Zhiyu Shen

Analysis of D-Q Small-Signal Impedance of Grid-Tied Inverters

This paper analyzes the small-signal impedance of three-phase grid-tied inverters with feedback control and phase-locked loop (PLL) in the synchronous reference (d-q) frame. The result unveils an interesting and important feature of three-phase grid-tied inverters - namely, that its q-q channel impedance behaves as a negative incremental resistor. Moreover, this paper shows that this behavior is a consequence of grid synchronization, where the bandwidth of the PLL determines the frequency range of the resistor behavior, and the power rating of the inverter determines the magnitude of the resistor. Advanced PLL, current, and power control strategies do not change this feature. An example shows that under weak grid conditions, a change of the PLL bandwidth could lead the inverter system to unstable conditions as a result of this behavior. Harmonic resonance and instability issues can be analyzed using the proposed impedance model. Simulation and experimental measurements verify the analysis.

Small-Signal Stability Analysis of Three-Phase AC Systems in the Presence of Constant Power Loads Based on Measured d-q Frame Impedances

Small-signal stability is of great concern for electrical power systems with a large number of regulated power converters. In the case of dc systems, stability can be predicted by examining the locus described by the ratio of the source and load impedances in the complex plane per the Nyquist stability criterion. For balanced three-phase ac systems the same impedance-based method applies, for which this paper uses impedances in the synchronous rotating reference (d-q) frame. Small-signal stability can be determined by applying the generalized Nyquist stability criterion (GNC). This approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this paper investigates the small-signal stability of a three-phase ac system using measured d-q frame impedances. The results obtained show how the stability at the ac interface can be easily and readily predicted using the measured impedances and the GNC, thus illustrating the practicality of the approach, and validating the use of ac impedances as a valuable dynamic analysis tool for ac system integration, in perfect dualism with the dc case.

Dual Active Bridge-Based Battery Charger for Plug-in Hybrid Electric Vehicle With Charging Current Containing Low Frequency Ripple

Manufacturers want high power density for the on-board battery chargers of plug-in hybrid electric vehicles. Wide bandgap devices can be used to shrink other passive components by increasing the switching frequency, but the bulk dc link capacitor of the ac-dc power factor correction stage, becomes one of the major barriers to higher power density, because its volume depends on the ripple power at the double line frequency in a dc current charging system. However, if this double line frequency ripple flows into the battery, the dc link capacitance can be significantly reduced. This charging scheme, named as sinusoidal charging in this paper, is analyzed and implemented based on a two-stage battery charging system, which is comprised of one full bridge ac-dc stage and one dual active bridge dc-dc stage. We further find that converter loss causes ripple power imbalance and bigger dc link capacitance. Therefore, the impact of converter loss on the ripple power balance is analyzed, and a feedback control on the dc link voltage ripple is proposed based on this analysis in order to further reduce the dc link capacitance. The effectiveness of the proposed solutions is verified in both Si-based and GaN-based charging systems.

Experimental verification of the Generalized Nyquist stability criterion for balanced three-phase ac systems in the presence of constant power loads

Stability is a great concern for power systems with relatively small sources and multiple regulated power converters. Addressing this need, this paper presents the experimental verification of the Generalized Nyquist stability Criterion (GNC) for balanced three-phase ac systems. This criterion, developed by MacFarlane and Postlethwaite in the 1970s, was proposed as a stability analysis tool for ac interfaces in the d-q frame in the late 1990s. Since then, however, very few papers have addressed the verification of this theory in a real power system given the intricacies of three-phase impedance measurement in the d-q frame. In this paper, a voltage source inverter feeding a boost rectifier is used to implement an experimental balanced three-phase system, where by adjusting their respective control bandwidths, stable and unstable cases are found at their ac interface. To this end, the d-q frame impedances of both converters are measured, and the GNC is applied, showing how both stable and unstable cases can be effectively predicted.

Influence of phase-locked loop on input admittance of three-phase voltage-source converters

A regulated power converter can cause instability issues at its terminals with its input filter or the source. The Generalized Nyquist stability Criterion (GNC), when applied to the product of the source impedance and load admittance in the dq frame, can be used to analyze the stability of balanced three-phase ac systems. This paper presents the influence of the Phase-Locked Loop (PLL) on the input admittance of three-phase Voltage-Source Converters (VSC) for open loop, current-feedback and voltage feedback control conditions. Analytical results show that the PLL introduces a parallel admittance to the open loop admittance of the VSC, and that the current and voltage control loop have additional effects on it. Both simulation and experimental results are presented to verify this analysis. The possible instability due to different PLL design is also shown.

Soft-switching capability analysis of a dual active bridge dc-dc converter

This paper analyzes the ZVS soft-switching region of a DAB converter with arbitrary operating waveforms. The effect of the junction capacitor of the device and the magnetizing inductance of the transformer are also analyzed. Through the analysis, a group of waveforms for different loading conditions are identified to maximize the ZVS operating region. The results are verified by simulation using real device models.

Three-phase AC system impedance measurement unit (IMU) using chirp signal injection

Impedance defined in synchronous coordinates is a useful tool to predict and evaluate stability of three-phase AC systems. For online impedance measurement in AC systems, the widely used frequency sweep method takes a long time and may not be practical in systems where the operating point cannot be maintained for a long time. A wide bandwidth signal could be used to significantly reduce the measurement time. This paper suggests a chirp signal and describes its advantages over other signals. Experimental results demonstrate the effectiveness of this approach.

An adaptive dead-time control scheme for high-switching-frequency dual-active-bridge converter

In high-switching-frequency isolated dc-dc converters, dead-time has a great influence on the switching transition behavior of switching devices. Fixed dead-time is determined by the longest required dead-time and results in missing zero-voltage-switching (ZVS) in some operation conditions, which causes undesirable switching stress and ringing, and efficiency degradation of converter. This paper proposes an adaptive dead-time control scheme for the high-switching-frequency dual-active-bridge (DAB) dc-dc converter. The proposed scheme feedbacks the Vds of primary side switch and can auto-tune the dead-time to make the primary side switching devices work under zero-voltage condition and to increase the converter efficiency. The scheme was implemented in a 5 kW 500 kHz DAB converter prototype, and the experimental results verified the effectiveness of proposed scheme.

Design and implementation of three-phase AC impedance measurement unit (IMU) with series and shunt injection

Electric power systems based on power electronics technology are prone to instability due to the constant power nature of power electronics converters. The impedances defined in synchronous coordinates are useful tools to predict and evaluate three-phase small-signal system stability. This paper presents the design and implementation of an impedance measurement unit used in three-phase systems up to 100kW. It extracts the system impedance online at any specific operating point. The unit utilizes both series and shunt injection modes to perform impedance identification for any load and source conditions, even if the source is very stiff. Experimental results demonstrate the effectiveness of the designed unit with both linear and nonlinear loads.

Inverse Nyquist Stability Criterion for Grid-Tied Inverters

A new impedance-based stability criterion was proposed for a grid-tied inverter system based on a Norton equivalent circuit of the inverter [18]. As an extension of the work in [18], this paper shows that using a Thévenin representation of the inverter can lead to the same criterion in [18]. Further, this paper shows that the criterion proposed by Middlebrook can still be used for the inverter systems. The link between the criterion in [18] and the original criterion is the inverse Nyquist stability criterion. The criterion in [18] is easier to be used. Because the current feedback controller and the phase-locked loop of the inverter introduce poles at the origin and right-half plane to the output impedance of the inverter. These poles do not appear in the minor loop gain defined in [18] but in the minor loop gain defined by Middlebrook. Experimental systems are used to verify the proposed analysis.