Bushings Online Monitoring

Dynamic Ratings, Inc www.dynamicratings.com

Electrical Diagnostic Innovations www.elec-di.com

7308 Aspen Lane N #137 +1 612-963-0974

Brooklyn Park, MN 55428

ON-LINE BUSHING MONITORING AND COMPARISON TO OFF-LINE TESTING

INTRODUCTION

To better understand the aspects of on-line bushing monitoring, review of typical construction of a bushing, power factor and capacitance, and typical off line testing theory would be prudent. This will allow comparisons to be made between the methods and technology.

CONDENSER BUSHING CONTSTRUCTION

The key components of a condenser bushing are shown in Figure 1.

Figure 1

Capacitance LayersC1 C2 C3 C4 C5

IfC1=C2=C3=C4=C5

ThenV1=V2=V3=V4=V5

Foil/Conductive Ink PaperFilled with Oil

Tap

Center Conductor

2 Experts in Diagnostic Monitoring Technologies

The purpose of the capacitive layers allows the energized central conductor to penetrate the ground plane. By having equal capacitances, the voltage is stressed equally across each layer.

There are two major capacitances in a bushing, C1 and C2 as shown in Figure 2. The C1 capacitance is the capacitance between the center conductor and the tap. The tap is usually connected to the last foil and in some cases, to the second to last foil. The C2 capacitance is the capacitance from the tap to ground. Typically, the tap is grounded, therefore, the C2 capacitance is not in the circuit during normal operation.

Figure 2

BetweenLast Foil&Flange

Test/Capacitance/ Potential/VoltageC1

C2


WHAT IS A CAPACITOR

A capacitor is two conductive plates that are separated with a dielectric. In the case of a bushing, the conductive plates are the foil and the dielectric is the paper and the oil.

All dielectric materials have some sort of loses except for a perfect vacuum. Comparisons of dielectric constants are as follows:

Material Dielectric Constant Vacuum 1.0

Air 1.00549 Paper 2.0

Oil 2.2 Porcelain 7

Water 20

The higher the dielectric constant, the higher the losses of the dielectric material.

DIELETRIC LOSSES

Dielectric losses have a unit of watts. Heat is generated due to these losses. Losses are created by the following causes:

• Natural resistance of the material • Type of the material • Polar molecules, such as moisture • Ionization of gases (Partial Discharge)

Dielectric Conductive Plates


Losses will vary by the amount of dielectric materials. Since bushings are not the same size and composition, comparison of watt losses between different manufacturers, sizes, etc., would be difficult. Therefore, power factor is used to assess the condition of the insulation system. As losses increase due to any of the above causes, the power factor will also increase.

CAPACITANCES

In bushings, there are a several capacitors in series. The total capacitance is the sum of the inverse of each capacitance as shown in formula below.

𝐶 𝑇𝑜𝑡𝑎𝑙 = 1/𝐶𝑡 = 1𝐶1

+1𝐶2

+1𝐶3

+1𝐶4

+1𝐶5

+1𝐶6

+1𝐶7

+1𝐶8

+1𝐶9

When a capacitor layer shorts out, the value of the capacitance will always increase. For example, if one has three capacitors in series that are each 3 pF, the total capacitance will be equal to 1 pF.

1/3 + 1/3 + 1/3 = 1/1 = 1 If one capacitor is shorted out, the sum is 1.51 pF.

1/3 + 1/3 = 1/.66 = 1.51

The capacitors in series are a voltage divider. If a capacitor shorts out, the voltage at the tap will increase in proportion. Also, as the voltage varies, the leakage current from the top will vary. Therefore, if the voltage increases, there will be an increase leakage current.

C1 C9C8C7C6C5C4C3C2

Tap

Flange


POWER FACTOR

Power Factor (PF) is the phase angle relationship between the applied voltage across a capacitance and the total current through the capacitance.

Power = Voltage (E) x Current (It) x Cosine(Θ).

Watts = E x Ir

Watts = E x It x Cosine(Θ)

PF = Cosine(Θ) = 𝑊𝑎𝑡𝑡𝑠𝐸 𝑥 𝐼𝑡

= 𝐸 𝑥 𝐼𝑟𝐸 𝑥 𝐼𝑡

= 𝐼𝑟𝐼𝑡

if Θ decreases, more resistive current will flow through the insulation and the power factor will increase. The table below shows the angle Θ and the calculated %PF. A Θ of 0.2 degrees gives a %PF of 0.349, which is a common value (0.25 to 0.50) for a power factor of a bushing.

Θ %PF (%COSΘ) 90 0

89.8 0.349 89.5 1.745 88 3.490 87 5.23 86 8.710

TYPICAL OFF-LINE TEST SET

The basic test circuit is shown below. A high voltage is applied to the test object and the leakage current is measured.

TestObject

EIrIc

It


The test circuit to measure the C1 capacitance of a bushing is as follows:

The high voltage is connected to main conductor of the bushing and the return lead is connected to the tap. The electronics in the test set can accurately measure leakage current, the applied voltage, frequency and the phase angle between the applied voltage, and the leakage current. From this data, the watts loss and PF can be easily calculated as discussed earlier in this document. Typically 10kV is applied to the main conductor.

Other connections of the test leads can be made to test the C2 capacitance and other characteristics, but C1 is only discussed since it is the only capacitance that can be monitored on-line.

ON LINE BUSHING MONITORING

THEORY

By far, the most common method to monitor bushings is the sum of current method. The following figure shows a basic block diagram of a bushing monitoring system that uses the sum of currents method. During commissioning, the null-meter is balanced to zero. The purpose of the balancing circuit is to take into account the differences in system voltages and phase fluctuations and bushing characteristics. As a defect develops, the complex conductivity of the bushing insulation changes, and the current and its phase angle in one of the 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 is experiencing the change.


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

2

0

2

0

)tan(

∆+∆≈

∆=∑ C

CIII δ

where: ∑ I - Parameter Sum of Currents,

δtan∆ - Tangent delta change,

0CC∆ - Relative change in bushing capacitance.

0C - Initial Capacitance Reading

0I - Initial Sum of Current Value

Ideally, the sum of the three bushing currents should be zero. In reality, not all parameters are equal from each phase. Therefore, during commissioning, the system is placed in a balancing mode and the system self-adjusts and attempts to set the null meter to zero.

c1

c2

c1

c2

c1

c2

BalancingUnit

SummationUnit Null Meter

A

B

C


The above diagram explains the method in vector format. Fig a. shows all three currents from the bushing test taps perfectly balanced and the sum equal to zero. If there is a change in tangent delta in the phase-A bushing, an additional active current will pass through the A-phase bushing insulation and

the new current 'AI , thus throwing the system out of balance. The consequent imbalance vector is equal

to the tangent delta change and directed along the phase-A voltage vector Fig b. A change in capacitance is shown in Fig c. This additional current is perpendicular to the A-phase voltage. The

consequent imbalance is now positioned along the vector 0AI .

The magnitude of the change is an indicator of the problem’s severity, and the vector change indicates which bushing is going bad and whether the power factor or capacitance is changing. The diagrams below shows a recent example of a bushing going bad.

b.

'∑I

0AI

0CI

0BI

'AI

AV

'AI∆

a.

0=∑I0AI

0BI

0CI c.

"∑I0

AI

0CI

0BI

"AI

AV

"AI∆

A Phase Power Factor

B Phase Power Factor


CALCULATION OF POWER FACTOR AND CAPACITANCE

For decades, the only method to determine the quality of the bushing insulation was to perform off-line tests and compare the measured power factor and capacitance to nameplate values or previous tests. Therefore, this is what maintenance personnel are used to looking at. When performing on-line monitoring, the key diagnostic factor is the sum of currents and the phase angle of the sum. Only estimates of the power factor and capacitance can be made since all the data required to calculate the absolute PF and capacitance is not available, as it is for off line tests.

For this reason, on-line bushing monitoring provides relative calculation of PF and capacitance. When the system goes out of balance, estimates are made on the change of PF and/or capacitance. These values are then added/subtracted to baseline values (nameplate or recent test values) entered into the system. For example, if the baseline PF is 0.35% and the algorithms change calculation show the PF increased by 0.50% the reported PF will be 0.35 + 0.50 = 0.85%.

It must also be noted the sum of currents value is not a calculated value, but is a measured value.

As stated earlier, when performing off line tests all quantities can be measured. The following table compares the available data inputs for off-line testing to the data available for on-line monitoring.

Parameter Off Line Testing On-Line Monitoring Applied Voltage X Leakage Current X X Phase angle between Voltage and Leakage current

X

Frequency X X

Let's consider a system of three bushings. Total variables required are 4 x 3 = 12. If one had a three phase off-line test set, all the data that is required is available.

For on-line monitoring the following conditions apply:

• The line voltage at the bushing terminals is assumed to be constant on all three phases. • The phase angles between the phase voltages are constant. • In additional to the leakage current, the phase angles between phase A and B and A and

C are also measured and frequency is measured.

In actual applications, the voltage is not constant and the phase angles between phases are not always exactly 1200. If all changes remained symmetrical, then the system would stay in balance and the PF and


capacitance calculations would be fairly accurate. Since this is not the case, small variations in the sum of currents will occur.

As a bushing deteriorates, the variations have less of an impact on the calculations. As can be seen in the chart below of the sum of currents, at lower values the variations are a lot larger than those when the sum of currents is higher. In fact, as the sum of current increases the accuracy of the PF and capacitance calculations will improve.

As stated earlier, a change in voltage at the tap (because the amplitude of the leakage current will change) will indicate a change in capacitance and a change in phase angle will indicate a change in PF.

The actual calculation of PF and capacitance is quite complex with extensive averaging, filtering and proprietary algorithms. Again, the accuracy of these calculations are based on the limited information available to make the required calculations.

OTHER ITEMS TO CONSIDER WHEN COMPARING OFF-LINE TO ON-LINE VALUES

Many defects are temperature and voltage dependent. When testing off-line, the bushing is at ambient conditions and only 10 kV is applied. With on-line monitoring, higher voltages are applied and elevated temperatures are present. As can be seen, there is significant temperature correlation.

The chart below shows the affects of top oil temperature (red) to that of the sum of currents (blue). Top oil temperature is used as a relative temperature measurement for the temperature of the bushing. Approximately 60% of the bushing temperature comes from the oil.


The following chart shows the PF calculations for the above chart. Diagnostics show the B phase bushing is deteriorating. The following charts shows the calculated PF for all three bushing. One can observe the changing values of the B Phase PF.

Changes in B Phase


OFF-LINE TESTS

The following are the offline tests of the three high voltage bushings discussed above. As can be seen, the offline power factor test on B Phase is out of specification.

There was also significant partial discharge occurring in the B phase bushing.


ADDITIONAL INFORMATION

The following chart show both the temperature and voltage dependency of a bushing.

SUMMARY

This paper reviewed the concepts of capacitance, dielectrics and off line bushing testing theory. Comparisons of offline testing to on-line monitoring were made and why absolute calculations of the power factor and capacitances cannot be made with traditional bushing monitoring systems.

When performing on-line monitoring of bushings, the sum of currents and the phase angle are the key diagnostics parameters. A paradigm shift must be made from focusing on the capacitance and power factor readings to that of the sum of currents.

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