A modern two dof controller for grid integration with solar power generator

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN
0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME
& TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 3, Issue 3, October - December (2012), pp. 164-174
IJEET
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2012): 3.2031 (Calculated by GISI) ©IAEME
www.jifactor.com

A MODERN TWO DOF CONTROLLER FOR GRID INTEGRATION
WITH SOLAR POWER GENERATOR
Sweeka Meshram1, Ganga Agnihotri2, Sushma Gupta3
1
(Deptt. Of Electrical Engg., MANIT Bhopal, 462051, India, sweekam@gmail.com)
2
(Deptt. Of Electrical Engg., MANIT Bhopal,462051, India, ganga1949@gmail.com)
3
(Deptt. Of Electrical Engg., MANIT Bhopal,462051, India, sush_gupta@yahoo.com)

ABSTRACT

This paper presents the design, analysis and implementation of the power grid
integration with the solar power generator using the two Degree Of Freedom (DOF)
controller. Micro grid distribution generation (DG) systems usually have inverters and these
interfacing inverter are directly connected to the power grid through passive filter. In this
paper, a two DOF controller is employed for controlling the DG inverter, which behaves as
an uninterruptible power supply (UPS) for its local community loads. The two DOF
controller is capable of connecting/disconnecting the power grid with the variation in the
solar power generation and load. The proposed system is a reliable because it is capable of
providing continuous smooth power flow with control. The interfacing inverter must have the
capability to regulate the AC bus voltage at the occurrence of nonlinearities and irregular
natures of the loads. To meet this requirement a fast two DOF controller is applied to the
proposed system. The effectiveness of the grid connected solar power generator system and
adopted control technique is verified.

Keywords: Power grid, Solar System, Two DOF Controller, PLL.

I. INTRODUCTION
With the increasing electricity demand, it is very necessary to promote the new form
of power generation using the renewable energy such as wind, solar and fuel cell. The power
generation systems generating the electricity from many small energy sources are known as
the Distributed Generation (DG) system. Most countries generate electricity in large
centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants
or hydropower plants. These plants have excellent economies of scale and allow collection of
energy from many sources and may give lower environmental impacts and improved security
of supply. Among the DG systems, the photovoltaic (PV) technology based DG is gaining
approval as an approach of maintaining and civilizing living standards without harming the
environment. Fig. 1 shows the PV installed capacity in the world. The PV installation has an

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exponential growth because the many grid connected PV system is supported by the
government and private companies.

Fig. 1 Installed PV Power Capacity
The power generation from the solar energy has two major tribulations. One problem
is the PV cell has very limited power generation capability. The series and parallel
combination of the PV cell/module may not generate the sufficient power required for the
application. Another problem is power generation is not possible when solar irradiation is not
available such as in the night and in the rainy days. Thus continuous power flow is not
possible when using the PV system in standalone mode. This problem can be solved by
connecting utility grid in parallel with the PV generator. For the parallel operation of the grid
and PV generator the frequency and amplitude of voltage of both the system should be same
and the generated voltage of the PV generator should be in phase with the grid voltage. But
with the variation in the load the amplitude/frequency of grid voltage may be disturbed. To
vary the PV generator voltage according to the grid voltage variation, an interfacing device is
required. In the grid connected DG system an inverter works as an interfacing device. For
generating the switching pulses for the DG inverter, a controller is required. This controller
tracks the phase angle of the three phase grid voltage and generates the switching pulses such
that the inverter will output the voltage waveform which in phase with the grid voltage. A
phase tracking system has been developed for tracking the phase angle of the grid voltage [1].
A new green power inverter for interfacing the fuel cell with the grid has been developed [2].
As the advancement is being continuously in power electronics, it is suggested that the
inverters based on the power electronics devices are efficient for interfacing purpose [3]. A
novel vector control system using deadbeat-controlled PWM inverter with LC filter has been
developed [4]. Using the LCL filter, a modified direct power control strategy has been
developed for connecting the inverter to the grid [5]. A two DOF controller is also developed
for controlling the DG inverter for the parallel operation of the fuel cell with grid [6]. The
performance analysis of these systems shows that these systems have harmonics and
therefore an adjustable harmonic mitigation technique for harmonic reduction of the
photovoltaic system utilizing the surplus capacity of the interactive inverter has been
developed [7]. The modeling, analysis and testing of the inverter based micro grid system has
been done [8]. The two DOF controller is the best controller because it can control the power
flow bi-directionally to/from the grid and enables the smooth parallel operation of the grid
with the DG [9-12].
In this paper, analysis of the grid connected PV generator based DG system using the
two DOF controller been done. In this paper, a DG inverter is designed, which can perform
the task of the Uninterruptible Power Supply (UPS). The solar power based DG system is

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designed for feeding the particular local load. In the rainy days/night, the solar power
generator is not capable to supply the required power demand. In that case, the power grid is
connected for meeting up the power demand. When the power demand is greater than the
solar power generation, then the grid is connected automatically using the two DOF
controller. Thus continuous power supply is possible for supplying to the local load.

II. STRUCTURE OF PV SYSTEM BASED DPGS
The structure of the Distributed Power Generation System (DPGS) based on the PV
system is shown in the fig. 2. The maximum power is extracted using the MPPT and stored in
the battery. The MPPT uses the Incremental Inductance technique. The task of the power
transformation from the PV system to the utility grid is carried out using the power
conversion unit. The power conversion unit is the combination of DC-DC converter and
PWM AC inverter. The stored DC power (generated by the PV array) in the battery energy
storage system is not able feed directly to the load/grid. The DC/DC boost converter is used
for increasing the level of the DC voltage up to the level reliable for synchronizing the grid.
The three level AC inverter works as a grid interfacing device and able to generate the AC
voltage matches the grid voltage level with reduced harmonics. The generated AC electricity
now can be fed to the local load or to the grid.

Fig. 2 Structure of the PV System based DPGS

The most important parts of the proposed system are the controllers. There are two types
of the controller is adopted for the purpose of controlling the DPGS i.e. PV generator side
controller and grid side controller.
a) PV side Controller: The task of this controller is to extract the maximum generated
power from the solar renewable energy sources. The protection of the PV side
converter is also handled by this controller.
b) Grid side Controller: This controller performs the many functions such as controls the
PV generated active power which is fed to the grid, controls the reactive power
transfer between DPGS and grid, controls the DC link voltage of the PV array,
enhances the power quality and the main function is to synchronize the grid with the
PV generator
.
III. TWO DEGREE OF FREEDOM CONTROLLER
The grid connected Solar Power Generator must have the high control bandwidth for
maintaining its AC voltage undistorted during load changeover. The two DOF controller
provides the high control bandwidth with reliable operation. The grid voltage is taken as
reference frame for phase synchronization of the PV generator with the grid. The controlling

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of the DG inverter is a necessary part of the system and it has been done using the PLL
(Phase Locked Loop). With the variation in the load and due to the harmonic component, the
grid voltage is usually distorted. If this distorted grid voltage is considered as a reference for
the synchronization, then the power quality of the local AC bus of the PV generator may be
poor. The quality may be improved using the PLL. The PLL calculates the phase angle (θ) of
the grid voltage (VG_abc). The PLL is generally a frequency controller. The three phase grid
voltage and capacitor voltage (VC_abc) across the filter capacitor is converted into two phase
quantity. The d-axis component of the capacitor voltage (VCd) is compared with the d-axis
component of the grid voltage (VGd) and the error signal is processed with the gain (Gm)
which is equal to 1 for the designed system. The output is then processed with the inverse
dynamic [G-1(s) = Lf.Cf + 1] and it is also compared to VCd. The error is processed with the PI
controller. The output of the PI controller and inverse dynamic gain is then added. The VCd is
also processed with the ω block and the output is added to ∆Id.

Fig. 3 Block Diagram of Two DOF Controller for DG Inverter

Similar process is adopted for obtaining the ∆Iq. These two quantities i.e. ∆Id and ∆Iq is
then converted into three phase and considered as a control signal for generating the PWM
switching pulses for DG inverter. In the tracking performance, the two DOF controller is
better than the conventional PI controller. Table.1 shows the comparison between the
conventional PI controller and the new Two DOF Controller.

TABLE.1 COMPARISON BETWEEN PI CONTROLLER AND 2 DOF CONTROLLER
Sr. Two DOF Controller Conventional PI Controller
No.
1. It Implies fast controlling of power. It has slow power controlling capacity.
2. It has larger bandwidth. It has smaller bandwidth.
3. It has smaller phase lag. It has larger phase lag.
4. Managing of power to/from grid is It is less efficient as compared to the 2
efficient using this controller. DOF Controller.
5. Able to prevent from an excessive With this controller, a large circulating
circulating current. current flows from grid.
6. It has fast disturbance rejection capability. It has no disturbance rejection
capability.

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IV. CONTROLLING OF POWER FLOW
The two DOF controller is able for grid synchronization as well as helps DG inverter to
work as a uninterruptible power supply (UPS). Fig. 4 shows the power circuit diagram in
which PV generator is connected with grid through the inductor LG. The grid voltage (VG)
and grid current (IG) is considered as a reference for controlling the DG inverter. It is
considered as the grid has unity power factor i.e. VG ∠0.

Fig. 4 Power Circuit of Grid Connected PV Generator

The phasor diagram of the DG system circuit is shown in the fig. 5. When the real power
is supplied to the grid from the PV generator the VC is leading VG by an angle δ which is
greater than zero. The voltage across the grid inductor VLG is having 900 angle to grid
voltage. The real power flow from PV generator to the grid can be mathematically expressed
as:
ܸீ . ܸ஼ ܸீ . ܸ஼
ܲ= sin ߜ =
߱‫ீܮ‬ ߱‫ீܮ‬
The power transfer depends on the angle δ. If δ is greater than 0, power flows from PV
generator to the grid. If δ< 0, PV generator receives power from the grid and if δ = 0, then
there will be dynamic isolation between PV generator and grid.

Fig. 5 (a) Fig. 5(b)
Fig. 5 (a) Vectors when DG Supplies Real Power to Grid,
(b) Vectors when DG supplies Reactive Power to Grid
For transmitting the reactive power VG and VC should be in phase and the grid current IG
will have 900 to VG. The transmitting VAR is given as:
ଶ
ܸ஼ ܸ஼ . ܸீ
ܳ= −
߱‫ீܮ‬ ߱‫ீܮ‬

V. RESULTS AND DISCUSSION

A 25 kV, 2500 MVA power grid is connected to 100 kW generating PV System. The
array consists of 66 strings of 5 series connected PV modules connected in parallel. One PV
module has 96 PV cells. The PV array is delivering the 100 kW at 1000 W/m2 at maximum
power. The open circuit voltage (VOC) of the one module is 64.2 V and voltage at the
maximum power point (Vmp) is 54.7 V. The short circuit current (ISC) of the one module is

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5.96 A and current at the maximum power point (Imp)is 5.58 A. At 1000 W/m2, solar array is
generating 400 V DC voltage and 321 A DC current. The generated DC voltage is converted
using the 3 level DC/AC inverter. The switching frequency of the inverter is 3 kHz. Fig. 6 (a)
shows the output voltage of the solar inverter before filtering and fig. 6 (b) shows the
enlarged waveform of the solar inverter output voltage before filtering.

Fig. 6 (a) Solar Inverter Output Voltage before filtering

Fig. 6 (b) Solar Inverter Output Voltage before filtering

For making the inverter output voltage into pure sinusoidal AC voltage a LC filter is used.
The numeric values of the filter inductance (LF) and capacitance (CF) are 20 mH and 350 µF
[12,13]. The resonance problem is avoided using the damping resistance RD , connected in
series with the filter capacitor and valued as 8 Ω. The damping resistor is able to absorb the
switching frequency ripple and the LC filter gives the pure sinusoidal AC voltage with fewer
harmonic. Fig. 7 (a) shows the solar inverter output voltage after filtering using the LC filter
and fig. 7 (b) shows the enlarged waveform of solar inverter output voltage after filtering.

Fig. 7 (a) Solar Inverter Output Voltage after filtering

Fig. 7 (b) Solar Inverter Output Voltage after filtering

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For analyzing the performance of the grid connected solar system with the
new fast two DOF controller variable load is connected. At t = 0 sec., the Solar
System is able to supply the power to community load of about 18 kW, 5 kVAR
at 0.9 lagging PF. The grid connection to the solar system depends on power
demand of the local load. At t = 0.5 sec. a step load of about 15 kW, 5 kVAR at
0.9 lagging PF is connected. The solar system is able to supply power to this
extra load, hence grid is connected to the solar system and ensures the
continuous power with the variation in the load. At t = 0.8 sec. again load is
increased by 12 kW, 4 kVAR at 0.9 lagging PF and results show that the system
is also able to supply this load also. At t = 1.2 sec. load of about 12 kW, 4
kVAR is disconnected. At t = 1.5 sec. 15 kW, 5 kVAR load is also
disconnected. Now the solar system is able to supply the power to the remaining
load, therefore the grid will be disconnected automatically.

Fig. 8 (a) and fig. 8 (b) shows the contribution of the active and reactive
power of the power grid respectively according to the variation in the load.
From 0 to 0.5 sec. and 1.5 to 2 sec. the solar system operates as a standalone
system and during those periods the grid does not supply power to the solar
system.

Fig. 8 (a) Active Power Variation of Grid

Fig. 8 (b) Reactive Power Variation of Grid

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Fig. 9 (a) and fig. 9 (b) shows the involvement of the active and reactive power of the
Solar System with the variation in the load.

Fig. 9 (a) Active Power Variation of Solar Inverter

Fig. 9 (b) Reactive Power Variation of Solar Inverter

Fig. 10 shows the waveform of load voltage.

Fig. 10 Load Voltage

Fig. 11(a) shows the waveform of the current through the load. From 0 sec. to 0.5 sec., solar
system supplies power to the load of about 18 kW, 5 kVAR. At this load the peak value of
the load current is 31.2 A. At 0.5 sec. load is increased and grid is connected. With the
variation in the load the peak value of the load current reached to 62.1 A.

Fig. 11 (a) Load Current due to step change in load and grid connection

171


Fig. 11 (b) shows the load current, when the load is again increased at t = 0.8 sec. by a
step of about 12 kW, 4 kVAR. The peak load current at t = 0.8 sec has been reached to 82.7
A.

Fig. 11 (b) Load Current due to again step change in load

From t = 1.2 sec the disconnection of the load is started. At t = 1.2 sec. 12 kW, 4 kVAR is
disconnected and the load current regain the peak value of load current of about 62.1 A. Fig.
12 (a) shows the variation in the load current due to step change in the load.

Fig. 12 (a) Load Current due to step load disconnection

Fig. 12 (b) shows the load current when the load is of about 18 kW, 5 kVAR and the grid
is disconnected. During this period the Solar System is again operating in standalone mode.

Fig. 12 (b) Load Current due to step load and grid disconnection

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Fig. 15 (a) shows the phase angle of the grid voltage. According to this phase angle PLL
generates the PLL output as shown in the fig. 15 (b). Using this output, the two DOF
controller generates the switching pulses for the solar inverter such that the solar inverter
output the voltage waveform which is in phase with the power grid voltage.

Fig. 15 (a) Phase Angle of the Grid Voltage

Fig. 15 (b) PLL Output

The variation of the load is taken into consideration to prove that the two DOF controller is
efficient and can control very fast with the variation in the load. With this controller connection and
disconnection of the power grid with the Solar System is possible with good performance.

VI. CONCLUSION

In this paper, performance analysis of the grid connected PV generator based DG system using
Two DOF controller has been done. The Two DOF controller enabled the control of power flow bi-
directionally to and from the grid and smooth parallel operation between the PV generator based DG
system and grid. This controller also enables the regulated inverter output voltage and trying to keep
constant amplitude and frequency of the grid voltage with the variation in the linear and nonlinear
load. The system is analyzed under the grid connected and stand alone mode. Initially the PV
generator is considered as a grid connected. When the PV generator voltage is equal to the grid
voltage i.e. (VC =VG), the grid will automatically disconnect from the DG and PV generator operates
in the isolation mode. Depending upon the VC, grid will receive/supply the power from the PV
generator. The results verify the operation of the adopted system for the grid connected/disconnected
mode and it is apparent that the system is able to put into the service feeding the load.

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A modern two dof controller for grid integration with solar power generator

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