STATCOM

Static compensator
 Synchronous compensator
System bus
V
System bus
V
Coupling
Transformer
Coupling
Transformer Transformer
leakage I
M/c Syn inductance
reactance + I
Transformer STATCOM
leakage
inductance DC_AC
switching
converter
Rotating
Synchronous
machine +

Exciter Vdc Cs
Rotating Synchronous compensator Static Synchronous compensator
 Statcom basics
• STATic COMpensator (STATCOM) is a second
generation shunt connected FACTS device
based on a voltage source converter (VSC)
using GTOs.
• STATCOM maintains the bus voltage by
supplying the required reactive power even at
low bus voltages and improves the power
swing damping.
• STATCOM has several advantages over the
conventional Static Var Compensation (SVC).
 Statcom Analysis
System voltage

VSC output
+Vd/2

S1
Vd C

S2
C
Filter
Compensator voltage
-Vd/2
 VSC basics

+Vd/2

VSC output
+Vd/2
0

S1 -Vd/2
Vd C

T/2 T/2

S2 S1 on S1 off
C S2 off S2 on

-Vd/2 Frequency f= 1/T

 VSC basics
+Vd/2

-Vd/2
+Vd/2

T/2 T/2
S1
Vd C S1 on S1 off
S2 off S2 on
N
S2 S1 Ean
S2 EaN
0 0 0* not used in two level
C VSC output 0 1 +Vd/2
1 0 -Vd/2
-Vd/2 1 1 Forbidden*, shorts battery
 VSC Analysis Em

Vd/2

VSC output
+Vd/2
-Vd/2

S1
Vd C

S2
C
Filter
Compensator voltage
-Vd/2
 Van=Vm Sinωt
Vm
VSC Vd/2

0 π
+Vd/2
T/2

S1 Practical conversion
Vd C RMS value of the fundamental
component

 2  Vd 
S2
E1 _ rms  {  sin d } / 2 
C
Filter
Ean   0 2 
-Vd/2 2
E1 _ rms  Vd  0.45Vd

Relationship between DC and AC voltage
 Fundamental component of VSC
output
+Vd/2 2
Eac _ rms  Vdc

2
eac (t )  Vdc Sin(t   )
VSC output
+Vd/2

S1
Vd C

S2
C
Filter
Compensator voltage
-Vd/2
 For single phase Half bridge

+Vd/2

v v
V

Fundamental
component
-Vd/2

2  1 1 
e(t )  Vd sin t  sin 5t  sin 7t  ............
  3 7 
 Practical conversion
VSC RMS value of the fundamental
component

 2  Vd 
E1  {
 
0 2
sin d } / 2 

2
E1  Vd  0.45Vd

+Vd/2
Ex: for 100 volt battery voltage
S1 Corresponding fundamental
Vd C component of ac voltage
E1 = 0,45 x 100 = 45 volts

100 V S2
C Van
Filter
45 V
-Vd/2

Relationship between DC and AC voltage

 VSC Instantaneous Inversion mode
EaN

Id
+Vd/2 Ia

G1
Vd C

G1
N

G2 Ia
C
EaN

-Vd/2 EaN Positive

Ia negative , from DC to AC
Capacitor C1 discharging through G1
 VSC instantaneous
EaN
rectifier mode
Id
+Vd/2 Ia

D1
S1
Vd C1 D1

S2 Ia
C2 D2
EaN

-Vd/2

Capacitor C1 charging through D1

 Van

VSC Instantaneous Inversion mode

+Vd/2 Id

G1
C1 G2 Ia
Vd

G2
Ia
C2
Van

-Vd/2

Capacitor C2 discharging through G2

 VSC instantaneous EaN
rectifier mode
D2

Id
+Vd/2

S1
Vd C1 D1

S2 Ia
C2 D2
EaN

-Vd/2

Capacitor C2 charging through D2

VSC current flow during positive half cycle of the voltage
EaN
Id
+Vd/2
Instantaneous
Inversion
S1
Vd C
Instantaneous
rectification

S2 Ia
C
EaN

-Vd/2
 VSC current flow during Negative
half cycle of the voltage EaN
Id
+Vd/2
Instantaneous
Inversion
S1
Vd C1
Instantaneous
rectification

S2 Ia
C2 D2
EaN

-Vd/2
 VSC Analysis H-Bridge
configurationVSC output
+
Vd

-Vd
S1 S3
Vd C

Va

S4 S2 S4 S3 S2 S1 Vab
Filter
C 0 0 0 0 0
0 0 1 1 Vd/2
1 1 0 0 -Vd/2
-

Compensator voltage
 GTO based single phase
+ full bridge Vd
VSC output

-Vd
G1 G3
C D3
D1

Vd a
Filter Vab
b
Compensator
C G2
G4 voltage
D4
D2

-
H-Bridge
 GTO based single
phase full bridge Practical conversion
RMS value of the fundamental
(SFB) component
+
 2  
E1  {  Vd sin d } / 2 
  0 
G1 G3 2 2
E1  Vd  0.9Vd
C

Vd
Filter
Vab
G2 Compensator
C
G4 voltage

H-Bridge VSC output

 3-phase GTO based VSC

G1 G3 G5
C
Vd
a
N
b
c

C
G4 G6 G2
Compensator
voltage
VcN
-
 V, 0

Voltage relation
X
Vc, -θ
Vs
VSC

V_comp
Vs I_comp
IX

Vc
θ
θ

Ic
 Six pulse STATCOM Analysis
System voltage
at the point of
connection

Va R L
a C
b
Capacitor
c
G
n

Va: system voltage

R: resistance, represents
losses in switches, coupling
transformer and inductor
L: coupling Inductance
G: loss in DC capacitor
 Six pulse STATCOM Analysis

System voltage
at the point of
connection

EaN
Va R L
a C
b
Capacitor
c
G
n

EaN
VnN
N

N : Converter neutral
.n:: ac power system neutral
 Six pulse STATCOM Analysis
Voltages with respect to Dc and AC neutral
System voltage
at the point of
+Vd/2
connection
C!
Ean

Va R L
a EaN

b N

c
n

C2
VNn

.n :AC system neutral -Vd/2

N ; DC system neutral
 Voltages wrt AC source neutral

Ean  EaN  VNn

Ebn  EbN  VNn
Ecn  EcN  VNn
Under balanced condition

Ean  Ebn  Ecn  0

voltage _ between _ DC _ and _ AC _ neutral
EaN  EbN  EcN
VNn  
3
Converter output phase voltage
with respect to ac system neutral

2 EaN EbN EcN

Ean   
3 3 3
2 EbN EcN EaN
Ebn   
3 3 3
2 EcN EaN EbN
Ecn   
3 3 3
 6-pulse VSC operation
• Switches must have turn on and turn off
capability ( use GTO or IGBT,IGCT etc in place
of conventional thyristor.)
• Switches are turned on and off cyclically
• For NON_PWM converter switches conducts
for a period of 180 degree
• Switches in upper and lower half of the bridge
are complementary( i.e when s1 is on s4 is off
and vice versa)
 3-phase 6-pulse VSC out put
voltages wrt DC neutral
Va
Vdc/2 system voltage
EaN

0 60 120 180 360

EbN EbN
0
Converter
output

EcN
 Voltage relations wrt dc neutral
Interval EaN EbN EcN VNn
0-60 +Vd/2 -Vd/2 +Vd/2 -Vd/6

60-120 +Vd/2 -Vd/2 -Vd/2 +Vd/6

120-180 +Vd/2 +Vd/2 -Vd/2 -Vd/6

180-240 -Vd/2 +Vd/2 -Vd/2 +Vd/6

240-300 -Vd/2 +Vd/2 +Vd/2 -Vd/6

300-360 -Vd/2 -Vd/2 +Vd/2 +Vd/6

 Voltage relations wrt ac neutral
Interval Ean Ebn Ecn VNn
0-60 +Vd/3 -2Vd/3 +Vd/3 -Vd/6

60-120 +2Vd/3 -Vd/3 -Vd/3 +Vd/6

120-180 +Vd/3 +Vd/3 -2Vd/3 -Vd/6

180-240 -Vd/3 +2Vd/3 -Vd/3 +Vd/6

240-300 -2Vd/3 +Vd/3 +Vd/3 -Vd/6

300-360 -Vd/3 -Vd/3 +2Vd/3 +Vd/6
 3-phase 6-pulse voltages
Ean
Converter
2Vdc/3 output

Vdc/3

-Vdc/3

-2Vdc/3 Va
system voltage
 3-phase 6-pulse voltages
Ean
Converter Interval Ean Ebn Ecn
2Vdc/3
output 0-60 +Vd/3 +Vd/3 +Vd/3
Vdc/3
60-120 +2Vd/3 -2Vd/3 -Vd/3
0 120-180 +Vd/3 +Vd/3 -2Vd/3
-Vdc/3

-2Vdc/3 180-240 -Vd/3 +2Vd/3 -Vd/3

Va 240-300 -2Vd/3 +Vd/3 +Vd/3

system 300-360 -Vd/3 -Vd/3 +2Vd/3
voltage
 Filtered output voltage
• The rms value of the fundamental component
of the 6-pulse converter

4  /2
Ea1 
 2 
0
Ean sin d

2
 Vdc  0.45Vdc

 Harmonics content

• Harmonics in converter output

• Rms value of hth harmonics voltage
Ea1 0.45Vdc
Eah  
h h
h  6k  1, k  1,2,3,.....
• The rms value of the fundamental component
of reactive current

V  0.45Vdc
Iq 
L
 Harmonic current
• .hth harmonic current
• = (hth harmonic voltage)/ hth harmonic
impedance

0.45Vdc 1
Ih  
h hL
0.45Vdc

h 2L
 3-phase 6-pulse voltages & current

2Vdc/3 Ean

Vdc/3

-Vdc/3
Va
-2Vdc/3
 AC current waveform
• Current through the coupling Inductance
• The instantaneous current in phase a inductor
neglecting losses (R=0)
• Under this Assumption ;
dia
L  VL (t )  va (t )  Ean (t )
dt
va (t )  Ean (t )
ia (t )   dt
L
 AC current waveform 2Vdc/3
Ean
Converter
output
Vdc/3

dia
L  va (t )  Ean (t ) 0
dt -Vdc/3

 2VSin t  E an(t ) -2Vdc/3 Va

system
1

ia (t )   { 2VSint  E an(t )}dt
L
 voltage

 initial _ inductor _ current

Interval 1, 0< wt< 60

V  Vdc 2 t 
ia (t )    2 cost  (  )
L  V 9 3 
Ref: KR Pdiyar
 AC current waveform
Ref: KR Pdiyar

Interval 2, 60< wt<120

V  Vdc  2t 
ia (t )    2 cos t  (  )
L  V 3 3 
 AC current waveform
Ref: KR Pdiyar

Interval 3, 120< wt<180

V  Vdc  t 
ia (t )    2 cost  (  )
L  V 9 3 
 STATCOM
• consists of a two-level Voltage Source
Converter (VSC),
• a dc energy storage device,
• a coupling transformer connected in shunt to
the transmission or distribution network
through a coupling transformer
 Operation
• The VSC converts the dc voltage across the
storage device into a set of three-phase ac
output voltages.
• These voltages are in phase and coupled with
the ac system through the reactance of the
coupling transformer.
 Statcom functions
• Suitable adjustment of the phase and
magnitude of the STATCOM output voltages
allows effective control of active and reactive
power exchanges between the STATCOM and
the ac system.
• Such configuration allows the device to
absorb or generate controllable active and
reactive power.
 Statcom functions
• The VSC connected in shunt with the ac
system provides a multifunctional topology
which can be used for quite distinct purposes:
• 1. Voltage regulation and compensation of
reactive power;
• 2. Correction of power factor; and
• 3. Elimination of current harmonics.
• 4.controlling power flow and damping power
oscillations.
• 5. improving the transient stability limit
 Basic control scheme
Vsys

Io
Vsys
Reactive
current Synchro
computer nizing
Io; (Iop + IoQ)
PLL
IoQ θ
Vo; Vo /_α
IQref Error Δα Θ + Δα Gate Converter
Amplifier Pattern
generator

Basic control scheme for the VSC type VAR Energy

storage
generator , controlling reactive power by unit
variation of DC capacitor voltage ( Indirect
output voltage control)
 STATCOM & SVC
V-I & V-Q Characteristics
• STATCOM voltage source behind coupling inductance
• Max capacitive or inductive current of the STATCOM
can be maintained independently of the system
voltage
• SVC becomes a fixed capacitive admittance at full
output
• Max attainable compensating current of SVC
decreases linearly with ac system voltage.
 STATCOM SVC Transient rating
Transient rating (t<1 sec)
(t<1 sec)

VT VT
1.0 1.0

0.8 0.8

0.6 0.6

BLmax
0.4 0.4
Bc
0.2 0.2

Icmax 0 ILmax Ic 0 IL
Icmax ILmax

Capacitive Inductive
STATCOM CONTROL
 Basic control scheme
Vsys

Io
Vsys
Reactive
current Synchro
computer nizing
Io; (Iop + IoQ)
PLL
IoQ θ
Vo; Vo /_α
IQref Error Δα Θ + Δα Gate Converter
Amplifier Pattern
generator

Basic control scheme for the VSC type VAR Energy

storage
generator , controlling reactive power by unit
variation of DC capacitor voltage ( Indirect
output voltage control)
 Internal control of the converter Vsys

Vsys

Synchro
nizing
Io; (Iop + IoQ)
IQref |vo
Output
| Gate Vo; Vo /_α
voltage
magnitud Pattern Converter
e & angle generator
Ipref computer α

The main function :

Operate converter power switches so as to Energy
produce a synchronous output voltage waveform storage
unit
that forces the reactive and real power exchange
required for compensation
 Basic control scheme
Vsys V

Io Io; (Iop + IoQ)

Vsys
Real &
Reactive Synchro
current nizing
computer PLL Vo
IoQ θ
Iop
IQref α Θ+α Gate Converter
Error Pattern
Ipref Amplifier |v| generator

Error
Amplifier
Energy
Vdc Direct control of VSC controlling reactive storage
output by internal voltage ( Mag & angle) unit
control at a sustained dc capacitor voltage.
Vdcref
SVC as Ideal voltage regulator
Vref = constant
Vref
Vsvc
System load
lines

Isvc ILmax
Icmax
Voltage at the point of connection is held strictly
constant
SVC as Ideal voltage regulator
Vref = constant
Vref
Vsvc

Isvc ILmax
Icmax
Voltage at the point of connection is held strictly
constant
SVC as Ideal voltage regulator
Vref Vref = constant

Vsvc
System load
lines

Isvc2

Isvc1 ILmax
Icmax
Voltage at the point of connection is held strictly
constant , needs a large change in Isvc. Even for
a small change in system bus voltage
 SVC Practical characteristic
Vfef* = Vref + k Isvc

Vsvc

Vref*

Icmax Isvc ILmax

 SVC charactristic
System Vref
1 Vsvc Vref Load line 1

2 2
∆V ∆V

∆Isvc
∆Isvc Isvc

Ideal , Vref= constant Practical, Vref*= Vref+k Isvc

 SVC Ch
∆I=∆V/k
Slope k=∆V/∆I

Vsvc

V*ref=Vref+kIo

Icmax Isvc ILmax

 SVC Control
• Controller characteristics form Hingorani
 Perfect Terminal voltage regulator Vref = constant

IL
VT
Voltage
Z(ω,t) Measuring
unit
Io

ΔV
Static VT
VAR IQref PI ΔVT -
V generato controller +
r Vref

Vref
System
load lines

Ic IL
Ic1 IL2
 General control scheme of SVG
IL
VT
Voltage
Z(ω,t) Measuring
unit
Io
Pm

V,δ Static IQref VT

VAR PI ΔVT -
generato controller
+
r
Z includes Gen & Transmission line V*ref
impedance that is the function of ω Direct Input
and time, the impedance variation is
due to faults line switching etc.

Vref
Aux Inputs
 General control scheme of SVG
• The output of SVG is controlled so that the amplitude of
the reactive current drawn from the power system follows
the current reference IQref.
• With the basic static compensation control the VAR gen is
operated as a perfect terminal voltage regulator.
• The amplitude VT of the terminal voltage is measured and
compared with the voltage reference Vref ;
• The error ∆VT is processed and amplified by a PI controller
to provide the current reference IQref for the VAR
Generator.
• Io is close loop controlled via IQref so that VT is
maintained precisely at the level of Vref in face of power
system and load changes.
 General control scheme of SVG

• If the proper compensation of the power requires

some specific variation in the amplitude of the
terminal voltage with time or with some other
variable , then an appropriate correcting signal,
derived from the aux inputs is summed to the fixed
reference Vref in order to obtain the desired
effective ( Variable) reference signal V*ref that close
loop controls the terminal voltage VT.
Practical terminal voltage regulator V*ref= Vref + kIo

∆I=∆V/k
Vref
V*ref

System load lines

Ic Ic1 IL2 IL
 Practical terminal voltage regulator V*ref= Vref + kIo
IL
VT
Voltage
Z Measuring
unit
Io

ΔV
Static VAR IQref VT
generator PI ΔVT -
V controller +
V* ref

Power System Current

measuring kIo.
+ +
& Vref
processing Vref

System
load lines

Ic Ic1 IL2 IL
 Practical terminal voltage regulator V*ref= Vref + kIo
IL

VT
Regulator
slope k Vref
X
Io
∆VT1
∆VT2
ΔV
Static Vref
VAR
V compens
ator

Power
System

Ic IL
Io
∆VT = VT -Vref Ioref_1
Ioref_2
Ioref= ∆VT /k
 The regulation slope
• ** from note
 SVC controller block diagram
∆V Ioref=∫∆I Io V

Vref +
- VT
+ G1 G2 X
- Regulator VAR GEN
System VT=V-IoX
impedance

1
k 1
G1   G2  e Td s H
1  T1s 1  T2 s
1 G1G2 X
VT  V  Vref
1  G1G2 HX 1  G1G2 HX
 Svc controller block diagram
∆V Ioref Io
V
VT=V-IoX
G1 - + VT
+ - G2 X
Regulator VAR GEN
Vref System
impedanc
H
e
1 1
k Td s H
G1  
1  T1s
G2  e 1  T2 s
T1 main time constant of PI controller (10-50 ms)
T2 time constant of amplitude measuring ckt (8-16 ms)
Td Transport lag of the Var generator(2.5 ms for TCR, 5 ms for TSC, 0.2 to 0.3
ms for converter)
X is reactive part of system Impedance,
k is a regulator slope ( typically 1-5%)
 ∆V Ioref Io
V
Vref
VT=V-IoX
G1 - + VT
+ - G2 X
Regulator VAR GEN
System
impedanc
H
e
1 G1G2 X
VT  V  Vref
1  G1G2 HX 1  G1G2 HX
Setting Vref = 0
VT 1

V 1  G1G 2 HX

Under steady state condition VT 1


V 1
X
For k =0 , ∆VT/∆V =0 ; k
No change in terminal voltage or a perfect terminal
voltage regulator
 ∆VT
V
Vref VT=V-IoX
X -
k e Td s + VT
+ - G1G2 X  
1  T1s

1
H
1  T2 s
Setting Vref = 0
VT 1

V 1  G1G 2 HX

Under steady state condition VT 1


V 1
X
For k =0 , ∆VT/∆V =0 ;
No change in terminal voltage or a perfect terminal S=0 k
voltage regulator
 ∆VT V
VT=V-IoX
Vref X -
k e Td s + VT
+ - G1G2 X  
1  T1s

1
H
1  T2 s
setting Vref = 0

VT 1

V 1  G1G 2 HX
Above eqn shows that the dynamic behavior of the compensator is a function
of the power system impedance ‘X’,ie the system is a integral part of the
feedback loop. For this reason control is normally optimized for the maximum
system impedance ( minimum short circuit capacity)
 ∆VT V
VT=V-IoX
Vref X
k e Td s + VT
+ - G1G2 X  
1  T1s

1
H
1  T2 s
setting Vref = 0

VT 1
 System data:
V 1  G1G 2 HX Xmin= 4.761 pu strong system
control is normally optimized for the Xmax= 9.522 pu weak system
maximum system impedance ( k= 0.846 with strong system
minimum short circuit capacity) TdTSC=5.55 ms(TSC/TCR/SVC)
Tdconv = 0.5 ms (STATCOM)
T2= 4 ms
With this data ∆VT/∆V =
 Statcom controller
Vs
ψ
Vac
I
Vac
_ Vac
+ PI AC voltage Converter Vd
regulator Dynamics
Vac_ref

Vd

Angle ψ
_
PI DC voltage Converter
+ regulator Dynamics
Vd_ref
• 7. Applications of STATCOM
• The STATCOM has the following applications in controlling power system
dynamics.
• Damping of power system oscillations
• Damping of subsynchronous oscillations
• Balanced loading of individual phases
• Reactive compensation of AC-DC converters and HVDC links
• Improvement of transient stability margin
• Improvement of steady-state power transfer capacity
• Reduction of temporary over-voltages
• Effective voltages regulation and control
• Reduction of rapid voltages fluctuations (flicker control)[4],[5],[9]