When machines are parallel, the magnitude of the field excitation cannot directly influence the output voltage.
It does however adjust the internal power factor at which a particular machine operates. For instance, an over-excited alternator will draw lagging current, whereas an under-excited alternator will draw leading current.
The total load, comprising a KW or active component and a KVAR or reactive component, must be shared by the systems with respect to their normal ratings (the KVAR component is a function of the alternator excitation).
The KW component is adjusted by purely mechanical means and requires relatively fine speed control. It is advisable to fit a limited range governor to avoid “misuse” of the speed control, actuated from the distribution panels.
If a difference in excitation exists, then circulating currents will flow, limited only by the internal machine reactance. This current will appear as a zero PF leading or lagging current, dependent on the machine excitation, and will either subtract or add to the total current that each machine supplies. Reactive current, either leading or lagging, is by virtue of the 90° phase displacement quite commonly described as being in quadrature. Means must therefore be provided to sense this reactive current and limit it to an acceptable level, hence a quadrature droop kit. This comprises a CT (current transformer) and a burden resistor .
Examination of vector diagrams shows that at 1.0 PF the small voltage produced across the burden resistor adds at right angles to the sensing voltage. This produces little change in the sensing voltage and therefore no change in the terminal voltage.
The effect is more marked at 0.8 PF but only marginally so. At zero PF however, the additional voltage is in phase with sensing voltage, producing a much larger change. The artificially high voltage now seen by the sensing circuit causes the machine excitation to be reduced.
Sufficient voltage is produced to ensure that when full load current at zero PF is flowing, the terminal voltage will droop by 5%. This should be sufficient to limit the circulating current to a satisfactory level, which in any case should not exceed 5% of the normal maximum current.
In short, the machine supplying more than its share of the reactive current has its excitation reduced. As an under excited alternator draws less reactive current, the excitation balance and hence power factor balance is restored.
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