system integrated with the doubly-fed induction generator (DFIG). Second, the equivalent damper and spring of EEES are used to model the controllable dissipated and potential energy for the converter controller respectively, so criteria are developed to evaluate the effectiveness of the converter controller for stabilizing the system. Third, the extended state observer-based sub-synchronous damping controller (ESSDC) is proposed to mitigate the SSCI, whose performance and parameters’ sensitivities are evaluated by the proposed EEES. Finally, the accuracy of the proposed EEES and the effectiveness of the ESSDC are validated on a DFIG-integrated power system and Guyuan wind farms. The results show that the proposed ESSDC performs better than conventional controllers with satisfactory damping and robustness under the variation of operating conditions.the effectiveness of the proposed EEES is validated on with different SSCI damping controllers.

Subsynchronous control interaction analysis between PMSG-based offshore wind farm and SVGs

With the rapid development of wind energy, the subsynchronous oscillations (SSO) involved with wind conversion systems, has been the focus of academic concern. To analyse the subsynchronous control interaction (SSCI) issue of multi-converters (wind-power converter and widespread SVGs), the state matrix of a typical PMSGs-based offshore wind farm system including SVGs is constructed . It is proposed that SVGs in wind farm may cause SSCI issue by their uncoordinated control targets. Under certain conditions, the SSCI between SVGs forms the dominant oscillation mode, delivered with variation of the SSO frequency. By calculating the influence factors along with different converters, it can be concluded that unreasonable parameter configuration of converters and variations of operating conditions affect the SSCI characteristics. At last, time domain simulation is used to verify the SSCI mechanism and characteristics of the PMSG-based offshore wind farm and SVGs.

On Mitigation of Sub-Synchronous Control Interactions in Hybrid Generation Resources

One major reliability concern regarding renewable energy resources connected in the vicinity of series-compensated power transmission lines or to weak power grids is sub-synchronous resonance (SSR). A particular type of interaction known as sub-synchronous control interaction (SSCI), with a purely electrical nature, has the potential to grow very rapidly and can cause severe damage to power grid infrastructure. This paper proposes an additional damping controller that allows mitigating the SSCI expeditiously. The proposed damping loop provides a very large impedance at the synchronous frequency and very low impedance at the sub-synchronous range. More importantly, it can be integrated into the control loop of the battery energy storage systems (BESS) without imposing a negative impact on the routine performance of the BESS. As BESSs are becoming more popular in power grids, thanks to their significant impacts on reliability and power quality, it is anticipated that the proposed approach will have a great implementation opportunity. Hence, the proposed mitigation solution is implemented in a hybrid plant and tested with a radial test system and a section of the Electric Reliability Council of Texas (ERCOT) power grid under a variety of operating conditions. The results show the efficiency and robustness of the mitigation solution even under different frequencies of oscillation and larges disturbances.

Improving grid strength in a wide‐area transmission system with grid forming inverters

This paper summarizes Electromagnetic Transient (EMT) simulation studies using PSCAD/EMTDC undertaken to evaluate the capability and suitability of commercially available large scale Grid Forming Inverters (GFMI) to dampen oscillations in a real bulk power transmission network. Faults and a range of grid voltage oscillation frequencies are tested on GFMI and synchronous condenser (SC) models using single source equivalent network model and comparisons of transient, post fault and oscillatory rejection tests are presented. A critical credible fault in the West Murray Zone (WMZ) was simulated on a wide-area EMT model of the Australian National Electricity Market (NEM) to show the effectiveness of GFMI in providing system strength services and improving damping of network sub-synchronous control interactions (SSCI). Two scenarios were examined: Direct replacement of existing centralized synchronous condensers in the WMZ of the NEM, and a decentralized distribution of GFMI in the transmission network (treated as expansion or repowering solution for existing grid following inverter equipped solar farms). Simulation results show that commercially available GFMI are a viable option for improving system strength in a practical transmission system with a high proportion of Inverter Based Resources (IBR).

MMC-STATCOM supplementary wide-band damping control to mitigate subsynchronous control interaction in wind farms

The interaction between the DFIG-based wind farm and series compensation can easily cause subsynchronous control interaction (SSCI). The oscillation frequency of SSCI varies with the actual wind farm situation, which is different from the fixed subsynchronous oscillation frequency of the thermal power unit. Due to the time-variant of the SSCI's oscillation frequency, this paper proposes a supplementary wide-band damping control strategy for MMC-STATCOM to mitigate SSCI. The method adopts a notch filter (NF) to filter only the fundamental frequency component in the current and obtains the corresponding subsynchronous component to generate the damping control signal. Additional damping control signal tracking subsynchronous component changes correct corresponding output voltage, making MMC-STATCOM present positive resistance in subsynchronous frequency. This strategy does not require phase compensation and can always extract the corresponding subsynchronous component compared with the existing research. Finally, both the impedance analysis and time-domain simulation have been involved in verifying the feasibility of the proposed suppression strategy for SSCI under various operating conditions.

Sub‐synchronous interaction damping controller for a series‐compensated DFIG‐based wind farm

Sub-synchronous interaction (SSI) phenomenon is one of the dynamic system problems that have an adverse influence on the safety and stability of the system.The use of supplementary damping controllers (SDCs) in the double-fed induction generator (DFIG) converter controllers is quite promising due to their simplicity,low costs, effectiveness, and easiness of tuning. This paper presents a new effective input control signal, rotor voltage, to the SDC for damping SSI in a series-compensated DFIG-based wind farm. The proposed SDC is embedded into both the q-axis and d-axis of the rotor-side converter inner current loops. Particle swarm optimization algorithm is used to identify the optimum parameters of the SDC which maintain the system stability at various operation conditions. Both eigenvalue analysis and time-domain simulations have been carried out to demonstrate the capability of the proposed SDC for enhancing the system stability and damping SSI. Compared to the conventional SDC, the proposed SDC has the best performance where it can quickly and robustly damp the SSI at different compensation levels, different wind speeds, and sub-synchronous control interaction.

Evaluation of the Sub-Synchronous Control Interaction Based on the Elastic Energy Equivalent System

Evaluation of the Sub-Synchronous Control Interaction Based on the Elastic Energy Equivalent System

An embedded VFAF in DFIG-based wind farms for SSCI mitigation

Due to the increasing penetration of wind generations, a new type of sub-synchronous interactions, the so called sub-synchronous control interaction (SSCI), has emerged in power systems in the past decade. Incidents of this type have caused severe consequences in real-life power systems, including equipment damage and loss of power supply. To overcome the impacts and minimize the adverse influences of such phenomenon, this paper proposes a novel varying-frequency adaptive filter (VFAF) in doubly fed induction generators-based wind farms for SSCI mitigation. The proposed VFAF utilizes a linear prediction model and coefficient-based polynomials to analyse the spectral components of the controller signals. In addition, singular value decomposition is used to identify the number of signal components in the controllers. Then, based on the analysis, the VFAF produces compensation signals to modulate the distorted controller signals and thus mitigates the oscillations during SSCI. Based on this mechanism, the VFAF is auto-adaptive to various oscillation frequencies, including those that are time-varying. The above feature is of particularly importance in modern power systems. Testing cases of the VFAF have been conducted in the Power Factory software, and the results verify the effectiveness of the proposed VFAF under relevant system conditions.

Mechanism and Mitigation Strategy of Sub-synchronous Oscillation Caused by Grid-connected DFIG with Series Compensation

Power systems are at risk of sub-synchronous oscillation (SSO) when connecting doubly-fed induction generator (DFIG)-based wind turbines to transmission lines with series compensation. This paper focuses on the mechanism and mitigation strategy of SSO. Firstly, the mechanism of SSO is studied from two aspects: induction generator effect (IGE) and sub-synchronous control interaction (SSCI). An equivalent circuit is developed to illustrate the negative resistance effect. Based on the result of the analysis, the SSO mitigation strategy using notch filter and virtual resistance control is proposed. In order to verify the result of analysis and the effectiveness of mitigation strategy, time-domain simulations are carried out based on PowerFactory DIgSILENT.