ABSTRACT
Measurement and instrumentation in power systems are often concerned with the analysis of voltage and current waveforms. Harmonic analysis and phasor estimation play an important role in several power system areas, from protection to power quality (PQ). PQ analyzers estimate the harmonic content using the Discrete Fourier Transform (DFT) and the Phasor Measurement Units (PMU) computes the time-synchronized phasor using modified DFT algorithm, or other signal processing tools. Independent of the power system application, a measurement system is composed of four main parts as illustrated in Fig. 1. The first block is the instrument transformers, which are used for “stepping down” the voltage or current of the system to measurable values. Typical values are 110V for voltage and 5A for current. The second block is the signal conditioner,
composed of surge suppress and analog filters. The third block is the sampling block, composed of a sample and hold (S/H) circuit and an Analog-to-Digital converter (ADC). The last block is the digital signal processing where signal processing-based algorithms are performed to produce the desired output that can be information to be recorded and analyzed off-line or to be used in real time, such as a trip for opening a recloser in protection application.
