JNTU B.Tech IV Year HVDC TRANSMISSION Previous Question Paper (February/March - 2022 - R18)

The advantages of DC transmission over AC include: 1. No skin effect, allowing full utilization of the conductor cross-section. 2. Reduced cost of transmission lines as only two conductors are required compared to three for AC. 3. No reactive power compensation is required for the line itself, as there is no charging current. 4. Ability to connect asynchronous grids and control power flow precisely. 5. Lower corona loss and radio interference for a given voltage level.

The disadvantages include: 1. High cost of converter stations at both ends. 2. Difficulty in circuit breaking due to the absence of natural current zeros. 3. Generation of harmonics, requiring expensive filtering equipment. 4. Limited overload capacity compared to AC systems.

There are three primary types of HVDC systems:

• Monopolar link: Uses one conductor (usually negative polarity) with ground or sea return. It is economical for long-distance subsea cables but causes issues with ground currents and electrolysis.
• Bipolar link: Uses two conductors, one positive and one negative. Each terminal has two sets of converters in series. If one pole fails, the other can continue to operate at half capacity using ground return, making it highly reliable.
• Homopolar link: Uses two conductors, both having the same polarity (usually negative), with ground return. It is rarely used due to the disadvantages associated with continuous ground current.

Merits: Bipolar systems offer the highest reliability and flexibility for power transmission. Monopolar systems provide significant cost savings for low-power or subsea applications. Demerits: The primary drawback for all systems is the complexity and cost of the converter stations, which require sophisticated control and protection schemes to manage harmonic distortion and commutation failure.