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Case Study


SVC for Dynamic Voltage Control and Prevention of Voltage Collapse on a 115 kV Power Transmission System

Since 2009, NSTAR Electric & Gas Corporation has been operating a Static Var Compensator (SVC) supplied by Hitachi Energy on their 115 kV transmission system in Lower Southeastern Massachusetts, feeding the Cape Cod area.

The SVC, rated at 115 kV, 0-225 Mvar capacitive for 2 seconds and 0-112.5.

Mvar capacitive for longer term supply, is located at a substation near Hyannis, Massachusetts. Its purpose is to supply fast-acting voltage support to the transmission system on Cape Cod in the event the two major supply sources to the area were to be out of service unexpectedly. If this event occurred with no other sources of generation available, a voltage collapse was predicted to occur, affecting Cape Cod and possibly other sections of the transmission system. The SVC senses degrading voltage and rapidly injects up to 225 Mvar of reactive power for 2 seconds followed by a longer term supply of up to 112.5 Mvar to stabilize and control transmission system voltage.

The system is fully automated to activate and inject reactive power when the transmission system voltage falls below a specified set point, to maintain a specified voltage range, and then automatically return to stand-by status.

The SVC was supplied under an Engineer-Procure-Construct contract, where Hitachi Energy also coordinated system installation with NSTAR who was simultaneously expanding the substation into which the SVC connected.

Main data
System SVC
Commissioning year 2009
Controlled voltage 115 kV
SVC rating Short-time (2 seconds): 0-225 Mvar capacitive, continuously variable
Long-time (6 hours): 0-112.5 Mvar capacitive, continuously variable
For 3 seconds: 112.5 Mvar inductive (fixed)
Control system Three-phase symmetrical voltage control by means of a closed loop voltage regulator
Thyristor valve PCT type thyristors, water cooled, indirect light firing.

Barnstable: SVC for Dynamic Voltage Control and Prevention of Voltage Collapse on a 115 kV Power Transmission System