Что мне удалось найти, так это : "A swing source is a utility supply or generator in which the real power flow is not given...",
"The swing or "slack" sources supply the difference between the given real and reactive power into the system at the other buses and the total system output plus the losses in the system...",
"In most large industrial plants, the utility supply is designated as the swing source."

Так что же это? "Компенсаторный источник питания", "дополнительный", "вспомогательный"?

Абзац полностью (речь идёт о UBLF-приложении):
"UBLF is designed to accommodate large scale distribution models extracted from GIS and provide fast solutions for real-time. The application can support distribution networks meshed or paralleled to any degree including multiple swing sources, electrical loops, and underground phase loops where phases split and come back together."

И ещё: можно ли в данном контексте перевести "underground phase loops" как "подземные силовые линии"? То, что речь идёт о фазах как составляющих многофазовых цепей, понятно из контекста.

И ещё: "initial or reversing control actions" - "начальные и обратные управляющие действия" - ?

"Once through units are generally less responsive than drum units. A tunable delay following initial or reversing control actions prevents control windup in these units."

Речь идёт о блоке автоматического управления генерацией (АУГ - AGC). Полный абзац:
"The AGC unit control subfunction contains various features and tunable parameters that permit accurate control of all types of generating units. For example, to account for the quick response typical of drum units after an extended period of operation in a steady state condition, AGC inhibits control actions when the unit actual response rate exceeds its tunable response rate limit. Once through units are generally less responsive than drum units. A tunable delay following initial or reversing control actions prevents control windup in these units."