EHV AC Transmission

EHV AC Transmission

AU : Dec.-04, 13,14, May-15

In recent years the electrical energy is generated and consumed at a very high rate throughout the world. There are many new trends and developments which have occurred in the field of transmission of electric power which leads to use high voltages extensively. Currently large amount of power is transmitted over medium and log transmission lines at the voltage of 300 kV and above. 

As per current terminology, voltages which are less than 300 kV are termed as High voltages. The voltages which are in the range of 300 kV and 765 kV are called Extra High Voltage (EHV) whereas the voltages above 765 kV are termed as Ultra High Voltages (UHV). In India, transmission voltages range from 66 kV to 400 kV rms (line to line) in three phase bulk power transmission.

 

1. Necessity for EHV AC Transmission

1. With the increase in transmission voltage, for same amount of power to be transmitted current in the line decreases which reduces I²R losses (or copper losses). This will lead to increase in transmission efficiency.

2. With decrease in transmission current, size of conductor required reduces which decreases the volume of conductor.

3. The transmission capacity is proportional to square of operating voltages. Thus the transmission capacity of line increases with increase in voltage. The costs associated with tower, insulation, and different equipments are proportional to voltages rather than square of voltages. Thus the overall capital cost of transmission decreases as voltage increases. Hence large power can be economically transmitted with EHV or UHV.

4. With increase in level of transmission voltage, the installation cost of the transmission line per km decreases.

5. It is economical with EHV transmission to interconnect the power systems on a large scale.

6. The number of circuits and the land requirement for transmission decreases with the use of higher transmission voltages.

7. Large amounts of power over long distances is technically and economically feasible only at voltages in EHV and UHV range. Thus Economics can be achieved in power generation.

 

2. Configuration of EHV A.C. Transmission

The typical configuration of a very long EHV / UHV three phase AC transmission system is shown in the Fig. 7.24.1.

EHV AC transmission line requires minimum two parallel three phase transmission circuits to ensure reliability and stability during a fault on any one phase of the three phase lines.

Similarly EHV line also requires one or more intermediate substations for installing series capacitors, shunt reactors, switching and protection equipment. Generally an intermediate substation is required at an interval of 250 to 300 km. 

 

3. Advantages of EHV Transmission System

Electrical energy is generated at a voltage about 11 kV using alternators. This voltage is then stepped up to 132, 220 or 400 kV for transmission purpose. For transmission of electric power high voltage is preferred because of following advantages,

1) Reduction in the current

Power transmitted is given by

P = √ 3 V_LI_L cos ϕ

where V_L = Line voltage, I_L = Load line current

cos ϕ = Load power factor

Hence load current is given by, I_L = P = √ 3 V_L cos ϕ

From the above expression it can be seen that for the constant power and power factor, the load current is inversely proportional to the transmission voltage. With increase in transmission voltage, load current gets reduced. As current gets reduced, size of conductor required also reduces for transmitting same amount of power, which reduces the cost.

2) Reduction in the losses

Power loss in a line is given by,

From the above expression it can be seen that power loss in a line is inversely proportional to square of transmission voltage i.e. greater the transmission voltage lesser is the loss in the line.

3) Reduction in volume of conductor material required

We have seen that,

Volume of conductor meterial required = 3 × Area of conductor × Length of line

It can be seen that with increase in the transmission voltage, volume of conductor material reduces.

4) Decrease in voltage drop and improvement of voltage regulation.

The voltage drop in the transmission line is given by,

Voltage drop = 3 I R

With reduction in current due to increase in voltage, voltage drop in the line reduces.

Voltage Regulation = Voltage drop / Sending voltage × 100

As voltage drop decreases, regulation of the line is improved.

5) Increase in transmission efficiency

Transmission efficiency is given by,

Transmission efficiency = Output power / Input power × 100

= Input power - Power loss / Input loss × 100 = (1 - Power loss / Input power  ) × 100

We have seen that with increase in transmission line voltage power loss gets reduced. Hence the transmission efficiency increases as losses in the line are reduced.

6) Increased power handling capacity

Power transmitted over a transmission line is given by,

P = V_S . V_R / X sin δ

Thus if we assume that V_S = V_R then power transmitted is proportional to square of voltage which increases power handling capacity of the line.

7) The number of circuits and the land requirement reduces as transmission voltage increases.

8) The total line cost per MW per km decreases considerably with the increase in line voltage.

9) The operation with EHV AC voltage is simple and can be adopted easily and naturally to the synchronously operating a.c. systems.

10) The equipments used in EHV AC system are simple and reliable without need of high technology.

11) The lines can be easily tapped and extended with simple control of power flow in the network.

 

4. Disadvantages or Problems involved in EHV AC Transmission System

The major problems that can be occurred with EHV transmission system are as follows

1) Corona loss and radio interference

The corona loss is greatly influened by choice of transmission voltage. If weather conditions are not proper then this loss further increases. There is also interference in radio and TV which causes disturbance.

2) Line supports

In order to protect the transmission line during storms and cyclones and to make it wind resistant, extra amount of metal is required in the tower which may increase the cost.

3) Erection difficulties

There are lot of problems that arise during the erection of EHV lines. It requires high standard of workmanship. The supporting structures are to be efficiently transported. 

4) Insulation needs

With increase in transmission voltage, insulation required for line conductors also increases which increases its cost.

5) The cost of transformers, switchgear equipments and protective equipments increases with increase in transmission line voltage.

6) The EHV lines generates electrostatic effects which are harmful to human beings and animals.

 

5. Environmental Considerations for EHV AC Transmission

The various environmental considerations for EHV AC transmission system are,

1. Corona effect and ozone gas discharge at the time of corona. It affects the sun and hence affects the environment. So corona effect must be reduced.

2. Radio and television interference is generated due to corona which causes disturbance in wireless signals and communication lines. In bad weather conditions the corona is more and radio interference is more. The radio interference plays an important role in designing of EHV AC lines.

3. For a large voltage, a hissing sound is generated due to corona which can be easily heard and affects the environment. The humming noise from transformers and other electrical equipments also create audible noise. The care must be taken to keep such audible noise as low as possible.

4. Practically EHV AC lines run through forests, farm lands and hilly areas. Thus clearing a path for these lines is an important aspect without affecting environmental balance. The possibility of fire due to the branches of dead trees near such lines is another issue. Such trees and branches must be cut and removed.

5. EHV AC lines are responsible to produce electromagnetic and electrostatic fields which are harmful to human and animals. These fields produce adverse effects of human health such as changes in immune system, changes to the functions of cells and tissues, inducing currents on the surface of the human body, changes in the heart rate and brain activity of human etc. The efforts must be taken to reduce such fields so as to restrict their biological effects.

6. Proper protective equipments must be provided to reduce the effects of lightning, storms and other adverse atmospheric conditions on the environment.

 

6. Standard Rated Voltages of EHVAC Lines

The standard rated voltages for AC transmission are given in the Table 7.24.1.

The choice for the transmission line voltage is made by referring this table. For a new line,  the chioce of voltage is made in such a way that the nearest existing system voltage is preferred.

In EHVAC lines additional parallel three phase line is always provided to maintain continuous flow of power and stability of transmission line.

Review Questions

1. Explain why EHV AC transmission is preferred? What are the problems involved in EHV AC transmission?

AU: Dec.-04, 13, 14, Marks 8

2. Draw and explain the configuration of EHV AC transmission system.

AU: May-15, Marks 13

3. Explain the necessity of EHV AC transmission.

4. Explain the various environmental aspects for EHV AC transmission.