Ece 476 power system analysis
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ECE 476 POWER SYSTEM ANALYSIS. Lecture 3 Three Phase, Power System Operation Professor Tom Overbye Department of Electrical and Computer Engineering. Reading and Homework. For lecture 3 please be reading Chapters 1 and 2 For lectures 4 through 6 please be reading Chapter 4

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ECE 476 POWER SYSTEM ANALYSIS

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Ece 476 power system analysis

ECE 476POWER SYSTEM ANALYSIS

Lecture 3

Three Phase, Power System Operation

Professor Tom Overbye

Department of Electrical andComputer Engineering


Reading and homework

Reading and Homework

  • For lecture 3 please be reading Chapters 1 and 2

  • For lectures 4 through 6 please be reading Chapter 4

    • we will not be covering sections 4.7, 4.11, and 4.12 in detail

  • HW 1 is 2.7, 12, 21, 26; due Thursday 9/4


Balanced 3 phase systems

Balanced 3 Phase () Systems

  • A balanced 3 phase () system has

    • three voltage sources with equal magnitude, but with an angle shift of 120

    • equal loads on each phase

    • equal impedance on the lines connecting the generators to the loads

  • Bulk power systems are almost exclusively 3

  • Single phase is used primarily only in low voltage, low power settings, such as residential and some commercial


Balanced 3 no neutral current

Balanced 3 -- No Neutral Current


Advantages of 3 power

Advantages of 3 Power

  • Can transmit more power for same amount of wire (twice as much as single phase)

  • Torque produced by 3 machines is constrant

  • Three phase machines use less material for same power rating

  • Three phase machines start more easily than single phase machines


Three phase wye connection

Three Phase - Wye Connection

  • There are two ways to connect 3 systems

    • Wye (Y)

    • Delta ()


Wye connection line voltages

Vcn

Vab

Vca

Van

Vbn

Vbc

Wye Connection Line Voltages

-Vbn

(α = 0 in this case)

Line to line

voltages are

also balanced


Wye connection cont d

Wye Connection, cont’d

  • Define voltage/current across/through device to be phase voltage/current

  • Define voltage/current across/through lines to be line voltage/current


Delta connection

Ic

Ica

Ib

Iab

Ibc

Ia

Delta Connection


Three phase example

Three Phase Example

  • Assume a -connected load is supplied from a 3 13.8 kV (L-L) source with Z = 10020W


Three phase example cont d

Three Phase Example, cont’d


Delta wye transformation

Delta-Wye Transformation


Delta wye transformation proof

Delta-Wye Transformation Proof


Delta wye transformation cont d

Delta-Wye Transformation, cont’d


Three phase transmission line

Three Phase Transmission Line


Per phase analysis

Per Phase Analysis

  • Per phase analysis allows analysis of balanced 3 systems with the same effort as for a single phase system

  • Balanced 3 Theorem: For a balanced 3 system with

    • All loads and sources Y connected

    • No mutual Inductance between phases


Per phase analysis cont d

Per Phase Analysis, cont’d

  • Then

    • All neutrals are at the same potential

    • All phases are COMPLETELY decoupled

    • All system values are the same sequence as sources. The sequence order we’ve been using (phase b lags phase a and phase c lags phase a) is known as “positive” sequence; later in the course we’ll discuss negative and zero sequence systems.


Per phase analysis procedure

Per Phase Analysis Procedure

  • To do per phase analysis

  • Convert all  load/sources to equivalent Y’s

  • Solve phase “a” independent of the other phases

  • Total system power S = 3 Va Ia*

  • If desired, phase “b” and “c” values can be determined by inspection (i.e., ±120° degree phase shifts)

  • If necessary, go back to original circuit to determine line-line values or internal  values.


Per phase example

Per Phase Example

  • Assume a 3, Y-connected generator with Van = 10 volts supplies a -connected load with Z = -j through a transmission line with impedance of j0.1 per phase. The load is also connected to a -connected generator with Va”b” = 10 through a second transmission line which also has an impedance of j0.1 per phase.

  • Find

  • 1. The load voltage Va’b’

  • 2. The total power supplied by each generator, SY and S


Per phase example cont d

Per Phase Example, cont’d


Per phase example cont d1

Per Phase Example, cont’d


Per phase example cont d2

Per Phase Example, cont’d


Per phase example cont d3

Per Phase Example, cont’d


Power system operations overview

Power System Operations Overview

  • Goal is to provide an intuitive feel for power system operation

  • Emphasis will be on the impact of the transmission system

  • Introduce basic power flow concepts through small system examples


Power system basics

Power System Basics

  • All power systems have three major components: Generation, Load and Transmission/Distribution.

  • Generation: Creates electric power.

  • Load: Consumes electric power.

  • Transmission/Distribution: Transmits electric power from generation to load.

    • Lines/transformers operating at voltages above 100 kV are usually called the transmission system. The transmission system is usually networked.

    • Lines/transformers operating at voltages below 100 kV are usually called the distribution system (radial).


Small powerworld simulator case

Small PowerWorld Simulator Case

Load with

green

arrows

indicating

amount

of MW

flow

Note the

power

balance at

each bus

Used

to control

output of

generator

Direction of arrow is used to indicate

direction of real power (MW) flow


Power balance constraints

Power Balance Constraints

  • Power flow refers to how the power is moving through the system.

  • At all times in the simulation the total power flowing into any bus MUST be zero!

  • This is know as Kirchhoff’s law. And it can not be repealed or modified.

  • Power is lost in the transmission system.


Basic power control

Basic Power Control

  • Opening a circuit breaker causes the power flow to instantaneously(nearly) change.

  • No other way to directly control power flow in a transmission line.

  • By changing generation we can indirectly change this flow.


Transmission line limits

Transmission Line Limits

  • Power flow in transmission line is limited by heating considerations.

  • Losses (I2 R) can heat up the line, causing it to sag.

  • Each line has a limit; Simulator does not allow you to continually exceed this limit. Many utilities use winter/summer limits.


Overloaded transmission line

Overloaded Transmission Line


Interconnected operation

Interconnected Operation

  • Power systems are interconnected across large distances. For example most of North America east of the Rockies is one system, with most of Texas and Quebec being major exceptions

  • Individual utilities only own and operate a small portion of the system, which is referred to an operating area (or an area).


Operating areas

Operating Areas

  • Transmission lines that join two areas are known as tie-lines.

  • The net power out of an area is the sum of the flow on its tie-lines.

  • The flow out of an area is equal to total gen - total load - total losses = tie-flow


Area control error ace

Area Control Error (ACE)

  • The area control error is the difference between the actual flow out of an area, and the scheduled flow.

  • Ideally the ACE should always be zero.

  • Because the load is constantly changing, each utility must constantly change its generation to “chase” the ACE.


Automatic generation control

Automatic Generation Control

  • Most utilities use automatic generation control (AGC) to automatically change their generation to keep their ACE close to zero.

  • Usually the utility control center calculates ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds.


Three bus case on agc

Three Bus Case on AGC

Generation

is automatically

changed to match

change in load

Net tie flow is

close to zero


Generator costs

Generator Costs

  • There are many fixed and variable costs associated with power system operation.

  • The major variable cost is associated with generation.

  • Cost to generate a MWh can vary widely.

  • For some types of units (such as hydro and nuclear) it is difficult to quantify.

  • For thermal units it is much easier. These costs will be discussed later in the course.


Economic dispatch

Economic Dispatch

  • Economic dispatch (ED) determines the least cost dispatch of generation for an area.

  • For a lossless system, the ED occurs when all the generators have equal marginal costs. IC1(PG,1) = IC2(PG,2) = … = ICm(PG,m)


Power transactions

Power Transactions

  • Power transactions are contracts between areas to do power transactions.

  • Contracts can be for any amount of time at any price for any amount of power.

  • Scheduled power transactions are implemented by modifying the area ACE:ACE = Pactual,tie-flow - Psched


100 mw transaction

100 MW Transaction

Scheduled

100 MW

Transaction from Left to Right

Net tie-line

flow is now

100 MW


Security constrained ed

Security Constrained ED

  • Transmission constraints often limit system economics.

  • Such limits required a constrained dispatch in order to maintain system security.

  • In three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3.


Security constrained dispatch

Security Constrained Dispatch

Dispatch is no longer optimal due to need to keep line

from bus 2 to bus 3 from overloading


Multi area operation

Multi-Area Operation

  • If Areas have direct interconnections, then they may directly transact up to the capacity of their tie-lines.

  • Actual power flows through the entire network according to the impedance of the transmission lines.

  • Flow through other areas is known as “parallel path” or “loop flows.”


Seven bus case one line

Seven Bus Case: One-line

System has

three areas

Area top

has five

buses

Area left

has one

bus

Area right has one

bus


Seven bus case area view

Seven Bus Case: Area View

Actual

flow

between

areas

System has

40 MW of

“Loop Flow”

Scheduled

flow

Loop flow can result in higher losses


Seven bus loop flow

Seven Bus - Loop Flow?

Note that

Top’s

Losses have

increased

from

7.09MW to

9.44 MW

Transaction has actually decreased

the loop flow

100 MW Transaction

between Left and Right


Pricing electricity

Pricing Electricity

  • Cost to supply electricity to bus is called the locational marginal price (LMP)

  • Presently some electric makets post LMPs on the web

  • In an ideal electricity market with no transmission limitations the LMPs are equal

  • Transmission constraints can segment a market, resulting in differing LMP

  • Determination of LMPs requires the solution on an Optimal Power Flow (OPF)


3 bus lmps overload ignored

3 BUS LMPS - OVERLOAD IGNORED

Gen 2’s

cost

is $12

per

MWh

Gen 1’s

cost

is $10

per

MWh

Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost


Line overload enforced

LINE OVERLOAD ENFORCED

Line from 1 to 3 is no longer overloaded, but now

the marginal cost of electricity at 3 is $14 / MWh


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