online monitoringon bushing

ON LINE MONITORING SYSTEMS FOR BUSHINGS

CLAUDE F. KANE, ALEXANDER A. GOLUBEV, ANATOLIY B. SELIBER, Predictive Diagnostics of Eaton Electrical, Minnetonka, USA,

ABSTRACT Industry statistics suggest that 80% of all plant and equipment failures occur on a random basis and only 20% of the failures are age related. This means that 80% of failures have not been detected with common test and maintenance practices and therefore these failures have not been prevented. Based on different sources up to 30-35% of large power transformer failures are attributed to bushing insulation failures. About half of these bushing failures result in an explosion and fire. In today's competitive environment, increasing demands are being placed on the management of physical assets. Advances in technology are allowing new approaches to maintenance. These include reliability-centered maintenance, predictive maintenance, condition monitoring, and expert systems. Trend setting organizations are increasingly taking advantage of the convergence of these new technologies to implement proactive maintenance programs.

INTRODUCTION During the late sixties and early seventies the former Soviet Union experienced a high rate of catastrophic HV bushing failures on their 500kV power transmission systems. Root cause of the failures was a combination of design, manufacturing and technological problems. The existing maintenance strategy based on periodical off line insulation tests were not effective in preventing failures due to the fast rate of defect development. On line bushing monitoring methods and technology were developed and introduced at that time by P.M. Svi and his colleagues [2,3]. Implementation of the technology quickly reduced the bushing failure rate by timely detecting developing insulation defects and replacing failing bushings. The instrumentation with some modifications is still in use in most of the 500kV and 750kV apparatus in the former Soviet Union republics. The basis of this on line monitoring method is to compare insulation characteristics of three-phase bushing system. Technology for several three-phase bushing sets have been developed and tested but has not been widely used due to more complexity despite of its better noise immunity.

Figure 1 During commissioning the null-meter is balanced to zero. As defect develops the complex conductivity of the bushing insulation changes and the current and its phase angle in one of phases also changes. Therefore, the null-meter will no longer be null. The amplitude of the change reflects the severity of a problem and the phase angle indicates which phase that is experiences the change.

2

The change can be approximately represented by the formula under the assumption of a single defective phase:

( )1)tan(2

0

2

0

−⎟⎠⎞⎜⎝

⎛ ∆+∆≈∆= CC

II δγ

- Parameter GAMMA,

δtan∆ - Tangent delta change,

0CC∆ - Relative change in bushing capacitance.

The subject of the paper will be mainly on the current version of the Eaton Electrical InsulGard TMG2+ monitor. The monitor has features including phase measurements and temperature correlation that provides for a new knowledge base for on line bushing insulation monitors.

INSULGARDTMG2+ The system continuously monitors the power frequency current through the insulation of a 3-phase set of bushings as well as top oil temperature. Ports are provided that allow the attachment of measurement equipment for making periodical partial discharge (PD) measurements [4]. The sensors are connected to the bushing capacitor taps, and an additional neutral PD sensor is installed on the available grounded neutral of the transformer or on the transformer tank ground for noise cancellation purposes when PD tests are performed. Each bushing sensor has internal protection for the test tap insulation which limits the residual voltage at the tap to a level of not more then 135 Volts RMS even if open circuited. In normal operation, the test tap voltage does not exceed 10 Volts RMS.

Figure 2. InsulGardTM G2+ and bushing sensors installed

FACTORS AFFECTING ACCURACY Factors affecting accuracy of on line technology may be divided in three groups: Device accuracy; Noise; System voltage variations.

3

DEVICE ACCURACY Accuracy of the device was specified to timely detect dangerous changes in power factor or capacitance that may lead to bushing insulation failure. Experience indicates that the change in Gamma is of several percent1. Therefore, the accuracy of 0.1% for relative magnitudes and 0.1O in phase angle were specified and achieved.

NOISE The device monitors power frequency currents from bushing potential or test taps. Higher power frequency harmonics are considered as noise, especially the odd harmonics. The odd harmonics from the three phases are not balanced in the balancing unit but are summarized. In order to suppress this noise, the device must have high quality low pass filters. These filters allow for the application on DC/AC converter substations where signals have a significant high content of power frequency harmonics. Other noise that exists to some extent is a common mode noise resulting from two grounding points in a measuring circuit. There are two protection issues requiring two grounds. The first is to provide “absolute” safety for personnel that may occasionally control the monitor from its keypad or touch its enclosure. Reliable grounding of the device and the enclosure to a local ground and isolating all interfaces provides for personnel safety. The second requirement is to protect the bushing itself “no matter what” even if all links to the device are broken and device protection is not functional. Sensor must be rugged and adequate protection must be built into a sensor body. Normally a sensor has impedance connected in parallel to the ground and a surge-protecting device. The impedance passes power frequency test tap current to the ground, keeping a voltage on the tap to an acceptable range in case the measuring scheme is disconnected for any reason. The internal surge protector suppresses surges and lightning strike currents. Therefore, the second path (normally relatively high resistance path) to the ground is built into the sensor2. Installing the device enclosure next to a transformer tank and grounding it to the transformer ground would normally resolve the issue due to a small AC voltage drop across the transformer tank. The only problematic connection is on three single-phase transformer banks. The banks have some AC voltage between their tanks and this will produce common mode noise for the summation circuit. Good grounding practice will keep the AC voltage between the tanks within a few hundred milli-volts is sufficient. For example, in a InsulGardTMG2+ installed on the middle phase of three single-phase 500kV transformers. Figure 3 shows the schematic of common noise influence.

RO

Ua

Ya

Rd

Ub

Yb

Rd

Uc

Yc

Rd

Vb

Vab Vcb

Ia+Vab/R

d Ic+Vc

b/Rd

Ib

Figure 3

1 In several decades of Soviet Union experience the warning and alarm setting were 3% and 5% respectively. 2 The idea to leave only surge protection in a sensor does not seems to provide reliable protection. In the case of disconnecting the measuring device at 750kV and 60Hz power frequency, surge protection should carry up to 150mA rms continuously at its residual voltage. Several different types of commercially available discharge gaps carrying such a current have been destroyed in our laboratory within weeks.

4

Common mode noise sources are applied between a sensor ground at test tap and measuring scheme ground in the device enclosure. These sources are labeled as Vab, Vb, Vcb since the device installed on the phase B bank. Vb voltage is negligibly small, but the other two were measured as 100 – 200 mV RMS. Attempt to provide additional low impedance connections between banks did not significantly change common mode voltage. This voltage creates additional noise current (as it shown in the fig. 4) that is measured by the device. Taking 200mV rms and bushing capacitance of 500 pF additional noise current impact can be estimated as below 0.1%, which is acceptable.

SYSTEM VOLTAGE VARIATIONS System voltage behavior is one of the main contributors to method accuracy as a whole. This issue becomes very critical when the precision of 0.1% is required. A variation in system voltage (magnitude or relative phase shift between phases) creates an unbalance and may be interpreted as capacitance or power factor change. Magnitude variation may be interpreted as capacitance change and phase shift variation – as power factor. System voltage (phase) variations have been observed at all locations where the InsulGardTMG2+ is installed regardless of region or country. Figure 4 below shows the A-B and A-C phase angles3 from four units in three locations across North America for a time period of several weeks. Figures 4a and 4b are for standalone units and 4c and 4d for two identical units connected to the same HV buses. Behavior of phase angle variations is very similar in the last two cases despite the two units were not fully synchronized in taking their readings (time difference between units taking readings was within 10 minutes).

-0.4-0.3-0.2-0.1

00.10.20.30.40.50.60.7

11/13/03 11/23/03 12/3/03 12/13/03 12/23/03 1/2/04 1/12/04 1/22/04Time

Var

iatio

ns [D

eg.]

a-b a-c

Fig. 4a Unit #1, Location #1

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/23/03 12/3/03 12/13/03 12/23/03 1/2/04 1/12/04Time

Varia

tions

[Deg

.]a-b a-c

Fig. 4c Unit #3, Location #3

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/20/03 11/30/03 12/10/03 12/20/03 12/30/03 1/9/04 1/19/04 1/29/04Time

Vari

atio

ns [D

eg.]

a-b a-c

Fig. 4b Unit #2, Location #2

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/23/03 12/3/03 12/13/03 12/23/03 1/2/04 1/12/04Time

Var

iatio

ns [D

eg.]

a-b a-c

Fig. 4d Unit #4, Location #3

FIGURE 4

For changes in all phases the formulas should be changed to vector summations. Gamma will react on asymmetric changes in system voltage only. All symmetric voltage changes will compensate each other (the same increase of all voltage magnitudes, for example, will not disturb a balance). Therefore, accuracy of the method depends upon the statistics of the asymmetric voltage variations in the particular location and statistical data processing procedure.

DIAGNOSTICS The technology as originally introduced and implemented was focused on producing timely alarms and then suspected bushing should be further evaluated with additional off line tests. This part remains unchanged and Gamma-parameter is a very reliable indicator of a dangerous trend in bushing insulation system. In addition, modern microprocessor based instrumentation allows for additional diagnostics performed on line while a unit is

3 Phase angles of 1200 and 2400 subtracted from A-B and A-C angles respectively to scale data to the same origin.

5

running. On line diagnostics gives additional valuable information and therefore advantages in maintenance strategy and as a result saves money. The main goal of on line diagnostics is to locate defective bushing, determine the predominant failure mode and finally predict timely critical insulation triggering shut down and bushing replacement. The diagnostics has three parts: time trend, temperature dependencies and defect identification. Defect identification requires determining the tangent delta and capacitance of all three bushings. In the worst case of a single stand-alone unit installed on one three-phase transformer (or three single-phase transformers) five independent quantities can be obtained: three current magnitudes from the test tap and two independent phase angles between the currents. The number of variables is twelve, three of each: tangent deltas, capacitances, system voltage magnitudes, and phase angle between system voltage vectors. The situation partially improves by learning the statistical behavior of the system voltage at the particular location for a period of time and assuming that the tangent deltas and capacitances are known at the time and equal to their off line values. Based on the voltage behavior statistics we can then compensate for the change in the various quantities over time.

PRACTICAL RESULTS Four units installed in North America have been selected for this paper. This selection provides good representation of rated voltages, apparatus, and field situations. The nameplate information is shown in Table 1 and the phasor gamma graphs and gamma trend are shown in Figure 5. Top oil temperature is recorded along with other data and is also available for analysis. A 100 Ohm Pt RTD is installed on the top of the transformer radiator or cooler header next to the transformer tank. The most challenging is the installation on unit #1 that has three single-phase transformers with all three having different bushings. The transformer operates in a peaking mode and generally has either full load or no load. Two clusters are observed in the phasor graph (Figure. 5a) reflecting different load modes: left – loaded and right – unloaded. Temperature variations during the observation period are from 150C to 640C. The second unit is most “quiet”. Gamma variations are very small over the recorded temperature variations of 200C to 490C. The unit is base loaded. Gamma variations in units #3 and 4 are moderate with temperature ranging of 200C and 300C.

TABLE 1. Nameplate Data

Unit Descript.

Phase MVA / Imp. %

Rated Voltage kV

Bushing Pow. Fact.

Bushing Capac.

Bushing Manuf. /Year

Tr-r Manuf.

A 105.3/ 16.56%

512.5/13.8 0.34 470.1 GE U-type/ 1986 Canadian GE

B 96.7/ 12.93%

512.5/13.8 0.46 507.9 Bushing / 1971 Pioneer Electric

Unit #1 Three single-phase step up transformers

C 105.3/ 17.1%

512.5/13.8 0.4 543.1 Trench / 2001 Pauwels

A 0.41 377 GE U-type B 0.47 389 GE U-type

Unit #2 Three-phase step up tr-r C

100/

230/13.8

0.43 386 GE U-type

ABB

A 0.26 439 ABB O+C B 0.26 441 ABB O+C

Unit #3 Three-phase Auto-tr-r C

400/

230/115/ 12.47

0.26 439 ABB O+C

GE

A 0.26 604 Haefely COTA B 0.26 606 Haefely COTA

Unit #4 Three-phase Auto-tr-r C

400/

230/115/ 12.47

0.33 640 Haefely COTA

GE

6

FIGURE 5

None of the units showed significant gamma trend at the time.

TREND All units were base lined for a month and then used the information on system voltage variations for further diagnostics. Diagnostics results for tangent delta and capacitance trend are shown in Figure 6. Note that the scales in the graphs representing trend of capacitance are different for different units. Trends of both the capacitance and tan delta do not indicate any essential insulation deterioration in all four units. Even in the worst case of system voltage instability on units #3 and #4 the variations in tangent delta are very small of about 0.1%. It is noticeable that the parameter’s variations on units #3 and #4 are very similar, which reflects system behavior rather than a change in insulation condition. As expected, unit #2 shows the best stability in both tangent delta and capacitance with overall variations within 0.05%.

Fig.5a Unit #1

Fig. 5b Unit#2

Fig. 5c Unit #3

Fig. 5d Unit#4

7

Bushing Tan Delta Trend

0

0.1

0.2

0.3

0.4

0.5

0.6

25-Nov-03 25-Dec-03 24-Jan-04 23-Feb-04Time

Tan D

elta %

Tan A Tan B Tan C

Bushing Capacitance Trend

460

480

500

520

540

560


Capa

citan

ce pF

CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6


Tan D

elta %

Tan A Tan B Tan C


375

380

385

390


Capa

citan

ce pF

CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6

25-Nov-03 10-Dec-03 25-Dec-03 09-Jan-04 24-Jan-04Time

Tan D

elta %

Tan A Tan B Tan C


438

440

442


Capa

citan

ce pF

CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6


Tan D

elta %

Tan A Tan B Tan C


600

610

620

630

640

650


Capa

citan

ce pF

CapA CapB CapC

FIGURE 6

TEMPERATURE DEPENDENCY Another very important diagnostic characteristic is temperature dependency, primarily in tangent delta and also in capacitance. During the learning period of 30 days this characteristic is also determined. Results of analysis are shown in Table 2. The table also shows the temperature range containing statistically reliable data (10 or more data points at the same temperature). Correlation to the temperature has been detected in the unit #1 with correlation coefficient of 0.8. Temperature variation range is sufficient. Some temperature dependency has been observed in phase A tangent delta which may explain the 0.5% Gamma variation over the temperature range. Correlation in units #2 and #3 is very low with a correlation coefficient of about 0.1. In unit #4 the correlation coefficient approaches 0.3, but both the linear approximation and temperature range are very questionable.

Fig. 6b Unit #2

Fig. 6c Unit #3

Fig. 6d Unit #4

Fig. 6a Unit #1

8

TABLE 3. Parameter’s Temperature Coefficients.

Parameter Tan Delta [%/0C] Capacitance pF/0C] Phase A B C A B C

Temp. Range

Unit #1 0.008 0.000 0.001 -0.001 0.066 -0.005 24-630C Unit #2 No Correlation 31-470C Unit #3 No Correlation 3-300C Unit #4 Correlation Questionable 15-340C

CORRELATION WITH DISSOLVED GAS ANALYSIS Table 4 shows the DGA on two of the bushings from Unit # 1. The Phase A bushing is 35 years old and Phase B is around d 25 years old. The Phase C bushing is only a few years old and no DGA data exists. The DGA analysis indicates some slight overheating in the Phase A bushing and supports the change in the Gamma Parameter.

Table 4

CONCLUSIONS High voltage bushing is one of the primary failure components of large power transformers. For over thirty years, technology has been in place to monitor the insulation condition of bushings on-line but it is not wide spread in North America. Many bushing defects occur very quickly and performances of periodic off-line tests may not be the answer. Recent advances in on-line monitoring technology have improved the accuracy, reliability, and the diagnostics capability of such devices. The described device not only monitors changes in Gamma parameter and provides timely alarm signal on a defect growth, it also performs diagnostics based on bushing temperature and provides trending of the power factor and capacitance of each bushing. It is a viable system to monitor critical and important units. Factors affecting the accuracy, such as noise (harmonics), voltage and phase variations of operating systems are addressed. The achieved accuracy allows for reliable information in planning and implementing predictive maintenance strategy. Monitoring a unit load current along with monitored parameters may allow for further improvement in diagnostics accuracy. Data is presented from four transformers installed in North America.

REFERENCES Lau, M. Y. et All “On Line Monitoring Systems For Bushings: 2004 Doble International Client Conference. [1] Lau, M. Y.; “500KV Bushing Failures and Bushing Oil Sampling Program”; 2002 Doble International Client Conference. [2] Svi P. M., “Diagnostics of Insulation of High Voltage Equipment”; 2-nd edition, EnergoAtomIzdat, 1988 (In Russian); [3] Svi P. M., “Methods and Techniques of Diagnostics of High Voltage Equipment”; 2-nd edition, EnergoAtomIzdat, 1992, 240pp, (In Russian); [4] Golubev A. A., Kane C. F., Seliber A. B., Blokhintsev I. D., “On-Line Predictive Diagnostics Technologies for Power Transformers”, The Proceedings of TechCon® 2003 North America, pp.263-279, February 5-6, 2003, St Petersburg, FL.

online monitoringon bushing

Documents