Session 2.

Synchronous Interconnections (or “Grids”) 1

Woody Allen

“I took a speed-reading course and was able to read

War and Peace

in 20 minutes. It’s about Russia.” 2

• • • • •

What is a “Grid”?

Battle of the Currents (late 1800s) DC (Edison) vs. AC (Westinghouse)

The concept of

economy exchanges

The concept of

load diversity

The concept of

emergency assistance

The concept of

reserve sharing

The concept of

INTERCONNECTION

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Grid = “Synchronous Interconnection”

• System or group of systems all of which are connected together with AC lines. Anything that happens anywhere is felt everywhere else. Sometimes called being “in synchronism” – whence the expression, “in synch.” (Are you listening, Justin Timberlake?) • An “interconnection” is a

single large machine.

• Sort of like

“The Borg”

(but no “Seven of Nine”).

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5

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Loehr’s Second Law

•

Power flows in an interconnection …

– Via all paths – Inversely proportional to impedance • All lines will be affected by every power transaction or contingency The effect of a transaction over one portion of the interconnection on other portions is often called “Parallel Path Flow.” 7

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Four “Interconnections” (“Grids”) in North America

• Synchronous interconnection: a single large machine. Everything’s connected with AC. • What happens

here

affects

there

.

• Eastern Interconnection • Western Interconnection 140,000 MW • ERCOT (Texas) 580,000 MW 60,000 MW • Hydro-Quebec 30,000 MW 9

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North American Interconnections

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Eastern Interconnection

• 101 Control Areas • Tightly coupled • Disturbances propagate • Today’s system highly stressed • Failures beyond criteria likely to result in widespread outages 13

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Control Area Major Responsibilities

• Tie Line Control Make certain that actual net interchange matches schedule • Frequency – minimize “Area Control Error” (ACE) – typically every 6 sec.

Contribute toward maintaining exact 60 Hz freq.

(Participation factors developed by NERC OC) • Time Error Correction Contribute toward corrections in clock time error caused by accumulated deviations from 60 Hz (In Eastern Interconnection, when gets to 2 sec.) 15

Control Area Major Responsibilities

All of above done by specified generating units placed on “Automatic Generator Control” (AGC)

ALSO

• Usually responsible for monitoring key interface flows compared to the established Transmission Transfer Capabilities, and monitoring conformance with reliability criteria in general • Often responsible for dispatching the outputs of individual generators within the Control Area (“economic dispatch” in old world order) 16

Control Areas

Asynchronous ties – HVDC, isolated generation or load HQ/TE MARITIMES Ontario NEW YORK NEW ENGLAND ECAR Areas PJM

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US/Canada Control Areas

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Control Area Size

• •

PJM, NYISO and ISO-NE represent 20% of the load in the U.S. portion of the Eastern Interconnection but have only 3 control areas; the other 80% have 91 control areas Average control area sizes, by region, Eastern Interconnection/U.S.

(based on forecast peak load 2001):

• • • • • • • • • •

ECAR FRCC MAAC MAIN 5,993 MW 3,359 MW 51,762 MW 3,663 MW MAPP/US NPCC/US 2,161 MW 27,055 MW SERC 9,037 MW SPP 2,308 MW .............................................................................................

Eastern Interconnection/U.S. 5,474 MW

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How Many?

• Some ISO/RTOs are single Control Areas – NY, NE, PJM • Others aren’t – key issue: each have 1 Control Area Western Interconnections

giving up control

• Eastern Interconnection has 100+ Control Areas • Western Interconnection has about 30 • ERCOT and Hydro-Quebec Interconnections • WHAT’S THE RIGHT NUMBER?

• My opinion: reduce number in Eastern & • Maximum 15-20, each interconnection 20

Why Not Just One? (Quebec, ERCOT)

• “Centralization” can get to be too much of a good thing in a large interconnection can’t keep track of everything knowledge of local conditions conformance monitoring • best to proceed in smaller, evolutionary steps 21

The Importance of Being Earnest

System Operator of the Control Area

• know all schedules & all changes, monitor all interfaces, assure TTCs not violated • authority to order changes in schedules & generation • authority to take any action necessary in an emergency, including tripping generation or disconnecting firm load • • exchange data with other Control Areas

Both judge and jury

• Military authority in emergencies 22

NYCA Major Emergency April 17, 2005

• L/O Millstone #3 in NE at 1165 MW • NY Central-East voltage collapse limit exceeded • NYISO declared major emergency @ 8:32 • • Terminated @ 8:35

Duration = 3 min.

• Compare/contrast with transcripts of conversations from Midwest during the Aug. 14, 2003 incident 23

10 Regional Reliability Councils

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ECAR

Reliability Regions

NPCC MAAC

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Power Pools, ISOs & RTOs

Traditional Power Pools

• Single control area, centralized dispatch, transmission monitoring • PJM, New York Power Pool, New England Power Pool

ISOs & RTOs – FERC 2000

• Open transmission system to all comers on a fair and non-discriminatory basis • Form independent associations to manage the the transmission system 26

SOME MODELS OF THE ELECTRIC GRID AND WHY THEY’RE WRONG

TELEPHONE SYSTEM • The telephone system is not “on” all the time. In fact, when you make a call, only you and the other party are connected. • The electric system, however, is all on all the time. You are connected to every generator and every other customer on the interconnection – every minute of every day.

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Models … Wrong (cont.)

GAS OR WATER PIPELINE • The electric system is several orders of magnitude more complicated than even the most complex gas or water systems. A gas or water pipeline (transmission) system is essentially point-to-point; an electric transmission system is essentially a network.

• You can’t make the power flow down certain lines and not others by opening and closing valves.

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Models … Wrong (cont.)

RAILROAD OR HIGHWAY • You can’t direct the power down

this

truck.

line rather than

that

one like a train or truck on a railroad or highway – or make the power stop like a train or • It takes several days or more for a train or truck to cross the country; it takes electricity less than a tenth of a second.

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Models … Wrong (cont.)

THE POWER LAKE

“The lake is an enormous reservoir from which you can draw a lot of water before you have to add some or run out.”

• Electricity cannot be stored; you have to generate it exactly as you use it.

• There are no constraints to water going from one end of the lake to the other. But there are major constraints on the electric network. 30

RELIABILITY CRITERIA (a.k.a. “STANDARDS”)

CRITERIA: The standards by which an electric system is planned and operated.

• • •

Generation Adequacy

meet the load -- having enough generation to

Transmission Security

-- getting power from one point on the grid to another without overloading the lines or causing system separations and blackouts

National? Regional? Local? Yes!!

Bill Richardson after 2003 Blackout: “

The power industry has never had reliability standards

.

” Horsefeathers!!!

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PROBABILISTIC VS. DETERMINISTIC CRITERIA

Generation Adequacy

• Probabilistic • “One day in ten years”

Transmission Security

• Deterministic • Experiments with probabilistic for over 30 yrs.

• Problem:

a x b

where: – # of things that can happen (

a

) approaches infinity – prob. of one thing happening (

b

) approaches zero 32

Contingencies

• A “contingency” is a sudden disturbance to the power system • Before the contingency: Steady state condition, often referred to as “base load flow,” “pre-contingency” or “zero minus” condition 33

TYPICAL TRANSMISSION CRITERIA (PLANNING & OPERATIONS)

• three-phase fault on any transmission circuit, transformer or bus section, with unsuccessful reclosure • loss of any generating unit, with or without a fault • double-line-to-ground fault on any two adjacent circuits of a multiple circuit transmission line • line-to-ground-fault on any transmission circuit, transformer or bus section, with a stuck breaker • Loss of both poles of an HVDC line

No overloads, low voltages, instability, loss of customer load

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EXTREME CONTINGENCIES

• loss of all lines on a right-of-way • loss of all units at a generating station • loss of all lines emanating from a switching station or substation • three-phase fault on any element with a stuck breaker

Test of system strength Reasonable measures should be taken to minimize consequences Moderate loss of load permitted

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After a Contingency Occurs

Loss of a line or other transmission element (without a fault): • Sudden change in configuration of the network • Generator power angles oscillate • Line flows (MWs & MVARs) oscillate • Voltages oscillate • After some seconds, things settle down into a new “post contingency” or “post disturbance” steady state condition (if it’s stable, of course!) • But, usually with the differences in generator power angles a little larger, line flows a little (or a lot) higher, and voltages a little lower 36

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After a Contingency Occurs (cont.)

Loss of a generating unit (without a fault): • System load now greater than generation; frequency declines; all generators connected to the interconnection increase their MW outputs roughly proportional to their capacities – they are converting the potential energy of their rapidly spinning rotors to kinetic energy • This increased MW generation flows into the area where the generating unit was lost • After some seconds, things settle down into a new “post contingency” or “post disturbance” steady state condition (if it’s stable, of course!) • But, usually with the differences in generator power angles a little larger, line flows a little (or a lot) higher, and voltages a little lower 38

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After a Contingency Occurs (cont.)

• At the same time, automatic equipment on generating units begin to open valves etc. to allow more fuel flow • “Spinning reserves” will be brought up, too, to restore net interchange to its scheduled amount These last two actions will tend to restore system frequency to 60Hz

If an electrical fault occurs with L/O either line or gen.:

• Load in the vicinity of the fault will be much reduced, tending to accelerate the generators and cause greater oscillations • A contingency with a fault is not always more severe and less stable than one without a fault 40

After a Contingency Occurs … We’re Not in Kansas Anymore!

• After a contingency, system is in a new “state” • New line/impedance or generation matrix • Have to reconfigure the system to prepare for possibility on next contingency • Double-contingency design? No!!!

• Once that first element is out, you’re at a new “zero minus,” or base condition 41

Albert Einstein

“Since the mathematicians got hold of the Theory of Relativity, even I don’t understand it.”

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