On-line Condition Monitoring of Power Transformers S. Tenbohlen, F. Figel ALSTOM T&D Abstract - The development in sensor and computer technology allows the realisation of on-line monitoring systems for application on power transformers, in order to use this most expensive transmission equipment in the optimum technical and economical manner. This means the controlled utilisation of overload and life capacity of the transformer, as well as early warning in case of an oncoming insulation fault and condition-based maintenance. In this contribution a state-of-the-art monitoring system for power transformers based on field bus technology and process control software is described. The presentation of experiences in operation shows considerable possibilities regarding optimisation of service and early fault recognition. Keywords: On-line Monitoring System, Power Transformer, Lifetime Assessment, Field Bus Technology I. INTRODUCTION
An on-line monitoring system is particularly suitable for utilisation with power transformers, with the aim of guaranteeing a reliable electrical power supply in connection with reduced maintenance expenditure and an optimum exploitation of the active part [1,2]. Furthermore the remaining operating life of the transformer can be estimated by recording important operating data. In order to enable a consistent utilisation of the technically possible load capacity of the transformer, statements regarding the current overload capacity, for example, can be made. Transformer outage rate statistics indicate on-load tap changer, bushings and winding insulation as the most frequent cause of long duration outages [3,4]. Therefore the installation of a comprehensive monitoring system to warn in case of an oncoming fault is advisable for strategically important power transformers [5]. Retrofitting a transformer which is currently in operation with a monitoring system should be achieved without
requiring large number of cables, and in the shortest time possible. Field bus and process control technology have become established in many technical applications due to their compact design, modular construction and flexibility and are now increasingly being used in the energy industry. If one views the operational transformer as a technical process the use of these technologies, especially in the field of power transformer monitoring seems the obvious choice. II. SENSOR TECHNOLOGY FOR EARLY ERROR DETECTION AND LIFETIME ASSESSMENT
A multitude of different measurable variables can be collected for on-line monitoring (Table 1). However, it is very rarely useful to use the entire spectrum. Therefore, sensor technology must be adjusted to the specific requirements of a particular transformer or transformer bank, depending on their age and condition. Transformers, for example, which came to notice through a Buchholz warning must be monitored by a Buchholz gas sensor, while this sensor certainly will not be absolutely necessary for a new transformer. The great modularity of ALSTOM MS2000 monitoring system can easily be focused on the customer’s needs. Depending also on their monitoring “philosophy” it is possible to propose a personalised set of sensors and functionalities. The customer may prefer to have a wide range of condition indicators or to concentrate on specifics (e.g. high frequency voltage sensor if the transformer is connected to GIS). For early error detection, the monitoring of the active part is of particular importance. Voltage surges for example represent a significant risk potential for the insulation of a transformer winding. Therefore the detection and evaluation of these transients with a voltage sensor on the measuring tapping of the capacitor bushing is of great importance. These voltage peaks can be detected through a peak value sampler. In connection with the
TABLE 1 - MEASURING QUANTITIES OF THE MONITORING SYSTEM MS2000 Active Part
Conservator
Bushings
Cooling Unit
Tap Changer
Gas-in-oil content
Oil level
Voltages, Overvoltages
Oil temperature in/ out cooler
Tapping position
Oil level in Buchholz relay
Humidity
Load currents
Air temperature in/out cooler
Power consumption of motor drive
Oil pressure
Ambient temperature
Moisture of oil Oil temperature, Hot Spot temp.
Operating condition of pumps and fans
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
1
volume of noxious gases which are dissolved in oil, which can be measured with a gas-in-oil sensor, one can draw deductions regarding possible damage to the insulation of the active part after the occurrence of voltage surges. For the gas-in-oil detection a Hydran sensor is used which reads a composite value of gases in ppm (H2 (100%), CO (18%), C2H2 (8%), C2H4 (1,5%)). As hydrogen is a key gas for problems in the active part, an increase in the output signal of the sensor is an indication for irregularities such as for example partial discharges or thermal overload [6]. The evaluation of this signal, together with the dependency on the temperature of the oil and the load current, provides a reliable basis for the continued operation of the transformer. In the event of an increase of gas-in-oil content, an immediate reaction can be effected via an off-line gas analysis to determine the concentration of the other components dissolved in the oil in order to limit the cause of the damage. After the absorption limit has been exceeded, the gases not dissolved in the oil are collected through the Buchholz relay. Even very small quantities of gas can be measured with a electronic gas sensor screwed onto the Buchholz relay with a high degree of sensitivity and temporal resolution. This device specially developed for Fig. 1: Electronic Buchholz gas easy retrofitting at sensor transformers in service is shown in Fig. 1. Due to the fact that water is formed during the decomposition of the oil/paper insulation, the water content of oil is also an important indicator for the condition of the winding insulation. The hot-spot temperature can be determined through the thermal image of the transformer. This enables the acquisition not only of information regarding the temporary overload but also of the lifetime consumption of the transformer. The acceptance of overloading the transformer can be necessary in case of critical conditions in the electrical network. Unusually heavy loading can cause high temperatures within the winding with a high ageing rate of the insulating paper. However, acceptance of this condition for a short time may be preferable to other alternatives. The aim of the calculation of the overload capacity is to ensure that the thermal ageing is not affected while operating with loading beyond nameplate rating. To guaranty this, the actual overload capacity is on-line
calculated by measurement of load current, oil and ambient temperatures. The implemented thermal model calculates on the one hand the normal cycling loading in dependence on the ambient temperature according to IEC 354. On the other hand the system gives additionally on-line details about the short-time emergency loading time with a load factor of 1.5. In the cooling plant, not only the switching state of the oil pumps and the ventilators is monitored but also the temperature values. For this purpose, the input and output temperatures of the air are measured in addition to the oil temperatures. The intention is to make selective statements regarding the state of the entire cooling plant from the measured values. Recordings of the tap changer position and the operating current help determine the number of switching operations of the tap changer and the total switched current. If the limiting value, pre-set in accordance with the maintenance instructions, is exceeded a message is generated. Due to the fact that serious damage to the transformer can be expected in the event of the failure of the tap changer, the monitoring of this mechanically and electrically highly stressed element is of great importance. In order to be able to obtain information regarding the mechanical state of the switch the power consumption of the tap changer drive mechanism is recorded during tap changing. III. ARCHITECTURE OF THE MONITORING SYSTEM
In conventional technology, the sensors are wired onto terminal strips right into a control cabinet situated centrally at the transformer. In the case of retrofitting in the field especially, this method is extremely material and time consuming due to the relatively long cable lengths and the usually relatively restricted working conditions around the transformer. The sensor signal is transmitted via a measuring transducer to an industrial PC for processing. In addition, an additional wiring (so-called parallel wiring) has to be carried out for the supply of the measuring transducer in the control cabinet. In comparison with this, the monitoring system MS2000 based on field bus technology stands out by virtue it exceptionally compact design. The sensors are wired with extremely short cables to de-centralised bus terminal boxes. These terminal boxes each consist of a bus coupler as a header station and electronic bus terminal strips which are connected up to the coupler and represent the decentralised input/output level of the control system. The dimensions of the control cabinet can be significantly reduced which is an essential advantage for the installation on the transformer. The connection to the control system and between the bus terminal boxes is carried out via the actual bus, which can be constructed either as a screened twisted copper cable or as an optical wave-guide. Instead of requiring a multitude of cable
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
2
connections, only the bus and the voltage supply of the bus terminal boxes have to be laid. In addition, the costly parallel wiring in the control cabinet can be dispensed with. The installation site of the control system can be selected at random in the substation. The industrial PC, for example, can be installed directly next to the transformer in a switch cabinet. This way a distinct allocation of the transformer and the monitoring system is achieved. The particular thermal requirements in the case of an installation directly adjacent to the transformer however necessitate the air-conditioning of the switch cabinet through cooling and heating. As a second variation, the control PC can also be erected in an operating building. Apart from the elimination of an expensive switch cabinet with an air-conditioning system, this also offers the advantage that several transformers can be monitored with only one control PC (Fig. 3). In this case only inexpensive monitoring modules are installed on the individual transformers which are connected to the control PC via the field bus. Server
Control Room
Client
Office
Modem
MS MS2000 2000 Field bus, Fibre Optic
Ethernet
Laptop Laptop
Client
Substation
Control ControlSystem System
Fig. 3: Architecture of monitoring system MS2000
As described above the monitoring cabinet is installed on the transformer while the industrial PC can be situated where required. This architecture with an independent monitoring cabinet is particularly adapted in case of retrofitting of old transformers. In this way it is possible to apply the monitoring system to a transformer quickly and without any major modifications to the existent equipment and control cabinet. Moreover an eventual failure of monitoring system will have no influence on transformer operations.
The measuring data can be recorded in the millisecond grid by means of the multi-tasking and real-time capable operating system QNX. When using a sample and hold converter this means for example that overvoltages can be measured as well. A protocol with a maximum time resolution is generated to track important process values during critical transformer states or in the case of malfunctions. A computer server (Soft-SPS) allows the implementation of complex computations and control sequences in a programming language standardised in accordance with IEC1131-3. This simplifies the logical operation of different process variables in order to be able to carry out an on-line diagnosis. In addition, it is possible for example to carry out via the monitoring system a loaddependent regulation of the cooling plant for the increase of the overload capability. 1) Data Storage Data storage is carried out in three stages (Fig. 4). Initially, the data are held in the RAM memory of the industrial PC at a high time resolution. This prevents a frequent access to the hard drive. After a time interval the mean values for each channel are filed in a historical database of the system, in order to guarantee an effective and very rapid on-line data access. The data are filed in accordance with their physical properties. On one hand this means the occurrence orientated filing of irregular events (e.g. switching events, voltage surges) and on the other a cyclic filing of continuous operating data (e.g. operating current and voltage values), for which the memory rate is adapted to the time constant of the individual parameter. In a third step, these data are reduced and filed in an SQL-capable relational database. On the one hand, this database has the function of a back-up, on the other it can also be edited off-line after being downloaded onto the work station computer. Furthermore, the application of this database guarantees that a purposeful data reduction can be carried out for long-term storage. In addition, the use of an SQL-capable database offers the possibility to edit the data with an operator-specific evaluation software or with MS Excel. Data Acquisition
IV. SOFTWARE FOR MONITORING AND DIAGNOSIS
The operational transformer with its constantly changing measured values, depending on the operating status, can basically be viewed as a technical process. Therefore, it seems the obvious conclusion to use a process control system for the execution of the multiple tasks of the software, such as event controlled data acquisition and processing, data storage, visualisation and communication.
Realtime Database RAM Transformer
Calculation Server
Historical Database Hard Disk Transformer
QNX 4.24
SQL-Database Hard Disk Transformer Hard Disk Office
MS Windows
Visualisation (online) Process Diagrams, Data Visualisation, Alarms, Messages
Records (offline) Backup, Statistical Analysis, Diagnosis, Standardized Output
Fig. 4: Data storage in three stages
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
3
Fig. 5 On-line screen of the transformer 2) Visualisation The visualisation software under MS Windows not only allows the user-friendly presentation of the measuring data with a multitude of combination and zoom options for evaluation (Fig. 5). The data stored in RAM can be analysed directly in the visualisation mode by mouse click. A constantly updated real-time trend is opened so that the user can carry out an immediate status evaluation. The data filed in the historical database can be analysed not only in form of a diagram but also in form of list information. By jointly presenting the different measuring values in one diagram the correlation between the individual process values immediately becomes clear. For condition assessment of the different parts of the transformer the user can get more detailed information by clicking on the corresponding buttons on top of the screen. 3) Alarms If limiting values are exceeded, the system generates an alarm message. Depending on the significance of the alarm status, messages are put down in different places. When incorporating the system in the security and process control engineering, messages can be transferred or the persons responsible can be informed specifically via modem, by fax message or via mobile telephone. The
alarms awaiting attention are displayed in colour in an alarm list, according to their status (awaiting attention, acknowledged or eliminated). By connecting the monitoring system MS2000 to the telephone network by means of a modem, remote diagnosis over large distances no longer presents a problem. V. PRACTICAL EXPERIENCE
The first operating data recorded by monitoring systems show the power handling capacity and options for an optimisation of the operational running and maintenance of the installation. Here, an attempt is made to document the possibilities of a monitoring system for the acquisition of the important operational data on the basis of the measured values of the gas-in-oil volume, the oil temperature curve with cooling output power calculation and the power consumption of the tap changer drive. The transformer is equipped with a heat recovery system. The energy obtained this way from the waste heat of the transformer is used to heat the building in the switchgear plant. The effect of the operation of the heat recovery system on the oil temperature and the different cooling output power is illustrated. During operation the oil temperature is almost completely independent of the ambient temperature and is kept at a level of between 60°C and 66°C by using the coolers in order to ensure an economic operation of the heat recovery. The output
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
4
power of the heat recovery system is approx. 40 kW. After it is switched off on 19/8, the transformer will again be operated at a lower oil temperature. The discharged output power from the cooling plants 2-4 has increased.
Oil Temperature 70 Heat Regeneration °C Cooler
Cooling Power
Heat Regeneration On
50
60
40
40
30 20
20
h
10
0 15.8.
17.8.
19.8. 21.8. Time
0 25.8.
23.8.
Diverter Switch Number of Tap Changings: 225 Sum of Currents: 16,2 kA
300
Operation Time
kW
Temperature
100
broken down according to the direction of switching for the month of August are illustrated in form of a bar graph in Fig. 8. During this time the tap changer was only operated in the range of the first seven steps. Step 3 was switched on most frequently with almost 300 operating hours. The number of switching operations for the tap changer was 225 at a switched current total of 16.2 kA per phase.
Fig. 6 Oil temperature and cooling power
200 150 100
5
29
36
27
11
4
50
5
28
35
26
11
4
00
The content of the gas components CO and CO2 dissolved in oil are dependent on the load, as Fig. 7 illustrates. The operation of the heat recovery system and the increased oil temperature in connection with this has a great effect on the gas content. In May, and at the beginning of June, the gas-in-oil content increases immensely due to the higher oil temperature. These results are confirmed by two oil samples which were taken on 26/5 and on 23/7. If one forms the cumulative gas-in-oil content from the gas concentrations determined by the DGA, in accordance with the above mentioned percentage, which is characteristic for the Hydran sensor, this value corresponds to the value of the display of the gas sensor. After switching off the heat recovery system and the lowering of the oil temperature below 50°C, the gas-in-oil content begins to decrease again.
Pos. 1
Pos. 2
Pos. 3 Pos. 4 Pos. 5 Tapping Position
Pos. 6
Fig. 8 Tap changer characteristic for one month
The power consumption of the tap changer drive is recorded in order to detect errors which occur during the maintenance intervals of the tap changer. Fig. 9 illustrates the characteristic curve trace of the power input for each individual switching. At the start of the switching process a power peak occurs due to the starting current. The intention is to draw conclusions regarding the mechanical state of the switch from the position and the amplitude of the following power peaks. In this matter, for example, it could be possible to determine envelopes which must not be exceeded during stepping. In the event that they are exceeded the monitoring system can trigger a warning. 2000
70
140
Temperature
50 40
80 60
Oil Analysis: 23 ppm H2 328 ppm CO
30 20 1.5.
100
Oil Temperature Gas-in-Oil-Amount
40
Gas-in-Oil-Amount
ppm
°C
20 21.5.
10.6.
30.6. 20.7. Time
9.8.
29.8.
Power consumption [W]
Transformer Off Heat Regenaration On
Switching of Selector Disturbance of contact opening Disturbance of contact closing
1500
1000 Opening of contacts
500
0
0
1000
2000
0
Closing of contacts
3000 t [ms]
4000
5000
6000
Fig. 9 Power consumption of motor drive
Fig. 7 Gas-in-oil content dependent on temperature
The history of the lifetime of the tap changer provides important information for maintenance procedures. The course of the tap changer position is used to determine the serviceable life of the individual steps. The operating time and the number of switching operations per step,
VI. CONCLUSIONS
This report introduced a comprehensive monitoring system for power transformers, using a field bus with intelligent bus terminals. Its software basis was taken over from the process control engineering. The modular-
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
5
ity and adaptability of a monitoring system to transformers which are in operation is increased significantly by the field bus. The installation in the substation for example, can be carried out quickly and simply. In addition, it is possible to monitor several transformers through one control PC which is installed in the control room resulting in a significant cost reduction. The ALSTOM monitoring system MS2000 for power transformers now provides a comprehensive tool for the continuous monitoring of transformers. A progressive, automatic on-line diagnosis of the state of the transformer however requires that in the future monitoring systems will have to collect and analyse additional operating data. VII. REFERENCES
[1] K. Feser, "Trends in the Insulation Monitoring of Transformers", 10th International Symposium on High Voltage Engineering, Montreal, 1997. [2] R. Malewski, "Continuous Versus Periodic Diagnostics of HV Power Apparatus Insulation", 10th International Symposium on High Voltage Engineering, Montreal, 1997. [3] D.F. Peelo, et al., "A Value Based Methodology for Selecting On-line Condition Monitoring of Substation Power Equipment", EPRI Substation Equipment Diagnostic Conference V, New Orleans, Louisiana, Feb. 17, 1997. [4] "An International Survey on Failures of Large Power Transformers in Service", CIGRE Working Group 12.05, Electra, No. 88, January 1983.
von Leistungstransformatoren", Elektrizitätswirtschaft Jg. 97 (1998), H. 10, S. 48-56. [6] "IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers", IEEE Std. C57.104-1991, New York, N.Y., 1992. VIII. BIOGRAPHY
Stefan Tenbohlen received his Dipl.-Ing. and Dr.-Ing. degrees from the Technical University of Aachen, Germany, in 1992 and 1997, respectively. Since 1997 he has been with ALSTOM Schorch Transformatoren GmbH in Mönchengladbach, Germany. He is responsible for the product development and in this function working in the field of on-line monitoring of power transformers. His address is: Alstom Schorch Transformatoren GmbH, Rheinstr. 73, D-41065 Mönchengladbach, E-Mail:
[email protected]. Federico Figel obtained his degree in electrical engineering from the Polytechnic Institute of Milan (Italy) in 1996. Moreover he graduated in engineering (energy systems) at the Electricity High School (SUPELEC) of Paris (France) in 1998. Since 1998 he has been working with ALSTOM Transmission & Distribution in the Transformers Research Centre in Saint-Ouen (France). He is responsible for the monitoring of power transformers. His address is: ALSTOM T&D S.A. 25, rue des Bateliers / BP 110 F-93403 Saint-Ouen Cedex (France) E-mail:
[email protected]
[5] U. Sundermann, St. Tenbohlen, "Der intelligente Transformator - Zustandserfassung und Diagnose
Presented at the IEEE Power Engineering Society Winter Meeting, Singapore, 2000
6
