Earth VZPS Polarization
With Earth VZPS polarization, the operating quantity is earth current (3 x IZPS) and 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 160
EF 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 in Figure 160.
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).
Earth Ir Cos (Ø) Characteristic Fault Angle
When a forward EF occurs on a compensated earthing system, VZPS and IZPS are assumed to be 180° out of phase.
Therefore the Characteristic Angle for Ir Cos (Ø) is fixed at 180°.
The Ir Cos (Ø) Fault Angle for a given fault is the Zero Sequence Current angle relative to the Zero Sequence Voltage angle VZPS → IZPS.
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 (VZPS → IZPS) = 135° would log an Angle at Fault of -45° (135°-180°) which indicates that the fault angle is lagging the Characteristic Angle by 45°.
Other events that are logged for the above fault include:
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Forward Pickup Earth
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Forward Earth Protection Trip
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Angle at Fault -45°
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Quantity at trip Earth/SEF IRCos Ø – xx.xx A
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Polarizing Voltage at trip VZPS
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Quantity at the maximum Earth/SEF – Maximum IRCos Ø - xx.xx A
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Polarizing Voltage at the maximum VZPS
Earth VZPS 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°.
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 VZPS Low Voltage Block
When the Earth 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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Earth 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 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:
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Earth Directional Low V Armed
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Polarizing Voltage Low VZPS
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Earth Protection Trip
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Earth Low VZPS
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Polarizing Voltage at trip VZPS
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Polarizing Voltage at the maximum VZPS
The default Earth Low Threshold setting is 500 VZPS configurable over the range 150...15,000 VZPS.
Earth 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.