NAME : Jürgen R. Weidner COUNTRY : Germany REGISTRATION NUMBER : 2817

GROUP REF. : SC A1 PREF. SUBJECT : 1 QUESTION N° : 1.6

U:\sbourneuf\SESSION 2012\PROCEEDINGS\A1\A1_PS1_Q1.6_Weidner.doc 10/11/2012

Direct Measurement of Copper Conductor Temperature at Generator Windings with Fiber Bragg Grating (FBG) Sensors

During test field run of a large air cooled generator different fiber optic sensors were used to measure vibration, strain, cooling air flow and temperature at areas where conventional sensors with metal parts and copper leads cannot be used due to high electrical and magnetical fields. This contribution deals with direct measurement of copper conductor temperature at generator stator winding bars with Fiber Bragg Grating (FBG) Sensors. These fiber optic sensors had been tested to interference immunity and long term stability before final installation in a large air cooled turbine generators. The following results were obtained: Measuring sensitivity not influenced by high AC magnetic fields up to 1,5 Tesla, Electrical strength of sensor and fiber leads proved up to

- 65 kV AC for high voltage acceptance tests - 30 kV AC for long term operation,

Unaffected by high transient surge voltage up to amplitudes of 120 kV, Withstand high transient load shocks and permanent dynamic mechanical stresses during

load regime, Thermal stability and long term endurance of fiber optic sensor system up to operating

temperature of 180°C.

The measurement principle of FBG sensors is based on stretching of grating distance d at the individual sensor by mechanical or thermal forces which results in drifting of the optical wavelength λ (see fig.1). The shift of wavelength is used in the measuring instrument to calculate the displayed results.

Figure 1: Principal of Fiber Bragg Grating (FBG) Sensor Arrays used at turbine generators

As FBG sensors measure the stretching between the gratings which could result from mechanical strain or temperature change it is very important for reliable measurement to distinguish between temperature and mechanical influence.

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Strain measurement FBG section directly glued to test object surface well fixed to take strain

Temperature measurement FBG section absolutely free moving in small tube only stretched by temperature

For application on the copper conductor of a generator bar a FBG sensor array was applied to an insulation material strip of 1,5 mm thickness with 12 FBG sensors in series in one fiber optic lead (fig.2)

Figure 2: Long insulation material strip of 1,5 mm thickness with 12 FBG sensors in series in one fiber optic lead

Fig. 3 demonstrate the application of FBG sensor array to the narrow side of the copper bar and the taping of the high voltage main insulation.

Figure 3: Fixing the FBG-Sensor-Array Strip at Copper Conductor of Stator Winding Bar

Six stator bars ( three bottom bars and three top bars) had been equipped with these FBG temperature arrays. After carefully taping these bars with normal mica glas fabric tapes the bars were inserted into the core slots together with the other bars and connected to each other. Core and stator winding obtained the standard Global Vacuum Pressure Impregnation (GVPI) process and the generator was prepared for test field run.

First results of temperature distribution at copper conductor of top and bottom bars in comparison to standard slot RTD and distributed fiber optic slot temperature measurement are given in fig. 4. Due to the symmetric cooling of the machine only one axial half of the stator core is shown.

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Figure 4: Temperature distribution at stator bar copper leads during test field run - comparison of axial measured copper lead temperature with FBG sensors at top and bottom bars with readings of slot temperature device RTD

The test results at this air cooled generator wit radial cooling zones can be summarized as follows: Maximum copper conductor temperature at top layer bars because of

- higher electromagnetic looses due to rotor flux field - warmer cooling stream near air gap due to mixture with warm rotor cooling air outlet

Readings of fiber optic slot temperature device correlates better with cooling air temperature than stator bar temperature - not suitable for precise bar temperature controlling.

Measured temperature with standard slot RTD is much lower than maximum copper conductor temperature at top bar - slot RTD not suitable for hot spot measurement.

FBG-Arrays reflect very well the real temperature distribution at copper conductor - hot spot monitoring of stator bar

Based on these promising results the following conclusions and next steps can be drawn:

1. It has been demonstrated that FBG temperature sensor arrays are an excellent tool to control the calculated thermal design model of the complete generator stator during test field run.

2. Direct temperature measurement at copper conductor on high potential could control insulation system hot spots and therefore optimize thermal utilization of indirect cooled generators.

3. Aging related decrease of thermal conductivity of winding insulation could be controlled by fiber optic copper temperature measurement. Operation condition could be adjusted to actual hot spot temperature.

4. Copper temperature measurement with FBG sensor arrays avoids thermal overstressing of high voltage insulation system at copper to insulation interface (local hot spot area).

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5. Direct copper temperature measurement enables maximum utilization of generator during complete design life time without progressive thermal stressing of the aged insulation system.

6. Temperature increasing gradient and maximum temperature of the aged insulation system could be controlled during load cycles dictated by the grid demands.

7. Different kind of fiber optic sensors could be applied for online condition monitoring of the generator when running in peak load operation with high stress categories.

8. Innovative monitoring systems with fiber optics could act as early warning systems and will help to avoid sudden machine damage in today's changing load demand conditions.