High Renewable Energy Penetration and Power System ......This type of response is called primary frequency response. It is used to restore power balance, and thus stabilise the frequency.

Centre for Renewable Energy andPower Systems

U N I V E R S I T Y O F TA S M A N I A

High Renewable EnergyPenetration and PowerSystem Security: New

Challenges and Opportunities

Prof Michael Negnevitsky

Chair in Power Engineering andComputational Intelligence

Director of the Centre for RenewableEnergy and Power Systems

School of EngineeringUniversity of TasmaniaPrivate Bag 65 Hobart

Tasmania, 7001 Australia

Michael Negnevitsky

ENERGY 2019June 2 - 6, 2019Athens, Greece

Centre for Renewable Energy and Power SystemsU N I V E R S I T Y O F TA S M A N I A

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Contents

• Concept of power system security.

• Operating reserves.

• Inertial and primary frequency response.

• Impact of renewable energy generation.

• King Island isolated power system.

• Risk-based security assessment.

• Conclusions.



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Introduction

• The word “security” in the context of a powersystem implies its security against a completecollapse, or a blackout. Secure operationinvolves practices aimed to keep the systemoperating normally when contingencies occur.

• An increasing penetration of intermittentrenewable energy generation introducesadditional uncertainties in power systems.



Power system security (cont.)

4

• Power systems are designed and operated towithstand contingencies selected on the basisof their probabilities.

• In practice, these contingencies are usuallydefined as the loss of any single majorcomponent in a power system.

• We cannot stop contingencies from happening,and we cannot predict when they will occur. Butwe can model potential contingencies andanalyse their consequences.




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• Because the specific times of failures areunpredictable, the system is operated at all timesin such a way that it will not be left in a dangerouscondition when any credible contingency occurs.

• Credible contingencies are most probablecontingencies. For example, we know fromexperience that the probability of failure of asingle component of the system is much higherthan simultaneous failures of multiplecomponents.



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• We must plan and operate power systems sothat even the worst credible contingency willnot result in an unacceptable performance ofthe system.

• We must also ensure that after the first crediblecontingency occurs, the system operator will beable to adjust the system and prepare it for thenext credible contingency.



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• To ensure the required level of security, apower system must have sufficient reserves.

• These reserves are needed to balance short-term variations in supply and demand, mitigatethe effects of load forecasting errors, handlepeak demand, and manage fluctuations inrenewable energy generation.

• But most of all, reserves are needed towithstand contingencies.

Operating reserves



888

• Power system equipment is subject to randomfailures, and thus additional generationcapacity is needed so that it can be called uponwhen a large generator or a heavily loadedtransmission line is suddenly taken out ofservice. This additional capacity is referred toas operating reserve.

Operating reserves (cont.)



Contingency analysis for System B

200 MW

600 MW

200 MW

200 MW

(a)

System A System B

150 MW

600 MW

300 MW

150 MW

(c)

System A System B

250 MW

600 MW

100 MW

250 MW

(b)

System A System B

600 MW

300 MW

300 MW

(c)

System A System B

99



1010

Contingency event and a typical system response

700

60.05

60.00

59.95

59.90

59.85

59.80-100 100 200 300 400 500 600

Time, s

A

B

C D

E

Frequency, Hz

0



1111

Inertial response• At the very instant of the generation loss,

synchronous generators that are still connected tothe grid release kinetic energy stored in theirrotating masses (turbines, shafts and rotors), andthereby slow down the frequency decline.

• This type of response is called inertial response.Large fast rotating generators have larger inertialresponse than smaller or slowly rotating machines.The system inertia, or the cumulative inertialresponse of all rotating machines, determines theinitial slope of the frequency decline.



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Primary frequency response• Following a disturbance, turbine governors detect

the frequency decrease and additional power isprovided to balance the load. This type of responseis called primary frequency response. It is used torestore power balance, and thus stabilise thefrequency. The primary frequency response isusually delivered within 30 s.

• Maintaining sufficient reserves of active power isnecessary for preventing under-frequency loadshedding and frequency collapse, but it does notguarantee voltage stability.



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Impact of Renewable Energy Generation• An increasing penetration of intermittent renewable

energy generation introduces additionaluncertainties in power systems.

• However, the impact of variable generation on thesystem security is often exaggerated. For example,in the Ireland and Northern Ireland power system,the system stability degrades only when non-synchronous variable generation exceeds 50% ofdemand.

• No significant mitigation measures are requireduntil the wind and solar penetration reaches 20%.



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• Wind speed variations within the range of 15-25 m/s donot cause any changes in power generation because inthis range, wind turbines operate at full output.

• On the other hand, wind variations within the range of4-15 m/s can result in a substantial change of theturbine output. This problem can usually be mitigatedby geographical dispersion of wind farms.

• For example, in Germany, a single wind farm canfrequently exhibit hour-to-hour power swings of up to60% of the installed capacity while aggregated powerswings of wind farms over entire Germany do notexceed 20%.

Impact of Renewable Energy Generation (cont.)



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The main challenge facing apower system with highpenetration of renewables is thedisplacement of conventionalsynchronous generation by non-synchronous generation.



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• Kinetic energy stored in the rotating masses ofsynchronous generators provides the systemrotational inertia.

• Wind power generators are decoupled from the gridby electronic converters – they do not provideinertia to the system. This reduces the total systeminertia.

• The system becomes more vulnerable tocontingencies – even contingencies that previouslywere considered “safe” can now lead to frequencyviolations and the system’s instability.

Impact of Renewable Energy Generation (cont.)



Relationship between the stored kinetic energy and inertia:

Kinetic energy and inertia

where E is the kinetic energy stored in the rotating masses,[MW·s]; J is the moment of inertia around the axis ofrotation, [kg·m2]; and ω is the angular velocity of the rotor[rad/s].

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Based on the amount of energy stored in the rotating mass,we can determine how long the generator can supply itsrated power solely from the stored kinetic energy:

Kinetic energy and inertia

where H is the inertia constant, [MW·s/MVA]; E0 the kineticenergy measured at the rated angular velocity of the rotorω0, [MW·s]; and S is the rated apparent power of thegenerator, [MVA].



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• The inertia constant is expressed in seconds. Itindicates the duration that the generator cansupply its rated power to the system using onlyits kinetic energy.

• For example, the inertia constant of 6 s meansthat the generator can supply its rated power tothe system for 6 s using only energy stored in itsrotating masses.

Inertia constant



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Inertia constant for conventionalmachines


Machine typeInertia constant,MW·s per MVA

Thermal3600 r/min (2-pole)1800 r/min (4 pole)

2.5 – 6.04.0 – 10.0

Hydraulic 2.0 – 4.0


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3.25 3.30 3.403.35 3.45 3.50

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

Time (in 15-min steps) ×104

Aggre

gate

din

ert

iaco

nst

ant

H,

s

Aggregated rotational inertia in the German power system(December 2012). Conventional generators provide inertia of 6 sand wind and PV generators do not contribute any inertia.



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• Six 200 MVA and ten 100 MVA synchronousgenerators are supplying a total load of 2000 MW.The inertia constant of each 200 MVA unit is 5.0 s onthe 200 MVA base, and the inertia constant of each100 MVA unit is 4.0 s.

• Determine the frequency deviation following asudden loss of one of the 200 MVA units.

• Examine the frequency dynamics of the system whenfive 100 MVA synchronous generators are displacedby non-synchronous renewable generation.

Case study



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The total system inertia constant following thecontingency:

The system frequency dynamics when five 100 MVAsynchronous generators are displaced by non-synchronous power generators:



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The MATLAB Simulink model of the systemfrequency dynamics

1

0.15 s+1

Governor

Controller

1

18 s + 2

_

Inertia andload dampingTurbine

1

0.70 s+1_

Sum1

+

Sum2

45

Step



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The MATLAB Simulink model for Example 9.3 when five100 MVA synchronous generators are displaced by wind

and solar PV generation

1

0.167 s+1

Governor

Controller

1

14 s + 2

_

Inertia andload dampingTurbine

1

0.733 s+1_

Sum1

+

Sum2

32.5

Step



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Frequency response of the power system

0 2 4 6 8 10 12 14 16

60.0

59.9

59.8

59.7

59.6

59.5

59.4

59.3

Fre

quency

,H

z

Time, s

Case 1

Case 2

Frequency in the “lighter” case declines much faster,although in both cases, the system has sufficient rotatingreserves to cover for the loss of the largest generator.



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• The frequency response of wind turbines is dependenton active controls. These controls are designed toprovide a response, which is functionally similar to thefrequency response of synchronous machines.

• Modern wind turbines are also capable of providing“governor” frequency response. This is accomplishedthrough blade pitch control. In order to provide“governor” response to low frequency events, windturbines must operate below the level of maximumpower output possible for a given wind condition.

Frequency response of wind turbines



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Battery energy storage systems?

• As penetrations of renewable energy increasewithin a system, conventional approaches maybecome unable to manage system security.

• Battery energy storage is a common solution.But it is an emerging technology and currentlyexpensive.

• Australian experience advocates approachesable to reduce both the system cost andcomplexity.



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King Island isolated power system

Image source: Google Earth

• Small, mostly agricultural islandbetween Tasmania andmainland Australia, ideallyplaced for wind generation.

• Electricity network owned andoperated by Hydro Tasmania –opportunity to implementwhole system changes.

• Wind energy currently covers65% of energy needed by thesystem.

• Instantaneous renewable energypenetration is very high (85%peak) but remains limited due tothe intermittent nature of wind.



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King Island power system



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• The King Island load varies between 1 MW and 3MW, with an average of around 1.5 MW.

• The wind resource on King Island suppliesapproximately 65% of the island’s power supply(2.4 MW wind farm).

• The target of this system is to use all the availablesolar and wind power to reduce diesel usage.

• The station has been designed to run unattended.The system has evolved progressively over a periodof 20 years, with system performance from 1998to 2012.




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• The step change observed in 2004 involves thecommissioning of the two Vestas V52 wind turbines(850kW each).

• In addition King Island was also the first MW scalesystem to achieve renewable penetrations above50%. This milestone was achieved via a range ofemerging technologies, installed from 2008 to 2014.

• Annual renewable energy penetrations hasexceeded 65%. The system is also able to operatefor with no diesel generation, achieving diesel offoperation for up to 20% of the year.




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King Island: technology portfolio

Technologies integrated:

•Dual axis solar PV tracking system (2008)

•Dynamic resistive frequency control (2008)

•Flywheel diesel uninterrupted power supply (2011)

•Biodiesel blending (2012)

•Demand side management (2012-2013)

•Battery energy storage system (BESS) integration(2014).



3434

Dynamic resistor

Flywheel

Battery storage

Demand response



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King Island: flywheel diesel technology

• The flywheel diesel technology consists of a largeflywheel generator, coupled to a diesel engine.

• The mechanical coupling allows for the generator toacts as a synchronous condenser, providing spinningreserve to the power system under high renewablepenetrations.

• In the event that system frequency drops belowacceptable limits the clutch engages to fast start thediesel engine.

• The technology provides the King Island power systemapproximately 30 s of inertia.



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King Island: Fast Demand Response

Example day on King Island

Surplus wind energy Short drops in wind resource



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Case for short-term power system support

Wind power

Diesel generation

System Load



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Demand Response GeneratorResidential

Residential

Commercial

SlaveController

Communicationsnetwork

MasterController

Ethernet TCP/IP

WiMAX

ZigBee

Power SystemController (PSC)

Demand ResponseGenerator



39

Installation example: ResidentialWiMax modem

Gateway

SmartSwitches



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Installation example: Commercial

15kW Air Condition unit



41

Installation example: Local School

School controllable loads:

• 3-Phase hot water

• 3-Phase Pool heaters

• 3-Phase Heaters

• 1-Phase Heaters



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10 20 30 40 50 60 70 80 90 100 1100

1000

2000

3000

Realpow

er

(kW

)10 20 30 40 50 60 70 80 90 100 110

-500

0

500

1000

Realpow

er

(kW

)

10 20 30 40 50 60 70 80 90 100 1100

5

10

15

20

25R

ealpow

er

(kW

)

Time (sec)

Wind power output

Power system Load

Spining reserve surplus

Available DR

Requested DR

Dispatched DR

Demand response operation

• Demand responseautomatically initiatedduring times of low spinningreserve.

• Demand response recordedin within 1 second from themoment of receiving acommand.



43

Demand response automated operation (I)July 04th 2018



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Demand response automated operation (II)

July 05th 2018



45

Demand response automated operation (III)

July 12th 2018



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Demand response automated operation (IV)

July 18th 2018



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King Island: battery energy storage system (BESS)

• BESS integration was commissioned in 2013. Atthat time the BESS represented the largest inAustralia – 3 MW / 1.6 MVh.

• The role of the battery is to extend the time forwhich the island can run diesel off, and tocapture some of the spilt renewable generationotherwise sent to the resistive load.



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-0.5

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35 40 45

Po

we

r(M

W)

Hours

WIND LOAD DIESEL RESISTOR BESS

King Island generation showing wind generation, islandload, diesel generation, resistor load and battery load



49

• Traditionally security assessment is performed based ondeterministic criteria. The N-1 security criterionrequires a power system to withstand an outage of anysingle system component without violating any systemoperating limits.

• It has satisfied the needs of the power industry fordecades. However, the deterministic approach tosecurity may not be adequate in modern powersystems with market driven dispatch and highpenetration of renewable energy and distributedgeneration.

Risk-based Security Assessment



50

• Traditionally security assessment is performed based ondeterministic criteria. The N-1 security criterionrequires a power system to withstand an outage of anysingle system component without violating any systemoperating limits.

• It has satisfied the needs of the power industry fordecades. However, the deterministic approach tosecurity may not be adequate in modern powersystems with market driven dispatch and highpenetration of renewable energy and distributedgeneration .

Risk-based Security Assessment



51

• Deterministic security criteria define a set of“credible” contingencies that the system shouldbe able to withstand.

• However, that the deterministic contingencyanalysis does take the probabilities ofcontingencies into account – the selection of“credible” contingencies implicitly implies thatthese contingencies are more likely to occur inreality.

Risk-based Security Assessment (cont.)



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• Security can be defined as the risk in thesystem’s ability to withstand randomcontingencies without interruption to customerservice. The higher the risk the lower thesecurity.

• Although risk cannot be eliminated fully due tounexpected faults and probabilistic behaviour ofa power system, it can be assessed and managedwithin an acceptable level in power systemplanning, design and operation.

Risk-based security definition



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• Risk-based security analysis is concerned withvoltage violations, overloads and frequencyresponse adequacy.

• Frequency response adequacy is defined as thecapability of frequency response resources toprevent frequency from dropping below a certainlimit.

• We need to assess the primary frequency responsebecause of the risk of under-frequency loadshedding, particularly in systems with low inertia.

Risk-based security definition



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• In risk-based security assessment, we do not use apredefined list of contingencies, but generatecontingencies at random based on theirprobabilities.

• Then, we assess the consequences of thesecontingencies to determine whether loads aredisconnected following voltage violations, overloadsand significant imbalance between load andgeneration. This allows us to measure the impact ofrandom contingencies in terms of loads not served.

Risk-based security assessment



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• Using a model of the restoration process, we obtainthe time needed to restore power, and thus,estimate the cost of the generated contingency.

• Finally, we repeat generating contingencies overand over, each time using a different set of randomvalues. This process is called Monte Carlosimulation – a problem solving technique used tocalculate the probability of certain outcomes byrunning multiple trials, called simulations, usingrandom variables.

Risk-based security assessment



The value of risk

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where Risk is the risk index,

p(Sn) is the probability of the pre-contingency operatingstate Sn,

p(Cnm) is the probability of contingency Cm occurring in thestate Sn,

Sev(Cnm) is the severity of the contingency Cm in the stateSn,

N is the total number of pre-contingency operation states,and M is the total number of contingencies considered ineach state.



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• The main challenge facing a power system withhigh penetration of renewables is thedisplacement of conventional synchronousgeneration by non-synchronous generation.

• Recent developments in wind turbine andbattery storage technologies offer a hybridsolution for mitigating the effect of fasterfrequency dynamics in power systems with highpenetration of renewable energy.

Conclusions



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• The deterministic approach to security may notbe adequate in modern power systems withmarket driven dispatch and high penetration ofrenewable energy and distributed generation.

• In risk-based security assessment, we generatecontingencies at random, based on theirprobabilities.

Conclusions (cont.)


High Renewable Energy Penetration and Power System ......This type of response is called primary frequency response. It is used to restore power balance, and thus stabilise the frequency.

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