CHAPTER 6

DESIGN, ESTIMATING AND DRAWING PROCEDURES Introduction

Design, estimating and drawing forms a major portion of the workload of ‘new works’ Elec-trical Engineers in Cawangan Elektrik. As such, this chapter is aimed at giving, a general terms, the procedures generally involved in these aspects of the workload, viz design, estimating and drawing of electrical installations for government buildings. After reading this chapter, the fresh graduate electrical engineer would hopefully have a fair idea on how to go about handling his first job. Needless to say, the material presented here is by no means complete and in the final analysis, there can be no substitute for experience gained on the job. 6.1 Design Procedure and Criteria

A good design is one that is safe, workable, economical, complies with the relevant regulations and is convenient to the user. The new electrical engineer is also encouraged to seek the advice and expertise of his more experienced colleagues.

All designs must comply with: a) NEB Electricity (Board Supplies) Rules (1949) b) Electricity Regulations 1951 c) Electricity Ordinance 1949 d) TEE Regulations for the Electricity Equipment of Buildings. e) Factories and Machinery Act 1967 f) JKR technical circulars g) British Codes of Practice/Malaysian Codes of Practice. h) British Standards/Malaysian Standards i) Illumination Engineering Society (UK) Codes of Practice

6.2 Coordination With Other Departments

It is essential for us to maintain a close liaison with the Building Section JKR or client and NEB, especially at the preliminary stage of a project. The architect or client is consulted to determine the exact and final layout and location plans for a building. Also the need if any for emergency lighting, expensive fittings, water heater points etc. should be confirmed. Finally, the amount of finance available for the job should be known.

A preliminary design is then effected to determine the load of the installation. Once load details and location of intake have been tentatively agreed upon, the NEB is approached for confirmation of availability of supply and tariff. The NEB is especially keen to know of the load requirement and the floor areas of the building to be agreed to by the NEB. Their policy is clearly spelt out in the (Board Supply Rules 1949).

If a substation is required, then it becomes necessary to negotiate with the architect to provide such a location or compartment to house the substation.

Once agreement has been reached with all relevant parties, detailed design and drawing can commence or in the case of standard buildings such as school or sub-health centers etc. the standard drawing are already available and the work is much simplified and can be handled by the technician.

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6.3 Design Proper The design proper is the design of the electrical installation for the particular building. In its

simplest terms the design of electrical installation involves the laying out of electrical equipments such as light fittings, socket outlets, switches, fans, etc. and making up the associated circuitry or schematic wiring diagrams showing how they are connected to the incoming supply and also how they are interconnected.

Since installation notes are readily available from standard JKR specifications, they are often omitted here. The reader is advised to obtain a project file and read the technical specifications. 1. Illumination and Power

The reader is referred to Chapter 13 for the methods used to obtain the number of light fittings of a particular type in order to achieve the required illumination levels. Illumination levels are broadly based on the IES (Illumination Engineering Society UK) Code of practice with a 20% to 50% reduction for Malaysian conditions, where appropriate.

Having decided on the number and type of fittings, the Electrical Engineer then arranges the fittings appropriately with respect to the layout of the floor. The light switches must be located in a reasonable position with ease of access. The same applies for 13A switch socket outlet (13A S/S/O), 15A water heater points, 20A water heater points and 30A air-conditioner point (30A/C pts.).

Heights of fixing for various types of typical fittings are given below. The heights are measured from the underside of the fitting to the finished floor level.

TYPE OF FITTING MOUNTING HEIGHT a) Suspended ceiling fittings 8’

b) Wall-mounted fittings 7’ c) Light switches 4’ 9 d) Socket Outlets (surface wiring) 4’ 9” e) Socket Outlets (concealed wiring) 1’

The structure of the wall must be noted before locating a S/S/O on it. It is not permissible to

place a 13A S/S/O on a cardboard partition wall.

Where sweep ceiling fans are to be installed, the slope of the roof beams must be gentle enough and the fan blades high enough to allow the fans to operate safely. Otherwise, wall-mounted fans will have to be used.

An awareness on much minor yet important details could prevent unnecessary alteration of plans at the site. Some contractors follow the details of the plan even when it is not practical to do so.

2. Wiring

To facilitate better understanding of this section, a single line diagram is given of a simple household electrical installation (low voltage) and another of a High Tension (H.T) electrical installation.

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3. Current-Carrying Capacities and Voltage Drops for Cables and Flexible Cords:

i) Basis of tables: The tabulated current-carrying capacities correspond to continuous loading and are also known as

the “full thermal current rating” of the cables, corresponding to the conductor operating temperature indicated in the headings to the tables. Cables may be seriously damaged, leading to early failure, or their service lives may be significantly reduced, if they are operated for any prolonged periods at temperature above those corresponding to the tabulated current-carrying capacities. The tabulated current-carrying capacities are based upon an ambient air temp. of 300 C. For other values of ambient air temperature it is necessary to apply a correction factor (multiplier) to obtain the corresponding effective current-carrying capacity.

ii) Voltage Drop:

Values of voltage drop are tabulated for a current of one ampere for a metre run i.e. for a distance of I m along the route taken by the cables, and then present the result of the voltage drops in all the circuit conductor. For any given run the values need to be multiplied by the length of the run in metres and by the current the cables are to carry, in amperes. The voltage drop for any particular cable run must be such that the volt, drop in the circuit of which the cable forms a part does not exceed 2.5% of the nominal voltage.

For cables up to and including 120mm2 they apply with sufficient accuracy where the power

factor of the load lies between 0.6. lagging and unity and for larger cables, where the power factor of the load is not worse than 0.8 lagging. In all other cases, the value may be unduly conservative.

iii) Determination of current-carrying capacity:

, In order to determine the current-carrying capacity of the cable, it may be necessary to apply one or more correction factors to the tabulated value given in the appropriate table for the cable. a) For ambient temp.

each table gives the correction factor to be applied depending on the actual ambient temp. in the installation.

b) For grouping where a correction factor for grouping has to be applied (see table 9B.)

c) For thermal insulation -_ For a cable installed in a thermally insulating wall or above a thermally insulated ceiling

the cable being in contact with a thermally conductive surface on one side, the rating factor to be applied may, in the absence of more precise information, be taken as 0.75 times the current carrying capacity for that a cable likely to be totally surrounded by thermally insulating material. The applicable rating factor may be as low as 0.5.

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TABLE 9B Correction factors for groups of more than three single-core cables or more than

one multicore cable

NOTES: 1. These factors are applicable to groups of cables all of one size, equally loaded, including groups bunched in more than one

plane.

2. Where spacing between adjacent cables exceeds twice their overall diameter, no reduction factor need be applied.

iv) Determination of the size of cable to be used: The following procedure enables the designer to determine the size of cables it will be neces-

sary to use in order to comply with the requirement for overload protection.

a) If protective device is a fuse or circuit breaker divide the nominal current of the protective device by the appropriate ambient temp.

correction factor given in the table for type of cable intended to use. then further divide by an applicable correction factor given in table 9B. the size of cable to be used must not less than the value of nominal current of the pro-

tection device adjusted as above. b) If protective device is a semi-enclosed fuse

divide the nominal current of the protective device by appropriate ambient temp. correction factor given in the table below.

Table iv (b)

then further divide by any applicable factor for grouping in table 9B then further divide by 0.725 the size of cable to be used must not be less than the value of nominal current of the

protective device adjusted as above.

The various gears in the electrical installation are connected by means of conductors in the form of wires or cables. Wires or cables of suitable size and type must be chosen for an installation. The criterion used in the voltage drop is permissible. The regulations stipulate that there must be no more than a 2.5% voltage drop of the nominal voltage from the Main Switchboard to any point in the installation. Thus for a 240V system, the maximum voltage drop permissible is 6 volts while 10 volts is permitted on a 415V system.

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Table in pages 132 to 192 of the IEE Regulations for the Electrical Equipment of Buildings give current ratings and voltage drop characteristics of various conductors. The voltage drop can be calculated from the formula:-

Voltage drop = Ic x Vd x 1 Ir 100 Where Ic = current normally carried by the conductor Where Ir = current rating of conductor Where Vd = volt drop per 100 feet and Where I = length of cable in feet

It is pertinent to point out that the cable rating must always be higher than that of the fuse that is

supposed to protect that part of the installation.

It is standard practice in Cawangan Letrik to use PVC/PVC wires for surface and concealed (not in conduit) wiring. For wiring in conduit PVC wires are used. The widely used underground cables are:

i) PILCDSTAS Cable Paper insulated, Lead covered, Double steel Tape Armored and Served cable). This is used in

conjunction with cable-box termination and is available only in the 3 phase 4 core type. It is normally used when the 3 phase line current exceeds 40A.

ii) PVC/SWA/PVC cable

(Polyvinyl chloride/steel wire armored/Polyvinyl chloride cable). This is used with cable gland terminations and is available both in the 3 phase 4 core variety as well as in the single phase 2 core variety. It is generally used when the single phase current demand is less than 60A.

Example: An immersion heater rated at 240V, 3kW is to be installed using twin-with-earth PVC insulated

and sheathed cable. The feed will be from an existing 15A spare way semi-enclosed (rewirable) fuse and will run for much of its 14m length in a roof space which is thermally insulated with glass fibre. Ambient temp. is expected to be 35°C. When leaving the consumer’s unit, the cable will be bunched with seven other twin-with-earth cables.

Solution: a) Check whether the 15A fuse will be adequate.

I = P/V= 3000/240 = l2.5A b) The protection rating (ISA), not the circuit rating, must be divided by each of

i) the group correction factor - from table 9B for eight multicore cables on a wall, the factor is 0.52 ii) the ambient temp. factor - from Table iv(b) of this chapter the ambient temp. factor is 0.97

iii) the thermal insulation factor - since the cable can be cooled on one side this the thermal insulation factor is 0.75 iv) the

further factor of 0.725 for using fuses (rewirable). c) The calculation then becomes:

Required Ratings = 15 0.52 x 0.97 x 0.75 x 0.725 = 54.7 Amps. d) The next step is to find from table 9D2 (Refer Page 154 of IEE 15th edition). Since the cable

concerned will be clipped direct to surface, it can be seen that 10mm cable must be selected with a current rating of 64A for one twin cable with protective conductor.

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NOTE: For other types of cable or conductor the relevant tables can be referred from the IEE 14th

Edition. The NEB needs to know the expected power demand of the installation in order to lay a

suitable cable for the incoming supply. This can be dealt with using the maximum demand plus diversity factor criteria together with the methods outlined in the section for estimating proce-dures. More detailed information on the installation aspect of wiring and cabling is available in JKR Specification.

The design of the final sub-circuits must be done in accordance with Section A of the IEE

Regulations (Control, Distribution and Excess Current Protection). 4. Fuse Ratings It helps a lot, however, to keep the following rules in mind:-

FUSE SIZE OF NO. OF POINTS PER CIRCUIT RATING WIRE*

5A 1.5mm2 10 Nos.of lighting or fan points or 1 Nos of 5A power point. 1 5A 2.5mm2 1 Nos. 1 3A switch socket outlet. 20A 2.5mm2 2 Nos l3A s/s/o. 30A 2.5mm2 10 Nos 13A s/sb provided they are all located (ring) within an area of not more than 1,000 sq. ft. 30A 4.0mm2 6 Nos 13A s/s/o.

(radial) A factor of 0.8 is normally applied to the specified rating of the distribution cable to limit

the load permissible, i.e. not more than a 4A load on a 5A sub-circuit. This ensures that the fuse rating is Standard HRC fuse Ratings available are (A):-

1,2,4,6,8,10,15,20,25,30,35,50,60,80,100,125,150,160 and 200. 5. Starting current, size of trunking and earth resistance Normally the fuse rating and cable sizes for A/C points (window unit) are chosen based on

the starting current. This starting current using a D.O.L. (Direct on Line) starter usually amounts to 2 or 3 times the normal running current. Generally, for an A/C unit with a compressor h.p. not exceeding 2 hp. and HRC fuse rating of 30A and a cable size of 2 x 6mm2 PVC/PVC are used. Also only one A/C point is permitted on one final sub-circuit.

For a multistory building, a rising mains in trunking is employed to provide power to the

various floors. Power is tapped from this mains at each floor. A system known as the Unit System is used to determine the size of the trunking.

Finally, the N.E.B. Consumer’s Section is very particular about earth resistance. During

supervision, it is good practice to get the electrical contractor to measure the earth resistance and drive appropriate number of earth rods to meet NEB requirements in accordance with the IRE Regulations. In certain solidly earthed installations where earth resistance is very poor, the NEB may permit the use of earth leakage protection for 3 phase loads. This will abviate heavy expen-diture on earthling.

6. Switchgears

The switchgears normally encountered are the ordinary lamp and socket outlet switches,

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isolators, fuse switches, switch fuses, circuit breakers (oil and air) changeover switches and con-tactors.

Given below are some useful notes on the more commonly used protective and isolating devices: i)Isolators these are used for local isolation only. They are not used for cable pro-

tection.

ii)Switch fuse this is a single unit made up of an isolator in series with an HRC Fuse, the HRC Fuse being retractable. It comes in both SP & N and TP & N configurations with standard ratings being 20A, 30A, 45A, 60A and 150A.

iii) Fuse-switch this is both an isolating as well as a protective device in that an HRC fuse is

lodged within the isolator arm forming part of the conducting path of the isolator. It is mainly used to control loads above 150A and has standard ratings of 100A, 150A, 300A, 400A and 600A.

iv)Air-circuit these together with over-current and earth leakage relays are used for

breakers busbar ratings of 400A and above. They are expensive but efficient tripping devices: (See fig. 6b).

v) Airr-circuit used for H.T. installations. (See fig. 6b). breakers

vi) Miniature circuit breakers (MCB) & moulded case circuit breakers (MCCB) —

MCBs and MCCBs are generally being introduced to replace h.r.c. fuses, switch fuses, fuse-

switches and even air circuit breakers on account of their compact size, ease of maintenance and neat appearance when incorporated into DB boxes and cubicles.

The main types are:- Westinghouse, Mitsubishi, Square D.Siemens, MEM’ Crabtree, F & G etc.

There is confusion among the JKR Electrical Engineers as to the short circuit KA ratings compared to h.r.c. fuses and air circuit-breakers. MCB/MCCB devices should be carefully used in circuits and adequate back-up protection may be necessary in the form of h.r.c. fuses. A JKR standard specification will be issued shortly.

The choice of rating for a circuit breaker must conform to the fault level designed for. The interrupting capacity of the circuit breaker (i.e. the maximum amount of fault current it can interrupt without damaging itself) must be equal to or greater than the amount of fault current that can be delivered at the point in the system where the breaker is applied. The maximum amount of fault current supplied by a system can be calculated at any point in that system. In selecting the circuit breaker to be used, the interrupting capacity of the circuit breaker is always the nearest higher or equal to the calculated value.

To conform to NEB standards when applying circuit breakers to an installation, the following interrupting capacities should be considered for the 240/415V system.

M.S.B. : 43 kA (31 MVA) to 32 IA (23 MVA)

S.S.B. : 22 kA (l6 MVA) to 14 kA(10 MVA) D.F.B. : lUkA (7 MVA)

For an 11kV system, the interrupting capacity should be 250 MVA. 44

vii) High Rupture Capacity Fuse The HRC fuse (High Rupture Capacity fuse) is in many ways superior to oil or air circuit

breakers in the medium voltage range of 400V to 600V. It is reliable with no maintenance required although replacement is necessary after operation. The reader is referred to manufacturer’s catalogues for the time-current characteristics of the various HRC fuses.

JKR Specifications on switchgear should be perused for further information on various H.V. and MV. switchgear.

The question of when to use SP & N or TP & N switchgear depends on whether the supply is single or three phase. Generally, a 3 phase supply is required for loads above 60A single phase.

7. Switchboards & Distribution Fuse boards

The main switchboard normally comprises of A.C.Bs, earth fault relays, ammeters, voltmeters,

power factor meters, current transformers, switch fuses, fuse switches, etc. It must be capable of withstanding certain fault and load conditions and should be positioned at a suitable location with respect to accessibility, ease of operation and length of cable run with adequate ventilation ensured. Only switchboards manufactured by CEI approved manufacturers may be used on the condition that the structure and material used for the manufacture of the switchboard comply with JKR Specification.

The mounting height for a distribution fuse board shall be 6’9” measured from the floor to the underside of the DFB. A table according to JKR Specification shall be provided with appropriate details of the circuitry such as cable sizes, number of wiring points, types of light fittings or socket outlets. Labels must also be fitted on the outside of all DFB’s.

One may sometimes have to decide between having a submain switchboard or a distribution fuse board in a project involving several separate buildings. The decision depends on whether it is cheaper to connect 2 pairs of 4 core cables to a submain switchboard plus associated cabling to the DB’s or connect a number of 4 core cables to the respective distribution fuse boards. 8. Maximum Demand and Diversity Factors

The term maximum demand refers to the expected maximum power requirement of an ins-

tallation. This value is given by the product of the Total Connected Load and the Diversity Factor i.e. M.D. = T.C.L. x D.F. Knowledge of this value enables the NEB to determine the size of its incoming cable.

The total connected load is the total load expected to be connected to the system while the diversity factor is a weighting factor that is used to simulate actual loading conditions.

Page 188 of the IEE Regulations provides a table which enables one to estimate the maximum current which will flow in an installation so as to enable to calculate the sizes of cables and switchgears.

It should be pointed out, however, that no diversity is to be allowed for when calculating the size of circuit conductors and switchgear for final sub-circuits. 9. Lightning and Fire Protection

The British Standard Code of Practice CP 326: 1965 titled “The Protection of Structures Against Lightning” together with JKR Specifications form the criteria for the design of lightning protection systems. Note should also be made of the existence of drawings for lightning protection system for standard buildings such as multistory quarters and office blocks. Fire Protection systems in the form of fire alarm systems are especially important for high

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rise buildings. The reader is referred to a recent JKR Circular on Fire Alarm Systems for High-Rise Buildings and the Fire Regulations. The Fire Regulations, for example, require the use of flame-proof (MICC) wiring of mains for the lifts and other essential loads and conduit wiring in ceiling spaces and voids, especially for lighting sub-circuits.

10. New Works or Additional Works (Extension)

The electrical installation work encountered can be divided into 2 types, viz: new installa-tions and additional installations (extension). The design for new installations is generally less complicated than that for additional installations. In the latter case one has to take into conside-ration factors like the existing load, capability of the existing system to support the required additional load, the need for additional generators or substations and how to go about obtaining the supply from the existing system.

11. Design and Maintenance

A good design in our case is one that is agreeable with the relevant codes of practice and regulations, economical for the government and requires a minimum of maintenance. While the first two criteria are readily satisfied the third criterion depends very much on the experience of the Electrical Engineer. However, problems can be avoided if the following points are kept in mind:-

i) use only good quality materials. ii) chokes, starters and poor quality wiring are the common source of trouble for fluorescent

fittings. iii) ensure that PVC wires in conduit are never used in place of PVC or Paper armored u/g

cables. iv) strict supervision of contractors’ work.

12. Standard designs

It is the practice in Cawangan Letrik to standardize electrical design for as many types of Government buildings as is reasonably practicable. This follows from the fact that projects can be implemented more speedily and economically when standard designs and drawings are available. Thus there are standard electrical installation drawing for standard government buildings like schools, health centers, police stations quarters and so on. One should therefore be aware if the job at hand is a standard one. However, various problems arise in trying to standardize the electri-cal design for the government buildings especially office blocks.

6.4 Estimating Procedures:

1. On receipt of drawings from the architect, the Electrical Engineer is required to mark --estimates of the lectrical load for the N.E.B. and costs of the electrical installation for the archi-tect or client. In order to speed up initial estimates there are several rules of thumb viz:-

Lighting Point : 0.1 kW per point Fan point : 0.06 kW per point l3A 3 pin s/s/o : 0.25 kW per point 1 SA s/s/o : 0.5 kW per point 5A s/s/o : 0.1 kW per point A/C point : 2.0 kW per point

The figures given above represent the maximum electrical demand for the particular installation if no diversification is applied.

2. The maximum demand in watts/sq. ft for a typical example is as follows: Type of load Multi-storey office block Lighting 0.9W/sq.ft. A/C 4,7W/sq.ft. Future load growth, say 20% of above l.lW/sq.ft.

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3. Rough estimates of the electrical load per sq. ft. and the unit costs of electrical installations per sq. ft. are tabulated below for a sample of buildings. Once again, it must be emphasized that the figures arrived at are only to be used for rough initial estimates or as a check on the detailed estimates. It should also be pointed out that the cost of installing a building evaluated as a ‘component project’ based on unit prices quoted for the entire project is always lower than that evaluated as a single individual project based on the nett tender price quoted for the project. 6.5 SELECTION OF STANDBY ELECTRICAL GENERATOR SETS 1. Introduction 1. There are rare occasions when the public electricity supply fails and a building is left without electricity. In some buildings the risk of being totally without electricity cannot be taken, and some provision must be made for an alternative supply to be used in an emergency. Many consumers have installed standby generating sets in their building to maintain a supply to their essential loads. 2. The majority of small standby sets installed will be powered by diesel engines, as these are the most readily available prime movers, and most economic in capital costs, operation and maintenance. Most emergency sets can be started either manually or automatically. A manual start is simple, but it involves a delay during which the building is without power. This delay can be avoided by automatic starting, initiated by a sensing unit which detects a drop in the mains voltage. 6.5.1 Load Assessment 1. It is necessary to determine to which loads one proposes to maintain a supply under emergency conditions. These essential loads may consist of static loads and motor loads. 2. Static loads consist of lamps, communication equipment, etc. — items which do not involve rotating machinery. Static loads are expressed in KW. Motor loads are those electrical motors which power such things as fireman lifts, pressurization fans and fire fighting equipment. These motor loads are also expressed in KW, but they also place an additional demand on the electric supply. This demand is the starting KYA (sKVA). The starting KVA for a motor of 5 KW or more is 5.5 to 6 times the rated KVA and it is 8 to 10 times approximately in the case of one of 5 KW or lower. 3. In starting the motor, therefore, a fairly large load is applied to the generator. Under the rated full load, most motors require about 1.4 KVA per KW. It is therefore easy to find out KVA values on the basis of output KW.

Thus to determine the total load,

a) Add up the static loads in KW to establish the total static load. b) Identify the individual motor loads and compute their total KW value. Then individually

calculate the starting KYA of each motor — as stated below. 6.5.2 American Design Motors

1. The motor horsepower (hp) and NEMA (National Electric Manufacturer’s Association) code letter will be shown. These two items allow you to determine quickly the motor power requirement in KW and the starting requirement in KVA. For KW requirement, multiply the nameplate horsepower by 0.85 (0.85 is derived from the conversion of 0.716 kw/hp divided by a typical motor efficiency of 0.88). The result closely approximates the motor KW demand at full load. For sKVA — refer to the NEMA code letter in Table 1. Multiply the related sKVA/hp figure by the motor nameplate horsepower.

Example: A 100 hp, code F motor has a sKVA of 100 x 5.5 = 550 kva.

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Table 1 Identification of code letters on typical US design 3 phase induction motors

Wound rotor motors have no code letters. 6.5.3 European Design Motors 1. The essential data is on the nameplates of European design motors. However, the method of

using this information may be different. The motor capacity (output) may be expressed in horse-power, or, in kw; and there will be no code letters. If the name-plate includes horsepower, multiply this figure by 0.85 to determine the kw requirement (same process as with American design motors). If the nameplate lists motor capacities in kw, multiply this mechanical kw figure by 1.15 to determine the electrical kw requirement from the generator set (1.15 is derived from the reciprocal of a typical electric motor effiency of 88% of 0.88:- 1/0.88 = 1.15).

2. The nameplate may list the starting kva (skva) or may list a figure, LRA, which means ‘Locked

Rotor Amperes’. To arrive at the skva, the following formula should be applied,

skva = LRA x rated voltage x 1.732 1000

c) Establish the total kw demand: The static load kw should be added to the motor load kw. When this exercise is completed for all static and motor loads, the total figure is the minimum generator set capacity of the selected generator set.

6.5.4 Check for voltage Dip

The next step is to establish if the generator set with the required kw capacity will be adequate to cope with the motor skva needs. Manufacturers usually provide tables listing voltage dip versus skva. Each motor is checked against the generator set skva capacity for the probable voltage dip. This figure is compared with the acceptable voltage dip. If the expected dip exceeds the acceptable dip, the generator size will have to be increased or the starting requirement of motors will have to be reduced.

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6.6 Power Factor Improvement 1. Introduction The effects of a low power factor in an installation is well known. These include:

i) A penalty charge in the electricity bill ii) Extra losses in the feeder cable iii) A significant voltage drop in the cables iv) A reduction of the effective capacity of the cables v) A reduction in the power available at the transformer vi) A significant voltage drop at the secondary of the transformer vii) Significant losses in the transformer There are basically two types of equipment for improving the power factor of an installation:-

a) By rotary phase advancers, synchronous condensers or synchronous motors b) Static capacitors.

For normal installations, the capital cost of rotating machinery, both synchronous and phase advancing, makes its use uneconomical and, in addition, the wear and tear inherent in all rotary machines involves additional expenses for upkeep and maintenance. Capacitors, on the other hand, have a very low initial cost, have minimal upkeep costs and can be used with high efficiencies on all sizes of installations. They are compact, reliable and convenient to install and thus is the more satisfactory equipment for power factor improvement. The sitting of the capacitors in an installation depends on whether each piece of equipment, example a motor, is being individually corrected or the plant/installation as a whole is being corrected as a block (bulk of coup correction). 2. Individual Correction This is used in small installations on motors constantly in operation or, in the case of kVA maximum demand tariffs, on certain motors known to be in operation at the time of maximum demand. It should not be applied where the motors are used for haulage, cranes, colliery winders or where “inching” or “plugging” and direct reversal takes place. Individual correction of tandem (or two speed motors) should be avoided. If correction is necessary, the capacitor should never be connected directly to the low speed component but a contractor arrangement installed using one capacitor for both windings.

In general, this method is-not profitable for motors less than 10 kW. -

i) Advantages of Individual Correction - This method reduces the current loading on the distribution system with consequent

improvement in the voltage regulation. Also no additional switchgear is required as the capacitor is connected directly across the motor terminals and, therefore, switched with the load by the motor starter.

ii) Disadvantages of Individual Correction

Several small capacitors installed at various individual loads may cost more than a single capacitor of total equivalent rating centrally installed. Also the capacitors have a low utilization factor as the capacitor operates only when the particular load is used. iii) General Considerations

1.One size of capacitor will give constant value of power factor over the normal load range since variations in motor kVAR are comparatively small.

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2. Since connection of the capacitor directly across the motor results in a lower load current, the overload setting on the starter should be reduced in order to obtain the same degree of protection.

3. When star delta starting is used, a standard three terminal, delta connected capacitor

should be employed, which gives maximum power factor correction at the start when the power factor of the motor is low.

4. To prevent auto-excitation, (ie. self excitation of the motor by the stored capacitor charge

— when the motor supply is switched off) ensure that the capacitor current is equal to or smaller than the motor magnetizing current. A commonly used value is 90% of the motor no load current.

5. The capacitor rating required is calculated as follows:- kVAR required = HP. (of motor) x 0.746 x % of full load x (tan Ø1 — tan Ø2).

efficiency (at the above % of full load)

where cos Ø1 is the original power factor and cosØ2 is the required power factor. The required kVar is the smaller of that obtained from (4) & (5) above. If motor characteristics are not available use Table 2.

6. Motors are usually corrected to a power factor of 0.98 at 75% load.

7. Welding equipment generally have a power factor of about 0.35 lagging but since

welding loads are intermittent and consequently have a low load factor, they are usually corrected to about 0.6 to 0.8 based on its continuous kVA rating.

8. The diagram below shows the method of connection for individual power factor

correction of motors.

3.0 Bulk or Central Correction This method is used when the total reactive load varies during the day, but is too small to

be compensated individually because of cost reasons. Also, it may sometimes be impossible to connect capacitors at the individual load locations due to high ambient temperatures, restricted space or presence of explosive gases.

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i) Advantages of Central Correction This is an ideal method of obtaining the full electrical and financial benefits of a capacitor

installation. The central location makes supervision easier and with automatic control, the resulting economics and convenience may outweigh the initial cost.

ii) Disadvantages of Central Correction Here, the load in the distribution lines are not lightened. The capacitors must also be provided with

protective and isolating gear. This can be done manually but manual switching requires surveillance which may not be convenient to provide. Alternatively, it can be done automatically (but at a higher cost).

iii) General Considerations (for central automatic power factor correction)

a) The equipment consist of a capacitor bank subdivided into two or more steps, each step or capacitor being controlled by a contractor.

b) In turn the contractors are controlled by a reactive relay. The reactive relay consists of a

potential coil connected across 2 phases of the supply load and a current coil taken from a current transformer on the third phase - so as to obtain a 90° phase displacement at unity power factor.

c) The number of stages installed is usually a compromise between the technical requirement

and cost. The aim is to have each contactor switching its maximum rated capacitance and, at the same time, have the capacitor bank divided into the most economic subsections, so that all variations in load can be corrected.

d) To determine the rating of the capacitor needed, the following formula is used:-

Pr = Pw (tan Ø1-tan Ø2). where

Pr is the rating of the capacitor required in KVAR Pw is the installation load in kW

Cos Ø1 is the initial power factor, and Cos Ø2 is the required power factor

The above formula is derived from the following diagram.

V: System Voltage I1, I2; Original & final system current Ic: Capacitor current

Alternatively, the required rating can be obtained from tables which give the required values of (tan Ø1 - tan Ø2). This is given as the ‘Multiplier’ in Table 1.

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e) It is necessary to set the relay to operate with the particular current transformer used. On most relays, this is usually achieved by means of a C/K setting, where C = size of capacitor being switched (in Kvar), and K = ratio of current transformer primary to secondary. Example: For C = 100 Kvar and K = 2000/5 = 400 C/K = 100/400 = 0.25

f) To prevent “hunting” ie. a continuous switching in and out of capacitor step, the

sensitivity limit of the regulator is set such that is greater than the current of one capacitor step. In practice the regulator is usually set to react to changes corresponding to about 2/3 of the current of one capacitor step.

g) A typical connection of an automatic relay is as shown below:-

54

55

4.0 General i) Selection of Current Transformer (for power factor relays)

a) The primary rating of the CT is based on the line current of the system at its original power factor.

b) The secondary rating depends on the current rating of the power factor relay used and is

usually 5 amps. c) The burden of the CT depends on the Volt-Ampere consumption of the relay and is usually

less than 5 VA. If the CT is far from the battery, a 10 VA CT may have to be used to take into account of the 12R losses in the cables.

d) In some cases, it may be necessary to summate the currents in more than one circuit.

Here a suitable auxiliary summation CT can be used.

Here the main current transformers (CT1 and CT2) are placed in the same phase of their respective circuits.

If, for example, the two main CTs have ratios 600/5 and 200/S respectively and the summation CT has a ratio of (S + 5)/5, the ratio of the total transformation will then be

600+200 =160 5

e) The CTs are usually of accuracy class C or D (Class I).

ii) Ratings of Switchgear The duty imposed on switchgear and fuse gear used with capacitors is heavier than that in normal circumstances due to:

a) At the instant of switching, a large transient current will flow.

b) High over voltage transients can occur when capacitors are disconnected by switching devices which allow restriking of the arc.

c) The switchgear has to carry continuously the full rated current of the capacitor at all times

the capacitor is in circuit.

56 d) At light loads when the voltage may be higher than normal, the capacitor currents will be

increased accordingly.

e) If harmonics are present in the supply voltage, the capacitor current will be increased.

In view of this, it is normal to have switchgear of ratings higher than the rated current of the capacitor. IEC 70 recommends a current rating of 1.3 x 1.1 times the rated current of the capacitor. Often, a factor of 1.6 times the rated current is used.

iii) General Considerations in the Installation of Capacitors a) The relevant Codes applicable for the installation of capacitors are IEC 70/70A and

British Standards Code of Practice CP321.102: on “Installation and Maintenance of Electrical Machines, Transformers, Rectifiers, Capacitors and Associated Equipment”.

b) Due to the maximum permissible ambient temperatures (not greater than 50° C as per IEC

70/70A and 45° C — mean over 1 hour — as per B.S. 1650), the capacitors must be installed in a well ventilated position.

c) Except for capacitors directly connected to motor terminals, all capacitor b4ks should

have a switching device equipped with an appropriate automatic trip.

d) A discharge device should be provided for every capacitor equipment unless it is connected directly to other electrical equipment providing a discharge path without a disconnecting switch, fuse cut-out, or series capacitor interposed. IEC Publication 70 states that the discharge device should reduce the residual voltage from the crest value of the rated voltage to 50V or less within a given time after disconnection of the capacitor from the supply. This time is 1 minute for capacitors of rated voltage up to and including 660V, and 5 minutes for capacitors of rated voltage above 660V.

The value of the discharge device resistance can be calculated as follows:- Example: for low voltage capacitors below 660V, to fulfill the above condition,

-60

50 = U√2 e RC

where C is the capacitance of the capacitor in Farad, R is the discharge device resistance in Ohm, and U is the r.m.s. value of the system voltage in Volt.

To include the effect of the tolerances in voltage and resistance values, the nominal voltage is usually increased by 10%. Thus the above expression is modified to

-60

50 = 1.1 U√2 e RC

To obtain the capacitance of a three phase capacitor rated at P kVAR, V volts, 50 Hz.

C= Farad P

3 x (2π x 50) x V2

e) Self healing dry type capacitors should normally be specified for all installation due to their superior qualities and ease of maintenance.

f) To reduced the current loading in the distribution system, the power factor correction

should be done as close as possible to the load.

g) Apart from the individual and the bulk/central corrections methods, other methods

57 are possible. These are in effect variations and combinations of the above mentioned two methods. They are:

Group compensation eg. One capacitor/capacitor bank for a group of motors. Fixed central compensation.

Combined compensation — a combination of fixed, automatic, group and individual compensations.

h) Very often at the time of installing the power factor correction system, the load has not build

up yet. The power factor correction system should thus either be designed to cater for future loads or provisions be made in switchgears, cables, accessories, space, etc. for easy extension of the system in the future.

iv) Maintenance of power factor correction capacitors.

a) Capacitors being static equipment, do not generally require the same degree of care as rotary machinery, but, nevertheless, require regular maintenance.

b) Capacitors should normally be inspected every 12 months — preferably 6 months. Inspection

time intervals are governed mainly by conditions on site. Humid atmospheres or those subjected to chemical fumes, dirt or dust require more frequent attention.

c) Before examination, the apparatus should be switched off and time allowed for complete

discharge as stated on the rating plates. Current transformers must never operate with the secondary circuit open. If it is not being used, the secondary terminals must be short-circuited.

d) Conditions of exterior finish and protective paint should be in good condition. If necessary, it

should be repainted. e) The terminal box cover should be removed and inspected for abnormalities. Special care should

be taken of i) Condition of cable ii) Condition of interior paint work (repainting if necessary) iii) Tightness of nuts and bolts — especially of earth connections iv) Removal of dust and other foreign matter v) Cleanliness — particularly of insulators and terminals