Office of Naval Research
The goal of this initiative is to investigate the specific energy delivery technologies for nonlinear dynamic loads/systems and investigate the integration and interface issues of these systems on the ship Integrated Power System (IPS), which will be accomplished through modeling and system simulations and scaled prototype testing. Through these efforts the capability and security of various system topologies and control schemes to operate the various nonlinear dynamic loads will be assessed. The results will provide the Navy and its ship builders with vital information that can be utilized to design and de-risk deployable ship IPS and nonlinear dynamic loads.
The U.S. Navy is developing the Next Generation Integrated Power System (NGIPS) warship that will be based on an all-electric platform of propulsion loads and electric power systems with rapid reconfigurable distribution systems. One motivation for NGIPS is to enable new nonlinear dynamic loads. On demand delivery of the large amounts of energy needed to operate these nonlinear dynamic loads presents challenging technical issues that must be addressed prior to implementing a combat ready system. These include the appropriate topology for the ship electric distribution system for rapid reconfiguration to battle readiness and the energy supply technology for the nonlinear dynamic systems.
One of the early rationales for the electric ship was the concept that the integrated power system (IPS), merging of propulsion and auxiliary systems into a common electric system, would make available a large reservoir of power that in need could be diverted to nonlinear dynamic loads as well as other energy consumers that require large amounts of power for short durations. Storage of electric energy is both space and weight intensive. However, the basic nature of large electric systems provides for storage. The rotational inertia within the generators (both the electric generator and the prime mover) and in the loads (particularly motors) provides a significant reservoir of energy that can be extracted for short durations of time. Moreover, a large number of small to medium sized energy storage devices are distributed throughout the ship already in the form of uninterruptible power supplies, local storage for power quality enhancements, local capacitive storage in power electronic converters, etc. In the case of the ship, there is one more available reservoir of stored energy: the forward motion of the ship. The inertia of a 30,000 ton ship moving at 20 knots represents a large reservoir of stored energy. The electric propulsion system, via regeneration, provides a mechanism to utilize this stored energy. The electric ship with an IPS provides an opportunity to consider nonlinear dynamic load, to improve performance and to reduce or eliminate high maintenance systems such as steam or compressed air launch systems that are difficult to control. The full advantage of these systems can be achieved only when these nonlinear dynamic loads are fully integrated with the design of the overall ship electric power systems.
The power electronics testbed was designed to create a scale model of a shipboard power system for student learning and research. The IEEE 1709-2010 Standard outlines recommendations for 1kV – 35kV MVDC power systems aboard ships. The power electronics racks are used to model each of two Main Turbine Generator (MTG) sets described in IEEE 1709-2010 along with the AC/DC converter to provide 3-wire DC for power distribution. Back to back, two-level, three phase converters provide a dynamic AC source to the three-level Neutral Point Clamped (NPC) rectifier to model terminal characteristics of the MTGs feeding the AC/DC converters.
The testbed converters are setup in a modular fashion so multiple experiments can be made using the same hardware. The two-level three phase inverter (INV) in each rack can also be used as three single phase inverters in parallel, or as DC-DC buck converters. The two-level three phase active front end (AFE) can also be used in single phase configuration or as a DC-DC boost converter. Each converter’s DC bus bar can be reconfigured for a split DC link capacitance and connection for measuring internal DC bus currents for different control methods.
Along with converter reconfiguration capabilities, the converters can be connected together in several ways. The active rectifier and inverter pairs can be used as a motor drive set for studying active power factor correction on the rectifier side as well as motor control on the inverter side. The NPC rectifier can also be easily rewired to connect to the two-level rectifier to become a three-level inverter for motor control or as a general AC distribution inverter to provide 60Hz AC power to a distribution zone.
Control of each two-level active rectifier and inverter pair is achieved by one DSP and the NPC rectifier on a second DSP. Each of the four total DSPs (two in each rack) can share information over CAN bus for studying coordinated control of the converters. The AFE/INV DSP is also configured for SPI communication to extend the computational power of the controller to external processing. All of the DSPs have SCI connections which can communicate over the RS232 protocol. Each DSP is connected to an interface board that houses signal conditioning for 16 analog to digital converters (ADC) and 4 digital to analog converters (DAC), digital input and output (DIO), and gate pulse outputs for 6 IGBT half-bridges.
The lab is also equipped with multiple real time simulators. RTDS and dSPACE systems allow for real time simulation of power systems along with analog and digital I/O in order to interface with hardware components in the lab. This allows the lab converters to be tested as if they were connected to a shipboard power system through Hardware-in-the-Loop (HIL) experiments. For example, the lab’s 2.4kW DC electronic load can be programmed for pulsed load operation, triggered in a real time power system simulation environment, to study the effects of these types of loads on the MTG and AC/DC rectifier dynamics.
National Science Foundation - Engineering Research Center - FREEDM
The main objective of this research effort is to study the nonlinear behavior of power electronic devices that are used in the FREEDM system. The FREEDM system seeks to take advantage of power electronics, controls, energy storage devices and renewable energy sources to enable a smart grid. With such a novel system, comes many uninvestigated operational scenarios due to nonlinearities that may potentially lead to undesirable or poor system performance, which are the antithesis of the FREEDM goal.
The goal is to build up an understanding of the various nonlinear behavior and complexity issues for power electronics (PE) dominated FREEDM systems. Time-scale differences between PE components and conventional grid-connected devices may lead to undesirable interactions. The development of component-level and system-level models, coupled with the fast speed of PE components, will facilitate tools necessary to embed intelligence at the component level and implement fast control actions to mitigate or thwart developing undesired non-linear and/or emergent phenomena.