Real-Time Peace of Mind

RTDS Testing Offers Protection Assurances

Would you wait until there’s a fire to find out if your smoke detectors are working properly?

Of course not. We regularly test smoke detectors to confirm that they are in good working order before there is an incident. What if we applied this same principle to our electric power grid?

Instead of waiting for power system faults to confirm correct operation of a protection system, what if you could proactively test your system against a wide variety of circumstances and see how it responds? This is the value provided by real-time digital simulation (RTDS).

RTDS testing upends the traditional approach to power system testing by allowing users to “lifetime test” their transmission or distribution systems in a matter of days. You don’t have to wait for months or years for an event to occur to confirm proper protection settings and relay behavior.

“An RTDS system runs thousands of fault simulations in a single day,” said Jordan Bell, power system simulation expert and protection engineer supervisor at Schweitzer Engineering Labs (SEL). “The protective relays provide event reports used to validate and optimize relay settings. This approach effectively condenses a lifetime of potential power system faults into a few days.”

Closed-loop RTDS testing connects physical protective devices to the virtual power system model, exposes the system to simulated faults, and verifies that the protective devices will operate as expected. Long before commissioning a new transmission line or implementing changes to an existing system, engineers can optimize protection settings and prove that these settings are going to work before the system goes into service.

Real-time digital simulation is an underutilized approach to power system testing, in part because of the required expertise and specialized computing environment, but also because of a lack of awareness of its potential benefits.

How It Works

Consider flight simulator training for pilots. Powerful software simulates a variety of dynamic flight conditions so that the pilot can practice making decisions and taking appropriate control actions without risking equipment or safety. After practicing in the virtual space, the pilot can address dynamic, real-world conditions with confidence. As technologies change, pilots can periodically return to the simulator to keep their skills sharp.

“RTDS modeling is like a flight simulator for the power system,” said Mike Bryson, a protection systems engineer in SEL’s Engineering Services division. “Powerful computing and signal processing allow modeling tools to deliver signals to the relay that are indistinguishable from the real thing.”

Relays are set and connected to the virtual power system, virtual power system faults are applied, and the relays respond with control actions. Events are analyzed to compare the control actions against the expected response, and relay settings are adjusted accordingly.

“Real-time power system modeling, as compared to traditional static power system modeling, is kind of like comparing a movie and a photograph. RTDS allows you to see the power system model in action,” Bryson said.

The key to successful RTDS testing is the ability to create an accurate virtual system model, which may require days or even weeks of modeling work, depending on the size and complexity of the system. Details of the model include line types, line lengths, physical configurations, nameplate data from primary equipment, locations of apparatus and more. In short, every detail required to accurately represent the real-world system. Once the model is reviewed and validated, the fault simulations begin.

Return on Investment

The investment in real-time power system modeling often has a quick payback.

“We know of several RTDS clients who have avoided costly outages, which more than paid for the service after just a single incident,” said Bell. “To see the value of this service, one must simply consider the cost of unintended outages. The benefits of this approach to testing are matched by the great peace of mind of knowing that your system is ready for any fault, anywhere.”


While the benefits of testing protection equipment prior to commissioning are obvious, the potential applications of RTDS testing are vast. A few important system modeling and closed-loop testing applications include:

  • Transient testing of series-compensated line protection to ensure that protective devices make the proper directional decision and phase selection during a system fault.
  • Building and testing custom relay logic to address phase-shifting transformer challenges and to ensure proper operation of the transformer protection.
  • Validating automatic generator control and automatic load-shedding algorithms in remedial action schemes to avoid blackouts at critical sites.
  • Testing the ability of distribution automation systems to automatically reconfigure circuits; implement fault location, isolation and service restoration (FLISR) when necessary; and provide voltage profile leveling by automatically controlling voltage regulators and shunt capacitor banks.

Peace of Mind

RTDS testing allows you to know exactly how your protection system will function and how to optimize its unique design before it ever goes into service. This prevents mistakes, optimizes system configurations and settings, improves system reliability, and provides peace of mind unique to this testing approach.

Ryan Urie is a technical writer in the technical communications department at Schweitzer Engineering Laboratories in Pullman, Washington. He holds a master’s of science degree in bioregional planning and community design from the University of Idaho.

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