Remotely Monitoring Electrical Power Signals with a Single-Board RIO
Electronics Design for sbRIO Mezzanine Card Combines Custom Needs with Flexibility
Client: A designer and manufacturer of leading-edge electrical power monitoring equipment.
Smart Grid investment is growing. Two important premises for Smart Grid design are access to local power sources and an understanding of loads and disturbances on the grid at various locations. These local power sources are typically alternative, such as solar and wind, which have intermittent power levels. Since the levels fluctuate, an important feature of proper Smart Grid operation is handling these erratic supplies. Optimal understanding of these disturbances and load changes increasingly requires measurements on individual AC power cycles.
Local power analysis systems typically have constraints in equipment cost, size, and power usage balanced against the need for simultaneous sampling front-end circuitry and custom data processing algorithms on the back-end. Furthermore, many of these systems are presently deployed as prototypes or short-run productions, requiring a combination of off-the-shelf and custom-designed components.
A custom RIO Mezzanine card was designed and built for the National Instruments Single-Board RIO platform to provide access to simultaneously-sampled signals from the 3-phase and neutral lines of an AC power source. Timing synchronization between physically-separated installations was provided by monitoring GPS timing signals. Custom VIs were developed to retrieve the sampled data points and GPS timing for subsequent processing and analysis.
Figure 1 – Power Line Data Acquisition sbRIO RMC Module with GPS Timing
We needed 8 channels of simultaneously-sampled analog inputs (AI), each capable of sampling at least 50 kHz. These AI channels sample the voltage and current of the neutral and three phase power lines. Furthermore, to coordinate power and load fluctuations across many measurement locations, a world-wide synchronization signal is needed.
The Single-Board RIO (sbRIO) platform from National Instruments offers an excellent balance between off-the-shelf capability and custom design needs in a reasonably small package. The sbRIO provides the processor, memory, and connectivity while the RIO Mezzanine Card (RMC) provides the I/O and signal conditioning needs. See our white paper, Developing Embedded Systems: Comparing Off-the-Shelf to Custom Designs, for a discussion of the benefits of using this approach.
We designed the RMC for the simultaneously-sampled analog inputs and a GPS receiver. The RMC was mounted to a sbRIO-9606. Some design specifications were:
- 8 analog input channels: simultaneous sampling at 50 kHz, ±10 V range, 16-bit resolution
- GPS receiver with Pulse Per Second (PPS) timing signal with 60 ns accuracy
- SMA Connector for external GPS active antenna
- 20 position terminal block for analog inputs and shields, removable for wiring
- Operates inside an enclosure with internal conditions -40 to 55 °C temperature
An image of the designed RMC and the sbRIO-9606 is shown below. Since the A/Ds reside on the RMC, the data bytes are accessed by sbRIO FPGA VIs code communicating through an SPI data bus designed into the RMC. The internal real time clock coupled with the GPS PPS signal allowed for timing accuracy within a GPS region well under +/- 1 uS of accuracy for all data sampled no matter the location, internally or from unit to unit within feet or 1000s of miles away.
The combination of the sbRIO off-the-shelf platform and the custom RIO mezzanine card (RMC) for I/O makes a powerful, cost-effective, and yet configurable solution for measurements of AC power signals. With the GPS component on the RMC, measurement units can be placed at dispersed locations while still providing adequate synchronization of acquired waveforms for localizing and understanding disturbances in power transmission and distribution, irrespective of any specific application. If you have an embedded monitoring application that you’d like help with, you can reach out to chat here. If you’d like to learn more about our circuit board design capabilities, go here.