A Study on the Harmonic Resonance during Energizing Primary Restorative Transmission Systems: Korean Power System Case

In this paper, a power system restoration study following a massive or complete blackout was performed. The power system restoration process from a complete shutdown system without the operating generation and load starts with energizing primary restorative transmission systems. During this primary restoration process, unexpected over-voltage may occur due to nonlinear interaction between the unloaded transformer and the transmission system. This is known as the harmonic resonance phenomenon that may cause the burning out of a transformer or other devices. So far, harmonic resonances have been reported in some extra-high voltage systems around the world. Since the harmonic resonance originates from the nonlinear characteristics of the power system components, it is very difficult to predict the occurrence of this phenomenon. This paper reports the analyses of the harmonic resonance that can occur in the Korean power system. In addition, through calculating the required buffer load compared to the length of the line, a solution that changes the length of the restoration path impedance considering the specificity of the Korean system was presented. The various analyses of harmonic overvoltage, including methodologies that are used internationally as comparison groups, are provided based on PSCAD/EMTDC simulations.

A gray-box harmonic resonance frequency identification method of multiple-inverter-fed power system oriented to system designers

This paper presents a gray-box harmonic resonance frequency identification method of multiple-inverter-fed power system, which enables modal analysis oriented to system designers based on only frequency response data provided by diverse vendors or measured by frequency scanning. First, admittance transfer functions of all grid-connected inverters (GCIs) are fitted using Matrix Pencil Method-Vector Fitting (MPM-VF) combined method. Then, node admittance matrix (NAM) is formed according to the topology of whole system. Finally, harmonic resonance frequency along with changes in number of GCIs are identified by NAM-based modal analysis (MA). The proposed gray-box identification method is implemented in a typical multiple-inverter-fed power system. The correctness of harmonic resonance frequency identification results and the effectiveness of the presented method are verified by simulation results obtained in Matlab/Simulink platform and OPAL-RT digital real-time simulation platform. Based on the identification results, a more stable and better power quality multiple-inverter-fed power system can be built by system designers though avoiding the appearance of harmonic sources with corresponding resonance frequency.

Natural Frequency and Resonance Study of FGM Turbo-Machinery Blade Using Campbell Diagram

Turbo machinery rotating blades are a vital component of aero-engines for transferring the energy of gas flow to kinetic energy. Rotating turbo-machinery blades are prone to fail under working fields with high temperature, high speed, high pressure and resonance of blades with prime engine. Vibration of the turbo-machinery blades need to be studied carefully for safe and reliable operation of the engine. Thermal barrier coatings (TBC) layers are used to protect the blade metal at higher operating temperature though these can be plagued due to phase instability, thermal instability and adherence problems. TBCs also tend to spall due to thermal expansion mismatch with the super alloy substrate or because of erosion/impact caused by particles in the high-velocity combustion gases. Functionally graded materials (FGMs) are a relatively new class of materials that can solve these problems in high-temperature environments and can be considered as potential material for making gas turbine blades. Unlike composite materials the functionally graded materials (FGMs) are free from delamination and debonding and have the ability to withstand high temperature without TBC coatings. In this present study natural frequency of the FGM turbine blade for various rotational speeds during actual operation is determined using finite element technique. As the aero-engine is complex in nature therefore harmonic resonance can always occur in the aero-engine system and it is difficult to determine the natural frequency at varying speed during actual operation. The chances of failure of the blade also increase because of harmonic resonance. Hence, the resonance property is required to investigate of the FGM blade in details. Various rotational speed and operating temperature is considered to determine the natural frequencies of the blade. The resonance studies for each case are performed using the Campbell diagram. Resonance margin is calculated for each case and safe operating speed is determined. This study could serve as ready reference for safe operation of turbo-machinery engine considering FGM blade for specific range of engine speed.

Active Damper Frequency Division Control Method for Cluster Resonance of Photovoltaic Inverters

Aiming at the harmonic resonance problem of photovoltaic inverter cluster system when it is incorporated into weak power grid, an active damper frequency division control method is proposed to suppress the harmonic resonance. Firstly, the voltage signal measured by the voltage transformer is separated according to the frequency, and then the harmonic conductance value of the frequency band is controlled respectively according to the harmonic voltage. Finally, the output current is feedback controlled by the generalized integral PI controller, so as to realize the impedance remolding of the photovoltaic inverter cluster system. This method can adjust the value of virtual conductance in different frequency band adaptively according to the harmonic voltage, so as to suppress the harmonic resonance problem of photovoltaic inverter cluster more effectively. The simulation results of Matlab/Simulink demonstrate the correctness and effectiveness of the proposed frequency division control method.

Amplitude resonance response and feedback control of cantileverbeams with tip-mass under aerodynamic load

The dynamic of a cantilever beam with tip mass is studied under an aerodynamic loading. The effects of coupling is investigated by tacking into account the fluid flow. Using the multiple time scale method, the approximative solutions are found and the study of their stability is made by the Routh-Hurwitz stability criterion. The influence of parameters on the system is studied at the harmonic and subharmonic resonances. The results show that, the effects of tip mass can be neglected in harmonic resonance case ,while they are more important in subharmonic resonance cases. The results equally demonstrate that an increase of the stable state fluid velocity reduces the amplitude of vibrations. In addition, the hysteresis phenomenon studies show that it is principally induced by nonlinearity coefficients. Finally, time-delay feedback control is applied and the effects of controlling are observed on amplitude response curve at the harmonic resonance, from where we note that optimized choice of control parameters can be useful in controlling vibrations.

Long period fibre gratings inscribed by sinusoidal intensity modulated femtosecond laser

To the best of our knowledge, this is the first study to propose and verify a novel and simple direct inscription method for long-period fibre gratings (LPFGs) with an arbitrary grating period using a sinusoidal intensity modulated femtosecond laser. This method induces a sine refractive index (RI) modulation, which makes the LPFGs have no higher-order harmonic resonance that exists in the normal square-wave modulation method. This is beneficial for reducing the insertion loss of the LPFGs and optimising the transmission spectrum. Using this method, we also conveniently realise the inscription of superimposed, apodised, and chirped LPFGs. This work presents a simple but effective inscription scheme for various types of LPFGs using a femtosecond laser.