Back to: UPS Basics
As covered in Benefits, Applications, and Risks, a UPS is a store of power that is able to sustain the load during times of supply disruption, not to act as a secondary supply. This lesson looks at the different types of UPS available and provides a clear definition for each to ensure specifications are unambiguous.
Static vs Rotary UPSs
There are two types of UPS available on the market, those that use batteries as the energy store (known as a static UPS or just UPS) and those that employ a rotary flywheel as its energy storage component instead of batteries. These are known as Rotary UPS. This course will explain the functions of the battery-based static UPS as they are by far the dominant type of UPS in use. Rotary systems are much more sophisticated and specialised installation that will be covered in a separate course.
Double Conversion vs Line Interactive
There are high-level principles of how a UPS is designed to work/interact with the mains supply, these are known as Double Conversion and Line Interactive. This blog series looks only at the double conversion approach. In our experience, line interactive is a rare method to provide power supply stability and therefore these are not covered.
For further information on interactive UPS systems, the Kohler UPS Handbook is an excellent technical guide.
Type of UPS
For the purposes of this blog series, we have categorised static UPSs into 3 types:
- Plug-in: under-desk,
- Plug-in: rack-mounted, and
- Fixed wired: Industrial.
Plug-in under desk UPS are generally provided to support a single or very small number of pieces of equipment, such as a tower PC that is acting as a server for a specific application. The UPS gets it power by being plugged directly into a socket outlet (general use socket or industrial socket). The UPS unit has multiple output socket on the unit itself to allow the equipment to be directly plugged in to it. The size of these UPS is typically quite small, are is sized to connected load of the equipment plugged in.
Plug-in rack mounted UPS are the same as under desk, but instead are mounted in a 19” rack alongside multiple pieces of equipment. To allow the UPS to serve all of the separate equipment housed within that 19” rack, they are provided with a high power output which a Power Distribution Unit (PDU) is normally plugged in. A PDU can be seen as a glorified extension lead. the UPS is also provided with multiple lower capacity sockets so equipment can also be plugged directly .
The rack UPS serve multiple pieces of equipment and are therefore sized to the collective maximum demand of the rack, not to one piece of equipment. It is not standard practice to use a rack mounted UPS to serve equipment outside of that rack at 230/400VAC. It is normal however for ELV supplies to leave the rack (power over ethernet for instance) and therefore these will be supported by the UPS.
Industrial UPSs are hardwired into the fixed wiring system and therefore serve all downstream loads, not just specific equipment (plug-in under desk) or equipment local to the same 19” rack (plug-in rack mounted). Industrial UPS are sized to the collective maximum demand of all downstream loads and are therefore typically larger than plugin. They also serve power resilience within the fixed wiring system, not from the demarcation socket (note: the socket is the end point of the fixed wiring system).
Compared to plug-in, industrial UPS are centralised and serve more loads making them larger. It is this inclusion on the fixed wiring system, rather than being outside/downstream of the fixed wiring system, this generally means there is a local switch panel to distribute the UPS power to the loads via the fixed wiring system as can been seen in Exhibit 3-3.
The cost of installation of plug-in UPS is low compared to the same kVA of an industrial UPS. Maintenance UPS increases in proportion both to the number of UPSs and UPS size but the number of UPSs is a much more dominant factor when establishing the cost of maintenance.
The graph shown in Exhibit 3-4 is a simplified approximation of the whole lifecycle cost of a UPS in terms of £/kVA and shows that the centralised system becomes more cost effective than distributed plug-in systems at a certain kVA. The exact point at which it becomes more cost effective to use a centralised industrial UPS is dependant on the specific project, but occurs typically between 75-125kVA assuming the loads are local to the UPS.
This course will mainly focus on fixed-wired industrial UPS but there will be a dedicated lesson will cover plug-in UPS too.
As with most things in electrical design, providing an objective definition between an industrial and non-industrial UPS is difficult as there is a lot of overlap. There are there are rack-mounted UPSs that can be classed as industrial and industrial UPSs that are rack-mounted, but for clarity, we use the following definitions:
- Plug-in, under-desk:
- Single-input with power from a socket outlet,
- Free-standing
- local to the single or small quantity of free-standing equipment served,
- Single phase,
- 0.1kVA – 5kVA output with 5 mins autonomy
- Does not form part of the fixed wiring installation
- Protection against back-feed voltages
- Plug-in rack-mounted:
- Single- or dual-input with power from an industrial socket outlet,
- Rack-mounted serving equipment within that local rack only,
- Single phase, and
- 2kVA – 10kVA output with 10 mins autonomy
- Does not form part of the fixed wiring installation
- Protection against back-feed voltages
- Industrial:
- Single- or dual-input
- Supply feeders from a dedicated input/output panel with maintenance bypass,
- Free-standing industrial enclosure,
- Serving a fixed wiring installation,
- Three-phase,
- 10kVA – 10MVA output with 10-60 mins autonomy
- No protection against back-feed voltages
An important rule as defined in the UPS product standard is a UPS designed to be plug-in, it must have back-feed voltage protection.
It is bad practice to connect a plug-in UPS to a fixed wiring installation downstream. It can therefore be inferred that if the UPS forms a part of the fixed wiring installation it can be assumed to be:
- Larger in terms of kVA
- Have no back-feed voltage protection and
- An industrial UPS
Summary
The source of energy that allows a UPS to ride through a short power outage or transition can be a battery or a rotary mass. Battery sourced UPS are known as static UPS and are the vast majority of installations. Rotary UPS are an important UPS class, but are not covered by this course.
UPS can be line interaction or double conversion. By far, double conversion is the technology used and therefore only double conversion is covered by this course. PrioirtyOne.energy do not provided a course on line interactive UPS as in our experience line interactive applications are too rare and niche.
It is difficult to clearly categorise all UPS into discrete categories and many people have different opinions on this topic. This course divides UPS system into two categories: industrial and plug-in, with the focus of this course being on industrial UPS hardware.
Industrial UPS are typically larger that plug-in in terms of kVA and form part of the fixed wiring installation. As they form part of the fixed wiring installation, under product standard, they do not need back feed voltage protection. A dedicated lesson is provided on plug-in UPS, however their application is less complicated than industrial.
The simplification in design/installation that is obtained in using plug-in UPS decreases the more plug-in UPS are needed. The tipping point where a single centralised industrial UPS becomes more cost effective than distributed plug-on units is specific to each site and client objective, however PrioirtyOne.energy have delivered multiple projects for clients to remove distributed plug-in UPS and replace with centralised industrial systems. Industrial UPS systems provide higher security of supply and lower maintenance costs. These factors drive the acceptance of the business cases despite the increase in capital costs.
