Consider the case where a lossless synchronous machine is run up to synchronous speed by an external prime mover. The field current is then gradually increased until the terminal voltage of the machine (the same as the internal generated voltage, as no current is taken) is made to be equal to the voltage of the local bus to which the machine is to be connected. Precise adjustments to the speed of the prime mover are made so that through some external instrumentation it is possible to detect an instant at which the internal voltage VA exactly matches VB in magnitude and phase. The synchronous machine can now be safely connected to the bus. This process is known as synchronisation and must be carried out each time a synchronous machine is to be connected to the mains.
Now arrange for the prime mover to apply an accelerating torque. This will result in a positive load angle 8 and according to Equation (4.8a) a negative active power, i.e. active power injected into the power system. As expected, the machine is generating. With a braking torque on the shaft, i.e. with the machine motoring, the load angle is negative and active power is supplied to the machine from the power system.
Returning to the idling state, consider now what happens if the field current is increased so that VA is made larger than VB , but no external torque is applied so that 5 and the active power are zero. Equation (4.8b) shows that the reactive power is negative; i.e. the synchronous machine injects reactive power into the system, and acts as a generator of reactive power. In this state the machine is said to be overexcited.
Conversely, if the excitation current is decreased so that VA < VB, the reactive power flow is reversed, the machine is a consumer of reactive power and it is said to be underexcited.
The synchronous machine is capable of operating in each of the four quadrants of the quadrant diagram shown in Figure A18 in the Appendix.
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