2009 West-North Stability Limit Study

2009 West-North Stability Limit Study

Reliability Assessment GroupERCOT System Planning

Phase II – Effects of Adding PSS on System Limits

Regional Planning Group Meeting, Jan 2010

2 Regional Planning Group MeetingJan 2010

Outline

• Study Objectives and Assumptions• Introduction

What is the West-North Stability Limit and how is it calculated? System Oscillation – how is it related to stability limits? How to improve system response to oscillations?

Power System Stabilizers (PSS) Will this improvement increase the system transfer limits on the

West-North interface?• Study Results and Observations• Discussion


Study Objectives and Assumptions


The West-North Stability Limit

• The 2009 West-North Stability Limit study identifies six 345kV transmission lines from the West-North interface to be monitored for power transfer. When the flow along this interface exceeds a set limit (dependent on the operating conditions), the system would experience low-frequency oscillations in the event of contingency, due to generators in one region swinging against generators in another region. This limit is defined as the West-North transfer stability limit.

• The six 345kv transmission lines monitored to maintain the WN transfer stability limit are

– Long Creek to Graham– Cook Field To Graham– Tonkawa to Graham– Bowman to Graham– Bowman to Jacksboro– Red Creek to Comanche

• WN transfer stability limit is decided based on the combination of – Line outage– Unit outage– Contingency


System Oscillations

• When perturbed from steady-state operation by fault/contingency, system shows an initial transient response. Assuming there is no angular separation, the system then attempts to settle around a new operating point.

• If in the post-transient operation, a natural mechanical mode of the system is poorly damped, then low frequency local or inter-area oscillations in the range of 0.2 – 2 Hz appear.

• These system oscillations and their associated damping, form the small-signal stability response of the network.

All limits in this study were determined either by angular instability (separation) or by the low-frequency oscillation damping ratio.

– If post-transient, one or more machines show angular separation then the system is assumed to be operating beyond limits.

– If post-transient, the small-signal damping ratio is less than 3.5%, the transfer is assumed to be beyond limit.


System Oscillations – Damping Coefficient

22


System Oscillations – Damping and Instability

(a) Angular Instabilityfirst-swing

(d) Well Damped

(b) Angular InstabilityNegative Damping

(c) Under Damped


System Oscillations – Modes of Oscillation

REFERENCES –1. V. Larsen and D. A. Swann, “Applying Power System Stabilizers Part II: Performance Objectives and Tuning

Concepts”, IEEE Trans. On Power Apparatus and Systems, Vol. 100, 1981, pp 3025-3033

• Types of System Modes of Oscillation

– Inter-area or Inter-Tie Modes

• The aggregate of units at one end of an inter-area tie oscillating against the aggregate of units at the other end.

– Local Modes

• Oscillations occurring at generating units connected to large power system through weak (mostly radial) transmission lines.


Power System Stabilizers

REFERENCES –1. V. Larsen and D. A. Swann, “Applying Power System Stabilizers”, IEEE Trans. On Power Apparatus and Systems, Vol.

100, 1981, pp 3017-3046

• Power System Stabilizer (PSS):– A device used to damp low-frequency oscillations by modulating generator

excitation and as a consequence, extending power transfers limited by these oscillations.

• A PSS uses shaft speed, AC bus frequency and/or electrical power as input signals.

NOTE: The PSS is a device that can efficiently improve system damping ratio for local and inter-area oscillatory modes. Other means are used to control/check first-swing transient instability.


Power System Stabilizers – Operating considerations

REFERENCES –1. V. Larsen and D. A. Swann, “Applying Power System Stabilizers Part II: Performance Objectives and Tuning Concepts”,

IEEE Trans. On Power Apparatus and Systems, Vol. 100, 1981, pp 3025-3033

• It is not unusual for a generating unit to participate in both local and inter-area modes of oscillation. Power system stabilizers must therefore be able to accommodate both modes.

– Since a single unit or small set of units is dominant in local modes, the PSSs can have a very large impact on damping the oscillation.

– In an inter-area mode a single unit experiences only a portion of the total magnitude of power oscillation. The PSS’s contribution to the damping of an inter-area mode is dependent on the extent of participation of the unit in that mode.

• A PSS should be designed to provide adequate local and inter- area mode damping under all operating conditions, with particular attention to conditions of heavy load and weak transmission.


PSS – Assumptions and Limitations of the Study

• Study performed for three conditions applied to every unit and line outage scenario –

– Morgan Creek CTs Offline– Permian Basin CTs Offline– Both sets of CTs Offline

• At least one of the units where additional PSS were placed was online in every scenario examined– Any PSS will only have an effect on system oscillations when the unit it is

connected to is online and the unit’s AVR is on.

• This study assumes that all existing stabilizers are already tuned to deal with local and inter-area oscillatory modes.


Effect of adding PSS – Choice of Units

• The units from the West CSC zone were chosen to study the effects of adding PSS on the system stability limits because clearly, they have the most positive impact on system oscillations.

Unit Bus # Unit Name Participation Unit Type Zone 1002 Permian Basin CT1 1 CT West1003 Permian Basin CT2 1 CT West1004 Permian Basin CT3 1 CT West1005 Permian Basin CT4 1 CT West1006 Permian Basin CT5 1 CT West1007 Permian Basin Unit5 1 RMR West1039 Morgan Creek CT1 1 CT West1040 Morgan Creek CT2 1 CT West1041 Morgan Creek CT3 1 CT West1042 Morgan Creek CT4 1 CT West1043 Morgan Creek CT5 1 CT West1044 Morgan Creek CT6 1 CT West1045 Cal Energy Unit1 1 Co-Gen West1046 Cal Energy Unit2 1 Co-Gen West1047 Cal Energy Unit3 1 Co-Gen West1069 Kinder Morgan 0.917 CC, SS West1320 Big Spring Carbon Plant 1 0.917 Co-Gen, SS West1479 Wichita Falls 0.17 CC West1482 Wichita Falls 0.17 CC West1433 Graham 0.14 Base Load NG West

Units with highest participation in System OscillationsPlot of the effect of various combinations of PSS

Highlighted units not chosen –

Permian Basin Unit 5 is RMR and likely to be phased out of operation.

Units at Kinder Morgan, Big Spring, Wichita Falls and Graham are too small.


Effect of adding PSS – Choice of Units (contd.)

• Limiting assumptions: The tuning of existing PSS as a means of

improving stability limits was not considered.• Existing PSS models should be validated

and tuned for the 0.2 to 2 Hz range to cover both inter-area and local modes of oscillation.

• It is assumed that all existing stabilizers are already tuned to deal with these modes.

The new PSS models added in this study utilize parameters of the most common PSSs in the model database and do not eliminate the need for tuning of the PSS upon installation.

Units chosen as candidates for addition of PSS – Permian Basin CTs Morgan Creek CTs Cal Energy Gas Turbine Units

(From DYRE 77)

Primary Mode Damping for several combinations of PSS

DYRE Additional Unit with PSSDamping (%)Primary mode

= 0.774Hz77 PB CTs, MC CTs & Cal Energy (1 & 2) 7.1275 PB CTs, MC CTs & Cal Energy 6.6566 PB CT 1 & Monticello 1 3.749 PB CT 2 3.6150 PB CT 3 3.5852 PB CT 5 3.5844 PB CT 1 3.5851 PB CT 4 3.5625 PB unit 5 & Monticello 1 3.520 None 1.37


Effect of adding PSS – Improvement in Damping

Unit Outage System Damping Improvement

(For the Limiting case without PSS) No Additional PSS PSS - DYRE 77

27 3.41 12.6836 3.26 12.535 3.04 9.7126 2.2 9.0229 3.38 8.628 3.12 8.3432 Unstable 8.2324 3.49 8.068 3.3 89 3.14 834 2.73 7.8633 3.41 7.7811 3.18 7.6925 2.35 7.6530 2.78 7.5420 2.17 7.57 Unstable 7.4931 0.5 7.485 2.81 7.1319 3.26 7.0817 3.19 718 2.33 73 2.41 6.814 1.95 6.761 2.44 6.6421 1.06 6.52 2.03 6.4722 Unstable 6.1810 1.32 5.9812 2.08 5.9314 -0.38 5.7313 -0.64 5.716 Unstable 5.615 Unstable 4.2916 Unstable Unstable 23 Unstable Unstable


Effect of adding PSS – Improvement in Transfer Limits

Unit Outage Transfer with No Additional PSS Δtransfer with PSS

9 116 14198 490 112736 434 112227 965 105426 1174 100318 75 99335 619 95417 282 8673 1090 34211 378 32512 555 30529 869 30314 718 29121 1654 27724 1923 27625 1638 2762 910 2674 1074 2551 772 24820 1406 24219 1283 21630 1122 1995 1282 19734 1232 19028 704 17513 554 1547 1325 1496 1475 14815 1007 14810 250 14722 1642 13732 1322 13231 1067 11033 1573 1623 1933 016 856 0

Stability Limits for Line Outage L13


Observations

• Addition of PSS drastically improved system damping – consequently, system small-signal stability limit also saw a dramatic increase.

• The PSS is the device most commonly used to improve the system damping ratio for low frequency local and inter-area oscillatory modes. There were several cases where transient instability from negative damping changed to a well-damped response after stabilizers were added.


Study Conclusions

• With additional power system stabilizers on the West Texas Permian Basin CTs, Morgan Creek CTs, and Cal Energy gas turbine units the West-North Stability Limits are found to increase under all study conditions (all of which have one or more of these units online) with few exceptions. There is no change in the Limits when these units are not online.

• The Permian Basin CTs or Morgan Creek CTs are peaking units rarely deployed.• The best units for addition of PSS would be at Cal Energy.

• When either PB or MC CTs are removed from the dispatch, the West-North Stability Limits would increase due to much lower small-signal oscillations. The highest small-signal stability limits are determined when the Permian Basin CTs as well as Permian Basin unit 5 (RMR) are not included in the system dispatch.

• Separate analysis is underway to determine how frequently these units are online when the Stability Limits are binding.


Documents

2009 West-North Stability Limit Study