Earth Wattmetric Polarising

Earth Wattmetric 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 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 following a fault 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°.

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

• Forward Pickup Earth

• Protection Group A Active

• Forward Earth Protection Trip

• Angle at Fault -45°

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

• Polarising Voltage at trip V_ZPS

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

• Polarising Voltage at the maximum V_ZPS

Earth V_ZPS Sector Width

The default Forward and Reverse Sector Width settings for earth protection are both 90°. These are ± settings and equate to actual sector widths of 180°.

Sector Width settings are identical for all elements and Decision settings. For more information, refer to Sector Width.

Earth Wattmetric Low Polarising Voltage Action

In order to determine the direction of an EF that caused a pickup when Wattmetric polarization is used, the controller requires 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...15,000 Volts. The default value is 500 V_ZPS.

When Use Forward or Use Reverse actions are selected, a fault with low polarising voltage will cause a fault detection 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 Low 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 fault detection. The pickup condition will remain active until the fault resets.

Earth Wattmetric 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 Wattmetric Voltage Memory

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

Consider a reverse EF where V_ZPS and I_ZPS are in phase and V_ZPS is above the Low Threshold setting.

If the EF 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 EFs, a Fault Hold Time between 0.01...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 EF 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 during the hold time.

The events recorded from pickup 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 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.