Earth Wattmetric Polarizing

Earth Wattmetric Characteristic Fault Angle

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

Therefore the Characteristic Angle for Wattmetric is fixed at 180°.

The Wattmetric 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 Trip of -45° (135°-180°) which indicates that the fault angle is lagging the Characteristic Angle by 45°.

When the Earth Tripping Direction is Trip Forward, the above fault will log the following events:

• Forward Pickup Earth

• Earth Directional Armed

• Earth Protection Trip

• Angle at Fault -45°

• Quantity at trip Earth/SEF Wattmetric – xx.xx kW

• Polarizing Voltage at trip V_ZPS

• Quantity at the maximum Earth/SEF – Maximum Wattmetric - xx.xx kW

• Polarizing Voltage at the maximum V_ZPS

Earth Wattmetric Sector Width

The default Forward and Reverse Sector Width settings for earth protection when the Decision is Wattmetric are both 90°.

These are ± settings and equate to actual sector widths of 180°.

The forward sector is centred on the Characteristic Angle V_ZPS → I_ZPS = 180° and the reverse sector is centred on V_ZPS → I_ZPS = 0°.

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 Wattmetric Low Voltage Block

When the Earth Low Block setting is On, tripping is blocked if the polarising voltage is below the Low Threshold setting.

In this situation the ACR will still record fault resetting events when the apparent fault goes away as shown:

• Earth Directional Low V Blocked

• Polarizing Voltage Low V_ZPS

• Quantity at the maximum Earth/SEF – Maximum Wattmetric xx.xx kW

• Polarizing Voltage at the maximum V_ZPS

When the Low Block setting is Off, the polarising voltage is disregarded and the action is entirely based on the calculated fault angle in relative to the Earth Characteristic Fault Angle.

So if the polarising voltage is below the Earth Low Threshold setting, a protection trip will occur but no fault angle will be logged.

The Event Log will record:

• Earth Directional Low V_Armed

• Polarizing Voltage Low V_ZPS

• Earth Protection Trip

• Earth Low V_ZPS

• Quantity at trip Earth/SEF Wattmetric xx.xx kW

• Polarizing Voltage at trip V_ZPS

• Quantity at the maximum Earth/SEF Wattmetric xx.xx kW

• Polarizing Voltage at the maximum V_ZPS

The default Earth Low Threshold setting is 500 V_ZPS configurable over the range 150...15 000 V_ZPS.

Earth Wattmetric Voltage Memory

When Wattmetric is selected as the Decision for directional earth faults, 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 reverse earth fault where V_ZPS and I_ZPS are in phase and V_ZPS is above the Low Threshold setting.

If the earth fault Tripping Direction is Trip Reverse, and the Voltage Memory setting is 2.00 s, this fault will cause a reverse pickup and Earth Directional Armed will be logged.

If V_ZPS then drops below the Low Threshold setting before the switchgear trips, this will not cause a Low V condition as the controller will continue to use the last sampled value of V_ZPS that was above the threshold as the polarising voltage while the pickup is active.

If the fault persists long enough to cause a trip, the memorized voltage and corresponding fault angle recorded in the Event Log will be identified by an asterisk ‘*’.

The Event Log will report the following events:

• Reverse Pickup Earth

• Earth Directional Armed

• Earth Protection Trip

• Angle at Fault* 180°

• Quantity at trip Earth/SEF Wattmetric xx.xx kW

• Polarising Voltage at trip* V_ZPS 514 V

• Quantity at the maximum Earth/SEF Maximum Wattmetric xx.xx kW

• Polarising Voltage at the maximum V_ZPS 601 V

Earth Wattmetric Fault Hold Time

When Wattmetric is selected as the Decision for directional earth faults, 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 protection 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 earth fault with a tripping time of 3.0 s that goes away after 1.5 s for 1.0 s and then returns will cause a trip after another 1.5 s.

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 protection timing is paused and the tripping 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 with relevant time stamps would be:

The fault was paused during the 1.0 s hold time so never reset. When the fault returned a Protection Element Timing event was logged.

The time between pickup and trip would be 4.0 s that is 3.0 s tripping time plus
1.0 s hold time.