Hassan Mohammadi Pirouz

Abstract: This paper discusses a transformerless shunt static compensator (STATCOM) based on the modular multilevel converter (MMC). It introduces a new time-discrete appropriate current control algorithm and a phase-shifted carrier modulation strategy to fast compensation of reactive power and harmonics, and also balancing of three-phase source side currents. Analytical formulas are derived to demonstrate the accurate mechanism of stored energy balancing inside the MMC. various simulated waveforms verify that the MMC based STATCOM has the capability of reactive power compensation, harmonic cancellation, and simultaneous load balancing procedure while controlling and balancing all the DC mean voltages even during the transient states.

Abstract: A new transformerless four-leg topology is suggested for shunt compensation, the modular multilevel converters (MMC) based on the half-bridge converters, to achieve higher performance as a STATCOM in a distorted and unbalanced medium-voltage large-current (MV-LC) system. Further, an extended MMC (EMMC) is proposed in order to manage more accurate compensation for high-power applications. Both proposals can be controlled for various purposes such as reactive power and unbalance compensation, voltage regulation and harmonic cancellation. Moreover, related control strategies are also suggested for both the MMC and the EMMC to ensure the source-end three-phase currents are sinusoidal and balanced. Also, the DC-link capacitors of the half-bridge converters are regulated. One interesting application for the EMMC-based STATCOM could be the improvement in power quality and performance of electrified railway traction power supply system. Both the MMC and the EMMC-based STATCOM along with their proposed control strategies were simulated; further, to verify the suggestions, these proposals were also implemented on a 30 kVA modular laboratory prototype. Experiments and simulations confirm the pre-defined objectives.

Abstract: The multilevel AC/AC converters have found various applications in high powers and voltages, nominated as the potential third generation of FACTS controllers. These devices have become more popular among researchers, engaging both half-bridge and full-bridge sub-modules. This paper presents a new control and modulation scheme for these converters that provide control of output ac voltage independent of the input ac voltage and the load current as well as input/output reactive power control. Like the advanced UPFC, it is also possible to control the transferred active power between the two sides. Moreover, the dc-link voltages are balanced using the proposed modulation scheme, while a closed-loop control adjust the dc levels at a reference value to secure the stable operation of the AC/AC converter. The whole circuit is simulated with PSCAD and a single-phase laboratory prototype (2kVA, 100V) was implemented, including the power circuits along with the control and modulation blocks to confirm the validity of the proposed schemes.

Abstract: To achieve higher performance in a distorted and unbalanced medium-voltage large-current system, a new shunt compensator is presented based on half-bridge cascaded converters (HBCC). Then, the transformerless HBCC based STATCOM can be controlled for various purposes such as reactive power of STATCOM, simultaneous harmonic cancellation and load balancing procedure. Compared to the conventional modular multilevel converters, the HBCC based STATCOM introduces lower ratings both for active and passive elements, improving the overall efficiency of the converter. A control strategy is proposed to ensure the source-end three phase currents are sinusoidal and balanced. At the same time, the DC capacitors voltages are controlled to operate under balanced condition. One interesting application for the HBCC based STATCOM could be electric traction system. Both power circuit and the proposed control strategy are simulated with PSIM and MATLAB, confirming the pre-defined objectives.

Abstract: This paper discusses a new transformerless shunt static compensator (STATCOM) based on the extended modular multilevel converter (EMMC), to achieve higher performance in a medium-voltage large-current network. It introduces an appropriate control algorithm based on a phase-shifted carrier modulation strategy, to ensure the source-end three-phase currents are sinusoidal and balanced, balancing the voltages of the dc-link capacitors of the modules as well. Compared to the conventional modular multilevel converters (MMC), the EMMC introduces higher reliability and better performance in the distorted large-current systems. Simulation results from a complete model of the proposed STATCOM are presented, confirming the pre-defined objectives. One interesting application for the EMMC-based STATCOM could be the improvement in power quality and performance of the electrified railway power supply systems.

Abstract: Cascaded multi-level converters have some noticeable advantages such as high reliability, efficient fault management capability, simple structure, and adaptability with different power levels. Furthermore, these converters could be connected to medium-voltage networks without an isolating transformer. However, the voltage balancing of the DC capacitors is a highly challenging task particularly in harmonically polluted and unbalanced systems. To overcome this problem, a new cascaded converter based on two level half-bridge modules for shunt compensators is proposed in this paper. The control algorithm of the compensator and its configuration are designed and optimized in order to achieve an efficient performance in unbalanced systems. This makes it possible to compensate the currents produced by the electric traction system as a popular of medium voltage unbalanced load properly.

Abstract: Modular multilevel AC/AC converters have been examined for high powers and voltages applications such as traction converters and interphase power controllers. It can also operate directly on a transmission line for voltage regulation and reactive power control . This paper presents a modulation technique aiming at an independent control over the AC voltages as well as reactive power of the AC/AC converter, having the capability of controlling the transferred active power from the input source to the load. Moreover, the DC-links of a modular AC/AC converter needs to be regulated for proper operation under various load conditions. The proposed modulation technique embraces a DC voltage balancing strategy based on the engaged switching instants. PSCAD simulation results are presented to validate the proposed modulation technique for both open-loop and closed-loop control

Abstract: Nowadays, modern medium voltage distribution systems supply nonlinear loads such as single-phase AC traction systems. These loads make the network to operate under undesired conditions, i.e. distorted, uncontrolled reactive power and significant unbalance enforcement. Therefore, the associated problems of reactive compensation and harmonic elimination as well as load balancing are inevitable and ought to be considered simultaneously to achieve acceptable power quality level. Meanwhile, mitigation of all these power quality problems by means of a single compensator is a challenging task in a medium-voltage network. This dissertation describes a new transformerless shunt compensator topology, based on the modular multilevel converters (MMC) for full compensation of a nonlinear unbalanced load in a distorted medium voltage network. The new MMC based static compensators (STATCOM) can be controlled for various purposes such as reactive power compensation, harmonic cancellation, and simultaneous load balancing procedure. The MMC introduces higher reliability and better performance in a distorted and unbalanced medium-voltage network as compared to the conventional multilevel converter. Furthermore, this dissertation introduces a new control strategy for the MMC based STATCOM to ensure the source-end three-phase currents are sinusoidal and balanced, balancing the voltages of the DC-link capacitors of the half-bridge converters as well. In this light, the control strategy based on instantaneous power theory is developed for extracting the compensating current signals. Then, a new real time current control technique is introduced for the MMC, based on the predictive control method. An appropriate switching modulation technique is applied to the MMC, maintaining saved electrical energy of all legs balanced, even if the converter currents are unbalanced and the network voltages are slightly distorted. Analytical formulas are derived to demonstrate the accurate mechanism of DC link voltage balancing. In addition, a discussion is presented based upon several aspects to evaluate the performance and practical capabilities of the MMC topology in the STATCOM applications, as compared with the conventional multilevel topologies. Various simulation results demonstrate the high performance of the new MMC based STATCOM topology and the proposed control strategy. In addition, the results of this effort, as well as the novel features of the MMC based STATCOM, are verified by experimental investigation of a 25KVA laboratory scale prototype. One interesting application for the MMC-based STATCOM could be the improvement in power quality and performance of the power supply network of electrified railway systems.

Abstract: The multilevel AC/AC converters have found various applications in high powers and voltages, nominated as the potential third generation of FACTS controllers. These devices have become more popular among researchers, engaging both half-bridge and full-bridge sub-modules. This dissertation presents a new control and modulation scheme for these converters that provide control of output ac voltage independent of the input ac voltage and the load current as well as input/output reactive power control. Like the advanced UPFC, it is also possible to control the transferred active power between the two sides. Moreover, the dc-link voltages are balanced using the proposed modulation scheme, while a closed-loop control adjust the dc levels at a reference value to secure the stable operation of the AC/AC converter. The whole power circuits along with the control and modulation blocks are simulated with PSCAD to confirm the validity of the proposed schemes. In addition, the results of this effort, as well as the novel features of the MMC based power electronic transformer, are verified by experimental investigation of a 5KVA laboratory scale prototype.