Showing papers by "Behrooz Bahrani published in 2011"

Multivariable-PI-Based $dq$ Current Control of Voltage Source Converters With Superior Axis Decoupling Capability

Abstract: This paper presents a linear direct-quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional-integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated.

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

Abstract: This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Nondetection Zone Assessment of an Active Islanding Detection Method and its Experimental Evaluation

Abstract: This paper analytically determines the nondetection zone (NDZ) of an active islanding detection method, and proposes a solution to obviate the NDZ. The method actively injects a negative-sequence current through the interface voltage-sourced converter (VSC) of a distributed generation (DG) unit, as a disturbance signal for islanding detection. The estimated magnitude of the corresponding negative-sequence voltage at the PCC is used as the islanding detection signal. In this paper, based on a laboratory test system, the performance of the islanding detection method under UL1741 anti-islanding test conditions is evaluated. Then, determining the NDZ of the method and proposing the countermeasure, the existence of the NDZ and the performance of the modified method to eliminate the NDZ is verified based on simulation results in PSCAD/EMTDC software environment and experimental tests.

Model predictive-based voltage regulation of an islanded distributed generation unit

Abstract: This paper presents a Discrete-time Model Predictivebased Control strategy for the voltage regulation of an islanded distributed generation (DG) unit. The DG unit includes a voltage source converter (VSC) interfaced to the grid through a switch and a coupling transformer. An RLC branch is also connected in parallel at the point of common coupling (PCC) at the DG-side of the switch. Assuming an open switch and an islanded mode of operation, the Discrete-time Model Predictive-based Controller (DMPC) is adopted to regulate the direct and quadrature (dq) components of the load voltages. The frequency of the load voltage is also regulated in an open loop. In this paper, the design procedure is detailed. Then adopting a laboratory scale setup, the performance of the proposed control strategy is experimentally evaluated. The test results verify the capability of the controller in regulating the load voltages in the nominal plant, and also in the presence of load parameters uncertainties and nonlinear loads.

Railway catenary parameters identification based on harmonic current injection

Abstract: Modern trains with active line-side converters are currently the state-of-the-art technology in the traction industry. In such systems, the active line-side converter of the drive system can be exploited for power quality improvement purposes. In this paper, adopting the active line-side converter, an identification strategy is proposed, which estimates the parameters of the catenary line through the injection of low-order harmonic current and monitoring the corresponding voltage at the connection point of the pantograph to the network.

Stability Analysis and Experimental Evaluation of a Control Strategy for Islanded Operation of Distributed Generation Units

Abstract: This paper presents stability analysis of a distributed generation (DG) controller, in an islanded mode, based on the Mapping Theorem and the Zero Exclusion Condition, and validates the results based on a laboratory scale experimental setup. The DG unit is interfaced to the host system through a voltage-sourced converter (VSC). The control strategy regulates the load voltage at the desired value in the islanded mode, despite uncertainties in the load parameters. The frequency of the island is controlled in an open loop manner by an internal oscillator. The experimental results show that the controller provides robust voltage control for a wide range of load parameters, and even maintains the voltage within the prescribed range during the start-up transients of a motor which is part of the island load.