his of course makes additional demands on a d.c. motor, (which is in effect idling and acting to all intents and the provision of sophisticated monitoring and synchronising equip- purposes as a flywheel), coupled in turn to an alternator whose ment, but state of art technology is making such plant design more output supplies the equipment load. On mains failure the d.c. ma- economic and reliable. chine is fully energised from a motor battery, and maintains the The increased use of directly operated a.c. powered switching or drive to the set. The problems with this arrangement are that is an computer equipment has created a situation in which even the on economical induction motor is used as the primary drive, the slip load testing of standby plant, (which introduces change-over breaks factor means that it runs a little below synchronous speed, and hence of just a few milli-seconds in switching from the mains to the engine the alternator output frequency is correspondingly lower than the generator supply, and vice versa), is operationally unacceptable. In nominal value. Figure 24.3 shows the basic principles of the system. these circumstances the ability to synchronise and work in parallel with the mains without causing a transient disturbance is attractive. However, it must be borne in mind that this form of operation, even 24.2.10.2 Two machine continuity set for short periods, is often unacceptable to some supply authorities whose concurrence to such operations is essential, especially at MV. A two machine set (Spurgin, 1970) comprising a d.c. motor-alterna- Even in the case of HV generators, the ability to operate in parallel tor set gets away from the sub-synchronous speed feature, drawing with the supply feeder has to be conducted with plant which is fully its energy from a rectifier floating an appropriate secondary battery. equipped with agreed synchronisation and protection devices. The basic plant configuration is shown in Figure 24.4. There are Nevertheless, such systems can, in appropriate cases, be vi