Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 19

The two‐wire AC circuits discussed above (called single‐phase circuits or systems), are commonly used in residential, commercial, low voltage, and low power industrial applications. However, all electric power systems to which industrial generators are connected are three‐phase systems. Therefore, any discussion in this book about the “power system” will refer to a three‐phase system. Moreover, in industrial applications, the voltage supplies are, for all practical reasons, balanced, meaning that all three‐phase voltages are equal in magnitude and apart by 120 electrical degrees. In those rare events in which the voltages are unbalanced, the implications for the operation of the generator will be discussed in other chapters of this book.

Three‐phase electric systems may have a fourth wire, called “neutral.” The “neutral” wire of a three‐phase system will conduct electricity if the source and/or the load are unbalanced. In three‐phase systems, two sets of voltages and currents can be identified. These are the phase and line voltages and currents.

Figure 1.4-1 shows the main elements of a three‐phase circuit. Three‐phase circuits can have their sources and/or loads connected in wye (also known as “star”) or in delta forms. (See Figure 1.4-2 for a wye‐connected source feeding a delta‐connected load.)

Figure 1.4-1 Three‐phase systems. Schematic depiction of a three‐phase circuit and the vector (phasor) diagram representing the currents, voltages, and angles between them.

Figure 1.4-2 A “wye‐connected” source feeding a “delta‐connected” load.

Almost without exception, hydro generators have their windings connected in wye (star) form. Therefore, in this book, the source (or generator) will be shown wye‐connected. There are a number of important reasons why hydro generators are wye‐connected. They have to do with considerations about its effective protection as well as design (insulation, grounding, etc.). These will be discussed in the chapters covering stator construction and operations.

On the other hand, loads can be found connected in wye, delta, or a combination of the two. This book is not about circuit solutions; therefore, the type of load connection will not be brought up herein.