Paper Ups Online vs Delta Convertion

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PERFORMANCES COMPARISON

OF DELTA-CONVERSION AND

DOUBLE-CONVERSION UPS

WhitePaper

032011

ANGELO BAGGINI,

Lecturer at the Engineering Faculty, University of Bergamo

MATTEO GRANZIERO,

Technical Communication Specialist, SOCOMEC UPS


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2 Performances comparison of delta-conversion and double-

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INDEX

1. INTRODUCTION ..................................................................................................................................................... 3

2. UPS TOPOLOGIES ................................................................................................................................................ 3

2.1. Line-interactive UPS topology .............................................................................................................................. 3

2.2. Delta-conversion topology .................................................................................................................................... 4

2.3. Double-conversion UPS topology (online UPS) ................................................................................................... 5

3. COMPARISON ........................................................................................................................................................ 6

4. APPLICATIONS ...................................................................................................................................................... 8

4.1. IT Equipment and control systems ....................................................................................................................... 8

4.2. Variable speed drives ........................................................................................................................................... 9

4.3 Electrical Motors .................................................................................................................................................... 9

4.4 High-pressure discharge lamps ............................................................................................................................. 9

5. CONCLUSION ...................................................................................................................................................... 10

REFERENCES .......................................................................................................................................................... 11


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1. INTRODUCTION

On the market, from many years delta conversion UPSs are present without imposing their peculiar technology.

But does this technology guarantee comparable performances to the ones of double-conversion? Or reduced costsare hiding limited performances?

This paper has the aim to compare the double-conversion technology with the delta-conversion one, trying tohighlight its limits. The main limitations of the delta-conversion topology are the impossibility of regulating frequencyvariations from power supply, the limited possibility of regulation voltage variation (for example voltage dip) on theload and the non zero transfer time.

From the technical point of view the double-conversion technology is still the most reliable solution and the lessvulnerable to disturbances coming from the power supply.

2. UPS TOPOLOGIES

UPSs can be classified into 3 main categories or topologies:

Offline ;

Line-interactive of which Delta-conversion can be considered as a sub-class;

On-line or double conversion.

The three categories are completely different and guarantee completely different performances.

In the following paragraphs the last 2 topologies are described, whereas the first is not subject of this paper.

2.1. Line-interactive UPS topology

Delta Conversion UPSs are a special type included in the more general class of line-interactive UPS topology.

So-called line-interactive UPSs are able to provide improved conditioning to utility power by interacting with theincoming electricity. They achieve this by placing inverter/battery charging circuitry or transformers in parallel withthe AC utility supply.

The line-interactive topology is illustrated in Figure 1.

Figure 1 Line-interactive topology.


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As with a standby UPS, a line-interactive UPS allows utility power to flow to the connected load in normal mode,but because of the parallel configuration of the inverter/battery charger in this topology, the UPS can conditionincoming utility power for over - or under - voltages by interacting with the AC main. Small perturbations in

frequency and common mode noise usually cannot be filtered out. The battery is continuously trickle chargedduring normal operation through the inverter/battery charger similar to the standby UPS topology.

This voltage regulation is accomplished through the use of either a tap-switching transformer, a ferroresonanttransformer, or inverter magnetic components. UPS output is dependent of input supply frequency variations.

When utility power is unavailable or reaches unacceptable limits, a line-interactive UPS, like a standby UPS, willenter stored energy mode. The UPS disconnects the load from utility power and reroutes this load with a staticswitch to backup AC power, provided by the battery through the inverter.

2.2. Delta-conversion topology

The delta-conversion configuration consists of two bidirectional converters, a battery bank, a static switch and a

series transformer. The delta-conversion topology is illustrated in Figure 2.

Figure 2 Delta-conversion UPS

Delta-conversion UPSs use a special transformer configuration to interface between the load utility power, with adelta inverter in the transformer secondary to regulate input current and power. With this configuration, the UPScan regulate the magnitude, wave shape, and power factor of the current supplied at the UPS input, while tillcontrolling the Voltage at the load. This results in effective power factor at the UPS input. UPS output isdependent of input supply frequency variations.

Under normal conditions, the Delta inverter and the Main inverter are both on, so that the delta inverter controls

the transformer secondary current, including its magnitude, phase and wave shape. Based on the turns ratio, theprimary current, (which feeds the load), is controlled and maintained. In the event the utility is removed, the deltainverter turns off, forcing the primary current to zero, and the battery discharges through the main inverter tosupport the load.

The main inverter is the usual inverter of the line interactive UPS. It is full rated and connected in parallel with theload. It uses PWM control to regulate the output voltage. On the other hand, the delta inverter is connected withthe normal supply by a series transformer and is rated at 20% of the ups power. Its function is to compensate forany voltage difference between the input and the output. Furthermore, it controls the charging of the batteries andthe input power factor. The load is fed by the normal supply whenever it is within the preset values. The powerconditioning is limited to 15% of the total power to make the input power factor unitary. Figure describes differentsituations of the power flow in front of normal power supply deviations. As the figure illustrates, control is rathercomplex and the load is not electrically isolated from the normal supply.


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Figure 3 - Delta-conversion UPS power flow

2.3. Double-conversion UPS topology (online UPS)

Online or double-conversion UPSs are the most commonly used static UPSs because they are capable ofcompletely isolating sensitive IT loads from unconditioned utility power. They receive their name because theyconvert unconditioned utility power two times under normal operating conditions: first from AC to DC electricityand then back again from DC electricity into a highly conditioned AC.

Figure 4 - Double-conversion topology

Double-conversion UPSs always provide the load with a high quality, conditioned AC signal, even during normaloperation when utility power is available. For this reason, double-conversion UPSs are more common in high-availability, high-power mission critical applications such as industrial facilities, data centers and medicalapplications. Typical output power ratings for these types of UPSs range from 10 to over 1.000 kVA.

The double-conversion topology is illustrated in Fig. 5.


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Fig. 5 - Double-conversion UPS

During normal mode, utility AC power is converted into DC power and is then converted back to a conditioned ACoutput for critical loads. During this phase of operation, the battery receives a continuous trickle charge to keep itat maximum charge.

When utility power goes beyond acceptable limits, the double-conversion UPS enters stored energy mode. Theinverter draws DC power from the battery and continues to put out a conditioned AC power to critical loads. Inother words, the output of a double-conversion UPS is always conditioned. This makes double-conversion UPSsthe best topology for highly sensitive loads in mission critical applications.

3.COMPARISONBoth technologies are now a day commercially available from watts to hundreds of kilowatts, and evenmegawatts if some units operate in parallel. However, their performance and structure, as well as costs,determine the choice.

The main differences between delta and double conversion UPSs can be summarized as follow:

According with the classification code provided by IEC 62040-3 in order to describe the quality of outputvoltages under the different operation modes conditions according to load point of view, delta-conversionUPS have to be classified as VI (Voltage Independent) while Double Conversion UPS are VFI (Voltage and

Frequency independent);Output voltage can be fully regulated in Double Conversion UPS while a Delta UPS does not guarantee acomplete regulation, e.g. frequency.

A synthesis of performance comparison between double-conversion versus delta-conversion UPS technologies isgiven in

Table 1.


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UPS technology Double-conversion

Delta-conversion

Classification VFI VI

Voltage regulation Yes Limited

Frequency regulation Yes No

Transfer time 0 ms 10 ms Offline

2. Sags / brownouts < 16 ms

LineInteractive

3. Dynamic overvoltage < 16 ms

4. Undervoltage Continuous

5. Overvoltage Continuous

6. Lightning Sporadic

Double

Conversion

7. Transients (Surge) < 4 ms

8. Frequency variations Sporadic

9. Voltage distortion Hf (Burst) Periodically

10. Voltage harmonics Continuous

Table 2 - Normal supply disturbance and UPS solutions

1The 96 % declared efficiency is the maximum reachable not conditioning the Energy. Compensations of voltage

related disturbances considerable reduces the efficiency.

VFD

Voltage Frequencyde endent

VIVoltage

indipendent

VFI

Voltagefrequency

indipendent


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Double Conversion is the best UPS technology in terms of performance, power conditioning and load protection,despite some minor disadvantages like efficiency. To solve this limit of efficiency, modern double conversion UPS

have usually the possibility to be set also in off line service if necessary and possible.These are the reasons why double-conversion is broadly installed in high-power applications.

4. APPLICATIONSIn this paragraph for different applications the delta-conversion UPS limits are analysed. The most important ofthem is probably the non zero transfer time causing practically a voltage dip.

Voltage dips cause the power needed for normal operation not be supplied to the equipment. This leads todegradation in the performance of the equipment and in extreme cases to a complete process interruption.Protective system are often implemented for the purpose of disconnecting the supply in the event of the voltagefalling below a set level. Such protection can have the effect of converting a voltage dip into a long supply

interruption. This long interruption is not caused by the voltage dip, but is the intended result of a protectivedevices response to the reduced voltage. If the voltage attains too low a value, or the duration of a dip isexcessively long, the equipment may be disconnected by a protective system or may operate in an impropermanner. The economic consequences of such an event can be of considerable significance. They include loss ofproduction, costs of restarting the technological process, damaged equipment and materials, delayed delivery,reduced customer satisfaction, decrease in the power delivered to the user, etc.

4.1. IT Equipment and control systems

Immunity levels of IT equipment (computers, components of computer networks, PLC, etc.) to changes in r.m.s.voltage value is provided by the so-called ITIC (Information Technology Industry Council) curve, shown in Figure

6.

Figure 6 - ITIC (Information Technology Industry Council) curve

As shown by the curve, the immunity of equipment (guaranteed for disturbance within the curve branches) isstrongly dependent on the duration of the dip. According to these characteristic, the IT equipment shouldobviously withstand steady-state voltage changes within 90-110% of the nominal value.

For example, considering the transfer time of delta-converter UPS, an overvoltage between 150 and 200% of therated value of the duration at least 1 ms, would turn out in the prohibited region for the IT equipment, with relative

failure of the same and probable loss of production or processes.


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4.2. Variable speed drives

Variable speed drives produce one of the most serious problems associated with voltages dips and short supply

interruptions. Unlike other disturbances, such as harmonics, unbalance, etc., the effect of dips and shortinterruptions is instantaneous.

DC and AC drives differ in the topology of their power part and control system (both in hardware and software)and therefore they respond to voltage dips in different ways. There are three main reasons for their susceptibilityto voltage dips:

The power supply of the control system. If the power supply cannot maintain the supply voltage at sufficientlevel, the drive must be switched off for fear of loss of control;

Possible irregular operation or failure of the power part of the drive resulting in the disturbance (e.g.commutation failure in a DC drive);

In many processes the loss of precise speed or torque control is not tolerated for technological reasons, evenfor a very short time. The drives response to a voltage dip is a function of the drive parameters and type ofload, as well as the quantities which define the disturbance. Some loads (fans, blowers, etc.) can toleratesignificant reduction of speed and torque, others do not allow such changes. A precise control of parameters,like pressure, temperature, flow, is required in many industrial processes. As most of these processes employelectric drives, the motor torque and speed directly influence the process variables.

Dips introduced by delta-conversion UPSs in case of transfer from the network operation to battery operationmake the adoption of delta-conversion topology absolutely inadequate.

4.3 Electrical Motors

Induction Motors

Induction Motors are very robust and reliable electrical machines. They are designed to resist to supplyinterruption with energy restoration in phase opposition.

The problems arise when the motors slow down during the supply outage. This is typical when the motors shaftor the load has not enough inertia to keep the speed almost constant. If the motors slow down and this happensoften it is like having repetitive startings: a critical phase for the motor that could drive to an over-heating and asubsequent burning of the machine. Therefore Delta Conversion UPSs are not indicated especially when theoutages could be frequent.Synchronous Motors

The most diffused Synchronous Motors are brushless and Variable Speed Drive fed. Refer, therefore, to theprevious paragraph.

4.4 High-pressure discharge lamps

High-pressure sodium lamps, actually the most popular, are extinguished by a supply interruption of about twocycles duration, or by a voltage dip that reduces the voltage to less than 45% of the nominal value. The lamprequires time, from one to several minutes, for cooling and restart. In the case of wear-out lamps a voltage dip ofmuch lesser depth (residual voltage of about 85% ofUN) is sufficient to extinguish the lamp.

Dips introduced by delta-conversion UPSs in case of transfer from the network operation to battery operation orpersistent voltage reduction can cause in some cases the switching off of the lamps. Because of the workingprinciple they can not be switched on immediately after.


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4.5 Electronic equipments in generalElectronic equipments in general are very sensitive to voltage spikes and overvoltages. Overvoltages canmanifest in various waveforms. It is not possible describe and standardize all the waveforms, therefore standardIEC 60071-1 define three type of overvoltages:

Standard short-duration power frequency voltage: a sinusoidal voltage with a frequency between 48 and62 Hz and duration of 60 s;

Standard switching impulse: an impulse voltage having a time to peak of 250 s and a time to half value of2500 s;

Standard lighting impulse: an impulse voltage having a front time 1.2 s and a time to half value of 50 s.On the basis of the characteristics of wave shape, overvoltage can be classified in Table 3 with reference toshape and duration; and cause.

Classification Overvoltage

type (cause)

MV-HV

overvoltage

coefficient

Term Steepness of

frequency

front

Damping

Internal* At powerfrequency

(insulationfault)

Switching(short-circuit

disconnection)

3

2 to 4

Long > 1 s

Short < 1 s

Powerfrequency

Medium, 1 to200 kHz

Low

Medium

External Atmospheric(direct lightningstroke)

> 4 Very short, 1 to10 ms

Very high,1000 kV/ms

High

Table3 - Characteristics of the various overvoltages types

Note: * Negligible in LV application

Delta-conversion UPSs are not able to filter to the load the major part of these kind of disturbs. The voltage spikereach the load with subsequent fault.

5. CONCLUSIONDelta-conversion UPS introduces three remarkable limits:

the impossibility of regulating the output frequency at loads, generating problems for all those loads that needof constant feeding frequency and the limited possibility of regulation output voltage;

unforeseeable and impulsive voltage as the spike coming from the feeding line, they cannot be in some waylimited or cancelled from the system delta-conversion. Electronic equipment presents in the data center or thePLC, is strongly susceptible to spike, being some often the root cause of fault;

the non zero transfer time in case of transition from network operation to battery operation can createsproblem to all the equipments sensitive to voltage dips. To be noted that often protection systems can havethe effect of converting a voltage dip into a long supply interruption.

The full decoupling between feeding and loads, guaranteed from the double-conversion UPS, allows a full filteringfrom the mains to the load and viceversa.


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REFERENCES

A. Baggini: Handbook of Power Quality. John Wiley & Sons, Ltd. Chichester 2008

Emiliano Cevenini: UPS Architectures for Power Quality improvement.

A. Sudri, E. Jaureguialzo, A. Sumper, R. Villaffila and J. Rull: High Power Ups Selection Methodology andInstallation Guideline for High Reliability Power Supply.

Brian Fortenbury: High Performance Buildings: Data Centers - Uninterruptible Power Supplies (UPS).

APC Legendary Reliability: The Ideal Green UPS.

Angelo Baggini, Franco Bua: Convegno Istituzionale Comitato Elettrotecnico Italiano 2008: Impianti diEmergenza - Scelta delle Apparecchiature, Milano.

______________________________________________________________________________________


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White Paper 03/2011

PERFORMANCES COMPARISON OF DELTA-CONVERSION AND DOUBLE-CONVERSIONUPS

Authors:

ANGELO BAGGINI, Lecturer at the Engineering Faculty, University of Bergamo

MATTEO GRANZIERO, Technical Communication Specialist, SOCOMEC UPS

Media & Marketing Department

SOCOMEC UPS

Via Sila, 1/3

36033 Isola Vicentina (VI) Italy

Media Marketing Coordinator: [email protected]

Head Offices

SOCOMEC UPS

11, route de Strasbourg

B.P. 10050

F-67235 Huttenheim Cedex France

Paper Ups Online vs Delta Convertion

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