Matislav Majstrović

FACTS-based reactive power compensation of wind energy conversion system

Voltage control and reactive power compensation in a distribution network with embedded wind energy conversion system (WECS) represent main concern of this paper. The WECS is of a fixed speed/constant frequency type that is equipped with an induction generator driven by an unregulated wind turbine. The problem is viewed from short-term (10 seconds) and mid-term (10 minutes) time domain responses of the system to different wind speed changes. Being disturbed by a variable wind speed, the WECS injects variable active and reactive power into the distribution network exposing nearby consumers to excessive voltage changes. In the FACTS-based solution approach, the unified power flow controller (UPFC) is used at the point of the WECS network connection to help solve technical issues related to voltage support and series reactive power flow control.

Statistical analysis of particles of conductor clashing

Fire ignition as a consequence of conductor clashing has happened in many countries all over the world. These fires can cause severe environmental (forest fires) and financial damage, and even be potential life-threatening. The goal of the article is to describe the processes which occur when two live conductor clash together. The most dangerous product of conductor clashing are particles (sparks), which fall to the ground, while being hot enough to potentially start a fire. Primary, by defining of those sparks we can do the first step which would lead to the answer: “Is conductor clashing the cause of fires?” Conductor clashing was simulated in two environmental conditions. The first one was in a live low voltage electricity distribution network as line-to-line short circuit and the second one was in laboratory conditions. Al/Fe conductors of the same characteristics were used in both simulations. Simulations were recorded with high speed camera. Statistical analysis of particles and their probability density function (PDF) are presented in this paper. PDF calculation may be a part of additional criteria on power system protection adjustment.

Current reduction factor of compensation conductors laid alongside three single-core cables in flat formation

The three-phase cable line consists of three single-core cables, which can have conductive sheaths, grounded at either one end only, or at both ends. If they are grounded at both ends, currents flow through sheaths of single-core cables during both balanced and unbalanced loads and the line-to-ground short circuit in a grounded network. During the line-to-ground short circuit, currents flow through cable sheaths and through the ground. When cable sheaths are grounded at one end only, currents do not flow through the sheaths and the influence of the short circuit current on neighboring electrical circuits is greatest. Compensation conductors are used to reduce this effect. Reduction effect of compensation conductors laid alongside three power single-core cables in flat formation is analyzed in this paper. Calculation methods of current reduction factor are also described.

Determining currents of cable sheaths by means of current load factor and current reduction factor

A new method for determining sheath currents as the consequence of electromagnetic coupling during line-to-ground short circuit is described in this article. This method is based on using both the current load factor and the current reduction factor. The model cables are selected to represent major constructions encountered in practice. The cable line consists of three single-core cables laid in trefoil formation and touching each other. Cable sheaths are grounded at both ends. Current load factor and current reduction factor are characteristic data of the analyzed cable line. The method presented in this paper is easy to use.

Calculation Of Losses In Electric Power Cables AsThe Base For Cable Temperature Analysis

Power losses refer to the heat generated in cable conducting parts (phase conductors and sheaths) and in cable insulating parts. It is necessary to know the exact data regarding heating powers for the calculation of heath transfer and cable temperatures. Heating power in phase conductors and sheaths mainly depend on current values. Exact calculation of those powers is very difficult. This paper develops a mathematical model of heating power calculation in three phase single-core cable conductors and sheaths. This model is used to determine filament currents and heating powers in phase conductors and sheaths. Geophysical features of the cable route are also considered. Three-phase single core electric power cables of 35 kV rated voltage are taken as an example. Two laying conditions (trefoil and flat formation) are considered. Sheathes are bonded and grounded at both ends. Calculation results for cables in flat configuration show that heating powers in cable sheaths do not have equal magnitude and increase with distance. The least heating power occurs in the sheath of the middle cable. Heating powers in sheaths of outer cables are of unequal magnitude too

Modified dynamic programming method for reactive power compensating device allocation

The main topic of this paper refers to reactive power compensating device allocation. By means of modified dynamic programming method, as proposed in this paper, optimal rated power and location of a compensating device are obtained in order to simultaneously satisfy voltage limit constraints, minimize active power loss and avoid voltage collapse in transmission network. In other words, it is necessary to unify all three criteria and find single optimal solution. Software package OPTLOK is developed on the basis of proposed method. At the end, an example given for the realistic transmission network of Croatia is exercised. Croatia, as well as some other South East Europe countries is expected to face serious reactive power compensation problem after the UCTE reconnection that happened recently.

Influence of earthing conductors on current reduction factor of a distribution cable laid in high resistivity soil

Various types of system circuit faults are possible. The most common is the phase-earth fault. During this fault, currents flow through earthing grids as well as through the earth. These currents are the consequence of electromagnetic coupling of conductors and earthing grid potentials and can be presented by current reduction factor. When earthing conductors are laid together with cables, the current reduction factor is very difficult to define. In this paper a mathematical determination model of current reduction factor is presented for the cable and earthing conductors laid in the same trench. Influence of both electromagnetic characteristics and earthing conductor geometrical characteristics as well as influence of geophysical features on the current reduction factor is analyzed in this paper.

Comparison of Aluminum and Copper Particle Critical Diameter Produced in Overhead Line Conductor Clashing

The issues of hot metal particle eruption due to overhead line conductor clashing and potentially ignition of cellulosic fuel beds under the transmission line have not been sufficiently explored although conductor clashing is often considered as fire cause. At the point where contact or electric arc between two conductors on different potentials is established, electric energy is converted into heat energy so the large amount of generated heat can cause melting and vaporization of conductor material. Some of ejected sparks will ignite and burn, while others will simply fall to the ground, cooling off on the way by convection and radiation. The critical diameter is the least diameter of the particle caused by conductor clashing that will be sufficient to ignite the biomass on the ground. The results show that the copper particles in the same conditions bring a greater ignition risk due to their higher heat capacity.

The impact of flywheel torque of refurbished generator units on the limit switching time in Croatian power system

In a refurbishment project, possible solutions for old generators in Zakucac and Peruca hydroelectric power plants (HPP), Croatia, are suggested. In the first step, transient processes caused by 3-phase and phase-earth short circuits on all connected transmission lines to Zakucac and Peruca HPPs were analysed on the different power system models. Many computer calculations of transient stability in Zakucac and Peruca HPP and dependent environment of the power system with refurbished generators in these plants were made. Since transient stability in the power system with recommended characteristics of refurbished generator units were satisfied, the impacts of smaller flywheel torque on limit switching time values were considered in a second step.