IEEE TRANSACTIONS ON INSTRUMENTATIONAND MEASUREMENT, VOL. 54, NO. 4, AUGUST 2005 1471 An Easy VI Program to Detect Transient Disturbances in the Supply Voltage Alessandro Ferrero, Fellow, IEEE, and Simona Salicone, Member, IEEE Abstract—The assessment of the “power quality” and “voltage quality” in a deregulated electricity market has become a more and more critical point. As far as the voltage quality is concerned, the first step toward its assessment is the availability of measure- ment systems able to detect and classify the possible disturbances affecting the voltage itself. This task is quite difficult when tran- sient disturbances are involved, and very complex algorithms have been proposed, mainly to classify these disturbances. This paper proposes a very simple detection algorithm, whose aim is to act as a real-time “trigger” for subsequent, more complex classification algorithms. A VI has been realized implementing this detection al- gorithm in real time, and the experimental results collected during several months of operation are reported. Index Terms—Disturbance detection, virtual instruments, voltage quality. I. INTRODUCTION T HE detection of the disturbances affecting the line volt- ages is one of the most qualifying points in the estimation of the “voltage quality” or “supply quality.” The correct assess- ment of the quality of the supplied voltage has become one of the key issues in the deregulated electricity market. Ensuring a “high quality” of the supply voltage is the main requirement for ensuring a high “power quality.” Great attention is therefore paid to the definition of suitable indexes of voltage quality and the definition of suitable measurement procedures to evaluate these indexes. As far as the definition of voltage quality is concerned, sev- eral recommendations have recently been issued by the stan- dards organizations to define the acceptable voltage character- istics [1]–[4], also in terms of the acceptable number and level of the disturbances superimposed to the voltage waveform and their characteristics. Among the possible disturbances that may affect the voltage quality, the most difficult to detect are the transient ones, such as spikes and sags. Several methods have been proposed in the lit- erature to detect these disturbances [5]–[10], and many of them are based on the wavelet analysis of the voltage signal [9], [10]. The wavelet transform is a complex, time-consuming algo- rithm very useful when the disturbance parameters have to be detected or the disturbances have to be classified into families, according to their characteristics. However, the most critical part of the whole detection and characterization process is the Manuscript received June 15, 2004; revised April 21, 2005. The authors are with the Dipartimento di Elettrotecnica, Politecnico di Milano, 20133 Milan, Italy (e-mail: alessandro.ferrero@polimi.it; simona.sali- cone@polimi.it). Digital Object Identifier 10.1109/TIM.2005.851078 detection part, since it must be performed on-line and in hard real time. On the other hand, all further processing can be done off-line, by employing a suitable algorithm for postprocessing the acquired disturbed signal. The most effective solution to this problem is therefore to use different methods for: 1) the preliminary detection part of the disturbance; 2) the subsequent analysis of the detected disturbance. Fast and simple algorithms should be used in the detection stage [5], while more complex algorithms are allowed in the charac- terization part [8]–[10]. This paper proposes a very simple algorithm for transient dis- turbance detection in the line voltage signals of a three-phase system and shows the result of the experimental work done on the low-voltage distribution network of Politecnico di Milano. II. DETECTION ALGORITHM According to the above introductory notes, the detection al- gorithm is required to be fast, robust, and simple, since it must execute its detection function on-line and in real time. No ac- curate measurements are required for this algorithm, since its only function is to act as a real-time trigger for subsequent, more complex classification algorithms, if needed. However, in many practical situations, the subsequent anal- ysis is very often left to the operator for diagnostic purposes in order to investigate on the possible reasons that caused the injec- tion of the disturbance itself. In this situation, the only additional function required to the detection algorithm is to “capture” the portion of the voltage waveform affected by the disturbance and store it for an off-line postanalysis. For these reasons, with measurement accuracy not being the main requirement, a very simple detection algorithm has been analyzed, which appears to be simpler than the simplest ones available in the literature [5]. This algorithm is based on the consideration that the supply voltage of a distribution network is quite stable, in the short term, unless a disturbance occurs. Therefore, the difference between the voltage signal in a pe- riod and the signal in the previous period (1) where is the period, should be zero for every value of time , unless a transient disturbance occurs. The operating principle of the proposed detection algorithm is hence based on (1). One period of the signal is acquired, converted into digital, and stored in the system memory. 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