Balanced Three-phase Power System Research Articles

Fixed switching frequency scheme for current predictive control of DFIG

This paper suggests a new current predictive control algorithm for the wind-driven doubly-fed induction generator (DFIG) based on space vector modulation (SVM) with fixed switching frequency. Combining the current predictive controller and SVM has the benefits of a predictive controller, and fixing the switching frequency improves the output waveform quality. However, if classical predictive control is used, the computational burden will still be high. The proposed algorithm uses SVM to fix the switching frequency of the rotor side converter. Additionally, by using an incremental algorithm, the proposed technique prevents examining all inputs over the prediction horizon. As a result, in the proposed current predictive algorithm the computational time is significantly reduced. The detailed state-space model of DFIG and rotor side converter are used to execute the predictive algorithm. The proposed algorithm uses the rotor current to predict the next behavior of the DFIG, and then, by using SVM, the duty cycles of 2-level voltage source inverter are obtained. Finally, the proposed controller's responses are compared to those of the SVM-based field-oriented control strategy. Simulation results of the proposed controller on a two-level voltage source inverter under a balanced three-phase power system illustrate the satisfactory active and reactive power tracking and improved quality of the inverter outputs.

Extending CIM Standards to Support Exchange of Ratings on Dynamically Rated Equipment

This paper describes the design of the ERCOT modeling extensions to the IEC 61968/70 standards to enable Common Information Model (CIM) schema support of dynamic ratings methodologies. Implementation of dynamic ratings was driven by current economic and environmental constraints placed on the construction of additional electrical transmission infrastructure, thus creating a need to operate the existing equipment closer to its physical limitations. In order to effectively utilize these dynamic limits, the necessary data constructs must be modeled, so ERCOT extended the CIM representation of the balanced three-phase power system model used to operate its various grid functions, in order to support the modeling and use of dynamic ratings in its operational real-time and congestion hedging market models. This paper also explains how the process utilized in the identification and extension of the CIM model for this purpose is flexible enough for generic application within the power industry.

Continuation Three-Phase Power Flow: A Tool for Voltage Stability Analysis of Unbalanced Three-Phase Power Systems

In this paper, a continuation three-phase power flow (CTPFlow) approach in polar coordinates, which can be used to analyze voltage stability of unbalanced three-phase power systems, is proposed. Using CTPFlow, the PV curves of unbalanced three-phase power systems can be obtained. It is found that the patterns of the PV Curves of unbalanced three-phase power systems are quite different from that of balanced three-phase power systems or positive-sequence power systems. The investigations indicate that a CTPFlow is needed where there are unbalanced network or loads existing in a system. Numerical examples are given to illustrate the approach.

A Novel Active Power Quality Compensator Topology for Electrified Railway

To improve the power quality of traction power system, a novel active power quality compensator (APQC) and a new compensating currents detection method are proposed. The APQC consists of a three-phase voltage source converter and a Scott transformer. The Scott transformer, taken as an isolation transformer, not only connects the three-phase converter to the traction power system, but also converts the traction power system to a nearly balanced three-phase power system. Therefore, a general three-phase converter could be used in APQC. Regarding the traction substation as a compensating object, the power quality of a traction substation can be improved integrally. Simulation and prototype experimental results show that the proposed APQC is able to compensate reactive power, harmonic, and negative-sequence currents in two feeders of a traction substation.