OPTIMIZING MOTOR PROTECTION

The Advantages of a Coordination Tool for Thermal Element Setting in SEL-710 Relays

By Wassim Mousa

Introduction

In the dynamic area of motor protection, the SEL-710 and SEL-710-5 Motor Protection Relays stand out for their innovative thermal model that provides comprehensive thermal overload protection.

This article explores the significant advantages of utilizing SEL Coordination Tool for setting the thermal element in these relays, focusing on the ANSI 49 overload thermal protection for induction motors.

Let’s go into this application guide, using a practical example with specific numerical information:

SF = 1.15, (Service Factor)

LRA = 6.5xFLA, (Locked Rotor Ampere)

LRTHOT = 17s. (Locked Rotor Time Hot)

Key Features of the SEL-710 Relay

The SEL-710 relay offers two distinct methods for setting the thermal element:

  1. Motor Rating (RATING, RATING_1): Ideal for scenarios with comprehensive motor data.
  2. Thermal Limit Curve (CURVE): Suited for situations with limited motor information.

Coordination Tool and Application Guide

SEL provides a Coordination Tool, along with an insightful application guide.

This tool facilitates the plotting of trip characteristics based on relay settings and preload current, offering a visual representation of thermal capability curves.

The application guide “Coordination Tool for Setting the Thermal Element in the SEL-710 and SEL-710-5 Relays” outlines the elements of the tool, instructing users on how to ensure coordination between relay settings and motor thermal capability curves.

Numerical Example: SF = 1.15, LRA = 6.5xFLA, LRTHOT = 17s

In this practical example, let’s consider specific numerical values:

  • SF (Overload Pickup) = 1.15
  • LRA (Locked Rotor Amperes) = 6.5 times Full Load Amperes (FLA)
  • LRTHOT (Hot Locked Rotor Time) = 17 seconds
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Figure 1 : Thermal Limit Curve   (Credit – SAL online)

Using the Spreadsheet

SEL Coordination Tool is provided by a Microsoft Excel spreadsheet, prefilled with example data and settings.

  • Motor Worksheet: The Motor worksheet contains an example motor data sheet with thermal limit and time-current starting curves. Tables 1 and 2 on the right allow users to input data for thermal limit curves and additional curves.
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Figure 2 : Motor Data Sheet   (Credit SEL online)

  • Table 1 – Data from Thermal Limit Curves: Entering data from the thermal limit curves at the rated voltage in Table 1. This typically includes running (hot and cold) and locked (hot and cold) curves. The provided example data corresponds to the thermal limit curves, Figure 1.
  • Table 2 – Optional Additional Curves: Users can enter data for up to two more curves in Table 2. This step is an option, plotting existing motor protection characteristics or starting characteristics provided by the manufacturer.
  • Rating Method: For the rating method (RATING or RATING_1), users input values for SF, LRA, LRTHOT, thermal overload acceleration factor (TD), and running time constant (RTC). These values, detailed in Appendix H: SEL-710 Manual and Appendix K: SEL-710-5 Manual, influence the stator and rotor trip characteristics independently. By using RATING setting with the above-mentioned example, we ca see below how the SEL-710 relay calculated and plotted the same Thermal Limit Curve of the motor.
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Figure 3 : Example Plot   (Credit SEL online)

  • Curve Method: For the curve method, users input SF and the curve number. The curve number determines the trip characteristic, with the relay internally calculating thermal model parameters. Curve selection is facilitated by a scroll bar in the spreadsheet.

Conclusion

SEL Coordination Tool, coupled with the comprehensive application guide, empowers users to optimize thermal settings for the SEL-710/5 relay.

By plotting thermal trip characteristics and motor curves, users can ensure seamless coordination. This invaluable tool validates the harmony between motor characteristics and relay trip curves, ultimately enhancing the reliability and efficiency of motor protection systems.

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