HVDC Transmission

High Voltage Direct Current (HVDC) transmission is a technology used to transmit large amounts of electrical power over long distances using direct current at very high voltage levels, typically ranging from 100 kV to 800 kV or higher. While the power grid predominantly uses AC, there are specific scenarios where DC transmission is technically and economically superior. HVDC systems are critical in modern power systems for interconnecting asynchronous grids, long-distance bulk power transfer, and offshore wind farm integration.

Core Concept of HVDC Transmission

The fundamental principle of HVDC is to convert AC power at the sending end into DC using a rectifier converter station, transmit this DC power at high voltage over the transmission line, and then invert it back to AC at the receiving end using an inverter converter station. The voltage is stepped up by a transformer before conversion and stepped down after inversion. Both converter stations are identical in hardware but operate in opposite modes.

The motivation for using DC instead of AC for long-distance transmission comes from the physics of transmission lines. An AC transmission line behaves like a distributed capacitance and inductance, which causes significant reactive power exchange along the line. For very long lines (above approximately 600 km for overhead lines or 50 km for underground cables), the charging current of the line capacitance becomes comparable to the thermal rating of the conductor. This means a large portion of the conductor capacity is used just to supply the reactive charging current and not for active power transfer. DC has no frequency, so there is no capacitive charging current, no skin effect, and no reactive power associated with the line. The full conductor capacity is available for active power transmission.

Another major advantage of HVDC is the ability to connect two AC systems that operate at different frequencies or that are not synchronized in phase. For example, the 50 Hz Indian grid and a 60 Hz Japanese grid cannot be directly interconnected with AC. An HVDC back-to-back station converts AC to DC and immediately back to AC, allowing two asynchronous grids to exchange power. This is also useful for interconnecting regional grids within India that may have different instantaneous frequencies.

Types of HVDC Systems

Line Commutated Converter (LCC-HVDC) uses thyristor-based converters and is the classical HVDC technology. Thyristors are switched on by gate pulses but cannot be turned off by the gate, so they rely on the natural zero crossing of the AC current for commutation. LCC systems require a strong AC network at both ends for commutation support. They absorb reactive power and require large AC filters and reactive compensation banks. Voltage Source Converter (VSC-HVDC) uses IGBT-based converters and can control both active and reactive power independently. VSC systems can operate with weak or even isolated AC systems, making them ideal for offshore wind farms and remote loads.

Mathematical Expression

For an LCC-HVDC system, the average DC output voltage of a 6-pulse thyristor bridge rectifier depends on the AC line voltage and the firing angle alpha of the thyristors. The firing angle is the key control variable in LCC-HVDC. The average DC voltage at the rectifier is Vd = Vd0 * cos(alpha) - (3/pi) * Xc * Id, where Vd0 = (3*sqrt(2)/pi) * VL is the ideal no-load DC voltage, VL is the AC line-to-line voltage, Xc is the commutation reactance, and Id is the DC current. The term (3/pi)*Xc*Id represents the voltage drop due to commutation overlap.

Given:
6-pulse thyristor bridge rectifier for HVDC
AC line voltage VL = 400 kV, firing angle alpha = 15 degrees
Commutation reactance Xc = 10 ohm, DC current Id = 2000 A

Why this formula applies:
LCC-HVDC rectifier output voltage follows 6-pulse bridge equation.

Formula:
Vd0 = (3 * sqrt(2) / pi) * VL
Vd = Vd0 * cos(alpha) - (3/pi) * Xc * Id

Substitution:
Vd0 = (3 * 1.4142 / 3.1416) * 400000 = 1.3505 * 400000 = 540,200 V
Vd = 540200 * cos(15) - (3/3.1416) * 10 * 2000
Vd = 540200 * 0.9659 - 0.9549 * 20000
Vd = 521,883 - 19,099

Final Answer:
Vd = 502,784 V = approximately 502.8 kV DC output voltage

Exam Tip: For GATE, the key HVDC formula is Vd = Vd0*cos(alpha) - (3/pi)*Xc*Id. The breakeven distance beyond which HVDC becomes economical over HVAC is approximately 600 km for overhead lines and 50 km for underground cables. Beyond this distance, saved transmission losses and conductor cost outweigh converter station costs.

Mechanism Explained

• The rectifier station converts 3-phase AC to DC using a 6-pulse or 12-pulse thyristor bridge. The firing angle alpha of the thyristors controls the output DC voltage.
• Power flow direction in LCC-HVDC is controlled by reversing the polarity of DC voltage (not current direction). The current always flows in the same direction.
• A smoothing reactor (large inductor) is connected in series on the DC side to reduce ripple in the DC current and to limit fault current rise rates.
• AC harmonic filters are installed at both converter stations to suppress the characteristic harmonics generated by the converters, which could distort the grid voltage.
• VSC-HVDC using IGBTs can independently control active power and reactive power at each converter station, making them far more flexible for modern grid applications.
• Bipolar configuration uses two conductors at plus and minus high voltage with a metallic or ground return, providing redundancy as each pole can operate independently.

Quick Revision

• HVDC principle: AC to DC conversion at sending end (rectifier), DC transmission, DC to AC inversion at receiving end (inverter).
• LCC-HVDC uses thyristors, controlled by firing angle alpha. VSC-HVDC uses IGBTs controlled by PWM.
• Key formula: Vd = Vd0*cos(alpha) - (3/pi)*Xc*Id, where Vd0 = (3*sqrt(2)/pi)*VL.
• Advantages of HVDC: no reactive charging current, no skin effect, asynchronous grid connection, lower line losses at very high power.
• Breakeven distance: overhead lines approximately 600 km, underground or submarine cables approximately 50 km.
• GATE trap: In LCC-HVDC, power flow reversal is achieved by reversing voltage polarity, NOT current direction. Current always flows rectifier to inverter.
• VSC-HVDC can provide reactive power support and operate with weak AC grids. LCC-HVDC requires strong AC grid commutation support.