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July 26, 2009
July 15, 2009
A Requirement for Redundancy
On Friday the residents of Taipei were left stranded when their newly built $1.6 billion transit system shut down. It seems that the cause of this, was due to the failure of the Uninterruptible Power Supply (UPS System) at one of the stations. This UPS System controlled the signalling and communication functions and without power to these systems the trains could not operate.
Now UPS Systems are designed to increase the reliability of connected systems, by providing power protection and back up power. If the UPS System should fail then it should revert to bypass and allow utility mains power through (this is for online double conversion Uninterruptible Power Supplies and particularly three phase systems). It’s unclear what happened in this case, but whatever the outcome the critical systems were left without power and the UPS System is perceived not to have done its job.
Here’s where redundancy is required. Basically, you have one more system than you actually need, therefore if one system should fail, the other can take over without any loss of UPS System support. Should the utility fail for an extended period, then you either need batteries to keep you going (or shut down gracefully) or an external generator to kick in and simulate utility power.
Redundancy is usually expressed as ‘n+1′, which means that if you need ‘n’ UPS Systems to power your load, then you install ‘n+1′. For example, if you have a 100KVA load, you can achieve this with 1x100KVA UPS System. If you want redundancy you will need to use n+1 = 1+1 =2 UPS Systems, i.e. 2x 100KVA UPS Systems. Alternatively, you may have achieved your load by using 2x50KVA UPS Systems in parallel, and redundancy can be achieved by installing an additional 50KVA UPS System i.e. 3x50KVA UPS Systems, which may be more cost effective.
I’ll blog more about this later, as the citizens of Taipei walk to work.
You can read about the Taipei metro power cut here
July 13, 2009
Overvoltage Protection
Here’s a neat article written by our friends at APC: Powercuts during summer months may damage data, albeit a confusing title for what the article is about.
They bring up a valid point about overvoltage leading to damage of equipment. Your normal mains supply is designed to operate at 230V±10%, which means a maximum voltage entering your building of 253V. However, the substation providing this voltage has to be able to do so during full power loading. Let’s say you’re on an industrial park and the substation is providing power to all the buildings – the IT infrastructure, the telecom systems, the lighting, the air conditioning, elevators, escalators etc. The load can be quite substantial, but let us take a figure of say, 1000Amps (equivalent to 10 houses). If the impedance on that line was half of one tenth of an Ohm – 0.05Ω the voltage drop across the cable using good old Ohm’s law would be 50V. This means that the substation needs to set its output voltage to around 280V so that when the power reaches your building it is 230V and within limits. However, if the load is suddenly removed – all the air conditioners are switched off, the buildings are empty and nobody is at home, all of a sudden you are hit with 280V, as the lower current causes less voltage to be dropped across the supply cables.
Some people call this a surge and think that surge suppression devices will protect them against it. In fact, this is not a surge but rather a voltage swell or overvoltage condition. (A surge is an overvoltage condition too, but of short duration -usually µseconds), and in order to safeguard your equipment you need to have some form of overvoltage protection. The only way to achieve this is by the use of either voltage regulators or by the Uninterruptible Power Supply (UPS).
A voltage regulator is a (usually mechanical) device that incorporates a tap changing, or continually variable transformer to keep the output voltage to a tight tolerance.
The Uninterruptible Power Supply, however will also provide overvoltage protection by keeping the voltage within limits. How well it does this depends upon the type of technology used:
- The Offline Uninterruptible Power Supply will provide overvoltage protection by dropping to battery as soon as the mains voltage is out of limits. This will protect your equipment but if this happens regularly or for prolonged periods, the UPS battery will drain and you will lose power.
- The Line Interactive Uninterruptible Power Supply will provide overvoltage protection by incorporating some voltage regulation. When the mains goes to high, the UPS System will “buck” the voltage downward by changing taps on a transformer. This has the benefit over the Offline UPS System in that there is no dropping to battery for marginal overvoltage conditions.
- The Online Uninterruptible Power Supply, (aka Online Double Conversion Uninterruptible Power Supply) provides the best possible overvoltage protection. It has a very wide input voltage window, which means it can take very high voltages (as well as very low voltages) without reverting to battery. What’s more the voltage supplied to your system is constant and unchanging regardless of what is happening to the input voltage.
It’s another string to the Uninterruptible Power Supply bow, as not all power problems are as obvious as the power cut. Give your equipment overvoltage protection with a Uninterruptible Power Supply from UPSMart.
July 8, 2009
Where just a small letter can have such a big difference
Scouring the web I came across this press release from Chloride North America announcing the launch of their Agility Uninterruptible Power Supply (UPS).
I was intrigued, as the release states that they have five high density models ranging from 750kVA to 3000kVA, all in a “standardised 2U chassis”!
Wow, Chloride must have discovered a working cold fusion reactor and superconductivity at room temperature in order to cram 3MVA of power into a 2U rack space. The worlds power problems are solved!
Unfortunately the unit specs tell a different story. Somebody has put in the letter “k” in front of VA by mistake. This means the sizes are out by a factor of 1000, 750,000VA instead of 750VA, and 3,000,000VA instead of 3000VA. A bit of a difference!
For their benefit I’ve included a few of the common SI units used in engineering today for reference:
G: Giga = x 1,000,000,000
M: Mega = x 1,000,000
k: Kilo = x 1,000
m: milli = ÷ 1,000
μ: micro = ÷ 1,000,000
n: nano = ÷ 1,000,000,000
July 7, 2009
How To Build An Uninterruptible Power Supply
I saw this crazy article about how to build your own Uninterruptible Power Supply. I’ll not put a link to it as I think it is possibly the most irresponsible thing we could do. It came complete with warnings such as:
- The AC Output Voltage Can kill You
- The DC Current from the battery can burn you
- There is enough DC current in a battery bank to stop your heart
- Shorting batteries can cause blinding flashes, blow wrenches into splinters, even cause the batteries to explode and spray sulphuric acid and hunks of plastic everywhere.
And my favourite:
- A ring that gets between ‘hot’ wires can amputate your finger
An Uninterruptible Power Supply consists of three main stages:
- A source of power – usually a battery.
- A Battery Charger – to charge the batteries
- An Inverter – to convert DC power to AC power
This article is proposing that you buy all the bits an pieces and assemble them yourself. And while you’re on, make sure that you’ve selected the correct type of battery, make sure that the battery charger is adequate for the battery bank, ensure that the inverter is adequately rated, then connect it all together with heavy duty cable, and there you have it – your very own Uninterruptible Power Supply.
Why would anybody in their right mind do this? Never mind the inherent danger with somebody with a little knowledge putting high voltage AC and DC together, but you can buy off-the-shelf Uninterruptible Power Supplies with all the design worked out at a fraction of the price that it will take to build your own.
Another misconception is that people assume that you can just use car batteries for the UPS. There’s two drawbacks for this. Firstly a car battery is not designed for deep cycling – that is constant charging and discharging. It is designed for providing instantaneous power to crank your engine. A UPS battery is designed to be discharged slowly. Secondly, no appreciation of the voltage conversion has been taken into account. For example, I often get asked for a UPS to power an entire house. What is the power consumption of an entire house? Well, excluding things like electric showers and electric ovens, and just covering some lighting and some electronic systems we could probably get by with 3KVA or so. 3KVA is about 13 Amps of current at 230V, but to generate this from a 12V battery you will need to draw 250A! This is why 3KVA UPS systems usually have a DC battery string of 96V.
My point is, is that articles like this which may be of interest to the hobbyist are actually giving people impractical and dangerous advice. People who may be a dab hand at wiring a few things together probably do not understand the complexities of the Uninterruptible Power Supply and how the individual parts should fit together to produce a fully functioning system that provides you with the backup system that you require, and probably at a fraction of the cost they would otherwise spend.
June 23, 2009
Now Wimbledon’s here
Well it’s now officially summer and of course Wimbledon has started and that can mean only one thing -rain. And lots of it. It’s not the rain we are concerned about though, it’s the lightning storms.
I love watching the lightning. As soon as a storm brews I’m at the window or the front door watching for the flash and waiting for the rumble. Once I was lucky enough to witness a fairly local strike, which took your breath away.
No wonder too. The average lightning strike heats the air to three times the temperature at the surface of the sun and packs an instantaneous hit of power of about one terawatt. That is 1,000,000,000,000 Watts. Wow. Your surge protection strips, rated at 2000 or 3000 Joules, pale into insignificance compared to an actual lightning strike of about 500 Million Joules of Energy, which means you’ll need about 166,000 to cope with it.
Of course, practically nothing except for lightning conductors cope with a direct lightning strike. Over 30,000 Amps can flow which will fry all but the most lowest of impedance conductors. What’s more, is that the extreme electric fields generated induce voltages onto telecom and power lines. These can be damaging and is what your surge strips and Uninterruptible Power Supply or UPS systems will protect (to a greater or lesser extent) you from.
Power lines are particularly susceptible, and lightning strikes to these cause protection relays to activate, momentarily cutting power, as well as sending all sorts of power line transients straight into your electronic systems.
So, unless you want to have to unplug everything at the first sign of dark clouds overhead, it makes sense to connect a UPS to your computer, or home entertainment system. With the prices of an entry level system here at UPSMart being about £30 it’s completely crazy not to do so.
June 2, 2009
Lost in Translation
This is a pretty good article on how to make an informed choice on your UPS System: New UPS Industry Prices Offer Generous Rewards to Savvy Buyers.
Unfortunately, the same people are responsible for this: Uninterruptible Power Supply – 7 Points to Smart Cost Effective UPS Selection.
If ever there was a reason never to use an automatic translation system this surely is it.
April 10, 2009
Local Vs Centralised UPS
I read this article about an Australian University to Save £200k by a variety of energy saving schemes, one of which was to use local UPS systems instead of a central UPS system.
There are course, pro’s and con’s for each approach, but I hadn’t considered the efficiency angle before. When you look at it as a whole, then there is no way lots of individual UPS systems can be more efficient than one big one. Most centralised UPS systems will operate at no more than 50% load (due to redundancy -so if one UPS fails the other can support 100% load), and this is where a lot of efficiency is lost. Most UPS systems will be more efficient at full load than at half load (or less).
With point of use UPS systems, if you wanted to maintain redundancy, the same effect would occur. You would have two systems running at half load. Since each UPS system also requires its own onboard controller, you would think that this power loss would add up throughout the data center, in order to make the data centre less, rather than more efficient.
However, the real gain with using local systems is that you can size them exactly. With a centralised system you need to define what the maximum power consumption will be now, and at any time in the future and put in the according UPS (or opt for a modular system – but this is another blog entry). It is likely therefore, that in most early data centres, the centralised UPS are running no where near their 50% loading, whereas with local point of use systems you can just add systems as and when needed, thereby ensuring that you’re not wasting power by not having the UPS operating at its sweet spot.
We’ve actually used this approach for a customer recently. He has a small computer room, that has been built up over the years and has no overall UPS support. We’ve gone in to help and look at the options. The simplest approach seemed to be to put in a 10KVA UPS and wire this in to the existing infrastructure. This would give him the UPS support he needed. However, as his data suite was provided power by several circuits we would need to run in a new power feed. We would need to add PDU’s at the output of the UPS. We would then need to wire these into the existing circuits. All of a sudden, the actual cost of the UPS started paling into insignificance with the added installation costs.
As a result, we looked at individual UPS to fit into each rack and power the server and associated equipment individually. All of a sudden the numbers started to make sense. The KR1000J is more than enough for his servers, and occupies only 2U of rack space. So the customer has opted for individual UPS systems, saving an astounding £5,000 on a centralised system!
April 2, 2009
New Improved Selection Guide
Our product list is now so comprehensive that we feel it’s a little difficult for the novice to know exactly which UPS meets his requirements. We’ve now improved our UPS selector tool and also placed it right on the shop home page.
The biggest change we’ve introduced is that we can now identify which UPS will meet your runtime requirements for any load. Of course we’ve had to introduce a number of approximations to do this, but we feel the result is a very good guess.
We’ve also added a new feature, that will highlight the UPS which is the best price, the one that gives you the best performance, and the one that provides the best value in terms of technology and price.
Note that not all products will show up on the selector, as we need to programme in certain details of the UPS and this takes time, however we do have Powerware, Kehua and Opti-UPS all present. Although we’ve tested this, it is inevitable that we may have made a mistake or two. Please feel free to let us know if you find any.
Full version of the selector is available at UPSMart.co.uk/shop/bo-selector.php
February 23, 2009
Interactive UPS Demo
We’ve tried several times to explain the difference between the different type of UPS but it just gets too technical with diagrams and really boring text.
What we need is a demonstration, where you can see the effect of each of the different types of UPS. OK simple, what you need is an oscilloscope, some decoupling device, a variac, a multimeter. Oh, and of course one of each type of UPS.
Or you can have a look at this flash file on UPS operation. It’s cool!
UPS Operation Screenshot