Abstract of over current Protection of Transformer (NEC 450.3)

ABSTRACT OF OVER CURRENT PROTECTION OF TRANSFORMER (NEC 450.3)

INTRODUCTION:

• The over current protection required for transformers is consider for Protection of Transformer only.Such over current protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer. * When voltage is switched on to energize a transformer, the transformer core normally saturates. This results in a large inrush current which is greatest during the first half cycle (approximately0.01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current. * To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for 0.1 second and 25 times for 0.01 second. Some small dry-type transformers may have substantially greater inrush currents. * To avoid using over sized conductors, over current devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller over current protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially. * Protection of secondary conductors has to be provided completely separately from any primary-side protection. * A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation. * Over current protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead. * It is important to note that the over current device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer

OVER CURRENT PROTECTION OF TRANSFORMERS > 600 V (NEC 450.3-A)

 1) UNSUPERVISED LOCATION OF TRANSFORMER (IMPEDANCE <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) UNSUPERVISED LOCATION OF TRANSFORMER (IMPEDANCE 6% TO 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) SUPERVISE…