Small-Signal Impedance Measurement of Power-Electronics-Based AC Power Systems Using Line-to-Line Current Injection

Abstract

Naval ships as well as aerospace power systems are incorporating a greater degree of power electronic switching sources and loads. Although these components provide exceptional performance, they are prone to instability due to their high efficiency and constant power characteristics that can exhibit negative impedance nature at certain frequencies. When designing these systems, integrators must consider the impedance versus frequency at an interface (which designates source and load). Stability criteria have been developed in terms of source and load impedances for both dc and ac systems, and it is often helpful to have techniques for impedance measurement. For dc systems, the measurement techniques have been well established. This paper introduces a new method of impedance measurement for three-phase ac systems. By injecting an unbalanced line-to-line current between two lines of the ac system, all impedance information in the traditional synchronous reference frame d−q model can be determined. For medium-voltage systems, the proposed technique is simpler and less costly than having an injection circuit for each phase. Since the current injection is between only two phase lines, the proposed measurement device can be used for both ac and dc interfaces. Simulation and laboratory measurements demonstrate the effectiveness of this new technique.Index Terms-Impedance measurement, power conversion, power system stability, power system testing.

Communication-less harmonic compensation in a multi-bus microgrid through autonomous control of distributed generation grid-interfacing converters

“…The parameters of the line impedance can be relatively accurate. Other methods discussed in literature [18][19][20], inject a perturbation into the grid and then measure the PCC voltage and current to analyze and calculate line impedance at each frequency, these methods can accurately and rapidly measure the line impedance at each frequency, and it is suitable for the applications which need to measure harmonic impedance rapidly. For instance, the real resistance and inductance of feeder in [20] are 1 X and 6.1 mH, meanwhile, the measured values are 1.05 X and 6.01 mH.…”

“…However, the large error between the real line impedance value and estimated value will affect the accuracy of the harmonic voltage calculation, and the complete compensation cannot be achieved. But when considering the closed control loop can achieve proper fundamental and harmonic current tracking performance and the estimated values are very close to the real values of the feeder impedance [18][19][20], the proposed harmonic compensation scheme can still significantly improve the power quality of the microgrid.…”

Frequency Domain Stability Analysis of MMC-Based HVdc for Wind Farm Integration

“…The ac-and dc-side d-q impedances of a two-level VSC-HVDC transmission system with the outer and inner control loops were derived, respectively, and then the ac-and dc-side resonances of the VSC-HVDC system were examined by using the impedance-based Nyquist stability criterion in [23], [24]. Furthermore, some methods for measuring the d-q impedances via impedance measurement circuits were proposed in [25], [26]. It is noted that the derivation of d-q impedances of threephase converters is relatively easy in the case that current controllers are implemented in d-q reference frame.…”

Frequency-adaptive grid-virtual-flux synchronization by multiple second-order generalized integrators under distorted grid conditions

“…If the grid impedance is large enough, it could seriously influence the performance of VSC. The stability of the grid-connected VSC can be analyzed by the impedance ratio and its Nyquist plot [25]. The impedance ratio can be expressed as:…”