Ajit A. Renjit

Graphical and Analytical Methods for Stalling Analysis of Engine Generator Sets

Reciprocating engine driven generator sets (or “gensets”) are a leading class of distributed energy resources (DERs) from the low kilowatt up to hundreds of kilowatt ratings. Natural-gas-based DERs are more likely to be favored by both industries and utilities in the near future because of their low cost due to the newly found shale deposits at many places in the world. The DERs such as gensets when operating under islanded conditions are susceptible to stalling and can bring down the entire industrial power system, particularly if they are loaded beyond 80% of the rated engine capacity. This paper presents graphical and analytical methods to determine the stalling conditions of the most common type of gensets that have a synchronous generator as the utility interface. A systematic approach is developed from the first principles of generator electromagnetic energy conversion. The engine fuel map limits have been integrated into the generator speed versus active power characteristics for this purpose. The stalling prevention by applying an underfrequency load relief (V/Hz) scheme is also discussed.

Modeling and control of a natural gas generator set in the CERTS microgrid

The recent addition to the CERTS microgrid was a commercial off-the-shelf natural gas engine generator set, also referred as gen-set. With this incorporation, the CERTS microgrid that earlier consisted of the same kind of inverter-based combined heat & power (CHP) distributed energy resources (DERs), has got transformed into a diverse/mixed microgrid. As the gen-set came equipped with an isochronous speed governor and ac brushless type exciter, a few enhancements became necessary for augmenting it with CERTS controls that enable capabilities of plug-and-play operation, and seamless islanding and reconnection with the utility grid. This paper describes the modeling and control of gen-set. The models developed are validated with experimental testing against large step changes in load. Performance of gen-set with CERTS controls is demonstrated in different modes of operation of the mixed microgrid.

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.

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.

Modeling and analysis of the CERTS microgrid with natural gas powered distributed energy resources

Distributed energy resources (DERs) powered by natural gas engines are becoming more popular in several nations. This paper presents the modeling and analysis of engine fuel map limits on natural gas powered DERs installed at the CERTS Microgrid. A physics-based approach is used for correct modeling of the limiting conditions in DERs. The distinguishing characteristics of both inverter-based and synchronous generator-based DERs are established. Furthermore, the performance of CERTS Microgrid under is shown for different case studies of islanded operation.

An analytical framework to design a Dynamic Frequency Control scheme for microgrids using energy storage

Microgrids with increased penetration of renewables have significant frequency excursions to power generation changes. A straight forward approach to solve this problem is by enhancing the inertia of the system using energy storage (ES). They provide Dynamic Frequency Control (DFC) support to the interconnected Distributed Energy Resources (DERs) in the microgrid. However, the amount of inertial support required from the ES varies based on the type of interconnected DERs, the amount of load change and many other factors. This paper proposes an analytical framework for calculating the inertia required from the ES systems during a generation change. A case study to corroborate the proposed framework for the DFC scheme has also been studied.

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.

Operation and impact of energy storage system in an industrial microgrid

This paper analyzes the performance of a stone crushing facility powered by an islanded microgrid comprising two natural gas engine driven generator sets (also known as gensets). The goal is to achieve the microgrid performance in accordance with the ISO 8528-5 Standard. For improving the system frequency response, different approaches are evaluated including applying underfrequency load shedding and increasing generator inertia. However, it is shown that integration of energy storage system with fast frequency controls offers the best solution for satisfying the ISO 8528-5 Standard. Results from modeling and simulation using MATLAB/Simulink are presented to show the impact on microgrid performance.

Flexible Distribution of Energy and Storage Resources: Integrating These Resources into a Microgrid

Industrial power systems in the mining and metal industries typically comprise several large and fluctuating loads, such as crushers, excavators, shovels, and steel mills. Flexible distribution of energy and storage resources (FDERS) is a proposed new framework for reliably regulating the quickly varying loads supplied by a network of multiple smaller-rated distributed energy resources (DERs), especially when power from the main grid is not available. It was inspired by the cooperative V-shape formation of flocks of birds and the peloton/echelon formation of cycling racing teams for extending their endurance limits. Similar ideas applied to integrating DERs can extend sustainability through increased resource lifetime, optimal energy storage deployment, enhanced controllability, and improved system robustness.

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.