Fixed-Speed and Variable-Slip Wind Turbines Providing Spinning Reserv

E. Muljadi, M. Singh, and V. Gevorgian 

● National Renewable Energy Labo

IEEE Power and Energy Society General Meeting 2013, Va

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory.The information contained in this poster is subject to a government license.

Introduction • By 2030, it is possible that up to 300 GW of wind power may be generated by wind 

power plants (according to the “20% Wind by 2030” study by the U.S. DOE)• Wind turbine technology must move from merely generating power from wind to taking a role in supporting the bulk power system• Wind turbines should have the capability to provide inertial response and primary frequency (governor) response. Wind turbine generators with this capability can support the frequency stability of the grid• Wind turbines should be able to generate less power than the available wind power and hold the rest in reserve, ready to be accessed as needed• In this work, we explore several ways to control wind turbine output to enable reserveholding capability. The focus of this paper is on fixed-speed (also known as Type 1) and variable-slip (also known as Type 2) wind turbines• The benefit of allowing these turbines to hold reserve power is enhanced grid stability 

at lower cost

Type 1 and Type 2 Wind Turbines Pitch Control to Allo

It is possible to operate wind turbinesat different pitch angles (or scheduledpitch angles). This allows the WTG toset aside the reserve power to becalled when needed—for example,when a large generator drops from thegrid because of fault occurring in thetransmission lines, the reserved powermay be called to substitute the loss ofgenerated power; thus, the powersystem frequency will not dropsignificantly. The operation follows thepath ABCDE. The shaded arearepresents the reserve power availablethat can be retrieved. Thedisadvantage of this algorithm is thatduring productive winds, the outputpower has to be shed; thus, there issome productivity loss.

Power vs. Speed Characteristics

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Dynamic Simulations

• The performance of the proposed controller was investigated

through dynamic simulations

• The induction generator is represented by typical fifth-order

dynamic representation

• Dynamic models were developed in PSCAD/EMTDC• Models tested using 150 s of real wind speed data• In each case, the rated power of the wind turbine is 3 MW

Proposed Pitch Con

Reserve Power Implementation: Type 1

A constant reserve power iscommanded (20% of ratedpower—i.e., 0.6 MW). The outputpower shows both the availablewind power (blue line) and theoutput power with reserve powerheld (green line). The pitch angleis shown to vary at a slower ratethan the wind speed variation.The pitch controller is slowerthan the power electronicscontroller when used to limit theoutput power; thus, only theaverage power is limited to therated output power. As shown inthe figure, there are times whenthe peak of the output powerbriefly exceeds the rated power (3MW) of the wind turbine.

In Type 1 wind turbines, thepower output is dictated by thecharacteristic of the inductiongenerator. There is no powerconverter connected to the statorwinding or the rotor winding. InType 2 wind turbines, there is anexternal rotor resistance that canbe used to control the outputpower; however, using the rotorresistance control may result inlosses. In the approach proposedin this paper, the pitch controlleris used to allocate part of theavailable aerodynamic power asreserve power and the turbinedelivers the remainder to thepower system grid

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References

The Type 2 WTG was simulated and the outputwas plotted against wind speed. This figureshows the output power comparison between thebase case and the operation with reservedpower. As expected, the actual output power wasscattered. However, when we used thepolynomial fitting of 7th order, the output power

characteristic was similar to the one predicted inFigure 7. The scattered points can be accountedto the changes of the kinetic energy stored in therotating mass (blades, generator, gearbox, etc.)because the rotational speed of the wind turbinechanges with time.Top figure: Base case vs. delta reserve power

case, Type 2 WTG, ∆Preserve = 20% of the rated

powerBottom figure: Base case vs. proportionalreserve power case, Type 2 WTG, ∆Preserve = 20%

of the rated power

Two Different Reserve-Holding Methods

To set aside some reserve power, a constant value of reserve power (constant reserve ) or a constant proportion of the available aerodynamic power (proportional reserve) may be selected. The figure on the left shows wind turbine power curves if the reserve power to be held is a constant value (20% and 40% of rated power); the figure on the right shows the power curves if the reserve power to be held is a constant percentage (20% and 40% of target power—i.e., maximum CP operation).

In this implementation, the rotor resistance iconstant value, according to the percentage oreserve power to be held is 20% of rated porated current by controlling the external rotor is much faster than the pitch controller; thus, effectively.The effect of fast control of external resistancpower fluctuations between Type 1 wind turturbines, the power output is practically clamturbine, which is equal to the rated power (3 Mpower sometimes exceeds the rated value of 3 

Reserve Power Imp

Conclusion

• This paper describes the reserve power implementation in a wind power plant for Type 1

and Type 2 wind turbines

• We chose Type 1 and Type 2 wind turbines because they are still available on the market

and because the reserve power is more difficult to control with pitch control

• The power converters in Type 3 and Type 4 wind turbines make it easier to implement

spinning reserves; however, as shown in this paper, spinning reserve power can beimplemented successfully even with only the pitch controller

• When scaled up to the plant level, these capabilities can offer significant frequency

support to the bulk power system grid and should be considered in a high-windpenetration scenario

• The low cost of Type 1 and Type 2 wind turbines make them an attractive choice if they

are able to provide grid support.

Top figure: Type 1 WTGThe normal operating point at rated wind speed is at Point A,where the output power is at 1.0 per unit. To allow power tobe held in reserve, the wind turbine is operated with anoutput power set point lower than what is actually available,e.g., at high wind speeds, the pitch can be controlled togenerate 80% of rated power although it is capable ofgenerating 100% of rated power. The operating point movesfrom A to B.Bottom figure: Type 2 WTGThe external rotor resistance can be adjusted so that theoutput power stays constant. This is indicated by A and A’.Alhough the rotor resistance can be varied, in practice thepitch controller is used to control the speed to minimize theexternal rotor resistance because deployment of rotorresistance generates heat that must be dissipated. To allowpower to be held in reserve, the wind turbine is operatedwith an output power set point lower than available poweras in the Type 1 WTG. The operating point moves from A toB.

0 10 20 0

12

Max Cp (k w^3)k w^3 at 20% Reservek w^3 at 40% Reserve

Wind speed (m/s)

Power (MW)

0 10 20 0

12

Max Cp (k w^3)k w^3 at 0.8 Gaink w^3 at 0.6 Gain

Wind speed (m/s)

Power (MW)

60 65 70 

01234

P

out (MW)

Type 1 deltaType 2 delta

Output Power Comparison