Arman Hassanpoor

Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application

The outstanding features of modular multilevel converters (M2C) make it attractive for high voltage direct current (HVDC) systems. In order to achieve high efficiency in HVDC converter stations, the switching frequency and the capacitor voltage ripple of the converter should be minimized. A suitable modulation algorithm should achieve an optimal tradeoff between these two requirements. This paper evaluates different carrier-based PWM algorithms and discusses the most challenging technical aspects of an efficient M2C. It is observed that decoupling the waveform synthesis from the selection of which cell to switch at each instant has beneficial impact on operation performance. The evaluation is done by time-domain simulation considering a grid connected, three-phase M2C converter and an advanced control system. Results of this study can be used for implementing more economical HVDC converters.

Technical Assessment of Load Commutation Switch in Hybrid HVDC Breaker

The development of a large scale high voltage direct current (HVDC) power grid requires a reliable, fast and low-loss circuit breaker. The load commutation switch (LCS) is an essential part of ABBs 1200 MW hybrid HVDC breaker concept which builds up a low-loss conducting path for the load current. The technical requirements for the LCS are expressed in this paper by studying the operation principle of the hybrid HVDC breaker. The voltage stress over the LCS is calculated and simulated based on a dc grid with 320kV and 2kA rated voltage and current. A system model of the hybrid HVDC breaker is developed in PSCAD/EMTDC software to study the design criteria for snubber circuit and arrester blocks. It is observed that conventional snubber circuits are not suitable for a bidirectional hybrid HVDC breaker as the current of snubber capacitors prevent the fast interruption action. A modified snubber circuit is proposed in this paper along with two more alternatives for the load commutation switch to overcome this problem. Moreover, the power loss model for a semiconductor device is discussed in this paper based on the 4.5 kV StakPak IGBT. The model is used to calculate the conduction power losses for different LCS topologies. Ultimately, a matrix of 3x3 IGBT modules is selected to provide a reliable LCS design which can handle several internal fault cases with no interruption of operation. A full-scale prototype has been constructed and tested in ABB HVDC Center, Ludvika, Sweden. The experimental test results are also included in the paper in order to verify the calculation and simulation study.

Tolerance Band Modulation Methods for Modular Multilevel Converters

Modular multilevel converters (M2Cs) are increasingly used in high-voltage direct current (HVDC) systems. The efficiency of M2Cs is influenced by the modulation and cell selecting methods, which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of the M2C is presented in this paper. Tolerance band methods are employed to obtain the switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few cells on one hand and also reduce the sorting efforts for cell balancing purposes of many cells converter on the other hand. Three different algorithms are also proposed to balance the cell capacitor voltages. The evaluation is done in time-domain simulation by which the performance of each method is studied in both the steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid-connected system. Eventually, the most promising tolerance band method has been implemented and verified in a real-time digital simulator, RTDS®. The average switching frequency of 70 Hz has been achieved for the system under study, while the capacitor voltage ripple is limited to 10% of the nominal cell voltage.

Optimization-Based Cell Selection Method for Grid-Connected Modular Multilevel Converters

Modular multilevel converters (MMCs) are widely used in different applications. Due to low-loss operation, compactness, and high modularity, MMC is extremely attractive for high-voltage direct-current (HVDC) transmission systems. The HVDC station loss is highly related to the converter switching pulse pattern, which is generated by modulation algorithm and cell selection methods. This paper formulates the switching pulse pattern generation, as a versatile optimization problem. The problem constraints and objectives are formulated for HVDC applications and compared with similar problems in the field of computer science. To overcome the computational complexity in solving the introduced optimization problem, a heuristic method is proposed for cell selection algorithm. The method utilizes the current level in order to obtain lossless switching at zero-current crossings. The study of the proposed method, in a time-domain simulation platform, shows that the method can reduce the switching converter losses by 60% compared to carrier-based modulation, maintaining the same capacitor voltage ripple. Eventually, the practical functionality of the proposed method is verified in a real-time digital simulator, RTDS, for a 512-level converter in a point to point HVDC link. Although this paper focuses on HVDC, the mathematical model is applicable for any MMC application.

Tolerance Band Adaptation Method for Dynamic Operation of Grid-Connected Modular Multilevel Converters

The use of modular multilevel converters (MMC) in high-voltage direct current (HVdc) transmission systems has grown significantly in the past decade. The efficiency, cell capacitor voltage ripple, and dynamic performance are three contradictory aspects of the MMC which are related to the converter switching scheme. Previously introduced tolerance band (TB)-based schemes enable efficient and simple control for grid-connected MMCs. This paper addresses the dynamic operation of TB switching schemes by proposing a dynamic boundary setting technique for steady-state operation and a switching scheme scheduling controller for transient fault handling. The performance of proposed methods are validated in a realistic point-to-point HVdc link, modeled in real-time digital simulator where two converters with 512 cells per arm are implemented. Utilizing the proposed methods will enable efficient implementation of TB-based schemes for different operating points, and also a robust transient fault handling.

Loss evaluation for modular multilevel converters with different switching strategies

Apparently, modular multilevel converter (MMC) has been extensively used in high voltage direct current (HVDC) transmission links in recent years. The efficiency of MMC stations are highly related to the switching methods and semiconductor devices. So, various switching methods and semiconductor devices have been investigated and introduced in the field. This paper settles a benchmark for an HVDC link, based on a real project, and investigates the impact of six different switching methods on the converter loss, utilizing a commercial semiconductor device. The evaluation indicates that switching methods which consider the current level at switching instants are more efficient in comparison with the other methods which only consider the number of switching events. The result of this study is essential for more efficient converter stations.

Tolerance-band modulation methods for modular multilevel converters

Modular multilevel converters (M2Cs) are increasingly used in high-voltage direct current (HVDC) systems. The efficiency of M2Cs is influenced by the modulation and cell selecting methods, which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of the M2C is presented in this paper. Tolerance band methods are employed to obtain the switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few cells on one hand and also reduce the sorting efforts for cell balancing purposes of many cells converter on the other hand. Three different algorithms are also proposed to balance the cell capacitor voltages. The evaluation is done in time-domain simulation by which the performance of each method is studied in both the steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid-connected system. Eventually, the most promising tolerance band method has been implemented and verified in a real-time digital simulator, RTDS®. The average switching frequency of 70 Hz has been achieved for the system under study, while the capacitor voltage ripple is limited to 10% of the nominal cell voltage.

Theory to practical implementation of full-bridge modular multilevel converter for HVDC applications

Modular multilevel converter (MMC) has become a prominent converter topology used in HVDC transmission systems over the past decade. The efficiency, size and system performance are the main factors of the HVDC converter stations which directly affect the converter performance end-cost. Replacing conventional half-bridge (HB) cells by full-bridge (FB) cells in the MMC arms offers new features such as the dc fault blocking capability for grid-connected voltage source converters (VSC) at the cost of higher number of devices and conduction losses. However, practical implementation of FB MMC has not been fully discussed so far. In this paper, the implementation challenges of FB MMC is addressed in both steady-state and dynamic operation modes of a point-to-point HVDC link. The work is supported with the analysis of design arguments for a realistic converter station.

Switching pulse pattern optimisation for modular multilevel converters

Modular multilevel converters (MMCs) are widely used in different applications such as high voltage direct current (HVDC) applications. The HVDC station loss is highly related to the converter switching pulse pattern which is generated by modulation algorithm and cell selection methods. This paper formulates the switching pulse pattern generation, as a versatile optimisation problem. The problem constraints and objectives are formulated for HVDC applications and compared with similar problems in the field of computer science. To overcome the computational complexity in solving the introduced optimisation problem, a heuristic method is proposed for cell selection algorithm. The method utilizes the current level in order to obtain lossless switching at zero-current crossings. The study of the proposed method, in a time-domain simulation platform, shows that the method can reduce the switching converter losses by 60% compared to carrier-based modulation, maintaining the same capacitor voltage ripple. Although this paper focuses on HVDC, the mathematical model is applicable for any MMC application.

A new automatic spark generation system for gasoline engines

This paper presents a new automatic spark system for gasoline engines by reshaping the electric field in the combustion chamber through repositioning high voltage electrodes and applying changes in the electrodes geometry. This system provides better and wider discharge resulting in higher efficiency and ignition rate. Furthermore, a much simpler timing system, relying exclusively on built in control parameters, i.e. chamber pressure, temperature and piston position is needed. Numerical simulation of this newly-proposed design is conducted using a time-dependent solver. In particular, electrical field is investigated along the course line of a single piston. It has been observed that ignition is easier to control at the right piston position and crank angle which will obviously lead to better fuel economy and environmental protection.