Equilibrium Point Analysis and Power Sharing Methods for Distribution Systems Driven by Solid-State Transformers

A solid-state transformer (SST) is one of the promising candidates to interface the renewable generation to a power distribution system. An emerging and attractive topology for the building block of the next generation SST is the multiple active bridge (MAB) dc-dc converter. The MAB converter technology has the inherent capability to reduce the number of the high-frequency magnetic links (HFMLs) and the overall complexity of the system, especially in multi-module-based SST configurations. However, an MAB converter suffers from the power unbalance issues due to the parameter mismatch of the HFML. As, several active bridges share a common HFML in an MAB converter, the controller architecture should ensure that the bridges share the same amount of power and the output port maintains a constant dc voltage. In this paper, an improved control architecture is proposed for the HFML-based MAB dc-dc converter for use in the SSTs. The simulation and experimental results show that the designed controller can effectively mitigate the power unbalance issues of the MAB converter due to the HFML parameter mismatch, while maintaining a constant dc voltage at the output.

Solid-state transformers (SSTs) are an emerging class of converters that have shown potential to replace traditional low-frequency transformers (LFTs). With cascading capabilities similar to modular multilevel converters, SSTs can efficiently handle higher voltage and power levels. A cascaded SST setup usually uses a decentralized hierarchical control architecture, with primary controllers overseeing operations at the module level and secondary controllers keeping an eye on operations at the aggregated phase level. Despite their potential to replace LFTs, reliability concerns hinder their widespread adoption, given their vulnerability to cyberattacks. Nonintrusive attacks, such as side-channel noise intrusion (SNI), threaten the SSTs' stability by corrupting the measurement feedback signals fed to the controllers. This letter proposes a novel technique for rapid intrusion detection and mitigation of radiative SNI threats to provide resilience to the secondary layer of the cascaded SSTs. The technique entails spectral decomposition of the noise-tampered signal, followed by noise-free signal reconstruction of the targeted frequency component. Experimental results validated the solution deployed in a cascaded SST experimental prototype's secondary control layer to deal with radiative noise on global input voltage sensor feedback.

This article proposes a Distributed-control Architecture (D-CA) and an operation sequence with start-up strategies for a Digital Signal Processor (DSP)-based Solid-State Transformer (SST). Although various control techniques for SSTs have been reported in earlier studies, there is still a lack of research covering comprehensive content, including hierarchical control architectures and operation sequences with start-ups considering the implementation of DSPs. Therefore, this article addresses the following factors of SST. First, the D-CA is described for the design of the hierarchy between control boards. With the D-CA, because sub-boards are in charge of their corresponding DC-link voltage balancing control individually, the computational burden on the master board can be reduced. Second, the operation sequence of the SST system is explained based on the SST with D-CA. The step of DC-link voltage balance is considered throughout the entire operation sequence for safe driving. Furthermore, the PWM start-up strategies for a Cascade H-bridge Multilevel (CHM) converter and Dual Active Bridge (DAB) converter are proposed to prevent switching pulse errors caused by DSP operating characteristics. These start-up strategies reduce the current surges. The validity of the proposed D-CA and operation sequence with start-up strategies are verified by experimental results.

This paper proposes a distributed digital control architecture of a modular-based solid-state transformer (SST) using a digital signal processor (DSP) and a field-programmable gate array (FPGA). In particular, the three-stage SST based on a modular structure is the topology of most interest because of its superior controllability. In order to make the modular-based SST, the digital implementation is inevitable to achieve higher performances, improved reliability, and an easy development. In addition, the modular-based SST requires enough capacity for implementing complex control algorithms, multiple interfaces, and a large number of internal variables. In this paper, a digital control platform for the modular-based SST is built using a floating-point DSP and an FPGA that operate cooperatively. As a result, the main control algorithms are performed by the DSP, and the simple logical processes are implemented in the FPGA to synthesize the suitable gating signals and control external devices, respectively. The proposed implementation method enables high-switching-frequency operation, multitasking, and flexible design for the modular-based SST. Experimental results are presented to verify the practical feasibility of the proposed technique for the modular-based SST.

Long distances in the vicinities of railways are not exploited in terms of wind energy. This paper presents a scalable power electronics approach, aimed to harness the wind potential in a railway infrastructure. The key aspect of this proposal relies on both using the wind energy in the location, and the displaced air mass during the movement of a train along the railway, in order to produce electrical energy. Vertical Axis Wind Turbines (VAWT) are used in order to take advantage of the wind power, and widely used and well-known power converter techniques to accomplish the goal, showing MPPT techniques, parallelization of converters and power delivery with a Solid State Transformer (SST). Results are shown according simulations of the whole system, with and without train activity, resulting that 30.6 MWh of the energy could be generated without the train, and the energy generated with the assistance of the train could reach 32.3 MWh a year. Concluding that almost the 10% of the energy could be provided by the assistance of the train.

This paper presents an adaptive back propagation learning scheme for three phase four wire grid integrated solar PV - battery microgrid feeding three phase and single phase nonlinear loads simultaneously. The proposed power quality improved 3 phase 4 wire solar PV - battery micogrid is capable of delivering highly non-sinusoidal currents to nonlinear and unbalance loads of different types such that single and three phase industrial and domestic loads, while the source currents in all three phases remains balanced and sinusoidal. An adaptive back propagation learning scheme is used to control the microgrid voltage control and power quality improvement under various loading conditions through mitigation of harmonic currents, reactive power compensation and active power balancing in the system. It also improve the system power factor in highly nonlinear and unbalanced loading conditions. The proposed scheme significantly improves the steady state and dynamic performances of the 3 phase 4 wire grid integrated solar PV - battery micogrid system.

We propose and investigate a new type of multiple access protocol that combines the characteristics of PRMA, CDMA, and the hybrid ARQ error controlling and retransmission scheme to reduce the retransmissions and keep the frame error rate low when channel experiences heavy traffic. It is an adaptive optimization of the balance between the CDMA processing gain and FEC coding gain to obtain a better throughput. We use both the tagged user approach (TUA) and equilibrium point analysis (EPA) method to analyze performance of the protocol. Simulation shows that using hybrid ARQ-II with code combing in preamble minislot channel access protocol CDMA (PMCAP/CDMA) can significantly improve the delay performance in AWGN. Code limitation strategies can decrease the multiple access interference (MAI) level and frame error rate, which in turn improve the system performance when the background noise is very high.

A novel stochastic fractal search algorithm for optimal allocation of distributed generators in radial distribution systems

The purpose of this study is to improve the P-I controllers of the voltage-source converters (VSC)-based multiterminal high voltage direct-current (MT-HVDC) networks. Since the VSCs are the non-linear elements of the MT-HVDC stations, the classical optimization methods, which approximately implement the linear model to optimize the P-I controllers of the VSCs, do not generate optimal results. Therefore, this paper presents a novel technique to optimize the VSC-based MT-HVDC grids’ P-I controllers by embedding the artificial bee colony (ABC) algorithm. The voltage-droop control method is employed at on-shore grids to ensure the active and reactive power balance within MT-HVDC networks. During an evaluation, achieved via a detailed four-terminal MT-HVDC model designed in PSCAD/EMTDC, the improved results obtained under different dynamic situations such as unbalance wind power generation, change in load demand at the on-shore side grids, and eventual VSC disconnection, respectively.

: This report gives the results of all Program tasks for the Advanced Inverter Technology Program. APL has three existing series resonant inverter-based power converters. The control and protection circuits are different for the three units. The purpose of this program was to produce a control and protection architecture and functional circuits that would work with all three power stage topologies. The requirements for the three existing power stages were determined. The three existing control and protection circuits were analyzed. An IC technology study was performed to determine the optimum technology and fabrication method for SRI control and protection circuits. A control and protection architecture utilizing common functional circuits that would work with the three existing topologies as developed and block diagrams drawn. The functional tasks were partitioned into five custom parts. The requirements for the custom parts were described. A study was performed to assess the potential present and future functions for a microprocessor in a series resonant inverter. A packaging and thermal study was performed to investigate the problems associated with space deployment of a 200-kW series resonant inverter. Keywords include: Control Architecture, Converter Control, Converter Protection, Gate Control, Generalized SRI Control and Protection, Power Converter, and Series Resonant Inverter Control.

Control architectures based on distributed controllers are favored in control of modular stacked converters because of their scalability, ease of installation and stable structure. However, the existing solutions are only applicable to traditional voltage source converters (VSCs) with high-inertia components, i.e., larger DC-link capacitance which are controlled with slow proportional-integral controllers. Therefore, there is no demanding requirement on the communication speed. This paper presents the challenges in the design of control architecture of low-inertia converters, such as soft-switching solid-state transformers (S4T), particularly using fast-dynamic control. It also proposes a methodology to realize the control architecture for such stacked low-inertia converters. The proposed control architecture is verified on a two-module S4T operating at 4 kV using hardware-in-loop simulation in the OPAL-RT platform and by experiments.

An optimal reactive power dispatch is a real power transmission losses minimization problem, while maintaining real and reactive power balances at all system buses and satisfying different system inequality constraints. It is a highly nonlinear complex optimization problem which requires a robust and effective solution technique for achieving global optimal solution efficiently. This paper proposes a principle component analysis based mutation operator for improving the performance of real coded genetic Algorithm to deal such type of problems. The proposed algorithm is tested for IEEE-57 bus system for optimal reactive power dispatch problem. The simulation results obtained are compared with various optimization methods available in literature.

In many applications of DC-DC converters, loads cannot be specified in advance, i.e., their amplitudes are suddenly changed from the zero to the maximum rating. Generally, design conditions are changed for each load and then each controller is re-designed. Then, a so-called robust DC-DC converter which can cover such extensive load changes and also input voltage changes with one controller is needed. Analog control IC is used usually for the controller of DC-DC converter. Simple integral control etc. are performed with the analog control IC. However it is difficult to retain sufficient robustness of DC-DC converter by these techniques. The authors proposed the method of designing an approximate 2-degree-of-freedom (2DOF) controller of DC-AC converter. This controller has an ability to attain sufficient robustness against extensive load and DC power supply changes. For applying this approximate 2DOF controller to DC-DC converter, it is necessary to improve the degree of approximation for better robustness. In this paper, we propose a method of designing good approximate 2DOF digital controller which makes the control bandwidth wider, and at the same time makes a variation of the output voltage very small at a sudden change of resistive load. The proposed good approximate 2DOF digital controller is actually implemented on a DSP and is connected to a DC-DC converter. Experimental studies demonstrate that this type digital controller can satisfy given specifications.

Suction based mechanical characterization of superficial facial soft tissues

In Ukraine, a gradual reduction of electricity production at thermal power plants is planned due to the development of renewable sources of electricity. Small hydraulic power plants are being restored, solar power plants and wind power plants are being built. This will allow solving the existing problems of the domestic energy industry regarding the shortage of fuel resources, energy security and reducing the level of harmful effects on the environment caused by the operation of traditional sources of electricity. There is a trend of transition from a purely centralized electricity supply to a combined one, when the number of local decentralized sources of electricity directly in distribution networks is increasing. In this way, distribution electric networks are gradually transformed into a network with features characteristic of a local electric system, which receives power both from its own distribution electric networks and from a centralized source - the electric power system. Renewable energy has a number of advantages compared to traditional energy, but there are also disadvantages. Among them, the complications of the functioning of electrical networks should be highlighted in the event of an increase in the capacities of renewable sources of electricity installed in them and the instability of generation due to their natural dependence on meteorological conditions, if we talk more specifically about technical shortcomings, then this concerns - sinusoidal of voltages and currents and voltage deviations, quality assurance of electricity, which directly depends on ensuring the balance of active and reactive power in the electrical system. Hence the need for coordinated power supply from renewable sources of electricity and substations of the power system. At the same time, there is a gradual transition from the wholesale electricity market of a single buyer to the balancing market of electricity and electricity supply under bilateral agreements, as well as the introduction of market management methods. This article discusses measures to reduce electrical energy losses, limit voltage deviation, improve the quality of electrical energy, and compensate for the reactive power of local loads due to the introduction of reactive power compensation devices together with renewable sources of electricity and facilitating their integration into the power grid.