Aleksei Agytin 

To conduct research using the physical model of an electric network with the motor load, the concept is shown in Figure 1. The scheme includes: an isolation transformer, voltage 0,4 / 0,4 kV power 100kVA, the model of supply cables, made in the form of RLC chains and real electric motors, ranging from 3 to 10 kW. The physical model is equipped with the playback device of arc fault in the network, made on the basis of the thyristor T9-200 and its control device.

Figure 1 — Schematic diagram of the physical model of the electrical network with the motor load.

In this circuit, replace the power supply is represented by phase emf, inductance L and resistance R. In the equivalent circuit network is taken into account capacitances (Ca, Cb, Cc) and resistance (Rua, Rub, Ruc) isolation of phases on the ground, inductance-capacitance (m, cm) line to line connections, the capacity of which has an active leakage resistance RT. In the neutral of the transformer can be connected to current limiting resistor RD or arcing reactor LD. High-voltage asynchronous motor is included in the equivalent circuit by phase subtransient leakage inductance L1 and the resistance R1. In one phase of the electric motor can change the location of a single-phase ground fault along the winding through the introduction of variable resistors R11, R12 and the inductance L11, L12. Circuit phase-to-ground fault in the winding engine simulates its capacitance Cz and active resistance of the arc Rz. Zinc oxide surge suppressors (ARF), established on the bus shinah or conclusions of engines, the nonlinearity of the dependence of their resistance to current or voltage [4].

As a result of extensive research conducted with the use of mathematical models for different parameters and the mode of neutral grounding networks sn TPS revealed that the main factor that determines the nature and magnitude of transient overvoltages in OZNZ in a network with isolated neutral is the capacity of phase with respect to land and interphase capacitance, inductance, power supply and transformer, the nature of the load resistance in place of the closure phase on the ground and etc. To limit the multiplicities of surges in the network with the specified parameters have a decisive importance: the value of the instantaneous voltage on the affected phase at the time of the primary arc ignition, the moment of arc extinction and the voltage at the second and subsequent ignition of the arc.

Studies have shown for the different parameters of electrical networks sn TPP maximum surge at the advanced phase after the breakdown of insulation reaches (2.4-2.5) Uf, and subsequent breakdowns of the overvoltage on the healthy phases grows. Escalation (gradual increase) surge in the network by arcing in the second scenario, due to increasing voltage across the neutral in the process of repeated ignition and extinction of the arc fault current in the arc gap. Network sn Thermal power plants, with their characteristic parameters, the magnitude of overvoltage can reach (3.2-3.5) Uf. When a voltage unbalance in the network in phases over-voltage can rise significantly, as studies have established that the multiplicity of the arc overvoltage grows roughly proportional to the displacement of the neutral.

The most common currently a way to reduce accidental effects of single-phase circuit is arcing reactor (DGR), which preserves the benefits of networks with isolated neutral. Effectiveness of the GDR is largely determined by the degree of its setting on the value of the capacitive earth fault current. In this connection the reactor to a neutral network promotes: a significant reduction of heat loss in the arc gap by reducing the earth fault current to the level of the active component of the higher harmonics and unbalance currents and, as a consequence, the willful extinction of the arc, reducing the number of re-ignition of the arc due to a significant reduction speed (up to hundreds of periods of industrial frequency) recovery voltage arc gap after arc extinction, with the exception of repeated breakdowns in the arc gap at a voltage greater phase, which prevents the escalation of overvoltages in the network with resonant neutral grounding, and allows you to save the multiplicity of the arc voltage surge at the first insulation fault ie in the range (2.4-2.6) Uf. However, as shown by our extensive studies, to achieve the expected results only in a strictly symmetric networks with a coefficient of adjustment of the GDR in the range (0.98-1.02), which is well illustrated The calculated waveforms of voltage and current in the network of the GDR in setting it up as close to the resonant.

Figure 2 — Processes for the closure phases C on the ground in a network with the GDR in setting it up as close to the resonant (phase fault current to earth - 30 A)

Detuning compensation significantly reduces the effectiveness of the use of the GDR. For example, the detuning compensation, more than 5% leads to a sharp increase in the multiplicity of surge, and when you set up the GDR in overcompensation mode (recommended PTE) to 25% surge in the multiplicity of the network of the GDR can already achieve 3Uf and more [3]. In the event of a network of voltage unbalance on phases of unbounded multiplicity surge could rise sharply, as here, as well as in networks with isolated neutral overvoltage increases proportional to the displacement of neutral. Growth surge contributes to the fact that the inaccurate tuning of the GDR process of aligning the stress phase after arc extinction is the nature of the beats, the amplitude and frequency of which is determined by the degree of detuning compensation and a quality factor of the oscillatory circuit. When fine-tuning of the GDR or in a small overcompensation, the occurrence of beats in OZNZ possible if you disable connection with a large capacitive current recharge. The danger lies in the beat that for paper-carnival cable insulation, which samovostanavlivaetsya, re-closure of the damaged phase can occur at a voltage close to 2Uf that will lead to a maximum overvoltages on healthy phases.

* List of results is not over, because the master work is being developed, the final version available from the author in January 2012.