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Session 3. Power Transfers on the Bulk Power System. 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.” .

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Session 3.

Power Transfers on the Bulk Power System

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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.”

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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

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Blackouts in Mirror

are Closer

than they Appear

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“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

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“All Gaul is divided into three parts. . . .”


  • “n–1”; or in some cases “n–2” (e.g., Con Ed area subs.)

  • Performance indices


  • “One day in ten years”

  • LOLE – Installed Reserve Margin (IRM)


  • “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??)

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Who Should HaveResponsibility/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

  • “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


    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


  • “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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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?

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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


A-2 -------------------------------------------------- 345

B -------------------------------------------------- 230

C ----------------------------------------------------------------------- 115

D ----------------------------------------------------------------------- 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


  • 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

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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” (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?

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“Doctor, it hurts when I do this”Henny Youngman


Whatever you do, don’t do this:

  • Lower the criteria

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Things to Remember

  • 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 … orThe Trouble with Tribbles

Congestion: no such thing

  • Another term we’re stuck with – like “loop flow”




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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!” Physics)

Lt. Commander Montgomery Scott (“Scotty”), aka James Doohan, Star Trek