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.
Figure 161
SEF Current and Voltage Phasors
This figure shows phasor diagrams for voltage and current during a hypothetical purely resistive A phase to EF 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 EF 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 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°.
+135° is the default Characteristic Angle for SEF Directional Blocking. So an A phase to EF where the residual current led the residual voltage by 135° would be a forward fault in the centre of the forward sector.
For this fault the Event Log would record:
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Forward Pickup SEF
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SEF Directional Armed
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SEF Protection Trip
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Angle at Fault 135°
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Polarizing Voltage at trip VZPS
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Polarizing 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 Voltage Block
When the SEF Low Block setting is On, tripping is blocked if the polarising voltage is below the Low Threshold setting. The Event Log will record the following events:
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SEF Directional Low V Blocked.
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Polarizing Voltage Low VZPS
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 SEF 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. Instead the Event Log will record:
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SEF Directional Low VArmed
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Polarizing Voltage Low VZPS
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SEF Protection Trip
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SEF Low VZPS
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Polarizing Voltage at trip VZPS
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Polarizing Voltage at the maximum VZPS
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.