Vipi ni Faides za HVDC Kulingana na HVAC katika Upeleka Nishati?

Vipi za HVDC kwa kumepaswa HVAC?

Umeme huenda umbali mrefu kabla ya kupata wateja. Vifaa vya umeme, mara nyingi vilivyoko mbali, hutoa umeme kupitia mita mingi na majukwaa mengi. Utumiaji wa kutuma umeme kwa kiwango cha juu unarejelea hasara katika mitindo, na kila moja ya AC na DC inatumika. Ingawa AC ni rahisi kuelewa kupitia migomba ya umeme na vitendo nyumbani, HVDC inatoa faida zisizohesabika katika kutuma umeme.

Lengo la kutuma umeme ni kuridhi hasara na gharama. Ingawa wote wanakubalika na viwango vya kusababisha, HVDC ina faida zaidi. Maandiko haya yatasimamia faida za HVDC kwa kumepaswa HVAC:

Gharama Ndogo za Kutuma
Gharama za kutuma zinategemea kwa vifaa vya kubadilisha kiwango cha mwisho, idadi/kiwango cha mitindo, ukubwa wa migomba, na hasara. HVAC hutumia vifaa vya kubadilisha (transformers) kwa ubadilishaji—rahasa na rahisi kuliko vifaa vya kubadilisha ya thyristor ya HVDC, ambayo ni gharama pekee inayofanana.

HVAC inahitaji kamili sita au tatu kwa kutuma tatu-phasi. HVDC, kwa kutumia dunia kama njia ya kurudi, huchukua mitindo moja (monopolar) au mbili (bipolar), kurejesha gharama. Hata mitindo tatu-phasi yanaweza kupeleka nguvu mara mbili kwa kutumia mitindo miwili ya HVDC bipolar.

HVAC inahitaji umbali mkubwa wa phase-to-ground na phase-to-phase, kuhitaji migomba mikubwa sana. Migomba ya HVDC hurejesha gharama za uwekezaji. HVDC pia ina hasara ndogo sana za kutuma, kufanya iwe rahisi zaidi.

Jumla ya gharama za kutuma zinaweza kugawanyika kwenye jamii mbili kuu: gharama za stesheni ya mwisho na gharama za mitindo ya kutuma. Iliyo ya awali ni gharama ya chaguo, isiyotegemea umbali wa kutuma, ingawa iliyo ya pili inabadilika kwa urefu wa mitindo. Gharama za stesheni ya AC ni chache, ingawa gharama za stesheni ya HVDC ni kubwa sana. Lakini, gharama kwa kila 100 km kwa mitindo ya HVAC ni zaidi sana kuliko kwa mitindo ya HVDC. Kwa hivyo, mstari wa jumla wa gharama kwa HVAC na HVDC unatengeneza sehemu inayojulikana kama umbali wa kutosha.

Umbali wa kutosha ni umbali wa kutuma unaoelekea kwa kutosha ambapo gharama za kutosha za HVAC zinazidi kwa HVDC. Umbali huu unabadilika kulingana na aina ya kutuma: takriban 400-500 maili (600-800 km) kwa mitindo ya juu, 20-50 km kwa mitindo ya chini ya maji, na 50-100 km kwa mitindo ya chini ya ardhi. Unapokosa umbali huu, HVDC hutoa chaguo rahisi zaidi na kwa gharama chache.

Kutuma umeme kwa HVDC huwasilisha hasara ndogo sana kuliko HVAC, na maendeleo muhimu katika masomo haya:

Ukimwi wa Nguvu Reactive

Kutuma umeme kwa HVAC huna hasara za nguvu reactive, ambazo zinategemea mrefu wa mitindo, eneo la kusafiri, na mizigo ya kusoma. Hasara hizi hurejesha upatikanaji wa nguvu na kukosa nishati, kusababisha mrefu wa HVAC kwa utaratibu. Kukabiliana na hii, mifumo ya HVAC hutumia ushauri na shughuli za kusambaza kureduku VARs (volt-ampere reactive) na kudhibiti ustawi.

Kwa upande mwingine, HVDC hufanya kazi bila eneo la kusafiri au nishati ya kusambaza, kunyima hasara za nguvu reactive. Hii kunyima hitaji kwa hatua za kusambaza.

Hasara ndogo za Corona

Wakati kiwango cha kutuma umeme kinapopita kwenye kiwango cha muhimu (kiwango cha kuanza cha corona), atomi za hewa zinavyoingia kwenye mitindo huionekana, kuunda sparks (corona discharge) zinazokosa nishati. Hasara za corona zinategemea kiwango cha umeme na eneo la kusafiri. Tangu DC haina eneo la kusafiri, hasara za corona za HVDC ni takriban sehemu ya tatu tu ya HVAC.

Ukimwi wa Skin Effect

Nishati ya AC ina skin effect, ambako nishati huchukua karibu na ngozi ya mitindo, kuacha kipa chini chenye nishati chache. Uchukuzi huu usio sawa wa nishati unaridhisha sekta ya mitindo, kubadilisha resistance (kwa sababu resistance ni reciprocally proportional to area) na kutoa hasara za I²R zaidi kwa mitindo ya HVAC. HVDC, na nishati zake direct, hunyima hii, kuhakikisha uchukuzi wa nishati ni sawa na kurejesha hasara za resistance.

Hakuna Hasara za Radiation au Induction

Mitindo ya HVAC yana hasara za radiation na induction kutokana na magnetic fields zao zinazobadilika. Hasara za radiation zinatokea kwa sababu mitindo ya AC yanaweza kutumika kama antennas, kuteleza nishati isiyo wezekanavyo. Hasara za induction zinatokea kutokana na currents induced kwenye mitindo yanayokuwa karibu na alternating field. Katika mifumo ya HVDC, magnetic field ni constant, hakuna hasara za radiation na induction.

Hasara ndogo za Charging Current

Cables za chini ya ardhi na chini ya maji hana parasitic capacitance, ambayo inahitaji charging kabla ya kupeleka nishati. Capacitance inabadilika kwa urefu wa cable, na hivyo charging current inabadilika kwa hiari.

Katika mifumo ya AC, cables zinacharge na kusafisha mara kadhaa kwa sekunde, kuhitaji current zaidi kutoka kwa chanzo kwa ajili ya kudhibiti mzunguko huo. Hii current zaidi hutoa hasara za I²R zaidi kwenye cable. Cables za HVDC, hata hivyo, huchukiwa mara moja tu wakati wa energization au switching. Hii hukataa hasara zinazotokana na charging currents.

Hakuna Hasara za Dielectric Heating

Alternating electric field katika mifumo ya AC hutoa mchanganyiko kwa materials za insulation katika mitindo, kuhakikisha zinaweza kuchukua nishati na kubadilisha kwa heat—phenomenon linalojulikana kama dielectric loss. Hii hukosa nishati na kutumia muda wa insulation. Mifumo ya HVDC hutoa electric field constant, hakuna hasara za dielectric na matatizo yaliyotokea.

3) Mitindo Fupi

Skin effect katika AC hutoa nishati kuwa karibu na ngozi ya mitindo, kuhitaji mitindo fupi zaidi kwa ajili ya kuongeza sekta na kupeleka nishati zaidi.HVDC, yenye nishati direct, hutoa nishati kuwa uniform across the cross-section of the conductor. Hii hutoa fursa ya kutumia mitindo fupi zaidi na kudumisha uwezo wa kutumia nishati sawa, kurejesha gharama za material na uzito.

4) Limitations ya Mrefu wa Mitindo

Mitindo ya HVAC yana hasara za nguvu reactive ambazo zinabadilika kwa mrefu wa mitindo. Hii hutoa limit critical kwa HVAC transmission distance: unapokosa umbali wa takriban 500 km kwa mitindo ya juu, hasara za nguvu reactive zinakuwa zaidi, kusababisha system kuwa unstable.Transmission ya HVDC, kwa upande mwingine, haina limits hizi, hii hutoa fit kwa ultra-long-distance power delivery.

5) Reduced Cable Rating Requirements

Cables zinategemewa kwa maximum tolerable voltage na current. Katika mifumo ya AC, peak voltage na current ni takriban mara 1.4 zaidi ya average values (ambayo zinawakilisha actual power delivered). Hata hivyo, conductors zinategemewa kwa hii peak values.Katika mifumo ya DC, peak na average values ni sawa. Hii inamaanisha HVDC inaweza kutuma nguvu sawa kwa kutumia cables na ratings voltage na current chache kuliko HVAC. Kweli, mifumo ya HVAC huwaste kiasi gani cha 30% ya capacity ya conductor kwa sababu ya requirements zao za peak.

6) Right-of-Way Narrower

"Right-of-way" inamaanisha corridor ya ardhi inayohitajika kwa infrastructure ya transmission. Mifumo ya HVDC hihitaji right-of-way narrower kwa sababu hutumia towers ndogo zaidi na mitindo machache tu.HVAC, kwa upande mwingine, hihitaji towers zifuata zaidi kusaidia mitindo mengi na insulators mkubwa (rated for AC peak voltages), ambayo hihitaji support structural imara. Footprint hii inaongeza gharama za material, construction, na ardhi—kufanya HVDC kuwa superior kwa urahisi wa right-of-way.

7) Superior Cable-Based Transmission

Cables za chini ya ardhi na submarine zinaweza kuwa na mitindo mingi zinazoseparated na insulation, kutoa parasitic capacitance kati yao. Cables hizi hazitaweza kutuma nishati hadi wakati wamecharge fully, na capacitance (na hivyo charging current) inabadilika kwa urefu.Mifumo ya AC hucharge na kusafisha cables mara nyingi (50-60 mara kwa sekunde), kuboresha hasara za I²R na kurejesha urefu wa cable. Cables za HVDC, hata hivyo, hucharge mara moja tu (wakati wa energization au switching), kurejesha hasara hizi na limitations za urefu.Hii hutoa HVDC kuwa chaguo bora kwa offshore, underwater, na underground cable transmission.

8) Bipolar Transmission

HVDC hutoa modes za kutuma mbalimbali, na bipolar transmission kuwa option widely used na cost-effective. Ina two parallel conductors with opposite polarities, their voltages balanced relative to the earth.Ikiwa line moja ifaihi au ikasikio, system inaswitch seamlessly kwa monopolar mode: remaining line inaendelea kutoa current, using the earth as the return path.

9) Controllable Power Flow

HVDC converters, based on solid-state electronics, enable precise control over power flow in AC networks. Their rapid switching capability (operating multiple times per cycle) enhances harmonic performance, dampens power swings, and optimizes the network's power supply capacity.

10) Fast Fault Clearance

Fault currents—abnormal currents from electrical faults—pose significant risks. In HVAC systems, high fault currents can damage transmission lines, stations, generators, and loads.HVDC minimizes such risks: fault currents are lower, limiting damage to specific sections, and its fast-switching operation ensures rapid fault response, enhancing system resilience.

11) Asynchronous Grid Interconnection

HVDC enables interconnection of asynchronous AC grids with differing parameters (e.g., frequency, phase).Regions often use distinct frequencies (e.g., 50 Hz in Europe vs. 60 Hz in the U.S.), and grids may have phase differences, making direct AC interconnection impossible. HVDC, operating without frequency or phase constraints, easily links these independent systems.

12) Enabling Smart Grids

Smart grids integrate small-scale generators (solar, wind, nuclear) into a unified network with intelligent power flow control.This is feasible with HVDC, which supports asynchronous interconnection of generation units and provides full control over power distribution, aligning with smart grid requirements.

13) Reduced Noise Interference

HVDC causes far less noise interference to nearby communication lines compared to HVAC.HVAC generates audible buzzing, radio, and TV interference, with intensity tied to its frequency. HVDC, with zero frequency, produces minimal noise. Additionally, HVAC noise increases in bad weather, while HVDC noise diminishes, ensuring more stable operation.