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2.2 Differential Protection

115

7UT613/63x Manual

C53000-G1176-C160-2

Non-earthed Trans-

former Starpoint

Figure 2-28 illustrates an example for a power transformer Yd5 (wye-delta with 150°

phase displacement) without any earthed starpoint. The figure shows the windings

(above) and the vector diagrams of symmetrical currents (below). The general form of

the matrix equation is:

) Matrix of the matched currentsI

, I

k Constant factor for magnitude matching,

) Matrix of the phase currents I

, I

On the left (delta) winding, the matched currents I

, I

are derived from the differ-

ence of the phase currents I

, I

. On the right (wye) side, the matched currents

are equal to the phase currents (magnitude matching not considered).

Figure 2-28 Matching the transformer vector group, example Yd5 (magnitudes not consid-

ered)

Since there is no point earthed within the protected zone, no considerable zero se-

quence current can be produced within the protected zone in case of an earth fault

outside the protected zone, regardless whether or not the system starpoint is earthed

anywhere else in the system. In case of an earth fault within the protected zone, a zero

sequence current may occur at a measuring location if the system starpoint is earthed

anywhere else or another earth fault is present in the system (double earth fault in a

non-earthed system). Thus, zero sequence currents are of no concern for the stability

of the differential protection as they cannot occur in case of external faults.

However, in case of internal earth faults, the zero sequence currents are practically

fully included in the differential quantity because they pass the measuring points from

outside. Even higher earth fault sensitivity during internal earth fault is possible by

means of the time overcurrent protection for zero sequence currents (section 2.4.1)

and/or the single-phase time overcurrent protection (section 2.7),

Siemens SIPROTEC 7UT613 series User Manual

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