CZT-Based Harmonic Analysis in Smart Grid Using Low-Cost Electronic Measurement Boards

This paper validates the use of a harmonic analysis algorithm on a microcontroller to perform measurements of non-stationary signals in the context of smart grids. The increasing presence of electronic devices such as inverters of distributed generators (DG), power converters of charging stations for electric vehicles, etc. can drain non-stationary currents during their operation. A classical fast Fourier transform (FFT) algorithm may not have sufficient spectral resolution for the evaluation of harmonics and inter-harmonics. Thus, in this paper, the implementation of a chirp-Z transform (CZT) algorithm is suggested, which has a spectral resolution independent from the observation window. The CZT is implemented on a low-cost commercial microcontroller, and the absolute error is evaluated with respect to the same algorithm implemented in the LabVIEW environment. The results of the tests show that the CZT implementation on a low-cost microcontroller allows for accurate measurement results, demonstrating the feasibility of reliable harmonic analysis measurements even in non-stationary conditions on smart grids

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Impedance measurement and detection frequency bandwidth, a valid island detection proposal for voltage controlled inverters

Anti-islanding detection methods have been part of a secure operation for distributed energy resource inverters, avoiding the creation of non-intentional energization when the mains are lost. These detection mechanisms were conceived historically for current-controlled inverters. New control possibilities have broken ground, and current- or voltage-controlled inverters are a reality; however, special attention must be paid to detection strategies when applied to the latter ones. This paper addresses two topics: it exposes the lack of effectiveness of those detection algorithms based on the voltage/frequency displacement concept under voltage-controlled inverters and evaluates the applicability limits of the others based on the impedance measurement (IM). The IM is presented as a valid mechanism to achieve the islanding detection, but the exploration of its limits drives the concept of detection frequency bandwidth (DFBW), introduced in this paper. The DFBW is suggested as a practical approach to select the proper injection frequency to measure. Therefore, an improved strategy based on the IM and DFBW is proposed to allow achieving the detection towards (non-)resonant loads considering low computational burden. The results were experimentally validated in a 90-kVA four-wire voltage-controlled inverter, offering detection times of less than 100 ms in any case.Postprint (published version

Grid-Connected Renewable Energy Sources

The use of renewable energy sources (RESs) is a need of global society. This editorial, and its associated Special Issue “Grid-Connected Renewable Energy Sources”, offers a compilation of some of the recent advances in the analysis of current power systems that are composed after the high penetration of distributed generation (DG) with different RESs. The focus is on both new control configurations and on novel methodologies for the optimal placement and sizing of DG. The eleven accepted papers certainly provide a good contribution to control deployments and methodologies for the allocation and sizing of DG

MatLab Simulink Modeling for Network-Harmonic Impedance Assessment: Useful Tool to Estimate Harmonics Amplification

The importance of the subject is given by the fact that harmonics are making their presence felt in electrical distribution networks, and the cheapest and most widespread solution for power factor correction is the capacitor banks. This chapter proves that the harmonic impedance is an efficient tool for assessing the state of distribution networks containing harmonics. The unfavorable operating conditions are anticipated based on the network harmonic impedance values, and the means of intervention are selected. Harmonic impedance monitoring and using it in expert systems for operating condition optimization will increase in the future. Power factor correction by shunt capacitor switching in electrical networks containing harmonics can lead to harmonics amplifications by harmonic voltage increasing and capacitors thermal overstressing by great values of the currents flowing through them. This chapter proposes a method for practical determination of harmonic impedance. Based on its values, a quick method is developed to anticipate the harmonic voltages and current amplifications that can occur when a shunt capacitor is installed for power factor correction. Amplification factors are calculated depending on the equivalent harmonic impedance of the network seen in the compensation bus. A distribution network containing harmonics is modeled using MatLab Simulink, and harmonic impedance is determined by simulation in different operating conditions. Using the values of the harmonic impedance and the capacitive reactance of the capacitor bank that is connected for power factor correction, the amplification of the harmonic voltages and currents is estimated by calculus. The results obtained by calculus are then compared with the values obtained by simulation after the connection of the capacitor bank to the network. In conclusion, the chapter proves that the network harmonic impedance is a useful tool to estimate the harmonics amplification caused by power factor correction using shunt capacitor banks

COMPRESSIVE SENSING-BASED METHODOLOGIES FOR SMART GRID MONITORING

Modern distribution networks, commonly known as Smart Grids, will be characterized by strictly requirements in terms of reliability and efficiency of the power supply. This will require a high empowerment in the management of the distribution, and transmission, networks by the system operators. Problems such as the identification of the prevailing harmonic sources and the fault location are characterized by criticality which must be appropriately taken into account, in order to fully exploit the capabilities of the Smart Grids. The analysis of both phenomena requires an appropriate monitoring of the networks, which are currently characterized by the availability of a limited number of measurements. This increase the complexity of the analysis of distribution networks, and the necessity of developing ad-hoc algorithms and solutions aimed at supporting the system operators while managing the networks. In this thesis, Compressive Sensing-based algorithms for detecting the main harmonic polluting sources, and for identifying the location of faults occurring in distribution systems have been presented. With reference to the identification of the main harmonic sources, two algorithms have been proposed: one for detailed analysis, with reference to a specific harmonic order, and one for more general analysis, which allows to investigate multiple harmonic orders simultaneously. The performed tests have proved how both methodologies are robust with respect to the measurement uncertainties, underlying the different capabilities of the two methods. Contrarily, the performance of the fault location algorithms are more influenced by the higher uncertainties in measuring the dynamic signals involved during the fault. The analysis performed considering the proper uncertainty scenarios have underlined how the use of modern devices for branch current measurements allow to increase the performance of the fault location algorithms; providing additional information which are useful for locating the fault

Neural network for estimating and compensating the nonlinear characteristics of nonstationary complex systems

Issued as final reportNational Science Foundation (U.S

Control of voltage source converters connected to variable impedance grids

The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second.The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second