Introduction

ATC also designs its system to meet stability criteria that are more stringent than NERC Standards. In the Planning Criteria section of this report, the Transient and dynamic stability performance assessment discussion gives details about the ATC’s criteria for assessing system stability.

Reviewing compliance with NERC Standards and ATC stability criteria is a continuous process. Each year ATC adds to its library of studies. There are two components to consider in assessing system stability. One component is the angular stability of the system or often more generally referred to as generator stability. The second component is the system’s voltage stability. Our approach to assessing both of the system stability components is described below.

Generator Stability

For each 10-Year Assessment, generator stability is screened or assessed at all major generator stations connected to the ATC system. Numerous generator interconnection studies add to our knowledge of the ATC system stability response to Category B, C and selected D outages. A MRO/RFC joint on-site review completed in December 2007 determined that ATC was fully complaint with NERC Standards that cover multiple outages (Category C), including the system’s stability response to multiple outages.

In the 2008 10-Year Assessment, we have revisited a select list of generator stations as described below. As generator stability concerns arise they are evaluated and appropriate corrective actions are developed and implemented. Generator stations with total net output above 100 MW and associated transmission lines operating above 100 kV are generally selected to assess system angular stabilities. 

The methodology used in screening or assessing the major generator stations includes a review to determine that no significant system topological changes have occurred near the generator stations other than local load growth.  In addition, the methodology includes a review of the parameter values and the model types used to represent the dynamic response of the units at the generator stations in system angular stability simulations to determine that no significant changes have occurred.  This methodology also includes a review of the date the last time a stability study was conducted for a major generator station to determine that the elapsed time does not exceed 5-years.  Considering the number of existing major generator stations shown in Table ZS-7 - ATC System Angular Stability Assessment this requires that at least 6 major generator stations be included in the system angular stability analysis for each 10-Year Assessment in order to complete a study of all major generator stations in a 5-year sequence.

If these criteria are confirmed, the generator stability results from the existing studies are still applicable and are acceptable in the following years with proposed system upgrades. If any of these criteria are not met then the generator stability is screened or restudied.

In the 2008 10-Year Assessment the power flow models were compared with the 2006 power flow models.  In addition, the parameter values and types of dynamic models (e.g. generator, exciter, power system stabilizer, governor etc.) currently used to represent the major generator stations in dynamic simulations were compared with those in the 2006 TYA studies.  The review identified seven (7) generator stations that had either: a) significant system topological change near the station, b) a significant change in the parameter values or type of dynamic model used to represent the unit responses during simulation, c) were at or approaching the 5-year elapsed time criteria or d) a combination of the criteria.

The seven (7) generator stations identified are: Paris, Port Washington Combined Cycle Block 1 (CC1) and Combined Cycle Block 2 (CC2), Fox Energy, Forward Energy, University, Concord and Presque Isle.  These stations are shown high-lighted in Table ZS-7 - ATC System Angular Stability Assessment.

Concord and Paris generator stations had parameter changes for the each of the exciter models.  Fox Energy had changes to the type of governor model for the two combustion turbine units as well as parameter changes for the generator, exciter and power system stabilizer models of one of the combustion turbine units.  University involved a correction in the representation of the generator step-up transformer from a single three-winding transformer to two two-winding transformers and parameter changes for steam turbine generator model.  Forward Energy had a change in number of units installed at the wind farm from 104 to 86 units and was within one year of the 5-year elapsed time criteria for the system angular stability analysis.  It also involved the change from the using a simple induction generator model to the new WT3 generic wind model for use in representing the unit in system dynamic simulations.  All these major generator stations were re-studied as part of the system angular stability analysis of the 2008 10-Year Assessment with the ATC criteria applied.

The Port Washington Generator Station CC1 (in service date 2008) and CC2 (in service 2005) were last studied in 2005 just after CC2 was placed in service.  The study used the “as built data” for CC2 units and original data for CC1 units from the 2003 study to evaluate the impact of an additional capacity increase primarily available in winter operating conditions.  The "as built data" for the CC1 units recently became available and needs to be studied to verify the results from the previous studies and include any update for the stability models or their associated parameters.  A separate unit verification study was conducted in 2008 and satisfies the requirement for a re-study of these stations as part of the system angular stability analysis of the 2008 Ten-Year Assessment.  Both studies are noted in the Table ZS-7.

The Presque Isle generator station involves changes to parameter values of each of the generator models; plus updates to the exciter and power system stabilizer models on 5 of the 7 units, as well as, the addition of a power system stabilizer on one unit.  In 2007 a separate study was conducted addressing the special protection system at Presque Isle and satisfies the requirement for a re-study of this station as part of the system angular stability analysis of the 2008 10-Year Assessment and study is noted in the Table ZS-7. 

As shown in Table ZS-7 - ATC System Angular Stability Assessment, all assessed generators in the ATC area meet the applicable NERC Category B2, C3, C4 and D2 criteria.

Voltage Stability

ATC is still developing a rigorous process for assessing voltage stability across the system. Currently we monitor single and multiple contingency voltages throughout the ATC system to screen for indications of where voltage stability may be an issue.

A detailed voltage stability analysis was last performed in the Rhinelander area of Zone 1 in 2003 that covered the period from 2003 to 2005.   As a result of this study, solutions were developed and implemented in the Rhinelander area to address the voltage stability concerns that were found.   These solutions included the following:

• Installation of two new, high speed 115 kV breakers (2 cycle) at Aurora Street on A-313 and Highway 8 on D-56 with maximum primary clearing time of 3.75 cycles in 2003.
• Installation of a 16.9 MVAR, 115 kV capacitor bank at Summit Lake Substation in 2003.
• Relocation of the Reedsburg 6.0 MVA D-SMES unit to the Clear Lake Substation in 2004.
• Conversion of the Pine to Eastom 46 kV line to 115 kV in 2004.
• Installation of a new 115 kV line from Skanawan to Highway 8 in 2005.

It was decided that a new detailed voltage stability analysis be performed in the Rhinelander area of Zone 1 for 2008 to cover the period from 2008 to 2013 to assess the impact of these improvements along with the addition of a new Cranberry-Lakota Road (Conover) 115 kV line in 2008 and the rebuild/conversion of the Lakota Road (Conover)-Plains 69 kV path to 138 kV in 2010.  The Lakota Road Substation includes a 138/115 kV transformer to interconnect the 115 kV and 138 kV lines together and includes a 138/69 kV transformer to connect to the Conover 69 kV system.  The Lakota Road (Conover) -Twin Lakes - Iron Grove segment of the Lakota Road (Conover)-Plains 138 kV line is scheduled for completion in 2009 and the Iron Grove-Aspen-Plains 138 kV segment is scheduled for completion in 2010.  In addition, the study also includes evaluation of the continued need for the D-SMES units and Undervoltage Load Shedding (UVLS) relaying in the Rhinelander area.  The work on this study is progressing and we will report on these results in either an update of this Assessment or in a future Assessment.

The MRO/RFC joint on-site review completed in December 2007 determined that ATC was fully compliant with the voltage stability assessment requirements in the applicable NERC standards.

Conclusion

Based on these assessments and numerous other studies, the ATC network will meet NERC System Stability Standards assuming reinforcements contemplated in this 10-Year Assessment, operating procedures, and special protection systems are implemented.