ABSTRACT
Islanding refers to a condition where distributed generators (DGs) inject
power solely to the local load after electrical separation from power grid.
Several islanding detection methods (IDMs) categorized into remote, active, and
passive groups have been reported to detect this undesirable state. In active
techniques, a disturbance is injected into the DG’s controller to
drift a local yardstick out of the permissible range. Although this disturbance
leads to more effective detections even in well-balanced island, it raises the
total harmonic distortion (THD) of the output current under the normal
operation conditions. This paper analyzes the power quality aspect of the modified
sliding mode controller as a new active IDM for grid-connected photovoltaic
system (GCPVS) with a string inverter. Its performance is compared with the
voltage positive feedback (VPF) method, a well-known active IDM. This
evaluation is carried out for a 1 kWp GCPVS in MATLAB/Simulink platform by
measuring the output current harmonics and THD as well as the efficiency under
various penetration and disturbance levels. The output results demonstrate that
since the proposed disturbance changes the amplitude of the output current, it
does not generate harmonics/subharmonics. Thereby, it has a negligible adverse
effect on power quality. It is finally concluded that the performance of the
sliding mode-based IDM is reliable from the standpoints of islanding detection
and power quality.
KEYWORDS
1.
Islanding
detection method (IDM)
2.
Power quality
3.
Sliding mode
controller
4.
Total harmonic
distortion (THD)
5. Voltage positive feedback (VPF)
SOFTWARE: MATLAB/SIMULINK
CONCLUSION
In this
paper, the influence of the classic VPF and modified sliding-mode IDM on the
GCPVS’s power quality and efficiency has been evaluated. The study has been
done for a 1 kWp PV system with string inverter. The simulation results show
that, while the THD of output current in the proposed IDM is smaller than the simple
VPF, both methods render acceptable power quality in a wide range of system operation.
This proper performance has been achieved due to the variation of the current
magnitude rather than the angle or frequency. This magnitude variation is
realized in VPF and the proposed method in the current and voltage control loops
(MPPT), respectively. The simulations also confirm that the acceptable THDI and
harmonics are guaranteed in multi-GCPVSs connection situation even at low power
generation levels as the worst scenario. Since the new technique tries to
deviate the system from its MPP condition, the effect of embedded disturbance
on the efficiency is also performed. In this regard, the simulations are
carried out and a negligible reduction in MPPT and inverter efficiencies (less
than 0.04%) has been demonstrated in the proposed method. This occurs since MPP
can be gained at a small bound around ref. It has been finally concluded
that the modified sliding mode controller has the advantages of the
conventional VPF scheme in islanding detection as well as a higher power
quality in the production of energy.
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