11Reliability of Electricity Supply:Structure – Case Study

Angelo Baggini, David Chapman and Francesco Buratti

This case study relates to a 10-floor office in Milan, Italy (hereafter referred to as the buildingfor confidentiality reasons). The building is the head office of a major financial institutionand is occupied by 500 employees using information technology equipment intensely.

After a description of the current status of the electrical installation in the buildingaccompanied by the results of power quality measurements, two design proposals arepresented that assure a resilient and reliable power supply. A cost analysis completes thiscase.

C11.1 DESCRIPTION OF INITIAL SITUATION

C11.1.1 Distribution Scheme

The building is connected to a 23 kV grid. The medium-voltage main power supply consistsof two 800 kVA transformers, 23/0.4 kV, 50 Hz. The low-voltage side of the installation isdesigned as a TN-S system.

The load is subdivided into standard, preferential and privileged loads, according tothe requirements for continuity of supply (this is discussed in greater detail later in the text).There is a second point of common coupling (PCC) to feed a small portion of the standardload. The two PCCs are fed from the same grid point and are not independent.

To ensure continuity of the power supply, two uninterruptible power supplies (UPS)(80+200 kVA) and a motor generator (250 kVA) are installed as shown in the layout design

Handbook of Power Quality Edited by Angelo Baggini© 2008 John Wiley & Sons, Ltd

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Figure C11.1 Previous distribution scheme

in Figure C11.1. Note that in such a scheme, it is imperative that the neutral conductor isconnected to ground only once, at the main earthing terminal, and not at each transformer.Otherwise, the TN-S wiring configuration, with all its benefits related to power quality andEMC, is lost.

The primary distribution is a compromise between radial and shunt schemes.1 Theinstallation has grown in a haphazard way and is not based on a predetermined design.This is a direct result of the many changes in power requirements experienced during thebuilding’s operating lifetime. Two distribution panels feed each floor. Each panel has twosections (standard and privileged) corresponding to the standard and privileged sections ofthe main LV power panel (Figure C11.2). Final distribution uses a single radial scheme

C11.1.2 Lines

The three-phase distribution is made with multi-phase copper cables. Neutrals are half-sizedfor phase conductors over 35 mm2.

1 Shunt scheme: a rising busbar or power line is shared for all floors; at each floor, a connection is made to the LVpanel at the floor. Radial scheme: each LV panel at each floor has a dedicated connection with its correspondingswitchgear at the main LV distribution panel in the basement.

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Figure C11.2 Present distribution flowchart: dark lines indicate privileged distribution; light linesindicate standard distribution

C11.1.3 Load

The rated load for the office building is typical and consists of:

• Lifts (approx. 80 kVA)• Services (approx. 100 kVA)• Air-conditioning (approx. 600 kVA)• Horizontal distribution for lighting and power in the open office space (approx. 35 kVA

per floor)

C11.1.4 Power Quality

To evaluate the quality of the power supply, current harmonic content was measured atthe main electrical lines feeding each floor and at the distribution panels for buildingservices.

Figure C11.3, Figure C11.4, Figure C11.5 and Figure C11.6 give examples of measuredcurrent and voltage waveforms and their harmonic content. It is important to highlight thefollowing points:

• Some phase conductors, particularly those for lighting circuits, have over 75 % totalharmonic current distortion (third, fifth and seventh harmonics) – Figure C11.6. Thereis significant third-harmonic current distortion in circuits serving IT and lighting equip-ment – Figure C11.4, Figure C11.5 (neutral conductor) and Figure C11.6. In some neutralconductors, the harmonic currents are more than twice the phase current (e.g. mainfeeding ground floor – Figure C11.6).

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Figure C11.3 Waveform and harmonic contents of phase current (phase L1) at main LV powerpanel in the line feeding elevators 1 and 2

• Both UPS show current distortion in the phase and neutral conductors – Figure C11.4and Figure C11.5.

• Even harmonics appear in more than one measurement (approx. 30 % in Figure C11.5).This means that the waveform of the current does not have the usual symmetry.

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Figure C11.4 Waveform and harmonic contents of phase current (phase L1) in the 80 kVA line tothe UPS (open office space)

• In some cases, the waveform produces more than two zero crossings per cycle of thesine wave (Figure C11.5).

• Significantly high permanent currents are detected in the ground conductor. This is atypical indication that the TN-S configuration has not been maintained, i.e. that there aremultiple connections between the neutral conductor and earth. It must be ensured that

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Figure C11.5 Waveform and harmonic contents of neutral current in the 80 kVA UPS line(open office space)

there is only one main earthing point with a connection between neutral and ground.On-site personnel need to be briefed not to make any inadvertent connection betweenthe neutral and ground in the LV distribution.

The instrument used to make these measurements was a Fluke 43 single-phase, 0–600 V,CT 600 A/1 mV/A power quality analyzer.

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Figure C11.6 Harmonic contents of phase L2 current at main LV distribution panel in the linefeeding the ground floor distribution panel (mainly lighting circuits)

C11.1.5 Events

The building occupant experienced a high and increasing number of events and faults,principally related to the overheating of lines and nuisance tripping of protectivedevices.

C11.2 ANALYSIS – INITIAL SITUATION

The current installation lacks organization and rationality in its approach. This is notcompatible with the resilient design that the company adopted at the start (supply LVdistribution through multiple transformers, UPS and generator).

Some elements do not comply with the prevailing standard. Even full compliance tostandards does not guarantee adequate performance for this building with its mission-criticalfunctions from a power quality and EMC viewpoint.

C11.2.1 Distribution Scheme

The distribution scheme is neither systematic nor rational, probably due to the numerousmodifications only to parts of the installation. There are important limitations relating toreserve capacity and independence. Some bottlenecks are present, e.g. at the level of themain LV busbar (Figure C11.1). The two transformers are not independent.

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C11.2.2 Line Overheating

The high density of IT equipment such as PCs, servers, etc., and electronic lighting produceshigh levels of harmonic current in many lines.

These phenomena result in the overheating of the neutral (see Chapter 7) as well asnuisance tripping of protective devices.

C11.2.3 Coordination Among Protective Devices and Lines

Some lines are not coordinated with their protective devices for overcurrents. The largenumber of lines in the same trunks makes the problem more critical.

Analysis of a faulted line showed failure due to constant overheating of the conductor,even if its protective relay was well coordinated.

In case of such a multiple feed with the TN-S configuration, the neutral current needsto be connected with the main earthing terminal. Procedures must be in place to avoidmaking any additional connection between neutral and ground. Where these neutral–groundconnections occur, they create alternative paths for the neutral current, thus eliminating allthe benefits of having a TN-S system.

C11.3 DESIGN APPROACH

The building occupant, operating in the financial sector, needs to upgrade the installationsince reliable power quality is considered mission-critical.

The problems shown by the analysis of the current situation and the power qualitymeasurements suggest consideration of an update of the electrical system at different levels:

– Rationalization of mains distribution.– Renewal of the electrical installation on the floors.

C11.3.1 Load Classification

To optimize the main distribution rationalization, the first step is the classification of theloads. All loads are classified into three groups:

• Standard• Preferential• Privileged

Standard loads are used for daily business; should they be unavailable, there is no risk topersonnel or damage to equipment and business processes. A simple radial circuit sufficesfor the supply and relatively long intervention times can be tolerated (Table C11.1).

Preferential loads need a redundant power supply, e.g. through a dual radial scheme,starting either from the risers or at the level of intermediate connections (Table C11.2).

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Table C11.1 Description, criteria, design and intervention requirements for standard loads

Standard load Type of power supply required Timing needed forintervention

Allows regularfunctioning of thebuilding, but theirunavailability does notresult in risk to personnelor equipment:– General services, e.g.

air-conditioning (butnot in server room)

– Normal lighting– Heating– Power sockets

StandardRadial circuitsResumption of service can waitfor some time without damageLoads can be switched off

NoneUnavailability of servicefor relatively long timeperiods can be tolerated

Table C11.2 Description, criteria, design and intervention requirements for preferential loads

Preferential load Type of power supplyrequired

Timing needed forintervention

Regular functioning of theload is required for comfortand security of personneland clients, as for ensuringsmooth business processesoperation. For example:– Lighting of staircases,

corridors and certainrooms

– Minimum lightingconditions to avoid panic

– Heating orair-conditioning of certainrooms

– Elevators– UPS

BackupDual radial primary supply,ensuring the functional andphysical independence ofthe risersTwo separate risers can beemployed, supported eitherby a generator or suppliedfrom two independent gridpointsSwitching off the load is notacceptable

A 20 s intervention time forthe generator group isacceptable for longinterruptions. Typical valuesfor a diesel group:– First attempt within 5 s– Second attempt within

10 s– Third attempt within 15 s

Privileged loads are mission-critical. Loss of service means grave danger to personnelor severe damage to the organization’s business processes. The level of independence needsto be determined for each load. At the very least, these loads must be supplied from twoindependent feeders with automatic switching (Table C11.3).

Based on cooperation, the building occupant’s briefings have been classified as shownin Table C11.4.

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Table C11.3 Description, criteria, design and intervention requirements for privileged loads

Privileged load Type of power supplyrequired

Timing needed forintervention

Essential services:– Security lighting– Servers– Telecommunication

systems– Personnel retrieval– Alarm and security

systems– Fire signaling and

fireproof systems– Closed-loop TV circuits– Certain auxiliary services

SecureDual radial scheme, withindependent risers. At leastone riser has to ensure highgrid reliabilityUse of UPSFor certain loads, adedicated UPS can beconsidered

Loads with interventionwithin 15 sShort-interruption loads,within 0.15 sSome loads need continuoussupply

Table C11.4 Classification of type of loads

Type of load Percentage

Standard 49Preferential 13Privileged 38

C11.3.2 Main Distribution Schemes

To avoid the existing bottleneck at the LV main bus bar, the primary distribution must bemodified as a dual radial distribution (Figure C11.7, left).

The rating of the transformers TR1 and TR2 must ensure that each can carry the fullload. Considering the defined waveforms of the current, transformers must be sized to takeinto account the harmonic content [8].

To reduce short-circuit currents, the system is normally managed with the main busbarswitch open, but parallel operation between the two main transformers is possible for ashort time.

To feed the thermal and HVAC services, the transformer section must be modified asshown in Figure C11.7 with a new 800 kVA transformer (TR3 – this in addition to theexisting two).

Standard loads are supplied from a single grid point. The same grid power cable, riserand radial distribution also supplies preferential and privileged loads.

Two generator groups supply preferential and privileged loads. Standard loads areswitched off through the switch at the extremity of the main busbar.

Two UPS supply privileged loads, in case of failure of normal and backup powersupply.

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Figure C11.7 New main distribution scheme

Primary supply and backup supply are wired TN-S. UPS can be wired either TN-S orIT. IT systems are excellent for continuity of power supply, but cannot guarantee protectionof personnel. Where IT is needed, proper security measures have to be taken to ensure thatonly authorized personnel can access the IT circuits.

The second LV PCC has been removed in Figure C11.7.Each floor is still supplied by two distribution panels, each having three sections

(standard, privileged and preferential) corresponding to the same sections of the main LVpower panel.

Final distribution could be done using a shunt (Figure C11.8) or single radial(Figure C11.9) scheme.

Figure C11.8 Solution with radial scheme (10 floors with three types of load = 30 dedicated risinglines): dark line indicates preferential distribution; gray line, standard distribution; light line,

privileged distribution

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Figure C11.9 Solution with unique riser lines (three types of load = three rising lines/busbar,shared by all floors). The dark line indicates preferential distribution; gray line, standard distribution;

light line, privileged distribution

The shunt scheme (shared line feeding all floors for each type of load) is cheaper andmore flexible in case of a rise in the load. Unfortunately it is limited due to the bottleneckrepresented by the main line in case of faults and maintenance. In addition, failure in theriser lines is totally unacceptable.

The single radial scheme (one line for each floor for each type of load) ensures:(i) minimum interference and voltage drop caused by loads; (ii) containing faults to thelines affected by fault loads; and (iii) reduced maintenance problems. The radial scheme istherefore the preferred scheme.

C11.3.3 Line Sizing

Table C11.5 shows the power-considered sizing of all the main sections of the system.The total installed load (columns 2 and 3) is multiplied by utilization and contemporary

factors (columns 4 and 5) to calculate the power requirements of the load (columns 6and 7). As a margin for future load growth, lines are sized (columns 8 and 9) consideringan additional factor equal to 130 % and 115 % for power and lighting circuits respectively.

Considering the measured waveform of the current, all the new lines have been sizedto take into account the harmonic profile and resilience requirements:

• Neutral equal to phase [7]• Derated cables [4], [7]

Special attention should be paid to neutral and phase conductor sizing to avoid overheatingand faulty tripping of protective devices. The adoption of UPS or a motor generator is notuseful if a line fault occurs after it.

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Table C11.5 Peak-rated and actual sizing of primary distribution system

Load Installedload (kVA)

Utilization andcontemporary factors

Power requirement(kVA)

Installedpower (kVA)

Power(1)

Light(2)

Power(3)

Light(4)

Power(5)

Light(6)

Power(7)

Light(8)

Secondunderground

7 10 0�7 1 5 10 6�5 11�5

Firstunderground

114 15 0�7 1 80 15 104 17�25

Ground andgeneral services

43 15 0�7 1 30 15 39 17�25

First floor 50 17 0�7 1 35 17 45�5 19�55Second floor 50 17 0�7 1 35 17 45�5 19�55Third floor 50 17 0�7 1 35 17 45�5 19�55Fourth floor 50 17 0�7 1 35 17 45�5 19�55Fifth floor 50 17 0�7 1 35 17 45�5 19�55Sixth floor 50 17 0�7 1 35 17 45�5 19�55Seventh floor 50 17 0�7 1 35 17 45�5 19�55Eighth floor 29 12 0�7 1 20 12 26 13�8Nine floor 3 2 0�7 1 2 2 2�6 2�3Thermal central 29 0 0�7 — 20 0 26 0HVAC mainstation

843 0 0�7 — 590 0 767 0

Boxes 14 5 0�7 1 10 5 13 5�75Elevators 114 0 0�7 1 80 0 104 0

Total 1546 178 — — 1082 178 1407 204�7

C11.4 COST ANALYSIS

The cost of the existing installation is compared to two possible alternative solutions(Table C11.6, Table C11.7). These alternatives differ only for risers, and hence for the costof the main LV panel.

Solution 1 is the shunt scheme, and Solution 2 is the simple radial scheme, which ispreferable for new buildings but difficult to implement as an installation upgrade.

In respect of this situation, it is important to highlight the following concepts:

• The percentages refer to the cost of the existing installation.• The extra cost of upgrading is in reality low, if addressed at initial design stage.• The cost of the best technical solution (i.e. Solution 2 – single radial scheme at final

distribution) differs only by 3 % from Solution 1 if addressed at the initial design stage.It differs significantly when addressed at the refurbishment stage only.

• Cost basis at 2001.

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Table C11.6 Cost when selected at initial design stage

Item Existing (E) Solution 1 (E) Solution 2 (E)

Main LV panel 32 000 35 000 45 000Risers 30 000 35 000 60 000Horizontal distribution 107 000 135 000 135 000Generator groups 87 000 107 000 107 000UPS 55 000 105 000 105 000Motive power 355 000 375 000 375 000Lighting 500 000 525 000 525 000Total 1 166 000 1 317000 1 352 000

(151 000, +13 %) (186 000, +16 %)

Table C11.7 Cost for installation upgrade

Total — (422 000, +36 %) (543 000, +46 %)

• The cost considered for the UPS takes only purchase and installation into account.It should be emphasized that operational costs (maintenance and replacement of thebatteries) are typically quite high.

Even if the evaluation of average costs related to a system designed according to goodpower quality (PQ) practice is difficult, it must recognize that:

• The cost estimates include the costs related to the practical difficulties of installing andrenewing a building in the center of a major city.

• The modification of the main distribution scheme is the most important and useful actionto undertake.

• The solution with unique riser lines is very difficult to install whilst the building isoperational.

C11.5 LESSONS LEARNED

Initial low cost does not necessarily mean good value. A PQ-compliant system, initially moreexpensive, can save significant amounts of money during its life. The case study analyzedhere shows that an electrical installation, designed without attention to PQ problems, resultsin a considerable amount of unnecessary expenditure, whether to resolve the issues or simplyto live with the inconvenience and lost time caused by them.

The cost/benefit analysis shows that resilience should be carefully considered at designstage. A mere increase of 16 % (typically less than 1 % of the total building costs) in theinstallation cost provides:

• Three lines of defense against power cuts for mission-critical loads (dual panels at eachfloor, generator, UPS).

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• A highly reliable system, with each floor supplied by two distribution panels. Each panelis independent from the other, and from all panels on the other floors.

• A highly resilient electrical system against future load growth.

Expensive though it may be, the highly resilient solution would typically add about 1 %to the cost of the building. For commercial buildings, where the running costs amount toinitial construction costs after 7–8 years, this initial investment will be amortized after aproductivity increase of 10 min per week.

A design according to present standards does not guarantee optimum performance fromPQ and EMC viewpoints and enhanced solutions have to be considered. At a Europeanlevel, better standards are at present under preparation.

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