4.2 Sensing of Transformer Primary and Secondary Currents

Figure 4-3. Sensing of Transformer Primary and Secondary Currents

The primary and secondary currents of the main power transformer are sensed using current transformers (CT). As these currents are AC, bridge rectification on the secondary side of the CTs is used. Burden resistors are used to adjust the voltages to levels appropriate for the dynamic ranges of the dsPIC33C ADCs and comparators (0V to 3.3V). At present, these signals are used only for overcurrent protection.

The primary current sensor has a sensitivity of 76mV/A. The secondary current sensor has a sensitivity of 60mV/A.

The CTs must have following characteristics:

Measure currents up to 40A peak, positive and negative at the primary and 50A peak at the secondary.
4kV reinforced galvanic isolation
Output voltage between 0 and 3.3V (this is the input voltage range of dsPIC33C ADCs and comparators).
Signal bandwidth 500kHz
Small signal propagation delay <200ns