Method and device for determining direction of current flowing through circuit breaker

A method for determining the direction of current flowing through a circuit breaker connected to a load having at least two power sources arranged at separate conductors, wherein the circuit breaker has a current reference direction from one of the at least two powers sources to a load, the method comprising:

• obtaining a sample value of current flowing through the circuit breaker to another circuit breaker and a differential value of current;
• obtaining a sample value of voltage at the circuit breaker;
• obtaining, in a present circuit in which the circuit breaker and the another circuit breaker lie, on the basis of a relationship between the voltage and the current at the circuit breaker at a present time and at a previous time, an equivalent resistance and an equivalent inductance in the present circuit;
• determining, independent of a frequency of the one of the at least two power sources supplying the present circuit in which the circuit breaker and the another circuit breaker lie and based upon the equivalent resistance and equivalent inductance both being greater than zero, that the direction of current flowing through the circuit breaker is the same as the current reference direction, and determining, based upon the equivalent resistance and equivalent inductance both being less than zero, that the direction of current flowing through the circuit breaker is opposite to the current reference direction, wherein the previous sampling time (tn-1) is a sampling time which immediately precedes a present sampling time (tn), and
• sending a directional zone selective interlock signal from the circuit breaker to the another circuit breaker based upon the determined direction of current flow, wherein
• the relationship between voltage and current in the present circuit at the present sampling time (tn) is:
• u(tn)=R×(tn)+L×i′(tn);
• the relationship between voltage and current in the present circuit at the previous sampling time (tn) is:
• u(tn-1)=R×i(tn-1)+L×i′(tn-1),
• wherein R is the equivalent resistance in the present circuit in which the circuit breaker lies;
• L is the equivalent inductance in the present circuit in which the circuit breaker lies;
• i(t) is the sample value of current flowing through the circuit breaker at time t, and i′(t) is the differential value of current at time t;
• u(t) is the sample value of voltage at the circuit breaker at time t;
• and
• tn is the time of a present nth sampling point, tn-1 is the time of an (n−1)th sampling point, and n is an integer greater than zero.