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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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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

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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. . . .”

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

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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

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 AWARDEverything 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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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

X X X X X

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

B -------------------------------------------------- 230C ----------------------------------------------------------------------- 115

D ----------------------------------------------------------------------- 115

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Uniform Standards/Criteria

Interface limit (TTC) based on:L/O A-1 2500 MWL/O A-2 2500 MWL/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 overheatVoltage -- VARsStability -- 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

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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“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

ALL OF THE ABOVE REQUIRE EXTENSIVE STUDY !!!

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 subtractedTwo 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”

• 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