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Control System for Automated Feed Plant 64.7 Software Design: Theory and Application 1135

One of the major tasks to be efﬁciently implemented

is the correct management of timing. The chosen solu-

tion was to use only one timer variable for all SFC-type

programs that is started when automatic mode is cho-

sen. In all SFC programs that need timing comparisons,

the solution adopted was to read this global timer as

necessary and make the desired calculations of time

passed. Other tested solution, using special timing func-

tions, like timer on (off) delay did not give expected

results in SFC programs, but performed well in LD dia-

grams. When the automatic mode ends, the global timer

is stopped and reset in order to avoid the situation in

which the timer could reach its maximum and overﬂow.

The timer is not needed in the manual mode.

Another special request was to minimize as much as

possible the production time, in order to increase pro-

ductivity. The solution chosen to achieve this goal was

to extract the cereals immediately after weighing ma-

chine 1 is unloaded into the transporting system. In this

way no time is lost; while the milling machine is pre-

pared to receive material, the weighing machine 1 is

emptied without any delay. In Fig. 64.4 an example is

shown of how the timer-off delay function block is used

in a LD program.

Because the impulseduration of variable b

SEL1 op

(starts a motor rotating clockwise) is too short, it was

extended by 500ms. In the second line of the diagram

you can see that the command is reset when:

•

500ms has passed or the conﬁrmation from the mo-

tor’s contactor arrived.

•

The motor reached the corresponding limit.

•

Conﬁrmation from the counterclockwise contactor

is not active (failure situation).

In thisway the operator must only start the movement of

a motor and the controller automatically stops it when

one of the conditions is fulﬁlled.

Another job to be correctly managed was the emer-

gency status. Because of the nature of the installations

of the factory, there are frequent situations when the

product process must be immediately interrupted (ex-

tractor clogging, transporters stuck, and so on).

When an emergency occurs (that could not be de-

tected automatically) the operator must stop all the

processes immediately by simply clicking a button, im-

plemented in the user interface. Inside the programs,

almost all the steps verify the status of the emergency

button and the status of several sensors. When an emer-

b_SEL1_op

l_sel1_cl

c_sel1_op o_sel1_op

c_sel1_cl

b_sel1_op

timrsib11

t#500ms

TOF

timrsib11.Q

Fig. 64.4 Automatically stopping a motor in an LD program

Step2

Step9

Step14

val_oil_co

TRUE

val_Prem_ emergency_button

Step10

Step14

TRUE

Step14

Step2

NOT

(emergency_button)

reset_but

Init

Fig. 64.5 Managing emergency situations in SFC

gency is detected, all SFC programs stop in their current

step. Depending on the gravity of the situation, the op-

erator has the possibility to resume the process from the

point where it was stopped or to reset all the programs

(Fig.64.5).

Once can see that step 2 has four conditions of tran-

sition. The ﬁrst is the normal behavior of the process,

conditions 2 and 3 appear when some material is miss-

ing in the bin, and the fourth is true when the operator

presses the emergency button on the interface screen.

The latter three transitions lead the program to an emer-

gency step 14, from wherethe graphcan evolve to step 2

(resuming the program) or the Init step, sending the

program back to its initial status.

Part G 64.7

1136 Part G Infrastructure and Service Automation

64.8 Communication

Usually PLCs communicate with a PC by means

of a serial port (RS232 or RS485) or an Ether-

net interface (usually available only in more ex-

pensive and newer PLCs). Various protocols are

implemented in the PLC in order to communi-

cate (such as Proﬁbus, transmission control protocol

(TCP)/internet protocol (IP) and others). Sometimes

the programmer must develop his own commu-

nication protocol if none is implemented in the

PLC.

ThinkIO controllers have already installed an OPC

server that can be used to exchange data between the

PLC and other computers connected in the same net-

work. This is a very useful feature because several PCs

can communicate with the controller and can be used

to monitor all the production process, to conﬁgure the

recipe, and even to send commands to the installations

(if the program allows this).

At this time one PC program was developed, with

the following behavior:

•

The operator can edit, save, load recipes, and send

them to the PLC on the network, acting as an OPC

client.

•

Reports are printed at the printer after each charge

ﬁnishes.

64.9 Graphical User Interface on the PLC

In a Codesys development program the user can create

as many visual interfaces as needd and run this inter-

faces directly from the ThinkIO PLC. Practically a PC

is not needed anymore to supervise the control process.

Raw material bins

Press bins

SGC

UMP_PMX

GOL_PMX

Product storage bins

Transporter 1 Transporter 2

L G L1 G1 L2 G2 L3 G3

F1 F2 F3 F4 F5 F6 F7 F8

Tank

empty

Grin-

ding

Premix

Time

40s

Left

15s

Weight

200kg

Oil

Time

25s

Left

10s

Weight

70kg

Mill

Mixing machine

Time

2m15s

Time left

1m10s

Status OK

Restart Auto

Resume

Start

Feeding

Load Recipe

Recipe Nr. 10

Charge Nr. 5

Press

Weight 1

1200kg

4.0

2.0

4.0

2.0

4.0

2.0

4.0

2.0

4.0

2.0

4.0

2.0

4.0

2.0

4.0

2.0

Fig. 64.6 Software view: main production area

For this projectﬁve visual screens weredeveloped, each

representing a speciﬁc area of the factory; for exam-

ple Fig. 64.6 shows the main area of the factory, with

its complicated transport system (formed by conveyors

Part G 64.9

Control System for Automated Feed Plant 64.11 Environment Control in the Chicken Plant 1137

and elevators), storage bins for raw material and ﬁnal

product, and other machines. Buttons on the screen can

be clicked with the mouse and will start/stop some mo-

tors. The color of the objects usually indicates its status

(started, stopped, full, empty, and so on), according to

the values of associated sensors.

In this screen the operator can see information from

all the sensors in the area, even the values from the

weighing machines. Also the operator can manually

command the execution elements from this area, e.g.,

can start automatic production or stop it by pressing the

emergency button.

64.10 Automatic Feeding of Chicken

All this fodder production obtained in the feed plant is

destined for feeding growing chicken in special farms.

Chicken growth requires two separate processes: the

supply of food and water to them and control of the en-

vironment. The chickens are prepared for sacriﬁce after

40days, which is the optimum period for the chickens

to be large enough and have tender meat. The control

system of the factory must be very efﬁcient in order

to minimize the cost per kilo of meat. Two factors in-

ﬂuence correct growth of chickens: food quality and

environmental parameters (temperature and humidity).

It is very important that the chicken do not waste energy

adapting to the environment, so temperature and humid-

ity control must be very accurate. Environment control

strategy is different from summer to winter.

The process of supplying food and drink is not very

complicated. There are sensors that check if there is

Fig. 64.7 The feeding system

food in the plate; when the plate is empty the control

system starts the motors of the conveying system and

stops them after a speciﬁc period of time. There is also

an alarm system when the tank reaches a lower limit

(Fig.64.7).

64.11 Environment Control in the Chicken Plant

Environment control has to be the most accurate. Tem-

perature and humidity must continually be maintained

inside a speciﬁc range of values that differs from one

day to another. To fulﬁl this, gas burners and fans are

used to ventilate the interior with fresh air. To achieve

a good distribution of temperature many small burn-

ers are used instead of fewer larger ones. Because the

outside temperature differs between day and night, not

all capacity of the fans is needed all the time. To ad-

just this, variable-frequency drive (VFD) speed control

is used. A special situation occurs on cold days when,

even with no fan speed, the air ﬂow is too strong to

maintain the temperature. There are three blowers and

normally all of them have to work for a good dispersion

of fresh air. Anyway there is the possibility to manu-

ally select the operation of each blower. Another option

is to use dumpers in each fan to control the opening of

the windows. The inside temperature is controlled by

a proportional–integral–differential (PID) software al-

gorithm combined with control of the dumpers when

needed. This is a continuous process and above it

there is a batch process that provides the set point for

temperature controller according to a temperature di-

agram. Usually this system is sufﬁcient also to keep

within prescribed limits, but it is not sufﬁcient for

humidity all of the time. The behavior of the indoors

environment is for increasing humidity, so controlled

opening of windows succeeds in maintaining the de-

sired conditions.

Here the processes are minor, but the combination

of a continuous process with a batch one does not re-

sult in a simple system. The performance of controlling

these parameters combined with the provision of appro-

priate food for each stage makes the difference between

companies. Another important inﬂuence on system per-

formance is the positioning of temperature sensors,

which should be uniformly distributed throughout the

area.

Part G 64.11

1138 Part G Infrastructure and Service Automation

64.12 Results and Conclusions

There were two important requirements to be accom-

plished by this automation system:

1. The ratio between different components contained

in a ﬁnal product had to be very close to the pro-

grammed values (maximum 5% error).

2. The quantity of ﬁnal product must also be close

to the programmed total quantity (2–3% error was

requested).

After several weeks of system testing the results

were as follows:

•

Condition 1 was fulﬁlled, and the error was very

small (2%).

•

Final quantity was about 5% more than the desired

quantity, because of estimated extraction, small

losses in transporting system, and so on.

The solution adopted was to slightly reduce the scales

for calculating the weight values for the weighing

machines until the resulted quantity was within ap-

proximately 2% of the desired value. At that time the

automation could be declared ﬁnished, with very good

results.

We believe that the solution chosen for this auto-

mated system is very modern and has several advan-

tages over what one might call classical solutions using

other PLC models [64.2]:

•

Process control and supervision do not require a PC,

as all programs and visual interfaces run on the

PLC.

•

Fast and reliable network communication with PCs,

by OPC server, in contrast to the widely used

serial RS232or RS485communication inother con-

trollers.

•

The short working cycle due to fast processor.

•

The extremely large number ofI/O modules that can

be managed (hundreds of digital signals monitored)

by a single controller.

Future work will be to design a supervising user inter-

face program that can access the controller directly the

from internet [64.3], so, e.g., the owner of the factory

can be informed about production ﬂow even when far

from the factory.

64.13 Further Reading

•

T. Borangiu, F.D. Anton, S. Tunaru, A. Dogar,

N.A. Ivanescu, N. Manu: High speed real-time

robot vision design for ﬂexible part feeding, Proc.

14th Int. Conf. Robotics, Alpe-Adria-Danube Re-

gion RAAD’05 (2005) pp.337–345

•

N. Ivanescu, S. Brotac, A. Rosu: Modern solution

for controlling combined fodder plant, IMS (2007)

pp. 236–241

References

64.1 T. Borangiu: Sequential Controllers and Micropro-

gramming (Polytechnical, Bucharest 1993)

64.2 N. Ivanescu, S. Brotac, T. Borangiu, A. Rosu: Mod-

ernizing a Fodder Plant, Proc. IEEE SMC Int. Conf.

Distrib. Hum.-Mach. Syst. (Atena 2008) pp. 543–

548

64.3 N. Ivanescu, S. Brotac, T. Borangiu: A distributed

and conﬁgurable architecture for controlling in-

dustrial processes, IMS, 132–137 (2001 )

Part G 64

1139

Securing Elect

65. Securing Electrical Power System Operation

Petr Horacek

Automation in power systems has a very long

tradition. Just recall the ﬂyball governor in a

steam engine and it becomes clear that power

people have been using control principles and

instruments for more than a century. There are,

however, new challenges in power generation

and transmission concerning the security and

efﬁciency of those services that require the at-

tentions of both theoreticians and practitioners.

These challenges are the subject of this chap-

ter. The power failures that affect large grids

from time to time show that system collapses

are not simply a subject of academic debate.

Power network operating reliability has become

an issue that any country must make a top

priority. The reliability of a power system de-

pends for the most part on the quality of the

decisions made, both automatically and manu-

ally. The term power system is very broad, and

we focus here on the power system backbone,

electric power transmission, and its operational

reliability, particularly when automatic control

plays a vital role. The content of the chap-

ter remains interdisciplinary, spanning power

systems, automation and economy, as changes

resulting from the opening of the markets and

permanent power system restructuring affect its

operation.

65.1 Power Balancing ..................................1141

65.1.1 The Problem .............................. 1141

65.1.2 Who Performs Power Balancing? .. 1141

65.1.3 Means of Balancing .................... 1142

65.1.4 Power Balancing Mechanisms

Within the Control Area............... 1143

65.1.5 System Frequency ...................... 1145

65.1.6 Primary Frequency Control .......... 1145

65.1.7 Secondary Frequency

and Power Control...................... 1148

65.1.8 Tertiary Reserve ......................... 1150

65.1.9 Quick-Start Reserve .................... 1150

65.1.10 Standby Reserve......................... 1150

65.1.11 Planning Reserves ...................... 1150

65.1.12 Performance Criteria................... 1151

65.2 Ancillary Services Planning ....................1153

65.2.1 Stochastic Model

of Area Control Error ................... 1154

65.2.2 Minimal Needs of AS ................... 1155

65.2.3 AS Bids

from Generation Companies ........ 1156

65.2.4 Procurement of Ancillary Services . 1157

65.2.5 Availability

of Self-Regulation Power ............ 1157

65.2.6 Monte Carlo Simulation

of Power Balancing .................... 1157

65.2.7 Control Performance Evaluation ... 1159

65.2.8 Case Studies .............................. 1159

65.2.9 Related Topics ........................... 1161

References ..................................................1162

There areseveral textbooks thatsurveythe power indus-

try – covering a regulated utility structure and the major

concepts of deregulation, the history of electricity, the

technical aspects, and the business of power – that are

recommended for those who would like to get a broader

picture of the industry [65.1].

Unlike other commodities, electricity generally can-

not be stored for later use; it must be used as it is

generated. Therefore, generation must be dispatched

instantaneously to respond to real-time changes in con-

sumers’ demand for electricity. To support these energy

ﬂows, grid operation encompasses scheduling and man-

aging ﬂows overtransmission linesand coordinatingthe

operation of the transmission network equipment.

This chapter introduces methods for securing the

operation of an electrical power system network, as

Part G 65

1140 Part G Infrastructure and Service Automation

Wind farm

High voltage

transmission lines

Distribution

substation

Generation

Thermal power plant

Homes and business

Distribution lines

Generation

Load

and

interchange

MW MW

System Operations:

Generation-load balancing

Maintaining scheduled interchanges

Supporting interconnection frequency

Transmission

station

Hydro power plant

50/60 Hz

Fig. 65.1 Area system control objectives

shown schematically in Fig. 65.1, in terms of active

power balancing through both automatic and manual in-

terventions. Unlike a traditional control problem, where

the task is to synthesize the control law for a given

plant/process and constraints, our problem is to deter-

mine the constraints for a given control law so the

control system has the right freedom in taking proper

actions. Constraints appear in the form of regulat-

ing/balancing power reserves that have to be planned

and reserved in advance so that the area control error,

the instantaneous difference between the actual and the

reference value for the power interchange of a control

area (also known as the unintentional deviation), is kept

within reasonable bounds. Balancing power is accessi-

ble in the form of ancillary services (AS), as provided

by power-generating companies and traded in the mar-

ket. Balancing power reserve activations are variables

manipulated by a system operator (SO) which is re-

sponsible for power balancing in a geographical area.

This is a complex task in a liberalized market environ-

ment. Power reserves, or rather the ancillary services

providing the reserves, must be carefully planned and

purchased so that the SO is able to use them when

required.

The chapter introduces a power-balancing prob-

lem, starting from a review of the basic concepts of

controlling synchronous generators connected to large

grids, such as the technical means for power bal-

ancing that are available, how the power balancing

is distributed throughout the network, and who is in

charge of the task. Next, criteria related to the suc-

cessful completion of the balancing task are revealed

for two large grids: the North American and Euro-

pean grids. Later, the principle of a decision-support

tool used by a SO for year-ahead planning and pro-

curement of ancillary services is described. A method

originally designed for the area of the UCTE,theUnion

for the Co-ordination of Transmission of Electricity,in

which 23 European countries are synchronously inter-

connected, can be made applicable to other conditions

after modiﬁcation. Some of the challenges associated

with continuously evolving power systems, where au-

tomation plays an important role, are discussed at the

end.

Part G 65

Securing Electrical Power System Operation 65.1 Power Balancing 1141

65.1 Power Balancing

In any electrical system, the active power genera-

tion and consumption must be balanced to prevent

major losses of material and even human lives, as

large or long-lasting power deviations may ultimately

lead to blackouts if the balance moves out of con-

trol. Disturbance to this imbalance, ﬁrst noticed as

a system frequency deviation, will be offset initially

by the kinetic energy of the rotating turbo genera-

tors and motors connected to the grid. The capacity

of kinetic energy storing elements is insufﬁcient to

maintain the power equilibrium in real time. Gener-

ation units must be manipulated to conduct power

balancing so that the network user is not affected

by demand changes or generation and transmission

outages.

65.1.1 The Problem

Secure operation of a transmission system and pre-

vention of blackouts is an issue of rising importance

for countries with deregulated electricity markets. The

liberalization of electricity markets is a process that

started a while back and is still in progress in many

countries [65.2–5]. The basic motivation for this liber-

alization is to enable more effective power generation

as well as investment and expansion planning than

would occur in a traditionally vertically integrated

electricity supply industry. On the demand side, end

users are free to choose their supplier and to negoti-

ate their contracts. On the supply side, producers can

sell their electricity to any other market players. It is

believed that this could possibly result in lower electric-

ity prices. However, links between physics and business

practices must be carefully maintained to ensure that

enough balancing power is available when required,

so the performance criteria of the system is guaran-

teed at the lowest cost possible. In another words,

there is the need for active power balancing mecha-

nisms, reserve planning and purchasing, which is the

task performed by a balancing authority (BA) responsi-

ble for power balancing in an area within the electrical

grid.

The BA is an electrical power system or com-

bination of electrical power systems bounded by

interconnection metering and telemetering.The BA bal-

ances the supply and demand within its area, maintains

the interchange of power with other balancing authori-

ties, and maintains the frequency of the electrical power

system within reasonable limits.

65.1.2 Who Performs Power Balancing?

All grid operators are charged with maintaining the

reliability of the systems under their control. Within

their footprints, SOs oversee and direct the high-

voltage bulk-power system and coordinate electricity

generation to maintain a reliable supply of electrical

power to electricity users. SOs provide critical reliabil-

ity services, including outage coordination, generation

scheduling, voltage management, ancillary services

provision, load forecasting, and more. They improve

reliability in part because of their large scope – by

consolidating control areas, they reduce the number of

decision makers managing the grid, which simpliﬁes

coordination and improves reliability.

System operators oversee grid functions and make

necessary corrections to ensure reliability on a minute-

to-minute basis around the clock. SOs may organize

wholesale markets for energy and ancillary services,

such as reserves, frequency and voltage regulation, and

voltage support. These ancillary services help ensure

reliability and help system operators react quickly and

effectively to changing conditions on the grid, such as

the loss of a generating unit or a transmission line.

Independent system operators (ISOs) and regional

transmission organizations (RTOs) in North America

and transmission system operators (TSOs) in con-

tinental Europe have the size, scope, scale, tools,

information and authority to be effective grid managers.

ISOs/RTOs/TSOs have near-identical responsibilities

for managing the grid over a large geographic scope and

operating markets to some extent.

Anyone who would like to get more details on

the deﬁnitions, terminology, responsibilities and pro-

cedures that a particular system operator should use

an internet search engine and look for the key word

Grid Code. The Grid Code is an industrial textbook

that every system operator must follow every day; it is

carefully and regularly updated.

North America

There are three synchronous areas in North America:

the Western, Eastern and Texas Interconnections. In

each interconnection thereare ISOs(BAs)and localbal-

ancing authorities (LBAs) that communicate. As part of

operating the grid, ISOs/RTOs meet or exceed the re-

liability standards set by the North American Electric

Reliability Corporation (NERC) and its regional coun-

cils. Adhering toNERC standards ensuresthat the entire

Part G 65.1

1142 Part G Infrastructure and Service Automation

Table 65.1 Classiﬁcation of ancillary services

Generic AS Description

Continuous regulation Provided by online resources with automatic controls that respond rapidly to

operator requests for up and down movements. Used to track and correct minute-

to-minute ﬂuctuations in system load and generator output.

Energy imbalance

management

Serves as a bridge between the regulation service and the hourly or half-hourly

bid-in energy schedules; similar to but slower than continuous regulation. Also

serves a ﬁnancial (settlement) function in clearing spot markets.

Instantaneous

contingency reserve

Provided by online resources equipped with frequency or other controls that can

rapidly increase output or decrease consumption in response to a major distur-

bance or other contingency event.

Replacement reserves Provided by resources with a slower response time that can be called upon to

replace or supplement the instantaneous contingency reserve in restoring system

stability.

Voltage control The injection or absorption of reactive power to maintain transmission system

voltages within required ranges.

Black start Generation is able to start itself without support from the grid and has sufﬁcient

real and reactive capability and control to be useful in system restoration.

grid in North America operates at appropriate levels of

reliability.

The ISO calculates the area control error (ACE;

described later in the chapter) for its balancing author-

ity area and measures the ACE against the applicable

balancing standards. There are also local balancing au-

thorities that are responsible for actual interchange.

To summarize, the ISO assumes the role of a BA,

planning authority, reliability coordinator, market oper-

ator and tariff and interconnection administrator. LBAs

take care of load and resource asset management, tie-

line checkout and emergency operation coordination.

ISO guaranteeseffectiveuse ofassets, includingthe dis-

patch of energy and the dispatch of reserves (including

contingency reserves). The ISO organizes the energy

and operating reserve market (also known as the ancil-

lary services market), regulation and response services,

and contingency reserves.

Europe

In continental Europe there is an equivalent to the

American ISO/RTO Council, the UCTE. The UCTE

also deﬁnes standards the members must adhere to, so

the UCTE plays the role of the NERC as well. The

difference is that members of the UCTE are system op-

erators that are each responsible for a single control

area (synchronously interconnected within the UCTE)

and own the transmission assets. In many countries, the

transmission system operator (TSO) plays the role of

the BA.

65.1.3 Means of Balancing

Power reserves are provided by generators and distribu-

tors in the form of ancillary services. By deﬁnition, an-

cillary services are interconnected operations services

inﬂuencing transfer of electricity between purchasing

and selling entities which a balancing authority must

include in an open access transmission tariff.

Ancillary services are a collection of secondary

services offered to help to insure the reliability and

availability of energy to consumers. These services

include regulation and contingency reserve (spinning,

supplemental – nonspinning). The contingency reserve

services are often referred to as operating reserves.

Ancillary services insure that capacity is available

when needed to maintain secure power system opera-

tions due to a loss of or increase in load and resources.

A contingency is a trip of a transmission line or gen-

erator, a loss of load, or some combination of these

events. This contingency in turn causes other prob-

lems, such as a transmission line overload, an over- or

undervoltage in an area, over- or underfrequency, or fre-

quency instability. Contingency reserves are a special

percentage of generation capacity resources held back

or reserved to meet emergency needs.

Part G 65.1

Securing Electrical Power System Operation 65.1 Power Balancing 1143

AS arrangements vary considerably across electric-

ity markets. Let us introduce six generic AS that are

necessary for maintaining system reliability and se-

curity in electricity grids. Ancillary services required

during normal conditions are continuous regulation and

energy imbalance management. Instantaneous contin-

gency reserve and replacement reserve are services

that are used during system contingencies. Ancillary

services that do not apply to active power balanc-

ing directly are voltage support and black start. The

generic AS categories are described more precisely

in Table 65.1.

Ancillary service markets are critical to power sys-

tem security and reliability and, given the value that

customers place on reliable electrical services, their

overall value to society is quite high.

Each market incorporates all six generic ancillary

services, although the nomenclature, technical require-

ments and procurement details vary signiﬁcantly. This

chapter assumes that we are working with a TSO, and

an area of the UCTE, in the European system, where

transmission ownership and system operation is man-

aged by a TSO as a single authority in the control

area.

Power

exchange

planned

Measured

Contracted

power reserves

Load

shedding

Transmission

system

operator

Domestic

production

Domestic

consumption

Grid

A, B, C

adjacent areas

synchronously

connected

production_plan

ΔP

balancing_by_producer

ΔP

TSO

Power balancing

control and

decision making

Fig. 65.2 Diagram showing the principle of power balancing ininterconnected areas (frequency control loops not shown)

The role played by the TSO in the real-time power

balancing can be understood from the overall block di-

agram in Fig.65.2, which shows the essentials of the

power balancing task while hiding the complexity of

the very large scale power system. The control area in

the center is synchronously connected to three adjacent

areas A, B and C, making an interconnection.

The task for the TSO is to acquire enough power

reserves in the form of ancillary services and activate

them in a timely manner in order to guarantee the lim-

its set on performance indices (standards) that evaluate

the quality of power balancing. The task must be solved

for the lowest cost possible. In this chapter, we de-

ﬁne performance indices, describe the methods used in

planning, and show how the planned reserves are val-

idated. This chapter considers one year as the horizon

for ancillary services planning.

65.1.4 Power Balancing Mechanisms

Within the Control Area

What Is a Control Area?

A control area is an electrical power system that is

managed under a common automatic control scheme

Part G 65.1

1144 Part G Infrastructure and Service Automation

that maintains frequency by balancing load with pro-

duction. Historically, a utility ran its own control area,

regulating frequency, balancing load and generation,

purchasing energy and capacity, and maintaining op-

erating reserves as needed. Many large utility control

areas served smaller in-area and nearby utilities as well.

Today, some control areas exist to manage only gen-

eration, but most balance both generation and load. In

North America, many control areas that existed histor-

ically have been consolidated into ISOsorRTOsand

offer system balancing and management services that

the control area no longer had to provide itself.

Power Balancing

To operate a large power system and to create suitable

conditions for commercial electricity trade, it is nec-

essary to schedule the power to be exchanged at the

interconnection borders between the system operators

in advance. During daily operation, the schedules are

followed by means of the load-frequency control in-

stalled in each control area. Despite the functionality

of load-frequency control, unintentional deviations oc-

cur in energy exchanges. For this reason, unintentional

deviations are compensated for through methods of the

TSO in every control area.

To plan and purchase

Inadvertent interchange

Net scheduled interchange

Scheduled import

Primary frequency

control

Power reserve activation

Auto

Man

Balancing power

domestic

Power

generation

Load shedding

Domestic demand

programmed

measured

Scheduled export

Balancing power

from other areas

Set-point of foreign exchange

(scheduled exchange)

Consumption

Production

Frequency bias obligation

Net actual interchange

metered

BA's ACE

ACE

meas

(Hz)

(MW)

Unscheduled import

Unscheduled export

Δf

Range of balancing power reserves

(purchased ancillary services)

TSO/ISO

EMS

AGC

Fig. 65.3 Power balancing in the area as a feedback control system

Real-time power balancing of a control area is usu-

ally performed by both automatic control and manual

interventions from the TSO’s dispatch center. The en-

tire feedback control loop of a particular control area is

showninFig.65.3.

From the control theory perspective, the controlled

variable is the instantaneous value of the cross-border

power exchange P

measured

, which should meet its

scheduled value P

programmed

. Manipulated variables are

represented by various categories of regulation power

available in the form of ancillary services. Disturbance

variables inﬂuencing cross-border power exchange can

be divided into two groups: deterministic, e.g., im-

balances caused by trading strategies; and stochastic,

mainly random demand ﬂuctuations and generator out-

ages.

The inputto the controller is the ACE, the difference

between the scheduled and actual cross-border power

exchange, corrected for the effect of the primary fre-

quency control so that the central controller does not

compensate for outages of generators located in adja-

cent control areas (for example).

The output of the central controller is to increase

or decrease power on the production side or, in the ex-

treme case, on the consumption side aswell. Despite the

Part G 65.1