11/11/22, 8:46 PM Surge Arrester Explained - saVRee Surge Arrester Load 3D Model Article Video Courses Online Course Courses Introduction Surge arresters are used to protect high-voltage equipment in substations, such as transformers, circuit breakers and bushings, against the effects of lightning and switching surges. Surge arresters are connected close to, and in parallel, with the equipment to be protected. Their purpose is to safely divert surge energy to earth and ensure that the resulting voltage across the terminals remains low enough so as not to damage the insulation of the associated devices from the effects of overvoltages. Surge Arresters Protecting Power Transformer Bushings Almost all surge arresters used in modern high voltage power systems, are of the gapless metal-oxide (MO) varistor type; this article focuses on this type. Insulation Coordination and Surge Arresters Insulation coordination is defined as the selection of dielectric strength of equipment in relation to the different types of overvoltages that can appear in the system. Surge arresters provide an indispensable aid to economical insulation coordination in electrical power systems. This is illustrated in the figure below, where in the absence of any surge protective devices, the https://savree.com/en/encyclopedia/surge-arrester 1/7 11/11/22, 8:46 PM Surge Arrester Explained - saVRee equipment cannot withstand the high dielectric stresses resulting from lightning and switching overvoltages. It is within this range that the surge arresters play their role in the system, by keeping the voltage at a level that is below the withstand voltage (the highest voltage that can be applied to an item without it incurring damage) of the equipment, by an adequate safety (protective) margin. On the other end of the spectrum, surge arresters cannot limit oscillatory power frequency temporary overvoltages (TOV) and must therefore be designed to withstand such temporary overvoltages, together with the maximum operating voltage of the system, without sustaining damage. Role of Surge Arresters in Power System Insulation Coordination As a point of interest, it should be noted that TOVs in a power system are limited by means of switched reactive compensation (such as by a shunt rector), or, through the application of flexible alternating current transmission (FACT) devices (such as SVC and STATCOM). Construction and Main Components At the heart of the surge arrester unit is the MO varistor column, which constitutes its active part. The column is composed of MO varistor blocks stacked on top of each other. These blocks are made of zinc oxide (ZnO) and other metallic powders mixed together and then pressed into cylindrical discs. The diameter of each disc determines the energy handling capability of the surge arrester. A diameter of 100 mm (3.9 inches) or more is usually required for high voltage systems. The required TOV endurance capability (governed by the arrester rated voltage), along with the desired switching and lightning impulse protective levels control the overall height of the MO varistor column. In most cases however, the porcelain housing of the surge arrester is designed to be considerably longer for dielectric reasons (clearance and leakage distance requirements) and is not controlled by the height of the active part. As a result, the column of MO varistors is installed in the housing of the arrester unit with the help of metallic spacers. The spacers consist of aluminium tubes with end covers to uniformly distribute contact pressure. Several supporting rods and holding plates manufactured from fibre-glass reinforced plastic (FRP) material encircle the MO varistor column in the form a cage; the cage mechanically secures the internal active part. On the upper end of the arrester, a compression spring provides the necessary axial pressure to press the stack of MO varistors together. Flanges are cemented to either end of the surge arrester porcelain housing; the flanges are usually manufactured from aluminium and enclose the sealing arrangement. In high voltage systems, instead of directly earthing the surge arresters, monitoring devices are connected in series with the arrester. In such instances, the bottom flange of the surge arrester is installed with insulating feet and an earthing connection (ground connection) is made through the monitoring device. https://savree.com/en/encyclopedia/surge-arrester 2/7 11/11/22, 8:46 PM Surge Arrester Explained - saVRee Cross Section view of a Porcelain Housed MO Surge Arrester The sealing system is one of the most critical components of a surge arrester. Firstly, it should prevent the ingress of moisture and contamination into the surge arrester housing. Secondly, it should act as a fast operating pressure relief device (PRD) in case of an arrester overload, which can result in a rapid pressure accumulation within the surge arrester housing. Lastly, it should provide a well-established point of contact for the transfer of current from the surge arrester external connection terminal to the MO varistor column. The sealing system of a surge arrester consists of a synthetic sealing ring and a pressure relief diaphragm, both of which are installed twice at either end of the surge arrester body. The very thin diaphragm (only a few tenths of a millimetre thick, or thousandths of an inch) is made of nickel or high-grade steel. The diaphragm is pushed against the sealing ring by means of a clamping ring screwed to the flange body. In the event of an arrester overload, an arc develops between the two flanges inside the housing. The thermal energy of this arc (which carries the full short circuit current of the network) results in a rapid build-up of pressure within the surge arrester. The resultant pressure is relieved by the relief diaphragm, thus averting a catastrophic failure of the arrester and possible resultant damage to the surrounding area. Hot gases that result within the arrester housing due to the overload, are directed through any of the two venting outlets. Outside the surge arrester, the gas streams meet, causing the arc that was burning inside the housing to shift (commutate) and continue burning outside the arrester until the fault clears. At higher voltages, due to insulation requirements and manufacturing economics, a complete surge arrester consists of several arrester units connected in series. Furthermore, a grading ring is installed at the high voltage terminal to control voltage distribution from the high voltage end to earth. https://savree.com/en/encyclopedia/surge-arrester 3/7 11/11/22, 8:46 PM Surge Arrester Explained - saVRee Multi-Unit High Voltage Surge Arrester Surge Arrester Condition Monitoring Modern MO surge arresters are highly reliable devices when configured correctly. They can be expected to have an almost maintenance-free service life of 30 years or more. Nevertheless, considering the high cost of the equipment which surge arresters protect, and the detrimental effects of arrester overloads, there is good reason to monitor the health of surge arresters. At normal service voltage, surge arresters exhibit a high impedance such that they act as an insulator for the majority of their working service life. Such behaviour is necessary to guarantee a long life for the arrester, as well as stability of the associated electrical system. It is therefore imperative to detect any deterioration of a surge arrester’s insulating properties before the situation becomes critical. Two types of monitoring devices commonly employed for high voltage MO arresters are: Surge counters that register the number of surge impulses. Leakage current monitors that measure the leakage current flowing through the arrester. The basic premise of using surge counters is to identify if a given transmission line or phase of system is experiencing an extraordinarily high number of overvoltages leading to arrester operation. Moreover, an abrupt increase in the surge count rate may also indicate an internal arrester fault. However, surge counters alone reveal only partial condition monitoring information. Most surge arrester monitoring devices register the number (count) of surge impulses, whilst also measuring any leakage current. Leakage current provides additional information about the magnitude of any surges and its relevance should a system overvoltage event occur. Using surge counters and leakage current measurement devices in conjunction with one another, allows for a more flexible means of monitoring and diagnosing the condition of the arrester. Operating Characteristics The voltage-current (V-I) characteristic illustrates how a surge arrester’s resistance varies with voltage, whilst also providing insights into its operation. The highly non-linear V-I characteristics of the MO varistor makes it a suitable candidate for surge protection application. The varistor is basically a variable resistor whose resistance depends inversely on the applied voltage i.e. the greater the voltage, the lower the resistance. The below image shows typical characteristics of a 420 kV rated MO surge arrester applied within a 550 kV rated system (phase-to-phase). https://savree.com/en/encyclopedia/surge-arrester 4/7 11/11/22, 8:46 PM Surge Arrester Explained - saVRee Operating Characteristics of a Surge Arrester with 420 kVrms Rated Voltage To develop a better understanding of a surge arrester’s operating characteristics, definitions of some important terms and parameters are needed: Maximum System Voltage (Us) The highest phase-to-phase rms power frequency voltage specified for a given system during normal condition. Continuous Operating Voltage (Uc) The maximum permissible rms power frequency voltage that can be applied across the arrester terminals for a continuous, or indefinite period; this is also sometimes designated as MCOV. In practice, the continuous operating voltage (Uc) of the arrester is set to be greater than the highest phase-to-ground system voltage ( ) with a margin of at least five percent. Rated Voltage (Ur) The maximum rms power frequency voltage that the surge arrester must withstand for a specified short duration (e.g. 10 or 100 seconds). It characterizes the ability of an arrester to endure system TOV. When a surge arrester is stressed to and beyond its rated voltage (Ur), leakage current will flow. Leakage current is defined as the unintentional flow of current to ground. This situation is not desired because as leakage current flows, a proportional increase in the operating temperature of the arrester will occur. If this condition is allowed to persist beyond the specified short duration, the arrester’s temperature will increase until it becomes thermally unstable, which may ultimately lead to failure of the arrester. Switching Impulse Protective Level (SIPL) The peak value of the residual terminal voltage of the surge arrester at nominal discharge of a switching current impulse with 30/60 µs waveshape and with a peak magnitude of 2 kA (in the case of extra high voltage systems). Lightning Impulse Protective Level (LIPL) The peak value of the residual terminal voltage of the surge arrester at nominal discharge of a lightning current impulse with 8/20 µs waveshape and with a peak magnitude of 20 kA. Selection and Configuration of Surge Arresters The general philosophy when selecting surge arresters for any particular system, entails matching the electrical and mechanical characteristics of the arrester with the system’s electrical demands and mechanical requirements. The following simplified flow chart demonstrates the general method and procedure for configuring an MO arrester. https://savree.com/en/encyclopedia/surge-arrester 5/7 11/11/22, 8:46 PM Surge Arrester Explained - saVRee Simplified Selection of Surge Arresters Flowchart The requirements for optimal and satisfactory selection of surge arresters dictate that arresters should provide an adequate protection margin and that they should also be suitable for stable continuous operation. An ‘adequate protection margin’ means that the device overvoltages are always below it’s withstand voltage, with a sufficient safety factor (safety margin). Whereas, ‘stable continuous operation’ refers to the arrester’s ability to handle all long-term, temporary, or transient stresses (which can be caused by system operation), whilst remaining electrically and thermally stable throughout its useful working life. Unfortunately, both of the adequate protection margin and stable continuous operation cannot be satisfied independently. A reduction in the arrester’s protective level (to provide a greater protective margin) inevitably results in higher electrical stresses during continuous operation. Also, the rated voltage of the arrester cannot be increased arbitrarily without raising its protective level (which results in a corresponding decrease in the protective margin). Thus a compromise is necessary, where both requirements are balanced to arrive at an optimal solution. Additional Resources https://en.wikipedia.org/wiki/Surge_arrester https://trimbox.com.tr/en/surge-arrester-working-principle Why saVRee? Privacy Policy (GDPR) LMS Services https://savree.com/en/encyclopedia/surge-arrester 6/7
