**1. Introduction**

380 Mechanical Engineering

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Considerable efforts have been developed to encourage the installation of independent electricity producers in distribution systems. Obvious example of such efforts is demonstrated by the attempt to standardize their interconnection and protection requirements. However, this standardization can be difficult due to variations in the design of distribution circuits, the various types of generators coupled to the network and the particular requirements of each utility. However, a series of questions points to the development of further studies to ensure the quality of electric power, the system transient stabilty and lifetime of the blades of steam turbines (Moura et al.,2011).

Distributed generation shows frequent use of steam turbines as a primary machine to produce electricity. Such units have increased considerably due to a restructuring of the energy sector worldwide.

Most of the process of converting thermal energy into electrical energy occurs in the steam turbine. This is due to the numerous advantages of such turbines over other similar technologies. Among the main advantages stands the balanced construction, relatively high efficiency, few moving parts, ease of maintenance, and availability in large sizes. The industries that typically employ the technology of cogeneration are the sugar and ethanol, the pulp and paper and oil refining (Anderson & Fouad, 2003).

Recently, special attention has been devoted to turbo-alternators under abnormal conditions of operation of the electrical system concerning the frequency (over or under frequency). In particular, major research efforts have been spent with the main aim of assessing the possible damage they are subjected to the steam turbine when in operation under conditions of prolonged under frequency, during a severe overload condition imposed on the system when occurring a deficit of generation (Kundur, 1994).

Contingencies common to the distribution of electricity systems can cause serious damage to the generators installed in parallel to the network as well as steam turbines responsible for providing mechanical power to their axis. Thus, it is necessary to assess the possible impacts of steam turbines of the IP (independent producer) electrical machinery in system abnormal operating conditions.

Steam Turbines Under Abnormal Frequency Conditions in Distributed Generation Systems 383

Indirectly, by feedback due to changes in the position of the connection point B ( *<sup>B</sup> x* ),

Historically, *load-frequency primary controls* are part of the so-called *characteristics of speed governors*. However, the term "regulator" implies a mechanical speed sensor mechanically connected to control variations in input power, and since today most of the load-frequency controls are electro-hydraulic, speed control term does not describe generically this control

Another simple mechanism of regulation is shown by figure 2. In this case, the deviation in speed, captured by the centrifugal body (*flyballs*), cause the displacement x in the pilot valve, which makes the oil to flow though the main servo motor. This servo motor, in turn,

It is important to observe that a displacement x in the pilot valve causes a rate of change of

opens or closes the valve or turbine blades, depending on the direction of x.

With reference to figure 2, can be written for the position of pilot valve (x):

2 <sup>d</sup> ( y)kx dt

<sup>1</sup> xkf (1)

(2)

the servo piston, or a rate of change in the valve position.

Fig. 2. "Isochronous" type speed governor

Also, for the turbine valve position (y), we have:

f = frequency deviation (f0 – f); k1 = proportionality constant.

where:

resulting from changes in speed.

function (Kundur, 1994).

**2.1 Isochronous governor** 

For this it is essential to develop a computer model able to represent accurately the phenomena experienced.
