International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 10, October 2013
ISSN 2319 - 4847
MULTI LEVEL STATCOM FOR HARMONIC
REDUCTION
Y.T.R.Palleswari, B.Kali Prasanna and G.Lakshmi
Assistant Professor, Shri Vishnu Engineering College for Women
Abstract
To improve the performance of power system, by reducing harmonics and managing reactive power, new power equipment is
required. During the last decade, many control devices called” Flexible AC Transmission Systems” (FACTS) have been
implemented. The new and dominant converters in FACTS controllers are synchronous condenser, static VAR compensator
(SVC) and static synchronous compensator (STATCOM). STATCOM is one of the devices used for compensation of reactive
power and harmonic reduction. This paper concentrates on harmonic reduction in two level, three level and five level diode
clamped inverter based STATCOM. To verify the effectiveness, simulation will be done using MATLAB/ SIMULINK.
Keywords: FACTS, Multi level inverter, STATCOM, Total Harmonic Distortion.
1. Introduction
Transmission lines when travels over large distances, they suffer with considerable losses. In order to compensate these
voltage losses, many FACTS (Flexible AC Transmission Systems) devices like static VAR compensator (SVC),
synchronous condenser, Static Synchronous Compensator (STATCOM). In which STATCOM is a shunt compensation
device also known as ASVG (Advanced Static VAR Generator) is coupled with a transformer and connected to a
transmission line [1]. It is capable of generating and or/ absorbing reactive power.
The STATCOM basically consists of a step down transformer with a leakage reactance, a three phase GTO or IGBT
voltage source inverter (VSI) and a DC capacitor. The AC voltage difference across the leakage reactance produces
reactive power exchange between the STATCOM and power system, such that the AC voltage at the bus bar can be
regulated to improve the voltage profile of power system.
Day by day, the demand for the power is goes on increasing. So, the converter/ inverter used within the STATCOM
should be able to withstand the increased power levels. In power electronics, the most basic controllable device is the two
level converter. But, this basic device injects unwanted harmonics into the power system [2]. So, in order to overcome this
problem, the term multi level is brought out in 1981[3]. The very important thing of using multilevel inverters is that
creating more output steps and to reduce the Total Harmonic Distortion (THD).
Three different multi level inverter topologies have been proposed. Those are diode-clamped, flying capacitor and cascade
H-bridge. Out of which diode clamped multi level inverter can compensate the reactive power and reduce the Total
Harmonic Distortion effectively even though with the higher levels, capacitor voltage balancing is a problem. This paper
introduces a STATCOM with three level and five level inverter and mainly concentrates on Total Harmonic Distortion.
2. Operation of STATCOM:
Fig 1.One line diagram of STATCOM
The STATCOM basically consists of a step-down transformer with a leakage reactance, a voltage source inverter (VSI), a
capacitor in its DC side and a control system. The inverter in conventional STATCOM, switched with a single pulse per
period and the transformer is connected in order to provide harmonic minimization and serve as a link between VSI and
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 10, October 2013
ISSN 2319 - 4847
the system. The leakage inductor limits the negative sequence currents. The capacitor is used to maintain DC voltage to
the inverter. The inverter itself keeps the capacitor charged to the required level.
The AC voltage difference across the leakage reactance produces reactive power exchange between the STATCOM and
the power system, such that the AC voltage at the bus bar can be regulated to improve the voltage profile of the power
system, which is the primary duty of the STATCOM [4]. If the system voltage is greater than the inverter voltage, then
the STATCOM absorbs the reactive power. If the system voltage is less than the inverter voltage, then the STATCOM
generates reactive power. If the system voltage is equal to the inverter voltage, then there is no reactive power
compensation [5].
3. Reactive current detection principle:
The main component of STATCOM is the voltage source inverter. The voltage source inverter operation depends upon
the working of semiconductor switches. Proper gating signals are required for the operation of these semiconductor
switches. These signals are generated by using a proper control circuit. Here the control circuit required for generating
pulses to the voltage source inverter is based on the synchronous reference frame theory. Before designing the control
circuit, there is a need of modeling the equations required for constructing the control circuit.
Reference current generation:
Fig.2. Reactive current detection principle
Fig 3 shows the basic block diagram for generating the reference currents required for producing gate pulses to the
switches. In this, shunt active power filter control is accomplished by monitoring the three phase line currents to the
nonlinear load and the three phase line-to-neutral voltages at the load bus and then generating the three phase reference
currents that should be supplied to the voltage source inverter [6].
Mathematical Analysis:
In a, b, c coordinates, the a, b and c axes are fixed on the same plane, apart from each other by 2π/3. The instantaneous
space vectors Vα and iα are set on the α- axis and their amplitude and direction vary with the passage of time. These space
vectors are easily transformed into α, β coordinates as follows
(1)
(2)
The instantaneous active and reactive power in the α, β coordinates are calculated with the following expressions:
P(t) = Vα(t).iα(t) + Vβ(t).iβ(t)
(3)
Q(t) = -Vα(t).iβ(t) + Vβ(t).iα(t)
(4)
It is evident that p(t) becomes equal to the conventional instantaneous real power defined in the a, b, c reference frame.
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Q = Vα.iβ + Vβ.iα
(5)
The expression of the currents in the α–β plane, as a function of the instantaneous power is given by the following
equation:
=
(6)
The values of p and q can be expressed in terms of the DC components plus the AC
Components, that is:
P=
Q=
(7)
(8)
(9)
The final compensating currents including the zero sequence components in a, b, c reference frame are the following:
(10)
4. Inverter Analysis:
Comparing two-level inverter topologies at the same power ratings, MLIs also have the advantages that the harmonic
components of line-to-line voltages fed to load are reduced owing to its switching frequencies [6].
Two level Inverter:
Fig 3.Two level Inverter
Table.1.Switching States and output voltages of two level inverter
S1
ON
OFF
S2
ON
OFF
S3
OFF
ON
S4
OFF
ON
Output Voltage
Vdc
-Vdc
From Fig.3, it can be observed that for two level inverter, the output voltages are Vdc and -Vdc. The switches turn on
and off for every period according to the mode show in table 1.
Simulation Results:
A system with inductive load having R=70Ω, L=0.2H.Voltage and current waveform before switching in STATCOM are as
shown in Fig 4.
Fig.4 voltage and current waveforms without STATCOM compensation with inductive load
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From Fig.4, it can be observed that as the load has inductive part, the current waveform lag behind the voltage waveform
with certain angle, therefore system power factor is not equal to unity. Hence there is a need of reactive power
compensation for the system.
A system with inductive load and after switching in STATCOM having R=70Ω, L=0.2H.Voltage and current waveform
of A phase after switching in STATCOM are as shown in Fig 5.
Fig .5.voltage and current waveforms with STATCOM compensation with inductive load
From Fig 5, it can be observed that current and voltage waveforms are in phase after STATCOM compensation. The
power factor of the system becomes unity as the reactive component is provided by the STATCOM.
Fig.6. THD analysis for 2 level with and without STATCOM.
Three level Diode Clamped Inverter:
Fig 7.Three phase three level Diode Clamped Multi Level Inverter
Diode clamped multilevel inverter is a very general and widely used topology. DCMLI works on the concept of using
diodes to limit voltage stress on power devices. A DCMLI typically consists of (m-1) capacitors on the DC bus where m
is the total number of positive, negative and zero levels in the output voltage [7]. From Fig.7 one can say that for three
level inverter, the output voltages are Vdc/2, 0, -Vdc/2. The switches turn on and off for every period according to the
mode show in table 2. Switches Sx1, Sx3(X=1,2,3) and Sx2, Sx4 mix in pairs.
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Table.2.Switching States and output voltages of three level Diode Clamped Multi Level inverter
Sx1
Sx2
Sx3
Sx4
Output Voltage
ON
ON
OFF
OFF
Vdc/2
OFF
ON
ON
OFF
0
OFF
OFF
ON
ON
-Vdc/2
Simulation Results:
A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase before switching in
STATCOM are as shown in Fig 8.
Fig 8.voltage and current waveforms before STATCOM compensation with capacitive load
From Fig.8, it can be observed that as the load has capacitive part, the current waveform leading behind the voltage
waveform with certain angle. Therefore system power factor is not equal to unity. Hence there is a need of reactive power
compensation for the system.
A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase after switching in
STATCOM are as shown in Fig 9.
Fig 9.voltage and current waveforms after STATCOM compensation with capacitive load
From Fig.9, it can be observed that current and voltage waveforms are in phase after STATCOM compensation. The
power factor of the system becomes unity as the reactive component is provided by the STATCOM.
Fig.10. THD analysis for 3 level with and without STATCOM
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Five level Diode Clamped Inverter:
Fig.11. Three phase five-level Diode Clamped Multilevel Inverter.
Fig.11 shows a three phase five level diode clamped inverter. The order of numbering of the switches for phase A is Sa1,
Sa2, Sa3, Sa4, Sa1’, Sa2’, Sa3’ and Sa4’ and likewise for other two phases. The DC bus consists of four capacitors C1,
C2, C3 and C4 acting as voltage divider. For a DC bus voltage Vdc, the voltage across each capacitor is Vdc/4 and
voltage stress on each device is limited to Vdc/4 through clamping diode. The middle point of the four capacitors ‘N’ can
be defined as the neutral point. The principle of diode clamping to DC-link voltages can be extended to any number of
voltage levels.
Table.3.Switching States and output voltages of five level Diode Clamped Multi Level inverter.
Simulation Results:
Fig.12. Simulink model of a system with STATCOM with capacitive load
A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase after switching
in STATCOM are as shown in Fig 13.
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Fig.13. Voltage and Current waveforms after STATCOM compensation with capacitive load
From Fig.13, it can be observed that current and voltage waveforms are in phase after STATCOM compensation.
The power factor of the system becomes unity as the reactive component is provided by the STATCOM.
Fig.14. STATCOM output currents.
Fig.15 THD analysis for 5 level with and without STATCOM
Table.4 Comparison table o total harmonic distortion for two level, three level and five level inverter based STATCOM.
Total Harmonic Distortion
Three level
without
with STATCOM
STATCOM
28.28%
11.44%
Two level
without
with STATCOM
STATCOM
29.63%
28.29%
Table 5. Switching losses and dv/dt stress
For 2 level
3level
Five level
without
with STATCOM
STATCOM
6.64%
0.8%
5level
Dv/dt stess
2Vdc
Vdc
Vdc/2
Switching losses
8Vdc
2Vdc
Vdc
Conclusion:
From the simulation results, it is observed that STATCOM can compensate the inductive and capacitive reactive power
irrespective of the levels in the inverter. Compared to two level inverter and three level inverter, five level inverter has
reduced harmonic distortion, less dv/dt stress and switching losses are reduced.
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Corresponding Author:
Y.T.R.PALLESWARI
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