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Identifying sources of subsynchronous resonance (SSR) is crucial for understanding and mitigating incidents. Recent studies have shown that SSR impedance and power are effective indicators that can identify an SSR source. However, the calculation of two indexes requires SSR voltage and current phasors as the inputs, which are not directly available from existing online monitoring systems. To address this issue, this paper presents a novel method to calculate SSR impedance/power based on synchrophasors provided by a wide-area monitoring system. As a result, the online detection of SSR sources is enabled. Through mathematical derivation, it is revealed that a synchrophasor under SSR is dominated by four modes: two from the fundamental frequency and two from the SSR frequency. The problem thus can be transformed to a modal parameter extraction (MPE). Two classic MPE algorithms, Prony analysis and the matrix pencil method, are employed in this paper to extract the SSR parameters and calculate the SSR impedance/power. Their performances are verified using synthetic signals and electromagnetic transient simulations. Furthermore, the method is applied to an actual SSR incident involving 23 wind farms. The findings suggest that 17 of 23 wind farms did not contribute negative damping to the oscillation. [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]

Given large-scale power networks with series compensation, the study of subsynchronous resonance (SSR) is a significant challenge, in practice, due to the model complexity and computation burden. To tackle these difficulties, this paper proposes a method to reduce a large-scale system to an equivalent model using a metric called damping sensitivity index. The proposed metric can quantitatively identify the boundaries of the equivalent model that ensure the accurate studies of SSR and reduce the computation burdens greatly. The effectiveness of the method is verified by its application to the SSR investigation of the modified IEEE New England 10-machine 39-bus system and 16-machine 68-bus system. The equivalent models obtained by the proposed method are compared with the original systems in terms of driving point impedances, electrical damping, and transient torques. These results obtained show that the equivalent models from the proposed method can reproduce the SSR dynamics of the original systems with a much shorter computation time. [ABSTRACT FROM PUBLISHER]

The emerging subsynchronous resonance (SSR) caused by the interaction of wind turbine generators (WTGs) with series compensation has aroused great concerns. For this particular issue, this paper is aimed to fill the gap between theoretical studies and actual observations. By analyzing the field data of 58 SSR events captured in a practical wind power system and examining the observed dynamics with previous theoretical results, the mechanism and characteristics of SSR are revealed in a more explicit and substantial way. The necessary conditions and dominant influential factors are identified and the underlying reasons are discovered. Theoretically derived as well as practically measured impedance models have demonstrated that the converter control of doubly fed induction generator (DFIG) produces negative resistance at the slip frequency and thus causes unstable SSR; while permanent magnet synchronous generators and self-excited induction generators are just passively engaged in those SSR incidents. The distribution of the oscillation frequency has also been examined with field measurements. It is discovered that WTGs at different locations participate into the same SSR mode and their frequencies are not fixed but keep changing with the time, the variation of grid topology, and the number of online generators. [ABSTRACT FROM PUBLISHER]

The subsynchronous resonance (SSR) problem may become more serious with the continuous evolvement of a series-compensated power system. Consequently, the existing SSR mitigation scheme might no longer keep the system stable. This issue has been encountered by Shangdu Power Plant, where the deployed supplementary excitation damping controllers (SEDCs) that well stabilized the system before cannot maintain torsional stability after the recent change occurred in the power system. Therefore, a combined mitigation scheme of SEDC and generator terminal subsynchronous damping controller (GTSDC) is proposed in this paper to regain system stability. SEDC provides electrical damping by modulating the excitation voltage at the rotor side. Meanwhile, GTSDC can damp SSR via injecting super-synchronous and subsynchronous currents into the generator stator. Field tests have proven the effectiveness of this combined scheme in addressing the deteriorated SSR issue. Eigenvalue analysis indicates that the combined scheme can provide enough positive damping for the system under any operating condition. Thanks to the low investment of SEDC and the flexibility of GTSDC, this combined scheme provides an economical and scalable solution for SSR mitigation, especially when the target system changes constantly. [ABSTRACT FROM AUTHOR]

For large turbine generators connected to transmission lines compensated by series capacitors, potential subsynchronous resonance (SSR) poses a big threat to the safe and stable operation of both the generator and the system. To protect them against the harm of SSR, a novel two-level protection scheme is proposed in this paper. It consists of unit-level torsional stress relays (TSRs) and a plant-level coordinator called TMaster. TSR is equipped for each generator to determine the stability of its torsional modes and to estimate the fatigue loss-of-life caused by system disturbances. TMaster summarizes the judgments of modal stability from TSRs and decides if and which generator should be tripped to suppress SSR. The whole protection scheme provides three types of tripping functions, namely, modal instability trip logic, fatigue over-limit trip logic and instant trip logic. They can protect the generator shaft from damage caused by unstable SSR, accumulated fatigue, and intolerable transient torque, respectively. The SSR protection system was applied in a practical series-compensated system and its effectiveness in a real SSR event was demonstrated with field data recorded during the event. [ABSTRACT FROM AUTHOR]

Supplementary excitation damping controller (SEDC) has been widely acknowledged as one of the effective measures to stabilize subsynchronous resonance (SSR). However, how to quantify its stabilizing ability is still an open issue raising widespread concerns. There are few in-depth studies or conclusions in this aspect yet. This paper presents a quantitative measure of SEDC's ability to damp SSR, namely stabilizing ability index (SAI). It is defined as the percentage margin of the effective gain relative to the critical stability gain (CSG). The CSG is the minimum gain of SEDC to guarantee SSR stability, while the effective gain is the control gain of SEDC actually functioning following system disturbances. A practical system is used to investigate the various factors determining the effective gain, the CSG and thus the SAI, based on eigenvalue analysis as well as time-domain simulation. The dominant factors identified include: the total equivalent series-compensation degree of system, the parameters and operating status of the generator, the response time and ceiling voltage of the exciter, and the severity of the disturbance. Why and how these factors affect the control capability of SEDC is examined and thus a practical method is offered for the assessment of SEDC's ability to stabilize SSR. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to analyze the mechanism of torsional interaction (TI) and its solution for series-capacitor-compensated power systems. The relationship between the oscillation of the generator shaft and the consequent electromagnetic torque is deduced by a time-domain analysis. It is revealed that the subsynchronous currents caused by torsional oscillations provide negative damping electromagnetic torque and are the cause of TI. A family of subsynchronous dampers (SSDs) is proposed, which is based on the controlled injection of supersynchronous and subsynchronous damping currents into the generator stator. The design procedure of subsynchronous damping controller (SSDC) of series SSD is elaborated. Eigenvalue analysis and simulations for the IEEE first benchmark model (FBM) have verified the effectiveness of the proposed SSDs in solving the multimodal subsynchronous resonance problem. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents a generator terminal subsynchronous damping controller (GTSDC) for mitigating multimodal subsynchronous resonance (SSR) that may occur in series-compensated power systems. The proposed GTSDC system is composed of a digital controller and a power electronic inverter with current tracking control. It uses the shaft speed deviation of the generator turbine as feedback to modulate the dynamic current reference of the inverter through multimodal control paths. By exactly tracking the reference value, the power electronic inverter generates current at frequencies complementary to the torsional modes. Part of the current flows into the generator and creates damping torque on the shaft system to damp SSR. The relationship between the output current of GTSDC and the resulting electromagnetic torque is derived based on the complex torque coefficient method. Furthermore, a nonlinear optimization method is implemented to obtain the optimal parameters for all modal-control paths, thereby effectively enhancing the damping of all torsional modes under different operating conditions. The case study on a real series-compensated power system through eigenvalue analysis and electromagnetic simulations validates the effectiveness of the controller. [Copyright &y& Elsevier]