Amrit S. Khalsa

Co-Optimization Scheme for Distributed Energy Resource Planning in Community Microgrids

Microgrids with distributed energy resources are being favored in various communities to lower the dependence on utility-supplied energy and cut the CO2 emissions from coal-based power plants. This paper presents a co-optimization strategy for distributed energy resource planning to minimize total annualized cost at the maximal fuel savings. Furthermore, the proposed scheme aids the community microgrids in satisfying the requirements of U.S. Department of Energy (DOE) and state renewable energy mandates. The method of Lagrange multipliers is employed to maximize fuel savings by satisfying Karush–Kuhn–Tucker conditions. With the Fourier transform and particle swarm optimization, the right mix of distributed energy resources is determined to decrease the annualized cost. A case study to test the proposed scheme for a community microgrid is presented. To validate its effectiveness, an economic justification of the solution and its comparison with HOMER Pro are also illustrated.

Analysis of limiting bounds for stalling of natural gas genset in the CERTS microgrid test bed

Natural gas powered distributed energy resources (DERs) are gaining more popularity due to the recent discoveries of huge natural gas reserves in several places. These DERs however suffer from the tendency to stall under large load fluctuations owing to the fuel map limits present in the engine governors and the inherent time lag in torque production. A critical analysis of the stalling causes is conducted for a natural gas engine powered synchronous generator (referred as genset) that was installed in the CERTS Microgrid. The grazing phenomenon is explored for analyzing the stalling behavior. Simulation studies are performed for different test case scenarios, and it is shown that such DER units can operate reliably near their rated capacity without stalling if the load changes are effected in a proper manner. The ability to handle load changes in such gensets without the need for any load reliefs is analyzed and the possible solutions are discussed for avoiding such stalling occurrence.

Design and Operation of Smart Loads to Prevent Stalling in a Microgrid

This paper investigates the benefits of smart loads in an iXndustrial microgrid. Such smart loads offer the microgrid greater flexibility to regulate frequency for large fluctuating load scenarios, commonly encountered in industrial plants. A case study of stone-crushing facility is analyzed when it is fed by natural gas engine driven generator sets (also known as “gensets”). The problem of genset stalling is addressed in this work, and a solution of deploying smart loads is presented. Furthermore, smart load algorithms are proposed to restore the services at an industrial site in a timely manner.

Modified Viterbi Algorithm Based Distribution System Restoration Strategy for Grid Resiliency

This paper presents a novel modified Viterbi algorithm to identify the optimal distribution system restoration plan for improving the grid resiliency. In the proposed algorithm, the switching operations performed for system restoration are the states with the minimum bus voltage being seen as the cost metric for each state and the extent of load recovery as the observed event. When full load recovery is spotted, the dynamic programming algorithm stops, thereby giving the least number of switching pairs necessary for system restoration. Moreover, an improved flexible switching pair operation is employed to maintain the radial nature of distribution system. Several case studies are presented for verifying the performance of the proposed strategy. Multi-fault conditions are considered in testing the system restoration scheme on 33-bus and 69-bus distribution systems. Furthermore, the effects of integrating distributed energy resources and microgrid systems are analyzed.

Evaluation of Control Methods to Prevent Collapse of a Mixed-Source Microgrid

For a microgrid with a mix of distributed energy resources (DERs), major challenges on its survivability are found in the islanded condition. In particular, a sudden loss of generation or a large and fluctuating load could force the microgrid to operate near its capacity limits. Such a situation can cause a cascading collapse of the mixed-source microgrid, even when the load demand is within the systems power rating. This condition was observed during several tests carried out at the Consortium for Electric Reliability Technology Solutions Microgrid Test Bed. This paper analyzes the root causes behind the collapse. It highlights that the capacity of a low-inertia system to support load changes is contributed by faster responding DERs initially. Therefore, the microgrid is particularly susceptible if the faster responding DERs do not have adequate reserve margin. Two control methods are evaluated for providing safeguards to these DERs and prevent the system collapse.

Analytical methods for characterizing frequency dynamics in islanded microgrids with gensets and energy storage

Microgrids with increased penetration of renewables experience serious challenges due to large frequency excursions under power system disturbances. An energy storage system can provide frequency regulation, but the effectiveness depends on whether it is configured for the grid-forming or grid-following mode of operation. For this purpose, two critical parameters are studied for frequency regulation in distribution systems; first, the initial rate-of-change-of-frequency (ROCOF), and second, the minimum value of frequency, also known as frequency nadir, following a load change. The aim of this paper is to identify analytical methods for accurately calculating the frequency parameters like ROCOF and frequency nadir. Reduced-order models are developed to determine the frequency deviation against power system disturbances. The results are verified against simulation models validated by testing at the Consortium for Electric Reliability Technology Solutions Microgrid test bed.

Distributed energy resource planning for microgrids in the united states

New measures for increasing energy efficiency and reducing CO2 emissions are being introduced by several nations to tackle the growing concerns on climate change. This paper presents optimal distributed energy resource (DER) planning for establishing microgrids. Such microgrid systems are devised for industrial sites and campus communities in the United States after evaluating location specific DER options. These are designed to maximize fuel consumption savings, besides meeting the U.S. Department of Energy (DOE) requirements and state renewable energy mandates. A qualitative evaluation of various types of DERs is presented cogitating factors like cost, environment, fuel consumption, controllability, longevity and government policy. Furthermore, the method of Lagrange Multipliers is used to determine the optimal allocation of DERs for microgrids. Due consideration is given to various factors such as climatic conditions, prevailing energy resources, and environmental regulations. The planning strategy presented in this paper can be extended to other conditions with minimal adjustments.

Evaluation of control methods to prevent prime-mover stalling in a mixed source microgrid

For a microgrid with a mix of distributed energy resources (DERs), major challenges on its survivability are found in the islanded condition. In particular, a sudden loss of generation or a large and fluctuating load could force the microgrid to operate near its capacity limits. Such a situation can cause a cascading collapse of the system, even when the load demand is within the DERs kW rating - as observed during several tests at the Consortium for Electric Reliability Technology Solutions (CERTS) Microgrid Test Bed. This paper analyzes the prime-mover stalling phenomena behind the system collapse. It highlights how the reserve margin of the system is lowered during transient conditions. Furthermore, two control methods are evaluated to resolve the microgrid collapse problem.

Survivability of synchronous generator-based distributed energy resources for transient overload conditions in a microgrid

The synchronous generator-based distributed energy resource (DER) is widely used nowadays due to its low cost and simple controls. However, the requirements for power delivery in a microgrid can be stringent due to the harsh demands. Frequent overloads can occur during the microgrid transition between grid-connection and islanded operation. Therefore, it is very important to have a clear understanding on the survivability of a synchronous generator-based DER for transient overload conditions. Recently, experimental tests on survivability of the DER were carried out in the Consortium of Electric Reliability Technology Solutions (CERTS) Microgrid Testbed at American Electric Power. In this paper, an in-depth analysis of the synchronous generator-based DER operation during transient overload conditions is presented. The key relationships governing the system dynamic behavior are investigated to design the load shedding scheme for survivability of the synchronous generator-based DER. Several case studies are presented to demonstrate the validity of the proposed load shedding methods.

Survivability of Autonomous Microgrid during Overload Events

Grid-forming sources are voltage sources that draw necessary currents to meet any load changes. A load step can cause part or all of these sources to become overloaded in a microgrid. This paper presents an overload mitigation controller that addresses the two overload issues in a microgrid by actively controlling the sources’ frequency. When part of the sources in a microgrid is overloaded, the controller autonomously transfers the extra load to other sources by rapidly reducing its frequency. The frequency difference between sources during transient results in a change of phase angle, which redistributes the power flow. When all sources in a microgrid are overloaded, each source keeps dropping the frequency. Therefore, under frequency load shedding can be used to trip the non-critical loads resulting in the survival of microgrid. The advantages of these concepts are that communications between sources are not needed during transient, and the robust voltage control is maintained. Simulation and field tests from Consortium for Electric Reliability Technology Solutions/American Electric Power microgrid test site verify that the control strategy is effective in both purely inverter-based microgrids and inverter and generator mixed microgrids.