Voltage transformers (VT) in electrical networks 6-10 kV ensure the safety of measurements, power supply of electricity meters, equipment insulation control, power supply of operational circuits, signaling circuits, automation and relay protection. Most of the settlement meters are installed at 6-10 kV connections. Therefore, to ensure the reliability and accuracy of operation, increased requirements are imposed on 6–10 kV VTs. First of all, this applies to insulation monitoring TN (TNKI), which, in addition to all the listed functions, must provide protection against damage during ferroresonant processes. Fulfillment of all these requirements leads to an increase in the size of the TNKI and to the complexity of the layout in small-sized switchgear cells. The hope for improving the accuracy of energy accounting through the use of electronic meters and automated measuring systems did not materialize due to high systematic errors. In many respects, this applies to TNKI, the condition of which is the grounding of the neutral of the HV winding. Its earthing would eliminate the main cause of their damage (the flow of significant currents through the HV windings to the ground) and reduce the errors.
Thus, the most urgent task was to create a TNKI that does not have all the above disadvantages, and the most difficult problem was the problem of protecting HP from ferroresonant processes. We think that at the moment the most reliable and effective device turned out to be the device developed in 1985 and having the inventor's certificate for invention N1319158 of the USSR, MKI3 HO2N9 / 04 (Device for protection against resonant overvoltage of a voltage transformer in a network with an isolated neutral / Yu.A. Stepanov , A.P. Kuznetsov, M.N. Ignatiev // Discovery. Inventions. - 1987, N 23). In this device, an inductive element was used, made in the form of a single-phase zero-sequence voltage transformer (TNP), the primary and secondary windings of which are connected, respectively, between the neutrals of the primary and secondary windings of a three-phase voltage transformer and earth.
For transformers of the NAMI 6-10-35 kV and NAMI-10 / 6-95 series (produced since 1995 at the Ramensk Electrotechnical Plant), a zero sequence transformer is also used as an inductive element, which is connected in the same way as the device described above. The difference is that the additional secondary winding 3U0 is located on the core of the zero sequence transformer. On three rods of the first transformer, a compensation winding is placed, connected in a closed triangle without external leads, which contradicts GOST 1983-2001. These deviations from the scheme proposed in the specified inventor's certificate reduce the efficiency of the TNKI, and also determine the increase in errors both in the normal mode and in the single-phase circuit mode.
It is known that in any case, when the neutral is grounded at the HV winding, ferroresonant processes can occur. An example of their manifestation is the situation with transformers NAMI-6 at the Samara CHP and NTMI-6-10 at two step-down substations 35/6 and 110/10 kV in the Samara region.
The processes took place in a symmetrical mode, but without 6-10 kV power lines connected to the buses. They were accompanied by a false signal of a ground fault, strong heating of the resistors connected to the 3U0 winding in NTMI-6-10 transformers (the resistor is not installed in the NAMI transformer), an increase in phase voltages in NTMI-6-10 to 125 V, and 3U0 to 220 B. The values of line voltages of 100 V remained unchanged, the ferroresonance current in this case is less than the rated current of the high-voltage winding of the VT. After disconnecting resistors, relays and devices (switched on at Uph or 3U0) with Uн less than voltages of 125 or 220 V, the transformers worked normally for a long time and at increased voltage values. At the same time, there was no possibility of phasing with other sections.
Ferroresonance stopped after turning on one of the power lines, although according to the method described in the inventor's certificate, to stop ferroresonance, it would be enough to bypass or dehunt the secondary winding of the zero sequence transformer of the TNP.
In 1985-1990 at seven 35/10 kV substations of Samaraenergo OJSC with a total length of 10 kV overhead lines of 40–60 km per section, where NTMI-10 transformers were damaged during single-phase short circuits, TNPs were installed, connected according to the scheme described in the inventor's certificate. Voltage transformers NOM-6 were used as consumer goods. In this case, an automatic scheme of operational circuits was used. After the installation of the antiresonant device, the ferroresonant phenomena were almost instantly eliminated and the damage to the TNKI stopped.
Returning to the already considered transformers NAMI-6-10-35 kV (see Fig. 1) and NAMI-10 / 6-95, it should be noted that their compensation winding current, connected in a closed triangle, has a significant effect on increasing its errors in all cases of the appearance of voltage unbalance of this winding.
The zero operating point of the secondary winding, assembled into a star, is transferred to the point of the secondary winding of the zero sequence - TNP. As a result, the accuracy class, according to the factory data, when measuring phase voltages is reduced to accuracy class 3.
The absence of terminals of the secondary winding of the closed triangle does not allow assessing its technical condition during operation, determining the insulation resistance, the resistance of the windings to direct current, checking the absence of turn short circuits and open circuit of the windings.
This is a violation of the "Norms for testing electrical equipment". For example, there is no possibility of detecting a turn circuit during commissioning and operational checks, which increases the risk of VT ignition.
Voltage transformers NAMI-10 / 6-95 differ from NAMI-6-10-35 kV by the presence on the HV side of an additional compensation winding, the switching circuit of which is similar to the connection diagram of the HV windings of a voltage transformer of the NTMK type. In order to exclude the possibility of damage to the NTMK transformer, the neutral of its HV windings according to GOST 1983-2001 is not grounded, but for NAMI-10 / 6-95 it is grounded. In case of violation of the order of phase rotation on the high side of the VT, the compensation windings will not decrease, but increase the error. It should also be noted that the NAMI-10 / 6-95 factory diagram does not show the connection diagram of the compensation winding connected to a star.
In fig. 2 shows the electrical diagram of the connections of the NAMI-10/6 type insulation control transformer. This transformer is anti-resonant only for the windings of phases A and C, connected in an open delta circuit, since these high-voltage windings are not grounded. However, it is known that the considered circuit excludes the possibility of connecting the load to terminals a-c due to the occurrence of unacceptable errors. The operation of TNKI with unloaded pins a-c is practically unrealistic.
When using the open delta circuit, it is impossible to monitor the insulation condition of the high-voltage electrical network. To eliminate this drawback, an additional transformer is installed in the transformer housing, the high-voltage winding of which is connected to phase B, and its second terminal is grounded.
The low voltage open delta windings are assembled so that in normal operation the geometric sum of the secondary voltages is zero. This is achieved by the fact that the voltages on the ah and cz windings are equal respectively: Uab / 3 and Ucb / 3, and the voltage Ubo is applied to the by winding. Moreover, the polarity of the triangle winding cz is changed. As a result, the vector diagram of the NAMI-10 voltage transformer will take the form, where the sum of the voltages is zero: Uab / 3 + Ucb / 3 + Ubo = 0.
When any of the phases A, B, C is shorted to ground, one of the terms is excluded from this formula and a voltage will appear at the terminals of the open triangle, equal to the geometric sum of the vectors of the remaining two terms of the phases.
The occurrence of ferroresonance phenomena in the normal mode and in the mode of ground fault of phases A or C is, in our opinion, a significant drawback of the NAMI-10 transformer.
In normal mode, when the inductive resistance of the phase B winding and the total capacitive resistance of the electrical network are equal, a ferroresonant process occurs, which, due to low currents, does not cause damage to the voltage transformer, but predetermines an unacceptable increase in phase voltages and voltage 3U0 of an open triangle ** and the appearance of a false signal about earth fault in the electrical network. When a phase A or C earth fault occurs, when the inductive resistance of the phase B winding is equal and the capacitance between phases AB or CB, ferroresonance occurs with possible damage to the transformer.
In addition, it should be noted that the vectors of phase voltages Uao and Uco are obtained artificially - by adding two vectors of other phases, namely: Uao = Ubo + Uab and Uco = Ubo + Ucb. As a result, Uao and Uco will not correspond to their true parameters. The errors of TNKI NAMI-10 in normal mode and under normal load are presented in the table. For accuracy class 0.5, the voltage permissible error limit is + 0.5%, angular +20 '.
In strict accordance with the inventor's certificate for the above invention, a three-phase antiresonant TNKI brand NAMIT-10-2 (produced since 1997 at JSC "Samara Transformer") is produced. In fig. 3 shows the connection diagram of NAMIT-10-2, and Fig. 4 is an automatic diagram of operational circuits.
The normal operation mode of the TNKI is carried out with a closed secondary winding of the TNP by means of the SA switch or in the automatic mode by the contacts of the KL relay. In this mode, the high voltage winding of the TNP has only an active resistance of about 6 kOhm, which reduces the above negative effect of the TNKI operation in the open delta mode with a single-phase short circuit in the electrical network.
With a symmetrical three-phase voltage, due to the non-identity of the impedance of phases A, B, C from the HV side, an unbalance voltage Unb arises at the terminals ad - xd. When the secondary winding of the TNP is opened, the resistance of its HV winding increases to »300 kOhm and, as a consequence, due to an increase in the voltage drop across it, the zero bias voltage Uо of the HV winding increases. This will cause an increase in the voltage Unb at the terminals ad - xd.
When ferroresonance occurs (XL = XC), the secondary winding of the TNP is automatically opened. In this case, the resistance of the primary winding increases to 300 kΩ, the equality XL = XC is violated and the ferroresonance breaks down.
With a single-phase earth fault when shunting and unblocking the LV TNP winding, the voltage Unb at the ad - xd terminals is, respectively, "100 V and" 70-80 V. When the LV TNP winding is bypassed, the current in it reaches 7-8 A, which is less than the permissible value current of this winding. However, this current has no effect on the TNKI load, and, consequently, on its error. TNKI brand NAMIT-10-2, manufactured at JSC Samara Transformer in accordance with Patent N 1319158 for the invention "Device for protection against resonant overvoltage of a voltage transformer in a network with an isolated neutral ".
Authors: Stepanov Yu.A., Kuznetsov A.P., Ignatiev M.N.