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Description

This relay comprises functionalities of the 50P (instantaneous or definite-time phase overcurrent), 50N (instantaneous or definite time neutral overcurrent), 51P (inverse-time phase overcurrent) and 51N (inverse-time neutral overcurrent) elements.

Figure to be replaced.

With the 51X elements, there are two options which determine the tripping time: one is by using the inverse time overcurrent curves based on IEC and US standards. Their characteristic is explained further in the documents. The other option 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. The relay operates when the current exceeds the preset pickup values. The model works for 50 Hz or 60 Hz system.

The overcurrent relay trips whenever the monitored current exceeds the predefined setting. A time-delay is normally used to enable the relay to ride through momentary disturbances and thus prevent nuisance operation.

The figure below shows the tripping logic of the relay.

Table of Contents


Details on the Overcurrent Model Functionality

The 50P and 50N elements are defined as instantaneous phase overcurrent element and instantaneous neutral overcurrent element, respectively, if the time delay is set to zero.  When setting a non-zero time delay, the element is set as a definite-time overcurrent element. If and only if the measured current becomes larger than the pickup value for a longer duration than the preset time delay, an overcurrent situation is asserted and the relay sends a trip signal through 50P or 50N. Otherwise, the element's internal timer is reset and the relay does not trip.

The time delay and reset mechanism of the 51P and 51N elements are applied similarly.

A tripping signal is sent through 51P or 51N, if the measured current is above the pickup value for a longer duration than a delay time. Both the pickup value and delay time can either be set by the user (custom curves) or determined by the standardized time curves (IEC or US time curves).

If either of the 50P (each phase), 50N, 51P (each phase), and 51N send a tripping signal, the model trips.

50P and 51P modules use the phase current while the operating quantity of 50N and 51N is the zero sequence current which is calculated as the following:

Inverse Time Curve Characteristics

Inverse Time Overcurrent Curves Based on IEC and US Standards

By using a time dial multiplier (TD) and choosing the curve type, the operating and tripping time of the relay can be determined. Setting the multiplier to zero results in instantaneous response to all current levels. The pickup level for 51P and 51N is in per unit. If overcurrent is above the pickup level and custom curves are not being used, then the tripping time is calculated based on this equation.

T = TD * [a + b/(I/Ipickup)c-1]

Equation 1: Trip Time Calculation     

The parameters shown in Equation 1 differ based on the curve type. This model has 5 curve types from the US standard and 5 from the IEC standard as shown in the table below. The US standard coefficients are based on the values used by SEL relays [1] 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.

Curve Name

Curve Shape

a

b

c

US (U1)

Moderately Inverse

0.0226

0.0104

0.02

US (U2)

Inverse

0.180

5.95

2

US (U3)

Very Inverse

0.0963

3.88

2

US (U4)

Extremely Inverse

0.0352

5.67

2

US (U5)

Short-time Inverse

0.00262

0.00342

0.02

IEC (C1)

Standard Inverse

0

0.14

0.02

IEC (C2)

Very Inverse

0

13.5

1

IEC (C3)

Extremely Inverse

0

80

2

IEC (C4)

Long-time Inverse

0

120

1

IEC (C5)

Short-time Inverse

0

0.05

0.04

The following curve shows the U Series Inverse Curve types.

The next figure shows the C Series Inverse Curve types.

Inverse Time Overcurrent Custom Curves

The custom curve settings for 51X elements have five levels of settings, using five data points. The resulting curve delimits the inverse-time overcurrent operation characteristics using a linear interpolation method between the data points. This means that if an overcurrent scenario occurs and the current measurement is between Level 5 and Level 4, the time delay for such overcurrent level will be linearly calculated between the corresponding Time 5 and Time 4 settings.  The pickup current values and delay times are all tunable parameters. The inverse time overcurrent characteristic of 51P or 51N element is presented in the figure below.

Mask and Parameters

General parameters

NameUnitDescription

Frequency

HzThe frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.
Base currentABase value for current
Sample TimesBlock sample time

Definite-time phase 50P

NameUnitDescription

Enable 50P

-Enable/disable definite-time phase overcurrenet element.
Pickup current in 50P relaypuThe pickup current in 50P element.
Delay time in 50P relaysThe time delay in 50P element.

Inverse-time phase 51P

NameUnitDescription

Enable 51P

-Enable/disable inverse-time phase overcurrenet element.
Pickup current 51PpuThe pickup current in 51P element.
Time Dial-Time Dial Multiplier
Curve types-Curve types based on IEC or US Standard
U1U.S. Moderately Inverse 
U2U.S. Inverse
U3U.S. Very Inverse
U4U.S. Extremely Inverse
U5U.S. short-time Inverse
C1IEC Class A Standard Inverse
C2IEC Class B Very Inverse
C3IEC Class C Extremely Inverse
C4IEC Long-time Inverse
C5IEC Short-time Inverse
Customized curve-Choice to use custom curves instead of standard time curves
Pickup currentpuLevel 1The pickup current of Level 1 in 51P customized curve
Level 2The pickup current of Level 2 in 51P customized curve
Level 3The pickup current of Level 3 in 51P customized curve
Level 4The pickup current of Level 4 in 51P customized curve
Level 5The pickup current of Level 5 in 51P customized curve
Pickup timesTime for level 1The time delay for Level 1 in 51P customized curve
Time for level 2The time delay for Level 2 in 51P customized curve
Time for level 3The time delay for Level 3 in 51P customized curve
Time for level 4The time delay for Level 4 in 51P customized curve
Time for level 5The time delay for Level 5 in 51P customized curve

Definite-time neutral 50N

NameUnitDescription

Enable 50N

-Enable/disable definite-time neutral overcurrenet element.
Pickup current in 50N relaypuThe pickup current in 50N element.
Delay time in 50N relaysThe time delay in 50N element.

Inverse-time neutral 51N

NameUnitDescription

Enable 51N

-Enable/disable inverse-time neutral overcurrenet element.
Pickup current 51NpuThe pickup current in 51N element.
Time Dial-Time Dial Multiplier
Curve types-Curve types based on IEC or US Standard
U1U.S. Moderately Inverse 
U2U.S. Inverse
U3U.S. Very Inverse
U4U.S. Extremely Inverse
U5U.S. short-time Inverse
C1IEC Class A Standard Inverse
C2IEC Class B Very Inverse
C3IEC Class C Extremely Inverse
C4IEC Long-time Inverse
C5IEC Short-time Inverse
Customized curve-Choice to use custom curves instead of standard time curves
Pickup currentpuLevel 1The pickup current of Level 1 in 51N customized curve
Level 2The pickup current of Level 2 in 51N customized curve
Level 3The pickup current of Level 3 in 51N customized curve
Level 4The pickup current of Level 4 in 51N customized curve
Level 5The pickup current of Level 5 in 51N customized curve
Pickup timesTime for level 1The time delay for Level 1 in 51N customized curve
Time for level 2The time delay for Level 2 in 51N customized curve
Time for level 3The time delay for Level 3 in 51N customized curve
Time for level 4The time delay for Level 4 in 51N customized curve
Time for level 5The time delay for Level 5 in 51N customized curve

Inputs and Outputs

Inputs

NameDescription
IabcThree-phase currents
ResetThe reset signal for the relay. The relay will be reset when this signal becomes 1.

Outputs

NameDescription
TripThe output of overcurrent relay. By default, it is 0, if an overcurrent scenario is detected, it becomes 1.
mThe output is a vector of 4 signals, to monitor the response of each protection element.
_50PA three-dimension output of 50P, in the order of phase A, B, C
_51PA three-dimension output of 51P, in the order of phase A, B, C
_50NOutput of 50N
_51NOutput of 51N

References

  1. SEL-451-5 Protection, Automation, and Bay Control System Instruction Manual

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