Dialogue Session II: Modeling and Control

Monday, April 10, 2017


D2.1 Equivalent Electric Circuit for a wave energy converter

Chien-an Chen
Changwei Liang
Khai D. T. Ngo
Lei Zuo

Abstract
A comprehensive and accurate model is essential for reliability estimation and output power optimization. Wave energy converters (WECs) involve hydrodynamic, mechanical structure, and power electronics converter. An equivalent circuit provides a faster and reliable way to estimate the characteristic of WEC as electrical components. A general approach is shown in this paper to derive a total circuit model from mechanical–electrical subsystem i.e. from buoy to generator. Least-square approximation and Brune network synthesis method are applied to derive a RLC circuit network for a buoy-wave behavior. The Mechanical-Motion-Rectifier (MMR) set based on the mechanical disengagement is analogue to the combination of transformers and diodes. Additional with the generator circuit model, the WEC equivalent circuit is developed. Two different applications based on the equivalent circuit are demonstrated to show its capability of the reliability analysis and the output power optimization. The methodology provides electrical engineers a complete perspective of a multidisciplinary system, and an electrical controller can be developed based on the model.


D2.2. Modeling of Voltage-Controlled Capacitor (VCC) in Dc-Dc Converters

Lujie Zhang
Andrew Ritter
Craig Nies
Suman Dwari
Ben Guo
Shashank Priya
Rolando Burgos
Khai D. T. Ngo

Abstract
This paper presents a Voltage Controlled Capacitor (VCC) that varies from 20% to 100% of the rated capacitance (1 μF) with a control voltage from half of the voltage rating to 0 V. Capacitance, self-resonant frequency, and equivalent series resistance (esr) were measured with respect to the control voltage. An equivalent circuit and a nonlinear model derived from relationship between permittivity and electric field were created and implemented in SPICE based on previous measurements. A buck converter with input of 12 V, output of 5 V, and switching frequency of 500 kHz was built to demonstrate the change from 85% to 40% of the rated capacitance of VCC. The error between the simulation and experiment was limited within 10%, which verifies the model.


D2.3. A Three-Terminal Switch Model of Constant On-Time Current Mode With External Ramp Compensation

Shuilin Tian
Jian Li
Fred C. Lee
Qiang Li
Pei-Hsin Liu


Please note: This paper will be presented by Syed Bari (pictured), as Shuilin Tian has graduated.

Abstract
Multiphase constant on-time current-mode control based on pulse distribution structure is widely used in voltage regulator application for microprocessor. To minimize ripple cancellation effect, external ramp compensation is used in commercial products. However, external ramp will introduce dynamic to the system and stability margin will be suffered without considering its effect. This paper first studies the effect of external ramp by deriving small-signal transfer function based on describing function method. It is found that external ramp brings additional dynamic, with time constant related with switching period. Then, a simple equivalent circuit model based on three-terminal switch concept is proposed, which considers the effect of external ramp by adding an additional R–L branch. The equivalent circuit model can be reduced to previous unified three-terminal switch model when external ramp is zero and can be reduced to model of constant on-time voltage mode control when external ramp is much larger than inductor current ramp. The proposed three-terminal switch model is a completemodel, which can be used to examine all transfer functions and is accurate up to half of switching frequency. The analytical transfer functions are provided for easy reference. The model is verified by SIMPLIS simulation and experimental measurement.


D2.4. An Enhanced Adaptive Frequency Locked Loop for Variable Frequency Controls

Syed Bari
Qiang Li
Fred C. Lee

Abstract
These days, variable frequency current mode controls are widely used in voltage regulators (VRs) for their higher light load efficiency, better small signal property and better transient response than fixed frequency current mode controls. One major issue for variable frequency control is its large switching frequency (fSW) variation with duty cycle. This variable fSW creates unpredictable noise in the system which is difficult to filter out, and hence not preferred by many frequency sensitive applications like mobile applications. While using variable frequency control e.g. constant on-time control (COT) in these applications, people use ‘Adaptive COT’ control which slowly changes the TON with duty cycle at steady state operation to keep the steady state frequency constant. VIN/VREF Feed-forward method and ‘Phase Locked Loop (PLL)’ method are very widely used to realize Adaptive COT control in the industry. Normally, VIN/VREF feed-forward method is very simple and fast but naturally suffers inaccuracy issue for its feed-forward approach; while PLL method is very accurate but suffers from complicated additional loop adjustment. Therefore, this paper proposes a new accurate but simple frequency control loop for variable frequency controls. This paper also proposes two additional techniques for performance improvements in proposed method, namely, faster loop settling time by using feed-forward signal and better loop stability with duty cycle variation by adding adaptive loop gain control.


D2.5. Impedance-Based Analysis of Active Frequency Drift Islanding Detection for Grid-Tied Inverter System

Bo Wen
Dushan Boroyevich
Zhiyu Shen
Rolando Burgos
Paolo Mattavelli


Please note: This paper will be presented by Zeng Liu (pictured), as Bo Wen has graduated.

Abstract
Islanding detection is critical to the safety of grid-tied inverters. This paper presents an impedance-based analysis of the active frequency drift (AFD) islanding detection method. To study this method, the output impedance of a grid-tied inverter is modeled. The Zqq of the inverter impedance features a negative incremental resistor, whose magnitude is equal to the dc impedance magnitude of the local load defined by the test standard. With a large value of feedforward gain N in the AFD method, the phase of Zqq is shown to drop below 180°. Under an islanding condition, due to a lack of phase margin, the inverter system becomes unstablewith a frequency drift away fromits steady state. The islanding condition is then identified by a frequency detection unit. Experimental results verify the analysis.


D2.6. Analysis of Sigma Converter for 48V Server Voltage Regulator

Virginia Li
Qiang Li
Fred C. Lee

Abstract
For the new-generation of server voltage regulators (VRs), companies are employing the use of 48V bus to improve system efficiency and to reduce the cost of datacenters. For 48V to 1V VR, a quasi-parallel topology, the sigma converter, is shown to have 93.4% maximum efficiency and 420W/in3 power-density. In order to design the sigma converter to meet the VR requirements, the large-signal performance as well as the small-signal behavior of the sigma converter must be studied. In this paper, the small-signal model of the sigma converter is examined for server application.


D2.7. Study of the Predictive Capability of Modular Multilevel Converter Simulation Models under Parametric and Model Form Uncertainty

Niloofar Rashidi Mehrabadi
Rolando Burgos
Christopher Roy
Jianghui Yu
Dushan Boroyevich

Abstract
The deviation of real system behavior from the predictions made from modeling and simulation is inevitable due to variations in different model input parameters as well as inaccurate modeling. The potential sources of uncertainty in modular multilevel converters (MMCs) is significant in medium- and high-voltage applications where each arm consists of several power electronics building blocks (PEBBs) connected in series. Therefore, assessing the predictive proficiency of the model, in the presence of various uncertainties, is critical in gaining confidence in modeling and simulation results. This paper investigates the predictive capability of MMC simulation models when adding more PEBBs. The relationship between the total uncertainty in modeling and simulation, and the number of PEBBs in each arm is presented for different model outputs. The results reveal an interesting feature of MMC—despite the fact that the number of potential sources of uncertainty increases by adding more PEBBs in each arm, the total uncertainty in the prediction of a system response quantity remains the same or decreases, depending on the selected model output response.


D2.8. Low-Frequency Terminal-Behavioral Modeling of Three-Phase Converters

Igor Cvetkovic
Zeng Liu
Dushan Boroyevich
Rolando Burgos

Abstract
New electronic power distribution systems built for airplanes, ships, electric vehicles, data-centers and modern homes, dominantly comprise a variety of power electronics converters with very different dynamic characteristics. If their behavior is not examined carefully before system is integrated, instability becomes one of the major concerns. This paper addresses low-frequency terminal-behavioral modeling of three-phase converters which dynamics can be captured on-line, in a non-intrusive way, and later decoupled from the source and load in order to get un-terminated model of a particular converter (or a system). Experimental results of the modeling methodology applied to the voltage source inverter, and active rectifier are given at the end to verify modeling and decoupling procedure.


D2.9. Small-Signal Terminal-Characteristics Modeling of Three-Phase Droop-Controlled Inverters

Zeng Liu
Jinjun Liu
Dushan Boroyevich
Rolando Burgos
Teng Liu

Abstract
Droop-controlled inverters are widely employed as power sources in three-phase AC power electronics system, such as distributed generation, while the interaction between the source and the load may lead overall system to be unstable. Terminal-characteristics based stability criteria are very attractive for analyzing the stability of three-phase AC power electronics system. However, the systems, composed by droopcontrolled inverters, exhibit the dynamical variation of fundamental angular frequency, and existing stability analysis approaches are just suitable for systems with constant fundamental angular frequency. To overcome this problem, this paper proposes small-signal terminal-characteristics model of the three-phase droop-controlled inverter covering the dynamic of fundamental angular frequency, and both current mode operation and voltage mode operation are taken into account. Finally, the proposed model is verified in frequency domain.


D2.10. Stability Criterion of Droop-Controlled Parallel Inverters Based on Terminal-Characteristics of Individual Inverters

Zeng Liu
Jinjun Liu
Dushan Boroyevich
Rolando Burgos

Abstract
Droop-controlled parallel inverters are widely adopted in micro-grids due to its high reliability and low cost. However, the interaction among the inverters will cause whole system to be unstable even though each individual inverter can operate stably in stand-alone mode. A lot of work has been done for analyzing the stability of parallel inverters with droop control through state-space approach originally used in conventional electrical power system. For this existing approach it is necessary to have the knowledge of inside parameters of each individual inverter in advance, while in practical applications it is not so convenient to obtain the internal structure and parameters. To overcome this problem, this paper proposes a stability analysis approach for the droopcontrolled parallel inverters based on the terminal-characteristics of individual inverters. At beginning, the terminal-characteristics of individual inverter with droop control are defined in system synchronous reference frame (SRF), which can be represented by the transfer function between output current and output voltage, and the transfer function between output current and fundamental angular frequency. Then the terminal-characteristics of the whole parallel system are derived out with the ones of each individual inverter. Furthermore, a stability criterion is proposed for the parallel inverters based on the terminal-characteristics of each individual inverter according to the Generalized Nyquist Criterion. Finally, the proposed terminal-characteristics and stable criterion for droop-controlled parallel inverters are verified in frequency domain.


D2.11. Decoupled αβ Model of Modular Multilevel Converter (MMC)

Yi-Hsun Hsieh
Fred C. Lee

Abstract
The modular multilevel converter (MMC), is widely adopted in high voltage applications because of its simplicity and modularity. However, the current and power flow is very complicated. As a result, methods proposed for capacitor reduction, which is required to stored line frequency related circulating energy, all went through complicated mathematical derivation. C. Li proposed a state plane analysis to visually illustrate the convoluted current and power flow. The circulating energy related to source and load as well as the circulating energy swapping between capacitors was explained for the first time. Based on the state plane analysis, this paper proposed a decoupled αβ model of MMC, which clearly identifies input and output relationship. In addition, the power flow and the causes for each component of circulating energy are well explained with the proposed model. Therefore, the proposed model paves a good way for an advanced control and a systematically understanding of MMC.

Video Nugget


D2.12. Modeling and Analysis for Input Characteristics of Line-Frequency Rectifiers

Xiaolong Yue
Dushan Boroyevich
Rolando Burgos
Fang Zhuo


Please note: This paper will be presented by Chi Li (pictured), as Xiaolong Yue has graduated.

Abstract
Impedance is very important for power electronic systems because of the close relationship between impedance and system stability. The line-frequency rectifiers are commonly used ac-dc interfaces in electrical power systems. For line-frequency rectifier, with a sinusoidal voltage perturbation excitation at input terminal, input current contains not only perturbation frequency, but also multiple additional frequency components. Therefore, input characteristics of line-frequency rectifiers are actually single input multiple output (SIMO) in frequency domain. However, in tradition, input current perturbation is simplified into a sinusoidal signal at perturbation frequency by ignoring other additional frequency components and the obtained impedance models mainly focus on frequency regions below line frequency. To describe the SIMO input characteristics of line-frequency rectifiers and to develop an impedance model that could be applied beyond switching frequency, this paper proposes a new model by using harmonic balance method. Detailed analysis indicates that input admittance of line frequency rectifier in high frequency regions acts like a resistor rather than inductor. The simulations validatethe accuracy and effectiveness of the proposed model.


D2.13. Small Signal Analysis of V2 Control Using Equivalent Circuit Model of Current Mode Controls

Yingyi Yan
Fred C. Lee
Paolo Mattavelli
Shuilin Tian


Please note: This paper will be presented by Syed Bari (pictured), as Yingyi Yan has graduated.

Abstract
In V2 control, the direct feedback contains the in- formation of the inductor current, the capacitor voltage, and the load current. In this paper, by separating the current feedbacks and the capacitor voltage feedback, an equivalent circuit of V2 control with ESR dominant output capacitor is proposed. The proposed equivalent circuit is based on the unified equivalent circuit of current mode controls. It is a completed frequency domain model for V2 control with a clear physical insight. V2 control can be interpreted as an advanced implementation of current mode con- trol with a proportional voltage feedback and an additional load current feedback. The load current feedback dramatically reduces the output impedance of a current mode controlled converter. The model is extended to the enhanced V2 control.The proposed model is applicable to both variable frequency modulations and constant frequency modulations. The modeling results are verified by the Simplis simulation and the experimental results.


NP2.1. Communication between Power Electronic Building Blocks (PEBB 2.0) with synchronous operation for High Voltage Application

Ashrarul Haq Sifat
Jun Wang
Dushan Boroyevich
Rolando Burgos

There is no paper associated with this session number. Click below to view the poster.


NP2.2. Inverse Charge Constant On-time (IQCOT) Control and Its Hardware Design Improvement

Luxing Wang
Syed Bari
Qiang Li

There is no paper associated with this session number. Click below to view the poster.