Abstract
Over the last decades, with the increase in the use of harmonic source devices, the filtering process has received more attention than ever before. Digital relays operate according to accurate thresholds and precise setting values. In signal flow graphs of relays, the low-pass filter plays a crucial role in pre-filtering and purifying waveforms performance estimating techniques to estimate the expected impedances, currents, voltage etc. The main process is conducted in the CPU through methods such as Man and Morrison, Fourier, Walsh-based techniques, least-square methods etc. To purify waveforms polluted with low-order harmonics, it is necessary to design and embed cutting frequency in a narrow band which would be costly. In this article, a technique is presented which is able to eliminate specified harmonics, noise and DC offset, attenuate whole harmonic order and hand low-pass filtered signals to CPU. The proposed method is evaluated by eight case studies and compared with first and second order low-pass filter.
Highlights
Disturbances are a main power quality issue in power systems
The harmonic sources can be divided into the following categories in a power grid [5,6,7]:
The Moving Average Window (MAW) technique in two schemes is compared with the common first and second order low-pass filters which are applied in protective relays
Summary
Disturbances are a main power quality issue in power systems. Voltage and current waveform distortions such as harmonics and noises are the main disturbances. The 8th clause of this report points to relay malfunction by harmonics [5] This topic is notable when the decision-making process depends on the rate of waveform distortion. The harmonics can cause mis-operation of the microprocessorbased relays [14] This issue has notably been investigated in case of power distribution system protection conducted on static under-frequency and over-current relays [15]. Figure 1shows the basic components of the typical digital relay which LP filter situates after surge protective circuits the prevalent low-pass filtering might be suitable for high-frequency harmonics, the performance and cost are considerable especially in filtering low-order harmonics [24]. The MAW technique in two schemes is compared with the common first and second order low-pass filters which are applied in protective relays. The design simplicity and the low computational burden is the two main advantages of the proposed method
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