Selection of MCCB for primary side of transformer

Magnetizing inrush current of transformer

When power is turn on to a transformer, significantly largemagnetizing current may flow into the transformer. The magnetizing current may have a peak value of 10 times or more the rated current and may cause malfunction of MCCB, and the transformer circuit may not be closed. This current is called magnetizing inrush current. 

The magnetizing inrush current varies depending on at which circuit voltage the transformer has been turned on and in which state the core residual magnetic flux was. The magnetizing inrush current is maximized when the transformer is turned on at point P in Fig. 5. 6. The magnetic flux changes by 2_ᴓm in 1/2 cycle after the transformer is turned on. Since the magnetic flux starting point is the residual magnetic flux fr in the center of the core before the transformer is turned on, the magnetic flux will be 2_ᴓm + fr after 1/2 cycle and considerably exceed the saturated magnetic flux of the core, and, as the result of this, large magnetizing current will flow. This magnetizing inrush current attenuates with time. There is a tendency that the higher the transformer capacity, the larger the attenuation time constant. Table 5. 9 shows the approximate values of magnetizing inrush current. The values shown in Table 5. 9 are larger than the actual magnetizing inrush current values because the values were determined not in consideration of current limiting due to electric circuit impedance. If the value is unknown, the value in Table 5. 9 should be used. It is recommended to refer to the transformer manufacturer for details.

Notes 

1.  Multiple: The first peak value of magnetizing inrush current for rated current peak value.
2. Since the magnitude of magnetizing inrush current considerably depends on the applied voltage, making phase and residual magnetic flux of core, normally, the magnetizing inrush current changes every time a transformer is turned on. The above table shows the maximum values. Note that the magnetizing inrush current caused when the rated voltage is applied to the rated tap may be larger if overvoltage is applied.

Selection of MCCB for primary side of transformer

The magnetizing inrush current stated in 5. 4. 1 attenuates with time, and, lastly, only the magnetizing current flows. However, the instantaneous trip of MCCB reacts to transient current. Therefore, it is necessary to select MCCB having sufficiently higher instantaneous tripping current than the magnetizing inrush current of transformer. Thermal magnetic MCCB are more suitable than hydraulic magnetic MCCB because thermal magnetic MCCB with high magnetic tripping current can be manufactured easier. Example of selection of MCCB for primary side of 3-phase 420V 50 kVA

The rated current I (RMS value) can be obtained as shown below.

The magnetizing inrush current peak value I_ᴓ is 22 times the rated current peak value.

Accordingly, MCCB having an instantaneous tripping current peak value of 2137A or more should be selected.

select the model which respective to the instantaneous tripping current peak value. 

Follow below instructions to know the instantaneous tripping current peak value of model at 150A.

Therefore this model meet the requirement.