Improve support protection devices against electrical corrosion
Moscow Road Experience
Various protective devices have been used for a long time to protect the contact network supports from electrical corrosion. The main requirements for them are reliability in the presence of traction current in rail circuits and operation in the event of a short circuit in the contact network.
Previously, it was believed that these requirements would be fully met with a breakdown voltage of protective devices in the range of 800 — 1200 V. However, until recently, the protective devices did not meet the listed requirements.
For example, spark gaps were triggered by switching overvoltages in the rail circuit at a breakdown voltage of 800 — 1200 V. In most cases, the copper electrodes were then "welded", i.e. they passed traction current to the reinforcement of the support or foundation. Such cases on the Moscow road were noted annually, with an average of 25% of the spark gaps identified.
When the working insulation broke down after several voltage supplies to the contact network, the spark gaps usually ruptured, the support was disconnected from the traction rail, followed by irreversible damage.
Widespread ZD-1 diode earthing devices on three class 8-10 fans connected in parallel worked satisfactorily when the working insulation broke, they broke through from switching overvoltages in the rail circuit and passed current to the support.
The ZD-1 + 2IP diode-spark protection device has the disadvantages of both protective devices due to the low breakdown voltage. In addition, it is expensive, cumbersome and inconvenient to maintain.
The danger to low—resistance supports of currents flowing through group earths was also underestimated - they exceeded the permissible standards tenfold.
Almost all the fractures of the supports in the underground part on the Moscow and Kuibyshev roads in recent years have been caused by overflowing currents with full serviceability of protective devices. Such jars must be removed from the group earthing.
The protective devices currently being manufactured (thanks to the experience gained on the Moscow Road) have been significantly improved. Spark gaps are equipped with aluminum electrodes, which are more difficult to weld with an electric arc if they have two gaps formed in series. Exhaust holes are provided in the spark gap (Fig. 1). Therefore, when short-circuit currents are passed repeatedly, the gap does not break, does not break the circuit onto the rail, although it fails, losing its primary operability.
Theoretical studies and special tests performed at MIIT have confirmed the need to increase the breakdown voltage for these protective devices to 1,500 V In order to gain experience, such an increase is allowed so far only on the Moscow Road. The absence of spark gap breakdowns for several years confirms the need to at least extend a useful recommendation to the road network.
The correctness of this measure is also confirmed by 20 years of experience in calibrating spark gaps on a Moscow road with a voltage of only 1200 V. It gave a quick and steady result: the number of failures of spark gaps due to "welding" of electrodes and switching overvoltages decreased from 25 to 0.5%.
After many years of impeccable operational tests of a batch of diode earthing devices on one 320-amp tablet valve of the 15th class (Fig. 2), and their special testing at the Moscow Power and Mechanical Plant in a short-circuit circuit, they began to manufacture multiple-acting earthing devices on one 1000-amp tablet valve of the 16th class in a depressurized housing. However, by this time, about 600 earthing devices had already been put into operation on the Moscow Road out of 320 ampere valves of the 15th class that had already exhausted their life in rectifier installations, and the plant's choice of such an expensive solution seemed unjustified.
It follows from the above that the need for a protective device ZD-1 + 2IP for group grounding has disappeared after increasing the breakdown voltage to 1200 V and increasing the thermal stability of spark gaps. At the same time, if necessary, it can be made very compact — in the same depressurized housing (Fig. 3). This is all the more important because the ZD-1 diode earthing device has become a "bait" for vandals due to its high content of non-ferrous metals.
And one more requirement for protective devices. Low-resistance supports with disconnector drives that are deafly grounded on the rail are absolutely unacceptable. They are well protected from electrical corrosion by single-vent earthing devices of at least the 15th class.
The poles of the contact network, which have been installed en masse since 1997 on new and updated electrified DC lines, can be connected to the rail tightly. To do this, it is necessary that the insulation resistance of the embedded parts (the same for the clamp reinforcement of the supports) is at least 10 kOhm.
With that said, experts have recognized that it is advisable to focus the efforts of the support protection groups against electrical corrosion on improving the reliability of protective devices and tightening control over its execution. For this purpose, it is necessary:
- set the breakdown voltage for the bulk of the spark gaps to 1200 V, for the pilot batch (up to 100 units) to 1500 V, they must have exhaust ports. ZD-1 earthing devices should be gradually replaced with devices with tablet valves of at least the 15th class. The scope of application of diode earthing devices should be limited to supports with manual disconnector drives, in publicly accessible places and in group earthing devices with low-resistance supports (Fig. 4);
- every year after the thunderstorm season, it is necessary to replace the spark gap liners, marking the replacement date on them (a measure of strict performance control). For low-resistance supports, the serviceability of protective devices should be checked quarterly. After replacing the insert, the spark gap in the assembly must be "molded": with a 1000 V megohmmeter, burn out the dust in the discharge gap, reducing the breakdown voltage;
- to focus the inspection of protective devices, their repair, breakdown voltage calibration, and maintenance in the groups protecting supports from corrosion of repair and revision sections, subordinating them to the deputy heads of the contact network distances. Cases of non-compliance with these measures should be reviewed by the road's power supply service.
The Rules for the Device and technical operation of the contact network are currently being prepared for republication. In our opinion, it would be advisable to include organizational and technical solutions that have proven themselves in operational practice.
Eng. V.A. SAVCHENKO, Moskovskaya Doroga,
Doctor of Technical Sciences V.I. PODOLSKY, VNIIZHT
24.11.2022, 08:58
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