Construction of Resource-Saving Differential Protections for Converter Units with Transformers with 2N Secondary Windings and 2N Rectifiers

Relevance of the development of resource-saving differential protections without current transformers for converter units with transformers with 2n secondary windings (n is the number of star- and delta-connected secondary windings of the transformers) and 2n rectifiers is justified. Two new techniques are suggested for the construction of such protections. The first technique consists in measuring EMF at the leads of inductance coils mounted near the busbars of all phases from the low voltage side of the transformer of a converter unit, equalizing and subtracting EMFs induced by currents in these busbars obtained from the coils for the same phases, and the comparison between the resulted differences with the reference value. In the second technique, in addition to inductance coils mounted like as in the first technique, a reed with a control winding is suggested to mounted near the DC conductor busbar. The difference between the magnetic fluxes generated by the current in this busbar and by the current in the reed winding, proportional to the total rectified EMFs received from the coils mounted near the busbars of the same phases, is compared with the magnetic flux sufficient to trigger a reed which indicates a damage when its contacts are closed and sends a signal to disconnect the protected unit from the network. Models of the devices which implement the protection techniques suggested are described, and their behavior in different converter units operating modes is analyzed.