Converter Arm Research Articles

Optimization of Cell Voltage and Circulating Current With Zero-Mean Current Command Injection in Modular Multilevel Converters

One of the modular multilevel converter (MMC) applications with scalability is renewable energy generation. Nowadays, renewable energy is especially the source of momentum, which stimulates the energy transition and reduction of greenhouse gas emissions. However, its existing control schemes may not precisely reveal the inherent mannerism of MMCs. The aim of this article is to optimize the cell voltage, arm current, and circulating current of converter arms in MMCs based on the Lagrange multiplier of an energy cost function and arm current tracking control law. Meanwhile, determination of the capacitors and switch ratings in each cell still has a big challenge, and it can result in cell-voltage fluctuations under unbalanced grid-connection conditions. This article presents unique approaches to derive cell-voltage expression in terms of arm currents, which reduces cell-voltage fluctuations by injecting zero-mean current, and the duty-ratio control laws. Moreover, tradeoff between cell voltage and circulating current can be also conducted. Experimental and simulated results from a 3-kW single-phase MMC have verified the functionality of the proposed control method, analysis, and comprehensive discussion.

A Submodule-Capacitor Voltage Balancing Strategy for Alternative-Arm Converters in HVDC Systems

The standard alternative-arm converters (AAC) with the sweet spot or short overlap operation have faced a challenge of balancing submodule (SM) capacitor voltages, where this issue has not been paid much attention so far. In this paper, a new control strategy for the SM-capacitor voltage balancing of the AAC applied in high-voltage dc transmission systems is proposed. The suggested control technique is composed of three regulators, where a control for the average voltage of all SM capacitors, the voltage balancing among the converter arms, and the voltage balancing of the individual SM capacitor are performed. Based on the operation principle of the standard AAC, which activates only three of six converter arms simultaneously, the energy balancing of the three activated arms is performed every one sixth of the fundamental cycle by the converter common offset voltage. Band-stop filters with the central frequencies at the fundamental, second-, and third-order components of the line frequency are applied to obtain the average values of the SM capacitor voltages for the feedback controllers. The simple proportional and integral controllers are employed for this scheme and the voltages of SM capacitors are mostly balanced during the steady states and kept closely to the rating with overshoot of less than 7% under the transient conditions. The proposed control method is verified by simulation results for a 33-level AAC under both normal and fault conditions.

Optimality in the Sense of Arm Current Distribution of MMC VSC-HVDC

This letter investigates the optimality of the modular multilevel converter (MMC) voltage source converters high-voltage direct current (VSC-HVDC), which constitutes a linear time-invariant (LTI) system for sufficiently large number of submodules per valve. The contribution of this letter is the mathematical proof that MMC VSC-HVDC is the optimal LTI topology in the sense of arm current distribution. Namely, the findings of this work show that MMC VSC-HVDC attains an optimal limit in the way the ac and dc components of the converter arm currents are distributed in the field of linear converter topologies, and may only be outperformed by nonlinear or time-varying VSC-HVDC topologies.

The Asymmetric Alternate Arm Converter: A Compact Voltage Source Converter Design for HVDC

The Modular Multilevel Converter (MMC) has become one of the most practical topologies for integration of renewable energy power plants into the grid, through submarine HVDC cables and HVDC lines. Nonetheless, the MMC requires significant large DC storage capacitors to mitigate for large power, low frequency pulsations due to the single-phase like AC to DC conversion process in their arms. This work addresses the recently introduced Asymmetric Alternate Arm Converter, a hybride MMC topology which uses modular converter arms only on the lower side of the main converter whereas the upper side uses only standard ON-OFF valves topology. This converter is similar to the Alternate Arm Converter (AAC) but dispenses of the modules on the upper side and of the director switch on the lower side. Like the AAC, the Asymmetric AAC offers a significant reduction in DC energy storage requirements and in the number of modules. In addition, the Asymmetric AAC offers a less restricted operation than the AAC, in terms of operating over a wide range of AC to DC voltage ratio, and the possibility of achieving both distortion-free AC current and ripple-free DC current. The principle of operation of the Asymmetric AAC is revisited here and a suitable control strategy to demonstrate its operation is developed. In addition, requirements for energy storage, voltage and current rating of converter modules and converter losses are examined by means of simplified analyses. Proposals are verified through simulation studies and experiments conducted on a low power prototype having three H-bridge modules per arm. Results confirm good operation of the proposed converter and control scheme.

Gradient-Based Energy Balancing and Current Control for Alternate Arm Converters

The alternate arm converter (AAC) is an emerging fault-tolerant multilevel converter topology from the same family of multilevel converters as the modular multilevel converter (MMC). Due to the alternate operation of the converter arms, energy balancing in the AAC is not continuous, but restricted to small time intervals. This paper develops a gradient-based current control and energy-balancing method for the AAC. The proposed strategy enforces the dynamic limits on the redundant submodules (SMs) during the overlap period and allocates effectively the maximum available number of redundant SMs to control the circulating current. The choice of the gradient as the circulating current control parameter improves the energy regulation capability of the AAC, and the enforcement of dynamic limitations avoids distortions of the output voltage. Results from an AAC-based HVDC converter model derived from the CIGRE benchmark MMC system demonstrate that the proposed strategy delivers improved energy control and balancing with good harmonic performance compared to existing current control methods for the AAC while also maintaining zero-current switching of the director switches of the AAC arms.

A Transformerless High-Voltage DC–DC Converter for DC Grid Interconnection

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal.

Analysis and Control of M3C-Based UPQC for Power Quality Improvement in Medium/High-Voltage Power Grid

To enhance the power quality in the medium/high-voltage distribution power systems, a single-phase unified power quality conditioner (UPQC) based on the modular multilevel matrix converter (M3C) is presented in this paper. The M3C-UPQC is comprised of four identical multilevel converter arms and associated filtering inductors. According to the established equivalent circuit of M3C-UPQC, its operation principle and power balance of each arm are analyzed theoretically, and the parameters’ design for the arm inductance as well as submodule capacitance is studied. Then, an integrated control method for M3C-UPQC in which the dc circulating current is used to balance the instantaneous active power of each arm is proposed to prevent the capacitor voltages from divergence inter- and intra-arms, so as to achieve voltages balance of M3C-UPQC. Finally, the effectiveness of the proposed control method is verified by a prototype rated at 8 kVA.

A novel MMC control scheme to increase the DC voltage in HVDC transmission systems

This research investigates third harmonic injection applied to a modular multilevel converter (MMC) to generate a higher DC voltage. This is achieved using a proposed novel control scheme that activates existing submodules (SMs) in the converter arms. The technique is fundamentally different to, and does the reverse of, the well-known third harmonic injection techniques utilized to increase the AC output voltage in three-phase converter systems for a given DC link voltage. In the proposed scheme, the number of inserted SMs in each converter leg is greater than the number of SMs per arm, whence the MMC can operate with a higher DC link voltage while the SM number per arm and their capacitor voltages remain unchanged. This lowers the DC current and the DC transmission loss is significantly reduced by 22%. Station conduction losses with the operational scheme are lowered by 2.4%. The semiconductor current stresses are also lowered due to the reduced DC component of arm currents. Additionally, the phase energy variation is reduced by 18%, which benefits circulating current control. The operating principles are presented in detail and mathematical models for conduction losses, energy variation, and circulating voltage are derived. Simulation of a point-to-point HVDC system demonstrates the effectiveness of the proposed MMC operational scheme.

Reliability Analysis and Redundancy Configuration of MMC With Hybrid Submodule Topologies

Modular multilevel converter (MMC) has become the most promising converter technology for high-voltage direct current (HVdc) transmission systems. MMC submodule (SM) topologies with dc fault ride-through capabilities are emerging which are suitable for overhead line applications. The hybrid SM design of each converter arm can get the compromise of higher capability of handling dc fault and lower capital investments and losses. In this paper, the initial hybrid SM numbers design method for supporting the dc-link voltage and riding-through dc faults and the optimized hybrid SM redundancy configuration strategy for effectively increasing the reliability of MMC are proposed and calculated. In contrast with the previously proposed redundancy configuration for MMC with the single SM topology, this approach solves the curvature of three-dimensional surface to calculate the recommended redundant hybrid SM numbers which takes both the semiconductor device utilization rate and the reliability of MMC into consideration.

Continuous Operation of Radial Multiterminal HVDC Systems Under DC Fault

For a large multiterminal HVDC system, it is important for a dc fault on a single branch to not cause significant disturbance to the operation of the healthy parts of the dc network. Some dc circuit breakers (DCCBs), for example, mechanical type, are low cost and have low power loss, but have been considered unsuitable for dc fault protection and isolation in a multiterminal HVDC system due to their long opening times. This paper proposes the use of additional dc passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy dc network can continue to operate without disruption during a dc fault on one dc branch. Two circuit structures, using an additional dc reactor, and a reactor and capacitor combination, connected to the dc-link node in a radial HVDC system, are proposed to ensure that overcurrent risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of dc fault current by dynamically regulating the dc components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method.

Optimized Control Strategy Based on Dynamic Redundancy for the Modular Multilevel Converter

Considering the synthetical effects of the ac system and the dc system, this paper proposes two novel indexes, the dynamic redundancy and the utilization ratio of the submodules. On this basis, the nearest level control is applied as the modulation method and an optimized control strategy based on the dynamic redundancy for the modular multilevel converter (MMC) is proposed. One of the main innovations is that the reference value of the capacitor voltage is derived according to the maximum output voltage of each converter arm and the safety margin which can be adjusted artificially. Unlike previous strategies, the redundancy can be adjusted dynamically, and the utilization ratio of the submodules can be effectively improved. In addition, the capacitor voltage and the inner stress are reduced, and the fault ride-through capability of the system can be enhanced. In particular, the strategy under abnormal operating conditions is also detailed in this paper. A model of two-terminal MMC-high-voltage direct current system is built in PSCAD/EMTDC, and the simulation result proves the validity and the feasibility of the proposed strategy.

Half- and Full-Bridge Modular Multilevel Converter Models for Simulations of Full-Scale HVDC Links and Multiterminal DC Grids

This paper presents an improved electromagnetic transient (EMT) simulation model for the half- and full-bridge modular multilevel converters (MMCs) that can be used for full-scale simulation of multilevel high-voltage dc (HVDC) transmission systems, with hundreds of cells per arm. The presented models employ minimum software overhead within their EMT parts to correctly represent MMCs behavior during dc network faults when converter switching devices are blocked. The validity and scalabilities of the presented models are demonstrated using open-loop simulations of the half- and full-bridge MMCs, and closed-loop simulation of a full-scale HVDC link, with 201 cells per arm that equipped with basic HVDC controllers, including that for suppression of the second harmonic currents in the converter arms. The results obtained from both demonstrations have shown that the presented models are able to accurately simulate the typical behavior of the MMC during normal, and ac and dc network faults.

Control and protection strategy for MMC MTDC system under converter-side AC fault during converter blocking failure

This paper investigates a control and protection strategy for a four-terminal modular multilevel converter (MMC) based high-voltage direct current (HVDC) system under a converter-side AC fault. Based on the system operating condition, a control and protection strategy against the fault with normal blocking of the converter is proposed. In practical, applications encountering such a fault, the MMC at the fault side may experience different conditions of blocking failure. The blocking failures may occur on: ① the whole converter; ② one converter arm; ③ one sub-module (SM)/several SMs of one converter arm; ④ other conditions. The phenomenon of the multi-terminal HVDC (MTDC) system following the fault is analyzed under the first three conditions with real-time simulations using the real-time digital simulator (RTDS). Based on the impact of different conditions on the MTDC system, the necessity of utilizing special control and protection is discussed. A special control and protection strategy is proposed for emergency conditions, and its effectiveness is verified by real-time simulation results.

Optimal Selection of the Average Capacitor Voltage for Variable-Speed Drives With Modular Multilevel Converters

Variable-speed drives have reduced voltage requirements when operating below the base speed. In a modular-multilevel-converter-based (M2C-based) motor drive, it is then possible to operate with reduced voltage in the submodule capacitors, than at the base speed. In this sense, a greater capacitor-voltage ripple can be accommodated, without exceeding the maximum peak-capacitor voltage. This paper presents an analytical investigation for the optimal selection of the average capacitor voltage for M2Cs, when the motor is operating with rated torque, below the base speed. This method does not require any power exchange between the converter arms, so it keeps the conduction losses at the minimum level. Additionally, the method decreases the switching losses, due to the decreased capacitor-voltage level. The overall ratings of the converter remain the same as in the base-speed operation. It is shown that this method can be applied at a speed range between the base speed and down to approximately one third of it, i.e., an operating range that covers the requirements for typical pump- and fan-type applications. The results obtained from the analytical investigation are experimentally verified on a down-scaled laboratory prototype M2C.

Global Asymptotic Stability of Modular Multilevel Converters

Modular multilevel converters require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Assuming that the individual capacitor-voltage sharing is managed effectively, an open-loop strategy has been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable. Experimental verification on a three-phase 10-kVA prototype is presented.

Predictive Control of a Modular Multilevel Converter for a Back-to-Back HVDC System

The modular multilevel converter (MMC) is one of the most potential converter topologies for high-power/voltage systems, specifically for high-voltage direct current (HVDC). One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents of converter arms while the capacitor voltages are maintained balanced. This paper proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system. A discrete-time mathematical model of the system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization a defined cost function associated with the control objectives of MMC units and the overall HVDC system. The proposed predictive control strategy: 1) enables control of real and reactive power of the HVDC system; 2) achieves capacitor voltage balancing of the MMC units; and 3) mitigates the circulating currents of the MMC units. Performance of the proposed MPC-based strategy for a five-level back-to-back MMC-HVDC is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate a satisfactory response of the MMC-HVDC station operating based on the proposed MPC strategy, under various conditions.

Open-Loop Control of Modular Multilevel Converters Using Estimation of Stored Energy

The internal control of a modular multilevel converter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternating output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure.

Directional Converter Arm Method for Surface and Interfacial Tension Measurements with a Top-Loading Balance

A method is described for utilizing a top-loading balance, with a directional converter arm, in vertical-pull surface force measurements. The Padday rod-pull technique, the du Noüy ring method, and the Wilhelmy plate method are utilized with rods, thin-walled tubes, wire rings, and plates either rigidly attached to the converter arm or hanging freely from a hook at the end of the arm. The robustness, large weighing capacity, and accuracy of top-loading balances make them ideally suited for a variety of types of surface and interfacial tension measurements. The converter arm method can be used with a stainless steel rod (3–7 mm in diameter) in vertical-pull surface tension measurements, with samples having volumes of only a few tenths of a milliliter. Measurements on very small liquid volumes are feasible because the rod is firmly attached to the converter arm rather than hanging freely as in measurements with balances mounted above the sample; therefore, the rod cannot swing toward and attach to the wall of small sample tubes. Automation of force and height measurements with the converter arm/top-loading balance method is straightforward.