Murat E. Balci

Power conditioning using dynamic voltage restorers under different voltage sag types

Voltage sags can be symmetrical or unsymmetrical depending on the causes of the sag. At the present time, one of the most common procedures for mitigating voltage sags is by the use of dynamic voltage restorers (DVRs). By definition, a DVR is a controlled voltage source inserted between the network and a sensitive load through a booster transformer injecting voltage into the network in order to correct any disturbance affecting a sensitive load voltage. In this paper, modelling of DVR for voltage correction using MatLab software is presented. The performance of the device under different voltage sag types is described, where the voltage sag types are introduced using the different types of short-circuit faults included in the environment of the MatLab/Simulink package. The robustness of the proposed device is evaluated using the common voltage sag indices, while taking into account voltage and current unbalance percentages, where maintaining the total harmonic distortion percentage of the load voltage within a specified range is desired. Finally, several simulation results are shown in order to highlight that the DVR is capable of effective correction of the voltage sag while minimizing the grid voltage unbalance and distortion, regardless of the fault type.

Optimal passive filter design for effective utilization of cables and transformers under non-sinusoidal conditions

Transformers and cables have overheating and reduced loading capabilities under non-sinusoidal conditions due to the fact that their losses increases with not only rms value but also frequency of the load current. In this paper, it is aimed to employ passive filters for effective utilization of the cables and transformers in the harmonically contaminated power systems. To attain this goal, an optimal passive filter design approach is provided to maximize the power factor definition, which takes into account frequency-dependent losses of the power transmission and distribution equipment, under non-sinusoidal conditions. The obtained simulation results show that the proposed approach has a considerable advantage on the reduction of the total transmission loss and the transformer loading capability under non-sinusoidal conditions when compared to the traditional optimal filter design approach, which aims to maximize classical power factor definition. On the other hand, for the simulated system cases, both approaches lead to almost the same current carrying (or loading) capability value of the cables.

Optimal Design of C-type Passive Filters Based on Response Surface Methodology for Typical Industrial Power Systems

Abstract In this article, a response surface methodology based approach is proposed for the optimal design of C-type passive filters that can be employed in typical industrial power systems. The purpose of the optimization process is to minimize voltage and current total harmonic distortions. According to IEEE Standard 519-1992, both indices and displacement power factor are handled as constraints for the optimal filter design problem. To show the validity of the proposed approach, numerical results are presented.

Optimal C-Type Filter Design to Maximize Transformer's Loading Capability under Non-sinusoidal Conditions

Abstract The main purpose of this article is to employ the C-type passive filters for maximization of the loading capability of transformers under non-sinusoidal conditions. For this aim, an optimal passive filter design approach is proposed regarding minimization of the harmonic loss factor (FHL), which is defined in IEEE standard C57.110 to represent the loading capability of the transformers supplying non-sinusoidal load currents. With respect to IEEE standard 519, displacement power factor and individual harmonics & total harmonic distortions of the voltage and current are considered as constraints of the proposed optimal filter design approach. In addition, numerical results are presented to figure out that the proposed approach provides higher loading capability of the transformers under non-sinusoidal conditions when compared with the other two optimal filter design approaches based on minimization of the current total harmonic distortion and minimization of the voltage total harmonic distortion.

Optimal design of single-tuned passive filters using Response Surface Methodology

This paper presents an approach based on Response Surface Methodology (RSM) to find the optimal parameters of the single-tuned passive filters for harmonic mitigation. The main advantages of RSM can be underlined as easy implementation and effective computation. Using RSM, the single-tuned harmonic filter is designed to minimize voltage total harmonic distortion (THDV) and current total harmonic distortion (THDI). Power factor (PF) is also incorporated in the design procedure as a constraint. To show the validity of the proposed approach, RSM and Classical Direct Search (Grid Search) methods are evaluated for a typical industrial power system.

Hosting capacity assessment and improvement for photovoltaic-based distributed generation in distorted distribution networks

In this paper, the hosting (or maximum allowable) capacity of a photovoltaic (PV)-based distributed generation (DG) unit for a typical two bus distorted distribution system, is analyzed. The harmonic constraints, total and individual harmonic distortion limits stated in IEEE Standard 519, and the conventional hosting capacity constraints, bus rms voltage limit and the current carrying capability limit of the supply cables are taken into account. In the analysis, various simulations are carried out to show the effect of the nonlinearity degrees of the loads on the systems hosting capacity. It is clearly seen from the analysis results that the harmonic distortion limits significantly constrain the PV-based DG unit hosting capacity for the higher nonlinearity levels of the consumer. Accordingly, a C-type filter is designed to maximize the hosting capability of the studied system while providing desired power factor and satisfying the harmonic and conventional hosting capacity constraints. Besides, the numerical results are given to point out that a higher allowable hosting capacity is obtained with the proposed filter design approach compared to two traditional filter design approaches, which aims to attain minimization of voltage total harmonic distortion and minimization of current total demand distortion by considering the same constraints of the proposed approach. It is found that passive filters, already planned in the original system, helps in improving the PV-based DG hosting capacity of distribution feeder.

An analysis into the effect of voltage harmonics on the maximum loading capability of transformers

In this paper, some exemplary cases are presented to show the effect of voltage total harmonic distortion (THDV) and its spectrum on the harmonic loss factors (FHL and FHL-STR) of the six-pulse rectifiers accompanied with a constant power dc load and a battery. Accordingly, the winding losses and maximum loading capability of a transformer supplying both non-linear load types are investigated for sinusoidal and non-sinusoidal bus voltage conditions. The obtained numerical results clearly shows that the current harmonics of the considered non-linear load types are highly dependent on the THDV level and spectrum of the supply voltage, thus, the voltage harmonic profile should be considered in the design stage of the transformers, which are under influence of particular to supply them.

An algorithm for optimal sizing of the capacitor banks under non-sinusoidal and unbalanced conditions

In non-sinusoidal and unbalanced systems, optimal sizing of the capacitor banks is not a straightforward task as in sinusoidal and balanced systems. In this paper, by means of qualitative and quantitative analysis, it is interpreted that the classical capacitor selection algorithm widely implemented in Reactive Power Control (RPC) relays does not achieve optimal power factor improvement in non-sinusoidal and unbalanced systems. Accordingly, a computationally efficient algorithm is proposed to find the optimal capacitor bank for smart RPC relays. It is further shown in a simulated test case by using Matlab software that the proposed algorithm provides better power factor improvement when compared with the classical algorithm. It is also figured out from the simulation results that both algorithms cause almost the same harmonic distortion and unbalance deterioration levels in the system.

A fixed speed induction generator model for unbalanced power flow analysis

This paper proposes a model of the induction generator for the phase-domain power flow analysis of fixed speed wind turbine generating systems connected to balanced or unbalanced three-phase distribution systems. The proposed model is established by using a modified bi-quadratic equation, which is generally used for the calculation of node voltages in the power flow analysis of distribution systems. The main advantage of the proposed model is that it does not require slip information. Thus, it has computational efficiency when compared with the slip calculation based phase-domain induction generator model. In addition, for several unbalanced voltage cases, the numerical results of the proposed model, slip calculation based phase-domain model and time-domain d-q model are comparatively evaluated to show the validity of the proposed one.

On the no-load loss of power transformers under voltages with sub-harmonics

Power transformers are traditionally designed for their utilization in sinusoidal voltage and current conditions. However, the non-linear loads are largely proliferated in the modern power systems. Thus, pec (point of common coupling) voltages and line currents usually have harmonically distorted or non-sinusoidal waveshapes. This study focuses on the parametrical analysis of transformer no-load loss under the excitation voltage with several sub-harmonic contents. For this aim, a computationally efficient technique is developed by combining both the harmonic-domain model of transformer windings and Finite Element Method (FEM) based modeling of transformer core. The obtained simulation results figured out that effect of sub-harmonic voltages on the transformer core loss is negligible. However, it is also seen that small amount of sub-harmonic voltages may highly contribute the winding loss under the no-load condition, and they should carefully be handled for the derating of power transformers.