FIELD OF INVENTION
This invention relates to the method of nondestructive testing of end retaining rings of turbo generator. This technique can be used in-situ, during over haul to detect discontinuities/defects/cracks which are originated due to fatigue, stress corrosion cracking, etc. Using this technique, it is possible to scan the complete outer surface of the end retaining rings in in-situ condition and the signals which are recorded can be used for easy analysis. Defects found, if any, can be traced back/located on the surface of the retaining ring using the encoder feedback system.
BACKGROUND & PRIOR ART OF THE INVENTION
Turbo generators are the main equipment coupled with turbines for generating electricity in fossil fuel power plants. It has a rotor with rotor windings, including end turn bends. These rotors windings are held in place by two ring like components on each axial ends of the rotor. These components are known as End Retaining Rings (ERRs) which support the rotor winding end turns against centrifugal force generated due to rotation. The ERRs are subjected to annularly oriented stress during the operation of the generator continuously. These retaining rings are designed to be tolerant to high stress levels with adequate toughness. The retaining rings are generally constructed of non-magnetic material, in order to minimize rotor end leakage flux, which also reduces rotor operating temperature. 18Mn-18Cr austenitic steel is

generally used due to its higher corrosion resistance and reduced risk of stress corrosion cracking during service. Generally, the retaining rings are shrink-fitted (by application of heat to expand the rings and subsequent shrinkage after cooling) onto the end of the generator rotor body, with internal annular groove for interlocking with rotor. This axial interlocking, prevents ring separation from the generator rotor body as the rotor windings and their end turns expand in the axial direction as they are internally heated by the electromagnetic forces generated within the generator. The resultant shrink fit also creates an additional mechanical stresses within the retaining rings, which is anticipated. However, degradation of the retaining ring structure during service operational heating is more difficult to anticipate and compensate for in the ring design. Therefore, it is required to assess the healthiness of the ERRs during overhaul using reliable nondestructive testing methods to prevent catastrophic failures.
Dye penetrant testing can be used for inspection of ERRs for detecting surface defects. In this method, the surface area will be treated with a dye penetrant so that the dye enters into defects/cracks which are present on the surface due to capillary action. The surface is then cleaned, and treated with a developer that causes the dye remaining inside these openings to spread into the developer making the cracks/defects visible. An ultraviolet (UV) light source is used to illuminate the defects in case of fluorescent dye penetrant testing. This technique is highly inspector dependent and it can only detect defects which are open to surface. Also this method is not environmental friendly.

Magnetic particle inspection (MPI) method, is highly sensitive for detection of surface/sub surface defects. Since it uses the principle of magnetic flux leakage due to surface discontinuity, only ferromagnetic material such as iron, cobalt, nickel or their alloys only can be inspected. Austenitic stainless steel materials are used for construction of ERRs which makes it impossible to test using MPI.
In the patent titled "Apparatus and method for non-destructive testing using multi-frequency eddy currents" (Patent No: US 5,237,271 dated 17/08/1993), a method for improving eddy current flaw detection by eddy current array probe is mentioned. This invention generally relates to multi-frequency eddy current non-destructive testing and more particularly to driving a plurality of eddy current probe elements with a multi-frequency excitation to simultaneously. This patent does not cover the method of applying this eddy current array test method for inspection of ERRs. Also the application of position encoder for improving the reliability in pin pointing the defect location is not considered.
In the US patent titled "Generator retaining ring and other component thermal degradation evaluation by eddy current non-destructive examination (Patent NO: US 9.488,618 B2, Dated: Nov. 8, 2016) the inventors created a method for non-destructive examination (NDE) and evaluation of the physical properties of generator retaining rings after it experience potentially degrading thermal exposure during any stage of manufacture, assembly and service use. The method is meant for identification of degraded components for replacement without wasting additional service or manufacturing expense. Another separate object of the invention is to create a method for in situ non-destructive examination (NDE) and evaluation of the generator retaining.

Electrical conductivity measurements made on the Surface of components, such as generator rings, is an indicator of the thermal exposure experienced by the ring. This method does not cover use of eddy current array method for surface crack inspection which can generate a 2D view of the surface. Also this method works by measuring conductivity rather than measuring impedance difference.
In US Patent titled "Eddy Current Array inspection device for shaped holes" (Patent No: 5,903,147, Dated: May 11, 1999), it provides a device for inspecting a component, such as a closed surface area of a gas turbine engine or the like, using an eddy current array circuit. The eddy current inspection device provides capability for the rapid inspection of any closed surface feature similar to holes of any shape, diameter or depth, in a single pass through the shaped area. This patent covers only closed surface area and the testing is carried out without the use of a position encoder.
Nondestructive examination of Generator ERRs are crucial in assessing the healthiness after prolonged service. In service damage due to fatigue or stress corrosion cracking will originate on the surface as cracks and eventually propagates during operation. If the surface cracks go undetected, it will lead to catastrophic failures/accidents and therby power outage. Therefore, it is required to test the ERRs in-situ during overhaul for detection of surface/sub surface defects. Dye penetrant testing is generally used for detecting surface cracks/efects on the retaining rings. This technique can detect only those defects which are open to surface. Since it make use of chemicals, dye penetrant testing is not environmental friendly. Conventional eddy current testing

can be used to overcome the difficulties of dye penetrant testing and can be used reliably for detecting surface/subsurface minute cracks/defects.
These end retaining rings are highly stressed components which are shrunk fitted to both the ends of the generator rotor. Austenitic stainless steel material such as 18Mn-18Cr steel is used for manufacturing of these retaining rings, which makes eddy current testing the only suitable method for surface and sub-surface defect detection. Defects in the form of fatigue cracks/stress corrosion cracking is expected on the outer surface of the retaining ring which may lead to failure, if undetected during NDT. Generally NDT is performed in in-situ condition during overhaul of the generator which follows years of continuous operation.
OBJECTS OF THE INVENTION
The prime object of the invention is to carry out in situ Eddy current array testing of generator ERRs for detection of surface and sub-surface defects.
Further object of the invention is to effectively test the ERRs with non-conducting coating on the surface without removing the coating.
Also the object of the invention is to use a manual / semi-automatic probe encoder for obtaining location data.

SUMMARY OF THE INVENTION
This invention proposes to the method of nondestructive testing of ERRs of turbo generators using Eddy Current Array testing technique. In this method, an array eddy current probe with a plurality of sensor elements is used for scanning the retaining ring surface. The scanning can be done manually or using a semi-automated mechanism. A position encoder is used for gathering the probe location information which also acts as a trigger for data acquisition during scanning. While scanning, if there is a lift off signal or spurious signal due to jerking of the probe or surface bumps, it is possible to move the probe in the reverse direction to overwrite the bad data. Once the probe scan the complete circumference it is shifted along axial direction of the ERR along the surface till there is minimum 20% overlap with the previous scan area. The circumferential scanning is started again. This process is repeated till the entire surface of the retaining ring is covered. The signal data acquired during the scanning can be analyzed for detecting the presence of any surface defects/discontinuities/cracks. Using this novel method, it is possible to assess the healthiness of the retaining ring in a fast and reliable manner.
The novelty of the present invention is that it uses Eddy Current Array testing with a position encoder for surface scanning of the ERR. The data acquisition is triggered by the movement of the encoder. The scan resolution, therefore, depends on the encoder resolution. Signals received by individual coil/sensor elements are recorded and processed using an image processing algorithm to produce and image of the defects on the surface. During scanning if any lift off signal/spurious signal is recorded, it is

possible to overwrite the data by moving the probe-encoder assembly backwards till it covers the spurious signal. Using this technique, it is easy to interpret the signals for defect detection/characterization. Once a defect is detected, its location can be precisely traced back to the surface of the ERR. Advantage of this system is that, unlike conventional eddy current testing of ERR, this testing technique can be semi-automated. The test data can be analyzed by a qualified personnel for identifying the defects. This technique is meant to be used at site during overhauling of the generator in in-situ condition. In this technique, an array eddy current probe is used which is having a plurality of elements/coils which collects data over a wider area and also improves defect detection. This probe and encoder is placed on the surface of the ERR and moved manually for collecting the signals. The data acquisition is triggered by the movement of the encoder wheel. Since the probe cannot cover the entire outer surface at one go, for every complete scan, the probe is shifted along the axial direction and repeated the scanning process such that there is some overlap between the scan areas. This way the entire surface area of the ERR can be covered effectively without losing valuable information.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is described with the help of Figures 1 to 4, where: Figure 1 depicts generator rotor (One end) with End Retaining Ring.

Figure 2 depicts probe-encoder assembly (Side view).
Figure 3 depicts ECA Probe bottom view.
Figure 4 depicts shifting the probe assembly in the axial direction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
The issues with the conventional eddy current testing using a single element probe is that, it is highly operator dependent and requires a lot of time to carry out inspection of the entire surface. Also it is not possible to record the inspection data with the corresponding location which can be later used for interpretation.
To overcome the aforesaid problems, a novel NDT technique, Eddy Current Array (ECA) testing is used for nondestructive evaluation of End Retaining Ring (1). Figure 1. Shows one end of the rotor (2) of a turbo generator. The ERR is shrunk fitted around each end of the rotor to hold the copper conductor (4) end windings against centrifugal force. Therefore, each turbo generator rotor will have two such ERR components. The eddy current array examination is performed in in-situ condition during overhaul of the unit when the rotor is in stationery position. Probe-encoder assembly as shown in Figure 2 (3) is placed on the end retaining ring surface towards one end to start the scanning. The equipment settings are such that the data acquisition is triggered by the movement

of the encoder wheel (7). This encoder wheel is attached to the encoder housing (6) which is mounted on the probe housing (5) so that any movement of the probe along the scanning direction will be sensed by the encoder. Eddy current probe is connected to the ECA acquisition unit via probe cable (8). Also there is encoder cable (9) which is also connected to the same acquisition unit. In order to perform the scanning, the rotor can be made to rotate while the probe is held stationary in contact with the ring surface. Where provision is not available for movement of the rotor, the probe assembly (3) can be moved manually on the surface of the retaining ring. During scanning, it is important that the probe is always in contact with the surface so that no signals are missed. The ECA probe contains number of coil/sensor elements (10) which are used for excitation and collection of signals during scanning. These coils are placed on a flexible backing material (11) for proper contact with the surface and also for protection of the delicate coil elements. After the data is acquired during a full rotation, the probe-encoder assembly is shifted in the axial direction along the surface of the retaining ring such that the new location will give 20% overlap in the coverage area (as shown in figure. 4). This axial shifting is repeated till the other end of the end retaining ring so that the complete area is scanned.

WE CALIM
1. A method for non-destructive evaluation of condition for end retaining rings (ERR) (1)
of turbo-generators through relative movement of the probe end of eddy current array (ECA) probe-encoder assembly (3) and retaining ring (1), either by way of rotation of the rotor (2) or by movement of the probe along the outer surface of the retaining ring (1), characterized by the combination of eddy current testing with a position encoder for surface scanning of the ERR.
2. The method as claimed in claim 1, wherein eddy current signals along with position
data are recorded to generate c-scan result for interpretation of defect.
3. The method as claimed in claim 2, wherein exact location of the defect traced back on
the rotor using encoder location and position.
4. The method as claimed in the preceding claims, wherein the data acquisition is
triggered by movement of the encoder, resulting scan resolution through encoder
resolution
5. The method as claimed in claim 4, wherein, signals received by the individual
coil/sensor elements (10) are recorded and processed to produce the image of the
defects.

6. An arrangement for non-destructive evaluation of condition for end retaining rings (1) of turbo-generator comprising:
- Generator rotor (2) with end retaining ring (1) to hold the copper conductor (4) against centrifugal force;
- Probe encoder assembly (3) placed on the end retaining ring surface comprising, encoder wheel (7) attached to the encoder housing (6), mounted on the probe housing (5);
- Eddy current probe having number of coil/sensor elements (10), placed on a flexible backing element (11), connected to ECA acquisition unit via probe cable
(8);
- Encoder housing connected to ECA acquisition unit via encoder cable (9);
- as substantially described herein through the description and figures.