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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
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.
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.
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.