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Voltage Balancing of Capacitors Using Kalman Filter in Modular Multilevel Converters without Current Sensors
Abstract
Modular multilevel converters (MMCs) play an essential role in power conversion for medium and high voltage industrial applications. These types of converters use many sensors for the process of measuring and sorting the voltage of their capacitors, but this increases cost and hardware complexity, as well as decreases reliability. One of the most widely used methods for voltage sorting of capacitors in MMCs is based on current measuring and detecting the current sign of each arm. This paper uses the capacitor voltages estimation scheme by the Kalman filter (KF) algorithm at each sampling time. Moreover, with the difference between the total voltage of the capacitors in each arm during two consecutive processes, their sorting is done without the current sensor. Therefore, by the proposed scheme, current sensors are entirely removed from the arms, and the MMC control process is implemented by only two voltage sensors in a phase branch. The simulation results in the MATLAB Simulink environment confirm the accuracy of the proposed method.
... Furthermore, its ability to operate with reduced complexity and lower switching losses enhances the NNPC converter's appeal within the industry, positioning it as a key technology in the advancement of sustainable energy projects [11], [12]. Since introducing NNPC converters, various control and modulation methods have been developed to enhance capacitor voltage balancing and achieve an output voltage waveform that approximates a perfect sine wave [13], [14], [15]. For instance, [16] proposes a capacitor voltage balancing method for the NNPC converter designed for MV applications ranging from 2.4 to 7.2 kV. ...
The Nested Neutral Point Clamped (NNPC) converter, functioning as a voltage source Converter (VSC), provides an effective solution for applications requiring Medium-Voltage and High-Power (MVHP). Earlier implementations of this converter typically required many sensors to maintain capacitor voltages equally in all series Half-Bridge Sub-Modules (HB-SMs). This research introduces two methodologies to minimize sensor requirements for computing capacitor voltages using novel algorithms based on estimation methods. These strategies simplify the converter control procedure by eliminating individual current and voltage sensors specified for HB-SMs. The proposed approaches ensure precise voltage balance with minimal estimation error by continuously adjusting capacitor voltages using estimated values from previous iterations and switching signals. Extensive MATLAB Simulink tests verify the effectiveness of these techniques across diverse practical scenarios. The results highlight significant simplification of sensor complexity while maintaining strong performance in NNPC Converter applications, emphasizing the importance of sensor implementation in the cost and operational effectiveness of VSCs.
... Despite these innovations, FC and NPC converters faced limitations due to their complex structural designs, high computational demands, and reliability concerns [5], [6], [7]. Modular converter topologies have been developed to address the earlier challenges, providing a more robust option for high-voltage and power applications [8]. During these converter evolutions, the Modular Multilevel Converter (MMC) has gained significant attention, offering improved scalability, efficiency, and fault tolerance in HVDC systems [9], [10], [11]. ...
A promising solution for operations in the moderate voltage and high-power ranges, the Alternate Arm Multilevel Converter (AAMC) distinguishes itself with a distinctive topology as an AC-DC Voltage Source Converter (VSC). However, traditional implementations require numerous sensors to maintain capacitor voltage balancing across series Full-Bridge Sub-Modules (FB-SBs). Introducing an innovative approach to enhance AAMC efficiency, this study proposes two sensor reduction methods for calculating capacitor voltages with estimation algorithms, demonstrating the current paths within each method. These streamlined approaches simplify the control structure and eliminate the requirement for individual current sensors in each phase and voltage sensors per FB-SBs. By continuously adjusting capacitor voltages based on estimated values used by data from executed last steps and switching signals, the provided methods achieve precise voltage balance with minimal estimation error. The methods’ effectiveness is validated through extensive simulations achieved by MATLAB Simulink software across various operational conditions. The results demonstrate notable reductions in measurement component requirements while maintaining robust performance in AAMC applications. This research highlights the potential of optimizing sensor utilization to improve the performance of voltage source converter technologies in the future.
... Other advantages of this converter include a high-quality output voltage waveform, the elimination of the need for multiple DC sources, and fault tolerance [21]. Despite all the outstanding advantages of AAMC, the inclusion of floating capacitors in the converter configuration has created challenges in controlling the capacitors' voltage balancing [22]. In conventional control methods, implementing voltage balancing control of capacitors necessitates using a large number of voltage sensors and related circuits to collect data and monitor the voltage of SMs. ...
The alternate arm multilevel converter (AAMC) is a new type of voltage source converter topology introduced as an attractive option for medium voltage and high-power applications. In AAMC, many series full-bridge sub-modules (FB-SM) are used to achieve high-voltage applications. Despite this, a large number of voltage and current sensors are needed in this converter to balance the capacitor voltage of the SMs. This paper proposes a new method to estimate the SM capacitor voltage that needs only one voltage sensor in each arm and a current sensor in the converter’s output to balance capacitor voltages. By using a proposed method based on a Kirchhoff voltage law and circuit relations govern on AAMC, the capacitor voltages can be determined. Contrary to voltage balancing control schemes based on individual sensors, the number of voltage sensors in the proposed method has been substantially reduced, and arm current sensors are eliminated. In addition, there is low estimation error since the capacitor voltages are calculated with high accuracy and updated depending on the arm voltage sensor and switching signals. Analysis of the proposed method in MATLAB Simulink for different scenarios indicates its validity.
... The development of multilevel converters has been accelerated by the increasing use of renewable energies [1]- [3]. One of the newest multilevel converter topologies recently developed is nested neutral-point clamped converters, which were first introduced in 2014 [4]. ...
The nested neutral point clamp (NNPC) converter is one of the most promising solutions for medium voltage motor drive applications. However, reliability due to the presence of semiconductor switches in the structure of this converter is one of the most critical concerns. The semiconductor switches are always one of the main components prone to failure. Switch failure can result in adverse effects, including system shutdown, poor system performance, and a reduction in overall system stability. Therefore, fault-tolerant converters are required to raise system availability in industrial applications. This research investigates and proposes the configuration of a fault-tolerant NNPC converter that uses a redundant phase to create a fault-tolerant converter. The redundant phase is in the bypass mode under normal operating conditions. After detecting the fault, the redundant phase will replace the faulty phase by utilizing a simple control scheme. Also, the proposed fault tolerance method is based on the cold reservation method, which will not increase the switching losses in the converter. The proposed fault-tolerant method’s feasibility and effectiveness are confirmed by simulating the NNPC converter under various scenarios in the MATLAB/Simulink environment.
... Using too many semiconductor switches challenges MMC reliability. Semiconductor switches are one of the most fragile components in the structure of multilevel converters [4]. In general, there are two types of faults on semiconductor switches: Short-circuit fault and open-circuit fault. ...
The modular multilevel converter (MMC) is one of the most commonly used power electronic converters in medium and high-power applications. However, reliability is one of the most critical challenges because of the many power switching devices applied in the MMC. Also, many voltage sensors in the converter topology have increased cost and hardware complexity. In this paper, a method for detecting IGBTs open-circuit fault is proposed. In the proposed method, each converter arm is divided into sets. Each set includes two sub-modules and a voltage sensor. These voltage sensors are responsible for monitoring their set. By comparing the output voltage of each set, in normal operation mode and after the open-circuit fault, the fault is detected with great speed and accuracy. Finally, the performance of the proposed method in various scenarios is examined by the simulation results in MATLAB/Simulink.
In multilevel converters (MCs), system components such as capacitors and semiconductor switches are very susceptible to damage. Furthermore, utilizing voltage sensors to balance capacitor voltages increases the system’s cost and complexity while decreasing reliability. This paper presents methods for voltage balancing of capacitors, capacitance monitoring, and open-circuit fault detection in a nested neutral point-clamped (NNPC) converter with a reduced number of voltage and current sensors. In the proposed method, converter capacitors’ voltage and current sensors are eliminated, and only the output sensors are employed to control the converter. In order to balance the voltage of capacitors, their voltages are estimated in three cases without using individual sensors. Comparing the capacitors’ measured and estimated voltage change ratios is used to assess their health in the monitoring method. Thus, the capacitors’ status can be checked by considering the system’s health indicator during each period. Furthermore, the open-circuit fault detection method can identify defective switches within one processing cycle by comparing the converter’s measured and standard output voltages in all switching states. The proposed methods have been evaluated in an experimental study of the system, and the results confirmed the effectiveness of the methods based on the new system configuration.
Modular Multilevel Converters (MMCs) have established their position as the choice of next-generation high voltage converters. These converters, however, require a bulk number of sensors in each phase for Sub-Modules (SMs) voltage balancing. A limited number of studies are found in the literature where the number of sensors is reduced by incorporating observers to estimate the SM voltages. Nonetheless, most of these methods require accurate measurements of the arm voltage. Thus, the dependency on sensors is not completely eliminated. In this paper, a Kalman-filter-based estimation of SM voltages is presented, which does not require any voltage sensor to measure the arm voltage. Therefore, this method considerably simplifies the implementation of MMCs. Extensive simulations and experimental test cases are conducted to validate the performance of the proposed SM voltage estimation method. The performance assessments verify the accuracy and robustness of the proposed method for various test scenarios performed over a wide range of sampling frequencies.
This paper presents a new voltage estimation method for the submodule (SM) capacitor in a modular multilevel converter (MMC). The proposed method employs a Kalman filter (KF) algorithm to estimate the SM voltages of the converter. Compared with sensor-based methods, this scheme requires only one voltage sensor to achieve the voltage-balancing of the converter. This sensor is connected to the total arm voltage; the proposed algorithm requires also the switching patterns of each upper SM switch which are provided from the processor used without the need for extra sensors. This impressive reduction in the number of voltage sensors improves system reliability and decreases its cost and complexity. Extensive simulation and experimental analyses carried out to validate the proposed estimation scheme under different conditions include steady-state analyses and investigation of the effect of variations in capacitance and inductance and the effect of low carrier and effective switching frequency on the accuracy of the estimation, step changes in load conditions, faults and a range changes in DC voltage. The results obtained are validated based on a single-phase MMC.
This paper presents some popular multilevel converter topologies and associated modulation techniques considering advantages for electric transportation (ET) applications including electric vehicles (EVs), charging stations, wireless charging, etc. Cascaded H-Bridge (CHB), Flying Capacitors (FC) and Neutral Point Clamped (NPC) are analyzed. Moreover, mostly used switching techniques in multilevel converters including Sinusoidal Pulse Width Modulation (PWM) and Space Vector Modulation (SVM) are explained and investigated comprehensively. Various advantages of multilevel converters such as generating low harmonic waveforms, low common mode voltage and low switching frequency would be reviewed to show their capability in medium and high power ET applications such as car or train.
The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. However, reliability is one of the most important issues for MMCs those are made of large number of power electronic submodules (SMs). This paper proposed an effective fault detection and localization method for MMCs. An MMC fault can be detected by comparing the measured state variables and the estimated state variables with a Kalman Filter. The fault localization is based on the failure characteristics of the SM in the MMC. The proposed method can be implemented with less computational intensity and complexity, even in case that multiple SMs faults occur in a short time interval. The proposed method is not only implemented in simulations with professional tool PSCAD/EMTDC, but also verified with a down-scale MMC prototype controlled by a real-time digital signal controller in the laboratory. The results confirm the effectiveness of the proposed method.
Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.
The internal control of a modular multilevel con- verter 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 alternat- ing 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. Index Terms—Energy balance, modular multilevel converter (M2C), modulation, open-loop control, prototype.
This paper presents a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission. The fundamental concept and the applied control scheme is introduced. Simulation results of a 36 MW-network intertie illustrate the efficient operating characteristics. A suitable structure of the converter-control is proposed.
This letter proposes a currentless sorting and selection (SAS) based capacitor-voltage-balancing method for modular multilevel converters (MMCs). Without the knowledge of arm-current signals, this method has almost the same performance as the conventional SAS method while reducing the sampling signals, compacting the control system and saving the overall cost. In this letter, the derivative of the total capacitor voltage of an arm, instead of the arm current, is employed to determine which sub-modules (SMs) should be inserted or bypassed. Furthermore, the efficacy of the proposed method is verified by experimental results.
This paper deals with the use of MEMS accelerometers to improve the performances of positioning control systems equipped with low-resolution positioning sensors. A kinematic Kalman filter (KKF) is used to combine the position and acceleration measurements and get a smooth estimate of the kinematic variables, even in the presence of a coarse position quantization. Compared to similar schemes existing in literature, the state of the proposed KKF is augmented, to include the accelerometer output bias/drift among the variables estimated by the filter. In this way, the intrinsic robustness of the KKF scheme is further improved, by making the estimation process of the kinematic variables practically insensitive to the variation of the sensor bias/drift. The proposed KKF is used to provide a smooth and robust estimate of the kinematic variables to a positioning control system consisting of a two degrees-of-freedom (DOF) proportional-derivative (PD) position control combined with an acceleration-based disturbance observer (ADOB). Compared to a solution based on a conventional KKF, not accounting for the accelerometer output disturbance, the proposed solution exhibits better positioning performances, and insensitivity to the accelerometer output bias/drift. This feature is validated through several experimental tests on a positioning system based on a linear motor.
This paper presents a new technique for measuring the capacitor voltages in a modular multilevel converter using a reduced number of voltage sensors. With this technique, the minimum number of voltage sensors per arm is two. Each sensor measures the output voltage of a set of submodules (SMs) connected in series and acquires a new measurement when there is only one SM activated within the set. The acquired value corresponds to the capacitor voltage of the activated SM minus the voltage drops produced in the switches. A simple mathematical model is used to estimate all the SM capacitor voltages, and it is then updated whenever there is a new measurement available. An algorithm that enforces the periodic update of the voltage measurements is also presented. The proposed measuring technique highly reduces the number of voltage sensors; hence reducing the complexity and costs of the signal conditioning and data acquisition stages. Simulation and experimental results are presented to demonstrate the efficiency of the proposed technique.
The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.
A modular multilevel converter (MMC) is an attractive solution for power conversion without transformers. The MMC consists of cascade connections of floating dc capacitors. In this paper, an adaptive observer design has been proposed to estimate the capacitor voltages from the measurement of arm currents. This work introduces the capacitance value of the cell capacitors as a parameter uncertainty for making the system performance robust with unknown constant parameters. It may be used for predictive control, condition monitoring for capacitors, and diagnosis check for capacitor health. In addition, a pulsewidth modulation (PWM) technique for MMC has been explored. The PWM technique is performed using a carrier-based level-shifted PWM strategy. It does not necessitate the calculation of duty cycles, and can be easily implemented in a DSP. By using the PWM technique, harmonics in the phase voltage is shifted to twice the switching frequency. Theoretical analysis is included in this paper for showing stability and convergence of the proposed observer. Analytical expressions are verified by simulation and experimental results.
This letter presents a fault detection method for modular multilevel converters which is capable of locating a faulty semiconductor switching device in the circuit. The proposed fault detection method is based on a sliding mode observer (SMO) and a switching model of a half-bridge, the approach taken is to conjecture the location of fault, modify the SMO accordingly and then compare the observed and measured states to verify, or otherwise, the assumption. This technique requires no additional measurement elements and can easily be implemented in a DSP or microcontroller. The operation and robustness of the fault detection technique are confirmed by simulation results for the fault condition of a semiconductor switching device appearing as an open circuit.
In this paper we shall provide new analysis on some fundamental properties of the Kalman filter based parameter estimation algorithms using an orthogonal decomposition approach based on the excited subspace. A theoretical analytical framework is established based on the decomposition of the covariance matrix, which appears to be very useful and effective in the analysis of a parameter estimation algorithm with the existence of an unexcited subspace. The sufficient and necessary condition for the boundedness of the covariance matrix in the Kalman filter is established. The idea of directional tracking is proposed to develop a new class of algorithms to overcome the windup problem. Based on the orthogonal decomposition approach two kinds of directional tracking algorithms are proposed. These algorithms utilize a time-varying covariance matrix and can keep stable even in the case of unsufficient and/or unbounded excitation.