SEFs Wattmetric Polarising

SEF Wattmetric Characteristic Fault Angle

When a forward SEF 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°.

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

• Forward Pickup SEF

• SEF Directional Armed

• SEF 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

SEF Wattmetric Sector Width

The default Forward and Reverse Sector Width settings for SEF 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.

SEF Wattmetric Low Polarising Voltage Action

In order to determine the direction of a SEF that caused a pickup when Wattmetric polarization is used, the controller expects Zero Sequence Voltage to be above the SEF Low Threshold setting.

The SEF 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 SEFs, 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 SEF Low Threshold setting can be configured for between 15 and 15 000 V_ZPS. The default setting is 500 V_ZPS.

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 SEF Low Threshold setting.

When No Action is selected, a SEF 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.

SEF Wattmetric Low Voltage Reset

Once the polarising voltage for SEFs 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.

SEF Wattmetric Voltage Memory

When Wattmetric is selected as the Decision for directional SEF, 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 SEF at the Characteristic Angle with V_ZPS above the SEF 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 logged following the trip will be identified by an asterisk ‘*’.

The Event Log will report the following events:

• Forward Pickup SEF

• Protection Group A Active

• Forward SEF Protection Trip

• Angle at Fault* 0°

• 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

SEF Wattmetric Fault Hold Time

When Wattmetric is selected as the Decision for directional SEFs, 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 SEF 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 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.