Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of ground fault indication
Patent
1997-12-18
1999-12-21
Do, Diep N.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of ground fault indication
324510, 361 44, G01R 3114, H02H 316
Patent
active
060053933
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND INFORMATION
The present invention is directed to a method of determining a ground-fault current in a multi-phase system by performing a vectorial summation of the output signals of each current transformer allocated to each phase of the system and of a neutral conductor.
European Published Patent Application 0 407 310 B1 requires the use of current transformers having great accuracy. Spurious tripping can occur if this requirement is not satisfied. For instance, if the ground-fault trip is to occur at 20% of the nominal current and increases by 5% when there is a fault in the transformer, then the measured ground current at 4-times nominal phase current has a fault of 100%. According to this reference, spurious tripping can only be avoided by employing current transformers that have great accuracy, but such transformers are heavy and require more space for installation. Space is limited because modern circuit breakers also place great value on compact dimensions.
Other methods for determining a ground fault current comprise specific features in terms of the requirements placed on the current transformers. In particular, ground currents can be measured directly, and this can be done by a ground transformer in the transformer neutral point or by a summation current transformer; this method is shown in U.S. Pat. No. 5,195,009. An obstacle to the high accuracy of such measuring methods can be increased complexity or conflicts with the system configuration.
SUMMARY OF THE INVENTION
An object of the present invention is to determine a ground-fault current in a multi-phase system by, first, using relatively inexpensive current transformers, and second, avoiding spurious tripping in order to achieve a certain level of operational reliability in the equipment to be protected.
This object is achieved by decreasing the vectorial sum of the current formed from the phase currents and the current of the neutral conductor by the product of the greatest current measured at the same point in time that flows in one of the phases or in the neutral conductor and the linearity error of the current transformer used.
This approach does not eliminate the measuring error of the ground current, but rather limits its effect by forming a single-sided tolerance band. This prevents spurious tripping caused by the measuring errors of the current transformers. The outlay required to realize the invention is slight in comparison to using current transformers having greater accuracy. The new method is equally suitable for current transformers having iron cores and for air-core transformers having a great dynamic range.
The object of the present invention can be achieved advantageously using a non-volatile memory for storing the linearity error of all current transformers used as a percentage. In order to achieve this object, it is not necessary to examine the linearity error of each current transformer individually. Rather, it is sufficient if the linearity error of the current transformer is determined and stored as a percentage quantity with respect each type of current transformer used.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example of the curve of the linearity error of a current transformer as a function of the current level and of the associated tolerance band.
FIG. 2 illustrates two tolerance bands of the vectorial error sum over the primary current.
FIG. 3 illustrates a flow diagram and circuit diagram in accordance with the present invention.
DETAILED DESCRIPTION
In FIG. 1, the straight line G represents the linear transfer of a primary current I.sub.prim into a secondary current I.sub.sec as effected by an ideal current transformer. In contrast, the relationship is in reality non-linear and correlates to the curve K. This means that the measurement values are sometimes less than and sometimes greater than the correct value, depending on the level of the primary current I.sub.prim. The linearity error F is therefore a function of the current. This results in a tolerance band T having a percent
REFERENCES:
patent: 5195009 (1993-03-01), May
Do Diep N.
Siemens Aktiengesellschaft
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