Voltage Regulation in Distribution
Systems with
Distributed Generation (DG)
Presented by: Hao Liang
2012.11.7
Broadband Communications
Research (BBCR) Lab
Smart Grid Research Group
Outline
2
•
Introduction
•
The Voltage Rise Problem
•
Optimization Based Voltage Regulation
•
Estimation Based Voltage Regulation
•
Combined Measurement and Estimation Based Voltage Regulation
•
Remote Terminal Unit (RTU) Coordination Based Voltage Regulation
•
Discussions
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Smart Grid Research Group
Introduction
3
Broadband Communications Research (BBCR) Lab
Smart Grid Research Group
4
•
In
the
recent
years,
distributed
generation
(DG)
has
experienced
significant
growth
all
over
the
world
•
Potentially
powered
by
renewable
energy
sources
such
as
wind
and
solar,
DG
units
are
able
to
supply
the
electricity
demands
in
an
more
economical
and
environmentally

friendly
way
as
compared
with
the
conventional
centralized
generators
•
However,
the
adoption
of
DG
units
(which
are
intermittent
in
nature)
poses
new
challenges
on
distribution
system
engineering
•
One
of
the
most
challenging
issues
is
the
voltage
regulation
,
which
is
aimed
at
keeping
the
voltage
level
of
the
distribution
system
within
a
certain
range
•
Distributed Generation (DG) Integration
5
•
WAN
: Fiber optics, microwave, cellular (e.g., GPRS, 3G, HSPA+, and LTE)
•
HAN: ZigBee, WiFi, power line communications (PLC)
•
NAN: ZigBee, WiFi
Picture: http://www.fluidmesh.com
•
Smart Grid Communications
Focus of This Talk
The Voltage Rise Problem
6
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7
Broadband Communications Research (BBCR) Lab
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•
Causes of Voltage Rise
•
The
voltage
(
V
2
)
at
a
remote
point
on
a
feeder
can
be
approximately
calculated
as
V
1
–
Source
voltage
R
+
jX
–
Impedance
of
the
line
between
the
source
and
the
remote
point
P
and
Q
–
Active
and
reactive
power
flows,
respectively
•
Model
of
a
3

bus
feeder
(no
DG)
or
in per unit
8
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Smart Grid Research Group
•
Causes of Voltage Rise (Cont’d)
•
If
the
DG
unit
injects
active
power
into
the
system
(i
.
e
.
,
the
DG
unit
is
working
at
unity
power
factor),
we
have
•
Model
of
a
3

bus
feeder
(with
DG)
•
If
the
output
of
the
DG
unit
is
large,
(
P
−
P
G
)
becomes
negative
.
As
a
result,
V
2
can
be
larger
than
V
1
,
which
causes
the
voltage
rise
problem
9
Broadband Communications Research (BBCR) Lab
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•
Possible Solutions for Voltage Rise Problem
•
Network
Reinforcement
–
The
network
reinforcement
method
aims
at
reducing
R
to
mitigate
the
voltage
rise
effect
.
However,
the
cost
is
relatively
high
for
upgrading
the
feeders
10
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Smart Grid Research Group
•
Possible Solutions for Voltage Rise Problem (Cont’d)
•
Generation
Curtailment
–
When
the
load
demand
is
low,
some
of
the
generation
capacity
(
P
G
)
can
be
curtailed
to
reduce
the
voltage
rise,
at
the
cost
of
a
reduction
in
the
profit
of
DG
.
The
significance
of
the
cost
is
heavily
dependent
on
the
electricity
price
and
may
be
less
when
the
electricity
price
is
low
as
the
load
demand
is
low
11
Broadband Communications Research (BBCR) Lab
Smart Grid Research Group
•
Possible Solutions for Voltage Rise Problem (Cont’d)
•
Reactive
Power
Compensation
–
The
reactive
power
compensation
method
requires
the
generator
to
absorb
a
certain
amount
of
reactive
power
(
Q
)
.
However,
this
method
is
not
very
efficient
(especially
for
cable
feeders)
since
the
X/R
ratio
in
distribution
systems
is
typically
much
smaller
than
that
in
transmission
systems
.
At
the
same
time,
this
method
results
in
higher
losses
in
distribution
systems
12
•
Possible Solutions for Voltage Rise Problem (Cont’d)
•
Voltage
Regulator
–
The
voltage
rise
effect
can
also
be
addressed
by
decreasing
the
source
voltage
V
1
.
Remote
terminal
units
(RTUs)
are
deployed
at
strategic
locations
across
the
network
for
voltage
and
power
flow
measurements
.
The
measured
information
is
transmitted
to
the
voltage
regulator
at
the
primary
transformer
(
via
certain
communication
networks
in
the
context
of
Smart
Grid
)
for
voltage
control
Cost of Communications?
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Optimization Based Voltage Regulation
13
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•
Basic Idea
•
A
linear
programming
(LP)
based
formulation
of
the
optimal
power
flow
(OPF)
is
considered
.
The
objective
is
to
minimize
the
annual
active
generation
curtailment
cost,
while
satisfying
voltage
and
thermal
constraints
.
The
decision
variables
are
the
generation
curtailment,
reactive
compensation,
and
coordinated
voltage
regulation
(area
voltage
control)
using
OLTC
15
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•
Problem Formulation
Minimize the annual active generation
curtailment cost
Active power injection
Reactive power injection
Load flows of the branch
ij
Tap setting of the tap

changer
k
Reactive power curtailment may be
correlated with the active power curtailment
Limits
16
•
Discussions
•
The
main
drawback
of
the
optimization
based
voltage
regulation
scheme
is
that
it
can
hardly
be
implemented
in
real
time
as
it
is
not
possible
to
read
the
information
of
all
nodes
in
the
distribution
system
.
How
to
reduce
the
number
of
measurements
is
the
main
issue
to
be
addressed
Broadband Communications Research (BBCR) Lab
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Estimation Based Voltage Regulation
17
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18
Broadband Communications Research (BBCR) Lab
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•
Basic Idea
•
A
reference
feeder
without
DG
unit
is
needed
•
The
key
technique
is
the
estimation
of
the
output
of
generator
which
is
connected
at
a
remote
point
on
the
feeder
19
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•
The Voltage Regulation Strategy
•
An
additional
current
measurement
I
FG
is
used
.
The
ratio
E
ST
represents
the
load
share
between
feeders
with
embedded
generation
to
those
without
generators,
given
by
where
I
TL
is
the
summation
of
transformer
currents
.
The
factor
E
ST
is
calculated
before
the
connection
of
the
DG
unit
or
when
the
output
of
the
DG
unit
is
zero
20
•
The Voltage Regulation Strategy (Cont’d)
•
Then,
during
the
operation
of
the
DG
unit,
the
generation
output
can
be
estimated,
given
by
•
Based
on
the
value
of
I
G
,
the
voltage
rise
at
the
connection
point
of
the
DG
can
be
calculated
as
This
value
corresponds
to
the
necessary
voltage
reduction
at
the
substation
in
order
to
bring
the
voltage
level
at
the
point
of
DG
connection
within
statutory
limits
Broadband Communications Research (BBCR) Lab
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21
•
Discussions
•
This
scheme
does
not
take
advantage
of
the
instantaneous
measurements
obtained
from
a
communication
network,
which
is
expected
to
be
deployed
in
the
next
generation
power
grid
(also
referred
to
as
the
smart
grid)
•
This
scheme
requires
a
reference
feeder
without
DG
•
In
addition,
the
scheme
is
used
to
solve
the
voltage
rise
problem,
while
the
low
voltage
point
(below
the
statutory
limit)
can
hardly
be
detected
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Combined Measurement and Estimation Based
Voltage Regulation
22
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23
•
Basic Idea
•
Investigate
both
real

time
measurements
and
statistical
estimation
•
Specifically,
based
on
the
information
of
real

time
measurements
and
load
profile,
the
voltage
magnitude
at
each
network
node
(supplied
by
the
primary
substation)
can
be
estimated
with
certain
accuracy
Broadband Communications Research (BBCR) Lab
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24
•
Voltage Estimation
•
A
weighted
least
squares
method
is
used,
where
the
state
variables
are
defined
with
respect
to
the
node
voltage
magnitudes
and
relative
phase
angles
,
given
by
where
x
j
represents
the
state
variables,
N
s
is
the
number
of
state
variables,
z
i
denotes
the
i
th
measurement,
N
m
is
the
number
of
measurements,
f
i
is
a
function
relating
i
th
measurement
to
state
variables,
and
σ
i
is
the
variance
of
the
i
th
measurement
•
Three
types
of
function
f
i
is
used,
which
relates
the
state
variables
V
i
,
V
k
,
θ
i
,
and
θ
k
to
the
measurements
P
inj
,
Q
inj
,
and
V
i
,
given
by
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25
•
Voltage Estimation (Cont’d)
•
For
state
estimation
on
a
distribution
network,
we
have
N
m
≪
N
s
.
However,
the
necessary
condition
for
a
weighted
least
squares
state
estimation
algorithm
to
have
a
unique
solution
is
N
m
≥
N
s
Power flow equations, which
can be solved by using the
Newton

Raphson
method
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26
•
Voltage Estimation (Cont’d)
•
Therefore,
the
pseudo
measurements
which
are
derived
from
offline
data
,
can
be
used
to
provide
unmeasured
quantities,
so
that
N
m
≥
N
s
•
In
this
work,
the
unmeasured
quantities
are
P
inj
and
Q
inj
at
load
nodes
and
so
pseudo
measurements
are
used
for
these
.
The
measurements
are
assumed
to
be
independent
normal
random
variables
,
with
variance
being
greater
for
pseudo
measurements
than
for
real

time
measurements
Power flow equations, which
can be solved by using the
Newton

Raphson
method
Broadband Communications Research (BBCR) Lab
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27
•
Discussions
•
The
combined
measurement
and
estimation
based
voltage
regulation
scheme
is
very
complicated
which
requires
the
solution
of
non

linear
equations
.
How
to
reduce
the
complexity
and
apply
the
scheme
in
real
time
is
still
an
open
issue
•
Moreover,
the
performance
of
the
scheme
heavily
depends
on
the
load
models
.
How
to
improve
the
modeling
accuracy
based
on
load
profiles
is
interesting
and
requires
future
research
•
In
addition,
the
voltage
controller
used
in
this
work
is
not
optimized,
which
potentially
degrades
the
efficiency
of
voltage
regulation
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RTU Coordination Based Voltage Regulation
28
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29
•
Voltage Profile Estimation
•
RTUs
are
deployed
at
the
DG
and
capacitor
connecting
buses
•
Theorem
1
.
For
the
voltage
profile
of
a
feeder,
maximum
voltage
can
happen
only
at
the
DG
connecting
buses,
capacitors
connecting
buses,
and
the
substation
bus,
provided
that
the
R/X
ratio
of
the
feeder
is
constant
along
the
whole
feeder
•
Theorem
2
.
There
exists
a
minimum
voltage
point
in
between
two
DG
connecting
buses
if
and
only
if,
for
both
DGs,
the
voltage
of
the
DG
neighboring
bus,
in
the
direction
of
the
other
DG,
is
less
than
the
voltage
of
the
DG
bus
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Directly Measurable
Need to be Estimated
30
•
Voltage Profile Estimation (Cont’d)
•
Assume
the
load
between
two
DG
units
is
concentrated
halfway
between
them
.
The
minimum
voltages
calculated
by
DG
1
and
DG
2
are,
respectively,
given
by
•
In
order
to
get
a
better
estimation
of
the
minimum
voltage,
we
take
the
average
:
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31
•
Voltage Profile Estimation (Cont’d)
•
In
order
to
achieve
voltage
regulation,
each
RTU
measures
the
following
information
1
)
The
voltage
of
its
element
bus
2
)
Active
and
reactive
power
flow
in
lines
connected
to
its
element
bus
3
)
The
voltages
of
the
immediate
neighbor
buses
of
its
element
bus
•
Based
on
the
measurements,
each
RTU
can
perform
the
following
calculation
1
)
Measure
a
maximum
voltage
point
2
)
Check
one
part
of
the
condition
for
the
possibility
of
the
existence
of
a
minimum
voltage
point
between
its
element
and
any
neighbor
element
3
)
Estimate
the
value
of
the
minimum
voltage
point
on
each
side
of
its
element,
if
it
exists
Broadband Communications Research (BBCR) Lab
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32
•
Voltage Profile Estimation (Cont’d)
•
A
communication
network
is
established
for
the
RTUs
to
pass
the
calculated
information
to
the
voltage
regulator
•
Data
passing
starts
from
the
farthest
RTU
towards
the
voltage
regulator
•
Along
the
data
passing
path,
each
upstream
RTU
updates
the
maximum
and
minimum
voltages
based
on
the
information
received
from
its
downstream
RTU
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33
•
Voltage Regulator Operation
•
When
the
information
with
respect
to
the
maximum
and
minimum
voltages
are
received
by
the
voltage
regulator,
the
following
operations
are
performed
:
1
)
If
the
absolute
maximum
voltage
is
greater
than
maximum
permissible
voltage,
then
the
voltage
regulator
will
decrease
the
current
tap
position
till
the
maximum
voltage
of
the
feeder
is
within
the
permissible
range
2
)
If
the
minimum
voltage
of
the
feeder
is
below
the
minimum
permissible
voltage,
then
the
voltage
regulator
will
increase
the
tap
position
to
bring
the
minimum
voltage
into
the
permissible
range
Broadband Communications Research (BBCR) Lab
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34
•
Discussions
•
The
RTU
based
voltage
regulation
can
efficiently
utilizes
the
instant
measurements
at
the
generation
buses
.
However,
how
to
integrates
the
estimates
of
the
loads
to
improve
the
voltage
regulation
performance
is
still
an
open
issue
•
The
scheme
is
not
optimized
for
certain
objective
functions
•
The
reliability
of
the
communication
network?
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Thank you!
35
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