Session 3.
Power Transfers on the Bulk
Power System
1
Dave Barry, Bad Habits
“The electric company sends electricity
through a wire to a customer, then
immediately gets the electricity back
through another wire, then (this is the
brilliant part) sends it right back to the
customer again.”
2
Criteria
The standards by which an electric system
is planned and operated
Transmission Security – getting power from
one point on the grid to another without
overloading the lines or causing system
separations and blackouts
How much power can you transfer from A
to B
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Criteria (cont.)
Transmission Transfer Capability (TTC)
The maximum flow possible – at which the
“limiting facility” reaches its appropriate
rating, either:
• without a contingency, or
• following the worst single contingency
4
Blackouts in Mirror
are Closer
than they Appear
5
“Gallia est omnis divisa in partes
tres. . . .”
• Distribution – about 90% of outage MW-hr.
– your house, your neighborhood
• Generation – what everybody thinks of
when they hear the word “reliability.”
– brownouts, price spikes
• Transmission – several orders of
magnitude more complicated.
– blackouts, often covering large areas
6
“All Gaul is divided into three
parts. . . .”
Distribution
• “n–1”; or in some cases “n–2” (e.g., Con Ed area subs.)
• Performance indices
Generation
• “One day in ten years”
• LOLE – Installed Reserve Margin (IRM)
Transmission
• “n–1” criteria – tho sometimes means >1 element
• Since 1965, Regional Reliability Councils
• Voluntary compliance? – mandatory in NPCC – incl. NY
• Trend toward weaker criteria
• Trend toward uniform national criteria (LCD??)
7
Who Should Have
Responsibility/Authority?
• Local level (individual systems, Control
Areas)
• State/ISO/RTO etc. (NY: NYSRC, NYISO)
• Regional level (RRCs/RROs)
• National level (U.S. & Canada) –
NERC/NAERO
All of the above!
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Costs of Outages
•
•
•
•
•
•
•
•
•
•
Injury/loss of life due to accidents (e.g., no traffic lights);
Injury/loss of life, ill and elderly;
Loss of productivity by industry;
Loss of sales by business;
Loss of wages of labor;
Damage to equipment in industry;
Fires and explosions;
Riots and thefts;
Increased insurance rates;
Higher fees for consulting engineers.
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Consumer Reaction “R”
R is a function of F, T, P, t
where: F = frequency
T = duration
P = amount of load interrupted
t = time when it occurs
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Top Excuses for a Blackout
3.
“A tree did it” – July 2, 1996 Western U.S., 2003
Midwest/Northeast, 2003 Italy, etc.
Birnam Wood to Dunsinane
2. “It was an act of God” – Con Ed CEO following July
13-14, 1977 New York City blackout
1. “A UFO drained power from an Upstate New York
power line” – November 9, 1965 Northeast Blackout
SPECIAL AWARD
Everything you heard on TV Aug. 14-15, 2003
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Transmission Transfer Capability
Concept: system will be intact:
1) without a “contingency”
2) following the worst single contingency
Philosophy
• “All in” condition – pre-disturbance
• Worst single contingency or “n -1"
• Single element
• Multiple elements from a common contingency
• “Extreme contingencies”
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13
Should Standards (Criteria) be
Uniform?
• Same in Albuquerque as Manhattan?
• Same in New Mexico as New York?
• Consequences of a blackout
• 20 story buildings
• But what about the transmission user?
14
Uniform Standards/Criteria:
Example
Interface with 2 – 345 kV, 1 – 230 kV, 2 – 115 kV
The 345 kV circuits are on the same towers – i.e.,
it’s a double circuit line
A-1 -------------------------------------------------- 345
X
X
X
X
X
A-2 -------------------------------------------------- 345
B -------------------------------------------------- 230
C
D
----------------------------------------------------------------------- 115
----------------------------------------------------------------------- 115
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Uniform Standards/Criteria
Interface limit (TTC) based on:
L/O A-1
2500 MW
L/O A-2
2500 MW
L/O A-1 and A-2
1800 MW
Lower TTC if use L/O both lines on a double
circuit tower as a contingency
But, all the user needs to know is the TTC
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Parallel Path Flow Constraints
An interconnection is a single large machine
• Every power transaction affects every line
• Every contingency (L/O of line, L/O
generator, etc.) affects every line
• What you do will affect your neighbors
• What your neighbors do will affect you
Golden Rule: “Do onto others as you would have
done onto yourself” – or, “Don’t do onto others
anything worse than they can do to themselves”
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Transmission Constraints
Thermal Capability -- conductors overheat
Voltage -- VARs
Stability -- system dynamics / machine angles
WHO DEVELOPS, MONITORS COMPLIANCE, etc.?
• NERC
• Regional Reliability Councils
• ISOs/RTOs – Other reliability entities
• Control Areas
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Increasing Transmission
Capability
• Add new line(s) ---------- [duh. . . .]
– note: these may be in some other system
• Cases where something other than conductor
rating is limiting – fix it !
• Reconductor existing line(s)
• Add series capacitors to lines not loading up
• Add series reactors to lines loading up too much
– note: not good if voltage or stability is limiting
19
Increasing Transmission
Capability (cont.)
• If voltage is limiting, put in VAR source(s)
• If stability is limiting, look into power system
stabilizers, generator control systems, etc.
• Add phase angle regulator(s) – or FACTS
device(s)
• Use sophisticated relay scheme – e.g. “Special
Protection Systems” (SPSs)
• Add HVDC link (in parallel with existing AC
system)
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Special Protection Systems
Special Protection Systems (SPSs)
• Require no intervention on the part of system
operators for their operation.
• Designed to do more than the simple removal
from the system of a faulted element – e.g.,
tripping lines, generators or other elements
many miles removed from the point of
disturbance. But …
• Potential problem: unintended consequences,
mutual effects – especially if large number.
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FACTS
“FACTS” (Flexible A C Transmission System)
• Largely experimental devices which promise
near instantaneous control of steady state power
flow, or system response in a dynamic situation.
• Many believe FACTS technology will permit
increased power transfer capabilities, without
having to construct new lines, by allowing
optimization of flow distribution. But …
• Potential problems similar to those of SPSs.
• How are you, Joe Friday?
22
“Doctor, it hurts when I do this”
Henny Youngman
ALL OF THE ABOVE REQUIRE
EXTENSIVE STUDY !!!
Whatever you do, don’t do this:
• Lower the criteria
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Things to Remember
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•
•
•
•
•
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Limiting contingency not always on interface
Limiting facility not always on interface
Limiting interface not always on “seam”
TTL in one direction not same in other
Assumed base conditions very important
Location of “sources” & “sinks” very important
Failure to observe TTLs will be dangerous to
your health – i.e., blackouts in mirror may be
closer than they appear
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OASIS
(Open Access Same-time Information System)
Internet based system for the display of available
transfer capabilities on transmission interfaces
Total Transfer Capability (TTC)
Maximum allowable transfer across an interface
[ Note: TTC is consistent with older FCTTC ]
Available Transfer Capability (ATC)
Transfer capability remaining after commercial activity
already committed is subtracted
Two other amounts may also be subtracted:
Transmission Reliability Margin (TRM)
Capacity Benefit Margin (CBM)
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Congestion … or
The Trouble with Tribbles
Congestion: no such thing
• Another term we’re stuck with – like “loop flow”
• MORE POWER WANTS TO GET ACROSS AN
INTERFACE (OR FLOWGATE) THAN ITS TTC
or
• “NORMAL” SCHEDULING RESULTS, OR WILL
RESULT, IN A CONDITION UNDER WHICH
THE TTC WILL BE EXCEEDED
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Congestion
The Physics
• Calculated transfer limits based on criteria
• “Congestion” occurs when sum of desired
transactions over an interface exceeds the
transfer limit
• Competition among potential transmission users
for the (always) finite capability of the system
• Expressed as total power flow on identified
groups of transmission lines known as
interfaces or flowgates
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Congestion (cont.)
• All potential solutions boil down to one of the
following:
– curtail transactions
– reschedule generation
– allow auctions or reselling at profit
• But, these are physically the exact same thing!
• The difference is not where “there” is, but how
you get “there”
• e.g., NERC’s TLR (Transmission Loading Relief)
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Curtailment & Redispatch are the
Same (in terms of the Physics)
• You get to the same place, but by different
institutional & procedural routes
• In both cases, as well as in any others likely to
be proposed, you’re reducing generation in one
area, & increasing it in another
• Note: the only way you can control
the transmission system is by
controlling the generation or load
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“I canna change the Laws of
Physics, Captain Kirk!”
Lt. Commander Montgomery Scott
(“Scotty”), aka James Doohan, Star Trek
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