1 Description
- 1.1 Inverse Time Overcurrent Curves
- 1.2 Inverse Time Overcurrent Custom Curves
2 Mask and Parameters
- 2.1 General Parameters
- 2.2 Inverse curve parameters
- 2.3 Custom curve parameters
3 Inputs, Outputs and Signals Available for Monitoring
- 3.1 Inputs
- 3.2 Outputs
- 3.3 Sensors
4 References

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 .

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 types.

The next figure shows the C Series Inverse Curve types.

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.

Mask and Parameters

General Parameters

Name	Description		Unit	Variable = {Possible Values}

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

Name	Description		Unit	Variable = {Possible Values}

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]}
Reset parameters	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

Name	Description		Unit	Variable = {Possible Values}

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

Name	Description	Unit
I	The per unit or actual RMS value (in A) of actuating current	pu or A

Outputs

Name	Description	Unit

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

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

HYPERSIM Documentation

51- Inverse Time Overcurrent (1-ph)