The document discusses STATCOM (Static Synchronous Compensator), which is a shunt connected reactive power compensation device capable of generating and absorbing reactive power using a voltage source converter. It can improve power system performance by providing dynamic voltage control, damping power oscillations, improving transient stability, and controlling voltage flickering and reactive/active power. The principle of operation involves using a voltage source converter to generate a balanced set of three sinusoidal voltages to exchange reactive power with the system by varying the amplitude and phase angle of the output voltage.

1. STATCOM
SWITFHING CONVERTER TYPE
VAR GENERATOR
Himal Chaulagain
Institute of Engineering
Paschimanchal Campus
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2. INTRODUCTION
• The Tennessee Valley Authority (TVA) installed the first ±100MVA STATCOM in 1995 for reducing
the TVA’s need for load tap changers, thus achieving savings by minimizing the potential for
transformer failure.
• STATCOM is a shunt connected reactive power compensation device that is capable of generating
and/or absorbing reactive power and in which the output can be varied to control the specific
parameters of an electric power system.
• It is a solid state switching converter capable of generating or absorbing independently
controllable real and reactive power at its output terminal when fed from a energy source.
• A STATCOM is analogous to an ideal synchronous machine which generates a balanced set of
three sinusoidal voltage at the fundamental frequency with controllable amplitude and phase
angle.
• STATCOM controller provides voltage support by generating or absorbing reactive power at the
point of common coupling without the need of large external reactors or capacitor banks.
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3. STATCOM can improve power system performance in the following aspects:
• Dynamic voltage control in T&D system.
• Power-oscillation damping in a power transmission system.
• Transient stability
• Voltage flickering control
• Control of not only reactive power but also active power, if necessary, in the connected line
requiring of DC energy source.
• Occupying small footprint as it replaces passive banks of circuit elements by compact electronic
converters.
• Offers modular, factory built equipment so that it reduces site work and commissioning time.
• Uses encapsulated electronic converters, thereby minimizing its environmental impact.
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4. PRINCIPLE OF OPERATION
• Provides the desired reactive power generation and absorption entirely by means of electronic
processing of the voltage and current waveforms in a voltage source converter (VSC).
• VSC is connected to a utility bus through magnetic coupling.
• In figure (b) a STATCOM is seen as an adjustable voltage source behind a reactance meaning that
capacitor banks and shunt reactors are not needed for reactive power generation and absorption,
thereby giving a STATCOM a compact design, or small footprint as well as low noise and magnetic
impact.
• The exchange of reactive power between the converter and the ac system can be controlled by
varying the amplitude of the 3-phase output voltage, Es ,of the converter i.e if the amplitude of
the output voltage is increased above that of the utility bus voltage, Et , then a current flows
through the reactance from the converter to the ac system and the converter generates
capacitive reactive power for the ac system.
• If the amplitude of the output voltage is decreased below the utility bus voltage then the current
flows from the ac system to the converter and the converter absorbs the inductive-reactive
power.
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5. • If the output voltage equals the ac system voltage, the reactive power exchange becomes zero, in
which case the STATCOM is said to be in floating state.
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6. • Adjusting the phase shift between the converter output voltage and the ac system voltage can
similarly control real power exchange between the converter and the ac system.
• Converter can supply real power to the ac system from its dc energy storage if the converter
output voltage is made to lead the ac-system voltage. On the other hand, it can absorb real power
from the ac system for the dc system if its voltage lags behind the ac system voltage.
• Provides the desired reactive power by exchanging the instantaneous reactive power among the
phases of the ac system.
• The mechanism by which the converter initially generates and/or absorbs the reactive power can
be understood by considering the relationship between the output and the input power of the
converter. The converter switches connects the dc input circuit directly to the ac output circuits.
Thus, the net instantaneous power at the ac output terminal must always be equal to net
instantaneous power at the dc input terminal.
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7. Assuming that the converter is operated to supply reactive output power.
• In this case the real power provided by the dc source as input to the converter must be zero.
Furthermore, because the reactive power at zero frequency is by definition is zero: the dc source
supplies no reactive power as input to the converter and thus clearly plays no part in the
generation of reactive output power by the converter.
• The converter simply interconnects 3-output terminal so that the reactive output current can flow
freely among them. If the terminal of the ac system are regarded in this context, the converter
establishes a circulating reactive power exchange among the phases.
• The real power that the converter exchanges at its ac terminals with ac system must of course be
supplied to or absorbed from its dc terminal by the dc capacitor.
• Reactive power is generated internally by the action of converter, a dc capacitor must still be
connected across the input terminals of the converter. The primary need for the capacitor is to
provide a circulating current path as well as a voltage source.
• Not to violate the instantaneous power equality constraint at its input and output terminals, the
converter must draw a fluctuating current from its dc source.
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8. • The VSC has the same rated current capability when it operates with the capacitive or inductive
reactive current. Therefore, a VSC having a certain MVA rating gives the STATCOM twice the
dynamic range in MVAR (contributes to compact design). A dc capacitor bank is used to support
the controlled dc voltage needed for the operation of VSC.
• The reactive power of a STATCOM is produced by means of power electronic equipment of the
VSC type.
• The VSC maybe a two-level or three-level type, depending on the required output power and
voltage.
• Number of VSCs are combined in a multi-pulse connection to form the STATCOM.
• During transient conditions caused by the line faults, a PWM mode is used to prevent the fault
current from entering the VSCs.
• In this way, the STATCOM is able to withstand transients on the ac side without blocking.
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9. V-I CHARACTERISTICS
• A STATCOM is capable of yielding the full output of capacitive generation almost independently of
the system voltage (constant-current output at lower voltages).
• The capability is particularly useful for situations in which the STATCOM is needed to support the
system voltage during and after faults where the voltage collapse would otherwise, be a limiting
factor.
• The figure also illustrates that the STATCOM has an increased transient-rating in both the
capacitive and the inductive operating regions.
• The maximum attainable transient over current in the capacitive region is determined by the
maximum current turn off capability of the converter switches.
• In inductive region, the converter are naturally commutated; therefore the transient current
rating of the STATCOM is limited by the maximum allowable junction temperature of the
converter switches.
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10. • In practice, the semi-conductor switches of the converter are not lossless. So, the energy stored in
the dc capacitor is eventually used to meet the internal losses of the converter and the dc
capacitor voltage diminishes.
• When the STATCOM is used for reactive power generation, the converter itself can keep the
capacitor charge to required voltage level accomplished by making the output voltage of the
converter lag behind the ac system voltage by small angle 0.1-0.2. In this way, the converter
absorbs a small amount of real power from the ac system to meet its internal losses and keep the
capacitor voltage at the desired level. Same mechanism can be used to increase or decrease
capacitor voltage and thus the amplitude of the converter output voltage to control the VAR
generation or absorption.
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11. • Reactive and real power exchange between STATCOM and ac system can be controlled
independently of each other.
• Any combination of real power generation or absorption with VAR generation or absorption is
achievable if the STATCOM is equipped with an energy storage device of suitable capacity as
depicted in the following figure. With this capability extremely effective control strategy for
modulation of reactive and real output power can be devised to improve the transient and
dynamic system stability limits.
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12. HARMONIC PERFORMANCE
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13. • An elementary six pulse VSC STATCOM consisting of six self commutated semiconductor switches
with anti-parallel diodes.
• In this converter configuration, IGBTs constitute the switching devices.
• With a dc voltage source (which maybe a charged capacitor) the converter can produce a
balanced set of 3 quasi square voltage waveforms of a given frequency by connecting a dc source
sequentially to the three output terminal via the appropriate converter switches.
• The power quality embraces issues such as voltage flicker, voltage dip & voltage rise, as well as,
harmonic performance and high frequency noise.
• The power electronics devices distorts voltage and current waveforms in a power network
influencing power facilities and customers equipment in a diverse manner.
• Harmonic current induce abnormal noise and parasitic losses, and harmonic voltages cause a loss
of accuracy in measurement instruments and the faulty operation of relays and control system.
• Electromagnetic noise, caused by the noise of high frequency, EM waves emitted from power-
electronics circuits affects electronics devices used in business & industry and often induces
interfering voltage in communication lines.
• The corrective measure generally recommended for mitigating harmonics and high frequency
noise is to limit their generation at source.
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14. • In general, the STATCOM output voltage wave is a staircase type wave synthesized from the dc
input voltage with appropriate combinations of converter switches.
• For example: A six pulse converter as shown is operated typically with either a 120˚ or 180˚
conduction sequence for converter switches.
• For a 180˚ conduction sequence, three switches conduct at a time.
• For a 120˚ conduction sequence, two switches conduct at a time.
• Figure shows the 3-step staircase line voltage vab along with the fundamental component, Vfund for
a conduction sequence of 180 ˚.
The line voltage vab in terms of its various frequency components, can be described by the following
Fourier series equation:
vab= 𝑎0 + 𝑛=1
∞
𝑎 𝑛 cos ℎ𝑤𝑡 + 𝑏 𝑛 sin ℎ𝑤𝑡 ---------------(i)
Where coefficients a0 , an and bn can be determined by considering onefundamental period of vab
bn =(2/π) ∫ Vdc sin(h𝑤t) d(𝑤t) (0 to π) ----------------(ii)
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15. bn =(2/π) ∫ Vdc sin(hωt) d(ωt) (α to π-α)
= (4Vdc/ πh) cos(hα)
Therefore;
Vab = ℎ=1,3,5
∞ 4𝑉 𝑑𝑐
πℎ
cos ℎα sin(ℎω𝑡)
• For 180˚ conduction sequence, α=30˚. Hence, the triplen harmonics are zero.
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16. • It is also noted that the converter has harmonics of frequencies (6K±1)f0 in its output voltage and
6Kf0 in its input current where f0 is the fundamental output frequency and K=1,2,3…
• As evident, the high harmonic content in the output voltage makes the simple converter
impractical for power system applications.
• To reduce harmonic generation various converter configurations and converter switching
techniques are suggested.
• For example: The first installed commercial STATCOM has a 48 pulse converter configuration so
that the staircase ac line output voltage waveform has 21 steps.
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17. STEADY-STATE MODEL
• A STATCOM is always connected in shunt with the ac system through some magnetic coupling
namely the coupling transformer or interface reactor.
• A typical STATCOM connection is shown in fig. it consists of a VSC using either a GTO or IGBT as a
switching device and a capacitor Cs on the dc side as an energy storage device. The resistance Rp
connected in parallel with Cs represents both the capacitor losses and switching losses. The
STATCOM is connected to the ac system through magnetic coupling , represented by leakage
inductance, Ls and resistance Rs . The STATCOM improves the desired power-system performance,
including dynamic compensation, mitigating the SSR by modulating the reactive power at the
common-coupling point and so forth.
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18. 8/14/2017 STATCOM 18

19. The STATCOM is a shunt connected reactive power compensator device that is capable of
generating and/or absorbing reactive power. STATCOM consists of a voltage source converter that
form a given input of dc voltage produces a set of 3-phase ac output voltage each in phase and
coupled to the corresponding ac system voltage through a relatively small reactance. The device
voltage is provided by an energy storage capacitor.
From fig; I =
𝑣 𝑠<Ө0 −𝑣<0
𝑗𝑋0
SSTATCOM =|V0| < Ө0 *𝐼∗
= V0<Ө0 *[
𝑣 𝑠<Ө0 −𝑣<0
𝑗𝑋0
]∗
=
𝑉0
2
−𝑉0 𝑉<Ө0
−𝑗𝑋
=
𝑉0 𝑉𝑠𝑖𝑛Ө0
𝑋
+ 𝑗
𝑉0
𝑋
(𝑉0 − 𝑉𝑐𝑜𝑠Ө0)
Now,
QSTATCOM =
𝑉0
𝑋
(𝑉0 − 𝑉𝑐𝑜𝑠Ө0)
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20. For small Ө0 ;
QSTATCOM =
𝑉0
𝑋
(𝑉0 − 𝑉)
If V0 > V; QSTATCOM = +ve → it generates reactive power
If V0 < V; QSTATCOM = -ve → it absorbs reactive power
If V0 = V; QSTATCOM = 0 → no exchange of reactive power
1. Capacitive mode of operation
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21. Reactive power drawn by the load = Vs IL sinӨL
Reactive power supplied by the STATCOM = Vs Is cosӨ0
in ideal case (i.e. if there is no power loss in the STATCOM)
Ө0 = 0 i.e I0 leads Vs by 90˚.
But in real practice there is some power loss in the STATCOM branch so
Ploss = 𝐼0
2
𝑅0
= Vs I0 sinӨ0
For unity power factor
Qstatcom = Qload
VsIocosӨ0 = VsILsinӨL
cosӨ0 =
IL
Io
sinӨL
Also, Ploss(STATCOM)= I2R0=VsI0sinӨ0
I0=
𝑉𝑠−𝑉0
𝑅0+𝑗𝑋0
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22. I0
2Ro = VsI0sinӨ0
sinӨ0 =
𝐼0 𝑅0
𝑉𝑠
The model can be simplified as:
Xeq= X0-Xc
ZL = RL + j XL
Z0 = R0 + j Xeq
Ztotal =
𝑍0 𝑍 𝐿
𝑍0+𝑍 𝐿
=
(R0 + j Xeq )(RL+ j XL)
(R0 + j Xeq )+(RL+j 𝑋 𝐿 )
=
(R0 + j Xeq )(RL+j XL)
(R0 + j Xeq )+(RL+jXL)
*
(R0 +RL )−j(Xeq+XL)
(R0 +RL )−j(Xeq+XL)
Taking only imaginary parts and equating to zero
XLXeq
2 + (XL
2+ RL
2) Xeq + R0
2XL = 0
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23. Comparing above equation with Ax2+Bx+C = 0
A = XL, B = RL
2+XL
2 , C = R0
2 XL
The above equation gives two values of X, so Xeq = X0-Xc
I0 =
𝑉𝑠
𝑅+𝑗𝑋 𝑒𝑞
We get two values of 𝐼0 say, I01 & I02
Inverter output, 𝑉0 = 𝐼0 −𝑗𝑋0
= |V0|< Ө0
• Out of two values of ‘V0’, we need to select |V0|>|Vs| and realistic as well Ө0 which is small.
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24. 2. Inductive mode of operation
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25. Reactive power generated by load = VLILsin ӨL
Reactive power consumed by STATCOM = VSI0sin(90 – Ө0)
= VSI0cos Ө0
For unity pf, VsIocosӨ0 = VsILsinӨL
cosӨ0 =
IL
Io
sinӨL
Here, Xeq= X0+Xj
Zc = Rc - j Xc
Z0 = R0 + j Xeq
Zeq= Z0 || Zc
=
𝑍0 𝑍 𝐿
𝑍0+𝑍 𝐿
=
(R0 + j Xeq )(Rc − j Xc)
(R0 + 𝑅 𝑐 )+ j( 𝑋 𝑒𝑞 − 𝑋 𝑐)
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26. For unity pf, imaginary part = 0. Thus,
Xc Xeq
2 – (RL
2 + Xc
2) Xeq + R0
2XL = 0
Comparing with Ax2+Bx+C=0, we get,
A= Xc , B = – (RL
2 + Xc
2), C = R0
2XL
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27. Control Strategy of STATCOM
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28. • The main function of the STATCOM branch is to generate the reactive power demanded by the
load.
• The branch also draws some active power from the system supply to fulfill the power loss in the
internal resistance of the switching device and the step-down transformer.
• The active power through the STATCOM branch is given by:
P0= VS I0 sinӨ0
• At steady state operation, active power through the STATCOM branch P0 is equal to the power
loss in the branch I0
2R0.
• Let Өsteady be the steady state phase of the inverter output voltage at steady state operation. Then,
VS I0 sinӨsteady= I0
2R0
Өsteady = sin-1(
𝐼0 𝑅0
𝑉 𝑆
)
• When Ө0>Өsteady , active power through STATCOM branch becomes more than power loss in R0→
DC capacitor charging and increase in Vdc.
• When Ө0<Өsteady , active power through STATCOM branch becomes less than power loss in R0→
DC capacitor discharging and decrease in Vdc.
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29. • Hence, Vdc is utilized as the feedback signal to control the phase of inverter output voltage (Ө0).
• The current through the inverter branch is
I0=
𝑉𝑠−𝑉0
𝑅0+𝑗𝑋0
• The reactive power generated by the STATCOM is
QSTATCOM= VsI0cos Ө0
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