ELECTRICAL SYSTEMS Generator line current, simultaneous energisation 3.0 2.0 1.0 0 -1.0 -2.0 -3.0 0 0.5 1.0 1.5 Time (s) Figure 3: Generator line current during simultaneous energisation.


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: n Transformer characteristics (e.g., zero sequence impedance, core hysteresis), unknown by the generator manufacturer.


n The installation (e.g. load scenarios and grounding scheme, paralleled generators), with a complete picture of the sub-components 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.


Mitigation strategies for specifying engineers – sequential starting of transformers or loads 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 ‘on line’, 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. Figures 3 and Figure 4 show 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, while Figure 4 shows the generator line current with a delay of one second between energisations.


Simultaneous starting of generators and energisation of transformers 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.


2.0 2.5 3.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 0 0.5 1.0 1.5 Time (s)


Figure 4: Generator line current with a delay of 1 second between energisations.


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 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.


Conclusions 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 work their way through all aspects of how best to size and integrate a standby generating set(s) into building infrastructure.


Acknowledgement n 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. He joined Kohler in 2010, and has contributed to electric machine design, power system analysis, and power electronics development, during his career with the company. He is a member of the Institute of Electrical and Electronics Engineers, and has authored papers on electric machine design and optimisation.


2.0 2.5 3.0 Generator line current, delayed energisation


Geoff Halliday


Geoff Halliday, a Business consultant at WB Power Services, started his career as an apprentice working for Square D (later part of Schneider), before moving into the critical power sector, where he has now worked for over 40 years, splitting that time equally between both the UPS and standby diesel generation sectors. His roles have ranged


from Customer Service engineer, Project manager, Technical director, and Sales director, to MD. The critical power


market exposes the individual to a wide and diverse range of market sectors – from healthcare, life science, and water treatment, to manufacturing, process control, and data centres. Drawing on his management skills, product knowledge, and extensive application experience, Geoff now enjoys sharing his knowledge with others.


April 2025 Health Estate Journal 57


Line current (PU)


Line current (PU)


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68