Event under the auspices of the Ministry of Economy of Slovak Republic

Use of the nonlinearity of the arc resistance for the designing main and backup relay protection

*Vladimir Nagay
Department "Electric stations and electric power system

Sergei Sarry
Department "Electric stations and electric power systems" Platov South-Russian State Polytechnic University (NPI)

Ivan Nagay
Department "Electric stations and electric power systems" Platov South-Russian State Polytechnic University (NPI) - «NPP «RELDON» Ltd

Pavel Kireev
Department "Electric stations and electric power systems" Platov South-Russian State Polytechnic University (NPI)

Alexander Ukraincev
Department "Electric stations and electric power systems" Platov South-Russian State Polytechnic University (NPI)


     Last modified: 2017-06-16

Abstract
Often, faults in the electrical grids of modern power systems are accompanied by the appearance of an electric arc, which poses a significant danger for high-voltage equipment and maintenance personnel. The presence of a arc resistance at the fault location leads to a decrease in the short-circuit current and an increase in the resistance of the short-circuit and, accordingly, to a decrease in the sensitivity of the current and impedance protections. At the same time, the presence of a arc resistance, which has a non-linear character and its unstable parameters in time, makes it possible to use this as additional information signs when designing both main and backup protections. Taking into account the nonlinearity of the electric arc resistance in the algorithms of the protection function increases its technical perfection.
The successful solution of the problem of designing the protection of the frame structure (for example, indoor switch-gears) or overhead lines, transformers depends to a large extent on the choice of the principle of their operation and the optimal choice of information signs of a regime. The appearance of an electric arc is accompanied by a change in the parameters and characteristics of the electrical grid, a significant increase in temperature, pressure, electrical conductivity, and thermal (light) radiation at the point of arc fault. These characteristics can underlie the methods of identifying the electric arc, which can be conditionally divided into two groups. To the first group, it is possible to classify methods based on the control of parameters and characteristics of an electrical network with an arc: the spectral composition of the current or voltage, the modules and the arguments of the phase and symmetrical components of the currents, voltages, unbalance levels of currents or voltages, resistances, or current-voltage characteristics of the short-circuited circuit.
The second group includes methods based on controlling the parameters of the electric arc and its attendant phenomena: temperature, pressure, electrical conductivity of the environment (the degree of ionization of gases), optical properties of the space, electromagnetic radiation. The operation of the devices in this group is practically not affected by the operating regimes of the grid.
A detailed account of the arc resistance in the form of nonlinear resistance makes it possible to expand the information base of relay protection and design relay protection with higher information and technical perfection.
The representation of the arc resistance by a nonlinear characteristic makes it possible to use the control of the ratios of the symmetrical components of the positive and negative current sequences in the synthesis of the algorithms for the operation of relay protection.
The sensitivity of the main and backup protections should be checked taking into account the reduction of the short-circuit current, the increase in the resistance of the short-circuit and the voltage drop on the arc column. In this case, it is necessary to take into account the possible reduction of up to 25% of the short-circuit current and the increase in the resistance of the short-circuit to 25% when the voltage across the arc column drops to 30% of the rated voltage of the protected electrical equipment. The possible change in the arguments of the current and resistance can be as high as 30 °.

 

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