by Mike Rycroft, Now Media –
Large uninterruptible power supply (UPS) systems are common for IT installations. Unreliable grid power, together with ever increasing automation and the sensitivity of modern industrial processes, have ensured that UPS systems for industrial applications are becoming common. The requirements of an industrial UPS differs from those for IT installations, but the overall need for a reliable source of power remains the same.
The differences between industrial UPSs and IT UPSs come from the environment in which the systems operate. An industrial process may be spread over a wider area than an IT installation, with more distribution and protection devices, making it more important to bring reliable power close to the equipment. Continuity of power is important to industrial and commercial operations. Loss of power for even a few seconds could result in millions of rand in lost revenue for businesses.
The weakness of any power supply system is that not only can the grid supply fail, but the UPS system and other components which distribute power to the load can as well, and these failures need to be catered for if the required reliability standard is to be met. In the IT industry it has been determined that 95% of all power failures occur between the UPS and the computer load  and thus it is the configuration consisting of all components which would determine reliability.
The solution is to provide multiple UPS systems, with multiple connection paths between the UPSs and the load. The way in which these components are connected together in the supply path, known as the configuration, can affect the ultimate reliability of the system. Configuration is the way in which the UPS systems, associated components, switching devices, cabling and protection devices are connected to feed the load.
Uptime: how many nines?
The desired result of high reliability systems is uptime, which is defined as the amount of time an infrastructure is on, up and uninterrupted. Many factors affect uptime, including unreliable utility power, the maintenance environment, system failure, and human error.
Reliability requirements are expressed as a percentage of the time that power is available and useable. Figures in excess of 99% can be achieved. Reliability is often expressed in terms of the number of nines, e.g. 99,9 is three nines. Table 1 shows the effect of reliability on a 24 hour continuous process. Five nines (99,999) amounts to five minutes of downtime in a calendar year. That is considered the highest level which could be reasonably achieved.
|4||52,5||4,5||1 (60s)||0,144 (8,6s)|
|5||5,25||0,45||0,1 (6s)||0,0144 (0,86s)|
|99,9999||6||0,525||0,045||0,01 (0,6s)||0,00144 (0,086s)|
Table 1: The “nines” principle.
Most industries do not run continuously but have processes which require several hours of unbroken power to complete. When determining the uptime requirements, the question to ask would be how long a process can be interrupted without having to restart. Some processes can tolerate short breaks and continue operation, while others need to be abandoned or restarted with wastage of partly processed material,and a loss of production.
Industrial operations do not generally require the same uptime as large IT server installations, but there is nonetheless the need to ensure that power interruptions are as short as economically possible. Selection of the UPS and configuration involves balancing the cost of the UPS installation against the cost of loss of production due to power failures.
Although there are many and varied UPS configurations available today, they all can be divided into a number of standard configurations accepted by industry.
Parallel capacity(N) (Figure 1)
In this configuration several UPSs are connected in parallel to provide the capacity necessary to carry the load. There is no redundancy, so if a single UPS fails, the capacity of the system is reduced. Some UPSs feature modular designs which allow capacity to be added incrementally as requirements increase. Even the largest UPS systems can be made modular in 200 to 300 kW increments. This is a scalable and efficient approach to keeping up with growing power needs while also lowering initial capital spend.
Maintenance requires a shutdown of the complete installation, although single UPS maintenance may be carried out if load reduction is possible. Maintenance thus contributes to the downtime of the system. In an industrial situation, maintenance could be scheduled to coincide with process downtime (e.g. weekends) and need not contribute to downtime.
Parallel redundancy (N+x)
N+x configuration solutions are commonly used to protect critical processes at industrial sites and larger business operations.The main principle behind a parallel-redundant UPS system is the provision of at least one redundant UPS module in excess of the load requirement. This allows the system to support the critical load should one or more UPS module fail. This means it can achieve higher availability and mean time between failure (MTBF) figures. Figure 2 shows a parallel-redundant system.
This also enables UPS maintenance to take place without interrupting the load. Modules can be powered down for servicing while the remaining UPS continues to support the load.
Parallel-redundant systems require all UPS modules to be identical in capacity. All modules must be of the same design, same manufacturer, same rating, same technology and configuration.
The output of the modules is synchronised externally, but in some cases this function is embedded within the UPS module itself. The paralleling function may also control current sharing between the modules. The number of UPS modules which could be paralleled onto a common bus is limited, and this limit differs for various UPS equipment .
Problems with the parallel-redundant system are mainly the use of a common bus to distribute power to the load. Failure of the bus negates any gains from the N+x system. The configuration is still vulnerable to single points of failure upstream and downstream of the UPS system.
System efficiency is an important factor to consider in the design of a redundant UPS system. Lightly loaded UPS modules are less efficient than modules loaded closer to capacity. The efficiency of a UPS at low load varies from manufacturer to manufacturer and should be investigated during the design process .
Isolated redundant configuration
The isolated redundant design concept does not require a paralleling bus, nor does it require that the modules have the same capacity, or even be from the same manufacturer. There is a main or “primary” UPS module which normally feeds the load. The “catcher” or “secondary” UPS feeds the static bypass of the main UPS module(s) should the main module fail. The secondary UPS runs in “hot” standby mode, synchronised to the main UPS output (Figure 3).
This configuration provides a way to achieve a level of redundancy for a previously non-redundant configuration without completely replacing the existing UPS.
A distinct disadvantage of isolated redundancy is that the entire load resides on a primary UPS until the point of failure. This means that a stand-by or “catcher” UPS must accept a 100% step load. Not only must the primary module’s static bypass switches operate properly to obtain power from the reserve module, but also (should an output overload have occurred) both static switches must properly detect the situation and supply current from the utility source.
Variations on the isolated redundancy configuration have included a single reserve module as backup to several independent primary UPSs. The reserve UPS is generally sized to support only one of the primary modules. This requires such complexity in the switch gear configurations and associated controls that it usually offsets any reliability gains.
Distributed redundant dual UPS bus system (Figure 4)
Several distributed redundancy configurations have been devised over the years. In its basic form, distributed redundancy involves creating dual, full capacity UPS system busses and redundant power distributed systems. This eliminates as many single points of failure as practical, all the way up to the load equipment’s input terminals.
Figure 4: Distributed redundant UPS bus system.In order to provide fault tolerance, some method of allowing the load equipment to receive power from both UPS power busses must be provided. Protecting against fast power system failures, such as circuit breaker trips or a power system fault, requires a commensurably fast switching method. Static transfer switches (STSs) have been found to satisfy this requirement of extremely fast break-before-make transfers between two AC power sources.
It is important that the two AC power sources be designed as independently as practical to eliminate any common failures. Switching between the two power sources needs to be break-before-make for the same reason.
Redundancy needs to be as close to the load as possible to achieve its goal of keeping power available at the load equipment level. The ultimate distributed redundancy configuration would be two independent UPS power distribution systems with dual-input load equipment as redundant AC power is provided up to and inside the load equipment.
Dual input load equipment is becoming popular in IT applications, but not in industrial equipment, so the use of static transfer switches (STSs)at the equipment input is becoming more common.Distributed redundancy not only provides the best assurance of power reliability and availability, but it also paves the way for an easy migration path as more dual-bus loads become deployed.
System plus (2(N+x))
This dual power technology requires having at least two completely independent electrical systems. These dual systems supply power via diverse power paths to the process load, which moves the last point of electrical redundancy from within the UPS to within the industrial process hardware itself. There are a number of versions of this approach. A typical system is shown in Figure 5.
Reliability of configurations
Analysis by Schneider APC  of the reliability of the different configurations gave the figures listed in Table 2.
|Single module capacity configuration||99,92%|
|Isolated redundant UPS configuration||99,93%|
|Parallel redundant (N+1) UPS configuration||99,93%|
|Distributed redundant catcher configuration||99,998%|
|Distributed redundant UPS configuration||99,9994%|
|2(N+1) UPS configuration||99,99997%|
Table 2: Reliability Figures for various configurations (APC Schneider).
 K Macarthy and V Avelaar: “Comparing UPS system design configurations”, Schneider Electric White paper 75. https://download.schneider-electric.com/files?p_Doc_Ref=SPD_SADE-5TPL8X_EN
 W Turner and K Brill: “Industry Standard Tier Classifications Define Site Infrastructure performance”, Uptime Institute, 2001, http://uptimetechnology.ru/docs/Uptime – Industry Standard Tier Classifications.pdf
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