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Session 2.

Synchronous Interconnections

(or “Grids”)

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Woody Allen

“I took a speed-reading course and was able to read War and Peace in 20 minutes. It’s about Russia.”

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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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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.”

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

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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

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Control AreaMajor Responsibilities

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

• FrequencyContribute toward maintaining exact 60 Hz freq.(Participation factors developed by NERC OC)

• Time Error CorrectionContribute toward corrections in clock time error caused by accumulated deviations from 60 Hz (In Eastern Interconnection, when gets to 2 sec.)

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Control AreaMajor 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)

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Control Areas

HQ/TE MARITIMES

OntarioNEW ENGLAND

NEW YORK

ECAR Areas PJM

Asynchronous ties – HVDC, isolated generation or load

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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 5,993 MW• FRCC 3,359 MW• MAAC 51,762 MW• MAIN 3,663 MW• MAPP/US 2,161 MW• NPCC/US 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: giving up control• Eastern Interconnection has 100+ Control Areas• Western Interconnection has about 30 • ERCOT and Hydro-Quebec Interconnections

each have 1 Control Area• WHAT’S THE RIGHT NUMBER?• My opinion: reduce number in Eastern &

Western Interconnections • Maximum 15-20, each interconnection

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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

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The Importance of Being EarnestSystem 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

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NYCA Major EmergencyApril 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

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10 Regional Reliability Councils

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Reliability Regions

NPCC

ECAR

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

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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 line rather

than that one like a train or truck on a railroad or highway – or make the power stop like a train or truck.

• 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.

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

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

• Generation Adequacy -- having enough generation to meet the load

• 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

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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

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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 strengthReasonable measures should be taken to

minimize consequencesModerate 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

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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

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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

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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

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Albert Einstein

“Since the mathematicians got hold of the Theory of

Relativity,

even I don’t understand it.”