Measurement of 6 (10) kV Bus Sections Voltage Using Capacitive Voltage Dividers

One of the problems in an electrical network with an isolated (compensated) neutral is the selective determination of a damaged element during a single-phase earth fault. Selective operation of protections against such damage can be ensured on the basis of the analysis of zero-sequence currents, phase voltages and zero-sequence voltage of the bus section from which these feeders are powered. As operational experience has shown, selective operation of protection against single-phase earth faults in such networks can be ensured only with the use of digital devices. There are no voltage transformers at many substations and distribution points of the energy systems of Ukraine with a voltage of 6 (10) kV. Therefore, it is not possible to use modern digital protection devices at such substations, which require information about the voltages of the bus sections. In addition, at modern transformer substations, taking into account the compact placement of equipment in them, it is physically impossible to additionally install traditional electromagnetic voltage transformers. Thus, to measure high voltage, it is necessary to use other methods of voltage measurement. One of these methods of measuring high voltage is the use of capacitive voltage dividers. A physical model of high voltage measurement was developed at the Department of Electric Power Engineering and Control Systems of Lviv Polytechnic National University using capacitive voltage dividers used in voltage indication systems on bus sections of substations with a voltage of 6 (10) kV. These capacitive voltage dividers were developed by the RELSiS company. After successful testing of this system on a physical model, it was put into experimental and practical exploitation at the operating substation RP-10 of PrJSC "Lvivoblenergo". The system was used for the operation of digital protection against single-phase earth faults in grids with an isolated (compensated) neutral "Altra". In "Altra" device, analog signals (voltages of bus sections and zero-sequence currents of feeders) are converted by an analog-digital device into a digital code. After that, the signal is filtered at the digital level - the main frequency signal is selected. Fourier transformation is used for digital filtering of the signal. Zero-sequence voltage value is calculated directly in the digital device based on the sine and cosine components of the phase voltages of the Fourier transform. The positive experience of experimental and practical exploitation of capacitive voltage dividers at the operating substation of PrJSC "Lvivoblenergo" makes it possible to recommend their use at other similar substations and distribution points of electrical networks with a voltage of 6 (10) kV, where there are no traditional electromagnetic voltage transformers.