Dynamic Modeling and Validation Wind Power Plant Dynamic Modeling and Validation WECC Model Validation Working Group Denver, Colorado May 18-19, 2009 Eduard Muljadi Eduard.muljadi@nrel.gov National Renewable Energy Laboratory Golden, CO Abraham Ellis aellis@sandia.gov Sandia National Laboratories Albuquerque, NM

Wind Power Plant (WPP) Topology

Background (Dynamic Model of a Wind Power Plant) WTG wind turbine generator new line loss of Resizing Short Circuit Dynamic models are needed to study the dynamic behavior of power system. Users include system planners and operators, generation developers, equipment manufacturers, researchers, and consultants. Wind Power Plant (WPP) models are needed to study the impact of proposed or existing wind power plants on power system and vice versa (i.e. to keep voltage and frequency within acceptable limits). Models need to reproduce WPP behavior during transient events such as faults/clear events, generation/load tripping, etc.

Differences between Wind Power Plant and Conventional Power Plant Single Large (40MW to 1000MW+) generator Prime mover: Steam, Combustion Engine – non-renewable fuel Controllability: adjustable up to max limit and down to min limit. Located where convenient for fuel and transmission access. Generator: Synchronous Generator Fixed speed – no slip: Flux is controlled via exciter winding. Flux and rotor rotate synchronously. Wind Power Plant Many (hundreds) of wind turbines (1MW - 5MW each) Prime mover: wind turbine - wind Controllability: curtailment, ramp rate limit, output limit Located at wind resource, it may be far from the load center. Generator: Four different types (fixed speed, variable slip, variable speed, full converter) Type 3 & 4: variable speed with flux oriented controller (FOC) via power converter. Rotor does not have to rotate synchronously.

Wind Turbine Generator (WTG) Topologies Four basic types, based on the WTG technology: Type 1 – Fixed-speed, conventional induction generators Variable Slip WTG Type 2 – Induction generators with variable rotor resistance Variable Speed WTGs Type 3 – Doubly-fed asynchronous generators with rotor-side converter Type 4 – Asynchronous generators with full converter interface

Partial List of Different Types of Wind Turbines Reference: K. Clark, 2008 IEEE PES GM– Tutorial on Wind Generation Modeling and Controls – DPWPGWG 6

WPP Equivalent Representation Power Flow Representation of WPP in WECC WECC developed and adopted guidelines for WPP representation Based on single-machine representation Access to guidelines: www.wecc.biz -> Committees -> MVWG -> WGMG Major components of WPP Equivalent Representation: • Wind Turbine Generator (WTG) Equivalent and power factor correction (PFC) caps • Pad-mounted Transformer Equivalent • Collector System Equivalent branch. W Pad-mounted Transformer Equivalent Wind Turbine Generator Equivalent PF Correction Shunt Capacitors Collector System Equivalent Interconnection Transmission Line - Plant-level Reactive Compensation POI or Connection to the Transmission System Station Transformer(s)

Pad-mounted Transformer Equivalent #1 Multiple Turbine Representation In some cases, multiple turbine representation may be appropriate, for example: • To represent groups of turbines from different types or manufacturers • To represent a group of turbines connected to a long line within the wind plant • To represent a group turbines with different control algorithms. W Pad-mounted Transformer Equivalent #1 WTG Equivalent #1 of Type 3 Voltage controlled Collector System Equivalent #1 considered to be a long/weak line feeder 21 MW Station Transformer(s) Interconnection Transmission Line W Pad-mounted Transformer Equivalent #2 WTG Equivalent #2 Type 1 PF Correction Shunt Capacitors 34 MW Collector System Equivalent #2 POI or Connection to the Transmission System Total Output 100 MW W Pad-mounted Transformer Equivalent #3 WTG Equivalent #3 of Type 3 PF=1 45 MW Collector System Equivalent #3

Equivalent Collector System Depends on feeder type (OH/UG) and WPP size Zeq and Beq, can be computed from WPP conductor schedule, if available For radial feeders with N WTGs and I branches: Where ni is the number of WTGs connected upstream of the i-th branch This can be implemented easily on a spreadsheet 9

Equivalent Collector System Example with N=18 and I=21: 10

Equivalent Collector System Sample project data Some segments are overhead 11 11

Equivalent Pad-Mounted Xfmrs Assume identical ZT are effectively in parallel For N identical pad-mounted transformers, each with impedance ZT , the equivalent impedance ZTeq is: For 1.5 MVA to 3 MVA, 600V/34.5kV: ZT = 6% on transformer MVA base; adjustable (fixed) taps on pad-mounted transformer MVA base OR on N × pad-mounted transformer MVA base 12

Reactive Power Limits Type 1 and Type 2 WTGs (induction machines) At full output and nominal voltage, PF ~ 0.9 under-excited => Qmin = Qmax = Qgen = -½ Prated MSCs at WTG terminals maintain PF near unity at nominal voltage => Qcap = ½ Prated Example: 100 MW WPP, Type 1 WTG Pgen = Prated = 100 MW Qmin = Qmax = Qgen = -50 Mvar ~ Qcap = 50 Mvar

Reactive Power Limits Type 3 and Type 4 WTGs Line-side converter allows for PF adjustment at WTG terminals; MSCs at WTG terminals are not needed If WTG PF is fixed, Qmin = Qmax = Pgen × tan(cos-1(PF)) If WTG PF range is used for steady-state voltage control, set Qmin and Qmax according to PF range and Pgen WTG PF adjusted by plant-level controller. Patents may apply. Example: 100 MW WPP, Type 3,+/-0.95 reactive range, controlling POI voltage Pgen = Prated = 100 MW Qmin = -33 Mvar; Qmax = 33 Mvar Qgen depends on POI voltage ~ POI

Dynamic Representation

Power System Dynamic Time Scales Source: Dynamic Simulation Applications Using PSLF – Short Course Note – GE Energy

Example of Wind Turbine Model Objective of the model? (fault transient or long term dynamic, mechanical or electrical characteristics, power system transient or power quality of the wind power plant). • Major components (wind turbine type 1, 2, 3, 4, include aerodynamic or use simplified one, positive sequence or 3 phase representation, use complete generator and power electronics or simplified power conversion, include system protection?). For each component block, the equations governing the function of the block can be derived (assumptions may be made to simplify power converter, aerodynamic, saturation level, nonlinear circuits). Control algorithm will be formulated according to the wind turbine system to be modeled (WTG type 1 is different from WTG type 4 etc, different manufacturers may have unique algorithms). Choose the method of calculation and/or the software to be used (PSLF, PSSE for positive sequence representation, PSCAD, PSIM, EMTP if detail of the power electronics switching to be emphasized, MATLAB/Simulink, Mathcad may be considered for different aspects of simulation).

WECC Generic Models Generic model development in PSSE/PSLF Complete suite of prototype models implemented Type 3 model formally approved for use in WECC; others pending Current focus Model validation & refinement (e.g., freq. response) Identification of generic model parameters for different manufacturers (at NREL) PSLF PSSE The WECC WGMG work has produced a set of prototype generic models in PSSE and PSLF. They are not perfect, but they are better than the alternative (no models that are publicly accessible). Since the models are open source and supported by program vendors, the model refinement process can proceed. 18

WECC Generic Models Type 2 WTG Type 1 WTG Simplification in key areas. For example, pseudo-governor approach can emulate a wide range of approaches for turbine control. This simplification was possible by making sensible assumptions such as wind speed is constant during a dynamic simulation (20 seconds). However, models correctly initialize to any specified wind speed and power level. 19

WECC Generic Models Type 3 WTG Type 4 WTG The converter dominates dynamics in both Type 3 and Type 3 WTGs. Type 4 model does not require representation of the turbine. The models are good for most of today’s control approaches, but do not yet have advanced features such as inertial response. They will be refined. How to obtain data to represent a particular manufacturer is still an issue. We are working on that. 20

Method of WTG Model Validation Comparison against field test measurement Prepare the simulation carefully (i.e. the correct information must be used): type of WTG, collector system impedance, transformers, power system network, input parameters to dynamic models, control flags settings set-up, reactive power compensation at the turbine level or at the plant level. Initialize the simulation based on pre-fault condition (check v, i, p, q, f, if available). Recreate the nature of the faults if possible, otherwise use the recorded data to drive the simulation and compare the measured output to the simulated output (pre-fault, during the fault, post-fault). Represent the events for the duration of observation (any changes in wind, how many turbine were taken offline due to the fault?). Prepare the data measured to match the designed frequency range of the software used. Field data is expensive to monitor, public domain data is limited, difficult to get, and quality of data needs to be scrutinized Anticipate errors in the measurement and make the necessary correction The location of simulation should be measured at the corresponding monitored data.

Example of Dynamic Model Simulation versus Field Data (Type 3) Collector System Equivalent Pad-mounted Transformer Equivalent Interconnection Transmission Line Station Transformer(s) POI or Connection to the Transmission System W 91% WTGs stays “on” after the fault. Two Turbine Representation W 9% WTGs were dropped of line during the fault. Interconnection Transmission Line POI or Connection to the Transmission System Station Transformer(s) 136 WTGs were represented 9% WTGs were dropped of line during the fault. Complete Representation (136 turbines) 22

Example of Dynamic Model Simulation versus Field Data (Type 3) W Wind Turbine Generator Equivalent Input V and f A C B System Generator Compare P&Q measured to P&Q simulated Regulated Bus 23

Method of WTG Model Validation Comparison against other model (Benchmarking) Another method to validate new model is to use another model that has been validated against field measurement as a benchmark model. Several transient fault scenarios can be performed using both models, and the results can be compared. Parameter Tuning The new model and the benchmark model may have some differences in implementation, we may have to perform parameter tuning to match the output of the benchmark model. However, one should realize that the model may not be able to match the output of the benchmark model in all transient events. Parameter Sensitivity In order to limit the number of parameters that should be tuned, parameter sensitivity analysis may need to be performed. In general important parameters are varied one by one and the sensitive parameters can be tuned to match the bench mark model.

Example of Model to Model Comparison (Type 2 “Detailed” Model vs Generic Model) Terminal Voltage Real Power Turbine Speed Reactive Power 25

Model with the parameter Parameter Sensitivity The output of the simulation and the measured data can be used to find the total error of the measurement. Perr = |Pmeas.-Psimulated| Qerr = |Qmeas.-Qsimulated| The error and the sensitivity parameter k1 with respect to the error can be computed. Use the other parameters k1, k2, k3, k4 etc The parameter sensitivity can be observed from the results. The trend can be used to drive the changes of the parameters. Actual Wind Plant + Model with the parameter to be tuned - Input T(k) Parameter k

Parameter Sensitivity - Example

Sensitivity of P output to a range of parameters. Parameter Sensitivity - Example Response of P output. Qualitatively similar results for other outputs. Note that a lot of parameters have small and/or correlated influence. Sensitivities obtained as a by-product of running the simulation. Sensitivity of P output to a range of parameters.

Summary Wind power plant model is different from conventional synchronous generator modeling in different aspects: Different types of generators used (level of capability for reactive compensation, voltage controllability, and LVRT are different) Wind power plant represents hundreds of generators (i.e. the collective behavior of all turbines is more important than the behavior of individual turbines) Wind power plant covers a very large area. During faults, each generator may have different operating condition with respect to other generators due to diversity within the wind plant. The simplification or equivalencing wind power plant may compromise the accuracy of the simulation, however, a complete model requires to represent hundreds of turbines (impractical) In some cases, components of the system needs to be simplified for many different reasons (NDA, complexity, time constant of interest).