This article proposes a novel modular multilevel converter (MMC) named “T-type MMC” (T-MMC) for reliable, controllable, and efficient performance in energy conversion applications. Circuitry configurations and control structures are presented with respect to fundamental and additional functions. The T-MMC topology consists of two solid-state power stages (Stage-I and Stage-II), which are coordinated for ac and dc fault tolerance and increased ac-side voltage synthesis. Energy storage elements can be integrated into the submodules. As a result, the T-MMC based energy storage system can not only increase system inertia but also maintain continuous power transmission during faults. A thyristor forced-commutation technique facilitates actively controlled utilization of symmetrical thyristors in the conduction path; thus, T-MMC normal-mode operation losses can be equivalent to those of the conventional half-bridge MMC. Simulation and experimentation demonstrate the performance of the T-MMC.

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A T-Type Modular Multilevel Converter

Ultracapacitors (UCs) today are playing an increasing role in the energy storage systems (ESSs) of several power conversion applications. In this article, a design method for optimal sizing of a UC stack is proposed, which minimizes the overall cost of the ESS. The problem is cast with normalized cost parameters of the ESS subsystem, using which the discharge ratio of the stack is optimized for the given ESS specifications. An iterative design algorithm is proposed which systematically engineers the stack parameters of unit cell capacitance, the number of series cells and parallel strings along with the initial and final stack voltage levels that meet the specified ESS requirements while also supplying the equivalent series resistance (ESR) loss. The iterative sizing process also factors in the maximum power transfer constraint imposed by the ESR to guarantee stable operation of the cost-optimal design even as the stack voltage varies during discharge. The design considerations, optimization framework, ESR effects, and the proposed sizing algorithm are synthesized in a generalized formulation in order to capture the variations in cost and subsystem parameter values that typically vary based on scaling requirements as well as the UC and power converter technology. Experimental results on a 4 kWmin UC stack validate the proposed cost-optimal design algorithm and the ESR induced voltage collapse.

Design Considerations of Ultracapacitor Stack for Optimal Sizing of Energy Storage Systems in Contingency Applications

Modular multilevel converters (MMCs) have been recognized as the most attractive technology for high voltage direct current (HVDC) applications during the last years. Their advantages include the ability to freely control the internally stored energy in the distributed submodules (SMs) and to independently control AC and DC powers. Recent studies showed that an MMC can take advantage of this degree of freedom to supply ancillary services with low energy requirements such as frequency response or power oscillation damping by connecting energy storage systems (ESSs) to its SM capacitors. This paper proposes a modular DC-DC converter as an interface between the energy storage element and the SM capacitor. As it can be built with switches allowing a high switching frequency, it is shown that the volume of the passive components can be reduced in comparison with interface converters known in the literature. The proposed control strategy avoids to transfer the harmonics of a SM to the ESS that prevents to drastically oversize the system. The proposed control method also provides an appropriate energy exchange without affecting the control philosophy of an MMC and it addresses balancing issues. Simulations and a downscaled prototype confirm the relevance of the proposed control. As the proposed interface converter is able to track the power reference with fast dynamics, it provides the same system-level advantages as other interface converters known in the literature but with a reduced volume. 

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A Submodule With Integrated Supercapacitors for HVDC-MMC Providing Fast Frequency Response

The introduction of Renewable Energy Sources (RES) and the electricity market liberalization have pointed out the need to upgrade the current electricity transmission network. High Voltage Direct Current (HVDC) transmission system is potentially the key. Recent advances in power electronics have allowed the introduction of power converters in power system applications as the Modular Multilevel Converter (MMC). In the same perspective, grid scale Energy Storage Systems (ESSs) have broadly appeared. The dual purposing to combine both systems can provide a completely new solution. Even though each submodule (SM) within modular converter can have its own ESS, decreasing the number of energy storage submodules (ES-SMs) may be required to reduce the cost. This paper deals with the opportunities to introduce split storage into an MMC. The analysis is focused on the internal energy exchange to maintain the proper function of the converter by using circulating current. Analysis shows that, SM capacitor voltage ripple or semiconductors load are greatly influenced by additional circulating current injection. This study helps to design embedded ESSs in the converter to provide new function like ancillary services for power system operation. Finally, a simulation of an MMC with ESS confirms analytical calculations.

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Assessment of the Impact of Split Storage within Modular Multilevel Converter

This article presents a new state-of-charge (SOC) balancing method with parallel and series output connected battery power modules (BPMs) in an active battery management system (BMS.) To increase both the battery pack and system-level modularity, the BMS controls the average SOC of the entire battery pack by regulating the input currents of all BPMs to a common reference. A balancing loop introduces input current offsets to regulate all cells SOCs. The BMS functions are distributed between a central controller for battery pack SOC regulation, and a module controller that regulates the input current of the individual BPMs. Experimental results are presented on a 1.5 kWh 1C-rate prototype with five series output connected battery bricks. Each battery brick consists of three parallel output connected BPMs, which employ three battery cells and three 100 W dc/dc converters.

State-of-Charge Balancing With Parallel and Series Output Connected Battery Power Modules

At present, passive balancing methods dominate energy storage applications, however, they suffer from a long balancing duration. In this article, we took advantage of a modular architecture, where several modular power converters replace a central dc–dc converter for fast charging and balancing of a supercapacitor stack. A strategy has been proposed to control how power is shared among the converters during the charging period in order to balance the supercapacitors. However, some converters enter control saturation due to voltage differences between supercapacitors caused by their nonuniform conditions and characteristics. The originality of this article lies in taking into account the saturation by modifying an energy-based strategy to correct the power shares and make balancing the supercapacitors possible. Simulation and experimental case studies were used to demonstrate the strategy's performance and limitations.

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Balancing Supercapacitor Voltages in Modular Bidirectional DC–DC Converter Circuits

The interest for modular multilevel converter (MMC) with energy storage systems (ESSs) has increased quickly over the last decades. Since an MMC has several hundreds of sub-modules (SMs) and ESS performances usually deteriorate quickly, the ability of the converter to provide the expected service during its lifetime must be investigated. In this work, supercapacitors are used as ESS and the reduction of their capacity is first calculated through lifetime simulation of the ESS using aging models. Then, simulations are used to analyze the available energy of the proposed converter considering dispersions for the initial parameters of ESSs and for the aging rate. Finally, the results are compared with different levels of redundancy and maintenance intervals to discuss the viability of the solution. For the depicted case study, results show that oversizing the ESSs without carrying any maintenance operations during the lifetime of the converter minimizes the total number of cells required.

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Assessment of Aging and Performance Degradation of Supercapacitors Integrated into a Modular Multilevel Converter

Energy Storage Systems (ESS) are a necessity for the future power distribution network and so their use is expected to grow. Usually, the implementation of an ESS is achieved by massive battery storage made of long connected row of cells, potentially leading to reliability issues. This paper investigates how to integrate a scalable ESS unit taking advantage of the modularity of modular multilevel converters (MMCs). It aims at presenting a detailed sizing methodology of supercapacitor (SC) pack in a way that the overall cost of the solution can be minimised. The proposed converter topology is briefly introduced. The results reveal that introducing an additional power converter into a submodule (SM) is mostly interesting for designing the ESS and for managing the ESS energy.

Optimal supercapacitor pack sizing for modular multilevel converter with integrated energy storage system

Energy storage systems (ESS) can enhance the reliability of service in power systems with a high share of renewable energy sources. A converter topology that can integrate ESS directly into an HVDC system is presented in this work. The topology consists of a branch of energy storage submodules (ES-SMs) and an inductor. The ES-SMs are based on half-bridges, which are connected to a supercapacitor or a battery by means of a dc/dc converter. The topology is scalable to different voltage levels and since the energy storage elements are distributed among all submodules, it provides a high degree of redundancy. In this work, the converter topology is modelled using an average model and its control designed to regulate the injected dc power and the energy of the ES-SMs. A preliminary sizing of the main elements of the topology is also provided. The simulations show that the ES-SMs can both inject and absorb power from the HVDC system, while maintaining the desired energy in the ES-SMs capacitors. 

Florian Errigo

Papers

Assessment of the Impact of Split Storage within Modular Multilevel Converter

Balancing Supercapacitor Voltages in Modular Bidirectional DC–DC Converter Circuits

Assessment of Aging and Performance Degradation of Supercapacitors Integrated into a Modular Multilevel Converter

Optimal supercapacitor pack sizing for modular multilevel converter with integrated energy storage system

Single branch of energy storage submodules to integrate energy storage devices in HVDC systems