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| 1991–2001 | 2001–2023 | 2023–present |
1991–2001[]
2001–2023[]
2023–present[]
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Iberdrola |
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Iberdrola
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| Avangrid (United States): Berkshire Gas | Central Maine Power | Connecticut Natural Gas | Maine Natural Gas | NYSEG | Rochester Gas & Electric | Southern Connecticut Gas | United Illuminating Scottish Power (United Kingdom) Neoenergia (Brazil):  |