Presented by Rohan R.V Seale

1. “AN INVESTIGATION INTO THE TECHNICAL DESIGN,TRANSIENT STABILITY STUDIES AND MODELLING ISSUES FOR LAND BASED WF INTO SMALL ISLAND GRID" Presented by Rohan R.V Seale

2. PROJECT OBJECTIVES : • Develop grid connection requirements and standards for BLPC that will permit the development and operation of an efficient, safe, reliable and well coordinated transmission grid system. • Investigate the power quality impact of wind farm on BLPC network. • Perform relevant Transient studies to assess the impact due to 24 kv Wind Farm connection on island grid. • Assess the overall protection, design and SCADA requirements for integration of a land based wind farm into a small island utility.

3. BACKGROUND INFORMATION • CONNECTION OF WF TO NORTH SUB. • NORMALLY WEAK CONNECTION -POTENTIAL LOW VOLTAGE AT DIST. LEVEL • RADIAL LINE WITH SINGLE OHL FEED • SECOND UG CABLE FEED DUE IN 2007 • ADDITIONAL 2 X 132 KV UG CABLES - 2008 • POTENTIAL GEN. SITE DEVELOPMENT IN NORTH-ADDITIONAL 4 x 20MW (2009)

4. 175km of transmission lines • 175km of transmission lines • 3,828km of distribution lines REGENCY PK. REGENCY PK. REGENCY PK. Existing Facilities

5. BLPC TRANSMISSION SYSTEM

6. Wind generators connect to both the distribution & transmission networks . An emerging set of renewable energy generation is under construction or in planning phase in Caribbean. The transmission & distribution systems will require selective reinforcement to support the volume of renewable generation being planned. The technical connection requirements for generators connecting to the transmission or distribution systems are set out in Industry Codes and Intl. Standards. Framework agreements set out obligations and contractually binding arrangements between generators & Utilities. ADVENT OF WT GENERATION

7. ELEMENTS OF SYSTEM OPERATION

8. INTERNATIONALSTANDARDS The status of the IEC standards is provided in Table I.

9. International Standards Cont’d • G59/1 - Recommendations for The Connection of Embedded Generating Plant to The Public Electricity Suppliers’ Distribution Systems (1991) (≤20kV, ≤5MW) • G75/1 - Recommendations for the connection of embedded generating plant to public distribution systems >20kV or with outputs ≥5MW • G 83/1 – Recommendations for connection of Small Scale EG (<16 A per phase) in parallel with LV Dist. Network

10. IEEE P 1547-2003 STANDARD • Influential standard for interconnection of all forms of DR is IEEE 1547-2003, Standard for Interconnecting Distributed Resources with Electric Power Systems. • IEEE 1547 is the result of a recent effort by SCC21 to develop a single interconnection standard that applies to all technologies. • IEEE 1547 addresses all types of interconnected generation up to 10 MW. • The 1547 standard has benefited greatly from earlier utility industry work documented in IEEE and IEC standards (ANSI- C37 series for protective relaying)

11. RECENT CHANGES TO USA GRID CODE (AWEA): • Low voltage ride-through (LVRT) capability for wind plants and wind turbines:AWEA recommended adoption of an LVRT requirement developed by E.ON Netz. This is a German grid operator faced with a significant and growing penetration of asynchronous wind generation on the German grid. This standard requires that the machine stay connected for voltages at the terminals as low as 15% of nominal per unit for approximately 0.6 s. • Supervisory control and data acquisition (SCADA)equipment for remote control: AWEA recommended the requirement of equipment to enable remote command and control for the limitation of maximum plant output during system emergency and system contingency events. • Reactive power capability:AWEA recommended that wind plants connected to the transmission system be capable of operating over a power factor range of ±0.95. • Current wind turbine simulation models: AWEA recommended that major stakeholders (TSOs and WT manuf.) participate in a formal process for developing, updating, and improving engineering models and turbine specifications used for modeling the wind plant interconnection.

12. RECENT CHANGES TO UK GRID CODE The Grid Code incorporates the technical issues raised by the 3 Licensees with respect to the connection of windfarms: • Fault ride through: Requirement for generating units to revert to normal operation when a fault on the network is cleared. • Power/frequency characteristics:Requirement for generating units to be able to deliver power & remain connected to the network when the system frequency deviates from 50Hz. • Frequency control:Requirement for generating plants to be able to increase/decrease power output with falling or rising frequency. • Reactive range and voltage control:Requirement for generating plant to be able to supply lagging/leading reactive power and control the voltage at the grid connection point. • Negative phase sequence:The requirement for generating units to be able to withstand negative sequence currents caused by phase voltage unbalance and phase to phase faults.

13. WFPS1.4 FAULT RIDE THROUGH REQUIREMENTS WFPS1.4.1 A Wind Farm Power Station shall remain connected to the Transmission System for Transmission System Voltage dips on any or all phases, where the Transmission System Voltage measured at the HV terminals of the Grid Connected Transformer remains above the heavy black line in Figure WFPS1.1.

14. Connection process overview PROJECT PLANNING PHASE INFORMATION PHASE DESIGN PHASE CONSTRUCTION PHASE TESTING & COMMISSIONING PHASE

15. CATEGORIES OF WIND PLANTS • BULK WIND PLANTS: Consist of large wind farms connected to the Transmission System (USA) • DISTRIBUTED WIND PLANTS: refers to single Turbines / small groups of turbines dispersed along Distribution System (popular in Europe , Caribbean )

16. WIND TURBINE CATEGORIES WT divided into two categories which define their electrical characteristics • FIXED SPEED DEVICES – Simple & Cheap • VARIABLE SPEED DEVICES – Power Electronic Interface to Grid • M/C RATINGS FROM 600 KW – 1.5 MW

17. FIXED SPEED FSIG: Standard squirrel-cage induction generator connected directly to the grid: These machines have a gearbox to match the rotational speed of blades with that of the generator. Mechanical power may be regulated through an inherent aerodynamic stall characteristic of blades or with active control of blade pitch. WRIG: Wound-rotor induction generator with variable rotor resistance: These machines have a gearbox for coupling an electrical generator to a turbine hub. They also have pitch control of blades for maximizing energy capture and controlling turbine speed within range of the generator and a small range of variable speed operation (e.g., 10% of generator synchronous speed). VARIABLE SPEED DFIG: Doubly fed asynchronous generator: These are essentially wound rotor induction machines with variable frequency excitation of the rotor circuit, incorporating rotor current control via power converter. The rotor circuit power converter may be four-quadrant, allowing independent control of real and reactive flow in either direction (rotor to grid or grid to rotor), or unidirectional real power flow (grid to rotor). These machines have a gearbox for coupling the generator shaft to turbine hub, active control of turbine blade pitch for maximizing production and controlling mechanical speed, and variable speed operation depending on the rating of power converter relative to turbine rating (e.g., ±30% of generator synchronous speed). Synchronous or induction generator with full-size power converter: In these machines, the generator is coupled to the grid through a fully rated ac/dc/ac power converter. They also have a gearbox to match generator speed to variable rotational speed of blades and variable speed operation over a wide range, depending on electrical generator characteristics. WIND TURBINE TECHNOLOGY

18. CAPACITY FACTORS • The Declared Net Capacity (DNC) of a generation scheme is a measure of the expected average power output of the generation scheme. • DNC = (RATED POWER OUTPUT– POWER CONSUMED BY PLANT) X CF • Capacity factor for wave energy schemes = 0.33 • Capacity factor for wind energy schemes = 0.43 • Capacity factor for other types of C-2 generation schemes = 1.00

19. POW. SYS. ANALYSIS FOR DISTRIBUTED WIND: • Voltage Regulation • POWER QUALITY- FLICKER, HARMONICS • Short Circuit Contribution

20. Pow. Sys. Analysis Issues for Bulk Wind include: • Var Support • Capacity Constraints • Stability • Reserve Capacity Requirements

21. Factors Impacting System Voltage • Local Wind Profile (Speed, Turbulence, Shear) • Size of Wind Turbine to Short Circuit ratio • X/R Ratio of system • Type of WTG and associated Reactive Power Control • Loading on Distribution Feeder

22. VOLTAGE REGULATION: • Commercial WTG employ Reactive Compensation • Variable Speed Gen. use Static Var. Control to adjust current phase angle • Fixed and semi variable speed IG use switched capacitor banks • Need to Coordinate with other Voltage Reg. devices e.g. Regulator/switched Caps.

23. POWER QUALITY • VOLTAGE FLICKER - Refers to the rapid variations in voltage levels within a certain Mag. and Freq. range • Synonymous with light or lamp flicker • Arises due to abrupt changes in WT Pow. O/P- Wind Gusting & Variable Dynamic behaviour • Fluctuations at freq. close to 8 Hz cause most annoyance • Occurs on weak systems with low X/R ratio

24. VOLTAGE FLICKER IMPACT • Peculiar to Fixed speed devices • Variable speed WT less likely to cause flicker • Wind farm with several turbines less likely to cause flicker as variations of Pow. O/P tend to cancel out.

25. CAUSES OF FLICKER BY WTG • Blade passing of tower results in oscillations • Variations of wind speed • Switching operations- startup & shutdown • Recommended limits on flicker in Dist. Networks addressed in IEC 61400.21

26. SHORT CIRCUIT CONTRIBUTION • For WTG in 600 kw - 1.3 MW range must consider Fault Contribution • 80-90% of Dist faults are SLG which cause m/c to receive normal excitation voltage • For voltage > 60% treat IG as Synch. m/c. (Rule of thumb by W. Feero) • Obtain Static Ind. m/c model – more accurate

27. NETWORK EFFECT OF GEN. TECH.

28. EFFECTS DUE TO FAULT CONTRIBUTION • REDUCTION OF RELAY REACH • SYMPATHETIC TRIPPING OF BREAKERS/RECLOSERS • COORDINATION ISSUES- DELAYED AUTO-RECLOSING 3-5 secs.

29. Network Design for WTG Connection • CONNECTION VOLTAGE – 24 KV, 69 KV • NETWORK FAULT LEVEL • SYSTEM X/R RATIO • NETWORK CAPACITY AT PCC – THERMAL RATINGS, EXP. REQ’MENTS, VOLT. REGULATION

30. WF POWER EQUATION Where, • ρ = air density (nominally 1.22 kg/m3) • R = radius of area swept by the turbine blades • V = speed of moving air stream • Cp = “coefficient of performance” for the composite airfoil (rotating blades)

31. VAR SUPPORT • WIND FARMS TYPICALLY NOT USED TO PROVIDE VOLTAGE CONTROL • CAN PROVIDE LOCAL VOLTAGE REGULATION FOR WEAK SYSTEMS • FLUCTUATING WIND PLANT O/P MEANS AMT. OF REACTIVE POW. REQ’D VARIES • FAILURE TO MAINTAIN REACTIVE Q LEADS TO VOLTAGE COLLAPSE AS WTG O/P INCREASES

32. REACTIVE COMPENSATION TECHNIQUES: • CONSTANT PF - Switched Cap. Banks at each WTG provide constant PF over range of Gen O/P • VARIABLE PF – Real time control of each WTG reactive Q production or absorption • SUBS. SWITCHED CAP. BANKS – Large Cap banks located at interconnection Sub. • STATCOM – FACTS devices that use voltage source converters to provide reactive current.

33. POWER SYSTEM STABILITY • Full Assessment of Network performance requires study of STEADY STATE and TRANSIENT STABILITY operation • Characteristics of WTG must not compromise the stability of Power System following CONTINGENCY

34. DEFINITION OF STABILITY • STEADY STATE STABILITY - Ability of Pow. Sys. to remain stable after a small disturbance e.g load disturbance, switching • TRANSIENT STABILITY – ability of Pow. Sys. to maintain synchronism after a severe transient disturbance. E.g. Short Circuits, loss of load or Gen.

35. PURPOSE OF TRANS. STAB. STUDY • TO PREDICT ABILITY OF GEN. TO RECOVER AND REMAIN CONNECTED TO POWER SYSTEM AFTER A FAULT • TO ASSESS INTERACTION OF GENS. AND OTHER ROTATING PLANT (WTG) CONNECTED TO NETWORK AFTER FAULT • TO ENSURE MINIMUM VOLTAGE DISTURBANCE DUE TO LOSS OF SYNCHRONISM

36. Two forms of Instability occur under these conditions: Steady Increase in Rotor Angle due to lack of sufficient Synchronising Torque Rotor oscillations of increasing amplitude due to lack of sufficient damping torque SMALL SIGNAL STABILITY

37. TRANSIENT STABILITY CONSIDERATIONS • In large complex power systems Transient instability may not always occur as first Swing Instability, but may be due to superposition of several modes of oscillations. • Analysing TS - the study period is 3-5 secs. after disturbance. May be extended to 10 secs. or more.

38. CAUSES SHORT CIRCUITS LOSS OF TIE LINES IN UTILITY NETWORK LOSS OF GENERATION SWITCHING OPERATIONS OF LINES, CAPACITORS ETC. SUDDEN LARGE STEP CHANGE OF GENERATION CONSEQUENCES AREA WIDE BLACK OUT INTERRUPTION OF LOAD UNDER VOLTAGE CONDITION DAMAGE TO EQUIPMENT RELAY AND PROTECTIVE DEVICE MALFUNCTION STABILITY CHALLENGES

39. LOAD FLOW STUDY TRANSIENT STABILITY STUDY DYNAMIC SECURITY ASSESSMENT COMMERCIAL SOFTWARE PSS/E – PTI LTD. ERACS – ERA LTD. IPSA – IPSA POWER/UMIST ETAP – OTI INC. MATLAB / SIMULINK SYSTEM OR NETWORK STUDY

40. PS STUDY CONSIDERATIONS • LOAD FLOWS • CURRENT FLOWS IN EACH BRANCH OF NETWORK • REAL & REACTIVE POWER FLOWS • VOLTAGES AT EACH NODE • VOLTAGE BOOST AT CONTROL NODES • LOSSES

41. FAULT LEVEL STUDIES • TOTAL FAULT CURRENT AT FAULTED NODE • ANGLE OF FAULT CURRENT RELATIVE TO REFERENCE VOLTAGE • FAULT CURRENT DISTRIBUTION • (CRITICAL TO PROTECTION)

42. LOAD FLOW ANALYSIS

43. MATLAB/POWERSIM

44. ETAP 5.5.0

45. ETAP 5.5.0 FEATURES

46. MODELLING ISSUES • DFIG MODELLED – ALL PARTS MODELLED FOR DYNAMIC STUDIES • AERODYNAMICS • TURBINE • DRIVE TRAIN - GEARBOX • GENERATOR • CONTROL SYSTEM

47. WIND FARM MODELLING • Modelling WF requires grouping of m/cs of similar type into an equivalent single m/c • Large WF can be split into several equiv. m/cs • Layout of WG to be taken into account as this affects regulation and dynamic behaviour especially of variable speed m/cs. • Consider Loss of accuracy vs. simplified practical model

48. SIMULATION STUDY • TRANSIENT STABILITY STUDIES PERFORMED FOR (up to 7 secs): • CONTINGENCY ON OHL INFEEDS AND WF CKT • CONTINGENCY ON UG CABLES • LOSS OF GENERATION • BUS FAULT • WIND GUST

49. SYMETRICAL LV TOLERANCE CURVE

50. EQUAL AREA CRITERION: • Divide the machines in the system into two groups: • the critical machines that are responsible for the loss of synchronism, and the remaining non critical ones • Replace the two groups by two equivalent. machines. • Replace these machines by an equivalent single machine, infinite bus system. • Evaluate the system stability using the equal area criterion.