The laboratory test spans for conductor self-damping measurements at the VRTC comprises of apparatus similar to that shown in figure (6.2). This figure represents a schematic view of the actual experimental setup of the test span shown in figure (6.1), the experimental set-up layout at the VRTC. The conductor span length is about 86.4 m. The facilities at the VRTC was built in line with recommendations from the IEEE Std 563-1978 for the Measurement of conductor self- damping [1, 2, 91].The laboratory equipment consists of a tension application device, shaker (the TIRAvib shaker type), conductor clamp system, and measurement system for data collection and processing. The free span length was preferably suited to produce a number of loop lengths longer than the longest loop length used in the tests. The test span is equipped with temperature controlling devices used to control its ambient temperature. Due to its relatively long length, the influence of the end termination losses, minimized by the rigid clamps, is further reduced and the distribution of the tensile load between the conductor strands is more homogeneous.
For the experimental setup as shown in figure (6.2), there is a device for applying tension known as the constant tension device. This device makes it possible to vary the applied tension by applying a specified load on the device. This helped to introduce different tensions to the test conductor by adding or removing loads weights at this device. This constant loading device cannot control the conductor tension after the conductor is camped, what does affect the tension is the temperature of laboratory at which the test is being conducted.
6.4.1 Shaker and Shaker Conductor Connection
Figure (6.3), shows the position of the shaker, which was used to provide the power input to the conductor. A vibration shaker is usually located near one end of the span i.e. loading arm part of the test set-up. This form of exciter used at the VRTC is the electro-dynamic shaker having a light
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armature and linear bearings that was capable of exciting the conductor at its natural frequencies.
The shaker induced a periodic function or a sinusoidal force on the test span. The alternating movement provided by the shaker produces a simple harmonic motion representing a specific mode of excitation. Vibration amplitudes and frequencies were controllable to the required accuracy and were done by the function generator through the power amplifier.
Figure 6.3: A flexible Connection used in connecting the Shaker to the Conductor
The location of the shaker along the test span was chosen to facilitate the required test frequency range in which the various modes can be produced. The 0.8 m distance from the end rigid clamp was used; this ensured that loop length at the highest test frequency was not produced between the shaker and the rigid clamp.
The connection between the conductor and the shaker was done using the flexible connection as shown in figure (6.3). The flexible connection between the shaker and the conductor guarantee that, at resonance, the conductor can vibrate at amplitudes higher that the amplitude imposed to the shaker table without driving the shaker armature. The power input from the shaker is adjusted to the resonant frequencies of the conductor to ensure that the displacement at the anti-node represents the speed of the wave travelling through it.
6.4.2 The Span End Conditions
The test span is strung between two massive blocks made of concrete which on top of it houses the clamp where the conductor is placed. This span end termination equipment has the capability of withstanding and maintaining a constant conductor tension. The span is terminated at the ends by two square clamps which are used to maintain the constant tension at the test span ends as shown
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in figure (6.4). These heavy and rigid clamps end is inserted with groove diameters not exceeding more than 0.25 mm of the diameter of the conductor to hold the conductor in order to produce good results. A rigid clamp shown in figure (6.4) is used to minimize energy dissipation through the termination fixture, but they do not have any capacity in controlling the conductor tension. At VRTC test laboratory the conductor tension is kept constant by maintaining the ambient temperature.
Figure 6.4: The span end termination
Between the clamp and the tension application device is the load cell measurement system and shackles as shown in figure 6.5. The shakes, the end sleeves threaded bars and pivotal balance beams are connected to the loading device. This arrangement was used to successfully achieve the constant tension applied to the conductor ends. For reliable results, the terminating fixtures and rigid clamps must have sufficient stiffness to ensure that energy losses do not occur beyond them i.e. not beyond the termination point of the conductor at the clamp.
Figure 6.5: The connecting link between the clamp and the loading arm
153 6.4.3 Accelerometers and Force Transducers
In the collection of data from the conductor and also to feed control information to the shaker, the accelerometers and the force transducer was used. The accelerometers type used was the PCB Model 352A73, shown as figure (6.6). They were mounted to the conductor using wax at various positions along the span for the sweep test and by a clip during the hysteresis test. The clip was used for the hysteresis loop test because, the acceleration locations were chosen such that the vibration modes could be detected by placing it at the anti-nodes. The force transducer was placed at the Shaker-Conductor. The placement was also used to measure the node and the antinode amplitudes on the conductors.
The accelerometer and force transducer were expected to have a phase shift between the two signals because of the different response time of the two signals. This phase shift was frequency dependent and has taken into account in the determination of the phase angle between the measured quantities at each vibration mode. The computer controlled test system; the data acquisition software was set up to perform automatically the phase shift correction between the force transducer and accelerometers. In addition, load cell was used to determine the value of the axial tension.
Figure 6.6: An accelerometer