Download Swing Dynamics and Frequency Regulation

Swing Dynamics and Frequency
Regulation
Anu Kowli
University of Illinois
Tutorial
Jan 28, 2013
Outline
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Brief introduction to a power system
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Synchronous generators
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Principle of operation
Swing dynamics
Interconnection of generators
Frequency regulation
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Why is it needed?
How is it achieved?
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Power System Basics
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Major components:
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Generators: produce electricity
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Loads: consume electricity
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Lines (T&D): transport energy from generators to loads
Key Features
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Absence of large-scale storage capabilities
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“Just-in-time” type manufacturing system
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Constraints: Kirchhoff’s laws, physical limits
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Power flows through paths of “least resistance”
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Power System Overview
Power System Block Diagram
Electrical
Distribution
Generator
Water Wheel
(Hydroturbine)
Generation
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Electrical Energy
Transmission
(Long Distance)
Transmission
Motor
Motion
Bulb
Light
Heater
Heat
Distribution
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An Example Power System
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Common Questions
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Why AC and not DC?
Transformers, Insulation
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Why higher voltages?
Lower losses
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Why 3-phase?
More efficient
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Why interconnect
Reliability and economics
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Synchronous Generators
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A.K.A. alternators – producing alternating current at
frequency f = # poles/2 x mech speed in rpm/60
Most commonly used in power grids
Important when considering electromechanical
oscillations
For more accuracy, they should be referred to as
synchronous machines
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Principle
of Operation
A three-phase
synchronous
machine consists of an inner rotati
outer stationary housing called the stator as shown in Figure 5
and it is balanced
on bearings.
Electricity
is produced
when a coil is in motion with
respect to a magnetic field
stator
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Stator with Distributed A-Phase Winding
In a generator,Figure 5.3 - Slotted Synchronous Machine
rotor
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Instead of having to draw all of the slots and windings each time, we represent each distributed coil
by a concentrated coil located in the center of the distribution. This is shown in Figure 5.4. The
circle with a dot denotes that current is referenced out of the page while a circle with a cross
indicates that current is referenced into the page. We use a, b, and c to reference the three stator
phases represented in Figure 5.4.
Coils on stator
Magnet on the rotor
Typically, the rotor
moves w.r.t. stator
Figure 5.2 - Layout of a Synchronou
rotor
stator
Page 1 of 5
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ntrols the amount of mechanical power transmitted to the generator. The generator in
nverts the mechanical power to electrical power. The automatic voltage regulator (A
citer connected to the synchronous machine adjust the rotor field current to maintain
minal voltage. Cables, switchboards, transformers, and circuit breakers then route th
ase power to the many shipboard loads. On other ships, the prime mover may be a di
a steam turbine or some combination.
Generator’s Block Diagram
fuel
ref
set-point
prime
mover
governor
stator
+
rotor
exciter
voltage
regulator
typical
gasofturbine
generator
Figure 5.1 - A
Notional
Portion
a Shipboard
Electric Power Generation Syste
1 Principle of Operation
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EMF and Back-EMF
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A sinusoidal rotor field rotating at constant speed
induces sinusoidal voltages in the stator windings
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Sinusoid’s frequency f = rotor frequency x # pole pairs
Sinusoid’s amplitude E depends on rotor magnetic field
Sinusoid’s phase shift δ = angle made by rotor w.r.t. a
synchronously rotating reference frame
When the machine terminals are connected, the
currents flowing in the stator windings create a
second rotating electromagnetic field, which causes
a torque on the rotor that opposes its motion
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The synchronous machine models developed in the
form the basis for the derivation of the swing equation
The Swing Equation
P e electrical power,
T
generator load m
electromagnetic torque T e
Generator
mechanical power P m
Pm
mechanical torque T m ω m
rotor speed
ω
Figure 10.1. Schematic description of powers an
chronousdmachines.
ω
J
m
= Tm − Te
dt
dω m
ωm J
= Pm − Pe
dt
103
In steady-state, speed is constant
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If Load Changes …
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If load decreases,
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Accelerating torque
Speed increases
Frequency increases
If load increases,
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Decelerating torque
Speed decreases
Frequency decreases
dω m
speed ω m J
= Pm − Pe
dt
ωm
frequency f = # pole pairs ×
2π
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Interconnected Generators
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When you interconnect synchronous generators,
they behave as though they are (almost) connected
on the same shaft
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Holds for small deviations around nominal
Large deviations should, as such, be avoided!
The frequencies of interconnected generators are
nearly same
A synchronous generator just connected to the grid
will be dragged to synchronous speed by the grid!
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Frequency Regulation
Evolution of system frequency following loss of 2600 MW of generation
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Monitoring and Control
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Large and complex hardware-software systems are
used for real-time operations and control
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Energy management system (EMS)
Supervisory control and data acquisition (SCADA)
Frequency is closely monitored and maintained
around 60 Hz
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Necessary for physical considerations
Power quality implies bounds on the frequency of the
power delivered
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Supply, Demand and Frequency
System conditions
continuously change!!
A hierarchical control
architecture is
employed to regulate
the grid frequency
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Droop Control
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A.K.A. primary regulation or governor control
Control logic
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slope of the speeddroop characteristic: ρ
Instantaneous change
in a generator’s output
to stabilize the grid
frequency
Proportional control
Primary regulation is fully delivered within 10-30
seconds
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Droop Control in Action
156
loss of
generation
14. Control of Electric Power Systems
offset
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additional
system inertia keeps
droop control kicks in
control needed
Figure
14.4. The
after a production
the
frequency
fromtransient
andfrequency
restoresresponse
frequency
loss. The diagram is a recording from the Nordic system at a loss of a
dipping too low
close to nominal
1000 MW unit on the 24 November 1983.
* Source: ETH course notes
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Automatic Generation Control
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A.K.A. load frequency control, secondary control
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It is a part of the SCADA/EMS systems
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Control logic
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A quantity known as area control error (ACE) is
measured to take into account frequency excursions as
well as deviations from scheduled interchanges – ideally,
it should be zero
AGC implements PID (or some other logic) control to
keep ACE ~ zero; and, hence, frequency ~ 60 Hz
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Regulation Signal
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AGC sends raise/lower pulses every 2-4 seconds –
this constitutes the regulation signal
The regulation signal is derived from ACE:
ACE = I act − I sch + β ( fact − fsch )
freq. bais
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The regulation signal changes
the reference set-point of the
generator
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PJM Regulation Signal
Frequency (Hz)
60.02
60
59.98
Frequency drop
59.96
00:00
00:10
00:20
00:30
00:40
00:50
01:00
time
100
regulation
ACE (MW)
needed
500
50
0
0
−500
−50
Regulation (%)
1000
Negative ACE
−1000
00:00
00:10
00:20
00:30
00:40
00:50
−100
01:00
time
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Implementation of AGC/LFC
Pref = f (Ped , ACE)
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Frequency
control
ISO’s Version
of ACE
ERCOT SCADA
AGC
QSE Resources
Load
Frequency
control
SecurityConstrained
Economic
Dispatch (SCED)
QSE SCADA
ICCP LINK
ICCP
LINK
AGC
Transmission
SCADA
Gas Turbine
DCS
LDC
Gas Steam unit
DCS
LDC
Coal unit
DCS
LDC
Some ISOs deploy up/down regulation reserves
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exciter connected to the synchronous machine adjust the rotor field current to maintain the
terminal voltage. Cables, switchboards, transformers, and circuit breakers then route the th
phase power to the many shipboard loads. On other ships, the prime mover may be a diese
or a steam turbine or some combination.
Simplified Generator Model
fuel
ref
set-point
AGC
prime
mover
governor
stator
+
rotor
exciter
E∠δ
E∠δ-120°
E∠δ+120°
voltage
regulator
dδ
Rotor dynamics
= ω − ω nom
dt Figure 5.1 - Notional Portion of a Shipboard Electric Power Generation System
dω
Swing equation
M
= Pm − Pe
5.1 Principle of Operation
dt
" ω − ωmachine% consists of an inner rotating cylinder called the rotor
A three-phase
dPm synchronous
1
outer stationary
the 'stator
as shown
in Figure
5.2. A shaft
runs through the
Prime-mover
dynamics
= − housing
+
P
−
P
$$ callednom
m
' ref
and itdt
is balancedρon bearings.
ω
#
&
nom
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Network Impacts
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The load Pe,i on the generator connected to node i
of the grid is subject to network flows
(
Pe,i − Pd ,i = ∑ViVk Gik cos(θ i − θ k ) + Bik sin(θ i − θ k )
k
)
9. Synchronous Machine Mo
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Vn , θ n can be explained by Single-Machine-Infinite-
Bus model:
internal voltage
and angle of
the machine
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jX d
In
En , δEn
I
Zn
Vn ,Uθ n
voltage and
angle at the
grid connection
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Sequence of Operations
area
control
error
ON/OFF
decision
dispatch
signal
unit
commitment: MIP
economic
dispatch:
SCOPF
dayahead
forecast
minutesahead
forecast
days
minutes
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reference
set-point
automatic
generation
control
turbine +
governor
action
realtime
data
seconds
time scale for operations
real-time
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Operational Planning is Crucial!
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Email: [email protected]
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