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50-51 - Overcurrent Relay

Owned by Sylvain Ménard
Last updated: Mar 16, 2023


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.

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/((I/Ipickup)c-1)+b]

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 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

NameDescriptionUnitVariable = {Possible Values}
FrequencyThe frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.Hz

Freq_param= {50, 60}

Base currentBase value for currentAIbase_param = { [1, 1e5] }


Phase Overcurrent Parameters


NameDescriptionUnitVariable = {Possible Values}

Pickup current 50P

The pickup current in 50P element.pu

pickup50P_param= {[0, 30]}

Time delay 50P The time delay in 50P element.sdelay50P_param= { [0, 600] }
Pickup current 51PThe pickup current for 51P element.puIpck_51P_param = {[0, 30]}
Time DialTime Dial Multiplier-TD51P_param = {[0, 600]}
Curve typesCurve types based on IEC or US Standard
-curve_type_51P_param = {[0, 10]}
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
Use custom curves 51PChoice to use custom curves instead of standard time curves-cust51P_param = {0, 1}
Level 1, 51PThe pickup current of Level 1 in 51Ppulevel1_51_param = { [0, 30] }
Level 2, 51PThe pickup current of Level 2 in 51Ppulevel2_51_param = { [0, 30] }
Level 3, 51PThe pickup current of Level 3 in 51P.pulevel3_51_param = { [0, 30] }
Level 4, 51PThe pickup current of Level 4 in 51P.pulevel4_51_param = { [0, 30] }
Level 5, 51PThe pickup current of Level 5 in 51P.pulevel5_51_param = { [0, 30] }
Time for level 1, 51PThe time delay for Level 1 in 51P.stime1_51_param = { [0, 600] }
Time for level 2, 51PThe time delay for Level 2 in 51P.stime2_51_param = { [0, 600] }
Time for level 3, 51PThe time delay for Level 3 in 51P.stime3_51_param = { [0, 600] }
Time for level 4, 51PThe time delay for Level 4 in 51P.stime4_51_param = { [0, 600] }
Time for level 5, 51PThe time delay for Level 5 in 51P.stime5_51_param = { [0, 600] }

Neutral Overcurrent Parameters

NameDescriptionUnitVariable = {Possible Values}
Enable neutral overcurrentChoice to enable 50N or 51N
EnableSym_param = {0, 1}

Pickup current 50N

The pickup current in 50N element.pu

pickup50N_param= {[0, 30]}

Time delay  50NThe time delay in 50N element.sdelay50N_param= {[0, 600]}
Pickup current 51NThe pickup current for 51N element.puIpck_51N_param = {[0, 30]}
Time DialTime Dial Multiplier-TD51N_param = {[0, 600]}
Curve typesCurve types based on IEC or US Standard-curve_type_51N_param = {[0, 10]}
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
Use custom curves 51NChoice to use custom curves instead of standard time curves-cust51N_param = {0, 1}
Level 1, 51NThe pickup current of Level 1 in 51N.pulevel1_neutral_param = { [0, 30] }
Level 2, 51NThe pickup current of Level 2 in 51N.pulevel2_neutral_param = { [0, 30] }
Level 3, 51NThe pickup current of Level 3 in 51N.pulevel3_neutral_param = { [0, 30] }
Level 4, 51NThe pickup current of Level 4 in 51N.pullevel4_neutral_param = { [0, 30] }
Level 5, 51NThe pickup current of Level 5 in 51N.pulevel5_neutral_param = { [0, 30] }
Time for level 1, 51NThe time delay for Level 1 in 51N.stime1_neutral_param = { [0, 600] }
Time for level 2, 51NThe time delay for Level 2 in 51N.stime2_neutral_param = { [0, 600] }
Time for level 3, 51NThe time delay for Level 3 in 51N.stime3_neutral_param = { [0, 600] }
Time for level 4, 51NThe time delay for Level 4 in 51N.stime4_neutral_param = { [0, 600] }
Time for level 5, 51NThe time delay for Level 5 in 51N.stime5_neutral_param = { [0, 600] }

Inputs, Outputs and Signals Available for Monitoring

Inputs

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

Outputs

NameDescription
QThe output of overcurrent relay. By default, it is 0, if an overcurrent scenario is detected, it becomes 1.
Q_hThe complementary value of Q.
MScopes

Sensors

NameDescriptionUnit
Iabc(0,1,2)Three phase currentsA
QDirectional overcurrent relay output 50-51-
Q_hComplementary of Q-
S_50P(A,B,C)Output of three phase overcurrent 50P-
S_50NOutput of neutral overcurrent 50N-
S_51P(A,B,C)Output of three phase inverse overcurrent 51P-
S_51NOutput of neutral inverse overcurrent 51N-

Typical Values for Parameters

The parameter section shows the possible values for each parameter. This section specifies typical values for a few parameters for the custom curves which would be useful as a start to work with the relay model. These values would change to fit the situation of the 50-51 relay model so they cannot be taken as sole values for each parameter.

NameDescriptionUnitTypical Values
Level 1, 51PThe pickup current of Level 1 in 51Ppu60
Level 2, 51PThe pickup current of Level 2 in 51Ppu30
Level 3, 51PThe pickup current of Level 3 in 51P.pu15
Level 4, 51PThe pickup current of Level 4 in 51P.pu7
Level 5, 51PThe pickup current of Level 5 in 51P.pu2
Time for level 1, 51PThe time delay for Level 1 in 51P.s0.3
Time for level 2, 51PThe time delay for Level 2 in 51P.s0.5
Time for level 3, 51PThe time delay for Level 3 in 51P.s0.9
Time for level 4, 51PThe time delay for Level 4 in 51P.s3
Time for level 5, 51PThe time delay for Level 5 in 51P.s10
Level 1, 51NThe pickup current of Level 1 in 51N.pu8
Level 2, 51NThe pickup current of Level 2 in 51N.pu6
Level 3, 51NThe pickup current of Level 3 in 51N.pu1.5
Level 4, 51NThe pickup current of Level 4 in 51N.pu1.2
Level 5, 51NThe pickup current of Level 5 in 51N.pu0.8
Time for level 1, 51NThe time delay for Level 1 in 51N.s0.8
Time for level 2, 51NThe time delay for Level 2 in 51Ns1.2
Time for level 3, 51NThe time delay for Level 3 in 51Ns3
Time for level 4, 51NThe time delay for Level 4 in 51Ns5
Time for level 5, 51NThe time delay for Level 5 in 51Ns10

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