Peter Herpertz - Product manager
Electricity theft is a widely underestimated problem. The thieves see their actions as a minor crime, or even as a sporting challenge, but the resulting losses for power utilities are far from minor. In some countries, it is estimated that up to 50% of the electricity generated is stolen and it’s honest consumers who always end up covering the costs of this theft.
Until recently, detection of illegal consumer connections was almost impossible. Conventional reflection measuring (TDR) techniques can’t provide the resolution required to locate illegal supply taps, especially when these are close to the point of connection of the measuring instrument. The types of low voltage cable used in local distribution systems increase the difficulties because of their non-uniform nature and their tight bends. These create disturbances and reflections that prevent even experienced technicians from recognising illegal connections.
In addition, when making conventional TDR measurements, it is usually necessary to disconnect all of the consumers in the road from the supply network, in order to reduce interference. Disconnecting consumers for other than safety reasons is, however, generally considered to be unacceptable. Indeed, in many countries, the loss of supply will result almost immediately in penalties or compensation claims against the power utility company.
Additionally there are legal issues that make it very difficult to conduct investigations in the premises or on the property of private consumers, when acting only on unproven assumptions. These problems, reported by many power utilities, were seen as a challenge to find a better approach to the problem of detecting illegal supply taps.
Typically, the cable segment on which an illegal tap is installed is within the premises of the consumer. It is between the T-joint with the street cable or the overhead line connection box and the consumer’s meter box. This almost always means that the length of the cable segment is no more than 10 metres. Anyone who has tried to make reflection measurements on such a short cable will know that even the shortest measuring pulses cover a considerable proportion of this distance.
Consider, for example, a measuring pulse of just 5 ns. Impulse length [m] = Impulse width [µs] * V/2 [m/µs] = 0.005 µs * 80 m/µs = >0.4 m
However, the measuring pulse is subject to attenuation and dispersion, and thus will become wider, even over such a short distance, as shown in Figure 1. And for LV cables the propagation velocity V/2 is more likely to be in the region of 100 m/µs.
Figure 1: Normal reflection pulse response
Considering the shape of the pulse, which is somewhat similar to that of a sine wave, we can divide it into two basic components, the rising slope and the falling slope. When the measuring pulse comes to an impedance change, the rising slope will cause a corresponding response. This response is then immediately counteracted by the falling slope, which changes it in the opposite direction. This means that long before the reflection caused by an impedance change has reached an amplitude that corresponds to the value of the impedance change, it will fall back.
In practice, this means that all events in the cable, when measured over short distances using short pulses, will have similar looking reflections. As a result, it almost impossible to distinguish between them. And attempting to use longer pulses provides no detail at all, as the pulse width is then in the same range as the length of the cable.
For these reasons, a normal reflectometer (TDR), although it may provide some recognisable details, will not yield information corresponding to the real impedance change. As a result, the reflections visible in the trace cannot be assigned to any specific event – see Figure 2.
Figure 2: Differentiation between normal joints and T-joints is not possible
Now let’s consider a T-joint. Because of its physical properties, at a T-joint the impedance is half the normal cable impedance, since the impedance values of the two cables leaving the joint are effectively in parallel. This allows the reflection factor to be determined without the need for complicated equations. On arrival, the measuring pulse divides itself into three equal parts – see Figure 3. One third is reflected at the T-joint with negative amplitude, while the other two fractions of the signal continue into the two branches. As a result, T-joints should be displayed with significantly larger amplitude.
Figure 3: Signal distribution in a T-joint
The correct approach to detecting T-joints comes from knowing that it is the falling slope of the measuring pulse that causes problems by interfering with the measurement. The solution is, therefore, simply to eliminate the influence of this falling slope. And that’s exactly what is done in a “step TDR”. An instrument of this type generates a step pulse that rises in one direction only and does not return to zero – see Figure 4. The TDR trace therefore has plenty of time to respond to the complete impedance change, which results in a significantly larger reflection, the amplitude of which is proportional to the real impedance change.
Figure 4: A step pulse
This makes it possible to differentiate illegal T-joint taps from normal joints and impedance changes caused by sharp bends or cable non-uniformities. Even technicians who do not have extensive experience of LV cable measurements can easily recognise events on the cable that should not be there, and the inconvenient tasks of recording and managing reference traces are no longer needed.
A new reflectometer, the SebaKMT Teleflex LV from Megger, has been developed specifically for applications of this type. Its main features are very fast-rising measuring pulses to suit this short-range application and the use of step pulses instead of normal measuring pulses. With these features it is ideally suited to the detection of details that would otherwise remain undiscovered, at short ranges. In multiple field trials, the new instrument has already proved its worth.
To allow the instrument to be used on live cables, a new power blocking filter, which is particularly suitable for short-range and step TDR applications, has also been developed. In contrast with conventional TDR power blocking filters, its characteristics are optimised for fast-rising, high frequency step pulses. This filter has a CAT IV 600 V rating, in line with IEC 61010/VDE 0411. Fused test leads rated CAT IV 600 V are supplied with the instrument.
For easy tracing of cables installed within walls or in similar concealed locations, the instrument also has an integrated audio frequency transmitter that covers frequencies from 810 to 1100 Hz. These frequencies can be detected by a receiving system such as the Ferrolux Minin antenna.
A practical application
To prove the effectiveness of this new technology, a Teleflex LV instrument was used in real field trials on house connections with suspected illegal taps. An important aspect of these trials was that the measurements could be performed without disconnecting the consumers. This is particularly important as, in many cases, disconnection will affect more than one consumer and it can also act as an early warning to the user of the illegal tap.
In the case described here, an illegal tap was suspected, and the objective was to confirm its presence with the tests. The measurements were performed with and without the power-blocking filter. Figure 5 shows the arrangement for the supply of power from the network to the house. The house connection box is mounted on the post and is provided with a fused LV supply from the overhead line. A buried cable links the house connection box and the meter box, and this cable is where illegal taps are typically installed.
Figure 5: Power supply arrangements for house - post-mounted connection box
When the Teleflex LV instrument was used to produce a TDR trace for the cable between the house connection and the meter box, with the supply off and no blocking filter used, the result was as shown in Figure 6. The negative reflection is a clear evidence of a T-joint. On the normally undisturbed, homogenous connection path from the house connection to the meter, any significant reflection is suspicious. Such a reflection provides good reason for further steps to be taken.
Figure 6: Strong, clear reflection caused by the T-joint
With the information provided by the TDR measurement it was possible to deduce that the illegal tap was about 4.3 m from the connection box. As the box is installed at a height of around 3 m, this meant that the tap was around 1 m from the post. As can be seen in Figure 7, excavation revealed the illegal T-joint at the exactly the position predicted, giving perfect confirmation of the measured results.
Figure 7: The illegal T-joint revealed
This exercise, which provided absolutely undeniable evidence of electricity theft, resulted in rapid prosecution of the electricity thief and substantial additional payments to the power utility.
As we have seen, the Teleflex LV allows reliable detection of illegal connections at short range, with a relatively small amount of effort. The accurate measurement of distance allows precise pinpointing of the position of the connection, which minimises excavation efforts and costs. For power utilities, an instrument of this type is an essential tool not only to detect but also to deter the theft of electricity, because when details of prosecutions of the type described here become widely known, others who may be considering stealing electricity will think much more carefully about whether it really is worthwhile.