KOREA
Study on anti-islanding method development for utility
interactive inverter system
Byunggyu YU
KOSEF - 10709
Researcher,
Korea Institute of Energy Research,
Photovoltaic Research Group
Japanese Advisor : Mikihiko MATSUI
Professor, Tokyo Polytechnic University
Distributed generation (DG)
system like photovoltaic (PV) and
wind is growing larger and more
complicated in order to avoid
environmental problem and fossil
fuel shortage crisis. However, the
advent of DG may be the cause of
some problems to stability and
power quality in the adjacent utility.
Especially, the possible occurrence
of islanding phenomenon has been
one of the most issued problems in connection with ongoing growth of DG
generation. It can possibly cause safety problems to utility service personnel or
related equipments. In order to prevent islanding, many anti-islanding methods
have been studied. However, it is mostly based on the power perturbation which
impacts on the inverter low power quality. Therefore, it is significant to propose
the anti-islanding method with the least power quality degradation.
In response to this concern, this dissertation is focused on investigation and
development of effective and reliable islanding detection method with high power
quality for grid-connected inverter system. In order to have the least power quality
degradation with good islanding detection capability, the research is basically
approached into reactive power variation method and effective power variation
method.
As a reactive power variation method, the optimal design methodology for slip
mode frequency shift (SMS) method is presented based on the non-detection zone
concept of standard relays by corresponding to IEEE Std. 929-2000. As a result, it
shows the good islanding detection capability within 2 second and good power
quality with around 0.99 power factor.
However, the reactive power variation is naturally based on the power quality
degradation by changing the current phase. Thus, as a novel effective power
variation, the Periodic up/down Current Magnitude Variation (PCMV) method is
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proposed in the dissertation. In order to differentiate between islanding voltage
deviation and abnormal grid-connected voltage deviation, the proposed PCMV
method is combined with the conventional AFD method for practical application.
The proposed PCMV method has reliable islanding detection capability within
0.245 second with 0.99 power factor. For the implementation of reactive and
effective power variations in this dissertation, the direct current control method is
used not the voltage control method based on the complex power in order to have
faster control response.
As an improved effective power variation method, a novel correlation based
islanding detection method is presented. This correlation technique is applied into
the PCMV method and it works well without frequent nuisance tripping. Thus, this
method can be useful to detect islanding very quickly without using second stage
AFD method.
Then, as an anti-islanding method application for three phase grid-connected
inverter system, it is proposed that a novel current commands for only phase-a and
phase-c are distorted by the conventional AFD method. Asymmetrically injected
zero currents by the AFD method make some time difference between positive half
cycle and negative half cycle only after islanding occurs. The presented time
difference can be used an islanding detection indicator with good capability under
various local load conditions including induction motor load.
In conclusion, this dissertation is targeted to study good anti-islanding method
with good power quality. Thus, effective and reactive power variation methods are
proposed including correlation technique and time difference indicator. Especially,
since the proposed indicators don’t depend on the standard relays, it can reduce the
amount of power perturbation and it leads naturally to the very high power quality.
As a result of this research, it is expected to contribute to solve the modern
electric power system safety problem by islanding remarkably with good power
quality. As a further
research, it is expected
to apply the proposed
correlation technique
into the other active
methods like AFD
method.
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