A distance relay, which is widely used for protecting a power transmission line as the primary as well as remote backup device, may be affected by the power disturbances such as power swings and post-faults. Consequently, false trips of protection system may be resulted. Root cause analysis based on fault tree analysis has been used to identify disturbances which may lead to false trips. Once the critical root causes have been identified, conventional mitigation measures are used and then, new blocking function and digital filters are proposed to enhance the security of the system. The quantitative analysis of the improved model shows a significant increase in the security which implies an appreciable enhancement of the reliability of the considered protection system.
Measurements of the instantaneous amplitude and phase angle of the fundamental current or voltage components and frequency that may be needed in three-phase power grid instrumentations/meters and relays may be affected by several encountering disturbances. These disturbances may be due to an abrupt heavy load changes and post-faults resulting in frequency deviation, DC offsets decaying, and harmonics especially in the power grid including TCSC devices. To study these effects on the quality of measurements, this work presents a new approach using the data generated by Simulink/MATLAB software. The obtained simulation results show that the unwanted DC offset and harmonics as well as sub-harmonics can be removed completely by conventional filters such as anti-aliasing and DFT filtering but they can be eliminated by inserting before DFT algorithm a digital filter.
During a transient operation condition of power smart grid, line current may include unwanted components that may cause unnecessary tripping of protection system. The disturbance mainly appears in a form of harmonics and sub-harmonics. In this case of signal waveforms including harmonics, the low pass filter may be used. However, this type of filter does not provide the ability to reject sub-harmonics. This chapter presents the digital filtering design issue based on optimization approach for removing sub-harmonics and hence improving the measurement. The first point of view is to reach an unified accurate phasor measurement algorithm that is immune to nearly all disturbances (sub-harmonics) in power grid including FACT devices and renewable energy sources, simultaneously with required speed of convergence. The second point focuses on reducing the computational requirement and algorithm complexity through designing recursive digital filter with reduced order.
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