Nimish Soni

Improvement of Transient Response in Microgrids Using Virtual Inertia

Generation is shifting from a centralized power generating facility having large synchronous generators to distributed generation involving sources of smaller capacity. Most of these sources require inverters on the front end while being connected to the grid. Lower available kinetic energy, coupled with less short-circuit current ratio compared to large synchronous generators, compromises the transient stability of the microgrid when isolated from the main grid. Sources in the microgrid use droop control to share power according to their capacity without any form of communication. This paper proposes a novel controller for inverters to improve the frequency response of microgrid under disturbances involving large frequency deviations. It also discusses design of various parameters defined for the proposed control. The microgrid, which has two inverters and two synchronous generators, is simulated using Simulink/MATLAB software to test the proposed control strategy.

Inertia Design Methods for Islanded Microgrids Having Static and Rotating Energy Sources

Dynamic frequency regulation and effect of penetration of static and inertial sources on system stability are important issues for islanded microgrid power quality and reliability. This paper presents a novel strategy of utilizing an inverter-based source as a voltage source inverter or virtual synchronous generator (VSG). Electromechanical and power modes are critical for small signal stability of an isolated microgrid having static and inertial sources. Interaction of these modes is analyzed through eigenvalue analysis of microgrid model and differential equations describing respective modes. Inertia is important for providing fault current, determining steady state and transient stability, and better system frequency profile. A novel technique is proposed to include inertia virtually to the inverter-based sources by adding swing equation. Furthermore, inverter-based sources with traditional and modified droop controls and VSGs are compared with respect to inertia, energy, and stability. The proposed control and stability comparison are verified through experimental microgrid setup having three inverter-based sources, which can be alternately operated as VSGs.

Analysis of frequency transients in isolated microgrids

Microgrids are subjected to limits on system voltage and frequency deviations for transient and steady state operation. Similar to a conventional power system with rotating generators, inertia and damping of frequency are necessary for frequency profile and stability of an isolated microgrid. This work proposes an approach to analyze inertia and frequency damping coefficient for a system based on the fact that sources relatively close to each other observe similar transients in frequency within an isolated microgrid. This is validated through bode plots and simulation. The approach allows defining the system response to transients in frequency in terms of individual source response using standard frequency mode parameters such as damping and inertia. Further, to optimize the overall system response so that sources share the frequency regulation requirement, a novel strategy is proposed to design individual source inertia and damping coefficient based on the source capacity. An experimental setup consisting of a microgrid system having three inverter-based sources working as virtual synchronous generators operating in parallel has been set up to verify the analysis.

Transient analysis of an inverter based source operating in microgrids

Traditional microgrids have sources whose transient response is limited by process or mechanical time constants depending on the technology used. Inverter based sources provide the flexibility in designing and tuning the controller parameters so as to operate microgrid within specified constraint in voltage and frequency. For this it is imperative to investigate the effect of controller parameters and different operating conditions on inverter response and define the response in terms of standard transient parameters such as damping, natural frequency and inertia. In this work a methodology has been defined to model the inverter based source as voltage magnitude and phase transfer function. Effect of controller and filter parameters as well as different operating conditions on low, medium and high frequency inverter transients is then obtained and analyzed. Simulation of an inverter based source, is done using Simulink/MATLAB software to validate the analysis.

Inertia design methods for islanded microgrids having static and rotating energy sources

Dynamic frequency regulation and effect of penetration of static and inertial sources on system stability are important issues for islanded microgrid power quality and reliability. This paper presents a novel strategy of utilizing an inverter-based source as a voltage source inverter or virtual synchronous generator (VSG). Electromechanical and power modes are critical for small signal stability of an isolated microgrid having static and inertial sources. Interaction of these modes is analyzed through eigenvalue analysis of microgrid model and differential equations describing respective modes. Inertia is important for providing fault current, determining steady state and transient stability, and better system frequency profile. A novel technique is proposed to include inertia virtually to the inverter-based sources by adding swing equation. Furthermore, inverter-based sources with traditional and modified droop controls and VSGs are compared with respect to inertia, energy, and stability. The proposed control and stability comparison are verified through experimental microgrid setup having three inverter-based sources, which can be alternately operated as VSGs.

Small signal stability in microgrids with high penetration of power electronics interfaced sources

In this paper, effect of power electronics interfaced sources on small signal stability in the presence conventional sources (synchronous generators) in a microgrid is studied. A detailed mathematical model for small signal analysis is presented. The models are linearised around operating point and state space matrix is used to find the eigenvalues at that point. Eigen value analysis is done to study the effect of placement of inverter and/or synchronous generator in a system on its stability. The effect of droop system response is also studied. Steady state stability of microgrid with different combinations of inverters and synchronous generators is presented and maximum allowable droop gain and damping factor for these cases is determined. From the analysis it is found that inverters at some locations tend to make system critical. It is also found that replacing these inverters with synchronous generators increase the stability margin. The results is validated by simulation (eigen value analysis) in MATLAB.

Analysis of Frequency Transients in Isolated Microgrids

Microgrids are subjected to limits on system voltage and frequency deviations for transient and steady state operation. Similar to a conventional power system with rotating generators, inertia and damping of frequency are necessary for frequency profile and stability of an isolated microgrid. This work proposes an approach to analyze inertia and frequency damping coefficient for a system based on the fact that sources relatively close to each other observe similar transients in frequency within an isolated microgrid. This is validated through bode plots and simulation. The approach allows defining the system response to transients in frequency in terms of individual source response using standard frequency mode parameters such as damping and inertia. Further, to optimize the overall system response so that sources share the frequency regulation requirement, a novel strategy is proposed to design individual source inertia and damping coefficient based on the source capacity. An experimental setup consisting of a microgrid system having three inverter-based sources working as virtual synchronous generators operating in parallel has been set up to verify the analysis.

Short-term electrical load forecasting using predictive machine learning models

Availability of cheap power through alternative means such as energy exchanges and bilateral agreements is resulting in short-term load forecasting gaining importance among industries, residential complexes and corporate buildings. Short-term forecasting over an hour or a day requires non-linear predictive models. Machine learning algorithms such as neural networks are inherently non-linear and are suitable for accurate forecasting. This paper compares neural networks, decision trees and Conditional Restricted Boltzmann Machines algorithms for forecasting short-term demand. The algorithms are tested on power consumption data acquired from two test sites with different consumption profiles.

Analysis of motor starting in a weak microgrid

Induction motor loads form an integral part of any electrical load network and possess a unique characteristic. The high reactive power drawn from the system by motor during start up causes voltage drop which may affect other loads in the network. This drop is more pronounced in a weak microgrid, which do not have the capability to quickly compensate the drop. The only way to determine voltage drop due to starting current or vice-versa is simulation studies. The initial work is aimed at filling this gap by providing a methodology to determine a relationship between the starting current and the voltage drop as a function of short circuit current values at that point. The impact of motor starting on the voltage profile is studied for a standard network. Subsequently, a control methodology is proposed for an inverter based source to improve the power quality during motor starting. The methodology makes use of the existing infrastructure with limited additional investments, if any to cater to reactive power requirements during motor starting. For study, a network energized by inverter interfaced photovoltaic array and a generator is considered. The studies have been performed based on computer simulation carried out using PSCAD™/EMTDC™ software package.

Study on optimal harmonic energy harvesting from non-linear loads within enterprise buildings

Increasing penetration of non-linear loads, results in lines carrying harmonic currents in addition to the fundamental frequency current. Harmonics cause increased losses in distribution system components and malfunctioning of certain sensitive loads. The problem is generally addressed by installing passive or active harmonic filters. However, rather than just suppressing them, if filter configurations are modified to harness energy associated with harmonics and utilize it for suitable applications, the cost of filter installation can be paid back. This paper proposes a concept of harvesting energy associated with dominant harmonic frequency component of sites having non-linear loads and utilize the energy for supplying heating loads present within the same premises. To harness the dominant harmonic energy, a tunable filter with harvesting load and a variable resistor is dynamically tuned to the harmonic frequency having dominant power at that instant. The paper also describes the possible filter configurations which can be utilized to harness the energy according to spectral components of the non-linear load present within the facility. Apart from harmonics, possibility of harvesting energy associated with other spectral components such as inter-harmonics and sub-harmonics is also discussed. To test the possibility of harvesting harmonic energy available in enterprise buildings a test site with predominantly computer/printer and HVAC load is considered and analyzed.