Dionne Soto

Multifunctional megawatt scale medium voltage DC test bed based on modular multilevel converter (MMC) technology

The recent development of modular multilevel converters (MMC) provides new opportunities for medium voltage DC (MVDC) systems for all electric ship design and offshore wind parks. Therefore, the Center for Advanced Power Systems at Florida State University has recently commissioned a new MVDC power-hardware-in-the-loop laboratory rated at 5 MW at DC voltages between 6...24 kV. The new lab features four individual MMCs, each composed of 36 full-bridge cells, and capable of delivering 210 A at 0...6 kV. This paper describes the entire system in detail, including the advanced current and voltage control concepts along with the state of the art digital control hardware. Selected commissioning results demonstrate the performance of the system under dynamic conditions and provide comparison with simulations obtained from a corresponding controller hardware-in-the-loop setup which is also described in the paper.

Experimental verification of limiting fault currents in MVDC systems by using modular multilevel converters

Medium voltage direct current (MVDC) system poses an attractive technology for the development of high power all electric ships in the future. The Modular Multilevel Converter (MMC) is a potential candidate to be used as an AC/DC interface in fault current limited MVDC systems. A Controller Hardware in the Loop (CHIL) setup and the experimental setup have been designed for the initial verification of the fault current limiting function of MMC converters. A novel MMC averaged value model (AVM) is proposed in this paper for the purpose of emulating the DC side dynamics. The experimental results show that the MMC is able to limit the fault current under the investigated scenarios of load steps and arc faults causing an over current. However, the AVM requires additional features which are to be implemented in future works to better reproduce the observed behavior.

Multifunctional Megawatt-Scale Medium Voltage DC Test Bed Based on Modular Multilevel Converter Technology

Recent developments in modular multilevel converters (MMCs) provide new opportunities for medium-voltage dc (MVDC) systems for all-electric ship design and offshore wind farms. The Center for Advanced Power Systems at Florida State University has recently commissioned a new MVDC power hardware-in-the-loop laboratory rated at 5 MW at dc voltages from 6 to 24 kV. The new lab features four individual MMCs, each composed of 36 full-bridge cells, capable of delivering 210 A at any voltage in the range of 0–6 kV. This paper describes the entire system in detail, including the advanced current and voltage control concepts along with the state-of-the-art digital control hardware. Selected commissioning results are shown, which demonstrate the performance of the system under dynamic conditions and provide comparison with simulations obtained from a corresponding controller hardware-in-the-loop setup. The results indicate that an MMC-based MVDC system is a strong candidate for the ship power system because of its excellent fault management capability. The setup can be used for the understanding and design of fast fault management schemes in a breakerless MVDC system in the future all-electric ship.

Comparative study of intensive pulse load impact on active and passive rectification system in MVDC ship power generation unit

The rectifier plays a key role in connecting generation units to a medium voltage direct current (MVDC) bus on all electric ship power systems, where the rectification systems provide power continuation warranty including power quality criteria in order to meet standard requirements. Active rectifiers are able to present distinguished benefits over passive rectifiers such as: regulation, dynamic response to load change, output voltage ripple, input current total harmonic distortion (THD), power factor correction, and wave factor. On the other hand, passive rectifiers can demonstrate simplicity. They require fewer components resulting in a lower cost, reduced maintenance and installation effort. This paper purveys a thorough quantified and qualified analysis of neutral-point-clamped (NPC) and three-phase diode rectifiers as active and passive rectification modules, respectively, when they supply intensive pulse loads. The three-phase diode rectifier without line filter inductor and with output filter capacitor is directly connected to a synchronous generator including an automatic voltage regulator (AVR) that regulates the generator and the rectifier output voltage in the first two scenarios, respectively. In the third scenario, the NPC rectifier that is able to regulate the output voltage independent of input voltage alterations is demonstrated by a three-phase source with variable output amplitude. All three cases will be simulated thoroughly to validate the steady-state and transient performance of these test scenarios, while in each case the rectifiers output supplies an intensive pulse load.

Short circuit output protection of MMC in voltage source control mode

The MMC operated in Current Source Mode (CSM) with branch current control shows excellent fault limiting capability. With proper design, the same capability is achieved in Voltage Source Mode (VSM). The key is combining the VSM and CSM controls and having same state variables. Seamless transition between CSM and VSM is ensured by the change of references triggered either by the user or by internal limiters. Experimental results in the full scale 1.25 MW demonstrator proved that the MMCs can limit short circuit currents without turning off even when its shorted at its terminals.

MW-scale power hardware-in-the-loop experiments of rapid power transfers in MVDC naval shipboard power systems

A Medium Voltage DC (MVDC), modular multilevel converter (MMC)-type, MW-scale, power hardware-in-the-loop (PHIL) advanced test facility is being utilized to better understand the impact of fast power transfer between dynamic loads in a notional MVDC shipboard system. In these experiments PHIL simulations demonstrate the transfer of power between two MW-scale, MMC-based loads in a time frame of milliseconds in a 5 kV DC system. The current work also includes the first use of an AC variable voltage source (VVS) as a power source for an MMC. PHIL and controller hardware-in-the-loop (CHIL) test results include a rest-of-system (ROS) simulation of a notional set of ship components. Results will show a distinct aspect to MVDC systems, tolerance to AC side excursions while maintaining the DC bus. Ramp rates that far exceed MIL-STD-1399 are demonstrated in an MMC-based MVDC system architecture. Comparison of CHIL and PHIL results are also presented.

Mitigation of PHEV charging impact on transformers via a PV-APF harmonic compensation technique: Application to V2G integration

This paper addresses the impacts on distribution transformers due to increasing PHEV loads on the distribution infrastructure, while conducting PV compensation (harmonics compensation). The authors have addressed the utilization of a rapid-prototyping control methodology coupled with a real-time digital simulator to facilitate the integration of a Photovoltaic (PV) inverter with the grid. Multi-string PV modules with a centralized inverter are considered, and the PV inverter is controlled to function as an Active Power Filter (APF). The harmonics on the grid-side are mitigated by the PV inverter and harmonic extraction is done via a novel time-domain technique. All simulations are conducted utilizing PSCAD, and the transformer losses are calculated using MatLab/Simulink. The losses (core-losses and magnetizing current losses), aging, and temperature effects on the distribution transformers due to the increasing PHEV loads, and the utilization of PV compensation to mitigate the impact, are the focus of this work.

A new class of high speed disconnect switch based on piezoelectric actuators

With the adoption of power electronic converters in shipboard power systems and associated novel fault management concepts, the ability to isolate electric faults quickly from the power system is becoming more important than breaking high magnitude fault currents and the corresponding arcing between opening contacts within a switch. This allows for the design of substantially faster, as well as potentially lighter and more compact, mechanical disconnect switches. Herein, we are proposing a new class of mechanical disconnect switches that utilize piezoelectric actuators to isolate within less than one millisecond. This technology may become a key enabler for future all-electric ships.

Power balancing in multi-converter systems composed of modular multilevel converters (MMCs)

Multi-converter systems composed of series or parallel connected modular multilevel converters (MMCs) require a means to manage power sharing between the MMCs by balancing the load during operation. Series connected MMCs require proper voltage balancing while parallel connected MMCs require proper current balancing. The paper describes this imbalance phenomenon, illustrates the solution via simulation results, and presents field measurements obtained with a megawatt class medium voltage DC test facility.

Modular multilevel converter based test bed for mvdc applications — A case study with a 12 kV, 5 MW setup

The Modular Multilevel Converter (MMC) topology has a wide area of possible applications. Features like redundancy at the module level and fault current limiting capability makes it a robust solution for many power applications. Moreover, this topology is very useful as a test equipment for Power Hardware in-the-Loop simulations, especially as a DC grid simulator. This paper describes a recently commissioned MMC based test bed built with four MMCs capable to operate at up to 5 MW power at 6 to 24 kV, depending upon which system configuration is chosen. Test results for the 12 kV configuration are presented along with a detailed description of all the salient control modules. It is concluded that due to its flexibility, this MVDC laboratory can serve the research and development community in addressing and de-risking a broad range of system integration and technology challenges.