causes of negative dissipation factor

Session Thirteen: Condenser Construction and Awareness of Negative Power Factor on Dielectric Dissipation Factor Measurements

Electrical Safety & Power System Protection Forum 1

Session Thirteen:

Condenser Construction and Awareness of Negative Power Factor on Dielectric Dissipation Factor (DDF)

Measurements

Wayne Proud Technical Officer, Westernpower

Condenser Construction and Awareness of Negative Power Factor on Dielectric Dissipation Factor (DDF) Measurements

Do you have contingencies to manage a transformer fire? What if you lost your million dollar asset due to an accessory? The following paper discusses the principals and construction of a bushing and traps associated with measurement inconsistencies.

Introduction

As an owner of Transmission High Voltage transformers you will want to manage your plant on condition. One of the accessories of a Transmission Power Transformers is the bushings. A failure of this accessory will lead to a loss of supply from this asset and may even result in a transformer fire which would not only destroy the Transformer, but it may lead to the destruction of neighbouring plant. As an Asset Manager you will have a need to reliably measure the capacitance and Dielectric Dissipation factor (DDF) of the bushings to monitor their condition. As quality control results needs to be accurate and reliable, inconsistencies can lead to poor evaluation and premature replacement. This paper discusses the principals of condenser construction related to testing bushings and presents a case study of the phenomenon of negative results associated with DDF measurements which could influence trending.

Design and construction

A transformer being one of the most expensive assets in the Switchyard is assigned the function of transforming voltages for the ease of distribution. All network energy must pass through this Plant and continued serviceability is crucial. As insulation failure can lead to catastrophic/terminal damage and a possible transformer fire, the condition of this insulation is a key factor in the managing the longevity of the transformer.

The bushings being a key accessory of the transformer are assigned the task of passing the load current at system voltage through the earth plane of the external casing, so connections can be made to the internal coils within the



transformer. The bushing does this by using a condenser design construction to control stresses which would be associated with the high field stresses at the entry to the casing. To show this diagrammatically a two dimensional illustration is shown below;

In the diagram, each vertical line represents a circular foil around the conductor, which is separated with cellulose insulation; this creates a capacitive string with the potential along the length of the string divided in a uniform manner. This prevents a localized field stress in the bushing near the earthed casing of the transformer.

C

A

CB

CC

CD

CJ

CENTER CONDUCTOR

C

K

CE

CF

CG

CH

C

I

Figure 1 Diagrammatic illustration of a bushing



The combined capacitance of these foils can be measured by breaking a link which is commonly fitted between the earthed flange and the last foil of bushing, this is known as C1. The capacitive current is plotted against the

resistive current (90 phase shift) to achieve a resultant angle (representing the losses at 50hz). This angle can be expressed in percentage power factor or Tangent delta. Dielectric Dissipation Factor (DDF) measurements of this kind are now common place in the transmission industry and the technology is well proven with added features such as; frequency variation to suppress interference or to offer added evaluation.

Main Insulation C1

CA = CB = CC = CD = CE = CF= CG = CH = CI = CJ

V1 = V2 = V3 = V4 = V5 = V6 = V7 = V8 = V9= V10 Tap

Electrode

CK

Center Conductor

Line-to-Ground System Voltage

Grounded Layer/Flange



Negative influence of results

Due to environment conditions and/or contamination the resistive current component of the circuit can increase by a factor of a 100 (hundred). With low capacitive items like bushings/instrument transformers this can have a significant effect on the DDF result. In certain cases (influenced by design) in combination with the contamination/humidity, current in parallel circuits can induce a negative DDF result.

Z1 Z2

I

L

E

It' It

Grounded-Guard

Bushing Flange

Current & Loss

Meter

It'

High Voltage Lead

Bushing Porcelain

LV Lead

Test Tap

Surface Leakage

RL

IL

CL It'

X

It

Bushing Top Terminal

IL



As DDF is a comparative method which is related to type of plant, a negative DDF results or very low positive result affected by this phenomenon will not represent the true losses (at 50Hz) of the insulation and must be treat as invalid data.

Some onsite conditioning of the plant may improve results, reducing the significance in the circuit of the unwanted current paths. Techniques which may improve the validity of the results are:

Cleaning of the surface

Products that reduce surface leakage

Waiting for conditions to improve (testing at a different time)

Removing parallel items from the circuit (including ladders/scaffold)

Guarding techniques (with limited success)

NOTE: A method for evaluating external currents (influencing negative results) is to conduct an Insulation resistance test with and without guard. If a considerable variance is recorded a (factor of 100) result may not be representative of the insulation.

NOTE: Using a guard band to carry out an a.c. DDF test needs to be used with caution, as it induces a similar effect to the contamination. Confirm results with previous data before acceptance.

To further understand this phenomenon a study/simulation was carried out to artificially induce a negative loss or effect.

In an attempt to simulate negative power factor, I wrapped aluminium foil around sections of the sheds to create a capacitive plate for the applied voltage to couple too. On selected tests a resistor or short was applied to the foils to induce simulated effects of contamination and environmental conditions.

The sample unit used for the test was removed from a Substation in the Eastern suburb of the Perth Metropolitan area and had suspected PD due to High H2 content in the oil.

Test conditions were: temperature 24C and humidity 52%

The unit was fitted with a DDF tap.

The lower end of the bushing was immersed in transformer oil as in service. The bushing was placed in a tank with streamlined oil to ensure DDF results were not influenced by high water in oil content. Voltage was applied by a step up transformer connected to a overhead bus system fitted with a standard capacitor for the DDF reference and voltage monitoring.



Different configurations of external electrodes (aluminum foil) were fitted to the outside off the porcelain section, with results for each configuration recorded.

NO FOIL APPLIED

Test Voltage DDF Capacitance pF

14.5kV 0.0098 277.3

29kV 0.0111 277.4

43.5kV 0.0114 277.4

58kV 0.0111 277.5



.

Bushings are a critical part of a transformer allowing voltage and current to enter/leave the earthed casing.

If a bushing failure occurs the transformer will be inoperative and unable to supply load

APPLICATION OF FIRST FOIL

DDF

0.009

0.0095

0.01

0.0105

0.011

0.0115

0.012

14.5kV 29kV 43.5kV 58kV

Test Voltage

Tan de

lta

DDF




14.5kV 0.0093 277.9

29kV 0.0111 278

43.5kV 0.0114 278

58kV 0.0111 278.1

DDF

0

0.002

0.004

0.006

0.008

0.01

0.012

14.5kV 29kV 43.5kV 58kV

Test Voltage

Tan de

lta

DDF



APPLICATION OF SECOND FOIL


14.5kV 0.0099 282.8

29kV 0.0117 282.9

43.5kV 0.0132 283.2

58kV 0.0141 283.5



DDF

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

14.5kV 29kV 43.5kV 58kV

voltage

Tan

delta

DDF

APPLICATION OF 1k OHM RESISTOR




14.5kV 0.008 262.2

29kV 0.009 262.2

43.5kV 0.0096 262.2

58kV 0.0094 262.2

to control electrical stress where they enter the earthed plane.

This is done by embedding capacitive foils in the insulation.

To test this primary insulation a DDF tap is brought out.

Connecting a measuring bridge to the DDF tap whilst applying a voltage to the conductor, capacitive and resistive losses can be measured.

DDF

0.007

0.0075

0.008

0.0085

0.009

0.0095

0.01

14.5kV 29kV 43.5kV 58kV

Test Voltage

Tan

delta

DDF



APPLICATION OF SHORT TO GROUND


14.5kV 0.0079 262.2

29kV 0.0086 262.2

43.5kV 0.0095 262.2

58kV 0.0094 262.2



DDF

0.007

0.0075

0.008

0.0085

0.009

0.0095

0.01

14.5kV 29kV 43.5kV 58kV

Test Voltage

Tan

delta

DDF

COMPARISON OF RESULTS

Above is a graph showing all the results compared from the study. The data clearly shows that different results can be obtained from applying external influences. Foils that were applied without any reference to ground (floating) did not reduce (negative influences) the value obtained from the instrument. Foils when shorted to ground, providing a current path in the instrumentation circuit shifting the results numerically downwards.

After comparing the results it was clear that applying only the foil around the bushing had only a little effect on the DDF reading and the capacitance reading. Once the foils were linked to earth via a 1k ohm resistor or short, the reading showed a reduced value for the losses.

Although a negative figure could not be induced, a reduction in the DDF figure was simulated.

comparision

0.007

0.008

0.009

0.01

0.011

0.012

0.013

0.014

0.015

14.5kV 29kV 43.5kV 58kV

Test voltage

Tan

del

ta

no foil

one piece of foil

two piece of foil

foil with 1kohm resistor

foil with short to grnd



Conclusion

It could be concluded that negative influence was the cause of the varied results in this study. This highlights that readings may not go negative but are not valid values representing the losses of the insulation at 50Hz. Negative results in the field are typically associated with coastal regions and/or high contamination through the wetter winter months when high levels of humidity are present. Technicians onsite need to consider values obtained, evaluating the validity of the data before remedial action is taken to obtain representative information of the plant. Results may look to good for a typical piece of plant and may need to be questioned. Training, skill and access to previous data onsite will aid in the Technician making an informed decision. As trending is the key to DDF evaluation of plant, ongoing repeatable results are crucial to allow the Asset Manager to trend and evaluate the longer term condition of Plant in the field.

Due to the importance of such data to the Asset Managers, Technicians should be made aware of these influences and be encouraged to question results in the field and take remedial action to limit variations associated with periodical data obtained. Outages are a costly exercise and accurate data is critical to assess the ongoing health of High Voltage electrical equipment across its design life.

References

Diagrams courtesy of Doble Engineering

causes of negative dissipation factor

Documents

key factor

condenser design construction

possible transformer

bushing session

phenomenon of negative

system voltage

testing bushings

key accessory