Welcome to this module on surge arrester. Need, application and functioning of surge arresters. This topic explains need for surge arresters, applications of surge arresters, functioning of surge arresters. Need for surge arresters. Lightning strikes on or its nearby vicinity lead to lightning surges or lightning overvoltages. Lightning surges last for few micro seconds. Switching action, CB open or close, in the system may lead to switching surges or overvoltages. Switching surges last for milliseconds. Switching surges increase with an increase in operating voltages of the system. Switching over voltages are usually not predominant up to 33 kV. Certain operating conditions may lead to temporary over voltages or TOV. TOV may last for several seconds. Examples, sudden load rejection. TOV occurs on the healthy faces of a system, during an earth fault in one of the phases. A voltage rise will depend on the type of system earthing. Energization of unloaded lines. Lightning and switching overvoltages may exceed the voltage withstand capability of the equipment's insulation. Hence, surge arresters are provided to protect the electrical equipment from lightning and switching overvoltages as they absorb such over voltages and provide better insulation co-ordination. Note, surge arresters do not protect electrical equipment from TOV or temporary over voltage which last for several seconds. Rated voltage of surge arrester is carefully chosen to withstand TOV for the expected duration. Surge arresters are voltage limiting devices used to protect the equipment insulation from voltage spikes in a power system. They limit the high transient voltages generated during the system disturbance by diverting the excessive surges to the ground. They reduce the amplitude of the transient wave impressed across the equipment to a safe value based on the protection margin of the arrester. They are similar to how a fuse functions to protect an electrical system from damages due to over current conditions, but the fuse is required to be replaced after every protective action. But surge arresters don't need such replacements. Surge arresters were commonly called as lightning arresters earlier based on the primary objective of protecting the electrical insulation from lightning spikes. However, as these devices are used to mitigate the harmful effects of over voltages, such as switching overvoltages. In addition to lightning spikes, the term surge arrester is more appropriate. Functioning of surge arresters. The purpose of surge arrester is to restrict the potential that might otherwise appear at the terminals of the equipment protected below its BIL value by diverting the excessive surges to the ground. This is achieved by connecting the surge arrester in parallel to the terminals of the protective equipment to the ground. Surge arresters should not pass any current at normal power frequency voltage as well as transient overvoltage through it. Surge arresters should breakdown electrically through non linear resistors as quickly as possible after the abnormal high frequency voltage arrives. Surge arrester should be carefully designed so that it will not damage by itself while diverting excessive surges to the ground. After the surge is diverted to ground, power frequency follow current may pass through the surge arrester elements. Surge arrester should be capable of interrupting this power frequency follow current very early immediately after the surge dies down. Surgery arresters should safely withstand the vagaries of the environment that is ambient temperature, heavy rainfall, maximum level of pollution expected, etc. By not allowing the searches creep through its internal surface of the SA housing. Protection offered by SA will be effective only as long as the surge pass through internal components of SA. When surges greater than residual voltage level enter at the location, they are diverted through internal components of SA. This diversion of surges ensure that all protected equipment do not experience overvoltages about residual voltage level. Hence it shall be sufficient to provide the protected equipment BIL level equivalent to residual voltage level plus adequate safety margin. All electrical equipment connected in parallel with surge arresters experience overvoltages only up to residual voltage level in case of any lightning or switching surges. Surge arresters prevent insulation failure of connected equipment, power transformers, shunt reactors, etc. And improve availability and reliability of power system. Where surge arresters are required to be introduced in electrical power systems. Surge arresters are usually installed in medium voltage, high voltage, extra high voltage, HVDC substations. In substations they are generally connected in parallel with and closer to the equipment to be protected. That is power transformers, reactors, cables, etc. To protect all other switchyard equipment such as circuit breakers, isolators, instrument transformers etc from traveling lightning surges, surge arresters are connected at the entry point of substation. That is at all incoming and outgoing OHL lines. Overhead transmission lines are very vulnerable to lightning strokes. The majority of the falls on overhead lines are due to lightning induced surge voltages reaching the tava. It results in the line insulator to flashover causing an interruption. Installation of zinc oxide arresters across the line insulators at intervals would greatly reduce or eliminate the possibilities of insulation failure. It is also a normal practice to provide arcing horn or spark gap type of surge arresters for this purpose as these devices cost less. Surge arresters installed at the end of transmission line, allow the insulation level of the substation equipment, like a power transformer to be reduced as the surges are suppressed. The reliability of the system would be enhanced. The requirement of the number of surge arresters depends on line length and traveling wave surges. The image represents surge arrester installed at the OHL line entry. Distribution systems. They are installed in distribution systems near the terminals of large, medium voltage rotating machines, cables and capacitors. They are also used in conjunction with VCB up to 33 kV distribution system for all feeders. They are usually connected between phase to ground. In case of tertiary delta winding of transformers, they are connected between phases. A vacuum circuit breaker, VCB, despite advantages like lower cost and reduced maintenance, causes switching over voltages with steep fronts, resulting in motor insulation failures. Hence surge arresters are required in VCB installations for limiting over voltage surges at motor terminals to protect the motor insulation. Surge arresters are used with VCBs for transformer feeders as well, for limiting over voltages. Here is a quick recap of what you've learned thus far. Surge arresters are provided to protect the electrical equipment from lightning and switching over voltages as they absorb such over voltages and provide better insulation coordination. Surge arresters are voltage limiting devices. They protect electrical equipment from voltage spikes in a power system. Surge arresters limit the high transient voltages generated during the system disturbance by diverting the excessive surges to the ground. Surge arrester does not protect electrical equipment from temporary over voltages. Hence rated voltage of SA is chosen to withstand such TOV for the expected duration. Surge arresters are installed in substations to protect equipment like power transformers, reactors, etc by connecting parallel with and closer to it. SA is also connected at the entry point, that is at all incoming and outgoing OHL lines to protect all other switchyard equipment such as CB, isolator, CT and VT. Surge arrester is mostly connected between phase to ground but between phases at the power transformer, tertiary winding. With respect to distribution system, SA is connected or near the terminals of large or MV motors or generators, cables, all VCB outgoing terminals, capacitors. Surge arrester function is to safely withstand the vagaries of the environment that is ambient temperature, heavy rainfall, maximum level of pollution expected at the maximum surge level. Protection level of SA is defined by residual voltage level. When surges greater than residual voltage level enter at the location, they are diverted through internal components of SA. This diversion of surges ensure that all protected equipment do not experience overvoltages above residual voltage level. Hence it shall be sufficient to provide the protected equipment BIL level equivalent to residual voltage level plus adequate safety margin.
