Phase displacement of the current
Directional protection relay is often used in all network parts in which the direction of flow of power might change, notably in the instance of a short circuit between phases or of an earthing fault (single phase fault).
Directional protection is complementary to overcurrent protection, enabling good discrimination of the faulty network section to be achieved in the above mentioned situations.
Active or reactive power protection equipment is used to detect abnormal power flow other than the one due to a short circuit (e.g. In the event of the failure of the prime mover, a generator will continue to run as a synchronous motor, drawing power from the system).
This reference variable is called the polarisation quantity.
- Earth fault (e/f) directional protection
- Phase directional protection
It is sensitive to the direction of flow of the current to earth. It is necessary to install this type of protection equipment whenever the phase to earth fault current is divided between several earthing systems.
However, this current flow is not only due to the earthing of the network’s neutral, but also due to the phase to earth capacitance of the lines and cables (1 km of 20 kV cable causes a capacitive current flow of around 3 to 4 amps).
On these feeders, the phase to earth capacitance is sufficiently high for a zero sequence current to be detected by a nondirectional e/f protection as soon as a phase to earth short circuit occurs, wherever it may be on the network (see Figure 1).
In earth fault directional protection, the residual current is measured and the residual voltage is most often used as the polarisation quantity. The latter should not be confused with the zero sequence voltage.
Recall that in any three phase system F1, F2, F3, symmetrical component theory defines the zero sequence variable Fh as:
The residual variable Fr is three times greater than the zero sequence variable.
Measuring residual current
The residual current is either measured by three current transformers, one per phase, or by a coil (ring CT) around the three phases.
The use of three current transformers (see Figure 2) has certain advantages:
- CT’s are generally dependable,
- It is possible to measure high currents.
But it also has certain disadvantages:
- Saturation of the CT’s in the instance of a short circuit or when a transformer is switched on produces a false residual current
- In practice, the threshold cannot be set to under 10% of the CT’s rated current.
Measuring using a ring CT (see Figure 3) has the advantage of being very sensitive and the disadvantage of the coil (low voltage insulated) being installed around a non-clad cable to insulate it.
Measuring residual voltage
Residual voltage is measured by three voltage transformers (VT). Often VT’s with two secondary windings are used (see Figure 4) where one is star connected and enables both phase to neutral and phase to phase voltages to be measured and the other is open delta connected enabling the residual voltage to be measured.
If the main VT’s only have one secondary winding are star connected, and grounded, a set of auxiliary VT’s can be used to measure the residual voltage (see Figure 5).
This situation is often encountered when the protection plan for existing installations is upgraded.
It should be noted that certain protection equipment does not require auxiliary VT’s, the equipment itself providing the residual voltage value from the three phase to earth voltages.
The residual voltage is most often used as the polarisation variable for an earth fault directional relay. However, it may also be taken as the current in the installation’s neutral earthing arrangement.
See Figure 6.
In theory, both these ways of polarising the protection equipment are equivalent. If Zh is the transformer’s zero sequence impedance and Zn the impedance of the neutral point, the residual voltage, Vr, and the current of the neutral point, In, are related by the following equation (written in complex numbers!):
Vr = (Zh + 3Zn) × In
In this instance the current measurement is more accurate than that of residual voltage, which has a very small value. It can only be used in substations that are near to the neutral earthing connection.
To determine the direction of the fault, the protection equipment measures the phase displacement between the current and the polarisation variable. If the polarisation variable is not in the axis of symmetry of the wished relay’s action (the characteristic axis, see Figure 7), it is necessary to re-phase it.
This is done by adjusting the characteristic angle.
When designing the protection coordination, the characteristic angle of directional protection equipment must be determined so that any fault in the chosen direction causes a current that falls in the tripping zone and that any current in the other direction causes a current falling outside of this zone.
To be able to measure the phase displacement between the current and the polarisation variable, it is essential for the latter to be sufficiently large (generally u 0.5 to 2% of the rated value of the variable).
If the polarisation variable is less than this threshold then the protection equipment does not operate, whatever the measured current value.
Three principles of detection exist concurrently and they correspond to various requirements and sometimes to various practices:
- Directionalized overcurrent,
- Measurement of the projection of the current,
- Measurement of the residual active power.
The first two are used for phase and earth fault detection, the last one is specific to earth fault detection with a special neutral point arrangement.
This type of directional relay is made by combining overcurrent protection equipment with an equipment to measure the phase displacement between the current and the polarisation variable.
Tripping is subject to the two following conditions:
- The current is greater than the threshold, and
- The phase displacement between the current and the polarisation variable, defined by the characteristic angle, is in the zone between +90° and −90°.
This type of protection equipment calculates the projection of the current along the characteristic axis. The value obtained is then compared with a threshold in order to determine whether or not to trip.
See Figure 8 below.
This type of protection equipment actually measures the residual active power and the threshold is expressed in Watts. It must be designed to avoid any spurious tripping caused by measurement inaccuracy in the instance of a strong capacitive residual current (strong residual reactive power);
The operating zone is limited as shown in Figure 9.
To detect earthing faults, the most universal principle is that of measuring the projection of the current. The use of directionalized overcurrent relays is not suited to all neutral point arrangements.
Protection equipment measuring the residual active power is restricted to use with compensated neutral point arrangements in competition with current projection type relays.
It is installed to protect two connections operated in parallel, a loop or a network component connected to two power sources (see Figure 10).
More often than not, this type of protection equipment is two phase comprising two independent, single phase elements. Sometimes three-phase protection equipment must be used.
When the relay measures the current in phase 1, the polarisation voltage most often used is V2-V3. The protection equipment’s angle of connection is then said to be 90° (see Figure 11).
Similarly to a directional earthing relay, the characteristic angle of a directional phase relay defines the position of the angular tripping zone. It is the angle between the normal to the tripping plane and the polarisation variable.
In order to be able to measure the fault direction, the polarisation variable (the voltage) must have a sufficiently high value. In particular, a three-phase fault very close to a directional relay is not detected because all of the phase to phase voltages are zero.
To obtain the direction of this type of fault, the protection system must use a memory voltage.
Phase directional relays function either as directionalized (see Figure 12) overcurrent element, or by measuring the projection of the current on the characteristic axis.
Although relays functioning on both principles exist on the market, the directional overcurrent relay should be preferred.
Co-ordinating overcurrent protection equipment is much easier since the detection threshold is independent of the current’s phase.
The power measurement is not used to detect short circuits. Power is not a good fault detection criteria because in the instance of a fault between phases the nearer the fault is the lower its value.
Source: Directional protection equipment by P. Bertrand (Schneider Electric)