Switching Surges in Underground Distribution

Cable Switching

Although the initial focus of DSTAR projects was on the impulse performance and protection of underground cable systems, the research over the past few years has diversified to also include other areas of current importance. Lightning-generated impulses are but one type of transient to which an underground distribution system is exposed. Routine switching, whether by field crews operating elbows or capacitor banks switched on voltage control, also create transient overvoltages in underground cable systems. As part of a DSTAR project, field measurements of switching surges were made at several sites in a member utility's underground systems.

rschfig1.jpg The photo on the left shows the sophisticated instrumentation setup used to capture these very fast transients in the field. EMTP modeling techniques were refined by validating simulations against the field tests, and simulations were used to extend the scope of the research to many more system configurations than could be tested in the field. Guidelines for minimizing switching transient magnitudes were developed.

A number of utilities, particularly those operating at the higher distribution voltages, have experienced unexplained flashovers to ground when switching unloaded cables with loadbreak elbows. Building on the previous field switching investigation, another major research project has been performed to determine the causes for these elbow failures.

The primary objective of this project was to characterize underground distribution system switching-surges, in terms of:

  • The magnitudes of surges which can be expected
  • The waveshapes of the surges
  • The influence of circuit characteristics on the surges
  • The behavior of common switching devices

The switching-surge characterization performed in this project is based on full-scale field tests. The characterization was extended to system configurations not tested in the field by using digital simulation techniques. To maximize the confidence in these simulations, additional objectives of this project were to refine the simulation methodology and to correlate simulations of the field-tested systems with the actual test results.

Using the results obtained in this project, guidelines were developed for system design and field switching operations which can minimize the severity of distribution switching surges. Also, digital simulation guidelines are also provided which will allow member utilities to perform more detailed in-house investigations for their specific applications.

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This study was conducted in: P2-3