Second-order ripples occur in the voltage and current during any DC–AC power conversion. These conversions occur in the voltage source inverters (VSIs), current source inverters (CSIs), and various single-stage inverters (SSIs) topologies. The second-order ripples lead to oscillating source node currents and DC bus voltages when there is an interconnection between the AC and DC microgrids or when an AC load is connected to the DC bus of the microgrid. Second-order ripples have various detrimental effects on the sources and the battery storage. In the storage battery, they lead to the depletion of electrodes. They also lead to stress in the converter or inverter components. This may lead to the failure of a component and hence affect the reliability of the system. Furthermore, the second-order ripple currents (SRCs) lead to ripple torque in wind turbines and lead to mechanical stress. SRCs cause a rise in the temperature of photovoltaic panels. An increase in the temperature of PV panels leads to a reduction in the power generated. Furthermore, the second-order voltage and current oscillations lead to a varying maximum power point in PV panels. Hence, the maximum power may not be extracted from it. To mitigate SRCs, oversizing of the components is needed. To improve the lifespan of the sources, storage, and converter components, the SRCs must be mitigated or kept within the desired limits. In the literature, different methodologies have been proposed to mitigate and regulate these second-order ripple components. This manuscript presents a comprehensive review of different effects of second-order ripples on different sources and the methodologies adopted to mitigate the ripples. Different active power decoupling methodologies, virtual impedance-based methodologies, pulse width modulation-based signal injection methodologies, and control methods adopted in distributed power generation methods for DC microgrids have been presented. The application of ripple control methods spans from single converters such as SSIs and VSIs to a network of interconnected converters. Furthermore, different challenges in the field of virtual impedance control and ripple mitigation in distributed power generation environments are discussed. This paper brings a review regarding control methodologies to mitigate and regulate second-order ripples in DC–AC conversions and microgrids.
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MIP Formulation for Type-III Wind Turbines Considering Control Limits
Hitting control limits changes the behavior of the control systems and invalidates commonly adopted assumptions made when calculating the operating point of doubly fed induction generator (DFIG)-based wind turbines (WTs). The current computing methods rely on trials and errors and iterations. This paper proposes an optimization-based algorithm with control limits integrated into the problem formulation. Hitting or not hitting a control limit is modeled as a binary variable. Both rotor-side converter (RSC) current order limits and grid-side converter (GSC) current order limits are modeled in the proposed mixed-integer programming (MIP) formulation. For a WT with given terminal voltage and wind speed, the electrical and mechanical variables of the system can be computed directly from the optimization problem. The proposed formulation has included losses in the back-to-back converters and nonzero reactive power support through GSC. The computing results have been validated with the electromagnetic transient (EMT) simulation results. This formulation can help accurately detect whether control limits are hit for low voltage conditions and facilitate Type-III wind turbine fault ride through analysis.
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- Award ID(s):
- 2103480
- NSF-PAR ID:
- 10400314
- Date Published:
- Journal Name:
- 2022 North American Power Symposium (NAPS)
- Page Range / eLocation ID:
- 1 to 6
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract This paper presents the genetic algorithm (GA) and particle swarm optimization (PSO) based frequency regulation for a wind‐based microgrid (MG) using reactive power balance loop. MG, operating from squirrel cage induction generator (SCIG), is employed for exporting the electrical power from wind turbines, and it needs reactive power which may be imported from the grid. Additional reactive power is also required from the grid for the load, directly coupled with such a distributed generator (DG) plant. However, guidelines issued by electric authorities encourage MGs to arrange their own reactive power because such reactive power procurement is defined as a local area problem for power system studies. Despite the higher cost of compensation, static synchronous compensator (STATCOM) is a fast‐acting FACTs device for attending to these reactive power mismatches. Reactive power control can be achieved by controlling reactive current through the STATCOM. This can be achieved with modification in current controller scheme of STATCOM. STATCOM current controller is designed with reactive power load balance for the proposed microgrid in this paper. Further, gain values of the PI controller, required in the STATCOM model, are selected first with classical methods. In this classical method, iterative procedures which are based on integral square error (ISE), integral absolute error (IAE), and integral square of time error (ISTE) criteria are developed using MATLAB programs. System performances are further investigated with GA and PSO based control techniques and their acceptability over classical methods is diagnosed. Results in terms of converter frequency deviation show how the frequency remains under the operating boundaries as allowed by IEEE standards 1159:1995 and 1250:2011 for integrating renewable‐based microgrid with grid. Real and reactive power management and load current total harmonic distortions verify the STATCOM performance in MG. The results are further validated with the help of recent papers in which frequency regulation is investigated for almost similar power system models. The compendium for this work is as following: (i) modelling of wind generator‐based microgrid using MATLAB simulink library, (ii) designing of STATCOM current controller with PI controller, (iii) gain constants estimation using classical, GA and PSO algorithm through a developed m codes and their interfacing with proposed simulink model, (v) dynamic frequency responses for proposed grid connected microgrid during starting and load perturbations, (vi) verification of system performance with the help of obtained real and reactive power management between STATCOM and grid, and (vii) validation of results with available literature.
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This paper proposes a unified distributed secondary control for the grid-forming (GFM) and grid-feeding (GFE) converters in DC microgrids. An optimization problem is formulated for the secondary control and the objective function considers regulating the global average of the GFM and GFE converter output voltages and proportional current sharing among all GFM and GFE converters. A unified distributed control is then designed to generate voltage and current references respectively for GFM and GFE converters based on the formulated optimization problem. The dynamic model of the DC microgrid under the proposed control is also developed, and steady-state analysis is performed to show that the proposed distributed control can achieve the control objectives in steady state. The performance of the proposed control is validated through real-time simulations in OPAL-RT on an 8-DG DC microgrid system.more » « less
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Switched Tank Converter(STC) is one kind of Resonant Switch Capacitor(ReSC) that can be considered as a good candidate for data center application with high power efficiency and high power density. On the other hand, LLC converter can also realize very good performance for low voltage application. Although STC can realize relatively higher efficiency than LLC converter in the light load since the core loss is saved, LLC can keep relatively higher efficiency in the heavy load than STC does since the conduction loss of LLC is smaller. The main reason is because transformer’s winding resistance is smaller than semiconductor devices’ resistance, and this is very important for high current application.In order to utilize the benefits of the STC and the LLC converter together, this paper proposes a family of the novel Switch Capacitor based Integrated Matrix Autotransformer LLC Converters (SCIMAC). The proposed converters share the same high voltage side circuit of the STC with low voltage stress devices. Different from the traditional LLC converter with an isolated transformer, the proposed SCIMAC utilizes one autotransformer with only the secondary side windings similar to LLC's secondary side. There are several advantages that can be realized of the SCIMAC: 1). Low figure of merit (FOM) devices can be adopted to realize higher efficiency due to the low voltage stress of the SCIMAC. 2). Higher power efficiency can be realized when compared with STC converter in heavy load because the resistance of the autotransformer’s windings is lower than semiconductor devices’ resistance. 3). The primary side winding loss of the transformer is saved to further increase the efficiency. 4) ZVS turning on can be realized by the magnetizing current of the core.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1109/NAPS56150.2022.10012164
