Delta and Y connections for railway transmission and distribution

Transformer connections under the following three conditions are discussed: The railway; (1) purchases power; (2) builds and operates the generating and transmission systems; (3) purchases power and in addition builds and operates a secondary transmission system. The choice between delta and Y connection is frequently determined by purely economic considerations. However, it is common practise to use the delta connection on both the high- and low-tension sides of transformers, except in the case of six-phase converters, where the diametrical connection on the low-tension side is the rule. Synchronous converter substations employing two machines in series for high-voltage d-c. railways, are frequently operated from single banks of transformers, which have double windings on the low-tension sides. Methods of starting have little effect upon transformer connections except in the case of six-phase converters. Transformer connections are seldom important electrically in single-phase and three-phase railway systems. A secondary distribution system may at first be delta connected and later changed over to Y connection to obtain better voltage regulation.