A shaft generator (SG) is a synchronous machine directly coupled to a vessel's propulsion shaft. Its speed, and thus the frequency of the generated AC power, varies with the engine's speed. To produce a constant frequency output, regardless of engine speed, the SG utilizes a static converter.
This converter comprises two main sections:
- Rectifier: This section, typically a three-phase diode bridge rectifier, converts the variable-frequency AC output of the shaft generator into direct current (DC). A reactor smooths out the DC current.
- Inverter: This section converts the DC power back into AC power at a constant frequency. This is achieved using a controlled inverter, often employing thyristors switched in sequence. The switching sequence is precisely controlled by a gate signal to create the desired frequency. A crucial aspect here is that the thyristor current needs to be in phase with its voltage to ensure proper turn-off at the end of each AC half-cycle. If the load is inductive (as is typical in ships), a leading reactive power (kVAR) must be supplied to the busbar to achieve this phase alignment. This often involves a synchronous motor acting as a synchronous compensator, whose power factor is adjusted by regulating its DC field current.
The excitation system of the SG is designed to maintain full output voltage even at engine speeds as low as 60% of its maximum. Separate frequency and excitation controllers manage the generator's output as needed. This entire system allows the shaft generator to provide reliable and consistent AC power to the ship's electrical systems, even under varying engine speeds.
Advantages
- Efficiently extracts electrical power from the ship’s main engine, which operates on lower-cost fuel than auxiliary diesel generators (DGs).
- During sea passages, it can supply all of the vessel’s electrical power, allowing auxiliary generators to be shut down, reducing operational costs and wear.
Disadvantages
- High initial installation costs due to the integration of the SG and frequency correction system.
- Complexity in frequency control and power factor management increases system design and maintenance requirements.