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In modern dc microgrids, several feedback-controlled power electronic converters are connected to the common dc bus. Although the control loops of each individual converter are designed with good stability margins, the interconnection of multiple source and load converters can cause stability and performance concerns, due to potential interactions. Therefore, in order to ensure the desired dynamic performance of the interconnected power converter system, an interesting approach is to perform online stability monitoring of the dc bus, and to properly damp the dc bus impedance, which has been demonstrated to ensure system-level stability and performance. In order to accomplish that, this article first derives a representation of the dc bus impedance in terms of voltage (or droop) loop gain of the source-side converter. Second, under certain simplifying assumptions it provides an estimate for the peak value of bus impedance—an indicator of system dynamic behavior— based on the phase margin. Third, it proposes to continuously monitor the peak value of bus impedance by only injecting a single sinusoid in the voltage (or droop) loop. The monitored value can then be used to autotune the voltage regulator, in order to keep the bus impedance in a well-known allowable impedance region. The proposed monitoring and stabilization technique eliminates the need for the time consuming and memory intensive impedance measurement tasks. This technique is verified by simulation and experimental results on a laboratory prototype system of interconnected power converters. [ABSTRACT FROM AUTHOR]

Frequency-domain identification based on wideband techniques has become a popular method in the analysis and control of various dc power distribution systems. In the method, a single converter or a system is perturbed by an external wideband voltage or current injection, the resulting voltage or current responses are measured, and Fourier analysis is applied to extract the spectral information of the measured variables. Most often, the system or converter input and output impedances and the loop gain are the quantities of interest. One class of perturbation signals, pseudorandom binary sequences, has become widely used because most of such signals can be generated using simple shift-register circuitry. As the signals are deterministic and binary, they are well suited to perform measurements on power-converter systems in real time, allowing fast response to system variations through, for example, adaptive controllers. This article reviews the pseudorandom binary sequences applied to dc power distribution systems and discusses their advantages as well as limitations. The conventional maximum-length binary sequence, inverse-repeat binary sequence, discrete-interval binary sequence, and orthogonal binary sequences are considered. Several experimental results from various dc power systems are presented and used to demonstrate the applicability of the discussed methods. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Using both modal analysis and time domain simulations, it is shown that sub‐synchronous resonance (SSR) phenomena in a series‐compensated DFIG wind farms may cause system instability. In order to damp the SSR, both the rotor‐side converter (RSC) and grid‐side converter (GSC) controllers of the doubly‐fed induction generator (DFIG) are utilised. The objectives are to design an SSR damping controller (SSRDC) with an optimum input control signal (ICS), and to identify the optimum point within the RSC and GSC controllers where to introduce the SSRDC. The optimum ICS is identified using residue‐based analysis and root‐locus method. The criterion for choice of optimal ICS is that it should make the SSR mode stable without decreasing or destabilising the other system modes. Moreover, since the dynamics of the DFIG‐based series compensated wind farm change with varying operating point conditions, the operation of the SSRDC is optimised using the adaptive gain‐scheduling method. [ABSTRACT FROM AUTHOR]

This paper deals with subsynchronous resonance (SSR) phenomena in a capacitive series-compensated DFIG-based wind farm. Using both modal analysis and time-domain simulation, it is shown that the DFIG wind farm is potentially unstable due to the SSR mode. In order to damp the SSR, the rotor-side converter (RSC) and grid-side converter (GSC) controllers of the DFIG are utilized. The objective is to design a simple proportional SSR damping controller (SSRDC) by properly choosing an optimum input control signal (ICS) to the SSRDC block, so that the SSR mode becomes stable without decreasing or destabilizing the other system modes. Moreover, an optimum point within the RSC and GSC controllers to insert the SSRDC is identified. Three different signals are tested as potential ICSs including rotor speed, line real power, and voltage across the series capacitor, and an optimum ICS is identified using residue-based analysis and root-locus method. Moreover, two methods are discussed in order to estimate the optimum ICS, without measuring it directly. The studied power system is a \mathbf100\;MW DFIG-based wind farm connected to a series-compensated line whose parameters are taken from the IEEE first benchmark model (FBM) for computer simulation of the SSR. MATLAB/Simulink is used as a tool for modeling and designing the SSRDC, and power system computer aided design/electromagnetic transients including dc (PSCAD/EMTDC) is used to perform time-domain simulation for design process validation. [ABSTRACT FROM PUBLISHER]

Power-electronics-based dc power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability and dynamic performance caused by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for dc distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, the Energy Source Analysis Consortium (ESAC) Criterion, and the Three-Step Impedance Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of dc distribution systems, and the formulation of design specifications that ensure system stability. Moreover, the Passivity-Based Stability Criterion is discussed, which has been recently proposed as an alternative stability criterion. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain, which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. A meaningful simulation example is presented to illustrate the main characteristics of the reviewed stability criteria. [ABSTRACT FROM AUTHOR]

A positive feedforward (PFF) controller is proposed as a new active approach to improve stability of a three-phase voltage source inverter. In particular, the proposed approach solves the source subsystem interaction problem, i.e., a system stability degradation which is commonly observed when the inverter is connected to a dc voltage source subsystem that presents finite Thévenin impedance. The implemented strategy is to combine the PFF control with the conventional negative feedback (NFB) control. When properly designed, the PFF control modifies the inverter input impedance in the frequency range where the subsystem interaction occurs. As a result, the PFF control stabilizes the dc bus voltage ensuring overall system stability, while allowing the NFB control to maintain good output voltage regulation performance. The approach results in greatly improved stability and damping of the dc bus voltage with a slight reduction of output FB control bandwidth. Complete small-signal models are presented on a dq rotating frame using g-parameter representation. The design criteria for the PFF controller as well as the tradeoff between dynamic output performance and stability improvement are discussed in detail. An extended analysis of stability based on minor loop gain and passivity concept at the dc input bus is also presented. The approach is validated by experimental results. [ABSTRACT FROM AUTHOR]