CHAPTER 3: Experimental equipement & Set-up
3.3 Laboratory configuration and equipment
3.3.1 Equipment & instrumentation
38 3.2.2.2 Line post support
For the scenario in which a line post insulator would be tested, a line post support was also designed (Figure 3-2b). As previously, the commercial package SolidWorks was used for the design. The line post support could mechanically withstand a cantilever load up to 5kN.
3.3 Laboratory configuration and equipment
39
response to a certain input signal. Vibration generated by the shaker may be used for several types of vibration tests, such as sine, random, shock, sine-on-random, random- on-random, or diverse complex waveforms, as well as replicating data collected from real-world conditions. The excitation of the conductor may be achieved in either a vertical or horizontal position. The shaker has a cooling system composed of a compressor used to cool down the field- and the armature-coils. The control system of the shaker piloted by an amplifier, shuts down the entire system in case of an abnormal behaviour of the shaker.
3.3.1.2 Piezoelectric Accelerometers
Accelerometers of Brüel & Kjær type 4508-002 and 4507 B which rely on the piezoelectric effect of quartz or ceramic crystals, were used during the vibration testing to control the conduction motion. These accelerometers were selected for their high- sensitivity (100mV/g), wide-range frequency of measurement, and low inertia (mass).
The sine-wave signals of the accelerometers which were interpreting the sinusoidal behaviour of the conductor during the test, were collected by the Puma Vibration Control and Analysis System. The data from the accelerometers provided valuable information (amplitude, velocity, acceleration, and frequency) regarding the system integrity of the overhead-line span and operating mode shapes. The accelerometer cables were securely fastened to their connectors in order to avoid any potential noise passing into the vibration measurement (Figure 3-4) [47].
Figure 3-4: Accelerometers: on the conductor (a) and on the flexible connection (b) 3.3.1.3 Force transducer
The vibrational loads of the vibrating conductor at the suspension point (mid-span) were measured with a dynamic force transducer which is made of strain gauges (Figure
Accelerometers
ACSR Conductor
Shaker base Force transducer 8230
40
3-5). The direction of these forces may be provided by either traction or compression.
An HBM S9M-10kN force transducer with 2 mV/V as the nominal sensitivity was used, and mounted to the anchor tower at the top and the middle according to the measurement desired (Figures 3-2a, 3-5, and 3-21). Its signal was collected by a MP55 Module before sending it on to the NI-DAQ module.
The MP55 module made by HBM is a carrier frequency amplified which allows connections of different transducers (force, pressure, torque and displacement). The input parameters can be assigned with a keyboard visible by a display panel. It is a PME product line (Power Monitoring Expert). The NI-DAQ (data acquisition) module is made by National Instrument (NI) and allows the connection of different sensors (force, pressure, torque and displacement) to a cDAQ module (Compact Data acquisition) which is an interface between the data acquisition system and the PC (personal computer) with programmable software (LabView).
Figure 3-5: Force transducer in the blocked clamp (a) and the suspension insulator configuration (b)
3.3.1.4 Linear variable differential transformer (LVDT)
The bending amplitude measurements are usually taken in the field by vibration recorders at 89 mm from the clamp edge (CE). For this indoor vibration testing an LVDT, which operates as a linear variable differential transformer, was used to measure the peak-to-peak bending amplitude of the vibrating conductor at 89 mm on the conductor from the last point of contact between the conductor and the clamp. The LVDT HBM WA/10mm-T with a maximum displacement of 10 mm and a sensitivity of 80 mV/V was chosen. A plunger (magnetic core) provided a path for magnetic flux
Suspension insulator Top plate of the anchor
tower
Middle plate of the anchor tower
Force transducer
Suspension clamp
(a) (b)
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linkage between the coils by moving through a bobbin. Any variation of plunger position creates a variation of flux which is then converted into voltage and interpreted in units of length (mm). This displacement transducer was mounted on a stand independently of the anchor tower or the line post tower (support). The signal of the LVDT was also connected to the MP55 (Figures 3-6. 3-21 and 3-22).
Figure 3-6: LVDT Transducer on the conductor 3.3.1.5 Strain gauges
The metallic strain gauges are fine metallic foils arranged in a grid pattern to measure the strain either statically or dynamically (Figure 3-7). The strain gauges change their electrical resistance linearly and proportionally to the force applied on the test specimen (FRP rod of the composite insulator) to which they are attached. All strain gauges used in this project have a resistance of 350 ± 5% and a gauge factor of 2.07 ± 0.5 % at 24°C.
Strains on the core of the suspension insulator were measured at three different places on the insulator (in the middle and at each end on the FRP core, and close to the end Figure 3-7: Strain gauges stuck on the FRP rod of the suspension insulator (a) and line
post insulator (b)
(a) (b)
Strain gauges on FRP core of the
suspension insulator Strain gauges on
FRP core of the line post insulator
LVDT Force transducer
Articulated clamp Conductor
Blocked clamp
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fittings - Figure 3.14) while the bending strain of the line post insulator was measured on the FRP core at the end close to the end fittings of the insulator, using four strain gauges separated by an angle of 90 degrees (Figure 3-15). Strain gauges connected as Wheatstone bridges were necessary for measuring the small changes of resistance caused by the mechanical loads of the vibrating conductor on the composite insulators (suspension or line post) [48, 49]. The strain gauges system for the line post insulator used temperature compensation systems (dummy gauges: Figure 3-9).
Figure 3-8: Strain gauges on the line post insulator.
Strain gauges which were attached to the FRP of the rod of suspension insulator in four half-bridge configurations, assisted in measuring the strain (Figure 3-7a and 3-14). For the line post insulator four couples of strain gauges were used (Figure 3-15). For each couple of strain gauges system: an active gauge was attached on the FRP core of the line post insulator to measure the bending stresses (Figure 3-7b and 3-8), and the other strain gauge was stuck on a non-active FRP rod (an FRP rod separated mounted – Figure 3-9). Both the active strain gauge and the non-active strain gauge were connected in the quarter-bridge configuration.
Figure 3-9: Temperature compensating gauges for the line post insulator