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Use of Metrosil Silicon Carbide Varistors for the Purposes of Overvoltage Protection in Synchronous Generator Exciter Discharge Systems By Silicon Carbide Surge Protection a division of Dr. Jeff Robertson Metrosil Technical Manager M&I Materials Ltd Hibernia Way Trafford Park Manchester M32 0ZD United Kingdom t: +44 (0)161 864 5449 f: +44 (0)161 864 5444 www.mimaterials.com © Copyright 2006 M&I Materials Ltd Specialist Materials for Industry & Science 2 www.metrosil.com Introduction to Metrosil Mature technology with over 70 years of applications Used in Exciter Discharge applications for many years Used in large static excitation systems and also smaller permanently connected / brushless systems Adopted worldwide by leading OEMs : ABB, Voith Siemens, Alstom, VA Tech and used in key projects 3 www.metrosil.com Function of a Metrosil In an Exciter Discharge Application To prevent large back EMF’s from damaging components & insulation systems in the excitation systems during discharge events Field Winding During a Discharge Event If V uncontrolled & Metrosil not present V = - L di dt If V controlled through a Metrosil V = CIβ Energy Stored in Field Winding E = ½LI2 Switched in Exciter Discharge Metrosil Units For large excitation systems exciter discharge system is usually switched in via a field breaker or a crowbar circuit Resistors disconnected from circuit under normal operating conditions Exciter discharge units utilise unspaced disc units High current and energy ratings 4 www.metrosil.com Metrosil Electrical Characteristics The equation describing the V-I characteristics of a Metrosil can be approximated to V＝CIβ or For Units With Different β Values For Units With Different C Values Log I = log C + β log V 5 www.metrosil.com Specification for an Exciter Discharge Application For the design of an appropriate Metrosil unit for a switched exciter discharge application, it is important to specify the following factors: (a.) Maximum Discharge Current (b) Maximum Discharge Voltage (c) Maximum Discharge Energy to be absorbed 6 www.metrosil.com Benefits of Using Metrosil In Excitation Systems Discharge Time Characteristics - - Non linear V-I characteristics allow rapid discharge times Smaller β value, the faster the discharge time Sharing of Current and Energy within Exciter Discharge Systems - Log V Log V Voltage Across Unit Under Discharge Conditions - ‘Matching’ determines how discs in series and parallel within a unit share current and energy Smaller β value, the more critical matching is Inappropriate matching or shifts in the characteristics of the discs may lead to unit failure Matching Voltage - 5% . ß =0.4 Log I Log I Nominal Test Current 13% Current Spread Metrosil combines the ideal characteristics of non-linearity and energy absorption capability for exciter discharge applications 7 www.metrosil.com Metrosil in Chinese Hydropower Projects M&I Materials is very proud to have been involved in providing Metrosil for several of the large hydroelectric projects in China, including: Project Generator Size Three Gorges Hydro Power Plant 26 x 700MW Pengshui Hydro Power Plant 6 x 300MW Qinghai Laxwa Hydro Power Plant 5 x 700MW Longtan Hydro Power Plant 6 x 700MW Guizhou Goupitan 5 x 500MW Pubugou Hydro Power Station 6 x 600MW Sichuan Ertan Hydro Power Station 6 x 550MW Yunnan Jinghong Hydro Power Station 5 x 350MW Xiaowan Hydro Power Station 6 x 700MW 8 www.metrosil.com Metrosil in Chinese Hydropower Projects Three Gorges Hydro-power Station 700MW hydro-generator Used on a Siemens Thyripol Static Excitation System Energy rating of 18.9MJ Metrosil 600A/US252/126P/2S/ Spec 6361 discharge varistor units 9 www.metrosil.com Summary With over 70 years experience, M&I materials have a proven track record with the high quality of Metrosil and with it’s application to exciter discharge applications. Knowledge of the operational parameters and expertise in the manufacturing processes is key to the success of Metrosil The proven track record is reflected through the supply of Metrosils to the worlds leading OEMS, such as ABB, Alstom, Siemens, Areva and VA Tech as well involvement with several leading flagship projects, such as the Three Gorges Metrosil is a highly reliable, low maintenance product, which may be used to protect both large medium and small excitation systems. In comparison with other technologies, it has been shown that Metrosil offers the best solution to exciter discharge applications. M&I materials shares a belief in working with our customers to provide them the best solution for their applications 10 www.metrosil.com Xiexie Any Questions,please send an e-mail to [email protected].