Search

The complex interactions between renewables and AC/DC networks have brought new oscillation issues. To effectively monitor and suppress these oscillations, the observability and controllability of the oscillatory modes at different locations of the system must be investigated. To achieve this, the paper first defines the concept of modal observability and controllability using the responses of unit-pulse perturbations. Then an eigenvalue-decomposition based method is proposed to calculate these two indexes with the impedance network model (INM) as the input. The obtained modal observability(controllability) has many important applications and one of them is to facilitate the aggregation of the INM. The paper investigates this application by selecting an optimal aggregation port to ensure that the aggregated impedance can reflect the complete system dynamics. Finally, the proposed method is verified on a simple passive circuit and a practical wind power system that experienced actual subsynchronous resonance incidents. Theoretical results as well as EMT simulations demonstrate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

In the past decade, several subsynchronous control interaction (SSCI) incidents have been reported in various large-scale grid-interfaced doubly-fed induction generator (DFIG) based wind farms. The subsynchronous oscillation caused by the SSCI endangers the safe and reliable operation of wind power systems. The DFIG's converter controls play a crucial role in such interactions; therefore, several SSCI mitigation schemes have been reported, which modify the DFIG's converter controls to stabilize the subsynchronous oscillation caused by the SSCI. However, the effectiveness of such converter control modifications has not been verified through laboratory or field experiments. This paper considers a practical rotor-side subsynchronous damping controller (RSDC) and fills the gap by first validating the RSDC's performance through controller hardware-in-the-loop (CHIL) simulations, and then implementing it in a real-world DFIG connected to a series compensated line. The RSDC fundamentally reshapes the effective impedance of the DFIG to stabilize the subsynchronous oscillation. The CHIL simulations, as well as the field tests, successfully validated the RSDC's ability to suppress the SSCI. The results enhanced the practical confidence of the SSCI mitigation strategy based on RSC's converter control modification. [ABSTRACT FROM AUTHOR]

With the increasing penetration of converter based renewable generators into the modern power systems, subsynchronous control interaction (SSCI) phenomenon is jeopardizing the equipment safety and system stability. For the analysis and prevention, it is critically important to identify the source of such sub/super-synchronous oscillation caused by the SSCI. This article first develops three criteria for determining the SSCI sources based on sub/super-synchronous powers and impedances. One of the criteria takes into account the coupling effects between sub- and super-synchronous frequencies. Then, a general framework for the SSCI source identification is established, which is based on wide-area monitoring of the sub/super-synchronous power flows obtained from measured voltage and current phasors. Finally, the proposed identification criterion and framework are verified on the electromagnetic transient model of the Hami wind power system, which has experienced an actual SSCI incident. The results indicated that the wind farms based on type-4 wind turbine generators were the main sources of oscillation, while nearby thermal power plants were the sinks. [ABSTRACT FROM AUTHOR]

Abstract Subsynchronous control interaction (SSCI) associated with wind farms has become one of the major challenges for maintaining the stability and reliability of modern power systems. SSCI is mainly caused by the active participation of fast-acting wind turbine converter (WTC) controls in the phenomenon. It can damage the system equipment, reduce the amount of power generation, and degrade the power quality. Thus, it is urgent to develop practical mitigation techniques for the SSCI in order to achieve a smooth and reliable operation of grid interfaced wind farms. This study presents an overview of potential SSCI mitigation techniques at various stages of the power system, including system planning, operation, control, and protection phases at both network and generation sides. In particular, this paper reviews the active damping controls utilizing flexible ac transmission systems (FACTS) and WTC controls. Finally, it outlines the challenges and future work in studying SSCI mitigation techniques in practical wind power systems. Highlights • SSCI mitigation techniques: planning, operation, control and protection stages. • Review of active damping control techniques in network and generation sides. • Challenges and limitations of various SSCI mitigation techniques are discussed. • WTC control modifications and shunt-VSC using filters are recommended approaches. [ABSTRACT FROM AUTHOR]

Unstable subsynchronous control interaction (SSCI) between wind farms (WFs) and series-compensated grid has posed a big threat to the safety of equipment and the stability of system. Many studies have been conducted on its analysis and control. However, very little research has been done on the protection. To fill the gap, this paper proposes a system-wide protection against unstable SSCI in wind power systems with fixed series compensations. The protection is composed of a centralized protection coordinator (CPC) and several distributed protection relays (DPRs). Local DPRs measure the subsynchronous impedances of WFs in real time. These impedances are next collected by the CPC to calculate the sensitivities of tripping for each WF. The sensitivities are then used to determine the candidate WFs and the number of WTGs to trip at those WFs, so as to stabilize the SSCI. The effectiveness of the proposed protection is verified with detailed electromagnetic simulations on a wind power system that had experienced actual SSCI incidents. [ABSTRACT FROM AUTHOR]

Recently, sustained power oscillation at subsynchronous frequency was captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in Xinjiang Uygur Autonomous Region, China. This new type of subsynchronous interaction (SSI) detected in practical systems has never been reported and analyzed before. Therefore, its mechanism and characteristics are not yet clearly clarified. In this paper, a simplified but representative system model with multiple PMSGs interfaced with AC networks is established first based on the actual system and the PMSG model provided by the manufacturer. Then, small-signal eigenanalysis, time-domain simulation, and impedance model analysis are carried out to investigate the interactive dynamics between them. The results show that such interaction between direct-drive PMSG wind farms and weak ac grids characterized by low short-circuit ratio would cause negative-resistance effect for the SSI mode, leading to unstable oscillation. In such cases, the controller of PMSG would saturate soon, resulting in sustained power oscillation in the system. If unfortunately the oscillation frequency matches the torsional mode of any nearby turbogenerator, severe torsional vibration would be excited on the shaft of the latter. The analysis results are finally validated with field measurements of an actual SSI event. To address the problem, a supplementary subsynchronous damping control loop is attached to the controllers of PMSGs to reshape the impedance and thus to stabilize the SSI. [ABSTRACT FROM PUBLISHER]

Subsynchronous resonance (SSR) was observed in wind farms located in North China. These wind farms prevailingly consist of doubly-fed induction generators (DFIGs) and are connected to series-compensated transmission lines. The observed resonant frequency is about 6 \sim 8 Hz, which is much lower than that of the reported SSR occurred in Texas. The frequency varies during the occurrence and this phenomenon is observed for the first time. The output power is usually within a certain range, when SSR occurs. Based on the practical system, an equivalent simulation system has been established, in which wind farms are modeled as many identical low rating DFIGs. Then, the SSR event is reproduced by simulations. Analysis results indicate that SSR happens even when the equivalent transmission system compensation level seen from wind farms is only 6.67%. Eigenvalue analysis shows that this phenomenon is an electrical resonance, and could be affected considerably by wind speed, number and control of DFIGs. The number of in-service DFIGs has a nonlinear impact on the damping of SSR. An equivalent electric circuit is deduced to intuitively explain why SSR happens and how the above factors affect it. Considering its features, this phenomenon is recognized as DFIG control participated induction generator effect. [ABSTRACT FROM AUTHOR]

The subsynchronous control interaction (SSCI) occurs when the wind turbine converter (WTC) controls interact with the series-compensated or weak AC network. The mechanism and attributes of the emerging interaction phenomenon invilving wind turbine generators (WTGs) are quite different from the traditional subsynchronous resonance or oscillation (SSR/SSO) phenomenon in steam turbine-generators (STGs). The SSCI is characterized by various system-wide parameters, including the wind speed and its uneven distribution in a wind power plant, varying number of WTGs connected to the grid, type of the WTGs, parameters of the WTC controls, level of series compensation, stiffness of the grid, etc. Such system-wide parameters change over time and thus make the modeling and analysis rather challenging to conduct SSCI studies. An ideal modeling approach for the SSCI is expected to preserve the system topology as well as valid for a wide range of operating conditions and parameters of the system. On the other hand, an ideal stability analysis method is supposed to give key quantitative information, such as the magnitude, frequency, and origin of the oscillation, as well as the component level participation indices. This paper provides a comparative insight into the application of existing and emerging modeling and stability analysis approaches for SSCI investigations in large-scale wind farms. It highlights the strengths and weaknesses of various modeling methods and analysis criteria. Finally, the paper underlines the recent advancements and points out towards the research directions in small/large-signal impedance modeling and stability approaches for accurate and quantitative investigation of the SSCI. • SSCI is a system-wide phenomenon thus the modeling and analysis is challenging. • Strengths and weaknesses of the existing modeling and stability analysis methods in the time and frequency-domain are discussed. • An modeling method should preserve system topology and remains valid under a wide range of operating conditions. • An ideal stability analysis method should give key quantitative information: magnitude, frequency and source. [ABSTRACT FROM AUTHOR]

• BESS model which can accurately describe BESS's power regulation behavior and provide a correct state of charge. • Simplifying method to enable the proposed model to finish 24 h-simulation within a few minutes. • BESS-based strategy to improve the AGC performance of wind farms. Battery energy storage system (BESS) is being widely integrated with wind power systems to provide various ancillary services including automatic generation control (AGC) performance improvement. For AGC performance studies, it is crucial to accurately describe BESS's power regulation behavior and provide a correct state of charge (SOC). In addition, BESS model is required to maintain a sufficiently high simulation speed, since it usually is involved in hour- or day-scale simulations. Different from existing models, this paper develops a new BESS model which considers DC-AC converter control, DC link circuit and switching loss, thus providing an accurate power regulation behavior. In addition, it also considers the nonlinear relationship between open-circuit voltage and SOC of battery so that it provides a correct SOC. As a whole, the BESS is simplified as a controlled current source with fundamental frequency (50/60 Hz) and described by algebraic phasor equations to reduce computational burden. This makes the proposed model capable of finishing hour- or day-scale simulations within a few minutes. Time-domain simulations are used to validate and compare the simulation accuracy with the classical first-order transfer function model and electromagnetic model. Based on the proposed model, this paper also introduces and verifies a new BESS-based strategy to improve the AGC performance of wind farms. [ABSTRACT FROM AUTHOR]

This paper concerns with the emerging power system stability issues, classification, and research prospects under a high share of renewables and power electronics. The decades-old traditional power system is undergoing a fast transition with two most prominent features: 1) high-penetration of renewable power generators, utilizing intermittent renewable sources such as wind and solar, and 2) high-penetration of power electronic devices in the generation e.g., wind turbine converters and solar power inverters, transmission e.g., flexible ac or dc transmission system converters, and distribution/utilization systems e.g., electric vehicle and microgrid. The development of modern power systems with dual high-penetrations, i.e., high-penetrations of renewables and power electronic devices, influences the power system dynamics significantly and causes new stability issues. This paper first overviews equipment-level features and system-level stability challenges introduced under the dual high-penetration scenario of the modern power system. Next, the impacts of emerging stability challenges on various aspects of the classical stability issues and classifications are highlighted. Under this context, this paper discusses the validity and limitations of the existing classical and extended power system stability classifications proposed by different IEEE/Cigre Working Groups. Furthermore, a new power system stability classification framework is proposed, which not only maintains the inherent logic of the classical classification but also provides wide coverage and future adaptability of the emerging stability issues. Finally, various classification-oriented research prospects in the power system stability domain are highlighted. • Discussion on the device- and system-level features of power systems with dual high-penetrations. • Emerging stability issues due to dual high-penetrations and their impact on stability classifications. • A critical review of existing classical and extended power system stability classifications of 2004 and 2020. • Highlighting the classification-oriented future research directions. [ABSTRACT FROM AUTHOR]