ML17054A303

Introduction to Basic Elements of Control Sys for Large Steam Turbine Generators

ML17054A303
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Site:	Nine Mile Point
Issue date:	12/22/1983
From:	Eggenberger M GENERAL ELECTRIC CO.
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Text

INTRODUCTION BASIC ELEMENTS CONTROL SYSTEMS FOR LARGE STEAM TURBINE

~ GENERATORS by M. A. Eggenberger LARGE STEAM TURBINE GENERATOR OEPARTMENT I'

8312280485 8ggggg PDR ADOCK 050004l0 A

PDR GENERALO ELECTRIC

INTRODUCTION TO THE BASIC ELEMENTS OF CONTROL SYSTEMS FOR LARGE STEAM TURBINE-GENERATORS Mechanical-hydraulic systems

~. ~....

Eiectrohydraulic systems by M. A. Eggenberger I ~'

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gl CONTENTS Page INTRODUCTION

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5 MAJOR FUNCTIONS OF THE BASIC ELEMENTS OF CONTROL SYSTEMS 5

The Transducer The Summer The Differentiator The Integrator The Amplifier Overriding Devices (Gating)

The Function Generator 0

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5 7

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fI' REHEA'T TURBINE SYSTEM

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System Operation System Representation.................

Mechani'Cal Control System..............

Operating Speed/Load Control System Auxiliary Control Functions and Protection Power Supply...

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16 17 17 17 18 19 Electrohydraulic Control System (Mark I)

Speed Control Unit.........

Load Control Unit Load Reference......

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'ull-arc/Partial-arc Admission Power/Load Unbalance First-stage Pressure Feedbabk Load Limit Initial Pressure Limiter (IPL)

Intercept Valve Limit Valve Flow Control Units Operating Control System....

Power Supplies (Mark I).....

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Protection System (Mark I)...

Trip Anticipator (T.A. )

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'9 19 19 22 22 22 22 23 23 23 23 24 27 28 29

CONTENTS (Cont'dj ONCLUSION

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C Page

'29 APPENDIX I.

THE IMPEDANCE CONCEPT 30 APPENDIX II. SPECIAL FEATURES OF THE MARK-IISYSTEM

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31 Operational Amplifiers Relays and Logic Systems Power Supplies.......

Standby Mode of Operation Protection System.....

Cabinet Mark II Conclusion

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31 31, 32 32 32 33 33 ACKNOWLEDGEMENT 33 RECOMMENDED,LITERATURE

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35

INTRODUCTION A great deal of ingenuity has gone into the early designof mechanical-hydraulic control systems, which do most of their "thinking" by means of me-chanical components.

Common sense andengineer-ing intuitions were the most important ingredients in the design of these pioneer control systems.

In.recent years we have gained much more exact information on the process with which the devices accomplish their functions.

We have learned to ex-press their performance mathematically and to iden-tifytheir inherent limitations inmore precise terms.

We have also found that the expressions used to de-scribe mechanical systems have a complete set of analogs in electrical systems, Withincertain reas-onable limitations either one of the systems can do the same thing as the other one, ifwe are immune to complexity.

However, immune to complexity we are not.

As the control tasks become more complex we willsteer in the direction of the less complex system that ac-complishes the specified functions.

The picture of the complexity of the control task to be fulfilled is shown in a general way in Fig, 1.

The analog electrical system is more complex to start with (vre need an operational amplifier to add with minimum error), but its complexity in-creases more slowly; (if we want to add one more variable, we just need one more resistor).

The digital electrical system is still more com-plex for simple tasks, (we need computer elements);

but adding very complex functions is relatively easy, (we merely need to program them).

Fluidics, a fourth principle developed recently, appears to have high capabilities with a relatively moderate complexity of the solution, but practical experience is limited at this time.

Assuming that the curves in Fig. I representthe general picture sufficiently well, we could draw the conclusions that the simple control system would most likelybe mechanical, the system forintermedi-ate complexity would be predominantly analog elec-trical, and the highly complexsystemwouldprefer a

digital electrical solution.

Our present reheat turbine control systems would probably fall in the category of intermediate com-plexity.

MA30R FUNCTIONS OF THE BASIC ELEMENTS OF CONTROL SYSTEMS i

~ ~

PLEXITY

'F SOLUTION I

I I

MECH.

ANAtDGE 0

q,W

~etc pIGL'Tt'A<

EI 'EC I

I I

DIGITAL ELEC.

The basic elements of a control system can all be called computing elements and their functions can always be expressed in equations.

They can be classified according to their func-tions as follows:

COMPLEXITY OF CONTROL PROBLEM Fig. I ~

General philosophy of control systems In Fig. I:

The abscissa is the aontplexity of the problenI that has some relation to the number and the nature of the controlled variables and the neededperform-ance (steady-state accuracy and transient deviation).

The ordinate is the complexity of the solution that has some relation to the cost, the effort of ad-justment, and the effort of maintenance ofthe entire control system.

The ntechanical system starts out with the least omplex solution for a very simple task (a floating ever is a very simple summer), butas the problem gets more intricate, the complextty of the soiution increases rapidly.

~ Transducers

~ Summers

~ Differentiators

~ Integrators

~ Amplifiers (multiplies a variable with a given constant)

~ Overriding devices (gating)

~ Function generators Any one device may perform several of these functions simultaneously.

The functions of these devices are explained on the following pages, with typical examples for me-chanical and analog electrical systems.

1.

THE TRANSDUCER A transducer measures a certain quantity and produces an output that has a given relation to that quantity, probably including some limits.

A mechanical lransducer for rotational speed is the flyball governor shown in Fig. 2.

characteristic at rated speed a is used to compute Kl as:

POSITION (OUTPUT)

LIMIT

$ X A7%V LIMIT (2)

PERMANENT MAGNET ROTOR

= tna l

dn at rated speed An electrical transducer for rotational sPeed is a simple permanent magnet (a-c) generatoroperat-ing a high impedance load (Fig. 4).

SPEED SPEED (INPUT)

Fig. 2.

Mechanical speed governor Fig. 4.

Permanent magnet generator speed transducer The approximate transfer function of this speed transducer is:

The characteristic of the permanent magnet gener-ator is for practical purposes linear, Fig. 4a.

The transfer function of this speed transducer is X = Kl dn Actually, this particular speed governor is a func-tion generator with a characteristic as shown in

~

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Fig. 3.

Interpreting equation (1) on Fig. 3 points out the process of linearization, whereby the slope of the Er E()=Kn (I

)

LIMIT 0

n I

Fig, 4a.

Characteristic of permanent magnet generator speed transducer

+

I (UNITS)

I LIMIT I dn

)

K,= tna Another type of electrical sPeed transducer uses a tooth wheel and a magnetic pickup that sends pulses to a frequency-to-voltage converter which transforms the pulses into a d-c voltage propor-tional to the frequency of the pulses, Fig. 5.

MAGNETIC PICKUP FREQUENCY-TO-E(pc)

VOLTAGE CON VERTER l

0.8 0.9 I

I.O5 I. I SPEED n (UNITS)

TOOTH WHEEL Fig. 3.

Characteristic of mechanical speed governor

~

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Fig. 5.

Magnetic pickup speed transducer

This speed transducer is very linear downto ap-oximately five percent of rated;speed.,

Its trans-r function is, therefore, simQar to equation (3).

E(DC) = K "n (3a)

A mechanical pressure transduc'er is a spring-loaded bellows, Fig. 6.

The transfer function of this pressure trans-ducer is AV=K4 4P (6) where:

P = input pressure V = output voltage K4 =

= transducer gain AV 0

O00 0000 6

Transducers ofthis kind are usually very linear down to almost zero pressure.

Mechanical vibration can produce undesirable noise in the transducer output.

The mountingof the transducer must be done with this noise problem in mind.

A t

~

Fig. 6.

Mechanical pressure transducer (low pressure)

The transfer function of this pressure trans-ducer is The LVDT is actually a position-measuring de-vice that can be used in such applications as the measurement of valve positions.

2.

THE SUMMER A summer is a device that performs the summa-tion of two or more quantities. Ifany of these quan-tities is added with a negative sign, the operation amounts to a subtraction (computingof a difference).

The added quantities can be either variablesorcon-stant values.

bX = hP~A where:

P = input pressure A = effective bellows area G = system spring gradient (4)

In most cases the summer also multiplies each variable with some constant value before adding it.

The simplest mechanical summer is a "floating lever", Fig. 8.

An electrical Pressure transducer can be in the form of a Bourdon tube, operating a linear variable differential transformer (LVDT), (Fig. 7).

This transducer actually transforms pressure into me-chanical motion (X) with small forces and this mo-tion is changed to an electrical signal by the LVDT and the demodulator.

Fig. 8.

Mechanical summer (Hoating lever)

The following equation applies to Fig. 8.

EMOOULATO 2=X b

+Y '6) e~x ~LVOT In the particular case whexe a=1, the sum becomes z=~

X+Y (6a)

Fig. 7.

Electrical pressure transducer (high pressure)

The result of this summation will be reasonably accuxate only ifthe tiltof the lever is not excessive (30o should not be exceeded).

The addition of three quantities canbe performed echanically by using a triangular plate, Fig. 9.

X Fig. 9.

Mechanical summer for three variables (wobble plate)

If the plate is an equilateral triangle, the sum 2 is (7)

Note that the joints with which the rods w, x, y and z are connected to the plate must beball joints with two degrees of freedom.

A larger number of quantities can be added me-chanically by breaking the addition into severalpar-tial additions V + W + X + Y = (V + W) + (X + Y)

(8)

The following points should be noted and well re-membered for the understanding of the subsequent material.

1. The output of the operational amplifier is al<<

ways of reversed polarity.

(It can be said that a multiplication with -1 is inherent in this summer.)

2. The gain with which each input signal is added is proportional to the feedback resistance (Rf) and inversely proportional to the respective input re-sistance (Rl, R2p

~

Rn).

3. The possible number of inputs is almost un-limited.

4. The summing junction can be consideredtobe

'substantially at ground potential because of the 'ex-tremely high gain of the amplifier.

5. The simple resistors Rf, Rl... Rn canbe re-placed by any other kind of impedances (Z) in order to produce almost any desired transfer function of the circuit, Fig. 11.

Zf and performing the addition in several steps using floating levers to add each group of two quantities.

'The mechanical summation of more than three variables is usually difficultbecause of conflicting solutions to the problems of friction and backlash.

Electrically the summation of d-c voltages is performed by means of a highgaind-camplifier (A) called "Operational Amplifier", represented by a piece-'of-pie-shaped wedge, Fig. 10.

FEEDBACK Rt El Zi En n

Fig.

11.

Electrical summer (with impedances)

Eo INPUTS E

RI Ra E

Rn n

Eo OUTPUT SUMMING JUNCTION Here the output voltage is:

Zf Zf Zf Eo = -

E. + E2 ~ +...

E 1 Zl r2

Zn See Appendix for impedance concept.

(10)

UBSTANTIALLYGROUND r~

POTENTIAL Fig, 10.

Electrical summer (with resistors)

Here the equation for the output voltage is Rf Rf Eo El

+E2R

~ +E R1 R2 (9)

An impedance is in general an element in which the magnitude and the phase relation of the current with respect to an impressed voltage are functions of the time variations of the impressed voltage.

In most cases a combination of resistors (R:

~~) and capacitors (C: +4

) is usedto pro-duce impedances of different character.

However, it is possible to use inductances (L:~) as well.

In order to express impedances mathematically, t is necessary to use the complex representation means of the Laplace operator "s". For a sinu-idal input voltage the Laplace. operator can be in-terpreted as s =jeI gc RI RI Z = 2R (1 +~)

Rl 1+ (R2+~)C s (18) where and e = angular frequency (of impressedvoltage) in rad/sec The impedance can now be evaluated as follows:

R

1. Resistor Z=R 3.

THE DIFFERENTtATOR The rate of change of a certain variable can be measured by differentiating its value with respect to time:

The rate of position (X) change is a velocity:

The impedance of a resistor is notfrequencyde-pendent.

dX v

dt (20) c

2. Capacitor HE 1

1 Z==

C s Cjcu (13)

Actually a coil willalways have a finite resist-ance, so that the impedance ofthe real coil would have to be represented by The impedance of a capacitor is ~at to= 0 and zero at u ~.

I.

3. Inductance llmP-(ideal coil)

Z = sL= jcuL (14)

The impedance of,the ideal coil is zero at zero frequency and infinityat infinitelyhigh frequency.

The rate of a rotational speed(n) change is an angu-lar acceleration dn~-a (21)

(p)= sx(,)

or the rotational acceleration of equation (21) sn(s)

(22)

(23)

A dashpot, a meciranical device that acts very much like a differenlialor (at low inputfrequencies) is shown in.Fig.

12.

A differentiation of a variable canalsobeexpressed by the Laplace operator "s" (see Appendix), for in-

stance, the velocity of equation (20)

Z = R+ jeIL (14a) and would have the value of R for co = 0.

A selection of typical transfer impedances is:

R I.

C 1

Z = R+sL+-

Cs (15)

RI R

~1+R s

(16)

O O

O O00 Rz C.

1+R2Cs 11+

1+

2

....;...:..;<<....;...e...

(1V)

I'requency) 9.

e transfer function of this device fs

~YJ Ta K(s)

(+ Ts lt can be seen that, as long as Ts<<

1 (25)

(25a) the operational amplifier will saturate and give a wrong output.

This willhappen when the input sig-nal El is noisy.

Dffferentfators must be handled with a good analytical knowledge.

In general, it is not advisable to have an input capacitor connected directly, to the summing function of an operational amplifier.

or Y

= Ts

{Ts <<1)

{s = )o(- very small) the value of + is close to Y

4.

THE INTEGRATOR An integrator is a device that integrates the value of a variable with respect to time.

A good example of a mechanical integrator is the combination of a (25b) pilot valve and a piston, Fig. 14.

or Y{s) = T X s Y(t) = T~

dX (25c)

(25d)

X INPUT PILOT VALVE OUTPUT T is the time constant (sec) associated with>the dashpot configuration.

An electrical differentiator can be built with an operational amplifier using a capacitor as the input impedance, Fig. 13.

OIL PRESSURE (AUXIUARY POWER)

IIII IIII Fig 14 Mechanical integrator SUMMING JUNCT(ON Fig.

13.

Electrical differentiator The impedance of the capacitor C is (see equa-tion (13))

1 Zl Cs (26) and the transfer function of the cfrcuit is, assuming a perfect amplifier From this illustration it can be seen that ifthe pilot valve is lifted by the amount Xl above the neutral position, the piston will start moving im-mediately and keep on traveling at a given speed until the pilot valve is returned to the neutral posi-tion, or until the piston reaches a stop (which fs also called saturation).

The particular meaning of the term "integration" can be seen in Fig. 15.

Expressed mathematically, the integration per>>

formed in Fig. 15 is t

Y(t) = K j X(t) dt (28) 0 or E

=RfCs El

{2'i) v x

(2 ib) 10 Eo{s) = - Rf C s El

{27a) or dE1 0

E'o(t)

" Rf C This fs a pure differentiator at all frequencies g(d) of El but if the differential becomes too bfg, XI>>

aV( KX(d(

XI()

dt 0

I Y I()I I

I I

Fig.

15.

Integration

sing the Laplace operator s, this canbewritten as KX( )

(4)

(29) or El tt(44) 5(((:( 44 (32)

The operator s in the denominator signifies an in-tegration.(See Appendix).

(32a)

K is called the gain of this integrator.

It is a constant containing the fixed physical parameters such as oil pressure, port width, piston area and flow coefficients.

By rearranging equation (29) we can immediately see that the rate of change (the linear speed) of Y is proportional to X:

(s)

(s)

This simple example points out how the simplifica-tion of the control language using the Laplace oper-ator s works.

We can also see that the piston (Y) willkeep on moving until X becomes zero, which is one of the basic qualities of an integrator (as long as it is not saturated).

Similar to the mechanical integrator, 1

is called Rllf the gain of this integrator and its dimension is It is the rate at which Eo changes with one 1

sec

'oltat the input (El) (until saturationoccurs).

5.

THE AMPLIFIER Amplifiers cover a wide range of devices basic-ally intended to increase the level of a signal to a higher level in magnitude or in force, or in voltage or current --- in short, to transform a signal in a predetermined way to a higher level.

Mechanical strofre amPli%'er.

A simple example is a lever, Fig. I'I.

Other examples of integrators are the turbine shaft of which the rotational energy is the time in-egral of the sum of all torques applied to it, or the ssure in a steam vessel that is the time integral the algebraic sum of steam flows into the vessel flow out of.the vessel has a negative sign).

The electrical iutegrator can again be built with an operational amplifier, Fig. 16.

Ct Fig.

17.

Mechanical stroke amplifier Its transfer function is Y=KX= X b

(33) a This amplifier has no time lag; its gain K is the lever ratio.

It amplifies only the stroke, while the energy level of the output is substantially the same as the input.

MecfIanical-hydraulic a)nplifier.

The most con-mon one is the servomotor.

It uses hydraulic fluid under pressure for auxiliary power, Fig. 18, E,

Eo O

a b

d f Y Fig.

16.

Electrical integrator 1

Zf Cfs (31)

The impedance Zf of the feedback capacitor is again PRESSURE ~

FLUID FLOW RATE y(~"yizj PORT WIDTH:

WPIInI PISTON AREA:

APIlntI and the transfer function of the amplifier circuit is Fig.

18.

Mechanical-hydraulic power amplifier (servomotor)

he amplifier shown in Fig. 18 usually amplifies stroke and the energy level.;. the latter appre-ciably (about 1000:1)... and can be used to drive substantial loads.

The output (Y) follows a change in input (X) position with a time lag.

In this particular case the reversing lever in the feedback has been omitted so that Yhas the opposite direction of motion of X,'he transfer function of this servomotor is the same as equation (34) with The transfer function of this servomotor is b

K=--a (34c)

Y(s)

K X( )

1+Ts where K is the lever ratio:

(34)

Ap T =

(sec) gwp $

(34d) bd K"=

ac (34a)

T "-

(sec) ac

~a+

A step change of the input X is followed by a move-ment of Y as shown in Fig. 19. This response is de-scribed completely by the transfer function (34).

(34b) also called the steady-state gain, and T is the time constant of the servomotor (in seconds)

From this example it can be seen that sign re-versals in the chain of control components canvery easily be applied, and it is basically not important what a specific sign in the chain is.

The important thing is that the end effectof all sign changes makes the final element operate in the proper direction.

Occasionally the sign of intermediate steps can be chosen so that failure of a givenlink in the chain makes the system fail in the right direction.

If, for example, the connection at X in Fig. 18

breaks, the pilot valve willfall down and decrease Y which will close the valves, therefore constituting a failsafe system.

)00%

OF STEP I

(

)

GY(1 40)

If the connection at X in Fig. 20 breaks, and the pilot valve falls down, Y willincrease; ifthis would open the valves it would not be a failsafe system.

Fig.

19.

Response

of servomotor GX Therefore it can be said that the sign of inter-mediate control quantities can be significant in the failure analysis of the system.

Electrical amplifier.

Electrical. signals can be amplified in a number of ways.

A servomotor can also be built "single acting",

which means simply that the oil forces on one side of the piston are replacedby a strong spring, Fig. 20.

E =-E o

1R1 (35)

A <<olla'>amplifier where a sign reversal can be tolerated is the simple use of the operational amplifier, Fig. 21, where b

000 00 Wp Ap E)

R, Eo Fig. 20.

Single-acting servomotor l

Fig. 21.

D-c voltage ampli%er 12

The increase in energy level of this amplifier is significant.

Usually the amplifying function can be incor-porated in a device that performs other functions as well.

Occasionally, when a sign reversal is needed, this voltage amplifier is used as a reversing amplifier.

A current amplifier can be built with a transis-tor, Fig. 22.

Electrohydraulic ampli%'er.

An amplifier com-bining electrical and hydraulic components is used in electrical systems to drive substantial loads such as steam valves, Fig. 23.

While the transfer function of this amplifier is too complicated to be discussed here, itcanbe seen that the freedom of choice of Zl and Z2 makes it possible to design the loop so that itoperates under optimum conditions

{as shown in Fig. 24 for a step input change).

OUTPUT (Y)

T R ANS ISTOR Io 100 4/o OUTPUT STEP INPUT (EI) l00%

INPUT STEP Fig. 22.

Current amplifier This amplifier can produce a higher current that might be needed to drive the next stage.

It will change the voltage onlyslightlyanditwillnot change the sign.

ZI 0

Fig. 24.

Response

of electrohydraulic amplifier The new position is reached somewhat faster than on the mechanical system and the positioningaccur-acy is generally better.

INPUT EI (+)

R Et (-)

CURRENT AMPLIFIER TORQUE MOTOR~

OEMOOULATOR JET NOZZLE EXCITATION (~)

LYOT Y~ o OUTPUT H.P. FLUIO GRAIN PILOT STAGE RAM Fig. 23.

Electrohydraulic amplifier 13

OVERRIDING DEVICES (GATES)

An "overriding device" is a component of a me-'hanical or electrical system which makes a choice between two signals, causing only one of them to perform the controlling function.

either "low value gate" or "high value gate", de>>

pending on the'pplication.

A low valuegate circuit is shown in Fig. 2B.

EL(-)

RL In a mechanical system a typical example is the load limitwhich can call for the valves to be closed further than the position set by the speed governor.

The principle is shown in Fig. 25a for a single-acting relay (not exactly the speed governor load limit)and in Fig. 25b for a double-acting relay.

"(+)

E (y)

RI Rt EoH DIODE (ELEC. CHECK VALVE)

Xg(+)

P

( II IIII

, Fig. 26.

Electrical low value gate The voltage Ei, (-) can, for example, be pro-duced by a voltage divider (which is the case for the load limit) or by another control circuit, such as the initial pressure limitcircuit.

Fig. 25a.

Mechanical overriding device (single-acting relay, X, controlling

(~)

X(

(+)

Xa NOTE:

Because of the fact that these gating functions are all done in the electrical cir-cuits, there willbe no load-limit handwheel or IPR at the turbine front standard; these devices are all incorporated in the control panel and the control cabinet.

The output voltage Eo will be the less negative of either Ei. (-) or -El (disregardingthediode volt-age drop).

Df qD It is, of course, possible to give EI,(-) any num-ber of characteristics such as those used for run-backs or step-backs.

QD

(+) g Y J OUTPUT Fig. 25b.

Mechanical overriding device (double-acting relay, X, controlling)

One or both inputs can be either controlling sig-nals or manually or automatically adjusted limit signals.

Since there is noswitchingactionwhen one signal takes over from the other, the transfer is entirely smooth.

For the purpose of protection (trip devices) a series arrangement of mechanically, hydraulically, or electrically operated three>>way valves can be ed to obtain the desired trip action.

In electrical systems this function is called "gating" and the circuits that perform it are called 14 In the meclIanical system we use cams in the forward loop for the control valves, Fig. 27, CAM ROLLER 0

0 0

0 0

TO I

OPEN r

CAM Y j SHAFT (SERVOMOTOR OUTPUT)

CONTROL g

VAI.VE L (VALVE LIFT)

P (STEAM FLOW)

Fig. 27.

Mechanical function generator (cam-operated

, control valve) 7.

THE FUNCTION GENERATOR Function generators are used to compensate for the nonlinear flow characteristic of steam valves.

block diagram form this combination of func-enerators is represented in Fig. 28.

Y (SERVOMOTOR STROKE)

L (VALVE LIFT)

L STEAM FLOW Fig. 28.

Block diagram for corn shaft and valve function generators The effect of the series arrangementof these two function generators is an approximately linear re-lation between p and Y2, A function generator witha characteristic similar to the valve characteristic can be used in the feed-back of the pre-amplifier.

This is widely used on the intercept valve relay. that produces the input stroke Y'I to the intercept valve servomotor and compensates for the nonlinear flow characteristic of the intercept valve.

This feedback function gen-erator (cam) is shown in Fig. 29.

In Fig. 30:

Yl

= Intercept valve relay input X

= Pilot valve position (off neutral)

Yl

= Intercept valve relay output Y2

= Intercept valve lift ppf = Intercept valve flow In order to obtain an approximately linear rela-tion between the intercept valve flow (ply) and the input (Yl) the feedback function generator should have very much the same characteristic as the valve flow-liftrelation.

In the electrical system we use "electrical cams" in the feedback circuit of the valve positioning loop, Fig. 31.

YI (INPUT)

F)

O 0

O O0 0

O O

AMP.

LVOT OE MOD.

ULATCR Ee()

Re E,(t)

R, ErH Rq VALVECHARACT.

(ELECTR. CAMI SERVO VALVE RAM STEAM vALYE STEAM FLOW a KE, PRESSURE FLUID tx Fig. 31.

Electrohydraulic valve flow control with feed back function generotor 0 (OUTPUT)

This nonlinear feedback can be implemented as shown in Fig. 32.

Fig. 29.

Mechanical function generator in feedback (intercept valve relay)

YI INTERCEPT VALVERELAY X

YI' FEEOBACK CAM SERVO-INTERCEPT MOTOR VALVE Y'&~i P

I+Tg s Yi In block diagram form this intercept valve control is represented in Fig. 30.

-22V BIAS E (+)

RI GAIN Er(0 Rg INITIALSLOPE 0 6-

-22V OEMOOULATOR FINAL SLOPE BREAK POINT POWER AMPLIFIER To SERVO VALVE RAM POS(TION 30.

Block diagram of mechanical intercept valve flow control with feedback function generator Fig. 32.

Example of an electrical function generator in feedback circuit 15

The feedback circuit, Fig. 32, willapproximate valve characteristic by two straight lines, where-e transition from one slope to the other is gently rounded by the characteristic ofthe diode D, Fig. 33.

I00%

BREAK POINT FINAL SLOPE EO U

lO I->

~)

DIODE ROUNDING ACTUAL VALVE CH ARACTER ISTIC INITIALSLOPE INTERCEPT POINT (NEXT YALYE STARTS OPENING)

THE REHEAT TURBlNE SYSTEM The reheat turbine system we are concerned with is shown schematically in Fig. 34.

0 VALVE LIFT Fig. 33.

Approximation of valve characteristic by elec-trical function generator (two slopes)

SYSTEM OPERATlON The steam is admitted through main stop valves and subsequently through a set of control valves.

Each of these control valves admits steam through a different set of nozzle sections.

Since these sec-tions have been distributed around the periphery of the first stage, they divide the steam admission into partial arcs.

If only a portion of the control valves are open, steam is being admitted along a partial arc of the first stage rather than through all 360 degrees of the circumference.

This mode of operation is called "partial-arc admission."

During the startup process the thermal stresses can be reduced by symmetrically admitting steam to all nozzle sections of the first stage.

Turbine con-trol, up to a predetermined fractionof ratedload, is achieved by fully opening the control valves andcon-trolling the steam Qow by the stop valves. This mode of operation is called "full-arc admission."

During full-arc admission operation the speedof the unit can be controlledby some speed-controlling

means, either from auxiliary speed control equip-ment {mechanical control system) or by the main speed control {electrohydraulic system) acting on the stop valves.

STEAM GENERATORr I

I I

I OVERSPEED TRIP SPEED

) CO~~ROL HIGH PRESSURE TURBINE MAIN STOP VALVE (M.S.V.)

IN TERMED>>

PRESSURE TUR BIN'E LOW PRESSURE TURBINE GENERATOR

(

I' L

REHEATER CONTROL VALVES

( C.V.)

INTERCEPT VALVE (Z.V.)

REHEAT STOP YALYE (R.S.V.)

CONDENSER LOAD 16 Fig. 34 Reh'eat turbine flow diagram

Two independent lines of defense against exces-e speed are provided.'he first consists of the rmal speed control system operating the control alves and the intercept valves.

The second com-prises the overspeed trip system which closes the main and reheat stop valves onoverspeeds in excess of the preset trip speed and shuts the turbine down.

It is the task of the control system to use the previously described elements in order to control the four valve sets properly under any operating conditions and within the safety limitsofthe turbine-generator.

Sl +

S1-.S2 S2 is a summing point where the signal Sl and the sig-nal -Sp are algebraically added.

(s) 0 is a control function (transfer function F(s)) that ansforms the input signal I into the output signal and can either be described in words or as a SYSTEM REPRESENTATION A system is usually represented inblock diagram form where:

mathematical term (in most cases as a function of "s", the Laplace operator).

MECHANICAL CONTROL SYSTEM This system consists of an operating control system, supplemented by auxiliary controls andpro-tections.

Operating speed/toad control system For simplicity the operating speed/load control system shall be discussed separately... first,with-

'ut the auxiliary controls and protections.

This simplified system is shown in Fig. 35.

The speed reference (position of the speed/load changer pa) is compared to the speed (position of speed governor

$ ) and the difference (e) operates the speed relay, which is a small servomotor.

The speed relay signal (Us) is further amplified in the main servomotor (rt,) (see Fig. 18, equation 34) and produces a valve position through nonlinear cams (function generators).

The valves (also nonlinear) willproduce a steam flow (tv) Ulat produces the turbine flow ttsT) with a time lag of T3 caused by the bowl volume and drives the turbine with the fraction f (approximately 0.3) of its power in the high-pressure turbine and 1-f (approximately 0.7) in the intermediate and low-pressure turbine after the reheater time delay (TR) is taken into account.

Qo

~O O+

Cy

~O

~o e-4 CP O

0 O

Q V

~>

~O

~O

~O eO

~Q

~O

~O e~

e~

e~

e~

~~

ee o

I I

"j I

"a It'vrv z

I+Tj5 I+T 5 ~

I+Tss 4'tv jv rv A. D A. D

~e ~0 t

rHp

+sv

~O ~

I 4R I-f I+T s r IP ANOu>>

e ~>~o D

O rvQ +

e O

k

+

T tjT a

4.

ee e

z '

T45 CII Fig. M.

Black diagram of mechanical reheat turbine speed control 17

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(i Overspeed protections:

~ The speed(o) is compared to an overspeed trip reference speed (pT). If itexceeds pT the emer-gency trip system trips the stop valves and the reheat stop valves close directly.

Tripping of the bypass handwheel mechanism wiQ prevent re-opening of the stop valves untQ the operator has reset the bypass handwheel mechanism to zero.

~ On units originaQy shipped without stop valve bypass the load limitis tripped by the emergency trip system (dotted line).

Other protections:

~ Other trip signals that can shut the unit down through the emergency trip system include:

- low vacuum

- thrust-bearing failure

- generator protections

- high exhaust temperature

- excessive'ibration

- excessive differential expansion

- others.

ELECTROHYDRAULlC CONTROL SYSTEM MARK l The electrohydraulic control system has been organized into three major subsystems.

One pur-pose ofthese subsystems is to minimize interactions.

As shown in Fig. 37, the speed control unit com-pares actual turbine speed with the speed reference, or actual acceleration with the acceleration refer-

ence, and provides one speed error signal for the load control unit.

The load control unit combines the speed error signal with the load reference sig-nal and biases todetermine desiredsteam flow sig-nals for the main stop valves, control valves, and intercept valves.

FinaQy, the valve flow control units accurately position the appropriate valves to obtain the desired steam flows through the turbine.

Speed control unit The speed control unit, Fig. 38, pxoduces the speed error signal that is detexminedby comparing the desired speed with the actual speed of the tur-bine at steady-state conditions, or the desired ac-celeration with the actualacceleration during start-upo Trip Anticipator (T.A.)

+ Used on units withhighoverspeed on loss of load.

The T.A. closes the M.S. V. and the R.S. V. at a low enough speed to limit the overspeed to 20 percent in case of a failure of the C.V. or the I.V. toclose.

But if theunitdid not trip on over-speed;.it wiQre-openthe M.S. V. and the R.S. V.

again in time so that the I. V. can blow down the reheater and go back to no load rated speed on speed control.

Power supply The power supply for the hydraulicamplifiers of the mechanicalcontrol system is a 200 to 250 psi oil system supplied with oil from a shaft pump (auxil-iary motor-driven pump for startup).

The characteristic of the shaftpump is such that it can furnish large transient flow requirements with an acceptably small drop in pressure so that (in most cases) 'no accumulators are needed.

The emergency trip system is also supplied with pressure oil from the same system.

Power for electrical trip functions (remote trip, (i

thrust-bearing weax trip and.generator protections) is furnished by a 125 or 250 VDC station battery, When the desired speed signal is increased in a step, the acceleration control willtake over and ac-celerate theunitat the setrate up to the value of the new speed reference, where the speed control will take over again automatically.

Upon decxease of the speed reference, the unit wiQcoastdown with the valves closed.

They wiQ re-open only when the new set speed has been reached; there is no limitin deceleration.

During normal operation at rated

speed, the speed error signal is zero, regardless of load.

Because of the extreme importancein safeguard-ing against overspeed, the speed control unit has two redundant channels.

If both speed signals fail, the unit wQI shut down.

Load control unit The prime puxpose of the load control unit is to develop signals to which steam flow for the main stop valves, control valves, and intercept valves may be proportioned.

These outputs are based on a proper combination of the speed errorand load ref-erence signal modified by power/load unbalance, fuQ-arc and partial-arc transfer bias signals, the load limitand the initial pressure limiter.

The regulation is introduced to modify only the speed error signal; the input xepresenting desired I

19

SPEED REFERENCE ACCELERATION REFERENCE FULL-ARC PART;ARC TRANSFER.

LOAD REFERENCE L R LOADING RATE INITIAL PRESS.

LIMITER LOAD LIMIT INITIAL PRESS.

M.S.V. FLOW SIGNAL M.S.V, FLOW CONTROL UNIT M.S.V. FLOW ERROR SPEED CONTROL UNIT LOAD CONTROL UNIT C.V. FLOW SIGNAL C.V. FLOW CONTROL UNIT C.V. FLOW SPEED FIRST-STAGE PRESSURE I.V, FLOW CONTROL UNIT I V. FLOW SIGNAL I.V. FLOW Fig. 37.

Block diagram of electrical reheat turbine control system ACCELERATION OIFFERENTIATOR ACCELERATION g

ERROR ACCELERATION REFERENCE INTEGRATOR INTEGRATED ACCELERATION ERROR SPEED SIGNAL NO.I SPEED Z

SPEED SET SPEED ERROR ACCELERATION SET TRIP SIGNAL LOSS OF SPEED SIGNAL TRIP SPEED REFERENCE C I RCU IT SPEED REFER-ENCE ACCELERATION REFERENCE CIRCUIT LOW VALUE GATE SPEED ERROR SIGNAL SPEED SIGNAL N0.2

. SPEED SPEED ERROR BIAS ACCELERATION REFERENCE ACCELERATION INTEGRATED ACCELERATION ERROR 20 DIFFERENTIATOR ACCELERATION ERROR BIAS Fig. 38.

Electrical speed control umt I NTEGRATOR

SPEED REFERENCE Z

REGULATION I.OAD REFERENCE VALVE OPERATORS AND STEAM VALVES LOAD TORQUE POWER TORQUE ROTOR INERTIA SPEED sPEED SIGNAL SPEED ERROR Fig. 39.

Electrohydraulic control system: speed control loop load is added after this point.

Figure 39 is a basic block diagram of the speed/loadcontrol loop of this system.

It can be seen that the load reference signal needed to produce a certain load at constant speed is independent of the speed regulation.

This means that for a given load reference

change, the load change willbe of the same magnitude regardless of the speed regulation that can be set from 2.5 to 7 percent (required by IEEE 122 standard).

Therefore, it is now possible to calibrate the load reference dial in percent of full load at rated speed and maintain this calibration regardless of what the regulation is going to be.

Figure 40 is the basic arrangement of the entire load control unit.

LOAD REFERENCE L R POWER-LOAD LOAD GEN.CURRENT UNBALANCE REFERENC REHEAT PRESSURE LOADING RATE FULL-ARC, PARTIAL-ARC TRANSFER BIASES LOAD INITIAL INITIAL PRESS.

LIMIT LIMITER PRESSURE MODIFIER SELECTOR SPEED ERROR SIGNAL MSV. REG C.V. REG OUT IN ig/

I GAIN ADJ LOW MAIN STOP VALVE ALUE FLOW SIGNAL LOW CONTROL VALVE FLOW SIGNAL I

I.V. REG FIRST-STAGE PRESS.

ADJ LOW INTERCEPT VALVE VALUE GATE FLOW SIGNAL Fig. 40.

Electrical load control unit 21

he followingare the operating features-of the control unit.

When the speed increases over rated speed, the negative speed error will decrease the flow signal of the controlling valves at a rate defined by the particular valve regulation.

For example, in par-tial-arc operation, if the control valve regulation were five percent, a five percent speed error wouM cancel a 100 percent load reference signal.

This wouId result in reducing the control valve position from the fullyopen to no-load position, power load unbalance relay initiates this action simultaneously with the start of the speed increase.

The power-loadunbalancerelaycompares reheat pressure (a measure of steady-state power) with generator current (representing generator load).

It also determines if the currentdecrease is occur-ring rapidly (in less than 0. 05 sec. ). If the unbal-ance is over 40 percent and the current has de-creased rapidly the following actions are initiated:

1. The f'ast closing of the control valves is in-itiated directly.

Load reference The calibrated load reference can be set locaQy or remotely by jogging a push button switch, or automatically by a dispatching or computer system.

The rate-of-load increase is adjustable in discrete steps, thereby providing the. feature of programmed automatic. loading. Typical rates are 0. 5, 1,

3 and 10 percentper minute. Adecrease in load reference settings is followed substantially without delay, Since the load reference is in effect a speed vernier adjustment, it is used for synchronizing the turbine.

Full-arc/partial-arc admission Biases are used to hold any noncontrolling valves

~

~

~

~

~

he wide-open position.

When the unit is started control valve bias calls for wide open control ves and the main stop valves are controlling (M.S.V. bias = 0).

When transferring from full-arc to partial-arc operation, it is first necessary to remove the con-trol valve opening bias and then to apply an opening bias to the main stop valve.

By properly program-ming the shifting of the transfer biases, both load variations and thermal stresses willbe kept within acceptable limits. Although transfer is performed automatically at preselected valve openings, it is possible for the operator to modify or override the automatictransferaction, The bias to the intercept valve is proportioned so that on slow acceleration the intercept valve will, in any mode of operation, start closing just after the controlling valve set'as come to the closed position.

2. The load reference signal is switched to zero by the "MODIFIER"such that the slightest speed in-crease willstart closing the intercept valves.

3. The load reference motor is run back toward zero at full speed.

4. The automatic transfer to F.A. admission is temporarily inhibited until the reheater pressure has come down to approximately 10 percent of rated such that the transfer operation will not interfere with the blowing down of the reheater by the inter-cept valves.

Generator load current (instead of power = volt-age x current) is used to'avoid unnecessary closing of the valves by a clearable short circuit on the transmission system.

Inhibitingthe action in a case where the current is decreasing more slowly than the set rate avoids actuating of the power-load un-balance relay during the sometimes substantial power swings that follow clearing of a short circuit in the transmission system.

The power-load unbalance logic is shown in Fig.

40a.

First-stage pressure feedback The lift/flowcharacteristic of any steam valve is highly nonlinear.

A first order linearization is made in the valve positioning loops, as willbe seen

later, In order to refine the linearization in nor-m'al operation on the control valves, the first-stage pressure feedback can be used.

When this is done the gain of the control valve flow signal must be changed in order to retain the same overall gain of the control valve loop. This is shown in Fig. 40b.

Power/load unbalance When the generator loses the electrical load, it ecessary to quickly close the control valves and rcept valves to stop the steam flow to the tur-e, thereby limiting the speed increase.

The 22 The first-stage pressure feedback can be put in operation only when the unit is in P. A. admission operation and when the initial pressure is at least 0.95 (95 percent).

During the process of putting it into operation the pressure feedback signal gain will be increased from zero to0.67 gradually, while the gain of the control valve signal is increased from 1

e (I ~ FUI.L LOAD CURRENT)

I+Ts GENERATOR CURRENT Ts YES LATCH OUTPUT OF ANO ING RELAY P/L UNBAL.

REHEAT PRESSURE

+

(I % FULL LOAD PRESSURE)

YES

) 0.4 RESET Fig. 40a.

Power/load unbalance logic lr = 0.02 to 0.05 sec) to 3.

Some load changes will occur during this transfer, but if all circuits are properly adjusted they should be minor.

On loss of the pressure feedback signal (signal below 0. 2) the loop willswitch to the feedback "out" conditions.

A loss of load will also remove this feedback.

Load limit The load limit can be used to manually override the signal to the controlling valve set to a lower valve position thanwhat the speed/load control dic-

~

~

~

tates.

The "SELECTOR" willautomatically direct the load limitsignal to the main stop valve flow sig-nal in F. A. operation and to the control valve flow signal in P.A. operation.

When the load limitis in

control, the speed control is not modulating the controlling valve position; therefore, the unit is not participating in the frequency control of the power system.

Because of this fact, the load limit shouM not be used to control unless a malfunction in the boiler or turbine control makes it necessary to temporarily peg the. valve position until the mal-function can be corrected.

Initial pressure limiter (IPL)

The initial pressure limiter will take control away from the speed/load control (or the load limit if it was controlling) when the initialsteam pressure falls to 0. 9 (90 percent, normal setting) in normal operation at full load.

If the pressure keeps on falling itwillreduce the valve position down to no load by the time the initial pressure has fallen to 0.8 (80 percent).

For startup when the boiler pressure is low the pressure reference of the IPL can be set down to any desired value or the IPL can be switched off altogether.

For extended light load operation the setting can be raised above 90 percent in order to reduce the delay before the IPL starts closing the valves.

The "SELECTOR" will direct the output of the IPL to the proper valve signal depending on the op-erating mode (F.A. or P. A, ) of the turbine.

Intercept, valve limit SPEED ERROR LOAD SPEED REFERENCE REGULATION (0 TO I)

+

~0 E

FEEDBACK OUT~ IN

'Qo'AIN C.V. FLOW SIGNAL (0 TO I)

The intercept valve limitis intended to be used only for lowering the I.V. position in order to set reheat safety valves.

Its adjustment is done on a screwdriver pot in the cabinet.

During normal op-eration the pot must be setout of the way (not limit-s)g) so that the valves are stem sealing.

Valve flow control units I

FEEDBACK SIGNAL (0 TO O.GT)

GAIN 0~0.67 FIRST-STAGE PRESSURE (0 TO I)

Fig. 40b.

First-stage pressure feedback The purpose of the valve flow control units is to produce the steam flows that are commanded by the load control unit.

Because of the appreciably non-linear steam flowcharacteristic of the steam valves, compensation circuits must be introduced to obtain quasi-linear steam flow response with respect to steam flow signal.

23

0

BIAS HYORAULIC TRIP SIGNAL MAIN STOP VALVE MAIN STOP VALVE FLOW SIGNAL AMPL.

a

'ERVO VALVE HYDR.

TRIP VALVE RAM

>sv "sv M.S V.FLOW N0.2 VALVE IDENTICAL LOOP Fig. HI. Main stop valve flow control unit The main stoP valve flow control unit is shown in Fig. 41.

During full-arc admission operation it receives the flow signal that can be limited by the load limit or the initial pressure limit.

During partial-arc admission it receives a valves-full-en signalthat is independent ofspeed and the limit ctions.

The flow signal operates a servo amplifier whose output positions a servo valve, which in turn oper-ates the ram across a trip valve (low value gate).

The feedback is modified by a function generator having a characteristic similar to the valve (similar to Fig. 31).

The hydraulic trip valve can close the main stop valve on loss of emergency trip system

pressure, regardless of the flow signal.

Allmain stop valve loops are identical.

The intercept valve flow control unit (Fig. 42) is similar to the main stop valve flow control unit.

In order to close the intercept valves rapidly on loss of load, a trigger amplifier is used that wiQ actuate a dump valve on a large closing error signaL Allintercept valve loops are identicaL The control valve flow control unit (Fig.

43) nsists of as many similar positioning loops as ere are control valves.

The input to each servo 24 amplifier for the control valves has a bias for se-quencing the valve operation.

A nonlinear feedback and a trigger and dump valve are also used.

The trigger is operated by the power-load unbalance relay.

In order to further linearize the load response of the unit, a stage pressure feedback can be used on the control valve flow. signal.

This feature should be used as operating control only in cases where it is compatible with the boiler control system.

Operating control system The operating electrohydraulic control system wiih the basic transfer functions is shown in Fig.

44 (in partial-are admission, all auxiliary functions have been omitted).

The following nomenclature is used:

p(o)=

speed reference (0 to 1 + [per unit] dis-crete values and "overspeed")

a

=

speed (a = :0 to 1 + [per unit])

n n = speed Lrpmj, nr = rated speed [rpmj

HYDRAULIC TRIP SIGNAL BIAS TRIGGER 8

DUMP VALVE INTERCEPT VALVE FLOW SIGNAL AMPL.

SERVO VALVE RAM

'ltv INTERCEPT VALVE I.V.FLOW NONLINEAR FEEDBACK N0.2 VALVE:

IDENTICAL LOOP Fig. 42.

Intercept valve flow control unit POWER-LOAD NBALANCE RELAY'RIGGER a

DUMP VALVE CONTROL VALVE FLOW SIGNAL SEOUENCING BIAS AMPL a

SERVO VALVE RAM

'Icv NO. I CV.

NO.I CV. FLOW NON LINEAR FEEDBACK TOTAL C.V. FLOW E

CONTROL VALVE N0.2, 5 AND4 SIMILAR TO C.V. NO.I Fig. 43.

Control valve flow control unit 25

0

q SPEED REFERENCE 0

QI SPEED SUMMER m

ae)

REGULATION

+

W LOAD REFERENCK f FIRST STAGE L

TI- ~O PRESSURE

~ FEEDBACK ra I

I I I I

ml I

I Q RD

+

tO IPT

~'ALVE POSITIONING LOOPS

~A Pt O

CONTROL VALVES VALVE FLOWS TOTAL FLOW RE HEASTER

+~

REHEAT PRESSURE IP BLP TURBINE BOWL FIRST-STAGE FLOW (PER UNIT}

E FIRST-STAGE PRESSURE (PER UNIT)

II HP TURBINE P

I'0 TURBINE TORQUE

+

I M R v

LOAD TORQUE D

ACCELERATING TORQUE TURBINE ROTOR SYSTEM TURBINE SPEED 26 Fig. 44.

Operating EHC system basic transfer functions

0" G

EL E i sv TAi AT speed error speed regulation (~ ='0.025 to 0.07/per unit], 2.5 to r percent; normally'6= 5%)

load reference (-1-0-+1. 4 [per unit],

p+ = 1 for fullload at rated speed)

~

gain of first-stage pressure feedback (G = 0 no feedback, G = 0. 67 fullfeedback) total flow signal [per unit]

bias signal of valve i; typical values are:

81valve: Epl= 0 82 valve: Eg2-" -0.4 [per unit]

83 valve: Ep3= - 0.65 [per unit]

ti4 valve: Eg4= -0.85 [per unit]

servovalvetimeconstantg

= 0.016 sec) integration time constant of actuator i (for small changes)

TA = 0. 1 to 0. 2sec (for all four actuators appboximately equal).

For large changes in the opening direction the valve can go through full stroke in approximately 10 seconds.

(input current to the servovalve is limited to achieve this).

valve lift per unit of each individual con-trol valve (fulllift= 1) per unit maximum flow of each individual control valve; typical values are:

Kvl = 0.4 per unitof max. turbine flow Kv2

= 0. 25perunit of max. turbine flow K 3

= 0. 2 per unit of max. turbine Qow K 4

= 0.15 per unit of max. turbine flow ZKvi= 1 per unit valve flow[per unit of max. turbine flow]

of individual control valve total valve Qow fper unit]

bowl time constant (T3 = 0. 1 to 0. 5 sec) turbine steam flowfper unit]

first-stage pressure [per unit]: forsteady-state conditions f

= fraction of total power developedby the high pressure turbine f = 0,25 to 0, 32 [per unit J TR

=

reheater time constant [sec]

WR TR Q

WR =total steam weight [lb] at max. load in reheater system (including pipes to and from turbine)

QR Maximum reheater steam flow[lb/

secJ driving torque [per unit]

load torque [per unit]

rh.v' accelerating torque [per unit]

T4

= turbine time constant WR2 n

'i 2 T

= 5.98 x t

[sec]

4 3600 mm j

WR

= Weight moment of inertia [lb ft J

2 2

P

= Max. power fKW]

n

= rated speed [rpm]

The steady-state characteristic of the speed/load control system set at five percent regulation and a speed reference of 1 (rated speed) is shown in Fig.

45.

Power supplies (Mark l)

Electrical Power to the electrical control and protection system is supplied by. redundant power supplies at +30 VDC and -22 VDC for the control functions.

A +24 VDC system is used for relay cir-cuits.

These power supplies are fed by the 115V 60-cycle a-c line or by a permanent magnet gener-ator on the turbine shaft. Either one of these power sources can sustain operation of the electrical con-trol system.

SPEED (PERCENTI 95 96 97 98 99 100 101 102 103 104 105 106 107

+ IA

+I 0.8 (L) 0.6 0.4 0.2 o.e 0 6 o.

O~

0

~OI'h i'

ld M I-MS Z O IIJ TL O IIS '4 AEC~

0 OPERATION BELOW 0.99 TIME LIMITED 0.95 0.96 0.97 0.98 0.99 I.OI 1.02 1.05 1.04 1.05 1.06 1.07 o'~SPEED (PER UNIT)

Fig. l5.

Speed/load control characteristic of CHC system hydraulic prnveT is suppliedby a redundant high-pressure Quid pumping system at 1600 psi with fire-resistant fluid.

- the mechanical trip valve

- the solenoid-operated lockout valve Two full-size variable displacement pumps with a-c motor drive are provided.

One pump is norm-ally in operation pumping only the actuaQy used oil low. The other pump is on standby, with automatic starting provisions if the operating pump shouM not supply sufficient pressure.

For large transient flow requirements accumu-lators are used.

Filtering and fluid treatment equipment as well as heating and cooling provisions for the fire-re-sistant fluidare incorporated in the hydraulic power unit.

Protection system (Mark I)

Trip System Mechanism

- the dual solenoid-operated master trip solenoid valve

- the reset solenoid valve actuating the bearing oil-operated reset niechanism

~FRONT STANOARO BEARING OIL 25 PSI OVERSPEEO RESET l

po OIL TRIP SOL VALVE ORY POCKET RESET SOL VALVE AIR RELAY MECHANICAL TRIP VALVE TO CHECK MANUALTRIP VALVES MASTER TRIP SOL VALVE The trip system mechanism controls the emer-.

gency trip fluid system pressure which is needed to operate the main and reheat stop valves directly, and supplies power Quid for operating the control and intercept valves.

The tripsystem mechanism in the front standard, Fig. 46, consists of:

-the conventional overspeed trip device (ec-centric ring)

ILIBE OIL SPACE MECHANICAL TRIP SOL LOCK-OUT VALVE TRIP FMAO SYSTEM

- the solenoid-operated oil trip valve

-the manual trip with mechanical trip solenoid HYORAuuc INKIER FLIJIO 1600 PS I Fig. 46.

Trip system mechanism 28

The overspeed trip device can be tested during rmal operation at rated speed by locking out the ip action of the mechanical tripvalve temporarily y means of the lockout valve.

During this test the electrical back-up overspeed trip will assume the function of the second line of defense against over-speed.

An electrical trip test logic system is pro-vided to minimize the possibility of misoperation during this test.

-solenoid-operated fluid valves can be tested (exercised) regularly

- trip actions are redundant in their final action.

A FIRST HIT monitoring system is incorporated in order to help analyze trip incidents and diagnose malfunctions.

Electrical Trip System Allexternal trip signals are obtained by closing contacts at the station battery voltage level (125 VDC or 250 VDC)and operatingsealed relays in the electrical trip system (24 VDC).

Trip signals from within the electrical control system are obtained by closing contacts in the 24 VDC tx ip system directly or through sealed relays.

Any of these trip signals willenergize the mas-ter trip relay (24 VDC), which will initiate the fol-lowing redundant trip actions:

-~de-ener izingoi the two pilot soienoids oi the master trip solenoid valve (24 VDC)

-~enzr 'zin oi the mechanical trip solenoid (t25 VDC or 250 VDC from station battery).

Either one of these actions willtrip the system.

Since loss of the 24 VDC powerwould render the ctrical trip system ineffective, the master trip olenoid valve is heM in the reset position by having at least one pilot solenoid energized with the same 24 VDC power.

Loss of this power will, therefore, trip the unit (see "Electrical Power Supplies" ).

Trip anticipator (T.A.)

On units with high overspeed on loss of load the overspeed trip must be set high, in order not to trip on loss ofrated load.

On some units this would result in an overspeed above 20 percent in case the first line of defense against overspeed should fail to work properly.

If this is the case, an electrical speed signal is used to trigger a trip anticipator signal, that will operate the fast acting valves on the main and reheat stop valves at a speed suffici-ently low to limit the emergency overspeed to 20 percent if an anticipated trip shouM take place.

If, however, the first line of defense against overspeed has worked properly and theanticipated trip did not occur, the T.A. signal willreset in time to let the MSV and RSV re-open again and permit the inter-cept valves to blow down the reheater, and later the control valves to re-open and control rated speed-no load in order to be ready for synchronizing at the earliest possible opportunity.

The trip antici-pator signal is obtained from a third speed pick-up that produces the back-up overspeed trip signal on units that are not using a trip anticipator.

CONCLUSION The redundant pilot solenoid arrangement re-duces chances of false trip-outs and makes testing of the pildht-solenoid valves possible.

Important features of this protection system are:

- solenoid coils can be replaced while maintain-ing safe operation The electrohydraulic approach to turbine control has two important advantages:

(1) it allows us to incorporate a number offeatures that cannot reason-ably be obtained on mechanical systems; (2) it im-proves the performance of features that are pre-sent in some form in the mechanical system.

The development of an electrohydraulic control system is a logical step in the continuing advance of tur-bine-generator technology.

APPENDlX l THE IMPEDANCE CONCEPT Equation (A-1) can be rewritten as With the impedance concept it is possible to ex-press the transfer function of a frequency-sensitive element by the use of a differential operator nota-tion.

tf Y(t) dt = KX(t)

(A-7)

The transfer function of anelementdescribes the ratio of the output {0) ofthis element to its input {I),

whereby the magnitude and the phase relation ofthe output are determined by'the time variations of the input.

or from (A-5)

KX Y

s (A-8)

In a study of differential equations Heavyside formulated a method by which an equation such as The operator s in the denominator signifies an integration.

Y=K dX dt can be represented as (A-1)

As seen from equations (A-5, A-6 and A-8) the factor s can be manipulated according to thesimple laws of algebra.

or Y=KpX (A-2)

From this startingpointitisnowpossible to show that frequency-sensitive elements can be repre-sented by their transfer functions with the Laplace operator "s".

Y=Kp X

where p is an operator replacing ~.

d()

(A-3)

Laplace later introduced a new operator (s) de-ined by the equation This is exemplified by the capacitor C

El E2=0 F(s) = f f(t) e dt 0"

The voltage on the capacitor is the time integral

{A-4) of the current divided by the capacitance or Y=KsZ( )

(A-5) where F and f represent functions of the variables in parentheses (s = Laplace operator, t = time) and e is the base of the natural logarithm.

It can be shown that equation (A-2) is valid for zero initial conditions when written as E](t) = g fi dt or with the Laplace operator

'(s)

El(s)

C s

The transfer impedance is now (A-9)

(A-10)

=Ks Y

X(s)

(A-8)

E i(s)

C s (A-11)

It can be said that s as a factor (in the numerator) signifies a differentiation.

which is the same as stated in equation (26).

30

APPENDIX II SPEClAL FEATURES OF THE EHC MARK II SYSTEM The EHC Mark II System is a second generation analog control system built for greater reliability and serviceability.

A block diagram of the operating control system for a reheat turbine - EHC Mark IIis shown in Fig.

4V.

The basic concept of the Mark Isystem has been retained in Mark 11; however, extensive technology advances have been incorporated

which, in some

cases, resulted in an adaptation of the system.

In particular, two new groups ofequipment were added:

the standby control

, the plant communication system An electrical equipment numbering system was created for computer use that identifies every piece of electrical controlequipment witha coded number giving location, general function, type of circuit, an assembly identification and acomponent number.

Operational amplifiers Alloperationalamplifiers used in the MKII Sys-tem are integrated circuit operational amplifiers t made the following improvements possible:

(1) a rated signal level of 10 volts DC is used (2) the 1kHz oscillator for stabilization was eliminated and the central 3kHz oscillator was replaced by individual 3kHz oscillators for each position indicating circuit.

(3) the I.C. op. amps occupy only the space of a transistor (compared to a whole circuit board for the Mark I op. amp) and use only eight connections (compared to about 150 in the discrete component op. amp of Mark I)

I With the use of these integrated circuit opera-tional amplifiexs a functional packaging technique became possible that further reduced the number of connections and made servicing simpler.

Relays and logic systems BIfilar relay coils that reduce the duty on the coil operating contact used in sealed multi-contact

relays, together with functional repackaging on printed circuit-boards, made it possible to handle substantially more complex logic systems with only modest increase in wiring.

A two-out-of-three logic principle thatincreases the reliability of a function compared to a single FULL-ARC PARTIAL-ARC TRANSFER SPEED REFERENCE LOAD REFERENCE LOADINGRATE LIMITS MAIN STOP VALVEFLOW SIGNAL STANDBY CONTROL STANDBY SIGNAL MATCH STANDBY LOAD SET M S.V. FLOW STANDBY FA/PA TRANSFER CONTROL UN IT M.S.V. FLOW ACCELERATION REFERENCE PEED FRROR IGNA SPEED SPEED CONTROL UNIT LOAD CONTROL UNIT C.V. FLOW SIGNAL I.V. FLOW SIGNAL FIRST-STAGE PRESSURE C,V. FLOW CONTROL UNIT C V FI.OW I V. FLOW CONTROL UNIT I.V. FLOW Fig. 47.

Operating control system for reheat turbine EHC Mark ll 31

contact by several orders ofmagnitude has been fn-oduced for the most important switching functions.

Such techniques were extensively used in the TRIP and MONITORING SYSTEM that will be dis-cussed later.

Power supplies A 115V, 60 Hz, I. 5 KVApermanent magnet gen-erator (PMG) driven by the turbine is used.

Its output is sufficient to supply all operating and em-ergency circuits of the control system with power in a black-out condition, as long as the turbine is above 90 percent speed.

For smooth transfer the valve position signals are manually matched and the switch-over can only be performed afterproper signal matching has been confirmed by logic.

An emergency switch-over withunmatched valve signals is possible by pressing an override button, but this should be necessary only in cases of very rare emergencies.

It should be noted that for standby operation the portions of the system listed below still have to operate:

Power supplies, electrical and hydraulic The valve flow controls The power/load unbalance A 125 VDC system, supplied redundantly by the PMG or the 115 VAC line is used for the protection and test logic external to the cabinet.

A redundantly supplied 24 VDC is used for the analog logic interface in the control cabinet and for the redundant overspeed protection.

Also redundantly supplied are the analog voltages

+22 VDC and -22 VDC.

Logic powered by the 125 VDC of the customer' battery can still be used to interface with the EHC logic, if desired.

The trip and monitoring system including back-upoverspeed tripand other trip functions It is possible to start the unit in standby opera-tion, bring it to speed and synchronize it with an acceleration indicator (similar to stop valve bypass startup on mechanical-hydraulic control system),

load it part way in full-arc admission, transfer to partial-arc admission and load it further in partial-arcadmission, and also areversalof this sequence.

The loading and unloading rates in standby operation are not restricted by the system; the operator has full responsibi)ity.

Standby mode of operation A STANDBY CONTROL has beenadded that will permit operation of the unit by manually controlled valve position while the speed controlunit and most of the load control unit are disconnected for main-tenance and tests.

In this mode of operation the backup overspeed trip, usually set one percent above the mechanical trip speed, will be reset to 105 percent of rated speed, such that the backupoverspeed trip assumes the role of the firstline ofdefense against overspeed and the mechanical overspeed trip remains the second line of defense.

The power/load unbalance relay and,the trip anticipator remain in operation during standby operation.

The transfer to standby operation is manual:

a thorough failure analysis on Mark I has shown that the cases inwhichautomatic switch-over wouM have prevented a shutdown were practically nonexistent and that the additional complexity of an automatic switch-over system and its increased probability of false operation could not be justified.

Protection system The protection system was built using the fol-lowing new philosophy:

Three, degrees of importance for protections were defined and a concept of redundancy and/or testing was established corresponding to each of the degrees of importance:

VITALPROTECTIONS, the failure ofwhich couM result in a major catastrophe en-dangering people and equipment Shall use CONCEPTUAL REDUNDANCY:

sensors, transmission medium, path and output devices of different design and nature AND shall be COMPLETELY TESTABLE during normal operation.

IMPORTANT PROTECTIONS, the failure of which would result in increased main-tenance but would not endanger people 32

Shall use EQUIPMENT REDUNDANCY:

two-out-of-three or two-out-of-two logic using identical hardware for the three (or two) branches OR shall be FULLY TESTABLE during normal operation.

OPERATIONAL PROTECTIONS or LIMIT FUNCTIONS, the failure of which could cause minor damage or misoperation Shall use an ALARM when the condition becomes marginal AND a TRIPof a single function line, when the condition becomes intolerable.

Statistics and failure probability calculations have played an important role in deciding what the degree of importance of a given protection should be and what redundancy and testing features should be used to implement the protection.

The Mark II protection system is shown in block diagram form in Fig. 48.

It consists of the 125 VDC (EHC) trip and reset logic that handles all trip functions external to the cabinet the 24 VDC trip and reset logic functions in-ternal to the cabinetand the 24 VDC electrical trip solenoid valves in the front standard the cross trip functions the manual trip and reset push buttons the overspeed trip reset and test monitoring logic the MECHANICAL-HYDRAULICTRIP SYS-TEM at the front standard The mechanical trip pilot valve, the mechanical shut-off valve and the mechanicaltrip valve as well as the electrical trip pilot valves and the electrical trip valve have been designed such that there is no close. fitting sliding seal under pressure drop dur-ing normal operation.

Both the mechanical and electrical trip line are fully testable during opera-tion by way of automatic test sequences.

As in the Mark I system, loss of 24 VDC power willtrip the unit through the electrical trip valve.

A FIRST HIT monitoring system is providedto help analyze trip incidents and diagnose malfunctions in each one of three groups of'equipment; it then determines which of the three groups was first to trip.

Cabinet The cabinet has been designed for:

ample room for servicing strict circuit separation for elimination of noise II accommodating four different terminal board designs accommodating a standard option package that willbe sufficient for most users The four-bay cabinet is divided into:

the power supply bay the analog bay the relay bay the terminal strip bay (wide bay)

MARK II CONCLUSION The Mark II EHC system is the result of an in-tensive effort to build the most reliable electro-hydraulic controlsystem without departing from the well-proven basic functional principles of its pre-

decessor, the Mark I.

ACKNOWLEDGEMENT The author wishes to express his appreciation to Patrick C. Callen, George W. Kessler and Paul E.

Malone for their assistance in updating this paper.

43

TRIP RK SET BUTTON BUTTON 0

0 I CUSK

)

IOTHER l OPERATIONAL I

INPUTS VACUUM IMPCATAHT TRIP SIGNAL I SIGNALS I

CUSK OTHER

'fRIPS VACUUM IMPORT l2SVDC TRIP ANT TRI~

EHC TRIPS IZSV DC KHC TRIP AND RESET LOGIC OVERSPEKO-TRIP TEST, RKSET AND MONITORING LOGIC ISPKEO TEST STATION N TKST PANEL I COST.

ISPEEO ISPEED PINPUTS P Q

SPEED COST.

ViTAL BACK-UP TRIP IOSS OF TRIPS p

ANTICI BOfH BATTERY I2SVDC SoEED PaTOR SIG TRI M

Vl ETTa CC0 ET

~E I

o IL'J lh

~I~

MECH.TRIP SOLENOIDVALVE MTSV MECIL TRIP PISTON MTP RSPEEO TRIP OST OIL RESET PISTON ORP OIL TRIP SOL.

VALVE 0 ST OIL RESKT SOLENOID VALVE ORSV 2<VOC EHC TRIP ANO RESKT LOGIC TO SET SPEED REF.

TO ZERO 'WHEN TRIPPEO PRESSURE SWITCHES ELECTRICAL

'fRIP SOLENOID VALVE ETSV TRIP LATCH ASSEMBLY llECHANICAL TRIP PILOT VALVE MTPV MECH.

TRIP HANDLE MTH

~ ELECTIIICLLG GR L

~ IITGRLCLIC ~ IGRLL

~ IIECRLRICLLRGRLL

~ OTHER SIGNAL

~ MULTIPLE LOGIC l2 OUT OF J IN MOST CASES)

ELECTRICAL LOCK OUTVALVE ELV ELECTRICAL TRIII VALVE KTV MECHANICAL LOCK OUT VALVE MLV MECHANICAL TRIP VALVE MTV MECIL SHUT OFF VALVE MSOV l600 PSI HYDRAULIC FLUID EMERGENCY TRIP SYS'fKM KTS ll600 PSI HYDRAULIC FLUIDI DISC DUMP VALVE ON MAINSTOP VALVES DISC, DUMP VALVE ON CONTROL VALVES DISC DUMP VALVE ON REH. STOP VALVES DISC DUMP VALVE ON INTERCEPT VALVES AIR RELAY DUMP VALVE Fig. 48.

Mark II protection system TO POSI'flVE CLOSED EXTRACTION CHECK VALVES G

~

W R

RECOMMENDED LlTERATURE "SERVOMECHANISMS AND REGULATING SYS-TEMS DESIGN", H. Chestnut and R.

W. Mayor, John Wiley& Sons, Inc., New York 1963, Vol. 1 Cc 2 (second edition).

"ELECTRIC COMPENSATION OF VALVE FLOW NONLINEARITIESIN LARGE STEAM TURBINES",

P. C. Callan, ASME paper 63-WA-190.

"A SIMPLIFIED ANALYSIS OF THE NO-IDAD STABILITY OF MECHANICAL-HYDRAULIC SPEED CONTROL SYSTEMS FOR STEAM TUR-BINES", M. A. Eggenberger, ASME paper 60-WA-34 (G. E. Reprint GER-2048).

"BASIC ANALYSISOF PRESSURE CONTROL SYS-TEMS USED ON LARGE STEAM TURBINE-GEN-ERATOR UNITS",

M.

A.

Eggenberger, P.

C.

Canan, ASME paper 64-WA/PTC-I (GER-2170).

"APPLICATIONS OF OPERATIONALAMPLIFIERS

- THIRD GENERATION TECHNIQUES", J. Graeme, McGraw-Hill, New York, 1973.

"OPERATIONALAMPLIFIERS DESIGN ANDAP-PLICATIONS", G. Tobey, J. Graeme and L. Huels-man, McGraw-Hill, New York, 1971.

35

GEK-3 t941 VALVE TEST LOGIC (BWR)

GENERAL See Schematic Wiring Diagram Valve Test Logic following this publication.

The purpose of valve tests is to permit regular checking of the operationof all turbine steam valves during normal ON-LINE OPERATION.

The Schematic WiringDiagram Valve Test l.ogic also contains the controlling logic of all non servo-controlled (non positioning) valves.

VALVE TEST FOR CONTROL VALVES partially closed.

Test CV-l, CV-2, and CV-3 in sequence.

This procedure willminimize load s zing during CV-1, 2, and 3 test.

The unit may see some bypass valve acti( n to compensate for the pressure increase.

SINGLE ADMISSION UNITS: The followin!, CV tes ing proce ure is recommended for singli ad-mission units.

Decrease load to approximately 70% to allow for increased flowpickup in those CV's not being tested.

This willminimize load swing.

Test each valve in sequence.

a tL' flIMA

'll<fL flIO HO

, Pla l GIIAa O

0%

Na OQ

'IA0 The control valves are tested separately.

When the test button of the CV-1 valve is depressed, CV-1 will close at a moderate velocity until, at approxi-mately I/2-inch of stroke of the servomotor, the fast-acting valve is operated to close the valve rapidly for the remaining stroke.

Both the normal operating devices and the fast-acting devices are thereby tested.

When the control valve has

closed, and the op-erator releases the test button, the control valve willopen.

All other associated CV's are tested in a like manner.

NOTE: The operator should test only one control valve at one time.

When one test button is released to allow the tested valve to reopen, the opening movement should be observed and the next valve of the set should only be tested after the first one has again assumed normal operating position.

The CV's should be tested weekly.

MULTIPLE ADMISSION UNITS: The following CV testing procedure is recommended for multiple admission units.

Decrease load so that CV-4 is more than 901o closed.

Test CV-4 to check the fast-acting valve operation.

Tlds procedure will minimize load swing due to CV-4 fast closing.

Continue to decrease load so that CV-3 (3 or 4 admission) or CV-1, 2,

3 (2 admission) have just VALVE TEST FOR STOP VALVES INDIVIDUALTESTING The stop valves are tested separate)y.

Wh.n the test button on the SV-1 valve is depressed, SV-1 will close at a moderate velocity until, at apIiroxi-mately 1-inch ofstrokeof the servomotor, the fast-acting valve is operated to close the valve r.ipidly for the remaining stroke.

Both the normal oper-ating devices and the fast-acting devices are thereby tested.

When the stop valve has closed, and the operator releases the test button, the stop valve will open.

All other associated SV's are tested in a like manner.

NOTE:

The operator should test only one stop valve atone time.

When one test but-ton is released to allow the tested valve to

reopen, the opening movement should be observed and the next valve of the set should only be tested after the first one has again assumed normal operating po-

sition, Individual testing should be done daily.

MULTI-TESTING Any combination of 2 stop valves can be tested simultaneously by pushing the associat< d test buttons simultaneously.

The valves willslowly move to thc

GEK-37941, VAIVE TEST LOGIC

% closed position and stop.

When the buttons are

eleased, the valves willre-open.

a \\ Units with selector switch and fast acting op-eration For multi-testing procedures refer to APED specifications.

VALVE TEST FOR INTERCEPT AND INTERMEDIATE STOP VALVE The intercept valve and its associated inter-mediate stop valve are tested together.

When the test button of the IV-1/ISV-1 valve combination is depressed, IV-1 will close at a moderate

velocity, until, at approximately 1-inch of stroke of the servo-motor, the fast-acting valve is operated to close the valve rapidly for the remaining stroke.

Both the normal operating devices and the fast-acting devices are thereby tested.

When the intercept valve has closed, the associ-ated intermediate stop valve starts closing.

Again, as mentioned above, both the normal operating de-vices and the fast-acting devices are thereby tested.

When both valves are closed and the operator re-leases the test button, the intermediate stop valve willopen first and afterwards the intercept valve willopen.

These units are tested individually.

The op-erator chooses the bypass valve to be tested by turning a selector switch.

When the oper-ator depresses the valve test button, the se-lected bypass valve starts opening at a mod-erate velocity.

When the bypass valve is 90%

open a fast-acting solenoid valve is operated to open the valve rapidly during the remaining stroke.

When the valve is fuQy open, the op-erator releases the test button and the valve willclose.

A light willindicate when the next valve can be selected.

The selector switch is used on units having more than four (4) bypass valves.

The bypass valve test amplifier is a ramp generator, which opens and closes the bypass valve at a moderate velocity during test.

Units with Selector Switch.

Same as(a) but without fast opening operation.

NOTE:

The operator should test only one intercept/intermediate stop valve combi-nation at one time.

When one test button is released to allow the tested valve(s) to

reopen, the opening movement should be observed and the next valve of the set should only be tested after the first one has again assumed normal operation.

Combined valves should be tested daily.

The combined valves test circuits also contain the necessary logic to control the slave intercept valves.

The slave intercept valve is controlled by means of limit switches, which are mounted on the master intercept valve, and associated relay logic.

Units with Push Buttons and Fast Opening Operation These valves are tested individually by de-pressing the appropriate push button.

When the operator depresses the valve test button, the selected bypass valve starts opening at a moderate velocity.

When the bypass valve is 90% open, a fast-acting solenoid valve is op-erated to open the valve rapidly during the remaining stroke.

When the valve is fully

open, the operator releases the test button and the valve will close.

A light willindicate when the next valve can be selected.

VALVE TEST FOR BYPASS VALVES Push buttons are used on units having up to four (4) bypass valves.

See Sclicrrrulic 1Virirrg Diuinarn - ByPass Valve Tesl Logic included in this Tab, For bypass valve testing procedures refer to APED specifications.

The bypass valve 'test amplifier is a ramp generator, which opens and closes the bypass valve at a moderate velocity during test.

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