Fatigue Analysis Of Composite Cross Arm For Transmission Line Under Typical Operating Load

For a long time,domestic transmission towers have adopted steel as the main material.Along with a series of problems exposed during the long-term use of iron cross arm,the cross arm made of Glass Fiber Reinforced Polymer(GFRP for short)exhibits unprecedented advantages.Studying the load characteristics and fatigue performance of the composite cross arm under the typical operating environment,accurately predicting and evaluating the fatigue life of the composite cross arm can ensure the safe and reliable operation during service.Taking the double-column cable-stayed composite cross arm with intermediate nodes as the research object,this paper analyzes the force condition of the cross arm under the influence of the galloping and aeolian vibration of the transmission lines.With the help of finite element software ANSYS,the static and dynamic characteristics of the composite cross arm under the typical operating load are analyzed,and the fatigue life of the composite cross arm is finally evaluated.The main achievements of this paper are listed as follows:1.Based on the theories of "catenary" and "parabola",the static characteristics of overhead transmission lines are analyzed,the corresponding relationship between sag and initial horizontal tension of conductor is obtained,and the equation of change in conductor stress under different meteorological conditions is deduced.According to the equivalent relationship between the length change and the tension change of the single-span conductor caused by the wind-induced vibration,the expressions of the horizontal tension change and the vertical tension change of conductors in a vibration period are obtained.2.According to the shape-finding principle of the conductor,the finite element analysis of the galloping concerning the single-span conductor is carried out with the galloping of the conductor stimulated by the sinusoidal standing wave,and the variation in conductor tension during galloping obtained by line length method and finite element method is compared.Based on the energy balance method,the expressions of the input power of wind energy and the self-damping power of conductor are obtained,and with the help of the actual transmission line parameters,the relationship between the amplitude and frequency of the conductor during aeolian vibration is obtained.It is shown that the variation of conductor tension during galloping obtained by the line length method and the finite element method has good consistency.3.According to the load characteristics of the conductor during galloping and aeolian vibration,the hanging point forces of the tension tower and the straight tower are calculated under different conditions of galloping and aeolian vibration,and the results are compared with the equivalent static wind load of cross arm in the current transmission tower specifications.It is shown that under the same meteorological conditions,the hanging point forces of the cross arm during galloping is much greater than that calculated according to the specifications.Therefore,in the design and analysis of the pole and cross arm,galloping can be considered as an independent load condition.When the wind deflection effect of the insulator is considered,the unbalanced tension on both sides of the cross arm in straight tower is much smaller than that in tension tower,and the vertical load of the two is equivalent.4.The first 20 natural frequencies and corresponding vibration modes of the composite cross arm are extracted by the method of block lanczos.The dynamic time history analysis of composite cross arm is performed based on the transient dynamic analysis method.The analyzing results show that when there is a half-wave galloping in the transmission line,due to the large amplitude of the galloping,the composite cross arm generally has static failure damage.The vibration of the composite cross arm in the direction of the conductor is more dangerous than that in the vertical direction,which is in line with the actual situation.Thus,as a relatively weak part,the inner tie-rod of the cross arm should be focused on in the structural design.5.Based on the theory of linear cumulative fatigue damage,a method is proposed to evaluate the fatigue life of composite cross arm in time domain.The stress amplitude,stress mean and number of cycles are counted by rain flow counting method,and the fatigue life of composite cross arm is evaluated according to the S-N curve of the material and Miner criterion.The analyzing results show that without any consideration of the shock-proof system,the failure modes and fatigue life of the composite cross arm are completely different under different galloping conditions and the average service life of the composite cross arm can reach 30 years under the condition of aeolian vibration.