Monitoring TT low voltage distribution systems
Author: Andy Sagl
The traditional method used for detecting ground (earth) faults in TT distribution systems is to monitor the current in the neutral conductor. “However, this method is not entirely satisfactory”, says Andrew Sagl of Megger, who goes on to illustrate an alternative and much more effective approach based on the use of a nine-channel power quality recorder.
In low-voltage distribution systems, the earthing method is identified by a letter code, where T (from the French word ‘terre’meaning earth) denotes a direct connection to earth, N denotes a neutral connection, I indicates isolated from earth, S indicates separate and C indicates combined. Various types of earthing methods are in widespread use, with the TN-S and TN-S systems, for example, being particularly common in the UK. In many parts of the world, however, TT systems are the most widely used and, even in those countries where TT is not the dominant earthing method, it is still likely to be frequently encountered in rural areas and in temporary installations.
In a TT system, the transformer feeding the distribution system has a direct connection to earth, and the load also has a (separate) direct connection to earth, as shown in Figure 1.
Figure 1: A TT earthing system
Some of the benefits of TT systems are that they are simple and inexpensive to implement, and that faults in the LV and MV do not migrate to other customers connected to the same LV grid. TT systems do, however, have a number of disadvantages and one of these is the need for consumers to maintain their own ground electrodes, which means that complete reliability cannot be guaranteed. It is also possible for high voltages to appear between system components and the neutral conductor, and for these overvoltages to stress the insulation of equipment connected to the system.
In a TT system where the phases are balanced, where there are no ground faults and no zero-sequence harmonics, the neutral current will be zero, as the return currents corresponding to each phase cancel out in the neutral. In addition, there will be no current flow in the ground connection – see Figure 2.
Figure 2: In a fault-free system, the neutral current is zero
If a ground fault occurs, current from the fault will flow through the ground rod into the earth, through the earth, and back to the source (see Figure 3). Since the fault is very unlikely to affect all three phases equally, the phase currents will now be unbalanced and will no longer cancel in the neutral. A ground fault will therefore result in a current flow not only in the ground conductor but also in the neutral conductor (see Figure 4).
Figure 3: A ground fault causes current flow through the earth
Figure 4: A ground fault also causes current flow in the neutral conductor
Under fault conditions, the magnitude current that flows in the ground conductor depends on the integrity of the ground connection. The higher the impedance of the ground path, the lower the current that will flow in it. However, with a poor ground connection, the touch voltage on the system components will be higher as this voltage is directly proportional to the impedance of the ground connection. The touch voltage is the voltage a person would experience if, with a fault present, they were to touch a nominally grounded item of equipment while standing on an earthed surface. It is clear, therefore, that touch voltages in excess of 50 V are hazardous. This is a significant concern because, as already mentioned, one of the drawbacks of TT systems is that the reliability of earth electrodes cannot be guaranteed.
To guard against high touch voltages and to ensure that ground faults in TT systems are detected quickly, the traditional approach is to monitor the neutral current, typically with some form of residual current
device (RCD). Unfortunately, this approach alone is not sufficient to provide reliable protection with minimal nuisance tripping and to help with fault diagnosis. This is because other power quality phenomena, including unbalanced loads and zero-sequence harmonics, can also give rise to current flow in the neutral conductor.
Unlike ground faults, however, unbalanced loads and zero-sequence harmonics do not cause a rise in ground current. Therefore, monitoring both the neutral current and the ground current not only provides a way of identifying ground faults, but also of distinguishing them from other power quality events.
Going a little further, measuring the voltage between the ground and neutral provides an indication of the integrity of the ground connection. The worse the connection – that is, the higher its impedance – the higher will be the voltage between the ground and neutral conductors.
Of course, making instantaneous spot measurements – as an aid, for example, to determining why nuisance tripping of a residual current device occurs – may not detect or locate a ground fault because such faults are often intermittent. They often occur only when a faulty item of equipment is energized or in the presence of moisture. The solution is to use a recording instrument to monitor the affected circuits. A suitable setup, using a recorder with nine channels (four voltage and five current) is shown in Figure 5. This arrangement will provide comprehensive information, allowing speedy and reliable fault detection and diagnosis.
Figure 5: Arrangements for nine-channel power quality recording
With this set up, voltage channel 1 connects between phase A and neutral, voltage channel 2 between phase B and neutral, voltage channel 3 between phase C and neutral, and voltage channel 4 between ground and neutral. Current channel 1 monitors the current in phase A, current channel
2 in phase B, current channel 3 in phase C, current channel 4 in the neutral conductor and current channel 5 in the ground conductor. Low range current transformers (5 to 20 A) should be used in the current monitoring channels.
The results from a monitoring system of this type can be readily interpreted. If the ground current increases only when a particular item of equipment is turned on (with indication of the load coming online identifiable through individual phase voltage and current measurements), this would indicate that the ground fault is located within this item. If the ground current increases during wet weather, this most probably indicates that the ground fault is due to water ingress in an exposed cable. And if the ground-to-earth voltage rises excessively, this indicates that the ground electrode requires attention.
In conclusion, for a three-phase wye- (star-) connected TT-grounded system, a nine-channel power quality recording offers multiple advantages. It is the best way to identify ground faults and distinguish them from other power-quality phenomena, to identify poor earthing and dangerous safety conditions in the presence of a ground fault, to detect intermittent ground faults and to determine the sources of the faults.