Uongozaji wa Kamilifu wa Chaguo na Kikomo kwa Uhesabu ya Mipangilio ya Kitamaduni za Umeme

Jinsi ya Kuchagua na Kusakinisha Circuit Breakers

1. Aina za Circuit Breakers

1.1 Air Circuit Breaker (ACB)
Vinavyoitwa pia kama molded frame au universal circuit breaker, vitu vyote vya kutumika vinapatikana katika eneo la chapa chenye usafi wa mamba. Ni mara nyingi yasiyofungwa, inayoruhusu uharaka wa kurudia magawanyo na sehemu, na inaweza kuongezwa na zao mbalimbali. ACBs zinatumika kama funguo muhimu za umeme. Mifumo ya kupungua current zinajumuisha electromagnetic, electronic, na intelligent. Zinatoa usalama wa ngazi nne: long-time delay, short-time delay, instantaneous, na ground fault, ambayo zinaweza kurudishwa kulingana na ukubwa wa frame.

ACBs zinazotumika katika mitandao ya AC 50Hz yenye rated voltage ya 380V au 660V na rated currents tofauti kutoka 200A hadi 6300A. Zinatumika kwa utaratibu wa umeme na usalama dhidi ya overloads, undervoltage, short circuits, na single-phase grounding. Breakers hizi zinatoa huduma nyingi za usalama na usalama wa chaguo. Katika mazingira sahihi, zinaweza kutumika kwa mara chache tu kwa kutengeneza mkondo. ACBs zinazotumika hadi 1250A zinaweza kuimarisha motors dhidi ya overloads na short circuits katika mifumo ya 380V/50Hz.

Matumizi yanayozingatia ni kama switches muhimu katika upande wa 400V wa transformers, bus tie switches, high-capacity feeder switches, na large motor control switches.

1.2 Molded Case Circuit Breaker (MCCB)
Vinavyoitwa pia kama plug-in circuit breaker, viwanja, arc extinguishers, trip units, na operating mechanism viko ndani ya mfumo wa plastic. Auxiliary contacts, undervoltage trip units, na shunt trip units zinaweza kuwa modular, kufanya kujenga kwa njia imara. MCCBs zinazotumika mara nyingi hazijengwe kwa ajili ya tafsiri na zinatumika kwa protection ya branch circuit.

MCCBs mengi yana thermal-magnetic trip units. Models zisizozuri zinaweza kuwa na solid-state trip sensors. Overcurrent trip units zinaweza kuwa electromagnetic au electronic. Electromagnetic MCCBs zinaweza kuwa non-selective, zinatoa tu long-time na instantaneous protection. Electronic MCCBs zinatoa nafasi za protection nyingi: long-time, short-time, instantaneous, na ground fault. Baadhi ya models mpya zinajumuisha zone-selective interlocking.

MCCBs zinatumika kwa utaratibu wa umeme na usalama, kama switches muhimu katika transformers madogo, motor control terminals, na kama switches za umeme kwa machine mingi.

1.3 Miniature Circuit Breaker (MCB)
MCBs ni devices zenye matumizi zaidi katika mikakati ya umeme ya majengo. Zinapambana na single-phase na three-phase circuits hadi 125A dhidi ya short circuits, overloads, na overvoltage. Inapatikana kwa configurations 1P, 2P, 3P, na 4P.

MCB unajumuisha operating mechanism, contacts, protective devices (trip units mbalimbali), na arc extinguishing system. Contacts zinafungwa kwa mikono au kwa nguvu na zinahifadhiwa kwa free-tripping mechanism. Coil ya overcurrent trip unit na heating element ya thermal trip unit zinajulikana kwa series na main circuit, undervoltage trip coil zinajulikana kwa parallel na power supply.

Katika mikakati ya umeme ya majengo, MCBs zinatumika kwa overload, short-circuit, overcurrent, undervoltage, ground fault, leakage protection, automatic transfer of dual power sources, na kuanza na upimaji wa motors kwa mara chache.

2. Maegesho Muhimu ya Teknolojia ya Circuit Breakers

• Rated Operating Voltage (Ue)
  Volts nominal ambayo circuit breaker imeundwa kusafiri kwa muda mrefu kwa masharti maalum. China, kwa systems hadi 220kV, maximum operating voltage ni 1.15 mara system rated voltage; kwa 330kV na zaidi, ni 1.1 mara. Breaker lazima ishike insulation na kutekeleze switching operations kwenye maximum operating voltage ya system.

• Rated Current (In)
  Current ambayo trip unit inaweza kusafiri kwa muda mrefu kwenye temperature za ambient hadi 40°C. Kwa adjustable trip units, hii ni maximum adjustable current. Kwenye temperatures zaidi ya 40°C (hadi 60°C), derating inaruhusiwa.

• Overload Trip Current Setting (Ir)
  Breaker hutrip kwa time delay wakati current inapita Ir, ambayo inatafsiriwa kama maximum current ambayo breaker inaweza kusafiri bila kutrip. Ir lazima iwe zaidi ya maximum load current (Ib) lakini chini ya cable’s permissible current (Iz). Kwa thermal-magnetic breakers, Ir inaweza kurudishwa kutoka 0.7 hadi 1.0In; electronic trip units zinatoa range zaidi, mara nyingi 0.4 hadi 1.0In. Kwa fixed trip units, Ir = In.

• Short-Circuit Trip Current Setting (Im)
  Threshold ambapo instantaneous au short-time trip unit inatekeleza kwa haraka kutengeneza mkondo wakati wa high fault currents.

• Rated Short-Time Withstand Current (Icw)
  Current ambayo breaker inaweza kusafiri kwa duration iliyotolewa bila thermal damage.

• Breaking Capacity
  Maximum fault current ambayo breaker inaweza kusafisha salama, bila kuzingatia rated current. Values zinazozingatia ni 36kA na 50kA. Inachanganishwa kwa ultimate breaking capacity (Icu) na service breaking capacity (Ics).

3. Sera Muhimu za Chaguo la Circuit Breakers

• Rated operating voltage ≥ circuit rated voltage.

• Rated short-circuit making/breaking capacity ≥ calculated load current.

• Rated short-circuit making/breaking capacity ≥ maximum possible short-circuit current in the circuit.

• Single-phase-to-ground short-circuit current at the circuit end ≥ 1.25 × instantaneous (or short-time) trip setting.

• Undervoltage trip unit rated voltage = circuit rated voltage.

• Shunt trip unit rated voltage = control power supply voltage.

• Electric operating mechanism rated voltage = control power supply voltage.

• Kwa lighting circuits, set instantaneous electromagnetic trip current to 6 times the load current.

• Kwa single motor short-circuit protection: 1.35× motor starting current (DW series) au 1.7× (DZ series).

• Kwa multiple motors: 1.3× largest motor starting current + sum of other motors’ running currents.

• As main transformer low-voltage side switch: breaking capacity > transformer’s low-voltage short-circuit current; trip rated current ≥ transformer rated current; short-circuit setting = 6–10× transformer rated current; overload setting = transformer rated current.

• Baada ya chaguo msingi, coordinate with upstream and downstream breakers to prevent cascading trips and minimize outage scope.

4. Selectivity ya Circuit Breakers
Circuit breakers zinachanganishwa kama selective au non-selective. Selective breakers zinatoa two- au three-stage protection: instantaneous na short-time for short circuits, long-time for overload. Non-selective breakers zinazotumika mara nyingi ni instantaneous (short-circuit only) au long-time (overload only). Selectivity inapewa kwa kutumia short-time delay trip units na settings tofauti. Mambo muhimu:

• Upstream instantaneous trip setting ≥ 1.1 × maximum 3-phase short-circuit current at downstream breaker’s output.

• Ikiwa downstream ni non-selective, upstream short-time trip setting ≥ 1.2 × downstream instantaneous trip setting to maintain selectivity.

• Ikiwa downstream pia ni selective, upstream short-time delay time ≥ downstream short-time delay time + 0.1s.
  Maranyingi, Iop.1 ≥ 1.2 × Iop.2.

5. Cascading Protection
Kwenye mifano ya design, coordination between upstream na downstream breakers husaidia kushughulikia selectivity, speed, na sensitivity. Coordination sahihi inasaidia kutoa selective fault isolation, kuhifadhi umeme kwenye circuits safi. Cascading hutumia current-limiting effect ya upstream breaker (QF1). Wakati short circuit unategemea downstream (katika QF2), QF1's current-limiting action huondokana na actual fault current, kukuwa na QF2 ikisafisha current zaidi ya rated capacity yake. Hii kunaweza kutumia downstream breakers wenye gharama chache na breaking capacity chache. Masharti yanafaa kuwa hakuna critical loads kwenye circuits jirani (kwa sababu QF1 trip italeta blackout QF3), na matching sahihi ya instantaneous settings. Cascading data hutolewa kwa testing na inatoa manufacturers.

6. Sensitivity ya Circuit Breakers
Kusaidia kufanya kazi salama wakati wa minimum fault conditions, sensitivity (Sp) lazima iwe ≥1.3 per GB50054-95:
Sp = Ik.min / Iop ≥ 1.3
Ipopo Iop ni instantaneous au short-time trip setting, na Ik.min ni minimum short-circuit current at the protected line end under minimum system operation. Kwa selective breakers wenye both short-time na instantaneous trips, tu short-time trip sensitivity inaweza kutathmini.

7. Selection na Setting ya Trip Units

(1) Instantaneous Overcurrent Trip Setting.Must exceed the circuit’s peak current (Ipk) during motor startup:
Iop(0) ≥ Krel × Ipk
(Krel = reliability factor)

(2) Short-Time Overcurrent Trip Setting and Time
Iop(s) ≥ Krel × Ipk. Time delays are typically 0.2s, 0.4s, or 0.6s, set to ensure upstream operation time exceeds downstream by one time step.

(3) Long-Time Overcurrent Trip Setting and Time
Protects against overload: Iop(l) ≥ Krel × I30 (maximum load current). Time setting must exceed allowable short-term overload duration.

(4) Coordination Between Trip Settings and Cable Capacity.To prevent cable overheating or fire without tripping:

Iop ≤ Kol × Ial
Ipopo Ial = cable’s allowable current-carrying capacity, Kol = short-term overload factor (4.5 for instantaneous/short-time trips; 1.1 for long-time trip as short-circuit protection; 1.0 for overload protection only). If not satisfied, adjust trip setting or increase cable size.