Considering the 11kV/132kV network shown in Figure 1 and the system data given here, build the equivalent circuit to carry out an insulation coordination study using the ATP/EMTP program (for simplification purposes, you can use a source rated at 132kV instead of the 11kV source in series with the ideal transformer, but the equivalent capacitance of the transformer needs to be included). Give details of the models for each component (screen shots from ATPDRAW) and explain how the various data used in the models were obtained. [3 marks]

Carry out a lightning surge study (use a 1.2/50, 32kA lightning strike) and determine the surge voltage at the key nodes of the system following a direct lightning strike to the overhead line at 1500m away from the substation (assume cable is part of the substation). Identify the worst-case lightning overvoltage level at the substation (along the last 1500m of the overhead line, cables ends, and transformer). To simplify the problem, remove the AC source to perform this study. Explain the adopted Program settings and selections such as computation time step and maximum time for computations. [4 marks]

Identify the key points to be protected at the substation and explain why? [2 marks]

Using the supplied company specifications and manufacturer data (e.g., see buyers’ guide on Hitachi/ABB’s website), select and give details of an appropriate surge arrester(s) to ensure protection of all equipment at the substation. Use the supplied surge arrester specification for your selection including the ratings and protection margins. In your simulations, consider the cost issue when adding surge arresters at various locations. [4 marks]

Determine the switching overvoltage level for a simultaneous switching at the instant of peak voltage on phase A. The AC source will now require a series network equivalent impedance to correspond to a fault level of 30kA. Explain any changes required for the model or Program settings. [3 marks]

Using the supplied utility’s specification, select the required spark gap at various locations: 1000m away from the substation, at the cable-overhead line junction, and at the transformer terminals. [2 marks]

Write a short report summarizing your results/answers and show typical waveforms of the computed voltages for each of the cases above. Give recommendations for overvoltage protection. [2 marks]

Submission

Students are expected to submit a full report (approximately 10 pages) by the end of week 10 (Friday 26 April 2024 before 5pm). A short oral examination to discuss the model and results is envisaged during (weeks 11/12).