Electricity: electrical systems and devices – Safety and protection of systems and devices – Transformer protection
Patent
1997-01-02
1998-08-04
Gaffin, Jeffrey A.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Transformer protection
361 63, 361 93, 361115, H02H 704
Patent
active
057903573
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to a differential current protective device for a transformer in a multi-phase power supply system.
BACKGROUND INFORMATION
Power transformers such as those used in multi-phase power supply systems are wired differently on the high- and low-voltage sides with regard to their windings in individual applications. The different wiring of the windings is referred to as "vector groups" so each transformer has its own vector group. For example, the vector group of a transformer may be indicated as "Yd5" where "Y" denotes that the windings of the transformer are star connected on the high-voltage side, "d" indicates that the windings are delta connected on the low-voltage side and the number "5" indicates that the angle between conductor currents of a phase is 5.times.30.degree., i.e., 150.degree..
If a transformer in a multi-phase power supply system is to be monitored for internal faults by means of a differential current protective device, then for a comparison of the currents flowing on the high and low voltage sides, it is necessary to use current measurement parameters that are independent of how the windings of the transformer are wired, because otherwise differential current protective devices of different designs would have to be used for transformers having different vector groups. In comparing the currents on the two sides of the transformer to be monitored, the phase angle of the currents must also be taken into account. This matching of the differential current protective device to the vector group of the transformer to be monitored is known as vector group matching.
In vector group matching of a differential current protective device to a multi-phase transformer having a certain vector group, all the cases of faults that can occur must be taken into account, specifically so that the differential current protective device definitely will not respond when there is an external fault with respect to the transformer to be monitored, whereas a fault in the transformer to be monitored will definitely be indicated by the differential current protective device. A special problem in this regard arises when a differential current protective device is to monitor a transformer where at least one star winding has a grounded star point. With regard to such a transformer, an equation for calculating the asymmetrical components of the zero system current is provided for vector group matching for a state-of-the-art differential current protective device PQ 721 from AEG. With this device, the matching matrix shown below is used for the high voltage side of a transformer T having vector group Yd5 (see FIG. 1) for a transformer T to be monitored: ##EQU1## where I.sub.L1os =measurable current in the phase or in conductor L1 on the high voltage side of the transformer to be monitored; voltage side of the transformer to be monitored; voltage side of the transformer to be monitored; to be monitored; known device: ##EQU2## The meaning of the individual parameters in matrix (2) is mostly the same as the nomenclature used in equation (1) except that the letter "u" is used in matrix (2) to indicate the low voltage side.
As mentioned before, the parameter I.sub.0os is determined in a computing circuit of the known device according to the following equation:
If an internal single-phase-to-ground fault occurs in phase L1 of transformer T that is to be monitored, then essentially the following fault condition (4) holds: ##EQU3## where I.sub.F denotes a short-circuit current in phase L1.
Calculations show that differential currents I.sub.DIFF1, I.sub.DIFF2, and I.sub.DIFF3 occur with respect to the individual phases or conductors L1 to L3 in the prior-art differential current protective device, as described by the following matrix (5): ##EQU4## This means that a differential current I.sub.DIFF2 and I.sub.DIFF3 is also detected with regard to the phases or conductors L2 and L3, although for the assumed fault case, no differential current should result for these lines or phases at al
REFERENCES:
patent: 3675080 (1972-07-01), Bath
patent: 5172329 (1992-12-01), Rahman et al.
patent: 5627712 (1997-05-01), Wilkinson
Clemens, et al., "Schutztechnik in Elektroenergiesystemen", 1991, pp. 224-227.
Lobos, et al., "Digitale Filteralgorithmen zur Echzeitermittlung der Grundschwingung von Spannungen, Stromen und Symmetrischer Komponenten", Elektrie, Berlin 46 (1992) 1, pp. 29-34.
Schiel, et al., "Umfassendes Konzept fur den Transformatorschutz", Schutztechnik, pp. 296-498 and 500-502.
Gaffin Jeffrey A.
Jackson Stephen
Siemens Aktiengesellschaft
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