Load Flow Study

Eta Engineering Consultants was contracted to provide a load flow study to determine the effects of increased loading analyze to a utility generating plant. The client is planning to renovate and add approximately 5 MW of load. There were four alternatives on how to supply the additional power: from the existing substation, from the existing substation with a new 4 MW generator, from a new dedicated substation with a new utility feeder, and from a new dedicated substation with two new utility feeders. Each alternative needed evaluated for loading and reliability issues.

EEC previously modeled the facility using SKM’s PTW software for a short circuit, coordination and arc flash study. This model was used as a base for the load flow study. Utility data was analyzed for the past 12 months to determine the highest peak demand. Individual KWH meter data for the peak demand month were used as inputs into the model. Present and future load data for the renovation was obtained from design study calculations.
After the base system was modeled, four scenarios were created to evaluate the alternative ideas on how to feed the new loads. The new load had to adhere to what is known as “n-1” criteria. For multiple transmission lines delivering power to the same point, if one of the lines goes out of service, the remaining lines must be able to carry both the load they were carrying before the event, plus the load carried by the line that is out of service. For each scenario, n-1 conditions were simulated and load flow results evaluated.

The following evaluations were performed and results tabulated in spreadsheet format:
Cable Overloads: Load flow amperes for all 5 kV and 15 kV cables were compared to their ampacity ratings. This evaluation was performed for each scenario. Results were highlighted when load flow amperes were approaching overload or exceeding ampacity.
Client’s Feeder Evaluation: Load flow amperes for each of the four client feeders were compared to their cable ampacities as well as with the protective relay settings. This evaluation was performed for each scenario and under N-1 (feeder loss) conditions. Results were highlighted when load flow amperes were approaching overload or exceeding ampacity. N-1 results were highlighted for values exceeding 100% of the utility’s limit.
Utility Feeders to Client’s Substation Evaluation: Load flow amperes for each of the four existing and two proposed utility feeders were compared to their cable ampacities as well as with the protective relay settings. This evaluation was performed for each scenario and under N-1 (bus outage) conditions. The utility’s substation bus loading was compared with capacity under N-1 bus outage conditions. Results were highlighted when load flow amperes were approaching overload or exceeding ampacity. N-1 results were highlighted for values exceeding 100% of the utility’s limit.