J. R. S. S. Kumara

Over current protection coordination of distribution networks with fault current limiters

The connection of new distributed generator (DG) to the existing network increases the fault current and disturbs the existing over current (O/C) protection coordination. The main focus of this study was to investigate the possibility of using a fault current limiter (FCL) to address the above issues. A case study was carried out for a 20 kV distribution network in interactive power system analysis (IPSA) simulation package. Connection of a DG to the considered network increases the fault current closer to the rating of two switchgears and disturbs the O/C protection coordination. An FCL is introduced and varies its impedance and location of installation to investigate the limiting capability and effect on protection coordination. The modifications required to O/C protection scheme for proper coordination were also investigated. The study proved that when locating the FCL near the DG, it limits the fault current while minimizing the problems associated with the protection

Spectrum sensing in cognitive sensor networks

In cognitive sensor networks, the first cognitive task preceding any form of dynamic spectrum management is the sensing and identification of spectrum holes in wireless environments. Research in the physical and medium access control layers for DSA (Dynamic Spectrum Access) and DSS (Dynamic Spectrum Sharing) is currently at its early development. Aspects such as spectrum sensing and spectrum management greatly support the future improvement of DSA and DSS in cognitive sensor networks. This paper expresses the necessities of spectrum sensing and the spectrum sensing framework for cognitive sensor networks and it describes several methods used for evaluating specific spectrum sensing techniques. The sensing techniques describe in this paper provide an effective and efficient spectrum sensing architecture to identify and locate spectrum holes in the signal spectrum.

Ultra Wideband (UWB) antenna design for cognitive radios in the UHF TV band

Presently, Ultra Wide Band (UWB) radio technology has attracted much interest in academics, industrial and standardization (IEEE) activities. UWB characterizes transmission systems with instantaneous spectral occupancy of higher bandwidth or higher fractional bandwidth. The antenna is probably the most overlooked part of a RF (Radio Frequency) design. The range, performance, and legality of a RF link are critically dependent upon the antenna. During this work, the UHF (Ultra High Frequency) TV Band is specifically addressed which is a target band of IEEE802.22 standardization. The UHF TV band is 336MHz wider according to CCIR (Consultative Committee on International Radio) standards. One major challenge in designing a UWB antenna for UHF band is limiting the physical size of the antenna. The paper illustrates the successful design, implementation and testing of a UWB antenna for cognitive radios in the UHF TV Band. The antenna is simple, low cost and compact enough to be used in fixed transceivers.

Moisture estimation of transformer pressboard by micro-strip ring resonator at GHz frequencies

Increased moisture content (MC) is considered as one of the main factors for the failure of transformer insulation. This paper presents a method of estimating the MC of a transformer pressboard insulation based on the relative permittivity and loss tangent measurements performed at GHz using a multi-layer micro-strip ring resonator. It was fabricated on a material of known dielectric properties and the sample to be tested (i.e. pressboard) was placed on the ring structure. The dielectric properties, namely loss tangent and permittivity, of the sample were then estimated by measuring the S21 transmission characteristics of the ring structure using a vector network analyzer. The technique was validated using a test sample of known dielectric properties. Additionally the results were compared with traditional frequency domain dielectric spectroscopy (FDS) measurements conducted at 1 mHz -1 kHz. The measurements were performed on oil impregnated and non-impregnated pressboard samples having different MCs conditioned by drying and/or wetting processes. The actual MC of the pressboard samples were obtained by weight measurements. It was found that the resonance frequency increased when the MC was reduced. Furthermore, a positive non-linear relation could be observed between the MC and both the relative permittivity and the loss tangent. It can be concluded that the proposed method could be used to estimate the MC of pressboards.

Remote leakage current detector for identification of insulators discharges

Performances of high voltage insulators can be evaluated by analyses of leakage current (LC) patterns. When insulators are polluted, the development of the LCs may lead to discharges causing insulator flashovers. This paper proposes a simple method of identifying the state of discharge activities through remotely sensed LCs. A remote detector, which is capable of detecting LC by capturing resultant electromagnetic field, was first developed. Preliminary tests were conducted at the laboratory considering typical near-by interference effects on polluted insulators such as effects due to corona and line current in order to test the robustness of the proposed system under the said interference effects. The tests were extended by obtaining different LC patterns by conducting salt-fog spray tests on five different types of clean insulators (made of porcelain, glass and silicone rubber), at two salinity levels (10 g/L and 40 g/L) and six different voltage levels (50%–300% of rated voltage). LCs were also measured on five different field polluted insulators through clean fog tests. In order to verify the proposed method, LCs were measured directly for all the test cases. The directly measured LCs were classified in to three classes: sinusoidal, non-sinusoidal and discharge. The remotely sensed LCs were also classified to these three classes using a kurtosis analysis. The statistical measure of kurtosis was observed as a viable and robust parameter to distinguish these three classes based on the remote measurement. The sinusoidal class had kurtosis values less than 5; discharge group had kurtosis greater than 12; the non-sinusoidal class had kurtosis values between these two levels. Hence, this paper suggests the possibility of using kurtosis values of remotely sensed LCs to identify the state of discharge activity prior to the insulator flashover remotely while proving to be robust under typical interference effects.

Aging assesment of transformer pressboard insulation by micro-strip ring resonator at GHz frequencies

Condition assessment of insulation is widely used to identify the state of aging of power transformers. Traditionally, condition assessment is done using dielectric response measurements conducted at low frequencies. However, due to long testing time required for these methods, attention has now been paid towards measurements at microwave (above 1 GHz) frequencies. In this paper, we establish a correlation between dielectric properties and the state of aging of transformer pressboard samples. This is useful as it paves way to develop techniques to assess the state of aging by measuring dielectric properties at high frequencies. Dielectric measurements are made by using a microstrip ring resonator. The state of aging is determined by measuring the tensile index. Tests were carried out on dry and wet oil-impregnated pressboard samples thermally aged at laboratory conditions for durations up to 45 days. The results were compared with field aged samples collected from a 33 kV sealed type distribution transformer of 18 years in service. It was found that the permittivity values had a good correlation with respective tensile index values whereas loss tangent values showed limited correlation for both dry and wet samples. The results of field aged samples matched well with the established correlation curves. It can be concluded that the permittivity of pressboard samples estimated from the ring resonator can be effectively used to evaluate the state of aging of transformer insulation.

Design and Testing of an TCSC for Distribution Network Applications

Connection of distributed generator (DG) into very weak points of the electricity distribution network is increasing. These connections impose many operational issues and therefore utilities restrict DG connections to the grid. In this study an application of thyristor controlled series compensator (TCSC) to overcome some of these issues are investigated, especially voltage control at the point of connection and its fault limiting capability. The TCSC was designed for a prototype network and implemented in the laboratory. The expected performance of the model was investigated with PSCAD/EMTDCcopy simulation and verified with experimental results. Application of an TCSC with a distributed generator (DG) for a real scale network was studies using PSCAD/EMTDC simulation to investigate the performance of the TCSC as FCL and series compensator