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Measurement of residual currents on a distribution feeder

Measurement of residual currents on a distribution feeder

Residual currents (earth fault)

The measurement of the earth fault or residual currents on a distribution system feeder can be done by the so-calledsummation connection, where the three current transformers, one in every phase, are connected and the summatedcurrent is brought to the inputs of an earth fault protection relay.

This is a convenient method for cases where the level of the earth fault current is relatively high, i.e.directly earthed systems.

When the level of the fault current to be measured starts to decline heavily, the measurement sensitivity and accuracythat can be reached with this summation connection is no longer sufficient. The measurement accuracy is affectedby the fact that the three different current transformers, even with similar core data, start to introduce different kind oferrors when it comes to ratio and phase displacement.

Window-type cable current transformer (photo credit: k-electric-gmbh.de)

These errors are summated together in the summation connection, and in the worst case they are cascading on topof each other. The sensitivity is affected by the fact that current transforming ratio of the phase current transformers ismainly selected based on the maximum load currents,whereas the anticipated earth fault currents can be onlysome fractions of those.

To overcome the accuracy and sensitivity requirementswith low (earth fault) residual currents, a cable (or busbar),also called a window-type, current transformer can beutilized. Following figure describes the installation.

Figure 1 – Typical installation of cable current transformer

In the figure above, a cable current transformer is utilized for measuring the residual (earth fault) current of anoutgoing three-phase cable feeder. The CT type used is a one that can be installed even after the medium voltagepower cable installation is done, by opening the bolted connections and separating the CT into two parts.

It is important to notice that the medium voltage power cable screen earthings are drawn back via the CT’s windowbefore connecting them to the earthing bar. Similarly, it is important to notice that if a separate ‘escort earthing‘wire is installed alongside the power cable, it may not pass through the CT’s window.

The suitable current transforming ratio of the cable current transformer is depending on the actual anticipated earthfault current levels.

As an example, the ratio of 70/1A is commonly used for earth fault protection in Finland with unearthed and resonantearthed systems. The cable current transformer can also have several ratios by utilizing different tappings in thesecondary side, as shown below.

Figure 3 – A cable CT with several ratio possibilities

When the earth fault protection relay is connected to a certain secondary tapping of the CT, the remaining tappingsshould not be shorted. This applies also to the test winding tappings. On the other hand, if the cable CT has severalcores utilizing separate iron cores inside the CT, each of the cores has to either be connected to a secondary device orshort-circuited and earthed.

Download Schneider Electric’s ‘Current Transformers Selection Guide’ //

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Reference // Distribution Automation Handbook by ABB