Handbook of Large Hydro Generators. Geoff Klempner
grown in size over the years. Today, it is now possible to see these hydro machines with terminal voltages over 20,000 volts and ratings over 900 MVA as shown in Figure 1.6-2.
1.6.2 Principles of Construction
Synchronous machines come in all sizes and shapes, from the miniature permanent magnet synchronous motor in wall clocks, to the largest hydro generators of up to about 944.5 MVA. Synchronous machines are one of two types: stationary field or rotating DC magnetic field.
The stationary field synchronous machine has salient poles mounted on the stator, the stationary member. The poles are magnetized either by permanent magnets or by a DC current. The armature, normally containing a three‐phase winding, is mounted on the shaft. The armature winding is fed through three sliprings (collectors) and a set of brushes sliding on them. This arrangement can be found in machines up to about 5 kVA in rating. For larger machines covered in this book the typical arrangement used is the rotating magnetic field.
The rotating magnetic field (also known as “revolving field”) synchronous machine has the field winding wound on the rotating member (the rotor) and the armature wound on the stationary member (the stator). A DC current, creating a magnetic field that must be rotated at synchronous speed, energizes the rotating field winding. The rotating field winding can be energized through a set of sliprings and brushes (external excitation) or from a diode bridge mounted on the rotor. The rectifier bridge is fed from a shaft‐mounted synchronous generator, which is itself excited by the pilot exciter. In externally supplied fields, the source can be a shaft‐driven DC generator, a separately excited DC generator, or a solid‐state rectifier. Several variations or combinations of these variations are used.
The stator core is made of insulated silicon‐steel laminations. The thickness of the laminations and the type of steel are chosen to minimize eddy current and hysteresis losses, while maintaining required effective core length and minimizing costs. The core is mounted directly onto the frame. The core is slotted (the slots are normally open), and the coils making up the winding are placed in the slots. The most popular arrangements are lap and wave windings of various types. Modern large machines typically are wound with double‐layer lap windings.
A rotor field of salient pole construction is as shown in Figure 1.6-3.
In a large generator, the rotor magnetic field is generated by a coil wrapped around it with current passing through the coil. For simplicity, this figure shows a two‐pole rotor. Salient pole rotors normally have many more than two poles. When designed as a generator, large salient pole machines are driven by water turbines. The bottom part of this figure shows the three‐phase voltages obtained at the terminals of the generator, and the equation relates the speed of the machine, the number of poles, and the frequency of the resulting voltage.
Figure 1.6-3 Synchronous machine construction.
This figure includes all synchronous hydro generators, almost every synchronous condenser, and the overwhelming majority of synchronous motors.
Large salient pole rotors are typically made of laminated poles retaining the winding under the pole head. The poles are keyed or bolted onto the shaft (spider‐and‐wheel structure). The majority of salient pole machines have an additional winding on the rotating member. This winding, made mostly of copper bars short‐circuited at both ends, is embedded in the head of the pole, close to the face of the pole at its airgap. In synchronous generators this winding serves to dampen the oscillations of the rotor around synchronous speed and is, therefore, named the damping or damper winding (also known as amortisseur). In synchronous motor applications, this winding can be used to start the motor or condenser as an induction motor, and take it to almost synchronous speed, when the rotor is “pulled in” by the synchronous torque.
1.6.3 Rotor Windings
In hydro generators, the winding producing the magnetic field is made of a number of coils connected in a series circuit, energized with DC power typically supplied through the shaft from slip or collector rings on the shaft. In self‐excited generators, the shaft‐mounted exciter and rectifier (diodes/commutators) generate the required field current. The shaft mounted exciter is itself excited from a stationary winding. Alternately, static excitation eliminates the need for shaft driven exciters. This method is commonly applied to modern generators or as an upgrade for older machines where maintenance and performance are issues. Generators typically have field supplies of 125 or 250 V DC, and in some machines even higher depending on the design. A more elaborate discussion of rotor winding design and construction can be found in Chapter 2.
1.6.4 Stator Windings
The magnitude of the voltage induced in the stator winding is function of the magnetic field intensity, the rotating speed of the rotor, and the number of turns in the stator winding. An actual description of individual coil design and construction, as well as how the completed winding is distributed around the stator, is meticulously described in Chapter 2. In this section, a very elementary description of the winding arrangement is presented to facilitate the understanding of the basic operation of the machine.
As stated above, coils are distributed in the stator in a number of forms, each with its own advantages and disadvantages. The basic goal is to obtain three balanced and sinusoidal voltages having very little harmonic content (harmonic voltages and currents are detrimental to the machine and other equipment in a number of ways). To achieve a desired voltage and MVA rating, the designer may vary the number of slots and the manner in which individual coils are connected, producing different winding patterns. The most common winding arrangement is the lap winding, shown in Figure 1.6-4, for salient pole machines. The section shown is a portion of one of the three phases. It can be readily seen that the winding runs clockwise under a north pole, and counterclockwise under a south pole. This pattern repeats itself until the winding covers the number of pole groups in a parallel. A similar pattern is followed by the other two phases, but located at 120 electrical degrees apart. A connection scheme that allows great freedom of choice in designing the windings to accommodate a given terminal voltage is one that allows connecting sections of the winding in parallel, series, and/or a combination of the two as shown in Figure 1.6-5.
Figure 1.6-4 “Developed” view showing four‐poles, slots, and a section of the winding.
1.7 SYNCHRONOUS MACHINE: BASIC OPERATION
For a more in depth discussion of the operation and control of hydro generators, the reader is referred to Chapter 4. In this chapter, the most elementary principles of operation of synchronous machines will be presented. As mentioned above, hydro generators are almost always three‐phase machines. Thus, the best place to start describing the operation of a three‐phase synchronous machine is a description of its magnetic field.
Earlier, we described how a current flowing through a conductor produces a magnetic field.