Hitung Proteksi Arus Lebih Trafo (NEC 450.3)

Pendahuluan:

Proteksi arus lebih yang diperlukan untuk transformator dianggap hanya untuk Proteksi Transformator. Proteksi arus lebih tersebut tidak akan selalu melindungi konduktor primer atau sekunder atau peralatan yang terhubung pada sisi sekunder transformator.
Ketika tegangan dinyalakan untuk memberi energi pada transformator, inti transformator biasanya jenuh. Hal ini mengakibatkan arus masuk yang besar yang paling besar selama setengah siklus pertama (sekitar 0,01 detik) dan menjadi semakin berkurang selama beberapa siklus berikutnya (sekitar 1 detik) hingga transformator mencapai arus magnetisasi normalnya.
Untuk mengakomodasi arus masuk ini, sekering sering dipilih yang memiliki nilai ketahanan arus waktu minimal 12 kali arus pengenal primer transformator selama 0,1 detik dan 25 kali selama 0,01 detik. Beberapa transformator tipe kering kecil mungkin memiliki arus masuk yang jauh lebih besar.
Untuk menghindari penggunaan konduktor berukuran besar, perangkat arus lebih harus dipilih sekitar 110 hingga 125 persen dari nilai arus beban penuh transformator. Dan saat menggunakan proteksi arus lebih yang lebih kecil, perangkat harus berjenis time-delay (di sisi primer) untuk mengompensasi arus masuk yang mencapai 8 hingga 10 kali arus primer beban penuh transformator selama sekitar 0,1 detik saat awalnya diberi energi.
Proteksi konduktor sekunder harus disediakan sepenuhnya terpisah dari proteksi sisi primer.
Lokasi yang diawasi adalah lokasi di mana kondisi pemeliharaan dan pengawasan memastikan bahwa hanya orang yang berkualifikasi yang akan memantau dan memperbaiki pemasangan transformator.
Proteksi arus lebih untuk transformator di sisi primer biasanya berupa pemutus arus. Dalam beberapa kasus di mana tidak ada panel tegangan tinggi, ada pemutus arus sekering sebagai gantinya.
Penting untuk dicatat bahwa perangkat arus lebih di sisi primer harus berukuran berdasarkan peringkat KVA transformator dan tidak berukuran berdasarkan beban sekunder ke transformator.

Over current Protection of Transformers > 600 V (NEC 450.3-A)

1) Lokasi Transformator Tanpa Pengawasan (Impedansi <6%)

Over Current Protection at Primary Side (Primary Voltage >600V):
Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next higher Standard size. or
Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage <=600V):
Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage >600V):
Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. or
Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

Full Load Current At Primary side=750000/(1.732X11000)=39A
Rating of Primary Fuse = 3X39A= 118A, So Standard Size of Fuse =125A.
OR Rating of Primary Circuit Breaker =6X39A=236A, So Standard Size of Circuit Breaker =250A.
Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
Rating of Secondary of Fuse / Circuit Breaker = 1.25X1043A=1304A, So Standard Size of Fuse =1600A.

2) Lokasi Transformator Tanpa Pengawasan (Impedansi 6% hingga 10%)

Over Current Protection at Primary Side (Primary Voltage >600V):
Rating of Pri. Fuse at Point A= 300% of Primary Full Load Current or Next higher Standard size.
Rating of Pri. Circuit Breaker at Point A= 400% of Primary Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage <=600V):
Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage >600V):
Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next higher Standard size.
Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.

Example: 10MVA, 66KV/11KV 3Phase Transformer, Impedance of Transformer is 8%

Full Load Current At Primary side=10000000/(1.732X66000)=87A
Rating of Pri. Fuse = 3X87A= 262A, So Next Standard Size of Fuse =300A.
OR Rating of Pri. Circuit Breaker =4X87A=348A, So Next Standard Size of Circuit Breaker =400A.
Full Load Current at Secondary side=10000000/ (1.732X11000) =525A.
Rating of Sec. Fuse = 2.25X525A=1181A, So Next Standard Size of Fuse =1200A.
OR Rating of Sec. Circuit Breaker =2.5X525A=1312A, So Next Standard Size of Circuit Breaker =1600A.

3) Lokasi Pengawasan (hanya di sisi Primer) Transformator:

Over Current Protection at Primary Side (Primary Voltage >600V):
Rating of Pri. Fuse at Point A= 250% of Primary Full Load Current or Next higher Standard size.
Rating of Pri. Circuit Breaker at Point A= 300% of Primary Full Load Current or Next higher Standard size.

4) Lokasi Transformator yang Diawasi (Impedansi Hingga 6%):

Over Current Protection at Primary Side (Primary Voltage >600V):
Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next Lower Standard size.
Over Current Protection at Secondary Side (Secondary Voltage <=600V):
Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage >600V):
Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next Lower Standard size.
Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current or Next Lower Standard size.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

Full Load Current At Primary side=750000/(1.732X11000)=39A
Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A.
OR Rating of Primary Circuit Breaker =6X39A=236A, So Next Lower Standard Size of Circuit Breaker =225A.
Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

5) Lokasi Pengawasan Transformator (Impedansi 6% hingga 10%):

Over Current Protection at Primary Side (Primary Voltage >600V):
Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
Rating of Pri. Circuit Breaker at Point A= 400% of Pri. Full Load Current or Next Lower Standard size.
Over Current Protection at Secondary Side (Secondary Voltage <=600V):
Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
Over Current Protection at Secondary Side (Secondary Voltage >600V):
Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next Lower Standard size.
Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next Lower Standard size.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 8%

Full Load Current At Primary side=750000/(1.732X11000)=39A
Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A.
OR Rating of Primary Circuit Breaker =4X39A=157A, So Next Lower Standard Size of Circuit Breaker =150A.
Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

Perbedaan C.B antara Lokasi Terawasi dan Tidak Terawasi

Di sini kita melihat dua kondisi penting saat kita memilih Sekring/Pemutus Sirkuit di Lokasi Terawasi dan Lokasi Tanpa Pengawasan.
Kondisi penting pertama adalah Proteksi Arus Lebih Primer. Di Lokasi Tanpa Pengawasan, Sekring di sisi Primer adalah 300% dari Arus Primer atau ukuran Standar Lebih Tinggi Berikutnya dan di Lokasi Terawasi adalah 300% dari Arus Primer atau ukuran Standar Lebih Rendah Berikutnya. Di sini, Proteksi Arus Lebih Primer sama di kedua kondisi (300%) Namun, Pemilihan Ukuran Sekring/Pemutus Sirkuit Berbeda.
Mari kita Periksa dengan Contoh untuk Trafo 3Fase 750KVA, 11KV/415V. Arus Beban Penuh di Sisi Primer = 750000/(1,732X11000) = 39A
Di Lokasi Tanpa Pengawasan: Nilai Sekring Primer = 3X39A = 118A, Jadi Ukuran Standar Berikutnya yang Lebih Tinggi = 125A
Di Lokasi dengan Pengawasan: Nilai Sekring Primer = 3X39A = 118A, Jadi Ukuran Standar Berikutnya yang Lebih Rendah = 110A
Kondisi penting kedua adalah Proteksi Arus Lebih Sekunder meningkat dari 125% menjadi 250% untuk Lokasi Tanpa Pengawasan ke Lokasi dengan Pengawasan.

Summary of over current Protection for more than 600V:

aximum Rating of Over current Protection for Transformers more than 600 Volts
LocationLimitations	TransformerRated Impedance	Primary Protection (More than 600 Volts)		Secondary Protection
		Primary Protection (More than 600 Volts)		More than 600 Volts		Less than 600 Volts
		Circuit Breaker	Fuse Rating	Circuit Breaker	Fuse Rating	C.B or Fuse
Any location	Less than 6%	600%(NH)	300%(NH)	300 %( NH)	250%(NH)	125%(NH)
Any location	6% To 10%	400%(NH)	300%(NH)	250%(NH)	225%(NH)	125%(NH)
Supervisedlocations only	Any	300%(NH)	250%(NH)	Not required	Not required	Not required
	Less than 6%	600%	300%	300%	250%	250%
	6% To 10%	400%	300%	250%	225%	250%
NH: Next Higher Standard Size.

Over current Protection of Transformers< 600 V (NEC 450.3-B)

1) Only Primary side Protection of Transformer:

Over Current Protection at Primary Side (Less than 2A):
Rating of Pri. Fuse / C.B at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
Example: 1KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=1000/(1.732X480)=1A
Rating of Primary Fuse = 3X1A= 3A, So Next Lower Standard Size of Fuse =3A.
Over Current Protection at Primary Side (2A to 9A):
Rating of Sec. Fuse / C.B at Point A= 167% of Pri. Full Load Current or Next Lower Standard size.
Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=3000/(1.732X480)=4A
Rating of Primary Fuse = 1.67X4A= 6A, So Next Lower Standard Size of Fuse =6A.
Over Current Protection at Primary Side (More than 9A):
Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Next Higher Standard size.
Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=15000/(1.732X480)=18A
Rating of Primary Fuse = 1.25X18A= 23A, So Next Higher Standard Size of Fuse =25A.

2) Primary and Secondary side Protection of Transformer:

Over Current Protection at Primary Side (Less than 2A):
Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size.
Over Current Protection at Primary Side (2A to 9A):
Rating of Sec. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size.
Over Current Protection at Primary Side (More than 9A):
Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Lower Higher Standard size.
Example: 25KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=125000/(1.732X480)=30A
Rating of Primary Fuse = 2.50X30A= 75A, So Next Lower Standard Size of Fuse =70A.
Over Current Protection at Secondary Side (Less than 9A):
Rating of Pri. Fuse / C.B at Point B= 167% of Sec. Full Load Current or Lower Standard size.
Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=3000/(1.732X230)=8A
Rating of Primary Fuse = 1.67X8A= 13A, So Next Lower Standard Size of Fuse =9A.
Over Current Protection at Secondary Side (More than 9A):
Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Higher Standard size.
Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=15000/(1.732X230)=38A
Rating of Primary Fuse = 1.25X38A= 48A, So Next Higher Standard Size of Fuse =50A.

Summary of over current Protection for Less than 600V:

Maximum Rating of Over current Protection for Transformers Less than 600 Volts
ProtectionMethod	Primary Protection			Secondary Protection
ProtectionMethod	More than 9A	2A to 9A	Less than 2A	More than 9A	Less than 9A
Primary only protection	125%(NH)	167%	300%	Not required	Not required
Primary and secondary protection	250%	250%	250%	125%(NH)	167%
NH: Next Higher Standard Size.