【前言】

 
        事实上，我们过去一直忽视了如何增加UPS下游的电效率，现在才对其开始高度关注。问题的重点是要提高PUE（或其反向DCiE），因此实现UPS下游的效率与UPS上游的效率同等重要。请记住，UPS下游的电效率每损失1%，则PUE将增加1.2%至3%。

  
        因此，我们将如何提高UPS下游的效率呢？这里将重点从三个主要方面加以介绍：配电体系结构、变压器和电压。

 

 
       【配电体系结构】

 
        您应该去掉配电设备（PDU）上的静态总线切换开关（STS或SBTS）和尽可能去掉UPS下游远程电力配电盘（RPP）。除了有一些固有的电损失外，STS还会增加成本、减少可利用的占地面积，从而会降低利用率（如果它们成为单一故障点）和增加操作复杂性。如果必须使用STS，则必须将它们安装在计算机柜和机柜内的供电设备中。

 
       对于第Ⅰ、Ⅱ和Ⅲ级设施，您应考虑冗余的PDU（实际上，2N或系统＋系统配电）是否可行。通常，PDU变压器效率会随着负荷比率增加，因此，共享相同负荷的两台PDU的效率将会比提供相同负荷的一台PDU的效率低。此外，单线服务器比双线服务器的电能效率高。本文后面将继续介绍更多有关变压器效率的信息。

 
       关于UPS体系结构只有两种想法：如果需要使用2N或系统＋系统体系结构，则您应考虑用3N/2或4N/3体系结构替代。假设临界负荷为2000kW，则2N设备将需要两个UPS系统，每个的负荷量为2000kW。在3N/2设备中，您需要三个UPS系统，每个的负荷量为1000kW。3N/2设备的优势如下所示：

 
       1.降低初始成本，因为与4000kW的2N设备相比，您要购买UPS总负荷量为3000kW的设备。

 
       2.运行更有效，因为每个3N/2系统将以比每个2N系统更高的额定值百分比运行。满载时，每个3N/2系统将以额定值的67%运行，而2N系统则将以额定值的50%运行。

 
       与2N相比，3N/2或4N/3的劣势为：3N/2或4N/3需要更加注意其负荷管理。

 
       关于UPS体系结构的第二个想法是要考虑UPS系统中的多余模块是否对该项目有用。冗余模块会增加初始成本（CAPEX）和运营成本（CAPEX）。CAPEX增加的主要原因是运行效率降低，从而导致模块冗余。

 

      【变压器】

  
       通常，UPS下游的变压器计算在PDU中。我们每次转换电压时，在此过程中要损失1.5%-3%的能源。尽可能去掉变压器是一种提高能源效率的良好方法。

 
       如果你需要使用变压器，可考虑用较大的变压器代替较小的变压器。通常，较大负荷量的变压器要比较小负荷量的变压器的效率高。因此，有必要规定少数几个较大负荷量的变压器的效率。例如，一台75kVA变压器的最低“能源之星”（编注：美国最新发布的服务器技术规范）效率为98%，而一台300kVA的变压器的最低“能源之星”效率为98.6%。

 
       你应规定预期额定工作负荷时的变压器效率。考虑每台变压器可以在高达90%额定负荷下工作的一个普通Ⅱ级配置：效率峰值应规定为额定值的50%～90%。将该配置与一个变压器效率峰值达到额定值的20%～45%的普通Ⅳ级配置对比。

  

      【电压考虑因素】

 
       现在，大多数电脑电源的额定电压均为100～240伏，因此，他们不必担心输入电压是否是120伏、208伏或230伏。同样，他们也无需担心输入频率是否是50HZ或60HZ。

  
       在传统US设计中，为了向电脑设备配电，UPS向将电压降到208伏的下游变压器供应480伏电压。一条30安培、208伏、3极和4线分支电路可提供大约8kW的负荷。

  
       400Y230伏配电是世界大多数国家（美国除外）的标准配电，给电脑设备供应230伏线与中性点间的电压。一条30安培、400Y230伏、3极和4线分支电路可供应大约15kW的负荷，大约相当于传统208Y120伏系统可供应负荷的两倍。UPS下游的变压器数量少，因此通常US实践可以获得1.5%～3%的效率。

  
       在美国，480伏配电系统中使用400Y230伏电压，您可以在UPS上游安装一个大型、高效的变压器，并取消UPS下游的PDU变压器。上游变压器的成本和能量损失要比下游变压器的低得多。
在美国，在一个中等电压配电系统中使用400Y230伏电压比较简单——您只需将变电站变压器的次级电压设定为400Y230伏。该变电站变压器在400Y230伏时的效率应与相同规格的480伏变压器的效率相当。通常，您要指定一个较大负荷量的UPS，因为您要在400Y230伏而不是480伏电压下使用这个UPS，但您已经取消了UPS下游PDU变压器并提高了效率。简单地说，您要平衡减少的变压器损失和分支电路线成本来防止增加配电线成本。

 
       当我们讨论400Y230伏配电时，这里有一个预测。当电脑电源制造商将可接受的输入电压从240伏增加到277伏时，则在美国400Y230伏配电将变得毫无优势可言。那时，480Y277伏将是UPS的标准输出电压和新数据中心的配电电压。一个30安培、480Y277伏、3相和4线分支电路将供应18kW的负荷，它是相似大小的400Y230伏电路负荷量的1.2倍和相似大小的208Y120伏电路的2.3倍。

  
       另一个电压考虑因素是给大型低压配电使用575伏而不是480伏电压。575伏在加拿大是一个公共电压，但美国除外。你将会发现它在美国大型数据中心的使用率日益增加，因为它的初始成本和运行费用低。

 
       可以降低初始成本，因为相同大小的导线、总线或断路器在575伏时比在480伏时能多输送20%以上的电能。例如，3750kVA变电站在575伏时可连接一根4000安培的次级总线，而在480伏时需要一根5000安培的总线。

 
       笔者对575伏的预测是，一旦可以使用277伏电脑电源，575伏就会变得黯然失色。电脑电源制造商告诉我，他们无望能够将可接受输入电压增加到347伏（575伏系统中的线与中性点间电压）。因此，配送575伏大型低压电，再给UPS变压到480伏电压将行不通。

 
       总之，我们讨论了增加UPS下游电效率的三种容易且有效的手段，您不妨马上使用这些工手段来降低运行成本和增加效率吧，祝您好运！

 

作者简介：
Christopher M.Johnston,PE——Syska Hennessy集团的国家关键设施总工程师

 

---

  

Increasing Electrical Efficiency Downstream of the UPS  

   

         Written by Chris Johnston, PE  

  

 

         Increasing electrical efficiency downstream of the UPS has been virtually ignored in the past but is receiving much more focus today. With emphasis on improving PUE (or its inverse DCiE), the industry is realizing that efficiency downstream of the UPS is as important as efficiency in the UPS and upstream of the UPS. Remember, every 1% of loss downstream of the UPS increases the PUE by 1.2% to 3%.

 

 

         So, how do we improve the efficiency downstream of the UPS? We will focus on three key areas that offer opportunities: electrical distribution architecture, transformers and voltage.

 

 

         Electrical Distribution Architecture

  

 

         You should eliminate Static Bus Transfer Switches (STSs or SBTSs) at Power Distribution Units (PDUs) and Remote Power Panelboards (RPPs) downstream of the UPS wherever possible. In addition to having some inherent electrical losses, STSs add cost, reduce available floor space, can reduce availability (if they become single points of failure), and increase operational complexity. If STSs are necessary, they should be mounted in the computer cabinets and supply devices in those cabinets.

  

 

          In Tiers I, II and III facilities, you should consider whether redundant PDUs (in effect, 2N or System + System distribution) make sense. In general, PDU transformer efficiency increases with percentage of load, so two PDUs sharing the same load will have a lower efficiency than one PDU supplying the same load. Also, single-cord servers are more electrical energy efficient than dual-cord servers. Stay tuned for more about transformer efficiency later in this article.

    

 

          Just two thoughts about UPS architecture. If 2N or System + System architecture is required, you should consider 3N/2 or 4N/3 architecture as an alternate. Assuming that the critical load is 2000kW, a 2N arrangement would require two UPS systems, each of 2000kW capacity. In a 3N/2 arrangement you would need three UPS systems, each of 1000kW capacity. The advantages of the 3N/2 arrangement would be:

  

          1. Lower initial cost since you are buying a total of 3000kW of UPS capacity versus 4000kW with the 2N arrangement.

 

          2. More efficient operation since each 3N/2 system would operate at a higher percentage of rating than each 2N system. At full load each 3N/2 system would operate at 67% of rating while the 2N systems would operate at 50% of rating.

 

          The disadvantage of 3N/2 or 4N/3 versus 2N is that either 3N/2 or 4N/3 requires more attention to load management than 2N.

  

 

          The second thought about UPS architecture is to consider whether redundant modules in UPS systems make sense for the project. Redundant modules increase initial cost (CAPEX) and operating costs (CAPEX). The major component of increased CAPEX is the reduced operating efficiency resulting from redundant modules.

  

 

          Transformers

   

 

          Transformers downstream of the UPS are typically contained in PDUs. You should understand that every time we transform voltage we lose 1.5%-3% of the energy in the process. Eliminating transformers wherever possible is a good technique for increasing energy efficiency.

  

 

           If you need transformers, think of using fewer larger transformers instead of smaller transformers. A larger capacity transformer is generally more efficient than a smaller capacity transformer. So, it makes sense to specify a smaller number of larger capacity transformers than the reverse. For example, the minimum Energy Star® efficiency is 98% for a 75kVA transformer and 98.6% for a 300kVA.

 

 

           You should specify transformer efficiency at its intended normal operating load. Consider a generic Tier II arrangement where each transformer may operate at up to 90% of rating: peak efficiency should be specified at 50%-90% of rating. Contrast that to a generic Tier IV arrangement where peak transformer efficiency should be attained at 20%-45% of rating.

  

 

           Voltage Considerations

  

 

           Most present-day computer power supplies are rated for 100-240 volts, so they don’t care if the input voltage is 120 volts, 208 volts or 230 volts. Likewise, they don’t care if the input frequency is 50Hz or 60Hz.

   

 

           In a traditional US design the UPS supplies 480 volts to downstream transformers that reduce the voltage to 208 volts for distribution to computer equipment. A 30 amp, 208 volt, 3-pole, 4-wire branch circuit can supply about 8kW of load.

  

 

           400Y230 volt distribution is the standard in much of the world outside the US. Computer equipment is supplied at 230 volts, line-to-neutral. A 30 amp, 400Y230 volt, 3-pole, 4-wire branch circuit can supply about 15kW of load, roughly double what the traditional 208Y120 volts system can supply. There are seldom any transformers downstream of the UPS, so 1.5%-3% of efficiency is gained over the US practice.

  

 

           To apply 400Y230 volts in the US in a 480 volts distribution system, you can have a large, very efficient transformer upstream of the UPS and eliminate the PDU transformers downstream of the UPS. The upstream transformer cost and energy losses can be much lower than those of the downstream transformers.

 

 

           To apply 400Y230 volts in the US in a medium voltage distribution system is much simpler - you simply specify the substation transformer secondary voltage as 400Y230 volts. This substation transformer should be as efficient at 400Y230 volts as a 480 volts transformer of the same size. You will have specified a physically larger capacity UPS since you are operating the UPS at 400Y230 volts rather than 480 volts, but you have eliminated the PDU transformers downstream of the UPS and increased the efficiency. In a nutshell, you are balancing reduced transformer losses and branch circuit wiring costs against increased distribution wiring costs.

  

 

            Here is a forecast for the future as long as we are discussing 400Y230 volts distribution. Once the computer power supply manufacturers increase the acceptable input voltage from 240 volts to 277 volts, all advantages for 400Y230 volts distribution disappear in the US. At that time, 480Y277 volts will be the standard UPS output voltage and the distribution voltage for new data centers. A 30 amp, 480Y277 volts, 3-phase, 4-wire branch circuit will supply 18kW that is 1.2 times the capacity of a similar size 400Y230 volts circuit and 2.3 times the capacity of a similar size 208Y120 volts circuit.

  

 

             Another voltage consideration is using 575 volts rather than 480 volts for bulk low voltage power distribution. 575 volts is a common voltage in Canada but is not in the US. It is finding increasing use in large US data centers because it can offer economies in initial costs and operating expenses.

 

 

              Initial costs can be reduced because the same size conductor, bus or circuit breaker can carry 20% more power at 575 volts than it can at 480 volts. For example, a 3750kVA substation can have a 4000 amp secondary bus at 575 volts while a 5000 amp bus is required at 480 volts.

 

 

              Operating costs can reduced because of reduced conductor losses.

  

 

               My forecast for 575 volts is that it will sadly fall out of favor once 277 volts computer power supplies become available. I have been told by computer power supply manufacturers that they have no hopes of being able to increase the acceptable input voltage to 347 volts (line-to-neutral voltage in a 575 volts system). So, distributing 575 volts bulk low voltage power and then transforming to 480 volts for UPS would not make sense.

  

 

              In summary, we have discussed three readily available tools to increase electrical efficiency downstream of the UPS. Keep them at hand and use them to reduce operating costs and increase efficiency. Best wishes!

 

 

               About the Author: Christopher M. Johnston, PE is the National Critical Facilities Chief Engineer for the Syska Hennessy Group.