Modern Trends in Indian AC Transmission System

Long distance bulk power transfer:

The need to economize investments in generation reserves, sharing of benefits in utilizing variability in generation mixes and load pattern have given rise to interconnection of neighboring power systems and development of large power grids. 

Rapid increase of load (which is the case in developing countries like India), remote generation and system interconnections have made it necessary to transmit more power over longer distances efficiently and easily. Long distance bulk power transfer is possible only with Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines.  

Research and Development activities:

Priority is given to research and development activities for the optimization of cost of power transmission, efficient utilization of existing Right of Way (ROW) and increased power transfer capability. The recent technological developments and application of power semi-conductor devices, digital electronics, control equipments and satellite communication have increased the capabilities of EHV and UHV AC transmission lines.

Modern Trends in AC Transmission:

The modern trends in AC transmission are:

1.      To utilize Flexible AC Transmission Systems (FACTS) employing power electronic based and other static controllers to enhance the controllability and capability of AC transmission lines.

2.    To opt for higher transmission voltages exceeding the EHV range. Transmission voltage of 765 kV AC has been introduced in India in the year 2007. Research and development is going on for the next higher voltage of 1200 kV UHV AC transmission system. In this regard a 1200 kV test station and test line at Bina (Madhya Pradesh) is in advanced stages of research and development.

3.      To deploy satellite imagery technique (which is supposed to enhance the transmission line survey). Survey techniques are improved through GIS and Airborne Laser Terrain Mapping (ALTM) or Light Detection and Ranging (LiDAR). Laser mapping produces the detailed elevation measurements at a faster rate and accuracy. It can be used at locations where the approach is limited or restricted. With this technique, long transmission corridor can be mapped with speed to determine the exact location of towers. The same data can be used for monitoring the transmission lines, for it's repair and modification activities.  

4.     Tall and multi-circuit towers are increasingly being used to avoid deforestation, protection of wild life and effective utilization of existing ROW.

5.      Recent trend is to use multi-conductor bundled conductors having four, six, and eight sub-conductors.

6.   To use high temperature endurance conductors for increased loading and higher power transfer capability.

7.      More and more use of compact gas insulated substations of 765 kV and 1200 kV class.

8.      Use of high strength polymer insulators.        

Bundled Conductors for Extra and Ultra High Voltage transmission lines

Last updated: January 20, 2017

Transmission of large amount of power over long distances can be accomplished most economically using Extra and Ultra High Voltage lines. An increase in transmission voltage results in reduction of electrical losses, increase in transmission efficiency, improvement in voltage regulation and reduction in conductor material requirements.

Effect of Voltage Gradient on Atmospheric Air: 

In an overhead transmission line, the atmospheric air in between the conductors behaves perfectly like an insulator when the potential difference between the conductors is small. With the increase in system voltage, there is a corresponding increase in the electric field intensity. When the electric field intensity or voltage gradient reaches a critical value of 30 kV/cm, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting.

Corona phenomenon at High Voltage:

Corona phenomenon is the ionization of air surrounding the power conductors. Free electrons are normally present in the atmosphere. The free electrons will move with certain velocity depending upon the field strength.  These electrons on their movement collide with the molecules of air and liberate more electrons. The process of ionization is cumulative and ultimately forms and electron avalanche.

The electrical breakdown of surrounding air around the conductor is accompanied by-

1. a faint glow around the conductor, 
2. a hissing sound, 
3. vibration in conductors, 
4. formation of ozone and oxides of nitrogen, 
5. loss of power, and 
6. radio interference. 

Factors affecting Corona loss in an overhead Transmission line:

The important factors that affect the corona loss in an overhead transmission line are -

1. frequency and waveform of supply, 
2. spacing between conductors, 
3. condition of conductor surface, 
4. atmospheric conditions, 
5. conductor diameter and 
6. number of conductors per phase.   

Bundled Conductors

At voltages above 300 kV, corona causes a significant power loss and radio interference if a single conductor per phase is used. Instead of using a single conductor, it is preferable to use two or more conductors per phase, in close proximity, which is called bundled conductors. 

Thus, a bundled conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. The high voltage gradient is reduced considerably by the use of bundled conductors.

Number of sub-conductors:

The number of sub-conductors used per phase in an Indian transmission system are two (for 400 kV), and four (for 765 kV). The bundled conductors for the proposed 1200 kV Ultra High Voltage (UHV) transmission system in India will have 8 sub-conductors. Figure 1 shows Bundled conductors with twin, triple and quadruple conductor. Figure 2 shows a 765 kV Transmission line in India with quadruplex sub-conductors.

Fig.1: Bundled conductors with twin, triple and quadruple sub-conductors.

Fig.2: 765 kV transmission line in India with 4 sub-conductors per phase (Quadruplex bundle).

Bundle spacing:

The spacing between adjacent sub-conductors is called bundle spacing and is almost 30 cm or more. In figure 1, bundle spacing is denoted by 'B'. In almost all cases, the sub-conductors are uniformly distributed on a circle. The radius of the pitch circle on which the sub-conductors are located is called bundled radius.

The various advantages of using bundled conductors are reduced reactance, voltage gradient, corona loss, radio interference, and surge impedance.