Design Low Order Robust Controller for the Generator’s Rotor Angle Stabilization PSS System

The electrical system's problem stabilizes the electrical system with three primary parameters: rotor angle stability, frequency stability, and voltage stability. This paper focuses on the problem of designing a low-order stable optimal controller for the generator rotor angle (load angle) stabilization system with minor disturbances. These minor disturbances are caused by lack of damping torque, change in load, or change in a generator during operation. Using the RH∞optimal robust design method for the Power System Stabilizer (PSS) to stabilize the generator’s load angle will help the PSS system work sustainably under disturbance. However, this technique's disadvantage is that the controller often has a high order, causing many difficulties in practical application. To overcome this disadvantage, we propose to reduce the order of the higher-order optimal robust controller. There are two solutions to reduce order for high-order optimal robust controller: optimal order reduction according to the given controller structure and order reduction according to model order reduction algorithms. This study selects the order reduction of the controller according to the model order reduction algorithms. In order to choose the most suitable low-order optimal robust controller that can replace the high-order optimal robust controller, we have compared and evaluated the order-reducing controllers according to many model order reduction algorithms. Using robust low-order controllers to control the generator’s rotor angle completely meets the stabilization requirements. The research results of the paper show the correctness of the controller order reduction solution according to the model order reduction algorithms and open the possibility of application in practice. Doi: 10.28991/esj-2021-01299 Full Text: PDF

Slow flow solutions and stability analysis of single machine to infinite bus power systems

Nonlinearity is a major constraint in analysing and controlling power systems. The behaviour of the nonlinear systems will vary drastically with changing operating conditions. Hence a detailed study of the response of the power system with nonlinearities is necessary especially at frequencies closer to natural resonant frequencies of machines where the system may jump into the chaos. This paper attempt such a study of a single machine to infinite bus power system by modelling it as a Duffing equation with softening spring. Using the method of multiple scales, an approximate analytical expression which describes the variation of load angle is derived. The phase portraits generated from the slow flow equations, closer to the jump, display two stable equilibria (centers) and an unstable fixed point (saddle). From the analysis, it is observed that even for a combination of parameters for which the system exhibits jump resonance, the system will remain stable if the variation of load angle is within a bounded region.

A METHOD OF ECONOMICAL CONTROL OF A SYNCHRONOUS MOTOR WITHOUT A ROTOR POSITION SENSOR

The article considers a method of economical speed control of synchronous motor with permanent magnet excitation using autonomous voltage inverter. The method provides stable (without tilting) rotation of the rotor without using the rotor position sensor signals. An algorithm, which realizes minimal losses in the machine and in the inverter, is proposed. The assumption of insignificant influence of voltage drop in stator winding active resistance on processes in synchronous machine accepted in the analysis is confirmed by parameters of real motors from 7DVM series and results of the experiment. It is shown that the mode with cosφ = 1 differs little from the generally accepted mode with load angle θ = φ. The results of experimental verification of the method in electric drive with 7DVM250 motor of 150 kW power are presented, which showed high dynamic stability of the system in a wide range of speeds and loads and while maintaining the most economical energy exchange between the motor and inverter (that is with cosφ = 1).

The Influence of Thickness on the Tensile Strength of Finnish Birch Veneers under Varying Load Angles

The development of high-performance, veneer-based wood composites is a topic of increasing importance due to the high design flexibility and the comparable mechanical performance to solid wood. Part of this improved mechanical performance can be contributed to the size effect present in wood. Based on previous findings in the literature, this size effect can be either strengthening or weakening. The presented study investigates the influence of thickness and load angle on the tensile strength and tensile stiffness of peeled veneers compared to thin sawn timber. Veneers with thicknesses of 0.5 ± 0.05 mm, 1.0 ± 0.05 mm and 1.5 ± 0.05 mm as well as sawn wood with thicknesses of 1.5 ± 0.1 mm, 3.0 ± 0.1 mm and 5.0 ± 0.1 mm were tested in tension under different load angles (0°, 45° and 90°). The results only partly confirm a size effect for strength parallel to the grain. The strength perpendicular to the grain increased significantly between 0.5 mm and 1.5 mm, with a significant decrease between 1.5 mm and 5.0 mm. The presence of lathe checks diminished the strength perpendicular to the grain of the veneers by about 70% compared to solid wood, partly overshadowing a possible strengthening effect. It was concluded that a transition from a strengthening to a weakening behaviour lies in the range of multiple millimetres, but further investigations are needed to quantify this zone more precisely. The presented results provide a useful basis for the development of veneer-based wood composites with a performance driven layer-thickness.

The Mathematical Models of Converter Fed Permanent Magnet Motors

The mathematical models of converter fed permanent magnet motors written in the coordinate axes are considered. Owing to pulse-width modulation, the currents and voltages of the motors are assumed to be sinusoidal. In the models with two state equations, the stator current is represented only by its quadrature-axis component, and the variable load angle appears in explicit or implicit form. These equations are most close to the similar equations of dc commutator motors. Third-order equations contain both stator current components; the load angle in them is fixed as constant one or varies to maintain the unity power factor for an arbitrary load. The models constructed in the form of state equations of the appropriate order, which are implemented in the MathCad software, illustrate possible functional regimes of the motor and indicate the ways of their possible implementations using means known from the practice.