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How to evaluate generator protective relay CT saturation using ETAP

When examining a particular system, we recommend initiating the process using the ETAP calculation method, as it is the quickest and easiest way to determine whether you have a saturation problem with your CT selection.
By Jesse Hoffman, Engineering Manager at Energy Systems Group

In distributed generation facilities, protective relays must operate reliably under high-fault and low-fault conditions. One of the most common challenges is Current Transformer (CT) saturation, especially in compact metal-clad switchgear where physical space limits the use of high-accuracy CTs. This case study from Energy Systems Group (ESG) shows how the ETAP built-in CT Saturation Calculation tool provides a fast, accurate method to evaluate CT performance and compare results with traditional transient simulation tools such as EMTP-RV and ATP-EMTP.

The Energy Systems Group, LLC is headquartered in Newburgh, Indiana, and maintains a separate headquarters for the federal team in Eagan, Minnesota. Their offices are located throughout the U.S. and help companies achieve their energy efficiency, resiliency, and sustainability goals.

Location: Newburgh, Indiana, United States

Year: 2023

Selection of the current transformer in medium voltage

Challenges

1. Physical limitations inside MV switchgear

  • Distributed generation projects typically use compact metal-clad switchgear.
  • High-accuracy CTs are physically larger and often cannot be installed on breaker stabs.
  • Engineers must choose between:
    • two low-accuracy CTs
    • or one medium-accuracy CT

This makes CT selection critical for relay stability.

2. High fault currents from the utility grid

  • Although the generator produces only a small contribution, the utility system can deliver very high fault currents, causing potential CT saturation.
  • Saturation may:
    • distort secondary current
    • delay relay tripping
    • cause miscoordination

3. The need to evaluate multiple scenarios

  • Internal generator faults (low fault currents)
  • External grid faults (high fault currents)

Both conditions must be assessed.

4. Industry standards and accurate modeling

  • Engineers require tools that accurately model CT behavior, burden, and excitation characteristics to remain compliant with protection standards.

Which solutions did they choose?

Selected applications

ESG used:
  • ETAP base package & STAR to perform short-circuit studies
  • Current transformer saturation calculation to evaluate CT behavior under both low-and high-fault scenarios
ETAP simulations matched well with industry practice and provided a fast diagnostic process.

Why do they use ETAP?

Main customer benefits

1. Fast, intuitive evaluation

  • ETAP calculates CT saturation in minutes, compared to hours or days using EMTP-type tools.
  • Engineers can quickly test multiple CT ratios, burdens, and scenarios.

2. Simplified workflow

  • No need for manual digitization of excitation curves or complex model setup.
  • The STAR short-circuit results populate the CT calculation directly.

3. Clear visualization & warnings

  • Saturation curves and warnings are automatically generated, helping users immediately identify problem cases.

4. Accurate comparison with traditional methods

  • ETAP results closely match EMTP-RV and ATP-EMTP outputs in both low-fault and high-fault scenarios.
  • ETAP helps determine whether advanced transient simulations are necessary.

5. Supports better CT selection

Engineers can quickly determine whether a CT provides sufficient accuracy for relay pickup and timing.

 

Conclusion

ETAP provides a reliable, efficient first-step method for evaluating CT saturation in generator protection systems.
For most applications, ETAP’s saturation calculation is sufficient to confirm proper relay performance.
Engineers can then decide whether deeper transient simulations using EMTP-RV or ATP-EMTP are necessary - saving time, effort, and project cost.

What do they think about ETAP?

Customer perspectives

Changing this using the same CT calculation in ETAP's model is straightforward. This sequence list will show you when each breaker opens, and we can then conclude if we have appropriate coordination on those breakers or if there is a fault. 
By Jesse Hoffman, Engineering Manager at Energy Systems Group

Compared to EMTP, which may take 60 minutes or more to configure and match, ETAP takes only a few minutes to determine if there is a problem.
By Jesse Hoffman, Engineering Manager at Energy Systems Group



Videos

Explore the complexities of current transformer (CT) saturation's impact on protective relay performance in high fault current scenarios, examining the importance of accurate CT selection and contrasting ETAP's saturation analysis capabilities with traditional methods in a real-world scenario, providing insights to enhance protective relay operation and system reliability.


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