HTM standby generation derogations explained

WB Power Services has a long and proud history of supplying standby generation solutions to the NHS and wider healthcare sector. Over many years the company has installed a large number of generating sets, both large and small, in applications ranging from basic containers to complex plant room installations. During this period, the business has acquired a significant experience base within its sales, project installation, and service teams. For those outside the day-to-day working of this sector, the size and scale of standby generation in a hospital setting is an unknown quantity. For those working from within the sector, on the other hand, the installation of a standby generating set probably isn’t an everyday occurrence, so this article seeks — from the standpoint of a specialist standby diesel generator installer, to ‘highlight, explain and clarify’:

How the guidance lines up with the way in which diesel generators are designed and built, and how this can be at variance with the HTM.
How the guidance within HTM 06 can be practically applied.
How some of the most common apparent ambiguities or variances can readily be overcome.

Starting point

The starting point of any design should be with the relevant standards or guidance documents, which in this case — for Secondary Power Sources in healthcare settings — is HTM-06 (2017), and in the case of standby diesel generators, BS ISO 8528 (updated in 2018). As with any specification, BS ISO 8528 sets out the basic minimum standards for the equipment (some manufacturers exceed these), and often doesn’t move quickly enough in terms of recognising more cutting-edge product developments. For example, this can be across engine performance or market requirements, and has the tendency to look out of date even when newly published. It is likely to be a similar problem with some aspects of HTM 06, particularly as it is a guidance document, and by the nature of the review period, will not take into consideration more recent advances in technology and changes in the law; for example recent changes in emissions requirements.

Hospitals are of course true 24/7 operations, forming a critical part of the local, regional, and national infrastructure, and varying greatly in terms of physical size, bed capacity, building age, and scope of services offered in that location etc. In the UK healthcare sector the Health Technical Memoranda (HTMs) are there to ‘give comprehensive advice and guidance on the design, installation, and operation, of specialised building and engineering technology used in the delivery of healthcare’.

The primary reason that standby generators are included within a hospital is to provide a Secondary Power Source (SPS), or simply to provide back-up power in the event of a power utility failure. A utility power failure can be anything from just a few seconds, to hours, or possibly days. Uninterruptible Power Supplies (UPS) are one such SPS, installed mainly to provide power to highly critical equipment for relatively short periods of time, typically in the region of 5 — 30 minutes (up to 3 hours in some critical areas), which is more than sufficient to deal with any short-term ‘brown out’, or the 10 to 15 seconds required to bring the standby generator(s) online.

Diverse nature of equipment

It is likely, in the case of more recently constructed hospitals, newly constructed wings, or recently refurbished areas, that the electrical load connected to a standby generating set is now the entire hospital / wing. This is due in part to the very diverse nature of the equipment used in all operational areas. When viewed in a more general context, the electrical loads can be considered to be:

1 The general building load, i.e. lighting and small power.

2 The mechanical load.

3 Critical area loads (ICU/CCU).

4 Critical equipment in general ward areas.

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:

Greater than 15 s;
Less than 15 s, but greater than 5 s;
Less than 5 s, but greater than 0.5 s;
Less than 0.5 s;
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 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

Category 1 — 100% load acceptance.
Category 2 — 80% load acceptance.
Category 3 — 60% load acceptance.
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 required under such load acceptance or transient load conditions.

BS ISO 8528 does not require a 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 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 advantages that they bring. This is often the case when we look at the healthcare environment.

Governing Standards / Performance Class ISO 8528-1: 2018 — 8

The standard lays out four performance classes: G1-G4, with G4 being the most onerous. In simple terms the performance classes set out the maximum voltage and frequency deviations permitted on the application of a first set load, and the time over which the generating set needs to return to a steady state condition and other operational considerations. It is in this area where it is the author’s view that the standard, while seeking to provide an all-embracing minimum standard, is failing to keep up with product development.

With the harmonisation of standards across Europe, and to a lesser extent globally, the performance of electrical equipment generally has improved, particularly in relation to the efficient use of energy, and reduction of reinjected waveform distorting harmonics prevalent in non-linear devices. These older devices, such as UPS and inverter / soft start drives serving larger mechanical plant, can be the primary loads of any hospital.

Class G2:2 This applies to generating set applications where the voltage characteristics are very similar to those for the commercial public utility electrical power system with which it operates. When load changes occur, there can be temporary but acceptable deviations of voltage and frequency.3

Class G3 speaks more about the ‘connected equipment making more severe demands on the stability and level of the frequency, voltage, and waveform characteristics’,3 and then cites examples such as telecommunications and thyristor-controlled loads. Both rectifier and thyristor-controlled loads can need special consideration with respect to their effect on generator-voltage waveform.3

Class G4 similarly refers to ‘Data-processing equipment or computer systems’ as its examples.

Three categories

By inference, the standards suggest that a hospital operates equipment falling into all three categories: for example, performance classes G3 or G4 as it uses ‘telecoms and data processing equipment or computer systems’. While those statements are in essence true, in a vast majority of cases, the type of products referred to, i.e. thyristor-controlled loads, are a thing of the past. If they aren’t, they must now meet all of the current harmonic reinjection requirements of a modern world.

In many, if not all cases, a modern well-designed generating set equipped with the appropriate method of excitation can deal with the harmonic impacts of a given load. It should also be noted that all the critical equipment is fed via and protected from mains variation by a UPS package — the same UPS package available in the commercial market, and widely used in much less critical applications. The same is also true for the cooling plant — equipment widely used across many different applications exposed to the same mains or generator supplies.

As we highlighted above, HTM 06 section 9.74 directly relates to BS ISO 3046 (Engine only). BS EN 8528 G2 performance is aligned with HTM 06-01 for Category-3 / 60% load acceptance at the PRP rating, but frequency recovery will be within 5 seconds (as per G2 ISO 8528). Section 16.8 indicates that the generator terminal voltage on starting should not overshoot the nominal terminal voltage by more than 15%, and return to within 3% of the rated voltage within 0.15 s. The generator terminal voltage should not vary by more than 15% following a step load increase from 0% load to 60% load, and then return to within 3% of the rated voltage within 0.5 s. Only a grossly oversized machine will be able to meet this requirement.

It is key during the design phase that an assessment be made of the actual site-specific first step load acceptance requirements. The loadings of various categories of risk areas, and any BMS or EMS load management capabilities that might be available etc, need to be considered. These matters should fall within the realms of the project specification, and not be reliant on a ‘general standard specification’.

Understanding Generator Power Rating Categories — BS ISO 8528 — 1; 2018 Section 14.3

For commercial reasons, generating set manufacturers use the same engine (with model variations) across a range of power nodes. In addition, a single model of generating set can carry a number of different capacity ratings, i.e. 1000 kVA ESP, 1000 kVA PRP, and 700 kVA COP; the set rating is dependent on the operational duty / types of loading applied to the generating set, rather than the maximum engine capacity.

Section 14.3 of the ISO standard covers the five different ways in which a generating set can be ‘rated’. These are COP, PRP, ESP, LTP, and DCP. Some of these ratings, for example PRP (Prime) rated sets, have their power ratings set against their ability to deliver into a varying load.

Section 9.73 of HTM-061 states that ‘Engines should be specified prime-rated. They should be capable of operating at the rated load for a period of 12 consecutive hours inclusive of an overload of 10% for a period not exceeding 1 hour, the prescribed maintenance having been carried out. This is known as a Class A rating.’

In the case of a Prime rated set the maximum permitted output of the set should not exceed an average load of 70%1 of the Prime Rating over a 24-hour period (Load Factor). Within that 24-hour period the generating set is able to deliver a 10% overload for one hour in 12. They deliver an average load level over a 24-hour period while still being able to provide a 10% overload, for one hour in 12.

Section 14.3.3 of the standard describes Prime Power rating as2 ‘…being the maximum power which a generating set is capable of delivering continuously while supplying a variable electrical load when operated for an unlimited number of hours per year under the agreed operating conditions with the maintenance intervals and procedures being carried out as prescribed by the manufacturer’.

It is important to note that the way in which these ratings are allocated will vary from manufacturer to manufacturer, based on engine performance, connected rating of alternator performance, and maximum operating ambient temperature.

A daunting task

With this extensive range of generating set capacities and rating options, it can be daunting for the design team to fix the direction of this aspect of the design. For most European markets, the grid is a very stable source of power, and it is very unlikely that power will be off for more than a few hours per year (testing included in this number). This is why a PRP rated set should be an acceptable option. When designing a generating set package for any application, it is important to consider the likely maximum operating ambient temperatures required, as this does play a big part in how the generating sets are designed, and rating capacity determined. In a hospital / healthcare setting, the generating set solution must always be rated to supply the peak site load, on the hottest day of the year, unless there is clear alternative direction.

As we identified in an earlier section, the electrical load of a hospital is very diverse in nature, being dependant on many factors. Some of the critical areas often have an additional layer of protection such as EPS, UPS, or power via a battery-backed system of varying duration, all aimed at providing an uninterrupted supply to the critical area served.

An EPS, UPS, or DC system only looks for stability of input supply, be it mains or generating set. When returning from battery to either mains or generator, the UPS’s input power requirements slowly ramp up over a period of 5-10 seconds, not in one single ‘lump’ of load. This is an important factor when assessing first step load requirements, as none of these load types would be part of the first step load.

Central cooling systems have a UPS equivalent — a buffer or reservoir vessel of cooling water which can maintain water temperatures within range for around 30 seconds. This buffer vessel is there to provide time for the generators to start and pick up the cooling / mechanical loads essential for operation of a hospital. Some of the compressors and pumps within this system can be significant in size, and can present some quite large load steps to the system. Bringing these back online can be delayed and staggered; this should be catered for in the design process.

Sophisticated BEMS equipment

Most, if not all, modern hospitals have sophisticated building and energy management systems (BMS /EMS). The key points here are that these loads:

Can and will be managed on by the BMS/EMS in a controlled way.
When these loads are applied the generating set will already be ‘under load’, meaning that the application of the cooling load ‘isn’t a first step load’.

This means that the generator(s) never get close to seeing anything approaching a 60% load step, negating the need for this measure and the associated G3 or G4 requirements that accompany them.

Regular and thorough maintenance of all plant is key to a reliable M&E infrastructure. In the case of a generating set(s), this should include all control functions and testing the engine at load. See the WB white paper, Greening Standby Power Generation.

Acknowledgements, clarification, references, and bibliography

1 Thanks to the writers of HTM 06 for all direct quotes and references.

2 Information directly from BE EN ISO 8528.

3 Some Kohler generating sets offer 75% load factor.

4 Figure 4 — Information provided by KUP, a Kohler Power Systems Company.