Growing demand for reliable operation of high voltage power systems has driven adoption of online monitoring technologies for surge arresters.

This edited contribution to INMR by Mario Augusto Caetano dos Santos and colleagues at Itaipu Binacional (ITAIPU) presents an assessment of two monitoring devices (A&B) deployed as part of its predictive maintenance strategy.

Laboratory evaluations were conducted to compare total and resistive leakage current measurements against an offline reference under controlled conditions. In addition, a pilot study explored using one of the devices to monitor total leakage current in a 444 kV GIS surge arrester under real service conditions. Results emphasized the value of such work before full-scale deployment and provided insight into some of the challenges encountered when implementing condition-based monitoring in complex substation environments.

Station-class surge arresters play a fundamental role in ensuring the reliability and safety of high voltage installations. By limiting overvoltages caused by lightning or switching operations, these devices protect critical equipment such as transformers, circuit breakers, and instrument transformers from insulation failure and damage. Indeed, according to IEEE Std C62.11, proper selection, installation, and maintenance of surge arresters is essential to prolonging equipment life and preserving system stability.

ITAIPU’s hydroelectric power plant, located on the Paraná River along the border between Brazil and Paraguay, ranks among the largest in the world, with an installed capacity of 14,000 MW. The facility includes approximately 300 surge arresters installed in both outdoor and gas-Insulated substation (GIS) configurations, applied across systems operating at 66/69 kV, 220 kV and 500 kV.

ITAIPU has for several years been implementing a predictive maintenance strategy for surge arresters. This strategy includes infrared thermographic inspections, with extraction of temperature differentials (delta T) across arrester housings as well as measurement of resistive leakage current according to Method B2 of standard IEC 60099-5 and performed every 6 months.

Recently, both diagnostic techniques have been integrated into a Surge Arrester Diagnostic System (SDPR) that employs Paraconsistent Annotated Logic with Two Values (LPA2v) to deliver a combined and intelligent diagnostic output (see Fig. 1). This approach enhances decision-making by providing a robust assessment of arrester condition (as presented during the 2023 INMR WORLD CONGRESS in Bangkok).

Surge Arrester Replacement Plan

As part of its ongoing asset renewal strategy and commitment to operational safety, ITAIPU has initiated a comprehensive replacement plan for HV surge arresters at its main generation and transmission facilities. The objective is to modernize aged equipment, enhance safety for personnel and integrate advanced monitoring capabilities to support predictive maintenance practices.

The plan encompasses replacing 54 outdoor ZnO surge arresters rated at 420 kV, originally installed in the early 1980s, and currently in service across 8 transmission line bays and 2 auxiliary transformer banks (500 kV, 50 and 60 Hz) at the ITAIPU Powerhouse. Existing porcelain-housed arresters are being replaced with new units featuring polymeric housings, which offer reduced risk of fragmentation upon failure and better resilience in contaminated or high humidity environments.

In addition, 30 outdoor ZnO surge arresters rated at 240 kV (also with porcelain housings and installed in power transformers and line bays of the 220 kV Right Margin Substation) are being replaced with polymer-housed units. As in the case of the 420 kV replacements, these devices date back over four decades and are being upgraded as a preventative measure aligned with Itaipu’s risk mitigation policies.

Beyond replacement of equipment, the project also includes a significant technological upgrade through application of surge arrester monitoring systems. As part of the procurement specifications, the new arresters must be supplied with dedicated monitoring devices capable of measuring and/or recording the following parameters:

• Total leakage current;
• Resistive leakage current component;
• Ambient temperature;
• Discharge events, including timestamp, peak current, and/or energy values.

These monitoring capabilities are essential to support ITAIPU’s transition toward condition-based maintenance and improved diagnostic practices for high voltage equipment.

Procurement of the new surge arresters took place in 2019 for the 420 kV units and in 2022 for the 240 kV units. This resulted in contracts with different manufacturers for each voltage class. Consequently, the monitoring systems supplied differ in both technological approach and data acquisition methods.

It is important to note that, in the face of technology developments by manufacturers, the technical specifications were slightly different, specifically with regards to means of communication. The two different technologies, hereafter referred to as Device A (used in 420 kV arresters), and Device B (used in 240 kV arresters) are compared in Table 1.

Initial Tests at ITAIPU’s High Voltage Laboratory

With the goal of evaluating performance of the two surge arrester monitoring devices prior to installation, ITAIPU conducted a comprehensive series of laboratory tests at its high voltage laboratory. This initiative aimed to evaluate the device’s performance and verify agreement of the measurements with the traditional instrument used to measure resistive leakage current in the field (hereinafter referred to as Instrument X).

Table 2 outlines the specifications of this instrument. It is important to clarify that both Devices (A and B), as well as Instrument X, use the B2 method of IEC 60099-5.

Device B was tested with its complete configuration (datalogger unit, transponderpad and smartphone) and installed on the surge arrester (Ur = 60 kV, Uc = 48 kV, class 10 kA) as per Fig. 2.

Two main functionalities were evaluated:
1. Impulse Detection

The device successfully detected and logged impulse events applied by a pulse generator. The data uploaded to the cloud platform showed a majority of impulses in the 100 A to 1000 A range, consistent with oscilloscope measurements.

2. Leakage Current Measurement
Multiple instruments were used to compare total (oscilloscope and multimeter) and resistive leakage currents (oscilloscope and Instrument X) during a 40 kV/60 Hz phase-to-ground voltage test.

Total RMS leakage current values were consistent across methods, with the following results:
Device B = 486 µA,
oscilloscope = 493 µA,
multimeter = 490 µA and
Instrument X = 496 µA (peak, converted to RMS).

However, discrepancies were observed in resistive current readings: Device B showed values in the 9 to 15 μA range, whereas Instrument X reported values around 37 μA (peak, converted to RMS). Further analysis using voltage-current phase comparison and harmonic content by oscilloscope suggested RMS resistive current values between 11 and 17 μA, corroborating the Device B data.

Overall, Device B’s system demonstrated reliable performance in both impulse registration and leakage current measurement, including integration with NFC technology and cloud-based analytics.

Tests of Device A were performed using the same surge arrester, impulse generator and voltage source as previous tests on Device B. Several impulses were applied, and the device registered all correctly (current range and timestamp). Related to total and resistive leakage current, the following values were obtained, respectively: 800 µA (peak) and 45 µA (peak). Fig. 3 shows Device A under impulse test.

Comparing results from Device A with Instrument X, it became possible to see proximity between the values (peak), respectively, 800 µA versus 701 µA (total leakage current) and 45 µA versus 52 µA (resistive leakage current). Moreover, communication between Device A and the transceiver occurred correctly, as did upload and analysis of data in the manufacturer’s software.

Surge Arrester Monitoring Devices: Post-Installation Performance Assessment

Following the laboratory validation phase and the replacement plan outlined above, ITAIPU initiated phased installation of surge arrester monitoring devices in operational substations starting in early 2024. The objective of this deployment was not only to enhance equipment reliability through continuous condition monitoring but also to evaluate performance over time of the devices under real service conditions.

Field measurements began in Sept. 2024, with a targeted monthly frequency whenever operational and logistical conditions permitted. During these inspections, data stored in the monitoring devices was collected and parallel measurements were performed using Instrument X to compare the recorded values.

For the purposes of this article, field performance data are presented for 3 representative groups of surge arresters, which vary in application, rated voltage and monitoring device type:

ACY2: A set of 240 kV surge arresters equipped with Device B, installed at a transmission line bay (see Fig. 4);

T02: A group of 240 kV surge arresters, also equipped with Device B, installed at a 500/220 kV, 470 MVA three-phase power transformer (see Fig. 4);

TA01: A set of 420 kV surge arresters equipped with Device A, installed at a bank of single-phase transformers 500/13.8 kV, 15 MVA each (see Fig. 6).

Comparative analysis included total and resistive leakage current values (without corrections) obtained from both the monitoring devices and Instrument X, considering accuracy range as well as surge event records, where applicable. Again, it should be noted that values from Instrument X, when compared with Device B, were converted from peak to RMS values.

Figs. 6, 7 and 8 show total leakage current, per phase, from the monitoring devices and Instrument X at ACY2, T02 and TA01, respectively.

Figs. 9, 10 and 11 show resistive leakage current, per phase, from the monitoring devices and Instrument X at ACY2, T02 and TA01, respectively. For the latter, measurements presented were corrected by temperature and system voltage factors since Device A and Instrument X have the characteristic correction tables available in the software for the specific 420 kV surge arrester model.

Device B allows using the correction tables for the specific 240 kV surge arrester model, but it was not possible to add new correction tables in Instrument X. For this reason, comparative measurements here were not corrected.

In general, for total leakage current, Device B exhibited good agreement with Instrument X across all applications, particularly in the ACY2 line bay installation. Measured values were consistently within the accuracy ranges reported for both equipment, demonstrating reliable field performance and validating the laboratory findings that supported Device B’s deployment.

However, this favorable correlation was not observed with Device A. Significant discrepancies were noted between Device A and Instrument X measurements of total leakage current, with no clear explanation yet identified. This divergence had already been detected during high voltage laboratory testing and remains an open issue under investigation. The inconsistency appears to be independent of environmental or operational conditions. Further study will be needed to determine whether it is related to hardware limitations, calibration drift, or other factors intrinsic to Device A.

For resistive leakage current, some differences were observed between Device B and Instrument X. Nevertheless, considering the very low magnitude of current involved – often on the order of a few tens of microamperes, which is typical for new surge arresters – and the ±5 μA margin of error for Instrument X, these deviations were deemed acceptable. This behavior was also consistent with trends observed during laboratory evaluation, where Device B measurements closely tracked expected values within measurement tolerance.

In turn, Device A demonstrated better alignment with Instrument X in measuring resistive leakage current. Although not perfect, the values generally overlapped within the combined error margins of both devices. This indicates that, for this specific parameter, Device A shows reliable behavior.

Two additional observations merit considering when interpreting field data:

1. Measurements performed using Instrument X showed greater variability over time. This is understood to be an inherent feature of the measurement process, especially due to the variable positioning of the current clamp and electric field probe during each site visit. Despite the field team’s efforts to ensure consistency and repeatability, minor deviations in set-up can influence readings, particularly at low current levels;

2. A significant limitation encountered during the monitoring period was intermittent unavailability of monthly data from Device A. The primary issue was difficulty establishing reliable communication with the transceiver unit. Despite several in-field attempts, including re-positioning and use of an auxiliary antenna, this problem persists. As a result, gaps in the time series limit continuity of performance evaluation for this device.

To conclude, screenshots of the software on Device A and the web cloud solution on Device B are shown in Figs. 12 & 13, respectively. In both cases, it is possible to track leakage current trends over time. The major difference is the automatic time-based acquisition on Device B, which allows detailed tracking of the arrester’s behaviour, including detection of sudden changes (as shown in Fig. 13), which was later confirmed to be a switching overvoltage.

Pilot Testing of Device B on 444 kV SF6-Encapsulated Surge Arrester

As part of ITAIPU’s efforts to expand application of online monitoring to different surge arrester configurations, a specific test campaign was conducted to evaluate the feasibility of using monitoring devices on SF₆-encapsulated surge arresters.

The pilot installation was carried out on a 444 kV surge arrester, encapsulated in SF6 and exposed to outdoor conditions as installed on one of ITAIPU’s 500 kV/60 Hz line bays (see Fig. 12). This application was selected to represent typical operating conditions of GIS surge arresters in the plant’s infrastructure.

The underlying motivation for this evaluation stems from the large number of GIS surge arresters in service at ITAIPU, i.e. 114 units distributed across the 50 Hz and 60 Hz GIS systems and for which semi-annual measurements of resistive leakage current are currently performed manually using Instrument X.

Device B was considered a good candidate for this application because of a key technical advantage: it is self-powered by the leakage current of the surge arrester itself. This feature is particularly beneficial in indoor GIS environments, where both sunlight and electric field are absent.

Initial observations from the test (see Figs.13 & 14) showed that Device B presented excellent agreement of total leakage current with respect to Instrument X. The same did not occur for resistive leakage current, with variance in initial measurements and a representative difference between values.

The pilot project is ongoing and extended operation will help validate long-term performance and data stability under these specific conditions.

Conclusions

Evaluation of online surge arrester monitoring systems at ITAIPU revealed significant differences in performance between the two devices tested. Laboratory tests confirmed that Device B delivers reliable measurement of both total and resistive leakage currents, with values closely aligned to those obtained from the reference equipment, i.e. Instrument X. By contrast, Device A showed acceptable agreement only in resistive current measurements, while notable deviations were observed in total leakage current values.

Field data supported laboratory findings, particularly for Device B, which demonstrated robust performance across different substation environments. Its ability to consistently acquire and transmit data, combined with its intuitive cloud-based interface and NFC integration, proved advantageous for condition-based maintenance strategies.
Despite some minor deviations in resistive current values, these were within acceptable limits given the very low magnitudes involved and the inherent uncertainty of field measurements. Device A, while effective in capturing resistive current trends, faced issues with communication stability and occasional data loss, which affected continuity of long-term monitoring.

A notable outcome of this study was the pilot application of Device B in a 444 kV GIS surge arrester. This test case confirmed the device’s ability to operate effectively, thanks to its self-powering capability via leakage current. Although differences in resistive current values were observed compared to Instrument X, total leakage current readings matched closely, indicating a promising path forward for monitoring GIS configurations without major modifications to infrastructure.

Overall, findings highlight the value of combining laboratory validation and field testing for deployment of monitoring technologies in high voltage environments. While Device B appears better suited for broader implementation, especially in locations where continuous data acquisition is essential, further refinement and investigation are warranted for Device A.

Integration of online monitoring, particularly in GIS systems, offers significant potential for enhancing the diagnostic capability and operational safety of critical power infrastructure.

Ongoing Developments

As part of continuous work to enhance condition-based monitoring of surge arresters, ITAIPU initiated new developments in collaboration with a manufacturer and research partner. A key initiative, for example, involves field testing a new version of Device B, which incorporates LoRa communication via a gateway. This advancement would eliminate need for on-site visits to retrieve monitoring data, thereby improving operational efficiency and enabling near real-time data access.

A total of 6 upgraded Device B units are currently under evaluation. Three of these are installed on 240 kV surge arresters connected to a power transformer, while the remaining units are applied to 60 kV surge arresters installed on a transmission line bay. The goal is to validate long-term stability, communication reliability and improve capability for data analysis.

In parallel, with technical support from Itaipu Parquetec, an R&D Centre, and in partnership with the manufacturer of Device B, a new solution is being developed to automate data collection from the existing fleet of Device B units. This initiative aims to create a scalable interface capable of periodically aggregating data with no need for manual intervention, thereby further strengthening digitalization of HV equipment diagnostics.

References
[1] IEEE Std C62.11-2020, IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV), Institute of Electrical and Electronics Engineers, 2020.
[2] IEC 60099-5 Ed.2.0, Surge Arresters – Part 5: Selection and Application Recommendations, International Electrotechnical Commission, 2013.
[3] Santos, M. A. C. dos, Diagnosis of High Voltage Zinc Oxide Surge Arresters Using Paraconsistent Annotated Logic, Master’s Thesis, Institute of Technology for Development – LACTEC, Curitiba, 2022. (Document available in Portuguese only). Available at: https://repositorio.lactec.org.br
[4] Hitachi Energy, Excount-II – Surge Arrester Monitoring System: User Manual, Hitachi Energy, 2020. Available at: https://www.hitachienergy.com
[5] Tridelta Meidensha GmbH, smartCOUNT – Online Monitoring System for Surge Arresters: Operating Instructions, Tridelta Meidensha GmbH, 2021. Available at em: https://www.tridelta-meidensha.com
[6] Doble Engineering Company, LCM-500 Leakage Current Monitor: User Guide, Doble Engineering, 2019. Available at: https://www.doble.com
[7] Raschke, P.; Fotsing, S., Arresters with Brains: New Era IoT-Enabled Condition Monitoring Enhanced by Artificial Intelligence, INMR World Congress 2023, Bangkok, Thailand.