Designing a Lab View based Automatic Generation Control Training System

Designing a Lab View based Automatic

Generation Control Training System

Introduction

1.1 Background

Power generation control aims to deliver power in an interconnected system

as economically and reliably as possible while maintaining the voltage and

frequency within permissible limits. Change in real power affect mainly the

system frequency, while reactive power is less sensitive to changes in

frequency and is mainly dependent on changes in voltage magnitude. Thus,

real and reactive powers are controlled separately.

The supplementary control, known as AGC is shown in Figure 1.1. It

accomplishes more than just frequency control. However, if the power

system is being maintained in economic dispatch, the AGC is responsible for

allocating generation changes in manner that the new total generation

matches the needed power for the system while being allocated in an

economic manner. In addition, the control of active and reactive power is

necessary in order to keep the power system in the steady state. Economic

dispatch optimizes the available mix of generation resources and maximizes

the use of low cost sources of electricity, while recognizing any operational

limits.

Karnataka State Open University



Figure 1.1: Schematic Diagram of Load Frequency Control System with Economic Dispatch

The LFC loop controls the real power and frequency while the AVR loop

regulates the reactive power and voltage magnitude.




The functions of the (AGC) are as follows:

Matching area generation to area load. That is, to match the tie – line

interchanges with the schedules and to control the system frequency.

Distributing the changing loads among generators minimize the

operation costs subject to additional constraints such as might be

introduced by security considerations.

The objectives of (AGC) are as follows:

A small change in the system load produces proportional changes in

the system frequency. That is, the Area Control Error (ACE)

provides each area with approximate knowledge of the load change

and directs the supplementary controller for the area to manipulate

the turbine valves of the regulating units.

The second objective is met by sampling the load every few minutes

(1–5 minutes) and allocating the changing load among different units

so as to minimize the operating costs. This pre assumes the load

demand remains constant during each period of economic dispatch.




Economic dispatch optimizes the available mix of generation resources and

maximizes the use of low cost sources of electricity, while recognizing any

operational limits.

1.2 Objectives

The objective of this project is to design a user friendly LabView based

training module to teach AGC. This simulink includes the

implementation of combination of LFC and AVR to compensate for load

demand variation and maintain both frequency and terminal voltage

within standard limits.

1.3 Problem Definitions

A large frequency deviation can damage equipments, reduce loads

performance and lead to lose of synchronism. For efficient and reliable

operation of the power system the voltage at the generator terminals has to

be maintained within acceptable limits by AVR. Teaching AGC using only

the simulation is not enough. There is a need for implementing the system

physically.




1.4 Methodology

The general solution approach is to research about background information

& industrial survey to familiarize team members about AGC system and

conducting research on the current approach for solving the AGC model

under normal operating conditions. Once the research was completed, a

model for AGC was simulated in MATLAB to test the performance of AGC.

Then, power circuit was built & LabView was programmed to be used to

control the performance of the system. After that, the AGC module was

constructed and the operation was tested to make sure it works as per

standards.

1.5 Limiting Factors

The main task was to develop a simulink/MATLAB based simulator and

design the experimental setup within a short period. Only single area AGC

was considered in this project. The proposed AGC could not be built like a

real power plant therefore we have developed a power plant simulator using

the equipment available in lab.

1.6 Main Findings

The load frequency & excitation voltage control was investigated

independently. Also, both the simulator and the experimental setup for




single area AGC system were designed. Analogue OP AMP regulator and

LabView based control technique were implemented.

1.7 The Report Structure

In chapter 2, a survey of the different commercial AGC modules is presented.In chapter 3, the design development of AGC/AVR system is detailed.

In chapter 4, the design procedure of the training system is detailed.

In chapter 5, the testing and validation of the proposed AGC training module

are detailed.

In chapter 6, presented a summary of main finding of the projects and

discuses different proposed for improving the proposed AGC

Survey of state of the art

2.1 What was published:

2.1.1 Automatic Power Generation Control Simulation:

AGC Simulation was published by Mr. Ravindrakumar Yadav.

Typical responses to real power demand were illustrated using the latest

simulation technique available by the MATLAB SIMULINK package [1].




The requirement of reactive power, voltage regulation and the influence on

stability of both speed and excitation controls with suitable feedback signals

were examined. An isolated power systems were simulated using AGC.

In an interconnected power system, LFC and AVR equipment were installed

for each generator. Figure 2.1 represents the schematic diagram for the LFC

and AVR loops. The controllers were set for a particular operating condition

and take care of small changes in load demand to maintain the frequency and

voltage magnitude within the specified limits.

Figure 2.1: Schematic diagram of LFC and AVR of a synchronous generator.

The change in frequency is sensed as which is the change in rotor angle

and the error. The LFC system with addition of the secondary loop is as




shown in Figure 2.2. The integral controller gain KI must be adjusted for a

satisfactory transient response.

Figure 2.2: Block diagram of AGC

Modern energy control centers (ECC) are equipped with online computers

performing all signal processing through the remote acquisition systems

known as “supervisory control and data acquisition (SCADA) systems”.

2.1.2 Load Frequency Control for Multiple-Area Power

Systems

A multi-area power system comprises areas that are interconnected by high

voltage transmission lines or tie-lines [2]. The deviation of frequency

measured in each control area is an indicator of the trend of the mismatch

power in the interconnection and not in the control area alone. The LFC

system in each control area of an interconnected (multi-area) power system




should control the interchange power with the other control areas as well as

its local frequency. The power flow on the tie-line from area 1 to area 2 is

(2.1)

Where X12 is the tie-line reactance between areas 1 and 2. 1, 2 are the

power angles of equivalent machines of the areas 1 and 2. V1, V2 are the

voltages at equivalent machine’s terminals of the areas 1 and 2.

In a multi-area power system, in addition to regulate area frequency, the

supplementary control should maintain the net interchange power with

neighboring areas at scheduled values. This is generally accomplished by

adding a tie-line flow deviation to the frequency deviation in the

supplementary feedback loop. A suitable linear combination of frequency

and tie-line power changes for area i, is known as the Area Control Error

(ACE).

(2.2)

Where i is a bias factor.

The block diagram shown in Figure 2.3 illustrates how supplementary

control is implemented using (2.2).




Figure 2.3: Control area i with complete supplementary control

2.2 What is available in the Market:

2.2.1 Open System International Company (OSI)

The AGC and economy dispatching software are used to control generated

power of subordinate power plants to ensure secure, effective and economic

operation of the power system [3]. Figure 2.4 shows the AGC data flow.

It aims at real-time balancing of power supply and demand to accomplish

the following targets:

Keep power network frequency within allowable error limits.

Keep interchanging power between controlled areas to present value.




Coordinate power plants under remote regulation to contribute based

on generation cost or bidding prices, so as to obtain maximum

economic benefits.

OpenAGC can easily be integrated into an existing control center

environment because it is based on open standards for software and database

implementation. OpenAGC is ideal for those searching for an upgrade from

an existing AGC product can be a primary or backup control center site.

Featured OpenAGC functionality includes:

Market operation

Multi-area Control

NERC performance monitoring

LFC

Online economic dispatching

Reserve monitoring (RM)

AGC performance monitoring

Unit generation schedule

Trade schedule and evaluation

Production cost analysis

Unit response test




Figure 2.4: AGC System data Flow

In today's deregulated environment, power producers need a means of easily

separating generation resources into control groupings (sometimes referred

to as multi-area). OpenAGC is specifically designed to meet this need,

allowing individual generator assignments to separate control groupings or

areas. The user interface makes it easy to view resources according to these

groupings because it consists of tabular and graphic displays that put the

operator “in control” of all resources.




Trends and plots are used to summarize vital system information and can

easily be customized based on information preference. When used in

conjunction with OSI's OpenView. NET based Graphical User Interface,

AGC information can be made available to the company enterprise via a

web browser, subject to individual permissions.

OpenAGC supports many features for tuning and control, yet it maintains its

simplicity through intuitiveness, making optimum performance and control

response easily realizable. Also, realistic system response is achieved

through proven non-linear filtering techniques for computing Area and Unit

Control Errors (ACE and UCE). Unit models are general-purpose, allowing

for any generator type to be modeled and numerous control modes and

regulation participations allow for tailored generation response as shown in

Figure 2.5.




Figure 2.5: AGC monitor for tailored generation response

2.2.2 Advanced Control Systems

AGC is a stand-alone subsystem that can be installed on any PRISM

SCADA Master as shown in Figure 2.6 [4]. AGC regulates the power

output of electric generators within a prescribed area in response to changes

in system frequency, tie-line loading, and the relation of these to each other.

This maintains the scheduled system frequency and established interchange




with other areas within predetermined limits. AGC monitors and controls

power generation with these overall objectives:

Minimize area control error.

Minimize operating costs in conjunction with ED calculation

software.

Maintain generation at fixed (base load) values.

Ramp generation in a linear fashion according to a schedule specified

by the operator.

In normal operation, the AGC subsystem adjusts the power of the generating

units automatically. This keeps the area's actual net interchange approximate

to the scheduled interchange and the actual frequency near the scheduled

frequency.

Figure 2.6: AGC installed on PRISM SCADA Master




All NERC performance calculations and reports are provided, including

alarming and event storage for post analysis. A time error correction

mechanism is also included. All tuning parameters are located in the Real-

time database and can be adjusted through screen displays without invoking

the database editor. For special filtering performance, the Real-time

database has a set of linear and non-linear data filtering functions that can be

selected and redefined.

AGC can communicate with any data source or data link through the Real-

time database. It supports primary and secondary data sources for important

variables, such as the system frequency, unit generation and tie-line power

and automatically switches the data source if one source fails to provide

reliable data.

Design Development

3.1 INTRODUCTION

As shown in Figure 3.1, the control of active and reactive power is necessary

in order to keep the power system in the steady state. The objective of the




control strategy is to generate and deliver power in an interconnected system

as economically and reliably as possible while maintaining the voltage and

frequency within permissible limits. Change in real power affect mainly the

system frequency, while reactive power is less sensitive to changes in

frequency and is mainly dependent on changes in voltage magnitude. Thus,

real and reactive powers are controlled separately. The LFC loop controls

the real power and frequency and the AVR loop regulates the reactive power

and voltage magnitude. LFC has made the operation of interconnected

system possible [5].





3.2 Frequency Control:

The operation objectives of the LFC are to maintain reasonably uniform

frequency to divide the load between generators and to control the tie-line

interchange schedules. The change in frequency and tie-line real power are

sensed, which is a measure of the change in rotor angle . The error signal,

i.e., f and Ptie, are amplified, mixed and transformed into a real power

command signal Pv, which is sent to the prime mover to call for an

increment in the torque.

The prime mover, therefore, brings change in the generator output by an

amount Pg which will change the values of f and Ptie within the specified

tolerance.

The first step in the analysis and design of a control system is mathematical

modeling of the system. The two most common methods are the transfer

function method and the state variable approach. The state variable approach

can be applied to design linear as well as nonlinear systems. In order to use

the transfer function and linear state equations, the system must first be

linearized. Proper assumptions and approximations are made to linearize the

mathematical equations describing the system, so the transfer functions

model are obtained for the following components.




3.2.1 Generator Model

The overall generator–load dynamic relationship between the incremental mismatch power (ΔPm−ΔPe) and the frequency deviation (Δw) can be expressed as:

(3.1)

Equation (3.1) can be represented in a block diagram as in Figure 3.2.

Figure 3.2 Generator block diagram.

3.2.2 Load Model

The load on a power system consists of a variety of electrical devices. For

resistive loads, such as lighting and heating loads, the electrical power is

independent of frequency. Motor loads are sensitive to changes in frequency.

How sensitive it is to frequency depends on the composite of the speed-load

characteristics of all the driven devices. The speed-load characteristic of a

composite load is approximated by:




(3.2)

Where PL is the nonfrequency-sensitive load change, and Dw is the

frequency-sensitive load change. D is expressed as percent change in load

divided by percent change in frequency. For example, if load is changed by

1.6 percent for a 1 percent change in frequency, then D = 1.6. Including the

load model in the generator block diagram, results in the block diagram of

Figure 3.3, eliminating the simple feedback loop in Figure 3.3, results in the

block diagram shown in Figure 3.4.

Figure 3.3: Generator and load block diagram.




Figure 3.4: Generator and load block diagram.

3.2.3 Prime Mover Model

The source of mechanical power commonly known as the prime mover. The

model for the prime mover relates to changes in mechanical power output

Pm to changes in the controller of the mechanical power Pv. The simplest

prime mover model for the motor can be approximated with a single time

constant T, resulting in the following function

(3.3)

Figure 3.5: block diagram for simple nonreheat steam turbine

3.2.4 Governor Model

When the generator electrical load is suddenly increased, the electrical

power exceeds the mechanical power input. The reduction in kinetic energy




causes the motor speed and consequently the generator frequency to fall.

The change in speed is sensed by the PI controller which acts to adjust the

mechanical power controller value to bring the speed to a new steady-state.

Figure 3.6: Governor steady-state speed characteristics.




The slope of the curve Figure 3.6 represents the speed regulation R.

Governors typically have a speed regulation of 5-6 percent from zero to full

load. The speed governor mechanism acts as a comparator whose output Pg

is the difference between the reference set power Pref and the power

as given from the governor speed characteristics, i.e.,

(3.4)

The command Pg is transformed to PV. Assuming a linear relationship and

considering a simple time constant g, we have the following relation

(3.5)

Figure 3.7: Block diagram representation of speed governing system for steam turbine.




3.2.5 The Complete Block Diagram Of the Load Frequency

gfgfControl

Figure 3.8: Load frequency control block diagram of an isolated power system.

Equations (3.4) and (3.5) are represented by the block diagram shown in

Figure 3.7. Combining the block diagrams of Figures 3.4, 3.5, and 3.7 results

in the complete block diagram of the load frequency control of an isolated

power station shown in Figure 3.8. Redrawing the block diagram of Figure

3.8 with the load change -PL as the input and the speed deviation W( s) as

the output results in the block diagram shown in Figure 3.9. The open-loop

transfer function of the block diagram in Figure 3.9 is

(3.6)

and the closed-loop transfer function relating the load change PL to the

frequency deviation W is

(3.7)




Figure 3.9: LFC block diagram with input ΔPL(S) and output Δw(S).

Utilizing the final value theorem on frequency system shown above results:

(3.8)

(3.9)

It is clear that for the case with no frequency-sensitive load (D=0) the

steady-state deviation in frequency is determined by the governor speed

regulation:




(3.10)

The system was tested with the following parameters:

T=0.5s, G=0.2s, H=5, R=0.05 & PL=0.2

Figure 3.10: Frequency Deviation Step Response.

Figure 3.10 shows that the frequency deviated at 0.2 pu and the steady state

value is almost 0.01 pu.




3.3 Automatic Generation Control

If the load on the system is increased, the prime mover speed drops before

the governor can adjust the input of the prime mover to the new load. As the

change in the value of speed decreases, the error signal becomes smaller and

the position of the governor gets closer to the point required to maintain a

constant speed. However, the constant speed will not be the set point, and

there will be an offset.

One way to restore the speed or frequency to its nominal value is to add an

integrator. The integral unit monitors the average error over a period of time

and will overcome the offset. Because of its ability to return a system to its

set point, integral action is also known as the rest action. Thus, as the

system load changes continuously, the generation is adjusted automatically

to restore the frequency to the nominal value this scheme is known as the

AGC.




In an interconnected system consisting of several pools, the role of the AGC

is to divide the loads among system, stations, and generators so as to achieve

maximum economy and correctly control the scheduled interchanges of tie-

line power while maintaining a reasonably uniform frequency. Of course, we

are implicitly assuming that the system is stable, so the steady-state is

achievable. During large transient disturbances and emergencies, AGC is

bypassed and other emergency controls are applied. In the following

sections, we consider the AGC in a single area system and with the primary

LFC loop, a change in the system load will result in a steady state frequency

deviation, depending on the governor speed regulation. In order to reduce

the frequency deviation to zero, we must provide a reset action. The rest

action can be achieved by introducing an integral controller to act on the

load reference setting to change the speed set point.

The LFC system, with the addition of the secondary loop is shown in Figure

3.11. The integral controller gain KI must be adjusted for a satisfactory

transient response and the proportional controller gain Kp must be adjusted

to provide fast response. Combining the parallel branches results in the

equivalent block diagram shown in




Figure 3.11: AGC for an isolated power system.

The closed loop transfer function of the control system with only -PL as

input becomes

(3.11)

The system response when KI =7 and KP=3.8 is




0 2 4 6 8 10 12 14 16 18 20-16

-14

-12

-10

-8

-6

-4

-2

0

2

4x 10

-3

Time, s

Fre

quen

cy D

evia

tion,

pu

Step Response

Figure 3.12: Frequency deviation step response

As observed in Figure 3.12 and 3.10 , steady state error is eliminated by the

controller.




3.3.1 Alternative Excitation Model

The generator excitation system maintains generator voltage and controls the

reactive power flow. The generator excitation of older systems may be

provided through slip rings and brushes by means of DC generators mounted

on the same shaft as the rotor of the synchronous machine. However,

modem excitation systems usually use AC generators with rotating rectifiers,

and are known as brushless excitation.

An increase in the reactive power load of the generator is accompanied by a

drop in the terminal voltage magnitude. The voltage magnitude is sensed

through a potential transformer on one phase. This voltage is rectified and

compared to a DC set point signal. The amplified error signal controls the

exciter field and increases the exciter terminal voltage. Thus, the generator

field current is increased, which results in an increase in the generated emf.

The reactive power generation is increased to a new equilibrium, raising the

terminal voltage to the desired value. We will look briefly at the simplified

models of the component involved in the AVR system.




3.3.2 Amplifier Model

The excitation system amplifier may be a magnetic amplifier, rotating

amplifier, or modern electronic amplifier. The amplifier is represented by a

gain KA and a time constant A and the transfer function is

(3.12)

Typical values of KA are in the range of 10 to 400. The amplifier time

constant is very small, in the range of 0.02 to 0.1 second, and often is

neglected.

3.3.3 Exciter Model

There is a variety of different excitation types. However, modern excitation

systems uses AC power source through solid-state rectifiers such as Silicon

Controlled Rectifier (SCR). The output voltage of the exciter is a nonlinear

function of the field voltage because of the saturation effects in the magnetic

circuit. Thus, there is no simple relationship between the terminal voltage




and the field voltage of the exciter. A reasonable model of a modern exciter

is a linearized model, which takes into account the major time constant and

ignores the saturation or other nonlinearities. In the simplest form, the

transfer function of a modem exciter may be represented by a single time

constant E and a gain KE, i.e.,

(3.13)

3.3.4 Generator Model

The synchronous machine generated emf is a function of the machine

magnetization curve, and its terminal voltage is dependent on the generator

load. In the linearized model, the transfer function relating the generator

terminal voltage to its field voltage can be represented by a gain KG and a

time constant G and the transfer function is

(3.14)

3.3.5 Sensor Model

The voltage is sensed through a potential transformer and, in one form, it is

rectified through a bridge rectifier. The sensor is modeled by a simple first

order transfer function, given by




(3.15)

3.3.6 Excitation Model

Utilizing the above models results in the AVR block diagram shown in

Figure 3.13

Figure 3.13: a simplified AVR block diagram.

The open-loop transfer function of the block diagram in Figure 3.13 is:




(3.16)

And the closed-loop transfer function relating the generator terminal voltage

Vt (s) to the reference voltage Vref (s) is:

(3.17)

For a step input using the final value theorem, the steady state

response is:

(3.18)

The system was tested with the following parameters:

G=1s, E=0.4s, A=0.1s, R=0.05s, KA=10, KE=1, KG=1, KR=1,




Figure 3.14: terminal voltage step response

Figure 13.4 shows that terminal voltage reduced to almost 0.91pu.

3.3.7 Automatic Voltage Regulator

One of the most common controllers available commercially is the

proportional integral derivative (PID) controller. The PID controller is used

to improve the dynamic response as well as to reduce or eliminate the




steady-state error. The derivative controller adds a finite zero to the open

loop plant transfer function and improves the transient response. The integral

controller adds a pole at origin and increases the system type by one and

reduces the steady-state error due to a step function to zero. The PID

controller transfer function is

(3.19)

In this subject only PI controller is used. The block diagram of an AVR with

a PI controller is shown in Figure 3.15

Figure 3.15: AVR system with PI controller.

The system response when KI and KP are 0.25 and 0.25 respectively.




Figure 3.16: Terminal voltage step response

Figure 3.16 shows that by using PI controller the steady state error becomes

zero. Excitation system performance is found to be highly improved.




3.4 AGC Including Excitation System

Since there is a weak coupling between the LFC and AVR systems, the

frequency and voltage were controlled separately. We can study the coupling

effect by extending the linearized AGC system to include the excitation

system so we obtain the following linearized equation.

(3.20)

Where K2 is the change in electrical power for a small change in the stator

emf. Also, including the small effect of rotor angle upon the generator

terminal voltage, we may write

(3.21)

Where K5 is the change in the terminal voltage for a small change in rotor

angle at constant stator emf, and K6 is the change in terminal voltage for a

small change in the stator emf at constant rotor angle. Finally, modifying the

generator field transfer function to include the effect of rotor angle, we may

express the stator emf as




(3.22)

The above constants depend upon the network parameters and the operating

conditions. For a stable system K2, K4, and K6 are positive, but K5 may be

negative. Including (3.22)-(3.21) in the AGC system of Figure 3.11 and the

AVR system of Figure 3.15, a linearized model for the combined LFC and

AVR systems is obtained and shown Figure 3.17.




Figure 3.17: Block diagram of AGC Including Excitation System

The system was tested with the following parameters

G=1.4s g=0.2s T=0.5s E=0.4s A=0.1s R=0.05s KT=1 Kg=1

KA=10 K6=0.6 KG=0.8 K2=0.2 KP1=6 KP2=0.25 KE=1 KR=1

K4=1.4 K5=-0.1 H=5 R=0.05 D=0.8




0 2 4 6 8 10 12 14 16 18 20-0.045

-0.04

-0.035

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005Step Response

Fre

quue

ncy

Dev

iatio

n, p

u

Time, s

Figure 3.18: frequency deviation step response




0 2 4 6 8 10 12 14 16 18 200

0.2

0.4

0.6

0.8

1

1.2

1.4

Ter

min

al V

olta

ge

Time, s

Step Response

Figure 3.19: terminal voltage step response

It is observed that when the coupling coefficients are set to zero, there is a

little change in the transient response. Thus, separate treatment of frequency

and voltage control loops is justified.

Figure 3.17 shows the proposed AGC block diagram for the project.




Description of Design

4.1 Detailed Design Schematics

The project design was constructed within two stages. First, the hardware is

development. Second, the software is development.

Figure 4.1 shows the block diagram of the system.





4.1.1 Hardware Development:

With sticking on the availability of components in the college's labs, a DC

motor has been chosen as a prime mover and a buck chopper circuit has

been chosen to drive the motor. Also, another buck chopper has been used to

control the current flows in the excitation winding.

Choppers use very fast semiconductor switches to convert DC voltages and

currents from higher to lower levels and vice versa [6]. The semiconductor

switches can be designed with bipolar transistors, metal-oxide

semiconductor field-effect transistors (MOSFETs), diodes, thyristors, etc.

As shown in Figure 4.2. A buck chopper built with a MOSFET (Q) and a

diode (D1), and some waveforms related to this circuit. When MOSFET (Q)

switch on (closed), the DC power supply voltage (E) is applied to the load,

diode D1 automatically switches off, and the current flowing in the load (Io)

starts to increase ( L starts storing energy).




When MOSFET (Q) switch off (opened), the DC power supply voltage is

no longer applied to the load, diode D1 automatically switches on and the

current continues flowing in the load through diode D1 and hence output

voltage will be zero, but the current starts to decrease. Diode D1 is free-

wheeling diode since it provides an alternative path for the load current,

which continues to flow when MOSFET (Q) is off.

Figure 4.2: Buck chopper power circuit

The semiconductor switches are controlled using Pulse Width Modulation

(PWM). The width of the pulse, that is produced to turn the switch on, can

be varied or modulated to adjust the output DC voltage as shown in the

Figure 4.3. The frequency of switching is kept constant and Ton is varied

over one cycle.




Figure 4.3: pulse width modification (PWM)

The output DC voltage of the buck chopper (V0) is proportional to the DC

power supply voltage (E) and the time MOSFET (Q) is on (Ton) during each

cycle. The on-time (Ton) is in turn proportional to the duty cycle (D) of the

switching control signal applied to the gate of the MOSFET (Q). The

equation relating output voltage (Vo) and input voltage (E) is given by the

expression V0 = D * E where D = Ton/T.

For the LFC circuit the chopper is connected to the variable power supply to

avoid high starting current which affects the MOSFET and the DC motor

operation.




Two module of resistive load have been used to represent two test steps of

loading. Push buttons and two contactors have been used to control the load.

The finalized power circuit is shown in the Figure 4.4.

Figure 4.4: Complete wiring diagram of the experimental setup

Figure 4.5 shows the circuit implementation in the lab.




Figure 4.5: Lab implementation for LFC circuit

4.1.2 Software Development:

The control system which was built in chapter three would be used for

software development using LabView.

LFC and AVR have been implemented using two PI controllers that are

designed on LabView. A PI controller calculates an "error" value as the

difference between a measured process variable and a reference. The

controller attempts to minimize the error by adjusting the process control

inputs. By tuning the two constants in the PI controller, the controller can

provide control action designed for specific process requirements.




Proportional control means that the system responds in proportion to how far

the system is from the set point. It is used to turn up the power to the set

point quicker. The magnitude of the contribution of the integral gain to the

overall control action is determined by the proportional gain KP.

The proportional term is given by:

(4.1)

where

Ut: Proportional term of output.

Kp: Proportional gain, a tuning parameter.

e: Error = Set Point – Process Variable.

t: Time (sec.).

The contribution from the integral gain is proportional to the magnitude of

the error. Summing the instantaneous error over time gives the accumulated

offset that should have been corrected previously. The accumulated error is

then multiplied by the integral gain and added to the controller. The

magnitude of the contribution of the integral gain to the overall control

action is determined by the integral gain Ki.

The integral term is given by:

(4.2)




where

UI: Integral term of output

Ki: Integral gain, a tuning parameter

e: Error = Set Point – Process Variable

τ: A dummy integration variable

The PI controller output voltage is expressed as:

(4.2)

LabView has been built to determine and show the online variations of the

generator power, frequency and voltage against load variations on graphs.

See Figure 4.6




Figure 4.6: LabView front panel chart

Figure 4.7 shows the horizontal pointer slides for adjusting proportional

gain, integral gain and generator speed.

Figure 4.7: LabView front panel pointer slide




Figure 4.8: shows the connections of the block diagram developed in

LabView.




Data acquisition (DAQ) NI USB-6215 has been used. It has 16 analog inputs

(16-bit, 250 KS/s), 2 analog outputs (16-bit, 250 kS/s), 4 digital inputs & 4

digital outputs and two 32-bit counters. Figure 4.9 shows the DAQ used and

the terminals [7].




Figure 4.9: (DAQ) NI USB-6215

The output voltage has been converted, using a transformer, rectifier and

regulator, to be within the DAQ capability range which is from -10 to 10V.




For the frequency reference, the motor is equipped with an internal

tachometer provides output voltage proportional to the motor speed in range

of 0-10 V.

The learning module has been constructed to hold the converting equipments

for DAQ and to provide signals for frequency, torque and voltage indicators.

Also, it has been provided with a current transformer to measure the currents

through the DAQ.

The front panel design is shown in Figure 4.10




Figure 4.10: training unit front panel

Testing and Validation

Refer to Figure 4.4 following procedure are used to explore the final result:

STEP 1:

The prime mover is started while contacts A & B are open.

STEP 2:

The buck chopper is fed by 120 V then the speed is adjusted to 1500 rpm (50

Hz) through the prime mover controller reference.

STEP 3:

Same procedure of step 2 is done for the exciter to adjust the terminal

voltage to 100V.

STEP 4:

The controller parameters are fine tuning in order to obtain acceptable

system response.

STEP 5:

While the system run at steady state (f=50Hz, Vt=100v), contacts A are

closed in order to connect a Y-connected resistive load to generator

terminals with R=240Ω. This step simulates P.

STEP 6:




Once the system reaches its steady state, contacts B are closed in order to

show an additional disturbance.

The system frequency decreases then returns to rated value. The prime

mover power is automatically increased by the AGC to match the speed with

the electrical power demand allowing frequency to remain within standard

limits.

The same observation is noticed on the generator's terminals voltage. The

AVR acts on the excitation current to compensate for voltage drop due to

load variation.

Figure 5.1 shows the results




Figure 5.1: output result

Conclusion and future workIn this project we have developed an AGC training system. The training

system include both hardware and software modules. The system allows

student to investigate both LFC and AVR. Then combine the LFC and AVR

to simulate a single area AGC. The proposed system in found to be user




friendly. Since it LabView based control module and simulink/Matlab

simulation module. The proposed system can be upgraded to:

Include an analogue power, voltage and frequency display unit.

Support a multiarea AGC experiment. Support a multiarea AGC with optimal dispatching unit.




References

[1]kkhttp://www.adit.ac.in/AditJournal/pdf_dec_2007/AUTOMATIC%20POWER%20G

uuuu ENERATION%20CONTROL%20AND%20SIMULATION.pdf

[2] Hassan Bevrani “Robust Power System Frequency Control”

[3] www.osii.com

[4] http://www.acsatlanta.com/pages/ems_agc.html

[5] Hadi Saadat, “Power System Analysis”, Second Edition, International

[6] Jubail Industrial College “EE414-Power Electronics II”

[7] http://sine.ni.com/nips/cds/view/p/lang/en/nid/203091


http://sine.ni.com/nips/cds/view/p/lang/en/nid/203091

http://www.acsatlanta.com/pages/ems_agc.html

http://www.osii.com/

http://www.adit.ac.in/AditJournal/pdf_dec_2007/AUTOMATIC%20POWER%20G%20%20%20uuuuENERATION%20CONTROL%20AND%20SIMULATION.pdf

http://www.adit.ac.in/AditJournal/pdf_dec_2007/AUTOMATIC%20POWER%20G%20%20%20uuuuENERATION%20CONTROL%20AND%20SIMULATION.pdf

Designing a Lab View based Automatic Generation Control Training System

Documents

Transcript of Designing a Lab View based Automatic Generation Control Training System