Day

Day
-
-
4
4
Custom Power
Custom Power
Compensating Devices
Compensating Devices
Arindam Ghosh
Arindam Ghosh
Dept. of Electrical Engineering
Dept. of Electrical Engineering
Indian Institute of Technology
Indian Institute of Technology
Kanpur
Kanpur
, India
, India
E
E
-
-
mail:
mail:
aghosh
aghosh
@
@
iitk
iitk
.ac.in
.ac.in
226
Load Compensation using
Load Compensation using
DSTATCOM
DSTATCOM
The primary goals of a DSTATCOM are
The primary goals of a DSTATCOM are

to cancel the effect of poor load power factor

to cancel the effect of poor load power factor
such that the current drawn from the source
such that the current drawn from the source
has a near unity power factor.
has a near unity power factor.

to cancel the effect of harmonic contents in

to cancel the effect of harmonic contents in
loads such that current drawn from the
loads such that current drawn from the
source is nearly sinusoidal.
source is nearly sinusoidal.

to offset the effect of unbalanced loads such

to offset the effect of unbalanced loads such
that the current drawn from the source is
that the current drawn from the source is
balanced.
balanced.
In addition, it can also eliminate dc offset in
In addition, it can also eliminate dc offset in
loads.
loads.
227
Ideal DSTATCOM Ideal DSTATCOM

Source voltage:
Source voltage:
v
v
s s
source current:
source current:
i
i
s s
load:
load:
Z
Z
l l
load current:
load current:
i
i
l l

The DSTATCOM: current sources

The DSTATCOM: current sources
i
i
fa fa
,
,
i
i
fb fb
&
&
i
i
fc fc
Point of
Point of
common
common
coupling
coupling
228
DSTATCOM DSTATCOM - - Function Function

Inject a set of currents such that the

Inject a set of currents such that the
source currents are
source currents are
balanced and
balanced and
sinusoidal.
sinusoidal.

If the load is unbalanced then the zero

If the load is unbalanced then the zero
-
-
sequence current circulates in the path
sequence current circulates in the path
joining the neutral of the load (
joining the neutral of the load (
n
n
) and the
) and the
compensator (
compensator (
n
n

) such that the current

) such that the current
flowing through the neutral (
flowing through the neutral (
N
N
) is zero.
) is zero.

Additionally power factor correction can also

Additionally power factor correction can also
be performed such that the DSTATCOM
be performed such that the DSTATCOM
supplies part or whole of the reactive power
supplies part or whole of the reactive power
required by the load.
required by the load.
229
DSTATCOM DSTATCOM
Reference Generation Reference Generation

Generate a set of reference currents

Generate a set of reference currents
i
i
fa fa
* *
,
,
i
i
fb fb
* *
and
and
i
i
fc fc
* *
.
.

These currents when injected into the

These currents when injected into the
system will simultaneously perform the
system will simultaneously perform the
following three tasks.
following three tasks.
a)
a)
Force the zero
Force the zero
-
-
sequence source current to
sequence source current to
be zero, i.e.,
be zero, i.e.,
0 = + +
sc sb sa
i i i

This guarantees that the zero

This guarantees that the zero
-
-
sequence
sequence
current flowing through the source neutral
current flowing through the source neutral
is zero in a 3
is zero in a 3
-
-
phase, 4
phase, 4
-
-
wire system
wire system
230
DSTATCOM DSTATCOM
Reference Generation Reference Generation
b)
b)
Force the angle between the source voltage
Force the angle between the source voltage
and current to be a specified value, i.e.,
and current to be a specified value, i.e.,
( ) ( )
( ) ( )
( ) ( )

+ =
+ =
+ =
sc sc
sb sb
sa sa
i v
i v
i v

In the above,
In the above,

is pre
is pre
-
-
specified.
specified.

If
If

= 0, the source voltage and load

= 0, the source voltage and load
current are in phase.
current are in phase.
This will be called
This will be called
the unity power factor (
the unity power factor (
upf
upf
) operation.
) operation.
231
DSTATCOM DSTATCOM Reference Reference
Generation Generation
c)
c)
The DSTATCOM must not absorb/generate
The DSTATCOM must not absorb/generate
any real power in the steady state, i.e., it only
any real power in the steady state, i.e., it only
supplies zero
supplies zero
-
-
mean oscillating power.
mean oscillating power.
232
DSTATCOM DSTATCOM Reference Reference
Generation Generation

The power condition can be written as

The power condition can be written as
lav sc sc sb sb sa sa
p i v i v i v = + +

In the above
In the above
p
p
lav lav
is the average power drawn
is the average power drawn
by the load.
by the load.

To calculated it on line, 3 Hall

To calculated it on line, 3 Hall
-
-
effect voltage
effect voltage
and 3 current transformers are required.
and 3 current transformers are required.

A low
A low
-
-
pass
pass
Butterworth
Butterworth
(or
(or
Chebyshev
Chebyshev
) filter
) filter
can be used. It will have longer settling time.
can be used. It will have longer settling time.

Alternatively a moving average filter settles

Alternatively a moving average filter settles
in just half (or full) cycle.
in just half (or full) cycle.
233
DSTATCOM DSTATCOM Reference Reference
Generation Generation

Since
Since
c b a k i i i
sk lk fk
, , , = =

The reference currents are

The reference currents are
( )
( )
( )
3 tan ,
2 2 2
2 2 2
2 2 2

+ +
+
=
+ +
+
=
+ +
+
=

lav
sc sb sa
sb sa sc
lc fc
lav
sc sb sa
sa sc sb
lb fb
lav
sc sb sa
sc sb sa
la fa
p
v v v
v v v
i i
p
v v v
v v v
i i
p
v v v
v v v
i i
234
DSTATCOM DSTATCOM Ideal Ideal
Compensation Compensation
In a similar way the algorithm can be applied to
In a similar way the algorithm can be applied to
3p, 3w systems with both star and delta
3p, 3w systems with both star and delta
connected loads.
connected loads.

=
=
-
-
30
30
0 0

= 30
= 30
0 0
235
DSTATCOM Structure DSTATCOM Structure
A DSTATCOM is realized by a VSC. The VSC
A DSTATCOM is realized by a VSC. The VSC
is supplied by a dc capacitor. The
is supplied by a dc capacitor. The
DSTATCOM must meet the following
DSTATCOM must meet the following
requirements:
requirements:

It should be able to inject distorted and

It should be able to inject distorted and
negative
negative
-
-
sequence currents.
sequence currents.

It must be able to provide a circulating path

It must be able to provide a circulating path
for the zero
for the zero
-
-
sequence load current such
sequence load current such
that it does not travel towards the source.
that it does not travel towards the source.

It must be able to regulate the voltage dc

It must be able to regulate the voltage dc
capacitor such that no external battery
capacitor such that no external battery
source is required.
source is required.
236
DSTATCOM Structure DSTATCOM Structure

A common dc capacitor makes the distribution

A common dc capacitor makes the distribution
of power easier.
of power easier.

The transformers provides isolation in addition

The transformers provides isolation in addition
to voltage step down.
to voltage step down.
My favorite
My favorite
structure
structure
237
DSTATCOM Structure DSTATCOM Structure

Suitable for dc compensation.

Suitable for dc compensation.

Two capacitors (
Two capacitors (
C
C
dc dc1 1
and
and
C
C
dc dc2 2
) required.
) required.

An additional chopper circuit containing

An additional chopper circuit containing
S
S
ch ch1 1
,
,
S
S
ch ch2 2
,
,
R
R
p p
and
and
L
L
p p
required.
required.
238
DSTATCOM Structure DSTATCOM Structure

Current
Current
I
I
P P
builds up in
builds up in
L
L
p p
in the direction
in the direction
shown.
shown.

Also the voltage

Also the voltage
V
V
dc dc2 2
falls.
falls.

Once
Once
I
I
P P
builds up to a desired level,
builds up to a desired level,
S
S
ch ch2 2
is
is
opened.
opened.

The current
The current
I
I
P P
discharges through
discharges through
D
D
ch ch1 1
charging the capacitor
charging the capacitor
C
C
dc dc1 1
.
.

The voltage
The voltage
V
V
dc dc1 1
rises.
rises.

Normally the switches

Normally the switches
S
S
ch ch1 1
are
are
S
S
ch ch2 2
open.
open.

Suppose
Suppose
V
V
dc dc1 1
drops and
drops and
V
V
dc dc2 2
rises.
rises.

S
S
ch ch2 2
is closed.
is closed.
I
p
239
DSTATCOM Structure DSTATCOM Structure

Zero sequence current is routed to path

Zero sequence current is routed to path
n
n

n
n

.
.

This must be supplied by the 4

This must be supplied by the 4
th th
leg.
leg.

Complexity and switching losses increase.

Complexity and switching losses increase.
240
DSTATCOM Control DSTATCOM Control

The reference current equations also get

The reference current equations also get
modified in which
modified in which
p
p
lav lav
is replaced by the sum
is replaced by the sum
p
p
lav lav
+
+
p
p
loss loss
.
.
loss lav sc sc sb sb sa sa
p p i v i v i v + = + +

The dc voltage supplying the DSTATCOM

The dc voltage supplying the DSTATCOM
must be held constant irrespective of the
must be held constant irrespective of the
losses in the circuit.
losses in the circuit.

This can be accomplished by drawing

This can be accomplished by drawing
power from the ac system to compensate
power from the ac system to compensate
for the losses.
for the losses.

The power equation then must be modified

The power equation then must be modified
to incorporate the losses as
to incorporate the losses as
241
DSTATCOM Control DSTATCOM Control

V
V
ref ref
is the reference dc voltage and
is the reference dc voltage and
V
V
dc dc, ,av av
is
is
the average dc voltage.
the average dc voltage.

Any deviation of the capacitor voltage from

Any deviation of the capacitor voltage from
the reference is due to losses.
the reference is due to losses.

The PI controller loop draws the loss from

The PI controller loop draws the loss from
the ac system to hold the voltage constant.
the ac system to hold the voltage constant.
av dc ref
I P loss
V V e
dt e K e K p
,
=
+ =

Here
Here
p
p
loss loss
is generated by dc capacitor
is generated by dc capacitor
control loop.
control loop.

A PI controller is used for generating this

A PI controller is used for generating this
242
DSTATCOM DSTATCOM
Hysteresis Hysteresis Control Control

The reference currents are tracked in a

The reference currents are tracked in a
hysteresis
hysteresis
band.
band.
243

Simulation results of the DSTATCOM operation.

Simulation results of the DSTATCOM operation.
DSTATCOM Operation DSTATCOM Operation
244
DSTATCOM Operation DSTATCOM Operation
Simulation
Simulation
Experimental
Experimental
245
DSTATCOM Connected to DSTATCOM Connected to
weak ac supply point weak ac supply point

The a load is supplied by a feeder.

The a load is supplied by a feeder.

The DSTATCOM is connected at PCC

The DSTATCOM is connected at PCC
(
(
terminal
terminal
) at the end of the feeder.
) at the end of the feeder.
246
DSTATCOM Connected to DSTATCOM Connected to
weak ac supply point weak ac supply point

The reference generation algorithm assumes

The reference generation algorithm assumes
balanced supply.
balanced supply.

The left figure shows when the measured

The left figure shows when the measured
terminal voltages are used and the right figure
terminal voltages are used and the right figure
shows when fundamental voltages are used.
shows when fundamental voltages are used.
247
DSTATCOM Connected to DSTATCOM Connected to
weak ac supply point weak ac supply point

To bypass the switch frequency harmonic

To bypass the switch frequency harmonic
generated, capacitor
generated, capacitor
C
C
f f
is used (left).
is used (left).

The
The
hysteresis
hysteresis
tracking of
tracking of
i
i
f f
results in an
results in an
unstable feedback system (right).
unstable feedback system (right).

Alternate control strategy required.

Alternate control strategy required.
248
DSTATCOM DSTATCOM Switching Control Switching Control

One approach is state feedback control using

One approach is state feedback control using
linear quadratic regulator (LQR).
linear quadratic regulator (LQR).

This will require on

This will require on
-
-
line generation of all the
line generation of all the
reference states.
reference states.

The LQR design is robust to parameter

The LQR design is robust to parameter
uncertainties.
uncertainties.

The output feedback approach is simpler but

The output feedback approach is simpler but
cannot guarantee stability.
cannot guarantee stability.

Bu Ax x + =

K
x
x
ref
+
249
DSTATCOM DSTATCOM Performance for Performance for
Non Non- -Stiff Systems Stiff Systems

With LQR control the source currents become

With LQR control the source currents become
sinusoidal.
sinusoidal.

To achieve this, the PCC (terminal) voltages

To achieve this, the PCC (terminal) voltages
are forced to be sinusoidal.
are forced to be sinusoidal.
250
DSTATCOM DSTATCOM Performance for Performance for
Non Non- -Stiff Systems Stiff Systems

The unbalance and harmonics in the source

The unbalance and harmonics in the source
voltages get reflected in the source currents.
voltages get reflected in the source currents.
251
DSTATCOM in Voltage
DSTATCOM in Voltage
Control Mode
Control Mode

The
The
Thevenin
Thevenin
equivalent of the system looking at
equivalent of the system looking at
Bus
Bus
-
-
3 is the same as those shown in Slides 245
3 is the same as those shown in Slides 245
or 247.
or 247.

We can then use a state feedback or output

We can then use a state feedback or output
feedback controller to hold the voltage of Bus
feedback controller to hold the voltage of Bus
-
-
3.
3.

Let us assume that

Let us assume that
we would like to
we would like to
correct the voltage
correct the voltage
of Bus
of Bus
-
-
3.
3.
252
Voltage Control Mode Voltage Control Mode
Reference Generation Reference Generation
( ) ( )

+ = dt v v K v v K p
dcav dref I dcav dref p shref

The magnitude of the bus voltage can be

The magnitude of the bus voltage can be
chosen arbitrarily.
chosen arbitrarily.

The phase angle (

The phase angle (

) should be controlled to
) should be controlled to
ensure the power flow.
ensure the power flow.

Note that the dc capacitor must be able to

Note that the dc capacitor must be able to
supply the DSTATCOM while maintaining its
supply the DSTATCOM while maintaining its
voltage by drawing power from the ac system.
voltage by drawing power from the ac system.

Two PI controllers are designed in an inner

Two PI controllers are designed in an inner
power and outer voltage loop configuration.
power and outer voltage loop configuration.

The outer loop regulates the dc voltage as

The outer loop regulates the dc voltage as
253
Voltage Control Mode Voltage Control Mode
Reference Generation Reference Generation

The inner loop set the angle by adjusting

The inner loop set the angle by adjusting
the power difference as
the power difference as
( ) ( )

+ = dt p p C p p C
shav shref I shav shref P

254
Voltage Control Mode Voltage Control Mode
Results Results

The source is
The source is
balanced but the
balanced but the
load is unbalanced
load is unbalanced
& distorted.
& distorted.

The uncompensated
The uncompensated
bus voltage (
bus voltage (
v
v
ta ta
) is
) is
distorted (top
distorted (top
trace).
trace).

The compensated
The compensated
bus voltage (
bus voltage (
v
v
ta ta
) is
) is
distortion free
distortion free
(bottom trace).
(bottom trace).
255
Voltage Control Mode Voltage Control Mode
Results Results
Load angle and power drawn from the ac system.
Load angle and power drawn from the ac system.
256
Voltage Control Mode Voltage Control Mode
Results Results

The source
The source
voltages are
voltages are
unbalanced (top
unbalanced (top
trace) and the
trace) and the
load is both
load is both
unbalanced and
unbalanced and
distorted.
distorted.

The DSTATCOM
The DSTATCOM
makes the bus
makes the bus
voltages balanced
voltages balanced
(bottom trace).
(bottom trace).
257
Dynamic Voltage Restorer
Dynamic Voltage Restorer
(DVR)
(DVR)

In addition, it can also tightly

In addition, it can also tightly
regulate the voltage at the
regulate the voltage at the
load bus.
load bus.

A DVR is a
A DVR is a
series
series
connected
connected
device.
device.

It protects
It protects
sensitive loads
sensitive loads
from sag,
from sag,
swell or
swell or
source voltage
source voltage
distortion.
distortion.
258
DVR DVR Voltage Support Voltage Support

A fault in either locations shown will results in

A fault in either locations shown will results in
a voltage sag in the feeder supplying the load.
a voltage sag in the feeder supplying the load.

The DVR adds a voltage in series to offset the

The DVR adds a voltage in series to offset the
sag.
sag.
SENSITIVE
LOAD
DVR
259
DVR Installations DVR Installations

In August 1996, Westinghouse Electric Corporation

In August 1996, Westinghouse Electric Corporation
installed world's first DVR in Duke Power Company's
installed world's first DVR in Duke Power Company's
12.47 kV substation in Anderson, South Carolina to
12.47 kV substation in Anderson, South Carolina to
protect to an automated rug manufacturing plant.
protect to an automated rug manufacturing plant.

The next commissioning of a DVR was done by

The next commissioning of a DVR was done by
Westinghouse in February 1997 in
Westinghouse in February 1997 in
Powercor's
Powercor's
22 kV
22 kV
distribution system at Stanhope, Victoria, Australia.
distribution system at Stanhope, Victoria, Australia.

This was done to protect a diary milk processing

This was done to protect a diary milk processing
plant.
plant.

The saving that may result from the installation of

The saving that may result from the installation of
this DVR is estimated at over $100,000 per year.
this DVR is estimated at over $100,000 per year.

The first ever platform mounted DVR was installed

The first ever platform mounted DVR was installed
to protect Northern Lights Community College and
to protect Northern Lights Community College and
several other smaller loads in Dawson Creek, British
several other smaller loads in Dawson Creek, British
Columbia, Canada.
Columbia, Canada.
260
The Study System The Study System
The DVR is represented by voltage sources
The DVR is represented by voltage sources
v
v
fa fa
,
,
v
v
fb fb
and
and
v
v
fc fc
.
.

N
Z
la
Z
lb
Z
lc
n
v
sa

v
sb

v
sc

v
fa

v
fb

v
fc

v
t

v
l
i
sa
i
sb

i
sc
R
X
R
X
R
X
+
+
+
+
+
+
+ +
PCC
261
DVR Structures DVR Structures

The dc storage is one important issue in the

The dc storage is one important issue in the
DVR design.
DVR design.

Schematic diagrams of (a) rectifier supported

Schematic diagrams of (a) rectifier supported
and (b) capacitor supported are shown.
and (b) capacitor supported are shown.

Other possibility is battery back up/support.

Other possibility is battery back up/support.
262
Capacitor Capacitor
Supported DVR Supported DVR

Assume that source voltage and desired load

Assume that source voltage and desired load
voltage to be 1.0 per unit.
voltage to be 1.0 per unit.

Then the injection must be such that

Then the injection must be such that
OA
OA
represent the source voltage.
represent the source voltage.

The DVR should

The DVR should
not absorb any
not absorb any
real power in the
real power in the
steady state.
steady state.

Voltage injection
Voltage injection
must be in
must be in
quadrature
quadrature
with
with
line current
line current
263
Capacitor Capacitor
Supported DVR Supported DVR

The compensator power is zero in the steady

The compensator power is zero in the steady
state.
state.
264
Capacitor Supported Capacitor Supported
DVR Implementation DVR Implementation

The DVR on the left has a filter capacitor in

The DVR on the left has a filter capacitor in
parallel with the transformer secondary.
parallel with the transformer secondary.

The DVR on the right has an LC filter connected

The DVR on the right has an LC filter connected
in the primary side of the transformer.
in the primary side of the transformer.
265
Capacitor Supported Capacitor Supported
DVR Limitations DVR Limitations

Without active power exchange with the ac

Without active power exchange with the ac
system, the capacity of the DVR to compensate
system, the capacity of the DVR to compensate
a deep sag gets limited.
a deep sag gets limited.
266
Rectifier Supported Rectifier Supported
DVR DVR

Power flow is unidirectional, i.e., from the

Power flow is unidirectional, i.e., from the
rectifier to DVR and not the other way
rectifier to DVR and not the other way
round.
round.

Difficult for the DVR to absorb power

Difficult for the DVR to absorb power
transiently.
transiently.

During deep sag, if the rectifier output

During deep sag, if the rectifier output
capacitor voltage exceeds the peak of the
capacitor voltage exceeds the peak of the
input ac voltage, the rectifier will cut off.
input ac voltage, the rectifier will cut off.

The rectifier generates harmonics at its

The rectifier generates harmonics at its
input terminals.
input terminals.

Solutions: Use UPQC.

Solutions: Use UPQC.
267
Unified Power Quality
Unified Power Quality
Conditioner (UPQC)
Conditioner (UPQC)
268
UPQC UPQC

A UPQC has a shunt inverter and a series

A UPQC has a shunt inverter and a series
inverter, both connected together through a
inverter, both connected together through a
dc storage capacitor.
dc storage capacitor.

The dc capacitor facilitates power exchange

The dc capacitor facilitates power exchange
between the two inverters.
between the two inverters.

The UPQC can independently or jointly

The UPQC can independently or jointly
perform the tasks of both the series and
perform the tasks of both the series and
shunt compensators.
shunt compensators.

No much research done.

No much research done.

Whether the device justifies its high cost.

Whether the device justifies its high cost.

Possibly the star of the future.

Possibly the star of the future.
269
Custom Power Park
Custom Power Park
270
Mini Custom Power Park Mini Custom Power Park
L-1 L-2
L-3
DSTAT
1 s
v
2 s
v
PF
AF
STS
CPP Bus
B1
B2
B3
B4
DG
C
f

STS is GTO based.

STS is GTO based.

B1, B2 are
B1, B2 are
SCBs
SCBs
, while B3, B4 are mechanical.
, while B3, B4 are mechanical.
271
Mini Custom Power Park Mini Custom Power Park
Loads L
Loads L
-
-
2 and L
2 and L
-
-
3 and the CPP bus voltage are
3 and the CPP bus voltage are
balanced sinusoidal irrespective of L
balanced sinusoidal irrespective of L
-
-
1 being
1 being
unbalanced and distorted.
unbalanced and distorted.
272
Thank you
Thank you