Workshop on Grid Integration of Variable Renewable Energy: Part 1 System Impact Studies March 13, 2018
Agenda Introduction Methodology Introduction to Generators 2
Introduction All new generators have impact RE generation in particular wind & solar have impacts in power system reliability and efficiency The impact is both positive as well as negative and depends on the time scale Different time scales mean different models for studying impact on the grid 3
Time-scale and Scope of Impact 4
RE (wind and solar PV) Integration Issue RE has intermittency, non controllable variability, partial unpredictability and is location dependent. Wind speed/solar radiation may vary from moment to moment affecting moment-to-moment power output. Even with multiple forecast scenario s, the actual RE generation would be differ from the forecast Unlike conventional sources like coal, gas, oil or nuclear, these resources cannot be transported and hence generation must be colocated with the resource itself which may be far from load centers 5
Key Issues The key issues for integrating large capacity of RE generation into the grid are on the system planning and operation These issues were less significant when the capacity and penetration level is very low. But, with high capacity, grid behavior would change significantly with change in their generation as the characteristics of RE plants are quite different from the conventional power plants 6
Timescales for Impact 7
Steps in the System Impact Study Data Collection and Quality Check Development of Assumptions Develop scenarios and study cases Create model of the grid Other Analysis System Operations Analysis Power System Stability Analysis Steady-state analysis: Load flow & Shortcircuit analysis Develop Recommendat ions Evaluate Cost and Timeline Develop Detailed Roadmap 8
Illustration of Methodology Tools Power Flow Analysis Short-Circuit Analysis Power System Stability Analysis Objectives: Study impact of the proposed interconnection Study if bus voltage and transmission line loading still meet grid code Study impact on existing protection system Study frequency, voltage and rotor angle response of grid to loss of proposed WPP & SPP Study impact on WPP & SPP due to disturbances on the grid Study impact of ramping and randomness of power output of WPP & SPP 9
Illustration of Methodology Tools Power Flow Analysis Short-Circuit Analysis Power System Stability Analysis This analysis influences: Generation & transmission planning Long & medium-term scheduling Unit commitment AGC regulation Governor response Inertial response 10
Types of Generators Type 1: Induction generator Type 2: Induction generator with variable rotor resistance Type 3: Doubly-fed induction generator (DFIG) Type 4: Full power converter generator Type 1 and 2 are not used in utility scale WTGs any more Type 3 is the most popular
Aerodynamics of WTG Source: P. Jain, Wind Energy Engineering, 2016
Optimum Aerodynamic Efficiency, Case for Variable Speed Turbines Source: P. Jain, Wind Energy Engineering, 2016
Type 3 Generator: DFIG ± 30% range around synchronous speed Both the stator and the wound-rotor deliver power, hence it is called doubly-fed Stator is directly connected to the grid Wound rotor produced current at slip frequency, hence there is need for power conversion About 30% of power has to be converted
Schematic of DFIG
DFIG Control Model In this model the wind turbine will be variable speed controlled by pitch angle regulation on broad range of speed (± 30%). The rotor end s slip energy control will be achieved by a power electronic control drive With the presence of this control drive optimization in energy captured and generator system efficiency are achieved The power electronic control drive will enforce the reactive power control under steady state, dynamic conditions Under dynamic conditions this power electronic control drive will superimpose the current limit control on the generator The power electronic control drive behavior will be represented with operating mechanical power vs operating slip and operating mechanical power vs operating wind speed. In this model it is possible to regulate the real power output below the maximum power limit for the respective wind speed.
Type 4 WTG Source: P. Jain, Wind Energy Engineering, 2016
Schematic of Direct Drive WTG
Type 4 Full Converter Generator This model is variable speed wind turbine with a synchronous generator The frequency of the power generated will be different from the grid The output of the machine is fed to a full converter The converter feeds power to the grid at standard voltage and frequency With the presence of this control drive, optimization in energy captured and generator system efficiency are achieved. The power electronic control drive enforces the reactive power control under steady state, dynamic conditions Under dynamic conditions this power electronic control drive will superimpose the current limit control on the generator. In this model it is possible to regulate the real power output below the maximum power limit for the respective wind speed, so for load flow studies the model is only real and reactive power limiter
Short-Circuit Response Type 3 and 4 generators will limit the current to 110/120% of rated current. Reactive power support response depends on the terminal voltage to facilitate maximum support for the system recovery This behavior is to satisfy the fault ride through capability of the Wind Power Plant The behavior should be checked against grid code
Transient Response The steady-state and dynamic characteristics of Type-3 and Type-4 WTGs are dominated by the power converter The converters allow the machine to operate over a wider range of speed, and control active and reactive power independently Only the converter and its controls come into play during grid disturbances Most Type-3 and Type-4 WTGs are designed to meet LVRT requirements without external reactive power support Converters are current-limited devices, and this plays a major role in the dynamic response of Type-3 and Type-4 WTGs to grid disturbances Control models provided by manufacturers allows accurate simulation of fault tolerance of these machines
DFIG and other induction generators Asynchronous generator works in a wide range of slip conditions. Wound rotor either delivers or is fed energy, depending on rotor speed. DDSG Rotor is fed DC excitation current. Variable-speed operation with large number of poles yields variable frequency power that is conditioned before delivering to grid. DDPM Rotor has permanent magnets. Variable-speed operation yields variable frequency power that is conditioned before delivering to grid. Power factor can be regulated Power factor can be regulated Power factor can be regulated. Additional circuits required to manage power factor Stator is directly connected to the grid. Grid disturbances can affect the generator. Output voltage is typically less than 1 kv Generator is isolated from the grid. Better ability to manage voltage ride-through Output voltage is high, can be order of tens of kilovolts. No transformer required if connected to distribution * Generator is isolated from the grid. Better ability to manage voltage ride-through Output voltage is high, can be order of tens of kilovolts. No transformer required if connected to distribution lines
DFIG and other induction generators Gearbox required DDSG No gearbox. Cost, vibration and noise associated with gearbox is eliminated No gearbox DDPM Compact generators Multiple pole generators are large Compact generators GE 1.5, Vestas (Opti Slip) and Siemens turbines use this Enercon and Emergya use this type of generator GE2.5, Clipper, Vensys and NorthWind use this Slip rings and brushes on rotor No slip rings or brushes. Same as DDSG winding, which leads to higher maintenance and lower reliability Operating wind speed range is Operating wind speed range is Same as DDSG narrower higher Ability to handle wind gusts is Gusts lead to increased speed Same as DDSG limited. Gusts cause significant increase in torque. while keeping torque within limit. Energy from gusts is converted to electric energy.
DFIG and other induction generators rpm of generator is large 1800/3600 rpm for 60 Hz or 1500/3000 rpm for 50 Hz; so torque is smaller Small power electronics converters are required to manage rotor circuit Compact generator, but complicated power electronics DDSG rpm is in the range of 15 to 30 rpm; so torque is large Large rectifiers and inverters are required to convert all power to DC and then to AC at grid frequency. High frequency harmonics are produced which may be mitigated by filters. Large generator requires precision manufacturing and assembly to maintain uniformly small air gap DDPM Same as DDSG Same as DDSG Material cost of magnets is high, and assembly is complicated by small air gap Source: P. Jain, Wind Energy Engineering, 2016
Thank You Contact Person: Pramod Jain, Ph.D., Consultant to USAID Power the Future President, Innovative Wind Energy, Inc., pramod@i-windenergy.com, +1-904-923-6489 25