Search

acceptedVersion Non

acceptedVersion

Energy-storage-equipped static synchronous compensator (E-STATCOM) plays an important role in a modern power grid. Such a compensator can be efficiently applied to control the active and reactive power thus improving the voltage and frequency stability especially in a power grid that is dominated by renewable energy resources. A challenge using an E-STATCOM is to minimize the adverse effect of the AC components present in the DC-link voltage especially during charging and discharging of an energy-storage system (ESS). This paper proposes a novel application of the dq- decoupled control to minimize the undesired dynamical effect of the ESS on the DC-link voltage of the E-STATCOM. In this method active and reactive power are decoupled by eliminating the dq- frame cross-coupling terms using reference current produced by the power controller instead of the measured inductor current. Experimental measurements are shown to demonstrate the effectiveness of the proposed method. acceptedVersion

Internal impedance of a Lithium-ion (Li-ion) battery is an important parameter in evaluating the battery characteristics such as the state-of-charge (SOC) and state-of-health (SOH). Recent studies have shown broadband methods based on pseudo-random binary sequence (PRBS) and Fourier techniques with which the battery impedance can be accurately and rapidly measured in real time. Applying the conventional PRBS requires, however, a relatively high injection amplitude that may easily interfere with the normal battery operation and produce nonlinear distortions. This paper proposes the use of a discrete-interval-binary sequence (DIBS) for a battery impedance measurement. The DIBS is a computer-optimized binary sequence in which the energy is maximized at specified harmonic frequencies to minimize the required level of signal injection. Otherwise the DIBS has the same attractive characteristics as the conventional PRBS. Experimental results based on a commercial Li-ion battery are presented to demonstrate the effectiveness of the proposed method. acceptedVersion Non

DC power distribution systems typically consist of several source converters supplying load converters through single or multiple dc buses. Such complex systems are prone to stability issues caused by the interactions among feedback-controlled converters. Additionally, dc interconnected systems typically undergo considerable changes in their operating points. This can cause a system that is well-stabilized at a certain operating point to go unstable for another operating condition. Therefore, a stabilization method that is capable of adapting to variations in system operating conditions is highly desirable. This paper presents an adaptive stabilization method implemented on the source-side. The system stability is periodically analyzed by real-time measurement of the system bus impedance, which is dominated by the source impedance. Accordingly, the source converter control is modified by the proposed method to guarantee stability and high performance of the entire system after system changes. The approach has several advantages, including ease of design and implementation, minimal deviation from the nominal controller, and robustness due to the real-time adaptive implementation. Simulation and experimental results are presented that confirm the effectiveness of the proposed method.

DC distribution systems typically consist of several feedback-controlled switched-mode converters forming a complex power distribution system. Consequently, a number of issues related to stability arise due to interactions among multiple converter subsystems. Recent studies have presented methods such as passivity-based stability criterion, where the stability and other dynamic characteristics of an interconnected system can be effectively analyzed using bus-impedance measurement. Studies have presented online measurement techniques, where the bus impedance is obtained by combining together the measurements of input and output impedances of single converters in the system. Since the converters are coupled, the presented measurement techniques require several measurement cycles in order to sequentially measure the individual impedances to be combined to obtain the overall bus impedance. This paper presents a measurement technique based on injection of orthogonal binary sequences. Applying this method, all the impedances in the system can be simultaneously measured during one measurement cycle. Therefore, the overall measurement time of the bus impedance is reduced compared to conventional measurement techniques. Furthermore, the method guarantees that the system dynamics do not change between measurements, and therefore, the computed bus impedance is not distorted. Experimental results are presented and used to demonstrate the effectiveness of the proposed method in the design of a stabilizing controller for a notional dc system using positive feed-forward control.

Identifying grid impedance at the point of common coupling is essential for the adaptive control and the online stability analysis of grid-connected converters. A balanced three-phase system is commonly modeled by d and q components in the synchronous reference frame. In identification of the synchronous reference-frame impedance components, errors may occur due to the coupling of the system impedances; for example, a measurement injection that is intended to perturb only the d -channel current may also perturb the q -channel current, thus distorting the impedance measurements. Traditionally, sequentially performed measurements, where different injections are performed one after another at the same frequencies, have been required to tackle the impedance coupling. However, the sequential measurements are prone to changes in the operating conditions between the measurements. This article proposes a method to simultaneously obtain all the grid-impedance components within a single measurement cycle with no coupling effect. In the method, two orthogonal binary injections are simultaneously injected into the d and q current references of the inverter controller. Then, a frequency-domain interpolation technique is applied to adjust the measured current and voltage responses. As a result, the impedance coupling is avoided in the measured grid impedance. The proposed technique is validated by experimental measurements. acceptedVersion

Inverter output impedance is an important parameter for assessing the stability of a grid-connected system. However, measuring the inverter output impedance is not straightforward because the impedance is affected by a nonlinear coupling over frequency. In a typical measurement setup the inverter output impedance is measured by applying an external perturbation injected by an additional (source) inverter. Under non-ideal conditions, the inverter under test produces undesired (mirrored) frequency harmonics which interact with the source inverter thus affecting the measured output impedance. This paper proposes a measurement technique that decouples the mirrored frequency harmonics during the impedance measurement. In the method, the impedance of the source inverter performing the measurement is shaped so that the coupling over frequency is minimized. By applying this method, the inverter output impedance can be accurately and reliably measured in non-ideal conditions. acceptedVersion

The impedance of a Li-ion battery is an important parameter for the battery state-of-health (SOH) estimation. The dependency of the battery impedance to the SOH can be monitored by fitting an equivalent-circuit-model (ECM) to the impedance data and observe the changes in the ECM parameters. The ECM is typically fitted by the complex-nonlinear-least-squares (CNLS) algorithm which requires accurately chosen initial conditions for the parameters to guarantee the consistent performance of the algorithm. In order to use the ECM parameters for SOH estimation in practical applications, the impedance measurements should be fast and simple to implement to the battery system. This paper demonstrates the utilization of practical and fast pseudo-random-sequence (PRS) impedance measurements to the SOH analysis of a nickel manganese cobalt Li-ion battery by observing the variations in the ECM parameters. The measured impedances are fitted to the ECM by using the CNLS with adaptively obtained initial conditions. It is shown that the ECM parameters are changing as the battery capacity degrades. In addition, it is observed that some parameters are able to indicate a drastic reduction in the battery capacity and SOH.

The amount of grid-connected power electronics is rapidly increasing and their effect to power quality becomes important especially in areas with high penetration levels. In weak grids with significant grid impedance, the inverter control performance usually decreases and the impedance-based stability issues may arise. A method to analyze the stability issues and to improve the inverter control performance is to consider the grid as a load effect in the small-signal model of the grid-connected inverter. Studies have shown that the bandwidth of the inverter current-control loop decreases in the weak grids through the load effect, and consequently, the controller ability to mitigate harmonics is weakened. This work introduces an adaptive method that keeps the bandwidth of the control loop constant under varying grid conditions. Additionally, an adaptive Notch filter is added into the current-control loop in order to avoid possible stability issues caused by the interactions with the LC-filter resonance and the grid impedance. Improved control performance and stability under various grid conditions are demonstrated through power hardware-in-the-loop tests with a kW-scale three-phase inverter. acceptedVersion

Grid characteristics have a drastic impact on the stability and control performance of grid-connected systems. Because the grid conditions typically vary over time, online measurements are most desirable for the stability assessment, protection design, and control-system optimization of the systems. Previous studies have presented methods based on Fourier techniques and broadband sequences with which the frequency responses of the grid-connected systems, such as the grid impedance or inverter control loops, can be measured. However, online measurements under unbalanced grids with harmonic voltages have not been comprehensively considered. The present paper demonstrates how the previously applied online-measurement methods fail in the presence of unbalanced grid and voltage harmonics due to the spectral leakage caused by Fourier transform. This work also proposes a simple signal-design method to avoid the leakage. Experimental results based on a high-power grid-connected system are shown to demonstrate the effectiveness of the proposed method. acceptedVersion

The rise of renewable electricity production has driven the power grid to a remarkable transformation, where a large share of the electricity production is integrated to the grid through power-electronic inverters. The inverters have fast internal dynamics and no inherent inertia, which makes the power grid prone to stability issues. The stability analysis to ensure system robustness can be performed based on the impedance ratio of the inverter and power grid. The grid impedance is often an unknown parameter, and methods for grid impedance measurements are required. Past studies have presented a number of measurement methods based on a broadband perturbation, such as pseudo-random binary sequence (PRBS), and Fourier techniques for obtaining the grid impedance. However, only a little attention has been paid to the injection-amplitude design, and most often, the amplitude has been selected based on trial and error. This work presents an algorithm based on the total harmonic distortion (THD) levels of grid currents and voltages for choosing a suitable perturbation amplitude. The proposed method makes it possible to fully automate the stability analysis of a grid-connected system. Experimental results based on a three-phase grid-connected inverter are presented and used to demonstrate the effectiveness of the proposed method. publishedVersion

Grid impedance affects the stability and control performance of grid-connected power-electronics devices, such as inverters, used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Wideband frequency-response-measurement techniques based on the pseudo-random binary sequence (PRBS) or impulse injection have been often applied for grid-impedance measurements. However, while using the PRBS or impulse, it is assumed that the system under study is linear. Because such an assumption does not typically hold for grid-connected systems, the measured impedances are usually prone to distortions caused by nonlinearities. This paper proposes the use of periodic ternary sequences for online grid-impedance measurements. Using appropriately designed periodic ternary signals, the linear component of the grid impedance can be identified, eliminating errors from even-order nonlinear distortions. In addition, the ternary sequences can be designed for a much wider range of sequence length compared to the conventional PRBS, enabling more efficient optimization of computing power and frequency resolution. Experimental measurements are shown from a grid-connected photovoltaic inverter.

Grid impedance has a major effect on the operation of inverter-connected systems, such as renewable energy sources. Stability of such system depends on the ratio of the inverter output impedance and the grid impedance at the point of common coupling. Because the grid impedance varies over time with many parameters, online grid-impedance measurement acquired in real time is most preferred method for observing the stability. Recent studies have presented methods based on multiple-input-multiple-output (MIMO) identification techniques, where the stability of grid-connected system is rapidly assessed in the dq domain. In the methods, orthogonal injections are used with Fourier techniques, and the grid impedance d and q components are measured. The Nyquist stability criterion is then applied to assess the stability. This paper extends previous studies, and presents a real-time implementation for the online stability analysis using MIMO-identification methods. The practical implementation is discussed in detail and experimental results based on a grid-connected three-phase inverter are provided to demonstrate the effectiveness of the methods.

Impedance-based analysis plays important role in assessing the stability and dynamics of grid-connected inverters and related systems. Measuring the inverter impedance is, however, not straightforward due to the load effect caused by the power grid. The power grid interacts with the inverter and produces additional (mirrored) frequency harmonics thus distorting the impedance measurements when applying conventional measurement techniques. This paper proposes an active-power filtering method to minimize the undesired load effect caused by the power grid in the measured impedance. Applying the method, the inverter output impedance can be reliably measured under non-ideal grid conditions. acceptedVersion

Grid impedance and the output impedance of grid-connected inverter are important parameters for the operation of grid-connected systems, such as solar, wind, and other distributed-generation resource systems. The impedance mismatch between the grid and the interfacing circuit often generates harmonic resonances that lead to reduced power quality and even instability. Since the impedances usually vary over time as a function of many parameters, online measurements are required for stability assessment and adaptive control of the inverters. Several methods have been proposed for quick, accurate measurements of impedances, but the use of multiple-input multiple-output (MIMO) identification techniques have not been considered. Applying the MIMO techniques, different components of the inverter output impedance or grid impedance can be simultaneously measured during a single measurement cycle. Therefore, the operating conditions of the system can be kept constant during the measurements, and the overall measurement time is significantly reduced. This paper shows the use of orthogonal binary sequences to simultaneously measure the “d” and “q” components of grid-connected inverter output impedance and/or grid impedance. Experimental results based on a three-phase grid-connected inverter are presented and used to demonstrate the effectiveness of the proposed methods.

Power electronic inverters are devices that are used to interface renewables, such as photovoltaic panels or wind turbines, with the electrical power system. The inverter should control its output current to follow a sinusoidal reference to avoid distorting the voltage waveform of the power system. However, in real inverters dead-time is used in the control signals which causes unwanted harmonics, leading to lower power quality. The amount of harmonics is difficult to predict due to nonlinear nature of the dead-time. Moreover, the harmonic distortion depends on many variables, such as inverter control parameters and power system impedance. This paper shows that frequency response from inverter control signal (duty ratio) to grid current is linked to the detrimental dead-time effect and poor power quality. The frequency response is identified by perturbing the inverter control signal with a maximum length binary sequence (MLBS). The magnitude of the identified frequency response is shown to follow the same trend as the amount of current distortion. Therefore, the effect of dead-time on power quality could potentially be modeled using a linearized model allowing optimal control design in the presence of dead time.

Many modern commercial ships are electric ships, i.e., they utilize integrated electric propulsion (IEP ), where electric motors with variable speed drives are used for ship propulsion. This provides a number of advantages, such as reduced size and weight, electric power availability aboard the ship, reduced vibration, and more flexibility in generation engine placement by eliminating the mechanical connection between engines and propulsion. Most cruise ships, many yachts, and even cargo ships utilize IEP. There is also significant interest in IEP for military ships.

The impedance of a Li-ion battery is an important parameter for the battery's state-of-charge (SOC) and state-of-health (SOH) estimation. Battery impedance is typically modeled by an equivalent-circuit-model (ECM) in which the variations in the specific model parameters can be used for estimating the SOC and the SOH. However, the fact that the battery impedance is highly non-linear complicates the parameterization of the model. The model is traditionally obtained by a complex non-linear least-squares fitting algorithm which comes with high complexity. This paper proposes a novel approach to extract all the ECM parameters of the battery impedance obtained with online-capable pseudo-random-sequence (PRS) measurements. Although the algorithm has low complexity, it still captures the desired variations in the ECM parameters as a function of SOC. The algorithm is validated for the impedance data from a lithium-iron-phosphate cell. acceptedVersion

Battery Energy Storage Systems (BESSs) play an important role in grid-connected renewable energy systems as they provide great flexibility in the energy production. For photovoltaic (PV) energy applications, the BESS may be connected to the dc-link of the converter system to inject the deficit or to absorb the surplus of generated power. One of the main issues in such systems is the voltage variation at the dc-link, especially under grid imbalance, distorting the current fed to the grid. This paper studies a method to control the BESS to emulate a capacitance, thus mitigating the dc-link voltage fluctuation, for example, under heavy imbalance. The derived model initiates impedance-based analysis at the output of the battery charger. acceptedVersion