Directional Over Current Relay [67]:

In this post operation of Directional current relay along with its settings explained. The detailed explanation about Directional relay settings with Characteristic angle, Maximum Torque angle and Polarisation voltages are given.

Operating Principle of Directional Over Current Relay:

Directional over current relays operate in either forward or reverse directions with over current protection.

If the directional relay set in Forward direction means if a fault occurs in forward zone then only relay operates. Conversely, if the relay set in Reverse direction means if a fault occurs in reverse zone then only relay operates.

Conventionally denotes that forward direction refers to power flow away from the busbar (towards the protected zone), while reverse direction refers to power flowing towards the busbar.

Directional Relay Characteristics:

The directional relays identify the zone of operation with the help of polarising voltage and characteristic angle.

• Polarising Voltage:

Directional relays needed reference voltage to identify the direction of currents. This is the voltage taken from Potential Transformers. The reference voltage is called polarising voltage. The relay operates for the defined direction when the fault current is in the same direction and exceeds the over current setting value.

• Relay Characteristic Angle: RCA

The characteristic angle is the phase angle by which reference or polarising voltage is adjusted such that the directional relay operates with maximum sensitivity.

 Characteristic Angle is also called Directional angle as it decides the direction of operation of the relay.

• Maximum Torque Angle: MTA

Maximum torque angle is the angle made by fault current with respect to its phase voltage. This is expected fault angle, and completely predictable. This angle is different for phase faults and earth faults.

• Center of Zone or Maximum Torque Line:

Center of zone is the line formed by Char Angle where relay exhibits maximum sensitivity. At this line relay current is in phase with the reference or polarising voltage.

• Zero Torque Line:

It is the boundary line which separates the plane into Operate and Inhibit regions.

• Forward Zone:

Forward zone is+/- 85° either side of the Maximum Torque line or Centre of forward zone line.

• Reverse Zone:

Reverse operate zone is the mirror image of the forward zone.

• Changes Occur during Fault condition:

Whenever, fault occurs on transmission line or busbars the voltage reduces proportional to the severity of fault and current increases larger than the normal currents. The fault current is lagging in nature. Healthy phase magnitudes and angles not changed.

Directional Over Current Protection Settings:

It is to be noticed that the directional over current operation is different for phase and earth faults. The variation is in selection of polarising voltages.

1.Directional Over Current Protection for Phase Faults [67]:

• Polarising Voltages:

The directional phase fault elements, work with a Quadrature connection to prevent loss of polarising quantity for phase faults. That is, each of the current elements is directionalised by a voltage derived from the other two phases.

This connection introduces a 90° Phase Shift (Current leading Voltage) between reference and operating quantities.

Each phase current is compared with the voltage between other two phases. Like I_L1 is compared with V₂₃.

These Polarising voltages are defined internally in the relay itself. No need to set externally.

• Maximum Torque Angle:

The Expected fault angle is called Maximum Torque angle in Electromechanical Relays.

(This angle is usually -30^o for overhead lines and -45^o for underground cables for phase faults.)

It is used to calculate Characteristic angle and identify center of zone. There is no setting for maximum torque angle in numerical relays.

• Characteristic Angle Setting:

The Relay Characteristic Angle RCA is given in relay Phase fault protection settings. The range is (-95 to +95 Degrees). The Char Angle setting must be specified while configuring Directional over current relay.

Example:

For Example consider a phase fault occurs at angle of -30^o. Fault current Lags the voltage by 30^o.[This is the Maximum Torque Angle]

Characteristic Angle= 90^o [Angle of Quadrature] – Maximum Torque Angle

Characteristic Angle = 90-30=60^o.

• Center of the forward zone:

It is the zone where the relay operates with maximum sensitivity.

Centre of forward zone = Vref Angle + Char Angle

                                            = 0 ^o + 60 ^o 

                                                           = 60 ^o

• Minimum Voltage Setting:

When the measured polarising voltage falls below this setting level, no directional output is given and hence direction protection will be inhibited.

Minimum voltage setting prevents mal-operation of the relay when fuse failure/MCB tripped conditions.

Directional Over Current Protection for Earth Faults:

Each earth faults in relay computed as either Measured or Direct earth faults. The Selection of Polarising voltage is different for these two types of earth faults.

To understand the difference between Measured Earth Faults and Derived Earth Faults Read here.

2. Directional Current Protection for Measured Earth faults: [67G]

• Polarising Voltages:

The directional earth fault elements, to sense Earth faults Zero phase sequence polarising technique is used. Voltage polarisation is achieved for the earth-fault elements (they are different from phase faults) by comparison of the appropriate current I₀ with its equivalent voltage V₀.

These voltages are available only during earth fault conditions. The voltage of faulty phase drops and other phases remain healthy.

These Polarising voltages are defined internally in the relay itself. No need to set externally.

• Maximum Torque Angle:

The Expected fault angle is called Maximum Torque angle in Electromechanical Relays.

 (This angle is usually varies from 0^o to -90^o depends on the type of Neutral Earthing employed.)

For Resisted Earth Systems = 0^o

Earthing Transformer with Resistor= – 15^o

Solidly Earthed Distribution System= -45^o

Solidly Earthed Transmission System= -65^o

Reactance Earthed Systems = -90^o

It is used to calculate Characteristic angle and identify center of zone. There is no setting for maximum torque angle in numerical relays.

• Characteristic Angle Setting:

The Relay Characteristic Angle RCA is given in relay Earth fault protection settings. The range is (-95 to +95 Degrees). The Char Angle setting must be specified while configuring Directional OC relay.

Example:

For Example consider an earth fault occurs at angle of -15^o on a resistance earthed system. Fault current Lags the voltage by 15^o. [This is the Maximum Torque Angle]

Characteristic Angle= 0^o– Maximum Torque Angle

Characteristic Angle = 0-15=-15^o.

• Center of the forward zone:

It is the zone where the relay operates with maximum sensitivity.

Centre of forward zone = Vref Angle + Char Angle

                                            = 0^o-15^o

                                                           = -15^o

• Minimum Voltage Setting:

This is the common setting for any type of faults. As mentioned earlier, when the measured polarising voltage falls below this level no directional output is given and hence direction protection will be inhibited.

Minimum voltage setting prevents mal-operation of the relay when fuse failure/MCB tripped conditions.

3.Directional Current Protection for Derived Earth faults: [67G]

To sense Earth faults Zero phase sequence or Negative Phase sequence polarising technique is used.

• Zero phase sequence polarising:

Zero sequence voltages are available whenever PT is a FIVE limb PT that can provide a zero sequence path or Open-delta Connected PT. This type of connection provides zero sequence polarising voltages.

In this case Zero sequence voltages V₀ and zero sequence currents I₀ are used for polarization.

• Negative phase sequence polarising:

When a two phase (phase to phase) PT or three limb PT is installed Zero Sequence voltages are not available. In this case Negative Phase sequence voltages V₂ and Negative sequence currents I₂ are used for polarization.

The selection of Relay characteristic angle, Maximum torque angle and Minimum voltage settings are same as Measured earth faults as explained above.

Let me know if you are interested, check the Application of Directional Over Current Relays in transmission lines explained by Mr. Chris in detail.