INCREASED OPERATION RELIABILITY THROUGH CONTINUOUS PD MONITORING IN THE JAVA-BALI REGION

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1 INCREASED OPERATION RELIABILITY THROUGH CONTINUOUS PD MONITORING IN THE JAVA-BALI REGION Andrea Piccolo, Laurentiu-Viorel Badicu, Wojciech Koltunowicz Omicron Energy Solutions Lorenzweg 5, D-12099, Berlin, Germany, Constant Citra Wahana Sekar Buana, Ugan Suganda, Wahab Winoto, Yohanes Mulyana, Wirawan PJB (PT Pembangkitan Jawa-Bali) ABSTRACT PJB (PT Pembangkitan Jawa-Bali), the main power utility in the Java-Bali islands, Indonesia, embarked in 2011 on an ambitious project to equip all of its important generating plants with continuous on-line partial discharge monitoring. At that time they were facing the fact that their equipment, installed mostly in the eighties and nineties, was approaching the period of major maintenance needs and needed a reliable tool to prioritize effort and improve reliability. Currently, PD monitoring systems are installed on 18 turbo-generators and on 8 hydro generators, and there are more to be installed in near future. This article describes the advanced hardware and software features of these systems, including elimination of disturbances and separation/identification of different types of insulation defects based on synchronous, multi-channel and multi-frequency techniques. Two case studies are also included, the first being the case of Gresik PLTU (thermal power unit) and showing the advantage of having a fully digital system, whose settings can be easily remotely adjusted. The second is that of Cirata Unit 7, a typical showcase of how to proceed with the assessment of the stator winding conditions, when clear PD activities are diagnosed by the monitoring system. KEYWORDS: Generator, Condition Based Maintenance, Stator Insulation, Partial Discharge, Monitoring. 1. INTRODUCTION PJB is the main power utility in the Java-Bali islands. Thanks to its installed 6793 MW, distributed in six main power stations (fig. 1), PJB is delivering yearly GWh to the Java-Bali grid, through 150 kv and 500 kv HV lines. Figure 1. Location of the power plants in Java-Bali islands

2 The base load generation is covered by the thermal power plants of Gresik, Paiton, Muara Karang and Muara Tawar (fig. 2), while the 19% of the installed power is based on the hydro-power plants of Cirata and Brantas, which cover the peak-load hours energy demand (7% of the yearly generated power). Figure 2. PJB installed power per power plant location Beside its power generation business, PJB develops and manages other diverse businesses related to power stations in order to maximize the potential of the company. On top of that, PT PJB created business areas in Operation and Maintenance, Engineering Procurement and Contracting, and entered a joint-venture to develop new power stations as well as to manage plant service Operation and Maintenance business [1]. Considering the fact that most of its installed generating power was installed in the early nineties, with some units even commissioned in the mid-eighties, PJB is continuously looking for enhancing its capabilities in Enterprise Asset Management. With reference to the popular survey of 1199 hydro-generators, carried out by the CIGRE Study Committee A1, WG A1.10 in 2009 [2], it is well recognized that the principal cause of generator failure (57%) is because of insulation damage. Considering an average life expectancy of years for a generator, most of PJB rotating electrical machines are approaching or are already in the critical period for major maintenance (refurbishment or rewinding). Aware of evidence that the peak of maintenance activity will probably be between the years of 2017 and 2020, PJB decided in 2012 to extend its capabilities in insulation condition-based assessment by installing PD monitoring systems on its generator fleet [3]. Since 2012, twenty one monitoring systems are operating, thirteen on turbo-generators (46% of the installed thermal power) and eight on the generators at Cirata hydro-power plant(77% of the available hydro power).thanks to the positive experience with the above mentioned machine, other five system has been recently installed and commissioned in Gresik Block 2 and Muara Karang. This following paper presents the achievements and challenges after more than two years since the first PD monitoring system was installed and commissioned. 2. MAIN CONTENTS 2.1 Challenges in Partial Discharge Monitoring Partial discharge (PD) monitoring is a widely accepted indicator of insulation condition for rotating machines. PD measurements are specified and strongly recommended for off-line testing and on-line monitoring [4,5,6,7]. Nevertheless, extensive application of PD monitoring for Enterprise Asset Management is still a challenging task based on the monitoring solution selected. Suggested requirements include:

3 The data acquisition and pre-processing stage must be flexible for installation to any kind of machine (hydro generators, turbo-generators, motors) and environmental conditions (measurement noise, distance of the sensors from the equipment under monitoring, et. al.); The monitoring system must be capable of trending and storing significant data for long periods; Remote access to the system must be granted to ease the access to the database; The monitoring software package must be able to provide tools both for basic measurement and advanced data analysis. The introduction of fully digital systems that include the synchronous multi-channel PD measurement technique allows sensitive measurements and accurate separation and classification of PD sources in a wide range of rotating machines [8]. With reference to the PJB experience, the example of integrating the above-mentioned technique within a server-based monitoring system/software is presented. 2.2 Monitoring System Lay-Out PJB represents the case of a typical utility, with a mix of different generating units, which have been installed and commissioned at different periods. Each monitored unit is equipped with a set of three 1.1nF capacitive couplers (one per phase), installed inside the Insulated Phase Bus and connected to the three phases (fig. 3). The capacity of the sensors, ten times higher than the more popular 80pF, guarantee high sensitivity to PD signals even in a frequency range lower than 1 MHz. Figure 3. Installation of capacitive couplers The sensors are connected to a three-channel data acquisition and pre-processing unit, which is based on FPGA technology (fig.4). Figure 4. Installation of data acquisition and pre-processing unit

4 Each channel is equipped with a 20 MHz high-sensitivity analogue-to-digital stage (14bits resolution and 130dB dynamic range the best in its class), which samples pulses at 65 MSsec. The digitized signals are then sent to real-time FPGA based system that accomplish the following functions: Digitally filters the pulses within a freely-selectable bandwidth, in order to reduce measurement noise; Populates the Phase Resolved Partial Discharge (PRPD) patterns; Generates the 3PARD map, in order to allow separation of noise and PD [3,8,9]. The actual advantage of having a fully-digital acquisition unit is that it can be easily customized with reference to the type of machine and installation conditions (i.e. distance of the sensors from the equipment under monitoring, measurement noise). The dialogue with the data acquisition and pre-processing units (usually one per machine), which include the timing periodic for acquisition and storage of the results, is managed by the server-based monitoring software. The monitoring software includes the following main functionalities: Application for real-time PD measurement; Storage of results in a database; Warning logic, based on the trend results; Web-based graphical user interface for monitoring data management. The advantage of a server-based system is to have a single remote access point for several machines that can be easily reached via the worldwide web whenever requested (fig. 5). Figure 5. Access to the server with monitoring software In fig. 6, the lay-out of a typical four generators-installation is shown. Figure 6. Lay-out of typical four-generator installation

5 2.3 CASE STUDIED The selected cases studied show a practical application of insulation condition-based assessment within the context of complex monitoring systems. In order to highlight the flexibility of the monitoring platform for different types of rotating machines, examples of turbo generators and hydro generators have been chosen respectively. Turbo and hydro generators are explicitly different in their construction. Even between turbo generators there are several technical solutions (i.e. cooling system and insulation technology), which can result in different PD activity. The direct comparison of results from different machines is not a meaningful way to evaluate data and should eventually often leads to a wrong diagnosis. Instead, the possibility to compare data from similar machines, which are stored in a common platform, is a powerful approach to achieve a reliable diagnosis and to build a solid inhouse know-how of the insulation conditions to efficiently support Enterprise Asset Management. The first case that refers to Gresik PLTU (thermal power unit) shows the advantage of having a fully digital system, whose settings can be easily adapted to environmental changes by remote adjustment of the monitoring settings. The second case that refers to Cirata HPP, represents instead a typical case of how to proceed with the assessment of the stator winding conditions when clear PD activities are detected GRESIK - PLTU Gresik power plant is the biggest generation facility of PJB and is located in the province of East Java, approximately 20 km to the northwest of the city of Surabaya. The power plant covers a surface of 78 hectares and is composed of a steam power plant (PLTU-600 MW of installed power), a gas power plant (PLTG-40 MW installed power) and three combined cycle blocks (Block 1, Block 2, Block 3) with 1578,78 MW installed power (fig. 7). In total, all sixteen generators are to be monitored for PD. Thirteen of them already have the monitoring system in full operation and on the remaining three generators, the PD monitoring system will be installed in The PLTU historically represents the core of the Gresik PP facility and is composed of four steam turbines: PLTU 1 and PLTU2, commissioned in 1981, are equipped with two twin 100 MW steam turbines coupled to 13.2 kv - air cooled generators; PLTU3 and PLTU4, commissioned in 1988, are equipped with two twin 200 MW steam turbines coupled to 15.0 kv - hydrogen cooled generators. Figure 7. Block 1 in Gresik PP

6 As mentioned before, the most efficient way to achieve a reliable insulation assessment of the stator insulation is by comparing the results from similar machines. The graph below shows the combined trend from PLTU 1 and PLTU 2 Figure 8. Combined PD data trend in PLTU 1 and 2 After more than one year since the commissioning, an increase of PD activity has been observed between September and October 2013 in PLTU 1 (blue, yellow and orange trends). The trend has been zoomed in order to investigate in detail the source of the steep increase in PD amplitude (fig. 9). Figure 9. Combined PD data trend in PLTU 1 and 2 related to the time period Sept Oct The increase is more evident in Phase S (yellow) and has a fluctuating behavior on all the three phases, increasing in average since the 18th of October. From the analysis of the recorded patterns, the cause of the increase has been identified in the appearance of a source of external noise or disturbances that periodically appears in the PRPD patterns. In order to restore the capability of the system to monitor PD exclusively, extra measurements have been performed by a team of PD monitoring specialists, with the target of evaluating an adequate system fine tuning. The results from the tests, carried out via remote connection between Germany (where the monitoring technical support is based) and Indonesia, showed that the monitoring band-pass filter central frequency can be moved from 2 MHz to 7 MHz, allowing the system to reject the noise while preserving adequate sensitivity to PD pulses.

7 In order to preserve the possibility to compare of results from PLTU1 and PLTU2 and validate the settings upgrade, the monitoring band-pass filter has been adjusted in both the machines. The results, showed in the fig. 10, clearly confirm, both for PLTU 1 and PLTU 2, that: The capability to reject the external noise has been increased; The sensitivity to the observed PD activity is preserved; The levels of PD activity are similar in the two machines, within an amplitude range of pC; The PD activities, investigated by means of patterns shape analysis, can be identified with regular internal PD activity. Original Settings 2 MHz central frequency Resetting to 7 MHz central frequency PLTU 1 PLTU 2 Figure 10. PRPD diagrams in PLTU 1 and PLTU 2 for two different central frequency of measurements: 2 and 7 MHz 2.4 CIRATA - Unit 7 PLTA Cirata is one of the largest hydroelectric power plants in Southeast Asia (1008 MVA) and the biggest hydroelectric power plant in Indonesia (fig. 11). Figure 11. Cirata Power Plant PLTA Cirata s power house consists of two main blocks Cirata I, commissioned in 1988, and Cirata II, commissioned in Each block is composed of 4x126 MW-16.5 kv hydro-generators with the same design and provided by the same manufacturer. The main contribution of PLTA Cirata is to deliver energy during the peak load hours and recover the Jawa-Bali electric system in case of black-out (Black Start-up PP). Installation of monitoring systems on all the generators was completed at the end of Since the first months of trending Cirata II-Unit 7, showed a steep increase of PD activity (fig.12).

8 Figure 12. PD trend in Cirata II-Unit 7 As all the four generators of Cirata II have the same design and were commissioned in the same period, they can be considered comparable. The first action taken was to compare Unit 7 with its neighbour Unit 8. Figure 13. CIRATA II PD trend in Unit 7 and in neighbouring Unit 8 From the trend diagrams of average charge for Unit 7 and Unit 8, it is possible to understand the deviation of Phase S-Unit 7. To confirm the activity in phase S, pattern acquisitions have been made periodically to identify the type of source and to confirm the phase location. In fig. 14, the results for Unit 7 and Unit 8 are presented, where each pattern is synchronized with its relevant phase voltage.

9 01/03/ /04/ /05/2014 Unit 8 Unit 7 Figure 14. PRPD patterns in Unit 7 and 8 acquired in different time The next step of the evaluation aims to isolate the investigated PD source from the internal PD activity. This task is accomplished by means of the 3PARD separation technology. 01/03/ /04/ /05/2014 PD Source Other PD 3PARD map Figure 15. PRPD patterns in Unit 7 and 8 acquired in different time The investigated PD source, identified by the selected cluster in the 3PARD, can be associated with surface tracking located in phase S. Considering the other sources separately, it is possible to have an indication of the machine from the point of view of internal discharges, which can be considered uniform in the three phases and comparable with the values recorded in Unit 8 (twin of Unit 7). The stator inspection carried out on September 2013 confirmed the presence of surface PD activity in six slots, belonging to the investigated phase S.

10 Slot 7 Slot 44 Slot 80 Slot 99 Slot 234 Slot 252 Figure 16. Evidence of surface PD activity in six different slots of Phase S 3. CONCLUSION Continuous PD monitoring of stator windings is proving to be an essential diagnostic tool for conditionbased maintenance of large generators operated by PJB, in order to improve their reliability and useful life. The main key to performing appropriate diagnosis of the state of the insulation is to accurately separate and identify different PD sources from external noise and disturbances, even when they are concurrent and produce similar symptoms. To achieve this, state-of-the-art synchronous multi-channel evaluation techniques are applied through an advanced automated system. Worldwide accessibility and possibility to cross-evaluate the monitoring data of the generators of whole power plants, stored in central databases, allows improved integration of generators PD monitoring within the Enterprise Asset Management system. The usefulness of the monitoring system has already been proven by the successful correct diagnosis of a problem in the end winding area of a large hydro generator, and subsequently fully confirmed by a visual inspection.

11 REFERENCES [1] PJB Pengbakitan Java-Bali website [2] CIGRE Technical Brochure 392 / Survey of Hydro Generator Failures CIGRÈ [3] Constant Monitoring in Electrical Rotating Machines Case Studied CEPED 2013, Bali [4] IEC , edition 1.0, On-line partial discharge measurements on the stator winding insulation of rotating electrical machines March 2012 [5] IEEE , IEEE Guide to the Measurement of Partial Discharges in Rotating Machinery [6] R. Bruetsch, M. Tari, K. Froehlich, T. Weiers and R. Vogelsang - Insulation Failure Mechanisms of Power Generators IEEE 2008 [7] C. Hudon, M. Belec and M. Levesque Study of Slot Partial Discharges in Air-Cooled Generators IEEE Transactions of Dielectrics and Electrical Insulation Vol.15, No. 6; December 2008 [8] L.V. Badicu, W. Koltunowicz, M. Koch and A. Piccolo - Return of experience from continuous PD monitoring of rotating Machines;ISH2013, Seoul [9] W. Koltunowicz, R. Plath - Synchronous Multi-channel PD Measurements - IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 6; December 2008.

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