Answer MET Question 31
Question: Sketch and describe a main engine shaft driven generator arrangement with an electronic system
for frequency correction.
At
the three-phase rectifier stage the a.c. generator frequency is
converted to a d.c. voltage. The three-phase controlled inverter
converts the d.c. back to a fixed output frequency by sequenced
thyristor switching. A d.c. link inductor coil is interposed between the
rectifier and inverter to smooth the normal current flow and act as a
current-limiter in the event of a short circuit fault.
An inverter thyristor switch is turned on by a positive current pulse to its gate when its anode is positive with respect to its cathode. The thyristor is only turned-off when its curreht is reduced to (approximately) zero. This is a problem for the inverter thyristors when driving
into the ship's inductive load (typically about 0.8 power factor lagging). In this case the current continues to flow in a thyristor after its voltage has gone through a zero point causing disruption of the inverter switching sequence.
To overcome this problem it is necessary to have the thyristor current in-phase-with its voltage so that turnoff is automatically achieved (line commutation) at the end of each a.c. half-cycle. The addition of leading kVAr compensation to the power system to create an overall unity power factor solves the problem. Hence, the SG/ converter must only supply true power P (kW). At every instant the leading kVAr ( + Q) must exactly match thelagging kVAr ( - Q) of the ship's load so the compensation must be automatically controlled. The practical solution is to include a synchronous motor, operating as a synchronous compensator, whose operating power factor is controlled by regulating its d.c. field current. Overall, the bus-bar voltage is fixed by the field flux in the shaft generator and the bus-bar frequency is regulated by the controlled inverter.
Answer:
A propulsion-shaft driven (SG) generaior- can be an efficient method for extracting electric power from the ship's main engine as the Power is derived from lower cost fuel than that used for an auxiliary DG unit. The SG may be fitted directly inline with the slow speed propulsion shaft or, more commonly, be gear-driven up to a higher speed.
Also, by using a shaft generator as the main source of electric power during long sea passages, the DG units operate for short periods only with a reduced maintenance requirement.
An apparent disadvantage of a shaft generator is that it has no direct frequency control as this is determined by the main engine which is set for the ship's full-away speed range (e.g. 70-100%). This means that the frequency must be separately regulated at the output of the shaft generator to maintain a constant 60 Hz to the ship's electric power consumers. Such a frequency regulator
utilises an electric a.c.-d.c.-a.c. converter as shown in Figure.
A propulsion-shaft driven (SG) generaior- can be an efficient method for extracting electric power from the ship's main engine as the Power is derived from lower cost fuel than that used for an auxiliary DG unit. The SG may be fitted directly inline with the slow speed propulsion shaft or, more commonly, be gear-driven up to a higher speed.
Also, by using a shaft generator as the main source of electric power during long sea passages, the DG units operate for short periods only with a reduced maintenance requirement.
An apparent disadvantage of a shaft generator is that it has no direct frequency control as this is determined by the main engine which is set for the ship's full-away speed range (e.g. 70-100%). This means that the frequency must be separately regulated at the output of the shaft generator to maintain a constant 60 Hz to the ship's electric power consumers. Such a frequency regulator
utilises an electric a.c.-d.c.-a.c. converter as shown in Figure.
An inverter thyristor switch is turned on by a positive current pulse to its gate when its anode is positive with respect to its cathode. The thyristor is only turned-off when its curreht is reduced to (approximately) zero. This is a problem for the inverter thyristors when driving
into the ship's inductive load (typically about 0.8 power factor lagging). In this case the current continues to flow in a thyristor after its voltage has gone through a zero point causing disruption of the inverter switching sequence.
To overcome this problem it is necessary to have the thyristor current in-phase-with its voltage so that turnoff is automatically achieved (line commutation) at the end of each a.c. half-cycle. The addition of leading kVAr compensation to the power system to create an overall unity power factor solves the problem. Hence, the SG/ converter must only supply true power P (kW). At every instant the leading kVAr ( + Q) must exactly match thelagging kVAr ( - Q) of the ship's load so the compensation must be automatically controlled. The practical solution is to include a synchronous motor, operating as a synchronous compensator, whose operating power factor is controlled by regulating its d.c. field current. Overall, the bus-bar voltage is fixed by the field flux in the shaft generator and the bus-bar frequency is regulated by the controlled inverter.
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