2.1.2 Current Network Adjustment

The PIC32CXMTSH-DB board includes two differential current inputs for two phases (A and B), and the PIC32CXMTC-DB board has four differential current inputs for three phases (A, B and C) and neutral current (N).

The hardware of the boards is configured by default to connect Current Transformers, with the same burden resistor value for all the phases (3.24 ohm). It is possible to re-configure the hardware to support other current sensor types, as it is explained in the Microchip PIC32CXMTSH-DB HW User Guide and in the Microchip PIC32CXMTC-DB HW User Guide.

The default CT sensor has a 2000:1 turn ratio. This default configuration allows currents up to 220A (Imax) to be applied. The maximum input current supported can be calculated according to the following equation:

Where:

  • . GPGAx is the gain setting of the programmable gain amplifier of the current channels.
  • Vpk is the peak voltage at the current channels metrology inputs of the metering unit when GPGAx = 1. Its typical maximum value is 0.5V.

The KIx values are configured into the metrology library by means of the control registers, K_Ix. Refer to the Microchip PIC32CXMTx Metrology User Guide, which includes configuration examples for current transformers, Rogowski coils, and shunt resistors. The K_Ix values are entered in the Demo Meter application using the terminal command DCW. The GPGAx values are configured into the metrology library by means of the control registers, ATSENSE ADC Control. If GPGAx values are changed, the systems needs to be reset for the AFE to apply the new values. Refer to 2.5 Demo Meter Application Commands for additional information.

Depending on the maximum current to be applied, the user can adjust the system by changing the current transformer ratio, the burden resistors value or the programmable gain amplifiers. This a highly flexible system allowing different configurations.

Tip: The kits offer a dynamic range of 3000:1. Outside of the dynamic range, the accuracy decreases. It is important to take advantage of the full dynamic range, especially in the case of the current inputs, having large variations. To adjust the dynamic range, it is recommended that the voltage swing across the converter input be close to the recommended maximum value when the maximum rated current is applied. Usually, a security threshold (for instance 20%) is considered.