Section 3: Reliability Issues and Solutions

3.1     Reliability issues identified

Figure 3-1 summarizes the transmission reliability issues identified as part of the 2009 planning process. These areas represent locations that require future transmission related upgrades or alternatives, such as local generation or energy efficiency, to meet the reliability standards. The following table provides a brief description of each deficiency and its causes.   

Figure 3‑1. Identified transmission reliablity issues.

Deficiency number	Name	Causes of Deficiency	Deficiencies
1	Georgia/ St Albans	* Loss of transmission line * Loss of E. Fairfax transformer * Loss of one or both St Albans transformer	Low voltage, voltage instability, voltage collapse, thermal overloads
2	Middlebury	* Loss of Middlebury transformer or breaker failure	Low voltage, voltage collapse
3	Blissville	* Loss of Blissville transformer	Low voltage, voltage collapse, thermal overload
4	Hartford/ Chelsea	* Loss of Hartford transformer or breaker failure	Low voltage, voltage collapse
5	North Rutland	* Loss of the North Rutland or Cold River transformers	Low voltage, voltage collapse, thermal overload
6	Ascutney	* Loss of Ascutney transformer	Low voltage, voltage collapse, thermal overload
7	Bennington	* Bennington breaker failure	Low voltage, voltage collapse
8	Blissville/ Ascutney	* Various scenarios involving loss of transmission lines	Low voltage
9	West Rutland/ Coolidge	* Light to moderate load levels	High voltage
10	St. Johnsbury	* Loss of St Johnsbury transformer, aggravated  when Highgate converter or Littleton autotransfomer is out of service * Loss of transmission line	Low voltage, voltage collapse
11	Vernon	* Loss of Fitzwilliam transformer	Thermal overload
12	Coolidge/ Ascutney	* Transmission line out of service	Thermal overload
13	Ascutney-Ascutney Tap	* New England power flows and New Hampshire load	Thermal overload
14	Vernon	* VY autotransfomer out of service	Thermal overload
15	Coolidge/ Cold River	* Loss of transmission line	Low voltage, voltage collapse, thermal overload
16	Coolidge	* Loss of Coolidge transformer	Low voltage, voltage collapse, thermal overload
17	Loop Flow	* Increase in load and transmission outages.  Note deficiency can be handled by operations at the subtransmission level	Thermal overload
18	Sub-transmission Voltage	* Load level and loss of transmission lines/transformers.  Note deficiency can be handled at the subtransmission level.	Low voltage
19	Barre	* Load level, loss of transformer	Low voltage, voltage collapse, thermal overload
20	Vermont Yankee	* VY removed from system.  Note overloads occur in southwestern New Hampshire.  Solution will need to be addressed at the regional/ISO-NE level.	Low voltage, voltage collapse, thermal overload
21	Plattsburgh-Essex	* Highgate converter removed and loss of transmission lines	Low voltage, voltage collapse, thermal overload
22	Highgate	* Highgate converter out of service and loss of transmission lines	Low voltage, voltage collapse, thermal overload
23	2028	* Year 2028 load levels, Highgate out of service, loss of transmission lines	Low voltage, voltage collapse, thermal overload

The range of reliability performance issues documented in the 2009 technical analysis center on these causes:

• Heavy use of transmission facility may overload the equipment beyond its rating.
• A poorly supplied area may suffer voltage far below or above acceptable levels.
• Voltage instability may occur in areas with weak transmission networks.
• In the extreme, very low voltage can lead to a voltage collapse, where the transmission system becomes unstable and sections automatically disconnect, potentially leading to widespread blackout.

These transmission system phenomena are examples of unacceptable system performance that must be resolved. The plan describes proposed transmission reinforcements to address these unacceptable transmission performance issues.