DFT-based recursive group-harmonic energy distribution approach for power interharmonic identification

AbstractThe Discrete Fourier Transform (DFT) is still a widely used tool for analyzing and measuring both stationary and transient signals in power system harmonics. However, the misapplications of DFT can lead to incorrect results caused by some problems such as the aliasing effect, spectral leakage and picket-fence effect. The strategy of a DFT-based recursive Group-harmonic Energy Distribution (GED) algorithm is developed for system-wide harmonic/interharmonic evaluation in power systems. The proposed algorithm can restore individual dispersing spectral leakage energy caused by the DFT, and thus retrieve respective real harmonic/interharmonic value. Every distribution of energy minimizing iteration procedure for harmonic/interharmonic evaluation can be convergent fast, and therefore guarantee each harmonic/interharmonic magnitude and respective frequency approaches its actual value. Consequently, not only can high precision in integer harmonic measurement be retained, but also the interharmonics can be identified accurately, particularly under system frequency drift. A numerical example is presented to verify the proposed algorithm in terms of robust, fast and precise performance

Online Detection of Fundamental and Inter-harmonics in AC Mains for Parallel Operation of Multiple Grid-Connected Power Converters

Parallel operation of multiple grid-connected power converters through LCL filters is known to have the potential problem of triggering oscillations in the ac mains. Such oscillatory frequencies are not integral multiples of the fundamental frequency and hence form a new source of interharmonics. Early detection of such oscillations is essential for the parallel power converters to move out of the unstable zone. This paper presents an online observer-based algorithm that can perform fast detection of interharmonics within a specified frequency band. The algorithm has been adopted in a specific and reduced form from an integral observer algorithm for detection of fundamental and interharmonic voltage components in the ac mains. A new method based on the kernel signal for fast interharmonic detection is proposed and practically verified. It has been implemented in a digital controller to detect oscillations such as those occurring between two grid-connected power converters. The practical results indicate that the algorithm can locate such frequency within the specific frequency band within 1 mains cycle

Protection of multi-inverter based microgrid using phase angle trajectory

This thesis presents a simple, yet a clever way of using the current phase angle to develop low bandwidth communication-assisted line protection strategies for medium and low voltage AC microgrids, particularly those with multi-inverter interfaced distributed generators. It is now a trend in both AC transmission and distribution segments of power network that inverters interface renewable energy to the system. Unlike synchronous generators the fault feeding, and control characteristic of these generators are different and mostly influenced by the topology, switching, control deployed in the power electronics interface. The limited and controlled fault current challenges the existing conventional protection schemes. Offering higher power supply reliability and system resilience than conventional radial distribution systems, multi-inverter based microgrids, particularly those with loop and mesh typologies, are characterised by bidirectional power flow. This further constrains traditional protections such that communication-less protection schemes become ineffective for such systems. So unit protection types, such as differential protection, become more technically suitable for such microgrids despite the necessity for a communication system. In this thesis, two current direction based protection schemes for medium voltage islanded microgrids have been developed. The change in current flow direction in a line is detected using the cosine of the positive sequence current phase angle. Expressing the change and no-change of the flow directions as binary states, a low bandwidth communication based protection scheme is proposed comparing the binary states from local and remote ends of the line. To further enhance the scope and reliability of this scheme, a second protection scheme is proposed in Chapter 7 whereby the cosine function is combined with the rate of change of the slope of the phase angle (ROCOSP). This combination allows the detection and isolation of a fault even with the failure of the communication channel between relays protecting a faulted line. Furthermore, these scheme can work together and share the communication infrastructure as primary and backup protections. The performance of these schemes was assessed through simulations of microgrid models developed in Matlab/Simulink.Open Acces

New advanced methods for the spectral analysis of time-varying waveforms in power systems

This thesis presents new advanced methods for the spectral analysis of time-varying waveforms in power systems. First, the main non-parametric, parametric and hybrid methods are presented in details under an analytical review of the state of the art, stressing both their advantages and their weaknesses. Then, a new advanced modified parametric method and three new advanced hybrid methods are presented in this thesis. All of the proposed methods guarantee an accuracy typical of the parametric methods, though with a significantly lower computational efforts

The estimate of amplitude and phase of harmonics in power system using the extended kalman filter

Nowadays, the amplitude of the harmonics in the power grid has increased unwittingly due to the increasing use of the nonlinear elements and power electronics. It has led to a significant reduction in power quality indicators. As a first step, the estimate of the amplitude, and the phase of the harmonics in the power grid are essential to resolve this problem. We use the Kalman filter to estimate the phase, and we use the minimal squared linear estimator to assess the amplitude. To test the aforementioned method, we use terminal test signals of the industrial charge consisting of the power converters and ignition coils. The results show that this algorithm has a high accuracy and estimation speed, and they confirm the proper performance in instantaneous tracking of the parameters

A survey of techniques applied to non-stationary waveforms in electrical power systems

The well-known and ever-present time-varying and non-stationary nature of waveforms in power systems requires a comprehensive and precise analytical basis that needs to be incorporated in the system studies and analyses. This time-varying behavior is due to continuous changes in system configurations, linear load levels and operating modes of nonlinear load / equipment and thus present conceptual and practical challenges. The objective of this paper is to provide a comprehensive bibliographical survey of the proposed techniques to deal with time-varying and non-stationary waveforms in power systems