Showing papers by "Marco Liserre published in 2018"

Junction Temperature Control for More Reliable Power Electronics

Abstract: The thermal stress of power electronic components is one of the most important causes of their failure. Proper thermal management plays an important role for more reliable and cost-effective energy conversion. As one of the most vulnerable and expensive components, power semiconductor components are the focus of this paper. Possible approaches to control the semiconductor junction temperature are discussed in this paper, along with the implementation in several emerging applications. The modification of the control variables at different levels (modulation, control, and system) to alter the loss generation or distribution is analyzed. Some of the control solutions presented in the literature, which showed experimentally that the thermal stress can be effectively reduced, are reviewed in detail. These results are often mission-profile dependent and the controller needs to be tuned to reach the desired cost-benefit tradeoff. This paper analyzes also the many open questions of this research area. Among them, it is worth highlighting that a verification of the actual lifetime extension is still missing.

Reliability of Power Electronic Systems: An Industry Perspective

Abstract: Power electronics systems are used increasingly in a wide range of application fields, such as variable-speed drives, electric vehicles, and renewable energy systems. These elements have become crucial constituents in the further development of such emerging application fields as lighting, more-electric aircraft, and medical systems [1]. Reliable operation over the designed lifetime is essential for any power electronics system [2], particularly because the dependability of power electronics is becoming a prerequisite for system safety in several key areas, e.g., energy, medicine, and transportation [3].

Thermal Analysis and Balancing for Modular Multilevel Converters in HVDC Applications

Abstract: The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages in hybrid networks. The MMC enables scalability to different power levels, full controllability provided by insulated-gate bipolar transistors, and can achieve very high efficiencies by using a low-switching-frequency method as the nearest level modulation. However, the nearest level modulation requires a capacitor voltage-balancing algorithm, which can result in unbalanced loading for the power semiconductors in the different submodules. Particularly at low-power-factor operation, which could occur in the case of low-voltage ride through and of reactive power injection, the conventional algorithm is no more effective. This paper provides thermal stress analysis of the MMC in operation and proposes a thermal balancing approach, which is embedded in the capacitor voltage-balancing algorithm. The purpose of the thermal balancing is to achieve similar stress distribution among the different submodules to enhance the lifetime. The junction temperatures in the different submodules are studied for HVDC applications, and this paper proves experimentally that the thermal balance within the submodules is significantly improved.

A Quadruple Active Bridge Converter for the Storage Integration on the More Electric Aircraft

Abstract: The More Electric Aircraft concepts aims at increasing the penetration of electric systems on aircrafts. In this framework, the electrical power distribution system (EPDS) is of high importance. In order to improve the utilization of the generators and face the peak power demand without disconnecting the loads, different technologies of storage are employed. This paper proposes the use of a quadruple active bridge converter, already employed in other fields, to interface a fuel cell, a battery, and a supercapacitor bank to the dc bus of the EPDS. This objective can be achieved by employing multiple dc/dc converters, which allow an individual control of the energy sources and a good efficiency. Obtaining the same power control and efficiency with a multiport power converter constitutes a challenge that is worth taking to reduce cost, volume, and weight and increase the system reliability. A novel control based on proportional integral (PI) controllers in conjunction with a decoupling system and current feedforward allow shaping the power request to each port. This, however, leads to an asymmetrical loading of each port, which could decrease the efficiency. A laboratory prototype is used to confirm that this asymmetrical kind of operation, where each port processes a different amount of power, does not imply a marked reduction of efficiency.

Failure Analysis of the dc-dc Converter: A Comprehensive Survey of Faults and Solutions for Improving Reliability

Abstract: In many industrial applications, power interruption is not tolerated, and a highly reliable power electronics system is required. In fact, a failure on the power converter penalizes not only the maintenance cost (to repair or change the converter), but also the operational cost, because the service is interrupted. Thus, this article investigates the failure in dc-dc converters, with the aim to identify the most vulnerable devices and discuss solutions for improving the converter's availability.

Load Control Using Sensitivity Identification by Means of Smart Transformer

Abstract: The higher variability introduced by distributed generation leads to fast changes in the aggregate load composition, and thus in the power response during voltage variations. The smart transformer, a power electronics-based distribution transformer with advanced control functionalities, can exploit the load dependence on voltage for providing services to the distribution and transmission grids. In this paper, two possible applications are proposed: 1) the smart transformer overload control by means of voltage control action and 2) the soft load reduction method, that reduces load consumption avoiding the load disconnection. These services depend on the correct identification of load dependence on voltage, which the smart transformer evaluates in real time based on load measurements. The effect of the distributed generation on net load sensitivity has been derived and demonstrated with the control hardware in loop evaluation by means of a real time digital simulator.

Reverse Power Flow Control in a ST-Fed Distribution Grid

Abstract: The massive integration of distributed generation in the grid poses new challenges to the system operators, like the reverse power flow from the low voltage (LV) to medium voltage (MV) grid. In the case of high DG power production and low load absorption, the voltage rises in the line reaching the upper voltage limit. At this regard the smart transformer (ST) offers a new possibility to limit the reverse power flow in the MV grids. The ST can adapt the voltage waveform modifying the frequency in order to interact with the local distributed generation, that are normally equipped with droop characteristic. However, when a fast change in the frequency is applied to avoid reverse power flow to MV grid, stability problems, so far not investigated, arise. In this paper, the stability analysis has been performed analytically and validated by means of control-hardware-in-loop in a real time digital simulator and with experimental results in laboratory.

Sizing and SOC Management of a Smart-Transformer-Based Energy Storage System

Abstract: A smart transformer (ST), which is a power-electronic-based transformer with control and communication functionalities, can be the optimal solution for integrating a battery energy storage system (BESS) in an electric distribution system. In fact, a comparison of energy efficiency for the conventional BESS and the ST-based BESS is carried out, which demonstrates that the ST-based BESS operation is more efficient during charging and discharging, as compared to the conventional BESS. It has been shown that by sizing the BESS appropriately for the peak load demand, the power rating of ST converters can be reduced. The ST-based BESS provides an enhanced fault ride-through capability, as compared to the conventional BESS. However, the state of charge (SOC) of the BESS shall be correctly managed to support the peak load demand when the ST converter reaches its maximum active power rating. Both simulation and experimental results clearly verify the developed SOC management.

Passivity-Based Control of Switched Reluctance-Based Wind System Supplying Constant Power Load

Abstract: This paper presents a passivity-based control (PBC) scheme for the switched reluctance generator (SRG) in small-scale wind energy conversion systems for dc microgrid applications. The main objective is to stabilize the output voltage in case the system supplies constant power loads (CPLs) and operates with maximum power point tracking (MPPT). Stability improvement and dc-link ripple reduction in the presence of CPLs is achieved using system-level modeling of SRG-based dc microgrid through the Euler-Lagrange system (ELS) from the viewpoint of the machine physical structure. Compared with other control methods, the proposed MPPT method based on passivity-based speed controller employs the back electromotive force (EMF) in the generation process as a position-dependent voltage source to overcome the major challenge of SRG complicated uncertain dynamic model. To deal with the time-varying inductance and back EMF of SRG, an adaptation mechanism is incorporated in the proposed adaptive PBC and the control design is constructed by using the Lyapunov theorem where the closed-loop stability is ensured. The effectiveness of the proposed method in avoiding instability effects of SRG and CPL with voltage ripple reduction and precise wind turbine speed tracking is investigated with simulation results and validated with experimental by using a four-phase, 8/6 SRG drive system.

Optimum Design of a Multiple-Active-Bridge DC–DC Converter for Smart Transformer

Abstract: The modular smart transformer (ST) is composed by several basic converters rated for lower voltage and power. In this paper, the quadruple active bridge (QAB) is used as the basic block for the modular ST. In this application, the efficiency and cost are the most important design parameters. Therefore, the paper focus on the design of the converter, with the aim to optimize its efficiency, taking the cost into consideration. To do so, the losses of all components are carefully modeled and a computer-aided design is used, where an algorithm to calculate the losses and cost is developed, allowing to perform multiobjective optimization. Additionally, silicon IGBTs and silicon carbide mosfet s are considered for the design and the performance of the converter using both semiconductors technology is compared. Experimental results obtained for the optimized 20 kW QAB converter has shown an efficiency of 97.5%.

Concurrent Voltage Control and Dispatch of Active Distribution Networks by Means of Smart Transformer and Storage

Abstract: Dispatching active distribution networks is expected to play an important role in the smart grid technologies. Voltage control is also starting to be widely proposed to avoid voltage violations in the electrical grid. The integration of the storage in the so-called smart transformer (ST), which is a solid-state transformer with control and communication functionalities, can help combine both services. The added value of such a configuration is that it allows the full decoupling of the reactive power flows between the medium voltage (MV) and low voltage (LV) networks. We show the augmented flexibility of such a configuration by proposing a control strategy where the storage system is used to achieve dispatched-by-design operation of the LV network active power flow, as well as the two ST power converters to control the voltage in both the MV and LV grid on a best effort basis. The control strategy is validated by simulations using the IEEE 34 nodes MV test feeder and the CIGRE LV reference network. Moreover, control performance is benchmarked against a conventional network setup, where the battery energy storage system (BESS) is connected to the LV network through a dc/ac power converter and the ST transformer is replaced by a conventional transformer.

Thermal Stress Based Model Predictive Control of Electric Drives

Abstract: The reliable operation of the power electronics system of an electric drive is a critical design target. Thermal cycling of the semiconductors in the power module is one of the main stressors. Active thermal control is a possibility to control the junction temperatures of power modules in order to reduce the thermal stress. In this paper, the finite control set model predictive control is designed for thermal stress based driving of electric drives converters. The optimal switching vector is selected using a multiparameter optimization that includes the current reference error, the additional thermal stress that a specific switching vector applies to each semiconductor, the temperature spread between semiconductors in the module, overall efficiency, and device constraints. This enables relieving the stress due to thermal cycles and reducing unsymmetrical fatigue of the modules chips while avoiding unnecessary losses. The approach is derived in theory and applied in simulation and experiment.

Interleaved Operation of Two Neutral-Point-Clamped Inverters With Reduced Circulating Current

Abstract: Parallel inverters are commonly adopted in high-power applications, for instance wind energy systems, smart transformers, and power conditioners. Meanwhile, interleaved pulse width modulation is usually considered as an optimal approach to reduce the current ripple and harmonics of the parallel inverters. However, in the case of a common dc link, the problem of circulating current emerges and leads to performance degradation. This paper aims at investigating the influence of different modulation techniques on the circulating current of two neutral-point-clamped (NPC) inverters under interleaved operation. Two modulation techniques, phase disposition (PD) and alternative phase opposite disposition (APOD), have been studied and compared in terms of current ripple, spectrum quality, and circulating current. Though the PD modulation was regarded as the optimum solution in most of the single-NPC cases, it offers worse performance in the two parallel NPC applications due to higher circulating current. Simulation and experimental validations are provided and show that the APOD leads to much lower circulating current and similar current ripple as well as spectrum quality compared to the PD.

Thermally Compensated Discontinuous Modulation Strategy for Cascaded H-Bridge Converters

Abstract: A widely adopted multilevel converter topology is the cascaded H-bridge (CHB), as it can provide voltage and power scalability. However, since the CHB converter can be composed by cells with different remaining lifetime, it could be useful to delay a fault of higher aged cells in order to prolong the entire converter's lifetime. In this paper, a thermally compensated discontinuous pulse width modulation (DPWM) strategy is proposed in order to reduce the thermal stress for power semiconductors in power modules of higher aged cells. Considering the thermal cycles as the most influencing cause of wear out of power modules, the proposed method compensates the thermal cycles under a varying power profile by manipulating the clamping angle of the DPWM. Moreover, the proposed method obtains a comparable total harmonic distortion performance to the phase-shifted carrier modulation.

Lifetime-Based Power Routing in Parallel Converters for Smart Transformer Application

Abstract: The use of a smart transformer-based electrical distribution could be an effective approach to reorganizing the electric grid and solving the problems and challenges of distributed generation systems and active loads. While lower efficiency and higher cost compared with the conventional transformer are well known and investigated limiting factors, the required higher maintenance, related to the use of electronics systems in the electric grid, is seldom targeted. In fact, high maintenance cost would make the use of smart transformers inapplicable even if new services could justify higher initial cost and new semiconductor development could increase the overall efficiency. This work proposes a modular repairable system based on condition monitoring, which aims at equalizing the lifetime consumption of the single modules to make possible prognostic maintenance.

A Family of Series-Resonant DC–DC Converter With Fault-Tolerance Capability

Abstract: The series-resonant dc–dc converter (SRC) is widely applied in a large range of voltage and power In most applications, fault tolerance is a highly desired feature and it is obtained through redundancy This paper proposes a fault-tolerance solution for the SRC, which could drastically reduce the need of redundancy Using the proposed scheme, the full-bridge-based SRC or multilevel-based SRC can be reconfigured in a half-bridge topology, in order to keep the converter operational even with the failure (open circuit or short circuit) of one switch Since the proposed scheme can be applied to the full-bridge-SRC and multilevel SRC, a family of fault-tolerant converter is proposed in this work The advantages of the proposed approach are minimum of additional hardware and no deterioration of the converter efficiency The proposed fault-tolerance solution was experimentally tested in a 10 kW SRC prototype with input voltage of 700–600 V A short-circuit fault in a semiconductor is tested and the results confirm the effectiveness of the proposed approach

Smart Transformer-Fed Variable Frequency Distribution Grid

Abstract: The smart transformer (ST), a solid-state transformer with control and communication functionalities, that interfaces medium voltage (MV) and low voltage (LV) grids, enables the control of the frequency in the LV grid independently from the MV one. In a ST-fed distribution grid, the ST can interact with the droop controllers of local generators and loads frequency characteristic to control the LV power demand. However, most of the existing controllers for power converters cannot guarantee good harmonic control under variable frequency condition. To address this issue, a frequency-adaptive control scheme based on the fractional-order repetitive control and the frequency-adaptive phase-locked loop are proposed in this paper. The proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be easily implemented in the power converters controllers of a ST-fed distribution grid. Moreover, the stability analysis of the frequency-adaptive system considering the effect of synchronization has been investigated in this paper. Simulation and experiments have been carried out to verify the effectiveness of the proposed scheme as well as the considered scenarios.

Analysis and Stabilization of a Smart Transformer-Fed Grid

Abstract: The stability of the grid-connected inverters adopted in distributed energy resources (DERs) highly depends on the characteristics of the grid impedance. Hence, various active damping methods and adaptive control algorithms have been proposed for the control of the grid inverters. Differently from those existing solutions, this paper proposes to use a smart transformer (ST) to shape the grid impedance to interact with the controllers of local inverters, aiming at improving the stability of the local controllers as well as the overall grid. A main advantage of this solution is that it offers a pervasive service to all the available DERs, reducing the cost and design effort of the local controllers. Three active damping methods are presented in this paper and the implementation issues in the ST LV-side voltage controller are given. Moreover, the design criteria of the active damping methods considering the control performance of ST and the stability requirements of local DERs are presented. Analysis and experiments are carried out to verify the effectiveness of the proposed solution.

A PV-Inspired Low-Common-Mode Dual-Active-Bridge Converter for Aerospace Applications

Abstract: In the framework of the more electric aircraft, the use of isolated dc–dc power conversion for the electrical power distribution system is one of the most investigated solutions. If the dc–dc converter produces a variable common-mode voltage, the leakage current can flow in the interwinding parasitic capacitance of the high-frequency transformer, leading to insulation deterioration and early failure. This paper proposes to use modified H-bridge structures, already employed in the photovoltaic system for dc–ac power converters, to enable a constant common-mode voltage for isolated dc–dc converters. The analysis shows that this solution can achieve the same efficiency as the conventional one, while simulations and experiments show a strong reduction of the common-mode current flowing through the transformer. A reliability analysis showed that the lifetime of the high-frequency transformer can be extended with the proposed solution.

Active Thermal Control of GaN-Based DC/DC Converter

Abstract: The new packaging technologies used for gallium nitride (GaN) devices avoid wire bonds and leads in order to completely utilize their switching performance. This also means that thermomechanical fatigue that used to exist within the device may no longer be a reliability problem. And one of the main bottlenecks for reliability will be the solder joints between the device and the printed circuit board (PCB). To limit the thermal cycling induced failures in these points, an active thermal control scheme using a two step gate driver designed for GaN is presented in this paper. In contrast to the active thermal control techniques employing variable switching frequency control, this method does not alter the converter operation frequency; instead, it merely controls the GaN device slew rates. A simple temperature control algorithm that actively varies the device losses with the objective to minimize thermal cycling is proposed. The solder fatigue due to thermal cycling has been discussed. The effectiveness of this active thermal control scheme has been analyzed also in comparison with losses and validated with analysis, simulation, and experimental results.

Investigation on Common-Mode Voltage Suppression in Smart Transformer-Fed Distributed Hybrid Grids

Abstract: High-frequency (HF) switching and ac-side unbalanced loads challenge smart transformer (ST)-fed hybrid grids (both ac and dc), causing common-mode (CM) voltage variations and dc-link oscillation. The HF switching introduces an HF CM voltage and the ac grid unbalanced loads introduce a fundamental frequency CM voltage in hybrid grids. The CM voltage in ST-fed distributed grids degrades the power quality, threatens the safety of the connected devices, and potentially constitutes a health risk for the operators of such devices. Therefore, this paper systematically analyzes the root causes of the ST CM voltage variations and the impacts on hybrid grids. Based on the two typical configurations (three- and four-leg converters), the performance and requirements of CM inductor filter and bypass CM filter on HF CM voltage suppression are studied in detail. By considering the CM voltage suppression and dc capacitor lifetime, a four-leg converter with improved modulation strategy and small dc bypass film capacitor is proposed. The simulation and experimental results clearly verify the feasibility and correctness of the proposed strategies.

Robust Active Damping in LCL -Filter-Based Medium-Voltage Parallel Grid Inverters for Wind Turbines

Abstract: LCL -filter-based grid-tie inverters require damping for the current-loop stability. There are only software modifications in active damping, whereas resistors are added in passive damping. Although passive damping incurs in additional losses, it is widely used because of its simplicity. This paper considers the active damping in medium-voltage parallel inverters for wind turbines. Due to cost reasons, only minimal software changes are allowed and no extra sensors can be used. The procedure must be robust against line-inductance variations in weak grids. Double-update mode is needed so the resonance frequency is under the Nyquist limit. The bandwidth reduction when using active damping is also required to be known beforehand. Moreover, the design procedure should be simple without requiring numerous trial-and-error iterations. In spite of the abundant literature, the options are limited under these circumstances. Filter-based solutions are appropriate and a new procedure for tuning the notch filter is proposed. However, this procedure requires that the resistance of the inductors is known and a novel filter-based solution is proposed that uses lag filters. The lag filters displace the phase angle at the resonance frequency so that the Nyquist stability criterion is fulfilled. Simulations and experiments with a 100-kVA prototype validate the analysis.

Analysis of DC-Link Current Influence on Temperature Variation of Capacitor in a Wind Turbine Application

Abstract: Back-to-back converters for wind turbine systems feature capacitors in the dc-link to maintain a stable voltage and to decouple a generator from the electric grid The electrolytic capacitors are typically chosen for their advantages; a higher energy density and a higher capacitance at lower costs Long-term field experiences and recorded failure data revealed that the capacitors are one of the most frequent failure reasons for the wind turbine system The current profile of the capacitors is highly responsible for this degradation, since it determines the dissipated power of the capacitor This paper analyzes the actual current profile in the dc-link capacitor of a back-to-back converter for wind turbine application A power converter is also designed to generate sinusoidal current at arbitrary frequency and arbitrary dc bias voltage for testing purposes The experimental results confirm that the proposed power converter enables us to derive the correlation between the current frequency and the temperature variation of capacitor

Performance Comparison of Variable-Angle Phase-Shifting Carrier PWM Techniques

Abstract: This paper analyzes the performances of different carrier phase-shifting pulse width modulation (PWM) techniques to be used with a multilevel cascaded H-bridge converter in case of unbalanced operational conditions. In fact, in many practical applications, the ideal condition of equal dc voltages and equal reference signals for each H-bridge cannot be achieved. In such conditions, the conventional carrier phase-shifting PWM technique loses its harmonic canceling capabilities and then the multilevel ac voltage harmonic quality is deeply affected. To overcome this limit of the original technique, different variations have been proposed. All of them still rely on the carrier phase-shifting concept and propose to use a different value of the shifting angle for each carrier (unlike the original technique) whenever unbalanced operational conditions occur. In this paper, the three main solutions proposed over the last years to extend the capabilities of the carrier phase-shifting PWM technique are compared. The analysis is focused on a three-cell cascaded H-bridge converter. Simulation and experimental results are presented.

Smart Transformer and Low Frequency Transformer Comparison on Power Delivery Characteristics in the Power System

Abstract: Smart transformer is a power electronics-based transformer, offering voltage regulation and DC connectivity. As a transformer, its basic function is still power delivery. Smart transformer with advanced controls can support MV gird voltage by absorbing/injecting reactive power while actively regulate the LV grid voltage. Due to the controllable voltage in both MV and LV side, the power delivery of smart transformer is flexible. This paper focuses on the power delivery characteristic of smart transformer and compares with the conventional low frequency transformer with the help of STACTOM at its primary side or on load tap changer at its secondary side, in the power system by means of maximum deliverable power and power-voltage curve analysis. The Simulink results validate that the smart transformer improves system voltage stability compared to the traditional low frequency transformer with load tap changer.

Smart Transformer for the Provision of Coordinated Voltage and Frequency Support in the Grid

Abstract: Considering the increase in renewable generation and the consequent reduction in power system inertia, the Virtual Synchronous Machine (VSM) control method has been proposed to control power converters to emulate the inertia and other the characteristics of the synchronous machine. However, to achieve the function of VSM control, an extra energy base, typically storage, is required to connect to the controlled converter. In this work we investigate the application of the VSM control to the distribution system demand through the use of a VSM controlled smart transformer. Through control of the demand in this way, the demand itself can be used to emulate inertia and provide frequency support. This paper presents the details of the flexible demand control applied to a smart transformer supplying a low voltage distribution grid. The operation of the control is validated on scaled hardware using real time simulation with hardware in the loop. Simulations on a 400 kVA, 400 V distribution network are used to quantify the demand flexible. IEEE 39 bus is used to verify the benefit of the proposed control in terms of voltage and frequency in the power system.

Active thermal control for delaying maintenance of power electronics converters

Abstract: Several studies have reported about power semiconductors and capacitors being the most sensitive components in power converters. The lifetime of these devices is associated with the mission profile and the resulting temperature profile. For preventing failures, it is of interest to estimate the Remaining Useful Lifetime(RUL) and several condition monitoring methods have been proposed for this purpose. Moreover, modular power converters consist of a high number of components and methods have been proposed to reduce the thermal stress and therefore extend the lifetime of a system with software, referred to as active thermal control. For power converters with limited accessibility, the RUL detected by the condition monitoring system may not fit to the scheduled maintenance of the system and devices may still have a significant RUL when their replacement is scheduled. Therefore, this work proposes to control the stress of the most deteriorated components in the system such that the failure probability of multiple building blocks is equalized when the next maintenance is scheduled. Moreover, this concept is proposed to extend the time to the next maintenance and reduce the number of maintenance instances without affecting the mean lifetime of the system.

Design of a Medium Voltage DC Fast Charging Station with Grid Voltage Regulation and Central Modular Multilevel Converter

Abstract: Fast, Superfast and Hyperfast Charging Stations (FCS) are now a reality and will become even more common in the next few years. Their integration in the medium voltage grid will impose new challenges and developments not only in the power electronics application range, but also in the grid analysis and dynamics. This paper presents a FCS structure based on a central AC-DC bidirectional converter, which for this study is the modular multilevel converter (MMC), with ancillary services provision capability. To evaluate the integration of the FCS on a standard distribution grid, a study case is described, in which an analysis on how load variations connected to it may affect the voltage regulation and how the FCS can contribute to reduce this effect. Once the requirements for the converter design are defined from the study analysis, a cost/losses optimization for the medium level MMC is provided. Finally, with experimental results presented, the current and future set-ups are discussed.

Smart transformer universal operation

Abstract: The Smart Transformer (ST) has been proposed for increasing the hosting capacity of renewable energy sources in the power system with advanced control and communication capability. Its capability to connect-and disconnect itself from the main grid and to provide power from the Low Voltage (LV) grid to the Medium Voltage (MV) grid in the grid forming mode are an opportunity to realize an islanded operation mode. This operation mode requires a suitable controller design, which is forming the MV grid and the LV grid at the same time. This work proposes a modular ST architecture consisting of a three phase Cascaded H-Bridge (CHB) converter connected to Quadruple Active Bridge (QAB) converters. The design of the converter and its magnetic components is examined as well as the controller design for the QAB for the capability to address different operation modes. Grid forming operation on the both AC grid sides and the resulting constraints by means of active power availability are examined as well as opportunities to influence the power consumption of the grids. An experimental setup is presented and selected operation modes are demonstrated.

Voltage-Based Load Control for Frequency Support Provision by HVDC Systems

Abstract: In the last years, the electrical grid has seen a deep change in the power production share between conventional generators, such as coal and gas turbines, to renewable ones, like wind and photovoltaic power plants. These resources are characterized by a power electronics-based interface to the grid. However, these resources do not offer any rotational inertia to the grid, that, in case of large disturbances, can suffer of large frequency deviation. Several strategies have been proposed in literature for damping the frequency deviation, mostly involving HVDC systems. The HVDC can request active power to the DC-connected AC areas in order to provide the needed energy to damp the frequency variation. However, all these strategies do not investigate the impact of the power variation on the frequency in the other DC-connected AC areas, that may result large if these system's inertia is low. This paper proposes a control strategy for minimizing the impact of the power request in these DC-connected AC areas varying the power consumption of near voltage-sensitive loads by means of a controlled voltage variation. As demonstrated in this work, the controlled voltage variation is able to reduce the frequency swing in these AC areas, matching the power variation request for frequency support with the power variation achieved from the voltage-sensitive loads.