Uninterruptible Power Supplies (UPS)

Uninterruptible power supply (UPS) is an automatic device, which enables the equipment being connected to it to operate for a short period of time with the power supply from batteries of UPS, when there is the miss of electric current or when the current parameters overrun its permissible limits. In addition, it is able to correct power supply parameters (voltage, frequency). It is frequently used to provide the trouble-free operation of computers. It can be combined with different types of electric power generators.

There are three types of UPS designing:

reserve is used to provide power supply to personal computers or work stations of local area networks. Practically all inexpensive low-powered UPS offered at the home market are designed according to the reserve circuit. When the power supply is beyond the voltage normalized values or it is absent, UPS automatically switches over the connected load to power supply from batteries (with the help of a simple inverter). When the normal voltage resumes, UPS again switches over the connected load to the power supply network. A shortcoming of this kind of UPS is the non-sinusoidal output and a relatively long time of switching over to battery operation. Due to the coefficient of efficiency equal to approx. 99 % uninterruptible power supplies are practically noiseless and have minimum heat release. They are not able to correct neither voltage, nor frequency (VFD according to IEC classification).
interactive is a device similar to the reserve UPS, but in addition it has a step-voltage regulator at the input, which makes it possible to get the controlled output voltage (VI according to IEC classification). Inverters of some models of linear-interactive UPS provide the voltage of both rectangular and trapezoidal form, as the previous variant, as well assinusoidal form. Switchover time is less than in the previous variant, because synchronization of the inverter is carried out with input voltage. Their coefficient of efficiency is lower than of the reserve UPS. The assortment of ТМ SVEN interactive UPSis given here.

SVEN Pro+ 1000 liner-interactive UPS
оn-line or double conversion (оn-line mode) is used to provide power supply to file servers and work stations of local area networks, as well as to any other equipment, which demand raise requirements to the power supply quality. The principle of operation consists of double conversion of a current type. Initially the input voltage is converted into direct, then back to alternating voltage by means of the inverse converter (inverter). Switchover time is identical to zero. UPS of double conversion has low coefficient of efficiency (from 80 % to 94 %), due to which they are notable for high heat release and sound level. Unlike two previous types of UPS, they are able to correct not only the voltage, but the frequency as well (VFI according to IEC classification).

Part 1 (prologue)

Poor power supply affects equipment extremely negatively: strong power surges are able to make power supply units and microcircuit inoperative and regular problems with electric power lead to premature aging of equipment. According to researches of the American consulting company Contingency Planning, a reason of data loss in 45% is poor power supply. The uninterruptible power supply (UPS) will help to avoid such problems.

So, the use of uninterruptible power supplies is needed to prevent equipment damages and failures in operation of systems. A top priority task of UPS is considered above all providing of normal and correct completion of operation at the sudden power failure. However, ideally UPS must protect equipment against all types of interruptions in power supply networks, the main of which are given below.

Types of electrical interferences

Let's get it clear right now what kind of failures in the power supply network are the most frequent and by what reasons they are caused:

Voltage impulse is an abrupt change in voltage, after which the voltage renewal follows to the ordinary level within 10 msec. Reason of impulses are elevators, air-conditioners, industrial equipment, lightning, etc. Impulse amplitude can reach 2000 V.
Voltage depression– is an abrupt considerable voltage deceleration with the voltage renewal within from several periods to tens of seconds. The reason is the limited power of a substation (especially in winter), starting and operation of powerful equipment. Blinking of light bulbs is a typical manifestation.
Brief power outage is power outage for less than 20 ms. The reason is the starting currents of Xerox machines, laser printers, powerful electric motors, etc. Such power outages are imperceptible for eyes.
Radio noise is high-frequency components of voltage, caused by the load connection, power generators, industrial equipment and radio transmitters.
Long-term power outage is power outage within from 20 ms to several hours. The reason is the disconnection of power supply lines from a substation due to overload, unfavorable weather conditions and physical damages.
Frequency drift– is short-term changes of frequency when powerful equipment being connected.

For informational purposes: according to research results conducted by Bell Labs and IBM Companies, every PC is monthly exposed to impact of about 120 abnormal situations related to power supply problems.

Functions of UPS

Main functions of UPS:

Absorption of moderate and electrical fast transients;
Supply voltage filtration, noise level reduction;
Providing reserve power supply of loading within some time after power outage in power supply network;
Overload and short circuit protection.

In addition to this, many models of UPS under the special-purpose software control can provide the following functions:

Automatic shutdown of the maintainable equipment at long voltage absence in the power supply network, as well as the equipment restarting after power supply renewal;
Monitoring and recording the status of a power supply unit (temperature, battery charge level and other parameters) in the log-file);
Display of voltage level and alternating current frequency in the power supply network, output voltage and capacity consumed by the load;
Monitoring of emergency situations and delivery of warning signals (audio signals, starting external programs, etc.);
Turning on-off of the load by the internal timer in the prescribed time.

Types of UPS

According to the principle of operation, uninterruptible power supplies are classified into three main types:
Off-line UPS – these uninterruptible power supplies serve for the backup of a main power supply unit (power supply network) in the case of an emergency (disconnection or voltage deceleration/rise higher of the prescribed value). If it takes place, a switch actuates and the load goes to the reserve power supply from the inverter being under battery operation. Under normal conditions, the power is supplied to the load directly from the power supply network, as a rule, via interference-suppression filter. Other names of off-line UPS: stand-by, backup, in-line.
Advantages of off-line UPS are simplicity and, consequently, cheapness; high coefficient of efficiency and, consequently, low operation expenses. Disadvantages of off-line UPS are the absence of voltage and frequency regulation as per normal conditions; long switching time to the battery operation (several ms) and, consequently, short-term power outage or voltage surge at the load; phase retardation at switching.
As a whole, UPS of this class can be characterized as a compromise between the acceptable protection level against failures in the power supply network and the price. Capacity of produced devices fluctuates from 220 to 2000 VA.
Linear-interactive UPS – during normal operation mode they supply the load with the voltage from the main power supply network, to some extent regulating voltage (autotransformer), but in event of a fault of the main power supply network the load is switched over to the inverter synchronously.
According to the operation principle, linear-interactive UPS are similar to off-line UPS: they also serve to backup the main power supply source “suppressing” small power surges and smoothing interferences. At the same time, they have some substantial distinctions. So, the inverter of UPS is connected in parallel with the power supply source and operates in the duplex mode, i.e. it monitors the power supply line and provides regulation and stabilization of output voltage of UPS within certain limits, as well as it charges batteries. In addition, many producers install additional units into UPS of this class (ferroresonant transformers or autotransformers), which make it possible to extend the output voltage range; at which the output voltage is supported at the acceptable level without passing to the battery operation.
Advantages of the linear-interactive UPS are quite high coefficient of efficiency and more reliable protection of power supply of the connected load as compared with off-line UPS.
Disadvantages of linear-interactive UPS are the output voltage instability in the normal operation mode, which depends on the input voltage range; the absence of frequency stabilization in the normal operation mode; the absence of the load isolation against the power supply network; inefficiency for the load during operation with a high nonlinearity degree; penetration of impulses and noises from the main network to the load; low information security (a possibility of unauthorized access to the equipment via power supply lines).

SVEN Pro+ 400linear-interactive UPS

SVEN Pro+ 800linear-interactive UPS

SVEN Reserve Home-1000linear-interactive UPS withprolonged reservation time

UPS in continuous operation or UPS with double-conversion (on-line, double-conversion)

Specifications of UPS

Let us enumerate main specifications of UPS briefly:

Input voltage range, at which UPS operates from the power supply network and does not switch over to the operation from built-in batteries. As everyone is well aware, the wide input voltage range reduces the number of changes to the battery operation and increases its operation life. In addition, UPS with wide input voltage range continue to operate from the power supply network and provide the power supply to the load, while UPS with low input voltage range have already entered to the battery operation and after discharging the battery they de-energized the load. This is particularly significant for our power supply networks, where long-term “voltage slumps”are frequent.
Output voltage change under the change of input voltage. “Responsibility” of UPS is to provide output voltage whereby the equipment protected by it can operate properly. The low voltage at the UPS output is able to cause failures of equipment in operation and data loss; considerable voltage rise causes the same results plus equipment breakdown. Voltage rises are rarer, but their consequences have more regrettable results. Off-line UPS do not correct voltage in any way; line-interactive models produce the voltage “shift” for a fixed value (which can have several values).
Output voltage parameters during the battery operation – voltage, frequency, signal waveform. These parameters determine the generation quality provided by UPS, due to which a field of application of a concrete device depends on.
Switching process of UPS to the battery and back. For normal operation of equipment connected to UPS, all switchings and transient processes must be “inconspicuous”. It means they must be performed for the minimum time and undergo correctly – in particular, must be accompanied by correct frequency synchronization of UPS with the external frequency of the power supply network.
Behavior of UPS in case of overload at the output. UPS turns off in case of overload during the battery operation (to prevent the breakdown). If there is an overload during operation from the power supply network (for example, when additional equipment has been connected to UPS), a user must know about it to decrease the load in time. Otherwise, the equipment will be deenergized instantly in case of voltage disappears in the power supply network. The combination of sound and light indications is the most effective, while some UPS provide only light indication or they have no indication at all.
”Cold start” availability, i.е. a possibility to turn on UPS in the absence of voltage in the power supply network. Such function can be useful, for example, if you have to turn on a computer or receive/send a fax during the long-time miss of power supply.
Frequency stabilization possibility (for on-line UPS). Some equipment can be critical to the power supply frequency. For example, rotating velocity of alternating current motors (tape recorder, vinyl record player, etc.) changes if the voltage supply frequency changes.

Part 2 (practical)

In this article we will dwell on the theme to select a uninterruptible power supply.

So, you have decided to purchase UPS. First and foremost, it is necessary to understand clear, what kind of above listed problems of power supply (described in part 1of the article) affect you directly, to choose UPS model suitable in all respects.

For house and small office. If you live in new districts of cities, the power supply network in your houses is quite stable and you do not leave from your computer in the “on” condition for a long time, then a simplest Off-line UPS of sufficient power is the best choice.
A standard PC is very «loyal» to the trapezoidal voltage and there is no point in spending the spare money for functions unnecessary for you. It is possible to select a model without the remote control and even without monitoring (the sound of a built-in beeper is quite enough in conditions of a small flat in order to solve what measures are to be taken). An ordinary surge protector is a good addition to the simplest UPS. Only you should remember: it is impossible to connect surge protectors to the outputs of UPS under no circumstances, because this substantially distorts characteristics of even not very sinusoidal wave.

For offices and local networks. There are at least three variants to solve the power supply problems. The first and simplest is to protect your server and store the most of information in it. A medium/high-powered Line Interactive UPS will be suitable for this purpose or, if you have very valuable information – On-line UPS. You should not be stingy in this case, monitoring support and other useful possibilities of software are practically obligatory.
The second solution: in addition to your server protection to protect work stations with simple UPS, the same, as for domestic use. There is a quite good possibility to save money – to power the nearby computers from one UPS. Remember only about the maximum power and do not use monitors of big size.
The third solution, the most optimum, but the most expensive as well – to power the whole power supply system via a high-powered On-line UPS. Application of these systems proves to be correct only in cases of very valuable information and with the availability of a permanent observer capable in the allotted time to de-energize correctly all turned on units of the power supply network or to shift them to the alternative power supply (emergency generator).

Selection of UPS

Once you have decided on an UPS type, it is worth paying attention to its following specifications:

output power, measured in volt-amperes (VA) or watts (W);
switching time, i.e. the shift time of UPS to the battery operation (measured in milliseconds, ms);
off-line operation time is determined by the battery capacity and by power of equipment connected to UPS (measured in minutes);
input (network) voltage bandwidth , when UPS is able to stabilize the power supply without shifting to the battery operation (measured in volts, V);
life time of batteries (measured by years, usually 5-10 years).

The main characteristic of an uninterruptible power supply is its power. In case of UPS, it is measured in volt-amperes (VA in abbreviated form). In most cases in order to convert volt-amperes into more common watts, it is necessary the power in VA to divide by 1.4 or multiply by 0.7.
The power consumed by the load is calculated as the product of output power of UPS (in volt-amperes, VA) by the power factor (PF).
It is essential to select such UPS, for which the following condition is met:

P = Wн x PF, where

Sum up powers of all devices to calculate the total power consumption. Select such a power from the line of UPS models, that the got total power will make not more than 80 % of the output power of UPS. Some specialists recommend selecting UPS with the power, which exceeds the total load at least 1.5 times. In this case, we can say about normal operation of the uninterruptible power supply.
As a rule, the power consumption value is indicated on the sticker located on the rear cover of devices. As an example, typical power consumption for ATX system unit makes 250-350 VA (180-250 W), for monitors 15-17” – 140-170 VA (100-122 W). So, UPS for 500-600 VA is sufficient for a personal computer of the standard home configuration (multimedia station with 15'' monitor).
The off-line operation time of a computer based on the battery operation of UPS is one of important criteria as well. This is the time during which UPS is able to support the normal power supply to the computer in the voltage absence in the dome power supply system. This time depends directly on the power capacity of UPS battery. The optimum time of off-line operation of a computer at home must be about 10-15 minutes, this time will be quite sufficient to finish your work and save any document.
A very useful additional feature is the “cold start” i.e. a possibility to turn on equipment connected to UPS in the absence of voltage in the external power supply network. It is necessary, for example, when you have to receive or send a letter by e-mail quickly.
While purchasing UPS you should also pay attention to the average service life of batteries for a specific UPS –the cost of battery changing can reach up to the third part of the total price of an uninterruptible power supply. In principle, the typical service life of batteries makes from 3 to 5 years, but of course, this index depends greatly on frequency of their use.

Connection of UPS

Connection of an uninterruptible power supply is a little bit complicated than connection of an extension cord. UPS has several similar jacks to connect power supply cables. The only thing – a PC and UPS are not to be connected with the help a computer cable, which has an ordinary plug on its end: it will not be suitable for jacks of UPS. Therefore, if special cables are not included into the package of UPS, then must be bought separately. Powerful UPS are connected with the help of special terminal blocks.
It is not recommended to connect to the output of UPS such devices as laser printers or Xerox machines, because during their operation they consume high peak power in separate moments, and that can lead to the inverter overload and power cut. That is why, many producers install additional sockets at the output of UPS, which provide protection against overvoltage and interferences only. Basically, it relates to low-powered devices.
After the first connection of UPS to the power supply network it is usually required from four to six hours to charge the battery in full. After that UPS is completely ready to operate. In common situations – with normal power supply voltage in the home power supply network – UPS does not approve itself in any way. However, as soon as UPS will have to be shifted to the battery operation, it will signalize you about this immediately, in several ways at that. The first thing is the alarm indicators on the case of UPS. The second thing is the piercing «beeping» of UPS. And, finally, the third thing is the software-based notification (special warning is displayed on the screen).

UPS operation

While selecting UPS it is important to take into account the simplicity of their operation and technical maintenance. Indicating means of UPS status and the connected load are of great importance for the end user. The simplest information display equipment is light-emitting diodes, but liquid crystal displays are more informative.
Every UPS is equipped with testing functions – checkout of its internal units. At that, contingencies (origin of overload or short circuit) are controlled, the state of batteries, degree of their discharge, as well as correctness of UPS connection are analyzed. When power is supplied to UPS, testing procedure stats automatically, which then is being repeated at stated intervals. This process can be started manually as well, by pressing a corresponding button (if such button is available on UPS).
If UPS has interface jacks, then it is possible to carry out the remote monitoring of power supply process to equipment, this is also considerably simplifies their technical maintenance. UPS can connect to PC with additional interface cable to control the state of the power supply network and batteries, as well as other units of the device with the help of special software. Usually the control software is included into the package content of UPS, if it is not available, it can be downloaded free of charge via Internet at the producer’s web site. Practically all devices are standard equipped with RS-232 serial interface.
A possibility of hot replacement of batteries (without UPS shutdown and power supply interrupt to the load) has been designed in many UPS. By this is meant that a user can buy batteries and replace old ones. If you do not rule out this, then you should pay attention to the bypass availability in UPS, i.e. a possibility to supply filtered voltage to the load by-passing the main circuit of UPS. UPS enters this mode automatically in case of troubles in its units or manually to carry out its maintenance, for example, to replace its batteries. All UPS with double voltage conversion, as well as some line-interactive devices, have this function.

About surge protectors

For a variety of reasons not every user needs UPS for protection against electrical interferences. In this case a surge protector is able to protect them against some problems.
Modern surge protectors in outward appearance are much like ordinary power extension cords with three and more sockets. They are equipped with a switch and alarm indicator. Their cable length can be from 1.5 to 5 and more meters. Such protectors are designed to protect computers against short-term voltage rise (surge), high-voltage spikes, as well as against different electromagnetic interferences. But unfortunately, these devices are absolutely powerless against such troubles, as voltage depressions (sag) and, moreover, its complete disappearance (blackout): all your unsaved data will be lost utterly.
A surge protector is connected similar as an ordinary electrical elongator: PC and monitor power cords – to the surge protector, and the protector power cord – into an ordinary socket.
A surge protector is also desirable with the availability of UPS because peak impulses in such power supply networks sometimes reach just monstrous values. A surge protector is considerably cheaper than UPS, and if it burns away, it hurts you not so much, as the loss of expensive UPS.

On-line UPS (continuously operating of double conversion)

Uninterruptible power supplies operate according to the “on-line” circuit, unlike the “off-line” and “line-interactive” systems give you confidence that you will not have interruptions in power supply, even in case of its absence in the power supply network.
The technology of double conversion of power supply is used for this purpose, which enables to convert continuously alternating current of the main network into direct current (which is used to charge the battery) before it is supplied through the inverter for back conversion into alternating current for power supply to PC.
This provides the absence of switching time in case of problems with the power supply from the main network, as well as this provides compensation of voltage “slumps” ( voltage reduction in the line) – a problem, which cannot be coped with the “off-line” type systems.
In most cases the power supply in the main network disappears not suddenly, but within several periods. As far as the voltage decreases the “slump” is compensated by the battery of UPS of “on-line” type. The inverter does not find out distinctions even in case of mixing of voltages of the battery and direct current supplied from the main power supply network.
If power supply from the main network is stopped completely, the whole load is switched over to the battery operation. As regards a computer, there are no changes in power supply to it and the sinusoidal signal at the output is constantly synchronized with the main network.
In UPS with actual double conversion “on-line” technology) the battery is connected to the DC bus through a diode or similar electronic device at all times.
Determination what is specifically an uninterruptible power supply (UPS) of «on-line» type was widespread in industry. However many producers do not adhere to this tendency, and though they can use double conversion, but they use a switch or relay for battery connection with the direct current unit of UPS. When there is the voltage loss, several milliseconds can be required for the relay to connect the battery to the inverter, but that can cause considerable voltage swings of output voltage.
Uninterruptible power supplies built according to the On-Line circuit have additional By-pass operation mode. By-Pass is a mode when the power is supplied to the load directly from an external network through a filter, in some models even through the isolation transformer mounted in UPS. By-Pass automatic and manual (technical) are recognized.
Automatic By-Pass is turned on, when UPS is overloaded, for example, when the load is turned on, the starting power of which is in 3-7 times higher than the rated power, when there are some failures inside UPS, during overheating, etc. Construction of some reserved uninterruptible power supply systems is impossible without automatic By-Pass. Inputs of UPS and By-pass must be separate while reserving. The input of the main UPS is powered from the power supply network, but the input of By-Pass – from a reserve UPS being in the hot mode. If there is a failure of the main UPS, it switches over to By-pass automatically and the load is powered from the reserve UPS. There are other architectures of uninterruptible power supply systems, which require the availability of automatic By-Pass.
Manual By-pass is required for repairs, preventive maintenance of UPS, to provide power supply continuity to the load.
The cost of batteries is 40-50% of the cost of an uninterruptible power supply of On-line type. Resource of batteries, as everyone is well aware, is determined by the number of charge-discharge cycles, environment temperature, optimality of charging and discharging current and their periodic “trainings”.
Battery intelligent control is not insignificant advantage of qualitative uninterruptible power supplies of “On-line UPS”. It is known that self-discharge processes occur in the battery continuously. Usually the floating charge of the battery with low current is carried out in UPS to compensate these processes. The light current continuously running through the battery causes changes of chemical composition of active substances, corrosion of grid and scaling of positive plates. This leads to the irreversible decrease of battery capacity, their service life shortens and the real time of battery support decreases. Algorithms of the battery charge control of different degree of complexity are realized in different models of UPS. The availability of the battery intelligent control mode provides their maximum partial operation and accordingly the battery service life extension.
The load powered through UPS of On-line type cannot be put out of action or damaged by way of “electrical subversive act” (deliberate action directed to an electrical appliance or a group of devices by bringing specially selected sequence of disturbances into the power supply network). On-line type UPS is also the only absolutely reliable protection against attempts to read-out information from a computer by the analysis of its reverse influence upon the power supply network.

Advantages of uninterruptible power supplies with double conversion “on-line” technology):
– maximum line voltage filtering against interferences and surges; interferences generated by the load are not let back to the power supply network;
– full stabilization of form and size of output voltage both during operation from the power supply network and during battery operation; sinusoidal output voltage;
– switching time to batteries and any transient processes during switching are absent.

Operation modes of UPS with double conversion:
Net operation – power supply mode to the load from the network. When the network voltage is within limits of permissible variation and the load not exceeding maximum permissible, UPS operates in the network mode. The following is realized in this mode:
• filtration of impulsive and high-frequency network interferences;
• conversion of alternating current energy of the power supply network into direct current energy with the help of rectifier and power factor correction circuit;
• conversion of direct current energy into alternating current energy with stable parameters with the help of the inverter;
• battery charging by a charger.
Off-line operation is the power supply mode to the load from the battery. When network voltage parameters deviate from permissible limits or when the power supply disappears completely, UPS instantly goes into battery operation through the step-up DC/DC transformer and inverter. After voltage renewal in the power supply network, UPS goes into net operation automatically.
Bypass mode is the power supply to the load directly from the network. If there is any overload in the power supply network or UPS is overheated, as well as if one of units in UPS fails to operate, then the load is automatically switched over from the inverter output directly to the network. After removal of Bypass mode reasons (overload or overheating), UPS automatically goes back to the normal network operation with double-conversion of energy. Please note that in the Bypass mode the load is not protected against low-quality voltage in the power supply network.

By-pass operation mode circuit

Cold start mode