Directional Blocking

Overview

AS are sometimes installed in network configurations where power flow could be in either direction through the switchgear.

Directional Blocking can be used when the ACR is only required to trip for faults on either the load or source side. When power flows from Source to Load, the power flow is deemed to be forward. Reverse power flows from Load to Source.

The Source and Load sides of the switchgear are configurable through the Power Direction setting.

The Power Direction setting can be found on the WSOS Measurements page or on the operator interface.

Directional Blocking is an optional protection feature that will only allow a fault to cause a protection trip when power flow is in the designated directions for the faulted element.

Each overcurrent element, Phase, Earth, SEF, and NPS if configured can be set for a Fault Detection Direction of:

Forward Fault
Reverse Fault
Forward and Reverse Fault (non-directional)

Directional Blocking uses only one active detection group that determines the pickup and detection timing for faults detected in the designated fault detection direction for each element.

If a fault is determined to be in the designated fault detection direction, detection will be armed.

If a fault is determined to not be in the designated fault detection direction, detection will be blocked.

Directional Blocking can only be turned on from the WSOS Feature Selection page.

It can’t be turned on from the Options pages on the setVUE O.I. or the Feature Selection pages on the flexVUE O.I.

Determining Direction

The user must set a Characteristic Fault Angle for each element.

The Characteristic Angle is essentially a prediction of the expected fault angle when a forward fault occurs.

The fault angle is defined according to the polarising method for each element and is effectively the phasor relationship between the fault current and the polarising voltage.

A forward fault is one where the fault angle is within ± 90° of the Characteristic Angle*.

For any given fault, the actual fault angle will be determined by network characteristics such as line resistance and reactance as well as fault characteristics such as the nature of the short circuit, solid or arcing etc.

It is not necessary that the fault angle matches the Characteristic Angle precisely. There are only two possibilities for direction, forward and reverse. As long as the fault angle is at the Characteristic Angle +/- 90° it will be recognized as a forward fault. Otherwise it is a reverse fault.

Each element takes 20 ms to determine the direction of a fault once it has been detected. This delay occurs prior to the fault detection timing and adds to the actual fault detect time.

The figure below demonstrates a Characteristic Angle of +45° with phasors representing possible fault angles on either side. Both of these fault angles represent forward faults as they are well within the forward fault region (135° to -45°)

Figure 207
Directional Blocking Forward and Reverse Sectors

Low V Block

In order to determine the direction of a fault that caused a pickup, the controller requires a Polarizing Voltage. If there is a bolted phase fault on the terminals of the switchgear there may not be sufficient voltage to determine the direction of the fault. Directional Blocking includes two settings per element to deal with low polarising voltage.

Low Block
Low Threshold

If the polarising voltage is below the Low Threshold setting for the faulted element, the Low Block setting will determine what action must be taken.

If Low Block is Off, the fault is assumed to be in the Fault Detection direction and fault detection will be armed.

If Low Block is On, Polarising voltage below the Low Threshold setting will cause fault detection to be blocked.

Sector Width

Each element will have either a Forward Sector Width setting if the Tripping Direction is Trip Forward, or a Reverse Sector Width setting if the Tripping Direction is Trip Reverse.

If the Fault Detection Direction is set for Forward and Reverse, Sector Width is meaningless and the setting is not displayed.

The Forward Sector Width setting is the angle that the deviation of the calculated fault angle from the Characteristic Angle needs to be equal to or less than to be deemed a forward fault.

The Reverse Sector Width setting is the angle that the deviation of the calculated fault angle from the Characteristic Angle plus 180° needs to be equal to or less than to be deemed a reverse fault.

V_zps Balance

ZPS Voltage (V_zps) is the polarising voltage for many of the polarising methods used by Directional Blocking.

Even in un-faulted networks the ZPS voltage is not likely to be zero because the three phase voltages are not going to be perfectly balanced.

This could lead to problems in high impedance fault conditions because the V_zps due to an earth fault might be dominated by the V_zps due to out of balance phase voltages, and this could lead to an incorrect determination of direction.

V_zps Balancing is provided to overcome this by continuously balancing the phase voltages under normal conditions and compensating for imbalance between the three phase voltage measurements.

V_zps Balance can be turned On in WSOS on the Directional Blocking page or on the operator interface.

When balancing is enabled, the zero sequence voltage is continually balanced at the rate of 0.6% of phase to earth voltage per second until zero V_zps has been achieved.

Balancing is paused when any of the following conditions occur:

Detection pickup
The SCEM data is not valid
The switchgear is open
Any of the bushings are dead

V_zps Balance status is displayed in WSOS under Controller Status on the Control page.

Figure 208
WSOS V_ZPS Balance Status

When balancing is enabled the status displays V_zps Balancing while the system is continually adjusting for balance, and V_zps Paused when the balancing is suspended for one of the reasons above.

When balancing is disabled the display shows V_zps Balance Disabled.

High V_zps Alarm

The Directional Blocking facility includes a High ZPS Voltage alarm.

This indication can be useful in unearthed neutral networks for earth fault detection.

The high V_zps alarm is asserted whenever the Alarm Set-point is exceeded for the Alarm Timeout period. Regardless of whether SEF protection is picked up or not.

The alarm is cleared when V_zps falls below the Alarm Set-point again.

The High V_zps Alarm can be turned on and configured via WSOS on the Directional Blocking page.