Comparison between Overhead Lines and Underground Cables

 

Comparison between Overhead Lines and Underground Cables

	Overhead Line	Underground cable
Fault location	As the overhead line is visible, it is easy to find the location of the fault.	As the underground cable is invisible, it is very difficult to find the location of the fault.
Initial cost	There is no requirement of digging, manholes, and trench. So, the overhead line system is cheaper than the underground system.	The initial cost of the underground transmission system is more compared to the overhead line because it needs digging, trenching, etc.
Chance of fault	As overhead line exposed to the environment, the chances of faults are more.	The cables are not exposed to the environment, there is less chance of fault.
Safety	This system is less safe as the conductors placed on the towers.	This system is safer as the cables placed underground.
Useful life	In this system, useful life is approximately 20 to 25 years.	Useful life is approximately 40 to 50 years.
Appearance	The general appearance of this system is not good because of all lines are visible.	The general appearance of this system is good because of all lines are invisible.
Maintenance cost	In this system, no need to dig at the time of maintenance. Hence, for the same number of faults, the maintenance cost is less.	In this system, to find the fault, digging is compulsory. It increases labor cost. Hence, for the same number of faults, the maintenance cost is more.
Flexibility	This system is more flexible. Because the expansion of the system is easily possible.	This system is not flexible. The expansion cost is nearly equal to the new erection of the system.
Conductor size	The conductors placed in atmosphere. So, the heat dissipation is better. Therefore the size of the conductor is small compared to the underground system.	Because of the poor heat dissipation, the size of the cables is more.
Interference with the communication line	The communication lines are run along the transmission line. In this case, it is possible to cause electromagnetic interference.	In this case, there is no chance of interference with communication lines.
Proximity effect	The distance between the conductor is very high. So, proximity effect does not affect.	As the distance between cables is very less, the proximity effect is very high.
Application	The cost of this system is low. Therefor overhead lines used in the long transmission system and in rural areas for the distribution system.	Because of the high cost, it uses in the short distance and in populated areas. Where space is a major problem for the overhead transmission line.

Standardization of Transmission System Voltage

 Standardization of Transmission System Voltage

There is much variation in transmission voltages in different countries. Each country have different voltage level as per there requirments. Earlier individual makes  an attempts to fix voltage levels for higher power transmission but such an attempt had resulted in wastage of time and higher cost because of designs of varied nature. Hence, the transmission voltages had to be standardized. The various advantages of standardization of transmission voltage are:

1.      Standardization provides better facilities for research and development.

2.      The equipments can be manufactured with greater economy and reliability.

3.      Systems are easily interconnected.

Hence standardization enables to carry out joint efforts to tackle Extra High Voltage (EHV) or Ultra High Voltage (UHV) problems. By standardizing, the voltage level can be adopted for a reasonable period of time before next change. The choice of the highest system voltage for a country is a matter of great significance. It is not merely the economic factors that influence the next higher voltage but the site of power station, location and density of load, and the technological developments are also kept in mind. The next higher voltage level should also be selected on the basis of future load enhancements. The interval between the existing and the proposed voltage level should be judiciously spaced, as too small interval between the voltages will result in a short life of the proposed voltage level. At the same time too large interval would lead to heavy expenditure. It is therefore desirable that the next voltage selected should be at least two steps higher than the existing one.  

The various AC voltages adopted by different countries above 220 kV are 275, 345, 380, 400, 500, 735, 765, 1000, 1100, 1200 kV etc. The AC transmission voltages adopted in India are 220 kV, 400 kV and 765 kV. The next higher AC transmission voltage selected is 1200 kV.

Smart Grids

     A Smart Grid is an electricity Network based on Digital Technology that is used to supply electricity to consumers via Two-Way Digital Communication. This system allows for monitoring, analysis, control and communication within the supply chain to help improve efficiency, reduce the energy consumption and cost and maximize the transparency and reliability of the energy supply chain.

    Smart grid is a large ‘System of Systems’, where each functional domain consists of three layers: 

(i) the power and energy layer, 

(ii) the communication layer, and 

(iii) the IT/computer layer.

     Layers (ii) and (iii) above are the enabling infrastructure that makes the existing power and energy infrastructure ‘smarter’.

It uses smart meters and appliances, renewable and efficient energy resources.

• The system delivers electricity via 2-way digital communication. It allows consumers to interact with the grid.
•  It reduces energy consumption and reduces cost to the consumers by smart means. Electric supply companies make efficient usage of energy and consecutively will be able to meet the varying load demands of the consumers.

Features of Smart Grid

    Smart grid has several positive features that give direct benefit to consumers:

• Real time monitoring.
• Automated outage management and faster restoration.
• Dynamic pricing mechanisms.
• Incentivize consumers to alter usage during different times of day based on pricing signals.
• Better energy management.
• In-house displays.
• Web portals and mobile apps.
• Track and manage energy usage.
• Opportunities to reduce and conserve electricity etc.

Benefits of Smart Grid Deployments

• Peak load management, improved QoS and reliability.
• Reduction in power purchase cost.
• Better asset management.
• Increased grid visibility and self-healing grids.
• Renewable integration and accessibility to electricity.
• Satisfied customers and financially sound utilities etc.

Smart Grid Architecture Components

The figure depicts generic Smart Grid Network Architecture components or modules with different reference points. As shown typical smart grid network consists of following components.

• Grid domain (Operations include bulk generation, distribution, transmission)

• Smart meters

• Consumer domain (HAN (Home Area Network) consists of smart appliances and more)

• Communication network (Connects smart meters with consumers and electricity company for energy monitoring and control operations, include various wireless technologies such as zigbee, wifi, HomePlug, cellular (GSM, GPRS, 3G, 4G-LTE) etc.

• Third party Service providers (system vendors, operators, web companies etc.)

Structure of power systems

        Electricity is generated at central power stations and then transferred to loads (i.e, Domestic, Commercial and Industrial) through the transmission and distribution system. A combination of all these systems is  known as an Electric Power System.

     A power system is a combination of central generating stations, power transmission system, Distribution and utilization system. Electric power is produced at the power stations which are located at favourable places, generally quite away from the consumers. It is then transmitted over large distances to load centres with the help of conductors known as transmission lines. Finally, it is distributed to a large number ofsmall and big consumers through a distribution network,

Energy is generated (transformed from one to another) at the generating stations. Generating stations are of different type, for example, thermal, hydro, solar power stations, nuclear. The generated electricity is stepped up through the transformer and then transferred over transmission lines to the load centres.

Electric power is generated at a voltage of 11 to 25 kV which then is stepped up to the transmission levels in the range of 66 to 400 kV (or higher). As the transmission capability of a line is proportional to the square of its voltage, research is continuously being carried out to raise transmission voltages. Some of the countries are already employing 765 kV. The voltages are expected to rise to 800 kV in the near future. In India, several 400 kV lines are already in operation. One 800 kV line has just been built. . The transmission of electric power at high voltages has several advantages including the saving of conductor material and high transmission efficiency. It may appear advisable to use the highest possible voltage for transmission of electric power to save conductor material and have other advantages. But there is a limit to which this voltage can be increased. It is because the increase in transmission voltage introduces insulation problems as well as the cost of switchgear and transformer equipment is increased. Therefore, the choice of proper transmission voltage is essentially a question of economics. Generally, the primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV.

Transmission System And Distribution System

The large network of conductors between the power station and the consumers broadly divided into two parts viz., can be transmission system and distribution system. Each part can be further subdivided into two — primary transmission and secondary transmission and primary distribution and secondary distribution.

Primary transmission.

The first stepdown of voltage from transmission level is at the bulk power substation, where the reduction is to a range of 33 to 132 kV, depending on the transmission line voltage. The electric power at 132 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. This forms the primary transmission.

Secondary transmission

The primary transmission line terminates at the receiving station (RS) which usually lies at the outskirts of the city. At the receiving station, the voltage is reduced to 33kV by step-down transformers. From this station, electric power is transmitted at 33kV by 3-phase, 3-wire overhead system to various sub-stations (SS) located at the strategic points in the city. This forms the secondary transmission.

Primary distribution

The secondary transmission line terminates at the sub-station (SS) where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important road sides of the city. This forms the primary distribution. It may be noted that big consumers (having demand more than 50 kW) are generally supplied power at 11 kV for further handling with their own sub-stations.

Secondary distribution

In the last stage in a Power System, the electric power from primary distribution line (11 kV) is delivered to distribution sub-stations (DS) or Distribution Transformer. A typical pole mounted distribution transformer is shown in Fig. 5. These sub-stations are located near the consumers’ localities and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution. The voltage between any two phases is 400 V and between any phase and neutral is 230 V. The single-phase residential lighting load is connected between any one phase and neutral.

EXHAUST FAN POWER CONSUMPTION CALCULATION

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 Let us assume that Exhaust Fan for Kitchen/Bathroom which runs 10 hours per day, 

 According to its nameplate details, the fan consumes 30 watts for one hour.

  Per day Electricity consume by the Exhaust Fan = 30 x 10 / 1000 = 0.3 kWh 

As we know that one unit is equal to kWh. 

 Per day = 0.3 units.

hence per day consumption will be, .3 unit

 Per month = .3*30=9 unit

Hence, the monthly consumption will be 9 units.

CABLE SIZE & CURRENT CARRYING CAPACITY OF CABLE

 

How to Find The Suitable Size of Cable & Wire for Electrical Wiring Installation? With Examples

•     

How to Find The Suitable Size of Cable & Wire for Electrical Wiring Installation? With Examples

Remember that it's far very crucial to choose proper wire size while sizing a wire for electrical installations. An inappropriate size of wire for heavy loads current can also create chaos which results in failure of the electrical system, hazardous fire and serious injuries.

Voltage Drop in Cables

Whenever the current flows through the conductor, there will be a voltage drops in that conductor. Normally, voltage drop may be neglected for small length of conductors but in case of a lower diameter and long length conductors we should consider a significant voltage drops for proper wiring installation and future load management.

According to Institute of Electrical and Electronics Engineers (IEEE) rule B-23, at any point between a power supply terminal and installation, voltage drop should not increase above 2.5% of provided (supply) voltage 

Example:

Let us assume , the supply voltage is 230V AC, then the value of permissible voltage drop should be;

• Permissible Voltage Drop = 230 x (2.5/100) = 5.75V

Similarly, if the supply voltage is 110V AC, the Permissible Voltage Drop  should be not more than 2.75V ( 110V x 2.5%).

In electrical wiring circuits, for sub circuits the value of voltage drop should be half of that permissible voltage drop.

Normally, the voltage drop is expressed in Ampere per meter (A / m)

Tables & Charts for Proper Cable & Wire Sizes

Below are the important tables which you should follow for determining the proper size of cable for Electrical Wiring Installation

How to calculate Voltage Drop in a Cable?

Step no. 1:         Calculate the maximum allowable  voltage drop.

Step no. 2:          Calculate the load current

Step no. 3:          After finding the load current select a proper cable from table 1

Step no. 4:          from table 1 find the voltage drop of the cable and multiply with the length of cable

Step no. 5:          Now multiply this calculated value of volt drop by load factor where;

Load factor = Load Current to be taken by Cable/ Rated Current of Cable given in the table.

                          This is the value of Volt drop in the cables when load current flows through it.

Step no. 6:          If the calculated value of voltage drop is less than the value calculated in step (1)                                   (Maximum allowable voltage drop), than the size of selected cable is proper

Step no. 7:          If the calculated value of voltage drop is greater than the value calculated in step (1)                               (Maximum allowable voltage drop), than calculate voltage drop for the next (greater in                             size) cable and so on until the calculated value of voltage drop became less than the                                 maximum allowable voltage drop calculated in step (1).

Example :

For Electrical wiring in a building, Total load is 5kW and the length of the cable from Main panel to sub circuit is 40 feet. Supply voltages is  230V and temperature is 40°C. Find the suitable size of cable which is going through conduits .

Solution:-

    Ø Total Load = 5kW

Ø                           Let us assume at max 20% overload occurs i.e. 1.2*5kW=6kW or 6000W

Now for 6000W load current will flow i.e. 6000/230= 26.08A

ØNow we have to select the size of cable from table 1 for 26.08A current which is 7/0.036 (28                      Amperes). It means we can use 7/0.036 cable according to table 1.

Ø                         Now check the selected (7/0.036) cable with temperature factor in Table 3, so the temperature                factor is 0.94 (in table 3) at 40°C  and current carrying capacity of (7/0.036) is 28A, therefore,                current carrying capacity of this cable at 40°C (104°F) would be

Current rating for 40°C  = 28 x 0.94 = 26.32 Amp.

                  Maximum current carrying capacity is 26.32 A and actual is 26.08A.Hence   this size of cable                  (7/0.036) is also suitable.

Ø                          Now find the voltage drop for 100 feet for this (7/0.036) cable is 7V, But in our case, the length of cable                 is 40 feet. Therefore, the voltage drop for 40 feet cable would be;

                Actual Voltage drop for 40 feet = (7 x 40/100) x (26.08/28) = 2.608V

                And Allowable voltage drop = (2.5 x 220)/100 = 5.5V

             Here The Actual Voltage Drop (2.608V) is less than that of maximum allowable voltage drop of 5.5V.                  Therefore, the most suitable cable size is (7/0.036) for that given load.