Matjaž Dolinar, Drago Dolinar, Gorazd Štumberger and Boštjan Polajžer
The majority of three‐phase dynamic transformer models used in commercially available electric power system transient simulation programs offer only saturated three‐phase…
Abstract
Purpose
The majority of three‐phase dynamic transformer models used in commercially available electric power system transient simulation programs offer only saturated three‐phase transformer models built from three single‐phase transformer models. This paper sets out to deal with the modelling and transient analysis of a saturated three‐limb core‐type transformer.
Design/methodology/approach
Three iron core models I‐III are given by the current‐dependent characteristics of flux linkages. In the first model, these characteristics are given by a set of piecewise linear functions, which include saturation. In the second model, the piecewise linear functions are replaced by the measured nonlinear characteristic. The more complex third model is given by a set of measured flux linkage characteristics.
Findings
The behaviour of transformers used in electric power applications depends considerably on the properties of magnetically nonlinear iron core. The best agreement between the calculated and measured results is obtained by use of the most complex iron core model III, which takes into account magnetic cross‐couplings between different limbs, caused by saturation.
Research limitations/implications
Measurement of the current‐dependent flux linkage characteristics of the 0.4 kV, 3.5 kVA laboratory transformer requires corresponding excitation of windings by three independent linear amplifiers. Current‐dependent flux linkage characteristics of the larger power transformer can be determined either by similar measurement with linear amplifiers of an appropriate power or by extracting them from the calculated magnetic field, which is done by the finite element method.
Practical implications
A three‐phase dynamic transformer model with the obtained iron core model III is suitable for the numerical analysis of nonsymmetric transient states in power systems.
Originality/value
This paper presents a three‐phase dynamic transformer model with an original iron core model III, which accounts for magnetic cross‐couplings between different limbs, caused by saturation.
Details
Keywords
Drago Dolinar, Petar Ljušev, Gorazd Štumberger, Matjaž Dolinar and Daniel Roger
The purpose of this paper is to analyze the impact of magnetic saturation on the steady‐state operation of the induction motor (IM) drive in regard to rotor field‐oriented control…
Abstract
Purpose
The purpose of this paper is to analyze the impact of magnetic saturation on the steady‐state operation of the induction motor (IM) drive in regard to rotor field‐oriented control (RFOC). The aim of the presented two methods is to obtain the required steady‐state torque with minimal stator current, which thus reduces stator coper losses considerably.
Design/methodology/approach
The first method is based on an analytic calculation of the peak torque‐per‐ampere ratio curve of saturated IM. The torque characteristics obtained at a constant stator current are used to calculate that value of magnetizing current which gives the minimal stator current for the required load torque. The second method directly searches the minimal stator current for the required load torque. Experiments completely confirm the efficiency of the proposed selection of a magnetizing current reference.
Findings
Operation of the IM drive strongly depends on a proper selection of the rotor flux linkage reference value, the selection of which represents an additional degree of freedom in control design. Therefore, it can be used to optimize some of those drive features subjected to voltage and current constraints. The proposed calculation procedure is simple so that can be easily implemented in practically application. However, some additional IM data like magnetizing curve, inertia moment, and coefficient of viscous friction are necessary.
Originality/value
The substantial impact of saturation on the stead‐state torque characteristics of IM, determined for the constant stator current and the constant d‐axis stator current, is determined analytically and numerically.
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Keywords
Matjaž Dolinar, Miloš Pantoš and Drago Dolinar
The purpose of this paper is to present an improved approach to reactive power planning in electric power systems (EPS). It is based on minimization of a transmission network's…
Abstract
Purpose
The purpose of this paper is to present an improved approach to reactive power planning in electric power systems (EPS). It is based on minimization of a transmission network's active power losses. Several operating conditions have to be fulfilled to ensure stable operation of an EPS with minimal power losses. Some new limitations such as voltage instability detection and generator capability curve limit have been added to the existing method in order to improve the reliability of reactive power planning. The proposed method was tested on a model of the Slovenian power system. The results show the achievement of significant reduction in active power losses, while maintaining adequate EPS security.
Design/methodology/approach
Optimal voltage profile has to be found in order to determine minimal possible active power losses of EPS. The objective function, used to find the optimal voltage profile, has integer and floating point variables and is non‐differentiable with several local minima. Additionally, to ensure secure operation of EPS, several equality and inequality boundaries and limitations have to be applied. Differential evolution (DE) was used to solve the optimization problem.
Findings
Corresponding reactive power planning can significantly reduce active power losses in EPS. However, such planning can affect the security of EPS, therefore, several additional constrains have to be considered. The presented constrains considerably improve the operational security of EPS.
Research limitations/implications
DE was used to solve the minimization problem. Although this method has proven to be fast and reliable, it is theoretically possible that the obtained solution is not global minimum.
Originality/value
Novel approach to voltage security constrained reactive power planning with additional nonlinear constrains, such as generator capability curves and voltage instability detection.
Details
Keywords
Gorazd Štumberger, Bojan Štumberger, David Stojan, Željko Plantić, Klemen Deželak, Matjaž Čemažar and Drago Dolinar
Many authors reported the decrease of performances when electric machines and electromagnetic devices were supplied by pulse width modulated (PWM) voltages. However, these…
Abstract
Purpose
Many authors reported the decrease of performances when electric machines and electromagnetic devices were supplied by pulse width modulated (PWM) voltages. However, these statements are rarely supported by measurements performed under fair conditions. The aim of this paper is to compare the performances of a single‐phase transformer and a three‐phase permanent magnet synchronous motor (PMSM) supplied by sinusoidal and PWM voltages and to find a way to evaluate the decrease of performances when PWM voltages are applied.
Design/methodology/approach
In order to perform a fair comparison between performances of the tested objects supplied by sinusoidal and PWM voltages, an experimental system was built. It contains a single‐phase and a three‐phase linear rectifier for supply with sinusoidal voltages and an H‐bridge inverter and a three‐phase inverter for supply with PWM voltages. The tests and measurements were performed on a single‐phase transformer and three‐phase PMSM, where different constant loads and different modulation frequencies were used. The test conditions were identical for the supply by sinusoidal and PWM voltages. The measured data, used for the evaluation of performances, were the input and output power and the time behaviours of currents and voltages together with their THDs.
Findings
The results presented in the paper clearly show that the efficiency of the singe‐phase transformer and three‐phase PMSM decreases with the increasing level of voltage THD. To properly determine the THD of PWM voltage, the sampling frequencies above 1 MHz and special equipment are normally required. However, if the modulation frequency is not too high, also the current THD, which can be easily determined, can be used to evaluate the decrease of efficiency in the case of supply by PWM voltages.
Originality/value
The results presented in the paper clearly show that the efficiency of the singe‐phase transformer and three‐phase PMSM decreases with the increasing level of voltage THD.