BLUEBOX TECHNOLOGY

BlueBOX Technology

Partial Discharge measurement is the most important technique for assessing the insulation condition of High Voltage systems and provides an effective solution for assessing the reliability of electrical installations.

The flagship of the 'BlueBOX Technology' consists of a new method of partial discharge measurement and analysis to assess the condition of the insulation of appliances, equipment and materials for high voltage installations, such as GIS, transformers and specially, insulated cables, either during operation in the network (on line) or out of service by mobile generators (off-line).

This technology is based on powerful techniques of the signal processing to discriminate electrical noise, locate partial discharges, separate different partial discharge producing sources that appear on the same equipment or accessory and assess the criticality of each in relation to the degradation of electrical insulation.

Powerful diagnostic tools:

  • Discrimination of the Background Electrical Noise
  • Partial Discharge Automatic Location
  • Identification and separation of different partial discharge sources
  • Assessment of the criticity of the PD sources identified

Discrimination of the background electrical noise

In the commercial, partial discharge measuring systems exists a band noise threshold below which it is not possible to detect PD signals.

Most of the systems for partial discharge measurements existing on the market address the problem of the electrical noise by filtering the recorded signal in a frequency band in which it is measured. This filtering technique also produces the attenuation of the partial discharge signals for the range of the filtered frequencies.

It should be noted that the concept of this filtering technique results in the elimination or attenuation of the noise together with the partial discharge signal for the filtered frequency range. Other techniques perform an analysis of the noise signal frequency spectrum in order to select a measuring frequency band where the noise amplitude is as low as possible. Sometimes the band of the lowest noise signal coincides with the band where the partial discharge signal is also weak in amplitude, so far the PD measurement is difficult and never free from noise threshold level. There are techniques that try to solve the noise problem measuring at a very high frequency (>30MHz) where the noise component conducted through the cable is negligible. The limitation of these techniques is that the PD signal for these measuring frequency ranges is greatly attenuated and it is very difficult to detect PD coming from hundreds of meters away, which prevents from a good PD location.

BlueBOX Technology gets the electrical noise elimination by doing the digital processing of the captured signals, consisting of applying the Wavelet transform to the signal acquired and statistically analyze its components, in order to find characteristic transient events of partial discharge signals that are distinguished from the statistical evolution of the electrical noise. With this technology it is possible to find partial discharge signals obscured by the noise level, even when the noise is random type.

Automatic Partial Discharge Location in insulated cables

The majority of the procedures used in the market treat the location of partial discharge sources by the technique of reflectometry, which consists of acquiring signals at a measuring station placed at one end of the cable and determine the time delay between the signal coming directly from the partial discharge source and the signal coming from the reflection at the opposite end of the cable where the circuit is left open.

The position of the partial discharge source along the cables is determined taking also into account the propagation speed of the partial discharge signal through the cable. The practical limitation of this technique for measures “on line” is that one end of the line must be open so that the PD signal could reflect, which prevents it to be applied in actual operating conditions (measures “on line”).

BlueBOX Technology solves this problem by using at least two sensors located at both ends of the cable and by doing periodic and synchronized captures. After each capture, the electrical background noise is removed and the time delay of the same partial discharge signal to the two sensors placed at the ends of the cable allows the identification of the position of its partial discharge source. This method can only be applied effectively by prior removal of the background noise signals to observe clearly the captured partial discharges, making possible the localization of partial discharge sources in severe electrical noise environments.

Identification and separation of different partial discharge sources

The efficiency of the conventional methods for identifying and separating different partial discharge sources is limited by the uncertainty in the location of the PD source. For example, the uncertainty in the partial discharge location along a cable is a few meters, so it is possible to have one or more partial discharge sources in the same location, especially in the open ends of the cables where they can appear corona PDs and outer surface PD type.

The identification of the PD source type is unsolved in most of the cable insulation diagnostic techniques, leaving this decision to the discretion of the operator. Some techniques use the phase shift of the partial discharge with respect to the applied voltage to generate a phase resolved pattern of the detected PD sources in order to assist the operator in making the decision, but as already indicated, patterns are produced.

When there are multiple faults in the same location, their corresponding patters are superimposed in the measuring point and can be easily confused with each other, without being easy to identify every defect, being sometimes the operator experience insufficient to make a correct diagnosis. In addition, the noise not eliminated in the commercial techniques difficult, even more, the identification of different sources of partial discharges through the simple observation of their patterns.

BlueBOX Technology overcomes this inconvenient generating automatically, after the background noise removal, a PD pattern associated with the partial discharges generated in each position along the cable in order to avoid the overlapping of patterns associated to the defects located at different locations.

To achieve the separation of more than one PD source located in a single position, BlueBOX Technology characterized the shape of each partial discharge source with three parameters: two parameters α and β correlated with the asymmetry of the pulse envelope and other parameter ω correlated with the frequency of the PD pulse. These three parameters allow a 3D representation.

By three cursor can be selected a PD cloud associated with each generated PD source whose phase resolved pattern corresponds to a different PD type generated (corona, internal defect, surface defect, etc.).

Criticality assessment of the DP sources identified

The PD diagnosis using BlueBOX Technology is performed analyzing separately the partial discharge sets generated by every PD source. This analysis can be done thanks to the powerful discrimination and PD identification tools. The conclusions of the PD Diagnostic with BlueBOX Technology feature a photo in detail of the current state of the system insulation, indicating which elements show some kind of failure in its insulation.

To perform the reception of an installation or the periodic maintenance by a PD diagnosis in a timely manner allows to detect the partial discharge sources in the insulation of the installation. BlueBOX Technology provides characteristic indicators of each detected PD source to assess its criticality: phase resolved pattern, PD pulse amplitude, PD rate per period, type of defect, location.

The continuous monitoring of a facility allows to detect the existence of PD sources from the first moment of its activity. The analysis of the indicators of the amplitude and PD rate per period evolution provides information about the insulation degradation speed. Additionally, the continuous monitoring ensures the detection of PD sources with intermittent activity due to the operating conditions of the installation.

 

Examples

Evolution of the located PD

Evolución de las DP localizadas

Amplitude and rate evolution of a PD source

Evolución de amplitud y tasa de una fuente de DP