A Shaft Generator System without Output Voltage Distortion and its Steady-State Characteristics
Shoji NISHIKATA*, Ken-ichiro YAMASHITA*, Fumiaki MITA*, and Teruo KATAOKA*
A shaft generator system feeds sufficient power to the electric apparatus in a ship by using a part of the power produced by the main engine. Conventional type of such system has a drawback of involving a large amount of harmonics in the output voltage waveform. A novel and simple shaft generator system without output voltage distortion is presented here. The proposed system is equipped with a duplex reactor to cancel out the subtransient inductance of the synchronous compensator. The steady-state characteristics of the system are then discussed, and it is shown that when the duplex reactor is used, the inverter dc input voltage and synchronous condenser terminal voltage should be increased for a larger system output to keep the output terminal voltage constant. A simple method of suppressing the increase in these voltages is also proposed. The effectiveness of the proposed system was confirmed with the experimental investigations.
Key Words: Ships, Generating system, Shaft generator system, Voltage distortion, Harmonics elimination, Duplex reactor, Capacitors
A shaft generator system supplies electrical power required in a ship by using a part of the power produced by the main engine. This type of generating system is widely used for power sources in large ships since it can reduce fuel and operation costs.
In Fig. 1 a typical shaft generator system is shown (e.g., Gundlach , and Nishiyama and Kotera ). As in the figure, the system consists of a shaft generator driven by the main engine, a thyristor rectifier, a current-source thyristor inverter, an ac reactor, and a synchronous compensator to provide reactive power for loads and for commutation of inverter thyristors. Since the speed of the main engine can vary over a wide range when the ship makes a voyage, the output voltage and frequency of the shaft generator can also be changed. Hence, ac power produced by the shaft generator is converted once into dc power with the thyristor rectifier, and then the dc power is converted again into ac power with constant voltage and frequency with the inverter.
Fig. 1 Typical Shaft Generator System.
In this system the output voltage distortion due to commutation of inverter thyristors is inevitable because of the existence of the subtransient inductance Ls" of the synchronous compensator. Although the insertion of an ac reactor with large inductance to the output side of the inverter can decrease the total harmonic distortion (THD) of output voltage waveform, the angle of overlap of inverter currents is increased for a large value of the inductance, reducing the operation range of the system as clarified by Nishikata et al. .Consequently, a satisfactory harmonics elimination in output voltage waveform is not achievable with this type of system in all cases, and eliminating harmonics included in the output voltage without reduction of maximum output power becomes an important issue.
*Tokyo Denki University
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