Determining The Ups System Size
An uninterruptible power supply is a product designed to protect against mains power failure and to provide a backup power source in the event of a mains power failure.
Most people have the following main questions when deciding on the most suitable UPS solution:
- What size UPS do I need and
- How long a UPS is needed.
- Single-phase UPS sizes
Uninterruptible power supplies are available in three standard topologies. They are called standby/off mode, interactive line and online mode. Each provides a different degree of current protection, with standby/off mode being the lowest and online UPS the highest.
Regardless of the UPS type, the system size is defined in VA or watts. VA is called ‘apparent power’ and is calculated by multiplying the total current (A) required by the load(s) by the voltage (V) to which the load(s) is connected.
Load 1 (amps) + load 2 (amps) + load 3 (amps), etc. = total load in amperes.
The apparent power (VA) of the load is related to the rated power in watts by a term called the power factor (pF). Most UPS systems have a power factor between 0,9 and 0,7, in which case the actual power (watts) required for the load can be calculated by multiplying the VA by the power factor if known, or by summing the load watts. The formula for calculating watts may be:
Load (watts) + 2nd load (watts) + 3rd load (watts), etc. = total load in watts.
UPS Battery Life
Once the total load has been calculated, the second question can be answered, i.e., “how long should the UPS provide backup power”.
The uninterruptible power supply is an energy storage device and the most common method of energy storage and delivery is a battery pack. In some UPS systems, supercapacitors may be installed as an energy storage component for short-term operation (milliseconds), but the more traditional method is a battery.
The most commonly installed battery, whether it is an uninterruptible power supply solution, emergency lighting, a generator starter motor or even a security alarm control panel, is a lead acid battery. Modern lead-acid batteries are valve-regulated lead-acid (VRLA) and maintenance-free. They have a typical design life of 5 years or 10 years and can be expected to last 3-4 years and 7-8 years respectively, with a full charge/discharge cycle of around 300-400 cycles.
The batteries in a UPS system are sized in ampere-hours (Ah) and are arranged in a battery string or battery string pack to provide a given Ah at the DC voltage required by the UPS inverter. This is more engineering intensive and to reduce the need for batteries, UPS manufacturers publish their data to show the lifetime of the UPS system at a given load.
For example, UPS manufacturer “A” may state that it’s 1kVA online UPS provides 7 minutes at full load. Manufacturer ‘B’ may state 10 minutes at 80% load. It is useful to read the small print and see if there are any objections to the specified battery life.
However, it should be remembered that a lead acid battery has a non-linear discharge curve. The lower the load, the longer the battery will provide power. If the UPS is correctly selected with a 20% reserve, the expected battery reserve life will be longer. Some line interactive and most online UPS systems can also be installed with extended run-time kits to extend the backup time from a few minutes to several hours.”
How Much UPS Backup Time Do You Need?
The electricity industry uses the term “blackout” to describe a loss of power. Unless there is a major power outage, most power cuts last minutes rather than hours. It is also common for power outages to occur several times in a short period, i.e., several short outages caused by a failure of the local substation or electricity distribution infrastructure and its automatic recovery, or by an overhead electrical storm and nearby lightning strikes.
In the event of short power outages (or blackouts), most UPS systems have a battery life of 10-30 minutes. This provides sufficient time for the UPS to interrupt the mains power supply, or to initiate a locally installed UPS shutdown software package and gradually shut down the local network server loads.
For larger loads, such as server rooms and data centers, the site may also have a local standby power generator that can automatically switch on and reach full power within 1-2 minutes after switch-on. This 10–30-minute period provides a safety margin in case the generator does not start immediately due to a starter motor battery problem, an open circuit breaker (after a maintenance visit) or air in the fuel lines.
Therefore, the lifetime of any UPS installation depends on the load to be fed and the available energy sources – battery or battery and local generator supply.
In some locations, load shedding is also applied in the event of a mains power failure. Critical loads remain powered, but some essential and all non-essential loads are ‘switched off’. The reduced UPS load increases the operating time available from the stored energy.
Summary
Sizing a UPS system is relatively straightforward, whether for single-phase or three-phase loads and configurations. It is important to identify all critical, essential and non-essential loads and determine those that need to be kept energized in the event of a mains power failure and for how long to allow the organization to continue operations.
Although most installations will be carried out with a lead acid battery pack, standby power generators can also be used to increase the UPS run time and provide several hours or even days of operation.
