Abstract
There is usually no information about permissible error of current transformers in modes succeed by large relative short-circuit current, at which microprocessor-based protections operate stably. By this reason, it is necessary to use data, defined for analog relays. It leads to value appreciation of a project because it is often essential to reduce current transformers’ error in case of a short circuit fault. Therefore, it is necessary to define the value of current transformers’ error, permitted for impedance relays. Conclusions of fundamental and applied sciences (mathematical analysis, theoretical foundations of electrical engineering, theory of simulation), analytical methods of researching nonlinear circuits and digital signal processing were used. A simulation model was created for setting overall tests of the current trans-former (CT) system. It was a relay protection device that reflected all the important properties of studied objects and allowed an analysis of digital distance protection’s operation at high levels of short-circuit currents. The factors influ-encing over digital distance protection’s operation in case of deep saturation of CTs were revealed, and a certain algorithm for definition of the permissible CT errors was proposed. Stable operation of digital distance protection was observed in case of a fault nearby the place of current transformers’ setting in all theoretically possible combinations of electrical system’s power and length of a protected electric power transmission line. It is valid if electric load choice is carried with account for stable protection’s operation in condition of a fault in the computational point and if voltage swell in secondary wirings is infeasible.
Highlights
To date, microprocessor-based digital protections are widely used [1, 2]
The essence of the problem is that permissible error of current transformers, valid for analog impedance relays, is not justified in relation to digital protection, but is often used due to the lack of other data
Such method leads to increasing of protection’s operation stability. By this reason, calculated secondary current trans-former (CT)-load may seem excess, but it is permissi-ble for digital protection if the calculation is carried as for analog relays
Summary
Microprocessor-based digital protections are widely used [1, 2]. Such protec-tions have better characteristics compared with analog protection systems. The essence of the problem is that permissible error of current transformers, valid for analog impedance relays, is not justified in relation to digital protection, but is often used due to the lack of other data. Such method leads to increasing of protection’s operation stability. By this reason, calculated secondary CT-load may seem excess, but it is permissi-ble for digital protection if the calculation is carried as for analog relays. There is no necessity to assume additional valuable measures to reduce current trans-formers’ error
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