Wind Power Generation Power Flow and Dynamic Modeling

Wind Power Generation Power Flow and Dynamic Modeling Vadim Zheglov EnerNex, Tennessee vzheglov@enernex.com Eduard Muljadi NREL, Golden CO eduard.muljadi@nrel.gov UWIG-EnerNex Modeling Workshop Albany, NY July 5-6, 2011

Background

Conventional vs. Wind Power Plant Load Other Conv. Generator Load POI or Point POI of or Interconnection to the grid Collector Collector System System Station GSU Xfmr Interconnection Transmission Line Line Large Synchronous Generator Prime Mover Individual WTGs WTGs Feeders and Laterals (overhead Feeders and Laterals (overhead and/or underground) and/or underground)

Power Generation Conventional vs Wind Power Plant Single or multiple large (100 MW) generators. Prime mover: steam, combustion engine non-renewable fuel affected by fuel cost, politics, and pollution restrictions. Controllability: adjustable up to max limit and down to min limit. Predictability: preplanned generation based on load forecasting, influenced by human operation based on optimum operation (scheduled operation). Located relatively close to the load center. Generator: synchronous generator Fixed speed no slip: flux is controlled via exciter winding. Flux and rotor rotate synchronously. Many (hundreds) of wind turbines (1 MW 5 MW each) Prime mover: wind (wind turbine) renewable (free, natural, pollution free) Controllability: curtailment Predictability: wind variability based on wind forecasting, influenced more by nature (wind) than human, based on maximizing energy production (unscheduled operation). Located at wind resource, it may be far from the load center. Generator: Four different types (fixed speed, variable slip, variable speed, full converter) non synchronous generation Type 3 & 4: variable speed with flux oriented controller (FOC) via power converter. Rotor does not have to rotate synchronously.

Power Generation Types of Wind Turbine Generator Four basic topologies based on grid interface: Type 1 conventional induction generator Type 2 wound-rotor induction generator with variable rotor resistance Type 3 doubly-fed induction generator Type 4 full converter interface Type 1 Type 2 Type 3 Type 4 generator Plant Feeders PF control capacitors generator Slip power as heat loss ac to dc Plant Feeders PF control capacitors generator ac to dc dc to ac Plant Feeders generator ac to dc dc to ac full power Plant Feeders partial power

Power Flow Modeling Wind Turbine Generator

WPP Representation POI or connection to the grid Collector System Station Interconnection Transmission Line Feeders and Laterals (overhead and/or underground) Individual WTGs Collector System Equivalent R eq jx eq B eq /2 B eq /2

Equivalencing

Equivalencing

Pad-Mounted Transformer

Reactive Compensations Represented by Separate Model Type 1 and 2 WTGs are induction machines: Several stages of capacitors banks at the WTG terminals are normally applied. Net power factor at bus 5 ~ 1.0 In power flow: modeled as fixed shunt devices WTG of type 1 is approximately PF=0.9 therefore the capacitor need is about to be ½ of the power output. example, for a 100 MW WPP at full output, Q min = Q max = -50 Mvar and add a 50 Mvar shunt capacitor at the WTG terminals. Plant level reactive compensation may still be installed to meet interconnection requirements and should be explicitly represented in power flow.

Reactive Compensations Type 3 and Type 4 WTGs (an estimate to start depending on the terminal voltage) These WTGs are capable of adjusting power factor to a desired value within the rating of the generator and converter. They are also capable of voltage control at the interconnection point or at its terminals. External reactive power compensation is often required to meet interconnection requirements If these WTGs do not participate in voltage control, the equivalent generator should be assigned a fixed power factor, typically unity. (i.e., Q min = Q max = 0). If the WTGs do participate in voltage control, then the equivalent generator should be assigned a reactive capability approximately equal to the aggregate WTG reactive power range (i.e., Q min = - S rated tan(cos -1 (0.90); and Q max = S rated tan(cos -1 (0.95)) ). For example, consider a 100 MW WPP that employs Type 4 WTGs with specified power factor range +/-0.95 at full output. In this example, Q min should be set to -33 Mvar and Q max should be set to +33 Mvar. At an output level below rated, the reactive limits should be adjusted according to the WTG capability curve.

Reactive Power Flow I + V A - ji X + V B - All Q comes from A A V A ji X I V A > V B ; B = 0 V B = 1.0 All Q comes from B B V A ji X I V A < V B ; A = 0 V B = 1.0 A = B V A = V B Q A =Q B = 0.5 I 2 X V A = 1.0 Q A =I 2 X ; Q B = 0 V A Q B =I 2 X ; Q A = 0 V A V A V A A B I ji X V B = 1.0 Equal Q (VAR) contribution I I I V B = 1.0 I V B = 1.1 Overexcited V A > V A ; Q A =I 2 X ; Q B < 0

Reactive Compensations POI or connection to the grid Collector System Station Turbine close to substation Interconnection Transmission Line Turbine far from substation Individual WTGs Feeders and Laterals (overhead and/or underground) X 100 > X 1 IX 100 > IX 1

Practical Limit of Reactive Power Output Due to collector system effects, some WTGs in the WPP will actually reach terminal voltage limits before reaching the nameplate reactive power limits. The net effect is that actual reactive power capability could be less than the nameplate. The reactive power capability can be determined by field test or careful observation of WPP performance during abnormally high or low system voltage. For example, Figure 7 shows the results of field tests to determine the practical reactive limits of a 200 MW WPP. All measurements were made at the interconnection point. Taking into account the effect of transformer and collector system impedances, the reactive power limits of the equivalent WTG can be established. Currently, there are no industry standard guidelines for testing WPP steady-state reactive limits.

Practical Limit of Reactive Power Output +95 and V A V POI V inf reactor I

Practical Limit of Reactive Power Output V A V POI V inf I

Detailed Vs. Single-Machine Representations 3-phase fault, all WTGs at 12 m/sec Q WT = 0.435 0-0.435 Validation P 34.5 kv Q 34.5 kv From «Validation of the WECC Single-Machine Equivalent Power Plant», Presented DPWPG-WG Meeting at IEEE PSCE, March 2009 - Jacques Brochu, Richard Gagnon, Christian Larose, Hydro Quebec

Validation Q WT = Detailed Vs. Single-Machine Representations 3-phase fault, different wind speed for each feeder 0.435 0-0.435 1 and 2 feeders P 34.5 kv 4 feeders = Typical 2 and 4 feeders = Typical Q 34.5 kv 1 feeder From «Validation of the WECC Single-Machine Equivalent Power Plant», Presented DPWPG-WG Meeting at IEEE PSCE, March 2009 - Jacques Brochu, Richard Gagnon, Christian Larose, Hydro Quebec

Wind Power Plant Network Infinite Bus Ideal Gen 230 kv Line 1 R1, X1, B1 1 230 kv Line 2 2 3 R2, X2, B2 34.4/230 kv station transformer Rt, Xt. Station level shunt compensation 34.5 kv collector system equivalent Re, Xe, Be 4 Turbine level shunt compensation 5 0.6/34.4kV equivalent GSU transformer Rte, Xte Gen 100 MW equivalent wind turbine generator

Power Flow Power Flow Prepare the network (line branches, transformers, generators, and the loads). Set the level of the loads and generations. For wind turbine generator: Type 1 and 2 set the level of real power and reactive power, set the maximum and minimum limits of the real and reactive power. set the level of capacitor compensation. Type 3 and 4 set the level of real power and reactive power, and set the maximum and minimum limits of real and reactive power Run the power flow program Observe the abnormal operation (over load lines, over/under voltage buses and make adjustments as necessary. Repeat the process for different scenarios: load change, generation change, line disconnected

Power Flow Data

Power Flow Assessment Infinite Bus Ideal Gen Fault Event 230 kv Line 1 R1, X1, B1 1 230 kv Line 2 2 3 R2, X2, B2 Pre fault Condition 34.4/230 kv station transformer Rt, Xt. Station level shunt compensation 34.5 kv collector system equivalent Re, Xe, Be 4 Turbine level shunt compensation 5 0.6/34.4kV equivalent GSU transformer Rte, Xte Gen 100 MW equivalent wind turbine generator Infinite Bus Ideal Gen 230 kv Line 1 R1, X1, B1 1 230 kv Line 2 2 3 R2, X2, B2 34.4/230 kv station transformer Rt, Xt. Station level shunt compensation Post fault Condition 34.5 kv collector system equivalent Re, Xe, Be Turbine level shunt compensation 0.6/34.4kV equivalent GSU transformer Rte, Xte 4 5 Gen 100 MW equivalent wind turbine generator

Power Flow Assessment Pre fault Condition Post fault Condition

Dynamic Modeling Wind Turbine Generator

Dynamic Modeling Needs G1 Short Circuit G2 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. loss of line new line 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). Resizing WTG wind turbine generator G3 Models need to reproduce WPP behavior during transient events such as faults/clear events, generation/load tripping, etc.

Dynamic Modeling Check List Check List: Prepare the power flow model and run the power flow to ensure that the pre-fault and post-fault condition results are acceptable and makes sense. For wind turbine generator: Prepare the wind turbine dynamic model to be represented If the wind turbine parameters (of the WECC generic models) are not available from the turbine manufacturers, use the default data provided by the generic models available from the WECC website If the wind turbine parameters (of the WECC generic models) of the turbines to be simulated are available from the turbine manufacturers, use the latest model parameters provided. Prepare the dynamic script of the scenario of interests and run the dynamic simulation for the contingencies fault, loss of lines, etc.) to be investigated.

Dynamic Modeling Time Scale Switching Transients Subsynchronous Resonance Transient Stability Oscillatory Stability Long-term Dynamics 10-6 10-5 10-4.001.01.1 1 10 100 1000 10 4 1 cycle 1 minute 1 hour Source: Dynamic Simulation Applications Using PSLF Short Course Note GE Energy TIME (seconds)

Dynamic Models WECC Generic Models Generic model development in PSSE/PSLF Complete suite of prototype models implemented PSLF PSSE Model Type Type 1 Type 2 Type 3 Type 4 Generator wt1g wt2g wt3g wt4g Excitation / Controller wt2e wt3e wt4e Turbine wt1t wt2t wt3t wt4t Pitch Controller wt1p wt2p wt3p wt4p Generic model WT1 WT2 WT3 WT4 Generator WT1G WT2G WT3G WT4G El. Controller WT2E WT3E WT4E Turbine/shaft WT12T WT12T WT3T Pitch control WT3P Pseudo Gov/: aerodynamics WT12A WT12A Current focus Model validation & refinement (e.g., freq. response) Identification of generic model parameters for different manufacturers (at NREL)

Dynamic Models WTG Type 1 and 2 Type 1 WTG generator Plant Feeders PF control capacitors Type 2 WTG generator Slip power as heat loss ac to dc Plant Feeders PF control capacitors

Dynamic Models WTG Type 3 and 4 Type 3 WTG generator ac to dc dc to ac Plant Feeders Type 4 WTG generator ac to dc dc to ac full power Plant Feeders partial power

Single Turbine Representation Major components of WPP Equivalent Representation: Wind Turbine Generator (WTG) Equivalent and power factor correction (PFC) caps Plant level reactive power compensation if applicable Pad-mounted Transformer Equivalent Collector System Equivalent branch. Interconnection Transmission Line Station Transformer(s) Collector System Equivalent Pad-mounted Transformer Equivalent W Wind Turbine Generator Equivalent POI or Connection to the Transmission System Plant-level - Reactive Compensation PF Correction Shunt Capacitors

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. Interconnection Transmission Line Station Transformer(s) Collector System Equivalent #1 considered to be a long/weak line feeder Collector System Equivalent #2 Pad-mounted Transformer Equivalent #2 34 MW W WTG Equivalent #2 Type 1 Pad-mounted Transformer Equivalent #1 WTG Equivalent #1 of Type 3 Voltage controlled W 21 MW POI or Connection to the Transmission System Total Output 100 MW Pad-mounted Transformer Equivalent #3 WTG Equivalent #3 of Type 3 PF=1 W PF Correction Shunt Capacitors Collector System Equivalent #3 45 MW

Dynamic Model Validation 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.

Dynamic Model Validation Example Example of Dynamic Model Simulation versus Field Data (Type 3) Interconnection Transmission Line Station Transformer(s) Collector System Equivalent Pad-mounted Transformer Equivalent 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. Complete Representation (136 turbines) Interconnection Transmission Line Station Transformer(s) POI or Connection to the Transmission System 136 WTGs were represented 9% WTGs were dropped of line during the fault.

Dynamic Model Validation Example V and f Real Power Comparison Reactive Power Comparison Voltag e (p.u.) 1.2 1 0.8 0.6 0.4 0.2 0 0.5 1 1.5 2 Time (s) V f 1.15 1.11 1.07 1.03 0.99 0.95 Freq uency (p.u.) Real Power (MW) 140 120 100 80 60 40 20 0 P-sim-1wtg (MW) P-measured (MW) P-sim-136WTG 0 0.5 1 1.5 2 2.5 3 3.5 4 Time (s) Reactive Power (MVAR) 80 60 40 20 0 0 0.5 1 1.5 2 2.5 3 3.5 4-20 -40-60 Time (s) Q-sim-1wtg (MVAR) Q-measured (MVAR) Q-sim-136WTG Compare P&Q measured to P&Q simulated V and f System Generator A C B W Wind Turbine Generator Equivalent Input V and f Regulated Bus

Dynamic 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.

Dynamic Model Validation Example Example of Model to Model Comparison (Type 2 Detailed Model vs Generic Model) Terminal Voltage Real Power Reactive Power Turbine Speed