1005pgu_hydroautomation

7/31/2019 1005PGU_Hydroautomation

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Hydro Automation:Aspects of Mechanical

Engineering Design


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Educational ObjectivesOn completion of this course, students will know how to:

OverviewIn todays increasingly competitive power market, utili-

ties are constantly looking for ways to reduce cost in order

to remain competitive. Because the fuel costs for hydro

generation are essentially zero, nancial gains must come

in the form of improved unit eciencies or reductions in

operating and maintenance cost. To this end, the Tennessee

Valley Authority (TVA) has embarked upon a program to

completely automate all of its 29 conventional hydro power

plants. While the majority of the modications required to

automate a hydro plant involve electrical controls, a substan-

tial amount of modications are required to predominately

mechanical systems.

BackgroundofTVAsAutomationProgram

TVA relies on hydropower for approximately 5,300 mega-

watts (including pumped storage) of its approximate 29,469

megawatts of generating capacity. Prior to the Automation

Program beginning in 1997, one-half of TVAs power pro-

ducing dams were controlled manually with operating sta

24 hours a day. The balance of the plants were operated by

remote control utilizing Supervisory Control and Data Ac-

quisition (SCADA) concepts.

In the 1990s, TVA management began a study to de-

termine the feasibility of completely automating the hydro

system. The purpose of the study was to determine the

associated costs of the program and compare those with

the benets that could be realized with its implementa-tion. The goal of the Automation Program is to completely

automate the control functions of all hydro facilities in the

TVA system. 1

TVAs Wide Area Network (WAN) which connects

plant and oce communications was chosen one of the

primary communications paths for automation. The plan

is to place intelligent control systems at most of the plant

sites such that all control functions can be generated locally.

The corporate WAN is used to transmit schedules from the

Hydro Dispatch Control Center (HDCC) centrally located

in Chattanooga, TN, to the individual plant control systems

to start, stop, protect, and load the generating units. Should

the WAN become disabled for any reason, the local control

system would continue to operate the plants in accordance

with the last scheduled received. Also, dedicated SCADA

communication channels will be utilized for manual control

if the WAN is unavailable. A number of dierent control

schemes are required which are determined by the plants

current operating systems and its value to the system. The

authors recommend Reference 1 for a more thorough dis-

cussion of the automation program.

Mechanical Design Features Required for Plant Automation

Although the majority of the design and equipment required

to automate a hydro plant is electrical by nature, a number of

predominately mechanical systems had to be either added or

upgraded in order to operate the plants without onsite per-

sonnel. For plants that had been previously remote controlled

under SCADA protocol, these systems had either been previ-

ously added during extended outages or were implemented

as a part of the plants initial construction. The challengebecame installing this new mechanical equipment on previ-

ously manned plants without requiring extended outages and

incorporating it into original plant design congurations.

Hydro Automation: Aspects of

Mechanical Engineering Design

1. Reduce system operating cost by reducing the

number of personnel required to operate the plants.

2. Optimize the use of water resources by schedulingand operating the units in the most ecient manner.

3. Improve system responsiveness to system changes

by utilizing automatic real-time dispatching.

4. Ensure system compatibility withany future enhancements.


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During the initial automation study, mechanical engi-

neers reviewed all plant systems to determine modications

that would allow aected systems to operate safely and

eciently without operations and maintenance personnel

on-hand to locally maintain and monitor their operation.

The systems were outlined, modication proposals made,

and cost benet studies were performed. Because of the

many dierent plant congurations, a small number of thesystems have designs that are unique to their situation. The

total cost to install the predominately mechanical systems

to automate plants was estimated at $7M.

Figure 1 Shear Pin Alarm System

Shear pin alarm and

air pressure control panel

Air supply

from plant

service air

system

Differential

pressure switch

High press

low press

1/2 ring header

around wheel pitFlexhose

1/4 drilled hole

Shear plane

Shear pin

Shear pin connectiontypical 24 gate pins

The following are the major four systems:

Shear Pin Alarm System

All 109 conventional hydro generating units within the

TVA automation program have a wicket gate operating

system that incorporates a breaking link (generally a shear-

ing pin) in order to protect the primary turbine operating

linkages in the event an obstruction prevents a gate from

closing. This protection system allows the remaining wicket

gates (typically 20-24 gates) to close and the obstructed gate

to remain partially open. When this event occurs, the unit

is either transferred into emergency condense mode or shut-

down depending on the units control scheme.

Under a manned system the local operator would notice

trouble by the units coasting down or hydraulically induced

vibration occurring during the unit restart. A shear pin

alarm system immediately alerts the operator through thelocal annunciation system that a pin or breaking link has

broken.

The shear pin alarm system employed by TVA utilizes

the plants station air system to supply pressurized air to the

shear pin (Figure 1). Station air is routed to a dierential

pressure switch located on a control panel that consists of

the switch and associated control valving.

This panel is generally mounted on the wall inside the

wheel pit to allow for local trouble-shooting. The air is then

routed from the panel to an air manifold that branches out

to each individual pin. The center of the shear pin is boredand tapped for the pressurized air supply system. When the

pin is broken, air pressure is decreased triggering the dif-

ferential pressure switch, which in turn transmits a signal

via the plant annunciation system.

One of the advantages of the pneumatic system over

other electronically activated systems is its ruggedness in

the often harsh environment found in turbine wheel pits

that typically contain oil and are subject to vigorous wash

downs.

The shear pin alarm system typically costs approxi-

mately $2,500 - $4,000 per unit. To construct and installusually takes 3-5 days depending on access to linkages. If

new pins are installed, instal lation requires that the unit be

unwatered.

Wicket Gate Latching Mechanisms

On conventional hydro wicket gate designs, an unbalanced

hydraulic moment typically exists on closed wicket gates

that, if left unrestrained, would have a tendency to open

the gates and rotate the unit. During a unit shutdown when

the gates are closed, this restraint is provided by the wicket

gate servomotors via the pressurized governor oil system.

The wicket gate latch provides a mechanical backup to this

restraint should the oil pressure to the wicket gate servomo-

tors become lost. While the unit is shut down, the gate latch

is engaged as a safety measure and stays engaged until unit

startup where it is disengaged by relays.

There are a number of dierent latching systems avail-

able on the market. Most modern turbine systems incorpo-

rate them with the OEM design of the turbine and governing

system. There are also a number of retrot designs available.

Most of these systems have one thing in commonthey

are designed not only to oppose the unbalanced hydraulic

moment imposed by the headwater, but also to resist the

greater force imposed by the governor servomotors should

they be accidentally operated while the latch is engaged.

Because the ratio of servomotor force to hydraulic opening

force is typically 3:1, these mechanisms are generally large

and expensive to retrot onto existing units.

In order to reduce equipment cost and, more impor-

tantly, reduce the outage time required to retrot a gate

latch to an existing turbine, TVA engineers developed a

new gate-latching device that was smaller and required ashorter outage to install.2 The design concept incorporated

the principle that the sole purpose of the latch is to resist

the hydraulic force imposed by the headwater, and not re-


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sist an accidentally imposed force by the servomotors. In

order to incorporate this concept into the design, a means of

controlled failure had to be engineered into the new design.

A breaking link was designed that allowed the mechanism

to fail should full force from the servomotors become ac-

cidentally applied (See Figure 2).

The result of the new design proved not only cost ef-

fective from a manufacturing standpoint but also reducedinstallation outage time for its installation from 2-3 weeks

to a couple of days.

Figure 2 Wicket Gate Locking Mechanism

Latched Position Un-Latched Position

Breaking Link

Assembly

located on

shift ring

Assembly

located on

headcover

Although this new latching mechanism is being used

on a number of our main river Kaplan unit plants where

the size of the operating ring, servomotors and linkages

are large, we are also using conventional latch designs on

smaller turbine units where cost eective. The cost to fab-

ricate and install a typical gate latch with a breaking link is

approximately $27k.

Automatic Back-ushing Strainers

Most hydro plants rely on raw water systems fed from head-

water for cooling, sealing, and some bearing lubricating

requirements. In all applications the raw water is strained

or ltered depending on its intended use. In the past, collec-

tion and disposal of the captured debris required personal

intervention. At many of our plants this manual cleaning

task may be required as seldom as monthly or as often as

hourly when shad runs occur.

Automatic back-ushing strainers were studied and

determined to be an economical and reliable method of

debris disposal. The ushing operation initiates when the

dierential pressure across the strainer basket reaches a pre-

determined set-point as measured by a dierential pressure

switch, which in turn operates a ush valve discharging the

debris to the tailrace.

The strainers that were chosen range from 4 to 14 inches

pipe diameter and typically pass between 200 to 1500 gal-

lons per minute (12.6 to 94.6 Liters per second). One of the

more challenging problems with incorporating the system is

installing the large strainers in conned pipe galleries. The

back-ush lines from each strainer (typically 2 strainers per

unit) are tied to a common ush header that discharges toan outfall downstream of the dam.

The estimated installation cost for a typical two strainer

system is $44K.

Raw Water Flow Transmitters

Because mercury is an environmental hazard if transported

to the river system, TVA management determined that

automation would be an appropriate program to replace

the meters, which contain mercury, with a standard non-

mercury design. Since there are a number of ow meters

on the commercial market today that could meet these

requirements, TVA engineering performed a cost study anddetermined that dierential orice plate devices and Rose-

mount, Inc., pressure transmitters would be the best choice

for all applications.

The typical installed cost for a owmeter and transmit-

ter is $3,500. Most plants have three per unit.

References1 Terry, William W., 1999, Tennessee Valley Authority Hydro

Automation Program, IEEE Presentation

2 Keith, Greg O., 1998, TVA Develops New Wicket Gate Latches,

Hydro Review, pp 68,69

ACKNOWLEDGEMENTS:This course is based on the technical paper entitled Hydro

Automation Aspects of Mechanical Design as presentedat Waterpower XII 2001. The authors for this paper areacknowledged as Greg O. Keith, Production Manager for theBoone Hydro Group in TVAs River Operations; Tracey C.Barnett, Senior Mechanical Engineer in TVAs OperationsEngineering Group; and Ronald G. Huaker, SeniorEngineering Technician in TVAs Hydro Automation Group.


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Questions

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1. Financial gains must come in theform of improved unit eciencies orreductions in operating and mainte-nance cost, because the fuel costs forhydro generation are essentially zero.

a. Trueb. False

2. TVA relies on hydropower forapproximately _____ megawatts ofits generation needs.

a. 5,100b. 5,300c. 3,500

d. 5,500

3. Prior to the Automation Programbeginning in 1997, ____ __ ofTVAs power producing dams werecontrolled manual ly with operatingsta 24 hours a day.

a. one-quarterb. one-eighthc. one-halfd. three-fourths

4. The objectives of the TVA Automa-

tion Program program are:a. Reduce system operating cost by

reducing the number of personnelrequired to operate the plants.

b. Optimize the use of water resourcesby scheduling and operating theunits in the most ecient manner.

c. Improve system responsivenessto system changes by utilizingautomatic real-time dispatching.

d. Ensure system compatibility withany future enhancements.

e. All of the above

5. TVAs Wide Area Network (WAN)which connects plant and ocecommunications was chosen as theprimary communications paths forautomation.

a. Trueb. False

6. Should the WAN become disabledfor any reason, the ____controlsystem would continue to operatethe plants in accordance with the

last scheduled received.a. remoteb. automaticc. locald. manual

7. The following are part of the major

four systems of the TVA Hydro-

power System that were updated:

a. Shear Pin Failure System

b. Wicket Gate Latching Mechanism

c. Automatic Back Flow Strainers

d. Raw Water Pressure Transmitters

e. All Of The Above

8. A breaking link (generally a

shearing pin) is used to protect the

primary turbine operating linkages

in the event an obstruction prevents

a gate from closing.a. True

b. False

9. One of the advantages of the

hydraulic system over other

electronically activated systems is

its ruggedness in the often harsh

environment found in turbine

wheel pits that typically contain oil

and are subject to vigorous wash

downs.

a. True

b. False

10. There are a number of dierent

latching systems available on the

market. Most modern turbine

systems incorporate them with the

OEM design of the turbine and

governing system.

a. True

b. False

11. There are a number of retrot

designs available for latching

systems. Most of these systems

have one thing in commonthey

are designed not only to oppose

the unbalanced hydraulic moment

imposed by the headwater, but also

to resist the _______ force imposed

by the governor servomotors should

they be accidentally operated while

the latch is engaged.a. greater

b. lesser

c. same

d. none of the above

12. The design concept for the wicket

gate latching system incorporated

the principle that the sole purpose

of the latch is to resist the _______

force imposed by the headwater,

and not resist an accidentally

imposed force by the servomotors.

a. velocity

b. hydraulic

c. rotational

d. lateral

13. Most hydro plants rely on raw

water systems fed from __ __ __ forcooling, sealing, and some bearing

lubricating requirements.

a. City water supply

b. sluiceways

c. headwater

d. tailwater

14. The ushing operation initiates

when the ___ ____ __ across the

strainer basket reaches a prede-

termined set-point as measuredby a dierential pressure switch,

which in turn operates a ush

valve discharging the debris to the

tailrace.

a. head loss

b. dierential pressure

c. ow drop

d. velocity head

15. The strainers that were chosen

range from 4 to 14 inches pipe di-ameter and typically pass between

____ __ __ ga llons per minute.

a. 150 to 1000

b. 225 to 1650

c. 175 to 1200

d. 200 to 1500

16. TVA engineering performed a

cost study and determined that

dierential orice plate devices

and Rosemount, Inc., pressure

transmitters would be the best

choice for a ll applications.

a. True

b. False

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