The 67 directional relay can detect the direction of the input current using impedance and torque which are calculated for their respective elements. The relay consists the following elements for detecting the current direction: Positive sequence voltage polarized phase directional (DIRP), negative sequence voltage polarized directional element (DIRQ) and for detecting ground current direction (DIRQG), zero sequence voltage polarized ground current directional element (DIRV), zero sequence current polarized neutral current directional element (DIRI), and zero sequence voltage polarized neutral current directional element (DIRN). The last 4 elements are used to detect the direction for ground currents. This relay can be used in conjunction with overcurrent relays (5051, or 50, 51) to either block a trip signal or permit it from them depending on the application. So, the output of this relay is only the current direction for the aforementioned elements.

That main source for designing this relay was the manual for SEL relays, specifically: SEL-351R [2], SEL-351-5-6-7 [3], SEL-751 [4].

Description

This section describes the different elements in the model: DIRP, DIRQ, DIRV, DIRN, DIRI.

Positive-sequence voltage polarized phase directional element (DIRP)

The phase directional element (DIRP) uses the positive sequence impedance () based on the ratio of () to determine the location of the fault. It is used for three phase faults. If the positive sequence impedance falls between  plus or minus 90 degrees of Z1ANG, which is the characteristic angle of the line, then the relay declares the direction as forward. The line-to-line currents must exceed a pickup current (50P32P) set by the user to enable this element. The unit for 50P32P is amperes and it has a range determined by the secondary transformer side current. The positive sequence voltage must be higher than 0.5 threshold value. If it drops, the value from 3 cycles ago will be used.

It should be noted that phase directional is disabled if there is a strong negative-sequence current and so that element (DIRQ) is enabled. The negative-sequence directional element has priority over phase directional [2]. 

The outputs of the directional element itself are: FDIRP and RDIRP which show forward and reverse phase current respectively.

The FDIRP and RDIRP are routed and compared with directions from the negative-sequence element (FDIRQ and RDIRQ). If either of them is 1, the direction for phase is detected as forward (DIRPF), or reverse (DIRPR). This will be explained more in the following sections.

• Equation for calculating the positive sequence impedance:

• Conditions for declaring the fault as forward:

• Pickup current condition:

• Forward and reverse areas in X-R plane based on Z1ANG are shown in the figure below:
  
  

Negative-sequence voltage polarized ground and phase current directional element (DIRQ, DIRQG)

The negative-sequence element calculates Z2 from negative sequence current and voltage to determine the fault. It is used for sensing unbalance faults.

This element controls the negative-sequence overcurrent elements. Along with positive-sequence voltage polarized directional elements, it controls the phase overcurrent directional element (DIRQ). Along with zero-sequence ground or neutral currents, it controls the ground or neutral directional elements (DIRQG). The negative-sequence element has priority over phase directional element and if it is enabled, it will disable the phase directional element.

Based on the settings and conditions, the direction of the current might be detected as forward (F) or reverse (R). That is true for both negative sequence directional element for phase direction and for ground direction. So the directions detected by the element are: FDIRQ, RDIRQ, FDIRQG, RDIRQG

Here are the features and characteristics of this element:

• To use the negative-sequence element for ground directional detection, the ground directional option has to be set to DIRQG.
• To enable the negative sequence element, first the negative sequence current, I2 must be higher than the pickup current for forward and reverse direction: 50QFP, 50QRP.

It should be noted the enabling of negative sequence element is for both phase directional and ground directional.

• As an extra security measure, two restraints are applied to the negative-sequence current: positive sequence restraint (a2) and zero-sequence restraint (k2). The following conditions have to be met so the element is enabled.

• The negative sequence impedance, Z2 is calculated from  which is shifted by Z1ANG. Z1ANG is the characteristic angle of the line. 

          The * sign means it is the conjugate of the complex value. 

• Z2 must be lower than the forward threshold calculated from the parameter Z2F to detect the current direction as forward. Z2 must be higher than the reverse threshold calculated from the parameter Z2R to detect the current direction as reverse. Z2F and Z2R are set by the user can based on them, the forward and reverse threshold are calculated as the following equations:

Forward threshold Z2 Forward direction FDIRQ, FDIRQG

Z2 Reverse threshold Reverse direction RDIRQ, RDIRQG

If Forward threshold  

If  Forward threshold

If Reverse threshold  

If  Forward threshold

• The operating areas based on Z2 and the thresholds are shown in the figure below:

• The directions FDIRQG and RDIRQG are routed along with other elements to determine the direction of the ground directional element (DIRGF or DIRGR) and neutral ground directional element (DIRN). 

Zero-sequence voltage polarized ground current directional element (DIRV)

This element calculates