100 Years’s Technical Achievements of High Voltage Bushings

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The definition in the IEC standard is as which you can check on IEC web: bushings serve to insulate conductors that are carrying high-voltage current through a grounded enclosure. To accomplish this safely is a challenge, especially at higher voltages, since the dimensions of the bushing are small compared with the dimensions of the equipment it is connecting. This article introduces the centennial history of bushings from 5 aspects.

1, Introduction

2, Historical review

3, Technical challenges in brief

4, Fault preventions and conditions assessment

5, Conclusions

Aspect 1:

Bushing is a device that enables one or several conductors to pass through a partition such as a wall or a tank and insulates the conductors from it, The means of attachment (flange or fixing device) to the partition forms part of the bushing. The conductor may form an integral part of the bushing or be draw into the central tube of the bushing.

Complex requirements: Different kinds of stresses. Different environments

Consequences of malfunctions: Because of high level electrical stress, failures tend to result in sudden and violent events. This applies in particular to internal phase-earth flashovers.

Primary components of the bushing are gfollowing:

An inner capacitance graded insulation for distributing the electrical field. #HeweiPower uses RIF capacitance core, APG or lipa-covered film bushing core.

An outer insulation for minimizing creepage currents and preventing external flashover. We use composite silicon rubber sheds and porcelain sheds mostly.

A conductor system for carrying the current. Draw-lead type or fixed rod type, copper rod or aluminum rod.

A flange arrangement for fixing the bushing to the transformer and the wall.

The bushing production process is roughly divided into three steps:

Winding: different technologies are used for different concepts and sizes.

Processing and assembly: dry insulated bushing requires several more steps compared to the oil impregnated technology.

Routine testing:

*#Tanδ and capacitance at ambient temperature;

*Dry lightning impulse voltage withstand test;

*Dry power-frequency voltage withstand test;

*Measurement of partial discharge quantity;

*Tightness test.

Aspect 2:

There are five main concepts of the bushing’s history:

1910~1960: RBP-resin boded paper;

Since 1940, OIP-oil impregnated paper;

Since 1960, RIP-resin impregnated paper

Since 2010, RIS/RIF-resin impregnated synthetics/fiberglass

Since 2000, Hybrid solutions for HVDC-capacitance graded/pressurized SF6.

Today insulation technologies for AC systems

#OIPbushing: 1kV-1200kV, kraft paper, dielectric oil.

#RIPbushing: 24kV-800kV, crepe paper, epoxy resin and hardener.

#RISbushing: 52kV-550kV, synthetic non-woven, epoxy resin and hardener.

#RIFbushing: 40.5kV-252kV, fiberglass, epoxy resin and hardener.

Aspect 3：

Foundation

Section1, Scope

Section 2, Normative references.

Section 3, Terms and definitions

Section 4, Ratings

Section 5, Operating conditions

Section 6, Ordering information and markings

Section 7, Test requirements

Section 8, Type testing

Section 9, Routing tests

Section 10, Non-capacitance graded bushing ≤52kV

Section 11, Transport, storage, operation and maintenance

Section 12, Safety

Section 13, Environmental aspects

Electrical: the floating equalizer screens are locked coaxially, resulting in the optimal balance between external flashover and internal puncture strength.

Thermal parameters are including: Heat conductivity, Thermal stability, #Overloading.

Mechanical parameters are including: #Seismic/ #ShortCircuit / #LinePull / wind/ ice, Variations with temperature, Nonlinear.

Production engineering must be under strict process control. Winding process includes placement of the conductive layers and Compensation. We will test #PartialDischarge and #DielectricLoss.

HVDC

Constant DC field needs longer external insulation distances

Combined voltage electric field requires in depth analysis and long-tern testing of different operating conditions.

Non-sinusoidal current shape result in increased thermal stress.

Aspect 4 Some typical failure modes

Electrical and thermal stresses exceeding design limits.

Loss of earth connection, or high inductance to earth.

Weakness in design/manufacturing

Ingression of moisture

Mechanical stresses

And more

Besides, bushing age is not a good indicator of the status.

Certain applications have an increased risk

High switching frequencies, e.g. Shunt reactors.

Very fast transients, e.g. GIS substations

Voltage fluctuations and harmonics, e.g. windfarms

Certain bushing design had an increased risk.

In order to avoid the above faults, we need to do the following tests:

High/low voltage testing:

A power frequency withstand voltage test (factory test) provides important information about the insulation status.

Testing of tanδ and capacitance is the most used method. Has limitations in early detection. The most reliable results is testing is done at operating temperature.

Dissolved gas in oil analysis (DGA):

DGA may give early indications of certain type of problems not detectable by other methods except DFR.

On the other hand, detection of moisture in the insulation is very uncertain.

Oil sampling and analysis should be carried out only after evaluating risks versus benefits.

Typical concentration levels exist in IEC, but no alarm concentration levels.

Dielectric frequency response analysis (DFR):

1.The low frequency spectrum offers a better validation of the insulation status compared to LV testing.

2.The high frequency spectrum offers a better validation of possible voltage potential issues, comparted to a LV test.

3.Interpretation of the results require experience.

4.IEEE guide.

On-line monitoring

1.Usually monitors the dissipation factor and capacitance at 50/60Hz.

2.The main advantage is that no outages are required, and the short detection time interval.

3.Measurements are carried out at operating temperature which provides more reliable data for certain type or detects.

4.Most monitoring systems connect to the main insulation via the test/voltage tap.

Aspect 5

Transmission voltage as well as the transmitted power has seen a tremendous increased over the past 100 years, where bushings are indispensable components.

Development in materials, calculation methodologies, and manufacturing processes made this possible. The improvements in product reliability are made possible through qualification of designs and products processes from standardized testing with a broad regional or international acceptance, as well as from knowledge sharing in the industry.

Future challenges are in the transition to dry technology for all voltage levels and applications, as well as new phenomenon related to applications and stress patterns from grids following the transition in the way electricity in generated and consumed.

The business of bushing insulator is our passion, as we strive for a comprehensive understanding of bushing and the production of top quality. Hewei Power warmly welcome inquiries and inspection!

#TransformerBushing #TransformerInsulator #BushingInsulator #WallBushing #PorcelainBushing #CondenserBushing #bushings #IECbushing #EpoxyResinBushing #RIFbushing #RISbushing