The Bits 2.0

In Malaysia where utility power is transient and noisy, protection of sensitive equipment such as servers, personal computers and peripherals is a must. Normally, we will use a voltage regulator to handle the job but nowadays it seems like blackouts and browouts are getting more common. The answer to the problem is a UPS but unfortunately, most Uninterruptible Power Supplies (UPS) sold in the market have very misleading specifications.

There are six basic UPS topologies:

• Standby
• Line Interactive
• Standby on-line hybrid
• Standby-Ferro
• Double Conversion On-Line
• Delta Conversion On-Line

Since On-Line UPSs are very expensive and we normal consumers will never need them, I will not talk about it. Standby Ferro UPSs are outdated and no longer produced.

Standby UPS

The most commonly sold UPSs to consumers are standby UPSs due to its cheap design and easy implementation. Standby UPS contains a sealed lead acid battery, a battery charger, an inverter and a transfer switch. Most units include power supply line filters and surge protectors. During normal usage, the transfer switch causes the load to draw power from the utility line, bypassing the battery. When a blackout occur, the transfer switch will switch the load over to the battery.

The fundamental problem with standby UPS is slow transfer time and inadequate protection. Since power is drawn directly from the utility line, line noises might still persist unless a high quality power filter is installed. Standby UPS might also suffer from unneccassary battery-hand over which causes the battery to fail prematurely. The transfer switch for standby UPS is normally a magnetic relay switch which could introduce 4~6ms of delay transfer delay.

Unless otherwise stated, UPS sold in the range of 400VA – 700VA are predominantly of this topology.

Line Interactive

Unlike standby UPSs, the battery of a line interactive UPS is always connected to the inverter (and then to the load). Therefore, transfer time is usually superior to standby UPS (2~5ms) and some advanced silicon rectifier based units might even have instant transfer times.

Line interactive UPS solves many utility power problems, including power noises and transient voltage. Line interactive power supplies provide nearly constant output frequencies and voltage, though inferior to a microprocessor controlled ( and more expensive ) Delta Conversion Online UPS. This makes LI UPSs ideal for entry level server environment or laboratories.

The major drawback of LI UPSs is that since the inverter and battery operates whenever there is a load, the components’ lives are greatly reduced (compared to standby UPS). Also due to its complexity, a LI UPS costs alot more than standby UPS.

UPS sold in the range of 600VA – 1000VA are normally line interactive. Some manufacturers might misleadingly lable it as “online”.

Here’s a good guide from wikipedia:

Choosing a UPS
The first question to ask when choosing a UPS system should be: is this unit going to be placed inside a controlled environment? If the answer to this question is yes, choose an indoor UPS. If the answer is no, choose an outdoor UPS. It doesn’t matter that you choose the right topology, power, backup time, etc. If you place an outdoor UPS in a controlled environment, you might be wasting money (exceptions to this could be found when powering small loads during an extended period of time, where outdoor UPS systems are sometimes the only available option). If you place an indoor UPS in a non-controlled environment, the useful life of this system will be considerably shortened, threatening the integrity and backup of the equipment you are protecting with the UPS.

Besides choosing a UPS design, there are 2 key ratings to be aware of when choosing a UPS unit. The first is the load rating, expressed as both volt amps (VA) and watts (W). Both the ratings represent the maximum amount of load that the UPS can support and the connected load typically should not exceed 80% of either. Special considerations must be made when connecting certain equipment such as printers or any type of motorized load.

The second factor in deciding which unit to purchase is the amount of runtime the unit will be able to provide when the power fails. This number will vary with the load amount that is plugged into the UPS. For example, a unit may run a single computer for 30 minutes, but with 2 computers it will generally last less than half that time. Larger units typically can provide more runtime for the same load than smaller units, however that is not always the case. Some UPS units are designed to provide extended runtime or have the ability to have external battery packs connected.

Another consideration is the anticipated usage. If the UPS is only intended to provide enough power to gracefully shut down the computers, serial or USB ports on the UPS and support software are essential. If the purpose of the UPS is to provide power until a standby generator kicks in (typically under a minute), the UPS input capabilities should be matched to the generator outputs. Specifically, most standby generators made for home use (15 kW or less) and most portable generators lack microprocessor voltage-and-frequency control and may not create a smooth sine wave. This can result in voltage and frequency fluctuating by 5% or more. While most UPS systems handle voltage fluctuations gracefully, most do not handle frequency fluctuations well. A UPS with a wide “frequency window” is essential in such cases. However, this can double the cost of the unit. Only a double conversion UPS can deliver a stable output frequency when powered by an unstable input frequency.

If the UPS needs to be quiet when running from battery, or will power AC motors (as found in air conditioners and fans) or other equipment requiring a clean sine wave (such as high-end computer power supplies), a UPS that outputs a smooth sine wave is needed. For some other uses, a block or quasi-sine wave waveform[5] is acceptable. UPS systems with square wave, or “simulated”, “approximated” or “stepped” sine wave output do not give smooth sine waves. In fact, their output voltage contains a lot of harmonic distortion. This is why this type of output exists only in small power ratings standby UPS, where electrical noise and excess heat in wiring is generally not an issue.

Another consideration should be based on the type of load or connected equipment the UPS will support. If the UPS is connected to “mission critical” equipment or sensitive electronics (like lasers), a rotary solution will be more suitable with 100% line to load isolation. This would not only protect the equipment from a power outage, but will also protect the connected equipment from any anomaly that comes from the utility feed.

Features to look for:

• Output frequency regulation within 0.5% (prevents connected equipment from over heating)
• Electromagnetic interference (EMI) AC noise suppression (noise filtering)
• Reasonable cost for replacement batteries
• If energy efficiency is important avoid “Standby On-Line Hybrid”, “Standby-Ferro”, and “Double Conversion On-Line” topologies
• If the UPS outputs a sine wave, a high quality unit will feature a voltage regulating transformer
• If the UPS outputs a square wave, a high quality unit will use Pulse-width modulation (PWM)

For more information, read this whitepaper from American Power Conversion, a leading UPS manufacturer:

ups-systems.pdf

This article originally appeared in Bits of Bytes (forefather of TheBits.info) on 5 July 2007.

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