White Paper. UPS Dimensioning. UPS topologies & classification

Transcription

1 White Paper UPS Dimensioning This w hite paper is about the right dimensioning of an uninterruptible pow er supply system w hich protects critical applications against pow er surges, failures and blackouts. UPS topologies & classification Mains voltage variations occurs more often than expected. Consequences are crashes, loss of data and cost intensive dow n times. Protection offers only an uninterruptible pow er supply (UPS). There three UPS topologies that offers different protection levels against all 10 mains disruptions and variations. UPS Uninterruptible pow er supplies are combinations of pow er converters, sw itches and energy storage media, e.g. batters, that form a pow er supply system guaranteeing ongoing supply to the load in case of a supply voltage failure. At the same time, voltage and frequency remain w ithin the static and dynamic limits defined for the load. The international product standard IEC puts UPS units into classes 1, 2 or 3 depending on these limits.

2 VFD (offline) UPS Topology Designations / classification The term VFD comes from "Voltage and Frequency Dependent from mains supply". The term offline UPS is also used, but is outmoded. The UPS output is dependent on changes in the mains voltage and mains frequency if the UPS does not have any improvement measures by tapping transformers, ENC filters or varistors. Otherw ise, almost all mains faults are passed on to the loads in normal "mains operation". In particular in UPS units of the VFD type, the voltage curve of the UPS output can be square-w ave or trapezoidal in battery mode, meaning that it differs significantly from the sine w ave. By no means all loads are suitable for this. VFD dynamics: If a pow er cut is detected, a mechanical sw itch is used for changing over to the inverter and thereby to battery mode w ith a typical gap of 4 8 ms. Therefore, it is clear that this UPS type can only achieve the tolerance range of class 3. Advantages of VFD technology VFD UPS units protect against 3 (of 9) voltage problems: Pow er failures Voltage dips Voltage spikes Compared to devices in the VFI/VI class, VFD UPS units have low er purchase and operating costs, w hich means they represent the ideal choice w herever non-critical applications or loads have to be protected. Disadvantages of VFD technology If a pow er cut is detected, a mechanical sw itch is used for changing over to the inverter and thereby to battery mode w ith a typical gap of 4 8 ms.

3 How VFD technology w orks (in an AEG PS UPS) The UPS is connected to a shockproof socket betw een the public utility's mains and the loads to be protected. Under normal operating conditions, i.e. if the UPS is supplied w ith mains voltage, the battery charger w ill ensure that the battery is alw ays completely charged. In a UPS of this design, the current is directly passed on from the mains to the connected devices during normal operation. If the mains supply fails then the UPS sw itches over to battery mode. VI (line-interactive) UPS Topology Designations / classification The term VI comes from "Voltage Independent from mains supply". Line-interactive UPS is also used, but this terminology is outmoded. In accordance w ith industrial standard IEC , w hen a UPS is classified in class 2, this means w ithin a time period from 100 ìs to 5 ìs, the output voltage is not allow ed to deviate from the tolerance range of +/- 30% under any conditions. This requirement is met w ith VFI technology. VI UPS units are therefore in class 2.

4 Advantages of VI technology Line-interactive UPS units protect against 5 (of 9) voltage problems: Pow er failures Voltage dips Surge voltages Undervoltage Overvoltage Compared to devices in the VFI class, VI UPS units have low er purchase and operating costs, w hich means they represent the ideal choice w herever non-critical applications or loads have to be protected. Disadvantages of VI technology There may be a brief supply gap, of up to 6 milliseconds, if there is a low -resistance mains short circuit, w hich is due to a sw itching operation w ithin the UPS during the changeover from mains to battery mode. How VI technology w orks (in an AEG PS UPS) The UPS is connected to a shockproof socket betw een the public utility's mains and the loads to be protected.

5 The UPS is connected to a fused socket betw een the mains and the loads to be protected. Under normal operating conditions, in w hich the UPS is pow ered w ith mains voltage, the battery charger keeps the battery fully charged. The loads connected to the UPS are supplied w ith voltage during this operating status via line filters that offer effective protection against mains surge voltage spikes and high frequency interference. If there is continuous mains undervoltage or mains overvoltage w ithin def ined ranges, the automatic voltage regulation (AVR) provides additional stabilisation of the load voltage. As a result, voltage fluctuations in the public utility's mains are reduced to a level w hich is acceptable for the loads. This is performed w ithout recourse to the internal energy reserve, w hich in turn has a positive effect on battery availability. The static bypass sw itch is activated in case of a pow er failure. The inverter then takes over the voltage supply of the connected loads, in order to prevent the risk of data loss or damage to the loads. The UPS continues to supply voltage until the battery is discharged or an IT system is shut dow n and sw itched off correctly. This standby time chiefly depends on the connected load. If the mains pow er supply is back to normal values, the UPS w ill sw itch back the loads to mains supply. The battery charger w ill then recharge the battery. For safety reasons (as required by German standards, VDE), the mains input in the unit w ill be disconnected by a tw o-pole sw itch in the event of a mains failure. Energy backfeed to the mains and voltage supply to the pins of the mains connector are thus reliably avoided. For safety reasons (as required by German standards, VDE), the mains input in the unit w ill be disconnected by a tw o-pole sw itch in the event of a mains failure. Energy backfeed to the mains and voltage supply to the pins of the mains connector are thus reliably avoided. Operating statuses in detail Normal operation During normal operation, i.e. w hen mains voltage is available, the built-in battery charger keeps the battery fully charged and the mains voltage monitoring system sw itches the inverter to standby mode. The connected loads are pow ered using the monitored and filtered mains voltage, w hich is additionally stabilised by the integrated A.V.R. regulator. The UPS capacity utilisation can be read off the bar chart of the operating panel. Battery operation / autonomous operation When there is a mains failure or if the input voltage moves outside the toleranc e range, the inverter automatically sw itches over to autonomous mode and supplies the loads w ith voltage from the battery. This drains the capacity of the battery and it is discharged. This status is signalled by the yellow LED BAT.MODE flashing as w ell as an intermittent acoustic signal. During the discharge process as the battery capacity consistently drops, the LED BAT.MODE flashes, accompanied by an intermittent acoustic signal (once every 4 seconds). When the battery undervoltage limit is reached (acoustic signal sounds every second before this), the electronics of the UPS sw itch off the voltage supply of the loads.

6 VFI (online) UPS Topology Designations / classification The term VFI comes from "Voltage and Frequency Independent from mains supply". Online continuous converter UPS is also used but is an outmoded term. In accordance w ith industrial standard IEC , w hen a UPS is classified in class 1, this means w ithin a time period from 100 ìs to 5 ìs, the output voltage is not allow ed to deviate from the tolerance range of +/- 30% under any conditions. This requirement can only be achieved using VFI technology. VFI UPSs are therefore in class 1. Advantages of VFI technology Online continuous converter UPS systems offer optimum protection against almost all possible kinds of mains fault. The permanent and double conversion of mains voltage from the input to the output of the UPS means that even frequency fluctuations, oscillations and voltage spikes are filtered out w ith an excellent level of effectiveness. The sinusoidal output voltage means that all kinds of load can be connected to the output. There are no internal sw itching times or supply gaps in the UPS during the changeover from mains to battery mode! A VFI UPS therefore guarantees complete shielding from current malfunctions as w ell as protection against all 9 voltage problems: Pow er failures Voltage dips Surge voltages Undervoltage Overvoltage Sw itching spikes Interference voltages Frequency modifications Harmonic distortion VFI UPS units are clearly the first choice for critical or very sensitive applications (significant damage potential, sensitive hardw are) and if the pow er system is subject to severe disruptions. Disadvantages of VFI technology How ever, this first-class protection is offset by some disadvantages. Due to their high electronic sw itching complexity, these systems are more expensive than offline or lineinteractive UPS systems. Also, this electronic sw itching complexity reduces the efficiency, and also results in increased heat generation w ithin the unit, meaning that the heat has to be dissipated w ith fans (noise).

7 How VFI technology w orks (in an AEG PS UPS) The UPS is connected to a shockproof socket betw een the public utility's mains and the loads to be protected. Explanation of the circuit diagram: Mains filter w ith overvoltage protection (appliance protection / class D) and mains energy backfeed protection Rectifier section w ith PFC logic (pow er factor correction unit) Separate battery charger w ith sw itch mode pow er supply technology Sealed, zero-maintenance battery system as energy storage medium w ith dow nstream DC/DC converter unit IGBT inverter for continuous supply of connected loads w ith sinusoidal AC voltage Automatic bypass as additional passive redundancy Microprocessor-controlled control unit The current flow s permanently through the UPS. The pow er section of the rectifier converts the mains voltage to DC voltage for supplying the inverter. The circuit technology used (PFC) enables sinusoidal current consumption and therefore operation w ith little system disturbance. A separate, second rectifier (charging REC set up using sw itch mode pow er supply technology) is responsible for charging or trickle-charging the battery connected in the intermediate circuit. The configuration of this charging REC means the harmonic content of the charging current for the battery is almost zero, w hich increases the service life of the battery even more. The inverter is responsible for converting the DC voltage into a sinusoidal output voltage. A microprocessor-driven control based on a pulse-w idth modulation (PWM) guarantees, in conjunction w ith a digital signal processor system and

8 extremely fast pulsating IGBT pow er semiconductors of the inverter, a voltage system of the highest quality and availability on the secured busbar. In the event of mains faults (e.g. current failures), the voltage continues to be supplied from the inverter to the load w ithout any interruption. From this point onw ards, the inverter draw s its pow er from the battery instead of the rectifier. Since no sw itching operations are necessary, there is no interruption in the supply to the load. For safety reasons (as required by German standards, VDE), the mains input in the unit w ill be disconnected by a tw o-pole sw itch in the event of a mains failure. Energy backfeed to the mains and voltage supply to the pins of the mains connector are thus reliably avoided. The automatic bypass serves to increase the reliability of the supply further. It sw itches the public mains directly through to the load if there is an inverter malfunction. As a result, the automatic bypass represents an extra passive redundancy for the load. Operating statuses in detail Normal operation Once you have connected the UPS to a suitable mains connection, you can start operation using the UPS main sw itch: Normally, the UPS operates continuously. The UPS now supplies the output w ith voltage, w hich is signalled by the mains symbol (Line LED) and the inverter symbol (INV LED) being lit. This is often referred to as online mode. It offers the greatest protection, in particular w hen there are mains fluctuations and mains failures, because the loads are supplied w ith voltage w ith no interruptions in this operating mode. The rectifier is pow ered from the mains and converts the AC voltage statically into a stabilised DC voltage. The inverter converts this DC voltage into a stabilised sinusoidal AC voltage and pow ers the connected loads. The battery is automatically charged or maintained at full charge by the battery charger. Battery charging is electronically controlled and monitored. Malfunctions are detected and result in charging being interrupted. A signal is also generated immediately. Battery Operation / Autonomous Operation The mains is not w ithin the required tolerance range or has failed. In this case, pow er is supplied to the inverter from the charged battery w ithout interruption. The pow er supply to the loads is therefore also ensured in the event of a mains failure. This drains the capacity of the battery and it is discharged. This status is signalled by the "BATTERY" symbol lighting up, as w ell as an intermittent acoustic signal. Depending on the expansion level, age and condition of the battery and in particular on the load to be supplied, the standby time can vary from a few minutes to several hours.

9 The inverter is sw itched off if the battery voltage drops below a factory -set minimum voltage value. Never store the unit in this condition! The discharged battery system should be recharged w ithin a w eek at the latest. When the voltage and frequency are w ithin the tolerance range once more, the rectifier and the battery charger sw itch back on automatically. The rectifier then continues supplying the inverter and the battery charger takes over charging the battery. Bypass Operation If the inverter is overloaded or if overtemperature is detected, e.g. also if an inverter defect is detected, voltage is supplied to the load via the bypass that sw itches on automatically. This is signalled by the "BYPASS" symbol. This so-called passive redundancy protects against the total failure of the voltage supply on the secured busbar; how ever, in this operating mode, mains faults w ould directly affect the load. As a result, the electronics continuously attempt to sw itch back to "online" / normal operating mode (e.g. w hen the overload or overtemperature goes aw ay). The bypass is a mechanical link that sw itches extremely rapidly. It is located betw een the load and the mains. The associated synchronisation unit in the bypass ensures that the frequency and phase of the inverter voltage is synchronised w ith the mains. Bypass mode is also adopted for a short period w hen the unit is starting up during the synchronisation phase betw een the inverter and the mains prior to the transition into normal operating status. This does not represent a system malfunction, how ever. Bypass operation is exclusively initiated fully automatically by the electronics in the UPS; it cannot be forced by manual sw itching operations.

10 UPS dimensioning The difference betw een w atts and VA Introduction This description is intended to help you clarify the difference betw een w att and VA, and explains how the terms in the specification of the overload protection are used correctly or incorrectly. Background The pow er consumption by computers is expressed in w atts or volt amperes (VA). The value in w atts designates the current that is actually consumed by the equipment. Volt ampere is referred to as the apparent pow er, and is the produce of the voltage and the r.m.s. value of the current that is received by the load. Both the w att and VA number have a specific purpose. The value in w atts defines the actual current that is draw n from the energy supply, and the heat that the unit generates. The VA value is used for establishing the dimensions of cables and cut-outs. The VA and w att numbers are the same for certain electrical products such as incandescent bulbs. In computer equipment, how ever, the w att and VA values can differ w idely, w ith the VA number being at least as large as the w att number. The ratio betw een w att and VA is referred to as the "pow er factor" and expressed either as a number (i.e. 0.7) or as a percentage (i.e. 70%). The w att number of a computer does not have to correspond to the VA number All IT devices including computers operate w ith electronic sw itch mode pow er supply units. There are tw o basic types of sw itch mode pow er supply units for computers, w hich are referred to as 1) PFC pow er supply units (Pow er Factor Corrected, i.e. pow er supply units w ith pow er factor correction) or 2) Capacitor pow er supply units. Even by checking the device, it is not possible to say w hich pow er supply unit it is using, and also the specification of the device does not normally provide any information in this regard. PFC pow er supply units w ere introduced in the mid-1990s and have the peculiarity that their w att and VA numbers are equal (pow er factor betw een 0.99 and 1.0). The w att value of capacitor pow er supply units is betw een 0.55 and 0.75 of the VA value (pow er factor of 0.55 to 0.75). All loads such as routers, sw itches, drive arrays and servers manufactured around 1996 or later use a PFC pow er supply unit, meaning that the pow er factor of these units is 1. PCs, small hubs and accessories for PCs typically operate w ith capacitor pow er supply units, w hich means the pow er factor here is less than 1. Normally, it is about Larger devices manufactured before 1996 are also typically equipped w ith these pow er supply units, and therefore have a pow er factor of less than 1.

11 Rating of the UPS UPS units are designed for maximum w att and VA values. Neither the w att number nor the VA number of the UPS is allow ed to be exceeded on a continuous basis. It is in line w ith the industry standard for the w att number of smaller UPS systems to be about 60% to 70% of the VA number. The ratio is expressed as the pow er factor cos phi = P/S (see also Physical principles). Example of a design problem: Consider a typical UPS w ith 1000 VA. The user w ants to supply a unit w ith 800 W using the UPS. This means the load has 800 VA and a pow er factor of 1. Although the load is 800 VA, w hich is w ithin the VA value of the UPS unit, the UPS is not suitable for a load of this type because the 900 W load exceeds the w att number of the UPS w hich is typically 60% of 1000 VA, therefore about 600 W. Avoiding rating errors The information on the nameplate is often given in VA. This means it is difficult to find out the w att number. Simply by looking at the nameplate of a unit alone might mean that the user w ill develop a system w hich is correctly rated in accordance w ith the VA number but w hich actually exceeds the w att number of the UPS. When selecting the UPS, use the available real pow er, i.e. the W value as the basis. Although this cautious approach to rating means that the UPS w ill normally be rated slightly too highly, how ever this offers the advantage of consumption reserves, better overload capability and, in particular, a longer standby time compared to that in new operation. Conclusion Information about current consumption is often not specified in a w ay that allow s the UPS to be configured straightforw ardly. This possibly leads to systems being developed w hich appear to be correctly rated, but w hich in effect overload the UPS. A UPS that is slightly over-dimensioned w ith regard to the data on the nameplate of the device ensures correct operation of the system. In addition, over-dimensioning offers the advantage that there is more time available for a backup.

12 Glossary of technical terms AVR (Automatic Voltage Regulation) Automatic voltage regulation to prevent mains voltage deviations DC/DC Booster Circuit technology for increasing a DC voltage to a higher voltage level EPO (Emergency Power Off) Emergency switch-off device PFC (Power Factor Correction) Switching technology for minimising system disturbances (particularly important when connecting non-linear loads) Appliance protection Term from surge voltage technology. Classic mains overvoltage protection consists of a lightning surge arrester (Class B), overvoltage protection (Class C) and, finally, what is referred to as appliance protection (Class D) IGBT (Insulated Gate Bipolar Transistor) High-performance transistors of the latest design with an ultra -low control power requirement (MOSFET structure) and ultra-low losses on the output side (structure of a bipolar transistor) Class D See: Appliance protection LED (Light Emitting Diode) Electronic semiconductor component, usually referred to by its acronym, used for optical signalling PWM (PulsWeitenModulation) Here: circuit technology for generating a sinusoidal voltage of the highest quality from an existing DC voltage SNMP (Simple Network Management Protocol) Frequently encountered protocol in networks for managing / handling components VFD (Output Voltage and Frequency Dependent from mains supply) The UPS output is dependent on mains voltage and frequency fluctuations. Earlier designation: OFFLINE VI (Output Voltage Independent from mains supply) The UPS output is dependent on mains frequency fluctuations, but the mains voltage is prepared by electronic / passive voltage control units. Earlier designation: LINE-INTERACTIVE VFI (Output Voltage and Frequency Independent from mains supply) The UPS output is independent of mains voltage and frequency fluctuations. Earlier designation: ONLINE