SEF VZPS Polarising
With SEF VZPS polarization, the fault angle is the zero sequence current angle with respect to the zero sequence voltage angle (VZPS → IZPS).
For demonstration purposes, the figure below substitute’s residual current and voltage for zero sequence current and voltage. Although they have different magnitudes, the angles are the same.
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Figure 166
SEF Current and Voltage Phasors
This figure shows phasor diagrams for voltage and current during a hypothetical purely resistive A phase to earth fault on an earthed neutral system.
The fault causes A phase voltage to reduce and the residual voltage phasor is 180° out of phase as shown. The current in A phase is increased and the residual current phasor is in phase with A phase current.
The fault angle for this fault is 180°. That is, the residual current is 180° out of phase with the residual voltage
The actual fault angle for an earth fault is determined by the network characteristics (line resistance and reactance, neutral earthing arrangement) and the fault characteristic (such as the nature of the short-circuit – solid or arcing, fault impedance and earth resistance).
SEF VZPS Characteristic Fault Angle
The default Characteristic Angle for SEF Directional Protection is +135°. So an A phase to earth fault where the residual current led the residual voltage by 135° would be a forward fault in the centre of the forward sector.
The Characteristic Angle setting for SEF VZPS polarising is configurable between -179° and 180°.
A typical Characteristic Angle for systems with solidly or impedance earthed transformer neutrals would be approximately +135°.
If this fault occurs when Protection Groups A and B are active the Event Log would record:
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Forward Pickup SEF
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Protection Group A Active
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Forward SEF Protection Trip
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Angle at Fault 135°
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Polarising Voltage at trip VZPS
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Polarising Voltage at the maximum VZPS
The same fault in the opposite direction would record:
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Reverse Pickup SEF
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Protection Group B Active
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Reverse SEF Protection Trip
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Angle at Fault -45°
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Polarising Voltage at trip VZPS
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Polarising Voltage at the maximum VZPS
SEF VZPS 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 VZPS Low Polarising Voltage Action
In order to determine the direction of a SEF that caused a pickup when VZPS polarization is used, the controller expects Zero Sequence Voltage to be above the SEF Low Threshold setting.
The 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 be taken.
The options are:
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Use Forward
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Use Reverse
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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 15000 VZPS. The default value is 500 VZPS.
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 VZPS event which would record a value lower than the Earth Low Threshold setting.
When No Action is selected, a SEF with low polarising voltage will cause a pickup followed by a Polarising Voltage Low VZPS 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 VZPS Low Voltage Reset
Once the polarising voltage 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 VZPS and a Low Voltage condition exists, VZPS will have to rise to 550 VZPS before the Low V condition is cancelled.