Monophase earth-faults (MEF) are the most widespread damage type in the middle voltage class electric networks. In most cases, in pole-to-pole short circuits or many-placed isolation breakdowns with group electric equipment failure are developed, being accompanied by the big financial damage. Therefore fairly it is considered, that the basic struggle direction for electric networks work reliability increases is struggle against single-phase ground short circuits. For this reason in the technically developed countries majority, including Ukraine, works on electric equipment functioning condition perfection in distributive networks now are intensively conducted. Thus various researchers offer approach ways rather contradicting each other to the specified problem decision. So one specify in existing directions perfection necessity with use compensating ground-fault neutralizer, and others suggest to transfer considered networks to other neutral grounding modes a etc. All it specifies in an insufficient processes scrutiny level at arc ground short circuits of a phase and offered decisions practical realisation problems misconception. To my mind, the specified problem optimum decision can be found only on the basis of transients course character electrophysical bases deep understanding at arc ground short circuits with the account all influencing factors.

          Devices of the relay relay protection produced by industry from earth-faults (relay of RTZ – 50, RTZ – 51, ZZP – 1), the excess-voltage suppressor(EVS) and executable on their basis projects do not decide foregoing problems, in this connection, for the work reliability increase the FDK power supply system the leadthrough of the proper calculation-experimental research and new effective decisions developments, directed on the marked higher problems removal is required. This work is devoted to these questions.

        Existing relay protections and their failings

        Electrical scheme of «Rudnichnaya» substation 6 kV class is a model for mine – metallurgical enterprises, which main users are walking excavators. Such schemes and set on them relay protections from earth-faults lacks are frequent earth-faults which are accompanied by disconnecting of one damaged line and two – three uncrippled. Damaged line searching and prefault scheme repeated renewal requires very long time. The indicated failings registered in works [12,13]. One of relay protections unselective work reasons is an ferromagnetic resonance origin after damaged equipment disconnecting, from – after sharp voltage transformers magnetizing currents increase. Resonance processes flow at 3Uo voltage 16 – 25 Hertzs frequencies, i.e. below than 50 Hertzs frequency, while the 3Io current has near to nominal frequency. As a result there is a of the sensible relay protections ZZP – 1 and other directed action loss. Nondirectional current relays are more steady to the resonance processes (RT – 40/0,2, RTZ – 50, RTZ – 51). However last relays are very hardness to co-ordinate wearing-outs on setpoints, if the separate joinings own capacity earth-fault currents strongly differ between itself.

        Before to examine the improved relay protection, I will mark the industry series-produced relays used on the enterprises features.

        Taking into account that in many cases the MEF currents values can piece out ampere units and even parts, in a ZS relay protection current, as a rule, the current relays with the Ioc.z.min small values are used, for example, electromagnetic current relays type of RT – 40/0,2, or the special relays for MEF protection: electronic type of RTZ – 50 (it is presently taken off from a production) and microelectronic type of RTZ – 51.

        Electronic current relay type of RTZ – 50 is developed by CHEAZ at the end of 60th for current electromagnetic relay type of ETD – 551/60 with the small wearing-out current (Ioc.z.min=0,01 A) replacement, used in the relay protection from MEF schemes. Principle RTZ – 50 relay scheme is established on a fig. 1.

        A relay activates second converter, consisting of a co-ordinate transformer T and rectifier VC1, twocascade direct-current strengthener UPT, executive branch (output intermediate relay K) and power source BP. A relay has three wearing-out ranges on a current: 0,01 – 0,02; 0,015 – 0,03 and 0,03 – 0,06 A.

        Entrance relay resistance Zp depends on an entrance current and is equal in the first settings range to 50 – 100 Ohm, in the second range to 30 – 50 Ohm, in the third range to 9 – 11 Ohm. Wearing-out range current settings switching is carried out by tap on a co-ordinate transformer. The smooth setpoints change within the every range limits is carried out by the amplification factor change UPT by the resistor R7. A spark-gap FV is intended for scheme elements protection from overstrains at double earth-faults. The resistor R1, included consistently with the co-ordinate transformer T primar, provides thermal stability of entrance chains of current at double earth-faults. The condenser C1 smooths out the voltage pulsations on the rectifier bridge VC1 output. The diode VC1 serves for UPT entrance signal level limitation. A relay feeding can be carried out from sources both permanent 110 220 V and variable ~100 V operative currents. A relay is executed on an electronic base.

        Taking into account all existent MEF protections relay dignities and lacks, and also Dokuchaevskiy FDK power supply system features, it is put improved relay protections development task with the serial produced current relay RTZ – 51 use.

       In durance vile it is desirable to mark that on the prepared master's degree go there will be done such work:

        1. The relay protections from earth-faults unselective work reasons analysis is produced and researches leadthrough directions are chosen.

        2. An algorithm is developed and the PC calculation program is created which allows to design transient and phase earth-faults steady state in networks with the insulated neutral.

        3. With the developed program use, as it applies to the substation «Rudnichnaya» power supply system, overstrains modes, SC currentsare calculated and capacity currents are certain at the 2th section 6 kV phase earth-faults, that allowed to carry out the earthen relay protections feeder action selectivity estimation.

        4. The substation «Rudnichnaya» 6 kV network on voltage 380 V physical model is created. By this model is possible to carry out relay protections from earth-faults feeders setpoints calculation verification and to check up the earthen relay protections action rightness.

        5. On the basis of the conducted calculation and experimental researches technical solutions are offered on selective earthen relay protections introduction and ferroresonant processes suppression in the substation «Rudnichnaya» conditions.

        6. . Calculations on a reconstruction and neutralgenerating transformers introduction for the effective ferroresonant processes removal, relay protections action selectivity increases and overstrains multipleness in a substation «Rudnichnaya» network 6 kV decline are executed..

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