An On-line Monitoring System for Gases Dissolved in Transformer oil Using Wireless Data Acquisition

Xuezeng ZHAO Electromechanical School

Harbin Institute of Technology Harbin, Heilongjiang Province, China

zhaoxz@hit.edu.cn

Yangliu LI Electromechanical School

Harbin Institute of Technology Harbin, Heilongjiang Province, China

LYLmaria@tom.com

Abstract –Dissolved gas analysis (DGA) is a certain method to diagnose incipient fault of transformers through the correlation between the content of gases dissolved in transformer oil and a particular malfunction. This paper developed an on-line monitoring system to detect the concentrations of H2 and CO dissolved in transformer oil. The system mounts Polyperfluoro ethylene-propylene membrane, electrochemical gas sensors, a wireless communication terminal based on RF transceiver, and data management software recording the concentration of H2and CO. Compared with off-line gas chromatograph results of concentrations of H2 and CO, the concentration trend of on-line detecting results is in agreement with off-line results approximately through 8 months. For hydrogen detection, the biggest error of the system is 4ppm when the on-line results range from 10ppm to 21ppm; for CO detection, the biggest error of the system is 26ppm when on-line results range from 136ppm to 189ppm. The whole system provides a low cost, simple way to on-line monitor H2 and CO dissolved in transformer oil. And the system which incorporated wireless data acquisition will definitely shorten a time gap between the changes of gas concentrations and dissolved gas analysis.

Keywords–transformers; DGA; on-line monitoring;electrochemical gas sensors; wireless data acquisition

I. INTRODUCTION

Transformers are vital components in electricity system. Certain electrical and thermal phenomena that occur in oil- filled transformers can lead to the generation of a number of fault gases[1]. By monitoring the amount of gases and their ratios, an incipient fault (arching, localized heating or sparking, overheating, and low-energy electrical discharge) can be diagnosed before it results in a forced outage. Therefore the quantification of the gases dissolved in transformer oil in past years has become very important for preventive transformers’ maintenance program[2]. It makes dissolved gas analysis (DGA) one of the most effective way for monitoring oil-filled transformers[3, 4].

Until now, off-line gas chromatograph (GC) is still standard way to quantify the gases dissolved in transformer oil[5]. Most of the electrical utilities use gas-tight syringes for manually sampling the transformer oil and then return the samples to laboratories for running off-line dissolved gas analysis[6, 7]. However, using off-line gas chromatograph for monitoring gases dissolved in transformer oil has some disadvantages:

(1) Sampling of gases (actually oil) may be considered very cumbersome and unsuitable for oil-filled transformers. Because the transformer has to allow the staff to safely reach the sampling valve and then send the sample to the laboratory [8].

(2) Present DGA procedures involve oil sampling at intervals of 6 to 12 months for transformers not supposed to be defective. But some incipient faults are thus likely to occur undetected during such long intervals between subsequent DGA tests.

(3) A time gap between oil sampling and its laboratory analysis is also of some importance. It may lead to deterioration of gas content in poorly secured oil samples making analysis results profitless. The oil extracted from the transformer may be contaminated, for some air is likely to be dissolved back into the oil sample. Moreover, H2 is very likely to leak from the sampling syringe through the process of sampling. And thermal conductivity (TCD) sensitivity to CO is low, which makes it necessary to use a catalytic converter. Such an approach is rather complicated; it requires a lot of expensive gases (Ar, H2, and O2). This procedure may also lead to larger measurement error due to the fact that the conversion is a typical heterogeneous-catalytic reaction and its rate can unpredictably vary in time. That is why there is a potential source of errors in the measurement of CO, which deteriorates the overall accuracy of the diagnosis by off-line gas chromatograph[9].

Considering the disadvantages of off-line GC, and easily making intervals of gas chromatograph shorter is not economically reasonable taking into account the present cost of a complete laboratory DGA analysis[10]. Nowadays, more and more utilities developed on-line apparatus to quantify the gases dissolved in transformer oil[11].

This paper described an on-line monitoring system which detected the concentration of H2 and CO. For H2 and CO are crucial gases for monitoring transformers’ working situation. Both overheating and arching can destroy the structure of oil molecule. So as the transformer oil is overheated, excessive H2is released[12]; and the increase of H2 concentration also testifies that partial discharging takes place in the transformers[13]. IEC60599 standard and IEEE C57.104 uses CO as key indicator for overheating of cellulose, and secondary indicator for arcing if the fault involves cellulose[14]. Therefore, for early fault forecast of transformer, concentration detections of H2 and CO are adequate and

978-1-4244-2487-0/09/$25.00 ©2009 IEEE

indispensable. Some systems measure all eight kinds of gases (CO, CO2, CH4, C2H2, C2H4, C2H6, H2, H2O), which makes the whole system complicated and expensive. In this study, the gases dissolved in the transformer oil are extracted by macromolecular membrane, and then H2 and CO are quantified by electrochemical gas sensors. Wireless data acquisition is developed to get the concentrations of gases instantaneously. Finally, all the concentration data are recorded in monitoring software. And we use the system as on-line monitoring system to provide instantaneous concentrations of H2 and CO of an oil-filled transformer in Shenyang, Liaoning Province, China.

II. METHODS AND EXPERIMENTAL

The whole system sketch map is shown in Fig.1.

Figure1. The whole system sketch map

Polyperfluoro ethylene-propylene membrane has been mounted to some of the on-line monitoring system for gas extraction. Compared with Polytetrafluoroethylene membrane, Polyperfluoro ethylene-propylene membrane has higher gas permeability coefficients[15]. The transport properties of Polyperfluoro ethylene-propylene have been testified in [16]. Under that circumstance, it takes less than 96 hours for H2 and CO to reach the equilibrium. So it is effective to use this Polyperfluoro ethylene-propylene membrane to extract H2 and CO from transformer oil.

The gas cell was relative small. Fig.2 shows the sketch of the gas cell. The inner diameter of the gas cell is 18mm. The gas cell is 68mm long. The Polyperfluoro ethylene-propylene membrane is 125 m thick. The external diameter of gas cell is 36mm, and the material is yellow brass which has low absorption affinity to gases.

Two electrochemical gas sensors are installed into the downside of the gas cell; one of them designed for hydrogen, the other for carbon monoxide[17]. Both of them are fitted with concentration-to-current converters producing current signals proportional directly to a concentration of active gases in sampled air stream. And the cross sensitivities of the sensors for H2 and CO have been eliminated mostly. The detection ranges of the electrochemical sensors for H2 and CO are both 0~2000ppm. And the detection limits of H2 and CO are both 1ppm. The H2 detection accuracy is ±3% of the range; the CO detection accuracy is ±1% of the range. The stability of the sensors is ±5% over 12 months. The temperature coefficient is 1%K-1, and the response time is less than 1minute.

Figure2. Structure of gas cell

The concentrations of H2 and CO detected by gas sensors are still raw data, for the concentrations of gases need to be converted into the concentrations of gases dissolved in transformer oil.

In most literatures, the relationship between those two concentrations is illustrated as:

59.87 [1 exp( 1.013 10 / )].g i o iC k C PSt dV= − − × × (1)

Where Cg is the concentration of gases in the gas cell, and Co is the concentration of gases dissolved in the oil in fact. And ki is coefficient which shows the dissolution of gases in oil, Pi the permeability coefficient of gas i, S the total membrane available for permeation, d the thickness of the membrane wall, t the time of permeation, V the volume of internal cavity. Pi can be calculated from the relation:

/ 76 .iP bdV S= (2)

Where b is corresponding to the steady-state transmission rates of the penetrant, and has been tested in former literature. Equation (1) and (2) can be used to compute the concentration of gases dissolved in the oil, and then the concentrations of gases dissolved in oil are acquired. According to standard IEC60599-1999, Ostwald coefficients of H2 and CO do not change when the temperature changes from 20 to 50 . In our situation, the transformer oil temperature changes within that range, so Co can be regarded as the eventual result of concentrations of gases dissolved in transformer oil.

The computer in the control centre is 200m far away from the transformer that installed the system. To acquire concentrations of gases instantaneously and exactly, the data should be transmitted to the computer in the control centre by wireless communication immediately. A wireless data terminal is designed and realized. Low-power wireless transceiver data module is used as the wireless data transceiver with the small size, small weight and low power consumption and good stability and reliability. Fig.3 shows the block diagram of wireless data acquisition terminal.

Gases dissolved in oil

Polyperfluoro ethylene-propylene membrane

H2 gas sensor CO gas sensor

Gas cell

Wireless Data Terminal

Transformer

Figure3. Block diagram of wireless communication module

The high voltage of 500kV may be considered as a disturber to the gas sensors’ output signal. So the gas sensors’ analog output signal is converted into digital signal. Then the converter is connected to the RF packet converter through RS-232. Every twenty minutes, the data is sent out by RF transceiver (SRWF-508; ShangHai TangRay Info-tech Co., Ltd, China), which can be used both as transmitter and receiver. The carrier frequency is 433MHz. Its communication style is based on the GFSK modulation mode, the high-efficiency forward error correction channel encoding technology is used to enhance data’s resistance to both burst interference and random interference and the actual bit error rate of 10-5 ~ 10-6 can be achieved when channel bit error rate is 10-2. The transmission speed can also be adjusted for the RS-232 connection. Another RF transceiver receives the data, and transmits the data to the computer in the control centre through RS-232 too. In the field test, the effective transmission distance of data acquisition terminal is 500m, which means the terminal can be used to receive the concentrations of gases.

To record and manage the data of H2 and CO concentrations, user-friendly Windows-based software is developed using VB programming. The frame of the software for the monitoring system is demonstrated in Fig.4. And Fig.5 shows the image of the software main form of the monitoring system.

The software implementation can be categorized into five parts. The main part is real-time display which consists of patrol check and spot check of the data. The data is presented in either the table or graphical format. The trend of gas concentration is also be calculated and presented. The communication part takes charge the communication between the computer and the RS-232 interface. The soft ware can also check the state of the RS-232 interface. Technical personnel can set interval time of data download, preparative alarm value and alarm level through the parameter setting part. Gas concentrations and events are recorded in database, and they can also be printed and backup. In the help system, some method of oil-filled transformer fault diagnosis is given. The temperature of environment and oil is also recorded for further evaluation of total transformer situation. The software is used to download and manage data to and from the data logger. The downloaded data can be viewed or analyzed in the table as well as graphical form. So the staff can be aware of the trend of the gas concentrations instantaneously. The software also allows the staffs check the real-time values from the sensing

module to be shown for immediate analysis purposes. The data file can be saved in an open-file format so that it may be exported to some common spreadsheet programs such as Microsoft Excel for other types of presentations. It offers the user the entire essential functions of controlling and optimizing the features offered in the system.

Figure4. The frame of software for monitoring system

Figure5. The image of the monitoring software main form

III. RESULTS AND DISCUSSION

The whole system has been mounted to a 500kV transformer in Shen Yang, Liao Ning Province, China, and has been working for 8 months since Jan. 2008. The experimental results are compared with the off-line GC data in Fig.6 and Fig.7. As can be seen, the on-line concentration trend of CO and H2 follow the off-line concentration detected by gas chromatograph correspondingly. But the concentration error is obvious. For CO detection the biggest relative error is 11.5%, and the biggest absolute error is 26ppm when the on-line results range from 136ppm to 189ppm. For H2 detection the biggest relative error is 18.7%, and the biggest absolute error is 4ppm when the on-line results range from 10ppm to 21ppm.

When considering b is dependent on temperature, the error may be reduced if the concentrations results are modified by

RS 232

RS 232

Gas sensor analog output

Analog to Digital

RF Transceiver RF Transceiver

Antenna

RF packet converter

Antenna

Control centre

Master Interface

Communication

Parameter setting

Database

Help system

Real-time display

Receive/send data

Set interval of data download

Send command

Set preparative alarm value

Set alarm level

gas concentrations

Event logger

Method of diagnosis

Data of patrol check

Data of spot check

Print

Backup

Figure of gas trend

Temperature of gas oil

Graphical display

calibrating the value of b at different temperature. The equilibrium time lags may also lead to the concentration error. The exposed surface of the membrane and the volume of the cavity are two factors that determine the rate at which equilibrium is attained. Therefore increasing the contact area of membrane and limiting the volume of gas cell can also decrease the error of the system.

Figure6. Comparison of on-line and off-line CO concentration

Figure7. Comparison of on-line and off-line H2 concentration

IV. CONCLUSION

An on-line monitoring system which incorporated macromolecular membrane, electrochemical gas sensors, wireless data acquisition and data management software is developed. Through 8 months working on a transformer, the results showed that the trend of on-line concentration detection of H2 and CO followed the off-line detection by Gas Chromatogram correspondingly. And the concentration trend is in agreement with that of off-line gas chromatograph approximately. For CO detection the biggest relative error is 11.5%, and the biggest absolute error is 26ppm when the on-line results range from 136ppm to 189ppm; for H2 detection the biggest relative error is 18.7% and the biggest absolute error is 4ppm when the on-line results range from 10ppm to

21ppm. The whole system is able to rapidly determine concentration of H2 and CO, and definitely provides the results instantaneously by using wireless data acquisition.

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