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CHK GridSense PTY Ltd. Unit 3 20-36 Nancarrow Ave. Tel: +61 2 8878-7700 Meadowbank, NSW 2114, Australia   Fax: +61 2 8878-7788

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Bushing Monitoring Guide

Facts and Projections on Transformers and Bushings

Transformers are a key capital intensive element in the Transmission and Distribution network.

The Installed base of transformers is aging.

The bushing is one of the leading causes of catastrophic failures in power transformers.

Time-based testing is insufficient and inadequate due to the increasing duration between off-line tests.

On-line trending with data collected at a high frequency is the best method to prevent catastrophic failure.

Successful predictive maintenance and smart grid asset management programs require smart IEDs.

Based on investigations of the installed base of transformers, industry experts project:

Increasing numbers of failures of the aging assets over the next two decades

Increasing periods between time-based testing due to cost pressures

Scheduled maintenance cycles of large power transformers moving from 1 to 2 years to 7 years and, in some instances, to 12 years

Problems resulting from outages required for standard transformer maintenance

Exposed transformer accessories like bushings are more vulnerable than ruggedized transformers and will have a higher incidence of failures.

Figure 1. BushingIQ sensor in a transformer bushing tap

   

  

Bushing Monitoring Guide 2  

Outwitting the Projections

Continuous bushing monitoring addresses all of the concerns raised by the industry projections, allowing testing:

Without an outage

At rated voltage

In true operating conditions

With trending based on data acquired every 5 minutes instead of every year or more

With diagnostics based on statistical analysis providing bushing condition the condition

With immediate communications of alarms and warnings 24/7.

The Value of On-line Bushing Monitoring

Until recently, bushing testing has been limited to a capacitance and power factor measurement off-line, during an outage, and on a day with moderate temperature and low humidity. (The IEEE and IEC define bushing test standards for off-line bushing testing [IEEE C57.19.00-1997 & IEC 60137/2003-08]). Off-line testing under such static conditions is not likely to detect incipient faults evident under operational conditions. In addition, several factors can influence off-line measurement, including voltage fluctuations, sediments on the bushings, moisture ingress, oil level, and damaged porcelain.

The obvious alternative is continuous monitoring of high voltage bushings, which the “IEEE Draft Guide for Application of Monitoring Liquid-Immersed Transformers and Components” recommends. Such a condition based, on-line monitoring system provides confidence in bushing insulation quality between off-line tests and helps pre-empt threats to valuable transformer assets.

The Methodology of On-line Bushing Monitoring

According to an Electric Power Research Institute (EPRI) study on accelerated transformer aging, relative power factor tests provided the earliest indication of incipient faults that could lead to failure. The voltage measurement technique used to derive relative power factor is far more sensitive than the technique used by the standard current sum method.

 

   

  

Bushing Monitoring Guide 3  

A bushing can be represented by several small capacitance values in series and in parallel (Fig. 2). As a fault occurs, some of these very small capacitors become resistive. The leakage current from deep inside the bushing is miniscule when measured to ground. However, a capacitive voltage divider can sense and measure the voltage drop due to these resistive components.

The relative power factor technique uses a Schering bridge method to measure the voltage from the bushing C1 test tap (Fig. 3). The voltage divider is used to create a low voltage output for the measuring circuit, which is a ratio of the power system operating voltage, the C1 capacitance of the bushing, and an appropriate value of capacitance in the bushing tap coupler.

The critical values that on-line monitoring provide include:

Test tap voltage - relationship to the power system operating voltage

Relative power factor - measured on each bushing monitored

Power factor - calculated in relation to each bushing monitored

Condition value - the result of a statistical analysis technique that indicates the condition of the bushing.

Testing Protocols

Power factor is the only test that provides a definitive clue to the integrity and health of bushing insulation. As mentioned, off-line power factor measurement is usually performed on a moderate day with low humidity. The bushing, however, operates under all environmental and loading conditions. These conditions impact the power factor readings and should be included when testing.

On-line monitoring systems provide the most accurate and the earliest indication of bushing condition and insulation integrity because they operate 24/7 and acquire data under all weather

Figure 2. Bushing represented by small capacitance values in series and parallel

Figure 3. Illustration of Schering bridge technique

   

  

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and environmental conditions. On-line power factor values can also be used to verify off-line test data.

Condition value is a proprietary statistical analysis algorithm used to calculate individual bushing condition values.

The statistical tests that create the condition value compare characteristics of the acquired data for each sample with that of a reference window, which is created for each bushing at the start of the installation. Every day, the data from every bushing monitored undergoes a 4-step statistical analysis. That data is then compared to similar bushings under test on the same or nearby transformers. The result determines the reference window.

Power factor/ tan statistical tests are performed specific to each bushing and its reference window. The process can be summarized as follows:

The data is selected for a specific bushing pair from the reference window

The data for the latest acquisition of the specific bushing pair is acquired

The values from both the reference window and the latest data is added to an array

Power factor/tan statistical tests are performed on the data in the array.

The 4 statistical analysis tools used in the 4-step statistical analysis to generate the condition value are:

Recent Changes (RC) test - While other tests look at several data records of the reference window, the RC test looks at the last third of these records and applies a Mann-Whitney and a Kendall Slope test to measure recent changes in the data.

Swing Indicator test - The standard deviation of the recent data is compared with the standard deviation of the reference window to predict trends in the magnitude of the variations in the data.

Seasonal Slope Estimator (Kendall Slope) - Essentially a non-parametric test for randomness against trend. In addition to identifying trends in the data, it is also desirable to estimate the magnitude of such trends. We have chosen to express this magnitude as a slope (but this does not imply that the trend takes the form of a linear function in the process).

Mann-Whitney Test - Strictly speaking, this is a test of the equality of two distributions. This is the most important test of the four and should give a clear indication of a relative shift in the data. For this test, the array containing all of the data is sorted (current data and reference data is mixed) and the sorted array is then tested for distribution.

 

   

  

Bushing Monitoring Guide 5  

Typical Monitoring Screen

Data Analysis

Naturally, every test has a weighting factor, based on the importance of the test, which is a part of the algorithm for calculating the condition values. Condition thresholds calculated for every unit range from 0 (good) to 100 (bad). BushingIQ uses a simple “green, yellow, red” code to indicate the condition of each

bushing under test (Fig. 4 below).

Figure 4. BushingIQ monitoring screen with bar graph provides an instant view of bushing status with clickable access to condition value

 

Analyzing the Data with On-line Graphs

Relative condition values show changes in pairs of data samples based on the "butterfly phenomena." Fig. 5 below provides an example: If the data from bushing A referenced to bushing B (red line and green line) show a power factor trend, and the data from bushing B referencing bushing C (yellow and blue lines) also show a power factor trend (the mirror image of the first trend), then the unit causing these trends must be unit B. Bushing B is being affected by high temperature.

The graph below illustrates actual data taken on six high voltage transformer bushings over a one year period. During the summer months, bushing B’s power factor value drifts 3% higher than in cooler months.

   

  

Bushing Monitoring Guide 6  

Figure 5. Graph of actual data taken on six high voltage transformer bushings over a one year period showing a 3% drift in Bushing B’s (red and green lines) power factor due to high ambient temperature

Communications

These days, anywhere/anytime access to information is essential. Similarly, network communications are essential to the smart grid efficiencies that will help utilities meet 21st century demands on 20th century manpower and budgets. To that end, BushingIQ supports multiple wired or wireless communications protocols, and can interrace with SCADA systems. The system also interfaces with other IED’s like temperature sensors and dissolved gas analysis monitors. When combined with GridSense TransformerIQ, it provides a complete transformer monitoring system.