How HIL Testing Supports Grid Reliability and Power Project Success

Hardware-in-the-loop (HIL) testing involves simulation of power plant behavior using the actual site-specific power plant controls before the commissioning stage. HIL testing can benefit all project stakeholders by minimizing risks while maximizing grid reliability.

Every member of the power industry recognizes reliability as a critical metric by which we can gauge our success. Cheap energy, high penetration of renewable energy resources, or any other success metric will be deprioritized by utility customers if it is accompanied by a noticeable drop in system reliability; the bottom line is that customers don’t care if their energy bill is low if that means their lights don’t turn on when they flip the switch.

Our resource mix is in the midst of a dramatic transformation, and maintaining system reliability through this transformation will require strategic effort and investment. High-fidelity testing of power plant behavior is a critical strategy, as well-tailored testing offers significant value for relatively low investment.

As the penetration of inverter-based resources (IBRs) increases, especially solar, wind, and battery energy storage, it will be critical to ensure that those resources are designed to support a reliable grid. Unfortunately, various grid instability events, including the 2021 Odessa Disturbances in the Electric Reliability Council of Texas (ERCOT) system and several disturbance events in the California Independent System Operator (CAISO) grid between 2016 and 2020, have highlighted gaps in the current treatment of IBRs. It is important that the industry take events like these as a learning opportunity.

Modeling and Testing Tactics

Currently, testing approaches vary depending on where a plant is interconnecting, but most locations have two principal required testing steps. First, during the interconnection approval process, plants are modeled and tested, typically using standard library models to represent the IBRs and plant controller; at this point, this is a logical approach as the plant controller vendor has typically not been selected at this stage. Later, detailed testing of the real plant is completed during the commissioning process while connected to the grid.

While both of these are critical milestones, waiting to test with the actual power plant controller code set until the commissioning stage leaves significant risk to the tail end of the project. We suggest that the industry is missing an interim milestone in which the full plant is tested with actual site-specific power plant controls before the high-risk commissioning phase.

1. This sample of a hardware-in-the-loop (HIL) test chart shows modeled and field data overlaid. Courtesy: Merit Controls

Hardware-in-the-loop testing, also known as HIL testing, offers an exciting opportunity to fill this gap. HIL testing involves creating a closed loop between the hardware under test, in this case the power plant controller with the code set that will be deployed onsite, and a model of the plant. By using real-time simulation of an electromagnetic transient–based high-fidelity plant model, HIL testing can offer very-high-accuracy results based on actual plant physics (Figure 1) rather than mathematical equivalencies. HIL modeling is typically completed around the factory acceptance testing (FAT) stage of a project, enabling any necessary changes to be made and re-tested before the equipment arrives onsite.

The benefits of HIL testing have already been identified and captured in many industries, including aerospace, automotive, and defense. Even in the power industry, HIL is already required to test inverter control systems. However, this testing looks at just one fraction of the overall site control systems, and capturing the dynamics of the full plant is critical for understanding how a plant may impact the grid’s functionality. The grid stability events that we have seen to date cannot be fully addressed without a comprehensive approach.

Evaluating Difficult-to-Simulate Situations

One of the most important things that HIL testing enables is the evaluation of how a power plant will respond to situations that cannot be simulated once the plant is connected to the grid. Appropriate response to voltage and frequency excursion events is critical to maintaining a reliable grid, but these events cannot be simulated on an operating power plant as the grid frequency and voltage at the plant is influenced by the many generators and loads on the bulk electric system.

For the plant undergoing testing, alternate tests can be performed by adding a bias signal or override to the metering measurements, but these are not ideal, nor able to fully test ride-through voltage or frequency duration curves. Additionally, this methodology cannot fully test the interaction between the power plant controller and IBRs when the IBRs are responding to a grid event and in a voltage or frequency ride-through mode. Thus, current standard testing approaches never evaluate the plant’s response to these events when using the actual controller code set. HIL testing would change that. It enables corner case testing for abnormal events to ensure that plants will behave predictably at the most important moments.

HIL testing can also simplify and potentially expedite the onsite commissioning process for IBR-based projects. Commissioning is a critically high-risk period for stakeholders, with completion dates, contractual obligations, high-cost labor and equipment, and more, contributing to pressure from all sides. Any reduction of risk or cost in this period is of high value.

Of course, identifying and rectifying potential system issues before equipment shipment can reduce commissioning period risk, but the impact of HIL extends beyond that. HIL’s high-accuracy modeling enables controls engineers to tune the power plant controller to the power plant and local grid’s specific parameters before deploying onsite. In some cases, onsite adjustments may still be needed, but the period required for that work will be decreased. By de-risking and accelerating the commissioning process, HIL can minimize exposure to liquidated damages, delayed completion dates, and other common penalties associated with late project completion.

With benefits across various stakeholder groups, including engineering, procurement, and construction (EPC) contractors; owners; utilities; and customers, HIL represents an opportunity for the power industry to take proactive action to minimize risk and maximize grid reliability. Now is the time to implement this approach as a standard across IBR plants, ensuring reliability and resilience as we continue the transition to the future grid.

—Tina Dornbusch is the Director of Product Engineering, Rohit Kumar is Grid Integration and Compliance Specialist, and Lars Johnson is Chief Technology
Officer, all with Merit Controls.