Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 31

To describe the fundamental principles describing the operation of a synchronous machine, it is convenient to use the constructs of an ideal salient pole rotor machine connected to an infinite bus. The infinite bus represents a bus which can deliver or absorb active and reactive power without any limitations and whose voltage and frequency are essentially constant. The ideal machine has zero resistance and leakage reactance, infinite permeability, and no saturation, as well as zero reluctance torque.

Figure 1.7-1 Production of stator rotating field.

Source: Courtesy of Voith.

The production of torque in the synchronous machine results from the natural tendency of two magnetic fields to align themselves. The magnetic field produced by the stationary armature is denoted as Φ_s. The magnetic field produced by the rotating field is Φ_f. The resultant magnetic field is

The flux, Φ_r, is established in the airgap of the machine. (Bold symbols indicate vector quantities.)

When the torque applied to the shaft equals zero, the magnetic fields of the rotor and the stator become perfectly aligned. The instant torque is introduced to the shaft, either in a generating or in a motoring mode, a small angle is created between the stator and rotor fields. This angle (λ) is called the torque angle of the machine, the angle (ψ) is called the internal power factor angle, and (β) is the space angle between the fundamentals of the Φ_f and Φ_r waves (see Figure 1.7-2) [1].

Figure 1.7-2 Phasor diagram of an unsaturated salient pole generator (lagging pf).

Due to saliency, the reactance measured at the terminals of the generator is a function of rotor positon and thus the two reactance theory can be applied [1]. The armature current I_a can be resolved into two components, namely, I_d and I_q representing direct and quadrature axis currents. I_d is in time quadrature with the internal excitation voltage E_f, where I_q is in time phase with E_f. The direct axis component of the armature I_d produces an armature reaction flux Φ_ad along the axis of the field poles. I_q however produces an armature reaction flux Φ_aq in space quadrature with the field poles. The magnetic effect of Φ_ad is centered on the axis of the field pole where the magnetic effect of Φ_aq is centered on the inter polar axis. The armature reaction flux Φ_s is the space phasor sum of the components Φ_ad and Φ_aq. It follows that the resultant flux Φ_r is a result of the space phasor sum of the main field flux Φ_f and armature reaction flux Φ_s.