essential guide to power protection design

The Essential Guide toPower Protection Design

CHLORIDEPOWER PROTECTION

CHLORIDE MI01/10030 - rev.1 - 18/014/1 - page 2 POWER PROTECTION

THIS DOCUMENT IS RESERVED FOR CHLORIDE POWER PROTECTION INTERNAL USE. IT MUST NOT BE DISTRIBUTED TO PERSONS EXTERNAL TO CHLORIDE POWER PROTECTION. ALL RIGHTS RESERVED UNDER CURRENT LAW AND INTERNATIONAL CONVENTIONS. First issue: 18 April 2001 Version 1 Silectron S.p.A. Via Fornace, 30 40023 Castel Guelfo (BO) Italy Web: http://www.chloridepower.com Email: [email protected] Tel: +39-0542 632111 Fax: +39-0542 632120


Introduction The main objective of this document is to assist in the diffusion, within Chloride Power Protection, of a style and analysis for UPS applications that are as uniform as possible. The information contained herein has been collected by using the considerable know-how of the personnel that work at Chloride. The personnel with most experience, both in terms of technical details and sales, as well as design and industrialisation, will be able to utilise this document to easily find the most commonly used information. Furthermore it is a valid technical training tool for new employees.

Franco Costa UPS Franchise Director

Authors' note The purpose of this publication is to collect basic information and experience concerning uninterruptible power supplies. It is aimed at those requiring more detailed information concerning UPS rather than specific product functioning. Topics under discussion range from product standards, and more generally those for special applications, to UPS sizing, with reference to problems that can arise when using this particular type of electrical equipment. The topics under discussion do not exhaust the material, and neither are they always examined in detail. This is because the document is aimed, as far as possible, at those dealing with UPS for the first time. We acknowledge the efforts of those who have contributed to this first edition. We welcome new contributions, criticisms and suggestions from them, and anyone else, to ensure that future versions of the text are better than the existing one.

Emiliano Cevenini Claudio Zucchini

CHLORIDE SUPORT & CONSULTING

E-Mail: [email protected]


CONTENTS 1 CLASSIFYING UNINTERRUPTIBLE POWER SUPPLIES ................................................ 7

1.1 Uninterruptible power supplies ................................................................................................ 7 1.2 Rotating units ........................................................................................................................... 8

2 PRODUCT STANDARDS ...................................................................................................... 9 2.1 The purpose of standards.......................................................................................................... 9 2.2 Standard EN 50091-1............................................................................................................. 10 2.3 Standard EN 50091-2............................................................................................................. 14 2.4 Standard ENV 50091-3 .......................................................................................................... 18

3 ELEMENTS OF ELECTROMAGNETIC COMPATIBILITY............................................. 25 3.1 Interface with the power supply ............................................................................................. 25

3.1.1 Disturbances from the network..................................................................................... 25 3.1.2 Disturbances injected into the mains power supply ..................................................... 28 3.1.3 Correct sizing of electrical energy sources in systems with uninterruptible

power supplies. ............................................................................................................. 32 3.2 Interfacing towards the load ................................................................................................... 37

3.2.1 Types of load ................................................................................................................ 37 3.2.2 Current in the neutral conductor ................................................................................... 39

3.3 Radio frequency interference (RFI) and unipotential connection.......................................... 39 3.3.1 Disturbance ................................................................................................................... 40 3.3.2 Unipotential connections .............................................................................................. 40 3.3.3 Outside earths ............................................................................................................... 42 3.3.4 The cabinet.................................................................................................................... 43 3.3.5 RFI filters ...................................................................................................................... 43

4 POWER SUPPLY SYSTEMS............................................................................................... 45 4.1 Classification.......................................................................................................................... 45

4.1.1 TN systems ................................................................................................................... 45 4.1.2 Protection against indirect contacts in TN systems ...................................................... 46 4.1.3 Using differential switches ........................................................................................... 48 4.1.4 TT systems .................................................................................................................... 50 4.1.5 Protection against indirect contacts in TT systems ....................................................... 51 4.1.6 IT systems ..................................................................................................................... 52 4.1.7 Protection against indirect contacts in IT systems (first fault) ..................................... 53 4.1.8 Protection against indirect contacts in IT systems (double fault) ................................. 53

4.2 Faulty UPS functioning cycles ............................................................................................... 56 4.2.1 Overloads ...................................................................................................................... 56 4.2.2 Short circuits ................................................................................................................. 58 4.2.3 Mains power supply failure .......................................................................................... 59 4.2.4 Floating neutral functioning ......................................................................................... 60

4.3 Permanent modifications to neutral status ............................................................................. 61 4.4 Differential selectivity components ....................................................................................... 62

5 SELECTION CRITERIA FOR UPS SYSTEMS .................................................................. 69 5.1 Emergency power supply ....................................................................................................... 69

5.1.1 Safety power supply ..................................................................................................... 69 5.1.2 Standby power supply................................................................................................... 69

5.2 UPS configurations for standby and safety services .............................................................. 70 5.3 Static switch assemblies. ........................................................................................................ 73 5.4 Using system static switch assemblies: two typical examples ............................................... 74 5.5 Service availability and communication with the UPS .......................................................... 76


5.6 Inverter power sizing.............................................................................................................. 78 5.7 Sizing examples...................................................................................................................... 81

6 BATTERY CAPACITY SIZING TECHNIQUES ................................................................ 83 6.1 Determining battery capacity for constant power discharging using current as a

parameter ................................................................................................................................ 84 6.1.1 Capacity sizing example for a VRLA battery using current as a parameter ................ 85

6.2 Determining battery capacity for constant power discharging using the power parameter for each cell ........................................................................................................... 87

6.2.1 Capacity sizing example for a VRLA battery using power per cell as a parameter ...................................................................................................................... 88

6.3 Battery recharging methods ................................................................................................... 89 6.3.1 Charging with characteristic I- V with voltage limit of 2.4 V/cell ............................... 90 6.3.2 Charging with characteristic I- V with voltage limit set at float voltage ...................... 91 6.3.3 Recharging at 2.7 Volts/cell ......................................................................................... 92

6.4 Recharging times .................................................................................................................... 92 6.5 Battery line protection............................................................................................................ 93

7 ANALYSING SPECIFICATIONS........................................................................................ 95 7.1 Cross references between obsolete standards and standards EN 50091 and IEC 62040 ....... 96 7.2 Standards for special applications ........................................................................................ 104


1 CLASSIFYING UNINTERRUPTIBLE POWER SUPPLIES An Uninterruptible Power Supply (UPS) is a system capable of supplying high quality electrical power without interruptions. An electricity generating unit (motor-alternator) cannot be considered as an uninterruptible power supply because it has a switch ON-time of several tenths of a second (automatic switch-ON when the mains power failure is detected). This time period can be considered as the time necessary to restore the power supply after a mains power failure. It is too long to enable normal functioning of virtually all devices that use electricity. Uninterruptible power supplies, as well as providing protection against all types of power supply failure, are also capable of filtering a vast range of disturbances found in the mains power supply thus providing more sensitive loads with a perfect power supply. Therefore uninterruptible power supplies perform two functions: They filter disturbances from the continuously functioning mains power supply. They supply power to the loads in the event that the mains power supply fails. Therefore a UPS must have a power reserve within its structure. The power can be stored in electrochemical or mechanical format. The former concerns uninterruptible power supplies whilst the latter concerns rotating units. This document deals exclusively with uninterruptible power supplies. The main characteristics of these two types of system are shown below.

1.1 Uninterruptible power supplies Uninterruptible power supplies use electrical accumulator batteries as a power reserve. The batteries are usually made of lead, and more rarely nickel-cadmium. Uninterruptible power supplies are widely used in the majority of applications. This is thanks to their reliability and flexibility. The main component is the DC/AC switch assembly, or inverter. This is needed to convert the continuous electrical power supplied by the rectifier or the batteries into alternating power at a set frequency. One of the architectures offering the best performance level, both in terms of power supply continuity and the high quality of the power supplied to the load, is that known as double conversion (see Figure 1-1). This architecture uses an AC/DC converter (rectifier) and an inverter. Under normal functioning conditions the power flows from the input to the output through the two converters. If there is no input power supply, the power is supplied to the inverter directly from the battery, without the intervention of any other device.

Figure 1-1 Double conversion UPS (main outline)

The performance levels of this architecture derive from the fact that under all functioning conditions the load is separated from the mains power supply because it is always powered by the inverter. Typical autonomy of these UPS (that is to say maximum battery functioning time until the stored power is exhausted) ranges from a few minutes to an hour. Usually, electricity generating units, capable of replacing the mains power supply, are used when longer periods of autonomy are required. See the following sections for further details.

LOAD


1.2 Rotating units

In this type of unit the power reserve is mechanical and stored in a flywheel kept in constant rotation. One of the most widely used architectures involves an electric motor-generator unit and an input frequency adjustment system. This system means that the rotation speed of the flywheel connected to the electric motor-generator unit is kept constant, and therefore the output frequency is kept constant. The load is powered by the generator output. In general the flywheel is able to withstand a mains power failure for a few seconds. Other versions use a synchronous regulatorgenerator system, an induc tion coupling and a diesel engine with a freely rotating clutch. The diesel engine is run at a speed that is equal to the rotation of the synchronous regulatorgenerator. When there is a mains power supply failure, the diesel engine keeps the generator rotating thanks to the intervention of the freely rotating clutch. In this case autonomy is limited only by the fuel supply. Rotating UPS have several disadvantages including noise level, long periods of downtime and maintenance associated with mechanical parts, considerable weight and size. Furthermore these devices are subject to legal requirements and standards that are different from those applicable to uninterruptible power supplies. For example they must adhere to DE 89/392/CEE (Machinery Directive) and they can only be installed in special rooms with specific features. Bibliography [1] IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and

Commercial Applications, IEEE Orange Book, 1996 [2] CEMEP, Uninterruptible Power Supply, European Guide, 1999


2 PRODUCT STANDARDS

2.1 The purpose of standards The main purpose of standards is to provide objective references to ensure a minimum acceptable safety level and, as with the latest standards, a reference for product performance levels. As is well known the strength of standards lies in the mutual recognition between nations (on a European and a wider international level) enabling products to be exchanged without specific restraints and without the need for specific adaptation. This means that there is a greater onus on manufacturers to pay more attention to specific problems, particularly in the area of personal safety. In return manufacturers can exploit the advantage of a more economic production process given by the standardisation of products, processes and performance levels. There is also more efficient technical communication given by the standardisation of symbols and icons. Finally, better consumer protection is ensured by a more transparent price/quality ratio. Standards are organised on the following three levels:

National European International

The European body is CENELEC (European Committee for Electrotechnical Standardization). Founded in 1973, CENELEC defines the conditions for free movement of goods within the EU to which the European Directives of today refer. CENELEC standards are identified by the suffix EN, and member states are legally obliged to enforce them. Modifications during translation into the language of the country that must adopt the standards are not permitted. There is a date (DOW Date Of Withdraw) by which time the national standards that contrast with the new ones must be withdrawn. There are numbers to identify the origins of the standard.

EN 50000 series: these are standards prepared by the CENELEC technical committees. The standard relating to UPS is part of this series because it was drafted by a CENELEC task-force called BT60-4.

EN 55000 series: these are drafted by CISPR and refer to radio interference. EN 60000 series: these are drafted by IEC and then adapted to European needs.

The international standards body is known as the IEC (International Electrotechnical Commission). All the countries in the world adhere to IEC standards. As has already been said, national standards are the exact equivalent of the standards used by the countries that make up CENELEC group. Therefore a British standard (BS) is the exact equivalent of the corresponding German standard (VDE) etc. That established by European directives must be added to this. The resolution known as the new approach, approved by the European Community Council in 1985, redefines the European Directive so that it no longer describes the technical requirements of a product, but exclusively the minimum safety requirements. Standards bodies are delegated the task of defining the technical specifications which must be met to show that a product conforms to said directives. European Directives relevant to UPS are as follows:

73/23/CEE (Low Voltage Directive). Council Directive 73/23/CEE of 19 February 1973 on the harmonization of the laws of Member States relating to electrical equipment designed for use within certain voltage limits. CE mark obligatory from 1/1/97.

89/336/CEE (Electro Magnetic Compatibility).


Council Directive 89/336/CEE of 3 May 1989 on the approximation of the laws of Member States relating to electromagnetic compatibility. CE mark obligatory from 1/1/96.

Table 2-1 shows an updated list of the publications which refer to the applicable European Directives.

European Directive References Amendments

EMC 89/336/CEE 92/31/CEE

93/68/CEE art. 5(4)

LVD 73/23/CEE 93/68/CEE art. 13

Table 2-1 Applicable European directives, amendments and Italian laws of recognition

Uninterruptible power supplies are exempted from application of the following European Directives:

89/106/CEE Construction Products Directive 89/392/CEE Machinery Directive

Specifically, the following standards have been issued with regard to uninterruptible power supply systems:

EN 50091-1 Uninterruptible power systems (UPS) - Part 1: General and safety prescriptions. This standard was ratified 9/12/92 with DOW 15/03/94.

EN 50091-2 Uninterruptible power systems (UPS) - Part 2: Electromagnetic compatibility (EMC) requirements. This standard was ratified 6/3/95 with DOW 01/03/96.

ENV 50091-3 Uninterruptible power systems (UPS) - Part 3: Performance prescriptions and test methods. This is a temporary standard, ratified 01/02/98, with a duration of three years.

Existing standards have then been amended giving rise to the following new standards: EN 50091-1-1

Uninterruptible power systems (UPS) - Part 1-1: General and safety prescriptions for UPS used in areas accessible to the operator.

EN 50091-1-2 Uninterruptible power systems (UPS) - Part 1-2: General and safety prescriptions for UPS used in limited access areas.

2.2 Standard EN 50091-1

This standard deals with the general prescriptions concerning the safety of UPS systems. The reference document (RD) is standard IEC 950, which corresponds to EN 60950: "information technology equipment including electrical equipment for the office: safety. We have used this standard (IEC 950), which to a large extent is not fully applicable to UPS, because it is an important benchmark for equipment, particularly limited power equipment, comparable with computer


equipment (PCs, printers, typewriters, etc.). This standard has been modified in some of its parts to adapt it to the specific features of UPS systems. We know that a UPS system has a very particular function within a power supply system: under normal functioning conditions it is simply a load for the mains power supply that powers it, with interface functions to filter and stabilise the electrical parameters of the network, to supply the load with perfect voltage and frequency. Whereas in the event of a mains power supply failure the UPS system behaves as an autonomous generator. Therefore the RD has been altered to ensure that it is a perfect tool for the task for which it was intended. Obviously, the work of those that write standards is an ongoing task. The working group is constantly making improvements and/or modifications. Particular attention has been given to the differences that may exist between different types of equipment particularly system power, which generally also gives an idea of the specific application type. For this reason standard EN 50091-1 was later divided into two separate standards:

EN 50091-1-1 EN 50091-1-2

As has already been said part 1-1 refers to installations accessible to all and therefore describes maximum safety level criteria, whereas part 1-2 refers to installations in locations that can only be accessed by specialist personnel. Clearly the aim of those that write standards is to allow the designer more room for manoeuvre. This means that although the designer must ensure that the vital personal safety performance levels are maintained, they can take different precautions compared to those for part 1-1. This is undoubtedly applicable to high power equipment where the system generally has different characteristics compared to a low power one. The main differences with EN 60950 are described in the section dealing with marks and instructions. In this case the power supply sources, the possible internal presence of batteries, the output power and relative load all had to be defined. Other specific features include the definition of qualified service and maintenance personnel responsible for installation and trained personnel. The main factors in the location of protection and/or the mains power supply switch, differentiating between installations with fixed and plug- in equipment, are also defined if they are not envisaged in the UPS. As far as backfeed protection is concerned, when the special circuitry is not envisaged, all the electrical cabinets that supply power to the UPS must be marked as such to protect the operator who may have to work on potentially live parts of the system. Furthermore the dispersion current is noted which is envisaged to be no greater than 0.05 (nominal input current). Given that the value of 3.5 mA has been exceeded then the earth conductor must be connected before any other conductor This factor must be highlighted. Annex N, defining the amount of air needed to ventilate the battery compartments, is of extreme importance (applicable also to battery rooms):

CInQ = 054,0

Equation 2-1

where:

0.054 result of the product (vqs). Value in Ahm 3

n number of elements (or cells) in the battery string

I AhA

1002,0

for valve regulated lead accumulators (VRLA)

C nominal battery capacity in Ah with autonomy of 10 hours.


Equation 2-1 enables calculation of the air flow rate (in m3 /h) needed to circulate through the ventilation holes so that the hydrogen concentration never reaches danger levels (4%). The annex also shows the calculation for the surface area of the openings, in cm2, given the necessary air flow rate, in the case of natural ventilation, defining the area sufficient to ensure an invective air flow rate to enable an exchange with the outside to guarantee the required flow rate. This is all valid in the absence of other air flow mechanisms that differ from natural motion. This calculation method ensures a sufficient level of safety against explosions given that the hot components (> 300C) or those that produce sparks are kept at a suitable distance from the ventilation holes (500 mm is sufficient in battery rooms). The formula also conforms with the new European standard EN 50272-2, Safety requirements for secondary battery installations. Part 2: stationary batteries (dop 01/04/2001), described in section 8. For natural ventilation this standard advises locating the air input and output holes on different walls, or to maintain a vertical distance of at least 2 m between them if the holes are on the same wall. Natural ventilation is preferred to forced ventilation. In the event that it is not possible to obtain the necessary air flow rate Q using only convective motion, then forced ventilation can be used. In this case ventilation functioning must be co-ordinated (interlocked) with battery charge functioning so that potentially dangerous situations do not arise (e.g. battery charge ON and ventilation OFF). Refer to section 6.3 for a description of recharging methods.


Figure 2-1 - Contents of standard EN 50091-1

CONTENTS 1 GENERAL 1.1 Scope 1.2 Normative references 1.3 Definitions 1.4 General requirements 1.5 General conditions for tests 1.6 Component 1.7 Power interfaces 1.8 Marking and instructions 2 FUNDAMENTAL DESIGN REQUIREMENT S 2.1 Protection against electric shock and energy hazard 2.2 Insulation 2.3 Safety Extra-Low Voltage (SELV) Circuits 2.4 Limited current circuits 2.5 Provisions for protective earthing 2.6 A.C. and D.C. power isolation 2.7 Overcurrent and earth fault protection 2.8 Protection of personnel - Safety interlocks 2.9 Clearances, creepage distances and distances through insulation 2.10 External signalling circuits 2.11 Limited power source 2.12 Protection of the applied load 3 WIRING, CONNECTION AND SUPPLY 3.1 General 3.2 Connection to power 3.3 Wiring terminals for external power conductors 4 PHYSICAL REQUIREMENTS 4.1 Stability and mechanical hazards 4.2 Mechanical strength and stress relief 4.3 Construction details 4.4 Resistance to fire 4.5 Battery location 5 THERMAL AND ELECTRICAL REQUIREMENTS 5.1 Heating 5.2 Earth leakage current 5.3 Electric strength 5.4 Abnormal operating and fault conditions ANNEXES A Test for resistance to heat and fire B Motor test under abnormal conditions C Transformer D Measuring instruments for earth leakage current test E Temperature rise of a winding F Measurements of creepage distances and clearances G Earth leakage current for UPS intended to be connected directly to it power systems J Table of electromechanical potentials K Thermal controls H Guidance on protection against ingress of water and foreign objects L Backfeed protection test M Examples of reference load conditions M.1 General M.2 Reference resistive load M.3 Reference inductive-resistive load M.4 Reference capacitiv e-resistive loads M.5 Reference non linear load N Ventilation of battery compartments N.1 Notes for guidance N.2 Application of lead-acid batteries N.3 Ventilation requirements (normative) P - Q A-Deviations


As far as the batteries are concerned, the recharging methods (external batteries), the location of the maximum current protection system, the value of the recharging current and all the information required to fully define the type of battery used must be defined. The variations for parts 1-2 are as follows:

Definition of accessibility and personnel. Safety instructions. Information on the manual rather than the mark. Protection against electric shocks. Design features of the case. Resistance to fire and dispersion current to earth.

2.3 Standard EN 50091-2

Part 2 refers to electromagnetic compatibility. This standard deals with all aspects relating to the applicability to UPS of existing standards that have in part already been experimented in the field, even though the specific difficulty of actuation and the importance of some types of measurement mean that further considerations must be made. The main aim however is to provide minimum criteria to guarantee sufficient electrical compatibility levels to ensure correct functioning both in an industrial environment and a residential one. It is for this reason that some emission limit categories have been introduced. Therefore UPS are classified according to the following rules:

UPS for unrestricted sales distribution. The most stringent emission limits are applied to this category of UPS in that the equipment can be used in any type of environment, both industrial and residential, without any restrictions. The UPS in this category are subdivided into two distinct groups: Class A: equipment suitable for use in non-residential environments, connected

directly to a mains power supply. This class is made up of all UPS that are permanently connected using an industrial type plug or socket. A warning concerning emissions is obligatory for this class: This product is a class A UPS. It may cause radio interference in residential environments. Refer to the user.

Class B: equipment suitable for use in all environments, including residential ones, and connected directly to a mains power supply. This class is made up of all UPS connected using a non- industrial type plug (IEC83).

UPS for restricted sales distribution.

The highest emission limits are applied to this category in that it is reserved for specialist users capable of assessing the necessary characteristics expected of the product. The reasons for this choice are linked mainly to economic and application aspects.

Equipment with a current greater than 25 A and a free surrounding space greater than 30 m belongs to this category. A warning concerning emissions is obligatory for this class: This product is restricted for sale to specialist installers or users. Installation restrictions or additional measures may be necessary to avoid disturbances. Paragraph 2 refers to the problems relating to driven type emissions for both unrestricted and restricted sales distribution UPS. Table 2-2 and Table 2-3 shown below describe the driven type interference voltage limits, to the network terminals, whereas Table 2-4 and Table 2-5 describe the irradiated type interference


voltage limits. The values shown in Table 2-2 and Table 2-3 increased by 14 dB are those for the UPS output driven interference voltage limits. As far as the low frequency driven emission limits are concerned the reference standard is EN 60555-2 which must be replaced by EN 61000-3-2 if its application field is relevant. Successive developments are currently being studied.

The measurement methods are described in the same paragraph.

Limits

[dB (mV)] Class A UPS Class B UPS

Frequency range (MHz)

Virtual peak Mean Virtual peak Mean from 0.15 to 0.50 79 66 (*) from 66 to 56 (*) from 56 to 46

from 0.50 to 5 73 60 56 46 from 5 to 30 73 60 60 50

(*) the limit decreases in line with the frequency logarithm

Table 2-2 Driven emission limits to network terminals for restricted sales distribution UPS

Limits [dB(mV)] UPS

nominal current

Frequency range (MHz) Virtual peak Mean

25-100A from 0.15 to 0.50 from 0.50 to 5.0 from 5.0 to 30.0

100 86

(*) from 90 to 70

90 76

(*) from 80 to 60

101-400A from 0.15 to 0.50 from 0.50 to 5.0 from 5.0 to 30.0

130 125 115

120 115 105

> 400A from 0.15 to 0.50 from 0.50 to 5.0 from 5.0 to 30.0

Under study Under study

(*) the limit decreases in line with the frequency logarithm

Table 2-3 Driven emission limits to network terminals for restricted sales distribution UPS

Figure 2-2 shows the driven emission limits to network terminals in graph format. Figure 2-3 shows, in graph format, the emission limits given by standards VDE 0871 and VDE 0875 normally used before definition of product standard EN 50091.


Figure 2-2 Driven emission limits to network terminals EN 50091-2

Figure 2-3 Driven emission limits VDE 0871 VDE 0875

0

20

40

60

80

100

120

140

0.1 1 10 100 MHz

Virt

ual p

eak

[dB

(mV

)]

RS 100 - 400A

RS 25 - 100A

Class A

Class B

0

20

40

60

80

100

120

140

0.1 1 10 100

Vir

tual

pea

k [d

B(m

V)]

VDE 0875 G

VDE 0875 N

VDE 0871 A - C VDE 0871 B

VDE 0875 K

MHz


The irradiated emission limits have been defined as shown below.

Virtual peak limits [dB(mV)] Frequency range

(MHz) Class A UPS Test distance

10 m

Class B UPS Test distance

10 m from 30 to 230

from 230 to 1000 40 47

30 37

Table 2-4 Irradiated emission limits for unrestricted sales distribution UPS

Virtual peak limits [dB(mV)] Frequency range (MHz) Test distance

30 m from 30 to 230

from 230 to 1000 Under study

Table 2-5 Irradiated emission limits for restricted sales distribution UPS

The irradiated emission limits are shown in graph format in Figure 2-4.

Figure 2-4 Irradiated emission limits EN 50091-2

Section 3 deals with immunity to disturbances. This is tested by taking into consideration the output characteristics (according to that envisaged by ENV 50091-3) and the various functioning modes. In general there are two criteria to satisfy, A and B.

1

10

100

10 100 1000 MHz

Virt

ual p

eak

[dB

(mV

)]

RS &

Class A

Class B


Performance criteria for immunity tests

Component Criterion A Criterion B Output characteristics Internal and external measurement indications Command signals to external devices Functioning mode

Static tolerances (ENV 50091-3)

Test variations

No variation

No variation

Dynamic tolerances

(ENV 50091-3)

Test variations

Variations depending on functioning mode

Only temporary variations

Table 2-6 Immunity levels

The prescribed tests are as follows:

Type of test Reference standard Prescription Performance Electrostatic discharges IEC 801-2 3 B Electromagnetic fields IEC 801-3 2 A

Quick transients IEC 801-4 2 A Overloads IEC 801-5 Under definition Under definition

Low frequency IEC 1000-2-2

Table 2-7 Immunity tests

2.4 Standard ENV 50091-3

This part attempts to cover the performance aspects of the product which up to now were only marginally touched on by an IEC document. Standards IEC146-4 and IEC146-5 deal with the problems more directly linked with the system and with specific refe rence to UPS. The EN standard is applicable to UPS in both mono-phase and three-phase configurations both in input and in output. Environmental functioning, storage and transport conditions are also defined. Section 3 defines the electrical functioning characteristics with reference to input and output performance levels. Three different groups are used according to the dynamic output characteristics. In particular these groups enable an initial selection of equipment type on the basis of specific application needs. The UPS offering the best dynamic output characteristics and therefore the best performance levels belong to Class 1 (Figure 2-5). Those UPS able to power not particularly sensitive loads belong to Class 2 (Figure 2-6). Those US with particularly low performance levels (no voltage for 10 milliseconds) belong to Class 3 (Figure 2-7).


Figure 2-5 Dynamic output performance for Class 1 UPS




The standard also applies to UPS with a non-sinusoidal wave form, and in this case describes the minimum characteristics of this wave form.

Figure 2-8 Non-sinusoidal wave form

0.1 Up

Up 0.9 Up

dUp

U

dt dt

Up

t T/2 T


Section 4 deals with the problem of UPS electrical testing and defines both type tests and those done in the factory. Section 5 describes the non-electrical tests including transport, environmental and noise level tests. Section 6 includes a classification of the UPS characteristics on the basis of performance. This enables comparisons under the same conditions and using the same measurement methods. This classification is shown below. V F I S S 1 1 2 ____________________ ___________ ____________________ Dependency of output on dependent input only in Normal mode

Output wave form Dynamic output performance

1st character = Normal or Bypass functioning

1st character = Performance during functioning mode changeover

VFI = UPS output voltage and frequency independent from input and within limits prescribed by standard ENV61000-2-2

2nd character = Battery functioning

2nd character = Performance with linear load step in Normal/battery (worst case scenario) functioning

VFD = UPS output frequency and voltage dependent on input

3rd character = Performance with reference non-linear load step in Normal/battery (worst case scenario) functioning

VI = UPS output dependent on input frequency variations whilst voltage is stabilised

S = Sinusoidal wave form with total harmonic distortion D


As far as the annexes are concerned, it is worth highlighting that D8 contains some of the commonest configuration examples with the aim of providing the user with information concerning the technology used by the various constructors. These are as follows:

Figure 2-9 Double conversion

Figure 2-10 Double conversion with bypass

AC output

Battery Battery mode

Normal mode

AC input

AC input

Inverter

Battery charger

Block diode

AC/DC converter

AC output


Normal mode

AC input

AC input

Inverter

Battery charger

Block diode

AC/DC converter

Static switch

AC input

Bypass mode

Bypass (main or standby)


Figure 2-11 - Interactive

Figure 2-12 Stand-by

AC output


Normal mode

AC input

Inverter

Power interface

Normal mode

Battery mode

Switch

Battery

Battery charger

Inverter

Ac output

Ac input

(optional connection)

AC switch


Bibliography [1] EN 50091-1-1 Uninterruptible power systems (UPS) - Part 1-1: General and safety

requirements for UPS used in operator access areas. [2] EN 50091-1-2 Uninterruptible power systems (UPS) - Part 1-2: General and safety

requirements for UPS used in limited access areas. [3] EN 50091-2 Uninterruptible power systems (UPS) - Part 2: Electromagnetic compatibility

requirements (EMC). [4] ENV 50091-3 Uninterruptible power systems (UPS) - Part 3: Performance requirements

and test methods.


3 ELEMENTS OF ELECTROMAGNETIC COMPATIBILITY An uninterruptible power supply is an electricity supply network conditioning device designed to maintain the correct level of power to electrical equipment. The electrical equipment is usually of vital importance in terms of its utilisation cycle. The UPS must also supply power when there is lots of disturbance in the mains electricity supply and even when there is a mains power supply failure. It is obvious that such a system will have a complex design in that on the one hand it must meet the specifications of the power supply source and on the other the specifications of the load, and of course these two factors may not be compatible. Both are variable and at the same time complex. The mains electricity supply can be that provided by the national grid or in some cases another electricity generating unit. The load varies and usually consists of process controllers, electric motors, remote control switch coils and system or emergency lighting. The load can be either mono-phase or three-phase and may involve IT systems such as PCs, main frames, etc. In terms of the electrical conditions, the most important factor to take into consideration is the level of disturbance from the mains power supply, the level of disturbance injected into the network and the type of load being supplied. Of course system conditions upstream and downstream of the UPS must not be overlooked and therefore the level of selectivity of these conditions.

3.1 Interface with the power supply

3.1.1 Disturbances from the network The biggest problems caused by the interface with the mains power supply involve voltage distortion, electromagnetic disturbances which can compromise correct system functioning and problems caused by the behaviour of other loads connected to the same line (the pickup of motors, triggering of maximum current fuses, distorting loads, etc.). The voltage is often subject to varying levels of disturbance which cannot always be controlled. These disturbances are produced and transferred throughout the circuit. It is obvious that a good UPS must be able to withstand these disturbances and consequently non not transfer the disturbance to the output terminals where the load is connected. Therefore pulsed voltage overloads, less sudden variations, wave form distortions and frequency variations (the latter are particularly frequent in the event of the intervention of an electricity generating unit) must be considered as important design parameters.

3.1.1.1 Voltage variations Given that electricity distribution lines are subject to continuous load variations, they cannot supply a perfectly constant level of voltage. In addition there is the problem of falls in voltage caused by undersized lines, overloads and voltage increases which occur when larger consumers do not absorb the expected level of power. These factors cause voltage variations which result in serious problems for the power supply stages of electronic equipment.


Figure 3-1 - Voltage variations

3.1.1.2 Pulsed variations These are brief surges. They are extremely dangerous for electronic and IT loads in that they can reach very high levels, sometimes several thousand volts. They are normally caused by switching on high voltage power lines, atmospheric phenomena, and the enabling or disabling of reactive loads.

Figure 3-2 - Pulsed variations

3.1.1.3 Radio disturbance This is very common and is generally caused by all those devices which, due to electrical size switching (voltage/current), produce radio frequencies. Typical examples include equipment using static switch assemblies, transmitters, commutator motors, etc. The presence of such disturbance in the distribution network can affect the IT load thus altering its functioning.


Figure 3-3 - Radio disturbance

3.1.1.4 Mains power supply failure This is the most obvious problem, even though it is the most infrequent, because a power supply failure is recognised by everyone. A power supply failure can be caused by faults in the production system or on the distribution lines. Power supply failures that last for only a short time are very significant, (statistics show that 90% of power supply failures do not exceed 100 milliseconds) and they are normally caused by short circuits or switching.

Figure 3-4 - Power supply failures

These disturbances can also be further divided as follows: a) External factors

Overhead power supply cables are often subject to a decrease in power quality caused by atmospheric phenomena e.g. lightning striking the line.

Factors reflected from activity in other geographically distinct areas electrically connected to the line. This means that accidental switching or short circuits in other parts of the line can be detected at the coupling point. Loads connected at the same point and characterised by high levels of current distortion causing


high levels of voltage distortion. The associated harmonics can cause resonance which can result in very dangerous overloads.

b) Internal factors

These include low voltage activity. Load connection and disconnection can cause overloads dangerous enough to damage the UPS input circuits.

3.1.2 Disturbances injected into the mains power supply In general the UPS can be considered a conversion system. The technology used for the input stage determines the type and quantity of harmonic pollution injected into the mains power supply upstream. For example, in double conversion UPS the converter is connected directly to the mains power supply and an AC/DC converter. If the converter is a controlled total jumper one consisting of thyristors then the harmonic pollution on the current ensures a THDI reading 30% during normal functioning. If current limitation functioning is reached then the current distortion is even higher. As is well known this can cause serious problems (refer to section 3.1.3). In general a current distortion total that causes a voltage distortion level at the common coupling point not exceeding 8% is considered reasonable (defined also by ENV 61000-2-2 article 2 for public supply networks, see Table 3-1, and EN 50160 article 2.11). The impedance value upstream, generator power and the level of harmonics created by the UPS must be known in order to obtain this value. These values can be used to identify the voltage distortion caused by the UPS at the common coupling point, if the UPS is considered as the only load powered by the upstream source. The existence of other loads powered by the source, as well as the UPS, means calculating the harmonic voltage distortion, assessing also the harmonic pollution level caused by these loads.

Odd harmonics not multiples of 3

Odd harmonics multiples of 3

Even harmonics

Harmonic order

Harmonic voltage

%

Harmonic order

Harmonic voltage

%

Harmonic order

Harmonic voltage

% 5 6 3 5 2 2 7 5 9 1.5 4 1 11 3.5 15 0.3 6 0.5 13 3 21 0.2 8 0.5 17 2 >21 0.2 10 0.5 19 1.5 12 0.2 23 1.5 >12 0.2 25 1.5

>25 0.2 0.5 25/n

Table 3-1 Compatibility levels for the individual harmonic voltages in low voltage networks ENV 61000-2-2 article 2


An interesting approach is the one established by the British standard which sets the maximum (absolute) acceptable current levels (G5/3). Therefore up to a power level of 100 kVA the UPS can be installed without taking any specific precautions. Whereas for higher power levels a system to reduce the level of harmonic current content injected into the network must be used. Table 3-2 shows the typical percentage current distortion values, for six-phase and twelve-phase rectifiers, compared with the absolute values (A) suggested by G5/3. Therefore beyond certain power levels it is possible to use twelve-phase conversion circuits when there is only one UPS. In the event of more than one UPS powered by the same source, system phase displacement circuits or out of phase twelve-phase systems can be used to ensure that the standard is met as effectively as possible. In general a good level of harmonic voltage distortion at the common coupling point can be obtained when the ratio between UPS/network impedance is greater than 5 for a six-pulse converter (six-phase) and greater than 3.5 for a 12-phase converter.

Harmonic order

Harmonic content (%)

G5/3 (A)

Six-phase Twelve-phase

2 3 0 48 3 0 1.7 34 4 0 0 22 5 26 3.6 56 6 0 0 11 7 5 2.3 40 8 0 0 9 9 0 0 8 10 0 0 7 11 4.5 4 19 12 0 0 6 13 3 7 16 14 0 0 5 15 0 0 5 16 0 0 5 17 0.5 2 6 18 0 0 4 19 1.6 2 6

Table 3-2 Acceptable maximum absolute levels according to standard G5/3

Some examples of the technological solutions currently used for AC/DC converters in three-phase input UPS are shown below. Six-phase rectifier (fully controlled), six-phase rectifier with fifth harmonic filter, twelve-phase rectifier.


Harmonic Six-phase Twelve-phase + 10% filter

1 100% 100% 2 0% 0% 3 0% 0% 4 0% 0% 5 26% 4% 6 0% 0% 7 5% 4% 8 0% 0% 9 0% 0% 10 0% 0% 11 4.5% 4.5% 12 0% 0% 13 3% 3% 14 0% 0% 15 0% 0% 16 0% 0% 17 0.5% 1.8% 18 0% 0% 19 1.6% 1% 20 0% 0%

THDi % 28% 8.5%

Table 3-3 Comparison between six-phase and six-phase with fifth harmonic filter

0

5

10

15

20

25

30

ampl

itud

e %

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20harmonic order

Six-phaseTwelve-phase + 10% filter


Harmonic Six-phase Twelve-phase

1 100% 100% 2 0% 0% 3 0% 1.7% 4 0% 0% 5 26% 3.6% 6 0% 0% 7 5% 2.3% 8 0% 0% 9 0% 0% 10 0% 0% 11 4.5% 4% 12 0% 0% 13 3% 7% 14 0% 0% 15 0% 0% 16 0% 0% 17 0.5% 2% 18 0% 0% 19 1.6% 2% 20 0% 0%

THDi % 28% 9.8%

Table 3-4 Comparison between six-phase and twelve -phase

0

5

10

15

20

25

30

ampl

itud

e %

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20harmonic order

Six-phaseTwelve-phase


3.1.3 Correct sizing of electrical energy sources in systems with uninterruptible power supplies.

This section provides a description of how to size an electrical energy source to be used in applications with an uninterruptible power supply (UPS). Guidelines for calculating the power of electricity generating units used in fail-safe uninterruptible power supplies are also shown. When an alternated electricity source has to be sized to power non- linear loads, as well as the active and reactive power, the harmonic voltage distortion must also be considered. This distortion is the effect of current non-sinusoidal absorption on the part of the non-linear load. According to the Fourier series, each non-sinusoidal wave form can be considered as the sum of the fundamental frequency multiple frequency sinusoids (harmonics). The harmonic currents generate multiple-frequency potential falls on the equivalent impedance of the power supply system (including the source internal impedance). The result is a distorted voltage wave form. If there is too much distortion then there are various negative effects on the system, including the possible incorrect functioning of the equipment connected to the common coupling point. The European reference standard is ENV61000-2-2 which imposes a maximum total voltage harmonic distortion value less than or equal to 8%. Under these conditions each source and each load must function correctly. The system sees uninterruptible power supplies as non-linear loads and therefore a method of sizing the electrical power must be found to limit the harmonic voltage distortion.

3.1.3.1 Sizing the electrical power of the source The model used for the calculation is that shown in Figure 3-5. The source is represented only by the impedance which is the equivalent of a series of outputs whereas the UPS input stage is represented by an ideal generator of harmonic currents.

ZSIn

Vnom

Sour

ce

UPS

Figure 3-5

To justify this it can be shown [3] that the harmonic currents do not contribute to the active power transferred to the load but only the reactive power, and therefore they are considered injected into the network by the load itself. Despite the minimum complexity of this model, the calculations and experience show that the model enables reliable sizing of the source if the equivalent impedance of the power supply line can be overlooked.


The following definitions are required to make the calculation: Normalised equivalent impedance SZ : this shows the voltage decrease, compared to the

nominal functioning voltage, that the equivalent impedance ZS would cause if it was traversed by the nominal current at the fundamental pulse w0=2pf0 (all currents and voltages shown are efficient values):

Equation 3-1

nom

SOnomS

S VIZ

Z

SOnom

nom

S 0 d

I nominal source current (efficient value)V nominal voltage (efficient value)Z ? L

==

=

Total harmonic voltage distortion THDV:

Equation 3-2

0

1

2n

V V

VTHD

0

n

V fundamental harmonic voltageV n harmonic voltage

==

On the basis of these definitions and used the model shown in Figure 3-5 it is possible to obtain Equation 3-3:

Equation 3-3

UPSmax

SOnom

V

UPSSUPSmax

SOnom

dim A A ithw 100,%THDKZ

AA

R =

where SOnomA means source apparent electrical power, where UPSmaxA means UPS apparent electrical

power, where Rdim means sizing ratio between source and UPS and where KUPS means a typical input stage parameter of the UPS. Typical values for the KUPS parameter are shown in Table 3-5 for some AC/DC converters used in UPS. Equation 3-3 links the total harmonic voltage distortion, the ratio between the nominal apparent electrical powers of the source and the UPS, the output feature of the source itself and the technology used to manufacture the UPS AC/DC converter. Indeed the KUPS parameter is a function of the normalised harmonic spectrum of the current absorbed by the UPS input stage. If a maximum total harmonic voltage distortion is set at the common coupling point, and assuming that the source only powers the UPS, then Equation 3-3 can be used to perfectly size the apparent power of the source.


UPS AC/DC converter KUPS

six-phase 1.7 twelve-phase 0.8 six-phase with filter1 1.0 PFC2 0.6

Table 3-5 Typical values for the KUPS parameter

If the source only powers the UPS then the apparent power of the latter to apply in Equation 3-3 is the maximum value shown in the technical specifications, that is to say the nominal apparent output power divided by the UPS AC/AC UPS performance plus, if relevant, the power required to recharge the batteries. It is worth noting that if the apparent power absorbed by the UPS is less than the maximum value then a portion of the power will be supplied to the nominal load by the UPS batteries. This functioning cycle is limited in time by the autonomy of the batteries for the load fraction that they must power. It will be seen in the next section that the latter consideration can be useful when the electrical energy source is an electricity generating unit. Still assuming that the source only powers the UPS under nominal conditions and that the ratio of the apparent powers in Equation 3-3 is virtually unitary, then the following approximation can be considered valid:

UPSSV KZ100%THD @ Equation 3-4 that is to say the voltage distortion at the UPS input is approximately equal to normalised equivalent impedance of the source for the KUPS if UPS and source sizing are in a ratio of one to one.

3.1.3.2 Some application examples If the source of electrical energy is a transformer then the normalised series equivalent impedance is equal to the normalised short circuit impedance3. The model used for the voltage distortion calculation is valid for single-phase transformers and three-phase transformers powered by three symmetric voltages and connected to balanced loads. The typical normalised short circuit impedance values vary from 4% to 7%. The impedance is mainly inductive. If the energy source is an electricity generating unit then the normalised equivalent impedance is the normalised sub-transient reactivity of the alternator. The values for this parameter are usually in a range between 8% and 20%. Table 3-6 shows, by way of example, some sizing ratios with the maximum harmonic voltage distortion set at 8%. It should be noted that the sizing ratio only refers to the apparent electrical power supplied by the source. Therefore in the event that the source is an electricity generating unit, the ratio refers to alternator power and not motor power. Keeping the harmonic distortion within acceptable values when the power is supplied by an electricity generating unit is of vital importance. Indeed, whilst the damage caused by the harmonic distortion from a transformer power supply may not be immediately obvious, that from an

1 The filter is passive and resonant on the fifth harmonic (L-C) 2 Three-phase input, boost converter (CHLORIDE Synthesis Twin series) 3 The normalised short circuit impedance value is more or less equal to the normalised short circuit voltage on the transformer secondary.


electricity generating unit may result in complete instability during voltage adjustment. This may cause the generator to be switched OFF and the loss of the load after the expiry of UPS autonomy. The following section shows a sizing example with an electricity generating unit. UPS AC/DC converter ZS=4% ZS=6% ZS=12% ZS=20% six-phase 1.0 1.3 2.5 4.2 twelve-phase 1.0 1.0 1.2 2.0 six-phase with filter 1.0 1.0 1.5 2.5 PFC 1.0 1.0 1.0 1.5 THDv% 8% 8% 8% 8%

Table 3-6 Rsizing as a function of source output equivalent impedance to obtain THDV= 8%.

3.1.3.3 Example of electricity generating unit sizing for UPS It is assumed that an uninterruptible power supply must be provided for a generic three-phase load Figure 3-6 absorbing an apparent Anom and active Pnom power.

M UPS Load

? G ?UPS

Anom, PnomAUPS, PUPSPM AGEN

Figure 3-6

The maximum power absorbed by the UPS input, both active and apparent, can be deduced from the technical specification4. However, the maximum power in input will be equal to Pnom absorbed by the load divided by UPS performance plus the level of active power required to recharge the batteries, if relevant. Given that AUPS is known, the electricity generating unit alternator can be sized by calculating the required AGEN value using the method described in section 3.1.3.1. Given the values for active power absorbed by the UPS and alternator performance, it is possible to calculate the mechanical power to the shaft that the motor must supply to the latter. Therefore the following equation is valid for motor sizing:

UPSG

nomM ??

PP = Equation 3-5

Consider the following example5: the maximum apparent power in input to a UPS with a six-phase AC/DC converter, at nominal voltage (400 V between phases), is 110 kVA with quick-charge batteries.

4 In this example (double conversion UPS) assume AUPS @ 1.25 Anom 5 The example is based on the technical specifications of the CHLORIDE EDP90 series


The electricity generating unit must be sized only to supply the UPS. From Table 3-6 it can be deduced that if the electricity generating unit alternator has a reactive sub-transient value of 12%, then the unit must be able to supply an apparent power of 250 kVA so that the total harmonic voltage distortion is equal to or less than 8%. It should be remembered that normal functioning of the electricity generating unit and the UPS, and indeed any other equipment powered from the common coupling point, cannot be guaranteed if this distortion value is exceeded. The maximum active power required at the UPS input is 72 kW in float charge (the power factor in input is 0.83 at nominal voltage). Therefore if the alternator has a performance level of 70%, the unit motor must be able to supply a maximum power of 95 kW. Now let us assume a UPS with the same power but a twelve-phase input converter. In this case the alternator must be capable of supplying 150 kVA of power. Note that motor sizing remains unchanged. Indeed, if alternator performance remains unchanged, the motor must supply the same level of active power in both cases. Table 3-7 illustrates the costs for a UPS and an electricity generating unit for the two scenarios described above.

AC/DC converter

UPS cost without batteries

(ITL millions)

UPS ANOM (kVA)

Generator cost (ITL millions)

Generator alternator

power

Total cost (ITL millions)

six-phase 42 100 75 250 117 twelve-phase 48 100 62 150 110

Table 3-7

It can be seen from this example that the saving on the generator, as well as compensating for the higher cost of the UPS (twelve-phase technology provides better performance levels and is therefore more costly than six-phase technology) enables an overall saving of approximately 6%. When a UPS mounting a six-phase input converter with a resonant L-C filter is used, the load percentage applied to the UPS plays a vital role. When the load applied to the UPS is 30% of the nominal power, the bank of filter condensers is designed to re-phase (at the fundamental frequency) all the reactive power at the rectifier input (which has an inductive power factor). If the load applied to the UPS should decrease further, the input power factor of the UPS becomes capacitive. Normally electricity generating units find it very difficult to adjust output voltage in the presence of capacitive loads. Therefore the use of six-phase rectifiers for applications in which the nominal power of the UPS is underused should be avoided.


3.2 Interfacing towards the load

3.2.1 Types of load The types of load requiring a more careful assessment are electric motors and distorting loads (typically variable frequency motor commands, DC motor commands, distorting loads and obviously all types of IT load). In the first instance problems derive from the characteristic current surge which causes the UPS to be oversized or the overall features of the load to be assessed very closely: sequencing when supplying the motors, start configuration type (star-triangle connection), etc. If the current surges are too high and/or last longer than that accepted by the inverter (causing current limit functioning with a natural decrease in the output voltage) then the standby network line can be used. The static switch assembly means that this line is able to supply very intense currents (10-15 In) for some periods (see section 4.2.1 for a more detailed description). The real problem arises when the mains power supply is not available because the tolerance is outside the accepted limits or even missing and therefore the inverter must bear the surge. In the latter instance the only alternative is to oversize the UPS. The Power supply to highly distorting loads consisting of six-phase rectifying circuits has a very low impedance level around the 5th and 7th fundamental harmonic. This causes the generator (UPS) to supply more current. Therefore the power supply system output impedance must adapt itself to these load conditions. As a general rule low impedance and good overload capacity are required. These must be able to meet the demands of the current surges without large variations in the output voltage. Another type of highly distorting load is the classic single-phase load with a powerful third harmonic component, which ranges from between 40% and 70% with total distortion levels which can reach 130%. These are classic IT loads with a crest factor6 of 3:1. They are most common in telecommunications and finance and typically linked to the problem of distributed rather than centralised architectures which are generally used in industrial contexts.

Single-phase distorting load

6 The crest factor (CF) is the ratio between the peak value and the efficient value of a wave form.

N

+

-

C L


Three-phase distorting load

Harmonic Single-phase distorting load

CF = 2 CF = 2,5 CF = 3 1 100% 100% 100% 2 0% 0% 0% 3 41% 56% 72% 4 0% 0% 0% 5 23% 42% 61% 6 0% 0% 0% 7 16% 25% 55% 8 0% 0% 0% 9 9% 10% 35% 10 0% 0% 0% 11 7% 10% 22%

THDI% 50.9% 77.2% 116.7%

Table 3-8 Harmonic analysis for distorting loads at different crest factor values (CF).

0

20

40

60

80

100

ampl

itud

e (%

)

1 2 3 4 5 6 7 8 9 10 11harmonic order

CF=2CF=2.5

CF=3

Figure 3-7 - Distribution of single-phase distorting load harmonics at different crest factor values

L1 L2 L3

+

-

C


3.2.2 Current in the neutral conductor The presence of single-phase distorting loads (therefore with a high 3rd harmonic %) overloads the neutral conductor with currents that are 1.5 - 2 times the line current as a result of the combination of line currents in the neutral conductor itself (phase/amplitude). IL1 = IL2 = IL3 IL_PICCO = 2,5IL_RMS

L_RMSL_RMS

N I1,72

I2,5I @

=

Figure 3-8 - 3rd harmonic current in the neutral conductor

3.3 Radio frequency interference (RFI) and unipotential connection

The electromagnetic compatibility of the individual devices, as with the relevant electrical systems, has today become a technical aspect that cannot be overlooked because of the increasing number of potentially disturbing sources. Electromagnetic compatibility should be addressed during the design stage of a system. Failure to do so will almost certainly result in problems that will not be easy to rectify. Therefore, regulation bodies throughout the world have drafted a set of rules. These include immunity and emission levels beyond which an electrical device may not be compatible with the electromagnetic environment surrounding it.


In Europe the general reference standard is Directive 89/336/CEE, followed by the more specific publications for each production sector, that is to say EN 55022 and EN 50091-2. All electrical and electronic devices emit disturbance and, at the same time, all are susceptible to the disturbance emitted by other devices in the surrounding environment. The standards require that such devices only emit a certain level of disturbance and that they can coexist with the environment in which they are located without suffering outside influence, that is to say they continue to function correctly.

3.3.1 Disturbance Electromagnetic waves emitted by electrical and electronic devices cause disturbance which can vary in intensity across the whole frequency spectrum. This disturbance causes interference which in turn causes any faults to the equipment involved. These faults can manifest themselves in reading errors in the case of data transmission, or even genuine functioning errors. There are two types of electromagnetic disturbance: either transmitted along cables or irradiated into the air. In the latter case electrical devices act as aerials thus causing all equipment to act as either a receiver or an aerial.

3.3.2 Unipotential connections Earth connections and optimised earth connections are important aspects in terms of compatibility. Earth means a unipotential point or surface which acts as a reference for the system. In the first place it is important to have a single good earth connection, and secondly, if possible, a mesh earth network (see Figure 3-9). This, together with the optimised mass, helps to ensure a unipotential connection for either the site or device in question. This also ensures considerable advantages at both low and high frequencies. The mesh earth system, using the appropriate PE or PEN cables, ensures excellent personal safety levels. As far as high frequencies are concerned, a dense unipotential mesh network ensures good performance in terms of system and equipment compatibility. Unfortunately PEN earth cables are only effective with low frequencies and therefore can only be used to ensure safety.


Figure 3-9 An example of a mesh network

On the other hand, the maximum possible number of earth connections must be made to ensure good functioning at high frequencies. The connections must be made using cables with a diameter that is less than the PE used as a reference. These connections must be located near to the earth of the various devices, cable ducting, existing or specially added metal structures. Therefore safety and good compatibility can be obtained simultaneously by giving serious attention to the multitude of connections between the earth and unipotential factors.

Figure 3-10 An incorrect example of connections between earths

However the connections between masses must be made with care. Star connections between devices and an earth connection should be avoided at all costs (see Figure 3-10). Therefore a mesh network should be used to directly connect the relevant equipment, ensuring that the live cables run as closely as possible to the network elements. Furthermore, the path of the earth connections must be designed carefully to ensure the correct layout of earth rings and rings between earths (see Figure 3-11).

LIVE CABLES

EARTHS

NO

NO NO

NO NO

NO

YES

YES


Figure 3-11 An example of earth rings

Earth rings are surfaces between a functional cable (power supply cable, command cable, connection network) and the conductor or the closest mechanical earth. Rings between earths are surfaces between two or more earth cables. The number of earth rings is equal to the number of functional cables. These rings are a dangerous means of coupling irradiated disturbance. Therefore, their surface must be reduced. That is to say the communication cables must be passed as close as possible to the earths.

3.3.3 Outside earths The conductors, both live and not, are components that when acting as aerials receive signals. If they conduct high-frequency current they can also emit signals. To ensure the least number of problems involving outside earths, the indications concerning mesh networks and unipotential factors should be followed. Furthermore a good mesh connection for the metal structures, metalworking and the earth networks is very important. In light of this, it should be remembered that the connections should ensure maximum contact and minimum connection length. Observe the following recommendations:

Use plaits. Bolt the ducts together. Ensure that the connection points are unpainted so as to maximise the contact.

All forms of insulation should be removed from the connection points. Observe the following recommendations:

NO

YES

OK

OK OK

!

! !

!


- Signal cables that can cause interference must be separated by a distance proportional to their length. This does not apply to screened cables which can, for example, be laid in the same duct.

- The conductors not used in a cable must be connected to the earth at both ends. - If cables must be crossed for different signals that can cause interference then the angle

between them must be 90. - The use of metal ducting is a good form of shielding. In particular the internal angles of

metal ducts have little exposure to electromagnetic disturbance. - Earth plate continuity must be ensured. Connection must always be perfect i.e. bolted

metal ducts, connection points should be neither painted or coated.

3.3.4 The cabinet A cabinet or rack for electrical equipment must be made. This should be designed with an unpainted earth plate on its base. The plate should be connected to the cabinet frame at various points. The frame should then be connected to the earth network. When designing the cabinet, the power components must be kept separate from those with low emission levels (logic, etc.). Earth plate continuity must be ensured in the event of two or more cabinets. The star connections to supply power to two or more devices should be designed so that they are as close as possible to the power source (transformer or mains power supply) so that the cables can be laid at a certain distance from each other. Two distinct power supplies should be used in the event of particularly sensitive equipment. System components emitting higher levels of disturbance should be connected close to the power supply whereas those causing less disturbance can be moved further away. The earth connections of any transformers inserted in the cabinet mus t be as short as possible.

3.3.5 RFI filters The following recommendations should be observed for RFI filters:

- The filters should be mounted at the front of the cabinet, and then bolted to the cabinet frame or earth plate.

- Any cables must be located as close as possible to the cabinet earth plate. - It is important that the filter input and output cables are laid separately and that they do

not have connections likely to cause extensive earth rings.


Bibliography [1] F. Costa, Generating sets matching rectifiers: comparison between 6 and 12 pulse

rectifiers CHLORIDE Applications document, 1989

[2] Le armoniche negli impianti di potenza, RL 88.01.15 CHLORIDE Applications document, 1988

[3] J. Arrillaga, Power sys tem harmonics John Wiley & Sons, 1985 [4] E. Cevenini, Il corretto dimensionamento delle sorgenti di energia elettrica in impianti con

gruppi statici di continuit CHLORIDE White Papers, 2000


4 POWER SUPPLY SYSTEMS

4.1 Classification Standard HD 384 requires that a power supply is cut automatically if, in the event of a fault, there is a risk of personal injury due to the size and duration of the voltage contact. This safety measure requires co-ordination between the system earth connection method and the characteristics of the protection conductors and the protection device. Furthermore standard HD 384 states that the earths must be connected to a protection conductor under the specific conditions of each earth connection. Protection against indirect contacts depends on the type of neutral distribution adopted in the system. It should be remembered that the UPS in no way alters the neutral distribution system because it is a "through" system. Some UPS models ensure full isolation between the input and the output. Therefore the input neutral conductor is isolated from the output one. This feature must be expressly indicated in the UPS technical specifications. The following rules should be used to determine the type of neutral distribution: First letter Power supply system status to earth.

T Direct earth connection of a point (usually the neutral one).

I Isolated from earth, or earth connection using an impedance. Second letter Status of earths to earth.

T Earths connected directly to earth.

N Earths connected to the earthed point in the power supply system.

4.1.1 TN systems The TN system has the neutral cable connected directly to the earth in one point, and the system earths connected to the same point by a protection conductor (Figure 4-1).


Figure 4-1 - TN system (HD 384-3)

There are three types of TN system depending on the layout of the neutral and protection conductors. TN-S The neutral and protection conductors are separated. TN-C The neutral and protection functions are combined in a single conductor (PEN). TN-C-S The neutral and protection functions are combined in one part of the system.

4.1.2 Protection against indirect contacts in TN systems The fault ring consists exclusively of metal components (see Figure 4-2), in which Ig = fault current).

(TN-C) L1

L2

L3

PEN

(TN-S)

PE

N


Figure 4-2 Fault ring in the TN system (HD 384-3)

As far as the type of protection against indirect contacts is concerned, the following equation must be adhered to (HD 384-4-41):

0UIZ as Equation 4-1

where: Ia is the current level that triggers the automatic protection device within the preset time limit. U0 is the nominal AC voltage, the efficient value between the phase and earth. Zs is the impedance of the fault ring. Given that the fault impedance Zs is always very low, the above ratio is always correct. A protection device with an inverse delay time trigger is required. Distribution circuits or terminals supplying power to fixed equipment can have a maximum protection device trigger delay time of 5 seconds. Whereas circuits supplying power (with or without a socket) to class 1 electrical components and mobile equipment have a drastically reduced protection device trigger time, as shown in Table 4-1.

Nominal voltage Uo (V) Delay (sec) 120 0.8 230 0.4 400 0.2

> 400 0.1

Table 4-1 Maximum stop delay times for TN systems (HD 384-4-41)

Ig

L1

L2

L3

PEN Ig

N

M

0 PE (TN-C) (TN-S)


In systems where the conventional contact voltage limit is 25 V (prHD 384-7-704; HD 384-7-705) the values shown in Table 4-3 must be replaced by those shown in Table 4-2.

Nominal voltage Uo (V) Delay time (sec) 120 0.4 230 0.2 400 0.06

> 400 0.02

Table 4-2 Maximum protection trigger delay times for TN systems with a conventional voltage limit of 25 V (IEC 364-4-481)

In all cases however the following equation must be correct:

( )KtI

St21

2

> Equation 4-2

where: I is the fault current. t is the protection device trigger delay time. K is the coefficient depending on the type of conductor insulating agent. St is the protection conductor section. The following example should help to clarify the concept.

AZU

I g 23001,0230

0

0 ===

t = 1 sec

( ) 221

2

16143

12300mmSt =

=

In the case of TN distribution the fault current is always very high because the fault ring impedance is much reduced. For this reason maximum current protection devices which are triggered within the delay time are sufficient. The use of the following protection devices is recognised in TN systems:

Maximum current devices. Differential current devices.

These are not permitted in TN-C systems. PEN conductors are not permitted downstream in TN-C-S systems.

4.1.3 Using differential switches Differential switches must be used in place of maximum current protection devices in each of the following situations:

Extensive fault rings (high Zs). Later system extensions (high Zs). Development faults (low fault currents).


The best solution when installing differential type switches envisages:

The use of highly sensitive rapid differential switches (0.01A and 0.03A) to protect all the system terminals accessible to the user.

A second level of selective protection (total or partial). Calibrations must be made with care to ensure that the interventions are spaced correctly.

A delayed reaction differential switch completes the system protection devices. In general the protection system can be co-ordinated as follows:

Overload selectivity. Differential overload selectivity. Differential selectivity.


4.1.4 TT systems In the TT system the neutral cable is connected directly to the earth in one point, and the system earths are connected to an earth that is electrically independent from the neutral one (Figure 4-3).

Figure 4-3 - TT system (HD 384-3)

In effect two completely independent earth circuits must be designed. It should be remembered that this type of distribution system is the one most widely used in low voltage systems and by the mains electricity supply companies.

L1

L2

L3

N


4.1.5 Protection against indirect contacts in TT systems The fault ring usually includes the earth in its circuit (seeFigure 4-4), where Ig = fault current).

Figure 4-4 Fault ring in the TT system (HD 384-3)

Even if the neutral earth and the system earths are not separate, as for example in the case of buildings that also contain the mains electricity supply company transformer, the system is still considered to be a TT one. In other words, unintentional connections between the earths are not taken into consideration when determining the protection conditions. This distribution system satisfies the protection conditions for indirect contacts when the ratio is correct (HD 384-4-41).

50 AA IR Equation 4-3

where: RA is the sum of earth plate resistance and the resistance of the earth protection conductors. IA is the current level causing the protection device to be triggered. 50 is the conventional contact voltage limit (UL). In some specific applications and

environments (prHD 384-7-704) UL is equal to 25 V. The use of the following protection devices is recognised in TT systems:

Differential protection devices. Overload protection devices.

When a differential device is used IA represents the differential nominal current Idn. For reasons of selectivity, S type differential current devices can be used in series with general type differential current protection devices. A maximum trigger delay time of 1 second (HD 384-4-41) is permitted to ensure selectivity using differential current protection devices in distribution circuits. When the device is an overload switch IA is the In value causing the device to trigger within 5 seconds. Given that it is generally difficult to obtain a very low earth resistance value, the best protection is undoubtedly the differential type.

L1

L2

L3

N

M

Ig RB RA

Ig


With this in mind it is also important to remember that type A differential current protection devices (unidirectional pulsed currents) are required for single-phase electrical circuits and type B for three-phase electrical circuits (EN 50091-1-1 section 1.8.11, IEC 364-5-53).

4.1.6 IT systems In IT systems all live parts are isolated from the earth, or the neutral conductor is connected to the earth at a certain point by an impedance (Figure 4-5), whereas the system earths can be:

Connected to the earth separately. Connected to the earth together. Connected to the system earth together (the system earth is the one that could be connected

to neutral using an impedance).

Figure 4-5 - TT system (HD 384-3)

L1

L2

L3

N

PE


4.1.7 Protection against indirect contacts in IT systems (first fault) The Id current caused by the first earth fault is of little importance because it is enclosed by the distribution capacity of the system towards the earth and, if relevant between neutral and earth.

Figure 4-6 - Fault ring in the IT system (HD 384-3)

The following ratio must be correct for protection against indirect contacts (HD 384-4-41):

50 dT IR Equation 4-4

where: RT is the resistance of the plate to which the earths are connected. Id is the fault current for the first earth fault. 50 is the maximum contact voltage. In this type of distribution system, the triggering of a protection device to deal with the first fault is not required in that the aforementioned condition is always satisfied. This is because the fault ring resistance is always very high thus allowing only very low fault currents if the system is not very extensive. The standard does however envisage that the first fault is signalled (visually and in audio) using a system to continuously monitor isolation status between the live parts and the earth.

4.1.8 Protection against indirect contacts in IT systems (double fault) Dangerous situations can arise in the event of two earth faults when the fault current, involving two circuits, reaches a value that is lower than the short circuit one in a single circuit. Furthermore if one of the two live conductors in contact w

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