Impedance Measurement

NREL has the hardware and software capabilities to assess with high detail how renewable energy devices will affect grid stability and strength.

NREL researchers work with the Flex manufactured battery enclosure. — *Photo by Dennis Schroeder, NREL*

As renewable energy and power electronics proliferate on the grid, it is critical to understand how these devices will affect grid stability. But there are far too many devices and commercial varieties to assess each device analytically and individually. Instead, the industry needs a versatile and tech-agnostic approach to determine device impacts quickly and confidently.

NREL has a software tool to perform impedance scans of devices, which evaluate how renewable energy resources interact with one another and the grid at multiple timescales and for various operations. This software is compatible with any renewable energy device and can prevent early mistakes in project planning.

For renewable energy deployments that require more rigorous field validation, NREL can also plug devices into its Advanced Research on Integrated Energy Systems (ARIES) platform, which has the ability to deliver real power for custom grid scenarios and analyze the devices’ electrical response.

Illustration of the United States showing the Flexible and Scalable Grid Impedance Scan Tool (GIST) for stability analysis on wind plants, PV plants, Storage, and Feeder with DERs all using GIST before going onto the grid. — *Illustration by Taylor Henry, NREL*

Software Capabilities

Grid Impedance Scan Tool
NREL has developed the Grid Impedance Scan Tool (GIST), software to scan the impedance of any inverter-based resource (IBR), such as offshore and land-based wind power plants, solar power plants, battery energy storage systems, and high-voltage DC converter stations. GIST can scan the impedance of all types of device models (e.g., black-box, real code, and generic electromagnetic transient models) to evaluate their impact on grid stability and does not require analytical models of devices. GIST can also scan impedance of large power system networks to evaluate its stability modes. It can also be used to estimate the grid-forming capacity required or assess the stability impact of various control modes for devices.
Automated System-Wide Strength Evaluation Tool
The Automated System-Wide Strength Evaluation Tool (ASSET) analyzes system strength in terms of short-circuit ratio at the points of interconnection of inverter-based resources, considering user-defined contingencies. The tool is also capable of identifying critical N-1 and N-2 contingencies for each renewable energy plant that can result in lowest grid strength condition at the plant interconnection point. It serves as a screening tool to identify scenarios for which more comprehensive grid reliability studies are needed.

Hardware Capabilities

Impedance measurement system
NREL has developed an in-house hardware testing system to measure impedance responses of multi-megawatt wind turbines and solar and battery inverters for characterizing their stability properties at different frequencies. The system is being used by several Department of Energy- and industry-sponsored projects to evaluate the stability impacts of wind turbine and inverter products.
Medium voltage impedance network
This hardware unit can emulate real weak grid conditions to test device-grid interactions under widely diverse scenarios, including weak grid conditions and series-compensated transmission lines.
Controllable grid interface
This hardware unit customizes power conditions for a 19-MW research bus.

Collaborations

The Australian Energy Market Operator has teamed with NREL to analyze large-scale renewable energy additions in its service territory. It has used GIST to identify the root cause of oscillations in its territory with high levels of inverter-based generation from renewables.

“GIST was able to identify the devices and controls causing the instability problems under certain operating conditions, which narrowed down the source of oscillations and enabled the Australian Energy Market Operator and network operators to explore appropriately targeted solutions with generators,” said Michael Gatt, the Australian Energy Market Operator’s chief operating officer.

As Florida Power and Light adds hundreds of megawatts of solar energy, it seeks to assess new plant stability. The utility is using ARIES hardware to recreate stability issues and GIST to analyze them and estimate required grid-forming capacity.

General Electric and NREL are partnering to demonstrate the ability of the company’s Type III wind turbine to run in grid-forming mode, which provides primary frequency and voltage support, restabilizing the surrounding grid by adjusting its power in response to momentary electrical disturbances.