WHAT IS HVDC ?• In a HVDC (High voltage D.C.) transmission

system, electricity is taken from an AC power network, converted to DC in a converter station and transmitted to the receiving point by a transmission line (overhead) or cable(underground).

• It is then converted back to AC in another converter station and injected into the receiving AC network. HVDC enables the power flow to be controlled rapidly and accurately, and improves the performance, efficiency and economy of the connected AC networks.

REASONS FOR AC GENERATION AND REASONS FOR AC GENERATION AND TRANSMISSIONTRANSMISSION

• Due to ease of transformation of voltage levels (simple transformer action) and rugged squirrel cage motors, ALTERNATING CURRENT is universally utilized.—Both for GENERATION and LOADS and hence for TRANSMISSION

• Generators are at remote places, away from the populated areas i.e. the load centers

• Voltage is boosted up to 220 or 400 KV by step-up transformers for transmission to LOADS.

• To reach the loads at homes/industry at required safe levels, transformers step down voltage.

COMPARISION OF HVAC & HVDC SYSTEMS

CONVENTIONALLY POWER TRANSMISSION IS EFFECTED

THROUGH HVAC SYSTEMS ALL OVER THE WORLD.

HVAC TRANSMISSION IS HAVING SEVER LIMITATIONS LIKE

LINE LENGTH , UNCONTROLLED POWER FLOW, OVER/LOW

VOLTAGES DURING LIGHTLY / OVER LOADED

CONDITIONS,STABILITY PROBLEMS,FAULT ISOLATION ETC

CONSIDERING THE DISADVANTAGES OF HVAC SYSTEM

AND THE ADVANTAGES OF HVDC TRANSMISSION ,

POWERGRID HAS CHOOSEN HVDC TRANSMISSION.

AC

DC

HVDC BIPOLAR LINKS IN INDIAHVDC BIPOLAR LINKS IN INDIA

NER

ER

SR

NR

NER

ER

SR

NR

RIHAND-DELHI -- 2*750 MW

CHANDRAPUR-PADGE – 2* 750 MW

TALCHER-KOLAR – 2*1000 MWER TO SR

SILERU-BARASORE - 100 MW EXPERIMENTAL PROJECT ER –SR

HVDC IN INDIA• In India presently 6 HVDC systems are in operation.• India- Pioneer developer of HVDC since 1990 with the first HVDC set

up of 1000MW 814Km Rihand-Dadri line between U.P & Rajasthan.• 2000MW 1440Km Talcher-Kolar link is biggest so far connecting four

states-orissa,andhra pradesh,tamilnadu and karnataka.• In march 2000,Power-grid(largest power transmission utility of India)

awarded a new long distance HVDC transmission project to SIEMENS,Germany.

• More greater plans are cooking to add 1lakh MW of power by 2012 in which HVDC will play vital role to transmit these powers to all corners of India.

HVDC IN INDIABipolar

HVDC LINK CONNECTING REGION

CAPACITY (MW)

LINE LENGTH

Rihand – Dadri

North-North 1500 815

Chandrapur - Padghe

West - West 1500 752

Talcher – Kolar

East – South 2500 1367

HVDC IN INDIABack-to-Back

HVDC LINK CONNECTING REGION

CAPACITY (MW)

Vindyachal North – West 2 x 250

Chandrapur West – South 2 x 500

Vizag – I East – South 500

Sasaram East – North 500

Vizag – II East – South 500

COMPARISON OF AC AND DC TRANSMISSION

The relative merits of the two modes of transmission (AC and DC) which need to be considered by a system planner are based on the following factors

1. economics of transmission2. technical performance3. reliability

Economics of Power TransmissionThe cost of a transmission line includes the investment and operational costs. The investment includes costs of Right of Way (RoW), transmission towers, conductors, insulators and terminal equipment. The operational costs include mainly the cost of lossesFor lines designed with the same insulation level, a DC line can carry as much power with two conductors (with positive and negative polarities with respect to ground) as an AC line with 3 conductors of the same size.

DC line requires less RoW, simpler and cheaper towers and reduced conductor and insulator costs. The power losses are also reduced with DC as there are only two conductors (about 67% of that for AC with same current carrying capacity of conductors)

The absence of skin effect with DC is also beneficial inreducing power losses marginallyThe dielectric losses in case of power cables is also very less for DC transmission.

The corona effects on DC conductors tend to be less significant than for AC and this also leads to the choice of economic size of conductors with DC transmission

DC lines do not require compensation but the terminal equipment costs are increased due to the presence of converters and filters.

Cost of HVDC and HVAC Links with 5 GW Capacity

Cost of 5 GW Overhead Link

0

500

1000

1500

20002500

3000

3500

4000

4500

0 1000 2000 3000 4000

Distance [km]In

vest

men

t [M

€]

1150 kV HVAC ± 800 kV HVDC

Figure 1.1 shows the variation of costs of transmission with distance for AC and DC transmission. For distances less than ‘break even’ distance, AC tends to be economical than DC and costlier for longer distances. The break even distances can vary from 500 to 800 km in overhead lines depending on the per unit line costs.