Fault Passage Indicators as dedicated device

As it has been mentioned previously the FPIs are distributed in overhead network and underground network. There are different models about overhead FPI, someone can wrap the MV wire and others in the electrical tower without any contact with wires. The underground FPI can be installed in the MV-LV substation through CTs in the wire [76]. Following there is an explanation about the constructive features of the FPIs.

The current and voltage magnitude can be captured by overhead FPI with contact or not with the distribution overhead network. In case of contact the current measurement can be done by CTs such as toroidal which wraps the wire and the voltage magnitude can be detected through VTs.

Other economic possibility is to use the generated electromagnetic field by the wires in order to read these magnitudes. This last possibility can be done due to a coil which is excited by the electromagnetic field. The figure 5.14 shows the individual FPI by phase which allowing to know in which phase the fault has passed. This kind of devices is known as clip-on mounted or phase mounted and its installation is in the wire by means of a hook which allow to the operator a correct installation avoiding a direct contact with the wire.

Figure 5.14 Clip-on mounted or phase mounted FPI in MV cable, courtesy of Schneider Electric

There are other FPIs, known as pole mounted, which are prepared to install in an electrical pole or tower. These FPI receives the influence of the electromagnetic field analysing current and voltage although in these cases is not possible to know the phase where the fault is. Unlike to the clip-on mounted, this kind of FPI does not need a complex installation as it is showed in figure 5.15.

Both kinds of FPI have a small battery with long duration or the set battery and rectifier to load the battery through solar panel or auxiliary supply. Normally if the FPI has a small exclusive battery only indicates the pass of the fault locally, then DSO’s patrol will travel the line to identify the information of every FPI. On the other hand, if there is a solar panel with rectifier the FPI will have the possibility to communicate with control centre. In this case the clip-on mounted FPI, several manufacturers have associated a local Remote Terminal Unit (RTU) in order to communicate the fault detection to the control centre according to [77] and [78].

Figure 5.15 Pole mounted FPI with solar panel, courtesy of Schneider Electric

Another important skill of these devices is the possibility to identify if the fault is in the substation’s outgoing where it is installed or is a capacitive current return to the substation. Then this kind of devices have the possibility to analyse the direction of the current in order to indicate correctly the position of the fault [76].

The FPIs for underground network are in MV-LV substation, delivery centre or distribution centre mentioned before. This kind of FPI has associated a toroidal or CT to get the current measurement, regarding the voltage measure it is possible to use some capacitive or resistive sensors even a set of Low Power Voltage Transformer (LPVT).

As economic option, these FPIs have toroidals to wrap the wires of the MV cubicles. Normally it is used one per phase although there are combinations with two toroidals by phase and other to wrap the three cables to detect earth fault with accuracy adding the three phases. These toroidals are opened in order to install comfortably in the existing cubicles.

The figure 5.16 shows a FPI on the wall inside MV-LV substation. This FPI is connected with two phase toroidals and one zero sequence tore in a switchgear cubicle. The phase toroidals are wrapping the cables in the bushings and the earth fault tore is wrapping the three cables. Also, the capacitive sensors of the cubicle are linked to the FPI to indicate the absence or voltage presence. This indication can be done if the FPI receives an external supply from the MV-LV substation, in front of a fault if there is not supply in MV the FPI will not receive supply and then it will know the absence presence. Obviously, this kind of FPI will associate a rechargeable battery, which is supplied by the monophasic alternating current (AC) of the MV- LV substation, or simple battery. Others devices are self-supply and receive this energy from the toroidals joint to the measurement.

It is important to remark that the algorithm detection of this kind of devices is based in the comparison of magnitudes of current and voltage. In fact, when the current pass some set level in the FPI and there is an absence of voltage during a specific period of time, the device will send a signal by communication or by light local indication.

Other algorithms in this kind of detectors are based in rate of change of the current, 𝑑𝐼

𝑑𝑡 by validation of

absence voltage [76]. Some of these devices validate the indication of fault with these algorithm, verifying also the direction of the earth fault by means of residual and voltage current. In next chapter the directional protection and detection will be explained.