Power Grid and Energy Management System - Lezione 15 · Power Grid and Energy Management System...

Power Grid and Energy Management SystemLezione 15

5 maggio 2015

Ing. Chiara [email protected]

Sistemi di Controllo per l’Automazione IndustrialeIngegneria Gestionale

A.A. 2014 - 2015Università degli Studi di Cassino e del Lazio Meridionale

mailto:[email protected]

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Definition of PowerGrid

Historical Notes

Structure of the PowerSystemGenerating Units

Substations

Transmission andDistribution Network

Energy ManagementSystem

State Estimation

Automatic GenerationControl

Sistemi di Controllo perl’Automazione Industriale

Ingegneria GestionaleA.A. 2014 - 2015

Agenda

Definition of Power Grid

Historical Notes

Structure of the Power SystemGenerating UnitsSubstationsTransmission and Distribution Network

Energy Management System

State Estimation

Automatic Generation Control

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State Estimation




Purpose of this lesson

The aim of this lesson is:1. understand the history of power grids and their evolutions2. detect the main components of electrical grids3. see the Energy Management System functionalities4. get confidence with state estimation and automatic

generation control modules

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Reading Materials

http://burnanenergyjournal.com/the-electricity-grid-a-history

http://en.wikipedia.org/wiki/Electricity_sector_in_Italy

Power System State Estimation: Theory and Implementation,by Ali Abur, Antonio Gómez Expósito

Power System Dynamics: Stability and Control, by JanMachowski, Janusz Bialek, Dr James Bumby

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4Definition of PowerGrid

Historical Notes


Substations



State Estimation




Introduction

Source:U.S. National Aeronautics and Space Administration (NASA);

http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=79765; Latestaccess: March 2013

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5Definition of PowerGrid

Historical Notes


Substations



State Estimation




What is Power Grid?

The electric power grid can be defined as the entire apparatusof wires and machines that connects the sources of electricity(i.e., the power plants) with customers.When most people talk about the “grid”, they are usuallyreferring to the electrical transmission system, which movesthe electricity from power plants to substations located close tolarge groups of users. However, the grid also encompasses thedistribution facilities that move the electricity from thesub-stations to the individual users.

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Substations



State Estimation




Historical Notes I

1882: The Pearl Street Station in New York City was the first ofthese complete systems, connecting a 100-volt generator thatburned coal to power a few hundred lamps in theneighbourhood. In a direct current, the electrons flow in acomplete circuit, from the generator, through wires anddevices, and back to the generator.

1888: William Stanley, Jr. built the first generator that usedalternating current (AC). Instead of electricity flowing in onedirection, the flow switches its direction, back and forth.Westinghouse Corporation bought the patent rights to Tesla’sAC equipment.

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Historical Notes II

1930s: Regulated electric utilities became well-established,providing all three major aspects of electricity, the power plants,transmission lines, and distribution. This type of electricitysystem, a regulated monopoly, is called a vertically-integratedutility

1978: The Public Utilities Regulatory Policies Act was passed,making it possible for power plants owned by non-utilities tosell electricity too, opening the door to privatization,demonstrated that traditional vertically integrated electricutilities were not the only source of reliable power.

1992: The Federal government stepped in and created rules toforce open access to the lines (Energy Policy Act-EPACT), andset the stage for Independent System Operators, not-for-profitentities that managed the transmission of electricity in differentregions.

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Historical Notes III

1999: Federal Energy Regulatory Commission’s (FERC’s)issued Order 2000 calling for the creation of regionaltransmission organizations (RTOs), independent entities thatwill control and operate the transmission grid free of anydiscriminatory practices.

Today: From the very beginning of electricity in America,systems were varied and regionally-adapted, and it is nodifferent today. Some states have their own independentelectricity grid operators, like California and Texas. Otherstates are part of regional operators, like the MidwestIndependent System Operator or the New EnglandIndependent System Operator. Not all regions use a systemoperator, and there are still thousands TK of municipalities thatprovide all aspects of electricity.

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Italian Case I

1883: in Milan, the first power plant was built to power theillumination of Scala Theatre.

1904: The first geothermal power station in the world was builtin Larderello, Tuscany.

1962: The electricity sector, private until then, was nationalizedwith the creation of the state-controlled entity named ENEL,with the monopoly on production, transmission and distributionof electric energy.

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Italian Case II

1992: ENEL was made a joint-stock company, however stillowned by the Ministry of Economy, following Europeandirectives. It was based on the adoption of different regulationsfor production and transmission: production and trading shouldbe free and managed by private companies, while transmissionand distribution, being natural monopolies, should be regulatedby the state.

1999: the Italian legislative decree 79/1999 (”Decreto Bersani”)created a path towards a complete liberalization of the marketthrough gradual steps.

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Italian Case III

1999: The network was transferred to a new company, Terna,responsible for the management of the system. Moreover, thelimit on Enel property share of Terna was set at 20%. ENELeventually sold its remaining share of the company in January2012.

In order to improve competition and to develop a free marketfor production, Enel was also forced to sell 15,000 MW ofcapacity to competitors before 2003. Following this, three newproduction companies were created: Endesa Italia, Edipowerand Tirreno Power. A new European directive, 2003/54/CE of2003, and a subsequent Italian decree, requested a freeelectricity trading for all commercial clients from July 2004 and,eventually, a complete opening of the market for privatecustomers from July 2007.

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Historical Notes

12Structure of the PowerSystemGenerating Units

Substations



State Estimation




Electric Power System

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Generation

Traditionally power system operation has been based around arelatively small number of large power plants connected to thetransmission system. Those plants are usually thermal orhydro plants in which electricity is produced by converting themechanical energy appearing on the output shaft of an engine,or more usually a turbine, into electrical energy. The mainthermal energy resources used commercially are coal, naturalgas, nuclear fuel and oil.The conversion of mechanical to electrical energy in traditionalthermal or hydro plants is almost universally achieved by theuse of a synchronous generator. The synchronous generatorfeeds its electrical power into the transmission system via astep-up transformer.

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Renewable Resources

Generally there are three main ways the industry can reduceits CO2 emissions:

1. by moving from the traditional coal/gas/oil-basedgeneration to renewable generation (wind, solar, marine);

2. by moving towards increased nuclear generation which islargely CO2-free;

3. by removing CO2 from exhaust gases of traditional thermalgeneration using for example carbon capture and storagetechnology.

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Transmission

Since the energy lost in a transmission line is proportional tothe current squared, transmission lines operate at high or veryhigh voltages. The electrical network connects all the powerstations into one system, and transmits and distributes powerto the load centres in an optimal way. Usually the transmissionnetwork has a mesh structure in order to provide manypossible routes for electrical power to flow from individualgenerators to individual consumers thereby improving theflexibility and reliability of the system.The transmission network makes the power system a highlyinteracting, complicated mechanism, in which an action of anyindividual component (a power plant or a load) influences allthe other components in the system. This is the main reasonwhy transmission remains a monopoly business, even underthe liberalized market structure, and is managed by a singlesystem operator.

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Distribution

Most of the electrical energy is transferred from thetransmission, or subtransmission, network to distributionhigh-voltage and medium-voltage networks in order to bring itdirectly to the consumer. The distribution network is generallyconnected in a radial structure as opposed to the meshstructure used in the transmission system. Large consumersmay be supplied from a weakly coupled, meshed, distributionnetwork or, alternatively, they may be supplied from two radialfeeders with a possibility of automatic switching betweenfeeders in case of a power cut. Some industrial consumersmay have their own on-site generation as a reserve or as aby-product of a technological process (e.g. steam generation).Ultimately power is transformed to a low voltage and distributeddirectly to consumers.

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Bidirectional Distribution

Power flows in distribution networks may no longer beunidirectional, that is from the point of connection with thetransmission network down to customers. In many cases theflows may reverse direction when the wind is strong and windgeneration high, with distribution networks even becoming netexporters of power. That situation has created many technicalproblems with respect to settings of protection systems,voltage drops, congestion management and so on.

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Demand

The demand for electrical power is never constant andchanges continuously throughout the day and night. Thechanges in demand of individual consumers may be fast andfrequent, but as one moves up the power system structurefrom individual consumers, through the distribution network, tothe transmission level, the changes in demand become smallerand smoother as individual demands are aggregated.Consequently the total power demand at the transmission levelchanges in a more or less predictable way that depends on theseason, weather conditions, way of life of a particular societyand so on. Fast global power demand changes on thegeneration level are usually small and are referred to as loadfluctuations.

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Historical Notes

Structure of the PowerSystem

19Generating Units

Substations



State Estimation




Power Generation Unit

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Historical Notes


20Generating Units

Substations



State Estimation




Generating Units I

Electrical energy is produced by a synchronous generatordriven by a prime mover, usually a turbine or a diesel engine.The turbine is equipped with a turbine governor which controlseither the speed or the output power according to a presetpower–frequency characteristic.

The generated power is fed into the transmission network via astep-up transformer. The DC excitation (or field) current,required to produce the magnetic field inside the generator, isprovided by the exciter.

The excitation current, and consequently the generator’sterminal voltage, is controlled by an automatic voltage regulator(AVR). An additional unit transformer may be connected to thebusbar between the generator and the step-up transformer inorder to supply the power station’s auxiliary servicescomprising motors, pumps, the exciter and so on.

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Historical Notes


21Generating Units

Substations



State Estimation




Generating Units II

The generating unit is equipped with a main circuit-breaker onthe high-voltage side and sometimes also with a generatorcircuit-breaker on the generator side. Such a configuration isquite convenient because, in case of a maintenance outage ora fault, the generator circuit-breaker may be opened while theauxiliary services can be fed from the grid. On the other hand,with the main circuit-breaker open, the generator may supplyits own auxiliary services.

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22Substations



State Estimation




Substations

A substation can be regarded as a point of electricalconnection where the transmission lines, transformers,generating units, system monitoring and control equipment areconnected together.Consequently, it is at substations that the flow of electricalpower is controlled, voltages are transformed from one level toanother and system security is provided by automaticprotective devices.These incoming and outgoing circuits are connected to acommon busbar system and are equipped with apparatus toswitch electrical currents, conduct measurements and protectagainst lightning.

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Substations

23Transmission andDistribution Network


State Estimation




Transmission and Distribution Network

The transmission and distribution network connects all thepower stations into one supplying system and transmits anddistributes power to individual consumers.The basic elements of the network are the overhead powerlines, underground cables, transformers and substations.Auxiliary elements are the series reactors, shunt reactors andcompensators, switching elements, metering elements andprotection equipment.

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Lines and Cables

Overhead lines are universally used to transmit electricalenergy in high-voltage transmission systems whileunderground cables are normally only used in low- andmedium-voltage urban distribution networks.For practical reasons there is a standardization of voltagelevels within different regions of the world. Unfortunately thesestandard voltages tend to vary slightly between regions but arenot too dissimilar. Typical transmission voltage levels are 110,220, 400, 750 kV for Continental Europe, 132, 275, 400 kV forthe United Kingdom and 115, 230, 345, 500, 765 kV for theUnited States.Distribution networks generally operate at lower voltages thanthe transmission network. For example, there are 12 differentstandard distribution voltages in the United States, in the rangebetween 2.4 and 69 kV. In the United Kingdom the distributionvoltages are 6.6, 11, 33 and 66 kV.

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Transformers

Transformers are necessary to link parts of the power systemsthat operate at different voltage levels. In addition to changingvoltage levels, transformers are also used to control voltageand are almost invariably equipped with taps on one or morewindings to allow the turns ratio to be changed.Power system transformers can be classified by their functioninto three general categories:

I generator step-up transformers (which connect thegenerator to the transmission network) and unittransformers (which supply the auxiliary service);

I transmission transformers, which are used to connectdifferent parts of the transmission network, usually atdifferent voltage levels, or connect the transmission anddistribution networks;

I distribution transformers, which reduce the voltage at loadcentres to a low voltage level required by the consumer.

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Tap-Changing Transformers

Controlling the voltage transformation ratio without phase shiftcontrol is used for generator step-up transformers as well as fortransmission and distribution transformers. The easiest way toachieve this task is by using tap changers to change thetransformation ratio.

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Shunt Elements

Generally, reactive power cannot be transmitted over longdistances and should be compensated for close to the point ofconsumption. The simplest, and cheapest, way of achievingthis is by providing shunt compensation, that is by installingcapacitors and/or inductors connected either directly to abusbar or to the tertiary winding of a transformer.Shunt elements may also be located along the transmissionroute to minimize losses and voltage drops. Traditionally, staticshunt elements are breaker switched either manually orautomatically by a voltage relay.

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Series Elements

Series capacitors are connected in series with transmissionline conductors in order to offset the inductive reactance of theline. This tends to improve electromechanical and voltagestability, limit voltage dips at network nodes and minimize thereal and reactive power loss.Normally series capacitors are located either at the lineterminals or at the middle of the line. Although fault currentsare lower, and line protection easier, when the capacitors arelocated at the mid-point, the access necessary formaintenance, control and monitoring is significantly eased ifthe capacitor banks are positioned at the line terminals.

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29Energy ManagementSystem

State Estimation




Power System Operation

The main goal of the system operator is to maintain the systemin the normal secure state as the operating conditions varyduring the daily operation. Accomplishing this goal requirescontinuous monitoring of the system conditions, identificationof the operating state and determination of the necessarypreventive actions in case the system state is found to beinsecure. This sequence of actions is referred to as thesecurity analysis of the system.

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State Estimation




EMS Configuration

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EMS Primary Functions I

1. Network Configuration / Topology Processor2. State Estimation3. Contingency Analysis4. Three Phase Balanced Operator Power Flow5. Optimal Power Flow6. Dispatcher Training Simulator

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EMS Primary Functions II

1. System Load Forecast: Every hour for a period of 1-7 days2. Start up and shut down times for most economic operation

of thermal units for each hour3. Fuel Scheduling: Economic choice, fuel purchase contract4. Transaction Evaluation: Purchase and sale of energy with

neighbouring companies5. Transmission loss minimization: Controller actions for

minimization of loss6. Security Constrained Dispatch: Ensuring economic

dispatch without violating network security7. Production cost calculation: Actual and economical for

each generated unit on hourly basis

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EMS Applications

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Real-Time Analysis Sequence I

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Real-Time Analysis Sequence II

I Contingency: Event that causes important component tobe removed from service. A list of contingency areprocessed as applicable to current state

I Topology Processing: Building a network model based onreal-time measurements

I State Estimator: Determining ”best” estimate fromreal-time measurements

I Power Flow: Load flow analysisI Contingency Analysis: Impact of a set of contingencies to

identify harmful onesI Optimal Power Flow: Optimization of a specified objective

function with constraintsI Preventive Action: to prevent contingencyI Short Circuit Analysis: Determines the fault current and

locations across the power network

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36State Estimation




State Estimation

In order to identify the current operating state of the system,state estimators facilitate accurate and efficient monitoring ofoperational constraints on quantities such as the transmissionline loadings or bus voltage magnitudes. They provide areliable real-time data base of the system, including theexisting state based on which, security assessment functionscan be reliably deployed in order to analyse contingencies, andto determine any required corrective actions.Power system state estimator constitutes the core of theon-line security analysis function. It acts like a filter betweenthe raw measurements received from the system and all theapplication functions that require the most reliable data basefor the current state of the system.

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37State Estimation




State Estimation Functions I

I Topology processor: Gathers status data about the circuitbreakers and switches, and configures the one-linediagram of the system.

I Observability analysis: Determines if a state estimationsolution for the entire system can be obtained using theavailable set of measurements. Identifies theunobservable branches, and the observable islands in thesystem if any exist.

I State estimation solution: Determines the optimal estimatefor the system state, which is composed of complex busvoltages in the entire power system, based on the networkmodel and the gathered measurements from the system.Also provides the best estimates for all the line Hows,loads, transformer taps, and generator outputs.

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38State Estimation




State Estimation Functions II

I Bad data processing: Detects the existence of grosserrors in the measurement set. Identifies and eliminatesbad measurements provided that there is enoughredundancy in the measurement configuration.

I Parameter and structural error processing: Estimatesvarious network parameters, such as transmission linemodel parameters, tap changing transformer parameters,shunt capacitor or reactor parameters. Detects structuralerrors in the network configuration and identifies theerroneous breaker status provided that there is enoughmeasurement redundancy.

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39State Estimation




Measurement Model I

The phase angle of the reference bus equals zero. The stateestimation problem is therefore to estimate the n phase anglestate variables x = (x1, x2, . . . , xn)′ based on the m activepower measurements z = (z1, z2, . . . , zm)′, where

z = h(x) + e (1)

Where h is the vector of functions, usually non-linear, relatingerror free measurements to the state variables;e = (e1,e2, . . . ,em)′ ∼ N (0,R) is independent measurementnoise, where R is the diagonal covariance matrix. The matrix Hrepresents the topology of the considered power grid.In a static context, more measurements are taken into accountthan the number of state variables to be determined, i.e.m > n. In this case, the set of equations 1 represents anoverdetermined set of non linear equations.

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40State Estimation




Measurement Model II

The following assumptions are commonly made, regarding thestatistical properties of the measurement errors:

1. E(e) = 0;

2. measurement errors are independent, i.e. E(eiej ) = 0.Hence cov(e) = E[e · eT ] = R diag

{σ2

1 , σ22 , . . . , σ

2m}

The standard deviation σi of each measurement i is calculatedto reflect the expected accuracy of the corresponding meterused.

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41State Estimation




Weighted Least-Squares Problem I

The state estimation problem can be solved as anunconstrained weighted least-squares (WLS) problem. TheWLS estimator minimizes the weighted sum of the squares ofthe residuals, expressed as

J(x) =m∑

i=1

(zi − hi (x))2

Wii

= [z − h(x)]T R−1 [z − h(x)] (2)

where R = diag(Ri ) is the weighting matrix.At the minimum, the first-order optimality conditions must besatisfied. This can be expressed as:

g(x) =∂J(x)

∂x= −HT (x)R−1 [z − h(x)] = 0 (3)

H is the m × n measurement Jacobian matrix.

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42State Estimation




Weighted Least-Squares Problem II

The first order necessary condition for a minimum are that

∂J(x)

∂x= −H(x)T R−1[z − h(x)] = 0 (4)

Expanding the nonlinear function g(x) into its Taylor seriesaround the state vector xk yields:

g(x) ∼= g(xk ) + G(xk )(x − xk ) = g(xk ) + G(xk )∆xk+1 = 0

G(xk )x = G(xk )xk − g(xk )

x = xk −[G(xk )

]−1g(xk ) (5)

where k is the iterative index; xk is the solution vector atiteration k . The matrix G(xk ) is called gain matrix and it iscalculated as

G(xk ) =∂g(xk )

∂x= HT (xk )R−1H(xk ) (6)

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43State Estimation




Weighted Least-Squares Problem III

The gain matrix is sparse, positive definite and symmetricprovided that the system is fully observable. The matrix G(x) istypically not inverted, but instead it is decomposed into itstriangular factors and the following sparse linear set ofequations are solved using forward/back substitutions at eachiteration k :

G(xk )∆xk+1 = HT (xk )R−1 [z − h(xk )] (7)

where ∆xk+1 = xk+1 − xk . This equation is also referred to asthe Normal Equations. Iterations are terminated when anappropriate tolerance is reached on ∆xk .

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44Automatic GenerationControl



Automatic Generation Control I

The large, slow changes in demand are met centrally bydeciding at regular intervals which generating units will beoperating, shut down or in an intermediate hot reserve state.This process of unit commitment may be conducted once perday to give the daily operating schedule, while at shorterintervals, typically every 30min, economic dispatch determinesthe actual power output required from each of the committedgenerators.

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Automatic Generation Control II

Smaller, but faster, load changes are dealt with by AGC so asto:

I maintain frequency at the scheduled value (frequencycontrol);

I maintain the net power interchanges with neighbouringcontrol areas at their scheduled values (tie-line control);

I maintain power allocation among the units in accordancewith area dispatching needs (energy market, security oremergency).

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Primary Control I

When the total generation is equal to the total system demand(including losses) then the frequency is constant, the system isin equilibrium, and the generation characteristic isapproximated by

∆ffn

= −ρT∆PT

PL(8)

where ρT is the local speed droop of the generationcharacteristic and depends on the spinning reserve and itsallocation in the system.

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Primary Control II

System loads are also frequency dependent and an expressionsimilar to previous one) can be used to obtain a linearapproximation of the frequency response characteristic of thetotal system load as

∆PL

PL= KL

∆ffn

(9)

where KL is the frequency sensitivity coefficient of the powerdemand.

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Primary Control III

In the (P, f ) plane the intersection of the generation and theload characteristic defines the system equilibrium point. Achange in the total power demand ∆PL corresponds to a shiftof the load characteristic.The increase in the system load is compensated in two ways:firstly, by the turbines increasing the generation by ∆PT ; andsecondly, by the system loads reducing the demand by ∆PL.

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Secondary Control I

If the turbine-generators are equipped with governing systems,then, following a change in the total power demand, the systemwill not be able to return to the initial frequency on its own,without any additional action.In order to return to the initial frequency the generationcharacteristic must be shifted. Such a shift can be enforced bychanging the Pref setting in the turbine governing system, theload reference set point. Changing more settings Pref ofindividual governors will move upwards the overall generationcharacteristic of the system. Eventually this will lead to therestoration of the rated frequency but now at the requiredincreased value of power demand. Such control action on thegoverning systems of individual turbines is referred to assecondary control.

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Secondary Control II

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Secondary Control III

In an isolated power system, automatic secondary control maybe implemented as a decentralized control function by adding asupplementary control loop to the turbine–governor system.This modifies the block diagram of the turbine governor wherePref and Pm are expressed as a fraction of the rated power Pn.The supplementary control loop, consists of an integratingelement which adds a control signal ∆Pω that is proportional tothe integral of the speed (or frequency) error to the loadreference point. This signal modifies the value of the setting inthe Pref circuit thereby shifting the speed–droop characteristic.

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AGC I

In an interconnected power system consisting of a number ofdifferent control areas, secondary control cannot bedecentralized because the supplementary control loops haveno information as to where the power imbalance occurs so thata change in the power demand in one area would result inregulator action in all the other areas. Such decentralizedcontrol action would cause undesirable changes in the powerflows in the tie-lines linking the systems and the consequentviolation of the contracts between the cooperating systems. Toavoid this, centralized secondary control is used. Ininterconnected power systems, AGC is implemented in suchaway that each area, or subsystem, has its own centralregulator.The power system is in equilibrium if, for each area,the total power generation PT , the total power demand PL andthe net tie-line interchange power Ptie satisfy the condition:

PT − (PL + Ptie) = 0 (10)

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AGC II

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AGC III

The area control error (ACE) is defined as

ACE = −∆Ptie − λR∆f (11)

The choice of the bias factor λR plays an important role in thenon-intervention rule.The central regulator must have an integrating element in orderto remove any error and this may be supplemented by aproportional element. For such a PI regulator the output signalis:

∆Pref = βR(ACE) +1

TR

∫ t

0(ACE)dt (12)

where βR and TR are the regulator parameters.

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AGC as a Multi-Level Control

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Conclusion

In this lesson we see the history of the power grid, withparticular attention to the Italian case. We are take a look atpower operations and its main devices.Then we define the Energy Management System functionality,with a special focus on the state estimation and on generationcontrol.

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