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Dynamic interactions introduced by grid-connected wind farms are multivariable. This is why wind farms’ models need to be simplified when the impedance-model-based analysis is applied to examine whether negative resistance is supplied by the wind farms to cause subsynchronous oscillations (SSOs). In this paper, we examine and reveal the mechanism of torsional SSOs caused by grid-connected wind farms from the standpoint of near strong modal resonance (NESMOR). The examination and revelation consider fully the multivariable dynamic interactions introduced by wind farms in a complex multi-machine power system without simplifying the models of wind farms. We carry out the analysis based on the multi-input multi-output (MIMO) closed-loop model with two interconnected subsystems. The analysis indicates that the open-loop modal proximity (OLMP) of interconnected subsystems is a special case of NESMOR. When the NESMOR happens, growing torsional SSOs may be excited by the grid-connected wind farms. Hence, the analysis unveils the connection between the modal condition of OLMP and NESMOR, shedding a deeper insight into the mechanism of torsional SSOs caused by wind farms. In addition, we propose a method to detect the NESMOR. An example multi-machine power system with two grid-connected wind farms is presented to demonstrate the analysis and method.

This paper introduces a new approach to obtain the generator rotor angle difference in the multi-machine power system for transient stability studies. It aims to replace the center of inertia (COI) for on-line transient stability assessment. Compared with COI, the proposed method can give more reasonable estimations of rotor angle difference between the individual generator and the multi-machine power system. In addition, it allows generators to have their own references for evaluating transient stability, which is more reasonable and accurate than using COI. Moreover, the proposed technique calculates the rotor angle difference by using the local generator's active and reactive power output. This does not add any extra burden to the communication channel.

This paper presents a global tuning procedure for conventional PSSs in a multi-machine power system using a non-linear optimization algorithm implemented in a simulation program. The main objective of this procedure is to simultaneously optimize pre-selected parameters of those PSSs having fixed parameters in coping with the complex non-linear natures of the power system. By minimizing a non-explicit target function in which the oscillatory rotor modes of the generators involved and swing characteristics between areas are included, overall power oscillations at transient conditions in the multi-machine system are improved. A four-machine power system is applied to demonstrate the efficiency and robustness of the presented tuning procedure. The results obtained from simulations validate the improvement in damping of overall power oscillations in the system. The simulation also shows that the optimized PSSs are robust in providing adequate damping for a range of conditions on the system.

This paper presents a design procedure for a fuzzy logic based power system stabilizer (FPSS) and investigates the robustness of the FPSS for a multi-machine power system. Speed deviation of synchronous generator and its derivative are chosen as the input signals to the FPSS. A two area, five generator power system is used to illustrate the robustness of the FPSS. A normalized sum-squared deviation (NSSD) index is used for designing the FPSS and demonstrating its robustness. Nonlinear simulation tests for different disturbances and their results are discussed.

The paper presents an approach based on integral of squared error (ISE) technique for tuning of the parameters of power system stabilizers (PSS) in a multi-machine power system. The PSS are tuned sequentially, depending on which machine needs stabilization most. However, later it has been establishes that if the tuning algorithm is iterated twice or more, no particular tuning sequence is necessary. Dynamic performance with PSS tuned by the proposed method is compared with a previously reported approach in which all the stabilizers are tuned simultaneously. The proposed approach provides satisfactory system dynamic performance and the method is computationally much simpler.

The control signals, needed for the optimal stabilization of an interconnected Multi-machine system, are obtained using a two-level control strategy. At the lower level, local control signals are derived assuming no interaction between different machines. Then a global control signal is generated at a higher level to minimize the effect of interactions.