TRANSMISSION LINES DESIGN and ELECTRICAL ENGINEERING HUB

From the consumer’s point of view another important parameter of the supply system is its impedance as viewed from his terminals. On the one hand, the lower the impedance the greater will be the stress on his switchgear and protective devices, but on the other hand, the higher the impedance the greater will be the risk of annoyance due to distortion caused by either the consumer’s own load or by that of a nearby consumer.  High network impedances are troublesome to installation designers because they result in low values of fault current, which severely limit the number of series graded protection devices and cause an increase in the I2t energy let through of inverse characteristic devices such as fuses. The 16th edition of the IEE Wiring Regulations, BS 7671, requires installation designers to have a knowledge of the limits of system impedance to which the supply will be kept in order that they may install the necessa...

The main protection system for a given zone of protection is called the primary protection system. It operates in the fastest time possible and removes the least amount of equipment from service. On Extra High Voltage (EHV) systems, i.e., 345kV and above, it is common to use duplicate primary protection systems in case a component in one primary protection chain fails to operate. This duplication is therefore intended to cover the failure of the relays themselves. One may use relays from a different manufacturer, or relays based on a different principle of operation to avoid common-mode failures. The operating time and the tripping logic of both the primary and its duplicate system are the same. It is not always practical to duplicate every element of the protection chain. On High Voltage (HV) and EHV systems, the costs of transducers and circuit breakers are very expensive and the cost of duplicate equipment may not be justified. On ...

Cross arms are now almost limited to carrying polyphase circuits in areas where appearance is not of paramount importance. They are also used where lines cross each other or make abrupt turns at large angles to each other. They are used as alley or side arms in which the greater part of their length extends on one side of the pole to provide adequate clearances where pole locations may be affected by limited-space rightsof- way. Cross arms are shown in Figure below. Uses of cross arms: (a) line arm; (b) side arm; (c) buck arm; (d) double arms. Loadings The cross arm acts as a beam, supported at the point of attachment to the pole, and must be capable of being subjected to vertical loadings from the weight of the conductors (encased in ice) and a 225-lb worker (specified as an additional safety measure). It is also subjected to horizontal loadings stemming from winds and from tension in the conductors where the tensions ...