Stray Current Protection for Individual Pipelines

Frequently, measures for dc installations are not sufficient to limit stray currents. This applies particularly to dc railways. In many cases, additional protective measures for the affected installations are advisable [1] or even necessary [11]. 

The pipe/soil potential can therefore become very negative. The pipe currents become very large with poorly coated pipelines. The pipe current can be limited by 

The curves in Fig. 15-5 are greatly simplified and only apply to one instant of time. In practice there are always several trams with varying current draw on a stretch of track. Currents and potentials are then subjected to wide time-dependent variations. Without protective measures, the pipe/soil potentials are usually always more negative in the outer regions of a tramway system and more positive in the vicinity of the transformer substation. In a wide intermediate region, potential changes occur in both directions. The current entrance and exit areas can be deter- 

With forced stray current drainage, the current is returned from the pipeline to the rails by means of a grid-fed rectifier. The transformer-rectifier is connected into the stray current return conductor, the negative pole is connected with the installation to be protected and the positive pole is connected to the rails or the negative side of the bus bar in the transformer substation. 

Various arrangements for the protection rectifier and its possible applications are described in Chapter 8. Large fluctuations of the protection current can be reduced by a current-reducing resistor, R (see Fig. 15-7). This means that the pipe/ soil potential in the middle becomes less negative. A current smoothing action can be achieved with rectifiers connected to the network or by reducing losses by an inductive resistance between the transformer and bridge rectifier (see Section 8.4). 

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