inter-turn rotor defect

Rotor Tester

Model Inuo-Rotor

• This Rotor tester detects shorted

turns in rotor windings based on the

recurring pulse phenomenon and

uses the RSO testing technique.

• Suitable for generators with

capacity greater than 100 MW

• In-built oscilloscope

• Determines inter-turn insulation

effectiveness

• Analyses all types of faults

• Locates position of shorted

windings

• Provides trending analysis of

insulation faults

Features

1. Determines insulation effectiveness

Tests and analyzes insulation effectiveness of symmetric generator rotor

windings through analysis of wave characteristics, using the recurring pulse

phenomenon. This device uses high-frequency low-voltage pulses which is

non-destructive to the rotor windings under test.

2. Analysis of faults

Analyzes qualitatively the inter-turn faults, distinguishing between metal

and non-metal short-circuits. Locates position of windings with insulation

fault. Provides trending of insulation faults.

Address:

5 Upper Aljunied Link, #02-07 Quartz Industrial Building, Singapore 367903

Phone: +65 6276 6733 Fax: +65 6276 6793

InuoSys Solutions Pte Ltd

E-mail: [email protected]

3. Testing Scope

Suitable for testing generators of capacity greater than 100 MW.

4. Test Equipment characteristics

This tester comes with an in-built Repetitive Surge Oscilloscope. It has a

large storage capacity as well as an USB output port. This is a user-friendly

device that works with both Windows 2000 and Windows XP. The device

casing is made of high strength incombustible plastic.

Specifications

Address:

5 Upper Aljunied Link,

#02-07 Quartz Industrial Building, Singapore 367903

Phone: +65 6276 6733 Fax: +65 6276 6793



Power Input 250W, 100V~240V single-phase AC,

95V~250V DC

Working Temperature -10°°°°C - +55°°°°C

Storage Temperature -40°°°°C - +65°°°°C

Working Humidity 5% - 95%

Impact Survivality 1.5 G in any direction

Vibration Resistivity 1.5 G @ 10 – 100 Hz in any direction

MTBF 50,000 hours

Operating Altitude Sea Level ≤≤≤≤ 4,000 m

Equipment TMFZ-1 device, a set of test wires, ABS high

strength plastic casing

Dimensions 400××××291××××213 mm3

Weight 10 Kg

Principle of Operation

1. Cylindrical Rotor Structure of Turbine Generator

The cylindrical rotor of a turbine generator has axial slots on the rotor body for

embedding rotor windings that are held in place by wedges. Figure 1 shows the

cross-sectional view of one of the slots. Each slot contains 6 to 8 turns of field

windings. These field conductors are insulated from the rotor body by the slot

insulation, from the slot wedge by the wedge insulation and from each other by

the inter-turn insulation.

Fig. 1 Rotor Insulation Structure

To prevent vibrations when the rotor is rotating at high speeds, the

cross-sectional placement of the embedded windings, main insulation and

inter-turn insulation must be magnetically balanced. Figure 2 shows an

expanded view of the rotor windings, which is strictly symmetrical from an

electrical circuitry viewpoint.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



Fig. 2 Rotor Windings Diagram

Based on circuit analysis theory, the rotor windings of a generator can be

modeled just like a full loopback circuit, that is, equivalent to the high frequency

response circuit shown in Figure 3.

Fig. 3 High Frequency response equivalent circuit for rotor windings

2. Conventional Testing Methods

Many conventional test methods are available today for detection of shorted

turns in rotor windings, such as: DC winding resistance measurement,

Generator no-load test, short-circuit characteristics curves, AC impedance and

power loss tests, flux probe analysis etc.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



(a) DC Winding Resistance

According to DL/T596-1996, during each overhaul, the rotor DC winding

resistance should be measured off-line, and these readings are then compared

to a baseline set of data. After adjustments to the readings to compensate for

temperature differences, the variation of the readings from the baseline should

not exceed 2%. In the DC Winding resistance test, as the rotor is off-line, the

inductance and capacitance of the windings have no effect and the equivalent

circuit is as shown in Figure 4.

Fig. 4 Equivalent Circuit in DC Winding Resistance Test

The rotor of a generator usually has about 6 to 8 slots per pole, each slot

containing 6 to 8 turns, giving a total of about 110 turns and the DC winding

resistance is in the mΩΩΩΩ range. The structure of the rotor is such that there is a

very low probability of inter-turn insulation breakdown and short circuit due to

metallic contamination between remotely positioned turns. Insulation

breakdown usually occurs between adjacent turns and when this happens, the

total DC winding resistance remains unchanged. Even if a short circuit due to

metallic contamination does occur between adjacent turns, the change in total

resistance is less than 1%. On top of this, if a different test instrument is used

each time for measurement, there is no basis for comparison. Hence, the

sensitivity of the DC winding resistance test in detecting inter-turn faults is very

low.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



(b) Impedance and Power Loss

This method is commonly used to detect inter-turn short circuits in a rotor

(sttaic status). Normally, the test power source used is at 50 Hz, but at times, a

100 Hz power source is used because of the need to reduce power supply.

When using a 50 Hz power source for measuring the rotor impedance, the

winding inter-turn capacitance Czj1 ~ Czjn and the winding capacitance to ground

Cj_1 ~ Cj_n are viewed as open circuit, while the values of the inter-turn

insulation resistance Rzj1 ~ Rzjn and the winding resistance to ground are

considered as infinity, resulting in the equivalent circuit shown in Figure 5.

Fig. 5 Equivalent Circuit in AC Impedance Test

This test is conducted for several conditions: before the rotor is moved out,

when the rotor is out, when the rotor is moved in, at different rotating speeds of

the rotor. The measurements taken are voltage, current and power loss while

the impedance, usually in the range of 4~8 ΩΩΩΩ, is calculated from the relevant

measured values. From analysis of the impedance values, under the effect of a

50 Hz supply, the rotor is purely inductive, so any change in the inter-turn

insulation resistance would not cause changes to inductance. A change in

inductance will only occur if there is short circuit caused by metallic

contamination and this change should be less than 1%. Upon conversion to

impedance value, the change should also be less than 1%. The power loss is due

to the resistance of the conductors, that is copper loss, reflecting the change in

resistance. This change in resistance thus measured is not as accurate as when

measuring the DC resistance directly.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



The accuracy of the impedance and power loss measurements are affected by

many factors, for example, rotor position, air-gap between rotor and stator,

rotating speed of rotor, short-circuit resistance and fault location, level of test

voltage, slot wedge and retaining ring structure etc. Hence it is difficult to

stipulate a standard for measurements, analysis is limited to a comparison of

the current set of measurements against a previous set of measurements. A

combination of other test methods is required to reach conclusive results.

(c) Measuring Generator No-Load and Short-circuit Characteristics Curve

This method is limited by measurement precision. Generally, the number of

rotor shorted winding turns has to exceed the total number of turns by 3 ~ 5%

in order for the condition to be reflected in the no-load and short-circuit

characteristics curve. As a result, sensitivity of detection is low and this method

is used together with other tests in order to reach a conclusive result.

(d) Flux Probe Measurements

This technique is based on measuring the main rotor flux and the rotor slot

leakage flux caused by the exciting current flowing through the rotor field

windings. This method requires a micro coil (flux probe) to be mounted on a

stator wedge and positioned in the air-gap of the generator. The probe detects a

combination of fluxes – the leakage flux in each rotor slot that is proportional to

the current flowing through the effective turns in that slot and the flux that

crosses the air-gap. The nearer the probe is to the rotor surface, the higher the

sensitivity. This method is superior to the three earlier mentioned methods for

the detection of short circuits due to metallic contamination, but it is unable to

detect insulation status for short circuits not caused by metallic contamination.

Obviously, this method cannot be used if generators do not have the air-gap flux

probes installed. Installing probes would require the generator to be taken out

of service.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



3. Theory of RSO technique

The recurrent pulse testing technique is called RSO (Repetitive Surge

Oscillograph) method. This technique applies travelling wave transmission

principles and is implemented based on neural network signature and high

frequency waves transmission symmetry through the same medium. This

method has been successfully tested in the laboratories and on generators.

The RSO method uses the time domain reflectometry theory to detect a short in

field windings. In essence, when a pulse is applied to the rotor winding circuit,

a shorted turn would cause reflections that cane be analyzed. A signal generator

is used to send a succession of step-shaped low voltage pulses along the rotor

field windings, and at the fault location, there will be a sudden impedance

change that will cause a reflected wave and a transmitted wave to occur,

resulting in a response characteristics curve that is different from that of a

normal loop back circuit that does not have sudden impedance variations. The

RSO technique uses the variation in impedance along the field windings

experienced by the travelling wave to detect anomalies. The extent of the

inter-turn short circuit is indicated by the degree of variation in the wave

impedance. The resultant reflected signals could be viewed on a dual-channel

oscilloscope screen as two separate waveforms or as a single summed trace.

The non-flatness of the combined trace is indicative of faults; blips on the

combined trace mean anomalies in the field windings and the wave amplitude of

a blip indicates the extent of damage. By analyzing the position of the anomaly

on the oscillogram, the approximate location of the fault can be inferred.

Therefore, even if there is only a single shorted turn in the field windings, the

RSO testing technique is sensitive enough to detect it.

Techniques for detecting inter-turn short-circuit in the generator rotors

involves two aspects: the early discovery of shorted turns and the location of

the inter-turn short circuit.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



The RSO technique is able to bring about early discovery of inter-turn metallic

contaminated short circuits and detection of one or more shorted turns caused

by non-metallic short circuit and insulation failure. In addition, this technique is

more accurate in locating the slot where the fault lies and the location of the

shorted turns.

Figure 6 shows a typical connection arrangement for RSO testing.

Fig. 6 RSO Testing Connection

4. Case Studies

(a) Short-circuit due to metallic contamination

During maintenance of a 1000 MW generator set for a certain power generation

station, impedance, air-gap flux waveform and RSO measurements were made.

Under the same controlled test environment, the impedance and waveform

measurements indicated normal conditions, however, the RSO Tester indicated

that the eighth turn of the rotor’s field winding is short-circuited by a metallic

contamination.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



The baseline normal operating condition data for this generator set:

(1) Impedance Values

Table 1 1000MW Generator Set AC Impedance (1)

(2) Flux Probe Waveform

Fig. 7 Flux Probe Waveform

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



(3) RSO Tester results

Fig. 8 RSO Measurement: Waveform (1)

The measurements for this generator set during overhaul:


Table 2 1000MW Generator Set AC Impedance (2)

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



(2) Flux Probe Waveform

This data is not available during overhaul of this generator set.


Fig. 9 RSO Measurement: Waveform (2)

The conclusion based on the RSO Test data and waveform is:

There is a coil-to-coil short-circuit in the rotor field windings of this generator.

The degree of short circuit-ness is one shorted turn, and the short-circuit

location is at the eighth turn of the rotor field windings at the excitation end.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



Fig. 10 Rotor of 1000MW Generator Set

Upon removing the retaining ring on-site, it was observed that the insulation on

the bridge between the 7th and the 8th coils showed evidence of overheating,

and that under the bridge between the 7th and 8th coils, two turns of field

winding were already welded together.

(b) Short-circuit not caused by metallic contamination

During maintenance of a 300 MW generator set for a certain power generation

station, impedance and RSO measurements were made. Under the same

controlled test environment, the impedance readings indicated normal

conditions, however, the RSO Tester indicated a short circuit caused by non-

metallic contamination between the 3rd and 4th coil of the rotor’s field winding

on the exciting side.

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793




Current I (A) Power P (W) Impedance Z ()

20 11.515 167.5 1.736

40 22.185 525.0 1.802

60 31.935 1217.0 1.878

80 41.340 2022.5 1.934

80 41.340 2022.5 1.934

60 32.170 1140.0 1.864

40 22.300 370.0 1.793

20 11.340 95.0 1.763

Table 3 300MW Generator Set AC Impedance


Fig. 11 Rotor of 300MW Generator Set: Before internal sweep Waveform

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



Fig. 12 Rotor of 300MW Generator Set: After external sweep Waveform

Fig. 13 Rotor of 300MW Generator Set

Address:


Phone: +65 6276 6733 Fax: +65 6276 6793



inter-turn rotor defect

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