Massimo Aiello

Synchronization techniques for power quality instruments

The measurement of voltage characteristics in power systems requires the accurate estimation of the power supply frequency and signal synchronization, even in the presence of disturbances. The authors developed and tested two innovative techniques for instrument synchronization. The first is based on signal spectral analysis techniques performed by means of the Chirp- transform analysis. The second is a phase-locked loop (PLL) software based on a time-domain coordinate transformation and an innovative phase-detection technique. To evaluate how synchronization techniques are adversely affected by the application of a disturbing influence, experimental tests were carried out, taking into account the requirements of the standards. The proposed techniques were compared with a standard hardware solution. In this paper, the proposed techniques are described, the experimental results are presented, and the accuracy specifications are discussed.

Theoretical and experimental comparison of total harmonic distortion factors for the evaluation of harmonic and interharmonic pollution of grid-connected photovoltaic systems

Grid-connected photovoltaic systems are increasingly used in electrical distribution systems. However, they inject distorted currents. Therefore, special attention must be paid to harmonic and interharmonic measurements. The new edition of IEC 61000-4-7 introduces the concept of harmonic and interharmonic groups, which implies new expressions for total harmonic distortion (THD) factors. In this paper, a theoretical and experimental comparison is made between the different THD factors in order to show which of the currently defined distortion factors is best suited to detect harmonic and interharmonic pollution. Experimental tests were carried out first by means of a calibrator and subsequently in a single-phase grid-connected photovoltaic system. In both cases, measurements were carried out with a PC-based instrument developed by the authors and able to calculate the distortion factors according to IEC 61000-4-7.

A Chirp Z transform based synchronizer for power system measurements

Power system analyzers are required for performing an accurate evaluation of harmonic pollution, voltage and current synchronous sampling in order to limit leakage error and to assure reproducible results. Moreover, the increased interest about power and voltage quality, forced international working groups to define new standards for testing and measurement techniques. In particular, harmonic and interharmonic measurement and instrumentation standards require the correct voltage or current signal synchronization and the recognition of the same condition. In this paper a new synchronizer algorithm is proposed, able to respect in force and draft standards for its accuracy and characteristics. Theoretical aspects are discussed and different experimental tests are reported, comparing the proposed method with a similar well-known one based on interpolated fast Fourier transform.

An induction motor speed measurement method based on current harmonic analysis with the chirp-Z transform

This paper presents a new method to measure motor speed by means of frequency estimation of rotor slot spectral components in the supply current of squirrel single-cage induction motors. The novelty of the method consists in the harmonic analysis of the supply current by means of the chirp-Z transform (CZT). The advantages are improved accuracy due to better spectral resolution and resolvability. Moreover, a shorter observation window is required, thus reducing errors related to nonstationary current signals. The experimental results are presented to validate the proposed method and to make a comparison with a similar method based on the fast Fourier transform (FFT).

A self-synchronizing instrument for harmonic sources detection in power systems

The energy markets deregulation holds a new contractual perspective between customers and utilities, in which energy price can depend on voltage quality and load characteristics, as well as on responsibility for disturbances caused on supply voltage in power systems. In this contest, harmonic sources detection is one of the most urgent problems, because of the proliferation of loads which draw nonsinusoidal currents. In this paper, the authors present a new instrument based on a time domain method for harmonic active power detection in three phase systems. Amplitude and sign of harmonic powers can be measured directly and no spectral analyses are required for the evaluation of amplitudes and phase angles of currents. Moreover, the instrument is able to synchronize itself to the input signal, to measure voltage and current total distortion factors and harmonic voltages amplitudes. Theoretical aspects are discussed, measurement accuracy is evaluated and experimental results are presented.

A comparison of spectrum estimation techniques for nonstationary signals in induction motor drive measurements

This paper compares different spectrum estimation techniques applied to nonstationary signals in order to determine which is the most suitable in analyzing the spectral harmonic content of signals in a very close frequency band. The comparison examines a specific application in which spectral analysis is applied to nonstationary signals, such as currents or voltages in induction motor drives. A test signal and some performance parameters are defined. Experimental tests were carried out by applying different spectral analysis algorithms in order to compare their behavior in detecting defined harmonic frequencies. The results show that the chirp-Z transform outperforms other techniques especially when a restricted frequency band has to be analyzed.

A comparison of spectrum estimation techniques for periodic not stationary signals

This paper compares different spectrum estimation techniques. The aim is to determine which is the most suitable to analyze a signals spectral harmonic content in a very close frequency band in the case of periodic non-stationary signals. A test signal and performance parameters have been defined. Experimental tests have been carried on different spectrum estimation techniques in order to define their behavior in detecting harmonic frequencies of periodic non-stationary signals. The result is that the chirp-Z transform is well suited for a restricted range spectral analysis of a non-stationary signal.

An induction motor speed measurement based on current harmonic analysis with Chirp-Z Transform

The paper presents a new method to measure motor speed by means of frequency estimation of rotor slot harmonics present in the supply current of squirrel single cage induction motors. The novelty of the method is the use of Chirp-Z Transform as supply current harmonic analysis. The advantages are an improved spectral resolution and accuracy. Moreover a shorter sampling time window is required reducing errors related to not stationary current signal. Experimental results are presented to validate the proposed method and to perform a comparison with FFT based one.

A fully-automated procedure for measuring the electrical parameters of an induction motor drive with rotor at standstill

The paper presents an automatic procedure to measure at standstill the electrical parameters of an induction motor fed by a PWM voltage source inverter. The proposed procedure executes automatically three tests using only the available PWM inverter control technique to obtain the required motor supply voltages. It allows the measurement of all the T-form circuit electrical parameters starting from the nameplate data as data-entry. It uses only a current sensor and no voltage sensor and process on line the collected data samples with a fast and easy to implement recursive least squares algorithm. Effectiveness of the automated procedure has been proved both by simulation and experimental tests carried out by means of a dedicated test bench.

A low cost and easy re-configurable instrument for power quality survey

In this document the development of a low cost and user friendly PC-based instrument for power quality measurement is proposed, according to IEC 61000-4-7 and standard IEC 61000-4-30. It is an easily re-configurable instrument able to measure harmonics, interharmonics and amplitude disturbances and supply unbalance. In order to verify its accuracy, the instrument has been tested according standard test procedures, by means of a built-up calibration test bench.