- Calculate Size of Circuit Breaker or Fuse on Primary and Secondary side of Transformer having following Detail
- Transformer Details(P)= 1000KVA
- Primary Voltage (Vp)= 11000 Volt
- Secondary Voltage (Vs)= 430 Volt
- Transformer Impedance= 5%
- Transformer Connection = Delta / Star
- Transformer is in unsupervised condition.
Calculations:
- Transformer Primary Current (Ip)= P/1.732xVp
- Transformer Primary Current (Ip)=1000000/1.732×11000=49Amp
- Transformer Secondary Current (Is)= P/1.732xVs
- Transformer Secondary Current (Is)=1000000/1.732×430=71Amp
- AS per NEC 450.3, Max.Rating of C.B or Fuse is following % of its Current according to it’s Primary Voltage,% Impedance and Supervised/Unsupervised Condition.
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Max Rating of Over current Protection for Unsupervised Transformer More than 600 Volts (As per NEC) |
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| %Imp | Primary | secondary | |||
| >600Volt | >600Volt | <600Volt | |||
| C.B | Fuse | C.B | Fuse | C.B/Fuse | |
| Up to 6% | 600% | 300% | 300% | 250% | 125% |
| More than 6% | 400% | 300% | 250% | 225% | 125% |
|
Max Rating of Over current Protection for Supervised Transformer More than 600 Volts (As per NEC) |
|||||
| %Imp | Primary | secondary | |||
| >600Volt | >600Volt | <600Volt | |||
| C.B | Fuse | C.B | Fuse | C.B/Fuse | |
| Up to 6% | 600% | 300% | 300% | 250% | 250% |
| More than 6% | 400% | 300% | 250% | 225% | 250% |
|
Max Rating of Over current Protection for Transformers Primary Voltage Less than 600 Volts (As per NEC) |
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| Protection | Primary Protection | Secondary Protection | |||
| Method | More than 9A | 2A to 9A | Less than 2A | More than 9A | Less than 9A |
| Primary only protection | 125% | 167% | 300% | Not required | Not required |
| Primary and secondary protection | 250% | 250% | 250% | 125% | 167% |
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Size of Fuse / Inverse Time C.B as per NEC (Amp) |
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| 1 | 25 | 60 | 125 | 250 | 600 | 2000 |
| 3 | 30 | 70 | 150 | 300 | 700 | 2500 |
| 6 | 35 | 80 | 160 | 350 | 800 | 3000 |
| 10 | 40 | 90 | 175 | 400 | 1000 | 4000 |
| 15 | 45 | 100 | 200 | 450 | 1200 | 5000 |
| 20 | 50 | 110 | 225 | 500 | 1600 | 6000 |
For Primary Side:
- Transformer Primary Current (Ip) =52.49Amp and impedance is 5%
- As per above table in not supervised condition Size of Circuit Breaker= 600% of Primary Current
- Size of Circuit Breaker = 52.49 x 600% =315Amp
- If Transformer is in supervised condition then Select Circuit Breaker near that size but if Transformer is in unsupervised condition then Select Circuit Breaker next higher size.
- Rating of Circuit Breaker =350Amp (Next Higher Size of 300Amp)
- Size of Fuse = 52.49 x300% =157Amp
- Rating of Fuse =160Amp (Next Higher Size of 150Amp)
For Secondary Side:
- Transformer Secondary Current (Is) =1342.70Amp and impedance is 5%
- As per above table in not supervised condition Size of Circuit Breaker= 125% of Secondary Current
- Size of Circuit Breaker = 1342.70 x 125% =1678Amp
- If Transformer is in supervised condition then Select Circuit Breaker near that size but if Transformer is in unsupervised condition then Select Circuit Breaker next higher size.
- Rating of Circuit Breaker =2000Amp (Next Higher Size of 1600Amp)
- Size of Fuse = 1342.70 x125% =1678Amp
- Rating of Fuse =2000Amp (Next Higher Size of 1600Amp)
Results:
- Size of Circuit Breaker on Primary Side=350Amp
- Size of Fuse on Primary Side=160Amp
- Size of Circuit Breaker on Secondary Side=2000Amp
- Size of Fuse on Secondary Side=2000Amp
April 01, 2019 at 08:00AM by Department of EEE, ADBU: https://ift.tt/2AyIRVT
