Analyses of transformer electromagnetic vibration noise are presented in this study. A finite element model is established which combines transient electromagnetic field analysis, mechanical field ...analysis and acoustic analysis to calculate the sound pressure level of the radiated noise around the transformer. Transient electromagnetic field analysis is performed to get the Lorentz, reluctance magnetic forces and magnetostriction according to Maxwell theory and virtual displacement principle. Mutual influence of strain and magnetisation has been considered. The main frequency components of harmonic electromagnetic excitations are analysed by Fourier transformation so as to carry out the harmonic response analysis and obtain the nodes displacements of the windings and core. The noise distribution is further calculated by acoustic analysis based on the achieved vibration data. Comparison of calculated results and measured data verifies that the combined noise calculating model is applicable for transformer noise prediction.
As the requirements of economical operation and reliability on power grid are enhanced gradually nowadays, the existing under frequency load shedding (UFLS) scheme is not quite fit for the modern ...power system that integrates high wind power. In this paper, the impacts of high wind power penetration on the UFLS are discussed thoroughly. A novel adaptive load shedding (LS) scheme is presented taking the high wind power penetration into account. In the proposed scheme, the equivalent inertia constant (EIC) is calculated accurately to improve the power deficit accuracy so as to reduce the error of LS. The dynamic correction of power deficit is able to solve the negative effects of the wind power output random reduction/the wind generator tripping. Besides, the locking criterion is capable of avoiding the influences of the wind power output random increase on the LS, thus cutting down the LS costs and even preventing the frequency overshoot. Moreover, in terms of the LS parameters setting, the coordination of the low frequency protection of the wind generator and the frequency threshold is addressed. The location and capacity model of LS, which is based on the load characteristics, can ameliorate the frequency recovery process. Finally, the validity and robustness of the proposed scheme are verified in the simulations on the IEEE-39 bus system with high wind power penetration.
Under the background of complicated interconnected network, the splitting criterion for accurately capturing the electrical center in real time is the prerequisite of power grid splitting. This paper ...studies the features of electric quantity in the electrical center in aspect of the instantaneous frequency, and proposes the out-of-step splitting criterion for power systems based on bus voltage frequency. Firstly, through the establishment and solution to the out-of-step model of the power grid, the analytical expression of the voltage frequency at any position is obtained in the out-of-step oscillation, and the voltage frequency features of electrical center and non-electrical center are analyzed in details. Then, this paper constructs the typical scene of migration of electrical center to study the change rules of voltage frequency. Finally, the splitting criterion based on bus voltage frequency is proposed as well as the instruction for use. This criterion is easy to be realized and can adapt to the migration of electrical center. Also it is free from the limits of power network structure and operational mode. Simulation results of CEPRI-36 system and interconnected network example of one actual region verify the accuracy and the effectiveness of the proposed criterion.
Under the background of clean and low-carbon energy transformation, renewable distributed generation is connected to the distribution system on a large scale. This study proposes a probabilistic ...assessment method of hosting capacity considering wind–photovoltaic–load temporal characteristics in distribution networks. First, based on time series of wind, photovoltaic, and load demands, a discretization–aggregation technique is introduced to generate and filter extreme combinations. The method can effectively reduce the scenarios that need to be evaluated. Then a holomorphic embedding method considering generation and load scaling directions is proposed. The holomorphic function of voltage about an embedding variable is established, and it is analytically expanded in the form of series. The hosting capacity restrained by the voltage violation problem is calculated quickly and accurately. Finally, the proposed stochastic framework is implemented to evaluate hosting capacity involving renewable energy types, penetration levels, and locations. The hosting capacity of single energy and hybrid wind–solar renewable energy systems is evaluated from the perspective of probability analysis. The results verify the outstanding performance of the hybrid wind–solar energy system in improving the hosting capacity.
Aiming at studying the influence of the charging uncertainty and load fluctuation characteristics in power system with electric vehicles (EVs) plugged in, a static voltage stability assessment scheme ...which considers both of the charging characteristics and load fluctuation limits is proposed. First, a mixed approach based on continued power flow (CPF) and probabilistic load flow (PLF) is employed to search for the critical states of the power system voltage instability. Then, the marginal operating states of power systems can be analyzed statistically. Thirdly, several evaluation indicators are presented to determine the weakened areas in the voltage stability. These weak regions will usually in advance experience voltage violation when system voltage collapse happens and they can be served as important monitoring objects. Finally, the analysis results of IEEE 30-bus system verify the accuracy and effectiveness of the presented scheme.
This paper proposes a modified temporary over-production inertial control. It consists of two parts: the deceleration phase and the acceleration phase. In the deceleration phase, the reference active ...power maintains constant first. Then it starts to decrease linearly with the decrease of the rotor speed when the DFIG has released half of the available energy stored in the rotating masses. In the acceleration phase, the reference active power switches to the mean value of mechanical power and electromagnetic power under maximum power point tracking control. Compared with the temporary over-production inertial control, the modified temporary over-production inertial control is simplified with one parameter and there is no over-speed. Moreover, the performance of the second frequency dip is improved with less power reduction. The simulation results show that the modified temporary over-production inertial control has the same ability to arrest the frequency nadir as temporary over-production inertial control while improving the performance of the second frequency dip.
In the case of high-permeability wind power access power system, the virtual inertia control (VIC) help the variable speed wind turbine (VSWT) release the rotational kinetic energy of the rotor and ...help the wind power participate in the system frequency regulation. However, there are few literatures that apply the VIC in the transient stability control (TSC) of power system. This paper proposed a VIC strategy of VSWT, which help to prevent new swinging modes. And the complicated swinging modes might lead to the migration of the out-of-step oscillation center. In the out-of-step process of a system, additional active power can be injected into or absorbed from the system by VSWTs under the VIC to reduce the unbalanced power of the system. It inhibits the complex swinging modes and frequencies, thereby inhibiting the migration of the out-of-step oscillation center and helping to maintain system stability. Simulation results of the Western Systems Coordinating Council (WSCC) 9-bus system containing 45% of wind permeability verify the proposed virtual inertial control on inhibiting the migration of the out-of-step oscillation center.
Transient stability is of essential concern in power systems. With the widespread of wide area measurement system, the method using geometric features of trajectory in phase plane to detect power ...system transient instability online were proposed. This paper provides a graphical method based on phase plane. The dimension of the power angle trajectory was reduced in power-angle space so the trajectory was projected in a one-dimensional coordinate, to intuitively show stability characteristics, a new phase plane is defined based on the motion parameters obtained by dimension reduction and the transient stability could be judged by the convexity of trajectory in the new phase plane. The simulation of the New England 39-bus system shows that the proposed method provides an intuitive and effective approach for multi-machine system transient stability analysis and is useful in field applications.
When power system is subjected to severe disturbances and then will be out of step. The interconnected power grid should be split into several islands to avoid blackout. The controlled islanding is ...an active control technique based on the information from Wide Area Measurement System (WAMS) to accomplish the islanding task. This paper proposes a fast islanding strategy for large power networks. Firstly, the grid structure is mapped to point sets and edge sets in the graph theory. Secondly, the model is built by adopting the minimum power flow impact as the objective function and considering several constraints which includes generator coherency constraint and connective constraint. Finally, the optimal splitting strategy is searched by the improved Dinic maximum flow algorithm. Simulation results on IEEE 118-bus power system show that the proposed strategy is correct, effective and fast.
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