Introduction

This is a a generic inverse-time over-current relay based on positive sequence measurements. It operates with a time delay when the current magnitude is more than a minimum (pick-up) current value.

The typical characteristic of this relay is as shown below where operating time is t_n when the ratio of current magnitude with pickup current is I_n. When the current magnitude is large enough, the operating time tends to become instantaneous t_inst.

Model Description

Input

The overcurrent relay model has two inputs as shown in the figure above. This model is based on positive sequence measurements of I_mag (p.u.). Thus, this relay is compatible with the I_mag I/O pins available for Positive Sequence Components.

When I_mag < I_pickup, then a Reset = 1 will reset Trip and Trip' outputs to the Normal Condition as shown in Relay Logic table below.

Output

The output Trip and Trip' are inverse to each other. When a fault is detected, the relay trip signals change from normal to fault condition as per the following Relay Logic table. These signals stay at Fault Condition until Reset =1 and I_mag < I_pickup.

Relay Logic

State	Trip	Trip'
Normal Condition	0	1
Fault Condition	1	0

Settings

The overcurrent relay is based on the IEEE C37.112-1996 standard curve equation:

Where, TDS, I_mag and I_pickup are the time delay setting, current magnitude and the pick-up current, respectively. The coefficients (a, b, and c) defines the Relay Characteristics curve to be used. There are ten characteristics curves available in this relay model based on U.S. and I.E.C. standard as shown in the figure below.

The overall settings of the relay mask is summarized as follows:

Simulation Time-Step (s)	Simulation time-step in seconds.
Number of Relays	Specify the number of relays to be modeled. The relay model takes vector inputs, and thus, can be used as more than one relay.
Relay characteristics (a, b, c)	Select the desired inverse time characteristic. Only one type can be selected for one mask.
Pickup Current (p.u.)	The minimum I_pickupcurrent is a vector with values specified for each relay.
Time Dial Setting (s)	The TDS is a vector with values specified for each relay.

Example

See inside the subsystem Power System Protection Relays in demo PHASOR-02.

Inverse Time Overcurrent Relay