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Table of Contents

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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.

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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.  

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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:

Mathblock
alignmentleft
\Large T(Trip) = TD*(\frac{A}{M^P-1}+B) + K

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

Mathblock
alignmentleft
\Large T(Reset) = TD*(\frac{T_r}{1-M^P})

Mathblock
alignmentleft
\Large TD = Time Dial,     M = \frac{I}{I(pickup)}

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 C37IEEE C37.112-2018 equation. The IEC curves use the same equation and coefficients stated by the IEC 60255 standard for inverse time overcurrent relays. [1]

Cfm footnote
textSEL-751 Relay Feeder Protection Relay Instruction Manual, Schweitzer Engineering Laboratories Inc., 2017
idycrwa803rh
.

Curve Name

Curve Shape

A

B

P

K

Tr

Q

US (U1)

Moderately Inverse

0.0226

0.0104

0.02

0

1.08

2

US (U2)

Inverse

0.180

5.95

2

0

5.95

2

US (U3)

Very Inverse

0.0963

3.88

2

0

3.88

2

US (U4)

Extremely Inverse

0.0352

5.67

2

0

5.67

2

US (U5)

Short-time Inverse

0.00262

0.00342

0.02

0

0.323

2

IEC (C1)

Standard Inverse

0

0.14

0.02

0

13.5

2

IEC (C2)

Very Inverse

0

13.5

1

0

47.3

2

IEC (C3)

Extremely Inverse

0

80

2

0

80

2

IEC (C4)

Long-time Inverse

0

120

1

0

120

1

IEC (C5)

Short-time Inverse

0

0.05

0.04

0

4.85

2

The following curve shows the U Series Inverse Curve Inverse Curve types.
Image Removed

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The next figure shows the C Series Inverse Curve types.
Image Removed

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Inverse Time Overcurrent Custom Curves

The custom curve settings for the 51 element 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 the 51 element is presented in the figure below.

NOTE: The custom curve only affects the trip time. Timed reset still follows the inverse curves and their parameters. Image Removed

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Mask and Parameters

General Parameters

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Name

Description

Unit

Variable = {Possible Values}

Pickup current

Pickup current for 51OC

pu or A

Ipck_51P_param = {[0, 30]}

Time Dial

Time Dial Multiplier

-

TD51P_param = {[0, 600]}

Reset type


Type of relay reset

-

Reset_set_param = {[0, 1]}

Timed

Timed Reset

Instant

Instantaneous Reset

Inverse curve parameters

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Name

Description

Unit

Variable = {Possible Values}

Inverse curve options


Option to use standard curve parameters or user-defined ones for inverse curves

-

User_coef_param = {[0, 1]}

Use standard parameters for inverse curve

Standard inverse curves

Use custom parameters for inverse curve

User-defined inverse curves

Standard IEC and US curve types

Curve types based on IEC or US Standard


-

curve_type_51P_param = {[0, 10]}

U1

U.S. Moderately Inverse 

U2

U.S. Inverse

U3

U.S. Very Inverse

U4

U.S. Extremely Inverse

U5

U.S. short-time Inverse

C1

IEC Class A Standard Inverse

C2

IEC Class B Very Inverse

C3

IEC Class C Extremely Inverse

C4

IEC Long-time Inverse

C5

IEC Short-time Inverse

Inverse curve with user-specific parameters 

Trip parameters



A

'A' characteristic for user-defined inverse curve

-

A_param = {[0, 200]}

B

B' characteristic for user-defined inverse curve 


B_param = {[0, 200]}

P

'P' characteristic for user-defined inverse curve 


P_param = {[eps, 2]}

K

K' characteristic for user-defined inverse curve


K_param = {[0, 50]}

Reset parameters

Tr

'Tr' characteristic for user-defined inverse curve 

-

Tr_param = {[0, 50]}

Q

'Q' characteristic for user-defined inverse curve


Q_param = {[eps, 50]}

NOTE: Since P and Q are powers in the denominator, if they are 0, the denominator becomes 0 which means the equation becomes infinity. To prevent this situation, if the value is set to 0 by the user, it is not taken by the relay and instead epsilon, which is a very small value, is used by the relay. 

Custom curve parameters

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Name

Description

Unit

Variable = {Possible Values}

Use custom curves 51P

Option to use custom curves instead of standard time curves

NOTE: The custom curve is only applicable to relay's trip time. Timed reset still follows the reset parameters in inverse curves tab.

-

cust51P_param = {0, 1}

Level 1, 51P

The pickup current of Level 1 in 51P

pu/A

level1_51_param = { [0, 30] }

Level 2, 51P

The pickup current of Level 2 in 51P

pu/A

level2_51_param = { [0, 30] }

Level 3, 51P

The pickup current of Level 3 in 51P.

pu/A

level3_51_param = { [0, 30] }

Level 4, 51P

The pickup current of Level 4 in 51P.

pu/A

level4_51_param = { [0, 30] }

Level 5, 51P

The pickup current of Level 5 in 51P.

pu/A

level5_51_param = { [0, 30] }

Time for level 1, 51P

The time delay for Level 1 in 51P.

s

time1_51_param = { [0, 600] }

Time for level 2, 51P

The time delay for Level 2 in 51P.

s

time2_51_param = { [0, 600] }

Time for level 3, 51P

The time delay for Level 3 in 51P.

s

time3_51_param = { [0, 600] }

Time for level 4, 51P

The time delay for Level 4 in 51P.

s

time4_51_param = { [0, 600] }

Time for level 5, 51P

The time delay for Level 5 in 51P.

s

time5_51_param = { [0, 600] }

Inputs, Outputs and Signals Available for Monitoring

Inputs

Name

Description

Unit

I

The per unit or actual RMS value (in A) of actuating current

pu or A

Outputs

Name

Description

Unit

Trip

The output of 51OC relay. By default, it is 0, if an overcurrent scenario is detected, it becomes 1. When the input signal falls below the pickup value, it resets.

Binary

Sensors

Name

Description

Unit

I

The per unit or actual RMS value (in A) of actuating current

pu or A

Trip

The output of 51P overcurrent relay. By default it is 0, if an overcurrent scenario is detected, it becomes 1

Binary

References

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Cfm footnotes display