Capacitor Voltage Balance Control Research Articles

A Hybrid Multiobjective Control Strategy Based on Combination of Modulated Model Predictive Control and Phase‐Shifted Modulation for a Unidirectional Five‐Level Rectifier

ABSTRACTThe unidirectional multilevel rectifier has immense potential in high‐power medium‐voltage industrial applications. This paper proposes a multiobjective control method based on the combination of optimal‐vector‐pair modulated model predictive control (OVP‐M2PC) and a phase‐shifted modulation strategy for a unidirectional five‐level rectifier. This strategy takes both of the inductor current tracking and capacitor voltage balancing as the control objectives. First, an optimal vector pair is determined based on the AC‐side current variation rate, and then, through combining the calculated action duration of OVP with carrier phase‐shifted modulation, inductor current tracking is achieved. Second, capacitor voltage balancing control is carried out by compensating for the duty cycles which also decouples the input current tracking and output capacitor voltage balance control. Compared with traditional FCS‐MPC, under the proposed control strategy, complex cost function calculation and weighting factor tuning are not required, which much reduced computational burden. Multiobjective control is achieved with better performance in both steady‐state and dynamic conditions. The superiority of the proposed strategy over traditional FCS‐MPC are validated through simulations and hardware experiments.

A Discrete-Time Current Control Method for the High-Speed Permanent Magnet Motor Drive Using the Modular Multilevel Converter

High-speed permanent magnet motors (HSPMMs) have received attention in direct drive systems from the industry. Modular multilevel converters (MMCs) have become the most practical solution for HSPMMs in industrial applications due to the advantages of their highly modular structure, high-quality output and high reliability. In this article, the symmetrical discrete-time model of the MMC for the HSPMM drive is presented. Based on this symmetrical discrete-time model, the discrete-time current regulator has been designed. Moreover, the submodel (SM) capacitor energy balance models for MMC have been described, and the energy balance control scheme is introduced. Finally, the simulation and experimental results show that the presented discrete-time current regulator has a strong dynamic response at high speed (15 kr/min), and the SM capacitor voltage balance control is also achieved. Compared with the conventional PI current controller, the proposed discrete-time current regulator improves the current loop dynamic performance at high-speed, and the energy balance control scheme can ensure the SM capacitor voltage balance of the MMC.

Optimal Decoupled Discontinuous Modulation for StatComs Based on the Cascaded H-Bridge Converter

Conventional discontinuous pulse width modulation (DPWM) strategies for cascaded H-bridge static compensators suffer from poor inter-phase capacitor voltage balance control during unbalanced grid voltage conditions. Specifically, the logic-based algorithm that calculates the zero-sequence voltage (ZSV) for discontinuous operation interacts with the closed-loop capacitor voltage control. This paper proposes an optimal DPWM strategy based on a finite-set optimization that chooses the best ZSV candidate that minimizes the control interactions while clamping a converter voltage. Furthermore, the effect of the different ZSV candidates on the switching loss is also considered within the optimization. Extensive experimental study is provided to demonstrate the effectiveness of the proposed approach.

A Novel DC-Link Capacitor Voltage Balance Control Based on Coordination of Rectifier–Inverter Stage in a Modified Unidirectional Five-Level Converter for AC Motor Drives

A Novel DC-Link Capacitor Voltage Balance Control Based on Coordination of Rectifier–Inverter Stage in a Modified Unidirectional Five-Level Converter for AC Motor Drives

Detailed Wideband Impedance Modeling and Resonance Analysis of Grid-Connected Modular Multilevel Converter

Modular multilevel converter (MMC) tends to cause resonance instability in interconnected systems. Current studies on the stability of MMC mainly focus on high-voltage and large-capacity power systems, while the impedance modeling process of MMC ignores the harmonics of the submodule (SM) capacitor voltages and the influence of voltage feedforward control. The applicability of the MMC impedance model with fewer submodules is doubtful. According to the circuit structure and control mode, the AC-side sequence impedance model of the MMC is established, which takes into account the capacitor voltage balance control of submodules and voltage feedforward control. Based on the RT-Lab control hardware-in-loop (CHIL) test platform, the MMC impedance frequency scanning was carried out to verify the accuracy of the impedance modeling method. The influence of control parameters in different frequency bands on the impedance characteristics of MMC was studied. The AC terminal voltage feedforward causes phase lag in the mid-/high-frequency range and increases the risk of resonance. In the low-frequency range, the dynamics and control of capacitor voltages reduce the impedance magnitude of MMC. Using the resonance phenomenon of an MMC connected to a weak grid as an example, the high-frequency resonance mechanism caused by control parameters is analyzed from the perspective of the negative damping effect. The simulation results show that the detailed wideband impedance model can improve the accuracy of the stability analysis results.

Control method of an AC-AC MMC for open-end stator winding wind turbine generation system

With the increasing of power rating and voltage level of wind turbine, the traditaional low voltage converter is being replaced by multilevel converter. This paper proposes a new ac-ac modular multi-level converter (MMC) connecting the open-end stator winding permanent magnet synchronous generator (OW-PMSG) to AC power grid. The working principle of the proposed topology is analyzed. Under different grid conditions, a unified capacitor voltage balance control method and a current control method are proposed. Aiming at the low grid voltage ride through problem, a distributed braking chopper circuit and the corresponding control strategy is given. Simulation is conducted for verification. Compared with the back to back (BTB) MMC for OW-PMSG, the number of capacitors and arm inductors of the proposed topology are reduced by 43% and 33%. Moreover, the number of the state variables of the average capacitor voltages and currents of the proposed topology are reduced by 33% and 47%, reducing the control complexity. The proposed topology also has higher efficiency than the BTB MMC for OW-PMSG. Compared with the other ac-ac MMC topologies for OW-PMSG, the combined power rating of the power devices of the proposed topology is much lower and thus the proposed topology has higher power density.

Control of a Hybrid Modular Solid-State Transformer for Uninterrupted Power Supply Under MVdc Short-Circuit Fault

Modular multilevel converter (MMC)-based solid-state transformers (M-SSTs) show promise in hybrid ac/dc distribution grids where there is need for a medium-voltage dc (MVdc) interface. It can be used to interconnect several distribution networks with different voltage forms and levels and enable flexible power flow among them. In an M-SST, the MVdc short-circuit fault is a critical issue due to the rapidity of the fault current, which can cause device damage and power failure of the low-voltage (LV) grids. Aiming at addressing this issue, this article proposes a fault-mode control strategy applied to a hybrid M-SST topology for uninterrupted power supply of its LV ports. The topology consists of a hybrid MMC with both half- and full-bridge submodules and isolated bidirectional dc–dc converters. Through the proposed control, the fault current is eliminated when an MVdc short-circuit fault occurs. The hybrid M-SST can keep uninterrupted power interaction between the MVac port and the LVdc port during the fault, thereby improving power supply reliability. Moreover, an improved capacitor voltage balance control and an optimized modulation scheme are developed and included in the uninterrupted operation scheme to maintain the harmonic performance of the ac port. The feasibility and effectiveness of the proposed topology and control are verified by simulation and experimental results.

Capacitance balance control strategy for three-phase four-switch open-winding permanent magnet synchronous motor

The open-winding permanent magnet synchronous motor (OWPMSM) system is a novel high-cost system that uses dual-inverters to control both sides of the stator to improve the performance of the motor. To save the system cost, the three-phase four-switch open-winding permanent magnet synchronous motor (TPFS-OWPMSM) system is proposed to replace the general three-phase six-switch OWPMSM system, but it brings the problem of bridge arm capacitor voltage offset. To solve the instability problem caused by capacitance, this paper carries out the state analysis and offset source analysis of the inverter capacitor voltage, and proposes a dual-inverters capacitor voltage balance control strategy. The capacitor balance control converts unstable capacitor offset voltage into current and injects the currents into the stator current loop through closed-loop correction. And this strategy includes the optimal control of different capacitors under dual-inverters with the goal of system stability. The effectiveness of the balancing strategy is verified by the simulation of various motor working conditions.

Hybrid injection strategy of sub-module voltage ripple suppression for MMC under low frequency operation

Energy crisis and climate changes have brought great challenges to traditional power generation and transmission. Nowadays, the power transmission system of VSC-HVDC attracts more and more attention. The reactive and active power can be separately controlled, which greatly improves the flexibility of power transmission. Moreover, the harmonic content of this system is extremely low. As an important topology of HVDC system, modular multilevel converters (MMC) have significant advantages. However, there is a problem of excessive circulating current between phases in MMC, which easily causes the fluctuation amplitude of the capacitor voltage in each sub-module to be extremely large, or even unstable. An improved method to suppress the circulating current is proposed in this paper. In order to balance the voltage of each capacitor, the average capacitor voltage control of the outer loop is used. Meanwhile, an independent controller of voltage balance and a controller of arm voltage balance are applied. In order to eliminate the circulating harmonics of currents, a controller of quasi-PR is adopted for the inner control loop. Then, a simulating model of MMC system was established to prove the effectiveness of the proposed method. Finally, an experimental prototype was built and the corresponding tested results are given. It is proved that the proposed method has obvious advantage on suppressing harmonic circulating current and capacitor voltage balance control.

Variable Voltage Variable Frequency Modular Multilevel AC/AC Converter With High-Frequency Harmonics Filtering Capability

This article proposes a variable voltage variable frequency modular multilevel ac/ac converter (VVVF-MMC) with a high-frequency isolation link, which can realize voltage and frequency conversion simultaneously with insulated-gate bipolar transistors (IGBTs) of the same rated voltage and current level. The parallel connection of multiple modules is realized by coupling reactors, which can realize large-scale voltage and current transformation and save volume. It combines parallel multiplexing and serial multiplexing technology of submodules (SMs) to increase the equivalent switching frequency, thereby canceling nonzero sequence high-frequency voltage and current harmonics, so it has good power quality in the steady-state. Under the improved carrier phase-shifted pulsewidth modulation (CPS-PWM) strategy, the pulse flip circuit can realize soft switching operation, thereby improving energy conversion efficiency. Furthermore, dc capacitor voltage balancing and parallel SM current balancing control strategies are designed. With the strong filtering ability, the proposed ac/ac converter can be used as the onshore converter station of fractional frequency offshore wind power system. Due to the advantages of small size and high efficiency, it also has broad application prospects in distribution networks and electric drive. Simulation and experimental results validate the effectiveness of the proposed converter topology.

Capacitor voltage balance control strategy based on MMC-MTDC

The MMC is composed of a large number of sub-modules. The operating conditions of each sub-module are different, which will inevitably affect the capacitor voltage of each sub-module, resulting in the difference of the capacitor voltage of each sub-module. The problem of uneven voltage of the sub-module capacitor voltage of the MMC system is unavoidable, and the uneven voltage of the capacitor will greatly affect the stable operation of the MMC system. This paper studies the multi-terminal flexible DC transmission system (MMC-MTDC) based on modular multi-level converters. The multi-terminal MMC-MTDC system model is established through the MMC-MTDC converter station level and the MMC-MTDC system level controller, and then according to the shutdown process of MMC-MTDC system, several optimal control methods are proposed to realize energy feedback and sub-module discharge to achieve capacitor voltage balance. Finally, the effectiveness of the proposed method is verified by building a simulation model.

Study of Modular Multilevel Converter in AC/DC Hybrid Distribution Network

The AC/DC hybrid power system plays an important role in the intelligent distribution network with the advantages of large power supply radius and capacity, low line cost and convenient distributed access. In this paper, a modular multilevel converter based on AC-DC side of AC/DC hybrid distribution network is exploited. An improved MPC sub-module capacitor voltage balance control method is proposed to reduce the calculation burden of MPC. The cost function is defined for phase currents, circulating currents, and sub-modules capacitor voltages. The weight factors of the cost function are eliminated, and the sub-module capacitor voltage control algorithm is improved to reduce the computational complexity of MPC. Not all cost functions for possible switching states combinations of MMC are calculated using improved method. The effectiveness of proposed improved method is evaluated based on 11 levels MMC system using Matlab/Simulink.

A Photovoltaic-Fed DC-Bus Islanded Electric Vehicles Charging System Based on a Hybrid Control Scheme

Electric vehicle (EV) charging stations fed by photovoltaic (PV) panels allow integration of various low-carbon technologies, and are gaining increasing attention as a mean to locally manage power generation and demand. This paper presents novel control schemes to improve coordination of an islanded PV-fed DC bus EV charging system during various disturbances, including rapid changes of irradiance, EV connection and disconnection, or energy storage unit (ESU) charging and discharging. A new hybrid control scheme combining the advantages of both master–slave control and droop control is proposed for a charging station supplying 20 EVs for a total power of 890 kW. In addition, a three-level (3L) boost converter with capacitor voltage balance control is designed for PV generation, with the aim to provide high voltage gain while employing a small inductor. The control techniques are implemented in a simulation environment. Various case studies are presented and analysed, confirming the effectiveness and stability of the control strategies proposed for the islanded charging system. For all tested conditions, the operating voltage is maintained within 5% of the rated value.

DC Capacitor Voltage Balance Control Method for High-Power Single-Phase Cascaded H-Bridge Rectifier to Extend the Regulation Range

Single-phase cascaded H-bridge rectifier (CHBR) plays an important role in high-speed trains. The dc capacitor voltage balance control is essential for a single-phase CHBR. However, existing dc capacitor voltage balance control methods suffer from lacking regulation range. To tackle this issue, a balance control method for dc capacitor voltage to extend the regulation range that is readily implemented is proposed in this article, whose regulation range is the widest. The steady-state operating region (SOR) of a single-phase CHBR is analyzed, and the operation loci of different methods in SOR are derived. According to the geometric relationship of operation loci, a simple method to evaluate the regulation range of different balance methods is introduced. Meanwhile, regulation ranges of different balance methods are compared based on this method, from which it can be known that the proposed dc capacitor voltage balance control method has the widest regulation range. Finally, experimental results verify the effectiveness and superiority of the proposed method.

Resonance overvoltage control algorithms in long cable frequency conversion drive based on discrete mathematics

AbstractIn order to solve the problem that the long cable variable voltage and variable frequency (VVVF) system does not adopt an effective capacitor voltage sharing control method, resulting in a poor effect of resonance overvoltage control, the resonance overvoltage control algorithm of the long cable VVVF system based on discrete mathematics is studied. First, the long cable frequency conversion drive system is established. In order to ensure voltage loss in the range of motor requirements, a frequency converter–cable–motor (ICM) system connection mode is used to maintain the system operation. Based on the research of the capacitor voltage balance control strategy of a long cable frequency conversion drive system, the discrete mathematical model of the AC side of the ICM system is established by using this control strategy. The improved constant active power controller is obtained by establishing the mathematical model, and the resonant overvoltage in a long cable frequency conversion drive is realized by using the constant active power controller. The experimental results show that the algorithm can effectively control the resonance overvoltage phenomenon in the long cable frequency control system, and the control accuracy is over 97%. It has good performance and can be applied in practice.

Carrier phase shift modulation and voltage equalization control strategy in modular multilevel converter

In order to solve the problem of voltage fluctuation of sub-module capacitors in Modular Multilevel Converter, based on carrier phase-shifted sinusoidal pulse width modulations, this paper analyzes the effect of different carrier phase shift modulation and the number of sub-modules on a single bridge arm on the number of output levels and uses proportional control to make capacitor voltage balance and PI control to suppress bridge arm circulation. The method has a good stability effect on the sub-module capacitor voltage.A 31-level model is built in MATLAB/simulink to analyze the capacitor voltage fluctuation, output current and voltage distortion rate, and bridge arm current of the sub-module. The voltage and circulation fluctuation range is compared when there is no circulation suppression. It is verified that the method is simple and effective.

HB and FB MMC Based Onshore Converter in Series-Connected Offshore Wind Farm

In contrast to the traditional high-voltage direct current (HVdc) based offshore wind farms, series-connected offshore wind farms do not require transformers, voltage source converters (VSCs), and platforms in offshore, thereby significantly decreasing costs. In this study, the onshore converter, which is based on the half-bridge (HB) modular multilevel converter (MMC), and full-bridge (FB) MMC, is proposed to reduce the costs and power loss of the series-connected offshore wind farm. In this type of onshore converter, the dc voltage of the HB-MMC is controlled to be the rated dc voltage, while the dc voltage of FB-MMC can vary from negative rated dc voltage to positive rated dc voltage. With the variable dc voltage of the FB-MMC, the dc-link current of the series-connected offshore wind farm can be controlled at will. Moreover, the dc-link current control strategies are proposed to reduce the power loss of the offshore wind farm. The capacitor voltage balance control, which is suitable for FB-MMC with variable dc voltage, is proposed to ensure the normal operation of the onshore converter. Finally, simulation and experimental results verify the feasibility of the proposed onshore converter and the corresponding control algorithm.

Series-Connected-Based Offshore Wind Farms With Full-Bridge Modular Multilevel Converter as Grid- and Generator-side Converters

Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm.

Capacitor Voltage Balance Control of Hybrid Modular Multilevel Converters With Second- Order Circulating Current Injection

The capacitor voltage imbalance between the full-bridge sub-modules (FBSMs) and half-bridge submodules (HBSMs) may occur in the hybrid modular multilevel converters under the overmodulation operation. In this paper, the sub-module output voltage model under overmodulation is established first to explore the mechanism of capacitor voltage imbalance. With the established sub-module output voltage model, the complementary second-order voltage component in FBSMs and HBSMs is disclosed, which makes the second-order circulating current a novel control freedom to balance the capacitor voltages. With proper design of the magnitude and the phase angle of the injected second-order circulating current, the energy between the FBSMs and HBSMs is transferred and the capacitor voltage imbalance under overmodulation is eliminated. Moreover, the relationship between the sub-module capacitor voltage ripples and injected circulating current is also investigated to optimize the capacitor voltage ripple. Finally, the simulation and experiment results show that the proposed second-order circulating current injection method can not only balance the capacitor voltage but also reduce the voltage ripples of capacitors and current stress of semiconductor devices.

Capacitor Voltage Balancing and Current Control of a Five-Level Nested Neutral-Point-Clamped Converter

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors.