Can a synchronous generator produce reactive power? When and why?
A synchronous generator is capable of producing active as well as reactive power. When the rotor is over excited, a synchronous Generator delivers reactive power. In general a synchronous generator works at 0.9–0.97 lagging power factor.
Prime Movers, Voltage Regulators, And Power Generation-Part 1
AT A GLANCE: AC power generation is no longer the exclusive domain of utilities. Many contractors and non-utility maintenance people now have responsibility for generator systems. Suppose you see fluctuations in frequency, voltage, kW, or kvar. Do you know how to isolate the problem quickly?
Fourthly, a reactive power limit with respect to a generator over-voltage limit and a generator under-voltage limit cannot be calculated. The generator reactive power limit is affected by the generator over-voltage limit and the under-voltage limit as well as the OEL limit and the UEL limit. Generally, a generator over-voltage and a generator under-voltage are respectively 105% and 95% of a rated voltage. Accordingly, a method of calculating the reactive power limit with respect to the set generator over-voltage limit and the generator under-voltage limit is needed.
To determine the real capabilities of aparticular regulatory synchronous generator its certification is done, the result of which is an official report issued by a certification authority. It also includes a certified chart (operation diagram) which is then used for technical and commercial representation of regulatory capability of the synchronous generator.
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When a generator is directly connected to the grid, the grid voltage VG is fixed and cannot be controlled by the AVR. Any requirement for reactive power from the generator will result in the AVR internal voltage set-point V to change to meet the new demand. So, in the diagram of figure 4, the increased reactive power demand QL causes the AVR set-point to increase from VG to VL because of the droop compensation control.
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This allows changing taps and thus the transformation ratio of the step-up transformer. It is common practice to use on-load tap changers to adjust the reactive power output of a generating unit when it is synchronized to the grid. The output of reactive power is often requested by the transport grid operator, and may be specified e.g. in a grid code, or on day-to-day basis.
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Synchronous machines can be used as generators or motors. They are of 3-phase construction, even though some special exceptions can be found. A bulk of the applications are within power ranges roughly varying from a megawatt level to several tenths (or even hundreds) of megawatts with rated voltages from 3 kV to 15 kV.
Stand Alone Phasor Diagrams
In the case of stand-alone operation, it can easily be shown that if Exciation is not adjusted, the terminal votlage of a generator is a function of the magnitude of the load and the phase angle of the load. Increased inductive reactive power demand reduces the terminal voltage, while increased capacative reacrive power can actually result in an higher terminal votlage, under some circumstances.