Uninterruptible Power Supply

     An uninterruptible power supply or UPS, in simplistic terms is a device which provides battery back-up power to IT equipment should utility power be unavailable, or inadequate. UPSs provide power in such a way that the transition from utility power to battery power is seamless and uninterrupted. UPSs can range in size and capacity to provide power to small individual desktop computers, all the way up to large megawatt data centers. Many UPS systems incorporate software management capabilities which allow for data saving and unattended shutdown should the need or application warrant it.

     There are many different types of UPSs on the market, and choosing the correct one can often be a confusing endeavor. For example, it is a very common belief that there are only two types of UPSs – standby and on-line. In reality, there are actually five different UPS topologies, or designs. These are:

1. Standby
2. Line Interactive
3. Standby-Ferro
4. Double Conversion On-Line
5. Delta Conversion On-Line

Understanding how these various UPS designs work is critical to choosing the best UPS for a particular application.

• The Standby UPS Is the most common design configuration used for personal computers. The operating principal behind the standby UPS is that it contains a transfer switch which by default, uses filtered AC power as the primary power source. When AC power fails, the UPS switches to the battery by way of the transfer switch. The battery-to-AC power converter, also known as the inverter is not always on, hence the name ‘standby.’

The primary benefits of this type of UPS are high efficiency, small in size and low in cost. Some models are also able to provide adequate noise filtration and surge suppression. The limitations are that this type of UPS uses its battery during brownouts, which degrades overall battery life. Also, it is an impractical solution over 2kVA.

• The Line Interactive UPS, is the most common design used for small business, Web, and departmental servers. In this design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Operating the inverter in reverse during times when the input AC power is normal provides battery charging. When the input power fails, the transfer switch opens and the power flows from the battery to the UPS output. With the inverter always on and connected to the output, this design provides additional filtering and reduces switching transients when compared with the Standby UPS topology. In addition, the Line Interactive design usually incorporates transformer which adds voltage regulation as the input voltage varies. Voltage regulation is an important feature when variable voltage conditions exist, otherwise the UPS would frequently transfer to battery and then eventually down the load. This more frequent battery usage can cause premature battery failure.

The primary benefits of the Line-interactive UPS topology include high efficiency, small size, low cost and high reliability. Additionally, the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA power range. The Line-interactive UPS is ideal for rack or distributed servers and/or harsh power environments. Over 5kVA, the use of a line –interactive UPS becomes impractical

• The Standby-Ferro UPS was once the dominant form of UPS in the 3-15kVA range. This design depends on a special saturating transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch, through the transformer, and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load. In the Standby-Ferro design, the inverter is in the standby mode, and is energized when the input power fails and the transfer switch is opened. The transformer has a special “Ferro-resonant” capability, which provides limited voltage regulation and output waveform “shaping”. The isolation from AC power transients provided by the Ferro transformer is as good as or better than any filter available. But the Ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection. Even though it is a standby UPS by design, the Standby-Ferro generates a great deal of heat because the Ferro-resonant transformer is inherently inefficient. These transformers are also large relative to regular isolation transformers; so standby-Ferro UPS are generally quite large and heavy. Standby-Ferro UPS systems are frequently represented as On-Line units, even though they have a transfer switch, the inverter operates in the standby mode, and they exhibit a transfer characteristic during an AC power failure.  

The primary benefit of this design are high reliability and excellent line filtering. The limitations include very low efficiency combined with instability when used with some generators and newer power-factor corrected computers, causing the popularity of this design to decrease significantly.

• The Double Conversion On-Line UPS is the most common type of UPS above 10kVA. The design configuration is the same as the Standby UPS, except that the primary power path is the inverter instead of the AC main. The Double Conversion On-Line UPS converts AC power to DC and then converts the DC back to AC to power the connected equipment. The batteries are directly connected to the DC level. This effectively filters out line noise and all other anomalies from the AC power. Failure of the AC Power does not cause activation of the transfer switch, because the input AC is charging the backup battery source which provides power to the output inverter. Therefore, during an AC power failure, on-line operation results in no transfer time.

There are certainly benefits and limitations of this UPS. A benefit is that it provides nearly ideal electrical output performance, with no transfer time. But the constant wear on the power components reduces reliability over other designs. Additionally, both the battery charger and the inverter convert the entire load power flow, resulting in reduced efficiency and increased heat generation. Additionally, the inefficiency of electricity energy consumption is a significant part of the life-cycle cost of the UPS.

• The Delta Conversion On-Line UPS design was introduced to eliminate the drawbacks of the Double Conversion On-Line design and is available in sizes ranging from 5kVA to 1.6MW. Similar to the Double Conversion On-Line design, the Delta Conversion On-Line UPS always has the inverter supplying the load voltage. However, in this configuration the primary power source is blended with power from the additional Delta Converter. As the primary power varies away from its normal value the inverter comes to life to make up the difference. The Double Conversion On-Line UPS converts the power to the battery and back again whereas the Delta Converter moves components of the power from input to the output. In the Delta Conversion On-Line design, the Delta Converter acts with dual purposes. The first is to control the input power characteristics. This active front end draws power in a sinusoidal manner, minimizing harmonics reflected onto the utility. This ensures optimal utility and generator system compatibility, reducing heating and system wear in the power distribution system. The second function of the Delta Converter is to control input current in order to regulate charging of the battery system. This input power control makes the Delta Conversion On-Line UPS compatible with generators and reduces the need for wiring and generator over sizing. Delta Conversion On-Line technology is the only core UPS technology today protected by patents and is therefore not likely to be available from a broad range of UPS suppliers.

The benefits of Delta Conversion On-Line UPS include high efficiency, excellent voltage regulation, and overall reduction in life-cycle costs of energy in large installations. It is impractical in installations under 5kVA.

Availability about Power

          In this post, we will continue with the discussion of power in the data center by specifically addressing aspects of power redundancy including availability, uninterruptible power supplies , system design configurations, dual-corded equipment, generators, and finally a summary.

          A key element relative to all data centers is the need for power. In most countries, the public power distribution system is fairly reliable. However, studies have shown that even the best utility systems are inadequate to meet the strict operating needs of critical nonstop data processing functions. Most companies, when faced with the likelihood of downtime, and data processing errors caused by faulty utility power choose to implement a back-up strategy for their mission-critical equipment.

          These strategies may involve the inclusion of additional hardware such as Uninterruptible Power Supplies (or UPSs) and generators, and system designs such as N+1 configurations, and dual-corded equipment. This course will address various strategies to consider when planning for redundancy in the data center. In our rapidly changing global marketplace, the demand for faster, more robust technologies in a smaller footprint is ever-increasing. In addition, there is a further requirement that these technologies be highly available as well.

           Availability is the primary goal of all data centers and networks. Five 9’s of availability of a data center is a standard most IT professionals strive to achieve.  Availability is the estimated percentage of time that electrical power will be online and functioning properly to support the critical load. It is of critical importance, and is the foundation upon which successful businesses rely. According to the National Archives and Records Administration in Washington, D.C., 93% of businesses that have lost availability in their data center for 10 days or more have filed for bankruptcy within one year. The cost of one episode of downtime can cripple an organization. The availability of the public power distribution, while sufficient for many organizations, is ill-equipped to support mission-critical functions. Therefore, planning for redundancy, or the introduction of alternate or additional means of support is a necessity. Redundancy can be thought of as a safety-net or Plan B should power utility fail, or be inadequate. One of the ways in which to increase data center power availability is through a UPS.

About Power Distribution Components

          

              Electricians often refer to one line diagrams. One line can be very simple to very complex. At a minimum, it should illustrate the primary electrical components of the electrical system and illustrate how they link and interact with each other. This one line lets us see how electrical power is distributed in the data center from a server plug to outlet strips to Power Distribution Units (PDU) to UPS and bypass to Automatic Transfer Switch to the primary power source (Utility) to the emergency power source (Generator).

               The utility provides the primary electrical power source for the data center. Ideally, multiple utility feeds should be provided from separate sub-stations or power grids. While not essential, this action will provide back-up and redundancy. An emergency, back-up power source, in the form of a generator, can be positioned to bear the load of both data center components, as well as all essential support equipment, such as air conditioners, in case of power disruption.

               A circuit is a path for electrical current to flow. A branch circuit is one, two, or more circuits whose main power is connected through the same main switch. Each branch circuit should have its own grounding wire. All wires must be of the same gauge. An uninterruptible power supply, or UPS, is a device or system that maintains a continuous supply of electric power to certain essential equipment that must not be shut down unexpectedly. The UPS equipment is inserted between a primary power source, such as a commercial utility, and the primary power input of equipment to be protected, for the purpose of eliminating the effects of a temporary power outage and transient anomalies. An automatic transfer switch is a switch that will automatically switch the power supply from one power source to another, in case of power disruption or bypass mode. For example, if the utility fails, the automatic transfer switch would immediately switch to UPS or generator power.

             A Power Distribution Unit (PDU) is a device that distributes electric power by usually taking high voltage and amperage and reducing it to more common and useful rates, for example from 220V 30A single phase to multiple 110V 15A or 110V 20A plugs. It is used in computer data centers and sometimes has features like remote monitoring, and control, down to plug level. (Please note: In many countries, such as in parts of Europe and Asia, voltages such as 220-240V and 400V are also common.) An outlet strip is a strip of sockets which allows multiple devices to be plugged in at one time, and usually includes a switch to turn all devices on and off. In a few cases, they may even have all outlets individually switched. Outlet strips are often used when many electrical devices are in close proximity, especially with audio/video and computer systems. A server plug is the power plug or other type of electrical connector which mates with a socket or jack, and in particular, is used with electrical or electronic equipment in the data center.

To summarize, let’s review some of the information :

1. Power infrastructure is critical to the uptime of any data center.
2. Understanding basic power terms helps to better evaluate the interaction between the utility, standby power  equipment and the load.
3. Failures can occur at various points in the power infrastructure, but special care should be given to the condition and coordination of circuit breakers.
4. Numerous power anomalies exist that can impact the uptime of data center equipment.
5. Understanding the threats and applying practical power solutions such as uninterruptible power supplies and generators can help to minimize the risk.