Experiences with the Application of TZD-type Low-Voltage Arresters in an Open-Circuit Earth-Fault Protection System

In the late 1990s, the company KOLEN developed the unidirectional low-voltage arresters TZD-1N and TZD-1R, which were implemented in the group earthing system of an open-circuit traction network. The solutions applied in these arresters were patented (patent no. 170994).

years of research on the group earthing system, conducted by CNTK (now the Railway Institute – IK), resulted in the development of guidelines in 2001 titled: “Guidelines for the Design and Operation of an Earth-Fault and Electric Shock Protection System with Group Earthing in an Open-Circuit Configuration on Railway Lines.”

According to these guidelines, three arresters are used in one group earthing section on railway lines: on one side, two arresters (TZD-1N and TZD-1R) are connected in parallel, and on the other side, a single TZD-1N arrester is used. At railway stations, four arresters are used in a group earthing section, with two on each side.

The results of the research conducted by CNTK led to the approval of these arresters for use in the open-circuit group earthing system by GIK (now the Office of Rail Transport – UTK), which was confirmed by the issuance of certificate no. B/2001/0308.

Operational experience with the unidirectional TZD-1N arresters quickly showed that they cannot operate independently in an open earthing system, for instance, in the protection of engineering structures. The primary risks were disturbances in the return network related to a lack of continuity in transverse and longitudinal connections (e.g., theft of a track impedance bond), which could cause damage to the arrester.

As a result of these experiences, the bidirectional TZD-2NR arrester was developed for the protection of engineering structures whose resistance to earth (the structure’s natural earth connection) is greater than 2 Ω. If this resistance is lower, the arrester may be damaged in the event of disturbances in the return network.

The TZD-2NR arresters became widely used for protecting engineering structures, often regardless of the protected object’s earth resistance. For some time, additional artificial earth electrodes began to be used for these structures, further lowering the earth resistance. When analyzing the earth resistance of an engineering structure, one must consider not only the measurement taken for a new object but also how this resistance value may change over time.

An example is a reinforced concrete viaduct over tracks connected to an overpass several hundred meters long. The span over the tracks rests on insulated bearings, and the protective barriers are mounted using insulating materials. According to measurements (no data on the meter or measurement method), the resistance of the barriers was several hundred ohms. An additional earth electrode was installed, and TZD-2NR arresters were used. A few months later, during acceptance tests of the arresters, the earth resistance of the protective barriers was measured with an IMR-4 meter. The resistance was approximately 1 Ω – which begs the question of the purpose of the additional spike earth electrode (and its cost), which had a resistance of several tens of ohms. A measurement of the barriers’ earth resistance after a few months in winter conditions (snow) yielded a result of about 0.5 Ω.

In 2008, the bidirectional TZD-1NR arrester was developed and manufactured. It is equivalent to the parallel connection of two unidirectional arresters, the TZD-1N and TZD-1R.

The TZD-1NR arresters were tested at the Electrotechnical Institute in Warsaw, which was confirmed by the issuance of ATTEST No. 0976/NBR/09. Subsequently, these arresters were installed for trial operation in the Warsaw Cross-City Tunnel. After a successful trial period, during which control tests and short-circuit tests were performed, the arresters were approved for operation by PKP PLK SA.

The TZD-1NR arresters were included in the PLK traction network catalog under no. 7340 in the supplement titled: “Low-Voltage Arresters,” Warsaw 2010.

The TZD-1NR arresters can be used in all conditions, i.e., for the open-circuit group earthing system of the traction network and for the open earthing of engineering structures, regardless of the value of their earth resistance.

Using one TZD-1NR arrester on each side of a group earthing section, both on railway lines and at stations, is a simpler and more effective solution. This solution is fully compliant with the applicable PKP PLK SA regulations, namely: “Guidelines for the Design and Operation of an Earth-Fault and Electric Shock Protection System with Group Earthing in an Open-Circuit Configuration on Railway Lines” (point 3.17.).

The use of two TZD-1NR arresters in a group earthing section allows for:

• increased operational reliability of the arresters,
• reduced investment costs,
• simplified installation.

The increase in reliability stems from the fact that if the group earthing cable is broken (e.g., due to theft), a single TZD-1N arrester would remain at one end of the section, which could be damaged by disturbances in the return network. The use of TZD-1NR arresters eliminates this risk.

Using two bidirectional TZD-1NR arresters in a group earthing section is more economical than using three or four unidirectional TZD-1N and TZD-1R arresters. The installation of a single TZD-1NR arrester is simpler than installing two TZD-1N and TZD-1R arresters and making the connections between them.

The availability of several types of arresters creates numerous possibilities for selecting an incorrect solution at both the design and implementation stages, for technical as well as economic reasons.

Here are a few examples of improper selection or installation of arresters:

• Earthing engineering structures with a unidirectional arrester: In the event of disturbances in the return network (a break in the tracks due to theft, replacement of a railway switch without bypass connections, track repairs, etc.), such an arrester can be damaged.
• Earthing a platform canopy (island platform) with two TZD-2NR arresters to two adjacent tracks: In the event of a voltage disturbance, both arresters may be triggered, creating an inter-track connection through which an equalizing current of up to several hundred amperes can flow, causing damage to the reverse thyristor in one of the arresters. The failure of the reverse thyristor in the second arrester is then only a matter of time.
• Using TZD-2NR arresters to earth large structures (bridges, viaducts, overpasses) with low earth resistance: This can lead to damage to the arrester.

The TZD-1NR arresters can be used in all previously employed applications, i.e., in the open-circuit group earthing system and for the protection of engineering structures, regardless of their earth resistance value.

Recently, the TZD-1NR/T arrester was developed based on the TZD-1NR. The TZD-1NR/T is intended for tram traction systems and for facilities related to the maintenance of traction vehicles (repair depots, locomotive sheds, etc.) where electric traction is present. The TZD-1NR/T is constructed identically to the TZD-1NR. The difference is that the thyristor trigger circuit in the TZD-1NR/T activates at a voltage of 60 V, while in the TZD-1NR, it activates at 120 V.

Considering the operational experience to date, as well as discussions with PKP PLK SA, KOLEN assumes that from the beginning of 2014, the primary production will consist of the TZD-1NR and TZD-1NR/T arresters. Manufacturing a single type of arrester will allow for a reduction in their price. The production of other types of arresters will still be possible, but the prices for all types will be the same. The new price will be an average, taking into account the production scale of the last year.

Detailed information can be obtained directly from KOLEN.