David A. Hamil - Back-to-Back Converter Station

The David A. Hamil DC Tie, also known as the Stegall DC Tie, is a high-voltage direct current (HVDC) facility located near Stegall, Nebraska. It began operating in 1977 and serves as one of the interconnection points between the Eastern and Western electrical grids in the United States.

Purpose and Functionality

The primary function of the Stegall DC Tie is to facilitate power transfer between the Eastern and Western Interconnections, which are asynchronous grids. It converts alternating current (AC) electricity from one grid to direct current (DC), then back to AC for the other grid, allowing power to flow between the two systems without the need for frequency synchronization.

The Stegall DC Tie is part of a larger network of HVDC interties along the East-West divide in the United States. For power companies like Basin Electric, which serves members in both Eastern and Western grids, the Stegall DC Tie plays an important role in power distribution and management. It allows for the movement of excess generation from one area to another, helps in optimizing power sales, and provides backup power supply in case of outages in either interconnection.

Technical Specifications

The Stegall DC Tie is a back-to-back HVDC system, which means it doesn't have a transmission line but instead connects two separate AC grids at a single point. The tie uses thyristor-based LCC technology, operating at ±50 kV and with a power rating of 100 MW.

The converters were designed with 10% continuous overload capability. The converters consist of 2 thyristor valve structures housed indoors in one valve hall, with each valve having 3 modules in series connection, each module having 10 thyristor levels in series and each thyristor level has 4 thyristors connected in parallel. This makes a total of 120 thyristors per valve arm or 720 thyristors per 6-pulse converter unit.

The valves are air-insulated and air-cooled. The valve cooling system consists of two closed loops and an evaporative assisted outdoor cooler. The heat from the air is moved from the lower plenum heat exchangers by means of a pumped water-glycol loop to the heat-exchanger in the outdoor evaporative cooler.

DC Reactor Rating 0.06H at 2000A

There is 1x AC Harmonic Filter on each side, rated at 30Mvar, tuned to 11th harmonic. In addition, there is 1x 30Mvar shunt capacitor on each side.

The Converter Transformers are 3-phase, 3-winding units, connecting to the 230kV AC network voltage on the line side windings.

Transformer Data: 123.3 MVA, 230+16%-6%/23.8/23.8 kV

Technology and Challenges

The Stegall DC Tie uses solid-state technology that was new in the 1970s when it was built. The core of its operation relies on thyristor technology, responsible for converting electricity from AC to DC and back to AC. Despite its age, the tie has maintained a reliability record of over 99%.