Iaetsd analysis of output dc current injection in 100k w grid

1
Sreenath B
Asst Professor, Dept of EEE
Amal Jyothi College of Engineering
Kanjirappally, Kottayam
bsreenath@amaljyothi.ac.in
Analysis of Output DC Current Injection in 100kW grid
connected VACON 8000 Solar inverter
Sneha Sunny George
PG Scholar, Dept of EEE
Amal Jyothi College of Engineering
Kanjirappally, Kottayam
snehasunny2712@gmail.com
Robins Anto
Head of Department
Mar Baselios Christian College of
Engineering and Technology
Kuttikanam, Peermade
robinsantoiitb@gmail.com
Abstract— Solar energy technologies have gained much
importance in the recent scenario due to their ability to
produce clean, reliable, useful power. Grid connected
Photovoltaic system requires conversion from DC to AC to
harness the useful energy produced. A Photovoltaic inverter
directly connected to the grid can cause, besides the
generation of several current harmonics, a DC current
component injection. Excessive DC current injection into the
AC network can result in problems such as increased
corrosion in underground equipment and transformer
saturation. The paper aims at evaluating the output DC-current
injection in grid connected inverter used for a 100kW solar
power plant installed at Amal Jyothi College of Engineering,
Koovapally, through experimental analysis.
Keywords: Grid connected inverter, DC offset current
I. Introduction
With the increase in energy crisis concerns growing during
day by day, much recognition is being gained in the potential
of solar energy as a sustainable energy source . Solar energy
adds flexibility to the energy resource mix by decreasing the
dependence on fossil fuels, but the greatest barrier to the
technological expansion in this field is the costs of devices
used for converting sun`s energy in the form of radiation into
useful electrical energy, limited space and energy. Even
though there has been a massive downward tendency in the
price of PV modules, the price of grid connected inverters still
remains high thereby increasing the overall cost. The
efficiency of the plant plays a crucial role in the profit
obtained from sustainable energy resources being harnessed.
The major benefit of designing a reliable, stable, efficient and
lower cost photovoltaic power electronics system is the
availability of reliable and quality power without relying on
the utility grid. It also avoids the major investment in
transmission and distribution. To the nation, the major benefit
lies in the fact that it reduces greenhouse gas emissions,
responding to the increasing energy demands by establishing a
new, high-profiled industry.Therefore, it is required to
minimize the losses and improve the efficiency of power
electronic devices used. Use of multilevel inverters has
increased the quality of waveforms and thereby increasing
efficiency of the system. H-bridge multilevel inverters are
more suitable for renewable energy harvesting due to the
presence of separate DC sources.
II. Grid Connected Inverter and DC injections
Grid connected inverters are used to convert the DC power
thus obtained into AC power for further utilization. They are
directly fed solar electricity to the grid. As it does not have the
battery component, the cost of the system is low. The main
quality requirements / factors affecting these power
converters are total harmonic distortion (THD) level, DC
current injection and power factor, the Impulse Withstanding
ratio (or BIL), High Frequency Noise / Electromagnetic
compatibility (EMC), Voltage Fluctuations and Flicker of
Inverter System. Therefore, inverters connecting a PV system
and the public grid are purposefully designed, allowing energy
transfers to and from the public grid. [1-3]
Due to approximate short circuit characteristics of
AC network, a little DC voltage component can accidently be
produced by grid connected inverters which can create large
DC current injections. If output transformers are not used,
these inverters must prevent excessive DC current injection,
which may cause detrimental effects on the network
components, in particular the network transformers which can
saturate, resulting in irritant tripping. This may also increase
the losses and reduce the lifetime of the transformers, if not
tripped. Moreover, the existence of the DC current component
can induce metering errors and malfunction of protection
relays and can create an adverse effect on the overall
functioning of the solar power plant.
Therefore, there are stringent regulations in many countries
to prevent the network from the large DC current injection.
Since most Indian standards published by BIS are aligned to
IEC standards, DC injections up to ±1% is being proposed by
the BIS in the Indian standard keeping with IEC 61727. The
H-bridge or Multi Level inverter eliminates the DC
component of the current by adding switches on the DC side
to clamp the voltage during the zero voltage periods. This
method could be also applied by clamping in the AC side.
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Both methods could not guarantee elimination of DC
component as the unbalance due to forward power electronic
switch voltages and PWM control can not be removed. [4-6]
III. Experimental Result
The main objective of the work is to conduct an analysis
study of output DC injection in grid connected 100kW
VACON 8000 solar inverter installed at the 100kW solar plant
in Amal Jyothi College of Engineering, Kanjirappally, India.
Technical analysis of the plant is done to evaluate the effect of
environmental and climatic conditions on the performance of
the system. The analysis also evaluates the effects DC offset to
variations in its operating conditions. The International Energy
Agency (IEA) Photovoltaic Power Systems Program
describes, in its IEC Standard 61724, the parameters used to
assess the performance of solar PV systems.
Fig 1: Block diagram of 100kW solar plant
The block diagram shown above represents the entire grid
connected solar power plant installed at Amal Jyothi College
of Engineering. 100kW solar panels are used to trap solar
radiations. The energy obtained is converted to useful AC
supply using a VACON built inverter.The energy harnessed
from the sun is used to meet the requirements of an entire 7-
storey building block. When the energy harnessed is not
sufficient the required amount is taken from the KSEB grid.
Since Kerala is placed in the equatorial region, it has high
solar insolation and temperature. The normal ambient
temperature varies from 23-33degC. The variation in solar
insolation and temperature affects the panel PV panel
performance. Rise in temperature results in degradation of
efficiency and power output of the solar panel. The solar
insolation falling in the earth`s atmosphere has direct or beam
radiation,diffused radiation and albedo or reflected radiation.
In bright sunshine days, the beam and albedo radiations are
greater . But that during cloudier days the diffused
components are more. The data collection was made for sunny
and rainymonths ie in the months of February, march, April
and May. From, the analysis of the available data, it is found
that the panel has different behavior for varying
insolation,temperature etc. From the experimental Datas
collected, the efficiency of the inverter has reached up to 87%
during high radiation time i.e.the inverter is never operating
near at its full capacity and the average DC to AC conversion
is below 90%.The inverter efficiency very slowly declines
after peak value is reached.PV system at its best is operating
in the 20 to 40% range of rated output and hence is operating
in the 87 to 91% efficiency range during the sunniest periods.
Since the inverter is kept in a mechanical room under the roof
and the temperature differences was not as drastic as they
would be for inverters located outside.
Fig 2. Solar insolation vs Time on a rainy day (a) and clear
day (b)
From the analysis done, on a clear day with bright sunshine
the panel receives daily average solar radiation of about
4.56kW/m2 to 5.24kW/m2. During the rainy days the solar
radiation is about 3.13kW/m2 to 4.3kW/m2. Thus from the
data collection it can be classified into two groups. First, high
solar radiation groups that is available from January to mid
April. The average solar insolation available over the region
from the online satellite data the solar radiation available is in
the range of 5 to 6kW/m2
for sunny hours and is around
4kW/m2 for rainy hours. The plot describes the solar radiation
for sunny and rainy days.
The main sources of DC injections are power supply,
computer, network faults, geomagnetic phenomenon,
cycloconverters, lighting circuits/ dimmers, embedded
generators, AC and DC drives and photovoltaic grid inverters.
Due to approximately short circuit characteristics of an AC
network under a DC voltage excitation, a little DC voltage
component that can be accidentally produced by the inverter
will produce large DC current injection. This causes
detrimental effects on the network components, in particular
the network transformers which can saturate, resulting in
irritant tripping. This may also increase the losses in and
reduce the lifetime of the transformers, if not tripped.
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Moreover, the existence of the DC current component can
induce metering errors and malfunction of protection relays.
The effect of DC currents on the accuracy of Domestic
Electricity watthour meters is both an issue in relation to the
type of meter used and its method of connection to the supply
network. As a consequence, it is believed that the effect of DC
components acting upon watt hour meters merits further
investigation, (best undertaken by direct testing due to the
reluctance of manufacturers to discuss operation).There are
thus stringent regulations in many countries to prevent the
network from the large DC current injection.The solar
insolation Vs output DC injection graph for a period of
3months is shown in the following graph.
Fig 4. solar insolation vs DC offset on a rainy day & clear
day
The VACON 8000 SOLAR inverters use special digital
control techniques to limit the DC offset in the output obtained
from the inverter. When solar insolation is below the required
value ( i.e.DC output voltage of solar panels is less than 340V)
or when a fault occurs the inverter shutdowns and starts only
5mins after the fault condition is restored. This results in an
error the value (357.67 %)of DC offset measured using
FLUKE power analyzing meter. During March - mid April
2014 time period, the DC o_set varies between 0.04 (during
high solar insolation) -0.19(During low solar-insolation)
percent and during mid April- May 2014 time period,the DC
offset varies between 0.03 during high solar insolation to 0.15
during low solar radiation time period. The maximum allowed
DC offset in India is 1 % of the output obtained.
On analytic calculation of the PV inverter efficiency, it
was found that between during the course of the experiment
conducted between March to May on a sunny / clear day the
efficiency of the 100 kW inverter was found to be 82.6%.
Similarly for a rainy day the average efficiency of the inverter
is calculated to be 80.23%. This can even drop if the solar
insolation considerably drops. The daily average PV inverter
output generation and efficiency can be noted to be
360kWh/day at 82.26% efficiency for a clear sky and about
200kWh/day at 80.23% efficiency for a rainy day.
Fig 3. Efficiency Vs output power on a rainy day & clear day
The plots show the relation of DC inverter input power and
inverter efficiency for clear and rainy day. The inverter is
found to have a serious defect of frequent shutdown if the
voltage drops to a low value that lasts only for a few seconds.
The estimated fault is due to an error in one of the parameters
coded in the software installed in the inverter. This defect is to
be corrected and rectified otherwise it will result it complete
failure of the system.Solar insolation or irradiance was
measured using the pyranometer at an interval of 30 minutes
along with other details collected. The total energy harnessed
by the installed inverter upto 16th may 2014 is 123992kwh.
IV. Conclusion
The overall system efficiency can get effected if DC
current injections are not limited to, the standards specified by
Indian standards and IEEE standards. The installed control
technique in the 100kW inverter limits the DC injection to
standard values unless tripped. Performance evaluation
conducted shows relation of solar insolation on DC offset and
output power on the efficiency of the system
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