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Highlights • A wide-area monitoring and early-warning system of SSO is proposed. • The system incorporates locally deployed S-PMUs and a centralized DC&SDA. • A global view of SSO is established with the subsynchronous state estimation. • The subsynchronous power & current flow is formed to locate oscillation source/path. • Two stability criteria are used for the real-time evaluation of SSO stability. Abstract With the increasing penetration of renewable power generation (RPG) in modern power systems, subsynchronous oscillation (SSO) induced by the interaction between RPG and the grid is posing a great threat to the system stability and equipment safety. To address this issue, this paper proposes a wide-area monitoring and early-warning system of SSO. It incorporates the locally deployed subsynchronous phasor measurement units (S-PMUs) and a centralized data center & subsynchronous dynamics analyzer (DC&SDA). Several key functions have been developed, including: (1) the subsynchronous state estimation (SSE) to form global view or observability of SSO dynamics, (2) the establishment of subsynchronous power & current flow to locate oscillation source/path, and (3) the real-time evaluation of subsynchronous stability using two complementary criteria, one based on the identification of oscillation damping and the other based on the frequency characteristics of the impedance models. The practicality and performance of the proposed system have been demonstrated with electromagnetic transient (EMT) simulations on a practical system that suffered SSO incidents. [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]

With the increasing integration of power electronic converters into the power system, the interactions between converters and their adjacent transmissions bring emerging oscillation issues. A new type of sub- and super-synchronous interactions (S $^2$ SI) between STATCOMs and the weak AC/DC grid were detected in China Southern Grid. As an oscillation incident never reported before, as far as the authors know, its characteristics were quite different from the previous ones. Field measurements as well as simulation studies have revealed that the various factors contributing to the S $^2$ SI include the number of online STATCOMs, the strength of the AC system (in terms of effective short-circuit ratio), the status of series compensation, and the loading level of the HVDC. The mechanism of such oscillation has been clarified and it is caused by the S $^2$ SI between STATCOMs and weak AC grids, where sub- and super-synchronous components are involved in a coupled way. To fully represent such a coupling effect, the concept of sub- and super-synchronous coupled impedance model (IM), or S $^2$ SC-IM, is proposed and developed in order to establish a stability criterion for the S $^2$ SI. Based on the IM-based method as well as the time-domain simulations, the impacts on S $^2$ SI from the various factors are quantitatively analyzed. The results show good consistence among theoretical analysis, electromagnetic simulations, and field measurements. Therefore, the effectiveness of the developed models and the stability criterion has been well verified. [ABSTRACT FROM AUTHOR]

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

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]

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]

Blocking filters (BFs) were designed to solve subsynchronous resonance (SSR) problems induced by fixed series capacitors (FSCs) of the Tuoketuo transmission system. However, during the commission of BF, unstable self excitation (SE) occurred unexpectedly, resulting in suspension of the project. This paper first analyzes the mechanism of how BF causes SE. Then, the parameters' sensitivity is investigated and the quality factor (Q) of BF reactors is discovered to have opposite impact on SE and torsional interaction (TI). Thus, by appropriately lowering Q of BF reactors with supplemental resistors, an improved BF scheme is put forward to simultaneously mitigate the different types of SSR problems. As a key issue, the determination of supplemental resistors is formulated into a multi-objective optimization problem, which is then effectively solved with our proposed searching method. Finally, the effectiveness of improved BFs is verified with field tests. For the first time, actual measurements of filed tests on BFs are offered. What is more, their comparison with model studies demonstrates a good agreement. This work paves the way for putting both BFs and FSCs into service, thereby ensuring safe operation and transfer-capability enhancement of the system. [ABSTRACT FROM PUBLISHER]

The Shangdu power plant has four 600-MW turbine-generators connected to the North-China Grid through two 500-kV transmissions, which are compensated with 45% fixed series capacitors. Extensive studies conducted on the system model indicate that the system suffers from multimodal subsynchronous resonance (SSR). To solve the problem, a countermeasure is developed by combining the supplementary excitation damping control (SEDC) and the torsional stress relay (TSR). In this paper, the characteristics of the SSR problem are investigated. Then the developed SEDC is presented. To validate the effectiveness of the proposed SEDC as well as the results of model studies, field tests were conducted under various operating conditions. The tests fully exposed the realistic threat of SSR in the system. Meanwhile, it is demonstrated that the developed SEDC can improve torsional damping significantly, and thus solve the multimodal SSR problem effectively. This is the first time in China that practical SEDCs have been developed and their ability to mitigate multimodal SSR has been verified in a real series-compensated system. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an intelligently optimized multimodal supplementary excitation damping control (OMSEDC) to depress subsynchronous resonance (SSR) that may occur in series-compensated power systems. In the proposed OMSEDC, each torsional mode is controlled through an independent feedback path in order to achieve better and multimodal damping. To make OMSEDC feasible and robust, the control parameters are optimized on a number of linearized system models over a set of carefully selected operating conditions. During system modeling, the time-delay of the firing control in the thyristor-based excitation system is taken into consideration for the first time, making OMSEDC practicable in real systems. The task of simultaneously tuning for stable control parameters considering multiple torsional modes and under different system conditions is formulated into a constrained nonlinear optimization problem, which is generally difficult for traditional methods. In this paper, the genetic and simulated annealing algorithm (GASA) is adapted and implemented to obtain the optimal parameters with high efficiency. A practical series-compensated system is employed to evaluate the effectiveness of OMSEDC. Both eigenvalue analysis and nonlinear electromagnetic simulations have demonstrated a satisfactory damping performance of the controller on multiple torsional modes and in different system conditions. [Copyright &y& Elsevier]