NorthWestern Corporation as of 04/06/2011
Electric TCS and MBR
NorthWestern Corporation
Effective Date:		01/01/2011				Status:			Effective
FERC Docket:		ER11-02519-000	20
FERC Order:		Delegated Letter Order				Order Date:		04/01/2011
Attachment C, Methodology To Assess Available Transmission Capability, 0.1.0	A

ATTACHMENT C

Methodology To Assess Available Transfer Capability

This Attachment C contains the Transmission Provider’s methodology for determining Available Transfer Capability (ATC).

The Transmission Provider employs the rated system path (contract path) methodology in its determination of ATC.

(1)	Description of Mathematical Algorithm Used to Calculate Firm And Non-Firm ATC

To determine firm and non-firm ATC, the Transmission Provider uses the following algorithms for the scheduling, operating, and planning horizons:

Firm: ATCF = TTC - ETCF - CBM - TRM + PostbacksF + counterflowsF 

Non-Firm: ATCNF = TTC - ETCF - ETCNF - CBMS - TRMU + PostbacksNF + counterflowsNF

Acronyms

ATC = Available Transfer Capability

CBM = Capacity Benefit Margin

ETC = Existing Transmission Commitments

TRM = Transmission Reliability Margin

TSR = Transmission Service Request

TTC = Total Transfer Capability

Definitions

The Scheduling Horizon is defined as the real-time (same day or next-hour) period.

The Operating Horizon is defined as the day-ahead or preschedule period.

The Planning Horizon is defined as the period beyond the Operating Horizon.

Link to Website

This Attachment C, which contains the actual mathematical algorithms for calculating ATC, can be found at:
<http://www.oatioasis.com/NWMT/NWMTdocs/ATTACHMENT_C.pdf>

(2)	Process Flow Diagram Illustrating Various Steps Through Which ATC is Calculated

(3)	Detailed Description of How Each ATC Component is Calculated for the Operating and Planning Horizons 

(a)	Total Transfer Capability (TTC)

(i)	Definition

From the WECC ATC document: “TTC represents the reliability limit of a transmission path at any specified point in time. It is a variable quantity, dependent upon operating conditions in the near term and forecasted conditions in the long term.”

TTC may also be defined as the amount of electric power that can be transferred over the interconnected transmission network in a reliable manner while meeting all of a specific set of defined pre- and post- contingency system conditions.

As described below, the TTC is also dependent on transmission rights the Transmission Provider may have on a transmission path.

(ii)	Calculation Methodology

1.	For transmission facilities that will affect the Western Interconnection, the determination of TTC is accomplished through the WECC Path Rating Process. Seasonal Operating Transfer Capability (OTC) studies are completed to determine the limit at which a transmission path can be operated at and still meet reliability requirement under normal and outage conditions. The study results are reviewed and approved through Northwest Power Pool (NWPP) Northwest Operations Study Group (NOPSG) and WECC Operating Transfer Capability Planning Committee (OTCPC) regional processes.
	
2.	TTC is determined either prior to a new transmission component being brought into service or when a modification to a transmission component would affect the TTC.

3.	Once the TTC determination is made, it remains fixed and changes only if there is a physical or operational change to the transmission system or a transmission component, which requires a change to TTC.

4.	When transmission facilities are jointly owned, the capacity is allocated between the owners based on the joint ownership or participation agreement; therefore, the TTC of the jointly owned facilities will be based upon the capacity allocated to the Transmission Provider.

5.	If a WECC defined path must be separated into components to properly allow for the commercial use of the path and its components, the components’ TTCs will be based on the same studies used to determine the path OTC,including the thermal rating of the components. The sum of the components’ TTCs will not exceed the path OTC.

6.	For internal constraints, the net of local load and local generation may be used to determine TTC and/or ATC.

7.	Narratives explaining changes to monthly and/or yearly TTC are posted on OASIS in the Order 890 Folder.

(iii)	Databases Used in TTC Assessments

As required by the NWPP NOPSG, WECC OTCPC, and Good Utility Practice, the system base cases developed by the WECC members are used by the Transmission Provider in its TTC assessments.

(iv)	Assumptions Used in TTC Assessments

To ensure reliability is maintained within the Western Interconnection, the following variables are varied collectively as appropriate to yield flows that stress the transmission system. The variables include, but are not limited to:

1.	the network load level,
2.	resource/generation dispatch, and 
3.	import/export flows on paths to neighboring transmission systems.

The procedures of the Western Electricity Coordinating Counsel (WECC) for calculating TTC are followed and can be found at <http://www.wecc.biz/documents/library/OTC/OTCPC_HANDBOOK_06-29-07.pdf> and <http://wecc.biz/documents/library/OTC/PathSelection_Study_Guidelines_7-03-03.pdf>.  The  

Network load must be varied from light to heavy load conditions in order to determine the most stressful conditions on the transmission system.  Light network loads can produce high flow on export paths and heavy network loads can produce high flows on transmission serving local loads.  Generation within the network could be varied from 0 MW to maximum generation levels.  Generation level directly affects export path flows because there is more generation in the Balancing Area than there is load.  Import/export conditions are varied on paths to neighboring transmission systems to cause maximum stress on the transmission path that is being studied.  An example of how these variables might be combined and studied is:
i)	light local network  load, 
ii)	maximum generation in eastern Montana, 
iii)	maximum imports from eastern Montana paths
iv)	 and export conditions on the paths in western Montana.  

The outage conditions and performance criteria that are considered in these studies are defined by the NERC TPL Standards at  <http://www.nerc.com/~filez/standards/Reliability_Standards.html>) and WECC Reliability Standards TPL - (001 thru 004) - WECC - 1 - CR - System Performance Criteria at  (<http://www.wecc.biz/documents/library/Standards/Criteria/TPLstd001-004_4-28-08%20clean.pdf>).  These outages include all single and multiple line outages that could affect transmission system steady state and transient performance.  For paths that include the 500 kV lines that traverse Montana, all 500 kV single and credible double line outages are studied.  For lower voltage paths, all single and credible multiple outages that affect the path transmission system steady state and transient performance are studied.

(b)	Existing Transmission Commitments (ETC)

(i)	Definition

ETC is any transmission that is already committed for use and is calculated as follows:

Firm:  ETCF=NLF+NITSF+GFF+PTPF+RORF+OSF

Where: 
NLF is the firm capacity set aside to serve peak Native Load forecast commitments for the time period being calculated, to include losses, and Native Load growth, not otherwise included in Transmission Reliability Margin or Capacity Benefit Margin. 
NITSF is the firm capacity reserved for Network Integration Transmission Service serving Load, to include losses, and Load growth, not otherwise included in Transmission Reliability Margin or Capacity Benefit Margin. 
GFF is the firm capacity set aside for grandfathered Transmission Service and contracts for energy and/or Transmission Service, where executed prior to the effective date of a Transmission Service Provider’s Open Access Transmission Tariff or “safe harbor tariff.” 
PTPF is the firm capacity reserved for confirmed Point-to-Point Transmission Service. 
RORF is the firm capacity reserved for Roll-over rights for contracts granting Transmission Customers the right of first refusal to take or continue to take Transmission Service when the Transmission Customer’s Transmission Service contract expires or is eligible for renewal. 
OSF is the firm capacity reserved for any other service(s), contract(s), or agreement(s) not specified above using Firm Transmission Service as specified in the ATCID. 

Non-Firm:  ETCNF = NITSNF + GFNF + PTPNF + OSNF 

Where: 
NITSNF is the non-firm capacity set aside for Network Integration Transmission Service serving Load (i.e., secondary service), to include losses, and load growth not otherwise included in Transmission Reliability Margin or Capacity Benefit Margin. 
GFNF is the non-firm capacity set aside for grandfathered Transmission Service and contracts for energy and/or Transmission Service, where executed prior to the effective date of a Transmission Service Provider’s Open Access Transmission Tariff or “safe harbor tariff.” 
PTPNF is non-firm capacity reserved for confirmed Point-to-Point Transmission Service. 
OSNF is the non-firm capacity reserved for any other service(s), contract(s), or agreement(s) not specified above using non-firm transmission service as specified in the ATCID. 

.

(ii)	Calculation Methodology To Determine Transmission Capacity Set Aside for Native Load, Network Load, and Non-OATT customers:

Native load and network service type contracts are modeled using the megawatt quantity and other terms which are determined consistent with the OATT and the transmission customer’s loads and resources forecasts.

The Transmission Provider does not have any non-OATT customers, therefore a calculation methodology used to determine the transmission capacity to be set aside for non-OATT customers is neither necessary nor included in this Attachment C.

(iii)	Incorporation of Point-To-Point Transmission Service Requests

For existing, confirmed TSRs and to properly account for potential rollover
 requests, point-to-point type contracts are modeled using the specified megawatt quantity, point of receipt, and point of delivery.

(iv)	Accounting for Rollover Rights

In its determination of ATC, the Transmission Provider must assume that a Transmission Customer having the ability to do so, will rollover it’s existing long term TSR. To account for this, transmission in the amount of the confirmed TSR is set aside, thereby reducing the ATC be the same amount.

(v)	Process For Ensuring that Non-Firm Capacity is Release Properly

Transmission reservations that are not scheduled will be made available and posted on OASIS as non-firm ATC.

(c)	Available Flowgate Capacity (AFC) Methodology

The Transmission Provider does not use an AFC methodology to calculate ATC.

(d)	Transmission Reliability Margin (TRM)  

(i)	Definition

TRM is the amount of transmission transfer capability necessary to provide a reasonable level of assurance that the interconnected transmission network will be secure under a broad range of uncertainties in system conditions.  TRM accounts for the inherent uncertainty in system conditions and system modeling, and the need for operating flexibility to ensure reliable system operation as system conditions change.

(ii)	Calculation Methodology

The Transmission Provider does not use a calculation methodology in it determination of TRM.

(iii)	Databases Used in TRM Assessments

The Transmission Provider does not use any databases in its TRM assessments.
(iv)	Conditions Under Which the Transmission Provider Uses TRM

The Transmission Provider utilizes TRM for the following:

1.	Regulating Reserves (Load Following)

Transmission may be set aside to meet the Transmission Provider’s load following obligations. Because the full capacity of the load following service may be needed at anytime during any hour for reliability purposes, the capacity is not released for non-firm use.

2.	Operating Nomograms

On paths with operating nomograms TRM is utilized to reflect the difference between firm and non-firm transmission in its firm and non-firm ATC calculations for the path. The firm TTC is based upon the low point on the nomogram, with the TRM representing the amount of additional transmission that can be sold non-firm.  If this non-firm transmission is sold during preschedule, the transmission may be curtailed in real time based on system conditions.

3.	Operating Reserves and Contingency Reserves

TRM is not held out for operating reserves, but may be for contingency reserves depending on the location of the contingency reserves.
  
(e)	Capacity Benefit Margin (CBM) Practice

Because of Retail Access in Montana and the fact that the Transmission Provider does not own any generation to serve its native load, there currently is not any CBM set aside on any of the Transmission Provider’s transmission paths. Potential generation deficiencies are currently handled through Contingency Reserves. Should the generation situation change in Montana, or if any of the LSEs in the Transmission Provider’s Control Area request CBM be set aside (e.g., for load growth), these requests must be made on the OASIS via a Transmission Service Request, which will allow unused (i.e., unscheduled/un-tagged) CBM to be released for non-firm use. Requests for CBM will be evaluated on a case-by-case basis based on the transmission availability. The Transmission Provider will also reevaluate its own need for CBM annually.  Such reevaluation will take into account any changes in system conditions that might affect the need for CBM.

(f)	Capacity Benefit Margin (CBM)

(i)	Definition

CBM is the amount of firm transmission transfer capability reserved by Load Serving Entities (LSEs) on the host transmission system where their load and generation resources are located, to enable access to generation from interconnected systems to meet generation reliability requirements.  CBM is a uni-directional quantity with identifiable beneficiaries, and its use is intended only for the time of emergency generation deficiencies.

(ii)	Databases Used in its TRM Assessments

The Transmission Provider does not use any databases in its CBM assessments.

(iii)	No Double-Counting of Contingency Outages When Performing CBM, TTC, and TRM Calculations

Since TTC is based upon path rating studies that incorporate contingency outages, and the Transmission Providers determination and utilization of CBM and TRM are independent of contingency outages, there can be no double-counting of contingency outages.

(g)	Procedures for Allowing the Use of CBM

As described earlier, it is the Transmission Provider’s practice to not set aside transfer capability for CBM; therefore the Transmission Provider does not have procedures in place allowing for the use of CBM.

(h)	Postbacks 

Firm Postbacks (PostbacksF) will include the capacity values of Recalls, Redirects and Annulled Firm Reservations.

Non-Firm Postbacks (PostbacksNF) will include all Firm Postbacks plus the unscheduled firm capacity of confirmed Transmission Service.

(i)	Counterflows

The scheduled energy values of transactions utilizing either a Firm or Non-firm Transmission Service on the opposite path for which an ATC is being calculated, i.e. for the purposes of ATC counterflows are counter schedules.  Firm counter schedules will add capacity to the calculation of Non-Firm ATC in the Scheduling Horizon.