WORKSHOP: Frequency Control Schemes and Frequency Response of Power Systems considering the...

Francisco M. Gonzalez-Longatt, PhD www.fglongatt.org.ve

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OffShore windpark Egmond aan Zee (36 x3 MW)Egmond Aan Zee, Holanda


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1. Introduction

2. Frequency control in classical power systems

3. System Frequency Response

4. Synthetic or Artificial Inertia

5. Frequency Response of Wind Turbines/Farms

6. Conclusions


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• Process dynamic in electric utility system cover a very wide spectrum of phenomena and a wide range of disciplines.

• The dimensions and complexity of power system dynamics can well be appreciated when one realize that there are hundreds of interacting elements.

Block Diagram of a Synchronous generator control systemSource: P.M. Anderson “Power System and control


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• There are hundreds of interacting elements such ad generator with their controls, energy supply system and controls, and that the mathematical representation of each element generally involves many independent variables described by set of high order, non linear differential equations.


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• Dynamic process in electrical power system can be characterized by various areas of consideration and their characteristic time scales or frequency bands.


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• In general way, the transients in electrical power systems are classified according to three possible timeframes: – Short-term, or electromagnetic transients;

– Mid-term, or electromechanical transients;

– Long-term transients.Short Mid Long


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• Electromagnetic phenomenon require instantaneous value simulation (IVS) software, such as the Alternative Transient Program (ATP), an electromagnetic transient program (EMTP) or SimPowerSystems from Matlab must be used.

Short Mid Long

Electromagnetic


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• The phenomena of interest have a frequency of about 0.1–10 Hz.

• The typical problems that are studied when using these programs are voltage and angle stability.

Short Mid Long

Electro-mechanical


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• Stability is a problem of balance


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• Instability in a power system can be manifested in many different forms depending on the configuration and operation.

• Traditionally, the stability problem has been maintaining synchronous operation.


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• Power system stability may be broadly defined as that property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions ad to regain an acceptable state of equilibrium after being subjected to a disturbance [1].

• The robustness of a system is defined by the ability of the system to maintain stable operation under normal and perturbed conditions [2].

[1] P. Kundur, Power System Stability and Control. New York: McGraw- Hill, 1994.[2] PowerFactory User’s Manual DIgSILENT PowerFactory Version 14.0. DIgSILENT GmbH, Gomaringen, Germany 2008


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Classification of Power System Stability [1]

[1] P. Kundur, Power System Stability and Control. New York: McGraw- Hill, 1994.RECOMMENDED READ: P. Kundur, J. Paserba, V. Ajjarapu, G. Andersson, A. Bose, C. Canizares, N. Hatziargyriou, D. Hill, A.M. Stanković, C. Taylor, T. Van Cutsem, V. Vittal, "Definition and classification of power system stability IEEE/CIGRE joint task force on stability terms and definitions", IEEE Transactions on Power Systems, Vol. 19 , No. 3 , pp.1387 - 1401, Aug. 2004


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• IEEE-CIGRE classification (IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and Classification of Power System Stability”, IEEE Trans. Power Systems and CIGRE Technical Brochure 231, 2003):


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• IEEE-CIGRE classification (IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and Classification of Power System Stability”, IEEE Trans. Power Systems and CIGRE Technical Brochure 231, 2003):


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• “Frequency stability refers to the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load.”

• Frequency stability analysis concentrates on studying the overall system stability for sudden changes in the generation-load balance.


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• An example of a frequency stability problem is the Italian blackout of Tuesday September 28, 2003 (this was extracted from material provided by Prof. Alberto Berizzi, Politecnio di Milano):


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Interconnection to the rest of UCTE (2002):


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• At 3:01.22 (27.5 GW Italian loading), the Swiss Mettrlen-Lavorgo line, which was carrying 1320 MW, trips due to a tree flashover.

• There are 3 automatic reclosings that fail at 3:03.50, and at 3:08.23, manual reclosing also fails due to a standing phase angle (SPA) problem.


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3:10.47 ETrans (operator of interties between Switzerland and restof Europe) lets the GRTN (Italian operator) know of theMettlen-Lavorgo line trip (1320 MW) and the overloadingof the Sils-Soazza line (1650 MW), and requests a 300MW demand reduction to relief the overload.

3:18.40 ETrans contacts EGL (Switzerland operator) requesting thetripping of a transformer in Soazza.

3:21 GRTN reduces the power imports by 300 MW.

3:22.03 ATEL (Swiss energy company) changes the connection ofthe transformer at Lavorgo.

3:25.22 Protections trip the Sils–Soazza line (1783 MW) due to atree flashover. This is basically the beginning of thecascading events that follow, severing Italy from the rest ofEurope and leading to the collapse of the Italian system.


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• Started as a “typical” transient stability problem, with separation of Italy with respect to France due to large angle differences.

• Total collapse of the system was due to a frequency stability problem:– Under frequency relays in generators were not properly

calibrated, tripping before load was tripped.


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• It is the energy stored in generators because they are rotating….

They continue to rotate after the energy source is removed

Large spinning machines provide stored energy due to the rotating mass of their rotor, driving turbine shafts etc.

Engineers model this as a number of rotating masses.


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• The inertia constant of a rotating system (H), orindividual generator, is used to define the energy storedin its rotating mass (Ec0).

• It can be understood as the time, in seconds, that itwould take to replace this stored energy whenoperating at rated mechanical speed (sm) and ratedapparent power output (Sbase)

21

2sm

base

JH

S

where: J is the total moment of inertia in kg.m2, sm is the rated mechanical speed in rad/s, and Sbase is the selected base apparent power in MVA.


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• The relationship between the power imbalance at theterminals of the i-th generator in p.u. (pi) and itsfrequency (fi), can be expressed as:

where: pm,i is the mechanical turbine power in p.u., pe,i

is the electrical power in p.u., pi is the load generationimbalance in p.u., Hi is the inertia constant in s, fi is thefrequency in Hz, fn is the nominal system frequency inHz and dfi/dt is the rate of change of frequency in Hz/s.

2m e

n

H dfp p p

f dt


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• If the balance between generation and demand isnot reached, the system frequency will change at arate initially determinate by the total system inertia (H).

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 548.5

49

49.5

50

50.5

51

51.5

Time (s)F

requ

ency

(H

z)

25 MW50 MW75 MW100 MW

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-2

-1

0

1

2

Def

ivat

e of

Fre

quen

cy i

n ti

me

(Hz/

s)

Time (s)

25 MW50 MW75 MW100 Mw

2m e

n

H dfp p p

f dt

pdf

dt


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• The total system inertia (HT) comprises the combinedinertia of most of spinning generation and loadconnected to the power system.

• The contribution of the system inertia of one load orgenerator depend if the system frequency causeschange in its rotational speed and, then, its kineticenergy

21

2i sm

ibase

JH

S

T 1 2 nH H H H

T1

N

ii

H H


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• Inertia is the stored rotating energy in the system

• Following a System loss, the higher the System Inertia (assuming no frequency response) the longer it takes to reach a new steady state operating frequency.

• Modern Generation Technologies employing power electronic converters currently do not contribute to System Inertia.

• Further growth is expected in HVDC which currently does not contribute to System Inertia

• The implications of this are a substantial erosion in System Frequency Control and consequent required increases in additional reserve and response.


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• an automatic change in active power output or demand in response to a frequency change

• to maintain system frequency within statutory and operational limits

Demand: 47318MW17:05:00 GMTFrequency: 49.996Hz17:07:45 GMTSystem TransfersN.Ireland to Great Britain: -444MWFrance to Great Britain: -310MWNetherlands to GB: 0MW07/03/2011 17:00:00 GMT

North-South: 7567MWScot - Eng: 215MW07/03/2011 17:10:00 GMT

http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Frequency/Freq60.htm

Real Time Frequency Data - Last 60 Minutes

National Grid has a license obligation to control frequency within the limits specified in the 'Electricity Supply Regulations', i.e. ±1% of nominal system frequency (50.00Hz) save in abnormal or exceptional circumstances.


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Standards


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• All licensed generators in accordance with Grid Codemandatory requirements generators offering enhancedcommercial services

• Demand tripping by low frequency relay

• Unlicensed generators with a commercialagreement.


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• Frequency Control processes


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INERTIAL RESPONSE: The speed of the synchronous generators also reduces and some of the kinetic energy stored in the rotating mass is released as electrical energy. This is a fast response and proportional to the rate of change of frequency.

Frequency Control Phases


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GOVERNOR ACTION: The automatic droop control loop of the governor acts on the change in frequency and opens the governor valve to increase the turbine’s output.This is a slower response and depends on the dead band of the governor and time lag of the prime mover

Frequency Control Phases


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• Primary Control: The action of turbine governors dueto frequency changes when reference values ofregulators are kept constant.

• Secondary Control: The restoration of the ratedfrequency followed to the primary control action, butnow at the required increased value of power demand.

• Tertiary control: Objective depends on theorganizational structure of a given power system andthe role that power plants play in this structure


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nf

freff

r

f

tieP

ACEPI

ref

refP


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• Low order System Frequency Response (SFR) modelthat can be used for estimating the frequencybehavior of a large power system, or islanded portionthereof, to sudden load disturbances.

• The SFR model is a simplification of other modelsused for this purpose, but it is believed to include theessential system dynamics.


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• When there is a demand change (Pmec), it is reflectedinstantaneously as a change in the electrical PowerOutput Pe of the generator.

dt

dMPP elecmec

Ms

1

mecP

elecP

+ -

Transfer function relating speed and power


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Load response to frequency deviation

• The overall frequency-dependent characteristic of acomposite load may be expressed as:

• The system block diagram including the effect of theload damping is:

loadPD

D loadP

Ms

1

mecP

loadP

+ -

D

-


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• System Frequency Model of the Typical ReheatTurbine Governor Model.

1

2Hs D

1

1m H R

R

K F T s

R T s

dP aP

mP

FH = Fraction of total power generated by the HP turbineTR = Reheat time constant, secondsH = Inertia constant, secondsD = Damping FactorKm = Mechanical Power Gain Factor


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• System frequency response variation of H incrementsof 0.5

0 2 4 6 8 10 12 14 16 18 20

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Fre

quen

cy (

Hz)

Time (s)

H=5.00TR = 0.05 Pd= -0.20D = 1.00 TR = 8.0FH= 0.30 KM= 0.95

H= 3.00

Frequency Response for Varying Values of H


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• System frequency response variations of R withincrements of 0.01.

0 2 4 6 8 10 12 14 16 18 20-2.5

-2

-1.5

-1

-0.5

0

0.5

Fre

quen

cy (

Hz)

Time (s)

R= 0.10

H = 4.00 Pd= -0.20D = 1.00 TR = 8.0FH= 0.30 KM= 0.95

R=0.05

Frequency Response for Varying Values of R


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• System frequency response variations of TR withincrements of 1.0

0 2 4 6 8 10 12 14 16 18 20

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Fre

quen

cy (

Hz)

Time (s)

K = 0.05 Pd= -0.20D = 1.00 H = 4.00FH= 0.30 KM= 0.95

TR= 10.0

TR = 6.0

Frequency Response for Varying Values of TR


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• During a system frequency disturbance the balancebetween generation-demand is not reached, then thesystem frequency will change at a rate initiallydeterminate by the total system inertia (HT).

Fre

quen

cy (

Hz)

df

dt

2m e

n

H dfp p p

f dt


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• The contribution of the system inertia of one load or generator depend if the system frequency causes change in its rotational speed and, then, its kinetic energy.

Restorative power

smaller

smaller

T1

N

ii

H H

21

2i sm

ibase

JH

S


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• The power associated with this change in kinetic energy is fed or taken from the power system and is known as the inertial response.

0 1 2 3 4 5 6 7 8 9 10

0.5

1

0 1 2 3 4 5 6 7 8 9 1048.5

49

49.5

50

Kinetic Energy

Inertial Response


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• The objective of the synthetic inertia control isextracting the stored inertial energy from themoving part on WTGs.


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• There are several names for this control system that enable inertial responses on a WTG: Artificial, Emulated, Simulated, or Synthetic Inertial.

• Examples of synthetic inertia controlled commercially available for WTG are: General Electric WindINERTIA™, ENERCON Inertia Emulation.


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Increasing wind power PenetrationWind turbines provide small or even no response to

frequency changes

3-level frequency control

1. Primary Control: <30s, automatic, local2. Secondary Control: 30s~30min, artificially or by AGC, global3. Tertiary Control: economic dispatch

13

2

A. Wind TurbineB. Wind Farm LevelC. Power System Level

Maintain 3 frequency indices

3

21

1. ROCOF2. Transient Frequency Nadir3. Steady state Frequency deviator


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• WT Technologies

S

S

S

S

GridIsUs

Double-fed induction generator

Gearbox

ac/dc dc/ac

DC link

Ir Ic

Ur

Variable SpeedPMSG

Direct Drive

Variable Speed Inductiongenerator

Variable SpeedDouble fed


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• Wind turbines provide small or no response to frequency changes

• DFIG(doubly fed induction generator): the rotor is connected to the grid through an AC/DC/AC converter which decouples the rotor from the frequency.

GridIsUs

Double-fed induction generator

Gearbox

ac/dc dc/ac

DC link

Ir Ic

Ur


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• PMSG (permanent magnet synchronous generator):contains a multi-pole magnet rotor and an AC/DC/ACconverter attached to the stator.

• Consequently the generator is fully decoupled from thegrid.

S

S

S

S

STOP


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• Fixed speed wind turbine: can provide inertialresponse to the frequency fluctuation, however, theinertial response is generally smaller and slower thanthat of synchronous generators due to allow normal slipof 1%-2%.


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• Traditional wind turbines store no power reserves.

• They always operate at the maximum power point tracking(MPPT).

Additional controllers are needed!

P

r

min 0, wv

1 0, wv


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1. Inertial Control

2. Droop Control

3. Deloading Control

Hidden Inertia Emulation

Fast Power Reserve Emulation

Pitch Angle Control

Rotational Speed Control

Controller

Wind

Externalturb set

P

meas


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• Functions: – For variable speed wind turbines.

– To reduce the maximum frequency change rate.

– To increase the transient frequency nadir.

d

dtsys

measP ,r ref

,r meas

r MPPTP

2 sysin sys

dP H

dt

r

P

21

2sm

base

JH

S

refP

• Comparison with fixed speed wind turbines and conventional generators: – Releasing considerably larger kinetic

energy.

– Responsing faster because of the PWM technology


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• Similar functions to “hidden” inertia emulation – To compensate the power loss for a short period and save time for other

slower generators to participate in the frequency control

measP,r ref

r

r

PrefP

,r meas

constPt

,0r

2, 0 2 const

r ref rt r

Pt

J 2 2

0

1 1

2 2const r rtP t J J


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• Functions:– For variable speed wind turbines.

– To increase the transient frequency nadir.

– The droop controller should be ended on time to avoid the stalling or

– coordinated with the deloading control.

1

WTRf

measP ,r ref

,r meas

r MPPTP

P

r

P

MPPT

refP

nomf

P1P0P

measf

1

WTR

1 0WT

fP P P

R


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• Deloading definition: – not MPPT, i.e., abandon some

wind energy.

• Advantages: – Provide sufficient reserves for

systems.

– Save investment of the reserves and storages although the wind energy may be partially lost.

– The wind power regulation is faster than the thermal power regulation due to the PWM control technology.

– The system will be more stable than MPPT condition.

P

r

min 0, wv

1 0, wv

•Deloading possibilities: Overspeeding: increasing the rotor rotational speed over the MPPT speed. Pitching: controlling the pitch angle.


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• Functions:– For both fixed and variable speed wind turbines.

– Providing power reserves.

– Slower than the convertor control due to the mechanical time constant.

Traditional Pitch Angle Controller

,r meas

ref

max

1

servoTd

dt

1

s

min

maxmin rate

max rate

Modified Pitch control

PIref

1

servoTd

dt

1

s

min

maxmin rate

Conterter

measP

Control Signal from wind farm

wtP

Droop control

f P

max rate


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• Functions:– For variable speed wind turbines.

– Providing power reserves.

– Faster than the pitch control due to the PWM technology.

– Storing more kinetic energy, like a big flywheel energy storage.

– Pitching increases wear and tear, overspeeding does not.

– Some research showed that overspeeding can improve the small signal stability.

,r ref

,r meas

r

r

PwtP

f P

refP


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1. Local Control

Central Control

Wind turbine deloadingcontrol

Energy storage system (ESS) control

turb

,1uP

*,1uP

,2uP

*,2uP

,3uP

*,3uP

cmmP

,1MPT

aP ,2MPT

aP ,3MPT

aP

1/ N

maxP

0

*WFP

WFP PCC


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WFPCentral Control

Local Control


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ESS technologies:• Aggregated ESS: battery, compressed-air, pumped hydro.

• Distributed ESS: battery, flywheel, supercapacitor

• For frequency control, fast and medium-term ESS is suitable.


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Control methodsPrimary Control

Secondary Control

Reduce ROCOF

Increase fmin

ReduceDf

Wind turbine level

Inertial

Droop

Deloading

Wind farm level

Local

Central

Power System level


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WORKSHOP: Frequency Control Schemes and Frequency Response of Power Systems considering the...

Engineering

Transcript of WORKSHOP: Frequency Control Schemes and Frequency Response of Power Systems considering the...