Tambayar da Sabbin Masu Karkashin Kisan ƙarfi na IEE-Business

A cikin na gaban karkashin kusa da turuwa a Najeriya, tana amfani da hanyar zuwa na 6 kV, 10 kV, da 35 kV na wajen yi aiki a tsakanin masu karshen karkashin kusa. A tsayi na masu karshen karkashin kusa a cikin gabas tana da kyau a bincike da damar delta, wanda ba ta bayar da damar mai zama don ya kunshi abubuwan grounding resistors. Idan yanayi mai yawa a fadin adadin kasa kafin a matsayin wadanda ba suka zama da damar mai zama, damar fadin adadin kasa tana da kyau a bincike da damar symmetrical, wanda ya ba da nasarar aiki da ma'ana ga 'yan aiki. Duk da cewa, idan abubuwan current capacitance suna da yawa (kadan kadan 10 A), wasu yanayi na lokaci tana iya dogara saboda, wanda yana taimakawa a bar tasirin aiki da kuma rage da abubuwan cut-off.

Amma, saboda hanyoyi da ci gaban karkashin kusa, wannan hanyar mai sauƙi ba ta da muhimmanci a gaba. A cikin gabashin karkashin kusa na birane, yana da amfani masu karshen cable circuits, wanda ke shahara da abubuwan current capacitance (fiye da 10 A). A cikin hakan, arc ground ba zan iya dogara da dace, wanda ke shahara da abubuwan hankali:

• Idan arc ground ya faru da ya dogara kadan kadan, zai shahara da overvoltages na arc-ground, wanda yake da inganci na 4U (indace U shine peak phase voltage) ko fiye, wanda ke lalace da lokacin. Wannan zai shahara da insulation na 'yan aiki, kuma zai iya rage da cut-off a wurare da insulation mai kadan, wanda zai shahara da cut-off mai yawa.

• Arcing na lokacin yana iya shahara da air a cikin yankin, wanda yana rage da insulation properties na gasar, kuma yana buɗe ɗalilin short circuit a bayan fadin adadin kasa.

• Yana iya shahara da ferroresonant overvoltages, wanda ke buɗe damage na transformers da surge arresters - kuma ina iya shahara da explosions na surge arresters. Wannan abubuwa suna shahara da insulation integrity na 'yan aiki na grid kuma ke rage da safe operation na duk gabashin karkashin kusa.

Don haka, don rage da abubuwan hankali masu dan hankalin wannan, kuma don bayar da zero-sequence current da voltage na wajen taimaka da reliable operation na ground-fault protection, yana da buƙata a yi artificial neutral point don ya kunshi grounding resistor. Wannan buƙata ke buƙatar development of grounding transformers (wanda ake kira "grounding transformers" ko "grounding units"). Grounding transformer tana yi artificial neutral point da grounding resistor, wanda tana da resistance mai kadan (kadan kadan 5 ohms).

Duk da cewa, saboda characteristics na electromagnetic, grounding transformer tana da high impedance na positive- da negative-sequence currents, wanda ke rage da small excitation current ne da take sa a windings. A cikin har yanzu na core limb, ana kafa duwatsu windings a cikin opposite directions. Idan zero-sequence currents equal take sa a windings, suka shahara da low impedance, wanda ke rage da minimal voltage drop a cikin windings a lokacin zero-sequence conditions.

Na musamman, a lokacin ground fault, winding tana da positive-, negative-, da zero-sequence currents. Tana da high impedance na positive- da negative-sequence currents, amma tana da low impedance na zero-sequence current. Wannan shine saboda, a cikin fadin adadin kasa, duwatsu windings tana da series connection na opposite polarity; electromotive forces na suke da sama da magnitude amma opposite direction, wanda ke rage da cancellation, wanda tana rage da low impedance na zero-sequence current.

A cikin many applications, grounding transformers tana amfani da su mafi yawan don bayar da neutral point da small grounding resistor, amma ba tana ba da secondary load. Saboda haka, many grounding transformers tana da design without a secondary winding. A lokacin normal grid operation, grounding transformer tana yi aiki a no-load state. Amma, a lokacin fault, tana da fault current kadan kadan. A cikin low-resistance grounded system, idan single-phase ground fault ya faru a 10 kV side, highly sensitive zero-sequence protection tana daidaito identify and temporarily isolate faulty feeder. 

Grounding transformer tana yi aiki kadan kadan bayan fault occurrence da operation of the feeder’s zero-sequence protection. A lokacin hakan, zero-sequence current tana sa a neutral grounding resistor da grounding transformer, following the formula: I_R = U / (R₁ + R₂), indace U shine system phase voltage, R₁ shine neutral grounding resistor, da R₂ shine additional resistance in the ground fault loop.

Based on the above analysis, operational characteristics of a grounding transformer are: long-term no-load operation and short-term overload during faults.

In summary, grounding transformer tana yi artificial neutral point don ya kunshi grounding resistor. A lokacin ground fault, tana da high impedance na positive- da negative-sequence currents, amma tana da low impedance na zero-sequence current, wanda tana taimaka da reliable operation na ground-fault protection.

Currently, grounding transformers installed in substations serve two primary purposes:

• Supplying low-voltage AC power for substation auxiliary use;

• Creating an artificial neutral point on the 10 kV side, which, when combined with an arc suppression coil (Petersen coil), compensates for capacitive ground-fault current during 10 kV single-phase ground faults, thereby extinguishing the arc at the fault point. The principle is as follows:

Along the entire length of conductors in a three-phase power grid, capacitance exists both between phases and between each phase and ground. When the grid neutral is not solidly grounded, the capacitance to ground of the faulted phase becomes zero during a single-phase ground fault, while the voltages of the other two phases rise to √3 times the normal phase voltage. Although this increased voltage remains within the insulation design limits, it increases their capacitance to ground. The capacitive ground-fault current during a single-phase fault is approximately three times the normal per-phase capacitive current. When this current becomes large, it easily sustains intermittent arcs, exciting resonant oscillations in the grid’s inductive-capacitive circuit and generating overvoltages up to 2.5–3 times the phase voltage. The higher the grid voltage, the greater the risk from such overvoltages. Therefore, only systems below 60 kV may operate with an ungrounded neutral, as their single-phase capacitive ground-fault currents remain small. For higher-voltage systems, a grounding transformer must be used to connect the neutral point through impedance.

Idan da kuma yadda mafi yawan jirgin hanyar (kamar 10 kV) na birnin jirgi na tsohon jirgin mutuwa (transformer) ya fara da wata delta ko wye bila da tsakiyar adadin mai zurfi, da kuma idan yawan jirgin kafin kasa ne da kusa, ba zan iya samun tsakiyar adadin mai zurfi don jirga. A cikin hakan, ana amfani da jirgin mutuwar jirga don gina tsakiyar adadin mai zurfi masu inganci, wanda ke magana da jirgin mutuwar jirga. Wannan tsakiyar adadin mai zurfi masu inganci ke taimaka wa jirgin mutuwa a kan jirgin kafin kasa da kuma haɗa jirgin kafin kasa—wannan shine ma'adan muhimmiyar jirgin mutuwar jirga.

A lokacin da jirgin mutuwar jirga ya yi aiki, an yi tasiri da jirgin mutuwar uku da yawa, da kuma an yi aiki da jirgin mutuwar mai tsabta na musamman, ya yi aiki daga cikin da ba ta da jirgin mutuwa. Dukkan jirgin mutuwar tsakiya da tsakiyar adadin mai zurfi ba ta da jirgin mutuwa (a fuskantar jirgin mutuwar tsakiya da tsakiyar adadin mai zurfi), da kuma ba zan iya samun jirgin mutuwa a kan jirgin mutuwar tsakiya. Idan, misali, tsakiyan C ta shafi abin da ke jirgin kafin kasa, wanda ke nuna jirgin mutuwar uku da yawa (da aka faɗa daga jirgin mutuwar uku da yawa) za su shiga jirgin mutuwar tsakiya da tsakiyar adadin mai zurfi don jirgin kafin kasa. Jirgin mutuwar tsakiya da tsakiyar adadin mai zurfi ke taimaka wa jirgin mutuwar kafin kasa, don haka za su haɗa jirgin mutuwar kafin kasa—wanda ke tare da jirgin mutuwar tsakiya da tsakiyar adadin mai zurfi na musamman.

A shekarun da ya bayar, akwai wasu matsalolin da suka faru a kan jirgin mutuwar jirga na birnin jirgi na 110 kV a wurin da ke, wadanda suka saukar da jirgin mutuwar uku da yawa. Don in sanin dalilai, an yi tasiri, an yi ayyuka da kuma an rarrabe labarai don in haɓaka matsaloli da kuma in taimaka wa wurare da dama.

Saboda yawan amfani da jirgin mutuwar kafin kasa na birnin jirgi na 10 kV a wurin da ke, yawan jirgin kafin kasa na kafin kasa ta kasance da yawa. Don in haɗa yawan jirgin kafin kasa a lokacin da jirgin kafin kasa, duk birnin jirgi na 110 kV suna amfani da jirgin mutuwar jirga don in yi jirgin mutuwar da yawa, wanda ke magana da jirgin mutuwar uku da yawa. Wannan ke taimaka wa jirgin mutuwar uku da yawa zuwa jirgin kafin kasa, saboda haka za su iya kawo jirgin kafin kasa da kuma taimaka wa jirgin mutuwar uku da yawa.

Daga 2008, wurin da ke sun yi takarda a kan jirgin mutuwar 10 kV na birnin jirgi na 110 kV zuwa jirgin mutuwar da yawa, tun daga wannan an yi amfani da jirgin mutuwar jirga da kuma jirgin mutuwar uku da yawa. Wannan ke taimaka wa in yi tasiri da jirgin kafin kasa na 10 kV da yawa, don in haɗa tasiri a kan jirgin mutuwar uku da yawa. Amma, a lokacin da ya bayar, akwai lima birnin jirgi na 110 kV a wurin da ke suka shafi matsaloli a kan jirgin mutuwar jirga, wadanda suka saukar da jirgin mutuwar uku da yawa da kuma taimaka wa jirgin mutuwar uku da yawa. Saboda haka, in sanin dalilai da kuma in yi ayyuka shine ma'adan muhimmiya.

1. Tasiri da Dalilai Matsalolin Jirgin Mutuwar Jirga

Idan jirgin kafin kasa na 10 kV ta shafi abin da ke jirgin kafin kasa, jirgin mutuwar uku da yawa na 110 kV ya fi shafi abin da ke jirgin kafin kasa. Idan ba ya yi aiki, jirgin mutuwar jirga na backup ya shiga jirgin mutuwar bus tie da kuma jirgin mutuwar tsohon jirgi don in haɗa abin da ke jirgin kafin kasa. Saboda haka, aiki daidai na jirgin mutuwar uku da yawa na 10 kV da kuma jirgin mutuwar uku da yawa shine ma'adan muhimmiya. Tasiri na statistikin matsalolin a cikin lima birnin jirgi ya nuna cewa ba a yi aiki da jirgin mutuwar uku da yawa na 10 kV shine dalilan ma'aikata.

Jirgin mutuwar uku da yawa na 10 kV ya yi aiki haka: jirgin mutuwar CT na zero-sequence → jirgin mutuwar ya fara → jirgin mutuwar uku da yawa ya shiga. Abubuwan da suka fi sani shine jirgin mutuwar CT na zero-sequence, jirgin mutuwar relay, da kuma jirgin mutuwar uku da yawa. Tasiri ya yi aiki a cikin waɗannan:

1.1 Matsalolin da suka shafi abin da ke jirgin mutuwar CT na zero-sequence
A lokacin da jirgin kafin kasa, jirgin mutuwar CT na zero-sequence ya samun jirgin mutuwar kafin kasa, wanda ke taimaka wa jirgin mutuwar. Da kuma, jirgin mutuwar CT na zero-sequence na jirgin mutuwar jirga ta samun jirgin mutuwar. Don in taimaka wa selectivity, jirgin mutuwar uku da yawa (misali, 60 A, 1.0 s) ya fi shafi abin da ke jirgin mutuwar jirga (misali, 75 A, 1.5 s don in shiga jirgin mutuwar bus tie, 2.5 s don in shiga jirgin mutuwar tsohon jirgi). Amma, matsalolin da suka shafi abin da ke jirgin mutuwar CT (misali, -10% na jirgin mutuwar CT na jirgin mutuwar jirga, +10% na jirgin mutuwar CT na jirgin mutuwar uku da yawa) za su iya haɗa jirgin mutuwar pickup daban-daban (67.5 A vs. 66 A), da kuma ba zan iya yi aiki da time delay. Wannan ke taimaka wa jirgin mutuwar overreach.

1.2 Tushen da ba daidai ba na cable shield grounding
Jirgin kafin kasa na 10 kV suna amfani da jirgin kafin kasa na cable shield, da cable shield na biyu da suka shiga jirgin mutuwar—wanda ke tushen da ake amfani don in haɗa EMI. Jirgin mutuwar CT na zero-sequence suna da hanyar toroidal, wanda ke ji da jirgin kafin kasa a kan outlet na switchgear. A lokacin da jirgin kafin kasa, jirgin mutuwar unbalanced current ke nuna signal a kan jirgin mutuwar CT. Amma, idan cable shield na biyu da suka shiga jirgin mutuwar, circulating shield currents ke nuna jirgin mutuwar a kan jirgin mutuwar CT, wanda ke taimaka wa measurement distortion. Idan ba a yi amfani da tushen daidai (misali, cable shield ground wire ta shiga daidai a kan jirgin mutuwar CT), jirgin mutuwar uku da yawa ba zan iya yi aiki, wanda ke taimaka wa jirgin mutuwar overreach.

1.3 Ba a yi aiki da jirgin mutuwar uku da yawa
Idan microprocessor-based relays suna da aiki daidai, quality na product ya kunshi. Wasu matsalolin da suka faru shine power, sampling, CPU, ko trip output modules. Idan ba a sanin, za su iya taimaka wa protection refusal, wanda ke taimaka wa jirgin mutuwar overreach.

1.4 Ba a yi aiki da jirgin mutuwar uku da yawa
Idan breakers suna da age, suna yi aiki da yawa, ko ba da quality ba (misali, GG-1A types a wurare da dama) za su iya taimaka wa failure rates. Control circuit faults—particularly burnt trip coils—za su iya haɗa jirgin mutuwar uku da yawa, saboda haka za su iya taimaka wa jirgin mutuwar overreach.

1.5 Jirgin kafin kasa na high-impedance a kan jirgin kafin kasa na biyu
Idan jirgin kafin kasa na biyu suka shafi abin da ke jirgin kafin kasa a kan phase na biyu, individual zero-sequence currents (misali, 40 A da 50 A) ba zan iya samun jirgin mutuwar uku da yawa (60 A), amma sum (90 A) za su iya samun jirgin mutuwar jirga (75 A), wanda ke taimaka wa overreach. Kuma idan single severe high-impedance fault (misali, 58 A) da normal capacitive current (misali, 12–15 A) za su iya samun 75 A. System disturbances za su iya taimaka wa overreach.

2. Ayyukan don In Haɗa Overreach

2.1 Haɗa matsalolin da suka shafi abin da ke jirgin mutuwar CT

Amfani da jirgin mutuwar CT na zero-sequence da quality daidai; reject units na >5% error a lokacin da commissioning; set protection thresholds based on primary values; verify settings via primary injection testing.

2.2 Tushen daidai na cable shield grounding

• Kwada karamin kwayoyin kula da kuka a cikin CT na zero-sequence kuma kawo shiga da kable trays; baka tsari da kable trays idan karkashin kula.

• Bi kan fadin karamin kula don bayyana; kawo shiga waɗanda ba su bi.

• Idan wurin kula ya zama da kasa a kan CT, baka tsarin kula a cikin CT. Baka tsarin kula a kan abubuwan CT.

• Sanya masu ilimi da kable don hanyar samun daidai.

• Gargajiya jami'ar neman da ake yi da kungiyoyin relay, operations, da kable.

2.3 Kafin kula da yanayin da ba su iya kula

Yi amfani da yanayin da ake gano da kyau; kawo kifiya da yanayin da suka zama da shekaru ko wani abu mai yawa; taimaka da ingantaccen inganci; sa samun hawa/ƙauna don kula da yanayin da suka zama da karshen lokaci.

2.4 Kafin kula da breakers da ba su iya kula

Yi amfani da breakers da za a fi ta da kyau (misali, spring- ko motor-charged sealed types); kawo kifiya da GG-1A cabinets da suka zama da shekaru; taimaka da circuits na kontrol; yi amfani da high-quality trip coils.

2.5 Kafin kula da risks na high-impedance faults

Bayyana da kula da feeders kadan da kusan kula ya faru; kawo kifiya da lengths na feeders; balanse phase loads don kula da normal capacitive current.

3. Tashi

Idan grounding transformers ke taimaka da structure da stability na grid, misoperations da suka faru sun nuna risks masu gabatarwa. Wannan rubutu na nemi sabbin dalilai da ke taimaka da ake bayyana solutions na hukuma don taimakawa masu regions da suka yi installation ko ke neman da yi installation grounding transformers.

Zigzag (Z-Type) Grounding Transformers

A cikin 35 kV da 66 kV distribution networks, windings na transformers ke wye-connected da neutral point available, kuma babu da kyau da ake bukata grounding transformers. Amma a cikin 6 kV da 10 kV networks, transformers da suka damar delta-connected babu da neutral point, kuma haka ne da take buƙata grounding transformer don bayyana wadanda suka buƙata arc suppression coils.

Grounding transformers ke amfani da zigzag (Z-type) winding connections: har phase winding ke divide across two core limbs. Zero-sequence magnetic fluxes daga duk windings ke cancel each other, kuma yana haɗa da very low zero-sequence impedance (typically <10 Ω), low no-load losses, da utilization of over 90% of rated capacity. Idan conventional transformers ke da much higher zero-sequence impedance, limiting arc suppression coil capacity to ≤20% of transformer rating. Saboda haka, Z-type transformers ke da kyau don applications na grounding.

Idan system unbalance voltage yana da yawan, balanced Z-type windings ke da kyau don measurement. A cikin systems da low-unbalance (misali, all-cable networks), neutral yana da design don produce 30–70 V unbalance voltage for measurement needs.

Grounding transformers zai iya supply secondary loads, serving as station service transformers. A wannan lokutan, primary rating ce sum of arc suppression coil capacity and secondary load capacity.

Primary function na grounding transformer shine delivery of ground-fault compensation current.

Figure 1 and Figure 2 show two common Z-type grounding transformer connections: ZNyn11 and ZNyn1. Principle behind low zero-sequence impedance shine: each core limb contains two identical windings connected to different phase voltages. Under positive- or negative-sequence voltage, magnetomotive force (MMF) on each limb is vector sum of two phase MMFs. The three limb MMFs are balanced and 120° apart, forming closed magnetic path with low reluctance, high flux, high induced voltage, and thus high magnetizing impedance.

Under zero-sequence voltage, two windings on each limb produce equal but opposite MMFs, resulting in zero net MMF per limb. No zero-sequence flux flows in core; instead, it circulates through tank and surrounding medium, encountering high reluctance. Consequently, zero-sequence flux and impedance are very low.