Difference Between HVAC and HVDC
Electricity generated in power plants is transmitted over long distances to electrical substations, which then distribute it to consumers. The voltage used for long-distance power transmission is extremely high, and we will explore the reasons for this high voltage later. Additionally, the transmitted power can be in either alternating current (AC) or direct current (DC) form. Therefore, power can be transmitted using either HVAC (High Voltage Alternating Current) or HVDC (High Voltage Direct Current).
Why is High Voltage Necessary for Transmission?
Voltage plays a crucial role in reducing line losses, also known as transmission losses. Every electrical conductor used for power transmission has a certain amount of ohmic resistance (R). When current (I) flows through these conductors, they generate thermal energy, which is essentially wasted energy or power (P).
According to Ohm’s Law
As evident, the energy wasted in a conductor during transmission depends on the current rather than the voltage. However, we can adjust the current magnitude through voltage conversion using specialized equipment.
During voltage conversion, power remains conserved and unchanged. The voltage and current simply vary inversely by the same factor, following the principle:
For example, 11KW power at the voltage of 220v has 50 Amps in it. In such a case, the transmission line losses will be
Let’s increase the voltage by a factor of 10. So the same power of 11KW would have a voltage of 2200v & 5 Amps. Now the line losses would be;
As you can see, increasing the voltage reduces the power losses significantly in transmissions lines.So in order to decrease the current in the transmission cables while maintaining the same amount of power transmission, we increase the voltage.
The War of the Currents (AC vs. DC)
In the late 1880s, during the so-called "War of the Currents," direct current (DC) was the first to be deployed for power transmission. However, it was deemed highly inefficient due to the lack of practical voltage conversion equipment—unlike alternating current (AC), which could be easily stepped up or down using transformers. Early low-voltage DC power stations could only supply electricity within a radius of a few miles; beyond that, voltage dropped drastically, requiring multiple generating stations in small areas—a costly approach.
While high-voltage DC transmission inherently incurs lower losses than AC, early DC systems relied on mercury arc valves (rectifiers) to convert high-voltage AC to DC for long-distance transmission. These terminal devices were bulky, expensive, and required frequent maintenance. In contrast, AC transmission depended on transformers—more efficient, affordable, and reliable—making AC the dominant choice for long-distance power transmission at the time.
When selecting between high-voltage AC (HVAC) and high-voltage DC (HVDC) for transmission, several critical factors must be considered. This article explores these factors in detail.
HVAC & HVDC
HVAC (High Voltage Alternating Current) and HVDC (High Voltage Direct Current) refer to voltage ranges used for long-distance power transmission. HVDC is typically preferred for ultra-long distances (usually over 600 km), though both systems are widely used worldwide today, each with its own advantages and drawbacks.
Transmission Costs
Long-distance power transmission requires high voltages, with power transferred between terminal stations that handle voltage conversion. Total transmission costs thus depend on two components: terminal station costs and transmission line costs.
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Terminal Stations
Terminal stations convert voltage levels for transmission. For AC systems, this is primarily done using transformers, which switch between high and low voltages. For DC systems, terminal stations use thyristor or IGBT-based converters to adjust DC voltage levels.
Since transformers are more reliable and cheaper than solid-state converters, AC terminal stations are less costly than their DC counterparts, making AC voltage conversion more economical.
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Transmission Lines
Line costs depend on the number of conductors and the design of transmission towers. HVDC systems require only two conductors, while HVAC systems need three or more (including bundled conductors to mitigate corona effects).
AC transmission towers must support heavier mechanical loads, requiring stronger, taller, and wider structures compared to HVDC towers. Line costs increase with distance, and per 100 km, HVAC lines are significantly more expensive than HVDC lines.
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Overall Transmission Costs
Total costs are determined by terminal costs (fixed, independent of distance) and line costs (variable, increasing with distance). Thus, the overall cost of a transmission system rises as distance increases.
Break-Even Distance
The "break-even distance" refers to the transmission length beyond which the total investment cost of HVAC exceeds that of HVDC. This distance is approximately 400–500 miles (600–800 km). For distances beyond this threshold, HVDC is the more cost-effective choice; for shorter distances, HVAC is more economical. This relationship is visually illustrated in the graph above.
Flexibility
HVDC is typically used for point-to-point long-distance transmission, as tapping power at intermediate points would require expensive converters to step down high DC voltages. In contrast, HVAC offers greater flexibility: multiple terminal stations can utilize low-cost transformers to step down high voltages, enabling power extraction at various points along the line.
Power Losses
HVAC transmission incurs several types of losses, including corona losses, skin effect losses, radiation losses, and induction losses, which are largely absent or minimized in HVDC systems:
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Corona Losses: When voltage exceeds a critical threshold, air surrounding conductors ionizes, creating sparks (corona discharge) that waste energy. These losses are frequency-dependent—since DC has zero frequency, HVAC corona losses are roughly three times higher than those in HVDC.
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Skin Effect Losses: In AC transmission, current density is highest at the conductor surface and lowest at the core (the "skin effect"), reducing the effective cross-sectional area used for current flow. This increases conductor resistance and amplifies I²R losses. DC current, by contrast, distributes uniformly across the conductor, eliminating this effect.
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Radiation and Induction Losses: HVAC’s alternating magnetic field causes long transmission lines to act as antennas (radiating irrecoverable energy) and induces currents in nearby conductors (induction losses). HVDC’s steady magnetic field avoids both issues.
The Skin Effect
The skin effect, directly proportional to frequency, forces most AC current to flow near the conductor surface, leaving the core underutilized. This reduces conductor efficiency: to carry larger currents, HVAC systems require conductors with increased cross-sectional area, driving up material costs. HVDC, unaffected by the skin effect, uses conductors more efficiently.
Thus, to carry the same current, HVAC requires conductors with a larger diameter, whereas HVDC can achieve this with smaller-diameter conductors.
Cable Current and Voltage Ratings
Cables have rated maximum tolerable voltage and current. For AC, peak voltage and current are approximately 1.4 times higher than their average values (which correspond to actual delivered power or equivalent DC values). In contrast, DC systems have identical peak and average values.
However, HVAC conductors must be rated for peak current and voltage, wasting approximately 30% of their carrying capacity. In contrast, HVDC utilizes the full capacity of conductors, meaning a conductor of the same size can transmit more power in HVDC systems.
Right-of-Way
"Right-of-way" refers to the land corridor required for transmission infrastructure. HVDC systems have a narrower right-of-way due to smaller towers and fewer conductors (two for DC vs. three for three-phase AC). Additionally, AC insulators on towers must be rated for peak voltages, further increasing their footprint.
This narrower corridor reduces material, construction, and land costs, making HVDC superior in terms of right-of-way efficiency.
Submarine Power Transmission
Submarine cables used for offshore power transmission have stray capacitance between parallel conductors. Capacitance reacts to voltage changes—constant in AC (50–60 cycles per second) but only occurring during switching in DC.
AC cables continuously charge and discharge, causing significant power losses before delivering power to the receiving end. HVDC cables, charged only once, eliminate such losses. For more details, refer to content on submarine cable construction, characteristics, laying, and joints.
Controllability of Power Flow
HVAC systems lack precise control over power flow, whereas HVDC links use IGBT-based semiconductor converters. These complex converters, switchable multiple times per cycle, optimize power distribution across the system, improve harmonic performance, and enable rapid fault protection and clearance—advantages unmatched by HVAC.
Interlinking Asynchronous Systems and Smart Grids
A smart grid allows multiple generating stations to feed into a unified network, leveraging small-scale grids for high-power generation. However, connecting multiple asynchronous AC grids (with differing frequencies or phases) is highly challenging.
Interlinking Asynchronous Grids
Power grids worldwide operate at different frequencies—some at 50 Hz, others at 60 Hz. Even grids with the same frequency may be out of phase. These are classified as "asynchronous systems" and cannot be connected via standard AC links.
DC, however, is unaffected by frequency or phase. HVDC interlinks resolve this by converting AC to frequency- and phase-agnostic DC, enabling seamless integration of asynchronous grids. At the receiving end, HVDC inverters convert the DC back to AC with the required frequency, facilitating unified power transmission.
Circuit Breakers
Circuit breakers are critical in high-voltage transmission, responsible for de-energizing circuits during faults or maintenance. A key requirement is arc-extinguishing capability to interrupt power flow.
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HVAC Circuit Breakers: AC current reverses direction continuously, creating natural zero-current moments (50–60 times per second) that automatically extinguish arcs. This "self-extinguishing" feature simplifies HVAC breaker design, making them relatively straightforward and cost-effective.
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HVDC Circuit Breakers: DC current is unidirectional with no natural zero crossings. To extinguish arcs, specialized circuitry must artificially generate zero-current points. This complexity makes HVDC breakers more intricate and expensive than their AC counterparts.
AC’s alternating current produces a constantly varying magnetic field, which can induce interference in nearby communication lines. In contrast, DC’s steady magnetic field eliminates such interference, ensuring minimal disruption to adjacent communication systems.
