23 stability and control

7/28/2019 23 Stability and Control

1/88

Room 6.2

.Wednesday, 14th September(8. 30 - 10.30)

Rpt: Costas Vournas

231 Volt/Var control for active distribution networks - L.A. Kojovic, C.A. Colopy, D.

Arden

232 Stability enhancement and blackout prevention by VSC based HVDC -Y. Jiang-Hfner, M. Manchen

233 Effectiveness of a supplementary MLQG power oscillation damping controllerinstalled at an HVDC line within a meshed network - R. Preece, A.M. Almutairi,O. Marjanovic, J.V. Milanovic

234 Voltage stability, loss reduction and dynamic stability studies of an integratedsystem - P. Pesoti, S.O. Souza, F. Terra, A.C. Zambroni de Souza, B. Isaias Lima

Lopes, R. Coradi Leme

mar a gor ms o accommo a e s r u e genera on n e gr -H.T. Yip, C. An, G. Millar, G.J. Lloyd

236 Optimization strategy applied to DG reactive power for decentralized voltage

- . . , . , .


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BOLOGNA 2011

Volt/Var Control

for

Active

Distribution

Networks

LjubomirA.KOJOVIC CraigA.COLOPY DanielARDEN

CooperPower

Systems

USA


3/88

DistributedVoltVar AutomationandOptimization

CapacitorBanks

Voltage

Regulators

Substation

LoadTapChanger

Voltage

RegulatorsDistributionTransformer

LTCome

DisadvantagesofDistributedAutomation

DistributedAutomation

LTC,CapacitorBanks,andVoltageRegulators

Controlbasedonlocalmeasurements:

Isnotcontinuouslymonitored

Doesnotadequatelyrespondtochangingconditions

outonthedistributionfeeders canmisoperate

Voltage

Current

Power

real

reactive

followingautomaticreconfiguration

Operationmaynotbeoptimalunderallconditions

Cannotoverridetraditionaloperationduringpower

systememergencies

Temperature,and

Timeofday

Maymisoperate whenDGarepresent reverse

powerflowfromDGcancausestandalonecontrolsto

believefeederhasbeenreconfigured


4/88


5/88

IntegratedVoltVar AutomationandOptimization

YukonIVVC

Server

SCADA

GIS/mapping

CustomerCare&Billing

OMS

EngineeringAnalysis

Administrativeand

Operations

InternalOperationsand

CustomerService

ExternalOperationsor

TCP/IP

(BPL,FiberOptic,WiFi)

Cellular

(GSM/GPRS,

x

AMINetworks

900MHz

VHF

Voltage

Sensors

LTC

Controls

Meters

CapacitorBank

Controls

VoltageRegulator

Controls

Recloser

Controls


6/88


FeederVoltageManagementSoftware

Capacitor

Control

Var orPowerFactorManagement

Software

Minimizing Var Flow

FlatteningtheVoltageProfile

ConservationVoltageReduction

CommunicationsNetworks

TCP/IP(BPL,fiber,WiFi)

DNP3.0(RadiosviaRS232port)

900MHz,VHF,PLC

CommunicationtoFeederCapacitorBanks: 1 or2waycommunicationnetwork


7/88


Upper

Voltage

Limit

FlattenedVoltageProfile

agnitude

LoweredVoltage(afterflattening)

Vo

ltageM

FeederLength

Regulated

by

100

%]

90

we

rFactor

Unregulated80P

o

TwoDayMeasurementData12

a.m. 12

p.m. 12

a.m. 12

p.m. 12

a.m.


8/88


BusinessDrivers Business

Objectives

Regulationimposesnewrequirementswiththe Unitypowerfactorminimizeslosses

Energypurchasesavings

IncreasedkWh

sales

on

improved

voltage

profile

ReducedvoltageminimizespurchasedkW:

1%reductioninvoltageresultsin0.5to0.8%

reductioninkW

Stimulusbilldrivenenergyefficiencyprojects

FunctionalObjectives Technical

Requirements Monitorandcontrolsubstationvoltageregulators

orLTC

Managefeedervoltagedowntosettabletargets

Monitorandcontrolsubstationandfeeder

capacitorbanks

Monitorend

of

line

voltages

to

avoid

excursions

Flatvoltageprofile

fromthespecifiedranges


9/88


thatneedrepair

Traditionallyutilitiesvisiteverycapacitorbanksiteannuallytofind

the20%thatneedrepair


10/88


11/88


CurrentCapacitorBankStatusReport


12/88

C

opyrightyearABB.

All

rightsreserved.-1-

9/23/2011,YingJiang-Hfner

Stabil ity Enhancement and

Blackout Prevention by

VSC Based HVDC

2011 08 14, Italy

Ying J iang-Hfner (speaker)

ABB AB, HVDC, Sweden

Manfred Manchen

NamPower, Namibia


13/88

C

opyrightyearABB-2-

9/23

/2011,

YingJiang-H

fner

Summary

Caprivi link project

Technical Challenges

Field experiences and Factory tests

Comparison between interconnected grids with AC and DC

Conclusions


14/88

C

opyrightyearABB-3-

9/23

/2011,

YingJiang-H

fner

Caprivi link, a VSC HVDC transmission,

is a 300 MW HVDC Light with OH-line 950 km

interconnects the national grids

of Namibia and Zambia

has trading of electricity as main purpose

Enhances stability of two national grids


15/88

C

opyrightyearABB-4-

9/23

/2011,

YingJiang-H

fner

330 kV AC400 kV AC

Gerus

Converter

station

Nambia

HVDC Line 950 km

Zambezi

Converter

station

Zambia- 350 kV DC

+ 350 kV DC

Electrode Lines50 km

Capriv i link, a planned Bipole


16/88

C

opyrightyearABB-5-

9/23/2011,

YingJiang-H

fner

- Overhead line and long distance,Re-start after DC-line faults within 500 ms after clearing

- Configured as a bipole, starting with monopole

- Connecting two changable AC network configurations. Any one of them can be completely out of generation. Any one of them can be a small islanded grid. Both of them can be extremely weak SCR


17/88

C

opyrightyearABB-6-

9/23/2011,

YingJiang-H

fner


18/88

C

opyrightyearABB-7-

9/23/2011,

YingJiang-H

fner


19/88

C

opyrightyearABB-8-

9/23/2011,

YingJiang-H

fner


20/88

C

opyrightyearABB-9-

9/23/2011,

YingJiang-H

fner

Conclusion


21/88

C

opyrightyearABB-10-

9/23/2011,

YingJiang-H

fner

Conclusion

Impact to Interconnected Grids


22/88

C

opyrightyearABB-11-

9/2

3/2011,

YingJiang-H

fner

Impact to Interconnected Grids

Interconnected by AC

+ Increased spinning capacity+ Increased availability by

coordinating maintenance+ Energy trading- Complexity in operation- Risk of uncontrolled power flow- Risk of spreading failures- Stability related to SCC

Interconnected by DC (VSC based)

+Increasedspinning capacity+ Increasedavailabilityby

coordinatingmaintenance+ Energy trading+ Simple in Operation

- Risk of uncontrolledpower flow- Risk of spreadingfailures- Stability relatedto SCC

Conclusion


23/88

C

opyrightyearABB-12-

9/2

3/2011,

YingJiang-H

fner

Conclusion

Field experiences from Caprivi link project have proven HVDC Light

Operate in extremely weak AC systems with SCR down to zero

Enhance the stability of weak AC systems

Prevent black out under critical contingencies- function as a super UPS to feed passive loads- stabilize the voltage and frequency of small island grid


24/88

EFFECTIVENESS OF A SUPPLEMENTARY MLQG

POD CONTROLLERINSTALLED AT AN HVDCLINE WITHIN A MESHED NETWORK

R. Preece A. M. AlmutairiO. Marjanovic J. V. Milanovi

Manchester, United Kingdom

School of

Electrical &

Electronic

Engineering

Overview


25/88

Development and comparison of two Power Oscillation

Damping (POD) controllers for application with a

VSC-HVDC link.

Completed within a large heavily meshed network.

Completed a thorough probabilistic evaluation of the

controllers performances across a wide range of

operating points and outage contingencies.

Wide Area Measurement System (WAMS) based

controller shown to outperform local Power System

Stabiliser (PSS) design.

2

OverviewWhat has been done?

R ob in P re ec e | C ig r S ym po si um | B ol og na 2 01 1

P t ti O tli


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Aims and motivation.

Introduction of test network.

Why we need POD.

Discussion of POD controller designs (PSS & MLQG).

Probabilistic Assessment Method.

Results.

Comparisons and conclusions.

3

Presentation Outline


Aims & Motivation


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Future transmission grids will feature more HVDC links

in parallel with existing AC lines.

HVDC links can provide effective POD capabilities

with various controller designs available.

The robustness of these POD controllers to varying

network conditions is rarely assessed:

Varying generation and load operating points. n-1 outages on generators and key tie-lines.

4

Aims & MotivationWhat are we looking at and why?


Test Network


28/88

5

Test NetworkNETS & NYPS 16 machine, 68 bus with 400 MW VSC-HVDC

Mode 1 Mode 2 Mode 3 Mode 4

Frequency (Hz) 0.40 0.51 0.63 0.79

Damping (%) 4.66 4.18 4.21 4.87


POD Options: PSS & MLQG


29/88

Observer

Modal Form

Optimum

FeedbackMultiple

Inputs

Single

Output

Network

Modes

6

POD Options: PSS & MLQG

WashoutPhase

CompensationGainSingle

Input

Single

Output

Power System Stabiliser (PSS)

Modal Linear Quadratic Gaussian (MLQG)

Tuned for most controllable mode (Mode 1 lowest frequency)

Targeted action on specific network modes


Small Signal Analysis


30/88

7

Small Signal AnalysisFollowing the controller designs which looks better?

With no POD, all inter-area modes have damping less than 5%.

5% Damping



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Small Signal Analysis


32/88

9

Small Signal AnalysisFollowing the controller designs which looks better?

With MLQG POD, damping is vastly improved for all modes.

5% Damping

4.9%

4.8%

5.4%

7.1%18.9%

15.4%

14.3%

11.7%


Probabilistic Assessment


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10

Probabilistic AssessmentMethodology for Establishing Controller Robustness


Random variation in generation and loads.

Generators and loads are normally distributed.

Loads have constant power factor.

HVDC varies with uniform distribution (50 MW steps).

Generator outage contingencies.

Outages on small generators.

Large equivalent generators set to half power.

Line outage contingencies.

Outages on 4 key inter-area ties.

Probabilistic Assessment


34/88

11

obab st c ssess e tFull Process Flowchart


Generate operating point based upon current contingency

Loadflow?

Linearise open loop system (no POD controllers)

Identify

critical modes?

Evaluate closed loop dynamic equation with MLQG POD & shortlist eigenvalues

Create features sets and pass to Nave Bayes Classifier

Stable?

Save results and repeat at new operating point

Yes

No

Yes

Yes

Linearise closed loop system with PSS POD

Identify

critical modes?

Yes

No

No

No

Results: Modal Variation


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12R ob in P re ec e | C ig r S ym po si um | B ol og na 2 01 1

0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequenc

y,

Hz

Real part , p.u.

0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequency,

Hz

Real part , p.u.

MLQG PSS



36/88

13

Improvements over no POD


0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequenc

y,

Hz

Real part , p.u.

0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequency,

Hz

Real part , p.u.

MLQG PSS

100%

100%

95%

96%

94%

97%

90%

98%



37/88

0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequency,

Hz

Real part , p.u.

0.2

0.4

0.6

0.8

-1.5 -1 -0.5 0

Frequency,

Hz

Real part , p.u.

MLQG PSS

14

Improvements of MLQG over PSS


MLQG outperforms PSS: 100%

100%

98%

88%

Conclusions


38/88

POD controller performance can vary greatly withchanging network conditions or topology.

WAMS based design shown to outperform local-PSS design

significantly.

Probabilistic assessment can demonstrate strengths

and weaknesses of different controller designs.

Can be used to identify situations when performancemay be detrimental (e.g. particular generator or line

outages) in order to help with further mitigation.

15R ob in P re ec e | C ig r S ym po si um | B ol og na 2 01 1


39/88

Voltage Stability, Loss Reduction andVoltage Stability, Loss Reduction andDynamic Stability Studies of anDynamic Stability Studies of anIntegrated SystemIntegrated System

Paulo Murinelli Pesoti, Silas O. SouzaPaulo Murinelli Pesoti, Silas O. Souza, Felipe Terra,, Felipe Terra,

A. C. Zambroni de Souza, Isaias L. Lopes, Rafael C. LemeA. C. Zambroni de Souza, Isaias L. Lopes, Rafael C. Leme

Federal University at ItajubFederal University at Itajub,,

BrazilBrazil


40/88

MotivationMotivation

Interaction of a microInteraction of a micro--grid and a powergrid and a power

grid considering loss reduction, voltagegrid considering loss reduction, voltage

and dynamic stability.and dynamic stability.

Loss Sensiti it Based on Tangent VectoLoss Sensitivity Based on Tangent Vector


41/88

Loss Sensitivity Based on Tangent VectorLoss Sensitivity Based on Tangent Vector

Losses given by:Losses given by:

PPsyssys= Vi= Vikk VjVjkk (G(Gkk(cos((ij)(cos((ij)kk)+ cos((ji))+ cos((ji)kk))))--GGkk(V(Vikik22 +V+Vjkjk

22))

Deriving in relation to system parameterDeriving in relation to system parameter ::

k

nl

=

1

== GGkk (( VVjkjk ++ VVjkjk ) A +) A +

VVjkjk VVjkjk -- 2 G2 Gkk (V(Vjkjk + V+ Vjkjk ))

d Pd

system

k

nl

=1dVd

ik

dV

djk

dV

d

jk

dVd

ik

dA

d

L S i i i B d T VL S iti it B d T t V t


42/88

Loss Sensitivity Based on Tangent VectorLoss Sensitivity Based on Tangent Vector

Main points:Main points:

Shows losses variation with respect toShows losses variation with respect to ..

Derivatives all known from tangent vector.Derivatives all known from tangent vector.

Computationally cheap.Computationally cheap.


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Dynamic Stability StudiesDynamic Stability Studies Based on a unified computational tool recentlyBased on a unified computational tool recently

proposed:proposed:

Transient period is integrated by incorporating theTransient period is integrated by incorporating theintegration step into the Jacobianintegration step into the Jacobian (Newton).(Newton).

After this period, quaseAfter this period, quase--dynamic approach employed.dynamic approach employed.

( ) [ ]0 0 0, ( , ) ( , )2

i i i i i

hF x y x x f x y f x y= + + +


44/88

MethodologyMethodology The critical buses and the buses most sensitiveThe critical buses and the buses most sensitive

to loss reduction are identifiedto loss reduction are identified (tangent vector).(tangent vector).

A disturbance is considered, which consists ofA disturbance is considered, which consists ofthe outage of the wind farm generation. Thethe outage of the wind farm generation. The

radial part of the system becomes an importer.radial part of the system becomes an importer.


45/88

Test SystemTest System

The wind farm generation and the loadThe wind farm generation and the loadconnected to the radial system are given by:connected to the radial system are given by:

Load=20+j10 [MVA]Load=20+j10 [MVA]

Generation=40+j20 [MVA]Generation=40+j20 [MVA]

grid


46/88

Wind farm trips outWind farm trips out

Tangent Vector NorBefore 2.811After 3.051

Variation 8.562%

Losses variation shown in Table below.

Tangent vector norm increases after the contingency

Loss Active Power Reactive Power

Before 1.9251[pu] 192.51[MW] -2.7354[pu] -273.34[MVAr]

After 1.9830 [pu] 198.30[MW] -2.3346[pu] -233.46[MVAr]



47/88


Critical buses: Bus 43: connects the grid to the microCritical buses: Bus 43: connects the grid to the micro--

grid. Bus 119 contains the microgrid. Bus 119 contains the micro--grid load.grid load.

BusesRank

Before After

1 41 41

2 39 39

3 117 117

4 2 119

5 3 2

6 19 37 13 43

8 14 19

9 33 33

10 15 13


48/88

Long Term SimulationLong Term SimulationVoltage at load buses of interest.Voltage at load buses of interest.


49/88

Long Term SimulationLong Term Simulation Voltage level at the synchronous machinesVoltage level at the synchronous machines

ConclusionsConclusions


50/88

ConclusionsConclusions

The problem of connecting a grid to a radial microThe problem of connecting a grid to a radial micro--gridgridconsisting of a wind farm and a load is addressed.consisting of a wind farm and a load is addressed.

The wind farm tends to enhance the system voltageThe wind farm tends to enhance the system voltagesecurity. The outage of this source may change the voltagesecurity. The outage of this source may change the voltage

stability scenario, since the critical buses may change.stability scenario, since the critical buses may change.

The dynamic simulation is executed in order to completeThe dynamic simulation is executed in order to completethe analysis. This paper employed a recent methodologythe analysis. This paper employed a recent methodologythat combines the transient and long term studies into athat combines the transient and long term studies into asingle computational tool.single computational tool.


51/88

SMART ALGORITHMS TO ACCOMMODATE

Tony Yip, Chang An, Graeme Lloyd ALSTOM GRID UK

Introduction


52/88

Introduction

Wind farms tend to be located at the extremes of the

rated to carry the full output of the wind farm in allcircumstances causing the standard winter and summer

Instead of applying fixed summer and winter line ratings,load management based on a dynamically derived line

More wind means :

of the wind.

More generation

More current through the line

ALSTOM 2010. All rights reserved. Information contained in this document is provided withoutliability for information purposes only and is subject to change without notice. No representation or warranty isgiven or to be implied as to the completeness of information or fitness for any particular purpose. Reproduction, use or disclosureto third parties, without express written authority, is strictly prohibited.

P341 Dynamic Line Rating


53/88

P341 Dynamic Line Rating

The DLR calculates the rating ofthe line dynamically from some ora o e oca wea er measurements such as windspeed, wind direction, ambienttemperature and solar radiation

inputs based on CIGRE or IEEEstandard equations.

The rela has 6 definite time DLRstages which can be set as a

percentage of the line ampacity.

If the load current exceeds a DLR

used to send commands to theDG to reduce their output . As aback-up, in case for some reason

reduced, the relay can initiatelocal tripping of the DG or line.


Protection Algorithm


54/88

Heat balance equation in CIGRE 207/IEEE 738 Standards

where

wrCiSMJ PPPPPPP

PJ = Joule heating (due to current flow)

M

PS = solar heating

Pi = corona heating

Pc = convective cooling

s

Pr = radiative cooling

Pw = evaporative cooling


PM, Pi and Pw are neglected.

Dynamic Line Rating load management and protection -Site trial of a 132kV double-circuit line between Skegness and Boston (North


55/88

Site trial of a 132kV double circuit line between Skegness and Boston (North

Wind generation at Skegness is expected to increase

Generation can cause power flow from Skegness to Boston.Rating of 40km 132kV line may be exceeded

E.ON Central Networks decided to apply Dynamic Line Rating asa cos e ec ve a erna ve o ren orc ng e nes as analternative to spending 5m to reinforce the existing 132kVnetwork

50-70MW Skegness

180MW

Onshore

ratings are based onEngineering

Boston

P27 assumes - ambient

temperature =2C

n er ,(Summer), 9C

(Spring/Autumn), wind


spee = . m s, so ar

radiation = 0)

Skegness Dynamic Line Rating


56/88

Weather monitoring equipment at Skegness and Bostonrelays data back to the NMS control system. Wind

NMS to calculate a real-time line rating based on theCIGRE 207 methodology

Solar Radiation 890W/m2 (Eng. Recommendation P27assumes none)

TempSola


Skegness Dynamic Line Rating


57/88

S eg ess y a c e a g

Ambient Temperature

Wind

Speed

ne

Current

Local

Offshore 132kV

Wind Farms33kV Wind Farms relay


Curtail if >95% Trip if >99%

Skegness Dynamic Line Ratings


58/88

g y g During the first year of operation the actual line temperature

has been monitored by sensors (Power Donuts) fitted directlyto the conductors.

These sensors have been fitted on the bottom conductor at

three locations along the line. The results are sent by GPRScomms an compare wt t e ca cuate va ues n rea tme.This will allow the theory to be verified.


Comparison of conductor temperature (Tc) at Sk calculatedfrom CIGRE standard and measured by Power DonutTM for


59/88

from CIGRE standard and measured by Power Donut forthe continuous period of 1 Day 13 Min from 24/04/2009

Verification of Conductor Temperature (Tc)25 600

20

400

450

500Tc, CIGRE

10

15

oC

orm/s

200

250

300

350

Amps

, u

Ambient Temp

Wind Speed

5

50

100

150Line Current

24/04 04:00 24/04 08:00 24/04 12:00 24/04 16:00 24/04 20:00 25/04 00:00 25/04 04:00

The results show a very good match between Power DonutTM and CIGRE calculatedconductor temperature.

The minimum and maximum differences are -1.24oC and 0.96oC respectively and for 90%of the time the absolute difference is less than 1.00oC.


correctness of the CIGRE equations.

P27 vs. Ampacity with each weather parameter at SK 23,24/04/09


60/88

/ /

P27 vs. each weather parameter

1100

1200

Ampaci ty w ith

WindSpeed&WindAng

800

900

1000

mps

500

600

700

Amp aci ty with

Wind Speed

Amp aci ty w ith

all parameters

300

400

23/04 00:00 23/04 06:00 23/04 12:00 23/04 18:00 24/04 00:00 24/04 06:00 24/04 12:00 24/04 18:00 25/04 00:00

P27 Spring/Autumn RatingAmpaci ty w ith

Solar Radiation

Ampacity with

Amb ien t Temp

The graph shows that the weather parameters having a significant impact one ne ra ng, are n e or er rom owes o g es : so ar ra a on -

501A, max +0% compared to P27), ambient temperature (438-528A, max+5%), wind speed (450-985A, max +97%) and wind speed + wind angle (457-1161A, max +132%).

P27 assumes zero solar radiation which is why the solar radiation ampacity isalways below the P27 ampacity.


e prng u umn ra ng or e - s ne s .

Ampacity Level Distribution a from all weather parametersat SK Spring 2009, 20/04/2009 27/04/2009


61/88

at SK Spring 2009, 20/04/2009 27/04/2009

400

Ampacity Level Dis tr ibution, Spring. Avg=753.8A, stdev=117.73

300

350

200

250

(times)

100

150n

0

50

Amps

For the majority of the time the ampacity is greater than the


P27 Spring/Autumn rating of 501A.

Conclusions


62/88

A protective relay has been developed which calculates the dynamic line ratingbased on weather conditions

DLR could facilitate the widespread and cost-effective connection of renewableenergy sources thus allowing the associated environmental, social and costbenefits to be captured.

DLR maximizes the usable capacity of overhead line assets by 50% or more

DLR avoids upgrading or replacing existing transmission lines (save significantcapital investment) For example the cost of installing 40km of 132kV upgraded line is approx 5M.

DLR increases energy yields, so improving the cost effectiveness of renewableenergy projects and reducing greenhouse gas emissions

capacity to the DG project. Assuming that the project is a wind generation project andthat it achieves a 25% load factor the extra energy exported would be 11 GWh perannum (5907 tonnes C02saving) and the extra revenue to the developer could be up to600k per annum

The relay provides easy and flexible integration to the control system with a largechoice of industry standard communication protocols and easy schemecustomization with Programmable Scheme Logic (PSL)


Conclusions


63/88

Analysis of the data obtained from site shows a close co-ordinationbetween the actual and the theoretical calculations of the conductortemperatures

The analysis also shows that the weather parameters having a significant

im act on the line ratin are in the order from lowest to hi hest: solar radiation ambient temperature wind speed .

Variation of the weather parameters along the line is unavoidable and thisneeds to be considered for each application. To cope with this variation therela includes correction factors which can be a lied for each weatherparameter.

The conservative approach of using only the wind speed and ambient

tem erature as var in uantities and the assum tions made about winddirection and solar radiation (wind direction is 20, solar radiation is890W/m2) are shown to provide a good safety margin in the Skegness-Boston application while providing some increase in the line rating.



64/88

ACCUMULATED PHASE ANGLE DRIFT

MEASUREMENT

Loss of Grid Problem


65/88

Loss of grid is where a

inadvertently isolatedfrom the grid andcon nues o suppylocal load

oss o gr can e

caused by:

ro ec on rppng

Accidentally due to

reconfiguration


Loss of Grid Problem


66/88

Safety risk - for example, through personnel working

the network are energised

-

Loss of system earth where the earth is on the starwinding of a network transformer. This can causeproblems for existing earth fault protection to detectearth faults if the system is unearthed.

(frequency and voltage ) to local demand.


Existing LOG Methods Performance Assessment


67/88

Loss of grid performance can be assessed in terms of sensitivity and

Sensitivity

ma es poss e msma c e ween oca genera on an e eman a einstant of islanding.

Also referred to as non-detection zone

Stability

Stability for different fault types with varying duration and retained voltage at thepoint of measurement

When designing loss of grid method objective is to have a small nondetection zone and be stable for as many fault characteristics as

STABILITY

Network faults


Generator/demand Imbalance

Existing Loss of Grid Methods


68/88

Passive Methods

Under/over frequency and voltage

Df/dt rate of change of frequency

o age vec or s

Direct inter-tripping

Active Methods

Active frequency drift

Reactive Error export

There is an abundance of active methods proposed in the technicalliterature, however, their application in practice has been limited todate. The traditional protection philosophy of independence from other

systems ma es t e ntro ucton o t ese met o s cu t.


Novel Loss of Mains Protection - Phase Angle


69/88

Aim: improve LOM stability without sacrificing

PAD: frequency difference between grid and island

will ive rise to a measurable hase an le drift Local island frequency is measured by the relay Grid frequency estimated based on linear extrapolation of

recorded historical data The PAD algorithm is based on a threshold comparison of

an accumulated voltage phase angle derived from thedifference between the current measured frequency and the

fnest

fn-Df - -

f[n]

fn


nTW TD

RTDS Testing


70/88

Two main network case studies have been used

connected to a 33kV network The second scenario tests the operation of relays protecting a synch generator

connected to a 11kV network.

These models are sim lified versions of the full network su lied b a UKDNO with appropriate aggregations made to reduce their complexity wherenecessary. Each scenario consists of a grid source, simplified network, pointof isolation, local captured load and generator (including a step-up transformerwhere appropriate).

For DFIG generators shortly after disconnection from the grid and the loss of the

reference signal the controller becomes unstable and identification of the islandingevent is therefore relatively easy.

or ync ronous mac nes e con ro er oes no s gn can y mpac on edynamic response to a loss of grid, the inertia of the machine is the primary factor.

Islanding & fault conditions considered

RTDS testing include: Sensitivity for a range of settings 0%, 2.5%, 5%, 10% power imbalance Stability for a range of settings 1/2/3 phase faults with retained voltage of


, ,

RTDS Testing


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Fault Position A (20%Retained Gen Voltage)

Fault Position B (50%Retained Gen Voltage)

Fault Position C (80%Retained Gen Voltage)


11kV network model and fault locations.

RTDS 33kV SM Sensitivity Testing (PAD)PAD Sensitiv it Settin s fo r 30MVA SM Assume a max imum o eratin timeo f 500ms


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35

40

45

ree)

The protection exhibitsgood sensitivity for smallimbalances of down to

10

15

20

25

MaxSettings(Deg

PAD Active

PAD Reactive

.

To detect all imbalances asuitable setting is 10

0

5

10 5 2.5 0 -2.5 -5 -10

Imbalance Ratio (% of Gen Rating )

Max sensitivity settings

based on 500ms trip timePAD Sensitiv ity Settings for 1.5MVA SM (Assume a maximum operating time of 500ms)

35

40

increased if trip time criteriaincreased

15

20

25

30

ttings(Degree)

PAD Active

0

5

10

10 5 2.5 0 -2.5 -5 -10

MaxS


Imbalance Ratio (% of Gen Rating )

11kV SM Maximum Sensitivity Setting for PAD

RTDS 11kV SM Stability Testing (PAD)


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11kV Stabil ity Test Results - ABCG fault

250

150

200

m

s)

80% retained Voltage

50% retained Voltage

20% Retained Voltage

50

100

TripTime(

.

-50

0

0 5 10 15 20 25 30 35 40

PAD Angle sett ing ()

11kV SM Stability (Retained Voltage 20%/50%/80%, Three-phase

fault).

or e ne wor mo e e pro ec on s s a e or a au ypesthe setting is >100.


Conclusion


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The use of an accumulated phase angle drift method has beendemonstrated to be an effective method for detecting loss of grid

It has been shown to possess good levels of sensitivity at near

It has also been shown to possess a high degree of stability undersevere faultconditions

An open loop trial is planned in the near future to confirm itsperformance under practical conditions



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www.alstom.com


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1


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INTRODUCTIONVoltage control is an im ortant as ect to be ro erl accounted for in theinteraction of DG with the Smart Grid (SG) operation and control.

The aim of the voltage control is to assure an adequate

.

In existing MV distribution system, voltage regulation isperformed by HV/MV substation, using a On Load TapChanger (OLTC). Large DG devices, can be equippedwith local voltage/reactive power control system.

The interaction between OLTC and DG devices maycause over or undervoltages along the feeders.

To overcome such problems, new voltage control structures have been proposed,based on centralised and decentralised/coordinated approaches requiring:

additional measurements

additional data communication

Such an impact on the distribution system willbecome economically viable in the far-awayfuture. Nowadays, the main technical challengeis to define road maps to update the existing

2

control systems.


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In this paper a completely decentralised approach is adopted to develop aAIM OF THE PAPER

voltage/reactive power control scheme for DG devices connected todistribution networks

keeping the existing control structures of DG device

avoiding any real-time data exchange among

using only local measurements of voltage and currentat the Point of Common Coupling (PCC)

so as to improve the voltage profi le along the feeder in presence ofvariationsboth of loads and ofthe HV/MV substation o eratin conditions.

REFERENCE SYSTEM

Radial distribution network

HV/MV substation with OLTC regulator

DG device: synchronous generator

3

w reac ve power regu a or


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REACTIVE POWER CONTROL SCHEME

The classical

enriched with anOptimisationStrategy.

OPTIMISATION

STRATEGY

In this paper attention isfocused only on theSubstation Voltage &Load Estimation block.

4


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SUBSTATION VOLTAGE & LOAD ESTIMATIONThe ado ted scheme for the Substation Voltage & Load Estimation block isdeveloped in the following assumptions:

distribution network configuration is known;

su s a on s mo e e y a vo age genera or n seres w a ea age mpe ance; a echanges of the operating conditions of the HV/MV substation are modeled by variations of thevoltage generator;

nodal loads are modeled by shunt admittances and are subject to homothetic variations.

extract f rom time-varying

signals the corresponding

phasors at fundamental

estimates the Thevenin

equivalent circuit of the

distribution system as

determines

the shunt admittancesof the nodal loads

computes the value of

the vol tage of the HV/MVsubstation

5

frequency seen from the PCC

SUBSTATION VOLTAGE & LOAD ESTIMATION


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SUBSTATION VOLTAGE & LOAD ESTIMATION

How is it ossible to se arate the effects of the variations of loads from

In the above modeling assumptions the distribution system

the ones of HV/MV substation?

Once the Thevenin

equivalent circuit is known

- the equivalent impedance depends onunknown load admittances and the

presents the following electrical equivalent ci rcuit:

nown su s a on mpe ance;- the no-load voltage depends onunknown load admittances, the known

HV/MV substation impedance and the

6

unknown HV/MV substation voltage.

CASE STUDY


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The distribution system has been simulated by using PSCAD/EMTDC and the proposed

C S S U

Substation Voltage & Load Estimation block has been implemented in Matlab.

Radial feeder with 4 busbar: line data & loaddata

132/20 kV transformer with OLTC data

The OLTC has a vol tage

2.4 MW-3MVA synchronous generator data

re erence equa o . p.u.

DG is equipped with a

classical reactive ower

7

regulator with Qref(t)=0

CASE STUDY


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Three cases have been considered characterized with different values ofloads and ofHV vol tage at the substation:

rated loads and 1.0 p.u. HV voltage;Case A

rated loads and 1.01 .u. HV volta e.

load increased of 10 % and 1.0 p.u. HV voltage;Case B

Case C

CaseActual values Estimated values

A

B

termsofaccuracy,althoughthevariationsoftheloadsandoftheHV

C voltagearequitesmall!! The actual value of vsub does not change because the OLTC regulation acts to keep constant the

8

voltage at the transformer MV busbar.

CONCLUSIONS


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A reactive ower control scheme for DG devices has been ro osed that uaranteesan optimal voltage profile in MV distribution systems in presence of both load and HV/MVsubstation variations.

The proposed scheme keeps the existing control structures of DG device, uses onlylocal measurements of voltage and current at PCC and avoids any data exchange withOLTC regulator.

Attention has focused on the eculiar Substat ion Volta e & Load Estimation rocedureand its validity has been tested by numerical simulations in open loop configuration.

FUTURE RESEARCH STEP

completing the Optimisation strategy development with the Voltage Profile

Optimisation oc ; analysing the performance of the closed-loop reactive power control scheme;

investigating the possibility of including other DG devices and load models.

9


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10

Question 3


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,are, the higher the achievable performance is.

- reactive power: it is controllable for some electric machines andeec ronc n er aces.

- active power: it is controllable for DG devices using dispatchableprimary energy sources.

If the increase of DG penetration is essentially based on renewable energyconversion s stems, then the control of active ower must be achieved b

promoting Active Demand so as to introduce new command signals to beused by the control system.

Q ti 4


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Question 4

Information about the distribution networkand DG operating conditions are gathered

APPROACH

The control action is performed by local

rom e su s a on.

On the basis of such information, at theHV/MV substation level, an optimal

controllers and information exchange is onlyused forcoordinationpurposes.

On the other hand, this approach does notsys em con ro s ra egy s e ne anactuated by sending adequate commands

to the OLTC and to the DG devices.

optimise the overall system operation butonly guarantee that specific voltage

constraints are fulfilled.

Concerning the optimisation functions, they can include the classical technical-economical objectives (such as flat voltage profile, distribution loss reduction, control actionminimisation) and additional new objectives. In particular it should be accounted for the costs

rea e o e servces prov e y c ve eman an y or exampe, y e re uc onof generation from renewable energy in response to a specific request from the controlsystem).

The com arison between different control structures should account both for theperformance evaluated in terms of optimisation functions and for the investment costs.

Question 5


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reduces the power flows along the feeder.

Problems of volta e stabilit limit ma arise if the lar e enetration of DG fromrenewable energy sources:- causes significant reverse power flows towards the HV network,- is trusted as a substitution of the necessary reinforcement of the feeder whennew loads are connected.

Concerning dynamic stability of the voltage control scheme proposed in the paper#236, the new optimisation strategy is designed to change the reference value ofreactive power with a settling time equal to 60-100 s. Such loop is very slower thatthe reactive power control loop which presents a settling time of 5-10 s. In this way

t e two oops are ecoupe an t e r sta e operaton s guarantee .

Further studies will concern the interaction with the OLTC control loop and witho er con ro sys ems w en presen .

23 stability and control

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