Modular power protection in Industrial applications
Modular power protection and conversion technology, particularly in the form of UPSs, has long been used in com - mercial applications, but take-up in industrial applications has, to date, been relatively slow.
This relatively slow uptake is due, in part, to a limited understanding of the “ilities” (“Availability”, “Reliability”, “Scalability”, “Flexibility” and “Maintainability”) commonly associated with modular technology and how the various “ilities” complement each other.
The third of five articles within our 'ility' series focuses on availability and reliability. We will review the technical and mathematic backgrounds and explain how both terms relate to the increasingly popular modular technology.
Power protection systems for industrial applications are typically expected to have an operational life of 20-25 years, so ensuring the size of the system is correct at the time of installation is very important.
If the installed system proves to be undersized at some point in its operational life then an expensive system upgrade or replacement may be required (i.e. a capital expenditure, or “Capex”, problem).
If, however, the installed system proves to be oversized then initial Capex would have been wasted, the system will be running at less than optimum efficiently and maintenance costs will be higher (i.e. an operational expenditure, or “Opex”, problem).
Both Capex and Opex problems can have a significant impact on total cost of ownership. Ever changing site equipment (critical load) technology and operational requirements makes predicting what the system load will be, and hence what the system size should be, over a 20-25 year period almost impossible. To err on the side of caution, almost all systems are oversized at the time of initial installation, which is good news for the system manufacturer but bad news for the system operator as they will probably have wasted Capex at the initial system purchase and will be wasting Opex every year the system is in operation. The only way to ensure that Capex and Opex are optimised, and hence total cost of ownership is minimised, is to ensure that the system is always “right-sized” and scalable systems enable this.
Total cost of ownership (TCO)
For a power protection system the three main elements affecting cost of ownership are: 1. Up-front capital cost (purchase price); 2. Power losses (a function of system efficiency); 3. Ongoing maintenance costs. It goes without saying that everyone wants to minimise the total cost of ownership (TCO) for their power protection system. However, to achieve this it is necessary to first understand what the key elements affecting the total cost of ownership are, and then to understand how they can be optimised for a power protection system that may have an operational life of more than 20 years and that supplies a critical load that may well change significantly over time. The problem is that these three elements are all interconnected and a balance needs to be struck between all three elements if the lowest overall TCO is to be achieved. Considering each element in turn:
- Up-front capital cost
If the lowest cost system is purchased the lowest up-front capital cost will have been achieved. However, if the system is of an older, less efficient, technology then the system running costs may be higher than necessary. Furthermore, the lowest cost system may also have the highest ongoing maintenance costs because it probably uses low quality (i.e. cheap) components.
- Power losses
If the critical load is 100kW, for every 1% reduction in operating efficiency there will be an additional 1kWh of wasted energy. Because the power protection system is in operation 24 hours/day, 365 days/year there will be 24kWh wasted each day or 8,760kwH wasted each year or 175,200kWh wasted over the 20 years operational life. With this in mind:
1. Care should be taken to ensure that the power protection system is sized to ensure it operates on the optimum point on the efficiency curve;
2. System operating efficiency is more likely to affect TCO than the up-front capital cost.
Almost all modern “transformerless” UPS have an efficiency curve that is relatively flat. In the figure above, we can see that operating efficiencies of >95% are achieved with loads above 17% and the optimum efficiency (>96%) is achieved when the load is between 35% and 60%.
- Ongoing maintenance costs
All power protection systems contain electrical and mechanical components that have a finite useful working life and will require routine maintenance and/or replacement if the power protection system is to achieve the required availability levels for its 20 years’ operational life. If, as suggested above, the lowest cost system uses the lowest cost (and hence probably the lowest quality) components then the maintenance costs for the system over its operational life will probably be higher as the components will need to be maintained/ replaced more frequently. With this in mind, it is always worth asking the power protection system supplier for their ongoing maintenance recommendations and hence ongoing maintenance costs before purchasing the system.
For consistency across the articles in the “ilities” series, we will revisit the examples seen in the “Availability v Reliability” article, i.e. we will assume that the system design critical load is 120kW and that N+1 parallel redundancy is required. However, as it is not good practice to operate a UPS continuously at 100% load we will assume an operational load of 80% (i.e. 96kW).
Example 1: 120kW, 1+1 parallel redundant system using modular-block topology.
In this example, we are using two 120kW UPS cabinets. If the critical load is 96kW then each UPS cabinet will be 40% (48kW) loaded.
The system will be perfectly “right-sized” and the system will be operating in the optimum part of the efficiency curve. Both Capex and Opex are optimised. If, however, the critical load is 60kW then each UPS cabinet will be 25% (30kW) loaded so the system will be oversized, Capex will have been wasted on purchasing a system that is too large and Opex will be being wasted as the system will be operating less efficiently. If, the critical load suddenly needs to increase to, say, 125kW then the system will be overloaded and either the N+1 parallel redundancy will be lost or an additional 120kW UPS cabinet must be installed (assuming that the site infrastructure can accommodate another cabinet). In the worst case, the system will need to be replaced with another, more powerful, system. If the site infrastructure can accommodate another 120kVA UPS cabinet, making the system a 2+1 parallel redundant system then the new 125kW load will mean that each UPS cabinet will be 34% (41.6kW) loaded so the system will be oversized, significant additional Capex would have been wasted on purchasing a system that is too large and Opex may be being wasted as the system may not be operating at optimum efficiency.
Example 2: 120kW, 3+1 parallel redundant system using rack-mounted modular topology
In this example, we are using a single cabinet capable of housing five 40kW UPS modules but fitted with only four modules. There is, therefore, one “spare” slot for an additional 40kW module. If the critical load is 96kW then each of the 4 off 40kW UPS modules will be 60% (24kW) loaded. The system will be perfectly “right-sized” and the system will be operating on the optimum part of the efficiency curve. Both Capex and Opex are optimised.
If, however, the critical load is 60kW then each UPS module will be 37% (15kW) loaded so the system will be oversized and Capex will have been wasted on purchasing a system that is too large but Opex will still be OK as the system will still be operating on a good part of the efficiency curve. However, because the system is rack-mounted modular the system operator has two options: 1. Continue to operate the system with all 4 modules and benefit from N+2 parallel redundancy;
2. Remove one of the modules from service (either switch it off or physically remove it to create a second “spare” slot) to operate the system on a better part of the efficiency curve. If a module is removed each of the remaining three modules will be 50% (20kW) loaded.
If, the critical load suddenly needs to increase to, say, 125kW then the system will be overloaded and either the N+1 parallel redundancy will be lost or an additional 40kW UPS module must be installed. Because the system cabinet was correctly sized at installation and there is a “spare” slot available to accommodate an additional UPS module and because the system is rackmounted modular the installation of the additional module is quick, easy and relatively inexpensive, thereby Capex and Opex continue to be optimised.
A system that is perfectly sized for the load from day one and can increase or decrease its capacity in sympathy with the critical load will achieve the lowest total cost of ownership (TCO) for the system over its operational life. The up-front capital cost of the system, whilst important, is not the most important element in TCO. System operating efficiency and ongoing maintenance costs both significantly affect the system total cost of ownership and all three of these elements must be jointly considered. Installing a rack-mounted modular UPS cabinet with one or more empty slots at the time of initial installation will future proof the installation for very little increase in the initial capital cost of the system. The next article in the “ilities” series will discuss “flexibility” and how the selection of the right UPS topology can give parallel redundancy, changes in system capacity and the ability to rapidly locate critical power protection capacity wherever needed.