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Consolidated Utility Base Energy (CUBE)

National Renewable Energy Laboratory

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Technology Marketing Summary

In the area of power electronics control, Field Programmable Gate Arrays (FPGAs) have the capability to outperform Digital Signal Processors (DSPs) through their ability to implement true parallel processing and to facilitate higher switching frequencies, control bandwidth, and enhanced functionality. However, their use for energy applications is often limited due to the relative difficulty programming complex algorithms using Hardware Description Language (the native programming language for FPGAs) as well as the finite amount of FPGA resources. Tools which allow development of FPGA code using higher level graphical programming environments are gaining popularity. One of these tools is National Instruments (NI) LabVIEW. Researchers at NREL have leveraged the NI platform to develop a complete suite of software for control of a hybrid power system. 

Description

The referenced software was developed for NREL’s Consolidated Utility Base Energy (CUBE) system. The CUBE is an integrated power electronic platform for a photovoltaic (PV)-battery-diesel hybrid power system developed for the U.S. Army to reduce use of diesel fuel at Forward Operating Bases (FOBs). The CUBE is based on modular power-electronic building blocks and includes power distribution and protection components, an isolation transformer, magnetics and other filter components, a liquid-cooling system, a control platform based on field-programmable gate array (FPGA) and real-time controllers, and a touchscreen user interface. Software was developed to control the various power converters required to interface the PV, battery, and external grid sources onto a common AC bus. Higher level software was also developed to determine when and how to operate each of the sources and to facilitate transitions between different operating modes of the CUBE.  

NREL’s CUBE has been demonstrated to reduce fuel use by 31% and diesel run time by 42% relative to diesel-only cases while maintaining reliable, high-quality power. Furthermore, replacing the line-frequency transformer with a DC-DC transformer (DCX) and adding a fourth leg to the inverter and a flexible grid port reduces the size and weight of the CUBE, provides the ability to balance voltage under unbalanced load conditions, and provides connection to host-nation grids of varying voltage. The CUBE is also suitable for off-grid applications, such as disaster recovery.

The corresponding software records include code developed in both LabVIEW FPGA and LabVIEW RT.  All referenced FPGA code software fits on the Xilinx Virtex V LX110 FPGA embedded in the NI cRIO-9118 FPGA chassis, and with a 40 MHz base clock, supports a modulation update rate of 40 MHz, user-settable switching frequencies and synchronized control loop update rates of tens of kHz, and reference waveform generation, including Phase Lock Loop (PLL), update rate of 100 kHz.

Three-Phase Inverter LabVIEW FPGA Control Code

This software implements complete control algorithms in LabVIEW FPGA for a three-phase inverter.  The software includes Space Vector Modulated (SVM) gate-drive algorithms, feedback control loops allowing operation in AC current, AC voltage, or DC bus voltage control modes, advanced alarm handling capabilities, contactor control, debugging and tuning tools, and the ability to synchronize its AC output to the grid or other voltage-sources before connection. The inverter can operate as a stand-alone voltage source, connected to the grid, or in parallel with other controllable voltage sources as part of a microgrid or remote power system.

PV DC-DC Converter LabVIEW FPGA Control

This software implements complete control algorithms in LabVIEW FPGA for three DC-DC converters to connect three separate PV arrays to a common DC bus. The software includes pulse-width modulated (PWM) gate-drive algorithms, feedback control loops allowing operation in PV current, PV voltage, or DC bus voltage control modes, Maximum Power Point Tracking (MPPT) with power curtailment capability advanced alarm handling capabilities, contactor control, and debugging and tuning tools.  The software includes provisions to allow converter operation in parallel with other voltage regulating devices on the DC bus.

Battery and PV DC-DC Converter LabVIEW FPGA Control Code

This software builds on the PV DC-DC Converter LabVIEW FPGA Control code to connect a battery and a PV array to a common DC bus In addition to the PV DC-DC converter functionality, the battery converter feedback control loops allow operation in battery current, battery voltage, or DC bus voltage control modes. 

Three-phase Four-leg Inverter LabVIEW FPGA Control Code

This software implements complete control algorithms in LabVIEW FPGA for a three-phase four-leg inverter. The addition of a fourth-leg allows the inverter to serve single-phase loads in a microgrid or stand-alone power system and to balance the three-phase voltages in the presence of significant load imbalance. The software includes three-dimensional space vector modulation gate-drive algorithms, feedback control loops, advanced alarm handling capabilities, contactor control, and debugging and tuning tools. The inverter can operate as a stand-alone voltage source, connected to the grid, or in parallel with other controllable voltage sources as part of a microgrid or remote power system. In addition, as the inverter is expected to operate under severe unbalanced conditions, the software includes algorithms to accurately compute real and reactive power for each phase based on definitions provided in the IEEE Standard 1459:  IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions.

Dual Active Bridge based DC Transformer LabVIEW FPGA Control Code

This software implements complete control algorithms in LabVIEW FPGA for a DC Transformer (DCX) based on a dual active bridge (DAB).  A DCX is an isolated bi-directional DC-DC converter designed to operate at unity conversion ratio, M, defined by M = Vout/nVin where Vin is the primary-side DC bus voltage, Vout is the secondary-side DC bus voltage, and n is the turns ratio of the embedded high frequency transformer (HFX). The DCX based on a DAB incorporates two H-bridges, a resonant inductor, and an HFX to provide this functionality. In addition, the candidate software employs phase-shift modulation of the two H-bridges, a feedback loop to regulate the conversion ratio at unity, alarm-handling capabilities, and debugging and tuning tools.

Hybrid Power System Supervisory Control

This Supervisory Controller communicates with the local controllers for diesel generator(s), battery bank(s), and PV array(s) through control dispatch algorithms, which determine when to start and stop individual components in order to meet load requirements while minimizing fossil fuel use. The supervisory controller also incorporates mode handling algorithms to enable smooth transitions between different system operating modes, including the transitions between CUBE vs. external source as grid-forming device.  . The Supervisory Control code is written in LabVIEW RT.

Benefits
  • Reduces fuel use and diesel run time
  • Includes high-frequency transformer isolation
  • Balances voltage under unbalanced load conditions
  • Provides connection to host-nation grids of varying voltage, frequency, and power quality
  • Implements complete control algorithms in LabVIEW FPGA
Applications and Industries
  • Power electronics
  • Off-grid applications, such as disaster recovery
More Information

For further information, please read these NREL publications:

Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL SWRs 12-12, 13-11, 15-18, 15-19, 16-26, 16-25DevelopmentAvailable01/10/201701/10/2017

Contact NREL About This Technology

To: Jean Wheasler<Jean.Wheasler@nrel.gov>