What is Parallel Operation and Load Sharing in Tech Speak?
The content discusses the interaction of parallel machines in power systems, focusing on their field excitation and its effect on output voltage and internal power factor. Key points include:
Parallel Machines: The field excitation influences internal power factor but not the output voltage, with over-excited alternators drawing lagging current and under-excited ones drawing leading current.
Load Sharing: Total load needs to be shared according to machine ratings, involving KW (active) and KVAR (reactive) components. Mechanical speed control is used to manage KW, while differences in excitation can lead to circulating currents.
Reactive Current Management: Circulating currents depend on machine excitation and can either add to or subtract from total current supplied by machines. A quadrature droop kit is needed to sense and limit reactive currents.
Sensing Voltage: Vector diagram analysis reveals that at higher power factors (PF), changes in sensing voltage are minimal, with larger impacts noted at zero PF. It can artificially raise voltage seen by sensing, leading to reduced excitation of the machine supplying excess reactive current, thus restoring balance.
Call to Action: The text concludes with an offer for assistance in installing diesel generators.
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.
Load Sharing
The total load must be shared by the systems with respect to their normal ratings. This comprises of a KW or active component and a KVAR or reactive component. The KVAR component is a f unction of the alternator excitation.
The KW component is adjusted by purely mechanical means. This requires relatively fine speed control. A limited range governor fitted avoids “misuse” of the speed control, activated 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. Therefore 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. Also 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.
Sensing Voltage
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 seen by the sensing circuit causes the machine excitation to reduce.
Sufficient voltage is produced as the full load current at zero PF flows. Thus dropping the terminal 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 power factor balance restores.