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Wednesday, 15 January 2020

Different Types of HVDC Links- Monopolar HVDC System,Bipolar HVDC System, Homopolar HVDC System.


High Voltage DC Transmission System

Necessity

1) As transmission voltage increases current decreases therefore cross section of conductor reduces.
2) As cross section of conductor decreases its weight also decreases.
3) If weight of conductor decreases the design of tower becomes lighter in weight.
4) Transmission cost per kilometer decreases.
5) As voltage increases current reduces, therefore transmission line losses also reduces.
6) Transmission efficiency increases because of reduction in line losses.
7) Voltage regulation increases because of reduction in voltage drop.
8) Increases transmission capacity of line.
9) Increases surge impedance loading.
10) Possibility of interconnection of power system.
11) Flexibility of future load system increases.
12) Huge amount of power transmitted for long distance. 


High Voltage DC transmission system

  • The generating voltage is in the form of AC that is in 11KV this voltage is stepped up with the help of step up transformer. 
  • The increased voltage is given to converter station. The converter station consists of SCR (silicon controlled rectifier) and filter and other equipment.
  • The converter converts AC in DC power with the help of of SCR. 
  • Then DC power is transmitted through line known as HVDC (High Voltage DC) line after transmission of DC power it is again converted into AC power at receiving end with the help of inverter. Then the  voltage is decreases by step down transformer and ac power utilize in load station.

Classification of AC DC system

1. Monopolar HVDC system

2. Bipolar HVDC system

3. Homopolar HVDC system

1. Monopolar HVDC system

Monopolar HVDC System
Monopolar HVDC System


  • This system has only one pole is used and return path is provided through permanent earth or sea. Generally monopolar HVDC line is operated at negative polarity for transmission with respect to earth by using negative polarity for transmission we can reduce radio interference. 
  • This system is used for low power transmission up to 250 MV but it is economical because only one conductor is used for transmission so losses are reduces and cost is save.
  • The earth electrode are design for continuous flow of leakage current or fault current.

2. Bipolar HVDC system

Bipolar HVDC System
Bipolar HVDC System

  • This system is most widely used for overhead long distance HVDC transmission it is also called as back to back hd dc system. 
  • In this link two conductors are used it is operating with negative polarity and other is with positive polarity. 
  • There are two conductor which having equal  rating and connected in series the junction between converter may be grounded at one end or both the ends when it is grounded at both end each for operate independently.
  • The rated voltage of bipolar link is expressed as 500KV.

3. Homopolar HVDC system

  • Homopolar has to conductor used with having polarity normally positive polarity is used for transmission and negative polarity is always operated at grounded return conductor 
  • In case of faulty condition power can we transfer by using another set of conductor such a scheme is complicated therefore mostly bipolar HVDC link preferred.




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