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.

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 (Electric Characteristic Angle) 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 angle difference < 90  => forward direction

                                                          If angle difference > 90  => reverse direction

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.

Mask and Parameters

General Parameters

The parameters for 50P, 51P, and 51N are covered in the 50-51 relay documentation.

Phase directional overcurrent

Neutral directional overcurrent

Inputs and outputs

Inputs

Outputs

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)