There are 3 main UPS technologies – Offline, Line Interactive and Online Double Conversion – and two different types of inverter – square wave (also called pseudo-sine wave or modified sine wave) and sine-wave.

An Offline UPS provides basic levels of power protection. There is usually a degree of surge suppression incorporated and when the input mains voltage goes out of tolerance (that is too high/low or fails) the UPS inverter starts up and provides power to your equipment. There is a break in the mains supply when this occurs in the region of 10-20 thousands of a second which will generally go unnoticed by the majority of equipment. The inverter in offline UPS is nearly almost always a square wave.

A line interactive UPS is similar to an offline UPS but has the added benefit of voltage regulation. This means that it will reduce the mains voltage if it goes too high (called “buck”) or raise the mains voltage when it goes too low (called “boost”). It will do this without reverting to battery and hence conserve battery life. Since a line interactive UPS has an inline transformer, it also provides in-built filtering and hence a higher degree of power protection. Like the offline types, there is also a break during the transition from mains power to battery power. In higher quality line interactive units, this break may be as low as only 2 to 4 thousandths of a second. Line Interactive UPS Systems come with square wave as well as sine wave inverters.

The online double conversion UPS System is considered by many to provide the highest levels of power protection. The inverter is always on, and provides power to the load. This means that there is no deviation in output voltage and no break if the UPS reverts to battery power. The output waveform in an online UPS System will almost always be a sine-wave, generally of very high quality and can enhance the operation of certain equipment. An online double conversion UPS also has a bypass circuit, which allows power to be continually fed to your load even if the UPS develops a fault or is overloaded.

In choosing your technology you need to consider:

1. The power consumption of your load
2. The type of load you have
3. How critical your load is
4. The environment in which the UPS will sit
5. The required runtime
6. Your budget

Your normal UK socket outlet is rated at 13Amps which means the most power you can draw from a UK outlet is 3KVA, or 3KW. (Power Measurement will be covered in a separate paper). Above this level (for specialist equipment, or hardwired installations) most UPS will be online double conversion technology as the economies of scale start making other technologies non cost effective. Below this level, however all technologies are available, although above around 2KVA, line interactive systems start to become heavy and unwieldy due to the size of transformer that is required.

If your load is made up of computer type power supplies, then both square wave and sine wave products will power this equipment adequately. If your load contains motors, transformers, pumps or other inductive components (on the input power supply) then square wave systems are not suitable and you must opt for a sine-wave system.

Any load that is susceptible to mains disturbances such as in analytical equipment or audio applications should also choose a sine-wave system. Where mains distortions affect equipment performance then you need to opt for online double conversion where a pure sine-wave is always present.

If your load is critical for operation then the increased benefit of online double conversion technology should be used. This provides additional security against all power disturbances as well as the comfort of a fall-back bypass in case a fault develops with the UPS. You may wish to parallel together UPS and again, this can only be achieved with online double conversion UPS. (We’ll cover parallel systems and redundancy in a separate paper). If you have a PC where you are looking for simple battery backup to stop annoyance rebooting or tripping then an offline or line interactive unit would suffice.

Offline and line interactive UPS Systems are generally quiet in operation and do not utilise cooling fans in normal operation (usually). This means they are suitable to be placed in an office or home environment. Online Double Conversion UPS systems require forced cooling and can be quite noisy and therefore tend not to be suitable for use in an office environment.

If you are running your UPS in conjunction with an emergency backup generator then the benefits of online double conversion technology come to the fore, as the UPS will cover for any transition to generator operation and also provide a constant unchanging power source during generator start up and any shifts that may occur in frequency and or voltage during load switching.

Offline UPS tend to be the smallest of all technologies so can be useful to provide emergency ride through for areas where space is at a premium.

Long runtimes on UPS are better handled with online UPS Systems. This is because they are designed to operate continually from the inverter. In which case it does not matter if the input power is from the rectifier or the battery – the operation is the same. Offline and Line Interactive units tend not to be designed for this continuous operation. (Battery selection and runtime will be covered in a separate paper.) It is also unwise to power any equipment with a square wave inverter for any prolonged period of time as you could cause degradation to electronic components. Square wave systems are designed to allow basic computer systems to ride through brief power outages or time to shut down a system in the event of a prolonged power cut.

You may have already made up your mind which technology you need, but let’s take a look at relative costs. Offline UPS systems used to be by far the cheapest. However nowadays an offline UPS and a line interactive square wave unit are typically similar prices. Online Double Conversion used to be far more expensive than other technologies, but now is comparable to a high quality sine-wave line interactive UPS.

With regard to running costs, the offline UPS is the most efficient as in normal operation the input power goes straight through to the load, so the only power loss is to keep the battery float charged and power the UPS logic. Line Interactive units are similar in efficiency but experience more losses due to the transformer required for the buck and boost facility. Online Double Conversion however, has the drawback of being the least efficient of the technologies.

In summary, for simple low cost battery backup then the offline or line interactive square wave systems are suitable. For high grade protection in a quiet environment, or where your load type demands it, opt for a sine-wave line interactive unit. However, if you want the best power solution, then we recommend online double conversion technology as it is compatible with all load types and provides the highest degrees of power protection.

The first thing you need to know is that battery discharge is non-linear. For those of you who don’t understand the expression I’ll elaborate. A linear expression is one where, for example, you put two in, you get four out. So it follows that if you put three in, you get six out, or you put five in, you get ten out. EG. whatever you put in  you get twice out. In the non-linear world this doesn’t hold true, for example, you put two in, you get four out, but when you put three in, you get nine out etc.. This non-linearity makes the discharge characteristics very difficult to express mathematically.

Luckily, the battery manufacturers provide us with discharge tables that we can look up, but first we need to know some information about the UPS, the load and about the method.

End of Discharge Point

At what point will the UPS switch off? Your normal 12V lead acid battery contains 6 cells each of a nominal voltage of 2V (so you get 6x2V=12V). In practice the voltage is slightly higher than this and reduces as the battery is discharged. It is important not to allow the battery to become too discharged, so the UPS will monitor the cell voltage and cut off when it gets to a predetermined point. This is usually around 1.7V per cell or 10.2V for 12V battery.

UPS Efficiency?

Well, more precisely, the inverter efficiency. The inverter is used to convert the battery DC power into AC power. There will be losses associated with this. The better the inverter, the lower these losses are. If you’re unsure, use a worst case of say, 80% efficient. This means that for every 100W provided to the load, the batteries will need to provide 125W (simply 100/efficiency).

The Load Power Factor

Is the load purely resistive, or does it have a power factor? We’re only interested in the amount of WATTS that are needed.

Amps or Watts Method?

Firstly, there are two methods for calculating runtime, the Amps method, or the Watts per Cell Method. Generally, Watts per Cell is used for short term discharges and Amps is used for long term discharges.

Process

It’s easier to do this with an example, so let us take a standard server type load with a number of ancillary devices. We know from measurement  that the Ampere draw is 7Amps and we have mains voltage of 235V. Our Watts therefore (assuming unity power factor) is 1645W.

Our UPS has an inverter that is rated at 90% efficient, so the amount of power from the battery required to deliver 1645W is 1645/0.9 = 1828Watts.

Our UPS is a 3KVA, 2.1KW UPS that contains 8 batteries connected in series. Each battery is rated at 12V 7.2 Ah.

 I now need to look up the manufacturers data sheet and I find the following for a 7.2Ah battery:

Constant Power Discharge Method 

Our battery load is 1828W and we have 6×8=48 cells. Therefore our Watts per Cell is 38Wpc.

We know the FV (Final Value or End Of Discharge Point) is 1.7Vpc so looking along the Constant Power Discharge Table, we can see that 52Wpc would give 5 minutes, so we will get more than 5 minutes runtime. We can see that 35.16Wpc would give 10 minutes runtime, so we will get less than this.

So the calculated runtime for this example, based on constant power discharge is between 5 and 10 minutes.

 Constant Current Discharge Method

We have a total of 8x12V batteries in series, giving us a string voltage of 96V. We need to deliver 1828W so our Amperage is 1828/96 (from Power=VoltsxAmps, so Amps=Power/Volts). = 19Amps.

Now looking along the table above in the Constant Current Section, with our FV of 1.7, we see that a 26A discharge would give us 5 minutes, so we will get longer than this. A 17.6Amp discharge would give 10 minutes, so we will get less than this.

Therefore using the constant current discharge method we will get between 5 and 10 minutes runtime.

Working it out in reverse – I want 2 hours runtime – how many batteries do I need?

Using Watts Per Cell Method.

From the table under Constant Power Discharge, we can see that for our FV of 1.7, for a 2 hour runtime we need to have a WPC discharge of no more than 6.05. Our load is 1828W, so we need 1828/6.05 = 302 cells, which is 50.35 batteries. The battery requires a 96V string voltage, based on banks of 8, so we will require 6 banks to get close (that is 48 batteries), or 7 banks to be sure (that is 56 batteries).

Using Constant Current Method.

From the table above under Constant Current Discharge, you can see for our FV of 1.7V we need to have no more than a 3.05A discharge from each of our batteries to achieve a 2 hour runtime.

Our total current draw is 1828/96 = 19Amps (1828Watts load/Battery String Voltage = 96V)

Dividing the 19Amps total current by 3.05 gives us the number of strings needed to achieve 2 hours runtime which is 6.24. Obviously we cannot add in a quarter of a string so we need to round up. In this case we require 7 battery strings, or a total of 56 batteries to achieve a runtime of 2 hours.

Alternatively, you could of course opt for higher capacity batteries, and maintain the same number of batteries. The examples above were using 7.2Ah lead acid batteries but there are other choices available.

This is bad enough for many people, especially when used for work and you’ve just lost all the data on that report you’d been writing. But did you know that mains fluctuations can cause damage to your hard drive?

A common effect is to hear a repetitious clicking noise coming from your hard drive – the click of death (like the blue screen of death but more fatal if there’s such an analogy). This renders your hard disk inoperable. What does this mean?

Well, your PC cannot function. You cannot access any data that you have on it without specialist services, and you need to replace your Hard Disk Drive (HDD), it cannot be repaired.

The cost of a new hard drive is relatively cheap these days. Probably around the £30 for a no-frills device. But then you have to fit it, and reload all your software. Assuming you’ve got all the CDs/DVD’s, and your access codes for downloaded software, plus you backed up all your important memories from your digital camera (you did didn’t you?), then this should be a breeze. Only taking a full day or so. It’s no laughing matter loading a PC from scratch. You get used to all your settings, software, and although sometimes its good to have a purge you’ll be surprised how long this process can take.

If you needed to recover data from your hard drive, well that’s a different story. You will be charged anywhere upwards of several hundred pounds to have data recovered – if it can be recovered.

To avoid these issues you need to invest in an Uninterruptible Power Supply. These will allow you to work through brief power cuts and shut your computer down (after saving your work), if the outage is longer than a few minutes. Some people provide a degree of power protection for their PC’s by using surge suppressors. Note that these will NOT protect damage to your HDD because of power fluctuations. A surge strip can do nothing to protect against mains voltage variations and power cuts.

How much is an Uninterruptible Power Supply? It depends upon the level of protection you require and the amount of runtime you need, but a basic standard system can be bought for under £30 (plus VAT). Power Inspired’s VIX series Uninterruptible Power Supply fits the bill for a home office / small office PC and is such a low cost it’s a no-brainer to me.

Firstly – expandability. Let us suppose you are developing a data room. The plan is to eventually have, for example, 25 cabinets, each with a power consumption of 3KVA = 75KVA total load. However, at present you only need power for 5 (15KVA), with the remainder being added over the next few years or so.

The sensible approach using the standard Uninterruptible Power Supply would be to fit an 80KVA model. However in the early days it would only be operating at less than 20% capacity. So you’ve shelled out for an 80KVA system that wont be at capacity for a couple of years. For an 80KVA system (excluding battery and installation) you’d be looking at a cost in the region of £8,000, depending on options.

With the Modular UPS, you would fit a 100KVA carrier, and 2x10KVA Power Modules at a cost of around £6,000. You can then add the additional 10KVA power modules as and when required at around £1,500 each.

The benefit here is that the initial outlay is lower, however the total cost will be higher, as you need to add in another 6x 10KVA Power Modules units, making the total cost £15,000 as opposed to £8,000 for the standard Uninterruptible Power Supply.

However, let us now suppose that we want a n+1 redundant solution. So with our standard Uninterruptible Power Supply model, we would put in 2x80KVA UPS Systems, at an upfront cost of £16,000. With the Modular UPS we can put in the 1 extra power module that we need, so our initial upfront cost is 1x 100KVA carrier, and 3x 10KVA Power Modules at a cost of around £7,500.

However, the real benefit is to do with the fact that to achieve n+1 we only need 90KVA of UPS power, as opposed to 160KVA in the configuration above. When the data centre is fully operational we would require 1x 100KVA carrier, and 9x 10KVA Power Modules at a cost of around £16,500. So, slightly more expensive but in an equivalent ball park, however other important factors are that the Modular UPS is in one cabinet with a small footprint, occupying probably half the space of the 2x 80KVA Standard UPS Systems and the fact that the power modules can be easily swapped in the event of a fault – thereby improving on availability figures.

It would be remiss of me however, not to include a third scenario. N+1 Redundancy is achieved by having one more Uninterruptible Power Supply than is needed to do the job. Therefore, it is possible to use, for example 3x40KVA UPS Systems, or 4x30KVA UPS Systems, that too, can grow with demand. If we take the latter, we would need initially 2x30KVA UPS Systems at a £6,000 outlay. You can add another for another £3,000, and then finally have the last in, at a total cost of £12,000. Of course, this price excludes batteries and installation. However, in this instance you need to have room for 4 UPS Systems!

I have also not included the additional costs of switch gear needed for the standard Uninterruptible Power Supply Solution. So, taking this into account, along with the additional floor space needed, you would have to argue that the Modular UPS would be a good solution.

There is another factor that gives the Modular UPS a wholesale advantage over other methods and that is efficiency. Let us assume for a moment, that the Modular UPS and the Standard Uninterruptible Power Supply, all share the same efficiency at full load. It is clear that UPS systems operating at half load or less will be less efficient. With 2x80KVA UPS Systems on a 75KVA load, each UPS will be operating at 47% load, whereas the Modular UPS with 90KVA of power available, will be operating at 83% load. So there is probably some running cost calculation that you could also take into account.

Money makes the world go round as they say, so if I were looking for simple UPS support, I’d opt for the standard Uninterrupibtle Power Supply, however if I was needing to include some redundancy in there, the Modular UPS is starting to look like a great contender.

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.

Now SNMP adapters have enabled the UPS to be connected to your network and can be monitored and even controlled by anybody who has access to that network. Here’s where a lot of the text has been about. Not about the ability to be able to monitor the UPS, but rather the security issue it poses.

A network managers worst nightmare

Many systems come with default security settings which apparently many people overlook when setting up the system. What happens is that any hacker with a bit of UPS knowledge gets into your network and not only does he try to plant a few viruses hear and there or extract some data, decides to switch everything off just for the hell of it. A nightmare scenario for any network manager.

I’ve even heard of some clients who are so scared of the possibility that they refuse any communications with the UPS at all, instead relying on the automatic starting of the gen sets in case of mains failure and visible and audible UPS alarms. This seems a bit unnecessary as monitoring the UPS by a web browser is a useful tool and can be secure provided the correct protocols are followed. Just remember that once SNMP is enabled the UPS is part of the network and your network security protocols should cover for this.

What they do is basically refit the power supply with a battery, intelligent charger and some DC/DC conversion. This makes perfect sense, as you gain efficiency by removing the DC-AC inverter stage as required by all other UPS systems, raising the efficiency from low 90′s to over 99%.

The trouble with this however, is that you can’t actually post fit. You need to have your server built with this technology incorporated at the beginning, and Google custom build their own servers in any case.

There is one big drawback however, and that is they’ve completely ignored power quality. It’s all very well making systems more efficient, but to do so at the expense of power quality seems false economy to me.

An Uninterruptible Power Supply does more than provide battery backup, it should condition the utility power so that any transients, surges, harmonics and all power quality problems are eradicated before they hit your server. Google’s approach seems to ignore this and they may be leaving themselves open to power problems as a result.

I agree with the decentralised approach however, you put the UPS in, as and when needed, saving the upfront costs. Ensuring the UPS are at capacity also has the efficiency benefit. Any problems with the UPS can be easily rectified (and will only effect the server it’s attached too) and probably more importantly, the UPS makes sure that only clean power enters the server.

Not all the costs are due to obvious power quality issues, such as blackouts, in fact, short term interruptions were the main culprit, followed by transients and surges – then blackouts. A rising phenomenon is flicker, and the costs borne here aren’t equipment damage, but rather the effects on individuals working in an environment that is prone to flicker. It’s interesting I find, as mostly power quality cost are put in the “data lost cost” and “hardware costs”, but seldom are people included in the equations.

Flicker is caused by changes in the supply waveform amplitude and is noticeable particularly with CRT’s and lighting. Workers subject to environments where flicker is a problem, complain of headaches, eyestrain and fatigue. What’s more, a lot of people are completely unaware that they have a problem.

Flicker frequencies are relatively low, but fast enough to mean that a line interactive UPS can do nothing about them. In fact, the transformer in a line interactive system may make flicker even worse. The only way to cure flicker, is either to remove equipment that is causing the flicker, remove the equipment that is showing the flicker, or fit an online double conversion UPS system. This will provide smooth power to the systems, eliminating flicker, and of course, protecting against all other power anomalies along the way.

How ludicrous this is, and I’m here to put the record straight. The energy rating of your surge suppression has zero effect on how good this will protect your equipment when a surge hits it. A small surge suppression device will be just as good as a large device for 99.99%* of the transients it’s ever likely to experience, and for the 0.001% of the time you are hit with a large surge as the result of a localised lightning strike, then both surge suppressors will probably fry anyway.

Another thing you should be aware of, is that even though surge suppressor may divert hundreds if not thousands of Amperes of surge current away from your equipment (reading one spec states 40,000Amps, but doesn’t tell you it is only for micro Seconds – that is a few millionths of a second), they will still let hundreds of volts through to your equipment. And guess what? Most of the time it is this kind of let through that will damage your equipment, and it will leave no trace. You will just put it down to random equipment failure. If you’ve fallen for one of these “if your equipment is damaged when using one of our products we will pay for it” guarantees, you will see in the small print that the equipment must have physical signs of damage, or if it doesn’t say this directly, the manufacturer will want to inspect the equipment for such signs before they pay out anything.

So, are surge suppressors a waste of time? Not at all, they provide a useful function in removing the peaks and a lot of energy from large impulses, but these alone will not stop damage to your connected equipment. You need secondary lines of defence, such as filters and/or isolation transformers to really protect your equipment.

*Well Vic Reeves said 88.2% of statistics were made up on the spot, and he’s probably right. This data doesn’t really exist but obtained by careful conservative consideration.

This is different to the operation of line interactive and standby types. You press the OFF button, and then the load is switched off. Press it on again, and there it is, back again.

Now with online systems, generally they will start up in bypass. So you connect power and lo and behold, you have power to the load. Don’t leave it at that!! You must remember to switch on the inverter or you will not be protected.

The UPS will indicate Bypass operation, and will indicate inverter operation when switched on. It’s a feature of online systems that will extend reliability of power to the load, but only if the correct operation of the unit is observed. After all, what’s the point of protecting your system only to leave it switched off?