Lecture 11Power System Controls

Read: Chapter 12.1 - 12.4 ; Supplementary materials Homework 2 is due on Sep 26th.

Dr. Lei WuDepartment of Electrical and Computer Engineering

EE 431POWER TRANSMISSION AND DISTRIBUTION

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Final Project

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Topic 12 Transportation electrification Cain Ballard Graham Dittler Brian Sheridan

Topic 10 Prediction of Distribution System Outages Isaac Jandreau Andrew Sharpe Phillip Haught

Topic 9 Modeling the dynamic process of cascading failure in power systems Tyler Dellea Preston Jone Sabrina Sepowski

Topic 1 Business models for energy storage Braylin Sheppard Rachel Hansen Erin Zanni

Topic 2 Offshore wind Alysa Leong Emily Fabian

Topic 4 Microgrid energy management systems Jonathan Bower Broc Breen Maria Denton

Topic 14 Water-food-energy nexus Amil Vargas-Castillo

Deliverable I Each team should send the following information by October 5th 2018 to the

instructor for approval. The topic choice A list of team members A tentative title together with a ~200 words abstract to describe your

project

Outline Power system transient stability Mechanical model of synchronous machine – swing equation

Electrical model of synchronous machine

The equal-area criterion for two-machine problem

Numerical integration for multi-machine problem3

2

, , ,22

pu m pu e pu a pusyn

d tH t P t P t P tdt

'( ) sinbuse

d

E VP tX X

1 2

0 1

, , , , 0m pu e pu m pu e puP t P t d t P t P t d t

Equal Area Criteria Example IV

When a permanent 3Φ-ground fault occurs at bus 3. Fault is cleared by opening the CBs. If the CBs open after 3 cycles and then reclose when . Assume fault has been cleared. Calculate the maximum value of

4

35t

t

1 2

0 1

35

2 3 135

1 1 1 0e e eP d t P d t P d t

2 0.679rad

5

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Delta (rad)

Time (s)

Delta 1

Delta 2

375380385390395400405410415

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Omega (rad/s)

Time (s)

Omega 1Omega 2

Methods for Improving Transient Stability Improving steady-state stability, by increasing the maximum power

transfer in steady state. High-speed fault clearing devices Majority of transmission line short circuit are

temporary and can be extinguished by itself.High-speed enclosure of CBs can increase postfault transfer power.

Increase H can reduce angular accelerating, slow down angular swings, and increase critical clearing time

…

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Outline Generator voltage control Turbine governor control Load-frequency control Economic dispatch Electric energy market clearing price

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Power System Operation and Control 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). Opening a circuit breaker causes the power flow to instantaneously

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

changing generation we can indirectly change this flow.

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Power System Control Local control v.s. Central control

Local control includes Generator voltage control to hold system voltage Turbine governor control to hold system frequency

Central control include Area control areas to monitor information including area frequency,

generator output and power flow and determine interchange power between control areas

Load frequency control controls the area frequency at 60 Hz, net tie-line flow at the designed values, and the operation cost.

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Generator Voltage Control Generator voltage control is to adjust power output to control

the magnitude of the generator terminal voltage VT. Constant power factor Constant interval voltage Constant reactive power

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Generator Governors The other key generator control system is the governor, which

changes the mechanical power into the generator to maintain a desired speed and hence frequency.

Historically centrifugal “flyball” governors have been used to regulate the speed of devices such as steam engines

The centrifugal force varieswith speed, opening orclosing the throttle valve

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Photo source: en.wikipedia.org/wiki/Centrifugal_governor

Isochronous Governors Ideally we would like the governor to maintain the frequency at

a constant value of 60 Hz (in North America) This can be accomplished using an isochronous governor.

A flyball governor is not an isochronous governor since the control action is proportional to the speed error

An isochronous governor requires an integration of the speed error

Isochronous governors are used on stand alone generators but cannot be used on interconnected generators because of “hunting”

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Generator “Hunting” Control system “hunting” is oscillation around an equilibrium

point Trying to interconnect multiple isochronous generators will

cause hunting because the frequency setpoints are the two generators are never exactly equal One will be accumulating a frequency error trying to speed up

the system, whereas the other will be trying to slow it down The generators will NOT share the power load proportionally.

Turbine Governor Control The solution is to use what is known as droop control, in which

the desired set point frequency is dependent upon the generator’s output

The steady-stage frequency-power relation for turbine governor control

Where

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1m refP P f

R

: change in turbine mechanical power output (MW): change in reference power setting (MW)

: change in frequency (Hz)R : regulation constant (Hz a typicalva/ lMW ue is 4 or 5%. ),

m

ref

PP

f

Turbine Governor Control The solution is to use what is known as droop control, in which

the desired set point frequency is dependent upon the generator’s output

The steady-stage frequency-power relation for turbine governor control

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1m refP P f

R

Turbine Governor Control Example A power system is operated with 500 MWA, f=60Hz, and

R=0.05pu. If f is increased by 0.01Hz, what is the decrease in Pm with fixed power setting?

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0.0160puf pu

4,

1 1 0.01 3.33 100.05 60m pu pu

puP f pu

R

43.33 10 500 1.667mP MW

Turbine Governor Control of Multiple Generators

Multiple generators in one area can be combined together

Where

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1

11

1 1m m mN

ref refNN

ref

P P P

P P fR R

P f

: area frequency response characteristics

Multiple-Generator Turbine Governor Control Example

For a power system with three units, each unit is initially operate at 0.5 of its own rating. The power ratings of the three units are 1000 MVA, 750MWA, and 500 MWA, with R are all equal to 0.05pu. When load is increased by 200MW, calculate β based on 1000MVA rating ∆f and ∆ Pm of each unit

R is in the unit of Hz/MW, and

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Bact pu

B

FR RS

n o

n o B Bpu pu o n

B B

S FR RS F

110000.05 0.051000

npuR pu 2

10000.05 0.0667750

npuR pu

310000.05 0.1500

npuR pu

1 2 3

1 1 1 45n n npu pu pu

puR R R

Multiple-Generator Turbine Governor Control Example

For a power system with three units, each unit is initially operate at 0.5 of its own rating. The power ratings of the three units are 1000 MVA, 750MWA, and 500 MWA, with R are all equal to 0.05pu. When load is increased by 200MW, calculate β based on 1000 MVA rating ∆f and ∆ Pm of each unit

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31 4.44 10mf P pu

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1 0.088 88.88m npu

P f pu MWR

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1 0.067 66.67m npu

P f pu MWR

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1 0.044 44.44m npu

P f pu MWR