Integrated compact units
Inverter as a means for start-up of gas turbo-sets
The demand of public utility companies for dependable and readily available sources of energy for peak-load coverage is reflected by the use of gas turbine groups and pump-storage power stations. Moreover, increasing numbers of gas turbine power stations use gasand steam combination processes as a means to boost efficiency. As gas turbines cannot start without external means, a start-up device is necessary driving the main generator via a static frequency converter while the main generator is working as a motor. The approach of starting via static frequency converters is increasingly gaining ground because it offers a number of important advantages: Highdegree of availability Very good black start capability because the inverter power can be continuously adapted to the black start diesel generator, and because the power can be slowly increased from zero with practically any rate of rise, whereas other systems (e. g. electric motors) mean an immediate application of power. Almost maintenance-free operation (no parts subject to wear and tear)Space-saving design Free selection of the place of installation. This approach is particularly favourable and economical if several gas-turbo groups exist which then require only a single start-up device. Despite the high degree of availability, however, a minimum number of inverters should always be available for reasons of redundancy and start-up times (an inverter can start up only one gas-turbo Setat the same time). One inverter for two gas turbines with switch-over facilities to another inverter has turned out to be the optimal solution (see Fig. 1). For many years, static frequency converters developed and supplied by AEG have met these requirements in many gas turbine power stations with a high degree of reliability. The rapid progress in the field of components (CMOS techniques,microprocessors, etc.) has led to a considerable reduction in volume and costs of the necessary equipment and permitted the elimination of the rotor position transducer which was formerly attached to the generatordevelopments which have further increased the availability of these static frequency converters and thus of the gas-turbine units. Rotor position transducers in a strongly simplified form (viathree proximity initiators) are today only necessary if the following operation requirements must be fulfilled: High torque with the turbo-set idling (e. g. for plant without oil-lift bearings) or if turning Operation is required for the static frequency converter
1 Start-up of 4 gas turbines via 2 static frequency converters with change-over facility 2 Gas turbine manufacture Cover photo:Jurong gas turbine power plant, Singapore, generator units 1 and 2, erected by Siemens/KWU using AEG compact units. Generator power: base load 105.3 MW, peak load 116.0 MW
The speed controlled Synchronous machine
The principles of Operation of a static frequency converter can be explained via the working principles of a converter-fed synchronous machine. Fig. 3 shows the power part.The inverter normally consists of two three-phase bridge circuits whose AC voltage inputs and outputs are connected to the mains and to the synchronous machine to be fed. Their DC connections are interconnected via a DC link circuit. In this illustration, the thyristors are identified in the order of their firing by numbers on the mains side and by letters on the machine side. After firing of thethyristors 1 and 2 in the mains converter and the thyristors a and b in the machine converter, a current flows from the mains phase U via 1, the link circuit reactor, via a, through the motor phases U1 and W1 via b and 2 back into the mains phase W. A magnetic field Ø1 is thus generated in the motor. lf an excitation current flows at the same time, a magnetic field ØF generated in the rotor,...