Method for identifying an outgoing circuit having an earth fault in a three-phase power supply system

1. A method for identifying an outgoing circuit having an earth fault in a three-phase power supply system, the method comprising:

measuring and storing, at periodically recurring times, a zero voltage occurring at a neutral point and zero currents of the outgoing circuits;

determining a space vector representation of the zero voltage and an active component of the space vector representation of the zero currents based on the space vector representation of the zero voltage via a transformation;

determining, after an earth fault has been determined at a first time, a second time at which the space vector representation of the zero voltage has a local minimum and subsequently a third time at which the space vector representation of the zero voltage has a local maximum;

determining a trapezoidal sum of an active component of the space vector representation of the zero currents between a second time and a third time and utilizing said determined trapezoidal sum to determine an outgoing circuit having the earth fault such that the trapezoidal sum of the active component of the space vector representation of the zero currents is compared to a predefined variable threshold value and an earth fault is determined if said predefined variable threshold value is exceeded; and

performing an adaptive adjustment of the predefined variable threshold to a total capacity of the outgoing circuit connected to a busbar to set the predefined variable threshold value as a particular percentage of a total system capacity of the three-phase power supply system.

2. The method as claimed in claim 1 , wherein an earth fault is determined if an amount of the space vector representation of the zero voltage exceeds a predefined limit value.

3. The method as claimed in claim 1 , wherein the space vector representation of the zero voltage and the active component of the space vector representation of the zero currents is not stored until the first time.

4. The method as claimed in claim 2 , wherein the space vector representation of the zero voltage and the active component of the space vector representation of the zero currents is not stored until the first time.

5. The method as claimed in claim 1 , wherein a difference between a local minimum and a local maximum of space vector representations of the zero voltage must have a predefined minimum value.

6. The method as claimed in claim 1 , wherein the method is aborted if an amount of the space vector representation of the zero voltage exceeds a predefined range.

7. The method as claimed in claim 1 , wherein weighting function is utilized to evaluate the trapezoidal sum;

wherein the weighting function is increased upon occurrence of an earth fault on a corresponding outgoing circuit by a first value up to an upper maximum limit value;

wherein upon occurrence of a transient earth fault on another outgoing circuit the weighting function is reduced by the first value to a lower minimal limit value; and

wherein in the absence of a transient earth fault the weighting function is modified by a second, smaller value in a direction toward zero, an outgoing circuit being displayed as affected if the trapezoidal sum of said outgoing circuit evaluated using the weighting function exceeds a threshold value.

8. The method as claimed in claim 1 , wherein the predefined variable threshold value is determined from a total capacity of the outgoing circuits connected to the busbar.

9. The method as claimed in claim 8 , wherein the variable threshold value is determined as a particular percentage of the total capacity of the outgoing circuits connected to the busbar.