Ch 9 - l Controls Excitation

8/20/2019 Ch 9 - l Controls Excitation

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GENERATOR CONTROL AND PROTECTION

Supplemental Controls


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Outline• Basic Control Functions

• Supplemental Control Functions

– Take Over (HV/LV Gates)– Summing Point

• Reactive current compensators

• var/power factor controllers

• Limiter functions– Over / under excitation

– V/Hz

– Stator current– Field temperature limiters


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Basic Control Functions• Basic control functions “Manual” or

“Automatic”• Manual: operator control of field excitation

directly

• Automatic: maintaining generator terminal

voltage at a predetermined level without

operator action


4/40


Supplemental Control Functions• Reactive current compensators

• var/power factor controllers• Limiter functions

– Over / under excitation– V/Hz

– Stator current

– Field temperature limiters


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Supplemental Control Functions• There are two basic ways a supplemental

control function can interface with theAVR, take over and summing point

• Take over utilizes either a “High Value” or

a “Low Value” gate


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Supplemental Control Functions• The HV (LV) gate selects one of two

inputs, based on which signal is higher(lower), and provides it to the output of the

gate.

IN 1

IN 2

HV

GATE

LV

GATE

IN 1

IN 2

OUT OUT


7/40


Supplemental Control Functions

• A summing point implementation adds the

supplemental control signal into the AVRssumming junction or at some other point in

the AVR control path

ΣΣΣΣ

Vsupplemental input

Vsense

Vref error signal

output

+

-

+


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Static Exciter showing both Summing

Point and Take Over Style SupplementalControl Functions

Voel

HVGATE

LVGATECONTROLLERHVGATEVref

Vuel

Vsense

Vpss

Vuel

Voel

ΣΣΣΣΣΣΣΣ

+

+

Vpss

-

Vuel

+

-

OUTPUT+

+


9/40


Reactive Current Compensators

• Reactive droop compensation is used to

facilitate reactive current sharing betweenmachines tied in parallel

VC = | V

T +/- I

T (R

C + jX

C) |

VT

VC

IT


10/40


Var/PF Controllers

• Supervisory controls that act to modify the

reference to the AVR in order to maintainreactive power or power factor at a

predetermined value.

• Some implementations interface to the

AVR through the reference adjuster (IEEE

type 1)• Others feed a continuous signal into the

summing junction (IEEE type 2)


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Var/PF Controllers

• Reference adjuster: A device or control

function that modifies the Vref input to theAVR via raise and lower command inputs

• Older excitation systems - motor operated

control (MOC) or motor operated

potentiometer (MOP)

• Modern digital excitation systemsimplement this functionality in software


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Var/PF Controllers

• A type 1 var / power factor controller

implements the control in much the sameway as the operator would, implementing a

“dead band” where no control action

occurs, and an intermittent or continuousraise / lower command.


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Type 1 - Var/PF Controllers


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Var/PF Controllers

• A type 2 var / power factor controller adds

a signal into the AVRs summing point,providing continuous correction signals.

The PI style of control is normally adjusted

for slow response so it won’t impact theAVRs ability to control excitation during

system fault conditions.


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Type 2 - Var/PF Controllers

KIs

ΣΣΣΣ

Q

VVAR

+

+

To AVRsumming

pointVCLMT

VCLMT

-VCLMT

-VCLMT

QREF KP

ΣΣΣΣ+

-

EXLON

0


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Limiters

Limiters are used to limit generator operation

to the area bounded by the capability curve of

the machine. Q

P

StatorCurrent

Limit

Over

ExcitationLimit

Under

ExcitationLimit


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Over Excitation Limiter

A control function that limits the field

current of a synchronous machine or

excitation equipment to permissible

values with regard to thermal overload.

Action may be immediate or timedelayed. See field current/voltage limiter.

Also called a maximum excitation limiter.

(IEEE421.1)


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OEL

• Designed to protect the field from damage due toexcessive heating

• Field thermal limits defined in ANSI/IEEE C50.13• Allowable field voltage versus time

– 208% for 10 seconds

– 146% for 30 seconds– 125% for 60 seconds


• New standard will be defined with equation


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Rotor Heating Curve

0 10 20 30 40 50 60 70 80 90 100 110 120 100

110

120

130

140

150

160

170

180

190

200

210

Time (sec.)

E x c i t a t i o n

l e v e l % o f

r a t e d


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OEL

• Steady state limit value can be a function of

cooling air temperature, hydrogen pressure

or other parameter

• Field current measurement techniques

– Field shunt– Saturating field current transducer

– Hall Effect device

C bili C f S h


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ARMATUREWINDING

HEATING

LIMITATION

Capability Curve of a Synchronous

Generator+1.0

-1.0

1.0 1.2 1.4

+0.8

-0.8

0.8

+0.6

-0.6

0.6

+0.4

-0.4

0.4

+0.2

-0.2

0.2

0

0

KILOWATTS PER UNIT

L E A

D I N G

L A G G I N

G

R E A C T I V

E P O W E R

P E R

U N I T

FIELD WINDING

HEAT LIMITATION

-3V t2/X

s


22/40


Summing Point OEL Model

KI

s

ΣΣΣΣ

IOEL_REF

0.00833 KG

VREF

IFD

VOEL

ΣΣΣΣ +

+

+

-

ΣΣΣΣ

VREF

VC

To PIDController

0

0

VREF

-

+

-


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Under Excitation Limiter

A function that either overrides the voltageregulator action (takeover type) or adds to

terminal voltage setpoint (summing type), tomaintain synchronous machine excitation suchthat synchronous machine output remains abovea preset level. Various terms have been applied,

often descriptive of the measured variable;minimum excitation limiter, underexcitedreactive limit, and rotor angle limiter.

(IEEE421.1)


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UEL

• Prevents operation which jeopardizes

stability or could lead to loss of synchronism

• Designed to protect the armature core end

iron from damage due to excessive heating

• Prevents loss-of-excitation relays fromoperating


25/40


UEL

• Two styles, Summing Point or Take Over

– Summing point works through the AVR by

adding a signal into the summing junction

– Take over style uses a high value (HV) gate to

select the higher of the two control functions,

either the AVR or the limiter. The limiter acts

to override the normal action of the AVR


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UEL

• Summing Point

– Summing point adds a signal into the summing

junction of an AVR

– Provides smooth transition in and out of limiter

– Does not cause integrator windup– Does not provide direct control of field current

– Coordinates with PSS


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UEL

• Take Over

– Provide signal to high value gate

– The higher of two signals is passed to output– Transition in and out of limiter can cause

discontinuities in excitation

– Can cause integrator windup

– Can cause system instabilities due to removal ofPSS stabilizing signal when in limit

– Provides direct control of field current



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Generator

ARMATURE

WINDING

HEATING

LIMITATION

+1.0

-1.0

1.0 1.2 1.4

+0.8

-0.8

0.8

+0.6

-0.6

0.6

+0.4

-0.4

0.4

+0.2

-0.2

0.2

0

0

KILOWATTS PER UNIT

L E A D I N G

L A G G I N G

R E A C

T I V E P O W E R

P

E R U N I T

ARMATURE CORE

END IRON

HEATING LIMITATION


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Circular UEL Implementation

VUC

= |KUC

VT - jI

T| VUELΣΣΣΣ

+-

KUL

+ KUI

/ s

VUR

= |KUR

VT |

VT

IT


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Circular UEL Characteristic

Q

P

Under Excitation Limit

KUC KUR


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Volts per Hertz Limiter

A function that acts to prevent the ratio of

terminal voltage to frequency from

exceeding a preset level. The purpose isto prevent excessive magnetic flux in the

synchronous machine and connectedtransformers. (IEEE421.1)


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Regulator Volts/Hertz Characteristic

1 pu V/Hz

VT

f

VNOM

fCORNER

=fNOM

1.05 pu V/Hz


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Volts per Hertz Limiter

+

-K

VHZ Σ

-

Corner

frequency

generatorfrequency

AVRSumming

PointΣ+

-

0

1.0

VREF

VC


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Stator Current Limiter

A function that acts to prevent the stator

current from exceeding a preset value. If

the generator is operating overexcited, thelimiter will decrease excitation, while in

underexcited operation the limiterincreases excitation. (IEEE421.1)


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SCL

• Designed to protect the armature from damage

due to excessive heating

• Armature thermal limits defined in ANSI/IEEEC50.13

• Allowable armature current versus time



– 130% for 60 seconds– 116% for 120 seconds



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ARMATUREWINDING

HEATING

LIMITATION


Generator+1.0

-1.0

1.0 1.2 1.4

+0.8

-0.8

0.8

+0.6

-0.6

0.6

+0.4

-0.4

0.4

+0.2

-0.2

0.2

0

0

KILOWATTS PER UNIT

L E A D I N G

L A G G I N

G

R E A C T I V

E P O W E R

P E R

U N I T

RATED PF

LAGGING

0.95 PF

LEADING


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Field Temperature Limiter• The temperature of the stationary winding of a generator

may be measured with some form of a temperature-measuring device, like a thermocouple or a Resistance

Temperature Detector (RTD).• The temperature of the field winding, on the other hand,

is much more difficult to measure because it is revolvingon the shaft.

• One method of calculating the field temperature is tomeasure the effective “dc” resistance of the field andcomparing it to the value at a known temperature

(normally 25°C).


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Field Temperature Limiter

• Using the following formula, you can

calculate the field temperature.

( ) 5.2345.234 −+= COLDCOLD

HOT

HOT T R

R

T


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Summary• Basic Control Functions

• Supplemental Control Functions– Take Over (HV/LV Gates)

– Summing Point

• Reactive current compensators

• var/power factor controllers

• Limiter functions– Over / under excitation

– V/Hz

– Stator current

– Field temperature limiters


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QUESTIONS?

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Ch 9 - l Controls Excitation