CONCERNS WITH MEDIUM VOLTAGE ELECTRICAL EQUIPMENT

CONCERNS WITH MEDIUM VOLTAGE

ELECTRICAL EQUIPMENT FACILITATED BY

M.M. (Thys) BOTHA

FROM

MMB ELECTRICAL MACHINES CONSULTANT

TEL.: +27 (0)11 8498878 MOBILE: +27 (0)76 956 9063

FAX.: +27 (0)86 502 0823 Email: [email protected]

WHAT WE EXPERIENCE

1. MECHANICAL FAILURE OF COMPONENTS LIKE BEARINGS

2. ELECTRICAL INSULATION FAILURES

3. FAILURES AS A RESULT OF EQUIPMENT EXTERNAL FACTORS

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BEARING FAILURE

By far the most frequent cause of failure on electrical rotating machines,

but in most cases the result of mechanical, electrical and other external factors.

One common electrical thread found in electrical machine and transformer failure

is the occurrence of earth fault conditions. The earth fault is the most common

electrical fault seen by the user and that applies to all electrical equipment.

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1. MISALIGNMENT

MECHANICAL FAILURE OF COMPONENTS LIKE BEARINGS

MAIN CAUSES:

MISALIGNMENT –

ONLY RESPONSIBLE FOR 70% OF ALL FAILURES

ON ANYTHING THAT ROTATE.

Misalignment can easily increase the

load on a bearing by a factor of three

and in the case of anti-friction bearings

will reduce the bearing life by a factor of

27 !!!!!

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BEARING FAILURE (Cont.)

OVERLOADING DUE TO MISALIGNMENT WORN COUPLING HALVES

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BEARING FAILURE (Cont.)

CLEARANCE AND LUBRICATION MAGNETISED BEARING AND SHAFT COMPONENTS

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SHAFT CURRENT

What gauss levels are acceptable?

26 Gauss

40 Gauss

Proposal is 3 Gauss

Bearing shell and housing need

To be demagnetised

WAS THIS CAUSED BY MAGNETIC

BASE DIAL GAUGE? = Yes.

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FOUR TYPES OF CURRENT CAN FLOW IN THIS INVERTER FED MOTOR

CABLE

ROTOR

STATOR

INV

ER

TE

R

COUPLING

LOAD

STATOR TO ROTOR COUPLING CURRENT

ROTOR TO SHAFT CURRENT

STATOR WINDING TO FRAME SHAFT CURRENT

STATOR WINDING TO GROUND CURRENT

EARTH PATHS WILL HAVE DIFFERENT IMPEDANCES TO DIFFERENT CURRENTS

COMMON MODE

CURRENT

= C . dV/dt

Where C is the

Circuit element

Capacitance to earth

CURE:- USE CORRECT EARTHING METHOD FOR HIGH FREQUENCY.

IN ADDITION TO THIS, YOU MAY HAVE ALL

THE PROBLEMS ASSOCIATED WITH A NORMAL

3 PH. SUPPLY!

VSD INDUCED SHAFT CURRENT

Shaft currents induced by VFDs can lead to motor failures.

Without some form of mitigation, shaft currents travel to ground

through bearings, causing pitting, fusion craters, fluting, excessive

bearing noise, eventual bearing failure, and subsequent motor failure.

NORMAL APPEARANCE

OF THE BALL IN A

HEALTHY BEARING

DULL APPEARANCE OF A BALL

SUBJECTED TO SHAFT CURRENT

CAUSED BY VSD AND RUNNING

AT VARIABLE SPEED

FLUTING AS RESULT OF

RUNNING AT CONSTANT

SPEED FOR RELATIVE

LONG PERIODS

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HSCB

ARM.

MOTOR

CYCLO-

CONVERTER

THYRISTOR DRIVES

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R

C

R

C

SNUBBER CIRCUIT

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DEFECTIVE SNUBBER CIRCUIT EFFECT

(IN EXCESS OF 400Volt PULSES WERE MEASURED ON THE SHAFT)

Sin wave RMS value = 6600 Volt.

Spectrum of voltage

Real Time Wave Healthy Snubber40

0 V

olt

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2. COOLING FAILURES

MECHANICAL MAIN CAUSES OF FAILURE:

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3. INCORRECT ASSEMBLY

ROTOR

STATOR

EXCENTRIC AIR GAP

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4. SUBSTANDARD DESIGN ROTOR TOOTH

PRESSURE FINGER

NOT ENOUGH MATERIAL

Broken rotor bars

Bursting end rings

Blown-up slip rings

Brush lifting gear

malfunctioning

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64MW GENERATOR COUPLING FAILURE

2. ELECTRICAL FAILURES

Electrical failures make up 20% of all failures experienced on electrical machines and

transformers. (Thys Botha Experience)

The most common fault experienced is the earth fault.

In the majority of the cases the earth fault was caused by an inter-turn insulation

failure, especially in the case of transformers and low voltage rotating machines.

Medium voltage machines and distribution transformers suffers from partial discharge

fatigue where 10 out of 100 rotating machine failures are a direct result of PD and

30 out of 100 transformers die of the same cause. In the case of power cables it is

almost 80% of all joints and terminations, mainly due to poor workmanship.

The user expects to get at least 20 years service out of large machines and transformers,

but a very large percentage of them fail before they reach the age of 10 years. WHY?

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YEARS IN SERVICE

NU

MB

ER

OF

FA

ILU

RE

S

GE STATISTICS ON MV ROTATING MACHINE FAILURES

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54 OUT OF 223 FAILED BEFORE 10 YEARS WERE OVER! THAT IS NEARLY 25% OF THE SAMPLE

CIGRE study committee SC11, EG11.02

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Rudolf Bruetsch, Makoto Tari of Von Roll Isola Klaus Froehlich, Tilman Weiers1) and Ruben Vogelsang2

High Voltage Laboratory , Swiss Federal Institute of Technology Zurich

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Rudolf Bruetsch, Makoto Tari of Von Roll Isola Klaus Froehlich, Tilman Weiers1) and Ruben Vogelsang2


New hydro bar with delaminations in the mica insulation

37 Years in service bar showing delaminations and voids

40 Years in service showing degraded binder resin

We found that treeing is accelerated by defects in the

mica insulation such as delaminations, cracks, voids and

wrinkled or damaged mica layers.

Such defects are either manufacturing failures or they

result from ageing.

Reference:Insulation Failure Mechanisms of Power Generators

Rudolf Bruetsch, Makoto Tari of Von Roll Isola

Klaus Froehlich, Tilman Weiers1) and Ruben Vogelsang2


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What happened here?

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Don’t we see the same phenomena on the

stator core end laminations of some large

turbine generators as a result of over-excitation?

RE-SWITCHING DAMAGE

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Worst case

Torque can be 15 p.u

And current 5 – 7 p.u.

When applying plugging

Torque can be 15 p.u

and current 20 p.u.

M.G.Say:

Alternating Current Machines,

5th. Ed. p 339.

Sparking during a rotor test.

(Photograph courtesy of Notified Body).

Rotor bars can be swaged for a tight fit in the slot.


WHY NOT USE FITTED BARS ?

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1. Stator sparking can occur at any time during motor operation. The risk is

increased by transients from the network, surface contamination and ageing.

Partial discharge causes sparking due to high potential differences between

stator coils and their surroundings.

2. Rotor sparking results from the intermittent breaking of the contact between

the rotor bars and core. It occurs during starting only, and is limited to the first

sections of the rotor core.

Electrical discharges on the surface of a

cable connecting the terminal and

winding, at the point where it passes

through the frame. (Photograph courtesy

of test laboratory).

Rotor bars can be swaged for a tight fit in the slot.


11kV Cable ?

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BROKEN ROTOR BAR

Line current spectrum analysis done on a healthy rotor (a) and on a rotor with one broken bar (b)

CURRENT SPECTRUM ANALYSIS

I AM 26 YEARS IN SERVICE AS A DISTRIBUTION TRANSFORMER

I never had an oil change or oil filter job done on me, my oil has no problem with acidity, moisture or sludge.

My boss keeps me

Busy for 24/7 and

make sure my fever

never get over 75OC

My bushings got

a bit too much

suntan and needs

to be replaced.

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PARTIAL DISCHARGE DAMAGE.

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THE RESULT OF PARTIAL DISCHARGE

DO NOT CONFUSE THIS WITH THE STATIC DEPOSITS OF

ENVIRONMENTAL DUST SETTLING ON THE LINE COILS

WHITE GREYISH DEPOSITS

WITH AN ACIDIC TASTE

PD EFFECT AS A RESULT OF

INCORRECT STRESS GRADING

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SO WHERE DOES THE MOISTURE, ACIDITY AND SLUDGE COME FROM?

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56

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PD IS THE MAIN PROBLEM! H2 + O = H2O

ROTATING MACHINES AND TRANSFORMER INSULATION HAS THE SAME PROBLEM!!!

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OTHER CONCERNS 1. WHY DO WE DO AN INSULATION RESISTANCE TEST AS IR INSTRUMENTS CANNOT

DETECT CRACKS IN INSULATION, ARE VERY SENSITIVE TO INSULATION TEMPERATURE

AND THERE IS NO INTERNATIONAL STANDARD FOR IT?

2. WHY DO WE MEASURE THE DC RESISTANCE OF AC MACHINE AND TRANSFORMER

WINDINGS?

3. HAS ANYBODY EVER DONE A DIELECTRIC FREQUENCY RESPONSE ON ROTATING

MACHINES?

4. HOW CAN WE EVALUATE THE QUALITY OF THE TURN INSULATION TO MINIMISE EARTH

FAULTS?

WHY DID THE WINDING RESISTANCE

TEST MISSED THIS?

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M.M. (Thys) BOTHA

FROM

MMB ELECTRICAL MACHINES CONSULTANT

TEL.: +27 (0)11 8498878 MOBILE: +27 (0)76 956 9063

FAX.: +27 (0)86 502 0823 Email: [email protected]

Documents

CONCERNS WITH MEDIUM VOLTAGE ELECTRICAL EQUIPMENT