ELECTRICAL SYSTEMS Lagging power factor response 1.04
1.02 1.00
0.98 0.96
0.94 0.92
0.90 0.88 0.86
Frequency Voltage
1.25 1.20 1.15 1.10 1.05 1.00 0 1 2 3 Time (s)
Figure 1. The behaviour of voltage and frequency when lagging power-factor loads are introduced.
4 5 6 7 0.95 0 1 2 3 Time (s)
Figure 2: The system voltage and frequency response when leading power-factor loads are applied to the set.
regulator is already fighting against alternator over- excitation introducing a delay in engaging the designed load acceptance response, which is aimed at enhancing the peak dynamic response of the generating set. Figure 1 illustrates the behaviour of voltage and frequency when lagging power-factor loads are introduced, while Figure 2 shows the system voltage and frequency response when leading power-factor loads are applied to the set. These illustrations clearly demonstrate that the designer needs to consider how the system responds to application of leading power-factor loads, and how – in general – we need to re-evaluate our traditional thinking around load application (e.g., dip, rise, overshoot, undershoot, recovery time). It should be noted that any leading power-factor load condition should consider the reactive capability curve of the alternator to avoid regions of instability and/or potential alternator damage.
A Rehlko KD generator engine in situ inside a hospital plant room.
Application of mixed loads Leading power-factor loads bring with them their own unique set of problems, such as the way in which the generating set responds under transient/dynamic conditions (load application or rejection). The impact isn’t, however, solely confined to transient conditions, but can also impact operation under what we would ordinarily think of as being ‘steady state’ conditions. If the capacitive elements of the load are not equally balanced across all three phases, it will have the effect of worsening any
normal operational voltage imbalance the generator would present; hence creating an extension of the transient case, for example: n With an inductive load connected, the voltage regulator needs to increase excitation to return the output voltage to its nominal level.
n With a capacitive load connected, the voltage regulator needs to reduce excitation to return the output voltage to the nominal.
The role of the voltage regulator In a ‘typical’ mixed load application (e.g. inductive, resistive, and capacitive), the voltage regulator will do what it can to regulate all three phases to as close to nominal voltage as possible. Modern voltage regulators such as the Kohler APM802 use a full three-phase sensing system to achieve the optimum output voltage for the applied load. The average of the three RMS voltages will be regulated to ‘nominal’. With this solution in place, the capacitive element of the load will still be operating at a higher than optimal voltage, and the inductive element at lower than optimal voltage, with the balance of the load somewhere in between, but closer to optimal than the other two. The electrical consultant may choose to recommend an adjustment to the selected output voltage of the generating set, but this must be done with care, being sensitive to the requirements of all the connected loads, and especially those of a more sensitive nature. Adjustments can be made by expanding traditional limits on unbalanced voltages, balancing voltages with power factor as a consideration, and/or adjusting limits to be single-phase focused, with an emphasis on loads that are voltage-sensitive. The increased complexity of many modern facilities
has been mirrored by an increased complexity of the downstream electrical infrastructure, which frequently includes multiple transformers of differing ratings; many more than the generating industry has previously seen. The power rating of the downstream transformers can be significant, often exceeding the rating of the generating set supporting them, even though the actual connected load is well within the capability of the generating set itself. In such cases, care needs to be taken in the selection of the transformer so that the generating set can manage the magnetisation current (in rush current(s)). There is no simple answer to this question, and as such it will require some in-depth analysis, of which there are three major parts. The generator package designer and/or manufacturers are more than able to provide the
56 Health Estate Journal April 2025 4 5 6 7
Frequency Voltage
Leading power factor response
Voltage (PU)
Voltage (PU)
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