The medium-voletage distribution networks in Russia are regarded as low level in terms of the national power transmission and distribution systems. The 6-kV and 10-kV medium-voltage (MV) distribution networks form the basis of the rural electric networks. They are predominately overhead networks extending more than 2 million km (1.24 million miles). Moreover, the MV lines experience the highest fault rate, which has necessitated the need for detailed fault-cause analysis. This has resulted in the development of new designs for overhead lines to improve reliability.

The failure rate for various components of MV distribution lines (mean failures per annum per overhead line circuit or per component) are as follows:

• Overhead lines — 3.0
• Transformers — 0.12
• Bushings — 0.1
• Switchgear — 0.01
• Cubicles — 0.03.

Statistically, the fault rate of MV overhead lines is much higher than all other components. In fact, it is 25 times higher than that of transformers, the second least-reliable component. Even allowing for regions with complex geological and severe climatic conditions, the fault rate of MV distribution lines is the critical factor determining the overall network reliability.

Considerable progress has been made recently in the design features of MV distribution networks with advanced instrumentation that enables operational engineers to substantially improve the reliability of MV substations. Dry transformers, vacuum switchgear and overvoltage limiters have improved substation reliability, which has further highlighted the adverse performance of MV lines.

Two major factors contribute to the high fault rate on MV overhead lines. First, the overhead lines are several decades old. Second, their design and construction specifications have subsequently been proved to be substandard. Traditionally, these circuits were intended for rural networks with, according to Russian national standards, consumers categorized as “minor important.” Hence, minimum first cost, not reliability, was the motivating design criteria. Subsequently, these engineering solutions were adopted with minor modifications in the networks constructed to supply other industries, constituting the entire MV network.

The three principal causes of MV network supply interruptions are as follows:

• Support failure — 40%
• Insulator failure — 35%
• Conductor failure — 25%.

The MV overhead line networks predominantly use concrete supports with conductors mounted on porcelain or glass pin-type line insulators. The fabrication of the concrete tower supports uses concrete vibration techniques. The reasons for the failure of concrete support are summarized as:

• Heaving soils causing lateral deflection and subsequent fall in heavy soils. This deflection occurs because the overall support length is fixed at 11 m (36 ft), which limits the foundation depth to around 2 m to 2.5 m (6.5 ft to 8.2 ft) (Fig. 1).

• Corrosion fracture of the concrete. This occurs in watered or salt soils from crystallization of the concrete (Fig. 2).

• Impact fracture of supports. This usually results from damage inflicted during transportation, handling or installation.

• Bond failure of supports. This is likely caused by concrete deterioration from leakage-current corrosion of reinforcement.

• Concrete degradation under dynamic loads. This is caused by rigid pin-type insulators.

• Sudden conductor failure of insulators and the dynamic fracture of insulators. This is likely caused by unanticipated conductor motions.

• Insufficient conductor separation. This results in phase conductor clashing and flashovers.

The main oil and gas production plants are concentrated in West Siberia and other locations where the MV network on concrete supports had the worst reliability. The resulting interruptions to power supplies led to production suspensions, and industrial consumers faced unacceptable financial losses. The solutions included:

• Reducing span lengths to between 40 m to 45 m (130 ft to 147 ft) to reduce mechanical loading and eliminate conductor clashing.

• Installing steel tubes 5 m to 10 m (16 ft to 33 ft) long to provide a foundation to withstand buckling in areas of permafrost soils or swamps.

• Using pin-type insulators rated at 20 kV.

Figure 2 shows a section of a 6-kV overhead line built in accordance with these solutions. However, these measures do not eliminate the concrete degradation that results from crystallization in the concrete structure during thawing/icing cycles.

The use of steel supports implies the construction of overhead lines on towers manufactured from standard steel tubes (Fig. 3). In overhead line supports, the bending load decreases with height. Therefore, supports are designed with a steel cross-sectional area decreasing with height. In supports fabricated from steel tubes, the tube gauge or tube thickness must be selected for maximum base-bending load. In the remaining sections of the support, the cross-sectional area is underutilized, making the supports both heavy and costly. With supports designed in this manner, it is impossible to have long spans because of the low-torsional stability of long tubes, a factor that increases line cost.

A further method employed on circuits supplying oil and gas production enterprises is the construction of 35-kV overhead lines operating at 6 kV to 10 kV, an expensive solution for key industrial consumers. Although these alternatives improved circuit reliability for key industrial consumers, the capital cost is too high to consider these alternatives for all MV overhead lines.

The 35-kV and 110-kV lines in Russia have reliability figures 20 times greater than MV lines. Even in regions with complex geological and severe climatic conditions, 35-kV and 110-kV circuits have a nominal service life of 40 to 50 years. The service life of MV lines in such regions is normally less than five to seven years.

The ELSI Company Group proposed a new approach to improve the reliability of MV overhead lines by applying high-voltage engineering solutions with specially designed steel supports for these lines, coupled with the use of suspension-type polymeric insulators and increased inter-phase conductor separation. These development, material and design changes will improve the circuit-reliability statistics of MV lines to the values typical of 35-kV and 110-kV overhead lines. The ELSI Company Group's steel-tower design is now certified according to the Russian national standards.