Improving the reliability of the operation of measuring voltage transformers in networks with isolated neutral

Most electrical networks of 6-35 kV in the CIS countries are made with isolated neutral. These networks, at certain earth fault currents (for Un=35 kV - ≥10 A; Un=10 kV - ≥20 A; Un=6 kV - ≥30 A), as a rule, must have reactor or resistive neutral compensation.

The main advantage of networks with an isolated neutral is the ability to provide consumers with electricity for a long time even if there is a "ground" in the network without disconnecting them. At the same time, one of the main drawbacks is the risk of occurrence (at low earth fault currents equal to 0.5÷3.5 A) ferroresonance processes with subsequent damage to electromagnetic inductive voltage transformers (VT).

Ferroresonance processes (FRP) in such networks, as shown by the experience of operation and research conducted by many scientists, occur during the appearance and breakage of the "earth" in the network (activation of arresters, touching tree branches, breakage of the cable of power transmission line phases, dripping of dew drops over insulators, especially dirty, some switching operations leading to a change in capacitance in the network, etc.). In most cases, these ferroresonance processes take place at frequencies of 17 and 25 Hz and are accompanied by overcurrents flowing through the primary winding of the voltage transformer, which exceed the currents allowed for the voltage transformer by an order of magnitude or more, due to which the primary windings burn out within a few minutes. In operation, there are cases when initially two or three times (after replacement) a high-voltage fuse from 6 to 35 kV blows out, designed for a rated operating current ≤2 A (this is despite the fact that the permissible current of the primary winding of the voltage transformer does not exceed 60 mA) and damage the voltage transformer. Thus, there are repeated flows of high currents through the winding of the voltage transformer in excess of the allowable ones, which gradually, due to overheating of the internal layers, lead to the decomposition of the insulation and damage to the voltage transformers.

Currently, judging by the publications, a lot of work is being done to protect voltage transformers from damage in networks. However, each of the proposed methods has its drawbacks and is not able to completely solve the problem of protecting voltage transformers from the effects of FRP ferroresonant processes. In addition, there is no possibility of fixing the appearance of ferroresonant processes in the network section from the voltage transformer.

From this point of view, the most effective way to suppress (and most importantly fix the time and duration) of ferroresonance processes is a resonance suppression device (RCD) developed for electrical networks of the PZFR-1 type (Fig. 1, 2).
When ferroresonance occurs at the terminals of the “open triangle” winding of a three-phase voltage transformer (or a group of three single-phase voltage transformers), a zero-sequence voltage of 3U0≈100 V occurs with a subharmonic frequency (most often 20÷25 Hz). After the appearance of a voltage with a subharmonic frequency, the PZFR-1 device, with a specified time delay, once connects a 5÷6 Ohm resistor to the outputs of the “open triangle” winding for the time specified to extinguish the FRP. The connected resistor ensures the disruption (quenching) of ferroresonant oscillations for t ≤0.3 s, which eliminates the possibility of thermal damage to the HV VT windings by ferroresonant processes.

Figure No. 1 Schematic diagram of the PZFR-1 ferroresonance protection device


TV - voltage transformer;
T - step-down transformer;
D - operational amplifier;
BMK - microcontroller unit;
VT - optothyristors;
R is a resistor;
D - display;
BU - control unit;
BP - power supply;
KL1, KL2 - signal relays

The PZFR-1 device has a one-time switch on for a specified time with repeated readiness for operation after a specified time. In case of long-term ferroresonance, a repeated one-time operation of the device is provided, followed by prohibition (blocking) of the damping pulse until the ferroresonance is eliminated, after which the device will again be ready for operation.
This ensures the thermal resistance of the resistor during repeated frequent starts of the device (for example, with an alternating arc, frequent ground faults of the network wires with tree branches, gusts of wind, etc.). The device generates an archive and displays the last 5 ferroresonance modes on the display (device actuations). In the “accident archive” of the device, information is accumulated on the date and time of the emergency conditions that occurred, which gives the operation additional information about the state of the network in a particular mode, and by analyzing the “archive” it is possible to take measures to improve the reliability of the network as a whole.

Figure No. 2 Wiring diagram for the PZFR-1 ferroresonance protection device