Earth Ir Cos (Ø) Polarising

Earth Ir Cos (Ø) Characteristic Fault Angle

When a forward EF occurs on a compensated earthing system, V_ZPS and I_ZPS are assumed to be 180° out of phase.

Therefore the Characteristic Angle for Ir Cos (Ø) is fixed at 180°.

The Ir Cos (Ø) Fault Angle for a given fault is the Zero Sequence Current angle relative to the Zero Sequence Voltage angle V_ZPS → I_ZPS

The Angle at Fault recorded in the Event Log is the fault angle relative to the Characteristic Angle which is fixed at 180°.

So a fault with a Fault Angle (V_ZPS → I_ZPS) = 135° would log an Angle at Fault of -45° (135°...180°) which indicates that the fault angle is lagging the Characteristic Angle by 45°.

A forward EF with a fault angle of 135° will log the following events:

Fault On

• Forward Pickup Earth

• Protection Group A Active

• Forward EF

ACR Tripped

• Quantity at the maximum Earth/SEF – Maximum I_R Cos Ø - xx.xx A

• Polarising Voltage at the maximum V_ZPS xxx Volt

• Supply Interrupt

• EF Angle at Fault -45°

Earth Ir Cos (Ø) Sector Width

The default Forward and Reverse Sector Width settings for earth protection when the Decision is Ir Cos (Ø) are both 90°. These are ± settings and equate to actual sector widths of 180°.

The likelihood of a fault angle causing an incorrect direction determination may be lessened by either increasing or decreasing the Sector Width setting. The range of these settings is 45° to 135° which equate to actual sector widths of 90° and 270° respectively.

Earth Ir Cos (Ø) Low Polarising Voltage Action

In order to determine the direction of an EF that caused a pickup when
Ir Cos (Ø) polarization is used, the controller expects Zero Sequence Voltage to be above the Earth Low Threshold setting.

The Earth Low Threshold is one of two settings that deal with low polarising voltages. The other is the Low Polarising Voltage Action.

If the polarising voltage is below the Low Threshold setting for EFs, the Low Polarising Voltage Action setting will determine what action must be taken.

The options are:

• Use Forward

• Use Reverse

• No Action (default setting)

More information on the No Action option can be found here.

The Earth Low Threshold setting can be configured for between 15 and 15,000 V.

When Use Forward or Use Reverse actions are selected, a fault with low polarising voltage will cause a trip as though it was a normal fault in the selected direction. The only indication that the polarising voltage was low would be the Polarising Voltage at trip V_ZPS event which would record a value lower than the Earth Low Threshold setting.

When No Action is selected, an EF with low polarising voltage will cause a pickup followed by a Polarising Voltage Low V_ZPS event reporting the low value of polarising voltage. It will not cause a protection trip. The pickup condition will remain active until the fault resets.

Earth Ir Cos (Ø) Low Voltage Reset

Once the polarising voltage for EFs has been deemed to be Low, it will not return to Normal until either the fault resets or the measured polarising voltage exceeds the Low Threshold setting by 10%. So if the Low Threshold setting is
500 V_ZPS and a Low Voltage condition exists, V_ZPS will have to rise to 550 V_ZPS before the Low V condition is cancelled.

Earth Ir Cos (Ø) Voltage Memory

When Ir Cos (Ø) is selected as the Decision for directional EFs, a Voltage Memory time between 0.01 s and 10.00 s or OFF can be configured. The default setting is 2.00 s.

Consider a forward EF at the Characteristic Angle with V_ZPS above the Earth Low Threshold setting and an expected tripping time of 3.5 s. If this fault is applied for 2.0 s and then V_ZPS drops below the Low Threshold setting, the controller will retain the last V_ZPS value that was sampled above the threshold for the Voltage Memory time.

This means that the low voltage condition is effectively ignored and the protection trip will still occur in the expected tripping time of 3.5 s.

The memorized voltage and corresponding fault angle will be identified by an asterisk ‘*’.

The Event Log will report the following events:

Fault On

• Forward Pickup Earth

Low VZPS

• Forward EF

ACR Tripped

• Quantity at the maximum Earth/SEF – Maximum Ir Cos Ø –xxx A

• Polarising Voltage at the maximum V_ZPS xxx Volt

• Supply Interrupt

• Earth Angle at Fault* 0°

Earth Ir Cos (Ø) Fault Hold Time

When Ir Cos (Ø) is selected as the Decision for directional EFs, a Fault Hold Time between 0.01 s and 10.00 s or OFF can be configured. The default setting is 2.00 s.

The effect of the Fault Hold Time is to pause the detection timing for a set period after a fault that caused a pickup has gone away.

When the fault hold time is 2.00 s, a forward EF with a fault detect time of 3.0 s that goes away after 1.5 s for 1.0 s and then returns will cause a fault detection 1.5 s later. Because the hold time, that is the time that the fault went away for (1.0 s), is a shorter duration than the Fault Hold Time (2.0 s), the pickup will remain active while detection timing is paused and the fault detection time will be extended by the Fault Hold Time setting.

The Event Log will record a Paused event to mark the start of the hold time.

The events recorded from pickup through to when the upstream ACR trips with relevant time stamps would be:

The pickup was paused during the 1.0 s hold time so never reset. When the fault returned a Detection element timing event was logged.

The time between pickup and fault detection would be 4.0 s that is 3.0 s fault detect time plus 1.0 s hold time.