10 kV SiC MOSFET Based Medium Voltage Power Conditioning System for Asynchronous Microgrids

Medium voltage (MV) power converters using high voltage (HV) Silicon Carbide (SiC) power semiconductors result in great benefits in weight, size, efficiency and control bandwidth. However, challenges still exist on the components design considering HV insulation and noise immunity requirements in the MV SiC based power converter. A 5-level MMC based transformer-less grid-connected dc/ac converter is developed for 13.8 kV medium voltage grid using 10 kV SiC MOSFETs. The key components, including gate driver with high dv/dt immunity and fast reliable protection, isolated power supply with low parasitic capacitance, voltage/current sensors with high noise immunity, and passives following related insulation standard are provided. A 25 kV dc-link phase-leg is demonstrated, and the experimental results are presented.

A notable spread of Dispersed Generation (DG) power plants, connected at Medium Voltage (MV) distribution grids, takes place in the Italian power system, with a big impact on the operation, both in normal and in emergency conditions, especially during transients determining frequency variations. Enel Distribuzione, the biggest Distribution System Operator (DSO) in Italy, proposed to install some special devices, on its distribution grid, to improve the effectiveness of management of DG and loads, during frequency transients in automatic way. The paper describes the solution proposed by Enel Distribuzione, that gives the possibility to better manage the DG and loads connected in MV in favor of the whole power system in presence of frequency transients, as the Italian Transmission System Operator (TSO) with its Defense Plan (DP) requires. Some numerical results for demonstrating the need and effectiveness of the application of the proposed solution in the real operation of the system are reported.

Energy storage systems are pivotal for maximising the utilisation of renewable energy sources for smart grid and microgrid systems. Among the ongoing advancements in energy storage systems, the power conditioning systems for energy storage systems represent an area that can be significantly improved by using advanced power electronics converter designs and control techniques. Normally, the battery, flywheel, ultracapacitor and superconducting magnetic energy storage are the types of energy storage systems that typically require power conditioning systems for efficient bidirectional power flows. Compared to the previous review papers, this paper critically reviews the power conditioning system configurations and control techniques from the perspective of energy storage system applications in the power systems. First, the power conditioning systems are categorised into DC bus power conditioning systems and AC bus power conditioning systems based on the output voltage types at the connected point. Next, the topologies and control configurations of popular power electronics are comparatively analysed to highlight their advantages and power system applications. Moreover, the control configurations are discussed in terms of the popular applications of energy storage systems, that is, power backup smoothing, frequency regulation, voltage regulation and power quality applications. In addition, the latest developments in the energy storage system such as multi‐functional energy storage system stacking, artificial intelligence for power conditioning system of energy storage systems and security of control of energy storage systems are critically analysed. Finally, the review is concluded by discussing industrial applications and future research trends for the power conditioning systems of energy storage systems.

The present paper deals with the topic of the integration of Energy Storage Systems (ESSs) in Medium Voltage (MV) distribution grids, managed by Distribution System Operators (DSOs), in case of significant presence of Dispersed Generation (DG), typically based on Renewable Energy Sources (RESs). A real case of installation of a Li-Ion Batteries ESS, by the Italian DSO Enel Distribuzione, in a High Voltage (HV) / Medium Voltage (MV) station in the Italian power system, has been object of study. For this purpose, a MatLab software, made up of more tools, has been implemented. It allows interfacing a proprietary database of Enel Distribuzione to MatPower tool and performing multi-period Power Flow (PF) analyses. The software has been used to evaluate the performance of the real ESS in the real MV grid, for active power peak shaving and reactive power compensation functions, in respect to the HV grid. With a critical analysis of the results, the effectiveness of the installation of the ESS on the real network has been tested.

Increasing concern about the environmental impact of ships has made Shore-to-Ship (STS) power an attractive solution for ship owners and port authorities worldwide in reducing emissions at ports. Existing shore-to-ship solutions for 0.1 MVA to 5 MVA applications employ silicon (Si) IGBT based static frequency converters. Recent developments in high voltage silicon carbide (SiC) devices have facilitated improvement in efficiency and power density of medium voltage (MV) converters in various applications. This paper proposes an MV STS system enabled by series connection of three 3.3 kV SiC MOSFETs, which shows the potential for improved power density and efficiency compared to existing Si IGBT based solutions. A 100 kVA 3-phase two-level voltage source converter (VSC) with series connected 3.3 kV SiC MOSFETs is designed and demonstrated. Experimental results for the series connected 3.3kV SiC MOSFET based converter is shown at 6 kV dc link voltage to validate the design and operation of such a system. Successful demonstration of a MV converter system enabled by series connection of high voltage SiC MOSFETs can open up opportunities to replace conventional Si IGBT based converters with SiC MOSFET based converters in applications interfacing with medium voltage grid.

Grid codes around the world are requiring grid-support functions (GSFs) and standardized interoperability interfaces for distributed energy resources (DERs) to address the rapid increase of renewable energy. However, these new GSFs need to be assessed to ensure the desired power and communication capabilities exist in the field. The IEEE 1547.1 standard outlines the conformance test procedures for DER devices and is currently undergoing a major revision to align it with IEEE 1547–2018. Once it is published (anticipated in mid-2020), GSFs in commercial PV inverters in USA and Canada will be certified to the IEEE 1547.1 conformance test procedures. Several international research laboratories are collaborating to develop a versatile open-source DER testing platform that performs automated testing of DER devices. This community of laboratories is developing open-source IEEE Std. 1547.1 test scripts to lower barriers to DER vendor internal equipment evaluations, ease product compliance testing at certification laboratories, and provide research institutions a tool to study DER behaviors. In this work, test scripts were used for test verification of GSFs, including limit active power, constant reactive power, active power-reactive power (watt-var), and prioritization of GSF response for several DER devices. Sample test results for these DER GSFs and test protocol recommendations are presented in this paper.

This paper discusses the design, development, and testing of a flexible combined heat and power (F-CHP) system. It includes two parts: the power conditioning system (PCS) converter, which connects the system to the grid and is the key component to provide grid support services, and the F-CHP central controller, which accommodates the control of local sources, local loads, and the PCS converter. The functions and structures of the central controller are discussed. The central controller’s system-level performances are verified with hardware-in-the-loop (HIL) tests, and the mode transition and grid transient performances are validated with power electronic converter-based hardware testbed (HTB) tests. The PCS converter is designed with 10 kV SiC MOSFETs. The grid requirements are considered in the 13.8 kV/1 MW PCS converter design, and the impact of different requirements on the converter design is identified. Also, converter scalability is studied with the paralleling operation. Two 13.8 kV/100 kW converter prototypes are developed to verify the converter design and scalability. The second prototype is an improved design of the first one; the volume is reduced by 49% and the efficiency at the rated output power is increased from 96.4% to 98.4%. The test results of the converter performances, grid support functions, and parallel operation are also discussed.

Single-phase ground faults are the most common faults in medium-voltage distribution grids. High-impedance grid grounding using a special four-leg transformer is discussed in the paper. The transformer's zero-sequence impedance differs from positive-sequence impedance and it is designed to compensate the earth faults currents in Medium Voltage (MV) distribution grids when single-phase ground fault occurs. The designed transformer can replace the traditional arc suppression coils (ASC) in the case where no distribution transformer neutral is available or distributed compensation is required. Alternatively, the transformer can operate in parallel with the ASC.

The growing demand for the ultra-fast DC charging stations needs the direct connection to the medium-voltage (MV) distribution grid, which can be achieved through the solid-state transformers (SST) as opposed to solutions using line frequency transformers (LFT). By using the MV high current silicon carbide (SiC) modules, which are becoming more available than before, the architecture of SST can be significantly simplified with much less number of series-parallel modular cells. In this work, we investigate the feasibility and present the development of a 3.3 kV SiC based full bridge building block. Then a MV dual active bridge (DAB) with 2 kV DC input and 800 V DC output, which is an essential unit in the MV DC charger, is prototyped using the developed building block. Compared to the use of modules rated at 1.7 kV or lower, the presented DAB can reduce the number of series cells in half, since the DC bus voltage for each cell can be raised from 1 kV to 2 kV. Extensive characterizations, e.g., switching tests, thermal tests, and etc., have been carried out for the 3.3 kV SiC module to extract key information to guide the electro-thermal co-design of the MV DAB system.

In the context of new policies promoting the use of distributed energy resources (DER) the number of distributed generation (DG) units connected to the medium voltage (MV) and low voltage (LV) distribution grids has largely increased. Distribution companies offer different prices to DG owners depending on the signal quality and time-slot, because the price of sale of generated energy is influenced by the curve of the demand. The use of energy storage systems can help for a larger integration of DG controlling the power flow in the point of common coupling (PCC). From the economical point of view, the possibility to store the electricity when the price is low to sell when it is high makes energy storage an interesting investment for DG owners. In this article is discussed the use of distributed energy storage (DES), paying attention on the control requirements for a larger DG penetration and the economical benefits. The aim of the article is to develop a control strategy to increase economical profits assuring supply quality, for energy storage systems working in parallel to DG units.

Ancillary services e.g., voltage control, congestion management and frequency control, require to be compensated increasingly from the Distributed Energy Resources (DERs). DERs are predominantly wind and photo-voltaic power plants, the major share of which are installed at the distribution grid level. Therefore, the previously passive distribution grids require transformation towards a more active role. Provision of ancillary services from the distribution grid level, requires assessment of active and reactive power flexibility (PQ-flexibility) potentials. Furthermore, increased renewable penetration correlates to increased responsibility of the Distribution System Operators (DSOs) for assessing the flexibility potentials. An aggregation of distribution grid potentials, subject to technical grid constraints and technological power limitations, is termed as Feasible Operating Region (FOR) of the distribution grid. The FOR effectively serves as an interface between the DSOs and the Transmission System Operators (TSO), for flexibility exchanges and planning of ancillary services provision. The determination of the FOR is established in current research, using different algorithms e.g stochastic methods, meta-heuristic programming and mathematical optimization techniques. In this paper, an FOR determination algorithm using successive linear programming (sLP) is proposed and validated against established optimization approaches on a uniform medium voltage (MV) grid model. Comparisons reveal competitiveness with established methods and an added advantage of fast calculation times, suitable for real time assessments. Further enhancement of the FOR is proposed, by integrating discrete transformer tap-changing operational flexibilities using a successive mixed integer linear programming (sMILP). Results demonstrate an increase in the flexibility potential from the distribution grid.

The AGGREGATE project team successfully developed and validated various modules for outage management. Brief summaries of each module are provided to showcase their strength for outage management and restoration for a distribution system with a high penetration of connected distribution energy resources (DERs).In recent years, inverter-based DERs have been widely deployed in distribution system. A most of behind-the-meter (BTM) solar power generation is not visible to the utility. The data-driven DER and load estimation modules are using machine learning (ML) and artificial intelligence (AI) to manage this issue, which provides an opportunity for distribution system operators (DSOs) to operate systems and make decisions in real-time for a distribution system with a high penetration of DERs deployed. Also, the estimated DER and true load can be further leveraged in network aggregation and cold-load pick up estimation for reducing the computing complexity and providing for fast restoration. After load demand and DER power generations have been estimated, the information will support topology and state estimation(SE). The topology estimation module demonstrated the viability of mixed integer linear programming (MILP) formulation to estimate the most likely operational radial topology and outage sections using power flow measurements, historical/estimated load and DERs data and smart meter ping measurements. Formulation includes continuous (power flow, load and DERs data) and binary measurements (smart meter ping measurements) in a single formulation. Errors in continuous data and binary data are modeled as normal distribution and Bernoulli distribution, respectively. In the future distribution grid, the power injection from controllable DERs will be essential for efficient and resilient grid operation. However, determining the optimal DER injections and restoration actions is dependent on knowledge of the system states. State estimation (SE), already the cornerstone of transmission energy management systems, will become commonplace in distribution management systems as more measurements become available from deployment of automated metering infrastructure (AMI). Observability analysis is the first step in SE, as it determines the sufficiency of the available measurements for accurately estimating the current system states. A new type of pseudo-measurement called a Correlational Measurement (CM) is introduced in this module, to enhance the observability of the system to enable more accurate SE. CMs encapsulate knowledge of correlation between demand patterns for similar classes of loads as well as injection patterns for same-technology renewable DERs. During grid contingency scenarios, DERs have been traditionally disconnected, without any fault ride-through capabilities. However, with new regulations and better technology, it is feasible for these resources to contribute to the grid’s restoration after an adverse event and hence enhance resilience. The controllability module proposes a two-step restoration scheme for the power system restoration process by leveraging additional degrees of freedom in power electronics interfaced DERs for mitigating voltage problems. In a resilience mode without the utility system, the distribution grid relies on DERs to serve critical load. In such a severe event with multiple faults on the distribution feeders, actuation of various protective devices (PDs) divides the distribution system into electrical islands. The undetected actuated PDs due to fault current contributions from DERs can delay the restoration process, thereby reducing the system resilience. The Advanced Outage Management (AOM) and the Advanced Feeder Restoration (AFR) modules developed in this project provide improved system resilience with multiple DERs. AOM identifies the faulted sections and actuated PDs in a distribution system with DERs by incorporating smart meter data. The most credible outage scenario including fault locations, PD actuations, and fault indicator (FI) failures is identified by a set of binary integer linear programming incorporating hypotheses. The AFR module serves to restore a distribution system with available energy resources taking into consideration the availability of utility sources and DERs. By partitioning the system into islands, critical load will be served with the available generation resources within islands based on the solution of a MILP. When the utility systems become available, the optimal path will be determined by a spanning tree search algorithm that reconnects these islands back to substations and restores the remaining load. The transmission and distribution (T&D) co-simulation module was used to validate the effect of a control action performed on the distribution side assets as it propagates to the transmission side. This ensures that the control action performed results in a feasible operating point on both the transmission and the distribution system. In addition to validation, the team used the T&D co-simulation module to demonstrate how distribution system assets can be used to mitigate issues on the transmission system. Specifically, the team demonstrated that appropriate switching operations on the distribution side can alleviate the line overload condition on the transmission side without causing new operational constraint violations.

PCS(Power Conditioning System) is the necessary component in Microgrid, composed of multiple distributed generators and energy storage system. In this paper, the functions of PCS are defined and 10[㎾] PCS for PV and BESS are developed. To apply PCSs to Microgrid, this paper presents a 3-phase inverter with the decoupling current controller, voltage controller and DPLL control system. PCSs were applied to 120[kW] pilot plant and its performance tests were carried out. Test results of PCS at each operation mode show stable in Microgrid.

With the proliferation of Distributed Energy Resources (DER), Grid Support Functions (GSFs) have been widely deployed. With the object to maintain the frequency and voltage stability during the grid-connected operation, GSFs, however, has raised great concerns on the effectiveness degradation of islanding prevention. Focusing on it, this paper evaluates the performance of the widely used islanding detection method, Slip-Mode Frequency Shift (SMS) operating together with GSFs. First, a small-signal model for this control combination is firstly derived, facilitating the investigation of interaction mechanism between GSFs and SMS. Accordingly, the optimal range of SMS coefficient is identified to guarantee both a successful islanding detection when an island forms and a stable operation during the grid-connected condition. It is observed that several key parameters, especially the slope and the open-loop response time of GSFs, as well as the SMS coefficient, jointly dominate the effectiveness of the islanding detection and should be deliberately designed. Finally, extensive simulations and controller hardware-in-the-loop (HIL) tests are performed to validate the above analysis.

Distributed energy resources (DER) such as photovoltaic (PV) systems, when deployed in a large scale, are capable of influencing significantly the operation of power systems. Looking to the future, stakeholders are working on standards to make it possible to manage the potentially complex interactions between DER and the power system. In 2009, the Electric Power Research Institute (EPRI), Sandia National Laboratories (SNL) with the U.S. Department of Energy (DOE), and the Solar Electric Power Association (SEPA) initiated a large industry collaborative to identify and standardize definitions for a set of DER grid support functions. While the initial effort concentrated on grid-tied PV inverters and energy storage systems, the concepts have applicability to all DER. A partial product of this on-going effort is a reference definitions document (IEC TR 61850-90-7, Object models for power converters in distributed energy resources (DER) systems) that has become a basis for expansion of related International Electrotechnical Commission (IEC) standards, and is supported by US National Institute of Standards and Technology (NIST) Smart Grid Interoperability Panel (SGIP). Some industry-led organizations advancing communications protocols have also embraced this work. As standards continue to evolve, it is necessary to develop test protocols to independently verify that the inverters are properly executing the advanced functions. Interoperability is assured by establishing common definitions for the functions and a method to test compliance with operational requirements. This document describes test protocols developed by SNL to evaluate the electrical performance and operational capabilities of PV inverters and energy storage, as described in IEC TR 61850-90-7. While many of these functions are not currently required by existing grid codes or may not be widely available commercially, the industry is rapidly moving in that direction. Interoperability issues are already apparent as some of these inverter capabilities are being incorporated in large demonstration and commercial projects. The test protocols are intended to be used to verify acceptable performance of inverters within the standard framework described in IEC TR 61850-90-7. These test protocols, as they are refined and validated over time, can become precursors for future certification test procedures for DER advanced grid support functions.