Xtreme Power Deploys a Grid-Tied Energy Storage System Controlled Using NI RIO Hardware
Figure 1: The inverter cabinet has an embedded NI Single-Board RIO that performs local control and communicates to a central PXI system via a custom fiber-optic network.
"The openness of the FPGA interface helped us create this custom high-performance real-time communications link, enabling distributed real-time closed loop control."
- Richard Jennings,
Creating a scalable control system to manage megawatt-sized energy storage and provide digital power management systems.
Using a centralized NI PXI system with R Series reconfigurable I/O (RIO) hardware and distributed NI Single-Board RIO devices to continuously measure the power and power quality of the energy grid and control a network of power inverters and battery cells to manage the energy flow.
Richard Jennings - Xtreme Power
At Xtreme Power (XP), we design, engineer, manufacture, and operate integrated energy storage and power management systems called Dynamic Power Resources (DPR), for independent power producers, transmission and distribution utilities, and commercial and industrial end users. The DPR systems can time-shift power, as well as use fast-responding and configurable digital systems to simultaneously perform many ancillary services demanded by the energy market including VAR compensation, firm day-ahead schedules, frequency response, and ramp control/smoothing.
A DPR consists of safe and efficient XP PowerCells combined with high-performance power electronics and a configurable control system, with each component sized to address each customer’s individual power and energy needs. We integrate all key components into a large-scale, utility-ready system that operates with a customer’s existing or planned infrastructure.
To understand the role of a DPR, consider the details of the electrical grid. At any given moment, the electricity entering the grid (referred to as supply or generation) must be equal to the electricity exiting the grid (referred to as demand or load). Historically, this was managed by controlling a few large, centralized power generation locations. Each of these locations could regulate power output to balance supply and demand on the grid.
As new technologies grow in popularity, including utility-scale wind power and solar photovoltaic (PV), the electric grid equation (supply=demand in real time) gets complicated. Renewable energy sources such as wind and solar introduce generation variability and can often generate power during off-peak times, or times when the demand for energy is low. Energy storage systems can be incorporated to act as a buffer between supply and demand to maximize the potential of renewable energy sources and ensure the seamless delivery of electricity.
Additionally, an energy storage system can quickly respond to grid changes to help mitigate instabilities caused by harmonics or inductive loads and can provide fast responses to imbalances in demand and generation. These technical support roles are currently only provided by fossil fuel generation and are collectively referred to as ancillary services. Without these services, the grid would be much less reliable, and their importance is shown through higher market values. The DPR can be installed to provide these lucrative services with faster responses than traditional generation and fewer emissions.
To take advantage of the potential of a megawatt/megawatt hour-sized sized energy storage system, we needed to create a flexible, fast control system. The system had a variety of technical requirements including the following:
- Accurate high-speed measurements of three-phase voltage and current information from the power grid
- Advanced algorithms for automated generator control including high-speed, synchronized responses between multiple inverter/battery subsystems
- Scalability from 500 kW to multimegawatt
- Remote data access for system diagnostics and management from across the globe
In addition to needing to meet these technical requirements, we are an innovative, small, private company who had to develop and deploy our control system in a short period of time with a small staff of engineers.
To accomplish these goals, we designed a distributed energy storage system with a centralized measurement unit containing a master controller and multiple distributed inverter/battery stacks with a remote controller node. The master controller was implemented using an NI PXI controller with multiple R Series field-programmable gate arrays (FPGAs) and expansion C Series modules. This PXI system measures the power on the grid, runs algorithms to determine the required power flow in or out of the battery stack, and sends commands to the distributed nodes. It also passes operational data to a colocated server that logs the data in an SQL database and makes it available both locally and remotely via a web server. The PXI system sends control commands and exchanges real-time data with the distributed inverter/battery stacks.
Through the addition of more inverter/battery systems, we can scale installations from 500 kW to multiple megawatts. The control system mirrors this scalability and each inverter has an embedded NI Single-Board RIO controller. This NI Single-Board RIO controller communicates with the PXI system via both Ethernet and a custom fiber-optic connection. Ethernet user datagram protocol (UDP) is used for bulk data command and the custom fiber-optic connection is used for time-critical direct communication between an FPGA R Series module in the PXI chassis and the FPGA on each NI Single-Board RIO controller. The openness of the FPGA interface helped us create this custom high-performance real-time communications link, enabling distributed real-time closed loop control.
Each NI Single-Board RIO interfaces to a full four-quadrant inverter capable of simultaneously delivering real (watts) and reactive power (volt ampere reactive, or "VARs"), which enables the DPR to provide multiple services at the same time. These power electronics can stay active through grid interruptions, enabling low-voltage and zero-voltage ride through. The inverter registers commands in less than 15 µs with command response in less than 1 ms. In response to a control signal, the DPR can adjust from a fully rated charge (+MVA) to a fully rated discharge (-MVA) in less than a second. The NI Single-Board RIO also integrates with a battery health system that monitors the charge state of each battery and runs distributed protection algorithms to manage the battery cells.
Thanks to NI hardware and LabVIEW system design software, we were able to use a single integrated development environment for everything from FPGA and real-time targets to user interface and diagnostic PCs. The NI graphical system design approach helped us focus on our application instead of getting bogged down in low-level syntax and implementation details. We used the highly productive tools and the ability to rapidly prototype and iterate on our design to deploy sophisticated, reliable systems with a software investment of only two man-years. We estimate it would have taken a team of ANSI C programmers 10 years or more to do what we did. We used LabVIEW and NI embedded hardware to deliver a robust system to fill a critical need facing green energy today.
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