distributed generation operation for distribution system volt/var control

DG participation in DS control

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DG Operation for Distribution System Volt/VarControl

N. Daratha

guided by

Prof. J.D. Sharma and Prof. B. Das

Department of Electrical EngineeringIIT Roorkee

PhD Seminar Course

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Proposition

There is a need for an effective methodology of multi-objectivevariable-power-factor distributed generation operation for

distribution system volt/var control during normal and emergencysituation.

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Outline

Electric Distribution System

Distributed Generation

Volt/Var Control In Distribution System With DGs

DG Participation in Volt/Var Control

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Electric Power SystemDistribution System

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Elements of Distribution Systems

Excluding DG, ....I All DS must have feeders and transformer with On-load Tap

Changer.I Most of them have shunt capacitors and/or shunt reactorI fewer of them have SVC (static Var compensator)I even fewer of them have D-STATCOM.

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We Want Many Objectives

A distribution system mustI have good voltage regulationI be energy efficientI have wide stability marginI support transmission system reactive power needI of course, maximize overall profit.

However, achieving all of them at the same time is NOT possible.

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Feeders: Minimum Losses 6= Minimum Voltage Drop

I Feeders bring electricity to consumers.I A feeder power loss is minimum when when load is pure

resistive.I A feeder voltage drop is minimum when load capacitive reactive

power equals feeders requirement.

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Control Devices in Distribution Systems

For effective, secure, and safe operation of DS, utility control:I SwitchesI Voltage regulators (OLTC, SC, SR)I Distributed GeneratorsI Energy Storages

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Distributed Generation (DG)definition, altenative names

I a distributed generation (DG) is a small generation connected todistribution network

I IEEE Standard Dictionary Terms :Electric generation facilities connected to an Area EPS(Electric Power System) through a PCC (Point ofCommon Copling); a subset of DR (DistributedResources).

I alternative names: embedded generation, dispersed generation

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Distributed Generation (DG)International Energy Agency’s Definition 1

I Distributed generation is generating plant serving a customeron-site or providing support to a distribution network, connectedto the grid at distribution-level voltages.

I Dispersed generation is distributed generation plus wind powerand other generation, either connected to a distribution networkor completely independent of the grid.

1Distributed Generation in Liberalised Electricity Markets, IEA, Paris, 200211 / 62



DG Classifications

DGs can be. . .I renewable (wind,PV,hydro) or non renewable (diesel)I dispatchable (diesel, micro/small hydro) or not-dispatchable

(wind, PV)I intermittent (PV, wind, ocean wave) or steady (diesel, hydro, fuel

cell)I grid-connected or isolated

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DG-to-Power Grid Interface

DG Type Electric Machine Interface

ICE SG directlyIG directly

Gas Turbines SG directlyMicro-turbines PMSG rectifier+inverter or

AC/AC converterSquirrel cage IG directly

Wind DFIG rectifier+ inverterSG or PMSG rectifier + inverter

Photovoltaic inverterFuel cell inverter

2

2ICE=Internal Combustion Engine; SG=Synchronous Generator; IG= InductionGenerator; PMSG = Permanent Magnet SG; DFIG=Doubly Fed IG

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DG Impacts on Voltage RegulationBefore Fault

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DG Impacts on Voltage RegulationAfter Fault

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DG May Not Participate in Voltage Regulation

IEEE Standard 1547-2003:

4.1.1 Voltage regulationThe DR shall not actively regulate the voltage at the

PCC. The DR shall not cause the Area EPS service voltageat other Local EPSs to go outside the requirements of ANSIC84.1-1995, Range A.

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Some DGs Reactive Power CapabilityI Interface that can control reactive power :

I synchronous machine 3 (hydro,diesel)I voltage source converter (PV, DFIG4, Ocean Energy)

(a) Synchronous Generator (b) Doubly-Fed Induction Generator

3J. Y. Jackson, “Interpretation and use of generator reactive capability diagrams”,Industry and General Applications, IEEE Transactions on, vol. IGA-7, no. 6, pp. 729–732, nov. 1971

4S. Engelhardt, I. Erlich, C. Feltes, J. Kretschmann, and F. Shewarega, “Reactivepower capability of wind turbines based on doubly fed induction generators”, EnergyConversion, IEEE Transactions on, vol. 26, no. 1, pp. 364 –372, march 2011

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DGs Have Low Utilization Level

I PV depends on solar irradiance.I Wind generator depends on wind speed.I Both solar irradiation and wind speed is highly intermittentI There is significant fraction of the time when DG works much

below rated power.I During those time, DGs can provide reactive power service.

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Distributed Reactive Power Generation Control forVoltage Rise Minimization in Distribution Network5

I Prevent significant voltage rise because of DG presence.

Q∗G ≈

XR2 + X 2 −

√X

R2 + X 2

2

− P2G +

2RPG

R2 + X 2

I Compared with constant power factor approach.I Effective reactive power control with two consequences:

I increased stress on tap changers.I increased feeder losses.

I Voltage become almost independent of DG real powergeneration.

I Voltage dependence on load is almost unchanged.5P.M.S. Carvalho, P.F. Correia, and L.A.F. Ferreira, “Distributed reactive power

generation control for voltage rise mitigation in distribution networks”, Power Systems,IEEE Transactions on, vol. 23, no. 2, pp. 766 –772, may 2008

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Voltage Become Almost Independent of DG RealPower Generation

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Voltage Dependence on Load is Almost Unchanged

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Grid Interconnection of Renewable Energy Sources atDistribution Level with Power Improvement Features 6

I Some other functions that can be provided by DGs:I power transfer at unity power factorI local reactive power supportI harmonic mitigationI load balancing

I Those functions can be achieved simultaneously or individuallyI no additional hardware is needed

6M. Singh, V. Khadkikar, A. Chandra, and R.K. Varma, “Grid interconnection ofrenewable energy sources at the distribution level with power-quality improvementfeatures”, Power Delivery, IEEE Transactions on, vol. 26, no. 1, pp. 307 –315, jan. 2011

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Observation I

I DG can cause voltage rise on the feeder to which it is connected.I There is a method to mitigate the voltage rise

I variable power factor operationI increased number of switching and losses.

I Current grid code do not allowed DG to control its output voltage.I DGs is also potential to improve power quality.

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Works in which DGs are inconstant power factor mode.

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Optimal Distribution Voltage Control and coordinationwith distributed generation 7

I Minimize total losses and voltage deviationI Control OLTC, Shunt Capacitor (SC), Shun Reactor (SR), Step

Voltage Regulator (SVR), Static Voltage Controller (SVC)I Optimization methods : Genetic AgorithmI DGs = PVs with constant unity power factor.I Centralized control

7T. Senjyu, Y. Miyazato, A. Yona, N. Urasaki, and T. Funabashi, “Optimal distributionvoltage control and coordination with distributed generation”, Power Delivery, IEEETransactions on, vol. 23, no. 2, pp. 1236 –1242, 2008

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Optimal Distribution Voltage Control and coordinationwith distributed generation

I Objective: min∑

w1|Vn,ref − Vn|+ w2LossI Contraints:

I voltage limitsI tap position limits (OLTC)

I Optimization methods : Genetic Agorithm

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Optimal Distribution Voltage Control and coordinationwith distributed generationSVC Model

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Optimal Distribution Voltage Control and coordinationwith distributed generationSVR Model

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Works in Which DGs are in CONSTANT power factormode 1

I Alessandro Casavola, Giuseppe Franzè, Daniele Menniti, andNicola Sorrentino, “Voltage regulation in distribution networks inthe presence of distributed generation: A voltage set-pointreconfiguration approach”, Electric Power Systems Research,vol. 81, no. 1, pp. 25 – 34, 2011→ OLTC only

I Joon-Ho Choi and Jae-Chul Kim, “Advanced voltage regulationmethod of power distribution systems interconnected withdispersed storage and generation systems”, Power Delivery,IEEE Transactions on, vol. 16, no. 2, pp. 329 –334, April 2001→OLTC only

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Works in Which DGs are in CONSTANT power factormode 2

I D. Viawan, F.A.; Karlsson, “Combined local and remote voltageand reactive power control in the presence of induction machinedistributed generation”, IEEE Transactions on Power Systems,vol. 22, no. 4, pp. 2003–2012, 2007, cited By (since 1996) 10→OLTC and SC

I Miyoung Kim, R. Hara, and H. Kita, “Design of the optimal ultcparameters in distribution system with distributed generations”,Power Systems, IEEE Transactions on, vol. 24, no. 1, pp. 297–305, feb. 2009→ OLTC only

I all of them do not include SVC and D-STATCOM

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Observation II: Constant Power Factor Operation

Among paper considering DG constant power factor operation:I most include OLTC and DGI other also include SCI only one include SVR and SVCI none include D-STATCOMI single objective mathematical programming

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Works in which DGs are invariable power factor mode

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Minimizing Reactive Power Support for DistributedGeneration8

I Choosing power factor of DGs and setting of OLTCI Maximising DG reactive power generationI Reducing transmission system burdenI Enhanced passive approach vs active approachI Uses DG and OLTC only

8L. F. Ochoa, A. Keane, and G. P. Harrison, “Minimizing the reactive support fordistributed generation: Enhanced passive operation and smart distribution networks”,Power Systems, IEEE Transactions on, vol. PP, no. 99, pp. 1, 2011

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Multiagent Dispatching Scheme for DGs for VoltageSupport on Distribution Feeders9

I Each generator control its output based on local measurements.I Those measurements used to calculate sensitivity factors.I Coordination between DGs through a Control Net Protocol (CNP)I Reliable communication networkI Uses DG and OLTC only

9M.E. Baran and I.M. El-Markabi, “A multiagent-based dispatching scheme fordistributed generators for voltage support on distribution feeders”, Power Systems,IEEE Transactions on, vol. 22, no. 1, pp. 52 –59, feb. 2007

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Options for Controls of Reactive Power by DistributedPV Generators 10

I Local control of PV generatorsI Local measurements were sufficient for voltage regulationI Support the idea of Baran and Markabi (2007)I Uses DG and OLTC only

10K. Turitsyn, P. Sulc, S. Backhaus, and M. Chertkov, “Options for control of reactivepower by distributed photovoltaic generators”, Proceedings of the IEEE, vol. 99, no. 6,pp. 1063 –1073, june 2011

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Voltage and Reactive Power Control in Systems withSynchronous Machine-Based Distributed Generation11

I Minimize total losses.I Include OLTC and SC.I DG regulate voltage at point of common connection.I If SC is enough, DG participation does not reduce losses

significantly.I Excess reactive power can support transmission system (Ochoa,

et. al. , 2011).

11F.A. Viawan and D. Karlsson, “Voltage and reactive power control in systems withsynchronous machine-based distributed generation”, Power Delivery, IEEETransactions on, vol. 23, no. 2, pp. 1079 –1087, april 2008

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Short-Term Schedulling and Control of ActiveDistribution Systems with High Penetration ofRenewable Energy Resources12

I a day-ahead scheduler + intra-day (15 minutes) scheduler.I includes dispatchable and not-dispatchable DGs.I a day-ahead scheduler is a forecaster of generator and energy

storage.I intraday scheduler minimize generation deviation define by the

other scheduler.

12A. Borghetti, M. Bosetti, S. Grillo, S. Massucco, C.A. Nucci, M. Paolone, andF. Silvestro, “Short-term scheduling and control of active distribution systems with highpenetration of renewable resources”, Systems Journal, IEEE, vol. 4, no. 3, pp. 313–322, sept. 2010

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The Day-Ahead Scheduler

I objective is minimal energy cost

minR∑

r=1

N∑j=1

cj,r ∆tPrj

I constraints:I Electrical Load balanceI Storage unitsI Power and energy limitsI Thermal load balance

I inputs: load forecast, generation forecast, energy cost, limits ofgenerating units, initial status of storage units.

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The Intra-day Scheduler

I Multiobjective:

min∆x

[∑αSP + βPloss +

∑γSV

]I minimal voltage deviationI minimal generation deviationI minimal network losses

I Input: 15-minutes ahead forecast, state estimation resultsI output: control signal for OLTC, voltage regulators, DGs and

energy storagesI controlled variable: active and reactive power generation and

OLTC tap position

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What are missing?Further considerations are needed:

I switching seguence?I transition cost?I security?

Initial state ProposedOptimum State

Optimum path?Reachability?

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Reducing Number of Switching:1. Constraint Addition

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Importance of Switching Reduction

I switching may initiate transientsI device has limited total number of switchingsI DG’s variable power factor mode increase OLTC’s switching

numbersI slow mechanical switch vs fast load change and intermitent

renewables

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Reactive Power and Voltage Control in DistributionSystem with Limited Switching Operation 13

I Objective : min energy losses

min E =23∑

t=0

f (x1(t), x2(t), x3(t))

I x1 discrete variables: OLTCs and CapacitorsI x2 Q and VI x3 P and θ

13M.B. Liu, C.A. Canizares, and W. Huang, “Reactive power and voltage control indistribution systems with limited switching operations”, Power Systems, IEEETransactions on, vol. 24, no. 2, pp. 889 –899, may 2009

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Reactive Power and Voltage Control in DistributionSystem with Limited Switching Operation

Constraints:I power flow equationsI tap positions limitsI capacity limitsI additional constraints : Maximum Allowable daily switching

operation (MADSON)

h(x1(0), x1(1), ..., x1(23)) =23∑

t=0

|x1(t+1) − x1(t)| ≤ Sx1Cx1

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Proposed optimization method:I discrete variables are treated as continous variablesI inequality constraints are converted into equality constraints with

help from slack variables

x1(t) + su1(t) = x1(t)max

x1(t) − su1(t) = x1(t)min

x2(t) + su2(t) = x2(t)max

x2(t) − su2(t) = x2(t)min

h(x1(0), x1(2), ..., x1(23)) = Sx1Cx2

su1(t), sl1(t), su2(t), sl1(t) ≥ 0

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Proposed optimization method:I interior point method was usedI KKT are derived and solved with Newton-Raphson method.I compared with Genetic Algorithm, BARON and DICOPTI test cases: Baran and Wu 69-buses system and chinese

14-buses systemI the proposed method is faster than other methods.

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Alternative Approach:Rule-based Control

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Reasons for Alternative Approach

I Our problem is NP-hard MINLP unless some simplification isassumed.

I Distribution system is largeI Slow voltage controller movement and changing load and

generation profileI minimum switching is favorableI some switching action are mutually exclusive

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Configurable, Hierarchical, Model-Based Control ofElectrical Distribution Circuits14

I objective : close and better operating state; minimize change ofstate

I preference-based multi objectives and constraints:I voltage regulationI Capacity constraintI lossesI priority is adjustable

I control devices : SC, OLTC, SVR, DGI single step (SS) : SC, DG (on-min-on)I multi step (MS) : OLTC, DG (min - max discretized)

14J. Hambrick and R. P. Broadwater, “Configurable, hierarchical, model-basedcontrol of electrical distribution circuits”, Power Systems, IEEE Transactions on, vol.PP, no. 99, pp. 1, 2010

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CHMC Main Loop

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CHMC Main Loop

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Selection of New State

If voltage deviation is smaller than before, accept this newer state.55 / 62




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Ways to Reduce Number of Switching

Using previous methods, variable power factor DGs operationincrease number of switching. There are to ways to reduce thenumber:

I MADSON constraintI rule-based optimization

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Observation III: Variable Power Factor Operation

Among paper considering DG variable power factor operation:I most include only OLTC and DG (one include DG)I single-objective mathematical programmingI increased number of switching is expected

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Observation IV: Possible Gaps for Future Research

What is not available in literature is volt/var control strategy/methodwhich:

I include a rather complete types of (potential) voltage regulatorI is multi-objective optimization plus higher information processing

In addition, optimum switching sequence needed to reach theoptimum state has not been well studied.

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Summary

Summary

I DGs reactive power capability is not fully utilised.I Grid codes require constant-power factor operation.I Most published research follow the grid codes.I Some works consider the variable-power factor (VPF) operation.I VPF operation increase number of switchings of voltage

regulatorsI two ways in limiting switching number: MADSON constraint and

a rule-based approach

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Summary

Thank You Very Much

DG Operation for Distribution System Volt/VarControl

N. Daratha

guided by

Prof. J.D. Sharma and Prof. B. Das

Department of Electrical EngineeringIIT Roorkee

PhD Seminar Course

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distributed generation operation for distribution system volt/var control

Technology

utility control

ds controlpropositionthere

dgsdg participation

safe operation of ds

ds controldg operation

generation operation

wind power

electric generation