Impact of submodule faults on the performance of modular multilevel converters

Modular multilevel converter (MMC) is well suited for high-power and medium-voltage applications. However, its performance is adversely affected by asymmetry that might be introduced by the failure of a limited number of submodules (SMs) or even by severe deviations in the values of SM capacitors and arm inductors, particularly when the number of SMs per arm is relatively low. Although a safe-failed operation is easily achieved through the incorporation of redundant SMs, the SMs' faults make MMC arms present unequal impedances, which leads to undesirable internal dynamics because of unequal power distribution between the arms. The severity of these undesirable dynamics varies with the implementation of auxiliary controllers that regulate the MMC internal dynamics. This paper studied the impact of SMs failure on the MMC internal dynamics performance, considering two implementations of internal dynamics control, including a direct control method for suppressing the fundamental component that may arise in the dc-link current. Performances of the presented and widely-appreciated conventional methods for regulating MMC internal dynamics were assessed under normal and SM fault conditions, using detailed time-domain simulations and considering both active and reactive power applications. The effectiveness of control methods is also verified by the experiment. Related trade-offs of the control methods are presented, whereas it is found that the adverse impact of SMs failure on MMC ac and dc side performances could be minimized with appropriate control countermeasures

Comparison of conventional and improved two-level converter during AC faults

An improved two-level converter (I2LC) is a practical compromise between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), recently proposed for dc transmission system for relatively lower dc voltages and rated powers. The I2LC inherent the ac and dc fault behaviors of the MMC and relative simplicity of C2LC. Therefore, this paper presents a detailed quantitative comparison between the ac and dc responses of the C2LC and I2LC to symmetrical and asymmetrical ac faults. It has been showing that unlike the C2LC, the I2LC provides better controllability than the C2L at system level during asymmetrical ac faults, including two operational objectives simultaneously such as balanced output currents and ripple-free dc-link current. Index Terms—ac and dc faults, two-level converter, improved two-level converter, medium and high-voltage direct current (HVDC) transmission systems

Novel enhanced modular multilevel converter for high-voltage direct current transmission systems

This paper proposes an enhanced modular multilevel converter as an alternative to the conventional half-bridge modular multilevel converter that employs a reduced number of medium-voltage cells, with the aim of improving waveforms quality in its AC and DC sides. Each enhanced modular multilevel converter arm consists of high-voltage and low-voltage chain-links. The enhanced modular multilevel converter uses the high-voltage chain-links based on medium-voltage half-bridge cells to synthesize the fundamental voltage using nearest level modulation. Although the low-voltage chain-links filter out the voltage harmonics from the voltage generated by the high-voltage chain-links, which are rough and stepped approximations of the fundamental voltage, the enhanced modular multilevel converter uses the nested multilevel concept to dramatically increase the number of voltage levels per phase compared to half-bridge modular multilevel converter. The aforementioned improvements are achieved at the cost of a small increase in semiconductor losses. Detailed simulations conducted in EMPT-RV and experimental results confirm the validity of the proposed converter

Modular two-level voltage source converter for direct current transmission systems

This paper presents a modular two-level voltage source converter (M2L-VSC) suitable for short distance dc transmission systems with relatively low dc operating voltage. The proposed M2L-VSC consists of two sets of three-phase cells, with each cell has its own capacitor, and these capacitors do not discharge when converter is blocked during dc short circuit fault. The main attributes of the proposed M2L-VSC are absence of the 2nd order harmonic from the arm currents, and reduced energy storage requirement as the cell capacitor only experience high-order harmonic currents associated with the switching frequency as in conventional two-level converter (C2L). The technical feasibility of the M2L-VSC for dc transmission systems has been assessed using simulations and corroborated experimentally. It has been shown that the transient responses of M2L-VSC to ac and dc faults are similar to that of the modular multilevel converter (MMC)

A secure system integrated with DC-side energy storage for renewable generation applications

Massive energy storage capability is tending to be included into bulk power systems especially in renewable generation applications, in order to balance active power and maintain system security. This paper proposes a secure system configuration integrated with the battery energy storage system (BESS) in the dc side to minimize output power fluctuation, gain high operation efficiency, and facilitate fault ride through, which is suitable for unidirectional renewable power generation systems (power transfer from renewable sources to the grid). The system utilizes robust diode units (DUs) to protect receiving-end devices against dc faults. Also, the BESS and half-bridge modular multilevel converter (MMC) at the receiving end can operate safely and flexibly to achieve stable and high-quality power transfer, in both source power intermittency and dc-link fault cases. Depending on BESS sizing, the source system power fluctuation can be reduced (absorbed by the receiving-end BESS) when a receiving-end grid fault occurs. Topological configuration and control design of the proposed system are presented. Simulation results show the effectiveness of the proposed system in both dc and ac fault cases, with power fluctuation elimination functionality highlighted. The receiving-end operation losses are investigated, showing a high-efficiency system. In addition, key system implementation considerations regarding the proposed system are elaborated

Controlled transition bridge converter : operating principle, control and application in HVDC transmission systems

This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to operate a controlled transition bridge (CTB) converter as ac/dc and dc/ac converter terminals. With such an operation, the CTB converter may require small ac filters; thus attractive for high-voltage direct current (HVDC) transmission systems. To facilitate ac voltage control over a wide range and black-start capability, the injected 3rd harmonic allows the cell capacitor voltages of the CTB converter to be regulated independent of the modulation index and power factor. The insertion of 3rd harmonic into modulating signals achieves two objectives: extends the regions around voltage zeros so that the total voltage unbalanced can be distributed between the cell capacitors, thereby exploiting the bipolar capability of the full-bridge cells in each limb; and to ensure that each limb can be clamped to the positive and negative dc rails every half fundamental period independent of the modulation index to allow recharge of the cell capacitors from the active dc link. The suitability of the CTB converter for HVDC type applications is demonstrated using a two-terminal HVDC link that employs a 21-cell CTB converter, considering normal operation and ac faults

Hybrid converter topologies for dc transmission systems

This study presents types 1 and 2 hybrid converters with reduced power circuit complexity compared with the mixed cell modular multilevel converter (MC-MMC). The type 1 converter is formed by replacing the director switches of the alternate arm converter by high-voltage (HV) half-bridge (HB) cells rated for half of the dc-link voltage. Also, it resembles special case of MC-MMC, where the entire HB cells of each arm are lumped into a single HV HB cell, with both capacitors of the half- and fullbridge cells are exposed to fundamental current as in the conventional MMC. The upper and lower arms of the type 2 converter resemble a front-to-front connection of two three-phase hybrid cascaded two-level converters, where the cell capacitors of the three-phase two-level converters that act as director switches do not experience fundamental currents. Therefore, the type 2 converter offers compact design compared with type 1 converter and the MC-MMC. The technical viabilities of the proposed hybrid converters are assessed using simulations, with both converters modelled in MATLAB-Simulink using electromagnetic transient simulation approach, considering normal and transient conditions. Experimental results obtained from single-phase type 1 converter confirm the practical viability of the proposed converters

A new hybrid dual active bridge modular multilevel based DC-DC converter for HVDC networks

Multi-terminal high voltage DC transmission currently represents a leading technology in long-distance power transmission systems. Among the main technical challenges facing such technology, DC fault isolation, permitting different grounding schemes, providing interoperability, and high DC voltage stepping between different HVDC networks, and allowing high-speed power reversal without power interruption especially when connecting the pre-existing voltage source converters (VSC) and line commutated converters (LCC)-based HVDC networks. This paper introduces a new modular multilevel converter (MMC) based front-to-front DC-DC converter to interconnect two different types (LCC/VSC) of HVDC networks. The proposed topology comprises a voltage source MMC (VS-MMC) and a current source MMC (CS-MMC), while both are coupled via an AC link including the isolating transformer. The proposed topology can successfully provide an uninterruptible bi-directional power flow, high DC voltage stepping with a DC fault blocking capability, and low number of semiconductors due to the usage of only half-bridge SMs. The system design is provided with a detailed mathematical analysis. Furthermore, two active power control methodologies are proposed and compared. The first control technique is simpler and entails lower passive elements, while the second technique ensures a zero reactive power over the full range of active power flow. Furthermore, Losses analysis and comparison are provided between the two proposed control techniques. Finally, Control-Hardware-in-the-Loop (CHiL) test validation is employed to confirm the validity of the proposed system under healthy as well as different fault scenarios

Operational issues related to the integration of renewable generation in distribution networks

With the dramatic increase in electricity demand and the need to tackle climate change by reducing greenhouse gas emissions such as CO₂, renewable generation has been increased in recent years in power systems. However, highly integrating renewable generation in distribution systems raises several issues and challenges that need to be addressed and some of these issues are investigated in this thesis. This research focuses on investigating the fault ride through, the system transient stability, the system frequency response and the grid power factor of distribution systems with high renewable energy penetration based on wind and solar-photovoltaic (PV). Several proposed control techniques for wind generators and PVs are introduced in this research to solve these issues and mitigate the negative impact of these units on distribution systems. A modified control system of DGs based on wind and PV in different distribution systems is used to help to ride through faults and meet the low voltage ride through (LVRT) grid code requirements for voltage recovery. A three phase two stage transformerless PV grid connected system is proposed with a new technique to ride through faults and protect the power electronic converters. A DC chopper is added to the DC link of wind generators to enable wind generating units to ride through faults and protect their converters from overvoltages. The Transient Stability Index method is used to assess the impact of such renewable sources on the system transient stability of various distribution networks. Different frequency control methods published in literature are used for various DGs based on wind and PV to investigate the frequency response of different distribution networks with high renewable energy penetration, and a new frequency control technique is proposed for wind generators based on Doubly Fed Induction Generator (DFIG) to improve the system frequency response. The impact of high renewable energy penetration based on wind and PV with different power factor settings on grid power factor of various distribution systems is discussed in this research. A three phase two stage transformerless PV grid connected system with reactive power capability is proposed to operate under different solar irradiance conditions and improve the utility grid power factor. Simulation results validate the proposed systems in various distribution systems, including IEEE 13 bus, IEEE 37 bus and IEEE 123 bus systems.With the dramatic increase in electricity demand and the need to tackle climate change by reducing greenhouse gas emissions such as CO₂, renewable generation has been increased in recent years in power systems. However, highly integrating renewable generation in distribution systems raises several issues and challenges that need to be addressed and some of these issues are investigated in this thesis. This research focuses on investigating the fault ride through, the system transient stability, the system frequency response and the grid power factor of distribution systems with high renewable energy penetration based on wind and solar-photovoltaic (PV). Several proposed control techniques for wind generators and PVs are introduced in this research to solve these issues and mitigate the negative impact of these units on distribution systems. A modified control system of DGs based on wind and PV in different distribution systems is used to help to ride through faults and meet the low voltage ride through (LVRT) grid code requirements for voltage recovery. A three phase two stage transformerless PV grid connected system is proposed with a new technique to ride through faults and protect the power electronic converters. A DC chopper is added to the DC link of wind generators to enable wind generating units to ride through faults and protect their converters from overvoltages. The Transient Stability Index method is used to assess the impact of such renewable sources on the system transient stability of various distribution networks. Different frequency control methods published in literature are used for various DGs based on wind and PV to investigate the frequency response of different distribution networks with high renewable energy penetration, and a new frequency control technique is proposed for wind generators based on Doubly Fed Induction Generator (DFIG) to improve the system frequency response. The impact of high renewable energy penetration based on wind and PV with different power factor settings on grid power factor of various distribution systems is discussed in this research. A three phase two stage transformerless PV grid connected system with reactive power capability is proposed to operate under different solar irradiance conditions and improve the utility grid power factor. Simulation results validate the proposed systems in various distribution systems, including IEEE 13 bus, IEEE 37 bus and IEEE 123 bus systems

Multi-Port DC-DC and DC-AC Converters for Large-Scale Integration of Renewable Power Generation

Numerous research studies on high capacity DC-DC converters have been put forward in recent years, targeting multi-terminal medium-voltage direct current (MVDC) and high-voltage direct current (HVDC) systems, in which renewable power plants can be integrated at both medium-voltage (MV) and high-voltage (HV) DC and AC terminals; hence, leading to complex hybrid AC-DC systems. Multi-port converters (MPCs) offer the means to promote and accelerate renewable energy and smart grids applications due to their increased control flexibilities. In this paper, a family of MPCs is proposed in order to act as a hybrid hub at critical nodes of complex multi-terminal MVDC and HVDC grids. The proposed MPCs provide several controllable DC voltages from constant or variable DC or AC voltage sources. The theoretical analysis and operation scenarios of the proposed MPC are discussed and validated with the aid of MATLAB-SIMULINK simulations, and further corroborated using experimental results from scale down prototype. Theoretical analysis and discussions, quantitative simulations, and experimental results show that the MPCs offer high degree of control flexibilities during normal operation, including the capacity to reroute active or DC power flow between any arbitrary AC and DC terminals, and through a particular sub-converter with sufficient precision. Critical discussions of the experimental results conclude that the DC fault responses of the MPCs vary with the topology of the converter adopted in the sub-converters. It has been established that a DC fault at high-voltage DC terminal exposes sub-converters 1 and 2 to extremely high currents; therefore, converters with DC fault current control capability are required to decouple the healthy sub-converters from the faulted one and their respective fault dynamics. On the other hand, a DC fault at the low-voltage DC terminal exposes the healthy upper sub-converter to excessive voltage stresses; therefore, sub-converters with bipolar cells, which possess the capacity for controlled operation with variable and reduced DC voltage over wide range are required. In both fault causes, continued operation without interruption to power flow during DC fault is not possible due to excessive over-current or over-voltage during fault period; however, it is possible to minimize the interruption. The above findings and contributions of this work have been further elaborated in the conclusions