STMicroelectronics Releases an Evaluation Board For 3-Phase Energy Metering

The new evaluation board (EVB) from STMicroelectronics (ST) enables faster design of 3-phase AC watt meters. Galvanic-isolation technology and electromagnetic-immune shunt sensors assure superior robustness and reliability.

 

The EVALSTPM-3PHISO evaluation board. Image courtesy of ST

 

The EVALSTPM-3PHISO provides data, on both a per-phase and cumulative basis, for active wideband, active fundamental, reactive and apparent power as well as for energy. For each voltage and current signal, RMS and optional distortion (THD) calculations are performed. And for each phase, DC measurements, line period, phase shift, and phase-voltage delay calculation are also provided.

 

What is Galvanic Isolation?

Galvanic isolation is the concept of “isolating” two parts of an electrical system. It prevents current flow from one section to another, but at the same time allows information and or energy to nonetheless be exchanged.

 

A Transformer provides galvanic isolation. Image courtesy of All About Circuits

 

In the example above, a transformer is used, in this case to transfer energy, but notice that thanks to galvanic isolation, the two sides of the circuits do not share a common ground.

 

The STPMS2

One of the keys to the new evaluation board is the STPMS2, a two channel, 24-bit second order sigma-delta modulator. It serves to measure the voltage and current for each phase through a voltage divider and a shunt current sensor.

The STPMS2 oversamples the signal by means of a 4 MHz clock provided by an STM32 microcontroller. The voltage and current are transmitted as sigma-delta bitstreams, with 6 kV of isolation provided by an STISO621W. This galvanic isolator chip allows for a 100 Mbps data rate.

The unit converts the six bitstreams (voltage and current for each of the three phases) into metrology data every 200µs. A virtual com port is implemented through on-board firmware, enabling the modification of the board's internal configuration and for accessing internal parameters for reading metrology data.

 

The STISO621

The other star of the new EVB is the STISO621. This dual-channel digital isolator is the first member in a family of IC’s employing the company’s 6kV thick-oxide galvanic-isolation technology. The new series will be applied to a range of industrial applications requiring the transfer of data between isolated domains.

The STISO621 features two independent channels (receive and transmit) with Schmitt-trigger inputs that keep pulse distortion below 3ns and ensure high noise immunity. 

 

The chip features:

• Maximum data rate of 100Mbit/s
• 6000V impulse withstand voltage (VIOTM)
• 1200V maximum repeating isolation voltage (VIORM)

 

STISO621 delivers faster data transmission, longer lifetime, and higher reliability than the optical isolators often used to achieve optical isolation.

 

Beyond Utility Metering

The EVALSTPM-3PHISO EVB can be also used as a starting point for other multi-phase industrial metering applications. These can include:

• Electric-vehicle (EV) charging
• Solar inverters
• Servers 

 

Regulatory Considerations

The EVALSTPM-3PHISO meets the following standards for AC watt meters:

• EN 50470-x
• IEC 62053-2x
• ANSI12.2x 

As per IEC 62053-22, the EVB reaches accuracy Class 0.5 on 3-phase active/apparent power measurement.

For 3-phase reactive power measurement, accuracy class 1 is achieved as detailed by IEC 62053-21.