A Fundamental Study of Applying Wind Turbines for Power System Frequency Control

Transcription

1 A Fundamental Study of Applying Wind Turbines for Power System Frequency Control CFES Annual Conference February 26, 2015 Felipe Wilches-Bernal and Joe H. Chow Rensselaer Polytechnic Institute 2/26/2015 1

2 Presentation Outline Introduction WTG models Frequency control overview Frequency control and wind generation Control to enable WTG with frequency response Conclusions 2/26/2015 2

3 Introduction Wind generation is increasing rapidly in power systems through the world. Wind is clean, renewable, mature and economically ready to compete with conventional generation. 2/26/2015 3

4 Introduction Wind generation is increasing rapidly in power systems through the world. Wind is clean, renewable, mature and economically ready to compete with conventional generation. Total installed capacity in the US 2/26/2015 4

5 Introduction WTG fundamentally different than conventional generation - Power electronic interfaces decouples WT rotor inertia. - WT prime mover (the wind) is not dispatchable. Primary frequency control or frequency response is affected by wind integration. 2/26/2015 5

6 WTG Models Doubly fed asynchronous generator (DFAG) or Type-3 WTG is used in this work. Type-3 the most popular WTG device in the US. Power transfer occurs partially through power electronics (around 30% for Type-3). Device operates at a variable speed to maximize the wind energy capture. Doubly Fed Asynchronous Generator Gear Box AC/DC Converter DC/AC Converter Type-3 WTG 2/26/2015 6

7 WTG Models Type-3 seen from the grid is a controlled current source I t Power Grid I t Power Grid I s I s V t V t Type-3 Type-4 Power electronics interface has current control loop with fast dynamics that are neglected for power system transient stability studies. Power electronics allows for a separate control of active and reactive power. Modeled as a PV bus (generator type of bus) for loadflow studies. 2/26/2015 7

8 Frequency Control Overview Frequency is a global variable visible throughout the entire power system. Frequency depends on the balance of generation and load. Actions to control frequency are applied in different stages and time frames: 2/26/2015 8

9 Frequency Control Overview The important factors to take into account in frequency response for a loss of generation event. Rate of Change of Frequency determines how fast the frequency is decreasing. Frequency nadir maximum frequency excursion. Settling frequency the frequency to which the primary frequency control stabilizes the system. 2/26/2015 9

10 Frequency Control Test System Test system: Modified version of the two-area, four-machine system. G G1 PSS G3 PSS Swing Bus G2 L17 L19 G4 System described by 66 states 6 states each machine using a sub-transient model (30 states) TG and Excitation System 3 states each (30 states) PSS in Generators 1 and 3, 3 states each (6 states) 2/26/

11 Frequency Control Small Signal Analysis Test System Frequency Response Changes with machine parameters Changes with load modeling 2/26/

12 Frequency Control Small Signal Analysis Test System Frequency Response Eigenvalue most affected is a complex pair at ±j Referred to as frequency regulation mode. Participation factor calculation shows that all generators participate in this mode. 2/26/

13 Frequency Control and Wind Generation Integrating Type 3 WTG into the Test System Type-3 WTG replaced completely Generator 4. System consist of 71 states (17 of which correspond to the WTG) Same loss of generation event was simulated for different wind speeds. 2/26/

14 v w P gsig Frequency Control and Wind Generation Proposed Control Intentionally pitch the blades to spill power and create a headroom. Governing control: pitch angle made to respond to frequency variations. Wind power model Turbine/ Generator model Pitch delay and limiter f( ) reference speed g Pitch angle compensator ref P gsig Pitch angle control Torque Control PFC P setlf g Frequency Control P ord Active power delay and limiter f Transient Correction (WindINERTIA) Pord Add signal to the pitch compensator loop. f P wini._. V t I pcmd Natural approach: proportional control with a deadband f ref f bus Deadband K wi P FC 2/26/

15 Frequency Control and Wind Generation Proposed Control Validation in the test system at 25% wind penetration for different wind speeds From small signal analysis it can be seen that the proposed control loop interacts and destabilizes the frequency regulation mode. 2/26/

16 Frequency Control and Wind Generation Proposed Control Participation factor calculation shows that wind participates in the frequency regulation mode 2/26/

17 Frequency Control and Wind Generation Frequency regulation mode sensitivity analysis on the control parameters of the WTG Note that Kpp is the constant that has the most effect on the mode 2/26/

18 Frequency Control and Wind Generation Proposed Control Modifying Kpp Validation in the test system at 25% wind penetration for different wind speeds Small signal analysis shows that the frequency regulation mode is less sensitive to the proportional frequency feedback 2/26/

19 Frequency Control and Wind Generation Proposed Control Transient Correction To further improve RoCoF and frequency nadir a transient frequency controller is added. g f Active power delay and limiter Transient Correction (WindINERTIA) Pord P wini._. I pcmd Control signal added to the power order signal (to go to the converter model) to boost the WTG power output immediately V t PFC P ord P setlf Frequency Transient f frequency controller structure: Control f ref Deadband 1 1 st lpwi K wi st 1 st wowi wowi P wini f bus 2/26/

20 Frequency Control Overview Proposed Control Transient Correction Root locus analysis with the transient controller to make sure it does not interact negatively with the frequency regulation mode Parameter determination for transient control 2/26/

21 Frequency Control Overview Proposed Control Comparison Comparison of different control approaches in the test system for 25% of wind generation. 2/26/

22 Frequency Control Overview Proposed Control Comparison Comparison of different control approaches in the test system for 25% of wind generation. 2/26/

23 Frequency Control Overview Increased Wind Penetration Proposed control was validated in the test system for a wind penetration of 50% Generators 2 and 4 replaced by Type-3 WTGs G Swing Bus G1 PSS G3 PSS 2 4 WTG 2 System described by 76 states L L19 WTG 1 6 states each machine using a sub-transient model (18 states) TG and Excitation System 3 states each (18 states), PSS G1 and G3 (6 states) 17 states each WTG (1 mass model, reactive power control: voltage regulation) 2/26/

24 Frequency Control Overview Increased Wind Penetration Root locus analysis for 50% penetration Increasing Kpp damps the frequency regulation mode when wind is integrated independently in each area (25% wind penetration) and for the case when is present in both areas (50% wind penetration) 2/26/

25 Frequency Control Overview Increased Wind Penetration Proposed control implemented in each of the WTG 50% of wind unresponsive to frequency is deleterious to the system. Enabling frequency control in WTGs improves considerably the frequency response. Small signal analysis shows that the frequency regulation mode is less sensitive to the proportional frequency feedback. 2/26/

26 Frequency Control Overview Increased Wind Penetration Proposed control tested for a case where WTG at Bus 4 experiences a sudden drop of wind speed creating the generation shortage The remaining conventional generation combined with the WTG at Bus 2 are able to provide enough frequency response. 2/26/

27 Conclusions Frequency regulation for the test system is mainly determined by a system mode that is affected by integrating wind. Integrating wind generation without frequency control deteriorates the frequency response of the test system. Including proper frequency regulation controls in wind generation improves considerably the frequency response of the system. Root locus and linearization analysis can be effectively used to provide insight in control parameter tuning to achieve a stable controller design. 2/26/

28 Acknowledgment This research is supported in part by the Global Climate and Energy Project (GCEP) from Stanford University and in part by the Engineering Research Center Program of the National Science Foundation and the Department of Energy under NSF Award Number EEC and the CURENT Industry Partnership Program. 2/26/

29 Questions? 2/26/

30 Thank you! 2/26/