Transcript DC microgrids (droop control)

ECE 2795
Microgrid Concepts and Distributed
Generation Technologies
Spring 2015
Week #10
© A. Kwasinski, 2014
Introduction
• Synchronous generators
• Input:
• Mechanical power applied to the rotor shaft
• Field excitation to create a magnetic field constant in
magnitude and that rotates with the rotor.
• Output:
• P and Q (electric signal with a given frequency for v and i)
Field
Excitation
Q
© A. Kwasinski, 2014
Introduction
• Synchronous generators
• Open circuit voltage:
e  NS
d
dt
ERMS  4.44 K d K p fN S 
E  N S 

1
NR IR
l
A
E
Magneto-motive force
(mmf)
IR
© A. Kwasinski, 2014
Synchronous generators control
• Effect of varying field excitation in synchronous generators:
• When loaded there are two sources of excitation:
• ac current in armature (stator)
• dc current in field winding (rotor)
• If the field current is enough to generate the necessary mmf,
then no magnetizing current is necessary in the armature and
the generator operates at unity power factor (Q = 0).
• If the field current is not enough to generate the necessary
mmf, then the armature needs to provide the additional mmf
through a magnetizing current. Hence, it operates at an inductive
power factor and it is said to be underexcited.
• If the field current is more than enough to generate the
necessary mmf, then the armature needs to provide an opposing
mmf through a magnetizing current of opposing phase. Hence, it
operates at a capacitive power factor and it is said to be
overexcited.
© A. Kwasinski, 2014
Synchronous generators control
• Relationship between reactive power and field excitation
http://baldevchaudhary.blogspot.co
m/2009/11/what-are-v-andinverted-v-curves.html
• The frequency depends on the rotor’s
speed. So frequency is controlled
through the mechanical power.
• Pmec is increased to increase f
• Pmec is decreased to decrease f
Field
Excitation
© A. Kwasinski, 2014
Q
Voltage and frequency control
• The simplified equivalent circuit for a generator and its output equation
is:
Q, pE
LOAD
• Assumption: during short circuits or load changes E is
constant
• V is the output (terminal) voltage
pe 
E.V
E.V
sin  

X
X
Electric power provided to the load
XQ
E V 
E
© A. Kwasinski, 2014
Voltage and frequency control
• It can be found that
d
  (t )  syn
dt
• Generator’s angular frequency
• (Micro) Grid’s angular frequency
• Ideally, the electrical power equals the mechanical input power.
The generator’s frequency depends dynamically on δ which, in
turn, depends on the electrical power (=input mechanical power).
So by changing the mechanical power, we can dynamically change
the frequency.
• Likewise, the reactive power controls the output voltage of the
generator. When the reactive power increases the output voltage
decreases.
© A. Kwasinski, 2014
Voltage and frequency control
• Droop control
• It is an autonomous approach for controlling frequency and voltage
amplitude of the generator and, eventually, the microgrid.
• It takes advantage that real power controls frequency and that
reactive power controls voltage
f  f0  kP ( P  P0 )
V  V0  kQ (Q  Q0 )
V
f
f0
V0
P0
P
© A. Kwasinski, 2014
Q0
Q
Voltage and frequency control
• Droop control
•Then a simple (e.g. PI) controller can be implemented. It considers
a reference voltage and a reference frequency:
•If the output voltage is different, the field excitation is changed
(and, thus, changes Q and then V).
•If the frequency is different, the prime mover torque is
changed (and thus, changes P and then f).
V  V0  kQ (Q  Q0 )
f  f0  kP ( P  P0 )
V
f
f0
V0
P0
P
© A. Kwasinski, 2014
Q0
Q
Voltage and frequency control
• Operation of a generator connected to a large grid
• A large grid is seen as an infinite power bus. That is, it is like a
generator in which
• changes in real power do not cause changes in frequency
• changes in reactive power do not originate changes in voltage
• its droop control curves are horizontal lines
V
f
P
© A. Kwasinski, 2014
Q
Voltage and frequency control
• Operator of a generator connected to a large grid
• When connected to the grid, the voltage amplitude and frequency
is set by the grid.
• In order to synchronize the oncoming generator, its frequency
needs to be slightly higher than that of the grid, but all other
variables need to be the same.
V
f
f gen
VG
fG
P
© A. Kwasinski, 2014
Q
Voltage and frequency control
• Operator of a generator connected to a large grid
• After the generator is paralleled to the grid then its output
frequency and voltage will remain fixed and equal to the grid’s
frequency and voltage, respectively.
• Output power is controlled by attempting a change in frequency by
controlling the prime mover’s torque. By “commanding” a decrease
in frequency, the output power will increase.
• A similar approach is followed with reactive power control, by
controlling field excitation in an attempt to change output voltage.
Higher commanded
frequencies
f
Higher power output
Operating frequency
No load droop line
P1
P2
© A. Kwasinski, 2014
P
A brief summary
• In conventional ac grids, large machine inertia helps to
maintain stability.
• Since frequency needs to be regulated at a precise value,
imbalances between electric and mechanical power may
make the frequency to change. In order to avoid this issue,
mechanical power applied to the generator rotor must follow
load changes. If the mechanical power cannot follow the
load alone (e.g. due to machine’s inertia), energy storage
must be used to compensate for the difference. This is a
situation often found in microgrids.
•
Reactive power is used to regulate voltage.
•
Droop control is an effective autonomous controller.
© A. Kwasinski, 2014
DC microgrids (droop control)
• Consider a microturbine in a microgrid controlled by droop control.
• Primary control:
vref  vref , NL  I T RD vref , NL  vn  VR / 2
VR  I T ,max RD
• Secondary control (voltage deviation compensation)
 vref  K p (vMG ,ref  vMG )  K i  (vMG ,ref  vMG )dt
Depends on microgrid
bus voltage
vref  ( vref  vref , NL )  I T RD
V [V]
Source Interface
NOTE: Based on Guerrero et al “Hierarchical Control
of Droop-Controlled AC and DC Microgrids—A General
Approach Toward Standardization”
vref,NL
400
390
380
370
360
δvref
Converter
rating
vn
IμT
IμT,max
0
© A. Kwasinski, 2014
ΔVR
DC microgrids (droop control)
• Tertiary control (associated with a grid tie):
 vref  K p ( I g ,ref  I g )  Ki  ( I g ,ref  I g )dt
Depends on current
to or from the grid
vref  ( vref  vref , NL )  I T RD
• Could be the input for a grid interface converter or the input for the distributed
generation sources interface. The latter applies when there is a direct
connection to a stiff grid because the stiff grid fixes the microgrid voltage. When
there is a grid outage, the tertiary control is replaced by the secondary control.
When the grid is present the secondary control is replaced by the tertiary
control.
Grid interface converter
V [V]
vref,NL
Converter
rating
δvref
Converter
rating
400
390
380
370
360
NOTE: Based on Guerrero et al
“Hierarchical Control of Droop-Controlled
AC and DC Microgrids—A General
Approach Toward Standardization”
-Ig,max
0
© A. Kwasinski, 2014
Ig,max
Ig
Secondary control
Tertiary control
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IμT
DC bus (360 to 400 V)
V [V]
Droop slope
(virtual dc output
resistance)
Microturbine
0
V [V]
0
© A. Kwasinski, 2014
LOAD
Microturbine
V [V]
Current Limit
“Power”
demand
Converter rating
400
390
380
370
360
Grid interface
converter
IL
Voltage range “to allow for power sharing and
voltage regulation using droop control”
Converter rating
Set by the utility
company
IμT
Current Limit
Ig
I μT
0
I μT
DC microgrids (droop control)
MICROTURBINE
IuT,1
uT
DC bus (360 to 400 V)
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
VDC bus [V]
400
390
380
370
360
MICROTURBINE
DC bus voltage
IuT,1 IμT,2
0
© A. Kwasinski, 2014
IL
LOAD
IμT,1+IμT,2 = IL
0
DC microgrids (droop control)
MICROTURBINE
MICROTURBINE
IuT,1
uT
DC bus (360 to 400 V)
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
VDC bus [V]
400
390
380
370
360
IL
LOAD
IμT,1+IμT,2 = IL
0
When the load increases, current is shared
between the two microturbines with the one with
the highest capacity providing more current to
the load
IμT,2
0 IuT,1
© A. Kwasinski, 2014
DC microgrids (droop control)
MICROTURBINE
MICROTURBINE
IuT,1
uT
DC bus (360 to 400 V)
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
VDC bus [V]
IL
LOAD
IμT,1+IμT,2 = IL
0
400
390
380
370
360
As the load increases, the voltage drops so
current output from the microturbines can
increase. Still, the microturbine with the highest
capacity providing more current to the load
0
IuT,1 IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT,1
uT
Ig
DC bus (360 to 400 V)
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
VDC bus [V]
IL
LOAD
Ig+IμT,1+IμT,2 = IL
0
400
390
380
370
360
When the load increases even further the
grid needs to provide the extra current in
order to prevent voltage collapse
0
IuT,1 IμT,2
© A. Kwasinski, 2014
Ig
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT,1
uT
Ig
DC bus (360 to 400 V)
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
VDC bus [V]
IL
LOAD
Ig+IμT,1+IμT,2 = IL
0
400
390
380
370
360
Current from the grid can be used to reduce
the current from the microturbines and
increase the dc bus voltage (see the voltage
in the case with the same load in slide #19)
Ig
0
IuT,1 IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT,1
uT
Ig
IμT,2
μT
Voltage range “to allow for power sharing and
voltage regulation using droop control”
DC bus (360 to 400 V)
VDC bus [V]
IL
LOAD
Ig+IμT,1+IμT,2 = IL
0
400
390
380
370
360
When the load is light, extra power
being generated by the
microturbines can be injected back
to the grid (see slide # 18)
IuT,1
Ig
0
IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
Grid interface
converter
Microturbine
V [V]
V [V]
Converter rating
Converter rating
Now, vref,NL can
be adjusted with
a δvref
400
390
380
370
360
Primary control is combined with a secondary
control to compensate voltage deviations
0
IL
LOAD
Microturbine
V [V]
Current Limit
DC bus (380 V)
IuT
Current Limit
IuT
Ig
0
I μT
0
I μT
Now, vref,NL can be adjusted with a δvref
© A. Kwasinski, 2014
DC microgrids (droop control)
MICROTURBINE
MICROTURBINE
IuT
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
VDC bus [V]
400
390
380
370
360
IL
LOAD
IμT,1+IμT,2 = IL
Nominal
Adjusted with δvref
IuT,1 IμT,2
0
© A. Kwasinski, 2014
0
DC microgrids (droop control)
MICROTURBINE
MICROTURBINE
IuT
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
VDC bus [V]
IL
LOAD
IμT,1+IμT,2 = IL
0
400
390
380
370
360
Notice that the currents are the same than in
the case with no secondary control (slide #18)
but now the voltage is kept at 380 V
IμT,2
0 IuT,1
© A. Kwasinski, 2014
DC microgrids (droop control)
MICROTURBINE
MICROTURBINE
IuT
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
Notice same δvref for
both microturnines
VDC bus [V]
400
390
380
370
360
0
IuT,1 IμT,2
© A. Kwasinski, 2014
IL
LOAD
IμT,1+IμT,2 = IL
0
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT
Ig
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
Notice lower δvref
than previous slide
VDC bus [V]
400
390
380
370
360
0
IuT,1 IμT,2
© A. Kwasinski, 2014
Ig
IL
LOAD
Ig+IμT,1+IμT,2 = IL
0
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT
Ig
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
IL
LOAD
Ig+IμT,1+IμT,2 = IL
VDC bus [V]
0
400
390
380
370
360
Ig
0
IuT,1 IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
MICROTURBINE
MICROTURBINE
GIC
IuT
Ig
DC bus (380 V)
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
IL
LOAD
Ig+IμT,1+IμT,2 = IL
VDC bus [V]
0
400
390
380
370
360
Ig
0
IuT,1IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
GRID
WIND
TURBINE
SOLAR
ARRAY
ENERGY
STORAGE
Paper: Boroyevich et al “Future
Electronic Power Distribution
Systems – A contemplative view –”
GIC
Ig
Is
DC bus (360 – 400 V)
V [V]
Ib
IL
Voltage range “to allow for power sharing and
voltage regulation using the droop control”
Droop slope
(virtual dc output
resistance)
Solar
converter
Wind
converter
V [V]
LOAD
Battery storage
converter
V [V]
V [V]
0
Ig 0
© A. Kwasinski, 2014
Is
0
Iw
Ibdsoc
0
Operating range
Converter rating
Actual
MPPT
Converter rating
Ibcsoc
Actual
MPPT
“Power”
demand
Converter rating
400
390
380
370
360
Grid interface
converter
Iw
Converter rating
Set by the utility
company
NOTE: Slide prepared by Prof.
Dushan Boroyevich from VT
Ib
DC microgrids (droop control)
• In the presence of constant-power loads, regulators in source converters
cannot use PI controllers. From a static perspective, regulators designed
for constant-power loads will make the source converter output
characteristic to look like MPP trackers.
• Battery interfaces have different characteristic depending on the state of
charge of the batteries. For example, at the float voltage, the battery may
take no current (if the state of charge is 100 %) or may take some current
if the state of charge is less than 100 %.
Microturbine with
Constant Power Load
Battery storage
converter
V [V]
V [V]
0
IμT
© A. Kwasinski, 2014
Ibdsoc
0
Operating range
Constant
Power
Output
Converter rating
Ibcsoc
Ib
DC microgrids (droop control)
GRID
SOLAR
ARRAY
WIND
TURBINE
ENERGY
STORAGE
Paper: Boroyevich et al “Future
Electronic Power Distribution
Systems – A contemplative view –”
GIC
Ig
DC bus
NOTE: Slide prepared by Prof.
Dushan Boroyevich from VT
Is
Iw
Ib
IL
360 – 400 V
VDC bus [V]
Iw+IIws+IIgsw+I
+Ig0=
bs= IL
400
390
380
370
360
Ig Ib
0 Iw IIwIswIww IIbsIIss IIsg
© A. Kwasinski, 2014
Ig
LOAD
0
DC microgrids (droop control)
MICROTURBINE
IuT
Ig
DC bus (380 V)
With a stiff grid
there is no limit
to Ig
MICROTURBINE
IuT
Primary control is combined with a secondary
control to compensate voltage deviations
Grid interface
converter
Microturbine
V [V]
V [V]
0
0
LOAD
Microturbine
V [V]
Current Limit
400
390
380
370
360
IL
Current Limit
DC GRID
I μT
0
I μT
Ig is regulated by adjusting δvref
© A. Kwasinski, 2014
© A. Kwasinski, 2014
DC microgrids (droop control)
DC GRID
MICROTURBINE
IuT
Ig
DC bus (380 V)
MICROTURBINE
IuT
Voltage is kept fixed by the stiff grid so no
voltage regulation is necessary
IL
LOAD
Ig+IμT,1+IμT,2 = IL
VDC bus [V]
0
400
390
380
370
360
IuT,1
Ig
0 IμT,2
© A. Kwasinski, 2014
DC microgrids (droop control)
DC GRID
MICROTURBINE
IuT
Ig
DC bus (380 V)
MICROTURBINE
IuT
Voltage is kept fixed by the stiff grid so no
voltage regulation is necessary
IL
LOAD
Ig+IμT,1+IμT,2 = IL
VDC bus [V]
0
400
390
380
370
360
Ig
0
IuT,1IμT,2
© A. Kwasinski, 2014
AC microgrids revisited (droop
control)
• Sources with a dc output or an ac output with a frequency different from
that of the microgrid main bus need to use an inverter to be integrated
into an ac microgrid. When implementing droop control, droop regulators
are used to emulate the inertia of an ac machine.
• Issues when implementing conventional droop control in ac systems with
inverters:
– Droop current-sharing methods are affected by harmonic content created by
non-linear loads. These issues can be solved by distorting the voltage signal
intentionally which leads to further issues.
– Frequency is dependent on load levels in the same way that voltage levels
depend on load levels. Also, frequency goals for two inverters with different
capacity may be different. Frequency deviations dependant on load levels
may lead to loss of synchronization when attempting to connect the microgrid
directly to a main grid. Hence, it is only applicable to islanded operation and
makes transition into grid connected operation complicated.
– In islanded mode there is both frequency and voltage deviations leading to
tradeoffs inherent to droop control in islanded mode.
• Secondary controls have been proposed in order to solve these issues
without the need for communication links.
© A. Kwasinski, 2014
Now secondary control
depends on microgrid
bus voltage and
frequency
   * GP ( s )( P  P*)
E  E * GQ ( s )(Q  Q*)
- GP(s) and GQ(s) represent
PI or P controllers.
- ω*, E*, P* and Q* are
reference signals, so when
P=P*, ω=ω* and when
Q=Q*, E=E*
Now tertiary control
depends on real and
reactive power flow from
or to the grid
NOTE: Figure from Guerrero et al “Hierarchical
Control of Droop-Controlled AC and DC
Microgrids—A General Approach Toward
Standardization”
© A. Kwasinski, 2014
Additional comments about droop
controls in ac microgrids
• It has been suggested by some researchers to consider energy stored in
dc link capacitors (i.e. their voltage) of ac micrgrids with inverters
interfacing sources and loads as analogous to rotating kinetic energy in
ac power grids with generators directed connected to the distribution
system.
• Zs depends on the inverter circuit components and on how the inverter is
controlled
© A. Kwasinski, 2014
Additional comments about droop
controls in ac microgrids (Review)
• Output real and reactive power of an inverter (or any source) equal
Pout 
VG E  RSL cos   X SL sin    VG2  RSL
RSL 2  X SL 2
Qout 
VG E  X SL cos   RSL sin    VG2  X SL
RSL 2  X SL 2
• In conventional power grids XL>>RL, so, without considering ZS (and
small angles
Pout , X
VG E sin 

X SL
Qout , X 
VG E  VG2 
X SL
– So, P relates to f and Q relates to V
• In microgrids, it could be expected that XL<<RL, so, without considering
ZS and small angles
Pout , R 
VG E  VG2 
Qout , R  
RSL
VG E
RSL
• So, droop relationships in microgrids may be inverted
© A. Kwasinski, 2014