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The 51OC element implements the inverse-time overcurrent protection logic. The input operating quantity can be the RMS of phase current, ground current or symmetrical components.

Description

With the 51OC element, there are two options which determine the tripping time: one is by using the inverse time overcurrent curves based on IEC and US standards or based on parameters which are specified by the user. Their characteristic is explained further in the help file. The other option to determine the tripping time is by defining a customized curve using five data points in the settings. Each data point corresponds to a current pickup and an associated trip delay. The complete inverse-time overcurrent characteristic is defined using linear interpolation between the five data points using an integrator. The relay operates when the current exceeds the preset pickup values for a corresponding time delay.

The relay resets once the input current drops to below the pickup current value. The reset can be instantaneous, which means the reset time is not according to an inverse curve and is an instant zero. Or it can be timed, which means the reset time is according to a reset inverse curve and its defined characteristics.  

Inverse Time Overcurrent Curves

If the input current is above the pickup level and custom curves are not used, the relay's tripping time is calculated based on the following equation:

The reset time of the relay can be calculated based on this equation in an inverse curve:

TD is a time dial multiplier, M is the value of the input current divided by the pickup current, and the rest of the parameters are characteristics of the inverse curve that define it. The parameters can be from standard curves such as the US standard and IEC standard curves, or they can be specified by the user to create their own inverse curve. As mentioned before, the reset curve only applies if the relay's reset setting is timed and not instantaneous. 

The US standard coefficients are based on the values used by SEL relays and it follows the IEEE C37.112-2018 equation. The IEC curves use the same equation and coefficients stated by the IEC 60255 standard for inverse time overcurrent relays .