Sean Brosig, PE

Electrical Engineer

Maggie Watkins, PE

Electrical Engineer

USACE, Hydroelectric Design Center

Date: 09 April 2021

THE IMPACT OF

SEASONAL/ANNUAL

VARIATION IN WATER LEVELS

ON HYDRO MODELING

2

OUTLINE

• Definition of head

• Different turbine types

• Case studies: Example dams with low and high head variation

- Historical head data

- Power/gate characteristics at different heads

- Speed step simulations at different heads

• Conclusion

3

DEFINITION OF HEAD

Head is the difference between the

water elevation at the forebay and the

water elevation at the tailrace.

The available potential energy is directly

proportional to the height of the head.

4

DIFFERENT TURBINE TYPES: FRANCIS TYPE

Francis Turbine185 Units

Spiral Case

PSLF – h6e Model GuidanceActual Data with Model Points

5

DIFFERENT TURBINE TYPES: KAPLAN TYPE

Kaplan Turbine162 Units

Semi-Spiral Case

PSLF – h6e Model Guidance

Actual Data with Model Points

6

LOW HEAD VARIATION

CASE STUDY: JOHN DAY DAM

7

Spring Summer WinterWinter

8JOHN DAY: TURBINE/GOVERNOR CHARACTERISTICS AT

DIFFERENT HEADS

9

JOHN DAY: AGGREGATE REAL POWER ERROR

In one possible scenario the error between

modeled and actual real power output varies by

11 MW per generator.

– Scenario:

Head Hdam Real Power Per Generator

Modeled 94 1.0 154.3 MW

Actual 107 1.14 165.3 MW

This plant has 16 generators and could have an

aggregate real power error of 176 MW for this scenario.

Looking back at the 10 years of head data, this scenario

may not be very likely to happen, so let’s look at one

that’s more realistic.

10

JOHN DAY: AGGREGATE REAL POWER ERROR

In a more likely scenario, the error between

modeled and actual real power output varies by

1.8 MW per generator.

– Scenario:

Head Hdam Real Power Per Generator

Modeled 101 1.07 160.7 MW

Actual 103 1.10 162.5 MW

This plant has 16 generators and could have an

aggregate real power error of 29 MW for this scenario.

This scenario is plausible and could happen regularly.

Does this matter to a transmission planner? Maybe.

11

HIGH HEAD VARIATION

CASE STUDY: DETROIT DAM

12

Spring Summer WinterWinter

13DETROIT: TURBINE/GOVERNOR CHARACTERISTICS AT

DIFFERENT HEADS

“SPEED MODE” or “GATE DROOP”

14DETROIT: TURBINE/GOVERNOR CHARACTERISTICS AT

DIFFERENT HEADS

“LOAD MODE” or “MW DROOP” “SPEED MODE” or “GATE DROOP”

15DETROIT: TURBINE/GOVERNOR CHARACTERISTICS AT

DIFFERENT HEADS

“LOAD MODE” or “MW DROOP”

If the conditions are right, load mode

could still result in difference between

expected and actual MW response.

With low head, gates may reach

maximum opening early in response

and MW response will be limited as a

result.

16

CONCLUSION

17

COMMUNICATING IMPACT OF HEAD IN REPORTS

Excerpt from Detroit 2018 Model Validation Report:

• Important to note what head value is

“nominal” (what does “Hdam” = 1

mean in terms of feet?)

• Selecting the head at which to report

the model should be done

intentionally.

• Providing ideas of alternative head

values to use for different seasons

may be good practice, if reasonable.

o This information isn’t useful

unless used, ultimately.

18

DOES HEAD REALLY MATTER?

Probably not too substantially

for a dam with low head

variation like John Day or

Chief Joseph.

Although, aggregate error can

add up.

Spring Summer WinterWinter

19

DOES HEAD REALLY MATTER?

It might for a high head

variation dam like

Dworshak with relatively

high power output

(~500MW), but the model

is still likely to be wrong a

lot because of the extreme

variations in head during

the spring and summer.

Sometimes head is much

more predictable than

other times.

Spring Summer WinterWinter

20

DOES HEAD REALLY MATTER?

It might also matter for a

high head variation dam

with low power output like

Hills Creek (~35MW), but

maybe it doesn’t matter

much in the scheme of

things.

Spring Summer

WinterWinter

21

DOES HEAD REALLY MATTER?

The way the models are used likely answers this question!

- How accurate do you need to be?

- How many different cases per year are you willing to simulate?

- How good is the water-year forecasting for the year you’re interested in?

22

“SPEED MODE” VS. LOAD MODE”(GATE DROOP VS. MW DROOP)

“SPEED MODE” or “GATE DROOP” “LOAD MODE” or “MW DROOP”

Variation in MW

response to frequency

events may be reduced

by using MW droop

instead of gate droop.

Some governors don’t

have MW droop option.

Governor operating

mode is an operational

decision – powerhouse

owners/operators have

final word.