ECEN 667 Power System Stability Lecture 26: Renewable Energy Systems Prof. Tom Overbye Dept. of Electrical and Computer Engineering Texas A&M University, [email protected] 1 Announcements Read Chapter 9 Final is as per TAMU schedule. That is, Friday Dec 8 from 3 to 5pm here

2 Renewable Resource Modeling With the advent of more renewable generation in power systems worldwide it is important to have correct models Hydro systems have already been covered Solar thermal and geothermal are modeled similar to existing stream generation, so they are not covered here Coverage will focus on transient stability level models for wind and solar PV for integrated system studies More detailed EMTP-level models may be needed for

individual plant issues, like subsynchronous resonance Models are evolving, with a desire by many to have as generic as possible models 3 Growth in Wind Worldwide Source: Global Wind 2016 Report, Global Wind Energy Council 4 Growth in Wind Worldwide Source: Global Wind 2016 Report, Global Wind Energy Council 5 Vestas Wind Systems Stock Price

Vestass stock has increased by more than 15times from their 2012/2013 lows! Their price fell significantly in November due to increased competition in wind power markets 6 Growth in US Wind

Production tax credit of $24/MWh being phased out 100% in 2016, 80% in 2017, 60% in 2018, 40% in 2019 Source: American Wind Energy Association 2017 Third Quarter Market Report 7 2016 Installed Capacity by State: Texas Continues to Dominate! Source: American Wind Energy Association 2017 Third Quarter Market Report 8 Wind Farm and Wind-Related Plant Locations id=eed1ec3b624742f8b18280e6aa73e8ec 9 State Renewable Portfolio Standards Texas has a goal of 10 GW by 2025, but that has already been achieved (by more than double!)

Image source: (see for updated information) 10 US Wind Resources Source: 11 Global Wind Speed 50m Map 12 Wind Map Texas 80m Height 13 Power in the Wind The power in the wind is proportional to the cube of the wind speed Velocity increases with height, with more increase over rougher terrain (doubling at 100m compared to 10m for a small town, but only increasing by 60% over crops or 30% over calm water)

Maximum rotor efficiency is 59.3%, from Betz' law Expected available energy depends on the wind speed probability density function (pdf) 14 Wind Turbine Height and Size The current largest wind turbine by capacity is the Vestas V164 which has a capacity

of 8 MW, a height of 220 m, and diameter of 164 m. Source: 15 Extracted Power WTGs are designed for rated power and windspeed For speeds above this blades are pitched to operate at rated power; at furling speed the WTG is cut out

16 Example: GE 1.5 and 1.6 MW Turbines Power speed curves for the GE 1.5 and 1.6 MW WTGs Hub height is 80/100 m; cut-out at 25 m/s wind Source: 17 Wind Farms (or Parks)

Usually wind farm is modeled in aggregate for grid studies; wind farm can consist of many small (1 to 3 MW) wind turbine-generators (WTGs) operating at low voltage (e.g. 0.6kV) stepped up to distribution level (e.g., 34.5 kV) Photo Source: 18 Economies of Scale

Presently large wind farms produce electricity more economically than small operations Factors that contribute to lower costs are Wind power is proportional to the area covered by the blade (square of diameter) while tower costs vary with a value less than the square of the diameter Larger blades are higher, permitting access to faster winds, but size limited by transportation for most land wind farms Fixed costs associated with construction (permitting, management) are spread over more MWs of capacity Efficiencies in managing larger wind farms typically result in lower O&M costs (on-site staff reduces travel costs) 19 Wind Energy Economics

Most of the cost is in the initial purchase and construction (capital costs); current estimate is about $1690/kW; how much wind is generated depends on the capacity factor, best is about 40% Source: 20 Offshore Wind Offshore wind turbines currently need to be in relatively shallow water, so maximum distance from shore depends on the seabed

Capacity factors tend to increase as turbines move further off-shore Image Source: National Renewable Energy Laboratory 21 Offshore Wind Installations The first US offshore wind, Block Island (Rhode Island) with 30 MW, became operational in December 2016; Cape Wind in Massachusetts was just officially cancelled this month Source: EIA August 14, 2015 and

22 Offshore: Advantages and Disadvantages All advantages/disadvantages are somewhat site specific Advantages Can usually be sited much closer to the load (often by coast) Offshore wind speeds are higher and steadier Easier to transport large wind turbines by ship Minimal sound impacts and visual impacts (if far enough offshore), no land usage issues

Disadvantages High construction costs, particularly since they are in windy (and hence wavy) locations Higher maintenance costs Some environmental issues (e.g., seabed disturbance) 23 Types of Wind Turbines for Power Flow and Transient Stability Several different approaches to aggregate modeling of wind farms in power flow and transient stability

Wind turbine manufacturers provide detailed, public models of their WTGs; these models are incorporated into software packages; example is GE 1.5, 1.6 and 3.6 MW WTGs (see Modeling of GE Wind Turbine-Generators for Grid Studies, version 4.6, March 2013, GE Energy) Proprietary models are included as user defined models; covered under NDAs to maintain confidentiality Generic models are developed to cover the range of WTGs, with parameters set based on the individual turbine types Concern by some manufacturers that the generic models to not capture their WTGs' behavior, such as during low 24 voltage ride through (LVRT) Types of Wind Turbines for Power Flow and Transient Stability

Electrically there are four main generic types of wind turbines Type 1: Induction machine; treated as PQ bus with negative P load; dynamically modeled as an induction motor Type 2: Induction machine with varying rotor resistance; treated as PQ bus in power flow; induction motor model with dynamic slip adjustment Type 3: Doubly Fed Asynchronous Generator (DFAG) (or DFIG); treated as PV bus in power flow Type 4: Full Asynchronous Generator; treated as PV bus in power flow New wind farms (or parks) are primarily of Type 3 or 4 25

Generic Modeling Approach The generic modeling approach is to divide the wind farm models by functionality Generator model: either an induction machine for Type 1 and 2's or a voltage source converter for Type 3 and 4 Reactive power control (exciter): none for Type 1, rotor resistance control for Type 2, commanded reactive current for Type 3 and 4 Drive train models: Type 1 and 2 in which the inertia appears in the transient stability Aerodynamics and Pitch Models: Model impact of changing blade angles (pitch) on power output 26

Wind Turbine Issues Models are designed to represent the system level impacts of the aggregate wind turbines during disturbances such as low voltages (nearby faults) and frequency deviations Low voltage ride through (LVRT) is a key issue, in which the wind turbines need to stay connected to the grid during nearby faults Active and reactive power control is also an issue 27

Low Voltage Ride Through (LVRT) The concern is if during low voltages, such as during faults, the WTGs trip, it could quickly setup a cascading situation particularly in areas with lots of Type 3 WTGs Tripping had been a strategy to protect the DFAG from high rotor currents and over voltages in the dc capacitor. When there were just a few WTGs, tripping was acceptable Standards now require specific low voltage performance

Image from California ISO presentation 28 Type 3: Doubly Fed Asynchronous Generators (DFAG) Doubly fed asynchronous generators (DFAG) are usually a conventional wound rotor induction generator with an ac-dc-ac power converter in the rotor circuit Power that would have been lost in external rotor resistance is now used

Electrical dynamics are dominated by the voltagesource inverter, which has dynamics much faster than the transient stability time frame Image Source: Figure 2.1 from Modeling of GE Wind Turbine-Generators for Grid Studies, version 4.6, March 2013, GE Energy 29 Overall Type 3 WTG Model Transient stability models are transitioning Image Source: WECC Type 3 Wind Turbine Generator Model Phase II, January 23, 2014, WECC TSS 30

Type 3 Converters A voltage source converter (VSC) takes a dc voltage, usually held constant by a capacitor, and produces a controlled ac output A phase locked loop (PLL) is used to synchronize the phase of the wind turbine with that of the ac connection voltage Operates much faster than the transient stability time step, so is often assumed to be in constant synchronism

Under normal conditions the WTG has a controllable real power current and reactive power current WTG voltages are not particularly high, say 600V 31 Type 3 WT3G Converter Model Network interface is a Norton current in parallel with a reactance jX" 32 Type 3 Converters

Type 3 machines can operate at a potentially widely varying slip Example, rated speed might be 120% (72 Hz for a 60 Hz system) with a slip of -0.2, but with a control range of +/30% Control systems are used to limit the real power during faults (low voltage) Current ramp rate limits are used to prevent system stress during current recovery Reactive current limits are used during high voltage conditions

33 Type 3 Reactive Power Control 34 Aerodynamics Type 3 and 4 models have more detailed models that directly incorporate the blade angle, so a brief coverage of the associated aerodynamics is useful The power in the wind is given by 3 AvwC p ( , ) 2

where is the density of air, A is the area swept by the blades, vw is the wind velocity, is the tip to wind speed ratio. For a given turbine with a fixed blade length, =K b ( /v w ) P Modeling of GE Wind Turbine-Generators for Grid Studies, version 4.6, March 2013, GE Energy 35 Aerodynamics The Cp(,) function can be quite complex, with the GE 1.5 curves given below If such a detailed curve is used, the

initialization is from the power flow P. There are potentially three independent variables, vw, and . One approach is to fix at rated (e.g., 1.2) and at min Source: Modeling of GE Wind Turbine-Generators for Grid Studies, version 4.6, March 2013, GE Energy 36 Simplified Aerodynamics Model A more simplified model is to approximate this curve as

Pmech Pm0 K aero 0 2 where K aero is a constant, Pm0 is set by the initial Pmech ; 0 is the initial angle, either set to min (when the wind speed is below Theta2 1 rated), or 1 2 with Theta2 a 0.75 vw

constant equal to the angle at twice rated speed 37 WT3T Model (Drive Train and Aero) 38 WT3P Model (Pitch Control) 39 Type 3 Example Case Previous WSCC case, with the same line 6 to 9 fault, is modified so gen 3 is represented by a WT3G, WT3E, WT3T, and WT3P Mechanical Power (MW)

110 105 100 95 90 85 80 75 70 65 60 55 50 45 40

35 30 25 20 15 10 5 0 0 0.2 0.4 0.6 0.8

1 1.2 1.4 Time (Seconds) b c d fe g Mech Input_Gen Bus 3 #1 g 1.6

1.8 2 Graph at left shows a zoomed (2 second) view of the gen 3 real power output, with the value falling to zero during the fault, and then ramping back up MW_Gen Bus 3 #1 40

Type 3 Example Case Below graphs show the response of the WTG speed and blade angle 1.219 1.218 1.217 1.216 1.215 1.214 1.213 1.212 1.211 1.21 1.209 1.208

1.207 1.206 1.205 1.204 1.203 1.202 1.201 1.2 1.199 1.198 1.197 1.196 1.195 1.194 1.6 1.5 1.4

1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1

2 3 b c d fe g 4 5 6 7

8 States of Governor\TurbineSpeed_Gen Bus 3 #1 9 10 0 0 1 2 3 b

c d fe g 4 5 6 7 8 9 10

States of Stabilizer\Pitch, Gen Bus 3 #1 41 Type 4 Converters Type 4 WTGs pass the entire output of the WTG through the ac-dc-ac converter Hence the system characteristics are essentially independent of the type of generator Because of this decoupling, the generator speed can be as variable as needed This allows for different generator technologies, such as

permanent magnet synchronous generators (PMSGs) Traditionally gearboxes have been used to change the slow wind turbine speed (e.g., 15 rpm) to a more standard generator speed (e.g., 1800 rpm); with Type 4 direct drive technologies can also be used 42 Example: Siemens SWT-2.3-113 The Siemens-2.3-113 is a 2.3 MW WTG that has a rotor diameter of 113m. It is a gearless design based on a compact permanent magnet generator No excitation power, slip rings or excitation control system Image:

43 Type WTG4 Model Very similar to the WTG3, except there is no X" 44 Type 4 Reactive Power Control Also similar to the Type 3's, as are the other models 45 Solar Photovoltaic (PV)

Photovoltaic definition- a material or device that is capable of converting the energy contained in photons of light into an electrical voltage and current Solar cells are diodes, creating dc power, which in grid applications is converted to ac by an inverter For terrestrial applications, the capacity factor is limited by night, relative movement of the sun, the atmosphere, clouds, shading, etc A ballpark figure for Illinois is 18% "One sun" is defined a 1 kw/m2,which is the maximum insolation the reaches the surface of the earth (sun right overhead) 46

US Annual Insolation The capacity factor is roughly this number divided by 24 hours per day 47 Worldwide Annual Insolation 48 US Solar Generated Electricity US_Monthly_Solar_Power_Generation.svg 49 Solar PV can be Quite Intermittent Because of Clouds Intermittency can be reduced some when PV is distributed over a larger region; key issue is correlation across an area

Image: 50 Modeling Solar PV Since a large portion of the solar PV is distributed in small installations in the distribution system (e.g., residential rooftop), solar PV modeling is divided into two categories Central station, which is considered a single generation plant As part of the load model 51

Central Station PV System Modeling The below block diagram shows the overall structure Solar PV has no inertia, and in contrast to wind there is not even the ability to mimic an inertia response since there is no energy storage in the system Source: "Generic Solar Photovoltaic System Dynamic Simulation Model Specification," WECC Renewable Energy Modeling Task Force, Sept. 2012 (same source for figures on the next three slides) 52 Central Station PV System Modeling

The generator model is similar to the Type 4 wind model, which is not surprising since this is modeling the converter operation Source: "Generic Solar Photovoltaic System Dynamic Simulation Model Specification," WECC Renewable Energy Modeling Task Force, Sept. 2012 53 Distributed PV System Modeling PV in the distribution system is usually operated at unity power factor

There is research investigating the benefits of changing this IEEE Std 1547 now allows both non-unity power factor and voltage regulation A simple model is just as negative constant power load An issue is tripping on abnormal frequency or voltage conditions IEEE Std 1547 says, "The DR unit shall cease to energize the Area EPS for faults on the Area EPS circuit to which it is connected. (note EPS is electric power system) 54 Distributed PV System Modeling

An issue is tripping on abnormal frequency or voltage conditions (from IEEE 1547-2003, 2014 amendment) This is a key safety requirement! Units need to disconnect if the voltage is < 0.45 pu in 0.16 seconds, in 1 second between 0.45 and 0.6 pu, in 2 seconds if between 0.6 and 0.88 pu; also in 1 second if between 1.1 and 1.2, and in 0.16 seconds if higher Units need to disconnect in 0.16 seconds if the frequency is > 62 or less than 57 Hz; in 2 seconds if > 60.5 or < 59.5 Reconnection is after minutes Values are defaults; different values can be used through mutual agreement between EPS and DR operator 55

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