**3. Laboratory Aspects for Internal Fault Experiments in Power Transformers**

It is important to specify equipment, methods and parameters, which vary according to the type of defect that is to be analyzed. In simple terms, the monitoring system can be better understood through Figure 1.

Intelligent Systems for the Detection of Internal Faults in Power Transmission Transformers http://dx.doi.org/10.5772/51417 7

**Figure 1.** Laboratorial setup diagram.

be highlighted, since it is directly related to the insulation conditions of a power transform‐ er, which in turn trigger the occurrence of more severe faults. PD in high voltage systems occurs when the electric field and localized areas suffer significant changes which enable an

**•** Point to Point discharges in insulating oil: these PDs are related to insulation defects be‐

**•** Point to Point discharges in insulating oil with bubbles: this kind of fault is also caused by PD between two adjacent winding turns, but the condition of insulation degradation al‐

**•** Point to Plan in insulating oil: defects in the winding insulation system can cause PD be‐

**•** Surface Discharges between two electrodes: the most common kind of PD, occurring be‐ tween two electrodes insulated with oil-paper called triple point, where the electrode sur‐

**•** Surface Discharges between an electrode and a multipoint electrode: the PD relating to these elements differ from the previous one with regard to the intensity distribution of the

**•** Multiple Discharges on the plan: multiple damaged points in the winding insulation may

**•** Multiple Discharges on the plan with gas bubbles: the PD in this case occurs at various damaged points in the winding insulation and the grounded parts of the transformer

**•** Discharges caused by particles: in this case, the insulating oil is contaminated with parti‐ cles of cellulose fiber formed by the degradation process of the oil-paper insulation sys‐ tem, due to the aging of the power transformer. Such particles are in constant motion in

It is important to specify equipment, methods and parameters, which vary according to the type of defect that is to be analyzed. In simple terms, the monitoring system can be better

cause PD between it and the grounded parts of the transformer tank;

**3. Laboratory Aspects for Internal Fault Experiments in Power**

tank, but in the presence of gases dissolved in insulating oil;

electric current to appear [6].

6 Advances in Expert Systems

the oil, causing PD;

understood through Figure 1.

**Transformers**

According to [13], PD can be grouped into 8 classes:

lows the formation of gas bubbles;

tween two adjacent turns in the winding of a transformer;

tween it and the grounded parts of the transformer tank;

face is in contact with dielectric solids and liquids;

electric field. Both are insulated with oil-paper;

The structures highlighted (inside the black boxes) are those that present the greatest chal‐ lenges for configuration and parameterization, which are entirely dependent on the type of tests to be accomplished.

The most complete and detailed tests are (given their wide coverage of internal faults) more complex and expensive due to the various devices necessary used for the fault detection and location process, because more sensors and also data acquisition hardware are necessary.

#### **3.1. Electrical measurements**

Electrical parameters are also necessary for a correct characterization of internal transformer faults, especially when dealing with systems that require databases for normal operating conditions and with situations when a system has to be restored following a disturbance. This is the case of artificial neural networks, which require quantitative data for the learning process. It is necessary to measure voltages and three-phase primary and secondary cur‐ rents, totaling 12 electrical parameters. The acquisition frequency in this case must not be high, because the purpose is to investigate the most predominant harmonic components in the electrical system.

#### **3.2. Acoustic measurements**

The acoustic signals are captured by acoustic emission sensors distributed evenly through‐ out the tank, which are externally connected to the power transformer. Such sensors have several characteristics that require a correct specification:


**Figure 3.** Acoustic emission sensor fixed to the outside of the tank.

**Figure 4.** Device to produce partial discharges in the tank.

**3.3. Measurements of dissolved gases**

ley system.

Figure 4 illustrates a device made in order to produce partial discharges in the tank. The mechanism can also be moved within the tank, in all directions, by means of a rail and pul‐

Intelligent Systems for the Detection of Internal Faults in Power Transmission Transformers

http://dx.doi.org/10.5772/51417

9

Measurement of dissolved gases in insulating oil can be acquired from chromatographic analysis of the oil, which is often performed in the laboratory. However, there are now some

The experimental apparatus for supporting experiments aimed at testing computer systems developed for identifying and locating partial discharges in power transformers consists of a metal tank, in which all the devices responsible for the acquisition of acoustic and electrical signals are mounted. Figure 2 illustrates a tank specially prepared for this purpose.

**Figure 2.** Tank for experimental testing.

Figure 3 illustrates the attachment of an acoustic emission sensor mounted on the outside of the metal tank, whose signals are transmitted via cable to the acquisition system.

Intelligent Systems for the Detection of Internal Faults in Power Transmission Transformers http://dx.doi.org/10.5772/51417 9

**Figure 3.** Acoustic emission sensor fixed to the outside of the tank.

**•** Number of sensors per transformer: The number of sensors needed to detect internal faults in transformers varies according to the size of the equipment, amount of available channels and the type of fault to be detected. For the fault location task, for example, it takes a greater number of sensors, so that the entire volume of the transformer can be

**•** Pre-amplification: This item is extremely important because only the amplified acoustic

**•** Operating frequency: This is strongly dependent on the type of fault to be monitored. Me‐ chanical faults are associated with frequencies ranging from 20 kHz to 50 kHz, while elec‐

**•** Resonance frequency: This parameter defines the frequency where the signal gain is maxi‐ mum. For maximum performance, it is necessary for the resonance frequency of the sen‐ sor to be tuned to the phenomenon to be monitored. The most common sensors have a

The experimental apparatus for supporting experiments aimed at testing computer systems developed for identifying and locating partial discharges in power transformers consists of a metal tank, in which all the devices responsible for the acquisition of acoustic and electrical

Figure 3 illustrates the attachment of an acoustic emission sensor mounted on the outside of

the metal tank, whose signals are transmitted via cable to the acquisition system.

signals are mounted. Figure 2 illustrates a tank specially prepared for this purpose.

signals are sent to the acquisition hardware, which removes extraneous noises;

monitored. Thus, a total of 16 to 20 sensors is normally used [14];

trical ones vary between 70 kHz and 200 kHz;

resonance frequency of 150 kHz.

8 Advances in Expert Systems

**Figure 2.** Tank for experimental testing.

Figure 4 illustrates a device made in order to produce partial discharges in the tank. The mechanism can also be moved within the tank, in all directions, by means of a rail and pul‐ ley system.

**Figure 4.** Device to produce partial discharges in the tank.

#### **3.3. Measurements of dissolved gases**

Measurement of dissolved gases in insulating oil can be acquired from chromatographic analysis of the oil, which is often performed in the laboratory. However, there are now some commercial devices that sense some gases dissolved in the oil. These devices can be used to monitor a power transformer in real time. It is worth mentioning that, through the analysis of dissolved gases, it is possible to obtain a first indication of a malfunction, which is usually related to electrical discharges and overheating.

**3.5. Computer for receiving and processing data**

disk is unnecessary, since a SCSI bus can be used.

ance of processes involving the detection and location of faults [13].

**4. Data Analysis from Acoustic Emission Signals**

**Figure 6.** Acoustic emission signal resulting from the gauging process of sensors.

**3.6. Analysis and diagnosis**

registered in these time slots.

this test are highlighted in Figure 6.

The computer is responsible for storing acoustic, electrical and dissolved gas data coming from the hardware acquisition. The hardware bus speed and the disk storage capacity must also take into account the amount of planned experiments, although a high performance

Intelligent Systems for the Detection of Internal Faults in Power Transmission Transformers

http://dx.doi.org/10.5772/51417

11

The implementation of this structure is very challenging, because it consists of a combina‐ tion of techniques to efficiently identify and locate faults in power transformers. Among these techniques, those based on intelligent systems have efficiently increased the perform‐

Altogether, we collected 72 oscillograph records of partial discharges. Each of these records depicts a time window of one second. In general, many occurrences of partial discharge are

In addition to this phenomenon, the data acquisition system also recorded mechanical waves that were used to evaluate the gauging of acoustic emission sensors. These waves are the result of the break, near the surface where the sensor is installed, of graphite with speci‐ fications given by the manufacturer of acoustic emission sensors. The graphs resulting from

Figure 5 shows the installation (in the tank) of the gas sensor, which is responsible for ac‐ quiring information on the quantities of gases dissolved in the insulating oil in order to re‐ late them to internal defects.

**Figure 5.** Gas analysis sensor installed in the experimental tank.

#### **3.4. Equipment for data acquisition**

As seen above, the frequencies for electrical signals differ greatly from those found in acous‐ tic signals, whose acquisition hardware can be divided into two according to technical and financial aspects:


#### **3.5. Computer for receiving and processing data**

The computer is responsible for storing acoustic, electrical and dissolved gas data coming from the hardware acquisition. The hardware bus speed and the disk storage capacity must also take into account the amount of planned experiments, although a high performance disk is unnecessary, since a SCSI bus can be used.

#### **3.6. Analysis and diagnosis**

commercial devices that sense some gases dissolved in the oil. These devices can be used to monitor a power transformer in real time. It is worth mentioning that, through the analysis of dissolved gases, it is possible to obtain a first indication of a malfunction, which is usually

Figure 5 shows the installation (in the tank) of the gas sensor, which is responsible for ac‐ quiring information on the quantities of gases dissolved in the insulating oil in order to re‐

As seen above, the frequencies for electrical signals differ greatly from those found in acous‐ tic signals, whose acquisition hardware can be divided into two according to technical and

**•** Hardware for electrical signals: for power quality purposes established in the Brazilian standard PRODIST [15], the 25th harmonic is the last one of interest. Thus, according to the Nyquist criterion, a minimal acquisition rate of 3 kHz is required. For electrical pa‐ rameters it is also possible to use hardware with an A/D multiplexed converter, which re‐

**•** Hardware to acoustic signals: one of the factors that make this hardware expensive is the need to use an A/D converter for each channel. The sources of acoustic emissions also vary between 5 kHz and 500 kHz, where an acquisition frequency in MHz is necessary.

related to electrical discharges and overheating.

**Figure 5.** Gas analysis sensor installed in the experimental tank.

**3.4. Equipment for data acquisition**

duces the cost of equipment;

financial aspects:

late them to internal defects.

10 Advances in Expert Systems

The implementation of this structure is very challenging, because it consists of a combina‐ tion of techniques to efficiently identify and locate faults in power transformers. Among these techniques, those based on intelligent systems have efficiently increased the perform‐ ance of processes involving the detection and location of faults [13].
