POWER GENERATION ADDITION
Recovery time
LOAD REMOVAL LOAD
required under such load acceptance or transient load conditions. BS ISO 8528 does not require a
% Overshoot
generating set to be capable of achieving any specific level of first step load acceptance, as this capability is based on the Break Mean Effective Pressure (BMEP) that any particular engine is able to deliver. First step load acceptance is, in many cases, a key design performance requirement (e.g. pump starting). Although improved and refined over
% Dip
Recovery time
Mechanical transient Time
Figure 2: Mechanical Transient Response.
The HTM speaks clearly about the importance of fully risk assessing and having practical emergency contingency plans in place that are always available and ready to implement. It also makes clear that the design approach adopted ‘should be mindful of the need to maintain an electrical supply within specific time periods for the safety of patients and staff (Chapter 7)’. Depending on the area and its use, these times are set out and defined such that supply should restored within timeframes of: n Greater than 15 s; n Less than 15 s, but greater than 5 s; n Less than 5 s, but greater than 0.5 s; n Less than 0.5 s; n No-break. (Reference IEC 60364-555)
The above times need to be aligned with the distribution strategy (discussed in HTM-06 Chapter 7) and final circuit configurations (discussed in HTM-06 Chapter 15). Often one of the biggest challenges
for the designer is to arrive at a rating for the generator / SPS, as there are many factors to consider, not least of which is an allowance for future requirements. HTM 06 section 9.18 says “that electrical outages can be very short (less than a few minutes) or for many hours… all generator sets should be designed and rated to provide continuous full load for prolonged periods”. It adds: “Provision may require a manual or automatic control system with the ability to ‘load shed’ a limited number of the secondary services such as non- essential lighting.” A difficult task indeed, and once installed the loading on each generating set should be checked annually
42 Health Estate Journal August 2024
to ensure that the load remains within the design criteria. The balance of this article looks to support and inform the design process.
Generator First Step Load Acceptance Section 9.17 of HTM 06 states: “The design strategy and plant sizing should take account of the load to be supplied within 15 s of cold start.” One of the most common areas of
variance encountered when considering the sizing / rate of the standby generator required emanates from the way in which section 9.74 of HTM-06 is drafted and interpreted. When considering the installation of a new generating set there are of course many things to consider. Some of the critical ones, however, are the rating of the set (kVA/kW), potential future increases in load requirements, the types of load to be protected, and the transient conditions which sit around that. The HTM speaks about diesel or gas engines being manufactured generally in accordance with BS ISO 3046, which of itself is not usually a problem. It then, rightly, moves on to discuss ‘four categories of load acceptance’, set out in the standard for various types of engine operation on the basis of percentage load acceptance for the Class A rating being:1 n Category 1 – 100% load acceptance. n Category 2 – 80% load acceptance. n Category 3 – 60% load acceptance. n Category 4 – 25% load acceptance.
While these levels are true specifically in relation to engine performance, they do not directly relate to the way in which load acceptance is defined in BS ISO 8528; nor do they clearly define the recovery time
many years, many of the engines (and particularly the larger power ranges) used on generating sets in the general commercial market have been in production for many years. Those that are of a newer design typically offer many advantages – such as lower fuel consumption and, as a result, lower emissions. Those improvements, however, can come at a cost, and this can be the first step load acceptance capability of the generating set. It is important then to have a clear understanding of what is actually needed, and what can be optimally achieved in these areas.
How engine and alternator work together under transient conditions We identified earlier that the BS ISO standard does not require the generating set to achieve any specific level of first step load acceptance, as this is determined by the BMEP that the engine is able to deliver; hence there is a wide variation between manufacturers and the engines used at various power nodes. A 60% first step load acceptance for
‘any rating’ of generating set has for a long time been considered an ‘industry norm’, and is often written into many ‘standard’ consultant specifications. However, specific consideration is not given to the actual operating requirements of the infrastructure, or being qualified with the essential giving performance classification (G1-G4). The design of many engines, and particularly those used on the larger generating sets (+1000kVA) widely used in healthcare, water treatment, or data centres, has been in existence for a very long time. While the engines have been enhanced and improved over time (with more electronic engine management, improvements in the combustion process, high pressure fuel injection, and improved materials etc), there are few truly new engines. Those that are new typically offer lower fuel consumption – hence lower emissions, and have a more compact footprint, i.e. with higher power density. Those improvements can come at a
cost, and this can be the first step load acceptance capability of the generating set. Whilst these advancements are accepted within the standard, the ‘industry norms’ often do not keep pace with, nor understand, the changes, or embrace the
Engine RPM (Frequency)
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