Publisher Summary

This chapter discusses generator configuration of generator systems. The simplest configuration of a single generator is supplying a mixed electrical load at its terminals or busbars. The generator rating must meet the total specified load. When a single generator supplies a single dedicated static load, its precise range of possible duties can be specified precisely. It is essential that the steady-state and dynamic performances of both the electrical generator and motor are matched to ensure that optimum performance is obtained at a reasonable cost. The frequency control and excitation control systems can then be selected to give satisfactory performance of the load being driven by the motor. The Island generation systems comprise installations that consist of a multigenerator station, as provided on an offshore platform or in a pipeline pumping station, which may be single purpose or of a mixed application type. In practice, it is usual to permit a temporary reduction in motor voltage under such conditions. By raising the motor voltage slightly above the rated value, the effective motor voltage at the instant of starting can be improved, resulting in a higher value of motor-starting torque. These schemes have the disadvantage that the generator voltage is required to vary over a range and at higher values than the rated value; this causes increased losses and heating and may require increased excitation power.

Single generator system

The simplest configuration is that of a single generator supplying a mixed electrical load at its terminals or busbars, i.e. the effects of line impedance can be neglected (Figure 2.1(a)). The prime mover must provide an acceptable speed–frequency of system to meet the load requirements, whether steady state, normal, abnormal or transient. The generator rating must meet the total specified load, but this may not be the total connected load when a diversity factor is being used. The control logic requirements will be determined by the actual requirements of the load as previously described. When details of the components of the load are not identified, a generator designed to a suitable standard specification should prove satisfactory, but the design should be adapted, if necessary, to meet any specific requirements that may be known.

In such single generator installations, problems that can prove particularly difficult are associated with loads which are phase unbalanced or which involve considerable amounts of harmonic current. However, other aspects, such as fault levels, selection of suitable switchgear and protection systems, do not usually present any problems.

When a single generator supplies a single dedicated static load, its precise range of possible duties can be specified precisely and a suitable set selected or designed to meet all requirements. Thus a machine intended to supply a pulse load will have to be designed primarily for suitable transient, or possibly pretransient, parameters, and particular attention will have to be given to the dynamic electromechanical performance of the generating set, since these are the features mainly involved in such loading requirements. Fast-response control systems will probably be necessary to ensure rapid recovery of steady-state conditions following each pulse, reducing the interval before the next pulse load can be handled.

Two-machine system

A single generator supplying a single dynamic load such as an electric motor driving a specific load presents entirely different problems (Figure 2.1(b)). It is essential that the steady-state and dynamic performances of both the electrical generator and motor are matched to ensure that optimum performance is obtained at a reasonable cost. With such a two-machine system the normal control modes may be quite unsuitable and the complete prime mover–generator–motor package should be designed as a single unit to match the load–speed requirements of the driven load. The frequency control and excitation control systems can then be selected to give satisfactory performance of the load being driven by the motor.

A typical example of this is an electrical ship propulsion system, where the ultimate load is imposed by the ship propeller, which follows one of the well-known propeller characteristics, which have a common general form but where the magnitudes of the various parameters are determined by the interaction of the propeller blades with the water together with the hull contour drag characteristics. The effect of relative movement of the vessel and the sea is also relevant and manoeuvres such as turning, reversing etc. introduce another set of loading characteristics.

It is necessary, therefore, to use the prime mover speed-governing system to ensure satisfactory propeller speed control against the known load–speed characteristics, either directly if a synchronous propeller motor is being used, or in conjunction with some form of speed-adjusting device, either mechanical or electrical. The purpose of the electrical generator–motor link is to provide this control in a convenient and efficient manner.

When a synchronous two-machine system is used, i.e. with a synchronous generator supplying a synchronous motor, this link behaves in the same way as a gearbox and the propeller speed is always proportional to the prime mover speed up to the maximum power which can be transferred through the synchronous electrical link. This scheme requires a variation in generated voltage and it is convenient to control this to maintain a constant value of voltage per hertz of the generated frequency, since this results in both machines operating at constant flux and hence optimum condition of core loss together with constant torque capability. The copper losses in both machines can then be optimized by using the motor excitation control system to maintain the power factor at unity, which corresponds to minimum current required to meet the actual load demand. Such a system will, of course, be designed to meet abnormal conditions of load, within the maximum power limit of the two-machine link. The capabilities of the generator and motor are designed to be of the same order to minimize total cost for a specific value of peak load.

This is a typical case where a separately driven excitation set will be used to provide both machines in preference to two directly coupled exciters in order to reduce overall size and weight of the total installation.

A two-machine system using constant frequency generation requires some other means of controlling vessel speed and this can be done in various ways, such as:

With all these alternatives, the generator can be regarded as supplying a single dynamic load and must be designed on this basis.

There are many possible variations of the simple two-machine system in which tandem or multiples of generators and/or motors, either duplicates or having different characteristics, can be used to extend the effective and efficient range of operating conditions and render these more flexible. Such machines should be designed on the same principles as the single machine.

It is sometimes convenient to use a change-pole propeller motor (which gives effectively a ‘gear change’) in conjunction with a variable-pitch propeller, which enables the latter to be run at a lower fixed speed for low powers and hence operate at a much greater efficiency. In such a case it is necessary to design the generator to be compatible with either motor winding together with the protection and control systems.

Other forms of two-machine systems exist for particular applications, but they can all be dealt with in the manner described for ship propulsion schemes.

The choice of prime mover and generator type, enclosure etc. depends on many factors, as previously discussed, and can only be correctly made when all relevant information is available. In single generator installations the most important considerations tend to be tactical rather than technical, and questions of suitable fuel supplies, servicing and spares may be overriding. In some instances reliability is the most important criterion, whereas on board ship or platforms, factors such as weight, space, noise or vibration can also be very significant. However, the satisfactory dynamic performance of the complete generation system cannot be neglected when choosing suitable plant for a particular installation.

Island generation systems

These comprise installations which consist of a multigenerator station, such as provided on an offshore platform, or in a pipeline pumping station, which may be single purpose or of a mixed application type (Figure...