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1. INTRODUCTION
This section introduces some of the main principles involved with substation control building and switchyard services. Such work is often left to specialist building services, engineering groups, and architects. Therefore this section covers basic internal and external lighting design, heating, ventilation and air conditioning (HVAC) practice; and the different types of low voltage distribution systems in sufficient detail as likely to be encountered by the power engineer.
2. LIGHTING
2.1 Terminology
2.1.1 General
Light consists of electromagnetic radiation with wavelengths between 760 and 380 nm to which the eye is sensitive. This energy spectrum (red, orange, yellow, yellow-green, green, green-blue, blue and violet) lies between the infra red (700-2,000 nm) and the ultraviolet (200_400 nm) wavelength ranges.
Lighting schemes may be necessary for the following substation areas:
_ Indoor and outdoor schemes for control buildings and indoor switch rooms under both normal and emergency (loss of alternating current (AC) supply) conditions.
_ Floodlighting and emergency schemes for outdoor switchyards and door or gate access.
_ Security/access road lighting together with any supplementary lighting for video camera surveillance.
_ Enclosed transformer pen lighting.
Emergency lighting involves battery backup derived either from cells within the fittings or from the main substation DC supply. The units are designed to have a given 'autonomy' such that upon AC failure the lighting continues to operate for a specified number of hours from the battery source.
2.1.2 Types of Luminaires
The type of light source chosen for an unmanned substation control building or switchyard will be less influenced by aesthetic than technical characteristics such as colour rendering, glare, efficacy, life and cost. TBL. 1 gives the chief technical characteristics of common types of luminaires.
An overcast sky in temperate regions gives an illuminance of some 5,000 lux. Typical substation lighting levels are shown below:
Substation Area Standard
Illuminance (lux)
Limiting
Glare Index
(a) Indoor switch room 200 _ (b) Control rooms 400 25 (c) Telecommunication rooms 300 25 (d) Battery rooms 100 _ (e) Cable tunnels and basements 50 _ (f) Offices 500 19 (g) Entrance halls, lobbies, etc. 200 19 (h) Corridors, passageways, stairs 100 22 (i) Mess rooms 200 22 (j) Lavatories and storerooms 100 _ (k) Outdoor switchyard (floodlighting) 20 _ (l) Perimeter lighting 10 _ (m) Exterior lighting (control buildings, etc.) 15 _ Note that Division II (flameproof) lighting fittings may be necessary in the battery room because of fumes given off from unsealed batteries.
2.1.3 Harmonics
Discharge lamps generate harmonics. Red, yellow and blue phase components take the form:
Fundamental 2nd harmonic 3rd harmonic Normal 120_ (2p/3) phase relation in sequence RYB (this is also true for 4th, 7th, 10th ... harmonics) 120_ phase relation but in sequence RBY _ phase reversal (this is also true for 5th, 8th, 11th ... harmonics) All in phase and will produce zero-phase sequence (ZPS) system voltages and currents (this is also true for 6th, 9th, 12th ... harmonics)
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TBL. 1 Characteristics of Different Light Sources
Type of Luminaires Approximate Life (h) Luminous Efficacy (lm/W) Colour Rendering Illuminance (lm)
Notes:
(a) Tungsten filament (GLS) 1,000 10_20 Excellent, warm white 40W_430 lm Cheap, easy to replace, no control gear required. Zero restrike time 60W_730 lm 100W_1,380 lm (b) Tungsten halide (TH) 2,500 60_90 Excellent 75W_5,000 lm Relatively low initial cost. Moderate efficacy. Zero restrike time. 150W_11,250 lm 250W_20,000 lm (c) Fluorescent (MC) 7,500 30_90 Good, various types available 10W_630 lm Cheap tubes and relatively cheap control gear as integral part of fitting. 20W_1,100 lm 40W_3,000 lm 65W_5,000 lm (d) Low-pressure sodium (SOX) 10,000 70_150 Very poor, colour recognition impossible, monochromatic yellow/orange 35W_4,800 lm Long life and very high efficacy. Control gear required.
Historically used for road lighting. 7_12 min restrike time.
135W_22,500 lm 180W_33,000 lm (e) High-pressure sodium (SON) 9,000 50_120 Fair, warm yellow/white 50W_3,500 lm Generally favored exterior lighting source.
100W_10,000 lm 150W_17,000 lm Control gear required.
400W_48,000 lm 4_5 min restrike time.
(f) High-pressure mercury vapor (MBF) 6,000_9,000 30_90 Good, neutral white 80W_3,700 lm White light but less efficient than high-pressure sodium type. 125W_6,300 lm 250W_13,000 lm Control gear required.
400W_22,000 lm 4_5 min restrike time.
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TBL. 2 Details of Some Standards and Useful References Covering Indoor and Outdoor Lighting Schemes
Standard or Reference | Description
IEC 60598 Luminaires _ covers general requirements for the classification and marking of luminaires, their mechanical and electrical construction, together with related tests. Applicable to tungsten filament, tubular fluorescent and other discharge lamps on supply voltages not exceeding 1,000 V IEC 60972 Classification and interpretation of new lighting products CISPR 15 (International Special Committee on Radio Interference) Limits and methods of measurement of radio disturbance characteristics of electrical lighting and similar equipment Illumination Engineering Society (IES) Code for interior lighting Complete guide. Also, Lighting Division of the Chartered Institute of Building Services Engineers (CIBSE) Philips Lighting Manual Hand guide of lighting installation design prepared by members of the staff of the N.V. Philips' Gloeilampenfabrieken Lighting Design and Engineering Centre, Eindhoven, The Netherlands
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2.1.4 Definitions
Candela: The illuminating power of a light source in a given direction.
The unit of luminous intensity.
Color: rendering General expression for the effect of an illuminant on the colour appearance of objects in conscious or subconscious in comparison with their colour appearance under a reference illuminant.
Efficacy: Luminous efficiency of lamps measured in lumens per watt (lm/W).
Flicker: Impression of fluctuating luminance or colour, occurring when the frequency of the variation of the light stimulus lies within a few hertz of the fusion frequency of the retinal images.
Glare: Condition of vision in which there is discomfort or a reduction in the ability to see significant objects, or both, due to an unsuitable distribution or range of luminance or due to extreme contrasts in space or time.
Glare index: Introduced by the British Illumination Engineering Society to specify and evaluate the degree of discomfort glare for most types of working interior, for a range of luminaires with standardized light distributions.
Illuminance, E (lux, lx): The measure of light falling on a surface. The illumination produced by 1 lm over an area of 1 m2 measured in lux.
E=F/A, where A5area in m^2.
Intensity, I (candela, cd) -- A measure of the illuminating power of a light source in a particular direction, independent of the distance from the source.
Luminous flux, F (lumen, lm) -- Unit of light flux. The amount of light contained in one steradian from a source with an intensity of one candela in all directions.
The amount of light falling on unit area of the surface of a sphere of unit diameter from a unit source.
Luminance, l (cd/m^2 ) -- Measure of light reflected from a surface or in some cases emitted by it. A measure of brightness of a surface. The units of measured brightness are candela per square meter and the apostilb being the lumens emitted per square meter. Luminance and illuminance are not to be confused. For example, if a sheet of paper has a reflectance of 80% and an illuminance of 100 lux, its luminance will be 80 apostilbs.
1 apostilb=1/p cd/m^2 Luminance=0.318xilluminancexreflectance L=0.318xE3?
Luminosity -- Attribute of visual sensation according to which an area appears to emit more or less light. Luminance and luminosity are not to be confused. Substation switchyard floodlighting seen by day and night will appear different to the observer. The luminance is the same but the luminosity in daylight will appear low while at night may appear perfectly satisfactory.
Maintenance factor -- Ratio of the average illuminance on the working plane after a specified period of use of a lighting installation to the average illuminance obtained under the same conditions for a new installation.
Room index (RI) -- Code number representative of the geometry of the room used in calculation of the utilization factor. Unless otherwise indicated: RI5(lxb)/(Hm [l1b]), where l is the length of room, b is the width of room, and Hm is the distance of the luminaires above the working plane.
Stroboscopic effect -- Apparent change of motion or immobilization of an object when the object is illuminated by a periodically varying light of appropriate frequency. May be eliminated by supplying adjacent luminaires on different phases, or using twin-lamp fittings on lag-lead circuits, or providing local lighting from tungsten filament lamps rather than discharge types.
Utilization factor -- Ratio of the utilized flux to the luminous flux leaving the luminaires. TBL. 3 details utilization factors for various indoor lighting fittings.
2.2 Internal Lighting
The following design procedure is suggested:
1. Decide upon the level of illumination required. The illuminance required can be obtained from various guides or the values given in Section 2.1.2.
2. Determine the mounting height of the fittings above the working plane.
Note that a desktop height is usually taken as 0.85 m above floor level.
Note that the fittings should be mounted as high as possible in order to permit wider spacing between fittings and reduced glare.
3. Ascertain the minimum number of fittings to be employed from the spacing factor (normally taken as 1.5 for batten type fluorescent fittings) and the mounting height.
For Hm =2.25 and spacing factor 1.5 then minimum number of fittings for a 20 mx10 m room=(20/[2.25x1.5])x(10/[2.25x1.5])= 20/3.37x10/3.37=18
4. Calculate the room index. RI5(l3b)/(Hm [l1b]) where l5room length, b5room width, and Hm 5mounting height above the working plane. For the 20 m310 m room mentioned above, RI=20x10/2.25 (30)=2.96 or 3 approximately.
5. Having decided on the general type of fitting to be used, ascertain the utilization factor, UF, from manufacturers' tables, taking into account the reflectance factors of the room ( TBL. 3). Since accurate information is rarely available it’s common to take reflectance factors of 20%, 70% and 50% for the working plane, ceiling and walls respectively, assuming finishes normally found in a substation control room.
6. Decide upon the maintenance factor, MF, to be used; typically taken as 0.8. This allows for a reduced light output from the fittings due to ageing and formation of dust on the luminaires.
7. Calculate the total light input to the room necessary, F lumens, to give the illumination level, E lux, required. Total luminous flux F=(Exlxb)/ (UFxMF).
8. Calculate the number and size of fittings required. Note that there may be preconditions regarding the physical size and wattage of the luminaires to meet the electrical supply company standards. If the size of the fittings has already been decided then the number of luminaires required is obtained by dividing the total calculated lumens figure by the output per fitting. If the result obtained is greater than the minimum determined by the spacing requirement the higher number of fittings must be used even though this may not be the most economical arrangement. The actual number of fittings used may require adjustment in order to give a symmetrical arrangement.
A typical calculation sheet for such work is given in FIG. 1 and an example using this based on a 30 mx15 m control and relay room, 3.1 m ceiling height, 1.5 spacing factor, and required 400 lux illuminance is given in FIG. 2. Reflectance and utilization factors are taken from design guides.
Following the determination of the number of luminaires required, it’s essential to take into account the position of the equipment and air-conditioning ductwork in the room in order to produce a satisfactory lighting fitting layout.
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TBL. 3 Indoor Lighting (Utilization Factors) Description and Typical Downward Light Output Ratio % Typical Outline Basic Downward LOR % (M) Aluminum Industrial 70 reflector (72276) (T) High Bay Reflector Aluminum (72) or Enamel (66) (M) Reflectorized colour corrected mercury lamp 95 MBFR/U (80290)
(F) Enamel slotted trough louvered (45_55) (F) Louvered recessed (module) fitting (40250)
(F) Closed-end enamel trough (65283) (T) Standard dispersive industrial reflector (77) (T) Enamel deep bowl reflector (73) (F) Plastic trough louvered (45255)
(F) Plastic trough unlouvered (60270)
(T) Near spherical diffuser open beneath (50) (F) Bare lamp on ceiling (F) Batten fitting (60270)
(F) Injection-molded prismatic wrap-around enclosure (55265) (F) Enclosed plastic diffuser (45255)
(F) Shallow fitting with diffusing sides optically designed downward reflecting surfaces (55) (T) Industrial reflector with diffusing globe (50)
(T) Opal sphere (45) and other enclosed diffusing fittings of near spherical shape
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2.3 External Lighting
The illuminance EP lux at a location d meters from a point source of intensity I candela is given by the cosine law of illumination, EP 5I cos ?/d2
. The point calculation method of determining the illuminance at a point then consists of taking the sum of the contributions of partial illuminance produced by all the individual lighting fittings involved in the design (see Figs .3 and 4).
For the more general case, the illuminance, E? lux, at point P on any plane, the normal of which makes an angle ? with the direction of incidence of the light is given by (Figs .4 and 5)
E? 5 I?cos2 ?cos?/h2
For substation access road lighting calculations the reflective properties of the road surface must really be known. The surface luminance may there fore also be calculated by the point calculation method from a knowledge of the luminous intensity of the lighting fittings involved and the addition of the contributions of their partial luminance. Such hand computations would be time consuming and assistance may be obtained from the major lighting fitting manufacturers who have computer programs to determine the correct type, spacing and mounting height of floodlighting fittings to obtain the required average illuminance over a substation switchyard or luminance on a road surface. Manufacturers also produce isolux and isocandela diagrams for their different fittings in order to reduce the calculation work involved. For example the illuminance at a point is derived from the contour values given in an isolux diagram using formulae of the form:
FIG. 1 Interior lighting calculation sheet.
FIG. 2 Interior lighting calculation sheet (as completed for control room example in Section 2.2).
FIG. 3 Relationship between floor level, working plane, fitting mounting height, and spacing between each fitting.
FIG. 4 Illuminance from a point source.
FIG. 5 Illuminance on any plane.
FIG. 6a Substation boundary wall lighting example.
Consider the following example:
A client requires boundary or security wall lighting around his or her sub station. The arrangement of the high pressure mercury vapor (MBF/U) 125 W lighting fittings which each have a luminous flux of 6,300 lumens is given in FIG. 6a.
1. To find the illuminance at point P
(a) Determine the distance from the row of lighting fittings to the point P. Distance from the line of fittings to the point P=2.5 m=0.6h, where h is the mounting height of the fittings.
(b) Draw the line A_A on the isolux diagram this same distance from, and parallel to, the longitudinal axis of the lighting fitting.
(c) Determine the distance from the point P to the transverse axis of each fitting and represent these points on the relative isolux diagram as points L1,L2,L3 and L4 on the axis A_A. Ignore the contributions from more distant lighting fittings in this example.
(d) From the isolux diagram read off the relative illuminance at each of these four points and calculate the total illuminance at point P:
Total=105% of Emax Using the isolux diagram formula for this particular fitting Emax =0.2x2xluminous flux/h2 =0.23236,300/4.=2=72 lux.
(e) The illuminance at point P, EP =72x105/100=76 lux.
2. To find the average illuminance from the substation boundary or security wall to a distance 2.5 m from the line of the lighting fittings.
The simplest way of calculating the average illuminance from a long straight line of fittings is by using the manufacturers' Utilization Factor (UF) curves and the formula:
... where ...
W=width of area under consideration (m) S=distance between each fitting (m) f=luminous flux of each fitting (lumens) (a) The Utilization Factor of the area is taken from the manufacturers' UF curve as shown in FIG. 6(b).
TBL. 4 Spotlighting Calculation Results
FIG. 6b Boundary wall lighting utilization factor curve.
(b) Average illuminance,
(c) An overseas client requires security spot lighting from an observation post along the boundary wall of a substation site. Calculate the illuminance EP lux d meters away from the observation post if the spot light has a 300 W halogen lamp with a main lobe luminous intensity of 9,000 cd (see TBL. 4 for calculation results). The arrangement is shown in FIG. 7. Why would sodium or mercury vapor lamps be unsuitable in this application?
The illuminance at point P, EP =I/(x2 )=I/(h2 1d2 ) lux so EP 59,000/(4.52 1d2 ) lux (approximately).
2.4 Control
Adjustment of the brightness may be achieved by varying current, voltage or delay angle.
FIG. 7 Observation post spotlighting example. Illuminance at point P=Ep
lux.
Given sufficient voltage current control is applicable to fluorescent and other discharge lamps using variable inductance circuits. Such control is not normally applicable to AC tungsten filament lamp operation.
Voltage control is not particularly applicable to fluorescent and other discharge lamps because of the need to maintain a threshold voltage (typically 50% of rated voltage) below which the lamp is not extinguished and there fore to avoid erratic operation. A control range in terms of rated luminous flux from 30% to 100% over an operating voltage range of 70-100% of rated voltage is achievable for discharge lamps. Voltage control will dim incandescent tungsten filament lamps to zero light output at approximately 10% of rated voltage when some 30% of rated current will flow ( FIG. 8).
Delay angle control is suitable for both incandescent and discharge lamps. Control units consist of a pair of thyristors connected in inverse parallel (or a triac) so that each thyristor may conduct a proportion of each half cycle of the AC supply. The thyristor or triac trigger pulses are arranged to initiate conduction at any point in the half cycle. Particular attention has to be made to interference suppression because of the fast thyristor switching rise times involved with this type of device.
FIG. 8 Incandescent lamp light output variation with operating voltage.
3. THE CHARACTERIZATION OF LVAC DISTRIBUTION SYSTEMS
The characterization of LV building services AC distribution systems may be described by the type of earthing arrangements used. Effective earthing is essential:
1. To prevent the outer casing of the apparatus and conductors rising to a potential which is dangerously different from that of the surroundings.
Where there is an explosive risk there may be a danger from very small voltage differences causing sparking.
2. To allow sufficient current to pass safely in order to operate the protective devices without danger. This requirement may conflict with the necessity to keep potentials at a low level and restrict the available methods of protection. Regulations therefore require the 'earth loop impedance' of the completed system to be measured in order to ensure protection operation is not compromised.
3. To suppress dangerous earth potential gradients.
Earthing methods are defined by a three letter coding:
First letter _ defines the state of the supply system in relation to earth.
T=Directly earthed system at one point.
I=Either all live parts are insulated from earth or one point connected to earth through an impedance.
Second letter _ defines the state of the exposed conductive parts of the installation in relation to earth.
T=Exposed conductive parts connected directly to earth, independent of any earthing of a point on the supply system.
N= Exposed conductive parts connected directly to the earthed point of the supply system, normally the supply transformer neutral point.
Third letter _ defines the earthing arrangement of the system conductors.
C=Combined neutral and earth conductors.
S=Separate neutral and earth conductors.
Five common arrangements are shown in FIG. The TN-S system, using separate neutral and protective conductors throughout the network, is the recommended method for installations in hazardous areas such as substations feeding chemical plants. The neutral and protective conductors may also be combined into a single conductor on part of the system (TN-C/S) or combined into a single conductor throughout (TN-C). In the TT system, with exposed metal bonded directly to earth, quick acting sensitive earth leakage protection is required. The IT system employs a neutral conductor isolated from earth or earthed through an impedance. It may be utilized on high security lighting circuits where the first fault is monitored by a current detector in the transformer neutral and initiates an alarm. A second fault causes further current to flow and is arranged to initiate a trip.
Irrespective of what earthing system is used it won’t be effective unless it’s frequently checked to ensure that all earth bonds are mechanically strong and free from corrosion. Furthermore, earth impedance should be monitored and recorded so that any change can be detected and the appropriate action taken.