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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]

Recently, a high-frequency resonance (HFR) incident has been captured in an actual HVDC project of China Southern Power Grid, which is regarded as a new stability issue caused by the interaction between the modular multilevel converter (MMC) and the weak ac grid. To investigate this event, this paper proposes a stability analysis method based on the frequency-coupling impedance model (FCIM). First, the FCIMs of the MMC and the ac grid are established in abc-frame individually with the frequency coupling effect of the MMC taken into account. Next, they are aggregated into a two-by-two impedance matrix by numerical calculation. Then, based on the aggregated FCIM, a criterion is developed to locate the HFR modes and to evaluate their stability. Finally, the results obtained from FCIM-based analysis are verified by electromagnetic transient (EMT) simulations as well as the field measurements. [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]

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]

This paper analytically examines how the condition of weak grid connection affects the torsional sub-synchronous oscillations (SSOs) induced by a grid-connected DFIG in a power system. The examination focuses on the SSO mode of grid-connected DFIG, which is dominated by the phase locked loop (PLL) or DC voltage control. Theoretical analysis is carried out to prove that the condition of weak grid connection amplifies the impact of modal resonance between the DFIG and torsional dynamics of synchronous generators. The amplifying effect of weak grid connection exhibits as the decrease of damping of SSO mode of DFIG and increase of the associated residue when the grid connection weakens. Consequently, the DFIG is more likely to induce torsional SSOs under the condition of weak grid connection, leading to system instability in the worst case. Analytical examination presented in the paper reveals the mechanism about how the condition of weak grid connection plays a key role for the DFIG to induce the torsional SSOs. An example multi-machine power system with the grid-connected DFIG is presented. Results of modal analysis and non-linear simulation using detailed model of DFIG validate the analysis and conclusions made in the paper. [ABSTRACT FROM AUTHOR]

This paper proposes a novel bipolar modular multilevel converter (MMC) topology with dc fault ride-through capability. The proposed bipolar MMC topology is a hybrid of two types of arms: the half-bridge submodule (HBSM)-based arms (HBSM-arms) which are connected to the ground pole, and the unipolar-voltage full-bridge submodule (UFBSM)-based arms (UFBSM-arms) which are connected to the positive or negative pole. During a pole-to-ground or pole-to-pole dc fault, the hybrid-arm design offers the proposed UFBSM-based hybrid-arm bipolar MMC (UHA-BMMC) the capability to provide additional reactive power to support the ac grid while blocking the dc fault current. The topology, operation principle, dc fault mechanisms, and dc fault ride-through scheme of the UHAB-MMC are presented in this paper. Compared with other possible bipolar MMC topologies, the UHA-BMMC provides good dc fault ride-through capability with a small cost of power-electronic devices. The validity and features of the proposed UHA-BMMC are verified by simulation results conducted in PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]