Imperial College London

DrYunjieGu

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Honorary Lecturer
 
 
 
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Contact

 

yunjie.gu

 
 
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Location

 

1105Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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43 results found

Green T, Xiang X, Gu Y, yang Q, Zhang X, Li W, He Xet al., 2021, Resonant modular multilevel DC-DC converters for both high and low step-ratio connections in MVDC distribution systems, IEEE Transactions on Power Electronics, Vol: 36, Pages: 7625-7640, ISSN: 0885-8993

DC transformers based on power electronics are keyitems of equipment for medium voltage dc (MVDC) distributionsystems. Both high and low step-ratio dc-dc conversions arerequired to interface dc links at different voltages to form anintegrated dc distribution system. The transformer-coupledresonant modular multilevel dc-dc converter (RMMC) is wellsuited to high step-ratio connection between a MVDC network anda low voltage dc (LVDC) network but it is not suitable in low stepratio conversion for linking two MVDC networks with similar butnot identical voltages. This paper presents a circuit evolution of thehigh step-ratio transformer-coupled RMMC into its low step-ratiotransformer-less RMMC counterpart. These two RMMCsshare thesame structure and the same resonant process within the resonantSM stack (RSS) giving rise to the same operational advantages. Also,the circuit design experience can be readily transferred from one tothe other. From these two base RMMC circuits, a family of RMMCswith further configurations is elaborated that provide a wider varietyof connection optionsfor MVDC distribution systems. In this circuitfamily, each high step-ratio transformer-coupled RMMC has a lowstep-ratio transformer-less RMMC counterpart and one can betransformed into the other via the circuit evolution processpresented. The theoretical analysis for both high and low stepratio RMMCs has been verified through full-scale simulations ofmedium voltage examples and further verified through downscaled experiments on laboratory prototypes.

Journal article

Zhu Y, Gu Y, Li Y, Green Tet al., 2021, Participation analysis in impedance models: the grey-box approach for power system stability, IEEE Transactions on Power Systems, ISSN: 0885-8950

This paper develops a grey-box approach to small-signal stability analysis of complex power systems that facilitates root-cause tracing without requiring disclosure of the full details of the internal control structure of apparatus connected to the system. The grey-box enables participation analysis in impedance models, which is popular in power electronics and increasingly accepted in power systems for stability analysis. The Impedance participation factor is proposed and defined in terms of the residue of the whole-system admittance matrix. It is proved that, the so defined impedance participation factor equals the sensitivity of the whole-system eigenvalue with respect to apparatus impedance. The classic state participation factor is related to the impedance participation factor via a chain-rule. Based on the chain-rule, a three-layer grey-box approach, with three degrees of transparency, is proposed for root-cause tracing to different depths, i.e. apparatus, states, and parameters, according to the available information. The association of impedance participation factor with eigenvalue sensitivity points to the re-tuning that would stabilize the system. The impedance participation factor can be measured in the field or calculated from the black-box impedance spectra with little prior knowledge required.

Journal article

Li Y, Yunjie G, Yue Z, Junyent-Ferre A, Xin X, Green TCet al., 2021, Impedance circuit model of grid-forming inverter: visualizing control algorithms as circuit elements, IEEE Transactions on Power Electronics, Vol: 36, Pages: 3377-3395, ISSN: 0885-8993

The impedance model is widely used for analyzing power converters. However, the output impedance is an external representation of a converter system, i.e., it compresses the entire dynamics into a single transfer function with internal details of the interaction between states hidden. As a result, there are no programmatic routines to link each control parameter to the system dynamic modes and to show the interactions among them, which makes the designers rely on their experience and heuristic to interpret the impedance model and its implications. To overcome these obstacles, this paper proposes a new modeling tool named as impedance circuit model, visualizing the closed-loop power converter as an impedance circuit with discrete circuit elements rather than an all-in-one impedance transfer function. It can reveal the virtual impedance essence of all control parameters at different impedance locations and/or within different frequency bandwidths, and show their interactions and coupling effects. A grid-forming voltage-source inverter (VSI) is investigated as an example, with considering its voltage controller, current controller, control delay, voltage/current dq-frame cross-decoupling terms, output-voltage/current feedforward control, droop controllers, and three typical virtual impedances. The proposed modeling tool is validated by frequency-domain spectrum measurement and time-domain step response in simulations and experiments.

Journal article

Gu Y, Li Y, Green TC, Zhu Yet al., 2021, Impedance-based whole-system modeling for a composite grid via embedding of frame dynamics, IEEE Transactions on Power Systems, Vol: 36, Pages: 336-345, ISSN: 0885-8950

The paper establishes a methodology to overcome the difficulty of dynamic frame alignment and system separation in impedance modeling of ac grids, and thereby enables impedance-based whole-system modeling of generator-converter composite power systems. The methodology is based on a frame-dynamics-embedding transformation via an intermediary steady frame between local and global frames, which yields a locally defined impedance model for each generator or converter that does not rely on a global frame but retains all frame dynamics. The individual impedance model can then be readily combined into a whole-system model even for meshed networks via the proposed closed-loop formulation without network separation. Compared to start-of-the-art impedance-based models, the proposed method retains both frame dynamics and scalability, and is generally applicable to various network topologies (meshed, radial, etc) and combinations of machines (generators, motors, converters, etc). The methodology is used to analyze the dynamic interaction between generators and converters in a composite grid, which yields important findings and potential solutions for unstable oscillation caused by PLL-swing coupling in low-inertia grids.

Journal article

Xiang X, Gu Y, Chen K, Astolfi A, Green Tet al., 2021, On the dynamics of inherent balancing of modular multilevel DC-AC-DC converters, IEEE Transactions on Power Electronics, Vol: 36, Pages: 34-40, ISSN: 0885-8993

Modular multilevel dc–ac–dc converters (MMDACs) serve as an enabler for dc distribution systems. The modular multilevel structure enables flexible voltage transforms, but raises issues over balancing of the submodule (SM) capacitor voltages. This letter focuses on the dynamics of inherent balancing of MMDACs under circulant modulation. We provide an invariance-like result using a variant of Barbalat's Lemma and prove that the SM capacitor voltages converge to the kernel of the circulant switching matrix, which is the intersection of the invariant sets for each switching state. We further interpret the balancing dynamics as a permuted linear time-invariant system and prove that the envelop of the balancing trajectories is governed by the eigenvalues of the permuted state-transition matrix. This result extends previous full-rank criterion for inherent balancing in a steady state and provides new insight into the dynamic behavior of MMDACs.

Journal article

Xiang X, Zhang X, Gu Y, Chaffey G, Green Tet al., 2020, Analysis and investigation of internal AC Frequency to minimize AC current magnitude and reactive power circulation in chain-link modular multilevel direct DC-DC converters, IEEE Transactions on Circuits and Systems Part 1: Regular Papers, Vol: 67, Pages: 5586-5599, ISSN: 1549-8328

Chain-link modular multilevel direct dc-dc converters (CLMMCs) have attracted much interest recently in for dc power systems because they achieve higher device utilization, lower power losses and they are physically more compact than the alternative front-to-front modular multilevel dc-ac-dc converters (FFMMCs). The CLMMCs rely on circulating an internal ac current to manage energy balance of the sub-module (SM) stacks but this current inevitably contributes to extra current stresses for circuit components and can also lead to excess reactive power circulation within the converter. This paper presents a circuit analysis that there exists a frequency value for the internal ac components that may minimize the current stresses and avoid excessive reactive power circulation. For illustration, the circuit analysis is applied to one of the base formats, the buck-boost CLMMC as an example. The key relationship between the CLMMC and the standard dc-ac modular multilevel converter is explored and established. The equivalent circuits for their dc and ac components with consideration of SM capacitance and SM voltage ripples are created and analyzed in detail, and a full derivation is provided for the specific ac frequency. From this example, this analytical method is extended and applied to other base formats of CLMMCs. The theoretical analysis and results are verified by a set of full-scale simulation examples and down-scaled experiments on a laboratory prototype.

Journal article

Li Y, Gu Y, Green TC, 2020, Interpreting frame transformations in AC systems as diagonalization of harmonic transfer functions, IEEE Transactions on Circuits and Systems I: Regular Papers, Vol: 67, Pages: 2481-2491, ISSN: 1549-8328

Analysis of ac electrical systems can be performed via frame transformations in the time-domain or via harmonic transfer functions (HTFs) in the frequency-domain. The two approaches each have unique advantages but are hard to reconcile because the coupling effect in the frequency-domain leads to infinite dimensional HTF matrices that need to be truncated. This paper explores the relation between the two representations and shows that applying a frame transformation on the input-output signals creates a direct equivalence to a similarity transformation to the HTF matrix of the system. Under certain conditions, such similarity transformations have a diagonalizing effect which, essentially, reduces the HTF matrix order from infinity to two or one, making the matrix tractable mathematically without truncation or approximation. This theory is applied to a droop-controlled voltage source inverter as an illustrative example. A stability criterion is derived in the frequency-domain which agrees with the conventional state-space model but offers greater insights into the mechanism of instability in terms of the negative damping (non-passivity) under droop control. Therefore, the paper not only establishes a unified view in theory but also offers an effective practical tool for stability assessment.

Journal article

Gu Y, Li Y, Zhu Y, Green Tet al., 2020, Impedance-Based Whole-System Modeling for a Composite Grid via Frame-Dynamics Embedding, IEEE Transactions on Power Systems, ISSN: 0885-8950

The paper establishes a methodology to overcome the difficulty of dynamicframe alignment and system separation in impedance modeling of ac grids, andthereby enables impedance-based whole-system modeling of generator-convertercomposite power systems. The methodology is based on a frame-dynamics-embeddingtransformation via an intermediary steady frame between local and globalframes, which yields a locally defined impedance model for each generator orconverter that does not rely on a global frame but retains all frame dynamics.The individual impedance model can then be readily combined into a whole-systemmodel even for meshed networks via the proposed closed-loop formulation withoutnetwork separation. Compared to start-of-the-art impedance-based models, theproposed method retains both frame dynamics and scalability, and is generallyapplicable to various network topologies (meshed, radial, etc) and combinationsof machines (generators, motors, converters, etc). The methodology is used toanalyze the dynamic interaction between generators and converters in acomposite grid, which yields important findings and potential solutions forunstable oscillation caused by PLL-swing coupling in low-inertia grids.

Journal article

Xiang X, Qiao Y, Gu Y, Zhang X, Green Tet al., 2020, Analysis and criterion for inherent balance capability in modular multilevel DC-AC-DC converters, IEEE Transactions on Power Electronics, Vol: 35, Pages: 5573-5580, ISSN: 0885-8993

Modular multilevel dc-ac-dc converters (MMDAC) have emergedrecently for high step-ratio connectionsin medium voltage distribution systems.Extended phase-shiftmodulation has been proposed and was found to create the opportunity for inherent balance of SM capacitor voltages. This letter presents fundamentalanalysis leading toclear criterion for the inherent balancecapability in MMDAC. A sufficient and necessary condition,with associated assumptions,to guarantee this capability isestablished. Using the mathematics of circulant matrices, this condition is simplified to a co-prime criterion which gives rise to practical guidance for the design of an MMDAC. Experimentson down-scaled prototypesand simulations on full-scale examples both provide verification of the analysis and criterion for the inherent balance capability of MMDAC.

Journal article

Gu Y, Li Y, Yoo H-J, Nguyen T-T, Xiang X, Kim H-M, Junyent Ferre A, Green Tet al., 2019, Transfverter: imbuing transformer-like properties in an interlink converter for robust control of a hybrid ac-dc microgrid, IEEE Transactions on Power Electronics, Vol: 34, Pages: 11332-11341, ISSN: 0885-8993

In a hybrid ac-dc microgrid, stiff voltage sources may appear in either the dc or ac subgrids which gives rise to multiple operation modes as power dispatch changes. This creates a challenge for designing the interlink converter between the ac and dc subgrids since the different modes require different interlink controls. To solve this problem, this paper proposes the concept of a transfverter inspired by how transformers link ac grids. Like a transformer, a transfverter can react to the presence of stiff voltage sources on either the dc or ac side and reflect the “stiffness” and voltage stabilizing capability to the other side. A back-to-back converter with droop control is used as the underlying technology to implement this concept. A novel optimization method called model bank synthesis is proposed to find control parameters for the interlink converter that offer the best controller performance across the different microgrid modes without requiring mode-changing of the controller. The effectiveness of the proposed solution is validated through both simulation and experiments.

Journal article

Gu Y, Liu J, Green TC, Li W, He Xet al., 2019, Motion-induction compensation to mitigate sub-synchronous oscillation in wind farms, IEEE Transactions on Sustainable Energy, Vol: 11, Pages: 1247-1256, ISSN: 1949-3029

This paper presents a comprehensive solution to mitigate the sub-synchronous oscillation (SSO) in wind farms connected to series-compensated transmission lines. The concept of motion-induction amplification (MIA) is introduced to reinterpret the physical root cause of the negative resistance in doubly-fed induction generators (DFIGs). Based on this new interpretation, a novel control scheme called motion-induction compensation (MIC) is proposed to counteract the MIA effect. The MIC control eliminates the negative resistance in DFIGs across the entire frequency range, and makes the Type-III (DFIG) generator behave like a Type-IV generator in dynamics. The proposed solution provides wide-range SSO damping and also shows excellent robustness against model and measurement errors.

Journal article

Gu Y, Li Y, Yoo H-J, Nguyen T-T, Xiang X, Kim H-M, Junyent-Ferre A, Green Tet al., 2019, Transfverter: imbuing transformer-like properties in an interlink converter for robust control of a hybrid ac-dc microgrid

Matlab/Simulink models and codes for getting the results in the paper with the same title on IEEE Transactions on Power Electronics, DOI: 10.1109/TPEL.2019.2897460

Software

Xiang X, Zhang X, Chaffey G, Gu Y, Sang Y, Green Tet al., 2018, Analysis on circulating current frequency of chain-link modular multilevel DC-DC converters for low step-ratio high-power MVDC applications, IEEE ECCE 2018, Publisher: IEEE

The direct chain-link modular multilevel dc converter has raised great interest recently in medium/high- voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.

Conference paper

Gu Y, Bottrell N, Green TC, 2018, Reduced-order models for representing converters in power system studies, IEEE Transactions on Power Electronics, Vol: 33, Pages: 3644-3654, ISSN: 0885-8993

A reduced-order model that preserves physical meaning is important for generating insight in large-scale power system studies. The conventional model-order reduction for a multiple-timescale system is based on discarding states with fast (short timescale) dynamics. It has been successfully applied to synchronous machines, but is inaccurate when applied to power converters because the timescales of fast and slow states are not sufficiently separated. In the method proposed here, several fast states are at first discarded but a representation of their interaction with the slow states is added back. Recognizing that the fast states of many converters are linear allows well-developed linear system theories to be used to implement this concept. All the information of the original system relevant to system-wide dynamics, including nonlinearity, is preserved, which facilitates judgments on system stability and insight into control design. The method is tested on a converter-supplied mini power system and the comparison of analytical and experiment results confirms high preciseness in a broad range of conditions.

Journal article

Xiang X, Zhang X, Chaffey G, Gu Y, Green Tet al., 2017, The isolated resonant modular multilevel converters with extreme step-ratio for MVDC application, 2017 IEEE 18TH WORKSHOP ON CONTROL AND MODELING FOR POWER ELECTRONICS (COMPEL), Publisher: IEEE

The dc-dc conversion will play an important role in multi-terminal dc networks and dc grids. This paper presents two isolated resonant modular multilevel converters (IRMMCs) to fulfill the large step-ratio conversion for medium voltage dc (MVDC) networks. The conventional resonant modular multilevel converters (RMMCs) suffer the common problems of non-isolation and high current stress, which are solved in the proposed IRMMCs. They not only inherit the beneficial features of inherent sub-module (SM) voltage-balancing and soft-switching operation from RMMCs, but also develop multi-module configurations to neutralize the current ripples on both sides of the dc-links. The theoretical analysis is verified by a set of full-scaled simulations for different application examples in MVDC collection and distribution. The results demonstrate the proposed IRMMCs and its derived configurations have good potential for operation as large step-ratio MVDC transformers.

Conference paper

Xiang X, Zhang X, Chaffey GP, Gu Y, Green TCet al., 2017, The isolated resonant modular multilevel converters with large step-ratio for MVDC applications, 2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL), Publisher: Institute of Electrical and Electronics Engineers, Pages: 1-6

The dc-dc conversion will play an important role in multi-terminal dc networks and dc grids. This paper presents two isolated resonant modular multilevel converters (IRMMCs) to fulfill the large step-ratio conversion for medium voltage dc (MVDC) networks. The conventional resonant modular multilevel converters (RMMCs) suffer the common problems of non-isolation and high current stress, which are solved in the proposed IRMMCs. They not only inherit the beneficial features of inherent sub-module (SM) voltage-balancing and soft-switching operation from RMMCs, but also develop multi-module configurations to neutralize the current ripples on both sides of the dc-links. The theoretical analysis is verified by a set of full-scaled simulations for different application examples in MVDC collection and distribution. The results demonstrate the proposed IRMMCs and its derived configurations have good potential for operation as large step-ratio MVDC transformers.

Conference paper

Gu Y, Bottrell, Green, 2017, Reduced-Order Models for Representing Converters in Power System Studies

Matlab codes of reduced-order models for representing power electronic converters in power system analyses.

Software

Li W, Wang J, Yang H, GU Y, YANG H, HE Xet al., 2017, Power Dynamic Coupling Mechanism and Resonance Suppression of Synchronous Frequency for Virtual Synchronous Generators, Proceedings of the CSEE, Vol: 2, Pages: 006-006

Journal article

Gu Y, Li W, He X, 2016, Analysis and control of bipolar LVDC grid with DC symmetrical component method, IEEE Transactions on Power Systems, Vol: 31, Pages: 685-694, ISSN: 0885-8950

The dc symmetrical component method is introduced for the analysis and control of bipolar dc distribution systems under asymmetrical operation. This method is an extension of the classical symmetrical component theory in three-phase ac power systems. The asymmetrical voltage and current in the positive and negative poles are decomposed into symmetrical components in common and differential modes. The equivalent circuit for each mode is derived, which forms decoupled mode networks. Consequently, it allows for independent investigation of each mode, and provides an insightful view of the static and dynamic behavior of a bipolar dc power system. The dc symmetrical component method is a general approach which is applicable to different aspects of system design. As an example, an enhanced common-mode voltage regulation scheme is described. It suppresses common-mode LC resonance by adding active damping control, and reduces common-mode impedance to improve power quality and voltage stability. The major theoretical conclusions are verified by experimental results.

Journal article

WANG J, WANG Y, GU Y, LI W, HE Xet al., 2016, Synchronous Frequency Resonance in Grid-connected VSCs with Virtual Synchronous Generator Technology, Journal of Power Supply, Vol: 2, Pages: 004-004

Journal article

Gu Y, Li W, He X, 2015, Passivity-based control of DC microgrid for self-disciplined stabilization, IEEE Transactions on Power Systems, Vol: 30, Pages: 2623-2632, ISSN: 0885-8950

DC microgrids may have time-varying system structures and operation patterns due to the flexibility and uncertainty of distributed resources. This feature poses a challenge to conventional stability analysis methods, which are based on fixed and complete system models. To solve this problem, the concept of self-disciplined stabilization is introduced in this paper. A common stability discipline is established using the passivity-based control theory, which ensures that a microgrid is always stable as long as this discipline is complied by each individual converter. In this way, the stabilization task is localized to avoid investigating the entire microgrid, thereby providing immunity against system variations. Moreover, a passivity margin criterion is proposed to further enhance the stability margin of the self-disciplined control. The modified criterion imposes a tighter phase restriction to provide explicit phase margins and prevent under-damped transient oscillations. In line with this criterion, a practical control algorithm is also derived, which increases the converter's passivity through voltage feed forward. The major theoretical conclusions are verified by a laboratory DC microgrid test bench.

Journal article

Li W, Gu Y, Luo H, Cui W, He X, Xia Cet al., 2015, Topology review and derivation methodology of single-phase transformerless photovoltaic inverters for leakage current suppression, IEEE Transactions on Industrial Electronics, Vol: 62, Pages: 4537-4551, ISSN: 0278-0046

Single-phase voltage source transformerless inverters have been developed for many years and have been successful commercial applications in the distributed photovoltaic (PV) grid-connected systems. Moreover, many advanced industrial topologies and recent innovations have been published in the last few years. The objective of this paper is to classify and review these recent contributions to establish the present state of the art and trends of the transformerless inverters. This can provide a comprehensive and insightful overview of this technology. First, the generation mechanism of leakage current is investigated to divide the transformerless inverters into asymmetrical inductor-based and symmetrical inductor-based groups. Then, the concepts of dc-based and ac-based decoupling networks are proposed to not only cover the published symmetrical inductor-based topologies but also offer an innovative strategy to derive advanced inverters. Furthermore, the transformation principle between the dc-based and ac-based topologies is explored to make a clear picture on the general law and framework for the recent advances and future trend in this area. Finally, a family of clamped highly efficient and reliable inverter concept transformerless inverters is derived and tested to offer some excellent candidates for next-generation high-efficiency and cost-effective PV grid-tie inverters.

Journal article

Gu Y, Li W, He X, 2015, Frequency-coordinating virtual impedance for autonomous power management of DC microgrid, IEEE Transactions on Power Electronics, Vol: 30, Pages: 2328-2337, ISSN: 0885-8993

In this paper, the concept of frequency-coordinating virtual impedance is proposed for the autonomous control of a dc microgrid. This concept introduces another degree of freedom in the conventional droop control scheme, to enable both time-scale and power-scale coordination in a distributed microgrid system. As an example, the proposed technique is applied to the coordinating regulation of a hybrid energy storage system composed of batteries and supercapacitors. With an effective frequency-domain shaping of the virtual output impedances, the battery and supercapacitor converters are designed to absorb low-frequency and high-frequency power fluctuations, respectively. In this way, their complementary advantages in energy and power density can be effectively exploited. Furthermore, the proposed concept can be integrated into a mode-adaptive power management framework with autonomous mode transitions. The entire solution features highly versatile functions based on fully decentralized control. Therefore, both flexibility and reliability can be enhanced. The effectiveness of the presented solution is verified by experimental results.

Journal article

Gu Y, Chi Y, Li Y, Sun W, Li W, He Xet al., 2015, DC symmetrical component method for analysis and control of bipolar LVDC grid, Pages: 2802-2807

Conference paper

Wang J, Wang Y, Gu Y, Li W, He Xet al., 2015, Synchronous frequency resonance of virtual synchronous generators and damping control, Pages: 1011-1016

Conference paper

LI W, GU Y, WANG Y, XIANG X, HE Xet al., 2015, Control architecture and hierarchy division for renewable energy DC micro grids, Automation of Electric Power Systems, Vol: 39, Pages: 156-163

Journal article

Wuhua LI, Chi XU, Hongbin YU, Yunjie GU, Xiangning HEet al., 2015, Energy management with dual droop plus frequency dividing coordinated control strategy for electric vehicle applications, Journal of Modern Power Systems and Clean Energy, Vol: 3, Pages: 212-220

Journal article

Wang Y, Gu Y, Li W, He Xet al., 2015, Analysis of pre-compensator for disturbance signal elimination in single-phase inverters with virtual vector control, Pages: 157-162

Conference paper

LUO Q, YANG H, GU Y, LI W, HE Xet al., 2015, Hybrid LLC Converter With Input Voltage Auto-balance Ability, Proceedings of the CSEE, Vol: 2, Pages: 026-026

Journal article

Li C, Gu Y, Li W, He X, Dong Z, Chen G, Ma C, Zhang Let al., 2015, Analysis and compensation of dead-time effect considering parasitic capacitance and ripple current, Pages: 1501-1506

Conference paper

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