Voltage Regulation in Distribution Systems with Distributed Generation (DG)

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Dec 12, 2013 (3 years and 5 months ago)

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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

Broadband Communications Research (BBCR) Lab

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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

7

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group


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

Broadband Communications Research (BBCR) Lab

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

Smart Grid Research Group


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

Broadband Communications Research (BBCR) Lab

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?

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

Optimization Based Voltage Regulation

13

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

14

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group


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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group


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

Smart Grid Research Group

Estimation Based Voltage Regulation

17

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

18

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group


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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group


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

Smart Grid Research Group

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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

Combined Measurement and Estimation Based
Voltage Regulation

22

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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

Smart Grid Research Group

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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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

Smart Grid Research Group

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


Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

RTU Coordination Based Voltage Regulation

28

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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


Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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
:


Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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

Smart Grid Research Group

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

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

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

Smart Grid Research Group

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?


Broadband Communications Research (BBCR) Lab

Smart Grid Research Group

Thank you!

35

Broadband Communications Research (BBCR) Lab

Smart Grid Research Group