Source-linked AI summary
Solid State Transformer - Next Generation MV Interface for Native DC loads
Steffan Hansen, Marco Liserre, Mohamed Moharram
TL;DR
Electrification is increasing pressure on distribution grids, while batteries and DC-native loads are becoming important parts of energy systems. PowerNexa proposes a modular, multi-port solid-state transformer architecture integrating battery storage and multiple DC voltage levels. In the described peak-shaving comparison, charging efficiency is 98.5% for a three-winding transformer configuration versus 96.7% for the Figure 4(b) configuration.
Problem
The grid must accommodate growing electrification and DC loads, while the choice of SST building block must support varied DC voltage, power-flow, and storage requirements.
Source: §1 · p. 1, §2 · p. 2, Why the right building block matter · p. 4Method
PowerNexa combines a CHB-based medium-voltage front end with TAB cells interconnected across multiple TABs to provide multiple DC levels and battery integration.
Source: MV Stage topology · p. 4, §5 · p. 6Results
While charging the BESS, overall efficiency is 98.5% for a 3-winding transformer with multiple output levels versus 96.7% for the Figure 4(b) configuration.
Source: Multi-port SST · p. 6Takeaways & Limitations
Abstract
from arXiv · showhide
About PowerNexa PowerNexa is the project name for a start- up initiative driven by Marco Liserre and Steffan Hansen with the ambition to develop the next generation power grid interface. Battery integration and DC loads Ongoing electrification is fundamentally changing how electrical energy is produced and consumed. DC loads are becoming an integral part of the infrastructure while battery integration is increasingly needed to manage peak demand and address grid capacity constraints. The next generation grid interface PowerNexa aims to combine a fully controllable Medium Voltage grid interface with native battery integration and multiple DC voltage levels, based on an architecture designed for high efficiency, modularity and redundancy.
1. The Grid Is Reaching Its Limits
Electrification is increasing distribution-grid demand, while connecting high-power applications can require capacity that the grid may not readily provide. Battery storage can help sites use their available grid connection more efficiently.
1. The Grid Is Reaching Its Limits
- Electric mobility, industry, data centers, heating, and renewable generation are increasing demand at the distribution-grid level.Many new applications require several megawatts of connection capacity.Source: §1 · p. 1
- Grid reinforcement can take a long time, leaving customers facing limited connection capacity, costly upgrades, project delays, or operating-power restrictions.Planning, permitting, and construction can all contribute to the time required.Source: §1 · p. 1
- Battery storage can supply power during demand peaks and charge when grid utilization is lower, helping sites use their grid connection more efficiently.For many high-power sites, storage is becoming part of the core electrical infrastructure.Source: §1 · p. 1
2. The Energy System Is Becoming More DC-Based
Many growing energy applications are DC-based, yet conventional distribution systems connect them through AC networks. The resulting conversion stages add cost and complexity, prompting the question of how directly medium-voltage electricity can serve batteries and DC loads.
2. The Energy System Is Becoming More DC-Based
- Electric vehicles and truck charging, data-center power systems, variable-speed industrial drives, and converter-based renewables are examples of DC-based assets or assets relying on DC links.The passage identifies these as a growing set of applications.Source: §2 · p. 1
- A battery-buffered charging hub connected through a conventional AC network may require a transformer, battery PCS, charger rectifier, vehicle DC/DC converters, and separate control and protection systems.These are listed as five distinct components or systems.Source: §2 · p. 2
- Each conversion stage adds cost, losses, cooling requirements, footprint, and control complexity, motivating more direct distribution of medium-voltage electricity to batteries and DC loads.The section frames direct conversion and distribution as its central question.Source: §2 · p. 2
3. What a Solid-State Transformer Changes
A solid-state transformer adds actively controlled power electronics and medium-frequency isolation to voltage transformation. Its potential value includes both smart grid functions and the integration of transformation, conversion, and control in one interface.
3. What a Solid-State Transformer Changes
- A solid-state transformer uses actively controlled power electronics and a medium-frequency transformer for galvanic isolation, unlike passive grid-frequency voltage transformation.The passage says an SST has the potential for smaller size and weight and lower total losses.Source: §3 · p. 2
- Potential SST functions include active and reactive power regulation, voltage and frequency support, direct DC output, monitoring, communication, and coordinated energy management.The passage presents these as possible smart features, not universal properties of every SST.Source: §3 · p. 2
- An SST integrates voltage transformation, conversion, and control within one grid interface rather than serving only as a voltage transformer.The section describes this integration as the principal value of an SST.Source: §3 · p. 2
4. SST Architectures
SSTs encompass many topologies and configurations, so the benefits of one design should not be assumed to apply to all. This section surveys selected significant topology choices.
4. SST Architectures
- SST refers to a broad range of topologies for different applications, rather than one fixed architecture.The section says it will cover some of the most significant topology choices.Source: §4 · p. 2
- Different SST topologies and configurations do not necessarily provide all the advantages mentioned for the technology.Source: §4 · p. 2
- The section focuses on selected significant topology choices within that diverse design space.Source: §4 · p. 2
Classification by Conversion Stages
SSTs are classified by conversion-stage count and modularity level. More stages can provide greater controllability and flexibility, while fewer stages promise lower cost and potentially higher efficiency.
Classification by Conversion Stages
- More conversion stages are expected to increase controllability and flexibility, while fewer stages promise lower cost and potentially higher efficiency.The number of stages significantly influences SST capabilities.Source: Classification by Conversion Stages · p. 2
- A single-stage SST converts power without a separate intermediate DC link, offering a compact, low-component architecture but limited input/output controllability and voltage quality.It can in principle provide either DC or AC output.Source: Classification by Conversion Stages · p. 2
- A two-stage SST has a DC link on either the medium- or low-voltage side, improving controllability and voltage quality; it is preferred when DC output is required.Smart-grid features depend on the medium-voltage converter topology.Source: Classification by Conversion Stages · p. 2
- A three-stage SST has DC links on both sides of the medium-frequency transformer and is mainly relevant when AC output is required.The passage introduces this architecture for completeness.Source: Classification by Conversion Stages · pp. 2–3
Classification by Modularity
- Modularity can be introduced at cell, converter, or system level, combining repeated cells, isolated converter modules, or complete functional systems.Modules may include switches, a medium-frequency transformer, local control, and an isolated output stage.Source: Classification by Conversion Stages · p. 3
- Modular building blocks support scaling, configuration, and servicing, and can improve availability by allowing faulty modules to be isolated or replaced while operation continues.The passage identifies cell-level modularity as the highest level.Source: Classification by Conversion Stages · p. 3
Why the right building block matter
SST architectures vary in topology, basic cell, power-flow direction, and DC connections. Selecting a building block requires examining application needs and their associated design trade-offs.
Why the right building block matter
- SST architectures differ in topology and basic cells, and may support unidirectional or bidirectional power flow and one or multiple DC connections.These variations define the range of architectures under consideration.Source: Why the right building block matter · p. 4
- The building block must accommodate application requirements including DC voltage level and polarity, power rating, power-flow directionality, and energy-storage integration.Source: Why the right building block matter · p. 4
- Identifying an optimal building block requires systematic examination of application requirements and their associated design trade-offs.Source: Why the right building block matter · p. 4
MV Stage topology
The medium-voltage conversion stage strongly shapes subsequent stages, and modular designs narrow the practical choice to MMC or CHB. PowerNexa retains a CHB-based front end while enabling a synthesized MVDC voltage.
MV Stage topology
- The medium-voltage topology strongly affects subsequent conversion stages, and modular designs make MMC and Cascaded H-Bridge the two practical choices.Source: MV Stage topology · p. 4
- An accessible MV DC link favors conventional MMC architectures, while CHB offers lower complexity, fewer semiconductors, smaller AC filters, and lower overall cost.MMC is established in high-voltage applications such as HVDC transmission; CHB is widely used in industrial medium-voltage applications.Source: MV Stage topology · p. 4
- PowerNexa enables a synthesized MVDC voltage while retaining a CHB-based medium-voltage front end.Source: MV Stage topology · p. 4
Multi-port SST
Multi-port SSTs provide multiple outputs or voltage domains, but their implementation affects power paths and efficiency. A three-winding transformer configuration is reported to outperform the described DAB arrangements in the stated battery scenarios.
Multi-port SST
- A multi-port SST supplies several output ports or voltage domains, while transformer modularization can provide galvanic isolation and individually controlled ports.Source: Multi-port SST · p. 5
- In the described DAB arrangement, output-port groups limit each port to part of total SST power and inter-load power must pass through the primary circuit; a multi-winding transformer provides multiple levels and a direct inter-port path.The three-winding configuration is called a Triple Active Bridge (TAB).Source: Multi-port SST · p. 5
- Charging the BESS is 96.7% efficient with the DAB plus separate battery DC/DC converter versus 98.5% with the three-winding transformer; discharging is 97% versus 98.5%, respectively.The passage assumes 98.5% efficiency for DAB and TAB and 98.2% for the separate battery DC/DC converter.Source: Multi-port SST · p. 6
5. The PowerNexa Concept: the key building block as universal enabler for a SST-platform
PowerNexa proposes a modular multi-port SST combining a CHB-based medium-voltage front end with TAB building blocks. Their flexible interconnection supports multiple DC levels and battery integration.
5. The PowerNexa Concept: the key building block as universal enabler for a SST-platform
- The platform targets multiple controlled DC outputs, voltage extension without redesign, native battery integration, and modular power electronics.The architecture uses a CHB-based medium-voltage front end and TAB building blocks.Source: §5 · p. 6
- Interconnecting corresponding cells across TABs supplies added ports from the entire SST, preserving CHB balance while extending voltage range and enabling battery integration without an extra converter.The TAB uses three cells and is described as the simplest multi-winding topology, offering flexibility with minimum complexity.Source: §5 · p. 6
- Figure 5 contrasts two LVDC output ports in panel (a) with LVDC and MVDC outputs in panel (b).Source: Figure 5 · p. 6
- The platform’s stated system-level benefits include coordinated control, modular scalability, and potentially reduced installed footprint through shared conversion, control, and cooling.It is designed for applications beyond a single application focus.Source: §5 · p. 7
6. Conclusion
Electrification is increasing pressure on distribution grids, while batteries and DC-native loads are becoming central to energy systems. PowerNexa aims to integrate these assets in a controllable medium-voltage platform that simplifies the complete system.
6. Conclusion
- Electrification of transport, industry, and digital infrastructure is increasing distribution-grid pressure, while batteries and DC-native loads are becoming central energy-system elements.Source: §6 · p. 7
- PowerNexa aims to combine battery storage, charging infrastructure, and several DC voltage domains within one modular multi-port medium-voltage platform.Source: §6 · p. 7
- The commercial opportunity is framed as simplifying the complete system, not merely replacing the transformer.Source: §6 · p. 7