synthetic inertia - supergen wind consortium 6th educational seminar

8/12/2019 Synthetic Inertia - Supergen Wind Consortium 6th Educational Seminar

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SU PE R G E N WIND CONSOR TIU M 6THE D U CA TIONA L SE MINA R

L O U G H B O R O U G H U N I V E R S I T Y 1 2 T H

S E P T E M B E R 2 0 1 1

Synthetic Inertia


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Inertia

Inertia can be seen as the resistance to change

Inertia prevents the grid frequency suddenly changing andresults from synchronous machines having large, heavy,rotating generators

Asynchronous machines, such as variable speed windturbines, do not contribute to grid inertia

As the UK aims for 20% renewable electricity supply by 2020,power from wind turbines will make up a larger percentage ofthe produced power. Unless they can produce a syntheticinertia, the inertia of the grid will be reduced


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Synthetic Inertia

Inertia can be seen as the conversion of kineticenergy of the rotating mass to electrical energy,causing the frequency to drop.

Synthetic inertia can therefore be produced byincreasing power output during a frequency drop

Luckily, wind turbines have a large store of kineticenergy...

DemSupm

m PP

dt

dJ


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How much Energy is Required?

A National Gridinvestigation intosynthetic inertia

concluded that a powerincrease of 5-10% duringa grid frequency drop to49.2Hz in approximately

8 seconds would besufficient


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How much energy is available?

A typical measure used forcomparing the inertia ofdifferent generators is known asthe inertia constant,H

An average value for a largegenerator would beapproximately 6s

For the Supergen 5MWExemplar turbine this value

works out to be approximately6s

Hence the wind turbine has thepotential to supply a similarsynthetic inertia response as asynchronous machines inertialresponse.

RatedPower

EH

2

2

1

mJE

DemSup

m

m PPdt

d

J


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How do we Access this Kinetic Energy?

Power is rotational speed multiplied by torque

By artificially increasing the torque the power will

also increase in proportion.

This will cause the turbine blades to slow down

Effectively, the kinetic energy in the blades is beingconverted to electrical energy


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The Controller A Conundrum

The additional torqueneeds to be demanded bythe controller

The original (inner)controller will detect thereduction in rotationalspeed and decrease torquedemand

This will cancel out whatwe wanted to do!


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Controller Solution

In order to fool thecontroller, a dummyspeed input is required

The dummy speed (ad)cancels out the reductionin speed caused by the

additional torque (Tad)


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The Torque Controller

The torque controllergenerates a value of Tad,

which is added to the originalcontrollers demanded torqueTdem

Inertia is proportional to thechange in frequency As the turbine slows, more

torque is required tomaintain the same poweroutput

In order to avoid anunrealistic torque demand, amax power limiter is used


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The Dummy Speed Controller

The synthetic inertiacontroller must providea dummy speed to foolthe original controller

By looking at the Bodeplot from torquedemand to generatorspeed, a similar, simplertransfer function can bededuced


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Hysteresis Loop

The dummy speed controller is only required below rated wind speed

It is undesirable for the controller to turn on or turn off during a frequency dropso a hysteresis loop is put in place

The values of BdLower and BdUpper are set so that the likely hood of the turbinepassing into the other mode of operation is low


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Anti-Stall Control

An addition to thecontroller is required toensure that the machinedoes not enter stall

The red line is describedby T=K2

This value is normallyhigher than Ttot

On the line the values areequal and above the linethe Ttot is the higher value


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Minimum Generator Speed

It is desirable to preventthe wind turbine operating

below its minimumgenerator speed

This addition to thecontroller graduallyreduces the torquedemand as the generatorapproaches its minimumspeed

Once at the minimumspeed, no additionaltorque is requested


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Interaction With the Drive-Train Frequency

This technique requires asudden increase in torque

Interaction with thenatural frequency of thedrive train causes poweroscillation

Techniques to minimisethis cause an unacceptabledelay in producing theincrease in power


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Test Simulations

Simulations run usingSimulink software

Frequency dropmodelled as shown.Based on the worst case

detailed in the NationalGrid Report


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Graph Displaying Additional Power Generated

Red 13m/s windBlue 10m/s windGreen 7m/s wind

The additional powerpeaks at approximately500kW, 10% of the ratedpower

When below rated speedthere is a recovery periodof 40 60 seconds


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Graph Displaying Increase in Total TorqueDemand

Red 13m/s wind

Blue 10m/s wind

Green 7m/s wind

The increase in torque is,

as expected, higher atlower wind speeds


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Graph Displaying Increase in Generator Speed

Red 13m/s windBlue 10m/s windGreen 7m/s wind

Due to greater torqueincrease, the generatorspeed dips lower at 7m/sthan at 10m/s

At 13m/s there is an

oscillation caused becausepitch controller is slowerthan the torque demandcontroller


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Future Work

Additional work could be done to investigate the impactof this technique on the lifetime of the turbine and/or onthe power electronics

It would be interesting to see how this technique could beapplied over a whole wind farm, maybe some turbinesproducing more extra power than others depending onconditions?

It would also be of interest to investigate if wind turbinescould supply some of the primary response as well as theinertial response especially as they increase in size


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Any Questions

Any Questions?

synthetic inertia - supergen wind consortium 6th educational seminar

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