The Vermont electricity grid was experiencing significant load growth in the early 2000’s. During peak load conditions, up to 90% of the power was imported from sources outside the state, which resulted in voltage stability concerns, especially at key locations such as the Granite Substation. Simulations indicated that in the worst case, up to 180 MVAr of local reactive power was needed at the location. Various alternative approaches were evaluated as possible sources of this reactive power, including mechanically switched capacitors, SVC, STATCOM and synchronous condensers, as well as hybrid solutions involving more than one of these technologies.

As part of the Northwest Vermont Reliability Project, the Vermont grid operator, Vermont Electricity Company (“VEMCO”) determined that a synchronous condenser would provide the optimal result, due to its the low voltage ride through capability and the high short time overload characteristics.

Installed by GE in 2008, the project consisted of:

• four +25/-12.5 MVAr synchronous condensers;
• four 25 MVAr shunt banks (MSC);
• two Phase shifting transformers; and
• integrated control system.

The overload capability of the condensers provides enough time for the mechanically switched 115 kV shunt capacitors to be placed into service. The system is designed to provide low-voltage ride-through capability for 115 kV or 230 kV faults, and was the first project of its kind to be built in New England in over 30 years. The units were commissioned in November of 2008.

VELCO uses the synchronous condensers to provide both steady state and dynamic support to the critical transmission corridor located in and around the Granite substation, which has also supported the growth of customer demand in the area.