Oleg Wasynczuk

Analysis and average-value modeling of line-commutated converter-synchronous machine systems

Analytical relationships are established which can be used to predict the steady-state characteristics of line-commutated AC-DC-converter-synchronous machine systems. In particular, basic relationships are established in which the average DC voltage and the average electromagnetic torque are related to the converter firing delay angle. It is shown that these average-value relationships predict the steady-state performance with significantly higher accuracy than the classical converter average-value equations in which the d-axis subtransient reactance is used as the commutating reactance. >

An efficient and accurate model for the simulation and analysis of synchronous machine/converter systems

A new synchronous machine model is presented which is readily implemented in either circuit-based or differential-equation-based simulation programs. This model is well suited for the simulation and analysis of synchronous machine-power converter systems. It is based upon standard representations and no approximations are made in its derivation. However, the numerical implementation is shown to be significantly more efficient. An example is provided which demonstrates a 1700% increase in simulation speed with no observable loss in accuracy. The model includes provisions for an arbitrary number of damper or rotor windings and may be easily modified to represent synchronous or induction machines with an arbitrary number of stator phases.

A model-in-the-loop interface to emulate source dynamics in a zonal DC distribution system

A model-in-the-loop capability (MIL) has been developed to emulate the dynamics of alternative power sources in a hardware-based dc zonal electrical distribution system. Using this tool, models of the power sources are simulated in real-time and interfaced with hardware components at the voltage and current levels of the power system. Coupling between simulation and hardware is established through a dc/dc power converter using model/wall-clock time synchronization. The MIL capability is illustrated in the emulation of a synchronous machine/converter power source. Results of time-domain and frequency-domain studies are provided to validate the approach.

Automated state model generation algorithm for power circuits and systems

A new approach of automatically generating state-space models of power circuits and systems is presented. In this approach, the composite system state equations are established algorithmically given the standard node incidence matrix and elementary branch data (e.g. resistances, inductances, back emfs). The resulting state equations can be solved using a variety of numerical techniques or commercially available computer simulation programs. An example system consisting of a three-phase generator and rectifier load is used to illustrate this approach. Experimental verification is also provided.

An efficient multirate Simulation technique for power-electronic-based systems

A novel multirate method of simulating power-electronic-based systems containing a wide range of time scales is presented. In this method, any suitable integration algorithm, with fixed or variable time-step, can be applied to the fast and/or slow subsystems. The subsystems exchange coupling variables at a communication interval that can be fixed or varied dynamically depending upon the state of the system variables. The proposed multirate method is applied to two example power systems that include power-electronic subsystems. Increases in simulation speed of 183-281% over established single-rate integration algorithms are demonstrated.

Reference frame analysis of a slip energy recovery system

Until the late 1970s the performance of a slip energy recovery system of an induction motor was predicted analytically by means of steady-state power balance relationships. Here, reference frame theory is used to establish the equations which can be used to predict adequately the dynamic and steady-state performance of a slip energy recovery system. The steady-state performance as predicted from this analysis is compared with that predicted from computer simulation. The errors of a previously published approach are discussed and the inaccuracy in predicting the steady-state torque-speed characteristics is illustrated. The stage is now set to develop linearized, small displacement equations which can be used for control system design. >

An efficient method of simulating stiffly connected power systems with stator and network transients included

A method is presented for simulating interconnected power systems with the stator and network transients included. This method is well suited for stiffly connected power systems in which tie-lines interconnecting the system components are electrically short (i.e. the tie-line charging capacitance can be neglected). The respective state-space models of the network and of the electric machines, combined with the algebraic constraint equations imposed by their interconnection, comprise a set of differential-algebraic equations which describes the composite system dynamics. A systematic procedure has been developed in which these differential-algebraic equations are used to establish a conventional state-space model of the composite system. Simulations of an example system have shown that the computation time associated with this method is comparable to that established using reduced-order models in which the stator transients are neglected. >

A voltage control strategy for current-regulated PWM inverters

Alternative voltage control strategies for current-regulated PWM inverters are analyzed, including previously established feedforward and feedforward/feedback controllers and a newly proposed decoupling feedback control strategy. The steady-state and dynamic characteristics of each of these control methods are illustrated and compared for a selected inverter design. It is shown that the feedforward controller exhibits steady-state error and an undesirable overshoot of the output voltages during startup. The addition of a feedback loop eliminates the steady-state error and reduces the overshoot; however, the natural response is underdamped regardless of the choice of feedback gains. A decoupling feedback control strategy that eliminates the disadvantages of the feedforward and feedforward/feedback controllers is described. Using the decoupling feedback controller, it is possible to eliminate the steady-state error and place the closed-loop poles wherever desired. Moreover, if the closed-loop poles are selected appropriately, it is possible to eliminate the overshoot of the output voltages during startup transients.

Efficiency Evaluation of the Modular Multilevel Converter Based on Si and SiC Switching Devices for Medium/High-Voltage Applications

The modular multilevel converter (MMC) is the most promising converter topology for medium- and high-power applications. One of the main concerns in the operation of the MMC, particularly for high-power applications, is its efficiency, which should be maximized. Silicon Carbide (SiC)-based devices have the potential to provide significant efficiency improvement compared with silicon devices. However, the possibility and impact of using SiC-based devices instead of silicon devices for high-power conversion have not been thoroughly explored. This paper reports on the results obtained from a detailed study to evaluate the performance of MMCs based on medium-voltage SiC MOSFETs and diodes with hybrid MMCs that employ silicon IGBTs and SiC diodes. The results are based on detailed circuit simulations that use simple physics-based circuit models. The study suggests the potential for significant efficiency gain for MMCs based on SiC power devices.

Simulation of a zonal electric distribution system for shipboard applications

This paper describes a DC zonal electric distribution system (ZEDS) architecture that is being developed for future shipboard applications. This system includes alternator/rectifier power sources, a DC distribution system, and distributed DC/AC inverters for zonal AC loads. System stability and fault tolerance are of major concern. A detailed computer simulation has been developed that addresses these issues. This simulation is based on an automated state model generation algorithm wherein the computer automatically establishes the system state model from a topological system description. It is shown that the system dynamics stiffness imposes a significant computational burden. Methods of automatic model partitioning and model-order reduction are presented that increase the computational speed by several orders of magnitude.