The auxiliary services of an electrical substation are very important, but they could not control the situation Substation, which would not be available for control, signaling, control consumption ... But there is another handicap type the question: Which came first: the chicken or the egg? regarding AC: How do you control the AC if you need to cut it? With a generator! The best: With Continuous current, more stable and controllable than a generator. But we must send DC to items that require it, for it will build these cabinets Ancillary Current.