Krzysztof Solak

Fuzzy Adaptive Transmission-Line Differential Relay Immune to CT Saturation

This paper presents an enhanced current differential protection scheme with fuzzy inference system (FIS) support for high-voltage transmission lines. The proposed adaptive fuzzy scheme combines strengths of current and phase comparison protection criteria. The relay stabilization characteristic is adapted online depending on the output of the fuzzy reasoning scheme supplied with information from the phase comparison unit. The resulting protection is characterized by improved selectivity for external fault cases as well as when nearby transformer inrush currents cause the saturation of current transformers installed in the protected line. The performance of the fuzzy protection scheme has been tested with the signals generated with the use of Alternative Transients Program-Electromagnetic Transients Program and compared to the traditional solutions.

Differential protection of converter transformers

In this paper, development of the special converter transformer model (with 5 windings) and testing results of its differential protection is presented. The special converter transformer is mainly used for supply of rectifiers or converters, FACTS devices etc. This type of transformer can introduce non-standard fixed phase angle shifts such as 20o, 15o, 12o, 10o, etc. this is obtained by unique connections of the secondary windings (e.g. polygon, zig-zag, extended delta). The converter transformer must be protected by a relay being able to compensate for non-standard phase shifts. Otherwise the error magnitude would be very high (0.34 pu for 20o phase shift) and problem with sensitivity and stabilization of protection can take place. An appropriate simulation model of the converter transformer with five windings has been developed in MATLAB/Simulink environment. Next, the testing results of differential protection (7UT87 Siemens device) operation for various disturbances are presented. The testing was performed with use of Omicron tester and prepared simulation files in COMTRADE format.

Numerical modelling of ground faults in MV networks for reliable testing of protection algorithms

The paper addresses the problems of digital modelling of MV networks with the focus on phenomena having a significant impact on the reliability of resulting data subsequently used for testing protection relays. In ATPDraw program the Ferranti current transformer (core-balance current transformer) model was developed that allows taking into account the composite error that is due to discrepancy of magnetic coupling between the cable cores and the secondary coil as well as to the magnetizing current of the transformer. The impact of higher harmonics in the supplying voltage on algorithm performance in pre- and fault conditions has been investigated as well. The test results obtained for data without mentioned disturbances have been compared with outcomes for faults in the system with current transformation errors and harmonics in the pre-fault MV network signals.

Modeling and analysis of the single-core phase shifting transformer and its differential protection

In this paper, the analysis and simulation of a single-core asymmetrical phase shifting transformer (PST) is presented. PSTs are usually used for varying the voltage phase angle between the two systems, which provides controlling active power flow through transmission (tie) line. The essential part of this work was related to developing an appropriate simulation model of the single-core phase shifting transformer in MATLAB/ Simulink environment. In addition, this paper presents testing results of a selected PST differential protection scheme (proposed in the literature) for various disturbances. The testing outcomes provide useful information about limitations of the selected algorithm and create basis for further research.

Verification of new protection algorithm for compensated MV distribution systems

In the paper authors propose the application of optimization techniques to developing filters dedicated for computation of criterion quantities suitable for distribution feeders protection. The fault detection algorithm has been investigated on the basis of data acquired from the physical model of the compensated MV distribution network and data from computer simulations in EMTP. The investigated algorithm is based on computational scheme with linear filtration that is used for analyzing fundamental component and high frequency components induced in compensated MV network during arcing faults in faulty and sound feeders. Since proposed structure realizes linear filtration and is composed of only five computational units, it is easy to interpret the final decision on the state of the feeder. The relaying scheme requires much less computational units when compared to approaches proposed so far. It is highly selective and secure.

Differential protection of single-core symmetrical phase shifting transformers

The development of the single-core symmetrical phase shifting transformer (PST) model and testing results of its selected differential protection is described in this paper. Generally, a PST is used for controlling the active power between the two systems being connected with one or more parallel transmission lines (paths). Most types of PSTs allow for changing of phase shift in a certain pre-defined range and thus the differential protection must on-line compensate for the phase angle shift introduced by a PST. Otherwise the error magnitude would be very high (even 1 pu) and problem with sensitivity and stabilization of protection can take place. An appropriate simulation model of the single-core symmetrical phase shifting transformer has been developed in MATLAB/Simulink environment. Next, the selected PST differential protection (proposed in the literature) was tested for various disturbances. The simulative testing results bring information about limitations of the selected algorithm and create basis for further research.

EMTP testing of selected PST protection schemes

In this paper, testing results of selected procedures for protection of phase shifting transformers are presented. A phase shifting transformer (PST) is usually used for varying the voltage phase angle between the two systems which provides controlling of active power transfer. Some types of PSTs (e.g. dual-core symmetrical PST) allow to control phase shift in a certain defined range which means that phase shift between the two systems can be varied on-line. Therefore this type of phase shifting transformers must be protected by relays which are able to on-line compensate for additional phase shift introduced by PST. The main attention is paid to testing of differential protections for transformer energization and external faults for different phase shift values. It has been observed that for such situations standard differential relays may sometimes maloperate. Therefore, these situations should be analyzed and suggestions for protection improvement proposed. The comparison of two protection arrangements has been realized with the signals generated with use of EMTP program.

Fuzzy adaptive transmission line differential protection with improved stabilization for near-by transformer inrush cases

In this paper enhanced differential current protection scheme dedicated for HV transmission lines is described. The adaptive fuzzy relay scheme is proposed that combines strengths of both current and phase comparison protection criteria. The relay stabilization characteristic is adapted on-line depending on the output of the fuzzy reasoning scheme supplied with information from the phase comparison unit. The resulting protection is characterized by improved selectivity for external fault cases as well as when near-by transformer inrush currents cause saturation of CTs installed in the protected line. The performance of fuzzy protection scheme has been tested with the use of Real Time Digital Simulator (RTDS) and compared to the traditional solutions.