Publications
53 results found
Zheng Q, Gao F, Li Y, et al., 2024, Equivalence of Impedance Participation Analysis Methods for Hybrid AC/DC Power Systems, IEEE Transactions on Power Systems, Vol: 39, Pages: 3560-3574, ISSN: 0885-8950
Participation analysis based on whole-system impedance models enables the root-cause stability analysis of large-scale power systems when state-space models are unavailable. To extend participation analysis to the hybrid AC/DC system, especially for the interlinking converter, this article proposes two complementary methods, namely the direct method and the indirect method. The direct method treats the interlinking converter as hybrid AC-DC port apparatus and the participation factor can be defined with the terminal hybrid AC-DC impedance matrix correspondingly. As for the indirect method, system equivalence can be made at the AC or DC terminal of the interlinking converter when impedance models within subsystems are unavailable such that preliminary knowledge of systems can be simplified. Importantly, the equivalence of these two methods on the observability of system components is revealed through theoretical analysis. The proposed impedance participation analysis methods are illustrated with a 4-bus hybrid AC/DC system, a modified 28-bus hybrid AC/DC system and a modified 96-bus hybrid AC/DC system. Numerical calculations and time-domain simulations are performed to validate the theoretical analysis.
Ducoin E, Gu Y, Chaudhuri B, et al., 2024, Analytical design of contributions of grid-forming & grid-following inverters to frequency stability, IEEE Transactions on Power Systems, ISSN: 0885-8950
Most of the new renewable generation in power systems is connected through Grid-Following inverters (GFL). The accompanying decline of fossil-fuelled synchronous generation reduces the grid inertia. As these two trends progress, instabilities become more likely. To allow more renewables onto the grid, the use of combinations of GFL and Grid-Forming inverters (GFM) has been proposed, however, it is unclear how to parametrise these inverters for system objectives. This paper tackles the issue of parametrizing each GFM and GFL to ensure frequency trajectories at all buses, expressed in terms of frequency deviation, Rate of Change of Frequency and settling time, are stable, recognising that local frequencies can deviate substantially from the Center of Inertia (COI). The procedure to achieve this comprises simple closed-form equations, and yields the required values of droop slopes, GFM filter bandwidth and GFL Phase-Locked Loop bandwidth. These equations are derived from an analytical formulation of swing equations for GFM and GFL which are combined to describe the behaviour of not only the COI but also each bus. The detailed EMT simulations of a modified IEEE 14-bus network demonstrate that the simplifying assumptions made in the analysis are justified by the close correspondence between simulation and analytical projections.
Li J, Li Y, Gu Y, 2024, The Impact of Frame Transformations on Power System EMT Simulation, IEEE Transactions on Power Systems, Vol: 39, Pages: 1319-1328, ISSN: 0885-8950
This article investigates the impact of frame transformations on the accuracy of numerical discretization in power system transient and stability studies. As analysed, frame transformations influence the convergence of the numerical discretization. Specifically, for an explicit discretization method (e.g., forward Euler method), the stability of the original system is best preserved in the frame where the system eigenvalue is closer to the origin of the complex plane, e.g., in the stationary frame for inductors and capacitors, and in the synchronous frame for dq-frame controllers of inverters. Simulation results are given to validate the theoretical analysis.
Fan S, Xiang X, Gu Y, et al., 2024, Optimal circulant modulation for submodule voltage ripple minimization with inherent balancing capability in modular multilevel dc-dc converters, IEEE Transactions on Power Electronics, Vol: 39, Pages: 784-798, ISSN: 0885-8993
The modularity of the modular multilevel dc-dc converters (MMDCs) makes it as a competitive candidate in medium voltage applications but brings the submodule (SM) voltage balancing issue. This paper proposes an optimal circulant modulation method for minimizing the SM voltage ripples with inherent balancing capability proven at the same time, which allows smaller SM capacitors and avoids the high-frequency communication for SM voltage balancing. Firstly, the optimal switching pattern is strictly derived providing a general method to theoretically minimize the SM capacitor voltage ripple. Then the switching matrix of the optimal circulant modulation is formulated by introducing the generalized-circulant matrix. It verifies the circularity and full-rank feature of the optimal switching matrix, which promises the uniformity of SM actions and the inherent balancing of SM voltages. Finally, full-scale simulations and down-scaled experiments are both provided with the isolated LLC -based MMDC model and prototype. The results show that the proposed optimal circulant modulation can reduce the SM capacitor voltage ripple by 37% compared with the existed method, and it also promises the inherent SM voltage balancing and the SM uniformity.
Li Y, Green TC, Gu Y, 2023, Descriptor state space modeling of power systems, IEEE Transactions on Power Systems, Pages: 1-13, ISSN: 0885-8950
State space is widely used for modeling power systems and analyzing their dynamics but it is limited to representing causal and proper systems in which the number of zeros does not exceed the number of poles. In other words, the system input, output, and state can not be freely selected. This limits how flexibly models are constructed, and in some circumstances, can introduce errors because of the addition of virtual elements in order to connect the mismatched ports of subsystem models. An extension known as descriptor state space (also known as implicit state space, generalized state space, singular state space) can model both proper and improper systems and is a promising candidate for solving the noted problems. It facilitates a modular construction of power system models with flexible choice of ports of subsystems. Algorithms for mathematical manipulation of descriptor state space models are derived such as preforming inverse, connection, and transform. Corresponding physical interpretations are also given. Importantly, the proposed algorithms preserve the subsystem states in the whole system model, which therefore enables the analysis of root causes of instability and mode participation. Theoretical advances are validated by example power systems of varied scales including inductor or capacitor systems, and modified IEEE 14-bus, 68-bus, and 118-bus generator-inverter-composite systems.
Li Y, Green TC, Gu Y, 2023, The intrinsic communication in power systems: a new perspective to understand synchronization stability, IEEE Transactions on Circuits and Systems Part 1: Regular Papers, Vol: 70, Pages: 4615-4626, ISSN: 1549-8328
The large-scale integration of converter-interfaced resources in electrical power systems raises new threats to stability which call for a new theoretical framework for modelling and analysis. In this paper, we present the intrinsic analogy of a power system to a communication system, which is here called power-communication isomorphism. Based on this isomorphism, we revisit power system stability from a communication perspective and thereby establish a theory that unifies the heterogeneous power apparatuses of power systems and provides a bridge between electromagnetic transient (EMT) and phasor dynamics. This theory yields several new insights into power system stability and new possibilities for stabilization. In particular, we demonstrate that a system of 100% converter-interfaced resources can achieve stable synchronization in small-and large-signal sense under grid-following control which was commonly considered impossible.
Fan S, Xiang X, Li C, et al., 2023, Coprime polynomial based dual-circulant modulation for inherent submodule voltage balancing in MMDC, IEEE Transactions on Industrial Electronics, Vol: 70, Pages: 10134-10145, ISSN: 0278-0046
The modular multilevel dc-dc converters (MMDCs) have attracted much interest in medium voltage dc applications, but have to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and proves it possessing the inherent balancing capability for arbitrary operation cases. High-speed communication for real-time sensor data transfer could be avoided, which reduces implementation costs and also enhances the system reliability. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases. Then the switching patterns are reallocated and optimized to keep the SM uniformity and reduce the capacitor voltage ripple at the same time. Full-scale simulations on MMDC models and down-scaled experiments on prototypes are both presented, which validates the inherent voltage balancing capability and the optimization of SM capacitor voltage ripple with the proposed dual-circulant modulation.
Gu Y, Green T, 2023, Power system stability with a high penetration of inverter based resources, Proceedings of the Institute of Electrical and Electronics Engineers (IEEE), Vol: 111, Pages: 832-853, ISSN: 0018-9219
Inverter-based resources (IBRs) possess dynamics that are significantly different from those of synchronous-generator-based sources and as IBR penetrations grow the dynamics of power systems are changing. This article discusses the characteristics of the new dynamics and examines how they can be accommodated into the long-standing categorizations of power system stability in terms of angle, frequency, and voltage stability. It is argued that inverters are causing the frequency range over which angle, frequency, and voltage dynamics act to extend such that the previously partitioned categories are now coupled and further coupled to new electromagnetic modes. While grid-forming (GFM) inverters share many characteristics with generators, grid-following (GFL) inverters are different. This is explored in terms of similarities and differences in synchronization, inertia, and voltage control. The concept of duality is used to unify the synchronization principles of GFM and GFL inverters and, thus, established the generalized angle dynamics. This enables the analytical study of GFM-GFL interaction, which is particularly important to guide the placement of GFM apparatuses and is even more important if GFM inverters are allowed to fall back to the GFL mode during faults to avoid oversizing to support short-term overload. Both GFL and GFM inverters contribute to voltage strength but with marked differences, which implies new features of voltage stability. Several directions for further research are identified, including: 1) extensions of nonlinear stability analysis to accommodate new inverter behaviors with cross-coupled time frames; 2) establishment of spatial–temporal indices of system strength and stability margin to guide the provision of new stability services; and 3) data-driven approaches to combat increased system complexity and confidentiality of inverter models.
Li Y, Gu Y, Green T, 2022, Revisiting grid-forming and grid-following inverters: a duality theory, IEEE Transactions on Power Systems, Vol: 37, Pages: 4541-4554, ISSN: 0885-8950
Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on duality is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are duals of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks.
Li Y, Gu Y, Green T, 2022, Mapping of dynamics between mechanical and electrical ports in SG-IBR composite grids, IEEE Transactions on Power Systems, Vol: 37, Pages: 3423-3433, ISSN: 0885-8950
Power grids are traditionally dominated by synchronous generators (SGs) but are currently undergoing a major transformation due to the increasing integration of inverter-based resources (IBRs). The state space method with transparent apparatus models can be readily used. However, models of IBRs are usually not disclosed by manufacturers. Alternatively, the port-based approach represents dynamics by input-output transfer functions without exposing internal states. These transfer functions at various ports are normally configured with a particular focus: an SG-dominated grid is traditionally analyzed in a mechanical-centric view which ignores fast electrical dynamics and focuses on the torque-speed dynamics, whereas the emergent IBR-dominated grid usually takes an electrical-centric view which focuses on the voltage-current interaction. In this article, a new perspective called the port-mapping method is proposed to combine these two views. Specifically, the mechanical dynamics are mapped to the electrical impedance seen at the electrical port; and the electrical dynamics are also mapped to the torque coefficient seen at the mechanical port. The bidirectional mapping gives additional flexibility and insights to analyze the sub-system interactions in whole-system dynamics and guide the tuning of parameters. Application of the proposed method is illustrated in three cases with increasing scales.
Zhu Y, Gu Y, Li Y, et al., 2022, Impedance-based root-cause analysis: comparative study of impedance models and calculation of eigenvalue sensitivity, IEEE Transactions on Power Systems, Vol: 38, Pages: 1642-1654, ISSN: 0885-8950
Impedance models of power systems are useful when state-space models of apparatus such as inverter-based resources (IBRs) have not been made available and instead only black-box impedance models are available. For tracing the root causes of poor damping and tuning modes of the system, the sensitivity of the modes to components and parameters are needed. The so-called critical admittance-eigenvalue sensitivity based on nodaladmittance model has provided a partial solution but omits meaningful directional information. The alternative whole-system impedance model yields participation factors of shunt-connected apparatus with directional information that allows separate tuning for damping and frequency, yet do not cover series-connected components. This paper formalises the relationships between the two forms of impedance models and between the two forms of root-cause analysis. The calculation of system eigenvaluesensitivity in impedance models is further developed, which fills the gaps of previous research and establishes a complete theory of impedance-based root-cause analysis. The theoretical relationships and the tuning of parameters have been illustrated with a three-node passive network, a modified IEEE 14-bus network and a modified NETS-NYPS 68-bus network, showing that tools can be developed for tuning of IBR-rich power systemswhere only black-box impedance models are available.
Fan S, Xiang X, Sheng J, et al., 2022, Inherent SM voltage balance for multilevel circulant modulation in modular multilevel DC-DC converters, IEEE Transactions on Power Electronics, Vol: 37, Pages: 1352-1368, ISSN: 0885-8993
The modularity of a modular multilevel dc converter (MMDC) makes it attractive for medium-voltage distribution systems. Inherent balance of submodule (SM) capacitor voltages is considered as an ideal property, which avoids a complex sorting process based on many measurements thereby reducing costs and enhancing reliability. This article extends the inherent balance concept previously shown for square-wave modulation to a multilevel version for MMDCs. A switching duty matrix dU is introduced: it is a circulant matrix of preset multilevel switching patterns with multiple stages and multiple durations. Inherent voltage balance is ensured with a full-rank dU . Circulant matrix theory shows that this is equivalent to a simplified common factor criterion. A nonfull rank dU causes clusters of SM voltage rather than a single common value, with the clusters indicated by the kernel of the matrix. A generalized coprime criterion is developed into several deductions that serve as practical guidance for design of multilevel circulant modulation. The theoretical development is verified through full-scale simulations and downscaled experiments. The effectiveness of the proposed circulant modulation in achieving SM voltage balance in an MMDC is demonstrated.
Green T, Xiang X, Gu Y, et 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.
Zhu Y, Gu Y, Li Y, et al., 2021, Participation analysis in impedance models: the grey-box approach for power system stability, IEEE Transactions on Power Systems, Vol: 37, Pages: 343-353, 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.
Li Y, Yunjie G, Yue Z, et 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.
Xiang X, Gu Y, Chen K, et 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.
Xiang X, Zhang X, Gu Y, et 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.
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.
Xiang X, Qiao Y, Gu Y, et 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.
Gu Y, Li Y, Yoo H-J, et 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.
Gu Y, Liu J, Green TC, et 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.
Gu Y, Li Y, Yoo H-J, et 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
Xiang X, Zhang X, Chaffey G, et 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.
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.
Xiang X, Zhang X, Chaffey G, et 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.
Xiang X, Zhang X, Chaffey GP, et 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.
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.
Li W, Wang J, Yang H, et 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
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.
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