Vipi ni Viwango vya tofauti kati ya HVAC na HVDC katika Uhamiaji wa Nishati?

Tofauti kati ya HVAC na HVDC

Umeme unaojengwa katika viwanda vya umeme hutumakaziwa kwa umbali mrefu hadi steshoni za umeme, ambazo zinayofanikisha kwa wateja. Votu unaotumiwa kwa kutumakazi umeme kwa umbali mrefu ni wa kiwango cha juu sana, na tutajaribu kuelezea sababu za votu hii ya juu baadaye. Pia, umeme unaweza kutumakaziwa kwa aina ya current ya kuvunjika (AC) au direct current (DC). Kwa hivyo, umeme unaweza kutumakaziwa kwa kutumia HVAC (High Voltage Alternating Current) au HVDC (High Voltage Direct Current).

Kwa Nini Votu Vya Juu Vinahitajika kwa Kutumakazi?

Votu ina uwezo mkubwa katika kupunguza hasara za mstari, vilivyokujulikana kama hasara za kutumakazi. Kila mtandao wa umeme unatumika kwa kutumakazi umeme unaowekwa ohmic resistance (R). Wapopo (I) wanapopita kwenye mitandao haya, huundwa nishati ya joto, ambayo ni hasara tu ya nishati au nguvu (P).

Kulingana na Ohm's Law

Kama inavyoonekana, nishati iliyoharibika katika mtandao wakati wa kutumakazi inategemea wapopo badala ya votu. Lakini, tunaweza kuhamisha ukubwa wa wapopo kupitia mabadiliko ya votu kutumia vyombo vya kisana.

Wakati wa mabadiliko ya votu, nguvu husafishwa na haingegeme. Votu na wapopo tu huongeza kinyume kwa sababu sawa, kufuata sifa:

Kwa mfano, nguvu ya 11KW kwenye votu wa 220v ana amperes 50. Katika hali hii, hasara za mstari wa kutumakazi itakuwa

Hebu ongeze votu mara 10. Hivyo nguvu hiyo ya 11KW itakuwa na votu wa 2200v & 5 Amperes. Sasa hasara za mstari zitakuwa;

Kama unavyoona, kuongeza votu hupunguza hasara za nguvu kwa wingi katika misikitini za kutumakazi. Kwa hivyo, ili kupunguza wapopo katika misikitini za kutumakazi samaki na kukidhi nguvu ile ile, tunawezekana kuongeza votu.

Vita vya Umeme (AC vs. DC)

Katika mwisho wa miaka ya 1880, wakati wa vita viliyosimamiwa kama "Vita vya Umeme," direct current (DC) alikuwa yule wa kwanza kutekeleza kwa kutumakazi umeme. Lakini, alijulikana kuwa duni sana kutokana na kutokuwa na vyombo vya kusana votu yanayofaa - tofauti na alternating current (AC), ambayo ingeweza kuongezeka au kupunguzika rahisi kutumia transformers. Steshoni za umeme DC zenye votu chache zinazozaliwa mapema zingeweza kutumia umeme tu ndani ya eneo la mita chache tu; hapo ndipo, votu linapoganda kabisa, kuhitaji steshoni nyingi za kuzalisha umeme katika maeneo machache - njia ya gharama.

Ingawa kutumakazi umeme DC kwa kiwango cha juu kwa asili huna hasara chache kuliko AC, mifumo ya DC mapenzi yalitumia mercury arc valves (rectifiers) kubadilisha umeme AC kwa DC kwa kutumakazi kwa umbali mrefu. Vyombo haya vya mwisho vilikuwa makubwa, magharama na vilihitaji huduma mara kwa mara. Ingawa, kutumakazi umeme AC kilipendekezwa kutumia transformers - vinavyoefiki, rahisi na vya kuaminika - kufanya AC kuwa chaguo bora kwa kutumakazi umeme kwa umbali mrefu wakati huo.

Wakati wa kuchagua kati ya high-voltage AC (HVAC) na high-voltage DC (HVDC) kwa kutumakazi, vitu muhimu kadhaa yanapaswa kutambuliwa. Maandiko haya yanataraji hizi vitu kwa undani.

HVAC & HVDC

HVAC (High Voltage Alternating Current) na HVDC (High Voltage Direct Current) yanamaanisha viwango vya votu vinavyotumika kwa kutumakazi umeme kwa umbali mrefu. HVDC ni mara nyingi linachukua hatua kwa umbali mrefu sana (marani zaidi ya 600 km), ingawa mifumo miwili yamechukua hatua duniani leo, kila moja ime na faida na demu zake.

Gharama za Kutumakazi

Kutumakazi umeme kwa umbali mrefu hunahitaji votu vya juu, na nguvu inatolewa kati ya steshoni za mwisho zinazohusika kwa mabadiliko ya votu. Gharama za kutumakazi kamili kwa hivyo zinategemea mbili: gharama za steshoni za mwisho na gharama za misikitini za kutumakazi.

• Steshoni za Mwisho
  Steshoni za mwisho humpunguza au kuongeza kiwango cha votu kwa kutumakazi. Kwa mifumo ya AC, hii kwa kawaida hutenda kutumia transformers, ambazo huhamisha kati ya votu vya juu na chache. Kwa mifumo ya DC, steshoni za mwisho huchukua thyristor au IGBT-based converters kubadilisha kiwango cha votu DC.

  Kwa sababu transformers zinaweza kuaminika zaidi na ni rahisi kuliko solid-state converters, steshoni za AC zinaweza kuwa rahisi kuliko zao DC, kufanya mabadiliko ya votu AC kuwa rahisi zaidi.

• Misikitini za Kutumakazi
  Gharama za mstari zinategemea idadi ya conductors na ubora wa mitandao ya kutumakazi. Mifumo ya HVDC haina haja ya conductor wa pili tu, ingawa mifumo ya HVAC haina haja ya tatu au zaidi (ikiwa ni bundled conductors ili kupunguza corona effects).

  Mitandao ya AC yanahitaji mikono nyengine kubwa, yanahitaji mikono yenye nguvu, matare na mizito zaidi kuliko mitandao ya HVDC. Gharama za mstari huongezeka kwa umbali, na kwa kila 100 km, misikitini ya HVAC zinaweza kuwa zaidi ya gharama kuliko misikitini ya HVDC.

• Gharama Za Kutumakazi Kamili
  Gharama za kuu zinatumika kutoka kwa gharama za steshoni (fixed, independent of distance) na gharama za mstari (variable, increasing with distance). Kwa hivyo, gharama kamili za mifumo ya kutumakazi huongezeka kama umbali huongezeka.

Umbali wa Kugawa Gharama

"Umbali wa kugawa gharama" unamaanisha umbali wa kutumakazi unayopita ambapo gharama kamili ya uwekezaji wa HVAC ina zidi gharama za HVDC. Umbali huu unategemea karibu 400–500 miles (600–800 km). Kwa umbali zaidi ya thamani hii, HVDC ni chaguo bora kwa gharama; kwa umbali fupi, HVAC ni rahisi zaidi. Uhusiano huu unaelezwa kwa undani kwenye grafu yenyewe.

Uwezo wa Kubadilisha

HVDC mara nyingi hutumika kwa kutumakazi kwa umbali mrefu kati ya point-to-point, kwa sababu kutumia nguvu katika maeneo ya kati yanahitaji converters gharama sana kurekebisha votu DC vya juu. Ingawa, HVAC unatoa uwezo mkubwa zaidi: steshoni nyingi za mwisho zinaweza kutumia transformers rahisi kurekebisha votu vya juu, kunawezesha kutumia nguvu katika maeneo mengi kwenye mstari.

Hasara za Nguvu

Kutumakazi umeme HVAC hutolewa na aina nyingi za hasara, ikiwa ni corona losses, skin effect losses, radiation losses, na induction losses, ambayo zinazozunguka au kupunguzika sana katika mifumo ya HVDC:

• Corona Losses: Wakati votu hupita kiwango cha muhimu, hewa zinazozunguka conductors huiionizwa, kutengeneza sparks (corona discharge) ambayo huhasira nishati. Hasara haya huwasiliana na frequency—kwa sababu DC haina frequency, HVAC corona losses ni takriban mara tatu zaidi kuliko HVDC.

• Skin Effect Losses: Katika kutumakazi umeme AC, density ya current ni juu zaidi kwenye surface ya conductor na chini zaidi kwenye core (the "skin effect"), kupunguza cross-sectional area inayotumika kwa current flow. Hii hongeza resistance ya conductor na kuongeza I²R losses. DC current, kwa upande mwingine, hutokea uniform across the conductor, kupunguza hii 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.