Cascaded Power Converters Research Articles

A new grid side inertia support control method for cascaded power converters in Bi-directional EV chargers

A new grid side inertia support control method for cascaded power converters in Bi-directional EV chargers

Cascaded Multi-Input Single-Output Boost Inverter for Mismatch Mitigation at PV Submodule Level

Mismatched power generation is a serious issue in PV systems, resulting from unequal power generation between PV components. Solutions have been proposed to reduce or eliminate the mismatch concern. One practical strategy is individually harvesting the maximum power from each PV component; the more distributed MPPT is applied to a finer level, the more power can be obtained. This study proposes three-input single-output boost converters that are employed to effectively increase PV power generation and significantly reduce mismatch issues between the PV submodule (PV SM). Each boost converter will be controlled to harvest the maximum power from a group of PV cells inside a single PV module. The outputs of the three boost converters are connected in series to provide higher output voltage for grid integration. The cascaded power converters are linked with a forwarding diode to provide a protection feature for the system and prevent the reverse current from harming the PV module. On the grid side, a single-phase Voltage Source Inverter (VSI) is used to convert the DC power from the PV module to sinusoidal AC power. The performance of the suggested inverter has been confirmed through experimental tests.

Mismatched Disturbance Compensation Enhanced Robust H$_\infty$ Control for the DC-DC Boost Converter Feeding Constant Power Loads

A new control scheme to enhance the robustness and disturbance rejection ability of a DC-DC boost converter feeding constant power loads (CPLs) is proposed in this paper via introducing mismatched disturbance compensation into the robust H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{_\infty}$</tex-math></inline-formula> control. In cascaded DC power converter systems, the load side converters, when tightly controlled, behave as CPLs for their source side converters, where the nonlinearities could result in severe instability. Besides, for power converters in DC microgrids, both the uncertainties/disturbances and the unpredictable load access could result in severe challenges to achieving higher control performance. In particular, the unknown CPLs must be considered as part of disturbances that form the so-called mismatched disturbances. To counteract these negative factors, a practical exact feedback linearization method is used to transform the control problem of a nonlinear system into that of a linear system, and the compensation of mismatched uncertainties/disturbances is introduced into the robust H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{_\infty}$</tex-math></inline-formula> control design. Moreover, sufficient conditions to ensure the asymptotic stability and robustness of system are established. Both simulation and experimental results confirm the efficiency.

Stability Effect of Load Converter on Source Converter in a Cascaded Buck Converter

In a cascaded power converter, the input current ripple of load converter is fed forward to the output capacitor of source converter and results in reshaping the output capacitor current and output voltage ripples of source converter. To investigate the stability effect of feedforward current ripple of load converter on source converter, a cascaded power converter comprising two peak-voltage-ripple controlled buck converters is taken as an example. First, the stability effects of chosen circuit parameters of the source converter in cascaded operation and standalone operation are demonstrated and compared. Second, by describing three switching state sequences induced by the feedforward current ripple, a discrete-time map model is established for the cascaded power converter. Based on the map model, the instability mechanism of source converter with the variations of the circuit parameters is thereby expounded, and the stability boundaries are obtained in the circuit parameter planes. The results show that the stability effects of the circuit parameters of source converter with feedforward current ripple are essentially different from those in the standalone operation. The theoretical analyses are verified by the simulation results and experimental results.

Integrated Model Predictive Control of a Single-Phase Multilevel T-type Converter for a Photovoltaic Grid Connected System under Failure Conditions

Abstract The article presents two contributions: the first is an optimised control structure for photovoltaic grid connected systems (PVGCSs). The power chain is composed of two cascaded power converters, namely, a boost converter and a five-level T-type multilevel converter. Traditionally, each power converter is controlled by a separate mode control (SMC) from the other, which is computationally intensive since each converter requires its own control system, which is not practical. The suggested control, called integrated finite set model predictive control (IFS-MPC), allows controlling cascaded converters at the same time in one stage, instead of controlling them separately. Consequently, the overall implementation system is widely reduced. The second contribution of the article is a modified IFS-MPC called modified integrated finite set-model predictive control (M-IFS-MPC), which ensures the correct functioning of the grid-tied PV system under certain faults in converter components. Indeed, when one of the DC-link capacitors fails or when one of the auxiliary switches breaks down, by selecting an appropriate choice of the DC-link capacitors’ voltage reference, the proposed design allows a normal operation without intervention on the power circuit.

A Unidirectional Cascaded High-Power Wind Converter With Reduced Number of Active Devices

For large offshore wind turbines, cascaded H-bridge wind power converters can raise the wind generator voltage to a medium voltage level such as 10kV, which is favored for reducing the generator current, mitigating issues such as cable twisting in low-voltage, high-current configurations. In order to reduce the number of required active power devices, this paper presents a unidirectional-power-flow medium voltage high-power cascaded wind power converter. The proposed topology uses a unidirectional cascaded H-bridge rectifier as the generator-side converter to connect the medium-voltage permanent synchronous wind generator. The proposed topology reduces the number of active power devices and their associated gate driver circuits, control, etc. A zero deadtime modulation method is also presented for the generator side converter, which can eliminate the deadtime and improve the voltage and current waveform quality. Furthermore, the proposed topology will not increase the converter power losses as analyzed in the paper, or sacrifice dynamic control performance. Simulation and experiment results are presented, which validate the proposed unidirectional power conversion system.

Development of grid-side converter-based FRT control and protection in a grid-connected solar photovoltaic park system under different control modes

In a grid-connected solar photovoltaic system, voltage dips on the grid side, increased grid current, and overshoot in the inverter’s dc-link voltage are all noticed during grid disturbances. The grid code stipulates that in order to protect the cascaded power converter systems from high currents and overvoltages, the solar PV system disconnects from the grid anytime the voltage level at PCC falls below a particular standard nominal value. This study proposes combined GSC-based fault ride-through (FRT) and protection control strategies which can provide independent real and reactive power control for the inverter for effective FRT in photovoltaic parks interconnected with a large power system during grid faults. The proposed approach operates in three modes—voltage control, power factor control, and reactive power control modes. The developed protection modules in the PV system consist of over/undervoltage protection, voltage sag detection, and overcurrent detection. The inverter-fed real–reactive power control technique limits grid overcurrent by modifying active power injection into the grid and stabilizes grid voltage during faults by injecting reactive current. The performance of the proposed FRT and protection control strategy is studied through the simulation of 75 MW PV park in the EMTP platform and the experimental setup of a 5 kVA grid-connected inverter-fed solar PV system. The simulation results and hardware discussion presented in this study validate the performance of the proposed FRT scheme in comparison with conventional control schemes during grid faults.

Dual-Circuit-Based Test Bench Design for HVDC Circuit Breaker Verification

High-voltage direct current (HVDC) circuit breaker development and deployment strongly depend on the testing process, which ensures that the HVDC circuit breakers will satisfy design requirements. This article presents an HVDC circuit breaker test bench circuit configuration that can provide controllable large output currents to simulate different fault conditions for the current breaking test and high output voltage for the dielectric withstand test. The current breaking test circuit is based on multiple cascaded power converters connected in parallel to provide the necessary output current capability. Each cascaded power converter is composed of multiple cells that are operated by a phase-shifted pulsewidth-modulated signal for greater controllability and higher quality of the output waveform. The dielectric withstand test circuit is a simple high-voltage source with a low power rating that can also be used to charge the test bench and the internal circuitry of the circuit breaker that is to be tested. The proposed test bench ensures that fault conditions can be replicated accurately and offers greater flexibility by being able to test mechanical, semiconductor-based, or hybrid HVDC circuit breakers with different current and voltage ratings on the same hardware without any changes. The idea and the operating principle of the proposed test bench are verified experimentally on a downscaled system that consists of three cascaded power converters connected in parallel with three cells per cascaded power converter and with a total equivalent switching frequency of 92.5 kHz.

Analysis and Protection of Converter-Side AC Faults in a Cascaded Converter-Based MVDC Link: ANGLE-DC Project

Medium-voltage direct-current (MVDC) technology exhibits advantages over ac transmission due to its enhanced capacity, controllability, and flexibility. A practical example is the ANGLE-DC project from Scottish Power Energy Networks, which converts existing 33 kV ac circuits into dc operation to improve the system transmission capacity in North Wales, U.K. Specially designed cascaded power converters are employed to build up the ±27 kV dc link voltage. Within the converter stations, converter-side single-phase ac faults may threaten the safe operation of the converters. This paper presents an analysis of such faults for an MVDC link based on ANGLE-DC. It is shown that the capacitors of the converters will be overcharged without suitable protection in place. A protection strategy using a bypass thyristor-based branch is presented to relieve overvoltage issues following converter-side faults, and it is verified through simulations conducted in PSCAD/EMTDC. For completeness, case studies considering post-fault operation are also included.

Partial Discharge Behavior of Epoxy-Mica Insulation System under Superimposed AC and DC Voltage Stress

Recent technology advances in wind energy conversion systems include a new type of permanent magnet synchronous generator with a segmented stator that allows lightweight concepts with high efficiency and superior controllability. Each stator segment is driven by a separate power converter. The various cascaded power converters can be connected in series and in parallel, which ultimately leads to the superposition of a DC and an AC voltage stress in the insulation of the stator coils. The resulting DC-AC mixed voltage is an entirely new electrical stress for the epoxy-mica insulation system that is typically utilized as main insulation of rotating electrical machines. In order to resemble this stress, typical dielectric tests such as partial discharge (PD) measurements and long-term aging of model insulation systems have to be be carried out under DC-AC mixed voltage stress. The mentioned investigations are presented in this contribution. The results indicate that the AC voltage is dominant with regard to the PD inception and influences the PD parameters such as the apparent charge and the repetition rate substantially when compared to the DC voltage proportion. The accelerated aging tests with significantly higher AC-DC mixed voltage stress than is expectable under operating conditions revealed no measurable PD deterioration of the investigated insulation systems.

Scott-Tied Magnetically Linked Solar Multilevel Converter in Varying Environmental Conditions

Recently, multilevel converters (MLCs) have given promising solutions for medium-power and high-power renewable energy systems. However, voltage level enhancement leads to a higher device count, especially for three-phase configurations. A megawatt-scale three-phase 27-level converter with a Scott-connected arrangement is used for medium-voltage applications to address such issues. The Scott transformer uses 1:3:9 turn ratios between the windings to have 27-level output converter voltage with the advantage of two-phase to three-phase conversion. Moreover, the transformers enable the converter output voltage to medium-voltage levels (typically 11 to 33 kV). Traditional three-phase systems have cascaded power converter modules in each phase. However, this system provides an alternative solution for solar power injection into the grid with minimum cascaded voltage source converters forming a two-phase network. The quadrature displaced nearest level modulation strategy fundamentally switches phase A and B legs. Moreover, the solar photovoltaic (SPV) system is modeled at a megawatt-scale and tested in various environmental conditions. Thermal, cost, and leakage-inductance models are presented for the Scott-tied solar MLC. Obtained results demonstrate the satisfactory observation and control of the SPV system for renewable energy applications.

Development of Cascaded Power Converters for High-Voltage and Constant-Current Applications

Development of Cascaded Power Converters for High-Voltage and Constant-Current Applications

Stability Improvement of Cascaded Power Conversion Systems Based on Hamiltonian Energy Control Theory

It is well known that the interaction between cascaded individually designed power conversion systems can cause instability. To overcome this issue, a Hamiltonian energy control scheme is proposed, which is based on passivity control theory and port-controlled Hamiltonian framework. A complementary PI adjustment term is also included in the control algorithm to eliminate the steady-state output voltage error caused by the parameter uncertainty. The proposed control approach is applied to three different cascade structures. First, the cascade structure between dc/dc converters is considered, and the detailed controller design is given. Second, the cascade connection of a single converter and its LC filter is studied. By placing the L C filter into the Hamiltonian model of the controlled converter system, the dynamic and potential instability caused by the filter can be adjusted. Finally, the cascade structure between subsystems including filters and converters, which are common in microgrids, is studied. By using the Hamiltonian function (storage function) as the Lyapunov function candidate, the large-signal stability of each controlled converter system is proved. When the cascade structure contains multiple controlled converter systems, the stability of the entire cascaded system is guaranteed by the superposition of multiple Lyapunov functions. A 3.5 kW 220−270−350 V test bench is built in the laboratory to demonstrate the application of the proposed control approach to these three cascade structures.

Design and implementation of fuzzy logic based modified real-reactive power control of inverter for low voltage ride through enhancement in grid connected solar PV system

In a grid connected solar PV system, during grid faults, increase in grid current, dip in voltage at point of common coupling and overshoot in dc link voltage in the inverter side are observed. The solar PV system gets disconnected from the grid whenever the grid voltage goes below a certain nominal value in order to safeguard the cascaded power converter systems from high currents and over voltages. This paper proposes an intelligent fuzzy based real and reactive power control of inverter for effective Low Voltage Ride Through (LVRT) capability during grid faults. The proposed fuzzy logic based modified inverter real and reactive power control strategy, stabilizes the grid voltage by injecting reactive current and limits the grid over current by reducing active power flow into the grid. To regulate the overshoot in dc link voltage during grid faults, a braking chopper and dynamic braking resistor strategies are included in the proposed LVRT scheme. The performance of the proposed inverter control strategy is studied through simulation of 100 kW grid connected solar PV system in MATLAB/SIMULINK 2018a platform. The algorithm is validated in real time hardware of 1 kW grid connected solar PV system. The simulation and hardware results validate the superior performance of the proposed inverter LVRT control strategy in comparison with other conventional schemes during grid faults.

Decentralized Quasi-Fixed-Frequency Control of Multiphase Interleaved Hybrid Dickson Converters for Fault-Tolerant Automotive Applications

Emerging autonomous-driving systems necessitate a fault-tolerant power management system to ensure continuous power delivery to the safety processor and sensor loads, while the forthcoming 48-V bus system requires power conversion at high and wide-ranging input voltages. To address the challenges in future automotive applications, this article demonstrates a modular multiphase system with N+1 redundancy as the preregulator converter in a two-stage cascaded power conversion system. Each phase is implemented based on the 4-to-1 hybrid Dickson topology, which enables efficient high-ratio voltage conversion. This article proposes a new decentralized quasi-fixed-frequency control scheme based on the valley current mode and on-time control. In contrast to existing methods, the proposed scheme: 1) enables decentralized interleaving of the multiphase system using distributed phase-locked-loop-based synchronization in the event of a failure in any phase, which is advantageous in terms of modularity and scalability; 2) incorporates droop voltage control to achieve balanced current sharing and voltage regulation in a multiphase system; and 3) offers self-balancing of the flying-capacitor voltages, which eliminates the need for any additional control circuitry. The performance of the proposed scheme is verified using a 36-W four-phase experimental prototype.

A Single-Stage Capacitive AC-Link AC–AC Power Converter

A single-stage three-phase ac–ac converter benefiting from a high-frequency alternating link voltage is proposed in this paper. In this converter, a very small film capacitor can transfer the energy from the input to the output, owing to the high-frequency alternating voltage of the link. This eliminates the need for large electrolytic capacitors that are typically used in dc-link ac–ac converters. Moreover, a compact high-frequency transformer at the link can replace the bulky low-frequency transformers, in case isolation is required. These features increase the power density as well as reliability of the proposed converter in comparison with the conventional dc-link converters. The number of required switches in the proposed converter is 12, which is less than the number of switches needed in matrix converters, leading to lower switching and conduction losses. Despite being single-stage, the proposed ac–ac converter is capable of both stepping up and stepping down the voltage and also frequency transformation. This eliminates the need for using cascaded power converters. In this paper, the operation principles of the proposed ac–ac converter are investigated, and variable switching frequency and fixed switching frequency control methods for operating this converter are introduced. The performance of the converter is verified through simulation and experiment.

Virtual Resistor-Based Integrated DC Bus Voltage Conditioner for Stability Improvement of Cascaded Power Converters

A bidirectional integrated bus voltage conditioner (IBVC) for an isolated phase-shifted full bridge (PSFB) dc/dc converter is proposed to reduce the dc bus voltage oscillations in a cascaded power converter system, where a load converter is controlled tightly with high-control bandwidth. In the proposed method, unlike conventional dc bus voltage stabilization methods for which auxiliary switches are used, the multiplexing utilization of the full bridge switches at the primary side of a PSFB is adopted. In this paper, to improve the dc bus voltage stability, an equivalent virtual resistor is implemented by applying the duty cycle regulation for both legs of the PSFB. The effectiveness of the proposed method on the dc voltage stabilization is examined through simulations and experiments. The achieved results reveal that the dc bus voltage stability is significantly improved with the proposed method.

DC bus voltage stability improvement using disturbance observer feedforward control

The constant power load (CPL) property of a load converter with tight control results in DC bus voltage oscillations in interconnected power converter systems. In this paper, a disturbance observer (DOB) feedforward compensation scheme is proposed to stabilize the DC bus voltage of a cascaded power converter. An equivalent DC link voltage control model and a disturbance observer are used to reconstruct the dynamic changes of the DC bus current to correct the inner current loop reference of the load converter. Simulation and experimental results show that the proposed method is effective in stabilizing the DC bus voltage.

Hot-Swapping Analysis and Implementation of Series-Stacked Server Power Delivery Architectures

Current data center power delivery architectures consist of many cascaded power conversion stages, where the system-level power conversion efficiency is reduced each time the power is processed through the individual stages. Recently, series-stacked power delivery architectures have shown how the overall power conversion can be reduced through architectural changes, reporting above 99% system-level power conversion efficiencies for data centers. In this paper, we contribute to the development of the series-stacked power delivery architectures by addressing the important hot-swapping challenge, without sacrificing the high power conversion efficiency. We analyze the hot-swapped operation of the series-stacked architecture, and experimentally validate it on a testbed that includes four series-connected 12 V, 120 W servers and four custom-designed differential converters with associated circuitry for hot-swapping. The results show that continuous operation of the series-stacked servers can be maintained while a server is hot-swapped without a significant reduction in the high power conversion efficiency.

Storage-Less and Converter-Less Photovoltaic Energy Harvesting With Maximum Power Point Tracking for Internet of Things

Energy harvesting from natural environment gives range of benefits for the Internet of things. Scavenging energy from photovoltaic (PV) cells is one of the most practical solutions in terms of power density among existing energy harvesting sources. PV power systems mandate the maximum power point tracking (MPPT) to scavenge the maximum possible solar energy. In general, a switching-mode power converter, an MPPT charger, controls the charging current to the energy storage element (a battery or equivalent), and the energy storage element provides power to the load device. The mismatch between the maximum power point (MPP) current and the load current is managed by the energy storage element. However, such architecture causes significant energy loss (typically over 20%) and a significant weight/volume and a high cost due to the cascaded power converters and the energy storage element. This paper pioneers a converter-less PV power system with the MPPT that directly supplies power to the load without the power converters or the energy storage element. The proposed system uses a nonvolatile microprocessor to enable an extremely fine-grain dynamic power management in a few hundred microseconds. This makes it possible to match the load current with the MPP current. We present detailed modeling, simulation, and optimization of the proposed energy harvesting system including the radio frequency transceiver. Experiments show that the proposed setup achieves an 87.1% of overall system efficiency during a day, 30.6% higher than the conventional MPPT methods in actual measurements, and thus a significantly higher duty cycle under a weak solar irradiance.