Great River Energy engaged Teshmont Consultants LP to carry out the CU HVDC Transmission System Shunt-Filter Bank Replacement Study project, for the replacement of ac filter and shunt capacitor banks containing PCB-filled and aged non-PCB filled capacitors at two HVDC converter stations. Teshmont evaluated the performance of the existing ac filters and determined the performance requirements and ratings of the new ac filters. An innovative, cost-effective and practical solution was provided that met the needs of Great River Energy.
The harmonic contributions from the surrounding ac system for this project were found to be higher than normal at certain frequencies, thus, values typically assumed for the harmonic currents injected by the HVDC converters (5% to 10% is typical) could not be used for this study.
Analysis, interpretation and mitigation of increased levels of background harmonics in power systems is relatively new to the industry and, as such, there is no established standard available. As a result, Teshmont developed several unique methods of calculating the ac harmonic currents contributed from the surrounding ac system. The results of each method were evaluated based on whether enough margin was provided in the ratings to ensure that the filters could withstand the most onerous conditions expected without failure and were the most practical and cost-effective at the same time. Ultimately, one method was selected that best fulfilled these criteria and was subsequently used in the design of the ac filters.
The economic state of the United States at the time of the project resulted in an especially pronounced emphasis being placed on cost-effectiveness. Teshmont met Great River Energy’s needs by producing a cost-effective design for the new ac filters through the application of technology and engineering innovation, while meeting all of the project’s technical requirements and staying within the project budget.
Of further benefit to Great River Energy was the equipment arrangement that Teshmont designed for the circuit breaker, disconnect switch, and ground switch that replaced the existing circuit switchers. The new arrangement used the existing foundations to the maximum extent possible through the application of pantograph style disconnect switches. This allowed Great River Energy to reduce the capital expenditures for the project and the time required for the system to be at reduced power transfer due to the installation of the new equipment.
Once implemented, the shunt-filter replacement project will improve the security, reliability and performance of the electricity supply for the greater Minneapolis and St. Paul area and the larger transmission grid in Minnesota, North Dakota and the surrounding region.
The replacement of PCB-filled equipment with equipment using environmentally friendly material minimizes the environmental hazard at Coal Creek and Dickinson stations. The replacement of circuit switchers with circuit breakers on each ac filter and shunt bank greatly reduces the probability of outages that may cause reduced power flow into Dickinson sub-station. This contributes to the improved reliability and dependability of the system.