Black start in distribution grids through solid-state transformer

The paper proposes a simple structure of high-power solid-state transformers (SSTs) able to control the energy flow in critical lines of the medium-voltage (20 kV) distribution grid. With an increasing number of renewable intermittent sources connected at the nodes of the meshed distribution grid and a reduced number of nodes connected to large power plants, the distribution grid stability is more and more difficult to achieve. Control of the energy flow in critical lines can improve the stability of the distribution grid. This control can be provided by the proposed high-power SSTs operating a 20 kV with powers over 10 MW. This function is difficult to achieve with standard SST technologies that operate at high frequencies. These devices are made with expensive magnetic materials (amorphous or nanocrystalline cores) and a limited power by SST cells. The required total power is reached by assembling many SST cells. On the other hand, existing SST designs are mainly aimed at reducing the equipment’s size and it is difficult to design small objects able to operate at high voltages. The authors propose to use cores made with grain-oriented electrical steel (GOES) thin strips assembled in wound cores. Experimental results obtained, with GOES wound cores, show that the core losses are lower for a square voltage than for a sine one. This counterintuitive result is explained with an analytical calculus of eddy currents and confirmed by a non-linear time-stepping simulation. Therefore, simple converter structures, operating with rectangular voltages and low switching losses, are the best solutions. Experimental results also show that the core losses decrease with temperature. Consequently, high-power SST cells can be made with transformers whose GOES cores are hotter than coils for reducing core losses and keeping copper losses at low levels. The paper proposes an appropriate transformer mechanical structure that avoids any contact between the hot GOES wound core and the winding, with a specific cooling system and thermal insulation of the hot GOES wound core. The proposed design makes it possible to build SST cells over 1MW and full SSTs over 10 MW at moderate costs.

The power system experiences many changes, which are leading to new challenges. The electric energy will no longer be generated only by large power plants, but rather by numerous decentralized energy resources that are primarily installed in distribution grids. In particular, power system restoration processes after a blackout will face new challenges. However, new opportunities by the widespread use of renewable energy resources arise. In this paper, the black start capability of a real distribution grid in Germany, operated by SWW Wunsiedel, will be investigated, as it has a high share of decentralised energy resources and a BESS with black start capability. For this purpose, the medium voltage level of the distribution grid is investigated with static load flow simulations to evaluate the best black start strategy. The driving forces of this black start concept are an 8 MW BESS unit and a 13.5 MW CHP plant.

Distribution systems have traditionally received the least attention in the bulk picture of power systems; they are yet the most "critical component" as they directly impact customers’ perception and utilities’ reputation. While transmission systems have seen abundant literature, innovations, and applications of power electronic-based devices to optimize their performance, distribution systems still require more attention toward a flexible and dynamic scheme to cope with volatile energy consumption and production. As a replacement or enhancement of the traditional ones, Solid-State Transformers can play a central role in the energy management of distribution systems towards this goal, in addition to the mere voltage conversion. This manuscript, developed within the IEEE Task Force on Solid-State Transformer integration in distribution grids, offers a critical review of the Solid-State Transformer potential, services, and technical challenges for its integration in distribution systems. Particular attention is given to scientific trends and open points in the current research that must be addressed before utilities extensively integrate the Solid-State Transformer in distribution systems.

The large-scale integration of renewable energy sources (RESs) and the increasing number of energy -storage (FS) systems connected to the grid will create a big challenge for power flow management at the distribution level. In this new scenario, a solid-state transformer (SST) will be a highly efficient key element for providing highly efficient and controllable power flow in distribution grids. SSTs not only mimic the operation of conventional transformers, which scale the voltage level between primary and secondary terminals, but they also control bidirectional power flow, stabilize voltage, facilitate direct integration of ES, and mitigate harmonics and transients, which can cause the power quality of the grid voltage to deteriorate. This article surveys several proposed structures for SSTs in the literature based on different ways of interconnecting power converter topologies.

Recently, medium voltage SiC devices have been developed which can be used for grid tie applications at medium voltage. Two such devices - 15 kV SiC IGBT and 10 kV SiC MOSFET have opened up the possibility of looking into different converter topologies for medium voltage distribution grid interface. These can be used in medium voltage drives, active filter applications or as the active front end converter for Solid State Transformers (SST). Transformer-less Intelligent Power Substation (TIPS) is one such application for these devices. TIPS is proposed as a 3-phase SST interconnecting 13.8 kV distribution grid with 480 V utility grid. The Front End Converter (FEC) of TIPS is made up of 15 kV SiC IGBTs. This paper focuses on the advantages, design considerations and challenges associated with the operation of converters using these devices keeping TIPS as the topology of reference.

Along with the DC grid distribution issue, the Solid State Transformer (SST) is increasingly emerging in the field of electric power electronics, and the Hybrid SST (HSST) study is also emerging between the current AC-based Grid and future DC grid distribution. At present, several SST and HSST studies have discussed ways to implement the same topology in series to create high DC voltages. These cascade-style topologies not only make it difficult to implement controllers and designing gate drivers but also require a lot of elements, which drives up the overall system’s price. Meanwhile, IGBT and MOSFET series connections are actively being studied for high voltage topology, and various techniques are being studied to solve simultaneous turn on/off problems. This paper proposes by altering the PFC circuit used in existing LVs to switch device series-connected base for HSST and SST. In this paper, circuits were constructed using balancing resistance and passive component for voltage balancing of series-connected switches, and validity is verified through proper controller selection and formulas.

Modular solid-state transformers (SSTs) are a promising technology in converting power from a 10kV three-phase medium voltage to a lower DC-voltage in the range of 100…400V to provide pure DC power to applications such as electrolyzers for hydrogen generation, data centers with a DC power distribution and DC micro grids. Modular SSTs which can be interpreted as modular multilevel converters with an isolated DC-DC output stage per module, are designed with redundant modules to increase reliability. Usually, each of the three arms operates independently, and therefore, only a fixed number of faulty modules can be compensated in each arm, even if all modules are operational in the remaining two arms. With the proposed zero-sequence voltage injection, up to 100% more faulty modules can be compensated in an arm by employing the same hardware. In addition, module power imbalances are nearly eliminated by utilizing a fundamental frequency zero-sequence voltage. A dominant 3rd harmonic zero-sequence voltage injection in combination with the 5th, 7th and several higher order harmonics with adaptive (small) amplitudes minimize the required arm voltages at steady-state. For nominal operation or symmetrical faults, the proposed technique is equivalent to the well known Min-Max voltage injection, which already reduces the peak arm voltage by 13.4% (to $\frac{{\sqrt 3 }}{2}$) compared to a constant star point potential. A statistical analysis proves, that the expected number of tolerable faulty modules of the 1MW SST increases by 12% without the need for additional hardware.

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.

A review on solid-state transformer: A breakthrough technology for future smart distribution grids

In this paper, we propose a new control scheme for a wind energy conversion system connected to a solid-state transformer-enabled distribution microgrid. The system consists of a wind turbine, a permanent magnet synchronous generator, a rectifier and a load which is connected to the distribution grid dc bus. The scheme combines a classical PI placed, in a nested-loop configuration, with a passivity-based controller. Guaranteeing stability and endowed with disturbance rejection properties, the controller regulates the wind turbine angular velocity to a desired value – in particular, the set-point is selected such that the maximum power from the wind is extracted – maximising the generator efficiency. The fast response of the closed-loop system makes possible to operate under fast-changing wind speed conditions. To assess and validate the controller performance and robustness under parameter variations, realistic simulations comparing our proposal with a classical PI scheme are included.

Due to the continuous increase of distributed generation (DG) units at the distribution grid level, temporarily islanded microgrid (TIM) operation in case of a blackout becomes energetically feasible. DG units with black start capability and additional MVA-scale generation units in TIMs are a strong option for a fast intermediate supply. They create possibilities for a longer-term emergency supply for a large number of customers until the overall grid restoration (GR) is completed. To develop generalised concepts that are applicable for more than one individual medium-voltage grid, general requirements and conditions have to be analysed thoroughly and defined as generic as possible. Therefore, a modular concept is developed comprising individual modules for GR, energy management (EM), control, protection, communication and operation. The focus of this study is to present the developed modular concept with a special focus on EM and control approaches for TIMs. As a result, the exemplary application of the modular concept is successfully conducted. Finally, the modular concept provides recommendations for project planning and engineering for distribution system operators, which can easily be extended. Special requirements of the distribution grid operation can be defined and considered by this approach.

Solid State Transformers Design, Control and Impact on Future Distribution Grid