This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.

Reduced Switching-Frequency Modulation and Circulating Current Suppression for Modular Multilevel Converters

Reduced switching-frequency modulation and circulating current suppression for modular multilevel converters

The modular multilevel converter is a promising topology for high-voltage and high-power applications. By using submodules equipped with dc-capacitors excellent output voltage waveforms can be obtained at low switching frequencies. The rated energy storage of the submodule capacitors is a driving factor of the size, cost, and weight of the submodules. Although the modular multilevel converter has been thoroughly investigated in the literature, a more detailed analysis of the energy-storage requirements will provide an important contribution for dimensioning and analysis of modular multilevel converters. Such an analysis is presented in this paper. The analysis relates the power transfer capability to the stored energy in the converter and the findings are validated by both simulations and experimental results. The required size of the submodule capacitors in a 4.5 MW grid-connected converter is first calculated and the calculated operating range is then compared with simulation results. The experimental results show that if the average capacitor voltage is allowed to increase 10% above the nominal value an energy storage to power transfer ratio of 21 J/kW can be achieved. It is concluded that the presented theory can relate the power transfer capability to the energy storage in the converter and is thus a valuable tool in the design and analysis of modular multilevel converters.

On Energy Storage Requirements in Modular Multilevel Converters

This paper consists of the presentation of a regulation strategy capable of controlling the energy stored in the modular multilevel converter (MMC) in an HVDC configuration. This is achieved by regulating the positive, negative, and zero sequences in dqo coordinates of the differential current using two rotating reference frames: at once and at twice the fundamental grid frequency value. The active and reactive negative sequence components of the differential current at twice the fundamental frequency are used to eliminate the oscillations of the three-phased leg energy, reducing significantly the capacitor voltage oscillations, while the zero-sequence component is used to regulate the total energy stored at a given reference. Meanwhile, active and reactive positive sequence components of the circulating current are used for eliminating the average energy difference between the upper and lower arms in a three-phase MMC. In order to decouple efficiently the differential current components, the decoupled-double-synchronous-reference-frame current control strategy is used. Finally, simulation results validate the performance of the MMC in an HVDC configuration with the proposed control. Control equations are demonstrated, and cross-coupled leg-energy terms are introduced.

An Energy-Based Controller for HVDC Modular Multilevel Converter in Decoupled Double Synchronous Reference Frame for Voltage Oscillation Reduction

Review of grid applications with the Zurich 1 MW battery energy storage system

At the design framework, in this Thesis s the modular multilevel converter (MMC) is analysed and proposed for connecting the power grids In order to mathematically describe the converter behaviour, the linear time-varying model (LTV) is developed The system is considered linear between two switching instants Since the switching functions have a periodical nature, the steady state of the system is calculated Using the (LTV) model, both balanced and imbalanced operations of the converter are analyzed Since the calculation of the steady state requires a low computational effort, the proposed method showed a good alternative to the numerical iterative procedures Applying the LTV method to the modular multilevel converter, the different operating points are determined depending from the converter parameters Special attention was paid on finding the optimal operating point concerning the module current Since the presence of the second harmonic of the converter's arm current and its dependence from the converter parameters is determined, it is used to optimize the module conditions The resonance points of the converter's arm current were also detected and presented

http://ieeexplore.ieee.org/iel5/5254890/5278662/05278961.pdf

Optimization of the modular multilevel converters performance using the second harmonic of the module current

The first Electric Storage System connected to the MV Italian distribution network has been activated during 2012 by Enel Distribuzione. The installation is based on a lithium-ion battery solution by Siemens AG; the rated power is 1 MVA and the energy capacity is 0.5 MWh. This paper presents the results of site acceptance tests (evaluation of the efficiency, consumption of the auxiliary services etc.) and the performances observed in the first year of operation, with a particular focus on “Black Start” application.

http://ieeexplore.ieee.org/iel7/6676859/6683125/06683525.pdf

Performances of the first electric storage system of Enel Distribuzione

A submodule charges or discharges an energy store and contains a capacitor unit and a power semiconductor circuit having power semiconductors that can be switched on and off. The capacitor unit and the power semiconductor circuit are connected to each other such that, after actuating the power semiconductors, the voltage released at the capacitor unit or a zero voltage can be produced at output terminals of the submodule. In order to provide a submodule allowing individual adaptation of the charging operation to the requirements of the energy store which, is cost-effective, the energy store is connected to the submodule via a DC-DC regulator. The regulator is connected to the capacitor unit and is configured to convert a capacitor voltage into a charge voltage that is required for charging the energy store, and to convert a discharge voltage released at the energy store during discharge into the capacitor voltage.

Converter and submodule of a converter for charging or discharging an energy store

/pdf/performances-of-the-first-electric-storage-system-of-enel-25hu2x0nbs.pdf

Energiespeichervorrichtung (1), umfassend mehrere Speichermodule (6) fur elektrische Energie, die jeweils an eine Kondensatoreinheit (5) eines einem Umrichter (2) zugehorigen Umrichtersubmoduls (4) zum Laden oder Entladen wenigstens eines Speichermoduls (6) uber wenigstens einen ansteuerbaren Gleichspannungssteller (7) angekoppelt sind, welcher Gleichspannungssteller (7) zum Umwandeln einer an der Kondensatoreinheit (5) anliegenden Kondensatorspannung in eine Ladespannung, die zum Laden des Speichermoduls (6) erforderlich ist, und zum Umwandeln einer beim Entladen an dem Speichermodul (6) anfallenden Entladespannung in die Kondensatorspannung ausgebildet ist, wobei eine zentrale Steuereinrichtung (11) vorgesehen ist, die zur Ansteuerung der einzelnen Gleichspannungssteller (7) unter Berucksichtigung von Betriebsdaten aller Speichermodule (6) und externer Anforderungsdaten (12) ausgebildet ist, und/oder jeder Kondensatoreinheit (5) und/oder jedem Gleichspannungssteller (7) eine Steuereinheit (8) zugeordnet ist, die fur wenigstens zwei in einer ihre Lade- und Entladeeigenschaften beeinflussenden Eigenschaft unterschiedliche Speichermodule (6) modulspezifisch konfigurierbar ist.

Energiespeichervorrichtung, umfassend mehrere speichermodule für elektrische energie

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Optimization of the modular multilevel converters performance using the second harmonic of the module current

Performances of the first electric storage system of Enel Distribuzione

Converter and submodule of a converter for charging or discharging an energy store

Performances of the first electric storage system of Enel Distribuzione

Energiespeichervorrichtung, umfassend mehrere speichermodule für elektrische energie