Central MA Transmission Study

Long-term Needs and Solutions

Central MA Infrastructure Study Study Initiated in 2001 Identified Short-term and Long-term

Upgrades to ensure reliability of service

Study area covers region bounded by: Sandy Pond (NE) Pratts Jct (NW) Carpenter Hill (SW) Northboro Rd (SE)

Geographic Location of Central MA

Short-term Upgrades 18.4 applications approved in

February 2002 63 MVAr Capacitor Bank at Millbury 54 MVAr Capacitor Bank at

Northboro Road Also adjusted taps on Ayer 115-69 kV

transformers In-service for Summer 2002

Long-term Needs in Central MA Needs identified in three main areas

Millbury– 115-69 kV transformer loading– 115 kV voltage

Sandy Pond – 345-115 kV transformer loading

Ayer– 115-69 kV transformer loading– 69 kV voltage

Existing Problems

1. Millbury T1 & T2 Overload for loss of each other

3. Ayer T4 & T6 Overload for loss of each other

2. Sandy Pd T1 & T2 overload for loss of each other

5. Shrewsbury 69 kV Breaker overdutied

4. Ayer 69 kV Area voltage drops below 0.9 pu for K-137 + L-138W DCT contingency

Millbury 115-69 kV Transformers

Loss of either T1 or T2 overloads the remaining small transformer

Recommended Solution to Millbury T1 and T2 Overload

Millbury Station – Replace T1 and T2 (115-69 kV)w/ Larger Transformers Cost: $1.5MIn-Service Year: 2004

Millbury – Replace Two (2) 115-69 kV Autotransformers

Replace T1 and T2 (50 MVA) w/ 56 MVA Transformers

56 MVA

Millbury Substation

56 MVA

Millbury Alternatives Recommendation:

Replace existing T1 and T2 with larger transformers ($1.5M)

Alternatives: Add fourth 115-69 kV transformer into

dedicated bus position ($3.8 M) Add fourth 115-69 kV transformer tapped

off Q-143, relocate I-35 ($2.1 M) Convert 69 kV line X-24 to 115 kV to

unload existing transformers ($14.9 M)

Millbury Solution The recommendation is based on:

Lowest cost Simpler construction compared to

adding a 4th transformer Less permitting risk than 69 kV line

conversion to 115 kV Existing Millbury transformers can

be relocated to Wachusett substation

Ayer 115-69 kV Transformers Loss of either T1 or T2 overloads the

remaining transformer

Recommended Solution to Ayer T4 & T6 Overload

Transfer 69 kV Load from Ayer to Pratts Jct,

and Install two additional 115-69 kV transformers at Pratts Jct

Cost: $1.4M

In-Service Year: 2004

Pratts Jct Sub – Install Two (2) Additional 115-69 kV Autotransformers

56 MVA

56 MVATwo Additional 115-69 kV Autos at PJ

Transfer ½ of Prospect St Load from V-22E to U-21S

Ayer Alternatives Recommendation:

Transfer Prospect Street from 69 kV line V-22E to U-21S; install two (2) 115-69 kV transformers at Pratts Jct and replace eight (8) 69 kV breakers ($2.46 M)

Alternatives: Install (2) 115-69 kV transformers at Ayer

($4.5 M) Convert 69 kV line V-22 to 115 kV to

unload existing transformers ($13.7 M)

Ayer Solution The recommendation is based on:

Lowest cost Simpler construction at Pratts

Junction compared to Ayer Less permitting risk than 69 kV line

conversion to 115 kV

Sandy Pond 345-115 kV Transformers Loss of either T1 or T2 overloads the

remaining transformer Considered installation of new 345-115 kV

transformers at: Sandy Pond Tewksbury Pratts Junction Millbury Wachusett Quinsigamond Junction

Sandy Pond Alternatives Preliminary analysis indicates:

Tewksbury location– not effective at unloading Sandy Pond transformers– does not support Central MA voltage

Millbury location– not effective at unloading Sandy Pond transformers

Sandy Pond 3rd transformer– does not support Central MA voltage– design challenges to prevent loss of two

transformers for a breaker failure contingency Pratts Junction, Wachusett, and

Quinsigamond Junction locations– off-load Sandy Pond transformers– support Central MA voltage – these alternatives identified for further analysis

Sandy Pond 345-115 kV Transformers

The Pratts Junction, Wachusett, and Quinsigamond Junction sites were fully evaluated to provide: A list of associated upgrades based

on thermal and voltage analysis Study grade estimates for all

upgrades associated with each option

Sandy Pond Alternatives Install two (2) 345-115 kV transformers

at existing Pratts Junction substation and associated upgrades

Install two (2) 345-115 kV transformers at existing Wachusett substation and associated upgrades

Install two (2) 345-115 kV transformers at new Quinsigamond Junction substation and associated upgrades

Pratts Jct Alternative - Costs Woodside 69 kV Breaker 2004 0.700 M Pratts Jct Substation 345-115 kV 2006 10.400 M Pratts Jct Substation 115-69 kV 2006 4.390 M Replace Pratts J 115 kV Breakers 2006 1.628 M Replace Pratts J 69 kV Breakers 2006 0.840 M Reconductor N-40 2006 1.180 M Reconductor O-141N 2008 0.750 M Replace W-23E Disconnect 2008 0.020 M Pratts Jct Substation 345 kV 2010 2.700 M Convert N-40/W-23 to 115 kV 2012 18.300 M

Wachusett Alternative - Costs Woodside 69 kV Breaker 2004 0.700 M Wachusett 345-115 Substation 2006 25.750 M Wachusett 115-69 kV 2006 2.530 M Reconductor O-141N 2006 1.500 M Reconductor M-39 2006 1.000 M Reconductor N-40 2006 1.000 M Replace Rolfe Ave Breaker 2006 0.600 M Reconductor W-175 2006 0.475 M W. Boylston Upgrades 2006 0.670 M Replace W-23E Disconnect 2008 0.020 M Reconductor P-142N 2008 0.175 M

Quinsigamond Jct Alternative - Costs

Woodside 69 kV Breaker 2004 0.700 M Quinsigamond Jct Sub 2006 27.700 M Wachusett 115-69 kV Upgrades 2006 4.400 M Replace O-141S Terminal Equip. 2006 0.500 M Reconductor M-39 2006 1.180 M Replace Rolfe Avenue Breaker 2006 0.366 M Reconductor W-175 2006 0.475 M Replace Pratts J 69 kV Breakers 2006 0.840 M Reconductor N-40 2006 1.180 M Replace W-23E Disconnect 2008 0.020 M

Recommended Solution to: Sandy Pd T1 and T2 OverloadLow Voltage in Central MA

Wachusett Station – Install two new 345-115 kV Transformers (448 MVA)

Cost: $25.75M

In-Service Year: 2006

Wachusett – Install Two 345-115 kV Autotransformers (448 MVA)

Wachusett115 kV

P-142

G

G

G

G

G

G

O-141

P-142N

O-141N

A-53

Install 345-115 kV Transformers and Substation at Wachusett

G

G

G

G

G

M-39B-54 G

G

50 MVA

50 MVA

448 MVA

448 MVA

69 kV

Pratts Jct

Greendale Boylston

ReplaceBreakers

345 kV

Sandy Pd

Millbury

343N 314N

G

G

G

G

G

G

Associated Thermal and Voltage Upgrades

Reconductor N-40 & M-39 w/ 795 ACSR

Replace all 69 kV breakers at Pratts Jct for Short Ckt Duty.

Replace Rolfe Ave 115 kV Breaker

Reconductor O-141N [Quinsigimond Jct – Nashua St] w/ 636 ACSS

Reconductor P-142 [Wachusett – W Boylston] w/ 1113 ACSR in 2008

Preferred Sandy Pond Solution The Wachusett alternative was

identified as the preferred alternative based on: Lowest cost Better system performance

compared to Pratts Junction option Less permitting risk than Pratts

Junction option (W-23 conversion) or Quinsigamond Junction (new substation)

Transient Stability Analysis Analysis conducted for the preferred

alternative West Boylston substation becomes

a bulk power system substation High-Speed pilot protection

required for P-142 line from Wachusett to West Boylston

Incremental Cost = $0.890 M Does not change recommendation

Recommended Sandy Pond Solution Wachusett recommended as the preferred

alternative based on: Lowest cost

– Wachusett PW = $32.2 M– Pratts Junction PW = $32.3 M– Quinsigamond Junction PW = $36.1 M

(all estimates are study grade – ±25%) Better system performance compared to

Pratts Junction option Less permitting risk than Pratts Junction

option (W-23 conversion) or Quinsigamond Junction (new substation)

Wachusett Construction Issues Construction Grade estimate

developed for the preferred alternative

Identified wetland issues that limit use of existing Wachusett site

Requires construction of Gas Insulated Substation

Wachusett GIS Layout

Revised Wachusett Cost GIS design increases cost of

Wachusett substation by $5 M (+19%) Revised Comparison of Project Costs

Wachusett (Air): PW = $32.2 M Pratts Junction: PW = $32.3 M Quinsigamond Junction: PW = $36.1 M Wachusett (GIS): PW = $37.2 M

Assessment of Alternatives Wachusett Alternative

PW = $37.2 M NEP land at existing substation Balanced system flows Supports Millbury/Northboro Rd

voltage

Assessment of Alternatives Quinsigamond Junction Alternative

PW = $36.1 M Requires purchase of land

– Higher permitting risk

Balanced system flows Supports Millbury/Northboro Rd voltage Transient Stability analysis and detailed

site engineering may identify additional costs

Assessment of Alternatives Pratts Junction Alternative

PW = $32.3 M NEP land at existing substation Requires conversion of N-40/W-23 circuit from 69

kV to 115 kV– Higher permitting risk– Requires construction of a new 345-115 kV substation

near Millbury if line cannot be permitted Unbalanced system flows

– Eight (8) 115-69 kV transformers at Pratts Junction– Heavy reliance on converted N-40/W-23 circuit to

Northboro Road Support for Millbury/Northboro Rd voltage Transient Stability analysis and detailed site

engineering may identify additional costs

Recommendation The Wachusett location is still the

recommended option Technically superior and less permitting

risk than Pratts Junction option Similar cost and less permitting risk than

Quinsigamond Junction option Changing recommendation will delay

installation by at least one year Additional engineering minimizes

uncertainty with the Wachusett option compared to Pratts Junction or Quinsigamond Junction