electrical distribution system in office building

ELECTRICAL DISTRIBUTION SYSTEM IN OFFICE BUILDING

PRESENTED BY:

APURVA ANAND

IIIRD SEM, M.ARCH.(BS)

Introduction• Electricity is a secondary energy

source, i.e. it comes from the conversion of other sources of energy, such as coal, natural gas, oil, wood and nuclear power.

• The energy sources used to make electricity can be renewable or non-renewable, but electricity itself is neither. Nor it can be stored.

Electricity T & D consists of two infrastructure

The high-voltage transmission systems, which carry electricity from the power plants and transmit it hundreds of miles away, (Primary Side)

And the lower-voltage distribution systems, which draw electricity from the transmission lines and distribute it to individual customers (secondary side)

Transmission and Distribution lines Power plants typically produce 50 cycle/second (Hertz), alternating-

current (AC) electricity with voltages between 11kV and 33kV. At the power plant site, the 3-phase voltage is stepped up to a

higher voltage for transmission on cables strung on cross-country towers.

High voltage (HV) and extra high voltage (EHV) transmission is the next stage from power plant to transport A.C. power over long distances at voltages like; 220 kV & 400 kV.

Sub-transmission network at 132 kV, 110 kV, 66 kV or 33 kV constitutes the next link towards the end user. Distribution at 11 kV / 6.6 kV / 3.3 kV constitutes the last link to the consumer, who is connected directly or through transformers depending upon the drawn level of service.

The T & D network include sub-stations, lines and distribution transformers.

Sub-stations, containing step-down transformers, reduce the voltage for distribution to industrial users.

The voltage is further reduced for commercial facilities.

Transmission and Distribution lines

Generation Efficiency η1 Efficiency ranges 28-35 % with respect to size of thermal plant, age of plant and capacity utilization

Step-up Station η2 Step-up to 400 / 800 kV to enable EHV transmission. Envisaged max. losses 0.5 % or efficiency of 99.5 %

EHV Transmission & Station η3

EHV transmission and substations at 400 kV / 800 kV. Envisaged maximum losses 1.0 % or efficiency of 99 %

HV Transmission & Station η4

HV transmission & Substations for 220 / 400 kV. Envisaged maximum losses 2.5 % or efficiency of 97.5 %

Sub-transmission η5 Sub-transmission at 66 / 132 kV Envisaged maximum losses 4 % or efficiency of 96 %

Distribution Station η6 Step-down to a level of 11 / 33 kV. Envisaged losses 0.5 % or efficiency of 99.5 %

Primary Distribution η7 Distribution is final link to end user at 11 / 33 kV. Envisaged losses maximum 5 % of efficiency of 95 %

End user Premises Cascade efficiency from Generation to end user =

η1 × η2 × η3 × η4 × η5 × η6 × η7 = 83 %

Transmission and Distribution lines

Electrical Distribution System

Primary distribution system Between distribution substation and distribution

transformer. Made up of circuits called primary feeders or distribution

feeders. aluminum conductors have almost entirely displaced copper

for new construction. For underground primaries, size range from No. 4 AWG to 1000 kCMil.

The traditional rule of thumb is to allow a voltage drop of about 5% in the primary of urban and suburban systems at time of peak load. It is very probable,

However, that economic system designs have a primary voltage drop smaller than 3%. In rural systems, typically with long lines and light load densities, voltage drop maybe somewhat larger.

Primary distribution

systems

Secondary distribution

systemsSecondary distribution system Between the primary feeders and utilization equipment. Consists of step-down transformers and secondary circuits

at utilization voltage levels. Residential secondary systems are generally single-phase,

but commercial and industrial buildings get three phase In residential and rural areas the nominal supply is a 220

V, single-phase. In commercial or industrial areas, where motor loads are

predominant, the common three-phase system voltages are 220 V and 440 V.

Heavy machines with 440 V loads are connected directly to the system at 440 V and fluorescent lighting is connected phase to neutral at 220 V.

Secondary distribution

systems

Primary distribution

systems

PRIMARY DISTRIBUTION

SYSTEM

Radial distribution system

Ring distribution system

Spike distribution system

Spindle distribution system

Network distribution system

Selective distribution system

Primary distribution system

Primary distribution systemRadial Distribution System: In this system, separate feeders are

radiated from a single substation and feed distribution transformations from one end only. This is the simplest distribution circuit and satisfies most circuit requirements with the lowest initial cost but it has the following drawbacks:

The circuit is not very reliable as in the case of fault in any feeder section, the supply to consumers who are on the side of the fault away from the substation is interrupted.

Feeder end nearest to the substation will be heavily loaded.

The consumers at the distant end of the feeder would be subjected to voltage fluctuations when the load on the feeder changes.

Ring Distribution System:

In this system the primaries of distribution transformers form a loop.

The loop circuit start from the substation bus-bars makes a loop through the areas to be served and returns to the substation.

The circuit provides for quick restoration of service in the case of transformer or feeder fault.

This circuit is very reliable but has a relatively larger cost. Also each side of the ring should not be loaded more than 50% of its rating as to accept the load of the other side in the case of fault.


Spike Distribution System: In this system all secondary

substations are supplied by using radial feeders.

In addition a separate stand-by feeder is provided for emergency (to provide power supply in the case of a fault in any main feeder).

Each feeder in this case is designed to withstand its own load with no additional capacity. The circuit is reliable and could be used if the site permits.


Spindle Distribution System:

In this system all secondary substations are supplied by using radial feeders.

In addition a switching station is provided to connect the supply to the feeder with a faulty section .

This circuit could be implemented only if the site layout permits. It is more expensive but reliable.


Network (or Grid) Distribution System:

In this system the primary feeders are connected to form a grid ( or network ).

The network is then supplied at two or three locations. The network should be provided with sectionalizing and switching facilities.

The system provides a good supply security and flexibility but it has a large capital investments.


Selective Distribution System: In this system two separate

feeders are used and each distribution substation could be connected to either feeder.

In the case of a fault in any feeder, the load could be transferred (manually or automatically) to the other feeder.

The system is very reliable but has a large financial cost.


Vertical Supply system

Single rising main

Grouped

supply

Individual floor supply

Ring main

supply

Double feed

supply

Secondary distribution system

The horizontal

supply (Distribution at each

floor level)

The vertical supply system (rising mains).

Single rising main:- In this type of system only one main

line goes to the upper level and distribute at each floor respectively. Not very common in practice, generally used at place where high supply security system is not important.

Advantages The different loads on individual floors

are balanced out. Only a small main L.V board is required. Simple in construction and operation.

Disadvantage Low supply security (a fault in the rising

mains effect all floors).

Secondary Distribution System


Grouped supply- Another type of electrical system of

distribution, in these two or more main lines run together and serves different floors.

Grouped supply- applicable for the high rise buildings.

Advantages:- Easier mounting. Smaller size for rising mains.

Disadvantages:- A fault in any rising mains effect

several floors (relatively low Security). Loads are balanced only within each

group. Larger power distribution board.



Individual floor supply:- In high rise buildings were stories are let

separately (metering is at central point at ground floor).

Advantages:- Smaller size of cables can be used (easy

installation). In the case of a fault in arising main, only one

story is affected.

Disadvantages:- Different loading of the individual floors cannot

be balanced out. The rising main must be rated for the peak

load of each floor. Uneconomical – large number of cables

and the size of the rising main shaft is quite large.

Large low voltage distribution board with numerous circuits


The vertical supply system (rising mains). Ring main supply:- In large buildings when relatively

higher security is required.

Advantages:- Higher power supply security (in the

event of a fault, it is possible to switch off the faulty part and leave the majority of the building operational)

A small low voltage distribution board is required.

The differing loading of individual floor are balanced out (smaller sizes for rising mains)



Double feed supply: In large buildings with relatively

large loads at the top floors (lifts, Kitchen, air-conditioning).

Advantages:- Higher power supply security. The differing loading of individual

floors are balanced out. Smaller L.V. distribution board

required.


The horizontal supply (Distribution at each floor level) Normally HV switchgear and

substation transformers are installed at ground floor (or basement).

However, often there are appliances with large power demand installed on the top floors (converters and motors for lifts, air-conditioning equipment and electric kitchens).

The arrangement of the rising mains depends on the size and shape of the building and suitable size of shafts for installing cables and bus ducts must be provided in coordination with the building architect.


Transformer A transformer can accept energy at one

voltage and deliver it at another voltage. Transformers consist of two or more coils

that are electrically insulated, but magnetically linked. The primary coil is connected to the power source and the secondary coil connects to the load.

The turn’s ratio is the ratio between the numbers of turns on the secondary to the turns on the primary.

The secondary voltage is equal to the primary voltage times the turn’s ratio.

Power supply system

Types of Transformers

Classification A- Power Transformers and Distribution Transformers Power transformers are used in transmission network of higher

voltages, deployed for step-up and step down transformer application (400 kV, 200 kV, 110 kV, 66 kV, 33kV)

Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V).

Transformer

Classification B- Dry Type and Oil Based Transformers Classification based on cooling and insulating system

for transformers,.

Disadvantage, When operating at the same flux and current density,

more material for core and coil implies higher losses and higher costs.

Will be larger than liquid-immersed units for the same voltage and capacity rating.

Advantage Offers certain fire-resistant, environmental, and

application advantages for industrial and commercial situations.

Oil based Greater energy efficiency. Lower sound level. Smaller footprint. Much longer operating life, ease of maintainability and

repairability. Ease of recycling.

Power supply system

Configuration of Load center- Two transformer load Centre

Power supply system

Configuration of Load center- Single transformer load centre High-voltage supply line from

power company transformed to 440-volts; general lighting and power fed by the 440-volt line through a unit dry-type transformer, 440/ 220-volt,

Similar to the above “except” several units of smaller dry-type transformers are distributed in the areas or floors for general lighting and power system; these unit transformers are fed by 440-volt line or lines from the load center.

Power supply system

Emergency Generators Emergency generators are used

to provide critical loads with power supply in the case of mains failure (operating theaters & intensive care units in hospitals, computer buildings, etc).

Emergency generators are usually driven by diesel engines, and connected to the load in the following way :

When the generator is of the same size as the power supply transformer.

When the generator is of a smaller size as compared with the power

Power supply system

Mains Failure Panel (MFP) or Auto Mains Failure (AMF) The MFP is intended for automatic operation (Start & Stop) of the

emergency generator and in coordination with the mains incoming supply from the low voltage side of transformer. The MFP shall be provided with :-

The MFP or AMF Panel must have the following features:- To provide the output load distribution with supply from public mains

when it is available. To provide the output load distribution with supply from generating

set in the case of mains failure with adjustable time delay (10 - 15) sec.

It should operate the generating set also when the mains voltage falls in one or more phases below 80% of the standard value (adjustable).

When the public mains power returns to within acceptable limits, the contactors should change over positions and the generator stop and reset itself in readiness for further failure.

A built in time delay unit shall be provided to allow generator running for a short period (adjustable) after mains have returned to ensure power supply continuity, should further mains failure or fluctuations occur.

Power supply system

Light and fan points are connected in the same circuit but the total number ahould be 10 or the total load should be 800 watt whichever is less.

For light points the load is 100watt For power plugs the load considered is 500 watt Two power plugs can be joined in one circuit For AC, the load is considered as 2500 watt. AC circuit should go individually into the DB

OR Find out built up area in Sqft.of per flat per House/Dwelling unit. Multiply area in Sqft. by Load/Sqft. Type of Load Load/Sqft Industrial 100 Watt/Sqft Commercial 30 Watt/Sqft Domestic 15 Watt/Sqft

Guideline To Design Electrical Network

Only two-no of transformer at one location shall be acceptable. If there is more number of transformers HT shall be required to extend using underground cables to locate additional transformer.

Either VCB or Ring Main Circuit shall be used to control transformers. There cables should have metering arrangement at 11 kV.

On LT side of transformer, LT main feeder pillar shall be provided. The Incoming shall be protected by MCCB/ ACB.

The factors for cable loading shall be taken as 50%. The factor for multiplicity of cables from same cable trench shall be 80%.

The suggested maximum length of LT cable feeder shall be 250 Mtrs.

LT Cable Size Upto 50kW 3 ½ C x 150 sqmm Upto 100kW 3 ½ C x 300 sqmm Upto 150 kW 3 ½ C x 400sqmm AL, XLPE insulated armored cable. The entire system has to be designed for a voltage drop of 2.0%

from 11kV Side of transformer to metering equipment at end consumer premises.

Guideline To Design Electrical Network

For the analysis I have taken an office building under construction and calculated the load based on the guidelines mentioned above. Apart from those one enlisted some more consideration that has to take into account are listed below. Substation It is desirable to locate the substation into the different structure

apart from the main building. But if it is necessary to house it within the building it should not be

installed above ground floor or below first basement (with height not less than 3500mm)

Some consideration that are to made for the location of the substation are:

Load Centre of the building (electrical load). If possible, geometrical Centre of the building. Highest level of floor in vicinity of DG and AC plant room. Direct access to the road and street independent of the main

building Entrance Door opening outward. All doors should have adequate space and air passage along with

proper ventilation Substation room floor minimum 450mm high from the normal floor

level.

Analysis and Logical Implication

Transformer The transformers should be placed at a plinth of height 450mm and

if placed inside the room the minimum floor to ceiling height should be 4 meters.

All doors and windows should be openable outside. In multistorey/ high rise structures, 2 transformers of equal capacity

has to installed so that 120% of the peak load can be catered by the single transformer in case another is not working properly. This caters to the continuity and stability of the system.

If the transformer is oil filled, it should be installed at a minimum distance of 6 meters from the adjoining building as a precautionary measure to avoid the oil spillage (4.5- 5 meters).

LT and HT Switch room Minimum floor to ceiling height of the switch room has to be 3600

mm. HT switch room should not be clubbed along with the guard or other

service room near the entrance. LT Panel should be in separate room of building or near the entrance

of the building. Floor panels on the floor are to be located near the shaft.

Analysis and Logical Implication

Shaft Design The width of the shaft can be

calculated as

4 times the width of the cable X Total no. of cables. With the minimum width of

450mm. The spacing between the cables

will be equal to the diameter of the widest cable.

With the help of the cable size and spacing the length can be determined with minimum criteria of 600mm.

DB location Easily accessible. Facilitate the worker to repair. Middle of the house and load

center.

Analysis and Logical ImplicationSome more Considerations Every circuit should have its

separate earth and neutral wire. In a circuit live wire should be

independent of other circuit For the three phase supply,

separate conduit for separate circuit is mandatory.

After the normal run of 15 meters, we should use junction box for

Easy pulling Avoiding stress and strain on the

wires Providing the drainage path to

condensation Inspection box with suitable holes

to be provided on cover plate to dissipate hot gases and cooling the conductor temperature.

Analysis and Logical Implication- Circuit Diagram

TOTAL LOAD=18600 WATT PER FLOOR (CEILING AND WALLS)

TOTAL LOAD=18100 WATT PER FLOOR (UNDER FLOOR)

TOTAL LOAD PER FLOOR=36700 WATT (PER FLOOR)

CABLE SIZE FOR LIGHT POINTS CIRCUIT 6 AMP MCB THEREFORE 2 X 2.5 SQ.MM +1 X1.0 SQ.MM FOR POWER CIRCUITS 10 AMP MCB THEREFORE 2 X 4 SQ.MM +1 X1.0 SQ.MM FOR AC UNITS 6 AMP MCB THEREFORE 2 X 2.5 SQ.MM +1 X1.0 SQ.MM FROM DB TO LT PANEL TOTAL LOAD ON DB A= 15 KW THEREFORE 15 KW X 1.7= 25 AMP CONSIDERING SAFETY AND LAYING FACTORS=

25 X 2=50 AMP THEREFORE CABLE SIZE- 16 SQMM.

FROM LT PANEL TO TRANSFORMER LOAD- 36 X 8= 300 KW 300 X 1.7= 510 AMP CONSIDERING SAFETY AND LAYING

FACTORS= 510 X 2= 1000 AMP 300 SQMM X 2 CABLES. FOR TRANSFORMER CAPACITY LOAD PER FLOOR= 36 KW DIVERSITY FACTOR FOR COMMERCIAL- 0.9 THEREFORE 36 X 0.9= 33KW TOTAL LOAD OF ALL FLOORS 33 X8= 264

KW THEREFORE 264 X0.9 (DIVERSITY

FACTOR)= 238 KW EXTRA LOAD FOR WATER PUMPS, FIRE

FIGHTING ETC= 62 KW TOTAL LOAD ON TRANSFORMER= 300 KW THEREFORE 270 /0.8(SAFETY FACTOR)

X0.8 = 468.75 KVA TRANSFORMER REQUIRED= 500 KVA

Analysis and Logical Implication- Load Calculations

Analysis and Logical Implication- SLD

electrical distribution system in office building

Documents

total number

rising mains

high rise

power supply

design electrical

ht switch

output load

primary distribution