High Voltage Direct Current Transmission

High Voltage Direct Current Transmission

In early days the transmission, distribution and utilization of electrical energy was dominated by a.c. After the introduction of large, high powered mercury are rectifiers, d.c. is also considered for transmission of electrical energy economically.

1. Principle of HVDC Transmission System Operation 

A typical HVDC transmission system is shown in the Fig. 7.25.1. At sending end there is one rectifier unit whereas one inverter unit at the receiving end. The two ends are interconnected by a dc transmission line. The ac produced by generating stations after stepping up is converted to dc by rectifier whereas the inverter converts dc to ac.

The converter makes use of thyristor for controlled operation. Thus by varying the firing angle of the thyristor, the d.c. output voltage magnitude is controlled. The firing angle is between 0° and 90° in rectifier while in inverter the firing angle is between 90° and 180°. The converter and inverter station in HVDC uses three phase controlled bridge converters.

From the Fig. 7.25.1 current I_d is given by,

I_d = V_R - V_i / R

where          V_R = D.C. output voltage at rectifier side

V_i = D.C. input voltage at inverter side

The power transfer is given by,

P_d = I_d . V_i = (V_R – V_i / R) V_i

2. Advantages of HVDC Transmission

1. These systems are economical for bulk transmission of power for long distances as the cost of conductor reduces since d.c. system requires only two conductors or even one if ground is used as return. Similarly the cost of supporting towers and insulation is also reduced. Also the transmission losses are reduced.

2. There are no stability problems with d.c. system. Hence asynchronous operation of transmission link is possible.

3. The line length is not limitation as there is no charging current in d.c. systems. Cables in d.c. system does not suffer from high dielectric loss. The skin effect is also low in d.c. system.

4. Greater power transmission per conductor is possible with d.c. system.

5. There are no serious problems of voltage regulation as there is no reactance drop that exists in d.c. at steady state.

6. The corona loss is low in d.c. systems. The radio interference with HVDC is less.

7. The losses are less in transmission with d.c. 

8. The fault level increases with interconnections of ac grids through ac lines whereas interconnection of a.c. grids through d.c. links does not increase fault level to that extent.

9. With HVDC link there is easy reversibility and controllability of power flow.

10. Shunt compensation is not required in d.c. lines.

11. Intermediate substations are not required with HVDC transmission.

12. During fault with HVDC system, the grid control of the converter reduces the fault current significantly.

13. The transient stability of the power system can be improved by making parallel connection of HVAC and HVDC lines.

3. Disadvantages of HVDC Transmission

1. The power transmission with HVDC is not economical if length of transmission is less than 500 km as HVDC system additionally requires converters, inverters and filters .

2. With multiterminal d.c. the circuit breaking is difficult and expensive.

3. Considerable reactive power is required by converter stations .

4. Harmonics are generated with d.c. system hence Alteration is necessary

5. Overload capacity of HVDC converters is low.

6. There should be local supply of reactive power if required as HVDC will not transmit reactive power.

7. The maintenance of insulators in HVDC system is more.

8. There are additional losses in converter transformers and valves. These losses are continuos. Hence cooling system must be effective to dissipate the heat.

4. Types of HVDC Systems (HVDC Links)

Depending on the arrangement of pole and earth return, HVDC systems are classified in different types. The pole is nothing but the path of direct current which has same polarity with respect to earth.

Following are the different types of HVDC systems.

1. Monopolar HVDC transmission system

Monopolar HVDC transmission system is represented in the Fig. 7.25.2. This system has only one pole and the return path is provided by permanent earth or sea. The pole generally has negative polarity with respect to earth. 

Full power and current is transmitted through a line conductor with earth or sea as a return conductor. The earth electrodes are designed for continuous full current operation. The sea or ground return is permanent and of continuous rating.

2. Bipolar HVDC transmission system

This system has two poles, one positive and one negative pole with respect to earth. During fault on one pole the bipolar system is changed to monopolar mode. The system is represented in the Fig. 7.25.3.

This system is more commonly used for transmission of power over long distance. The mid points of convertors at each terminal are earthed through electrode line and earth electrode. Power rating of one pole is about half of bipoler power rating. The earth carries only small out of balance current during normal operation.

The normal bipolar HVDC system consists of two separate monopolar systems with a common earth. The two poles can operate independently. Normally they are operated with equal currents and hence ground carries no current. 

3. Homopolar HVDC system

This system consists of two poles of same polarity and the return is through permanent earth. It is shown in the Fig. 7.25.4.

4. Back to back HVDC coupling system

In this system there is no dc transmission line but the rectification and inversion is done in the same substation. It is shown in the Fig. 7.25.5.

5. Multiterminal HVDC system

It has three or more terminal substations. It is shown in the Fig. 7.25.6.

5. Standard Rated Voltages for HVDC System

The bipolar HVDC line has two conductors, one of positive polarity with respect to earth and other has negative polarity . The voltage between the poles is twice that of the pole to earth voltage. Hence bipolar HVDC system is given as ± 500 kV. The standard rated voltages are given in the Table 7.25.1. 

The following HVDC systems are present and are operating in India.

1. Vindyachal 500 MW

2. Chandrapur 2 × 500 MW

3. Visakhapatnam 500 MW

4. Sasaram 500 MW

Review Questions

1. With a neat schematic, explain the principle of HVDC system operation.

AU: Dec.-04, Marls 8

2. What are the advantages and disadvantages of HVDC system?

AU: Dec.-06, 08, 09, 12, 13, 17, May-10, 13, 15, 18, Marks 5

3. Explain the different types of HVDC links in detail.

AU: Dec.-06, 11, 12, 16, 17, May-08, 10, 16, Marks 9