CPS VS UPS FOR POWERING DATA CENTERS

Transcription

1 CPS VS UPS FOR POWERING DATA CENTERS HOW THE ADVANTAGES OF A NO BREAK CONVERGED POWER SYSTEM (CPS) PROVIDED BETTER POWER RELIABILITY, SYSTEM AVAILABILITY AND REDUCED COSTS OVER A UNINTERRUPTABLE POWER SYSTEM (UPS) Authors: Gary Walker

2 INTRODUCTION This paper outlines the differences in service levels and in design between a Converged Power System (CPS) based on a no break design and an Uninterruptable Power System (UPS). The static UPS is the common foundation of a data center power system because it is well known and been around for many years. It is a single input system that conditions the power with batteries, which support the load during the transition of input power during power failure. This transition to a second source is facilitated by an automatic transfer switch (ATS) external to the UPS. The second power source is normally a diesel generator that will be used during a complete black out or irregular power feed such as a brown out or low input power. To replace this workhorse power system requires a dramatically better solution. While lower cost is important, power systems are mostly judged on reliability and availability. Making a significant improvement in all three areas is the way to change data center operator s minds about a new system. The CPS is a power system that entered the data center market in 2013, with a background in the telco DC power market as well as in industrial applications. It is also an uninterruptable power source with batteries and functions in the same way as a UPS, smoothing the power during irregular power feed such as a brown out or low input power. However the CPS does not leverage an ATS for switching between two inputs, because the ATS is a single point of failure and breaks the input power supply for a short period of time during the transition. The CPS, instead, has direct connections to both power inputs offering a no break transition between inputs. CPS INPUT SUPPLY CONVERGENCE The CPS has dual inputs that function independently of each other. One input dedicated to the grid power source and one input dedicated to back-up power source, typically a diesel generator. This design feature is one of two key differentiators from a typical static UPS and makes the CPS a no break power solution. The CPS functions without the requirement of an ATS before the input to the system. With the removal of the ATS, the CPS has eliminated one single point of failure in the total power system on site. A second feature of the CPS solution is a dual, concurrent input design in which the load can be shared across the two input sources. The second feature is particularly advantageous for supply cost management allowing the use of lower-cost power inputs when the cost of grid power is higher than the cost of diesel, for example. The dual inputs also are beneficial when switching to the diesel generator for tests or for using it as a backup. In these examples, the diesel generator can be turned on and off remotely via software control, to either share the load or take the entire load in real time without putting the load at risk, and without the requirement of transferring the load via an ATS. Should the diesel generator fail during its test cycle, the grid is still connected through its independent input feed and can resume taking the load without the requirements of a transfer from an ATS switch first. Eltek White Paper CPS vs UPS Page 2 of 7

3 CPS OUTPUT LOAD CONVERGENCE Models of CPS offer two additional convergence features at the output: 1. The delivery of a converged VAC and VDC output to the load. 2. A modular design that provides the ability to vary the capacity of VAC and VDC by installing the required Inverters and rectifiers to support the load. This enables the data center operator to migrate load from one voltage type to the other as the load profile changes over time. FEATURES AND BENEFITS Advantages of Static UPS vs CPS Solution Due to its market longevity, many data center operators feel comfortable with a UPS because it is a known solution with a perception of having a lower risk. And, as the product has matured, multiple vendors have emerged to offer solutions. But when compared to a CPS based technology benefits and features, some weaknesses become apparent: CPS Solution Traditional Static UPS CPS system does not require ATS switch to connect to grid and generator, both sources are permanently direct connected to the system, resulting in a no break power feed The generator can run simultaneously with the grid power for cost or testing purposes Gen-set and grid power the UPS input through a transfer switch. The Transfer Switch is a single point of failure This configuration does not offer a no break power feed. The generator can be load tested while system is running on grid power offering an ongoing operational benefit The gen-set cannot be tested without disrupting the grid connection. The generators must either be switched with a transfer switch to take the load, or disconnected from the infrastructure. Both methods increase the risk to the load during the testing period. Gen-set will share load during brown outs and power fades which is an operational benefit The system batteries can continue to be charged via the gen-set ensuring the batteries can assist with supporting the load during the brown out or power fade During multiple brown outs, the gen-set cannot share the load, which can result in the batteries being fully discharged and therefore being unable to support the load. In this case, the UPS input must be switched to gen-set Eltek White Paper CPS vs UPS Page 3 of 7

4 The CPS system has a common earth (ground) in the plinth connecting all racks, controllers, modules and batteries Earthing of multiple UPS systems require care to ensure safety and noise immunity for racks powered from independent sources % availability % availability (with redundant modules) CPS supports partial loads with hot swappable modules to match load to peak data center load. Power efficiency is maintained at peak across all operating conditions Mono-block static UPS delivers poor partial load efficiency. UPS units are large and only upgradeable by adding large blocks which can require the UPS to be shut down during upgrade Some models of CPS provide both VAC and VDC from a single system. The load profile can be varied by balancing the mix of inverters and rectifiers UPS only provides a VAC output. A second VDC system and second set of batteries are required to support VDC CPS VDC output supports 48V, 240V and 380V UPS cannot support VDC output By placing inverters in data center server racks, CPS can support different capacity loads and resilience on a rack-by-rack basis With hot swappable modules, racks sitting side by side can have different power capacities, each with its own redundancy A static UPS is configured based on the average power required across multiple racks at a common resilience level. CASE STUDY There are a lot of operational, reliability and power availability benefits of a CPS when compared to a UPS. But each data center has different power needs and operational concerns. The following case study describes how one data center operator in New Zealand analyzed its power system needs, which led them to purchase a Converged Power Solution (CPS) for a new data center. Spark New Zealand, a mobile and fixed line telecommunications company and major ICT provider to New Zealand businesses, undertook a study to determine which power system it should deploy in a new data center. First, the company studied the product feature differences between the CPS VAC and a traditional UPS and chose a CPS from Eltek for the reasons outlined above. Once the technical benefits had been established, they looked at the comparative cost differences Eltek White Paper CPS vs UPS Page 4 of 7

5 between a UPS and a CPS. The last part of their analysis was to determine how the CPS reduced the overall size of the total power system, from supply to load, and how the space savings might reduce the total build cost of the building and a reduction of overall infrastructure cost. Operations and maintenance, staff cost, spare parts and maintainability was also a part of the overall analysis. Below is a summary of each aspect of their analysis: Design The electrical service was a 2N supply and the UPS the company chose to evaluate came from a leading supplier and was a mono block static 550 KVA UPS to support a 400 kw load. Power to the UPS is from a 400 KW / 550 KVA transformer and a 400 kw 500 KVA generator. On mains failure, the generator starts and is switched to power the UPS through an ATS. UPS input and output boards provide the ATS function for the transformer and generator supplies and the maintenance bypass for the UPS. Dual distribution boards support the A and B feeds to the data center equipment racks. Internal Arrangement The static UPS used a mono-block design and transfer switch all mounted into a single cabinet. It must be powered down to work inside the UPS cabinet with an external bypass switch used to maintain power to the load. The batteries were referenced to neutral which presents an electrocution risk. An input board was required to connect the UPS to either the grid or generator through a transfer switch and to power the maintenance bypass circuit. An output switch board is required to combine the UPS output with the bypass supply and to distribute power to the final loads. Alternatively, the CPS design is based on a chassis or shelf concept with an array of active rectifiers and inverters (converters) combined with multiple spare units. The converters are hot swappable and intelligently switch between themselves without requiring the cabinet to be powered down. There are two groups of converter cabinets, one for the grid power and one for the generator power. Simple fuses are used to connect the grid and the generator to their respective converter cabinets. Both groups of converters charge an array of batteries. Availability The customer rated highly increased availability of the no break power design of the CPS. The key elements that determine this was the array of hot swappable modules with multiple spare units. The ability to test the generators at any time in parallel with the grid input. The minimal number of circuit breakers in the total power solution meant less risk due to breaker malfunction, and the removal of all transfer switches which are common single points of failure. Building and Land Cost Savings With the deployment of the Eltek CPS, the client realized a cost saving in building construction time of 6 months. The smaller size of the system and its high efficiency also resulted in a 30% reduction in building size and a 10% less land requirement. The savings were realized through the reduction of building power requirements primarily due to not requiring multiple switchboards and dedicated power infrastructure rooms. All of Eltek White Paper CPS vs UPS Page 5 of 7

6 these savings also contributed to a cost of finance saving during the building process and reduction in the time to revenue by six months due to the reduced data center build time. Staged Capital Investment The small rectifier and inverter power size of the CPS allows deployment of modules to closely match actual data center loads. This meant the operator didn't need to overinvest in the CPS initially, but was able to stage its capital investment as the data center grew. Price Comparison The initial cost of the CPS was 5% less than the cost of the UPS. Prices for each system at the time of the project are shown below: Item Static UPS CPS Solution QTY Unit Cost QTY Unit Cost UPS module 1 USD145K USD145K 2 USD135K USD270K Input SB 1 USD73K USD73K 0 Output SB 1 USD73K USD73K 0 Battery pack 2 USD36K USD72K 4 USD18K USD72K Load Dist. 2 USD22K USD44K 2 USD22K USD44K Boards Total USD408K USD386K Cost savings for other for data center construction elements that benefited from the CPS (excluding land, power, telco and water services to site) are shown in the table below: Typical Build Build Based on Eltek CPS Power System Total Cost $62M Total Cost $48M $12M power system $8m power system Cooling system - $1M Building structure - $4M (due to reduced footprint) Engineering - $1M Capital costs - $4M (Labour saving, financing capital for project) TOTAL COST SAVING $14M Operations & Maintenance The CPS is a complete power solution with the functionality of many power solution components, such as switchboards and synchronization infrastructure, converged into a single CPS. Technicians learn to maintain one system in many sites. Therefore, operational and maintenance standard operating procedures (SOP) are simplified, or, in many cases, can be eliminated. The convergence of third-party systems eliminates many of the maintenance processes such as switchboard and transfer maintenance. The modules are standardized and common to all systems. The power system design is no longer a site-specific design utilizing different components thus requiring different procedures for each site. With the CPS, all modules and the systems are standardized and interchangeable across sites resulting in SOP that are common to all sites. Eltek White Paper CPS vs UPS Page 6 of 7

7 The no break design also simplifies maintenance. For example, when performing gen-set tests, a large number of engineers are not required to ensure the successful transfer of supply from grid to gen-set, because there is no transfer with a CPS. The gen-set can be tested more frequently because during the test process the grid remains connected to the dedicated input. Therefore, should the gen-set fail its test, the grid can immediately take the load without a transfer. The gen-set can be tested with the real load in part or in full - with no risk. Due to the dual-input, no break design, the gen-set can be tested at any time, day or night, week day or weekend. Standby engineers are not required to be on site and customers no longer need be advised because the grid is always connected, therefore there is no risk of a power supply break to servers when testing the gen-set. CUSTOMER CONCLUSIONS The static UPS and the CPS solution have similar total cost. Static UPS has acceptance in the market. CPS solution has significant advantages in the following areas o Total cost of data center o Speed to market o Return on investment o Availability o Flexibility o Staged capital investment o Simplified earthing o Higher utilization of power train, better matching load to supply o Simplified generator operations and maintenance o No transfer switching which removes single points of failure o Higher energy density in the available footprint. CONCLUSION As power becomes a significant cost to data center operations, and in some cases reduced power availability can limit growth, data center operators are increasingly considering DC power to increase overall power conversion efficiency. This change requires those operators to seek out new power systems to support DC power. The CPS adds to the benefits of DC power with its no break design, flexible support of partial loads, support for VAC and VDC, and its other operational and reliability benefits. With these advantages and a lower initial cost - data center operators can be assured that making the change from a UPS will provide a better overall solution for their data centers. Eltek White Paper CPS vs UPS Page 7 of 7