Apparao Dekka

Space-Vector-Based Synchronized Three-Level Discontinuous PWM for Medium-Voltage High-Power VSI

The high-power voltage source inverters are switched at low frequency to reduce the losses in the inverter. The discontinuous pulsewidth-modulation (PWM) sequences reduce the switching frequency to two-thirds compared to the conventional continuous PWM sequences. At low switching frequency, the output will be rich in harmonics, and synchronization is a must to avoid subharmonics. This paper presents four basic types of synchronized discontinuous PWM (DPWM) sequences that ensure synchronization, three-phase symmetry, and half-wave symmetry. The proposed synchronized DPWM sequences are verified through simulation and experiment on a constant- v/f open-loop induction motor drive. The performance of the proposed DPWM algorithms is studied in the entire modulation region, including the overmodulation region, and is compared with that of the conventional synchronized space vector PWM. A different type of DPWM with low common-mode voltage is suggested for low-modulation region and verified experimentally.

Dynamic Voltage Balancing Algorithm for Modular Multilevel Converter: A Unique Solution

The modular multilevel converter (MMC) has several submodules in cascade. To control the submodule capacitors voltage, a new generalized dynamic voltage balancing algorithm along with a simple pulse width modulator structure is proposed in this paper. With proposed pulse width modulator structure, the dynamic voltage balancing algorithm can be easily implemented with any type of carrier based pulse width modulation (including level-shifted and phase-shifted pulse width modulation) scheme without any modifications. This method does not require the sorting technique for selection of submodule capacitors voltage. The performance of the proposed voltage balancing algorithm is presented for MMC with three-level flying capacitor submodules (MMC-3L-FC). The proposed method is also applicable to the conventional two-level half bridge submodules as well. The effectiveness of the proposed voltage balancing algorithm is verified with the phase-shifted carrier-based pulse with modulation scheme. The results obtained from the MATLAB/SIMULINK simulations on 6-kV/ 2.5-MW system and the dSPACE DS1103-based laboratory prototype of 208-V/3-kVA MMC-3L-FC system are in close relationship, and thus, the proposed methodology is validated.

A Novel Modulation Scheme and Voltage Balancing Algorithm for Modular Multilevel Converter

The control complexity of modular multilevel converter (MMC) increases with the number of submodules per arm. In particular, the implementation of a pulsewidth modulation (PWM) scheme and a voltage balancing approach for MMC is a major challenge. This paper proposes a generalized low-cost less computational decoupled sampled average PWM scheme for MMC. In this approach, the reference output voltage is generated by averaging the two nearest voltage levels from the top and bottom arms in each sampling interval. In addition, a simplified voltage balancing approach is proposed for MMC. The proposed approach does not require a sorting technique to select the submodules. Instead, the relative comparison logic is presented. The effectiveness of the proposed modulation scheme and voltage balancing approach is evaluated on a single-phase MMC system with three-level flying capacitor submodules. The proposed approach is extended to a three-phase system, which results in dynamic balancing of the zero-sequence voltage. Hence, the zero-sequence current is minimized. The performance of the proposed three-phase approach is compared with that of the conventional phase-shifted carrier PWM scheme. The simulation and experimental studies show successful voltage balancing among submodule capacitor voltage, low harmonic distortion, and less ripple in output current.

A Space-Vector PWM-Based Voltage-Balancing Approach With Reduced Current Sensors for Modular Multilevel Converter

Arm voltage and submodule (SM) capacitor voltage balancing is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). The arm voltage balancing is achieved through a zero-sequence voltage controller in carrier pulse-width modulation (CPWM). In this study, a dual space-vector pulse-width modulation (SVPWM) technique is proposed for an MMC, which eliminates the external controller for arm voltage balancing. In this approach, the three-phase top and bottom arms are independently controlled using SVPWM. In addition, the capacitor voltage balancing can be achieved using redundant switching vectors. However, this will increase the computational load on the space-vector modulator. Therefore, an external capacitor voltage-balancing approach is proposed to minimize the computational complexity. The proposed approach uses the direction of load current instead of the arm current in SM selection process. As such, the required number of current sensors is reduced to 50% in a three-phase system. The proposed modulation and voltage-balancing approach are simulated and experimentally verified on the MMC system with three-level flying capacitor (3L-FC) SMs. Simulation and experimental results show the successful balancing of the arm voltage and SM capacitors voltage.

Dynamic voltage balancing algorithm for modular multilevel converter with three-level flying capacitor submodules

The modular multilevel converter (MMC) has several three-level flying capacitor (3L-FC) submodules in cascade. To balance the submodule capacitors voltage, this paper describes a dynamic voltage balancing algorithm based on the carrier phase shifted pulse width modulation (CPS-PWM) scheme. The submodules are controlled based on the instantaneous value of capacitor voltage and the direction of current. For the selection of the submodules, the maximum and minimum voltage submodule selection logics are designed. These voltage logics are generating an index number for each submodule based on the relative comparison of capacitors voltage. Finally the switching state of the submodules is generated by comparing the submodule index number with the dynamic reference index number. The performance of the proposed voltage balancing algorithm at different operating conditions is evaluated on 6kV/2MVA MMC system with the MATLAB simulation and the corresponding results are presented. In addition, the performance comparison of MMC with 3L-FC and conventional two-level half bridge (2L-HB) submodules is presented.

Model Predictive Control with Common-Mode Voltage Injection for Modular Multilevel Converter

Submodule (SM) capacitor voltage ripple is one of the major concerns in modular multilevel converters (MMCs). Capacitor voltage ripple leads to the double-frequency circulating current (CC) in legs, thereby resulting in a cascading effect of increased peak value of the arm current, semiconductor device stress, and power losses in MMCs. In this study, a model predictive control (MPC) with common-mode voltage (CMV) injection is proposed to minimize capacitor voltage ripple and the magnitude of CC. A discrete-time mathematical model of the MMC with CMV is presented to predict the future behavior of the control variables. The injection of CMV guarantees arm voltage balancing without CC control and long-term stability of MMC without large capacitors. The dynamic and steady-state performances of MPC with CMV injection are verified on an MMC with three-level flying capacitor SMs. A performance comparison between the proposed approach and the conventional MPC is also presented. Simulation and experimental studies show that CMV injection significantly reduces the capacitor voltage ripple and the CC in legs. The proposed approach also improves output voltage and current waveform quality.

Dual-stage model predictive control of modular multilevel converter

The control of modular multilevel converter (MMC) is quite challenging and demands a flexible approach to achieve multiple control objectives. The model predictive control (MPC) is highly effective and suitable to control the MMC. The control objectives are included in a single cost function and evaluated for all possible switching states. The switching state which minimizes cost function is selected and applied to converter. In MMC, the number of available switching states are quite high and drastically increase with number of submodules per arm. Therefore, the implementation of conventional MPC is difficult and contributes to unwanted switching of submodules. This paper proposes a less computational, dual-stage MPC with common-mode voltage injection for MMC. In this approach, the overall MMC control objectives are categorized into primary and secondary group. The primary group objectives are evaluated in first-stage and secondary group of control objectives are evaluated in second-stage of MPC. By doing so, the computational complexity can be significantly minimized without affecting the dynamic response of MPC. The proposed approach avoids unwanted switching of submodules and, minimizes the output voltage harmonic distortion and ripple in output current. The simulation study is conducted to verify the effectiveness of proposed approach and corresponding results are compared with standard MPC.

Dual-Stage Model Predictive Control With Improved Harmonic Performance for Modular Multilevel Converter

The model predictive control has an ability to achieve multiple control objectives for a modular multilevel converter. This approach has high computational complexity and causes the unwanted switchings leading to a higher ripple in output current and harmonic distortion in output voltage and current waveforms. To solve the issues of computational burden and unwanted switchings, this paper proposes a dual-stage model predictive control approach for a modular multilevel converter. In this approach, the primary objectives are evaluated in first stage corresponding to the redundancy of voltage vectors, while the secondary objectives are evaluated in second stage corresponding to the submodule redundancy. Therefore, the computational burden is significantly minimized without affecting the dynamic response. In addition, a discrete-time model of modular multilevel converter incorporating the common-mode voltage is proposed. The simulation and experimental studies are conducted on a three-phase modular multilevel converter to verify the dynamic and steady-state performance of the proposed approach. The performance comparison between the proposed and conventional model predictive control approach is also presented.

A novel modulation scheme and voltage balancing algorithm for modular multilevel converter

This paper presents a low cost, less computational pulse width modulation scheme for a modular multilevel converter (MMC). In the present control approach, the reference output voltage is generated by averaging the two nearest voltage levels. The switching states corresponding to the two nearest voltage levels and their duty cycles are obtained from the instantaneous value of reference output voltage. The proposed modulation scheme is easily extended to any number of submodules per arm without any modifications. In addition, a voltage balancing algorithm is proposed for the balancing of submodule capacitors voltage. The performance of the proposed modulation scheme and voltage balancing algorithm is evaluated on the 2MVA/6kV MMC system with three-level flying capacitor submodules. Also, the proposed modulation scheme is compared with the conventional phase-shifted carrier pulse width modulation (PSC-PWM) scheme and the results are presented.

Retrofitting of Harmonic Power Filters in Onshore Oil Drilling Rigs: Challenges and Solutions

The nonlinear loads in oil drilling rigs result in power quality problems such as harmonic distortion and low power factor. This paper addresses the harmonic problems in onshore rigs that are commissioned in early period of semiconductor technology and use dc drives. Different power filtering solutions are considered, and their performance is presented. The influence of practical constraints such as space limitations, reliability, cost, ease of maintenance, and operating conditions on the choice of filter is discussed. An optimal choice of filter meeting the objective and satisfying the practical constraints is presented.