Power You Can Trust


In this white paper, WB Business Consultant Geoff Halliday considers the issue of transferring complex loads onto a standby diesel generator. Historically, during the early decades of power generation, standby-generating sets were mostly used for applications such as emergency lighting, providing power to key military installations and other similar applications. As the capabilities of the diesel-powered standby generating sets increased, so also did the type of loads supported by standby power. In the 1970s and 1980s, the industry saw dramatic growth in motor and pump-type applications such as water treatment works, process control and lifts, etc. These types of loads placed additional requirements on the coordination of power transfer from utility and back to utility and visa versa. Simple double-throw contactor arrangements were no longer sufficiently reliable to transfer loads with high levels of electromechanical inertia. It became necessary to undertake a full assessment to understand what happened when transferring from one source to another, particularly when transferring from a standby source to a utility source.



Once again standby generating sets are seeing an increase in the scope and variation of loads currently being connected to standby power plant. The massive growth in use of switched electronic conversion techniques in things like battery charging systems, switch mode power supplies and other such systems used in PCs and IT servers have led to a massive increase in the level of capacitive loads being connected. The growing use of downstream transformers has become more prevalent as facilities are becoming more complex in their design and as the data service industry (Data Centres) continues to grow exponentially. The problem associated with both leading power factors and downstream transformer loads are well understood, such that the specifying engineer can analyse the application and correctly coordinate equipment.


The “traditional” electrical network of a commercial building or facility would largely comprise resistive (lighting, heating) or inductive loads (pumps, motors). In the last decade or so this has changed significantly with the introduction of switch electronic devices for example LED lighting, motor soft starters, variable speed drives, desk top PCs, file servers and UPS etc. These switched electronic devices predominantly operate at close to unity power factor but on “switch on” can present a largely capacitive/leading power factor load which when operating on a standby generating set brings some unique problems.

When applying a classical resistive/inductive load to a standby generating set it would respond with a dip in both output voltage and frequency (dip in engine speed). Where the rate of frequency change is beyond preset limits the modern electronic voltage regulator helps compensate by reducing the output voltage effectively reducing the active power being delivered by the set (sometimes called a load acceptance or V/Hz feature).

When a leading power factor/capacitive load step is applied to the generator it will respond by increasing the output voltage whilst at the same time the engine speed will reduce (reduction in frequency). The alternator on the generator set is changing from an under-excited state to an overexcited state whilst the engine is seeing an increase in active power and is responding by reducing speed (reduction in output frequency). In this scenario, the voltage regulator is already fighting against alternator over-excitation introducing a delay in engaging the designed load acceptance response which is aimed at enhancing the pearl dynamic response of the generating set.

Figure 1  illustrates the behaviour of voltage and frequency when lagging power-factor loads are introduced. Figure 2 (below) illustrates 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.



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 though isn’t just confined to transient conditions it 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 :-

  • With an inductive load connected the voltage regulator needs to increase excitation to return the output voltage to its nominal level.
  • With a capacitive load connected the voltage regulator needs to reduce excitation to return the output voltage to the nominal.

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 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, 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 however this must be done with care being sensitive to the requirements of all the connected loads particularly 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 are well within the capability of the generating set itself. In cases such as this, 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 information to complete the first part of the analysis: the source characteristics of the generator (e.g., sub-transient reactance etc.)

The project designer will then be able to complete the picture providing details for both the second and third part of the analysis which include :-

  • Transformer characteristics (e.g., zero sequence impedance, core hysteresis), and is unknown by the generator manufacturer.
  • The installation (e.g., load scenarios and grounding scheme, paralleled generators). With a complete picture of the subcomponents of the system.

The electrical engineer is then equipped to complete the analysis and understand the power dynamics of his or her system as the transformer(s) are energised.


If there are several transformers in the network the resultant inrush of simultaneous energisation could be difficult. Sequential starting of the transformers can mitigate or alleviate some of the stress imposed on the installation.

Once the first transformer is “online” and the generating set is operating with some load, the set will offer more stability when dealing with subsequent load applications. The time delay between each energisation can be as short as one second. Figure 3 and Figure 4 illustrate an example of three transformers with a cumulative rating of 1.9 per unit of generator kVA. Figure 3 shows the generator line current during simultaneous energisation; Figure 4 shows the generator line current with a delay of one second between energisations.



If any transformers can be energised as the generator starts, the effects of transient inrush current on the system will be mitigated. Modern voltage regulators are fitted with a load acceptance feature to ramp voltage as a function of frequency. This slope of voltage vs. frequency is normally utilised to assist with engine speed recovery on load application; however, it can be used in a simultaneous starting scenario as it enables a predictable ramp-up of voltage with engine speed. With this voltage ramp, energising the transformer(s) does not cause excessive inrush current.



If multiple generating sets are starting, synchronising and then being connected to the load then “deadbus synchronising” can be used. In this scenario, the output circuit breakers of the multiple sets are closed prior to a start signal being sent. As the sets pass through a critical speed the alternator excitation is switched on. This enables the sets to seamlessly synchronise but it also enables the downstream transformers to energise on a rising voltage, obviating any issues with inrush current.

Care is needed with this system as the total system characteristics will change based on the number of sets; the harmonic study should consider all possible sourcing scenarios.  This process can also be used in a single set situation.


The diesel standby generating sets of today are supplying a wider variety of loads than ever before. Some capacitive loads are defying many of the general assumptions of what a load application looks like and as a result, we may need to modify or rewrite our current specifications. Some of the larger power networks within an installation now involve the use of several different voltage levels both AC and DC and this requires careful consideration of how to manage not only the loads but also the equipment between the source and loads. With careful planning and consideration of available strategies, capacitive loads and transformer inrush can be successfully managed to keep installations in power. Here at WB Power Services, we have a highly skilled group of experienced engineers who are available to help the consultant or buyer to work their way through all aspects of how best to size and integrate a standby generating set(s) into building infrastructure.



The author would like to fully acknowledge the major contribution made by Adam Larson a Senior Staff Engineer at Kohler Co for his significant contribution to the writing of this piece.  Adam holds a Bachelor of Science degree (BSEE) from Milwaukee School of Engineering and a Master of Science degree (MSECE) from Purdue University. Adam joined Kohler in 2010 and has contributed to electric machine design, power system analysis, and power electronics development during his career with Kohler. Adam is a member of IEEE and has authored papers on electric machine design and optimisation.


G R Halliday

Business Consultant

WB Power Services Ltd