In this paper, a simple and, effective solution to eliminate output voltage distortion in the shaft generator system is proposed. The application of a suitably designed duplex reactor realizes the system having no output voltage distortion. The system configuration is clarified first and a set of steady-state equations is then derived. The steady-state characteristics of the proposed system are investigated, and a way of suppressing the voltage increases in the system for a larger system output is proposed. The effectiveness of the proposed system is verified through the experimental investigations.
Fig. 2 Measured Output Terminal Voltage Waveform of a Conventional System.
Fig. 3 Shaft Generator System with Duplex Reactor.
Fig. 4 Equivalent Circuit for Duplex Reactor.
2. SYSTEM CONFIGURATION WITHOUT OUTPUT VOLTAGE DISTORTION
Fig. 2 shows an example of output terminal voltage waveforms of a tested system, which is of conventional type shown in Fig. 1. The total harmonic distortion (THD) of the output voltage waveform was fairly large (THD=15% for this case) since there were voltage jumping and sinking due to the commutation of thyristors in the inverter due to the presence of subtransient inductance Ls" of the synchronous compensator . Accordingly, we can say that THD may become zero if we can produce a synchronous compensator with Ls"=0. In general, however, the realization such a compensator with no subtransient inductance is impossible.
We, hence, propose simple and effective solutions to eliminate output voltage harmonics as shown in Fig. 3. Two types of configurations are shown, where a duplex reactor located in the output side of the inverter is used for both types.
For both cases, if the coupling of the coils is perfect, we have
where, L1, L2: self inductances, M: mutual inductance.
The equivalent circuit for the system with the duplex reactor is shown in Fig. 4. This circuit is valid for both types of configurations.
If the inductances in the duplex reactor are selected such that
then the total inductance in the right side of the imaginary point 'μ', shown in the figure, becomes zero. In such cases the voltage distortion caused by commutation of inverter thyristors will be eliminated, resulting in no distortion in the output terminal voltage.
3. SYSTEM EQUATIONS
Here, the equations needed to discuss the characteristics of the proposed system are developed.