Description

The directional overcurrent relay (50-51-67) trips if and only if the fault current magnitude is above the threshold (50-51) and it is in a specific direction (67). The direction can be forward or reverse depending on the type of fault and type of directional element which is activated.

This document focuses on the 67 element: part of the 50-51-67 model and the directional element of the overcurrent relay. For information regarding the overcurrent model or the measurement block used in both 5051 and 505167 models, refer to 50-51 relay documentation.

The 67 model determines the direction of the fault current based on the phase difference between the phase currents and phase-to-phase voltages (67P) or the phase difference between zero-sequence current (derived from neutral current) and either zero-sequence voltage or ground current (67N). The direction enables the overcurrent relay to trip or acts as a blocking signal.

The 50-51-67 or Directional Overcurrent Relay trips whenever the monitored current exceeds the predefined setting and is in the desired direction. This is true for both phase overcurrent and neutral overcurrent.

Table of Content

The figure below shows the concept of how the relay operates.

The direction discrimination principle is based on phase angle comparison between a set of phasors, one of which is used as a reference. The reference value is called the polarizing quantity and the value which is being compared is the operating quantity. In the case of 67, the operating quantity is always the fault current. By shifting the quantities by a specified value called the Element Characteristic Angle (ECA) or Relay Characteristic Angle (RCA), the model can determine the operating regions of the relay in terms of fault current direction. The directional element can act as a tripping mechanism or blocking for the overcurrent model.

Directional Elements

The model contains two directional elements: Phase Directional (67P) and Neutral Directional (67N).

67P Phase Directional Overcurrent Element

The operating quantity in 67P element is the phase current and the polarizing quantity is the line-to-line voltage. During a fault, the phase voltage is low so to be able to determine the direction, the line-to-line voltage is considered as a reference . The voltage is shifted by an amount of the ECA specified by the user to find the operating regions of the relay. The phase difference between the operating quantity and polarizing quantity is compared with 90 degrees and it is determined whether the fault current is in forward (trip) or reverse (block).

If phase-to-phase voltage drops to lower than 0.1 pu, the voltage memorized from 3 cycles ago is used for direction discrimination. The user has a choice of sending a blocking signal when voltage memory expires by enabling or disabling a setting (Block after voltage memory expiration) in the relay. When the setting is enabled, the directional element will block the phase overcurrent operation under directional control when voltage memory expires. If it is disabled, the phase overcurrent functionality can trip under directional control when voltage memory expires.

If phase current is higher than the threshold of 0.05 pu and based on the directional discrimination, the output of the 67 element can be forward (1) or reverse (0).

Forward direction is used for tripping the phase overcurrent and reverse is used for blocking. So, the overcurrent relay trips if a phase fault current is above a certain threshold and the direction is in forward [1].

67N Neutral Directional Overcurrent Element

The 67N element has one operating quantity which is the zero-sequence current:

I₀ = (1/3) * I_N = (1/3) * (I_A+ I_B+ I_C)

The positive sequence current acts as a restraint for better relay performance and is subtracted from the zero-sequence current with a factor of K. So, the operating current would be:

I_op = 3 * (I₀ - I₁ * K) , K = 1/16

K is recommended to be set to 1/16 [2] for neutral directional overcurrent settings but can be changed.

Based on the polarizing quantity, there are different polarization modes for 67N. If Voltage Polarization is chosen by the user, the polarizing quantity is the zero-sequence voltage:

Polarizing Quantity = V_pol = V₀ = (1/3) * V_N = (1/3) * (V_A + V_B + V_C)

If Current Polarization is chosen, the polarizing quantity is the ground current which can be taken from the ground CT. (I_G)

Polarizing Quantity = I_pol = I_G

If Dual Polarization is chosen, both polarizing quantities are used for direction discrimination.

Direction discrimination is the same principle as it is for 67P. Based on the phase angle difference between the polarizing and operating quantities, direction of the fault current is determined.

Characteristic Angle (ECA) is used in voltage polarization. The user can specify the pickup current and limit angles which are used for directional discrimination in both forward and reverse directions. It should be noted that the pickup value is separate than the one used for 50N or 51N and is typically a very low value. The phase angle difference between the operating quantity and polarizing quantity is compared with Forward Limit Angle and Reverse Limit Angle which determines the operating regions of the 67N element.

if angle difference < forward limit angle => forward direction (1)

if angle difference < reverse limit angle => reverse direction (1)

Because of this method, there are two outputs for 67N. One shows the direction in forward which can be 0 or 1 and the other shows the direction in reverse which also can be 0 or 1. There are non-operation zones which the output for both forward and reverse direction is zero. So, if the output for reverse direction is zero, it does not mean the output for forward direction is 1 and vice versa.

There are areas where the direction could be forward for current polarization but reverse for voltage polarization. In Dual polarization mode, the forward direction is considered as the main direction of the 67N element. So if both forward and reverse directions are true, the 67N output shows 1 for forward and 0 for reverse.

The output of the neutral directional element is always a tripping signal which is sent to the neutral overcurrent element. The user can choose whether the tripping signal is sent from forward direction or reverse. This gives more options to users.

The overcurrent relay trips if a neutral fault current is above the threshold (for 50-51N) and its direction is either forward or reverse [3].

Table below summarizes the different directional elements in 67.

Relay Type	Polarization mode	Operating Quantity	Polarizing Quantity	Output

Relay Type	Polarization mode	Operating Quantity	Polarizing Quantity	Output
67P	Voltage	I_A	V_BC ∠ ECA	One output: 1: FWD 0: REV
		I_B	V_CA ∠ ECA
		I_C	V_AB ∠ ECA
67N	Voltage	I₀ ∠ ECA	-V₀	Two outputs: 0 or 1: FWD 0 or 1: REV
	Current	I₀	I_G
	Dual	I₀ ∠ ECA or I₀	-V₀ or I_G

Mask and Parameters

General Parameters

Name	Description		Unit	Variable = {Possible Values}

Name	Description	Unit	Variable = {Possible Values}
Frequency	The frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.	Hz	Freq_param= {50, 60}
Base current	Base value for current	A	Ibase_param = { [1, 1e5] }
Base voltage	Base value for voltage	V	Vbase_param = { [1, 1e8] }

Directional Overcurrent Parameters

Name	Description				Unit	Variable = {Possible Values}

Name	Description		Unit	Variable = {Possible Values}
Enable phase directional 67P	Enable the use of 67P for control of phase overcurrent		-	Dir_Enabled_67P_param = {0, 1}
ECA (67P)	ECA angle for 67P		deg	ECA_67P_param = { [0, 359] }
Blocking signal when voltage memory expires	Enable sending a blocking signal when voltage memory expires in 67P		-	Block_Vmem_param = {0, 1}
Enable neutral directional 67N	Enable the use of 67N for control of neutral overcurrent		-	Dir_Enabled_67N_param = {0, 1}
Choose directional element for tripping	Select either forward or reverse current as tripping signal for neutral overcurrent control		-	DIR_67N_param = {1, 2}
	Forward (1)	Forward direction
	Reverse (2)	Reverse direction
Positive sequence restraint factor (K)	The factor used for positive sequence restraint in calculating the operating current for neutral directional overcurrent 67N		-	K_param = { [0, 0.0625] }
ECA (67N)	ECA angle for 67N		deg	ECA_67N_param = { [-90, 90] }
Polarization mode	Polarization mode for 67N		-	Pol_Mode_param = {1, 2, 3}
	Current (1)	Current polarization
	Voltage (3)	Voltage polarization
	Dual (2)	Dual polarization
Enable ground calculation (current polarization)	Enable ground current and voltage calculations that are used for current polarization		-	EnableGround_param = {0, 1}
Forward direction pickup current	Pickup current for forward directional discrimination		pu	I_FWD_PKP_param = { [0.002, 3] }
Reverse direction pickup current	Pickup current for reverse directional discrimination		pu	I_REV_PKP_param = { [0.002, 3] }
Forward direction limit angle	Limit angle for forward directional discrimination		deg	FWD_LIM_ANG_param = { [40, 90] }
Reverse direction limit angle	Limit angle for reverse directional discrimination		deg	REV_LIM_ANG_param = { [40, 90] }

Inputs, Outputs and Signals Available for Monitoring

Inputs

Name	Description

Name	Description
I_abc	Three-phase currents
V_abc	Three-phase voltages
I_G	Ground current taken from the ground CT
V_G	Ground voltage taken from the ground VT
Reset	The reset signal for the relay. The relay will be reset when this signal becomes 1

Outputs

Name	Description

Name	Description
Q	The output of directional overcurrent relay. By default, it is 0, if an overcurrent scenario is detected, it becomes 1
Q_h	The complementary value of Q
M	Scopes

Sensors

Name	Description	Unit

Name	Description	Unit
Iabc(0,1,2)	Three phase currents	A
Vabc(0,1,2)	Three phase voltages	V
IG	Ground current	A
VG	Ground voltage	V
Q	Directional overcurrent relay output 50-51-67	-
Q_h	Complementary of Q	-
Reset	Reset signal for the relay	-
S_50P(A,B,C)	Output of three phase overcurrent 50P	-
S_50N	Output of neutral overcurrent 50N	-
S_51P(A,B,C)	Output of three phase inverse overcurrent 51P	-
S_51N	Output of neutral inverse overcurrent 51N	-
S_67P(A,B,C)	Output of three phase directional overcurrent 67P	-
S_67N	Output of neutral directional overcurrent 67N	-

Typical Values for Parameters

The parameter section shows the possible values for each parameter. This section specifies typical values for a few parameters which would be useful as a start to work with the relay model. These values would change to fit the situation of the 50-51-67 relay model so they cannot be taken as sole values for each parameter.

Name	Description				Unit	Typical Values

Name	Description	Unit	Typical Values
ECA (67P)	ECA angle for 67P	deg	30, 45, 60, 85
ECA (67N)	ECA angle for 67N	deg	-80, 80
Forward direction pickup current	Pickup current for forward directional discrimination	pu	0.02
Reverse direction pickup current	Pickup current for reverse directional discrimination	pu	0.02
Forward direction limit angle	Limit angle for forward directional discrimination	deg	65
Reverse direction limit angle	Limit angle for reverse directional discrimination	deg	65

References

[1] GE Industrial Systems, “D60 Line Distance Relay: UR Series Instruction Manual”, Section 5.5.7.e: Phase Directional Overcurrent, (2009 GE Multilin)

[2] Bogdan Kasztenny, Dave Sharples, Bruce Campbell, Marzio Pozzuoli, “Fast Ground Directional Overcurrent Protection: Limitations and Solutions”, Page 16, (2000 GE Grid Solutions)

[3] GE Industrial Systems, “D60 Line Distance Relay: UR Series Instruction Manual”, Section 5.5.8.d: Neutral Directional Overcurrent, (2009 GE Multilin)

67 - Directional Overcurrent Relay