Unsa ang mga Diferensya sa HVAC ug HVDC sa Transmision sa Kuryente

Kalibutan sa pagitan sa HVAC ug HVDC

Ang elektrisidad nga giproduktso sa mga planta sa kuryente gitransmit sa dako nga distansya tungod sa mga electrical substation, nga sumala mopasabot sa mga komsumidor. Ang voltag nga gigamit para sa transmision sa dako nga distansya kaayo ka taas, ug atong isulod ang rason alang niini nga taas nga voltag higayon. Sa wala pa, ang gipasabot nga kuryente mahimong alternating current (AC) o direct current (DC) form. Busa, ang kuryente mahimong matransmit pinaagi sa HVAC (High Voltage Alternating Current) o HVDC (High Voltage Direct Current).

Asa Nagkinahanglan og Taas nga Voltag ang Transmision?

Ang voltag nagluwas sa pagbawas sa line losses, usab nailhan isip transmission losses. Bawat electrical conductor nga gigamit para sa transmision adunay tiyak nga ohmic resistance (R). Tungod kay ang current (I) moadto sa tanang conductors, sila mag-generate og thermal energy, nga maoy sayop nga energy o power (P).

Sumala sa Ohm’s Law

Tungod kay ang energy nga nasayop sa conductor sa panahon sa transmision depende sa current, dili sa voltag. Apan, mahimo nato i-adjust ang magnitude sa current pinaagi sa conversion sa voltag gamit ang specialized equipment.

Sa panahon sa conversion sa voltag, ang power mahimong conserved ug wala masayran. Ang voltag ug current simple lang nag-vary inversely pinaagi sa sama nga factor, sumala sa principle:

Isip eemplo, 11KW power sa voltag nga 220v adunay 50 Amps. Sa kasagaran, ang line losses sa transmision mahimong

Pagtaas nato sa voltag sa factor nga 10. Busa, ang sama nga power nga 11KW adunay voltag nga 2200v & 5 Amps. Karon, ang line losses mahimong;

Tungod kay makita nimo, pagtaas sa voltag nagbawas sa power losses sa transmision lines. Busa, aron mapasabot ang current sa transmission cables samtang nag-maintain sa sama nga amount sa transmision, natugyan nato sa pagtaas sa voltag.

Ang Gubat sa Current (AC vs. DC)

Sa katapusan sa 1880s, sa panahon sa nailhan isip "Gubat sa Currents," ang direct current (DC) ang unang gi-deploy para sa transmision sa power. Apan, gi-isipan kini nga dili efektibo tungod sa lack sa practical voltage conversion equipment—usa ra ang alternating current (AC), nga mahimo easy na step up o down gamit ang transformers. Ang mga low-voltage DC power stations sa unang panahon mahimo ra pasabot sa kuryente sa radius nga wala pa lima ka miles; sa sulod sa mas dako, ang voltag nagdrop drastiko, nagpasabot sa multiple generating stations sa small areas—a costly approach.

Tungod kay ang high-voltage DC transmision inherently adunay mas baba nga losses kaysa AC, ang early DC systems gidepende sa mercury arc valves (rectifiers) aron converti ang high-voltage AC sa DC para sa long-distance transmision. Kini nga mga terminal devices bulky, expensive, ug gibutangan sa regular nga maintenance. Sa contrast, ang AC transmision gidepende sa transformers—mas efficient, affordable, ug reliable—nagbutang sa AC isip dominant choice para sa long-distance power transmision sa panahon.

Kung magpili taka tali sa high-voltage AC (HVAC) ug high-voltage DC (HVDC) para sa transmision, mahimong importanteng factors ang dapat isulod. Kini nga artikulo isulod kini nga mga factors sa detalye.

HVAC & HVDC

Ang HVAC (High Voltage Alternating Current) ug HVDC (High Voltage Direct Current) nagrefer sa voltage ranges nga gigamit para sa long-distance power transmision. Ang HVDC karanihon preferred para sa ultra-long distances (usualmente over 600 km), apan parehas nga mga sistema wide use worldwide ngadto sa yugto, bawat usa adunay iyang own advantages ug drawbacks.

Transmission Costs

Ang long-distance power transmision nagkinahanglan og taas nga voltag, uban ang power transferreha tali sa terminal stations nga handle voltage conversion. Ang total transmision costs thus depend on two components: terminal station costs ug transmission line costs.

• Terminal Stations
  Ang terminal stations converti ang voltage levels para sa transmision. Para sa AC systems, kini primary ginagamit ang transformers, nga switch tali sa taas ug baba nga voltag. Para sa DC systems, ang terminal stations ginagamit ang thyristor o IGBT-based converters aron adjusti ang DC voltage levels.

  Tungod kay ang transformers mas reliable ug mas barato kaysa solid-state converters, ang AC terminal stations mas barato kaysa sa ilang DC counterparts, making AC voltage conversion more economical.

• Transmission Lines
  Ang line costs depende sa number of conductors ug design of transmission towers. Ang HVDC systems kinahanglan lamang og duha ka conductors, apan ang HVAC systems kinahanglan og tulo o mas daghan (including bundled conductors to mitigate corona effects).

  Ang AC transmission towers kinahanglan support heavier mechanical loads, requiring stronger, taller, ug wider structures compared to HVDC towers. Ang line costs increase with distance, ug per 100 km, ang HVAC lines mas mahal kaysa sa HVDC lines.

• Overall Transmission Costs
  Ang total costs determine by terminal costs (fixed, independent of distance) ug line costs (variable, increasing with distance). Thus, ang overall cost of a transmission system rises as distance increases.

Break-Even Distance

Ang "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:

• 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.

• 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.

• 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.

• 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.

• 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.

Interference Generation

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.