In Figure 11a & b the positive and negative sequence values of current and voltage for different faults are shown together with the summated values of current and voltage.
Relays usually only operate using the summated values in the right-hand columns. However, relays are available which can operate with specific values of some of the sequence components.
In these cases there must be methods for obtaining these components, and this is achieved by using filters which produce the mathematical operations of the resultant equations to resolve the matrix for voltages and for currents.
Although these filters can be constructed for electromagnetic elements, the growth of electronics has led to their being used increasingly in logic circuits. Among the relays which require this type of filter in order to operate are those used ιn negative-sequence and earth-fault protection.