Mohamed Belkhayat

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

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.

New Techniques for Measuring Impedance Characteristics of Three-Phase AC Power Systems

Naval ships as well as aerospace power systems are incorporating an increasing amount of power electronic switching sources and loads. Although these power-electronics-based components provide exceptional performance, they are prone to instability due to their constant power characteristics that lead to negative impedance. When designing these systems, integrators must consider the impedance versus frequency at a power system interface (which designates source and load). Stability criteria have been developed in terms of source and load impedance for both dc and ac power systems, and it is often helpful to have techniques for impedance measurement. For dc power systems, the measurement techniques have been well established. This paper suggests several methods for measuring ac impedance including utilization of power converters, wound-rotor induction machines, and chopper circuits. Simulation and laboratory results on an example ac power system demonstrate the effectiveness of the proposed methods.

Online Synchronous Machine Parameter Extraction From Small-Signal Injection Techniques

This paper proposes using a novel line-to-line voltage perturbation as a technique for online measurement of synchronous machine parameters. The perturbation is created by a chopper circuit connected between two phases of the machine. Using this method, it is possible to obtain the full set of four complex small-signal impedances of the synchronous machine d-q model over a wide frequency range. Typically, two chopper switching frequencies are needed to obtain one data point. However, it is shown herein that, due to the symmetry of the machine equations, only one chopper switching frequency is needed to obtain the information. A 3.7-kW machine system is simulated, and then constructed for validation of the impedance measurement technique. A genetic algorithm is then used to obtain IEEE standard model parameters from the d -q impedances. The resulting parameters are shown to be similar to those obtained by a series of tests involving synchronous reactance measurements and a standstill frequency response.

Standard Tools for Hardware-in-the-Loop (HIL) Modeling and Simulation

This paper presents a cost effective hardware-in-the-loop (HIL) environment integrated into a Simulink real-time simulation. Using an inexpensive national instruments data acquisition card in conjunction with Simulink real-time simulator, a Schweitzer engineering laboratory (SEL) relay was integrated into a Simulink power system fault transient simulation. The sequence of events, from the fault initialization to the breaker opening, was captured through the Simulink model. During the simulation, the relay was fed generator terminal voltages and currents from the Simulink simulation. When the relay detects a fault condition, it sends an open breaker signal back to the simulation. The simulation opens the virtual breaker until the fault is cleared and the open-breaker signal from the relay ceases.

Analysis of Phase Locked Loop (PLL) influence on DQ impedance measurement in three-phase AC systems

AC small signal stability of three-phase systems can be analyzed using the load and source impedances in the d-q synchronous reference frame and many solutions have been recently proposed to measure d-q impedances. One fundamental part of the measurement system is the Phase-Looked Loop (PLL), which tracks the supply voltage phase in order to transform voltage and current terms from abc to dq coordinates. This paper analyzes the influence of PLL dynamics on the accuracy of the impedance measurement results, focusing on two errors: 1) on the generation of voltage/current perturbation, 2) on the measured voltage and current components used for impedance extraction. The analysis shows that PLL has a small influence on the voltage/current injection and a more relevant one on the voltage and current measurements. For this reason, the PLL bandwidth in existing systems is much smaller than the lowest frequency of the measured impedance, strongly increasing the measurement time. However, PLLs effect can be compensated off-line with the use of derived analytical expressions, enabling a higher PLL bandwidth and a significantly faster measurement system. The proposed approach is verified by simulation.

DC Link Stabiliy Design Tool

For most integrated power systems, system stability can be most affected at the DC link. There are various choices of how to achieve stability at the DC link, ranging from passive to active techniques. This paper discusses these choices then focuses on a graphical user interface (GUI) that implements the passive RC damper technique. The design GUI implemented in Simulink allows the designer to load the source impedance, which then calculates and displays the Nyquist plot as the RC damper values are changed in real time. This allows the designer to reach the optimum values of the RC damper for a pre-set Gain Margin and Phase margin thus achieving system stability designs.

Algorithm-based evaluation of ramp rate and pulsed power loading for AC electric systems

The naval electric ship is ushering in a new era of pulsed power sensors and weapons. Successful integration of these mission loads necessitates new requirements and new measurement tools. This paper focuses on measurement techniques only. Evaluation for compliance is discussed for a medium-voltage, 60Hz, three-phase dynamic electrical load with requirements governing ramp rate, pulsed loading, and harmonic content. The procedures discussed consider the discretizing effect of the measurement sampling rate while avoiding spurious declaration of violations as a result of noise or ripple. Presence of rich harmonic content in the measured power draw presents a challenge to distinguish and separately evaluate harmonic content versus physically meaningful ramp rates; this is also addressed by the evaluation methodologies discussed. In general, the approach must have broad applicability for integration of large electrical loads with host power system architectures. The solution(s) must possess several novel aspects: a) power ramp requirement thresholds must be explicitly matched with practical numerical approaches to evaluate compliance; b) pulse loading requirement thresholds must be paired with an unambiguous, actionable algorithmic process; and c) validated techniques must be used to screen harmonic content from the fundamental power draw.

DQ impedance of a regulated synchronous machine

Impedance based stability techniques have been developed since the 80s for stable filter and converter designs. More recently, similar impedance techniques have been used to impose stability requirements on power systems including distributed converters. Significant effort has been spent on the development of models and impedance characterizations for regulated DC-DC converters and AC-DC converters. For rotating machinery however, and in particular synchronous machines, very little has been published on impedance characterizations. In this paper, the DQ impedance of a regulated synchronous machine is developed and compared to a hardware-validated Saber model of the synchronous machine. The DQ impedance expressions are derived based on the fixed-field impedances, the droop parameter, and simplified transfer functions for the exciter and the regulator. The analytical results are compared to the numerical results obtained from the Saber model.