Parallel Converter Research Articles

Novel power flow analysis method based on impedance matching for UPQC with grid voltage fluctuations and unbalanced loads

A new perspective of impedance matching is proposed to study the power flow operation of the three-phase four-wire (3P4W) unified power quality conditioner (UPQC) system. Specifically, the UPQC system is equivalent to an adjustable impedance network, when external conditions change, such as grid voltage sag/swell and unbalanced loads, the node impedances in the network will be dynamically matched. On the one hand, the three nodes of the series converter (SC) will be matched as positive or negative impedances with the change of grid voltages, which means that the SC operates in a rectifier or inverter state to absorb or transmit the energy. On the other hand, to compensate the unbalanced load currents, the three nodes of the parallel converter (PC) will be matched as positive and negative impedances at the same time, which means that the PC operates in both rectifier and inverter state to absorb and transmit the energy. After matching the node impedances, the system's output node impedances are equal to the load impedances, thus achieving the power balance of the whole system. Experimental results of the power flow and impedance matching from a laboratory-scale prototype hardware are presented to evaluate the correctness of the impedance analysis.

An Adaptive Observer-Based Controller Design for Active Damping of a DC Network With a Constant Power Load

This article explores a nonlinear, adaptive controller aimed at increasing the stability margin of a direct-current (dc), small-scale, electrical network containing an unknown constant power load (CPL). Due to its negative incremental impedance, this load reduces the effective damping of the network, which may lead to voltage oscillations and even to voltage collapse. To overcome this drawback, we consider the incorporation of a controlled dc–dc power converter in parallel with the CPL. The design of the control law for the converter is particularly challenging due to the existence of unmeasured states and unknown parameters. We propose a standard input–output linearization stage, to which a suitably tailored adaptive observer is added. The good performance of the controller is validated through experiments on a small-scale network.

Investigation of low voltage DC microgrid using sliding mode control

As the requirement of power increases, the use of renewable energy resources has become prominent. The power collected from these energy resources needs to be converted using AC-DC or DC-DC converters. The control of DC-DC converters is a complex task due to its non-linearity in the converter introduced by the external changes such as source voltage, cable resistance and load variations. Converters are to be designed to obtain a well stabilized output voltage and load current for variable source voltages and load changes. Droop control method is the most abundantly used technique in controlling the parallel converters. The major limitations of the conventional droop control technique are circulating current issues and improper load sharing. The proposed work is to resolve these issues by integrating Sliding Mode Controller (SMC) with the converter in order to enhance the performance of DC microgrid. The entire control system was designed by taking the output voltage error as the control variables. Similarly, droop control with PI and PID were also performed and all these techniques were simulated and compared using MATLAB/Simulink. The experimental results show that the proposed sliding mode controller technique provides good overall performance and is suitable against variable voltage and load changes.

A Non-Dissipative Equalizer with Fast Energy Transfer Based on Adaptive Balancing Current Control

In this study, an active inductive equalizer with fast energy transfer based on adaptive balancing current control is proposed to rapidly equilibrate lithium-ion battery packs. A multiphase structure of equalizer formed by many specific parallel converter legs (PCLs) with bidirectional energy conversion serves as the power transfer stage to make the charge shuttle back and forth between the cell and sub-pack or sub-pack and sub-pack more flexible and efficient. This article focuses on dealing with the problem of slow balancing rate, which inherently arises from the reduction of balancing current as the voltage difference between the cells or sub-packs decreases, especially in the later period of equalization. An adaptive varied-duty-cycle (AVDC) algorithm is put forward here to accelerate the balance process. The devised method has taken the battery nonlinear behavior and the nonideality of circuit component into consideration and can adaptively modulate the duty cycle with the change of voltage differences to maintain balancing current nearly constant in the whole equilibrating procedure. Test results derived from simulations and experiments are provided to demonstrate the validity and effectiveness of the equalizer prototype constructed. Comparing with the conventional fixed duty cycle (FDC) method, the improvements of 68.3% and 8.3% in terms of balance time and efficiency have been achieved.

An adaptive coordinated optimal control method for parallel bidirectional power converters in AC/DC hybrid microgrid

Parallel bidirectional power converters (BPCs) play an important role in achieving mutual support between the two grids and in improving the power quality. One of the difficulties of the study is how to coordinated control BPCs to simultaneously realize the dual goals of economically distribute transmission power among BPCs and the high dynamic quality. In view of this, this paper proposes an adaptive coordinated optimal control method for the parallel BPCs. First, taking the minimization of the sum of the power losses of BPCs as the objective, the economically optimal distribution scheme for power transmission among parallel BPCs is calculated. Second, a primary voltage-regulated controller is designed, outer loop of the primary voltage-regulated controller can distribute the dynamic transmission power among BPCs according to the power margin of each BPC, to avoid overloading BPCs, and inner loop of the primary voltage-regulated controller can realize the decoupling of the output variable and the interference variable, to improve the dynamic voltage quality. Third, a secondary voltage-regulated controller is designed, it can make the dc bus voltage restore to the rated value quickly after power disturbance occurs, and ensure the economically optimal distribution of the transmission power among BPCs after the system reaches the steady state. Finally, the stability of the proposed control method is illustrated and conclusions are verified.

System-level reliability assessment for a direct-drive PMSG based wind turbine with multiple converters

System-level reliability of wind power converter has an essential effect on the operation performance and lifespan of a wind turbine system. In this paper, a wind turbine equipped with a 2 MW direct-drive permanent-magnet synchronous generator (PMSG) serves as a case study. Considering the maximum stator current limitation of the PMSG, several multiple-converter structures and their reliability block diagrams (RBDs) are constructed for the machine side converter (MSC). To investigate the reliability influence caused by the amount of semiconductor components and the current for each component, the structures with four and five bridges in parallel are both configured. Reliability evaluation between two major parallel structures, namely, bridges in parallel and converters in parallel are also compared. In addition, the effect of different wind classes on the MSC system-level reliability has been investigated. A detailed discussion regarding the system reliability cumulative distribution function (CDF) is presented, which could serve as reference for future MSC structure design. It is concluded that the component current dominates in the system-level reliability analysis of the MSC and the standby structure can also improve the reliability under the same current level. Besides, compared with the converter structure changing, various wind classes have a minor impact on the system-level reliability.

P.300 Neurocognitive endophenotypes for bipolar disorder identified by visual backward masking and wisconsin card sorting test

System-level reliability of wind power converter has an essential effect on the operation performance and lifespan of a wind turbine system. In this paper, a wind turbine equipped with a 2 MW direct-drive permanent-magnet synchronous generator (PMSG) serves as a case study. Considering the maximum stator current limitation of the PMSG, several multiple-converter structures and their reliability block diagrams (RBDs) are constructed for the machine side converter (MSC). To investigate the reliability influence caused by the amount of semiconductor components and the current for each component, the structures with four and five bridges in parallel are both configured. Reliability evaluation between two major parallel structures, namely, bridges in parallel and converters in parallel are also compared. In addition, the effect of different wind classes on the MSC system-level reliability has been investigated. A detailed discussion regarding the system reliability cumulative distribution function (CDF) is presented, which could serve as reference for future MSC structure design. It is concluded that the component current dominates in the system-level reliability analysis of the MSC and the standby structure can also improve the reliability under the same current level. Besides, compared with the converter structure changing, various wind classes have a minor impact on the system-level reliability.

A 6.5-μW 10-kHz BW 80.4-dB SNDR Gm-C-Based CT ∆∑ Modulator With a Feedback-Assisted Gm Linearization for Artifact-Tolerant Neural Recording

This article presents a Gm-C-based continuous-time delta-sigma modulator (CTDSM) for artifact-tolerant neural recording interfaces. We propose the feedback-assisted Gm linearization technique, which is applied to the first Gm-C integrator by using a resistive feedback digital-to-analog converter (DAC) in parallel to the degeneration resistor of the input Gm. This enables the input Gm to process the quantization noise, thereby improving the input range and linearity of the Gm-C-based CTDSM, significantly. An energy-efficient second-order loop filter is realized by using a voltage-controlled oscillator (VCO) as the second integrator and a phase quantizer. A proportional-integral (PI) transfer function is employed at the first integrator, which minimizes the output swing while maintaining loop stability. Fabricated in a 110-nm CMOS process, the prototype CTDSM achieves a high input impedance, 300-mVpp linear input range, 80.4-dB signal-to-noise and distortion ratio (SNDR), 81-dB dynamic range (DR), and 76-dB common-mode rejection ratio (CMRR) and consumes only 6.5 μW with a signal bandwidth of 10 kHz. This corresponds to a figure of merit (FoM) of 172.3 dB, which is the state of the art among the neural recording ADCs. This work is also validated through the in vivo experiment.

Modeling of $N$-Parallel Full-SiC AC–DC Converters by Four Per-Phase Circuits

Due to the limit in current-rating of SiC MOSFETs, a number of SiC-based modular ac-dc converters needs to be paralleled to increase power capacity. The number of modular converters (N) may vary depending on the customer's requirement. On the other hand, paralleling diversifies paths of current, and complex filter topologies such as integrated inductors may complicate analysis. To enhance understanding of N-paralleled ac-dc converters and to provide an insight for filter design, four per-phase equivalent circuits are proposed. Any voltage on the frequency spectrum can be classified into four groups by two criteria; first, positive sequence and negative sequence versus zero sequence, and, second, circulating, versus in-phase. By extending symmetrical component theory to the parallel converters, the N-paralleled three-phase converters can be represented by four single-phase circuits for any N. Thanks to the simplicity, complex issues such as an impact of the magnetic integration or change of harmonic attenuation for different N can be easily understood for any N by single-phase equivalent circuits.

A Distributed Control Strategy for Parallel DC-DC Converters

This letter addresses the problem of voltage regulation and balanced current sharing in a parallel connection of heterogeneous DC-DC converters sharing a common ZIP (constant impedance, constant current, and constant power) load. To this end, a distributed dynamic control approach is developed. The proposed control approach does not rely on the load profile and the number of active converters. This letter describes theoretical aspects in rigorous Lyapunov-based stability analysis, load-independent characteristic, scalability, and plug-and-play feature of the control design, and verifies the performance of the proposed control mechanism via simulation case studies in MATLAB/Simscape Electrical environment.

A Novel Interleaved Parallel Bidirectional Dual-Active-Bridge DC–DC Converter With Coupled Inductor for More-Electric Aircraft

Although conventional dual-active-bridge (DAB) dc–dc converters have been applied in more-electric aircraft, the maximum transmission power of conventional DAB converters is still limited by its structure. In order to increase the maximum transmission power, this article proposes a novel interleaved parallel bidirectional DAB converter. Two bidirectional buck/boost converters are applied in low-voltage side of the proposed converter. So the voltage of transformer primary side would not be limited by the voltage of low-voltage bus and can be controlled by the duty cycle according to the modulation strategies. In this article, three phase-shift control methods are used for the proposed topology. From the analysis of operation modes and power characteristics of the proposed topology, compared with the single phase-shift (SPS), the extended phase shift can reduce the reactive power and current stress. The pulsewidth modulation (PWM) plus single phase-shift (PWMSPS) can achieve more than twice of the maximum transmission power of conventional DAB converter. Finally, test results verify the theoretical analysis and the validity of the proposed converter.

A Sliding-Mode-Based Duty Ratio Controller for Multiple Parallelly-Connected DC–DC Converters with Constant Power Loads on MVDC Shipboard Power Systems

The development of powered electronic technology has made many aware of the design and control of ship power systems (SPSs), and has made medium voltage DC (MVDC) architecture the main research direction in the future. The negative impedance characteristic of constant power load (CPL) generated by the coupling of powered electronic converters will seriously affect the stability of the systems if these converters are not properly controlled. The conventional linear control method can only guarantee the small-signal stability of the system near its equilibrium point. When the operating point changes in a large range, linear control methods will be ineffective. More importantly, research for the large-signal stability of the multi-converter system with CPLs is still rarely involved. In this paper, a sliding-mode-based duty ratio controller (SMDC) is proposed for voltage regulation and current sharing of the multiple parallelly-connected DC–DC converters system loaded by CPLs. By controlling the output voltage of each converter with SMDC, large-signal stability of the coupled bus voltage is ensured. Meanwhile, proportional current sharing between the parallel converters is achieved by droop control integrated in the reference value of converter voltage. Simulation studies were conducted in MATLAB/Simulink, where two typical operating conditions, including the variation of load power and bus voltage, were designed to verify the effectiveness of the proposed method. Moreover, a traditional PID controller was used as a comparison to reflect the superiority of the former. Simulation results showed that the proposed method is able to guarantee large-signal stability of the system in the presence of large-scale variations in load power and bus voltage. The output current of the parallel converters can also be distributed in desired proportions according to the droop coefficient.

Reconsideration of Grid-Friendly Low-Order Filter Enabled by Parallel Converters

High-order filters, such as inductor-capacitor-inductor ( LCL) filter, have been popular in grid-tied power converters. Although featuring small size, LCL filters are not grid friendly due to inherent resonance, especially when a large number of converters are in parallel operation in todays electric grid to attain modularity, reliability, and redundancy advantages. Thus, this article reconsiders the low-order L filter in parallel converters to eliminate the resonance and in turn to simplify the control. It is found that by interleaving a certain number of converters, the L filter will be sufficient to meet the harmonic limit requirements of the standards, while the filter size can be even smaller than the LCL filter. This further contributes to cost reduction as a promising solution for grid-friendly converters.

Control of Parallel AC Voltage Source Converter High-Voltage DC System Using an Adaptive PI Evolutionary-Based Controller

High-voltage direct current (HVDC) transmission system based on voltage source converter technology gains attraction in recent years for the grid integration due to many advantages like connecting wind farms to power grids, underground power links, connecting asynchronous grids, and providing efficient long-distance power transmission, etc. To acquire the best output from the above topologies, it is crucial to design a better controller strategy for the converter control. With this objective, this paper presents an adaptive PI controller based on evolutionary optimization for a parallel AC voltage source converter HVDC system. Looking at the extensive application of constant gain PI controller type, an adaptively changing gain parameter PI controller based on tan hyperbolic (tanh) function is proposed in this study. To enhance further its performance, a Modified Teaching Learning-Based Optimization (MTLBO) is used to find optimally the parameters of the proposed control strategy. To justify the effectiveness of the proposed controller, different test cases in terms of faults, parameter variation, and load & reference power variation are studied and analyzed. Comparative analysis with a conventional tuned PI controller, PI-MTLBO controller, and proposed API-MTLBO controller is demonstrated for indicating a substantial improvement in the damping system oscillations with better controllability and stability of the proposed approach.

GaN-Based 1-MHz Partial Parallel Dual Active Bridge Converter With Integrated Magnetics

In this article, a partial parallel dual active bridge (PPDAB) converter is proposed to lower the current stress over the switching devices on the low-voltage side. Gallium Nitride (GaN) transistors are used to improve efficiency and power density at 1 MHz switching frequency. Moreover, in order to reduce the number of magnetic components and achieve more compact design, with a unique three-leg core geometry, a new method of integrating three transformers and their associated interfacing inductors is proposed. The integration is able to shorten winding length and therefore reduces winding dc resistance. Moreover, wounding the secondary windings on each outer leg in the core enlarges leakage inductance, which can serve as the interfacing inductor L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> in the PPDAB converter. Given that, a reluctance model to analyze coupling effect of the secondary windings as well as their current balance is derived and validated by finite element simulations. Based on both calculations and simulations, the proposed integrated transformer is found to be functionally equivalent to three discrete transformers, in which their primary windings are connected in series, and the additional L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> inductor. Additionally, given the same number of the turns of the windings in each parallel module, the ac currents in these three secondary windings can inherently balance. Finally, a fully GaN-based 1.2-kW 1-MHz 400V/50V prototype with a peak efficiency of 97.51% is built and tested to verify the theoretical analysis.

Coordinated Control of the DFIG Wind Power Generating System Based on Series Grid Side Converter and Passivity-Based Controller Under Unbalanced Grid Voltage Conditions

In order to solve the problem of excessive damage to doubly fed induction generator (DFIG) system under the condition of unbalanced voltage, this paper presents an improved coordinated control strategy based on doubly-fed induction generator (DFIG) wind power system, which can solve these problems well. The innovation of this paper is that the parallel grid-side converter (PGSC) uses a passivity-based controller (PBC) based on the Port Control Hamiltonian Dissipation (PCHD) model. Not only can four different control goals be achieved, namely, constant voltage of DC bus voltage, grid-side active power without second harmonics, grid-side reactive power without second harmonics, and grid-side current without negative sequence component, but also to ensure that the balance of stator and rotor current without distortion, the DFIG output power and electromagnetic torque without pulsation. The proposed coordinated control strategy has the characteristics of not changing the control strategy of the rotor-side converter and avoiding complex high-order matrix. The experimental results on the software platform and the hardware platform show that the proposed coordinated control strategy has the advantages of fast response, strong anti-interference ability, high stability, less control parameters.

Control analysis of parallel DC-DC converters in a DC microgrid with constant power loads

This paper proposes a general methodology for designing hierarchical control schemes for DC microgrids loaded by constant power loads. This paper addresses issues when interfacing sources with a wide voltage range allowing for any general choice for a power source. The control strategy consists of two levels. The lower level consists of droop-based primary controllers. This controller enables current-sharing among paralleled sources and also damps limit cycle oscillations due to constant power loads. The higher level is a secondary controller which compensates for voltage deviations due to primary controller and performs voltage control of the microgrid. It also maintains the current sharing obtained in the primary stage. In the proposed secondary control, high-speed communication links to the primary controllers is implemented. The stability conditions are explained using the equivalent circuit of converters. Using this approach, stability conditions can be derived for microgrids of an arbitrary size and converter topology. The proposed control scheme is shown to be scalable and robust. The results obtained for the three basic DC-DC converter configurations are compared based on a microgrid with a parallel configuration of buck-boost converters. Simulations and experimental results are presented to verify the validity of the proposed control schemes.

Photovoltaic Equalizer Based on Single-Input Multi-Output Push-Pull Converter

Component mismatch often happens in the module-series photovoltaic system(including centralized, string, multi-strings PV system) due to partial shadowing, which causes a large loss of power generation. Photovoltaic equalizer can process the differential power under the condition of mismatching through parallel power electronic converter without changing the existing photovoltaic system architecture, so that all the modules can work near their maximum power points, which can greatly improve the power generation of the system under the condition of mismatching. This paper proposes a photovoltaic equalizer based on single-input multi-output push-pull converter. The topology has the advantages of simple structure and less switching devices. Firstly, the paper introduces the partial shadowing problem of photovoltaic modules and the principle of photovoltaic equalizer. Then, the topological structure and working principle of the proposed photovoltaic equalizer system are analyzed in detail. Finally, the simulation and verification of the designed photovoltaic equalizer are carried out. The simulated results show that the proposed equalizer can greatly improve the generation capacity of photovoltaic system under mismatching conditions, and the maximum increment can be up to 41%.

Unified integration scheme using an N × N active switch for efficient generation of a multi-photon parallel state

A source to efficiently generate multiple indistinguishable single photons in different spatial modes in parallel (multi-photon parallel state) is indispensable for realizing large-scale photonic quantum circuits. "A naive scheme" may be to use a heralding single photon source with an on-off detector set at each of parallel modes and to select the cases where each mode contains one photon at the same time. However, it is also necessary to suppress the probability of generating more than two photons from a single-photon source. For this requirement, serial-parallel conversion and a multiplexed heralded single photon source (HSPS) have been proposed and demonstrated. In this paper, we propose and demonstrate a novel method to produce a multi-photon parallel state efficiently using multiple HSPSs and an N × N active optical switch. As an advantage over the simple combination of a spatial multiplexed HSPS and a serial-parallel converter, our method, called the "unified integration scheme," can generate a multi-photon parallel state with minimized optical losses in the switch. Using a 2 × 2 active optical switch and a fixed delay, we achieve an enhancement factor of 1.59 ± 0.14, compared with a naive scheme using two HSPSs, and better than the factor of 1.46 using the simple combination scheme. Furthermore, we confirm the reduction of multi-photon events to 62 % of that of the naive scheme.

Nonlinear coordinated control of parallel bidirectional power converters in an AC/DC hybrid microgrid

Parallel bidirectional power converters (BPCs) can support each other's power between the ac grid and the dc grid, and they play an important role in maintaining the dynamic power balance in an ac/dc hybrid microgrid. This paper proposes a nonlinear coordinated control strategy for parallel BPCs when the hybrid ac/dc microgrid operates in grid-connected mode. First, taking the minimization of the sum of the power losses of the BPCs as the objective, this strategy can obtain the economically optimal distribution scheme for power transmission among parallel BPCs according to the efficiency curves of the BPCs. Second, distributing the transmission power according to the economically optimal distribution scheme rather than distributing the transmission power equally among parallel BPCs can not only reduce the sum of the power losses of the BPCs but also increase the power margin of the BPC that is used to control the dc bus voltage after a power disturbance. Third, a nonlinear control method that is based on the state-dynamic feedback linearization theory is proposed for the BPC that is used to control the dc bus voltage. This method can quickly re-establish the dc bus voltage if a power disturbance occurs and realize high dynamic voltage quality. Finally, the performance of the proposed control strategy is verified in MATLAB/Simulink.