Noman Ahmed

HVDC SuperGrids with modular multilevel converters — The power transmission backbone of the future

In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a stronger high voltage transmission grid is anticipated. Due to limitations in ac power transmission the most likable choice for such a grid is a high-voltage dc (HVDC) grid. However, the concept of the HVDC grid is still under active development as different technical challenges exist, and it is not yet possible to construct such a dc grid. This paper deals with prospects and technical challenges for future HVDC SuperGrids. Different topologies for a SuperGrid and the possibility to use modular multilevel converters (M2Cs) are presented. A comprehensive overview of different submodule implementations of M2C is given as well as a discussion on the choice between cables or overhead lines, the protection system for the dc grid and dc-side resonance issues.

A Computationally Efficient Continuous Model for the Modular Multilevel Converter

Simulation models of the modular multilevel converter (MMC) play a very important role for studying the dynamic performance. Detailed modeling of the MMC in electromagnetic transient simulation programs is cumbersome, as it requires high computational effort and simulation time. Several averaged or continuous models proposed in the literature lack the capability to describe the blocked state. This paper presents a continuous model, which is capable of accurately simulating the blocked state. This feature is very important for accurate simulation of faults. The model is generally applicable, although it is particularly useful in high-voltage dc applications.

Efficient Modeling of an MMC-Based Multiterminal DC System Employing Hybrid HVDC Breakers

Summary form only given. The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc (HVDC) breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac-side of the converter is proposed.

Efficient modeling of modular multilevel converters in HVDC-grids under fault conditions

High-voltage direct current (HVDC) grids using modular multilevel converters (M2Cs) have strongly been considered for the integration of distant renewable energy sources and also as a backbone to the existing ac-grids. The dynamic performance of the M2C is of particular interest in these grids. For electromagnetic transient (EMT) programs, modeling of HVDC-grids using detailed M2C models is unrealistic, as it requires extremely high computational effort and simulation time. In this paper an HVDC-grid test system is developed using a continuous simulation model of the M2C. The model is also capable of describing the blocking events of the M2C. Using time-domain simulations in PSCAD/EMTDC, the dynamic performance of the M2C in HVDC-grids under fault conditions is investigated. Simulation results reveal that the continuous M2C model can efficiently be used to study the dynamic performance of the M2C in HVDC-grids with high computational speed, under different fault conditions.

Continuous modeling of open-loop control based negative sequence current control of modular multilevel converters for HVDC transmission

Negative sequence currents are obtained during ac-side asymmetrical faults of converters in high-voltage direct current (HVDC) transmission systems. Consequently, second order harmonics in the dc-side voltage and current, unbalanced ac-side currents, and power oscillations can be observed. This paper presents a negative sequence current control (NSCC) scheme that eliminates second order harmonic ripples in the voltage and current of the dc-side during unbalanced grid conditions. Controllers for this purpose are investigated using a continuous model of the modular multilevel converter (M2C). The proposed scheme utilizes an open-loop controller for lower level control of the M2C. The continuous model used also has the capability to model blocking and deblocking events which may be used during protective actions. Simulation results reveal that the proposed NSCC scheme is effective in suppressing dc-side voltage and current ripples. Moreover, it keeps the ac-side phase currents balanced during asymmetrical fault conditions.

Performance of the modular multilevel converter with redundant submodules

The modular multilevel converter (MMC) is the state-of-the-art voltage-source converter (VSC) topology used for various power-conversion applications. In the MMC, submodule failures can occur due to various reasons. Therefore, additional submodules called the redundant submodules are included in the arms of the MMC to fulfill the fault-safe operation requirement. The performance of the MMC with redundant submodules has not been widely covered in the published literature. This paper investigates the performance of the MMC with redundant submodules in the arms. Two different control strategies are used and compared for integrating redundant submodules. The response of the MMC to a submodule failure for the two strategies is also studied. Moreover, the operation of the MMC with redundant submodules is validated experimentally using the converter prototype in the laboratory.

Efficient modeling of an MMC-based multiterminal DC system employing hybrid HVDC breakers

The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed.