Milestones:Eel River High Voltage Direct Current Converter Station
Eel River High Voltage Direct Current Converter Station, 1972
Operating since 1972, Eel River, New Brunswick is home to the world's first commercial solid state High Voltage Direct Current converter station. This 320 MW interconnection facility, built by Canadian General Electric and NB Power, incorporates high current silicon solid state thyristors to convert alternating current from Hydro Quebec to direct current and back to alternating, allowing asynchronous, stable power transfers to serve New Brunswick's Power's customers.
The plaque may be viewed in the main lobby of the Eel River Dalhousie Generating Station in North Shannonvale, New Brunswick, where it can be viewed by employees and visitors.
This converter station was historically significant because it was the first that was designed and built from solid state high voltage, high current thyristors. Previously the medium was a plasma in a glass envelope using mercury vapor. These had more losses and were prone to re-strikes during transient fault events. Additionally, the mercury is considered a dangerous pollutant. Nelson River HVDC had used thyristors in a portion of the bridge to mitigate re-strikes but these were retrofits. The knowledge gained there served as a proving ground to give confidence for a full scale project at Eel River.
With this project, the march of solid state systems to higher current and voltage ratings was advanced another major step. In fact, the Eel River Station performance was such that the station ran at 10% overload for the first 15 years without difficulty. The project allowed surplus Hydro Quebec energy to flow into New Brunswick which is synchronized with the Eastern Interconnection, without the risk of loss of transmission if disturbances happened in either New Brunswick or Hydro Quebec. Indeed Eel River served to govern and stabilize both systems with its External Control System, a benefit to either system in case of disturbances. This economically advantageous energy imported to the benefit of New Brunswick customers, allowed more expensive NB Power generation to be exported to New England where energy costs were higher, an economic advantage to the parties on both side of the US border.
The electronic external control system enhancing governing and stability as a response to weighted system frequency and weighted system acceleration was a new feature which allowed both Hydro Quebec and NB Power to use the other system as a crutch during power system disturbances in either system. This feature has had a major beneficial impact on dynamic system performance. This is where a difference in frequency modulates the power transfer in a manner to assist the deficient system being governed back to safety. In the event of a difference in acceleration, the system slowing down is assisted by the other system while mitigating any power swings as quickly as possible to regain stability.
The triggering for the thyristors was achieved by using fibre optics to communicate the isolated trigger pulse to the thyristors at various voltage levels. This was an early application of another new technology, now ubiquitous.
References and Further Reading
In the EPRI project where Prabha Kundur wrote a comprehensive textbook on Power System Stability and Control, a reference to Eel River. .
What Wikipedia has to say about ER HVDC 
A public relations document about Eel River Converter station written by GE and NB Power .
Collection of bits of documents by Frank Ryder .