UPS System Configurations and Their Effects on Reliability

UPS System Configurations and Their Effects on Reliability

By G Bennett, Standby Systems

So you have a critical load and you need to ensure no downtime. Of course you need an uninterruptible power supply (UPS) – but what configuration should you choose? In fact, what are the options available to you?

The reliability of a UPS installation is largely dependent on its configuration. To achieve 100% reliability of a UPS installation it would be necessary to ensure that there are no single points of failure. This would entail duplicating not only the UPS systems but all reticulation and the load itself.

The above solution, although desirable, is relatively expensive to achieve and not many institutions are willing to pay the price.

The following is a brief analysis of the most common installations used to improve the reliability of supply to a critical load:

  • Standalone
  • Hot standby
  • Parallel redundant

1. Standalone Unit

The most common type of UPS installation is a single standalone unit, fitted with a static bypass switch which transfers the load to raw mains in the event of an inverter failure, as suggested in Figure 1. Such a configuration does however have drawbacks, in that to carry out routine maintenance effectively; the load will have to be fed by mains.

Standalone UPS Unit
Figure 1: A common standalone UPS

Thus the load is put at risk every time maintenance is done on the UPS. Generally most users cannot afford any downtime and these results in costly maintenance being done after hours or ‘taking a chance’ that mains will not fail. Furthermore should the UPS develop a problem the load is subjected to mains power for the period it takes to repair the UPS.

2. Hot standby configuration

In a hot standby situation the configuration consists of the following:

  • Two UPS units are in series with one another (see Figure 2);
  • The master unit runs at full load, while the slave unit is merely switched on (idling);
  • Should a failure occur on the master unit, the slave unit will supply the full load, via the master unit’s static bypass switch.

Hot standby UPS configuration
Figure 2: Plant in Hot Standby Configuration


There are a number of disadvantages of the hot standby configuration:

  • The component life of the master unit will be short in comparison to that of the slave unit, and thus the mean time between failure (MTBF) of the whole system is greatly reduced;
  • This type of configuration involves the one UPS feeding the load through the bypass of the other UPS thus making removal of a damaged unit impossible without the supply being switched off, and hence downtime for the load is a result;
  • When the master unit fails the slave unit is subjected to a full load step. The resulting stress on the slave unit, which had been merely idling at this point, often causes the slave unit to malfunction resulting in loss of power to the load;
  • Higher losses, lower efficiencies, and worse regulation of the output.


Two obvious advantages of the hot standby configuration should be emphasised:

  • The reliability is better than a standalone system;
  • The cost is less than that of a parallel redundant configuration.

3. Parallel Redundant

The configuration of a parallel redundant configuration is shown in Figure 3. For extremely critical loads, reliability of available power may be increased by paralleling two or more units

Parallel Redundant UPS
Figure 3: Parallel Redundant Configuration. Note how the parallel UPS’s share the load

In this configuration the load rating is made up of an ‘n’ UPS unit, each supplying 1n of the load rating. One additional unit, also of rating 1n th of the load is added for redundancy. For example, a 300 kVA load would be supplied by four, 100 kVA units, giving a redundancy of one unit. The units all run in a parallel configuration and are of equal power rating, sharing the total load draw in equal proportion.

In the event of a failure on one of the units, the faulty unit will be shut down and the full load will automatically be assumed by the remaining units. There will be no voltage fluctuation, frequency fluctuation or short breaks in the output voltage whatsoever.

A two block system can be equipped with one set of batteries, which will reduce the system cost and enable cross feeding, i.e. one rectifier can feed two inverters. Further improvement of reliability is obtained by adding a second set of batteries. In addition the battery can be configured as parallel strings, further increasing redundancy.


While the parallel redundant configuration clearly applies to critical loads, the user must be cognisant of the fact that initially such a system is more expensive than a hot standby configuration due to the complex control required.


The parallel redundant configuration offers numerous real advantages for providing a critical load:

  • The component life of the parallel UPS units can be expected to be the same, but as they are only subjected to half the loading compared to the master unit in the hot standby configuration, the MTBF of the whole system is dramatically increased;
  • Should a UPS fail, the remaining units will automatically assume full load enabling the damaged unit to be isolated and repaired without the load being on mains or any other risk to the load;
  • It is very easy to disconnect and remove a unit, with no need to switch off the supply and hence no down time for the load;
  • Component failure in the systems assuming full load is greatly reduced as these components are already being used;
  • The paralleling of inverters increases the fault clearing capability of the system and thereby ensures that short circuits are cleared twice as effectively without having to transfer to bypass, thereby avoiding unnecessary switching or tripping of supply switchgear. In a two block parallel redundant system single phase fault clearing capability is increased from about 2,5 to 5,6 In and three phase fault clearing capability is increased from about 2,1 to 4,2 In;
  • The system overload capability is doubled;
  • A parallel system allows for ease of maintenance without the load having to be fed by mains i.e. static bypass operation, which results in the risk of dropping the load every time maintenance is done on a single block system;
  • A redundant system ensures that should any unit fail, there will be sufficient capacity to supply the load with clean uninterrupted power until the faulty unit can be repaired;
  • The non-linear load capability of the system is improved with the parallel redundant system as the filter capacitors are doubled, therefore making the system more suitable to switch mode power supply loads i.e. computers as well as loads generating a high harmonic content;
  • A parallel system with a common battery allows for cross feed capability i.e. one rectifier can supply two inverters.


As a user requiring a UPS, you need to be aware of the various UPS system configurations available to you. While each has its own strengths and weaknesses that need to be considered, a parallel system will supply increased reliability and ensure an uninterrupted supply to the load, even during maintenance.