After a simplified analysis, we notice the tap adapter is merely a jack allowing the electric contact between the bushing and the measurement input of the bushing monitor. This simplified approach is both wrong and dangerous, because we need to take into consideration that the bushing tap should never be kept open. Among the electric characteristics the adapter must have to meet safety requirements and monitoring performance, the most important are the differentiated mechanical design and the protection against failure in the electric connection of the measurement circuit or in case an accidental disconnection occurs.

The tap adapter used has a protective circuit inside it which has a twofold role. The first application is the redundant protection against tap opening if the connection cables between the adapter and the measurement module of the bushing monitor are accidentally disconnected. This is an instant protection, limiting the voltage in any part of the secondary circuit of the tap adapter in ≈15  Vac.  The protections which were used may conduct for an indeterminate amount of time and are sized according to the atmospheric impulse test (BIL) 12,050 kV – 765kV bushing.  In addition to that, each adapter has a double protection layer, which, working in parallel, provides redundant protection.

These characteristics are ensured by a mechanical design that makes these protections inaccessible during the handling and installation of the adapter. The second function is the primary protection against associated overcurrents (several kA) and overvoltages which occur at the bushing tap when there are overvoltages caused by switching maneuvers or atmospheric impulses. This function is supplemented by the secondary protection inside the BM itself, so that the primary and secondary protections work together in a coordinated way to avoid damage to the BM and to the adapter.

Now, supposing a change in the capacitance and in the dissipation factor of the bushing in phase A, as shown in Figure 5(b), the change vector which expresses the displacement of current Ia from its initial value up to its final value is also reflected in the current total sum, which changes in relation to its initial value according to the same change vector.

This change vector has a virtually insignificant weight when compared to the magnitude of the leakage current of phase A. But this is not what happens when this vector is compared to the total sum current, which allows its detection and as a result the detection of the change in the bushing impedance under study. Therefore, the technique of the vectorial sum of the leakage currents allows an increase in sensitivity which makes it feasible to monitor the changes both in the capacitance and in the tangent delta of the insulation.

However, the bushing leakage currents are a function not only of their capacitances and tangents delta, but also of the phase-ground voltages of the system. Since the latter are often not available for measurement by the monitoring system, as explained above, the possible influence of variations in the phase-ground voltages on the measurements of the variations in capacitance and tangent delta is eliminated through advanced signal processing proprietary techniques, which include also statistic data treatment.

These statistic treatments assume the use of a data mass accumulated along the system’s operation time, which is continually updated with the most recent leakage current and total current sum measurements.. Due to that, the online monitoring system has some time to respond to the variations occurred in capacitance and tangent delta, varying from some hours to some days, in order to confirm that the capacitance change and/or tangent delta change is real and not caused by voltage oscillation in the electric power system.

The changes in capacitance and tangent delta are used to increase the initial values programmed in the BM. This is how the BM indicates the current value of the capacitance and tangent delta corresponding to the one obtained in an offline bushing test by using the initial value and the posterior changes.

During the period from 2005 to 2010, the monitoring system was in continuous operation. No alarm was registered for the bushing status, indicating that until then the bushing was working normally. Table 01 shows the initial parameters for bushing monitoring in T-2 and in the pedestal CT.

Initial values of the bushing monitoring in the T-2 and in the Pedestal CT

High capacitance and very high capacitancealarm values were parameterized for a variation of 3% and 5% in relation to the initial value. High tangent delta and very high tangent delta values were parameterized for a variation of 100% and 200% in relation to the initial value. Table 02 shows these values in pF and %.

Parameterization of the bushing monitoring alarms for the T-2 and the Pedestal CT

On 02/20/2011, a Sunday, the bushing monitor issued a high capacitance trend alarm for the 230 kV bushing, phase B. The Mesquita Substation operators notified the Maintenance Engineering department at Cemig about this alarm [6].

On 02/22/2011, Tuesday, Cemig’s Maintenance Engineering staff contacted Treetech so this alarm could be verified. On this same date, the alarm showing a capacitance increase trend indicated 9 days remaining to reach the programmed high capacitance alarm value. The BM also indicated that the time remaining for the measurement to reach the high tangent delta alarm value was of 63 days. See table 3 for the values measured by the bushing monitor at the moment of the analysis.

Bushing monitoring values on 02/22/2011, at the moment of the analysis

We can notice that there was a 2.08% variation in capacitance and a 26.11% variation in the tangent delta of the 245kV bushing in phase B. All pertinent analyses were made, and the trend towards an increase both in capacitance and tangent delta were confirmed. After this confirmation the Cemig’s engineering team scheduled a system shutdown for 02/27/2011, a Sunday, to perform offline tests that could confirm the bushingstatus. Cemig also prepared to replace the bushing, if the problem was confirmed.

From this point on, both Cemig and Treetech mobilized a team of technicians to follow the evolution of the measurements indicated by the system with the purpose of keeping operation safe.

See in graph 01 the measurements up to 02/22/2011.

Graph 01 – Evolution of the capacitance and tangent delta values for the 245 kV assembly

Until the Saturday before the scheduled shutdown, the capacitance and tangent delta values evolved according to what was planned by the monitoring, and there was no need to reschedule.

Still on Saturday, 02/26/2011, at 11:55 am, the BM bushing monitor showed a sudden increase in the value of the tangent delta of phase B of the 245 kV assembly. The monitor showed a variation in the tangent delta indication, within a very short time interval, from 0.490%, to 1.263%, above the very high alarm limits, while the measured capacitance varied very little, with a value of 530.1 pF.  Cemig’s and Treetech’s teams on standby at the site did analyses based on the indications issued by the system, to verify the existence of indication consistency.

Both Cemig’s Operation and Maintenance Engineering teams took immediate action and and started negotiations to shut down the transformer as an emergency measure.

Since it is a piece of equipment that is a part of the Basic Network of the National Interconnected System, the shutdown was negotiated with the ONS [National Electric Power System Operator], which authorized the operation of the T-2 autotransformer at 3:13 pm through CLM 748/201, since the maneuver took place in real time.

Cemig immediately sent a team to take the first offline measurements of the suspected defective bushing, as soon as the transformer was shut down, to verify the validity of the indications issued by the online monitoring. During this waiting period, both company teams followed the evolution of the tangent delta online, and it started to go up very quickly until the transformer was shut down, reaching 3.331% at the most critical moment. Graph 02 shows the critical evolution taking place in the moments just before shutdown.

Graph 02 – Detailed evolution of the values monitored in the moments before shutdown

Around 3:40 pm the necessary maneuvers for shutdown were over, and the transformer, duly insulated, had its suspected defective bushing submitted to the offline test. Following the recommendation by the Treetech team, Cemig’s team measured the values of the tangent delta as soon as possible, because tangent delta varies with temperature. The typical behavior is that the tangent delta goes down as the temperature goes down for the same condition of insulation deterioration. This is why it is so important that this measurement was made as soon as possible. At the moment of the shutdown, the bushing monitor indicated a capacitance value of 529.9 pF and 3.329% for the tangent delta. Table 04 shows the data obtained through the offline measurements and by the online monitoring system.

Values of the offline measurements for the suspected defective bushing

After the offline measurements confirmed the indications of the online monitoring, Cemig decided to keep the transformer offline until the defective bushing could be replaced. The replacement was scheduled for Monday, 02/28/2011.

On Sunday, 02/27/2011, with the arrival of the Cemig crew who would change the bushing on the following day, new offline measurements were taken with an instrument manufactured by a different company with the intention to confirm the data obtained by the first instrument. Table 05 shows the results of this new measurement.

Values of suspected bushing offline and online measurements

Figure 06: Oil leakage point at the bottom of the bushing

On 02/28/2011 the maintenance crew changed the defective bushing. Figure 07 shows some steps of the operation conducted by the Cemig’s crew.

Figure 07: Cemig’s  crew replacing the defective bushing

Figure 08: Duval Triangle and its diagnostic zones

This shows the decision to shut down was correct and the monitoring system worked as it should, avoiding a probable bushing explosion and serious damage to the transformer.

Responding to a growing demand for better quality indexes, availability and reliability, Cemig, motivated by the its suspicion in relation to a certain family of bushings with history of failure in the electric power sector, installed, in the Mesquita substation, the BM condensive bushing monitor by Treetech in the 550 kV and 245 kV bushings of the T-2 autotransformer and of a 550kV pedestal CT as well.

In addition to this motivating factor, online bushing monitoring is known to contribute to the reduction of maintenance costs through improvement in the maintenance planning process and intensive use of the maintenance paradigm under condition, reducing the number of shutdowns for routine tests.

Additionally, online monitoring has provided valuable data on the kind of defects which can appear in  this component and their evolution time. The installation of the BM condensive bushing monitoring system has offered several advantages, and the primary purposes listed below were completely fulfilled:

• The reduction in failure risk with real time bushing diagnosis;
• The increase in availability through predictive maintenance instead of preventive maintenance (less maintenance shutdowns).
• The lower maintenance costs due to avoiding unnecessary maintenance;
• The preservation and improvement of the corporate image with the reduction in shutdowns and catastrophic failures.

This article has demonstrated the importance and effectiveness of bushing capacitance and tangent delta online monitoring, highlighting the importance of the tangent delta monitoring for the detection of incipient defects, since in the described experience there was no significant capacitance change, although the tangent delta increased ten times in relation to the initial values of the offline tests.

Therefore, it has been proven that bushing failures may be detected in real time, avoiding severe damage to the equipment from explosion and possible fire, and reducing the losses caused by measurement failure in condensive bushings.

1. ELECTRA, “An International Survey on Failures in Large Power Transformers in Service”, Paris, CIGRE, Ref. no. 88, 1983.
2. Melo, Marcos A. C., Alves, Marcos, “Experiência c om Monitoração On-Line de capacitância e tangente d elta de Buchas Condensivas”, XIX SNPTEE – Seminário Nacional de Produção e Transmissão de Energia Elétrica. Rio de Janeiro, Brasil, 2007.
3. Alves, Marcos, Pena, Miguel C Medina, Severino, Cláudio “DEFEITOS DE EVOLUÇÃO RÁPIDA E MUITO RÁPIDA EM BUCHAS DETECTADOS POR MONITORAMENTO ON-LINE”, XXI SN PTEE – Seminário Nacional de Produção e Transmissão de Energia Elétric a. Florianópolis, Brasil, 20011.
4. The Institute of Electrical and Electronic Engineers, ANSI/IEEE Std C57.19.100-1995, “IEEE Guide for Application of Power Apparatus Bushings”, Março/1995
5. Melo, Marcos A. C., “Investigaç ão de Ocorrências em Buchas de Transformadores e Reatores Utilizando o Doble M-4000”, IV Conferência da Doble Brasil – Cemig. Belo Horizonte, Brasil, 2003.
6. Relatório Técnico de Ocorrência – SE Mesquita – 04/2011
7. IEC 60599 ed2.1– Mineral oil-impregnated electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis