Q4Power System - I

Why Line Commutated Converters (LCC) are Used in HVDC

Line Commutated Converters use thyristor (SCR) valves, which are robust, capable of handling very high voltage and current ratings (essential for bulk HVDC transmission at hundreds of kV and thousands of amperes), and relatively low cost and highly reliable compared to fully self-commutating semiconductor devices of equivalent power rating. Thyristors, once triggered, conduct until the current naturally falls to zero and a reverse voltage appears across them — this natural ('line') commutation is readily provided by the AC system voltage itself at each converter terminal, eliminating the need for complex forced-commutation circuitry. LCC-HVDC technology has been in commercial use since the 1950s-60s, is extremely well proven for very high-power, very long-distance (or submarine) bulk transmission schemes, and remains the most cost-effective choice for the highest power ratings (several GW) currently in service, even though newer Voltage Source Converter (VSC) technology offers other operational advantages for many applications.

Comparison of AC and DC Transmission Systems

• Losses: AC lines suffer from skin effect, corona and dielectric losses in addition to I²R loss, and reactive power flow adds to line current; DC lines have no skin effect (full conductor cross-section carries current) and no reactive power flow, giving lower losses for the same power transfer over long distances.
• Stability: AC transmission over long distances is limited by the steady-state and transient stability limit (related to the power-angle relationship and line reactance); DC transmission has no such angle-stability limit, since converter control directly sets the desired power flow independent of angle.
• Reactive power/charging current: long AC lines/cables draw substantial charging current due to line capacitance, requiring reactive compensation, and this effect is especially severe for submarine AC cables beyond about 50-80 km; DC lines have no charging current issue at all, since capacitive charging current only exists under AC excitation.
• Cost: DC requires expensive converter stations at each terminal (rectifier/inverter, transformers, filters) but has lower line/cable cost (fewer conductors, narrower right-of-way); AC has no expensive terminal equipment (only conventional substations) but higher line/tower cost for equivalent power. Beyond a certain 'break-even distance' (typically 500-800 km for overhead lines, much shorter for submarine cables), the lower line cost of DC outweighs the extra converter station cost, making DC economical.
• Interconnection flexibility: AC allows easy multi-point tapping along a line and interconnection with other AC networks operating at the same frequency and synchronism; DC (especially two-terminal HVDC) does not allow easy intermediate tapping, but critically enables interconnection between grids that are NOT in synchronism (different frequency, or same frequency but not phase-locked), which AC cannot do directly.
• Fault current and protection: AC circuit breakers exploit the natural current zero crossing for interruption, a well-established, mature technology; DC circuit breaking is inherently more difficult (no natural current zero) and DC circuit breakers are more complex and costly, historically a limiting factor for multi-terminal DC grids (though this has improved with modern hybrid DC breaker technology).

In summary, AC transmission remains the default choice for most transmission distances and for meshed, multiply-interconnected networks requiring intermediate tapping, while HVDC (using LCC or increasingly VSC technology) is preferred for very long-distance bulk point-to-point transmission, submarine cable crossings, and asynchronous interconnections between grids — the choice is fundamentally an economic and technical trade-off dependent on transmission distance, power level, and the specific interconnection requirement.

Detailed Operating Principle of the LCC Bridge

An LCC-HVDC converter is built from a 6-pulse (or, more commonly in practice, a 12-pulse configuration formed by series-connecting two 6-pulse bridges fed from star-star and star-delta transformer secondaries phase-shifted by 30°) thyristor bridge. Each thyristor valve is turned on by a firing pulse from the control system at a chosen firing angle α (measured from the natural commutation point), and conducts current until the next valve in sequence is fired, at which point the incoming AC line voltage naturally forces the outgoing valve's current to fall to zero and reverse-bias it, completing commutation without any need to actively force the outgoing device off. The average DC output voltage of the bridge is controlled directly by α:

where Vdo is the ideal (α=0) no-load DC voltage, Xc is the commutating reactance per phase (dominated by the converter transformer's leakage reactance), and Id is the DC current. For α less than 90°, the bridge operates as a rectifier (net power flow from AC to DC); for α greater than 90° (up to a practical maximum limited by the need to maintain adequate commutation margin), the bridge operates as an inverter (net power flow from DC to AC) — this is a key operational advantage of LCC bridges, since reversing the direction of power flow requires only a change in firing angle (and consequent DC voltage polarity reversal, since DC current direction cannot reverse in a conventional thyristor valve), without needing to physically reconfigure the converter.

The 12-pulse configuration is universally used in modern LCC-HVDC schemes because it cancels the 5th and 7th harmonics that a simple 6-pulse bridge would otherwise inject into the AC system (leaving only the 11th, 13th, 23rd, 25th, etc. as the lowest-order characteristic harmonics), substantially reducing the size and cost of the AC harmonic filters required at each converter terminal compared to what a 6-pulse-only scheme would need.

Reactive Power Consumption of LCC Converters

A fundamental characteristic of the LCC bridge, closely related to its need for natural (line) commutation, is that it always draws lagging reactive power from the AC system regardless of whether the converter is operating as a rectifier or an inverter — this arises because the firing angle α must always be delayed from the natural zero-crossing point (α>0 for rectification, and the extinction angle margin must be maintained for successful inversion), causing the fundamental AC-side current to lag the AC-side voltage. Typically, an LCC converter consumes reactive power amounting to roughly 50-60% of the real (DC) power being transferred, necessitating substantial local reactive power compensation (switched capacitor banks and/or AC harmonic filters that also supply reactive power) at each converter station — a major cost and space driver for LCC-HVDC substations that is largely eliminated in VSC-based schemes, since a VSC can independently synthesize its AC-side voltage phase and magnitude without needing a firing-angle delay for commutation purposes.

Extended Comparison: Environmental and Right-of-Way Considerations

Beyond the technical loss, stability and cost factors already discussed, AC and DC transmission also differ in environmental and land-use impact: for the same power transfer capacity, an HVDC bipolar line typically requires a narrower right-of-way and shorter, simpler towers than an equivalent-capacity double-circuit AC line, since DC transmission needs only two (or, with a metallic/ground return, sometimes effectively one-and-a-half) conductors compared to the six conductors (two 3-phase circuits) an equivalent-capacity AC scheme might require; this reduced footprint is often decisive in densely populated regions or environmentally sensitive corridors where acquiring a wide AC transmission right-of-way is difficult or prohibitively expensive, further reinforcing the specific niches (very long distance, submarine, urban in-feed, and asynchronous interconnection) in which DC transmission technology, whether via LCC or the more flexible modern VSC bridges, offers a genuine advantage over conventional 3-phase AC transmission.