Performance metrics for small-signal stability assessment of DC-distributed power-system-architecture comparisons

The objective of this paper is to provide performance metrics for small-signal stability assessment of a given system architecture. The stability margins are stated utilizing a concept of maximum peak criteria (MPC) derived from the behavior of an impedance-based sensitivity function. For each minor-loop gain defined at every system interface, a single number to state the robustness of stability is provided based on the computed maximum value of the corresponding sensitivity function. In order to compare various power-architecture solutions in terms of stability, a parameter providing an overall measure of the whole system stability is required. The selected figure of merit is geometric average of each maximum peak value within the system. It provides a meaningful metrics for system comparisons: the best system in terms of robust stability is the one that minimizes this index. In addition, the largest peak value within the system interfaces is given thus detecting the weakest point of the system in terms of robustness

Simplified small-signal stability analysis for optimized power system architecture

The optimization of power architectures is a complex problem due to the plethora of different ways to connect various system components. This issue has been addressed by developing a methodology to design and optimize power architectures in terms of the most fundamental system features: size, cost and efficiency. The process assumes various simplifications regarding the utilized DC/DC converter models in order to prevent the simulation time to become excessive and, therefore, stability is not considered. The objective of this paper is to present a simplified method to analyze small-signal stability of a system in order to integrate it into the optimization methodology. A black-box modeling approach, applicable to commercial converters with unknown topology and components, is based on frequency response measurements enabling the system small-signal stability assessment. The applicability of passivity-based stability criterion is assessed. The stability margins are stated utilizing a concept of maximum peak criteria derived from the behavior of the impedance-based sensitivity function that provides a single number to state the robustness of the stability of a well-defined minor-loop gain

Effect of control method on impedance-based interactions in a buck converter

All the interconnected regulated systems are prone to impedance-based interactions making them sensitive to instability and transient-performance degradation. The applied control method affects significantly the characteristics of the converter in terms of sensitivity to different impedance interactions. This paper provides for the first time the whole set of impedance-type internal parameters and the formulas according to which the interaction sensitivity can be fully explained and analyzed. The formulation given in this paper can be utilized equally either based on measured frequency responses or on predicted analytic transfer functions. Usually, the distributed dc-dc systems are constructed by using ready-made power modules without having thorough knowledge on the actual power-stage and control-system designs. As a consequence, the interaction characterization has to be based on the frequency responses measureable via the input and output terminals. A buck converter with four different control methods is experimentally characterized in frequency domain to demonstrate the effect of control method on the interaction sensitivity. The presented analytical models are used to explain the phenomena behind the changes in the interaction sensitivity

Impedance-based stability and transient-performance assessment applying maximum peak criteria

The impedance-based stability-assessment method has turned out to be a very effective tool and its usage is rapidly growing in different applications ranging from the conventional interconnected dc/dc systems to the grid-connected renewable energy systems. The results are sometime given as a certain forbidden region in the complex plane out of which the impedance ratio--known as minor-loop gain--shall stay for ensuring robust stability. This letter discusses the circle-like forbidden region occupying minimum area in the complex plane, defined by applying maximum peak criteria, which is well-known theory in control engineering. The investigation shows that the circle-like forbidden region will ensure robust stability only if the impedance-based minor-loop gain is determined at the very input or output of each subsystem within the interconnected system. Experimental evidence is provided based on a small-scale dc/dc distributed system

Stability and Transient Performance Assessment in a COTS-Module-Based Distributed DC/DC System

This paper introduces a method to analyze and predict stability and transient performance of a distributed system where COTS (Commercial-off-the-shelf) modules share an input filter. The presented procedure is based on the measured data from the input and output terminals of the power modules. The required information for the analysis is obtained by performing frequency response measurements for each converter. This attained data is utilized to compute special transfer functions, which partly determine the source and load interactions within the converters. The system level dynamic description is constructed based on the measured and computed transfer functions introducing cross-coupling mechanisms within the system. System stability can be studied based on the well-known impedance- related minor-loop gain at an arbitrary interface within the system

Systematic Method to Assess Small-Signal Stability of DC-Distributed Power-System-Architecture

The objective of this paper is to present a simplified method to analyze small-signal stability of a power system and provide performance metrics for stability assessment of a given power-system-architecture. The stability margins are stated utilizing a concept of maximum peak criteria (MPC), derived from the behavior of an impedance-based sensitivity function that provides a single number to state the robustness of the stability of a well-defined minor-loop gain. For each minor-loop gain, defined at every system interface, the robustness of the stability is provided as a maximum value of the corresponding sensitivity function. Typically power systems comprise of various interfaces and, therefore, in order to compare different architecture solutions in terms of stability, a single number providing an overall measure of the whole system stability is required. The selected figure of merit is geometric average of each maximum peak value within the system, combined with the worst case value of system interfaces

Load-resistor-affected dynamic models in control design of switched-mode converters

The application of a resistor as a load of the pulse-width-modulated DC-DC converters has dominated the dynamic modelling since the development of the modelling methods in early 1970s. In 1990s, the research in the source and load interactions was very active providing valuable information to justify the necessity to develop unterminated dynamic models for characterising the dynamics of the converters. The small-signal modelling can be performed always by using the ideal load, which is determined by the output-terminal variable to be kept constant, regardless of whether the actual open-loop converter can operate or not with the ideal load. This paper will review the feasibility of using the load-resistor-affected models in control design of switched mode converters. The best strategy is always to use unterminated models, which can be used to obtain different load-impedance-affected models if needed. A buck converter is used as the source of information.Peer reviewe

Grid-connected PV power plant induced power quality problems - Experimental evidence

This paper presents new findings on phenomena contributing to flicker and voltage variations caused by grid-connected photovoltaic (PV) inverters. The voltage variations caused by two different 6 kW single-phase grid-connected PV inverters were studied during climatic variations by varying their grid-coupling impedance. Two different methods for characterizing the PV-plant induced voltage variations were studied: the short-term flicker index (Pst) and the 10 minute very-short voltage variation value (VSV). The results clearly indicate that PV inverter power fluctuations induced by cloud shading and enhancement have a significant effect on the VSV value, but not on Pst. PV inverters have a clear effect on the Pst as well, but the main contributors are related to the inverter design rather than the power fluctuations caused by clouds. The main contributor in the elevated Pst values could be traced back to the poor design of the maximum power point tracking (MPPT) of the inverters. The MPPT caused subharmonic current variations at a frequency of approximately 8 Hz which is close to the most sensitive frequency of human eye. Another factor causing rapid voltage variations in low irradiance conditions was the current transients related to the inverter start-up and shut-down. Harmonic current distortion is also a potential PV inverter related power quality (PQ) issue. This study indicates that although the current total harmonic distortion (THD) may be very large at low power levels the total demand distortion (TDD) of the PV inverters is almost constant regardless of the output power and the harmonic current had only a very limited effect on the voltage quality even at the weakest network having a short-circuit current of Isc=250 A. Thus, voltage variations caused by the PV inverters were the main PQ issue in the studied networks. The investigations also clearly show that a part of the power quality problems found i- the PV plants are caused by the poor design of the PV inverters.acceptedVersionPeer reviewe