2014 International Conference on Control, Instrumentation, Communication and Computational Technologies (ICCICCT)

HYBRID MODULATION BASED STATCOM

WITH AN EXTERNAL PHOTOVOLTAIC
SOURCE
Rahul V G
Dr.Glan Devadhas
PG Scholar: Dept. of E&I
HOD: Dept. of E&I
Noorul Islam College of engineering
Noorul Islam College of engineering
Kumara coil, India
Kumara coil, India
rvgchempoor@gmail.com
glandeva@gmail.com

Abstract— This paper presents a hybrid modulation based STATCOM is mainly due to the tolerance of passive
multilevel static synchronous compensator (STATCOM) components, switching loss, resolution issues in the control
voltage control method. A multilevel STATCOM is a side etc. Cascaded H-bridge STATCOM has merits in point of
combination of high voltage converter and low voltage switching losses, output harmonics, and the number of circuit
converter with an energy source .STATCOM use either a components, but each cell has isolated DC capacitors. It
battery or storage element as the energy bank. Most creates balancing problem of capacitor voltage. So voltage
probably STATCOM consists of capacitors as reactive control methods are essential. One common method for
power supplying or reactive power absorbing element. voltage control is using zero sequence voltage and negative
During the operation of STATCOM, the capacitor voltage sequence current[7],but the response time is more than the
may vary from its value, there the importance of this conventional. In electrical distribution network there is
work. A new control strategy is introduced in this paper in balanced condition and unbalanced condition. Literature [8]
focus on the capacitor voltage. An experimental model is deals with the control strategy for maintaining the capacitor
developed by using MATLAB SIMULINK block and the voltage of cascade converter during unbalanced load
result shows that the STATCOM along with the new condition. In [9] a reactive power control method based on
control technique perform satisfactorily. DC voltage is proposed but this paper is purely based on
steady state condition and it only account the individual
capacitor voltage. Some modified control techniques based on
Keywords— static synchronous compensator, hybrid grid current and voltage were in [10] and [11]. The voltage in
modulation ,cascaded multilevel converter, cascade H-bridge the capacitor is a fluctuating quantity, so reliability of the
I. INTRODUCTION system is less. An additional backup source along with the
capacitor improves the reliability of the system. Non
Reactive power is an important criterion for every electrical renewable energy sources are good option as a backup energy
stability analysis. Voltage stability problems arise where source. The detailed description of photovoltaic array in
reactive power control is inadequate. The application of power simulation studies is given in [12].
electronics makes the electrical transmission system more In this paper the author choose a 100v 3kva
reliable and controllable. The Static Synchronous STATCOM for study. The control strategy proposed in this
Compensator (STATCOM) is coming under FACTS family. paper is based on DC voltage and current. It improves the
The working of STATCOM is based on a voltage-sourced STATCOM output voltage and maintains the capacitor voltage
converter. STATCOM along with voltage source converter in every condition. Section II presents the design of the
regulates system voltage by absorbing or generating reactive STATCOM. Section III carries out the design of controllers
power [2]. There are various techniques to produce a better and section IV gives the experimental result. Section V
sinusoidal waveform by increasing the number of output presents the conclusion.
voltage levels with minimum number of components. Those
approaches have been published in the literature [4]-[5].These
topologies are very effective II. DESIGN OF A 100V 3KVA STATCOM WITH
The basic switching technique of every voltage PHOTO VOLTAIC CELL
source converter is PWM, but a new switching technique was
A. STATCOM design
proposed in the literature [6], known as hybrid modulation.
There are various techniques for hybrid modulation .These The modeling of the STATCOM, is reviewed in the
techniques produce better output than the conventional but lines below. The modeling is done with the following
carry a problem of voltage control. The voltage imbalance in a assumptions:

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 2014 International Conference on Control, Instrumentation, Communication and Computational Technologies (ICCICCT)

There are six voltage sensors associated to the capacitor in

1) All switches in the system are ideal each cell for sampling the capacitor voltage in every instant
2) The source voltages are balanced for controlling . There is also an inductor connected to each
3) Rs represents the converter losses and the losses of the cluster which eliminates the switching ripples due to high
coupling inductor s R voltage converter operation.
4) The harmonic contents during the switching operation are
negligible. Cascade number:
Ls Rs Lac
AC
Cascade number is one of the most important design
parameters for the design of STATCOM. This design criteria
AC
is based on the blocking voltage of the IGBTs being used in
AC
the STATCOM. The 1.7-kV IGBTs are now commonly
100V,50
available. The dc mean voltage should be designed to be
Hz
CELL W1
around 1000 V so that the ac root mean square (rms) voltage
CELL U1 CELL V1
of each H-bridge converter should be around 625 V. This
gives the cascade number N as
N =3000/√3
Therefore, the cascade number can be assigned as N = 2.
CELL U2 CELL V2 CELL W2

B. PV cell
Solar energy is one of the most promising energy source. It
has numerous advantages than other energy source such as it is
clean, is inexhaustible and environment friendly. In this
Figure 1 : STATCOM model context, photovoltaic (PV) power generation has an important
role. PV cell convert solar energy to electrical energy. One of
Fig 1 shows the configuration of a 100 V 3Kva STATCOM.
the main drawback of solar cell is its output current is very low.
This STATCOM arrangement is based on hybrid modulation
A boost circuit in association with solar cell solves this
technique. In this arrangement Each phase consist of a high problem. Figure 2 shows the equivalent circuit of a PV cell. A
voltage converter and a low voltage converter. Each cell is solar cell consists of mainly a current source, two exponential
with a capacitor of 9400μf.Table 1 gives an idea about circuit diodes and a resistor.
parameters.
Rp

NOMINAL LINE-LINE RMS VOLTAGE Vsab, Vsbc, Vsca 100V

AC Inductor Lac 6mH

Current source D1 D2
Rs

Power rating P 3KVA

Starting resistor R 51Ω

Background inductance Ls 0.3mH

Figure 2: Equivalent circuit of a PV cell

DC Capacitor C 9100µf As per Kirchhoff’s current law

I  I source  I D 1  I D 2  I RS
Switching frequency for cell U1,V1 and W1 --- 50Hz

I  I ph
 I s ((exp( V  IR s ) / NV s )  (V  IR s ) ) / R p In
Switching frequency for cell U2,V2 and W2 5000Hz
this paper the author choose a W solar cell for study. Figure 3
shows the SIMULINK model of a solar cell and table 2 shows
the specification of the solar cell used in this paper.

Table 1: Circuit parameters

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 2014 International Conference on Control, Instrumentation, Communication and Computational Technologies (ICCICCT)

Figure 3 : Solar cell model

Parameter specification
Figure 4 : Boost converter
Open circuit voltage 0.5 V
III. DESIGN OF CONTROLLER
Short circuit current 6000 mA
STATCOM is a compensating device. It supply reactive
power to the grid when grid require reactive power and absorb
Energy gap 1.11 eV
reactive power from the grid when grid have enough reactive
power. A control strategy is proposed in [1].This strategy
Temperature 3 doesn’t ensure the reliability of the system. During the
exponent for Is operation there is a chance of variation of capacitor voltage
due to capacitor damage, there the importance of a backup
source. A non renewable energy source is used here as a
Temperature 0
exponent for Rs
backup source. Its action comes in active mode when
capacitor voltage goes below a value. It is necessary to find
out a coordination control between STATCOM and the
Fixed Temperature 250C backup source. For simplicity solar system is used here as a
backup source. Figure 4 shows the block diagram of the
control algorithm proposed in this paper.
Irradiance Ramp up-down
irradiance(200W/m2
to 1000 W/m2) The whole control algorithm consists of mainly two parts,
Table 2: solar cell parameters namely capacitor voltage control and solar cell control.
Capacitor voltage control is subdivided into mainly four parts,
decoupled current control, overall voltage control, clustered
C. Boost converter balancing control and individual voltage control. These
The output from a PV cell is of very low current. It is very control algorithms are in [1].Figure 4 shows the block
essential to increase that current level for applications. Boost diagram of capacitor voltage control. Clustered balancing
converter increase the current level to an above value than the control method maintain the total voltage of a cluster,
previous. PV cell usually produce output efficiently at a Individual voltage control maintain the voltage of every
particular unique point known as maximum power individual capacitor, Overall voltage control maintain sum of
point(MPPT). At MPPT voltage and current from the solar all capacitor voltages to a value and decoupled current control
cell is maximum, therefore a separate control algorithm is generates command signal to generate gate pulses based on
required for finding maximum power point. This control source and load current. Figure 6 shows the block diagram of
algorithm drive the thyristor used in the boost converter capacitor voltage control.
.Figure 4 shows the SIMULINK block of a boost converter
with solar cell.

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 2014 International Conference on Control, Instrumentation, Communication and Computational Technologies (ICCICCT)

CONTROL

CAPACITOR VOLTAGE
PV CELL CONTROL
CONTROL

1.CURRENT CONTROL

Figure 7: PV cell controller

IV. EXPERIMENTAL RESULTS
2.VOLTAGE CONTROL
Figure 8 shows the STATCOM output voltage. From figure it
Figure 5: Block diagram of the total control scheme is clear that a nine level voltage is produced from the
STATCOM.

CLUSTERED
ref1 BALANCING
CNTROL

First phase Command G

Vabc A
T
.
. E
Iabc
DECOUPLED CURRENT Second phase Command INDIVIDUAL VOLTAGE
.
.
P
CONTROL CONTROL . U
.
Vlabc . L
. S
Third phase Command E
Ilabc s

OVERALL
ref2 VOLTAGE
CONTROL

Figure 8:STATCOM output voltage

Figure 6: Capacitor voltage control Figure 9 shows the capacitor voltages of high voltage and low
voltage converter during its operation. It shows that the
The operation of PV cell is not requires in all times. It comes voltage of both high voltage and low voltage converter is at a
in active stage when individual capacitor voltage goes below a particular value all during its operation and PV cell is active
particular value. The controller continuously sample the when the capacitor voltage is below a particular limit.
capacitor voltage and generate a gate pulse for the switch if Figure 10 shows the gate pulses to the PV cell switch during
capacitor voltage goes below a particular value. Figure 6 its operation. It shows that PV cell output is available only
shows the SIMULINK model of the above concept. when the capacitor voltage goes below a particular value.

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 2014 International Conference on Control, Instrumentation, Communication and Computational Technologies (ICCICCT)

Figure 9: Capacitor voltage of A. high voltage converter,

B. low voltage converter Figure 11: A.PV cell output current, B .Output current from
boost converter, C. Voltage across boost converter(100Ω
Figure 10 shows the gate pulses to the PV cell switch during resistor)
its operation. It shows that PV cell output is available only V. CONCLUSION
when the capacitor voltage goes below a particular value.
This paper presents a new control method to maintain the
STATCOM DC link voltage to a minimum value. The
additional PV cell acts as a backup to the STATCOM DC link
voltage source. It serves as a source to the STATCOM DC
link capacitor when the capacitor voltage is below a particular
limit. This control method along with STATCOM improves
the output waveform quality and improves the reliability of the
system.

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