Modular Multilevel Converters with Integrated Split Battery Energy Storage

The power conversion system for a battery-energy storage system typically employs a conventional voltage-source converter with battery strings directly connected to the dc bus. This system configuration presents several issues, such as limited efficiency of two-level converter systems and the limited reliability associated with the use of long battery strings. This paper examines three viable multilevel converter solutions for integrating battery energy storage that offer the potential for enhanced efficiency and reliability. These solutions are the modular multilevel converter (MMLC) with battery energy storage distributed into its submodules, the cascaded converter, and the MMLC with battery energy storage centralized on its dc link. The three systems are compared in terms of efficiency, reliability, and module redundancy. It is determined that the MMLC with distributed battery energy storage must operate differently from conventional MMLC systems. Its operation is therefore studied in detail and validated through simulation to demonstrate its suitability for distributed energy-storage integration. The analysis shows that the MMLC with distributed battery energy storage requires the largest number of semiconductor devices for a given power level; however, it also provides the most efficient, reliable, and versatile solution of energy-storage integration.

This paper presents the modular multilevel converter (MMC), which is a new topology of voltage source converter (VSC). The converter is characterized by a modular arm structure consisting of cascade connection of half-bridge sub-module (SM) with floating dc capacitors. The topology and modulation strategy are analyzed. The applications of MMC are HVDC, FACTS and so on, which have higher and higher voltage. More and more SMs are connected in the bridge, so nearest-voltage-level (NVL) modulation strategy is taken in this paper. The transient mathematical model for MMC in dq0 reference frame is developed, and the corresponding decoupled controllers are designed to control the active and reactive powers respectively. In order to testify the proposed controllers, simulations are implemented with the PSCAD/EMTDC.

This paper presents the techniques for regulation of system balancing in modular multilevel converter (MMC) based HVDC system under the AC grid double line fault condition. Due to the special structure of MMC, the balancing of the cell capacitor voltage is crucial for any operating circumstance of the system. Also, the responses against the grid faults of HVDC system based on MMC are different to those based on the conventional voltage sourced converters. Therefore, even under the nonpermanent fault in AC grid, the balancing of the cell capacitor energy of MMC has to be guaranteed continuously. Among AC grid fault modes, the double line fault causes one of the most severe transients in MMC. In this paper, a balancing control method of MMC based on an offset voltage injection strategy in the output reference voltages is proposed. By the virtue of the proposed method, the HVDC system based on MMC can have the enhanced fault ride-through capability against the double line fault in AC grid. The validity of the proposed method is verified by simulation results of a ±200kV, 400MVA point-to-point HVDC system.

One of the main challenges of voltage source converter based high voltage direct current (VSC-HVDC) transmission systems is the AC faults at the grid side. This work introduces the integration of multiphase induction machine (IM) based flywheel energy storage systems (FESS) with VSC-HVDC systems for AC side fault ride through purposes employing modular multilevel converters (MMC). MMCs have become suitable candidates for medium/high power energy conversion systems due to the capability of simply extending the levels of the converter while retaining high levels of reliability. In order to enhance the storage system reliability, a dual three phase IM is used to drive the FESS due to its fault tolerance capability. In this paper, the performance of the FESS is investigated under the operation of a dual three phase IM being driven by two three-phase MMCs. To step-down the DC-link voltage of the HVDC system to a proper voltage level for IMs, the DC-link voltage is divided into two series connected capacitor, and each capacitor voltage is fed as an input DC voltage for each three- phase MMC. The control strategies of the MMCs and the IM are presented, in addition to the IM mathematical model. Simulation case studies are performed using MATLAB/Simulink to validate the proposed system.

Modular multilevel converter with integrated battery energy storage system (MMC‐BESS) has been proposed for energy storage requirements in high‐voltage applications with large‐scale renewable energy resources. The MMC‐BESS is essentially a three‐port converter which can transfer energy between any two of the ac port, dc port, and BESS, constituting multimode operation of the system. Different from conventional MMCs, the state‐of‐charge (SOC) inconsistency among batteries would magnify the submodule capacitor voltage unbalance issue in the MMC‐BESS. In this study, an improved capacitor voltage balancing method applicable for multimode operation of MMC‐BESS is proposed by adjusting ac and dc modulation indexes simultaneously. After that, the ratio of ac and dc modulation indexes is optimised to enhance the tolerance of unbalanced power among SMs within phase arm, greatly improving the SOC equalisation rate. Based on the proposed capacitor voltage balancing method, the control structure is given and the dynamic model is conducted for the analytic design of the closed‐loop controller. Finally, the simulations and experimental results validate the effectiveness and feasibility of the proposed control strategy.

To ensure the reliable operation of modular multilevel converters (MMCs) in their widespread applications, the isolation of the faulty components from the system is mandatory. Power switching devices are the components in MMCs with the highest failure probability. Among different failure types of switching devices, the diagnosis of open circuit (OC) faults is extensively instigated because it is the most common fault type of MMC. In this paper, the fault diagnostic techniques presented in the literature for MMC in case of switch OC failures are analyzed and summarized. The fault diagnostic strategies are studied from model-based perspectives, which has the advantages of easy implementation and lower cost.

Microgrids are growing at a right pace due to their advantages towards the economical environmental sustainability. Effective operation and management of microgrids can significantly help both the microgrid operator and customers to get economic and technical benefits. In addition to distributed generation (DG) and battery energy storage system (BESS), a microgrid can effectively utilize demand response (DR) strategy to better manage the energy and improve the performance of the network. In this paper, a mathematical formulation for day-ahead energy management of a microgrid is developed. The day-ahead scheduling of resources have been done with the available information of DG, load demand and electricity price. The simulation case studies with two DR schemes have been carried out for four bus test system and results are presented to compare the DR schemes.

Grid emulators provide efficient manners to test grid-connected distributed power generators. They test the power generator such as photovoltaics, wind turbines without hardware damages. Recent studies investigated conventional voltage source inverters such as a two-level voltage source inverter and a three-level voltage source inverter as grid emulators. However, they have limited voltage capabilities and weak transient responses. This paper presents a technique for the control of a modular multilevel converter as a grid emulator in fault scenarios, where voltage sags, harmonics distortions, and frequency deviations are emulated. The steady-state performance in these scenarios is analyzed and compared with that of a two-level voltage source inverter. Energy balancing between the sub-modules in a modular multilevel converter is always a crucial topic in terms of stability, particularly under fault conditions. In this paper, the transient response of the point of common coupling voltage and the sub-module energy in the modular multilevel converter with a delta-wye configuration is analyzed and compared with a two-level voltage source inverter. Simulations of a 24 sub-modules modular multilevel converter verify the control method with S70 V at the dc-link and a 400 V line-to-line ac voltage at the point of common coupling. The modular multilevel converter grid emulator with a delta-wye transformer configuration shows better steady-state performances than a two-level voltage source inverter. The energy stored in each sub-module is well balanced during the fault scenarios, which indicates strong stability.

Electrical power systems are currently facing a steady increase of converter-based generation and a loss of conventional power plants. In order to compensate the decrease of inertia and to retain grid restoration capability, the converters have to operate in grid-forming mode and act as a voltage source. While the grid-forming concept itself is applicable to many converter types, the challenge for Modular Multilevel Converters (MMC), in which power input and power output are decoupled, is the voltage control of the submodule capacitors.This paper deals with the laboratory implementation of a grid-forming MMC with submodule-integrated as well as DC-bus-connected batteries. The objective was to enhance the established grid-feeding control, which decouples power input and power output of the converter, towards grid-forming control and operation. Therefore, the inner current controllers and outer energy balancing controllers were extended by voltage and PQ droop controllers and virtual impedances. The two main features shown in the test cases were instantaneous active power support and parallel operation of the grid-forming MMC with other voltage sources, i.e. with the grid and with another grid-forming converter. Load switching in islanded mode proved the ability to provide active power instantaneously while maintaining the voltage balance of the submodule capacitors.From these results, important functions of a grid-forming MMC can be derived for future power systems, e.g. instantaneous frequency support and inertia emulation or grid restoration capability.

The modular multilevel converter (MMC) is emerged as the most favourable multilevel converter topology for medium to high-voltage applications. Reliability is one of the major concerns in an MMC due to high fragile component count. Due to low-cost and high energy density, electrolytic capacitors (ECs) are usually preferred as floating capacitors in the MMC. However, the ECs are gradually degraded over the time due to inherent chemical processes, which results in a decrease in capacitance value. This results in an increase in voltage ripple across the SM capacitors and would distort the output waveforms. Moreover, the prolonged use of these aged capacitors could disrupt the MMC operation. Therefore, monitoring and failure detection of submodule (SM) capacitors in an MMC are pivotal to enhance the reliability. This paper presents an SM capacitance estimation strategy for an MMC using nearest level modulation (NLM). A modified voltage balancing control structure with NLM is presented to reduce the unbalance in the capacitor voltages. The proposed approach uses available hardware for converter control with easy implementation in the same controller. Extensive studies are conducted on a three-phase MMC in PLECS simulation platform. Furthermore, experimental results are presented to substantiate the effectiveness of the proposed approach.

With an increasing number of renewable energy resources being integrated into the power system through power electronics, conventional power plants based on large synchronous machines tend to be reduced in the upcoming future. With the modular multilevel converter (MMC) as the state-of-the-art power electronics solution for power systems, it will be necessary to operate some of these converters in grid-forming mode, in order to either preserve the stability and robustness of the ac grid or as the only solution to generate the ac grid in cases with no available synchronous generation. This article aims to understand how the internal energy control of the MMC can be used to effectively mitigate the propagation of ac power disturbances to the dc side, thanks to its buffering capability. Different energy control structures are explored, and a comprehensive discussion and comparison among them are provided, including stability analysis, transient performance, and tuning guidelines. Simplified and linear models are used to address the analysis, whereas detailed nonlinear models implemented in MATLAB Simulink are used to verify the results via time-domain simulations.

The modular multilevel converter (MMC) has unique topological characteristics and has gained significant popularity in medium-voltage applications. However, during MMC operation, circulating currents inevitably arise, exacerbating arm current distortion, causing additional losses in the converter system, and thereby increasing system costs. This paper primarily addresses the circulating current issue in traditional half-bridge MMCs by introducing a control strategy combining model predictive control and proportional resonance controllers. First, a value function is established using a discrete prediction model of the output current, followed by a rolling optimization that combines a capacitor voltage sorting strategy to determine the duty cycles of each submodule in the arm. Secondly, a proportional resonance controller (PR) is designed to eliminate the second- and fourth-order harmonic components in the circulating current. Finally, the output of the resonance controller is used to correct the duty cycles, which are then compared with the PWM triangular carrier to generate more switching actions that suppress the circulating current. The effectiveness of the strategy is experimentally verified. The results show that the proposed method yields better output characteristics, smaller capacitor voltage fluctuations, and significantly suppresses harmonic components in the arm currents.

The modular multilevel converter (MMC) in high voltage direct current (HVDC) is equipped with hundreds of sub-modules (SMs), so the detailed model consist of hundreds of switching devices and consequently large computational effort but mathematical converter model with Arm Level Averaged (ALA) allows to know the dynamic and converter control decreasing simulation time. This paper presents a control strategy decou- pling the modulation index implemented in MMC-ALA model, it allows MMC operation with variable DC bus voltage but keeping constant the capacitor voltage in upper and lower arms, the evaluations consider the next operations: boost mode reducing DC bus voltage to 0.5.p.u., DC fault and SM voltage increased. The upper and lower capacitor voltages in all cases follow the reference independently to DC bus voltage value, the strategy is simulated in 150 MW converter using Matlab/Simulink.

Modular multilevel converter (MMC) is drawing more and more attention in medium-voltage microgrid applications. Large-scale switching-off loads and grid faults may lead to the voltage swell in microgrid, where MMC should have the ride-through ability. In this paper, a simple operation approach with the fundamental and irregular zero-sequence voltage (ZSV) injection is proposed for MMC under grid voltage swell condition. First, the fundamental ZSV injection is applied to adjust the modulation margin of three phases where the amplitude values of modulation waves in each phase can be the same. Then, if the amplitudes with fundamental ZSV injection are still in over-modulation status, the new irregular ZSV injection is added to further enlarge the operation region. In addition, the limitation of the irregular ZSV injection is analyzed. Simulation and experimental results of MMC under different grid voltage swell conditions verify the effectiveness of the proposed operation approach.

Renewable energy integration in the smart grid - including solar photovoltaic (PV) systems - presents stability and reliability challenges due to their intermittent behavior. Integrating battery energy storage systems (BESS) with PV systems is one of the key solutions to these grid challenges, which improves the grid-tied PV systems' performance. Due to scalable and modular structure, modular multilevel converters (MMC) have been recently considered as a promising power electronic technology for grid integration of PV-BESS. This paper proposes a novel full-bridge submodule MMC (FBMMC)-based PV-BESS, where each full-bridge submodule (FBSM) is interfaced with a PV module through a DC-DC converter to realize independent maximum power point tracking (MPPT) and full control over the MMC DC-link where a large BESS is connected to support grid ancillary services. This BESS is used to cover the mismatch between the PV system power output and the grid demand. The proposed topology and its control systems are tested for a 13-level MMC system under different solar irradiance conditions and grid power demand.