The economic choice of transmission voltage

The economic choice of transmission voltage with the distance of transmission | economic choice of transmission voltage

We know that if transmission voltage is increased, the quantity of conductor material required is reduced. This decreases the cost of conductor material. It may look like that the higher transmission voltage leads to lower the conductor material cost. However, it may be remembered that as the transmission voltage is increased, the cost of insulation,  transformers, switchgear and other station equipments also increases. Therefore, for each transmission line, there is optimal transmission voltage, beyond which there is nothing to be profited in the matter of economy.

The  voltage for which the cost of conductors,  insulators, transformers, switchgear and other station equipment is minimum is called economical transmission voltage.

The method of finding economical transmission voltage is as follows:-

• Power to be transmitted 
• Generation voltage
• And lengths of transmission line are assumed to be known.

(i) Transformers, used at the generating and receiving ends of transmission line. For a given power, the increase in transmission voltage its cost also increases slowly. 

(ii) Switchgear, This cost also increases with increase in transmission voltage.

(iii) Lightning arrestor, its cost increases quickly with the increase in transmission voltage.

(iv) Insulation and supports, its cost increases sharply with the increase in transmission voltage 

(v) Conductor, This cost decreases with increase in transmission voltage.

The total of all above costs gives the overall cost of transmission for the voltage considered. Similarly, the calculations are made for the transmission voltage.

A curve for total cost of transmission against voltage is shown below:

Capital cost against Transmission voltage

Point (P) :- is the economical transmission voltage

OA is the: - optimum transmission voltage

The Above method for finding the economical transmission voltage is old, and it not includes all the other related costs.

Presently there is a new method used for the calculation of economical transmission voltage that is an empirical formula:

The economic voltage between lines in a 3-phase a.c. system is (Economical voltage formula): -

`V=5.5\sqrt{0.62l+\frac{3P}{150}}` 

V = line voltage in kV

P = maximum kW per phase to be delivered to single circuit

l = distance of transmission line in km

From the above formula we can conclude that:

• If the distance of transmission line is increased, the cost of terminal equipment is decreased, resulting in higher economic transmission voltage.
• Also, if power to be transmitted is large, large generating and transforming units can be employed. This reduces the cost per kW of the station equipment.

Factors affecting choice of transmission voltage level:-

• Type and size of the load to be served.
• Distance to which power is to be transmitted.
• Future Load Growth.
• Equipment availability for fault isolation.
• Permissible voltage regulations.
• Cost of utilization and service system equipment.

Effect of system voltage on transmission power :

i. Power loss in the line is inversely proportional to the system voltage and power factor both.

ii. Percentage voltage drop in resistance decreases with the increase in the system voltage.

iii. Weight of the conductor material for the line will decrease with the increase in supply voltage and power factor.

iv. Efficiency of transmission, increases with the increase of supply voltage and power factor

v. Higher supply voltage also enhances system stability.

vi. The problem encountered with high voltages are the insulation of the equipment, corona, radio and television interference.

vii. The voltage level of a system is therefore governed by the amount of power to be transmitted and the length of the line.

Voltage Level:

• Low voltage :- 

1.  230 V (1-ϕ)
2.  400 V (3-ϕ)             

• High voltage:-

1.  11 kV
2.  33 kV

• Extra high voltage : 

1.  66 kV
2.  132 kV
3.  220 kV

• Modern EHV: 400 kV
• Ultra-high voltage:- 765 kV and above

What are the requirements of satisfactory electric power supply ?

There are a number of factors to be considered for satisfactory supply, few of them are given below:-

1. Voltage regulation
2. Dependability
3. Balanced voltage
4. Efficiency
5. Frequency
6. Sinusoidal waveform
7. Freedom from inductive interference

1. Voltage regulation:-

• It has a drastic effect on operations of both power system and loads. For balancing voltage regulation we generally use flexible ac transmission(FACTS) devices.

2. Dependability:- 

• The most important points of electric supply is to provide uninterruptible power supply to consumers.

3. Balanced voltage:- 

• This factor is also affect the working of industrial or any consumer equipments failure. Therefore, it is very important to balance system voltage using various methods.

4. Efficiency:-

•  The efficiency of overall operating power system should be better. For that the consumer who having low power factor equipments are either penalized or asked to change equipment with better power factor.

5. Frequency:- 

• This factor is also very important in power system. Change in system frequency can alter the consumers equipments speed and losses. It must be maintained constant.

6. Sinusoidal waveform:- 

• The power supplied to consumers must be in the form of sinusoidal wave so that there is no harmonics present in it. Harmonics affects the losses of equipments. Sometimes it leads to  equipment failure.

7. Freedom from inductive interference:- 

• Power lines running parallel to communication lines

produce electrostatic and electromagnetic field disturbances. Which leads to producing noise in communication apparatus.