02-07-2013, 03:17 PM
The Hybrid Method for On-line Harmonic Analysis
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Abstract
The novel hybrid method of the Discrete Fourier
Transform (DFT) and the Enhanced Phase-Locked Loop
(EPLL) has been presented. The original well-known methods
and the Hybrid method have been analysed and tested in
several simulations and experiments. The methods are
implemented on the Digital Signal Processor and compared in
a real time power quality application. Although it requires a bit
higher computational effort, the Hybrid method compared to
original methods shows advantages for on-line harmonic
analysis.
INTRODUCTION
The number of sensitive devices that require high power
quality, i.e. high quality of power supply is constantly
increasing [1]. High quality power supply has a perfect
sinusoidal signal with constant frequency and amplitude. On
the other hand, the use of nonlinear devices and the timevarying
loads in the electric power system is widespread and
is growing with even bigger pace. Such devices and loads
are consuming distorted current from the power system grid.
To preserve high power quality, the most important indices,
such as harmonics, sags, swells, short interruptions,
imbalances, and flickers, ideally have to be eliminated. In
recent years, researchers have been developing tools for
accurate power quality analysis, as well as devices for
mitigation of power supply imperfections.
THE HYBRID METHOD
It is well known that the DFT is a powerful tool for
harmonic analysis, but its algorithm suffers from several
drawbacks [12], [22], [23]. The EPLL is known as an
accurate harmonic estimator in non-stationary frequencyvarying
environments, but compared to the DFT, has a
slower dynamic response [18]. In order to use benefits of
these two methods, the new hybrid method of the DFT and
EPLL is presented. The idea to combine the DFT and phaselocked
loop to overcome known problems has been
mentioned as a possible way forward [12], [18], [22]. It can
be seen in fig. 3 that the EPLL is used to estimate the
frequency and the DFT to estimate amplitudes of harmonics.
It is already mentioned that the changes of the fundamental
system frequency are small and the EPLL can track those
changes fast and accurate. Several simulation tests were
conducted to compare accuracy of the DFT, EPLL and the
Hybrid method. Parameters for the EPLL were chosen from
[16].
CONCLUSION
In this paper, different methods for on-line harmonic
analysis have been compared and discussed in order to find
the best solution for the power quality problems. The DFT
and EPLL have been implemented and then tested on a realtime
application. The DFT is the most used method that
gives accurate information about the steady state harmonics.
The EPLL works even better in the case when the system
frequency is changing. Both these methods require one
computational unit per harmonic so that only a limited
number of harmonics can be tracked.
In order to overcome the drawbacks of the DFT and
EPLL in the power quality analysis, the new method which
is the hybrid method of the DFT and EPLL has been
proposed and implemented. It was tested together with the
above mentioned ones. The Hybrid method requires an
additional unit compared to the DFT units for harmonic
estimation.