11. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Laško 2013 CIGRÉ ŠK C1-05

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SWITCHGEAR MONITORING SYSTEM-ASSISTANCE IN MAINTENANCE AND CALCULATION OF THE COSTS

Ana Mik Končar-Electrical Engineering Institute

anamik@koncar-institut.hr

Krešimir Meštrović Končar-Electrical Engineering Institute

kmestro@koncar-institut.hr

Mario Dujmović Hrvatska Elektroprivreda d.o.o.

Mario.dujmovic@hep.hr

Abstract – The biggest concerns of the new generation of managers in electric power industry, especially in the private enterprises, are the costs. On the open market where the customers can choose between two or more energy providers it is also very important to assure a safe and reliable electrical energy delivery. Because of that it is important to find the best balance between the maintenance costs and the operational safety of the equipment in the transformer stations. On-line switchgear monitoring technique has become a driver for remarkable improvements in both reliability and cost efficiency (condition based maintenance). It provides constant insight into the state of the switchgear apparatuses: circuit breakers, disconnectors, earthing switches and instrument transformers. Based on the collected data intelligent application makes analysis and gives information about the state of the equipment i.e. if there is a problem with the condition of the equipment or if the problem could arise in the future. If the failure nevertheless happens, system gives the proposal for the solution. Also based on the information switchgear maintenance plan can be made, together with the investment plan and calculation of future costs. Installation of the switchgear monitoring system provides longer life span and helps managers in decision making process. Keywords: Switchgear, Monitoring system, Expert system, Life cycle cost calculation

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I. INTRODUCTION Monitoring systems are changing the usual way of maintenance: they represent the transition from periodic maintenance to the condition based maintenance. Based on the data obtained from monitoring system it is easy to get an insight into the condition of the equipment. How wide the insight into the state of the equipment is, depends on the values that are monitored i.e. on the type of the monitoring system which is also associated with the cost of the monitoring system. Price of the monitoring system depends on the sensors which are used for monitoring. For example, monitoring of the state of the Sulphur hexafluoride (SF6) is the most expensive because of the high price of the sensors. Most of the monitoring systems are modular. Therefore it is important to find the right balance between the price of the monitoring system and the benefits which can be achieved by using the monitoring system.

II. SWITCHGEAR MONITORING SYSTEM

Monitoring systems provide on-line continuous insight into the condition of the equipment. High voltage switchgear monitoring system monitors a various values of the switchgear apparatuses: circuit breakers, disconnectors, earthing switches and instrument transformers. The observed values are collected from the sensors and saved in the database. By accessing the data from the user interface, analysis and presentation of all phenomena and conditions of the system is carried out. Analysed results are presented in both graphical and numerical form.

Switchgear monitoring system can be installed to monitor the apparatuses of the air insulated switchgear (AIS) and also to monitor the gas insulated switchgear (GIS). On the equipment containing SF6 gas it is the most important to monitor the values which are indicators of the change of the gas. In this way, the leakage of the SF6 gas can be prevented. Because the SF6 is the greenhouse gas, the switchgear monitoring system can be considered as the environmentally friendly product.

Figure 1. Switchgear monitoring system-interface

The bay monitoring system (Končar-BMS) has been developed in the Končar-Electrical Engineering Institute in order to evaluate the state of monitored high voltage apparatuses and equipment at any time [1], and bring the right decision regarding the necessary maintenance activity and further exploitation. The monitored values are compared with the values from diagnostic testing (reference values). If there is a deviation between those two values and if that divergence is greater than a certain threshold, the system alerts the users and depending on the alarm, provides tips for the troubleshooting. Circuit-breaker monitored values are:

• Status: circuit breaker close/open, trigger, lockout,

• Phase voltage, • Main circuit current, • Close coil and trip coil currents, • Control circuit voltage, • Motor current, • Frequency and duration of motor operation, • Contact and/or drive travel, • Switching times (opening, closing, close-open), • Hydraulic drive pressure, • Oil loss in hydraulic cylinder, • Spring drive tension.

Those quantities give the information about current state of the circuit breaker mechanism and motor and his readiness for performing future operations. Beside values obtained from the sensor directly, there are values which are calculated by mathematical methods. By using calculation from the values above the following values are obtained:

• Main and control circuits operating times, • Contact travel and contact velocity during

closing and making operations, • Arc integral (I2t) during making operations.

These quantities are a good indicator of circuit breaker wear which has to be remedied to prevent the catastrophic consequences. Arc integral is a good indicator of the breaking chamber remaining lifetime. By observing coil current trip [2] it is also possible to make a conclusion about the circuit breaker health. The trip energy is stored in a latched spring mechanism. After the trip command, the trip coil becomes energized. The trip coil plunger motion can be characterized by the current flowing through the actuating trip coil [3]. The resulting trip coil current signature provides a chronological record of the operating sequence and timing associated with the various breaker components during trip operation. By comparing coil current curve with the reference curve (Figure 2.), important information about the

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state of the circuit breaker can be obtained. If there is a discrepancy between these curves it is possible to accurately determine which part of the circuit breaker mechanism is damaged. The difference between those two curves can be obtained by two ways: by finding the characteristic points on both curves regarding the time and value of current and observing the differences between them. Second way is to find the grey zone by calculating the area between curves. There are two approaches in selecting the curves that are going to be compared. One way is to determine the referenced curve, which can be the first recorded curve that was taken by putting the monitoring system in operation or specified by the manufacturer. Then this referenced curve is compared with other curves. Another approach is to compare the two consecutive curves.

Figure 2. Trip coil current signature

Figure 2. shows trip coil current grey zone with specific points which are:

1 Beginning of the movement of the circuit breaker operating mechanism

2-3 Plunger trip 3-4 Plunger strikes the latch mechanism 4-5 Plunger is stopping the movement 5 Plunger strikes buffer 6 Change of the coils inductance 7 Main contacts open 8 Auxiliary contacts open 9 Trip coil de-energised

Coil current grey zone [4] provides insight into the deterioration of a circuit breaker. This is innovative method for supervision of the circuit breaker and its condition.

Disconnector monitored quantities are:

• Status: disconnector close/open, trigger, • Motor current, • Contact travel, and

• Switching times (make, closing). For disconnector the following values are calculated:

• Main contact arms opening and closing time, • Travel and angular velocity of main contact

arms. All these values are good indicators of the failures in disconnector mechanism.

Instrument current and voltage transformers monitored quantities are:

• Hydrostatic pressure from which is calculated hydrostatic pressure at 20°C,

• Main circuit current, • Main circuit voltage.

Earthing switch monitored quantity is:

• Status close/open.

In circuit breaker and GIS measured values of the insulator sulphur hexafluoride (SF6) are:

• SF6 temperature, • SF6 dew point, • SF6 density, • SF6 pressure.

From those values the following values are

calculated: • SF6 pressure at 20°C, • SF6 percentage.

From the measured values of SF6 temperature, pressure and density, SF6 percentage is calculated which can warn about the leakage of the SF6 gas. Leakage has to be stopped before the contamination of the environment. Also, decreased percentage of SF6 can affect the ability of current interruption.

The above parameters are monitored through all three phases and they may be presented by single amount or by their wave forms. Except from the alarms which are alerting that the measured value is out of the range, system provides insight in trends, which is a good tool for monitoring of the changes in values through time (Figure 3.). It can also be used to compare the data, for example, between phases.

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Figure 3. Trend of hydraulic pressure

In gas insulated switchgears metallic defects cause a reduction in the local dielectric strengths of SF6 systems which can lead to catastrophic consequences [5]. To prevent such events the ultra-high frequency (UHF) technique is applied for partial discharge measurement in gas insulated switchgear. By observing each of these values, failures can be foreseen and consequences can be prevented on time.

III. CURRENT DEVELOPMENT OF THE SWITCHGEAR EXPERT MONITORING SYSTEM

In decision making process consulting with the experts from the specific field is recommended. Those experts or consultants base their advices on the knowledge, experience and other available sources of information. There is significant interest in the automation of the human thinking process. Such automated systems, known as the expert systems, connect inbuilt knowledge with the available data. The goal is to turn the available data, collected by the monitoring system, into useful information for the end user. As switchgear monitoring system collects large amount of data, it is difficult to manage all the information. The end user should gain only the information about what has to be done in order to achieve the maximum reliability and availability of the apparatuses. At the moment development team in Končar-Electrical Engineering Institute is upgrading the existing switchgear monitoring system to Expert monitoring system. The first step in the development was creation of the failure database. Data about the failures of the switchgear apparatuses are gained from the three resources:

1. Data about failures from the CIGRE enquiry on reliability of high voltage equipment [6], [7],

2. Data gained from the simulation in Matlab or other adequate program,

3. Data obtained from the experts in the field of the high voltage apparatuses: maintenance personnel of the electricity companies and

employees of the factories which develop and produce high voltage apparatuses.

The development of the expert system is based on the Bayes decision theory. The use of this method is typical for the modelling of the expert system based on the knowledge of the experts from the specific field of interest. The probability of the failure can be obtained from the statistical data or from the experience of the experts from the specific field [8]. If the data is obtained from the few sources, based on the reliability of the sources, data is given a certain weight.

IV. ASSISTANCE IN MAINTENANCE SCHEDULING AND COST ANALYSIS

As the monitoring system stores large amount of data, it can be also used for other purposes which can make substation management easier. For example, based on the collected data, maintenance schedule can be made. Monitoring system gives us data about the instantaneous state of the equipment. Based on that data, it can be concluded when the maintenance or replacement of the specific part is needed. That information is then used to make optimal schedule of the maintenance personnel and also helps in spare parts management (removal of the parts from the warehouse and ordering of the new parts). Based on that, cost calculation and investment plan can be made. The goal is to take the monitoring system to a higher level i.e. except from the standard technical analysis to answer also at the most important question of the power station management: how high cost will incur? Because of that the plan is, except for the conduction of the technical analysis, also to perform an economic analysis using methods such as Life cycle cost analysis and Decision making. The life cycle cost calculation assists in finding the optimal technologies, configurations and operative strategies. The usage of the life cycle cost calculation on various resource strategies results in the decrease of the maintenance costs considering all relevant parameters. It is also very important to choose the optimal maintenance method. Usage of the maintenance strategy based on reliability centred maintenance leads to the conclusion which part of the equipment is necessary to repair or replace. If the conclusion is that the part has to be replaced, new question arises: should only that part be replaced or the complete system. The answer to that question gives the Decision making process. The goal is to create an effective system which will help in decision making which will have the cost reduction as a consequence i.e. it will assist in finding

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the optimal method that minimizes the cost of the equipment service life and failure risk.

V. CONCLUSION

To prevent the failures and the consequences which can occur, it is necessary to improve the technical and physical security of the switchgear, and reduce the likelihood of adverse scenarios. Implementation of the switchgear monitoring system provides on-line, continuous insight into the state of the apparatuses. It also has the advantage that the monitoring can be done from the remote location, which is particularly important for unmanned substations. It is also possible to integrate data of the transformer, generator and switchgear monitoring system into one application. In that way is achieved insight into the state of the critical infrastructure important for the production and delivery of the electrical energy to the end users. With the additional applications like expert system, the maintenance together with the cost calculation becomes easier and more reliable. It can be concluded that benefits which are gained from the continuous insight into the condition of the equipment justify the cost of the investment into the monitoring system.

REFERENCES

[1] K. Meštrović, M. Poljak, M. Vidović, M. Furčić, M. Lončar, I. Maras, A. Mik, “New concept of highvoltage switchgear on-line monitoring system”, 16th International Symposium on high-voltage engineering – ISH 2009, South Africa, Cape Town, 24-28 August 2009.

[2] S. Strachan, S. McArthur, J. McDonald, W.

Leggat, A. Campbell, “Trip coil signature analysis and interpretation for distribution circuit breaker condition assessment and diagnosis”, CIRED, 18th International Conference on Electricity Distribution Turin, 6-9 June 2005

[3] Stephen Beattie, “Circuit breaker condition

assessment by vibration and trip coil analysis”, Printed and Published by the IEE, Savoy Place, London WCLR OBL. UK.

[4] M. Stanek, “Model-aided diagnosis for high voltage circuit breakers”, Doctoral thesis, Swiss Federal Institute of Technology, Zürich, 2000.

[5] W.J. Bergman, “Equipment monitoring selection as a part of Substation Automation”, IEEE Switchgear meeting, Pittsburgh, 1999

[6] Carvalho A., Cormenzana M. L., Furuta H.,

Grieshaber W., Hyrczak A., Kopejtkova D., Krone J. G., Kudoke, M., Makareinis D., Martins J. F., Meštrović, K., Ohno I., Östlund J., Park K.-Y., Patel J., Protze C., Runde M., Schmid J., Skog J. E., Sölver C. E., Sweeney B., Waite F., “Final Report of the 2004 – 2007 International Enquiry on Reliability of High Voltage Equipment”, Electra, 264 (2012.)

[7] Intermediate results from on-going enquiry on

reliability of high voltage equipment, CIGRE, Brasil, 2007.

[8] K. Meštrović, “Postupci određivanja

pokazatelja pouzdanosti visokonaponskih SF6 prekidača”, doktorska disertacija, Zagreb : Fakultet elektrotehnike i računarstva, 16.05. 2008., 174 str. Voditelj: Tešnjak, Sejid