High Penetration Hybrid Power System - An Emerging Energy Technology Grant Project
Diesel-off hybrid power systems represent the next generation wind-diesel systems. In traditional systems, the diesel gen-set regulates both the voltage and frequency of the grid. In order to maximize fuel savings the diesels need to shut off when other renewable resources are available, but to do so the power electronics must be advanced enough to meet the needs of the grid. Further development of large hybrid inverters is needed to advance this technology. This project sought to analyze state of the art inverters to control the systems in a diesel-off mode through both a product and literature review and laboratory testing. The project goal was to allow for significant savings on the capital cost of installed projects and increased overall simplicity of hybrid power systems.
As the cost of diesel fuel has risen sharply in the past few years, the incentive to replace expensive diesel electric power generation with less costly alternatives has also increased. Many remote Alaska communities have excellent wind resources, but the cost of installing utility-scale wind turbines in these locations is high. Even more challenging is the stochastic (random) nature of wind energy, which makes it difficult to provide utility-grade electricity from this resource.
High-penetration wind-diesel hybrid systems that use energy storage are one option, but they require several components: diesel electric generators (DEGs), wind turbines, batteries or flywheels for energy storage, and a control system. Inverters, devices that convert alternating current (AC) to direct current (DC), are usually required for transferring energy in and out of batteries as well as converting “wild AC” from wind turbines and flywheels into utility-grade 60-cycle AC. Often the inverter is at the center of the control strategy, allowing the system to collect excess energy when available (from high wind events), then store that energy (in the battery or flywheel) and release it later. Since fuel savings can be maximized if DEGs are off during wind events, in an ideal system the diesel would operate during calm periods but be turned off when sufficient wind energy is available. Conventional utilities depend on rotating generators to provide AC power for both energy and reactive power support, tasks that must be done by the inverter in the new hybrid systems.
This project proposed to analyze state-of-the-art power electronics to assess options for operating in a diesel-off mode. The review was to be broad, incorporating both a systemic analysis of diesel-off technology as well as a component analysis, which would look in-depth at flywheel, energy storage, and inverter dynamics.
This project incorporated two phases:
- Phase I: A detailed literature review of inverter technology, including a background study of power electronics and an in-depth power quality analysis of high penetration systems. A list of key research questions will be identified and a testing protocol will be designed for each topic.
- Phase II: The construction of the testbed and hardware testing.
The original project proposal submitted by UAF, “Evaluating NW100B Inverter to Support Diesel-Off Operation in Alaskan Wind-Diesel Systems,” was to investigate the use of advanced inverter technology (specifically, the Northern Power Systems Northwind 100 inverter) to control wind-diesel hybrid systems in diesel-off mode. Funding was to be spent primarily on purchasing two Northwind 100 nacelles to construct a wind turbine simulator that would be supported by existing UAF diesel test bed infrastructure. The total project request was for $860,000.
The Denali Commission selected the project for EETG funding, but at only half the requested funding level, i.e., about $430,000. The reduced funding level unfortunately precluded the purchase of the Northwind 100 nacelles, originally quoted at approximately $500,000. The project instead pursued an alternate approach developed by Sustainable Automation that proposed using an inverter and energy storage system (battery bank) to manage a village-scale power system, with the claim that this could be done with diesels off and without the need for a synchronous condenser.
Of note, the original project application, the Alaska Wind-Diesel Application Center, housed at ACEP and UAF, transitioned in name over the course of the project. Project activities were completed by the Power Systems Integration Program at ACEP.
Phase I activities were complete and resulted in a report that provides a review of technologies that are suitable for communities in Alaska that are operating wind-diesel hybrid systems, including aspects such as power electronics, energy storage, and control strategies. Additionally, key research questions were developed as well as testing protocols and experiment specifics, based on final equipment selection for the test bed.
Phase II activities, the construction of the testbed and hardware testing, revolved around the Sustainable Automation system. The heart of this system was a grid-forming energy storage power inverter, or GRIDFORM Power Converter, which Sustainable Automation represented as a near-commercial product. The following components were purchased through Sustainable Automation under this alternative project approach:
- A 100 kW wind simulator with LabView control interface ($100,000)
- A commercial valve-regulated lead-acid battery bank ($75,000)
- A GRIDFORM Power Converter ($160,000)
- An isolation transformer ($7,500)
- A Matlab/Simulink/PowerSim model ($10,000)
While most of the hardware purchased functioned as expected, including the wind turbine simulator and the isolation transformer, the grid forming inverter turned out to be less well developed than anticipated. The core of the Sustainable Automation GRIDFORM Power Converter is the American Superconductor (AMSC) PM3000, intended for wind turbine systems. The PM3000 appears to be a low-voltage ride-through wind turbine controller, designed to keep turbines on line during low voltage events in a grid-connected environment. Based on specifications available online, it appears that it is designed to handle at least 300 kW of power, with a 1150 V DC bus connection and a 690 V x 750 A AC connection. These devices were manufactured in China, at considerably lower cost than inverters assembled in other parts of the world.
An initial functional test of the hardware was conducted at Marsh Creek in November 2011, but a more complete series of tests was conducted at the ACEP wind-diesel test bed facility at UAF between June and October 2012. During testing at the UAF test bed, several deficiencies were noted in the assembly of the GRIDFORM Power Converter that caused concern about the safety of this device. These deficiencies were identified and conveyed to the individuals associated with the former Sustainable Automation company, but they have not been corrected or resolved because the company is no longer in business. During the same testing event it was observed that the inverter did not produce the rated power (200 kW); this information was also conveyed to the same individuals, who claimed that the inverter had performed at rated power during testing in the fall of 2011 and suggested that either the battery settings or the battery condition was at fault. Sustainable Automation also noted that the AMSC PM3000 had “inherent limitations” that the company “could not overcome” and that this conclusion led to the demise of the company in December 2011. It is apparent that the Sustainable Automation GRIDFORM Power Converter not a commercially available product since the company is no longer in business to support the unit or to provide other units to customers (see Appendix B for comprehensive reporting on testing activities and results).
Despite the fact that the Sustainable Automation GRIDFORM Power Converter is not a finished product, the October 2012 test did successfully demonstrate many of the desired functions needed for an inverter in a high-penetration wind system. First, the unit smoothly transitioned between a diesel-on to a diesel-off state with acceptable transient performance, maintained good power quality during the diesel-off period, including frequency, voltage, and VARS support, and then transitioned back to a diesel-on state, once again with acceptable transients. The unit functioned well under unbalanced load conditions. The major performance deficiencies noted were that the unit did not provide the rated power level of 200 kW and it was not a reliable source of power under changing supply scenarios (i.e., a shut-down of wind generation during diesel-off would cause the unit to fault).
Based on these results, it appears that a high-penetration wind-diesel hybrid battery system using an inverter configured for VARS support without the use of a synchronous condenser is technically achievable. However, it is recommended that the Sustainable Automation inverter be replaced with a commercially available unit constructed to industry specifications and supported by the supplier.
The wind-diesel hybrid test bed at ACEP presents a new opportunity since it can be used as a “hardware model” for performance of wind-diesel hybrid systems for shorter-term tests and could provide modelers with improved performance parameters. For example, diesel engine efficiency curves are provided by engine manufacturers and have been verified in steady-state testing, but in wind-diesel systems without storage, the diesel engines can be “flogged” by the need to follow the wind and become less efficient. Adding short-term storage (capacitors, flywheels, or high-current battery systems) would allow gentler ramp rates on the diesel engine and enable the diesel engine to operate closer to the steady-state efficiency curve. During testing, diesel consumption in the two cases could be compared and some estimate made of the value of the short-term storage. To use the wind-diesel hybrid test bed as a hardware model, the system would need to be operated under (as near as possible) the same patterns—the same load, the same wind pattern, the same diesel engine—with different hardware configurations.
High-penetration wind-diesel hybrid systems for energy storage offer hope for reducing the cost of energy delivered to residents of rural Alaska communities, but additional development is required before these systems are suitable for mass deployment. Both hardware testing and computer modeling are needed to understand the potential benefits of these systems. ACEP and the Denali Commission are continuing to work with technology suppliers, utilities, and rural residents to evaluate and demonstrate currently available hardware.
Photo 1: Inverter Testing at the ACEP PSI Laboratory. Courtesy of Jason Meyer, ACEP.
Photo 2: ACEP PSI Laboratory. Courtesy of Jason Meyer, ACEP.
Photo 3: Test Bed Wind-Diesel Simulator. Courtesy of Jason Meyer, ACEP.